Abstract
Abstract. -Titanium diboride coatings (20 or 30 pm thick), deposited by hydrogen reduction of TiC14 and BC13, are protecting CISiC substrates against oxidation up to 1100 OC. The oxidation of TiB2 leads to the formation of a protective low viscosity B2O3 glass filling the cracks of the primary CVD-Sic oxygen barrier. Above this temperature, the rapid consumption of the Bz03 glass by vaporization limits the duration of the protection system. A comprehensive description of the oxidation mechanism, based on a complementary experimental study of B203 evaporation, is presented.
Introduction.
The use of C/SiC composites, as structural materials for aerospace applications at high temperature involves the development of a reliable oxidation protection [l-31. The current oxidation protection system is based on the use of a CVD-Sic coating, as primary oxygen barrier, coupled with an internal inhibitor. Since coatings are normally obtained at elevated temperatures, microcracked barriers result from thermal mismatch stresses.
Therefore, in order to create a full range oxidation protection system, it is necessary to seal the cracks in the range from the oxidation threshold for the C-Sic composite (< 500 "C) to the microcracking temperature of the silicon carbide coating (700 < T < 1000 "C).
Boron has proved to be the most important constituent for providing self-healing capabilities in coating systems since it oxidizes to give B203 (m.p. 450 "C) which can fill cracks and reduce permeation of oxidants.
The main aim of this work was to test a new oxidation protection system based on the use of an outer layer of titanium diboride able to produce a B203 glass.
A previous publication [4] dealt with the chemical vapor deposition of TiB2 and presented preliminary results concerning the oxidation behaviour of CISiC samples.
The purpose of this paper is to analyse in more detail the oxidation mechanism of such coated specimens.
Experimental.
2.1 COATINGS. -Titanium diboride was deposited by hydrogen reduction of TiC14 and
BC13.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993987 T e m p e r a t u r e ('C) 2.2 SUBSTRATES. -5 x 5 x 5 mm3 cubes of alumina (inert substrate) or C/SiC composite (coated with a 30 pm thick layer of silicon carbide).
The oxidation kinetics of TiB2 coated and uncoated specimens in pure flowing (5 Vh) oxygen (Prodair, 99,998%) were determined as a function of temperature using a symmetrical continuous recording thermobalance (MTB 50, Setaram, France). Samples were placed in an alumina crucible.
After oxidation, the quenched oxide layers were examined by optical scanning electron microscopy (SEM) and characterized by X-ray diffraction (XRD). The non-isothermal oxidation curves (Fig. 1) show that reaction (1) becomes noticeable above 500 "C.
Results and discussion.
The isothermal kinetics indicate a passive behaviour below 1100 "C ( Fig. 2 ). Above this temperature, the curves exhibit a maximum due to the vaporization of B203(2). Only a slight behaviour difference is observed between alumina and CISiC substrates. Figure 3 , which compares the oxidation of the coated and uncoated substrates, confirms the protectiveness of the coating below 1100 "C. Indeed, the coating suppresses the weight losses due to the internal oxidation of the carbon fibers. At higher temperatures, the influence of the thickness of the TiB2 coating has been tested. For example at 1300 "C (Fig. 4) , the increase in thickness (18 pm + 31 pm) of the coating results in a slight efficiency increase of the passive oxidation behaviour.
Oxidation time (h)
Therefore, in order to obtain a comprehensive understanding of the oxidation behaviour in the explored temperature range, it is necessary to determine the exact contribution of the B 2 0 3 volatilization weight loss. The maximum rate of evaporation of component i from a free surface into a vacuum is given by the Hertz-Langmuir equation:
where: A is a constant, ai is the evaporation coefficient (ai < I ) , pi is the vapor pressure of i above the sample, and Mi is the molecular weight of i.
It should be emphasized that equation (6) is derived on the assumption that none of the evaporating molecules returns to the surface. As a rough guide, back reflection of escaping molecules will not be significant at pressures below 0.1-1 torr. Under these conditions the evaporation rate is controlled by the interface reaction rate.
A higher pressures, back-reflection reduces the vaporization loss relative to that predicted from the surface processes. The evaporation molecules may also form a boundary layer of thickness S next to the surface. If the rate of evaporation is controlled by transport in the boundary layer, the flux J away from the surface of the melt may be defined by [8] :
where D is the diffusion coefficient, Ci is the equilibrium concentration of component at the interface and C , is the concentration at the external interface of the boundary layer (C,#O) .
Because of this boundary layer of gaseous oxide molecules, the reaction conditions in general become a critical parameter in oxidation at atmospheric pressure where oxide volatilization occurs. Indeed, the evaporation rate becomes a complex function of the gas pressure, gas flow over the melt, geometry of the reactor. .. This explains why very different data may be found in the literature [B] .
For all these reasons volatilization experiments were made in the thermobalance in the same experimental conditions as the oxidation runs, in order to determine the rate of evaporation. A pure and anhydrous B 2 0 3 powder (99.998%, Aldrich chemical company, Inc) was placed in the same A1203 crucible as C/SiC substrates. Weight losses were plotted vs. time for several temperatures (Fig. 5) .
The volatilization of B 2 0 3 becomes noticeable above 900 "C. Up to 1100 "C, the rate of evaporation is approximatively constant for a given temperature. Above 1100 "C, the kinetic curves are decelerated suggesting either that the simple diffusion approach (Eq. (7) ) is no longer valid or that the thickness of the boundary layer is increasing with time.
The corresponding Arrhenius plot (Fig. 6 ) leads to an apparent activation energy of evaporation (E#180 kJ/mol) in accordance with Tsapuk's previous work [9] . Nevertheless, the evaporation rates determined in the present study are approximatively two orders smaller. 
CALCULATED OXIDE GROWTH KINETICS.
Injuence of the temperature. - We are now able to take into account the effect of vaporization on the global oxidation rate. Indeed, the contribution to the weight gain of reaction (1) can be calculated by summing point by point the weight losses derived from figure 5 and the experimental weight gain determined from figure 2:
For the two types of substrates, the calculated curves (Fig. 7) present a parabolic type shape (Fig. 8) , except around T = 800 O C where they exhibit a cubic form (Fig. 9) . Such a behaviour was previously observed by Miinster [lo] who attributed this evolution to a morphological transformation of the oxide layer. In the case of coated C/SiC substrates the transitory kinetic domain extends up to 900 O C (Figs. 8 and 9) . 
Nature and morphology of the reaction products.
Polished cross-sections of the reaction products are observed after 24 h oxidation.
4.1 ALUMINA SUBSTRATES. -Before the total oxidation of the TiB2 coating, we observe an internal compact polyphase oxide (B203 + Ti02) sublayer and an external non-protective porous rutile scale. This external layer results from the vaporization of B2O3, and is only observed above 600 "C. Its thickness, porosity and crystallinity (rutile form above 900 "C) increase with the oxidation temperature. Above 1000 OC, after the total consumption of TiB2 and volatilization of B2O3, only the porous and non-protective Ti02 scale is observed. A slight diffusion of boron in alumina was revealed by electron microanalysis and the formation of boroaluminate (2 A2O3.B2o3 and 9 A203.B203) phases was detected by XRD.
4.2 C/SiC SUBSTRATES. -The results, for different oxidation temperatures, are summarized on figure 1 1 for CISiC substrates.
The nature and structure of the oxide layers are the same as for alumina substrates as long as a thin TiB2 layer remains near the substrate. After the total consumption of TiB2 (T > 1000 "C and 24 h oxidation time), important modifications are observed. Rutile (TiOa) and titanium suboxide (Ti203) are detected. At 1300 "C, the polycrystalline oxide phase is surrounded by a borosilicate glass resulting from a slight oxidation of the CVD Sic sublayer (Fig. 11) .
Therefore, it is clear that at high temperature, due to the rapid oxidation of the TiB2 CVD coating (Figs. 2 and 4) , the kinetic behaviour is directly linked to the transport properties and the thickness of the B2O3 glass scale (Fig. 4) . After total vaporization of B2O3, the porous Tion layer, present at the surface of the CISiC substrates, no longer prevents the internal oxidation of carbon fibers (Fig. 4) . 
Conclusion.
This work provides a better understanding of the oxidation mechanism of TiB2 coated CISiC composites. Indeed, the protectiveness of CISiC composites coated with TiB2 occurs as long as B2O3 is still present in the oxide outer layer. Below 1100 "C, it acts as a low viscosity diffusion barrier having efficient healing capabilities of the primary CVD-Sic oxygen barrier. Above this temperature, the rapid consumption of the B2O3 glass by vaporization, limits the efficiency of the protection.

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