The Tribological Properties of Zinc Borate Ultrafine Powder as a Lubricant Additive in Sunflower Oil

Abstract

This article presents an investigation on the tribological properties of zinc borate ultrafine powder (ZBUP) employed as a lubricant additive in sunflower oil. The stable dispersions of 0.5, 1, and 2 wt% ZBUP in sunflower oil were achieved using an ultrasonic homogenizer. Both a four-ball tester and a pin-on-disc tester were employed to evaluate the antiwear and friction reduction capabilities of ZBUP. Tribofilms with a dark color were generated on the worn surfaces and showed good contrast with the substrate. The worn surface with different morphologies reflected as color alterations on the worn surface were observed when different lubricants were applied. The morphology and elemental analysis of the worn surfaces were studied using atomic force microscopy (AFM) and scanning electronic microscopy (SEM). Mechanical properties of the tribofilms and substrates were studied with a nano-indentation tester. Test results suggest that tribofilms generated on the worn surface have relatively low hardness compared with the steel substrate. The substrates on the worn surfaces lubricated in sunflower oil with the powder demonstrated higher hardness than the substrate lubricated with pure sunflower oil due to the possible tribochemical reaction between the zinc borate additive and substrate. The combination of sunflower oil with 0.5% ZBUP delivered the most balanced performance in friction and wear reduction. This study has demonstrated the possibility of application of this industrially applicable solid lubricant additive (zinc borate) with a decomposable vegetable-based lubricant oil.