Solid lubricity of WS2 and Bi2S3 coatings deposited by plasma spraying and air spraying

Lubrication is highly relevant for the reliability and longevity of any moving machinery. Machines are mainly lubricated by oils or greases but there are many conditions especially for low and high temperature applications, under vacuum or for hygienic reasons where conventional oil and grease lubrication cannot be applied. Under these circumstances, solid lubricants such as those based on layered materials (MoS2, WS2 or Bi2S3) are the right choice to maintain a safe and reliable operation of machines. However, the tribological performance of the solid lubricant does not only depend on the chemical composition but also the deposition technique used. In this research article, we deposited coatings of WS2 and Bi2S3 by atmospheric plasma spraying and a simple air spray coating process. The friction and wear behaviour was studied at three different temperatures: room temperature, 100 °C, and 200 °C. To gain insight into the acting lubrication mechanisms, the morphological and chemical properties of the resulting wear tracks were investigated by light microscopy and Raman spectroscopy. It was demonstrated that atmospheric plasma spraying leads to a thermal decomposition of the feedstock powders and is therefore not recommendable as deposition technique. In contrast, simple air spraying results in well-performing solid lubricant coatings with superior friction and wear properties for WS2 compared to Bi2S3 even at an elevated temperature of 200 °C.