Friction of Tungsten-Based Coatings of Steel under Sliding Contact
Introduction
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. Often (but not exclusively) friction is desired to be minimized between two surfaces in sliding contact, as friction can cause a reduction in mechanical efficiency, physical, and material damage to the surfaces, as well as result in damaging heat from friction energy losses. Metals are unique for having a high non-lubricated friction coefficient, and lubricants including oils, greases, and dry surface coatings [1–13] are often used to reduce the friction during sliding contact.
Often in engineering, a hard-steel surface will be coated with tungsten carbide (WC) [14–23], applied as a surface treatment, often using High Velocity Oxygen Fuel (HVOF) to apply the WC coating as described in the standard AMS2448A [24]. The WC coating serves to protect the steel surface. Inherently, 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 thus, due to the pattern of random asperities within the surface, the true contact area (Figure 1) is reduced. As the friction force is determined as the product of the shear stress and the true contact area [25–28]
F = τ·A, (1)
the WC coating will reduce the COF marginally due to reduced true contact area. This has been observed in published [29] COF for non-lubricated steel–steel contact (COF = 0.8) versus steel–WC contact (COF = 0.6).
Matthew David Marko
Naval Air Warfare Center Aircraft Division, Joint-Base McGuire-Dix-Lakehurst, Lakehurst, NJ 08733, USA; matthew.marko@navy.mil
† NAVAIR Public Release 2018-609 Distribution Statement A—”Approved for public release; distribution is unlimited”.