Tribology of MAX phases and their composites

Surojit Gupta

doi:10.17918/etd-875

Currently there is a need for triboactive materials for high-speed turbomachinery applications in industry, which possess: (a) adequate mechanical strength, both at room and elevated temperatures and, (b) low wear rates, WRs, and low friction coefficients, [mu], over a wide temperature range. If such materials can be found, the impact would be huge since they would result in increased efficiencies and reduced pollution. This is an outstanding problem that many in industry have been trying to solve for the past 20 years. In this work, the tribological behavior of the MAX phases, and their composites with Ag have been studied for foil-bearing application. Initially, the tribological behavior - at 26 °C and 550 °C - of the following layered ternary carbides: Ti₂AlC, Cr₂AlC, Ta₂AlC, Ti₃SiC₂, Ti₂AlN, Ti₄AlN₃, Cr₂GeC, Cr₂GaC, Nb₂SnC and Ti₂SnC, tested against Ni-based superalloys (Inc718 and Inc600) and alumina, Al₂O₃, were studied. The high temperature tribo-properties were acceptable and in some cases, exceptional; but at room temperatures, the WRs were too high. Since the addition of Ag is known to improve the tribological behavior at room temperature, it was used to liquid-phase sinter Ta₂AlC or Cr₂AlC composites. They were tested against a Ni-based superalloy (In718) and alumina. For foil-bearing applications, Ni-based superalloys are the best of choice for the foils. The tribocouples were tested for the most part using a force of 3N at 1 m/s at 25°C, 350 °C and 550 °C for at least 1 km of dry sliding. Over the entire temperature range, the WRs were [less than or equal to] 10⁻⁴ mm³/N-m and [mu] [less than or equal to] 0.5. Essentially similar results were obtained when the temperature was cycled between ambient and 550 °C. Finally, hot isostatically pressed Ta₂AlC/Ag and Cr₂AlC/Ag cylinders were machined was successfully tested in a foil-bearing rig test for 10,000 and 3,000 stop-start cycles, respectively. When processed in the presence of liquid Ag, Al from the basal planes of the MAX phases reacts with the Ag to form Ag₂Al. This grain boundary phase prevents the formation of abrasive third bodies. As a result, the WRs of the MAX/Ag pins were reduced by ~ 3 orders of magnitude as compared to the pure MAX phases. During testing at 550 °C and under thermal cycling conditions, the MAX/Ag lubricating tribooxides, comprised mainly of the elements of the Inc718 (Ni, Fe, Cr) and Al from the MAX phases, were formed. The presence of these layers is responsible for the low WRs of MAX/Ag composites WR ([less than or equal to] 10⁻⁵ mm³/N-m), and relatively high WR (~ 10⁻⁴ mm³/N-m) of the Inc718, accompanied by [mu]'s of ~ 0.5. The films formed after rig testing were essentially the same as those obtained in the pin-on-disk experiments. As important, the lab-scale results obtained with the tribometer correlated well with pilot testing of the samples under stringent rig-testing conditions.

Tribology of MAX phases and their composites

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Tribology of MAX phases and their composites

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