Abstract
Ion cleaning is often used as a method for treating substrate surfaces before coating in low vacuum deposition processes. It is generally believed that the sputtering action of the ion bombardment removes contamination and that good adhesion results. However, we show that good adhesion is often achieved by simply heating the substrate to a temperature of 300 °C, or more, and we think this is due to the removal of organic contamination by decomposition and evaporation. Even so, good adhesion is not always obtained. We find, in the several cases of thick coatings examined, that this loss of adhesion is caused by the propagation of a fracture at, or in the vicinity of the interface. Vacuum coating processes often take place at elevated temperature owing to the dissipation of heat in the substrates. Upon cooling afterwards stresses appear in the coatings and these may cause fractures leading to flaking if a brittle region is present. The thickness of the coating is an important factor since this governs the elastic energy available to propagate the fractures. It is important to match coefficients of thermal expansion of coating-substrate combinations as far as possible, to avoid spurious brittle regions and not to exceed the critical coating thickness in a given case. Preliminary ion bombardment may be used as a convenient means for heating, but there are some disadvantages which must be minimized if good, overall adhesion is to be obtained.
Published Version
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