TABLE OF CONTENTS The majority of overlay coatings deposited by a spray process use raw materials in the form of alloy powders of appropriate particle morphology, particle sizes, and size distribution. Arc processes, on the other hand, use materials in the form of ingots. The composition of the raw material is critical in achieving the desired composition of the final coating, and in maintaining microstructural stability and adequate oxidation and corrosion resistance.
The microstructure of NiCoCrAlY coatings consists of both ? and ? phases, the relative volume fractions being dependent on the actual composition. In the absence of Co, and minor elements such as Y, the composition of a typical coating can be represented on a Ni-Cr-Al ternary phase diagram. The desired composition at high temperature is located within the ? + ? field (Figure 6.18a, indicated by the inset square). The coating, when cooled from 1150 C to 850 C (2102 F to 1562 F), undergoes transformation from ? + ? to ?+ ??. The composition at low temperature is located in the phase field ? + ?? of Figure 6.18b. It is indicated by the inset square. The transformation ? + ? to ? + ?? is accompanied by a significant change in volume as shown by the kink in the thermal expansion profile (Figure 6.19) (Mevrel, 1989; Choquet, 1987). The phase change is, therefore, detrimental to the...
