CONTROL OF FRETTING FRICTION AND WEAR OF ROPING WIRE BY LASER SURFACE ALLOYING AND PHYSICAL VAPOR-DEPOSITION COATINGS

Surface alloying and thin coatings were applied to roping wire as a means of suppressing fretting wear and friction. Wires were treated by physical vapour deposition (PVD), electroplating and laser surface alloying (LSA). The wear and friction levels of treated wires under dry fretting were measured using a high frequency wear test rig. Chromium and zirconium were chosen as coating materials based on previous evidence for fretting resistance, corrosion resistance and possible strengthening of steel wire. A solid lubricant, molybdenum disulphide, was also deposited by PVD. It was found that LSA provided a more durable friction reduction than PVD coatings. The friction of molybdenum disulphide coating was dependent on coating thickness: a thin coating failed rapidly after an initially low friction, while thicker films afforded a gradual rise in friction from a similar low initial friction coefficient. LSA enabled friction to be raised as well as lowered and laser treatment without alloying had little effect on friction. When high levels of zirconium were used in LSA, a large reduction in wear resulted and laser hardening alone also caused a decrease in wear.