I

, Low grain growth (composite)

Figure 4. Detailed Illustration of the coating and how It satisfies the requirements.of the application.

microstructure) can be achieved if CVD is used as the coating technique. It has been demonstrated that this technique is capable of producing tough, chemically resistant coatings of a variety of materials with a broad range of material properties (18). Furthermore, the coating can be deposited uniformly on different types and shapes of substrates.

In the CVD process, coating/substrate bonding is typically obtained by chemical reaction or diffusion of elements to form a solid solution at the interface. In PVD and sol-gel processes, bonding is usually mechanical in nature. Such mechanical bonds are inherently weaker and result in poorer adherence. This can be overcome in PVD processes by heating the substrate. However, temperatures required for chemical reaction or diffusion in ceramic substrates such as SN and SC are high and hence difficult to attain in a PVD process. Furthermore, due to the mechanical nature of bonding in PVD and sol-gel processes, morphology of the substrate surface is critical and must sometimes be modified to obtain good adhesion. This can be problematicwhen complex shapes, such as turbine blades, have to be coated.

Sol-gel has other inherent problems, such as one dimensional and multicomponent shrinkage, which can lead to high residual stresses. It also requires a series of fabrication steps, resulting in lower efficiency and higher cost.

The CVD process can produce unique coatings for each substrate or application. This can be achieved by varying the process conditions to give deposits with different composition, thickness, and morphology, and hence coatings with a variety of properties. In addition, CVD can give uniformly thick coatings even on complex shapes. Finally, CVD can be automated and scaled-up. Commercialization has been demonstrated by numerous industrial processes for coating cutting tools and wear parts, for manufacturing thin films for microelectronic applications, and for producing bulk ceramic materials.

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