Development and characterization of interface coatings in molybdenum-reinforced NiAl matrix composites
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INTRODUCTION
IN recent years, there has been an increased interest in the development and utilization of intermetallic matrix composites (IMCs) for high-temperature structural applications. In these IMCs, a second phase is added to enhance the low-temperature toughness and/or the hightemperature strength and stiffness of the intermetallic matrix. Unfortunately, only marginal improvements in toughness have been achieved by compositing with second-phase ceramic particles, whiskers, or fibers.[~ A more successful toughening approach involves the use of ductile second phases, t~,2j The toughness improvement in such materials is attributed to crack bridging by the ductile reinforcements and the energy dissipated by plastic deformation of the reinforcements, t3,4,51 One of the factors that influences the contribution to toughness is the nature of the fiber/matrix interface. Previous studies have shown that the highest toughnesses have been achieved for ductile reinforcements with interfaces that are weak and exhibit partial debonding in the wake of a crack, t3,6,71 In addition, it is also apparent that in some systems, the weak interfaces and debonding are promoted by coatings placed between the matrix and reinforcements, t6,8,gj Thus, the ability to tailor interface mechanical properties by the use of coatings in order to achieve the desired composite toughness is a promising approach. Because there is essentially no limit to the number of matrix/reinforcement combinations that can be identified, a large number of IMC systems have been studied to date. Many such combinations are thermodynamically incompatible and/or have different thermal expansion coefficients (CTE). In the former systems, the matrixreinforcement reactions are deleterious as far as longterm thermal stability of the composite is concerned. These reactions may be controlled by using intermediate barrier layers; such diffusion barriers must be inert and compatible with both the matrix and reinforcement phases, tl~ Alternatively, intermediate compliant layers may be required in otherwise stable systems in order to reduce the residual stresses in the vicinity of the interface P. KR1SHNAN, Graduate Research Assistant, and M.J. KAUFMAN, Associate Professor, are with the Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-2066. Manuscript submitted May 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
and avoid degradation due to CTE mismatch effects (thermal fatigue). Based on the foregoing discussion, it is apparent that in order to achieve optimum properties, it is necessary to tailor and control the interface structure and properties. The most common approach for tailoring interface structure and properties is to coat the reinforcement prior to its incorporation into the matrix. Numerous coating methods such as sol-gel, sputter deposition, and physical and chemical vapor deposition (PVD and CVD) have been used in the past to produce interface coatings. These techniques tend to be expensive, processing inten
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