Studies on thermal contact conductance across contacts of metal matrix composites

Abstract

Thermal contact resistance plays an important role in many mechanical, aerospace, electrical, and electronic systems when heat transfer from or to the equipment is critical. Metal Matrix Composites (MMCs) are of interest in aerospace applications and gas turbines because of their high strengthtoweight ratio. However, a lack of knowledge concerning their thermal conductivity and thermal contact conductance often hinders wider utilisation. In the present work, an experimental investigation has been conducted to determine thermal contact conductance across nominally flat contacts between metals, MMCs, and their crosscombinations. Thermal conductivity of MMCs has also been determined as a function of temperature. As part of this investigation, a test facility was developed to measure thermal contact conductance over a wide range of parameters such as contact pressure, mean interface temperature, ambient pressure and temperature, interstitial gas medium, and heatflow direction. Using this facility, thermal contact conductance across nominally flat contacts formed between metals and metalmatrix composites was determined as a function of contact pressure and mean interface temperature in vacuum. The same facility was also used to determine the thermal conductivity of MMCs. The design, construction, and operation of the test facility are presented in detail. Various subsystems, their construction, functions, and calibration procedures are described. Experimental procedure, data acquisition, data reduction, and uncertainty analysis are also discussed comprehensively. Experiments were conducted with the following material combinations:

OFHC Cu - OFHC Cu (AlO/Al-AlN) - (AlO/Al-AlN) MMC (Al 12%Si 10 wt% SiCp) - (Al 12%Si 10 wt% SiCp) SS304 - (Al 12%Si 10 wt% SiCp) MMC OFHC Cu - (Al 12%Si 10 wt% SiCp) MMC Al - (Al 12%Si 10 wt% SiCp) MMC

Results are presented in dimensional and nondimensional form. The test surfaces were characterised using a stylus profilometer. Microhardness of the test specimens was measured as a function of indentation depth. Measured thermal contact conductance was nondimensionalised using surface parameters and thermal conductivity. Nominal contact pressure was nondimensionalised using contact hardness. The influence of contact pressure, mean interface temperature, cyclic loading, and cyclic heating on thermal contact conductance was investigated. Results on thermal rectification across metals and MMCs are presented. Creep effects across OFHC Cu - OFHC Cu interfaces were examined. The influence of deformation mode on observed hysteresis in contact conductance during cyclic loading is discussed. Thermalcontactconductance results as a function of contact pressure were compared with theoretical models available in the literature. The present work confirms the validity of these models even for MMC contacts during first loading.