Porosity control in zig-zag vapor-deposited films

Many of the physical, chemical and mechanical properties of vapor-deposited films are strong functions of the volume fraction and morphology of pores that are entrained during film growth. It has recently been shown that by growing films under low surface mobility conditions (at homologous growth temperatures [T/Tm] of 0.5 or less) and alternating the inclination of a substrate to the vapor plume, it is possible to grow columnar zig-zag structures. Films with such a microstructure are of significant interest for thermal protection applications because they reduce the through-thickness thermal conductivity and the in-plane elastic modulus. They therefore provide a means for increasing the coating's thermal resistance while retaining its ability to accommodate thermal expansion mismatch with the underlying substrate during thermal cycling. A kinetic Monte Carlo (KMC) method has been utilized to simulate void evolution during physical vapor deposition of zig-zag microstructures and is used to explore methods for controlling pore morphology. The pore morphology of zig-zag coatings is found to depend strongly on the angular distribution of the incident flux. As the flux incidence angle changes from highly collimated to a cosine distribution, the coating changes from a uniform columnar to a hierarchical structure incorporating many length scales of porosity. The hierarchical nature of this structure is accentuated by increasing the substrate oscillation angle. A more narrowly distributed flux is found to result in denser films. Simulations have also revealed that the widely observed competitive growth phenomenon responsible for intercolumn porosity depends strongly on the incident flux distribution and is amplified by oblique deposition.