Vapor phase codeposition of Cr and Si on Nb-base in situ composites by pack cementation process

In order to identify suitable halide activators and pack compositions for codepositing Cr and Si to form diffusion coatings on Nb-base in situ composites by the pack cementation process, thermochemical calculation was taken to analyze the vapor pressure of halide species generated at high temperatures. NH₄Cl, NaF and CrCl₃·6H₂O were selected as the halide salts. The results of thermochemical calculations suggested that the pack powder mixtures, which contained Cr, Si, halide salts and Al₂O₃, may be activated by NH₄Cl and NaF. According to the thermochemical calculations, the pack powder mixture of 12Cr–6Si–5NH₄Cl–77Al₂O₃ (wt%) activated by NH₄Cl was formulated and coating deposition experiments were carried out at 1200 and 1300 °C. With adequate control of pack compositions and deposition conditions, it was found that codeposition of Cr and Si could indeed be achieved at these temperatures. The coating has a three-layer structure, of which was mainly composed of Cr₂(Nb,X) (X represents Ti and Hf elements), Nb₅Si₃ and (Nb,Cr)₃Si. Then the kinetics of coating growth process affected by temperature was studied. The experimental results of the oxidation showed that the coating can efficiently prevent substrate from oxidizing.