Wear response of crystalline nanocomposite and glassy Al2O3-SiC coatings subjected to simulated piston ring/cylinder wall tests

Abstract

The present paper describes the major part of a recent research investigation whose main purpose was to develop a series of novel functional coatings based on the Al2O3-SiC system that ideally would have a nanocomposite microstructure. Such coatings should be well suited for the wear protection of steel and cast-iron piston rings used in automotive internal combustion engines. Two methods were investigated: (i) plasma spraying and (ii) magnetron sputtering. The former was applied to cast-iron and plain carbon steel, whereas the latter was applied to stainless steel piston rings. The plasma sprayed coatings were porous with a hardness of 900-1150 HV, whereas the sputtered coatings were glassy and dense with a hardness of 701-788 HV. High-speed reciprocation wear tests, which simulate the piston ring/cylinder wall environment, were used to assess all coatings. CKS-36 coated cast-iron rings, one of the more common industry standards for this application, were also evaluated. Results showed these to be the most wear resistant. They were smoothly worn, possibly by a microabrasion wear process. The plasma sprayed coatings displayed two main types of wear surface: rough and smooth. The former were mainly generated by a process of grain or particle pull-out, whereas the latter resulted from a combination of microabrasion and microflaking mechanisms. The use of diamond grinding, for finishing the plasma sprayed coatings, mitigated failure through grain pull-out, but wear rates still remained slightly too high compared with uncoated stainless steel and CKS-36 coated cast-iron rings. Most of the sputtered coatings displayed varying degrees of smooth wear which was mitigated as the coating hardness increased. However, this trend was masked by sporadic coating loss through adhesive and/or cohesive failure. The creation of a gradated sputter coating having a core composition based on the Al2O3-SiC variant containing most Si and C and incorporating a bond layer enriched in Cr, adjacent the substrate, eliminated cohesive and adhesive coating failures. Future work should concentrate on the development and evaluation of sputtered crystalline Al2O3-SiC coatings and HVOF sprayed Al2O3-SiC variants. There may also be significant potential in investigating glassy Al2O3-SiC coatings containing significantly higher quantities of Si and C to those reported here.