When two surfaces are in contact, the actual area of contact is much smaller than the apparent area of contact. These areas of actual contact occur where the asperities of one surface are in contact with the asperities of the other surface. Typically, there is some material or fluid in the interstitial spaces between the contacting surfaces, and heat is transferred through this interstitial material. If there is no interstitial material or fluid, then most of the heat transferred across the interface formed by the two surfaces is transferred through these small contact spots. The amount of actual contact area is also dependent on the physical properties of the contacting materials. Heat is transferred across the interfaces through some combination of three paths: conduction through contacting spots, conduction across the interstitial space through interstitial material, and radiation across the interstitial spaces. Heat transfer between surfaces of imperfect thermal contact occurs in numerous practical applications such as in thermal supervision of space vehicles and estimation of thermal contact conductance between the exhaust valve and its seat in an internal combustion engine.
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