Silver diffusion in silicon carbide coatings

We have studied the effect of microstructure on the diffusion of silver (Ag) in different silicon carbide (SiC) coatings in tristructural isotropic-coated particles. Silicon carbide was deposited at 1300°-1500°C by fluidized bed chemical vapor deposition. The SiC coatings have been heat treated at 1200°-1400°C for 240 h. Depending on the microstructure our results confirm that silver can diffuse by grain-boundary diffusion but that a high concentration of nano- and microporosity at the grain boundaries can strongly influence the diffusion process by increasing the diffusion coefficient by at least two orders of magnitude (from ∼10 ^-17 for standard coatings to ≥10^-15 m²/s at 1200°C). We also show that the presence of excess silicon or the formation of long columnar grains in SiC does not increase the diffusion of silver as previously thought. Our results do not support the idea that silver diffuses through SiC due to the formation of defects produced by mechanical damage (nanocracks). Silver is aiding the recrystallization of SiC when heat treated at 1400°C.