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.