Degradation of Materials in Combustion Environments

Degradation mechanisms in conventional refractories, structural ceramics, and metallic alloys were revealed by examination of materials exposed to industrial and synthetic flue gases. Deterioration of refractory oxides and oxide structural ceramics involved formation of new liquid, glass, and solid phases, which resulted in loss of structural integrity. Destructive stresses associated with volume changes in the case of new solid phases were sometimes increased by thermal cycling. In the case of silicon carbide ceramics the viscosity of the normally protective oxide film was lowered by reaction with alkali oxides, which resulted in enhanced oxygen diffusion and active oxidation. Silicon carbide was also attacked by halide-containing atmospheres. Degradation was also caused by excessive temperatures, thermal cycling, and stresses caused by adhering slags. Several types of degradation mechanisms were observed in metallic alloys including breakaway oxidation, intergranular oxidation and attack, and sulfidation-oxidation.