Evolution of nanocrystalline “glaze” layers and subsurface ultrafine grain layers in high-temperature sliding wear

The transition from severe to mild wear is an interesting phenomenon, which is usually attributed to the nanocrystalline “glaze” layer and subsurface ultrafine grain layer. However, the formation of “glaze” layers and subsurfaces and their effect on wear mechanism are still unclear. The high-temperature tribological behaviors of the DD5 single-crystal superalloy and the electro spark-deposited NiAlTa coating were investigated at 25~1000 ℃ by a high-temperature tribometer. The microstructures, chemical compositions, and grain orientations of the “glaze” layers and subsurfaces were studied. NiAlTa coatings show excellent wear resistance compared with DD5 superalloys, which is attributed to the excellent high-temperature softening resistance, high microhardness of the “glaze” layer, and good strain-hardening capacity of the subsurface ultrafine grain layer. Quantitative analysis reveals that whether the oxides on the wear surface play an antiwear lubrication role or not depends on the content and properties of the oxides. The 100-fold difference in wear rate indicates that the high-temperature wear resistance of material is related to the chemical composition and microstructure of the “glaze” layers and subsurfaces. A mechanistic model is proposed to describe the evolution of nanocrystalline “glaze” layers and subsurfaces ultrafine grain layer “glaze” layers.