Intermediate temperature tribological behavior of carbon nanotube reinforced plasma sprayed aluminum oxide coating

Tribological behavior of plasma sprayed carbon nanotube (CNT) reinforced aluminum oxide (Al2O3) composite coatings was examined at room temperature, 573 K and 873 K using tungsten carbide (WC) ball-on-disk tribometer. The weight loss due to wear of Al2O3 coating was found to be increasing with the temperature while Al2O3-CNT coating showed a decreasing trend in the weight loss with the temperature. Relative improvement in the wear resistance of Al2O3-CNT coating compared to Al2O3 coating was found to be 12% at room temperature which gradually increased to ∼ 56% at 573 K and ∼ 82% at 873 K. Protective layer as a result of tribo-chemical reaction was observed on the wear track of both of the coatings. The improvement in the wear resistance of Al2O3-CNT coating was attributed to three phenomena viz. (i) higher hardness at the elevated temperature as compared to Al2O3 coating, (ii) larger area coverage by protective film on the wear surface at the elevated temperature and (iii) CNT bridging between splats. The coefficient of friction (COF) of Al2O3 coating was nearly constant at room and elevated temperature whereas COF for Al2O3-CNT coating decreased at the elevated temperature (873 K).