[0001]    The present invention pertains to a process for improving the stability of yttrium stabilised zirconia for dental restorations, and the use of a substantially water-free aerosol for the manufacturing of a veneered dental restoration. 
       BACKGROUND OF THE INVENTION 
       [0002]    Zirconia holds an exclusive place amongst dental restorative materials compared with other oxide ceramics, such as alumina, because of its excellent mechanical properties as a consequence of transformation toughening that was identified in the mid-1970s. Pure zirconia can exist in three different crystal structures depending on temperature. At room temperature up to 1170° C., the symmetry is monoclinic, the crystals are tetragonal between 1170 and 2370° C. and cubic above 2370° C. up to the melting point. The transformation from tetragonal (t) to monoclinic (m) during a cooling process is accompanied by a volume increase (approximately 4%) and shear distortion, sufficient to cause catastrophic failure. Alloying pure zirconia with stabilizing oxides such as Y 2 O 3  allows the preservation of the meta-stable tetragonal structure at room temperature and therefore a potential control of a stress-induced t-m transformation, which can efficiently enhance resistance to crack extension leading to higher toughness compared to alumina. 
         [0003]    As a consequence of the metastability of tetragonal zirconia, stress-generating surface treatments, such as grinding or sandblasting, are able to trigger the t→m transformation with the associated volume increase leading to the formation of surface compressive stresses, thereby increasing the flexural strength. However such alloying also alters the phase integrity of the material and increases the susceptibility to aging. The low temperature degradation (LTD) of zirconia is a well-documented phenomenon, dependent upon the presence of moisture and modest heat. The consequences of this aging process are multiple and include surface degradation with grain pullout and micro-cracking as well as strength degradation. Although LTD has been shown to be associated with a series of orthopedic hip prostheses failures in 2001 and despite a well established definition of the conditions under which L TD occurs, there is currently no clear relationship between LTD and failure predictability when zirconia is used as a dental bio-ceramic. 
         [0004]    3Y-TZP is now widely used in dentistry for the fabrication of dental restorations, mostly processed by machining of partially-sintered blanks followed by sintering at high temperature. The mechanical properties of 3Y-TZP strongly depend on its grain size. Above a critical size, Y-TZP is less stable and more vulnerable to spontaneous t→m transformation than smaller grain sizes (&lt;1 μm). Moreover, below a certain grain size (˜0.2 μm), the stress-induced transformation is not possible, leading to reduced fracture toughness. 
       SUMMARY OF THE INVENTION 
       [0005]    One object of the invention was to provide a process avoiding the drawbacks of the prior art, in particular the generation of a transformation of the tetragonal to the monoclinic state during manufacturing of a veneered yttrium stabilized zirconia restoration. 
         [0006]    According to the invention this is achieved by a process for improving the stability of yttrium stabilised zirconia for dental restorations comprising yttrium stabilised zirconia, the dental restorations are veneered with a dental ceramic veneering material wherein the dental restoration comprising yttrium stabilised zirconia and to be veneered is provided; the dental ceramic veneering material is applied to the dental restoration by an aerosol which contains the dental ceramic veneering material and substantially water-free propellant; and the resulting arrangement fired to a resulting veneered dental restoration. In particular, the dental ceramic veneering material is applied by the aerosol in a thin layer on a framework of the dental restoration. Typically the layer has a thickness of from about 0.01 mm to 0.5 mm, in particular about 0.1 mm. 
         [0007]    The method of the invention avoids destabilization of the yttrium stabilised zirconia by introduction of moisture when the veneering material is applied to the yttrium stabilised zirconia dental restoration. 
         [0008]    In one embodiment of the invention the yttrium stabilised zirconia dental restoration comprises a composition with zirconium dioxide, yttrium oxide and optionally hafnium oxide, aluminium oxide, and silica. 
         [0009]    In another embodiment of the invention the dental ceramic veneering material comprises the composition comprising SiO 2 ; Al 2 O 3 ; K 2 O; Na 2 O; B 2 O 3 ; optionally BaO; TiO 2 ; CeO 2 ; ZrO 2 ; CaO; SnO 2 . 
         [0010]    In yet another embodiment of the invention the dental ceramic veneering material is present in a particulate form. 
         [0011]    Typically the aerosol is applied by spraying-on the yttrium stabilised zirconia dental restoration. For example, the dental ceramic veneering material is released from a container by a propellant. 
         [0012]    Still another embodiment of the invention is the use of a substantially water-free aerosol for the manufacturing of a veneered dental restoration the aerosol comprising a propellant and a dental ceramic veneering material for spraying-on an yttrium stabilised zirconia dental restoration to be veneered with the dental ceramic veneering material. 
         [0013]    The method of the invention avoids destabilization of the yttrium stabilised zirconia by introduction of moisture when the veneering material is applied to the yttrium stabilised zirconia dental restoration. 
         [0014]    Every kind of yttrium stabilised zirconia dental restorations can be veneered according to the invention. In particular bridges, crowns, inlays, onlays and veneers can be manufactured. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The yttrium stabilised zirconia dental restoration comprises a composition with of from about 93 to about 97% by weight zirconium dioxide, of from about 3 to about 6% by weight yttrium oxide, of from about 0 to about 3% by weight hafnium oxide, of from about 0 to about 1% by weight aluminium oxide, and of from about 0 to about 1% by weight silicon oxide. 
         [0016]    The yttrium stabilised zirconia dental restoration in particular comprises a composition with of from about 93 to about 95% by weight zirconium dioxide, of from about 4.8 to about 5.5% by weight yttrium oxide, of from about 0 to about 3% by weight hafnium oxide, of from about 0 to about 0.4% by weight aluminium oxide. 
         [0017]    In another embodiment of the invention the dental ceramic veneering material comprises the composition of from about 58 to about 70% by weight SiO 2 ; of from about 10 to about 18% Al 2 O 3 by weight; of from about 6 to about 12% by weight K 2 O; of from about 3 to about 8% by weight Na 2 O; of from about 1 to about 6% by weight B 2 O 3 ; of from about 0 to about 3% by weight BaO; of from about 0 to about 2% by weight TiO 2  ; of from about 0 to about 2% by weight CeO 2 ; of from about 0 to about 2% by weight ZrO 2 ; of from about 0 to about 3% by weight CaO; of from about 0 to about 2% by weight SnO 2 . 
         [0018]    In particular, the dental ceramic veneering material comprises a composition of from about 60 to about 64% by weight SiO 2 ; of from about 13 to about 15% Al 2 O 3  by weight; of from about 7 to about 10% by weight K 2 O; of from about 4 to about 6% by weight Na 2 O; of from about 3 to about 5% by weight B 2 O 3 ; of from about 1 to about 3% by weight BaO; of from about 0 to about 0.5% by weight TiO 2 ; of from about 0 to about 0.5% by weight CeO 2 ; of from about 0 to about 1% by weight ZrO 2  ; of from about 1 to about 2% by weight CaO; of from about 0 to about 0.5% by weight SnO 2 . 
         [0019]    In yet another embodiment of the invention the dental ceramic veneering material is present in a particulate form, in particular of a powder having in particular a particle size of from about 0.5 μm to about 150 μm, in particular a particle size of from about 0.5 μm to about 50 μm 
         [0020]    Typically the dental ceramic veneering material is released from a container by a propellant which is in particular selected from the group consisting of aerosols based on heptafluoro propane or propane, butane or mixtures thereof. 
         [0021]    Still another embodiment of the invention is the use of a substantially water-free aerosol for the manufacturing of a veneered dental restoration the aerosol comprising a propellant and a dental ceramic veneering material for spraying-on an yttrium stabilised zirconia dental restoration to be veneered with the dental ceramic veneering material. 
         [0022]    The application of the ceramic veneering material by a spraying-on method presents or avoids the transformation from t→m of the zirconia crystal at the surface of the framework, because no aqueous liquid can attack the crystals during the first veneering step. Furthermore, the surface of the zirconia framework material becomes protected by the applied thin layer of ceramic veneering material during following veneering steps and of the saliva after the insertion. After the thin protection layer of the sprayed ceramic powder is applied and fired; the dental technician can build up by using a brush and a mixture of dental ceramic powder with liquid a veneering layer in the right shape of the dental restoration. This technique is the known technique for building a dental restoration on a framework. After the application of the powder liquid mixture the technicians has to follow the recommended firing schedule of the manufacturer of the dental ceramic powder. Normally it is needed one build up dentin firing and a glaze firing step. 
         [0023]    After a milling process the yttria partially stabilised zirconia material has to be veneered with ceramic material to receive a natural look of the dental restoration. According to the prior art typically a powder of dental ceramic and modelling liquid or distilled water is mixed to form a slip. The ceramic material is then applied on the zirconia framework with a brush by a dental technician. The restoration is then fired to a firing end temperature above 700°-950° C. with a heating temperature speed of about 30° C.-60° C. per min following a preheating time and temperature. The liquid or its steam causes during the firing process a transformation process from tm of the yttria partially stabilised zirconia on the surface of the framework material. The tm transformation affects a volume increase of the zirconia crystals and with this energy impact a nucleation growth process which may cause cracks in the framework material with a destabilisation of the dental restoration. 
         [0024]    The application of the spraying-on method of the invention avoids or minimizes the transformation because no aqueous liquid can affect the framework material during the first firing process. Furthermore, the thin layer of porcelain will protect the framework of the transformation process by the application of the following veneering steps, which can be performed by a brush with a normal dental liquid powder mixture. 
         [0025]    The invention is further described by the following examples which do not limit the scope of the invention. 
         [0026]    The application of the ceramic veneering powder with the spraying-on method can be performed by an aerosol which contains the dental ceramic veneering material and substantially water-free propellant. The yttria stabilised zirconia framework has been produced by a milling machine and then sintered as recommended by the manufacturer of the framework material e.g. at 1380° C. or at 1530° C. 
         [0027]    From a distance of about 20 cm to the framework the spray of dental ceramic veneering powder has been applied on the outer surface of the framework by spraying a thin layer of approximately 0.1 mm. 
         [0028]    After this step the framework with the completely covered outer surface has to be fired in a first firing veneering step at the recommended end temperature with the recommended heating speed and the recommended preheating time and temperature for the veneering ceramic. Following this firing step the thin veneered surface of the yttria stabilised zirconia material has a brilliant glossy shine. Because of the absence of a aqueous liquid medium no transformation process from t→m occurs at the surface of the yttria stabilised zirconia. 
         [0029]    Onto this surface the veneering ceramic with the mixture of powder and liquid and with the method by a brush can be applied. The dental technician can build up the dental restoration in the way he is used. The surface of the yttria stabilised zirconia will be protected from the aqueous liquid influence in the following veneering steps and no transformation process of the zirconia crystals will occur.
A process for improving the stability of yttrium stabilised zirconia for dental restorations comprising yttrium stabilised zirconia, the dental restorations are veneered with a dental ceramic veneering material wherein--the dental restoration comprising yttrium stabilised zirconia and to be veneered is provided;--the dental ceramic veneering material is applied to the dental restoration by an aerosol which contains the dental ceramic veneering material and substantially water-free propellant;--the resulting arrangement is fired to a resulting veneered dental restoration.