Effect of Extrusion on the Microstructure and Tribological Behavior of Copper–Tin Composites Containing MoS2

This article deals with the effect of extrusion on the microstructures and tribological properties of powder metallurgy–fabricated copper–tin composites containing MoS₂ by optical microscopy, scanning electron microscopy (SEM), and tribotesting. The extrusion decreases the number of pores and increases the density and hardness and thus improves the tribological properties of the composites. Results demonstrated that abrasion is the dominant wear mechanism in all extruded composites, whereas a combination of adhesion and delamination appears to be the governing mechanism for prepared composites. The developed hot-extruded composites exhibited lower coefficient of friction and wear rates compared to prepared composites. Design Expert software was used to develop contour map.     

Theoretical Study on Tribological Mechanism of Solid Lubricating Films in a Sand–Dust Environment

A model based on energy dissipation was developed to describe the tribological behavior of solid lubricant films in sand–dust environment. A relationship between wear rate and coefficient of friction was obtained. Theoretical results kept well consistent with the experimental data in reported publications (Qi et al., Tribol Lett 38:195–205, 2010; Surf Interface Anal 43:836–846, 2011; Wear 271:899–910, 2011). It was pointed out that the absolute value of slope of the simulated straight line is closely related with mechanical properties of solid lubricant films. The results increased our understanding about the individual friction and wear mechanism for solid lubricant films in sand–dust environment.     

Environmental Effects on the Composition of Surface Films Produced by an Organo-Molybdenum Compound©

The influence, of environments on the properties of the surface films formed, with molybdenum dithiocarbamate (MoDTC) was examined under a reciprocating sliding condition. MoS₂ was formed on the rubbing surfaces in air and oxygen, and molybdenum compounds with an oxidation state lower than MoS₂ were produced in nitrogen and argon. The surface film composed of MoS₂ was effective in reducing the friction and wear, while the molybdenum compound formed in nitrogen or argon had no ability to prevent direct contact between the rubbing surfaces and to reduce the friction. It was a necessary condition for forming the surface film composed of MoS₂ that the environment to be rubbed contained oxygen at a concentration above a certain level.     

Structural and Tribological Performance of Solid NiCr-WSe2-BaF2·CaF2-Y-hBN and NiCr-WSe2-BaF2·CaF2-Y Lubricant Coatings Produced by Atmospheric Plasma Spray

NiCr matrix WSe₂-BaF₂·CaF₂-Y-hBN and WSe₂-BaF₂·CaF₂-Y powders were prepared by mechanical granulation and crushing, and composite coatings were fabricated by atmospheric plasma spray technology. The microstructures and phase compositions of the powders, as well as the coatings, were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The friction coefficient and the wear behavior of the coatings from ambient temperature to 800°C were evaluated using a ball-on-disk tribometer. From the investigation of the worn surfaces, it was concluded that brittle fracture and delamination were the dominant wear mechanisms of the coatings at low temperature. At higher temperatures, a dense and protective oxide layer (BaCrO₄ and NiO) is generated on the worn surfaces of the coatings. Layered hexagonal BN particles reduce the direct contact and severe adhesion between friction pairs. Thus, the friction coefficient of the NiCr-WSe₂-BaF₂·CaF₂-Y-hBN coating is stable at the evaluated temperatures relative to the non-hBN coating. These fluorides exhibit excellent properties in these coatings over a large temperature range.