Friction and Wear Properties of Spark Plasma Sintering NiCr–SrSO₄ Composites at Elevated Temperatures in Sliding Against Alumina Ball

Nickel-based self-lubricating composites with SrSO₄ as a high-temperature solid lubricant have been prepared by spark plasma sintering method. Their friction and wear properties were evaluated using a high-temperature friction and wear tester in dry sliding against alumina ball up to 600 °C. NiCr–10 wt% SrSO₄ composite exhibits satisfactory mechanical properties and tribological properties from room temperature to 600 °C (except for 200 °C) as contrasted with unmodified Ni–Cr alloy. The friction coefficients of NiCr–10 wt% SrSO₄ at 400 and 600 °C are 0.32 and 0.20, respectively. The corresponding wear rates are 1.44 × 10⁻⁵ and 2.4 × 10⁻⁵ mm³/(Nm). The unmodified Ni–Cr alloy exhibits friction coefficients of 0.41 and wear rates in order of 10⁻⁴ mm³/(Nm) in a temperature range of 400–600 °C. At elevated temperatures, a dense and protective glaze layer is generated on the worn surface of composites. The glaze layer was then investigated by means of scanning electron microscopy and XPS. Tribo-chemical reaction during high-temperature wear tests is responsible for the formation of the protective glaze layer. The newly formed oxides (NiO and Cr₂O₃) and some self-lubricating oxysalts (SrSO₄ and SrCrO₄) can promote the densification of oxide glaze layer, and finally generate a dense lubricating film to effectively reduce friction and wear at elevated temperatures.