Need for Microstructural Control in Coatings Exposed to Severe Environments

摘要

Very fine grained or amorphous microstructures often improve the performance of coatings exposed to severe environments. Several examples are discussed to illustrate the desirability of controlling processing conditions to achieve microstructural control. In some cases, better environmental resistance is obtained from amorphous materials that exist in a metastable state of equilibrium. Since metastable phases are typically in a higher energy state, this observation is not intuitively obvious. For example, metastable glassy metal alloys exhibit significantly improved aqueous corrosion resistance. Here, the absence of grain boundaries appears to be the predominant factor. Very fine grained and amorphous optical coatings demonstrate improved laser damage resistance and better stability in the presence of humidity. It is suggested that compositional homogeneity is a principal factor in this performance. At high temperatures, the formation of protective glassy SiO sub 2 provides better oxidation protection than the stable, higher melting point refractories because the concentration of lattice defects in the crystalline oxides allows unacceptably high oxygen diffusion rates. Amorphous silica performs better until higher temperatures lower the viscosity and offset its advantage. (ERA citation 12:035145)