Heat protective properties of NiCrAlY/Al2O3 gradient ceramic coating fabricated by plasma spraying and slurry spraying

Abstract

Al2O3 non-gradient ceramic coating (marked as 1# coating) and NiCrAlY/Al2O3 gradient ceramic coating (marked as 2# coating) were prepared with the plasma spraying and slurry spraying methods, respectively. The coating specimens were characterized by scanning electron microscopy and X-ray diffractometry. The high temperature oxidation kinetics curves of the coatings were examined; the bonding strength between the coatings and the metal substrates was studied, and analyses were conducted on the thermal shock resistance and the failure mechanism of the coatings. The results show that spraying of Al2O3 ceramic slurries can effectively fill the cracks generated during the process of plasma spraying of NiCrAlY coatings, and metallurgical bonds form between the coatings and the metal substrates. α-Al2O3 and rutile TiO2 are the main crystalline phase structures of the temperature resistant ceramic coatings. The oxidation rate constant of 2# coating was 1.323×10−4mg2cm−4h−1, which shows the best resistance to high temperature oxidation. The effect of high-temperature melting of the Al metal in the bonding layer was found to be beneficial to the bonding strength. Under the same temperature conditions, 2# coating had a higher bonding strength than 1# coating, and when the oxidation time was 96h, its bonding strength reached up to 26.9MPa. The thermal expansion coefficient of the NiCrAlY bonding coating fell between those of the metal substrate and ceramic top coating, thus reducing the thermal expansion coefficient difference between the two and decreasing the rate of propagation of the cracks to improve the thermal shock resistance of the coating. An integrated study indicated that 2# coating had a better heat protective performance for the metal substrates.