Volume 5, Issue 2 (February 2008)
Wear and Friction Testing of Hard and Soft Lubricious Coatings in Dry Sliding for Use in Small Arms Action Components
Use of liquid lubricants in dry and dusty environments can lead to malfunction or jamming of small arms weapon actions. To eliminate the need for liquid lubricants, dry or “lubricious” coatings are desired for various key interfaces of such components. The tribological performance of a variety of coatings was assessed under unlubricated sliding conditions to simulate a pin-in-slot component geometry. Coating screening tests were run using a cylinder-on-flat geometry and self-mated coated couples. Tests were run in both reciprocating and unidirectional modes, with sliding velocities up to 3.6 m/s, at room temperature and 150°C. Progressive wear of the coating was monitored using a replicating technique and the measured width of the wear scar on the cylindrical pin, from which the wear coefficient was determined. Failure of the coating was determined not by visual wear-through of the coating, which can be misleading, but by an increase in the coefficient of friction (COF) and/or the occurrence of galling. In the high speed unidirectional tests, friction could be measured directly. However, in the reciprocating tests, test rig design limited the speed at which the COF could be determined. An improved rig design resulted from this work. The screening tests ranked the tribological performance of the coatings using the coefficient of friction, the wear coefficient, and number of cycles to coating failure. The coating types assessed covered a broad range of hardness, composition, and deposition methods. Coating types investigated included hard vapor-deposited TiCN coatings, relatively soft organic PTFE-filled resin-bonded coatings, modified anodized coatings (for aluminum), electroless nickel alloy coatings, PTFE-filled electroless nickel coatings, phosphate pre-treatment combined with dry-film burnished MoS2, and a ferritic nitro-carburizing treatment. Substrates included medium carbon steel, D2 tool steel, and 6061-T6 aluminum.