103 Applications

10.3.1 Environmental Protection Systems

A large number of applications require component protection from elevated temperature or from a corrosive environment. These include aerospace vehicles, both for skin protection for reentry and for engine components such as turbine blades, stirrers, and nozzles for the molten glass and metal processing industry, nuclear reactor components (diverter and first-wall components), and for any cutting tools that experience harsh environments.

Thermal barrier coatings are commonly applied to turbine engine blades to protect against high-temperature and corrosive gases. A large variety of coating compositions, microstructures, and morphologies are possible, though any change to an existing turbine blade design is not trivial from the point of view of engine designers. The coating performance requirements are inherently multifunctional through the coating thickness, and thus it is logical to attempt to employ functionally graded materials.

Silicon carbide-carbon (SiC-C) and carbon-carbon (C-C) composites are widely used as protective shields on the outside of space reentry vehicles and also in the combustion chamber components. It has been shown that applying a graded SiC-C interlayer between a C-C component and a SiC coating improves the life time of a space vehicle nose cone exposed to 1900°C in an oxygen atmosphere at Mach 3. MoSi2-based compositionally graded coatings have been used for protecting components against corrosion in molten glass environments [29], and improvements based on a functionally graded design have been suggested.

10.3.2 Wear-Resistant Coatings

Cutting tools typically require the exterior to be hard and the interior to be tough and strong. Because WC-Co cutting tools have been used extensively in the past, they have been used in the design of graded cutting tools, and it has been shown that resistance to wear may be achieved by appropriately grading the composition of the tool. The compositional gradient should be designed with two objectives in mind. First, the material with highest hardness should be at the surface to maximize hardness there. Second, if the tool is processed at an elevated temperature, the material with the lowest thermal expansion should be at the surface so that upon cooling, compressive stresses develop, thereby increasing the effective hardness. Fortunately, harder materials (e.g., ceramics) typically have lower thermal expansion coefficients than softer ones. Graded Ti-TiN coatings on titanium alloys are common for wear resistance in biological environments [3]. In this case, magnetron sputtering may be used to deposit Ti-rich material near the titanium alloy substrate and TiN-rich material at the coating surface. Chromium nitride films (of varying stoichiometry) produced by cathodic arc deposition are being evaluated for coatings on metal working dies [30]. An optimum combination of adhesion, wear resistance, and intrinsic residual stress has been achieved using a trilayer deposition of 50 nm Cr, 0.4 ^m CrN-rich layer, and 3.6 ^m Cr2N-rich layer onto a tool steel substrate [30].

10.3.3 Joining

Joining of dissimilar materials is challenging due to differences in thermal expansion coefficients between different materials. Thermal residual stresses that develop due to this difference may result in premature joint failure. One of the most common solutions is to employ a braze, which is typically designed to promote good wetting and relieve stresses through plastic deformation of the interlayer. Another approach is to employ a graded interlayer so that the effect of the material mismatch is diffused over a greater distance. In addition, a graded layer may provide additional plastic deformation when metals are bonded to brittle materials. The technique of tailoring the glass thermal expansion coefficient in glass/metal seals has been utilized since 1912 [1, 2]. In this technique, the glass composition is varied so that the thermal expansion coefficient increases nearer the metal. Additionally, the thermal expansion coefficient of the metal may be chosen accordingly as well. General guidelines state that the thermal expansion of the glass should be no less than 10% smaller than that of the metal for glass/metal seals [31]. By forming a graded interlayer, the range of possible glasses increases. However, it is not possible to give more specific guidelines since the success of a glass/metal seal depends on many factors, including geometry, glass mechanical properties, and processing conditions among others. The most efficient approach is to develop a numerical model (e.g., utilizing the finite-element method) for the specific application. While the potential exists for utilizing graded materials to form joints for high-temperature structural materials, techniques have not been sufficiently developed. However, at room temperature, one application is in biological prosthesis [3]. The rationale behind using graded materials is that (1) material mismatch is minimized and (2) when graded porosity is created, the bone can grow into it and become an integral part of the prosthesis. In addition, it may be desirable to grade the aspect of the joint that accounts for biocompatibility. Dental implants have the additional requirement that the external surfaces must be hard and tough [3].

10.3.4 Energy Conversion

10.3.4.1 Thermoelectric and Thermionic Conversion

The effectiveness in the conversion of thermal energy to electricity for a thermoelectric device is typically expressed as a figure of merit, Z, where a2

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