Friction and wear properties of hot-pressed NiCr-BaCr₂O₄ high temperature self-lubricating composites

In recent years, the increasing potential for the use of high temperature self-lubricating composites as sliding components at extreme environments have attracted great attention on these materials. In the present work, NiCr-BaCr₂O₄ self-lubricating composites have been prepared using the hot pressing method. The friction and wear properties of NiCr-BaCr₂O₄ composites were investigated by using a ball-on-disk friction and wear tester in sliding against alumina ball up to 800°C. At room temperature, the NiCr-20BaCr₂O₄ composite has a friction coefficient of 0.72, which is clearly higher than that (0.44) of NiCr alloy. However, the composites incorporated with BaCr₂O₄ exhibit distinct improvements in effectively reducing friction and wear in a temperature range of 400-800°C as contrasted with unmodified NiCr alloy. At 800°C, the NiCr-20BaCr₂O₄ composite exhibits a friction coefficient of 0.27 and a wear rate of 4.5×10^-6mm³/(Nm), while the unmodified NiCr alloy has a friction coefficient of 0.41 and a wear rate of 9.2×10^-5mm³/(Nm), respectively. At elevated temperatures, a dense and protective oxide glaze layer is generated on the worn surface of composites. The glaze layer was then investigated by means of scanning electron microscopy, laser Raman spectroscopy and nano-indentation. Tribo-oxidation reaction of BaCr₂O₄ during high temperature wear tests is responsible for the formation of the protective glaze layer. The newly formed BaCrO₄ contains lubricious hexavalent-Cr⁶⁺, which can promote the densification of oxide glaze layer, and finally generate a dense lubricating film to effectively reduce friction and wear at elevated temperatures.