Tribological behavior of select MAX phases against Al2O3 at elevated temperatures

The layered M(n+1)AC(n) ternary carbides-MAX phases-Ta2AlC, Ti2AlC, Cr2AlC and Ti3SiC2 were tested under dry sliding conditions against alumina at 550 degrees C and 3 N load (for a stress of approximate to 0.08 MPa) using a pin-on-disk tribometer. Ta2AlC and Ti2AlC exhibited low specific wear rates, SWRs, (<= 1 x 10(-6) mm(3)/N m), while the coefficients of friction, mu, were similar to 0.9 and similar to 0.6, respectively. At 0.4, mu of Ti3SiC2 was the lowest measured, but the SWR, at approximate to 2 x 10(-4) mm(3)/N m, was high. With mu of 0.44 and a SWR of 6 x 10(-5) mm(3)/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. (C) 2007 Elsevier B.V. All rights reserved.