Effect Of Thermal Cycling On Electrical And Thermal Properties Of Al/SiC_p Composites

Metal matrix composites (MMCs) have been of interest in electronic packaging technology due to their good thermal properties, mainly the high thermal conductivity and low coefficient of thermal expansion. The resistance against thermal fatigue is also very important for such materials. The different properties between the matrix and the reinforcements will cause high residual stress, and thus make the composite material more prone to damage in thermal cycling. The variation of thermal properties of Al/SiCp composites in thermal cycling and the mechanisms were investigated.It is hard for Dry Process to prefabricate green bodies of complex shapes in net shape forming. Gel-casting combined with vacuum pressure infiltration technique was applied to prepare the high volume fraction SiC particulate reinforced Al composites with excellent mechanical and thermal properties.The microstructural observations showed that thermal cycling caused the relaxation of interfaces, and the failure behavior changed from fracture of SiC particles to debonding of SiC particles. The increase in thermal cycles or temperature range aggravated the damage of the composites. Thermal cycling led to the multiplication of defects, mainly the dislocations, in the matrix. The internal friction measurements confirmed the increase in dislocation density, which caused the rise of peak temperature, as well as that of the shear modulus and damping.The thermal expansion coefficient remained stable in thermal cycling with a largest increase less than 3%, while the plastic strain accumulation in the form of exponential growth caused the increase in the specimen volume. After 100 cycles, however, the plastic strain tended to saturation, and thus the composite size became more stable.The thermal cycling caused the degeneration of the electrical conductivity, and the process is composed of three stages: in the first stage, the thermal conductivity basically remained unchanged; afterwards, it drops rapidly; and in the third stage, it remains stable again. It dropped as high as 34%. The increase in the dislocation density of the matrix was substantially responsible for the degradation.The degeneration of the thermal conductivity showed the similar tendency as the electrical conductivity in 600 cycles. Over 15% of conductivity lost in 600 cycles. The multiplication of the dislocations in the matrix is also the main reason for the degeneration. The difference in particle size did not change the general feature of the degeneration process, but influenced the thermal conductivity values. Different metal matrices caused the change of each stage length. The expanded temperature range sped the degeneration of conductivity: the temperature range from 280℃to 496℃caused bigger degeneration, from 5% to 14.38%. The increased number of cycles led to the further degradation.Equations were established to predict the electrical and thermal conductivity of Al/SiCp composites. The influence of the reinforcement size is considered through the deformation zone in the matrix and the introduction of a parameter to characterize the effect of interfaces. With the equivalent inclusion method and the introduction of the interfacial thermal resistance, the equation was founded to forecast the thermal conductivity of the particulate reinforced composite.