Evaluation of hot corrosion behavior of plasma sprayed and laser glazed YSZ–Al2O3 thermal barrier composite

The evaluation of hot corrosion behavior of sprayed and laser glazed YSZ + Al2O3 composite coatings was conducted in the presence of 55% V2O5 + 45% Na2SO4 molten salt was conducted. Inconel 718 coupons were sprayed with a Ni–22Cr–10Al–1Y bond-coat and then with an YSZ–Al2O3 top-coat as composite coatings. The sprayed coatings were glazed using a Nd:YAG laser. The results displayed that both coatings illustrated similar mechanisms of hot corrosion. However, the reaction between corrosive molten salt with Y2O3 and production of YVO4, leaching Y2O3 from YSZ, led to a destructive phase transformation of tetragonal-ZrO2 phase (t-ZrO2) to monoclinic-ZrO2 phase (m-ZrO2) and production of YVO4, accompanied by 3–5% volume expansion during cooling which could lead to cracking and degradation of TBCs. Findings revealed that by comparing the destabilization fractions of coatings, the content of m-ZrO2 fraction decreased from 80% in sprayed coating to 47% in laser glazed YSZ + Al2O3 coating, which displayed double improvement of hot corrosion resistance in laser glazing process. Reduction of the reaction between molten salt and zirconia stabilizer was the result of a surface roughness decrease and a specific reactive surface gained by laser glazing process.