Tribological behavior of select MAX phases against Al2O3 at elevated temperatures

The layered Mn+1ACn ternary carbides – MAX phases – Ta2AlC, Ti2AlC, Cr2AlC and Ti3SiC2 were tested under dry sliding conditions against alumina at 550 °C and 3 N load (for a stress of ≈0.08 MPa) using a pin-on-disk tribometer. Ta2AlC and Ti2AlC exhibited low specific wear rates, SWRs, (≤1 × 10−6 mm3/N m), while the coefficients of friction, μ, were ∼0.9 and ∼0.6, respectively. At 0.4, μ of Ti3SiC2 was the lowest measured, but the SWR, at ≈2 × 10−4 mm3/N m, was high. With a μ of 0.44 and a SWR of 6 × 10−5 mm3/N m the Cr2AlC sample was in between. No visible wear of Al2O3 counterparts was observed in all the tribocouples. Tribofilms, which were mainly comprised of X-ray amorphous oxides of the M and A elements and, in some cases, unoxidized grains of the corresponding MAX phases, were formed on the contact surfaces. The correlations between observed tribological properties and tribofilm characteristics are discussed.