Microstructural alterations within thermal spray coatings during highly loaded diesel engine tests

The good wear characteristics of thermal spray coatings are related to structural, productional, and topographical properties. Finally, the alteration of the microstructure in different contact zones is essential for mild wear conditions that will guarantee a long-life cycle because it reveals the ability of the material to adjust to the current load situation. Previous studies have shown that in diverse tribosystems an in situ formation of a nanocrystalline layer together with mechanical mixing maintains long-lasting, wear resistant surfaces with low wear rates. This investigation exemplifies results from a cylinder that was run in a road test motor. The relevant piston ring positions that fully describe the tribosystem have to be defined at the combustion chamber (CC), top dead centre (TDC), stroke and bottom dead centre (BDC). Transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EFTEM) are used to analyse these contact zones in terms of microstructure and chemistry. Nanocrystalline surface layers up to a thickness of 100 nm occurred in all areas of contact whereas subsurface zones revealed differing microstructural changes. Thus the influence of thermal impact and different mechanical load conditions in a motor cylinder are evident. EFTEM elemental mappings verify the incorporation of elements that stem from lubrication or combustion residues.