Stainless Steel Thermal Spray Coating of Cylinder Bores for VC-Turbo Engine

Nissan’s variable compression turbo (VC-Turbo) engine has a multilink mechanism that continuously adjusts the top and bottom dead centers of the piston to change the compression ratio and achieve both fuel economy and high power performance. Increasing the exhaust gas recirculation (EGR) rate is an effective way to further reduce the fuel consumption, although this increases the exhaust gas condensation in the cylinder bores, causing a more corrosive environment. When the EGR rate is increased in a VC-Turbo engine, the combined effect of piston sliding and exhaust gas condensation at the top dead center accelerates the corrosive wear of the thermal spray coating. Stainless steel coating is used to improve the corrosion resistance, but the adhesion strength between the coating and the cylinder bores is reduced. Trial production of the coatings with different linear expansion coefficients was conducted, and the sensitivity of the linear expansion coefficient and adhesion strength was obtained. Consequently, the adhesion strength was found to have a maximum value with respect to the linear expansion coefficient. By contrast, the surface observation of this coating after the honing process showed an increase in surface porosity. The martensitic content in the coating was reduced, and the appropriate martensitic content and chemical composition with the optimal corrosion resistance, adhesion strength, and surface porosity were found. Using this coating (0.01C-12Cr-0.22Ni-0.35Mn), the corrosive wear at the top dead center was resolved. Consequently, the fuel economy was improved by more than 4% compared with that of the current VC-Turbo engine, which has already adopted a carbon-steel-based coating.