A Wear Equation for Bonded Solid Lubricant Films: Estimating Film Wear Life

Abstract

A review of the literature establishes that the wear of bonded solid lubricant films for conditions of low average contact stress (say 10,000 psi and below), obeys an equation of the form V = KPL, where V is the wear volume, P is the normal load, L is the total sliding distance, and K is a constant. The limits on allowable bonded film thickness are examined, in the light of this equation, and it is shown that for low contact stress situations optimum film thickness is on the order of 10−3 in., rather than the popularly believed value of 10−4 in. This latter figure is the unfortunate consequence of applying data obtained at very high contact stress to contact conditions which are fundamentally different. Practical aspects of using this equation are explored by the development of specialized expressions which allow a design engineer or engineering analyst to estimate film wear life in terms of hours of operation or number of operating cycles. To aid in this purpose, wear coefficients for common solid lubricant-binder combinations are tabulated. A series of examples show how these expressions can be applied to typical situations. Equations are derived for the contact stress, area of contact, and average contact stress, for solid lubricant film covered surfaces under cylindrical or spherical contact. The use of these equations should replace the use of the Hertz contact equations in analyzing solid lubricated contact conditions.