Synergism of h-BN and La ₂ O ₃ in improving the tribological performance of Al ₂ O ₃ coatings

Wear is a significant cause of material loss, contributing to surface degradation of crucial components, notably within engineering sectors. This article is dedicated to enhancing tribological properties through the implementation of Al 2 O 3 -based ceramic coatings, specifically targeting situations where reciprocating sliding wear predominates. This study introduces lanthanum oxide (La 2 O 3 ), a rare earth oxide, as a reinforcing material in Al 2 O 3 coatings in varying weight percentages of 1.2%, 1.6%, and 1.8% respectively along with 3% hexagonal boron nitride (h-BN). The coatings were developed by the high-velocity oxy-fuel (HVOF) surface modification technique, with SS304 serving as the substrate. The porosity, hardness, and chemical composition of the developed coatings were examined. Additionally, reciprocating tribological testing of the coatings was conducted at two loads (5 N and 15 N) under dry sliding conditions. Results indicated that coating containing 95.4% Al 2 O 3 –3% h-BN–1.6% La 2 O 3 reduced the porosity up to 50% and improved hardness by 46% compared to the 97% Al 2 O 3 –3% h-BN coating. The formation of the α-Al 2 O 3 phase leads to an increase in coating hardness and a reduction in porosity. Notably, the coating comprising 97% Al 2 O 3 –3% h-BN exhibited the lowest coefficient of friction among all developed coatings under both loads. Furthermore, an increase in load led to a decrease in the coefficient of friction for all developed coatings. Furthermore, the addition of La 2 O 3 (1.6 wt%) to the coating with 3 wt% h-BN exhibited the lowest wear at both loads. The findings also revealed increased wear at high loads for all developed coatings. SEM, EDS, and XRD analysis of the worn surfaces revealed the contribution of h-BN and La 2 O 3 in enhancing friction behavior through the formation of oxides such as α-Al 2 O 3 , B 2 O 3 , Al 8 B 2 O 15 , and Al 11 La 0.497 O 17 , along with alterations in wear mechanism and the size of wear debris. The composite coatings developed in this study shall be helpful in advanced engineering applications.