Abstract:
The effect of oxides on the topographies of various random engineering
surfaces and steady-state thermal contact resistances between oxidefree
and oxidized metallic surfaces in contact in high vacua have been
investigated. The results of many previous investigations indicated that
such problems cannot be dealt with without a detailed study of the surface
topographies, resulting in accurate surface characterisation. The distributions
of the real micro-contact zones and heat transfer phenomena across
interfaces could 'then be predicted.
Cylindrical specimens normally mild steel EN3B and commercially
pure copper of nominal area 4.908 x 10-4 m2 were oxidized under controlled
environmental conditions and subsequently pressed into contact at
their flat faces. The oxide film thickness measurements were taken using
a high resolution Stereoscan electron microscope. Surface topographies
were quantified before and after oxidation to determine the effects of
oxide films upon their topological characteristics.
Following a detailed analysis, two theories concerning the problem
of predicting thermal contact resistances of oxidized joints having Gaussian
distribution of surface heights have been developed. The developed
theories establish the surface parameters measured to design a predictable
thermal joint and apply for thin films of the order of naturally occurring
oxides.
From a statistical analysis of the experimental measurements for
freshly-assembled contacts, an empirical expression
(R = 66.0 p-0.945 a-0.128 X0.0346)
has been established relating the loading pressure, mean roughness of'the
contacting surfaces and oxide film thickness to the thermal resistance of
the contacts in high vacua.
Finally a method of producing joints with low thermal contact
resistance (and vice versa) is suggested.