Properties of three thermal sprayed coatings were compared to select the effective sprayed coating for giving the thermal barrier to the Al alloy components of internal combustion engines. Results for coating 1 (top coat, Al2O3; bond coat, Ni-Cr), coating 2 (top coat, YSZ; bond coat, CoNiCrAlY), and coating 3 (SUS316) are presented below. (1) After thermal cycle testing, no crack or delamination was apparent in coating 3.Partial cracks and delamination were observed in the top coating in coatings 1 and 2. (2) All coatings have sufficient adhesion strength. The adhesion strength of coating 2 was especially high. (3) The thermal barrier property of coating 2 was better than that of coating 1 or coating 3. (4) Comprehensive evaluation results show that coating 2 had a good thermal barrier property and that coating 3 is a reasonable material because of its low cost as a thermal sprayed coating applied to Al alloy components of an internal combustion engine.
Plasma electrolytic oxidation (PEO) is a surface treatment technology using anodic oxidation. Recently, PEO coatings have been developed industrially, mainly for application to surface treatment of light metals. This study was conducted to elucidate the effects of metal oxyacid salts such as AlO2−, SiO32− and [Zr(CO3)2(OH)2]2− on PEO coating formation. PEO coatings were formed from pyrophosphate electrolytes respectively containing metal oxyacid salts. PEO coatings showed high hardness as well as metal oxides sintered by molting and cooling. Each metal oxyacid salt was included in the coating, but surface morphology characteristics such as discharge channels and cracks mutually differed. Both the formation rate and contents of the metal oxyacid salts in the PEO coating increased in the following order: AlO2− <[Zr(CO3)2(OH)2]2−<SiO32−. Results show that addition of the metal oxyacid salts enhanced the PEO coating formation rate and uniformity. Presumably, this effect resulted from stability of the metal oxyacid salts in the electrolytes.