Mismatch, thermal expansion

Thermal expansion mismatch between the reinforcement and the matrix is an important consideration. Thermal mismatch is something that is difficult to avoid ia any composite, however, the overall thermal expansion characteristics of a composite can be controlled by controlling the proportion of reinforcement and matrix and the distribution of the reinforcement ia the matrix. Many models have been proposed to predict the coefficients of thermal expansion of composites, determine these coefficients experimentally, and analy2e the general thermal expansion characteristics of metal-matrix composites (29-33). 

Aluminide and sUicide cementation coatings such as TaAl on tantalum and MoSi2 on molybdenum oxidize at slow rates and possess some inherent self-repair characteristics. Fine cracks that appear and are common to these coatings can be tolerated because stable, protective oxides form within the cracks and seal them. Thermal cycling, however, accelerates faUure because of thermal expansion mismatch that ultimately dismpts the protective oxide coating. 

Fig. 9. Residual stresses owing to thermal expansion mismatch between a particle with radius a and thermal expansion coefficient and a matrix with thermal expansion coefficient The stresses illustrated here are for and P is the interfacial pressure.

Monolithic refractory coatings have been applied to metallic components in furnaces for fuel ash corrosion control. Results have been less than satisfactory because of the large thermal expansion mismatch between the metal and refractory. Failure usually occurs upon thermal cycling which causes cracking, eventual spalling of the refractory, and direct exposure of the metal to the effects of the fuel ash. 

Due to the large thermal expansion mismatch between barium chromate and the sealing glass or FSS (e.g., AISI446) [213], the extensive formation of barium chromate has been shown to result in crack initiation and growth between the sealing glass and alloy coupons. 

Brun, M.K. and Singh, R.N. (1988). Effeet of thermal expansion mismatch and fiber coating on the fiber/ matrix interfacial shear stress in ceramic matrix composites. Adv. Ceram. Mater. 3, 506-509. 

Kuntz, M., Meier, B. and Grathwohl, G. (1993). Residual stresses in fiber-reinforced ceramics due to thermal expansion mismatch. J. Am. Ceram. Soc. 76, 2607-2612. 

Solar systems are subjected to a unique set of conditions that may alter their stability and, hence, their performance and life-cycle costs. These conditions include UV radiation, temperature, atmospheric gases and pollutants, the diurnal and annual thermal cycles, and, in concentrating systems, a high-intensity solar flux. In addition, condensation and evaporation of water, rain, hall, dust, wind, thermal expansion mismatches, etc., may impose additional problems for the performance of a solar system. These conditions and problems must be considered not only individually, but also for synergistic degradative effects that may result from their collective action on any part of the system. Since these degradative effects may also reduce the system or component performance, protective encapsulation of sensitive materials from the hostile terrestrial environment is required to provide component durability. 

Niihara [1] considered the improved toughness was mainly attributed to the residual stress that results from differential thermal expansion coefficients of two phases. In Al203/SiC systems, the tensile hoop stress, thought to be over 1000 MPa around the nanoparticles within the matrix grains, was generated from the large thermal expansion mismatch. Thus, the material may be... 

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