Investigation of scuffing damage in aluminum engines with thermal spray coatings

The development of lightweight internal combustion engines using materials such as cast aluminum alloys represents one of the most significant technological developments in automotive industry. Our previous work has shown that iron-based thermal spray coatings deposited on aluminum cylinder bores exhibited good wear resistance under the laboratory testing conditions. Meanwhile, it is important to characterize the surface and subsurface damage in the actual engines to understand details of the microstructural processes involved in engine wear and scuffing. The micromechanisms responsible for scuffing damage in a combustion engine that failed under racetrack conditions were investigated. The inner surfaces of the engine blocks were coated with HVOF 1020–2.5% Al type low carbon steel thermal spray coating. The coating was produced by a high velocity oxy-fuel thermal spray process from an ASTM 1020 wire stock with 2.5 wt.% Al addition. Microscopic observations of the scuffed cylinder walls showed that the damage depended strongly on the position around the wall. The highest degree of scuffing damage occurred in the direction of the major thrust face. SEM observations have revealed the presence of mechanically mixed tribolayers. These tribolayers were composed of a matrix of deformed iron with fine particles of inclusions. The tribolayers had a nanocrystalline structure with grains as small as 20 nm, as revealed by the TEM. Plastic deformation beneath the mechanically mixed layer on the major face exhibited a unidirectional pattern. This was due to the high loads applied during the power stroke of the combustion cycle. The delamination of the tribolayers was the principal source of material removal during scuffing. This process was facilitated by crack formation at the FeAlO3 particles that were present in the HVOF 1020–2.5% Al coatings as well as at FeO veins between the iron splats.