New experimental data on the effect of metallic interstitial materials on thermal contact conductance of cylindrically shaped solid metal specimens is reported. The investigation herein focused on the dependency of the thermal contact conductance on factors like microhardness and surface roughness of test specimen, thickness, hardness and thermal conductivity of the interstitial material, and contact pressure. However, particular emphasis was on the verification of the existence of the optimum thickness of the interstitial material. The optimum thickness is such that the filling of the voids between the contacting surfaces at a given contact pressure is maximum without considerably separating the contacting surfaces. Depending on the hardness of the interstitial material, if the thickness is less than the optimum it would still leave many unfilled voids while if thicker it would tend to separate the contacting surfaces although it may maximise the filling of the voids. If the interstitial material is relatively hard and thick it will not easily conform to the contour of the contacting surfaces.
The experiment was conducted under vacuum. Different types and thicknesses of metallic foils were used. The results show that for a particular pair of contacting surfaces there is a corresponding thickness of a foil at which the thermal contact conductance is maximum. This optimum thickness of the foil was found to be around twice the effective roughness, σе, of the two surfaces ( σе = √ σ12 + σ22).