Effect of Stiff Coatings on EHL Film Thickness in Point Contacts

Coatings are widely used for interface performance enhancement and component life improvement, as well as for corrosion prevention and surface decoration. More and more mechanical components, especially those working under severe conditions, are coated with stiff (hard) thin coatings. However, the effects of coatings on lubrication characteristics, such as film thickness and friction, have not been well understood, and designing coating for optimal tribological performance is a grand challenge. In this paper, the influences of coating material properties and coating thickness on lubricant film thickness are investigated based on a point-contact isothermal elastohydrodynamic lubrication (EHL) model developed recently by the authors. The results present the trend of minimum film thickness variation as a function of coating thickness and elastic modulus under a wide range of working conditions. Curve fitting of numerical results indicates that the maximum increase in minimum film thickness, $Imax$⁠, and the corresponding optimal dimensionless coating thickness, $H2max$⁠, can be expressed in the following forms: $Imax=0.769M0.0238R20.0297L0.1376exp(−0.0243ln2L)$ and $H2max=0.049M0.4557R2−0.1722L0.7611exp(−0.0504ln2M−0.0921ln2L)$⁠. These formulas can be used to estimate the effect of coatings on film thickness for EHL applications.