A comparative study of the differences in microstructure and properties of tribo-layers of DD5 nickel-based single-crystal superalloys caused by external heat and friction heat in dry sliding wear

In high-temperature sliding wear, the tribo-layer of DD5 nickel-based single-crystal superalloy caused by external heat applied at high ambient temperatures has been well investigated, but the mechanical properties, microstructure, and chemical composition of the tribo-layer caused by friction heat are still not clear. The differences between the tribo-layers of DD5 nickel-based single-crystal superalloys caused by external heat and friction heat in dry sliding wear were investigated firstly by a high-temperature tribometer and a high-speed rubbing tester. The microstructures and chemical compositions of the two tribo-layers were characterized by a field emission scanning electron microscope (SEM), a transmission electron microscope (TEM), a Raman spectroscopy, and an electron backscattering diffraction (EBSD) technique. The mechanical properties of the tribo-layers were tested by a Vickers hardness tester. The results show that the tribo-layer caused by friction heat exhibits thinner thickness, higher wear scar coverage, lower microhardness, and poorer adhesion compared to the tribo-layer caused by external heat. There are significant differences in the microstructure and chemical composition of the tribo-layers caused by external heat and friction heat. The tribo-layer caused by external heat consists of a mechanically mixed layer, an internal oxidized layer, and an ultrafine grain layer due to plastic deformation. The tribo-layer caused by friction heat is composed of a single nanocrystalline layer. The top of the nanocrystalline layer exhibits a columnar crystal structure, and the bottom is an equiaxial crystal structure. In addition, there is a difference in the way of crack initiation and extension in the tribo-layers caused by external heat and friction heat.