The model does not consider Interfacial composition or reactions between substrate and coating. Assumed thermal expansion coefficient (a) were:

1 SIC = 4.3 X 10-tyC 3 SIC = 5.6 x 10-®/°C AIN = 4.9 x 10-®/°C

and AI2O3. As previously discussed, these additives are predicted to react with the CVD gas mixture to form a SiAION-type interface. The lack of cracking and the apparent stability of the AIN coating shown in Fig. 3 indicates that inherent residual stress problems (as predicted by the finite element analysis) can be overcome by proper system design and use of an appropriate coating deposition technique.

Oxidation Resistance — An important requirement of the protective oxide layer is resistance to oxidation and low permeability to oxygen. Both AI2O3 and ZrC>2 are extremely oxidation resistant, with AI2O3 being one of the most oxidation resistant ceramics known. However, diffusion of oxygen through the layer is still possible. The largest diffusion paths are pores and cracks, so a fully dense, crack-free coating is essential. Even without pores and cracks, diffusion can still occur along grain boundaries. This can be minimized by depositing a layer with small, equiaxed grains since large, columnar grains allow rapid "pipeline" diffusion taoccur. Microstructure tailoring of CVD coatings via process control has already been established (11). However, in this case, the composite nature of the protective layer can also be utilized to achieve fine-grained coatings via controlled dispersion of the Z1O2 particles.

Mechanical Properties — Although thin coatings (< 5 um) do not necessarily exhibit bulk predictions represent worst case behavior since the model does not consider the interfacial composition or chemical interactions between coating and substrate.

Preliminary CVD experiments for growth of AIN on SC and SN substrates have yielded strongly adherent coatings with no inherent cracks. A fractograph of an AIN coating on SN is shown in Fig. 3. This substrate consists of Si3N4 with 8% by weight of a glass phase that is a mixture of Y2O3

Figure 3. Fractograph of an AIN coating on silicon nitride.

material properties, the trend is assumed to be similar since limited property data is available for thin films. As a result, bulk material properties were used as a guide for designing the coating configuration. Some relevant bulk properties are listed in Table 1.

A significant increase in the critical stress intensity factor has been reported by the incorporation of tetragonal Z1O2 in a matrix of polycrystaliine AI2O3 <12,13,14,15). This has been attributed to a stress-induced martensitic transformation of the metastable tetragonal Zr02 to its stable monoclinic structure. On this basis, a composite protective layer of AI2O3 + ZrC>2 is proposed.

It has been shown that improvements in mechanical properties of coatings can be achieved via the composites route. Formation of highly dense ceramic comDosites by CVD has been researched and results indicated considerable improvement in toughness (16). Preliminary results for deposition of AI2O3 + Zr(>2 coatings on ceramic substrates have confirmed the projected benefits

(17). These composite coatings have demonstrated improved performance in high speed machining applications over the state-of-the-art AI2O3 coatings currently in use.

Coating Stability — Thé effects of extended usage at temperatures above 1200°C on the structural and mechanical stability of the coating configuration are not known. The major concern is grain growth of the protective layer. It is hypothesized that the presence of discrete ZrC>2 particles in the AI2O3 matrix should help minimize this effect.

Figure 4 schematically summarizes the coating configuration and demonstrates how it satisfies the five design criteria.

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