CRYSTAL DEFECTS IN COATINGS AND THEIR INFLUENCE ON COATING PROPERTIES

Utilizing the techniques of transmission electron microscopy and field-ion microscopy, crystal defects ranging from point defects to grain boundaries were observed and compared in thin films and coatings. Some specific examples are vapor-deposited platinum on face-centered cubic (f.c.c.) iridium and body-centered cubic (b.c.c.) tungsten field-emitter substrates, vapor-deposited aluminum on NaCl (001) single crystals, vapor-deposited silver on copper, electrodeposited copper on aluminum (f.c.c.) and iron (b.c.c.), electrodeposited nickel on copper substrates and electrodeposited black chrome on nickel. The microstructures of the various nuclei and larger clusters associated with coating growth on these substrates were investigated together with the microstructures associated with coatings of thicknesses up to 50 μm. The characteristics of crystal defects in coatings and their influence on the physical, mechanical and related properties are similar to those observed in bulk materials. Dislocations and point defects can affect coating resistivities. However, a much greater effect is observed for grain boundaries (grain size) and other interfaces associated with microtwins and twin faults, where the resistivities decrease with increasing grain size or decreasing volume fraction of twin faults. Dislocations can also influence the mechanical properties of coatings in a manner identical with bulk solids. Grain size effects in coatings are of overwhelming importance. The grain size influences the coating hardness according to ΔH ∝ D- 1 2, where ΔH is a hardness change based on an initial hardness reference and D is the average grain size. The grain size also influences residual optical properties in mirror and absorber coatings. The reflectivity (at a wavelength of 10.6 μm) is observed to increase with an increase in grain size, whilst the absorptivity of black chrome coatings on nickel is observed to increase with increasing coating thickness. The coating thickness is observed to be related to the residual grain size and the surface (facet) structure. The surface structure, size and crystallographic features, which are all dependent on the coating thickness and grain structure (size), are observed to be directly related to optical properties determined by the surface roughness. © 1979.