Abstract
An investigation was made to determine the effects of tungsten surface coating on the coefficient of friction of sliding contact between lubricated steel surfaces. The four-ball test was modified, using a tungsten carbide ball bearing in the spindle to cause sliding contact onto three hard steel ball bearings coated with tungsten disulfide lamellar dry lubricant coating, with a coating of grease lubrication applied to the ball bearings. The coatings, loads, speed, and grease level were varied to best understand the impact of different conditions on the friction coefficient.
Highlights
The ability to reduce the coefficient of friction (COF) during sliding contact between two steel surfaces is a capability with numerous applications in mechanical engineering design
The first series of test utilized the Light Grease (LG) configuration steel-on-steel, utilizing the hard steel ball bearings sold by Falex
The second series of test utilized the LG configuration steel-on-WS2, utilizing the hard steel ball bearings sold by Falex on top, and using the hard steel ball bearings sold by Falex and later coated with Dicronite WS2 as the three ball bearings on the bottom
Summary
The ability to reduce the coefficient of friction (COF) during sliding contact between two steel surfaces is a capability with numerous applications in mechanical engineering design. WC is harder than steel; it has a Young’s modulus of (typically) 500–700 GPa, significantly higher than the 200 GPa for steel With this increased stiffness, there is less expected deformation of the surface, and due to the pattern of random asperities within the surface, the true contact area (Figure 1) is reduced. Two hard surfaces in dry contact will have a small true contact area (m2 ), but the shear stress τ (Pa) will inherently be higher for hard materials, and the friction force F = A·τ will be high. This low true contact area and low shear stress result in a low friction force and reduced coefficient of friction Lamellar solids such as WS2 are defined as solids with a repeating molecular pattern, effectively sheets of molecules. Grease has its unique challenges in modeling and predicting the film thickness, as grease is a distinct lubricant that is a solid to semi-fluid mixture of a liquid lubricant and a thickening agent [44]
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