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

Abstract

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 Al₂O₃-based ceramic coatings, specifically targeting situations where reciprocating sliding wear predominates. This study introduces lanthanum oxide (La₂O₃), a rare earth oxide, as a reinforcing material in Al₂O₃ 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% Al₂O₃–3% h-BN–1.6% La₂O₃ reduced the porosity up to 50% and improved hardness by 46% compared to the 97% Al₂O₃–3% h-BN coating. The formation of the α-Al₂O₃ phase leads to an increase in coating hardness and a reduction in porosity. Notably, the coating comprising 97% Al₂O₃–3% 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 La₂O₃ (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 La₂O₃ in enhancing friction behavior through the formation of oxides such as α-Al₂O₃, B₂O₃, Al₈B₂O₁₅, and Al₁₁La_0.497O₁₇, 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.