Effect of Applied Load and Surface Roughness on the Tribological Properties of Ni-Based Superalloys Versus Ta2AlC/Ag or Cr2AlC/Ag Composites

The novel Ta2AlC-20 vol.% Ag (TaAg) and Cr2AlC-20 vol.% Ag (CrAg) composites were tribologically tested versus a Ni-based superalloy Inc718 (SA) by dry sliding at a sliding speed of 1 m/s at room temperature in air at loads from 3 N to 18 N. The TaAg composites were also tested at 8 and 18 N at 550 degrees C, and at a 3 N load against the SA with different surface roughnesses at 26 degrees C and 550 degrees C. At room temperatures, the coefficients of friction, mu's, decreased from similar to 0.8-0.9 to similar to 0.3-0.4 for both the TaAg and CrAg composites as the applied normal force increased from 3 N to 8 N. Further increases in load to 18 N did not change the mu's. The specific wear rates, sWR, increased with increased loads for the TaAg composite; they remained almost unchanged for the CrAg composite. This behavior was attributed to the formation of glaze tribofilms-similar to ones observed previously in these tribocouples at elevated temperatures and 3 N-promoted by the increased loads. Preconditioning of the SA surface by sliding against the TaAg composite at 550 degrees C and 8 N resulted in mu's of <0.2 and sWR < 10(-6) mm(3)/N-m in subsequent room temperature sliding at 3 N. Somewhat higher, but stable room temperature mu's of similar to 0.3 and sWR of similar to 3 x 10(-5) mm(3)/N-m were observed when the TaAg composites were slid versus a sandblasted SA surface at 500 degrees C and 3 N. It follows that in situ preconditioning of the tribo-surfaces is a powerful tool for improving the properties of the MAX/Ag-SA tribocouples. The relationship between sliding conditions, chemistries of tribofilms, and their properties are discussed.