Preparation And Characterization Of High-emissivity Coatings By Slurry Stacking For GH4099 Superalloy Sheet Surfaces

The high-emissivity coatings was prepared on GH4099 sheet using slurry method. The silica sol and tetraethoxysilane(TEOS)were solvent, respectively. SiC was filler as high-emissivity agent. The preparation process and characterization of the coatings were investigated.First of all transition layer ZrO₂ was prepared on GH4099 by plasma spraying. High-emissivity coatings was prepared on transition layer ZrO₂ by slurry method following. The best process of coating preparation was obtained by optimizing: The coating of silica solvent was brushed layer by layer according to the ratio 100:8:2:0.25, 100:4:2:0.25 and 100:0:2:0.25:0.8.The drying method was that firstly dried at room temperature and then dried at 300℃. The sintering process was that firstly impregnated with silica sol for 2 minutes and sintered for 1h, then impregnated with silica sol again, then sintered at 300℃for 0.5h and the heating rate was 1℃/min.The coating of TEOS solvent was brushed layer by layer according to the ratio 16:3:1:0.2, 16:1.5:1:0.2 and 16:0:1:0.2:0.4. The drying method was that dried in steam at 40℃firstly, dried at room temperature then.The sintering process was that pre-sintered at 300℃for 30 minutes. Then the layer was cooled naturally and then impregnated with silica sol for 2 minutes and finally it was sintered for 1h with 1℃/min as the heating rate.The results showed that micro-cracks appeared on coating of silica solvent during drying, and sintering temperatures had no significant effect on the morphology of surface and cross section. However, there was no cracks appeared on coating of TEOS during drying, and the density was improved with sintering temperature increasing. Glass phase appeared after the coating was sintered at 900℃.Component mutations existed at the coating interface. ZrO₂ composition gradient existed in the high-emissivity coatings. The XRD showed that there was only YSZ, m-ZrO₂ and ?-SiC on the top layer. Both hardness and elastic modulus of the coatings increased with the sintering temperature increasing. And no micro-crack appeared on the coatings with different sintering temperatures after the thermal shock tested for 100 times from 900℃to room temperature.The emissivity of the coating increased to 0.92 with the sintering temperature and measurement temperature increasing. The emissivity reduced with coating thickness decreased. However the emissivity of coatings with TEOS solvent was higher than that of coatings with silica solvent. Glass phase appeared on coating of TEOS solvent after holding 2h at 1050℃, which sealed micro-cracks on the coating surface.