Process for the preparation of expanded polyvinylarene particles in which polyvinylarene particles pre-expanded to an apparent density do ranging between 600 and 200 kg/m3 are impregnated by an inorganic gas, and the impregnated particles thus obtained are expanded to an apparent density of at least three times lower than do.

The present invention relates to a process for the preparation of expanded polyvinylarene particles and to a process for the preparation of foamed articles from these expanded particles.
For many years it has been known that particles of polyvinylarenes, such as polystyrene, can be rendered expandable and that the particles thus obtained can be used in the preparation of foamed articles. In this respect reference is made to, e.g., U.S. Pat. No. 2,681,321 which discloses a process in which polystyrene particles are exposed to liquid hydrocarbons and treated such that the liquid hydrocarbon is dispersed in the polystyrene particles. Particles thus prepared contain generally 4 to 8% by weight of such liquid hydrocarbon, such as butane, n-pentane or mixtures of pentanes. Also halogenated hydrocarbons have been used for this purpose. These particles can then be expanded to particles with a reduced density. Apparent densities for packaging particles typically are 20 to 60 g/l. Once expanded, the particles are fused in a steam-heated mould to yield a foamed article of a desired shape.
One of the factors that influence the expansion of the polystyrene particles is the amount of hydrocarbon blowing agent. From Kirk Othmer, Encyclopedia of Chemical Technology, third edition, Volume 21, page 838, it can be read that the density of particles containing 5.7% wt n-pentane is typically 1080 kg/m3, compared to a value of 1050 kg/m3 for pure polystyrene beads and compared with a calculated density of 1020 kg/m3 for a simple mixture in which the n-pentane is dissolved in polystyrene. If all pentane would be in voids the calculated density would be 1120 kg/m3. Thus it has been suggested that part of the hydrocarbon blowing agent is present in little voids in the polystyrene. The skilled man will appreciate that the above densities are particle densities, which can be recalculated to apparent densities. A particle density of 1080 kg/m3 corresponds to an apparent density of around 720 kg/m3.
A drawback of the present practice is that during the transport and storage of the unexpanded particles hydrocarbons may evaporate from the particles, in particular from the voids. When the particles are transported and/or stored at varying temperatures and/or duration, the amounts of e.g. pentane retained may vary significantly. Apart from extra safety measures that have to be taken during transport, like gas-tight packaging, it will be appreciated that such a variation may have an effect on the resulting foam obtained after expansion.
Furthermore, the expansion process itself also causes that hydrocarbons originally present in the unexpanded particles are emitted into the environment. In order to reduce the emissions complicated equipment has been developed to collect the emitted hydrocarbons for further handling, e.g. combustion. This equipment is to be installed in the facilities of the end-user of the particles, i.e. the customer who produces the foamed articles. This requires additional expertise and investments with these customers.
The present invention intends to eradicate the above drawbacks by providing a process for the preparation of expanded polyvinylarene particles in which polyvinylarene particles pre-expanded to an apparent density do ranging between 600 and 200 kg/m3 are impregnated by an inorganic gas, and the impregnated particles thus obtained are expanded to an apparent density of at least three times lower than do.
The advantages of the present invention are enormous. At the facilities of the polyvinylarene manufacturer, unexpanded polyvinylarene polymer particles containing a blowing agent are prepared and the latter particles are pre-expanded to a reduced apparent density in the range from 600 to 200 g/l. If this density reduction is obtained by the use of a hydrocarbon blowing agent, the emitted blowing agent can be collected and handled further, (e.g. re-used) in the plant of the polyvinyl-arene manufacturer.
Secondly, the polyvinylarene manufacturer has complete control over the product quality in the period between production and use by the customer. Using the process of the present invention, the polyvinylarene manufacturer will be in a position to extract at least the blowing agent from the voids. If he has done so, there will be no loss of blowing agent during transport and/or storage of the pre-expanded particles. This will ensure that the customer will always obtain pre-expanded particles with a consistent cell structure and with consistent foaming properties.
Because the pre-expansion has been conducted such that the apparent density do ranges from 600 to 200 kg/m3, the volume increase compared to the volume of the original unexpanded particle is from about 1.5 to less than 3 times. This increase is so small that the transportation costs do not become such that they outweigh the advantages. On the other hand, the volume increase does ensure that pores are present in the pre-expanded particle having such a size that the customer can impregnate a reasonable amount of inorganic gas into them, in order to render the particles sufficiently expandable.
The pre-expanded polyvinylarene particles of the present invention have an apparent density do of 600-200 kg/m3. Preferably, the apparent density do ranges from 530 to 250 kg/m3, more preferably from 500 to 300 kg/m3 and most preferably from 450 to 350 kg/m3.
The pre-expanded particles can be obtained by pre-expansion of any unexpanded polyvinylarene particle containing a sufficient amount of blowing agent to arrive at the apparent densities as defined. The unexpanded polyvinylarene particles may be prepared by various methods, including solution polymerisation, bulk polymerisation, suspension polymerisation or mixtures of these methods. The blowing agent may be added after the polymerisation, as described in U.S. Pat. No. 2,681,321. It is also possible to add the blowing agent during the polymerisation of the vinylarene monomers or to add it before the polymerisation to the monomers. Preferably, the blowing agent is added during the polymerisation of the vinylarene monomers, the polymerisation advantageously being conducted in suspension. The unexpanded polyvinylarene particles suitably have an average particle size of 0.2-3 mm.
Suitable particles for use in the present invention are e.g. described in U.S. Pat. No. 3,973,884, which describes polymer particles with a relatively high density. These high density polymer particles are obtained by pre-expansion of unexpanded polymer particles containing 5.8-7.0% by weight pentane. From GB patent No. 1 106 143 and PCT application No. WO 98/01489 it is known that water may also be used as blowing agent. Apart from hydrocarbons, e.g. C2-C6 hydrocarbons or halogenated hydrocarbons, or water, other blowing agents may be used as well. Examples are inorganic blowing agents (carbon dioxide) or so-called chemical blowing agents, i.e. compounds that release gaseous components upon heating. The latter compounds are usually solid and are particularly used in bulk polymerisation techniques. Examples of chemical blowing agents are carbon dioxide-or nitrogen-liberating solid compounds like azodicarbonamide.
U.S. Pat. No. 3,973,884, mentioned in the previous paragraph, specifically describes that for the production of particles for higher-density applications it is possible to use less of pentane or other blowing agent than has been used in particles for low-density applications. However, it continues by saying that the difference is small and that in particles for low-density applications the pentane content may be 6-7.2% wt, which is hardly more than the 5.8 to 7.0% by weight pentane content of the particles for high-density applications.
It has now been found that smaller amounts of hydrocarbon blowing agent also enable a density reduction to a level of between 600 and 200 kg/m3. The resulting pre-expanded particles are extra advantageous in view of environmental and safety concerns during transport and storage. Therefore, it is preferred to prepare the pre-expanded particles according to the invention by using a process in which expandable polyvinylarene particles containing from 0.5 to 4% by weight of a volatile organic blowing agent, based on polyvinylarene, are pre-expanded to an apparent density of 600 to 200 kg/m3. A most preferred particle for use in the present invention is the porous particle as described in Applicant""s co-pending application No. 98203099.1. This particle contains 2.0% by weight or less of a volatile organic blowing agent, based on the weight of polyvinylarene, preferably less than 1.5% by weight. This particle can be obtained by e.g. pre-expansion of an unexpanded polyvinylarene particle which is prepared by aqueous suspension polymerisation of vinylarene monomers in the presence of a nucleating agent and from 0.1 to 1% by weight of a free radical initiator, wherein a C2-6 hydrocarbon blowing agent is added before, during or after the polymerisation, wherein the amount of blowing agent is from 0.5 to 4% by weight, based on the amount of vinylarene. Alternatively, they may be prepared by pre-expansion of the particles obtained by a process as described in Applicant""s co-pending application No. 98203098.3. Herein, compact polyvinylarene polymer particles are impregnated by an inorganic N2- and/or O2-containing gas at a temperature below 95xc2x0 C. and at a pressure of 100 to 2,000 kPa gauge, to yield expandable polyvinylarene particles.
Pre-expansion of an unexpanded polyvinylarene particle into the pre-expanded particle can be conducted in any suitable manner. Suitable well known methods are the use of hot air, a hot oil bath, infra red radiation microwave radiation or steam. Steam may be used of temperatures of 100 to 168xc2x0 C. at pressures of 0 to 600 kPa gauge, depending on the presence of additives and/or other polymers in the polyvinylarene particle. In the case of polystyrene, it is preferred to use saturated steam of a temperature of 100-125xc2x0 C. at pressures of 0 to 230 kPa gauge. The unexpanded polyvinylarene particles may also be pre-expanded by exposing them to warm water. This method is preferred. In this embodiment the water suitably has a temperature of from 60 to 100xc2x0 C. and the exposure has a duration of 5 to 120 minutes. The most preferred method to pre-expand the unexpanded poly-vinylarene particles in the present invention is the use of hot air, having a temperature of from 90-200xc2x0 C., preferably from 95-160xc2x0 C. and most preferably from 100-140xc2x0 C. The exposure preferably has a duration of up to 3 hours.
A further preferred particle for use in the present invention is a particle that has been pre-expanded xe2x80x9cin-situxe2x80x9d. Hereto, styrene is bulk polymerised in an extruder in the presence of a blowing agent. When the hot polymer melt exits the extruder, the blowing agent will be released to effect pre-expansion to a density of 600xe2x88x92200 kg/m3. The slightly expanded strands of polymer are then cut to obtain the particles for use in the present invention. In this technique, it will be preferred to use carbon dioxide or chemical blowing agents.
The vinylarene monomer comprised in the polymer of the present process consists preferably mainly of styrene. The polyvinylarene may contain up to 10% mole of another vinyl-group containing monomer, such as acrylonitril, acrylic or methacrylic acid or esters, substituted styrene, such as chlorostyrene, or xcex1-methyl styrene, or divinylbenzene. However, preferably the vinylarene in the polyvinylarene consists for more than 99% mole of styrene. More preferably, the polyvinylarene is polystyrene.
The polymerisation per se is well known in the art. It may be initiated thermally, via free-radical polymerisation or via anionic polymerisation. Although both methods are equally possible, preference is given to free-radical polymerisation. Suitable free-radical initiators can be selected from conventional initiators for free-radical polymerisation. They include in particular organic peroxy compounds, such as peroxides, peroxy carbonates and peresters. Typical examples of these peroxy compounds are C6-20 acyl peroxides, such as decanoyl peroxide, benzoyl peroxide, octanoyl peroxide, stearyl peroxide, peresters, such as t-butyl per benzoate, t-butyl peracetate, t-butyl perisobutyrate, t-butylperoxy-(2-ethylhexyl)carbonate, hydroperoxides and dihydrocarbyl peroxides, such as those containing C3-10 hydrocarbyl moieties, including di-isopropyl benzene hydro-peroxide, di-t-butyl peroxide, dicumyl peroxide or combinations thereof. Other initiators different from peroxy compounds are also possible, e.g., xcex1,xcex1xe2x80x2-azobisisobutyronitrile.
The suspension polymerisation is suitably carried out in the presence of suspension stabilisers. Suitable suspension stabilisers are well known in the art and comprise poly(vinyl alcohol), gelatine, agar, polyvinyl pyrrolidine, polyacrylamide, inorganic stabilisers such as alumina, bentonite, magnesium silicate or phosphates, like tricalciumphosphate and/or disodiumhydrogen phosphate, optionally in combination with any of the stabilising compounds mentioned earlier. The amount of stabiliser may suitably vary from 0.1 to 0.9% wt, based on the weight of the aqueous phase.
The suspension polymerisation is suitably carried out at two temperature stages, in which the temperature in the first stage is from 85 to 110xc2x0 C. and in the second stage is from 115 to 140xc2x0 C.
It may be advantageous to polymerise the vinylarene monomers in the presence of other polymers such as polyphenylene oxide or elastomeric polymers. Suitable polyphenylene oxides have been described in EP-A-350137, EP-A-403023 and EP-A-391499. The polyphenylene oxide is preferably present in an amount between 1 and 30 wt %, based on the amount of vinylarene monomers, and may improve the rigidity of the polyvinylarene polymer. Examples of suitable elastomeric polymers have also been described in EP-A-350137 and comprise (block) copolymers of vinyl substituted aromatic monomer and a conjugated diene monomer. These elastomeric polymers are preferably present in an amount of 0.5 to 10 wt %, based on the amount of vinylarene monomers, and may improve the impact strength of the polyvinylarene polymer.
The unexpanded and/or pre-expanded polyvinylarene particles may contain various conventional additives. Such additives include chain transfer agents, cross-linking agents and nucleating agents. Suitable examples of chain transfer agents are C2-15 alkyl mercaptans such as n-dodecyl mercaptan, t-dodecyl mercaptan, t-butyl mercaptan and n-butyl mercaptan. Other agents are pentaphenyl ethane and the dimer of xcex1-methyl styrene. Examples of crosslinking agents are butadiene and divinylbenzene. Nucleating agents are agents that promote cell formation and are suitably used in an amount of 0.01 to 3% by weight, based on vinylarene, preferably in an amount of 0.05 to 2% by weight. Examples of nucleating agents are finely dispersed inorganic compounds, polymer particles and organic solids. Examples are carbonates compounds such as calcium carbonate, sulphate compounds such as barium sulphate and calcium sulphate, silicate compounds such as talc, clay, magnesium silicate, amorphous silica particles, zeolites, diatomaceous earth, oxides such as magnesium oxide, and titanium oxide, mixtures of sodium bicarbonate with citric acid, organic bromine-containing compounds, naphthalene compounds, polycyclic aromatic hydrocarbons, carbon black, cokes, chars, graphite and diamond dust, paraffin and fatty acid derivatives such as stearate and palmitate compounds. Examples of suitable polymer particles are polyvinyl-chloride, polypropylene, polyethylene, acrylonitril butadiene styrene rubber, styrene butadiene rubber, styrene/maleic anhydride copolymer and cellulose. Further examples include polar polymers as described in e.g. WO 98/01501 which comprise e.g. starch, and starch modified by esterification or etherification, emulsifiers as described e.g. WO 98/01488 and WO 98/01489 which comprise bisalkylsulphosuccinates, sorbitol-C8-C20-carboxylates, and C8-C20-alkylxylene sulphonates. Particularly suitable as nucleating agent are polyethylene waxes having a weight average molecular weight of 500 to 5,000, which are typically finely divided through the polymer matrix in a quantity of 0.01-1.0% by weight, based on the amount of vinylarene, preferably from 0.1 to 0.5% by weight.
The particles may also contain anti-static additives, flame retardants such as hexabromocyclododecane, dyes, filler material, stabilisers, plasticizers, such as white oil, and lubricants. The particles are suitably coated with coating compositions comprising silicones, silicates, metal or glycerol carboxylates. Suitable carboxylates are glycerol mono-, di- and tri-stearate, zinc stearate, and mixtures thereof. Examples of such compositions have been disclosed in GB patent No. 1,409,285. Instead of stearate, one may also use citrate or palmitate. The coating compositions have been applied to the particles via dry-coating in a ribbon blender or via a slurry or solution in a readily vaporising liquid.
The pre-expanded particles are impregnated with an inorganic gas to yield impregnated particles. The pores in the pre-expanded particle are of such a size that a reasonable amount of inorganic gas can be impregnated into the pores of the particle. By xe2x80x9cinorganicxe2x80x9d gas it is meant that the impregnating gases according to the present invention may contain at most 1% by volume, based on the volume of the gas, of organic compounds, preferably at most 0.5% by volume. Most preferably, the gases according to the present invention do not contain any organic compounds. An example of a suitable inorganic gas is carbon dioxide. However, this interferes with the polyvinylarene matrix. Like many commercial blowing agents it dissolves to some extent into the polymer matrix. This means that for some applications it will have to be removed with effort in view of potential negative effects, e.g. in the field of safety, health or toxicology. Carbon dioxide, also being a well-known green house gas, is therefore not preferred. Preferred are inorganic gases that have no such a negative effect and that show less interaction with the polymer matrix. Examples of such gases are inorganic N2- and/or O2-containing gases, helium, neon and argon. More preferably, the inorganic gas used for the impregnation is selected from inorganic N2- and/or O2-containing gases. These gases suitably contain more than 90% by volume, based on the volume of the gas, of N2 and/or O2, more suitably more than 95% by volume. Most preferably, the gas is nitrogen or air. Not only do these gases hardly interfere with the polymer matrix, but they are also effective and cheap and have no negative environmental or health impact.
The impregnation can be conducted in many ways. However, it is preferred to impregnate the pre-expanded particles by the inorganic gas by exposing the particles to the gas at temperatures ranging from 0 to 95xc2x0 C. Preferred temperature ranges are from 0 to 50xc2x0 C., more preferably from 10 to 30xc2x0 C. Most preferably, the temperature used is room temperature. This way the voids in the particles are filled with the gas without the polyvinylarene being heated such that it deforms. Such deformation might have a detrimental effect on the structure and properties of the voids and thereby it would have a negative impact on the expandability of the resulting impregnated particles. Moreover, the low temperature ensures that the particles remain free-flowing and do not stick to each other, which might occur if the impregnation would be conducted at higher temperatures.
The impregnation is suitably such that in the pores of the porous particle a pressure of 100 to 1,500 kPa gauge is achieved. Preferred pressures in the pores are between 200 and 1,000 kPa gauge, more preferably between 300 and 800 kPa gauge. Lower pressures than 100 kPa gauge would mean that the voids would merely be filled with gas, e.g. nitrogen or air, at about atmospheric pressure. Such a replacement would result in a insufficient expansion, if any. Pressures higher than 1,500 kPa gauge are possible, but these are undesirable for economical and safety reasons. The external pressure applied, needed to establish the desired pressure in the pores, is preferably from 100 to 2,000 kPa. Although it is possible to use higher external pressures, this would require better equipped pressure vessels and it would make sampling more difficult. Preferably, the maximum external pressure employed is 1,500 kPa. Suitably, the external pressure applied is the same as the desired pressure in the pores of the porous particle.
After the impregnation the impregnated particles can be taken to a conventional expansion unit and expanded further to at least 3 times the volume of the pre-expanded particle. There is no necessity to take any specific precautions in taking the particles to the expansion unit. However, it could be useful to do such via a pressure sluice.
In the expansion unit the impregnated particles are expanded to an apparent density which is at least 3 times lower than the original apparent density. Preferably, the impregnated particles are expanded to an apparent density of at least 5 times lower than do. Although the expansion can be conducted to any apparent density desired, it is practical to conduct the expansion to an apparent density which is up to 20 times, more preferably up to 40 times lower than do. It may be advantageous to carry out the expansion process in steps. Thereto, the expanded polyvinylarene particles obtained after the expansion are advantageously again impregnated (xe2x80x9cre-impregnatedxe2x80x9d) with an inorganic gas and the thus re-impregnated particles are again expanded. The process of re-impregnation can be repeated up to a number of times. However, the skilled artisan will strive to a balance between the duration of the impregnation in order to maximise the amount of impregnated gas on the one hand and a low number of repetitions of the impregnation and expansion sequence on the other. Suitably this will lead to a process in which the impregnation and expansion steps are repeated between 1 and 4 times.
As already indicated the expansion can be conducted in any conventional expansion unit. Suitable methods are the same as the well known methods as described for the pre-expansion. Preferred is the use of steam as expansion method.
As already indicated in the description above, expanded particles are suitably put into a mould and heated so that the expanded particles fuse together to produce foamed moulded articles. Therefore the invention also provides a process for preparing a foamed article in which expanded polyvinylarene particles obtained by a process according to this invention are heated in a mould till the polyvinylarene particles soften and stick together, and the heated mould thus obtained is cooled to yield a foamed article. The heating in the mould is conventional and is typically in the range from 110 to 125xc2x0 C.


An improved simulated authentic divided-light windows, includes an internal muntin grid structure sandwiched between the panes of a double-glazed sealed window pane, and an external muntin grid removably attached to each side of the sealed window pane. The internal grid is equipped with multiple neodymium magnets, which are embedded in the grid structure at regular intervals, while the back side of each of the outer muntin grid structures is equipped with multiple steel or iron inserts which align with the neodymium magnets when the outer grids are positioned, as intended, on the sash. Outer grids used on the inside of a building can be made of wood or plastic. Outer grids used on the exterior of building are preferably made of extruded aluminum or other weather resistant material. The steel or iron inserts are threadably secured or adhesively bonded to apertures in the back side of the grid.

A muntin is a strip of wood or metal separating and holding panes of glass in a window. Muntins can be found in doors and windows of certain styles of western architecture. The combination of muntins and glass creates a grid system dividing a single sash or casement into smaller panes, called "lights" or "lites". Until the middle of the 19th century, large panes of glass were so expensive to manufacture that it was economically advantageous to combine multiple smaller panes of glass in a grid for the manufacture of windows and doors having large light-transmissive expanses.
So-called true divided-light residential windows typically make use of thin muntins which range from ½-inch to ⅞-inch in width. In windows constructed with wood framing, a fillet is cut into the outer edge of the muntins so as to provide a seat for a pane of glass within each opening of the grid. Putty or thin strips of wood or metal are then used to hold each pane in place. The inner sides of wooden muntins are typically milled to traditional profiles. In the U.S., the thickness of window muntins has varied historically, ranging from very slim muntins for use in 19th-century Greek revival buildings to thick muntins for use in 17th- and early-18th-century buildings.
In spite of the fact that the muntins of divided-light windows interfer with the view, many consider such windows to be more architecturally attractive than those of the single-pane variety. In addition, divided-light windows are inextricably connected with particular styles of architecture, and any attempt to build in these styles using single-pane windows is likely to be viewed not only as an exercise in poor taste, but as a cheap substitute for the genuine article. It would be unimaginable, for example, that anyone having even a modicum of architectural sensibility would attempt to build a colonial style structure using single-light windows. In addition, restoration of period-built structures requires adherence to architectural rules of that period.
The problem of maintaining architectural authenticity was compounded by the oil energy crisis of the 1970's that resulted from an oil embargo initiated by certain petroleum producing countries in 1973 in response to U.S. support for Israel during the Yom Kippur War. Single-pane windows suddenly became obsolete because their insulative R value was a mere 0.9. For structures having large expanses of single-pane windows—and this characteristic typically applied to colonial style buildings—this abysmal R-value rating guaranteed unacceptable levels of heat transfer from the interior to the exterior during winter months and in the opposite direction during summer months. Although single-pane windows are well-adapted to traditional muntins, double-pane windows are more than twice as heavy because a thick gasket, positioned around the edges of the assembly, not only maintains the separation of the double panes, but also seals the dead space between them. A true divided-light, double-pane window would therefore require the use of much heavier and considerably more expensive muntins. In comparison to the muntins of a divided-light window made of single panes, such muntins would, indeed, appear ungainly.
In an effort to avoid the high cost and unattractive appearance of true divided-light double-pane windows, window manufacturers began to supply double-pane windows having bar grills positioned between the spaced-apart double panes. Alternatively, they produced removable contoured grills which, generally, were attachable to the interior side of the double-pane glazing. Under certain, unexceptional lighting conditions, fake muntins of both types can be detected from great distances by casual observers. In fact, fake muntins have come to be regarded as a halmark of tawdry construction.
The only reasonable alternative to prohibitively-expensive and ungainly true divided-light double- and triple-pane windows, on one hand, and fake muntin grills on a large, single-light window, on the other, is a window structure accurately described as a simulated-authentic, divided-light window. The structure consists of a double- or triple-glazed sash having an authentically-detailed muntin assembly adhered to each side thereof. Muntin assemblies exposed to the elements are generally made of aluminum extrusions or other non-weathering elements. A final and essential element of the simulated-authentic window structure is the inclusion of a spacer grid between each adjacent pair of glass panes. The spacer grid not only strengthens the multi-pane sash, but also makes it appear—even on close inspection—that both muntin assemblies and the spacer grid(s) are unitary and physically separate the individual lights. Any multi-light window having any less a structure appears cheap and fake.
A major drawback associated with true, divided-light windows of both single- and multi-pane varieties is the intensive manual labor required both to clean the panes of the individual lights several times each year, as well as to paint the muntins every five to ten years. Simulated divided-light windows suffer from the same drawback if the muntin assemblies are permanently adhered to the outer surfaces of the sash. It has been estimated that the time required to clean both sides of a divided-light window can be as much as ten times that required to clean a single-light windows of the same size. The spray painting of removable muntin grids is a simple task, whereas the painting of a muntin grid without removal requires extensive masking and/or careful painting with a small brush. Thus, simulated-authentic, divided-light windows having removable outer muntin assemblies have a tremendous advantage over windows which are not temporarily convertible to single-light windows, both from the standpoint of manufacturing cost and the cost of ongoing maintenance. Nevertheless, removable muntin assemblies must be reliably secured to the sash. Muntin assemblies attached to the exterior surface of a sash must withstand exposure to the wind and the elements. If a mechanical muntin securing mechanism is employed, it must not degrade significantly over time. If one were to fall off the sash, it could not only pose a danger to a nearby object or person, but could also be damaged by impact with the ground.
A number of methods for constructing simulated divided-light windows have been devised over the years which have transpired since the Yom Kippur War. U.S. Pat. No. 4,783,938 to Douglas J. Palmer, for example, discloses a simulated divided-light window structure that is compatible with insulated double-glazed windows. The structure makes use of an internal grid structure and two external grid structures to achieve a realistic divided-light appearance. This structure does nothing to reduce the labor required to clean such windows, as both external grid structures are adhesively bonded to the glass panes. U.S. Pat. No. 5,077,950 to Donald D. Bretches, et al., on the other hand, discloses a simulated divided-light window structure for single-glazed windows, in which a pair of simulated muntin grids are attached to opposite sides of a pane through mutual magnetic attraction. There are several problems associated with this structure. The first is that the method is inapplicable to simulated muntin grids fabricated from hollow extruded aluminum members, as the method requires a magnetic or ferromagnetic strip to be adhesively bonded within groove formed on the back side of each muntin member. The method is also subject to adhesive bonding failure over time. Furthermore, the method will not work with double pane windows, as insufficient magnetic force is generated by a permanent magnet to bridge the gap between the panes.
What is needed is an improved structure for authentically simulating true divided-light windows. As with the U.S. Pat. No. 4,783,938, the structure must employ an internal grid structure in combination with a pair of external grid structures in order to achieve authentic simulation. However, the external grids structures must not only be secured by means which will not degrade over time, but they must be readily and quickly removable to facilitate window cleaning operations.

Divided light windows having magnetically-attached grids


A circuit and method for reading a multiple-level floating-gate memory is provided. The reading is done by a gate bias voltage VP that is equal to the voltage needed to obtain a predetermined reference current Iref in the selected storage transistor. The decoding of the stored data element is done by the decoding of the bias voltage VP. Thus the circuit and method reduces the current flowing through the transistors during the reading and reduces the mean electrical stress undergone during each read operation.

The invention relates to a multiple-level floating-gate memory, and more particularly, the invention relates to a memory in which the basic storage element is a floating-gate transistor that must store a piece of information whose value can take one of at least three states.
Multiple-level floating gate memories are used to increase storage capacity without increasing the surface area of the storage matrix. This principle has been disclosed, for example, in the applications EP-A-0 340 107 (D1 here below) and EP-A-0 656 628 (D2 here below). The standard memories generally store one bit, namely two electrical levels, per storage element. The multiple-level memories store a larger number of electrical levels. Thus, when a multiple-level memory with four electrical levels is made, this allows the storage of two bits per storage element, the storage capacity in this case being doubled for one and being the same matrix size.
To obtain the different electrical levels, the floating gate of the storage transistor is charged in varying degrees. The gate that is charged in varying degrees has the effect of bringing about a variation in the conduction threshold voltage of the storage transistor. In order to write in such cells, a sequence of programming/reading/verification cycles is performed until the desired programming level is obtained. The reading is done by applying a read voltage to the storage transistor which makes this transistor more conductive or less conductive depending on the charge present at the floating gate. The channel of the storage transistor is connected to a load which converts the current flowing through the storage transistor into a voltage. The voltage is then compared to different thresholds to determine the stored state.
However this type of memory has drawbacks. Two of these drawbacks are, firstly, high power consumption and, secondly, risk of deterioration of the data. The high consumption is due, inter alia, to the voltages and currents used during reading operations. Indeed, during a reading operation, the transistors are biased to obtain different currents in their drain-source channel, with the current representing the stored information. Now, to have better readability of the different values stored, read currents are used with values three times greater than in the case of standard floating-gate memories.
The structure of a multiple-level memory matrix is the same as that of a standard memory matrix. FIG. 1 shows the current-voltage curve of a bit line with a given basic bias voltage. The curve of FIG. 1 results from different programming levels for a given bias of the bit line. The reading is then done either in current or in voltage. The currents IR1, IR2 and IR3 represent the current thresholds that differentiate the storage levels. For further details, those skilled in the art may refer, for example, to D1.
The risk of deterioration of the data arises from the use of several data thresholds which increases the risks of deterioration of the data stored in a floating-gate transistor. The risk of deterioration of the data is also increased by the greater electrical stress undergone by each storage transistor. Indeed, the use of several programming levels requires the use of a substantial gate bias voltage (of about 4 to 6 volts). This favors charge migrations. The phenomenon is further accentuated during the reading of a highly conductive storage transistor.
FIG. 2 shows an example of characteristic curves of current going through the channel of a storage transistor as a function of the bias voltage, for the storage thresholds. In this example, the threshold current IR3 corresponds substantially to the conduction current of the storage transistor at the beginning of the state when the conduction is linear. This current IR3 corresponds to the threshold current used for a binary type of memory. Those skilled in the art can see that the spacings between the characteristics corresponding to the different thresholds do not follow a linear pattern. This stepped feature is partly due to the stress on the memories. Indeed, the first threshold characteristic 1 corresponds to the characteristic curve used for a binary type memory. It is difficult to shift this characteristic because it is far too low. This leads to a continuous conduction of the storage transistor (giving rise to malfunctioning for Flash-EEPROM type memories).
The second and third characteristics 2 and 3 are shifted to have high readability between the programming levels. Furthermore, to obtain a reading, it is appropriate to bias the bases of the storage transistors at a voltage such that all the programming levels can be read. The bias voltage therefore corresponds to the voltage that produces the threshold current IR3 for the third characteristic 3. This bias voltage brings about a correspondence of the bigger currents for the first and second characteristics 1 and 2.
In the example shown, the bias voltage is set at 4.5 VT with VT corresponding to the conduction voltage of an N channel MOS transistor, namely about 1.1 V. Now this bias voltage corresponds to about half the erasure voltage (or programming voltage) of certain storage transistors. Those skilled in the art will appreciate that the current flowing in the channel of the transistors whose programming corresponds to the levels xe2x80x9c11xe2x80x9d or xe2x80x9c10xe2x80x9d is fairly great and that a low-amplitude Fowler Norheim effect can occur. The likelihood of this effect occurring increases as the programming level is low, and has the effect of lowering the programming threshold. To prevent this, the programming levels are further spaced out as they are low. The transistors whose programming level is lower than the first characteristic (level 11) are the most subject to reading stress. In the case of a Flash-EEPROM memory, a depletion phenomenon (giving rise to an always-on transistor) may make the memory unusable.
In D1 and D2, the different approaches are presented to limit the risks of use. D1 proposes the use of several reading thresholds, some of which can be used to verify that the writing has been rightly done while others can be used to carry out a reading operation. The thresholds that can be used for the verification of reading are more restrictive that they are for simple read operations. D2 proposes a memory refresh device to compensate for programming drifts.
To overcome these problems, the invention proposes to limit the bias voltage and the current flowing in a storage transistor. Thus, according to the invention, the reading is done by a gate bias voltage that is equal to the voltage needed to obtain a predetermined reference current in the selected storage transistor. The decoding of the stored piece of data being done by the decoding of the bias voltage. Thus, the invention reduces the current flowing through the transistors during the reading, and reduces the basic biasing electrical stress undergone during each reading.
An object of the invention is to provide an integrated circuit comprising a memory that includes a plurality of bit lines, a plurality of word lines, and a plurality of floating-gate transistors placed at the intersections of the word lines and the bit lines. Each floating-gate transistor stores an electrical state representing a stored piece of data. The memory also includes at least one circuit to compare a current flowing through a selected bit line with a reference current, a voltage generator circuit that generates a growing bias voltage to a gate of at least one floating-gate transistor so long as the current flowing through the bit line is smaller than the reference current, and a decoding circuit that outputs digital information representing the stored piece of data by decoding the value of the bias voltage.
Another object of the invention is also a method for the reading of a memory comprising a plurality of bit lines, a plurality of word lines, and a plurality of floating-gate transistors placed at the intersections of the word lines and the bit lines. Each floating-gate transistor stores an electrical state representing a stored piece of data, wherein the current flowing through a selected bit line is compared with the reference current. A growing bias voltage is generated as long as the current flowing through the bit line is smaller than the reference current. The value of the bias voltage is decoded to give a digital value representing the stored piece of data.


A combination wear protection film for components made of substrates susceptible to wear is described. The film has a wear-resistant surface film that contains one or more of the elements carbon, nitrogen, or boron, the proportion of this elementxe2x80x94or, if more than one of these elements is present, the sum of the proportions of the elements presentxe2x80x94is at least 30% by atomic weight, preferably at least 40% by atomic weight; and a support film, located between a substrate and the surface film and made of a plasma polymer, that exhibits a quasi-continuous transition in modulus of elasticity and optionally in hardness from a value of a base film to a value of the surface film. A method for producing a wear protection film of this kind, and objects equipped with the wear protection film is also described.

The present invention relates to a combination wear protection film and methods for generating and using the combination protection film in technical devices, in particular mechanical energy transfer mechanisms.
Practically all objects are subjected to a greater or lesser degree of wear due to interactions with their environment. As a rule, this wear results over time and decreases in the utilitarian value or functionality of the object. Efforts have therefore always been made to configure technical components, i.e. objects having a technical function, in such a way that when used as intended they exhibit as little wear as possible and thus can fulfill their purpose for as long as possible.
Depending on the manner in which they interact with their environment, components are subjected to very different wear stresses that are to be counteracted with very different measures.
In the case of components that do not need to transmit any appreciable energy, wear occurs essentially due to the action of light, air, climatic influences, and/or particle impact. The principal task here is to equip the objects with nonporous, scratchproof surfaces. One conventional method that equipped a wide variety of surfaces with polymer coatings is the plasma polymerization method. In this method, a plasma is generated by electrical excitation in a high vacuum using saturated or unsaturated compounds that can be vacuum-evaporated. From the monomer compounds, fragments such as gaseous radicals, radical ions, ions, and excited molecules form in the plasma and are deposited onto a substrate, on which they form a highly crosslinked polymer film that constitutes a sealed coating. Detailed information about implementation of this method is provided, for example, in Ullmanns Encyclopedia of Industrial Chemistry, Vol. A20, pages 755-756, 5th ed. and the literature cited therein, Comprehensive Polymer Science 4, pp. 357-375 and Encyclopedia Polymer Science and Engineering. 11, pp. 248-261.
An interesting possibility, based on this method, for depositing a transparent scratchproof coating onto plastics such as polymethyl methacrylate or polycarbonatexe2x80x94as used, for example, in the manufacture of automobile headlightsxe2x80x94is Schutzschichten durch Plasmapolymerisation (Protective Film Using Plasma Polymerization) of Bosch Technische Berichte (Bosch Technical Reports) 8 1986/87 published in Handbuch Plasmapolymerisation (Plasma Polymerization Manual), VDI-Bildungswerk (VDI Education Text), 1990. According to. this publication, plasma polymerization of HMDS(O) is used to produce on the plastic surface a plasma polymer film that is relatively soft directly on the plastic surface, but becomes harder and harder with increasing thickness and terminates in a hard, quartz-like surface film. In practical terms, this hardness gradient is produced by the fact that plasma polymerization is initially performed in the presence of very little oxygen, and the oxygen partial pressure is continually raised as the film thickness increases.
Conditions are even more complex in mechanical energy transfer systems, which have components that are exposed not only to the kind of mechanical stress that occurs in the case of headlight but also to lenses, for example stone impact, abrasion wear particularly high and complicated loads. Components of this kind are exposed to shear forces, impact stresses, high pressures, and in particular to sliding frictional forces as well as long-term vibratory effects, which act on the elements either individually or (as a rule) simultaneously, albeit to different degrees, and result in more or less rapid wear.
One solution that at first glance appears obviousxe2x80x94i.e. to manufacture all the functional elements from materials that can withstand the various wear stressesxe2x80x94runs into considerable and often insurmountable difficulties, either because such materials are not conventional for suitable materials are much too expensive or very difficult or impossible to shape or machine into the desired components.
Many attempts have therefore been made to manufacture the functional elements of mechanical energy transfer systems as well from easily shaped and (whenever possible) also inexpensive materials, and to impart the necessary wear resistance to them by way of a surface finish.
Different protective measures are necessary depending on the type of effect that causes the wear. It is conventional, for example, for drills not only to be equipped with hard metal cutting edges, but also vacuum-coated with hard-material coatings, for example titanium nitride. In highly stressed energy transfer elements, however, very high demands are placed on these surface protective coatings. Desirable properties include, for example, good adhesion to the component, good cohesion and minimal stress within the film, high hardness and load-carrying capability, minimal coefficient of friction, good surface smoothness, and minimal adhesion to the countermember. In general, homogeneous hard-material films on wear-affected substrates cannot meet this aforesaid combination of criteria. Methods have therefore already been described for producing combination films that can be better adapted to requirements.
International Published Patent No. WO 95/16799 describes components equipped, for protection against wear, with a hard-material film that is composed of an adhesion film in contact with the component, a functional film thereabove, and a surface film. The adhesion film comprises a titanium composition, in particular titanium boride, or, if the component comprises a metallic substrate, pure titanium. The functional film is made up in turn of three films of hard-material alloys (titanium nitrides, titanium carbides, and/or titanium borides) of various composition, for example titanium nitride, titanium carbonitride, and titanium carbide, or titanium boronitride, titanium boronitride carbide, and titanium borocarbide, with gradual transitions in composition between the films.
The individual films are obtained by successively vacuum evaporating the titanium compositions, in accordance with a predetermined schedule, onto the substrate while simultaneously allowing high-energy (and, in particular, heavy) ions to act on it. The coating temperature for this method is approx. 200xc2x0 C. The surface film comprises a metal-containing carbon film (i-C(Ti)), and optionally also a metal-free carbon film (a-C:H) thereabove.
Film deposition in combination with heavy ion bombardment solves adhesion and film separation problems.
For some time, efforts have been made to replace expensive and/or difficult-to-machine materials with more favorable materials, e.g. roller bearing steel, for the manufacture of mechanically stressed components. In the context of these efforts, many attempts have also been made to fabricate components with a technical function from plastics. Plastics not only have the advantage of economical accessibility, but are also available in many different types, can be made into almost any desired shape, and have advantageous physical properties, e.g. favorable internal damping or resonance, which means they have little tendency to generate, accept and transmit vibration. This property not only prevents fatigue wear, but also results in particular low operating noise.
In cases in which no appreciable mechanical loads occur, it has also been possible to create sufficient surface protection, for example by applying (as described above) a scratch protection film of SiO2.
In the case of mechanically stressed components, however, the wear properties of the more favorable materials are often inadequate. As discussed above, in such cases an attempt is made to achieve the necessary wear resistance for the components by way of a thin coating. In the case of plastics as an economical material for wear-stressed components (i.e. gears or friction clutches), hard films must be applied onto these components.
Since high-temperature tolerance plays a role in the coating process for many materials, and in particular for plastics, it is necessary to use coating methods that are performed at relatively low temperatures and in which the film deposition rates are great. Many coating methods that are used for finishing metal surfaces are therefore unusable for coating plastics.
It has been found that uniformly hard films are too brittle, and chip off from plastic substrates in response even to the small pressure stresses that accompany most wear stresses (this behavior is called the xe2x80x9ceggshell effectxe2x80x9d). To get around this shortcoming, it is necessary to create a hardness gradient within the film; in other words, the film hardness must be adjusted so that it gradually increases from the flexible plastic base to the hard surface film that is exposed to wear. This principle has been implemented in the aforementioned method, described in the Bosch Technical Reports, for coating plastics with a transparent SiO2 scratch-protection film.
These conventional plasma polymer films with a hardness gradient have the disadvantage of insufficient resistance to other wear stresses, in particular to friction wear stresses. Their advantage lies essentially in scratch protection for relatively soft plastic components and in the transparency of the films, so that the coating is also a possibility for optical applications.
Amorphous carbon films exhibit outstanding wear protection properties with high hardness. They also exhibit low coefficients of friction in sliding wear tests. These films cannot yet, however, be applied at deposition rates on the same order of magnitude as those known from plasma polymerization.
Metal-containing carbon films, e.g. i-C(WC), also offer good frictional and vibratory wear protection, but because the deposition rate is again much lower than in the case of a plasma polymer coating process, and the level of thermal stress is high, they can be applied to only a few (usually expensive) types of plastic. In addition, protection against impact wear stress would require particularly thick film systems, precisely adjusted in terms of modulus of elasticity, that cannot be applied onto ordinary types of plastic by pure carbon coating.
The deposition of hardness-gradient films is possible in carbon systems, but cannot be adapted to very soft substrates, in particular to plastics, since the very soft, graphite-like carbon films necessary for the purpose have insufficient mechanical load-carrying capability at the base of the film, i.e. exhibit poor adhesion under normal and shear loads.
It has now been found that, surprisingly, it is possible to join certain hard-material films, in particular those based on metal-containing and metal-free carbon, in immovable and permanent fashion to surfaces of soft materials, in particular plastic surfaces, by way of a plasma polymer support film having defined hardness and elasticity properties. The materials coated according to the present invention with hard-material films exhibit advantageous wear properties.
The surface filmxe2x80x94for example a carbon film that can be of metal-containing or metal-free configurationxe2x80x94predominantly determines the frictional, vibrational, and abrasive wear protection of the combination film, while the support film influences the impact wear protection of the film package, so that overall, very good protection is achieved against frictional and fatigue wear even on components subject to high levels of tribological stress. The surface films of the protective coatings according to the present invention have a very good affinity for lubricants because of their carbon content, so that lubrication film detachment is largely avoided. In addition, carbon films exhibit dry lubrication properties, i.e. film abrasion can produce a kind of lubricating bushing on the tribological partner. They can also be varied in terms of their topography (particle size, surface roughness, porosity) in such a way that on the one hand very good resistance to cavitation wear is achieved, and seizing of sliding surfaces is also prevented.
The use of the plasma polymer film in this multi-level film configuration makes it possible to adapt hardness and elasticity moduli between the substrate and wear protection film, which results in higher ductility and very good adhesion of the films even on soft plastics.
The high deposition rates possible with the plasma polymer process allow short process times, which in turn permit a greater unit throughput for a given basic capital cost. This means that amortization of the capital investment is distributed over more components, thus reducing the per-unit cost.
With the use of plasma polymer support films it is thus possible to work economically and at relatively low coating temperatures. Because of the coating costs associated with them, this is particularly important for mass-produced components, e.g. the plastic gears of actuating gear drives or electric tool drives.
The combination wear protection film according to the present invention for components made of substrates susceptible to wear, in particular of plastics or soft metals, comprises a wear-resistant surface film and a support film, located between substrate and surface film, that provides a transition over its thickness from the relatively soft base level of the support film resting on the substrate to properties of the surface film. The surface film contains one or more of the elements carbon, nitrogen, silicon, or boron, the proportion of the carbon, nitrogen, or boronxe2x80x94or, if more than one of these elements is present, the sum of the proportions of these elementsxe2x80x94is at least 30% by atomic weight, preferably at least 40% by atomic weight; and that the support film is made of a plasma polymer that has been deposited from a silicon-organic compound and/or an aliphatic and/or an aromatic carbon compound; and that the support film exhibits a quasi-continuous transition in modulus of elasticity and optionally in hardness from the value of the base film to the value of the surface film.
A xe2x80x9cquasi-continuous transitionxe2x80x9d for the purposes of the present invention means either that the modulus of elasticity and/or the hardness of the film forms a gradient that can be represented as a continuous curve, such that this curve need not have the same slope at all points but must not exhibit any discontinuity points relevant to strength (i.e. no substantial breaks); or that a multi-step transition from the soft base to the hard surface film is generated by way of a package made up of a plurality of individual films whose modulus of elasticity and/or hardness changes slightly and in directed fashion from one film to the next.
The composition of the surface film is selected so that it is hard and/or possesses friction-reducing and/or dry lubricating properties, and protects the substrate from mechanical wear.
The films and film levels can contain further elements, preferably hydrogen, oxygen, and metals, in addition to the aforesaid elements.
In the surface film in particular, metal dopants can be used to modify the film properties and thus for optimal adaptation to the intended area of application.
Preferably the surface film is made entirely or predominantly, in particular at a proportion of more than 60% by atomic weight, of carbon, and at a proportion of up to 40% by atomic weight of nitrogen, boron, silicon, and/or metals. Surface films that contain more than 75% by atomic weight, in particular more than 85% by atomic weight, of carbon are particularly preferred.
Also particularly preferred are surface films that contain at least 60% by atomic weight carbon in combination with at leas 2% by atomic weight, preferably at least 10% by atomic weight, in particular at least 20% by atomic weight of one of the aforesaid elements, nitrogen, silicon, boron, or metals.
The properties of the protective films according to the present invention that are so valuable in terms of application engineering result from the interaction of the mechanical properties of the support film and surface film, which in turn are based on their physical composition and the structure that is created under the deposition conditions.
The plasma polymer films of the wear protection films according to the present invention are made of highly crosslinked polymer compounds that exhibit a more or less statistical structure, meaning that in them, the atoms participating in their configuration do not, as in simple monomers or polymers, need to have a simple integral correlation with one another. Fractional and mutually independent atomic indices, such as SiC1.9N0.3B1.1, may therefore appear in the empirical formulae. This has the great advantage that it is possible to adjust the stoichiometric composition of the polymer film as required in the various levels, and to modify it as desired over the film thickness, either continuously or in small steps that do not impair strength.
The components including nitrogen, boron, silicon, and metals form, with the carbon and among themselves, compositions in which all the phases possible under the film formation conditions can be present. For example, nitrogen, boron, and silicon can be partially incorporated into the three-dimensionally crosslinked structure of the plasma polymer, or can be present, for example, as nitride or carbide phases or as mixed phases thereof.
Depending on the deposition conditions, metals can also be incorporated into the polymer or can be present as nitride, carbide, or boride phases, but also in unbound form.
In view of the large number of structures that are possible in the individual levels of the support film and the surface film, a person skilled in the art could not have predicted the outstanding properties of the wear protection films according to the present invention.
All metals, with the exception of the first main group of the periodic system, are in principle suitable for modification of, in particular, the surface film properties. Advantageously, metals of the second to fifth periods of main group 3, the metals of the fourth through sixth main groups, and the adjacent group metals, are used. Metals of the adjacent groups, in particular those of the fourth period, are preferred. Tantalum, titanium, tungsten, and chromium are particularly preferred.
Suitable metal compounds are, in particular, metal carbides, and/or nitrides and/or carbonitrides and/or borides and/or boronitrides of the aforementioned metals. The proportion of metal atoms and/or metal compounds, in terms of metal, in the surface film is up to 40% by atomic weight. Preferably it is in the range from 1 to 30% by atomic weight, in particular from 5 to 30% by atomic weight.
The plasma polymer support film also can contain metal atoms or metal compounds, in particular if it is advantageous for adaptation of its physical data to those of the surface film. The base level of the support film advantageously is made of a plasma polymer that exhibits high adhesion to the material of the substrate. This can be achieved, for example, by the fact that the base level of the support film is made of a plasma polymer that exhibits, in terms of its modulus of elasticity, hardness, and deformability, a coordination with the material of the substrate that is sufficient to attain good adhesion.
In order to achieve particularly good adhesion properties between the support film and the relatively soft substrate, it has proven advantageous for the base level of the support film to be made of a plasma polymer whose stoichiometric composition differs only slightly from the stoichiometric composition of the monomer being delivered. Preferably the base level of the support film is made from a plasma polymer of a silicon-organic compound.
Further criteria that promote good adhesion between the substrate and the base level of the support film include the fact that the base level of the support film is made of a plasma polymer that has functional groups identical or similar to the material of the substrate, or whose functional groups can interact with substituents on the substrate surface. If a soft metal is to be coated according to the present invention, it is advantageous if the base level of the support film is made of a plasma polymer that contains polar groups with an affinity for metal. It is of course also possible to apply, between the surface of the substrate and the base level of the support film, an additional adhesion-promoting film made of a material that has an affinity both with the substrate and with the material of the base level of the support film. An adhesion-promoting film of this kind advantageously has a thickness of 5 to 100 nm, preferably 10 to 50 nm.
In order to achieve particularly good wear protection properties in the surface film, it is advantageous for the support film to have, at least in a film level region directly below the surface film, a gradient in its stoichiometric composition that terminates in the stoichiometric composition of the surface film, since this generally results in a smooth transition in elasticity properties from the support film to the surface film.
For example, in the case of a surface film according to the present invention having a carbon surface film, an elastic, firmly adhering support film can be achieved by the fact that the carbon content, proceeding from the base level, increases with increasing film thickness up to that of the surface film. The transition from the stoichiometry and modulus of elasticity of the plasma polymer film to that of the carbon film is thus smooth or takes place in a plurality of small steps.
For example, the film base is made of a Sixe2x80x94C plasma polymer or a Sixe2x80x94Cxe2x80x94O plasma polymer; this is followed by a carbon gradient that continues to the carbon surface film. Of course the total content of the other film constituents then constitutes a gradient opposite to the carbon content, since the sum of all the constituents must always add up to 100%.
The most important criterion for a good wear protection film according to the present invention is the quasi-continuous transition in elasticity data (and optionally in hardness data) from the base level of the support film to the surface film. This transition need not necessarily be brought about by a continuous transition in carbon content or in the content of another individual film constituent extending from the base level to the surface film. It is also possible for several of the film constituents, for example carbon, oxygen, nitrogen, or boron, to exhibit content gradients running in the same direction, if the remaining constituents result in their totality in an opposite content gradient of equal magnitude. It is also possiblexe2x80x94and particularly advantageous depending on the composition of the base level and of the surface filmxe2x80x94to implement the continuous transition in elasticity properties from the base level to the surface film by way of a sequence of concentration gradients of different film constituents in different film level regions. Examples of such stepped content gradients are the following film structures:
The base level is constituted by a Sixe2x80x94Cxe2x80x94O plasma polymer that is followed by an oxygen gradient. Adjoining this is a transition to a high nitrogen content, and following that, via a carbon gradient, is the carbon surface film.
In another embodiment, the base level of the support film comprises a Sixe2x80x94Cxe2x80x94N plasma polymer and transitions in a gradient to a high nitrogen content; succeeding this is a transition to a high carbon content that ends in a carbon surface film.
In a further embodiment, the base level is made of a Sixe2x80x94Cxe2x80x94O plasma polymer or a Sixe2x80x94Cxe2x80x94N plasma polymer; this is followed by a gradient with nitrogen or boron that transitions into a boron nitride or Sixe2x80x94Nxe2x80x94B surface film with or without carbon. As already described above, all these variants can contain metal atoms or metal compounds, for example TiN, in the support film but in particular in the surface film.
Doping with Si, B, N, O, or any desired metal atoms in a carbon surface film is also possible.
The wear protection films according to the present invention are produced by conventional methods of plasma. If desired, a PVD deposition operation, for example using a sputtering process, can also be performed simultaneously or subsequently. According to the present invention, plasma polymerization is performed in such a way that, optionally after application of an adhesion-promoting film or after plasma finish cleaning (plasma etching) and/or plasma activation of the substrate surface, a monomer gas or a monomer gas mixture, comprising one or more gaseous silicon-organic compounds and/or aliphatic and/or aromatic carbon compounds, and optionally further compounds containing doping elements, is delivered to the plasma, resulting in plasma-induced deposition onto the substrate of a polymer film that becomes crosslinked. The polymerization conditions and the monomer gas composition are advantageously selected, on the basis of a predetermined program identified in preliminary tests, in such a way that in each film level produced in the structure of the film, the stoichiometric composition provided therefor is achieved.
In many it is advantageous to select the polymerization conditions at the beginning of the deposition process in such a way that the soft film base approaches, in its stoichiometry, that of the monomer that is used.
All compounds with sufficient vapor pressure at temperatures between 20 and 200xc2x0 C. are in principle suitable for building up the plasma polymer films; vapor pressures of approximately 10 to 0.001 mbar are to be regarded as sufficient.
Organosilicon compounds, including those containing oxygen, nitrogen, or boron, are generally preferred as monomers for building up the wear protection film according to the present invention. Suitable compounds of this kind are found in the class of the poly(organosilanes), poly(siloxanes), poly(carbosilanes), poly(organosilazanes), and poly(carbosilazanes). Examples of such monomers are tetramethylmonosilane, tetraethylmonosilane, methyldiphenylmonosilane, trimethylmonosilanol, diethyoxydimethylmonosilane, or hexamethylcyclotrisiloxane. Particularly preferred as the monomer material containing silicon are representatives of the poly(organosilanes) and poly(siloxanes), in particular hexamethyldisilane (HMDS), hexamethyldisiloxane (HMDSO), tetraethylorthosilicate (TEOS), and divinyltetramethyldisiloxane (VSi2).
Monomer gases suitable for the deposition of Cxe2x80x94H films are saturated or unsaturated, branched or unbranched aliphatic hydrocarbons, advantageously those with 1 to 8, preferably with 1 to 4 carbon atoms, or aromatic hydrocarbons, preferably with 6 to 14 carbon atoms. Examples of suitable hydrocarbons are alkanes, e.g. methane, ethane, propane, butane, isobutane, octane, isooctane; alkenes, e.g. ethene, propene; alkynes, e.g. acetylene; and aromatics, e.g. benzene, toluene, xylene. Preferred hydrocarbons are methane, ethane, and acetylene.
If elements other than C, H, and Si are to be incorporated into the plasma polymer, it is possible in principle to use monomers based on the aforementioned classes that contain these additional elements as substituents or as chain members. For cost reasons, however, it is much more advantageous to incorporate these elements statistically into the plasma polymer, once it has been produced, by adding simple gasifiable compounds of these elements to the gas flow that is delivered to the plasma. For example, oxygen and nitrogen can be introduced into the polymer by adding oxygen, nitrogen, ammonia, or nitrous oxide. Gradual adaptation of the stoichiometry of the support film to a carbon surface film can be accomplished, for example, by adding carbon-containing additional gases, for example methane, ethylene, or acetylene, to the monomer gas flow. Also suitable for incorporating other elements into the polymer and implementing the content gradients are, besides oxygen, nitrogen, ammonia, methane, ethylene, or acetylene, boron compounds such as boric acid esters, boranols, or boranes, and nitrogen in the form of compounds containing amino or amido groups, for example acrylonitrile, and oxygen in the form of water, or mixtures of the aforesaid substances, as well as gasifiable metal compounds, preferably from the series of lower metal alkoxides such as aluminum, zirconium, and titanium alkoxides, for example zirconium(IV) tert-butoxide, titanium(IV) tert-butoxide, aluminum triethoxide, and from the series of metal carbonyls, for example tungsten hexacarbonyl.
It is particularly advantageous, however, to achieve metallic and nonmetallic dopants in the wear protection film according to the present invention by way of a PVD deposition operation occurring concurrently with the plasma polymerization process or performed subsequent thereto, for which a sputtering process, for example magnetron sputtering, pulsed magnetron sputtering, DC sputtering, high-frequency sputtering, or hollow cathode gas-flux sputtering can be used.
Any metallic, nonmetallic, oxide, boride, carbide, silicide, or nitride target materials can be used in this situation.
The structure and composition of the wear protection films according to the present invention and their levels can be controlled not only by the nature and quantitative relationships of the monomer gases, but also by the process conditions. For example it is also possible, via a smooth change in the process conditions toward a higher carbon content, to achieve a continuous transition from the composition and structure of a base level to a carbon surface film. Process conditions that can be defined in order to control the thickness, structure, and composition of the film levels are the overall process pressure; the monomer partial pressure; the flow velocity of the gas mixture and, associated therewith, the monomer flow rate; the temperature of the gas mixture and substrate; the spacings and geometry of the gas inlet, plasma region, and substrate; the plasma energy and excitation frequency; and the coating time. The plasma energy, working temperatures, substrate spacing, and pressure conditions offer particularly effective opportunities for influencing the film structure. For example, elevating the plasma energy and the substrate temperature results in harder polymer deposits; elevating the process pressure results in softer deposits with less thickness. Selection of the spacing between plasma and substrate, the concentration of oxygen or reactive gas in the gas mixture, the plasma power, and the substrate temperature can also be used to establish a desired topography for the polymer films, in particular the surface film. Lowering the proportion of oxygen or reactive gas, the plasma power, and the substrate temperature, and elevating the substrate spacing from the plasma, each result in a coarsening of the film surface, and vice versa. If other conditions are kept constant, an elevation in plasma energy also results in polymer films with greater chemical resistance. It is normal to work with an energy input of 100 to 6000 W, preferably 200 to 1000 W, to establish the working pressure at values between 5 and 0.01 mbar, preferably 1 to 0.025 mbar, and to establish the substrate temperature at between 20 and 200xc2x0 C., preferably between 20 and 120xc2x0 C.
To assist film deposition, a bias voltage can be applied, capacitatively or by direct contact, to the substrate. The bias voltage can be operated in a pulsed or unpulsed fashion; in the pulsed case, monopolar or bipolar pulsing is possible. The pulse frequency can be varied within wide limits. It is advantageous to select a pulse frequency between 1 kHz and 100 MHz, preferably between 20 kHz and 50 MHz, in particular between 50 kHz and 20 MHz. For design-related reasons, pulse frequencies below 27 MHz, e.g. a radio frequency of 13.56 MHz, are advantageous.
The monomer compositions and process parameters necessary for the deposition of a polymer film with a specific stoichiometry, physical properties, and topography are ascertained on the basis of series of preliminary tests. These involve systematically performing plasma polymerization tests with various monomers and monomer compositions and different process parameters, and measuring the stoichiometric and physical data of the polymer deposits thereby obtained. This yields the properties of the polymer films for each monomer combination as a function of the process parameters; these can be represented, for example, in the form of calibration curves from which can be derived the control program for the process conditions necessary for creating a plasma polymer film having, for example, a specific elasticity gradient.
The stoichiometric composition of the plasma polymer films can be ascertained in known fashion by XPS analysis. Determination of the hardness and elasticity can also be accomplished using known methods: hardness determination is accomplished, for example, using the method described in P. Plein, xe2x80x9cPlasmapolymerisationxe2x80x9d (1989), pp. 112-114, in which the force that must be exerted on a diamond tip in order to produce a scratch on the test surface serves as the measured variable that allows, by way of calibration measurements, a determination of the hardness of the surface under test. The method is relatively imprecise, but when performed under conditions that are kept as constant as possible has the accuracy necessary for the preliminary tests to be performed here. The elasticity determination can be performed using a measurement method also cited in P. Plein, xe2x80x9cPlasmapolymerisationxe2x80x9d (1989), pp. 108-110 and disclosed by K. Taube (seminar presentation of Nov. 25, 1987, Philips Research Laboratory, Hamburg, xe2x80x9cMeasuring the Mechanical Properties of Thin Filmsxe2x80x9d). In this, a diamond tip is applied onto the film with a force such that it penetrates to a certain depth (not more than 20% of the film thickness), but does not produce any plastic deformation of the surface. The application force is then modulated in a sine-wave pattern and the corresponding changes in penetration depth are measured.
The values for penetration depth and change in force can be used to calculate the modulus of elasticity.
The plasma necessary for generating the wear protection film according to the present invention can be a pulsed or unpulsed microwave plasma with or without magnetic field support (ECR), or a plasma excited by high or medium frequency, in particular radio frequency (e.g. 13.56 MHz), or created by hollow cathode excitation. The use of a microwave plasma is particularly preferred, since it allows the highest deposition rates to be achieved. The method of pulsed or unpulsed magnetron sputtering is particularly suitable for implementing a simultaneous or subsequent sputtering process. The present invention furthermore relates to the objects equipped with a wear protection film according to the present invention; and the use thereof as components in technical devices, in particular in mechanical energy transfer mechanisms.