Dry and lubricated friction behaviour of thermal spray low carbon steel coatings: Effect of oxidational wear

Abstract

The development of thermal spray steel coatings for Al-Si engine blocks with wear resistant cylinder bores constitutes an important aspect of automotive lightweighting strategy. In this work tribological behaviour of low carbon steel cylinder bore coatings produced by a plasma transferred wire arc (PTWA) method were studied using samples sliding against CrN coated top compression ring samples. The focus of the work was to study the role of oxidational wear on the friction behaviour of spray coatings consisting of α-Fe splats delineated by thin FeO layers. In-situ Raman spectroscopy was employed to observe the oxide transformations that occurred during dry and lubricated sliding tests conducted using a reciprocating tribometer. Lubricated sliding tests were performed within a speed range of 0.02–0.92 m/s covering the boundary and mixed lubrication conditions using a Polyol ester, POE, base oil with and without zinc dialkyldithiophosphate (ZDDP) addition. The use of ZDDP containing POE led to the generation of an amorphous carbon tribolayer incorporating ZnS and FePO4 nano-crystals that inhibited sliding-induced Fe2O3 debris formation, and consequently resulted in a reduced wear rate, but the steady-state coefficient of friction (COF) was higher during the boundary lubricated sliding. According to the Stribeck curves generated, the prevention of oxidational wear by the formation of oil residue layer delayed the transition from the boundary lubrication to the mixed lubrication regime.