Effect of Serpentine Mineral on Tribological Properties of Titanium Alloy under Oil Lubrication

Serpentine mineral powder has a unique metastable layered structure and excellent interfacial reaction activity on the friction surface. As a lubricating oil (grease) additive, the mineral can form a self-repairing layer with good friction-reducing and lubricating properties on the metal rubbing surface, realize the in-situ self-repair of micro damage on the worn surface, and at the same time strengthen the mechanical properties of the contact metal surface. It is considered a highly promising green and efficient lubricating material. In this paper, the surface modified natural serpentine mineral powder was introduced into the lubrication system for titanium alloy as a lubricating oil additive. Friction and wear behaviors of the surface modified serpentine mineral as a CD 5W/40 lubricating oil additive on Ti6Al4V alloy against steel was studied using the SRV4 reciprocating friction and wear tester. The effects of serpentine concentration in oil, applied load and reciprocating frequency on the tribological properties of titanium alloys under oil lubrication were systematically investigated. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and nano indentation were used to analyze the micro morphology, element composition and chemical state, and nano mechanical properties of the worn surface under different lubrication conditions. The mechanism of serpentine mineral powder in reducing the friction and wear of titanium alloy was discussed. The results showed that the natural serpentine mineral as an additive could significantly reducing the friction, wear and frictional heat of titanium alloy when grinding with steel under oil lubrication. Compared with the pure CD 5W/40 oil, the friction coefficient at the titanium alloy/steel interface and wear volume of metal lubricated with oil containing serpentine was decreased by 11.1%~55.9% and 11.6%~76.7%, respectively, and the friction surface temperature decreased by 6~20 ℃. The efficiency of serpentine mineral in improving the tribological properties of titanium alloy under oil lubrication was closely related to the applied tribological test conditions. When the addition of serpentine was 0.3%, the load was 10 N, and the sliding frequency was 5 Hz, the friction coefficient and wear volume under oil sample lubrication with serpentine were reduced by 55.9% and 76.7% respectively compared with those under base oil lubrication, the optimal condition for best tribological properties of titanium alloy was obtained. During the friction process, complex physical and chemical interactions had taken place between serpentine mineral powder and friction surface. Through a series of complex processes such as physical adsorption, mechanical polishing, and tribochemical reaction, a composite self-repairing layer composed of metal oxides, ceramic particles, serpentine, forsterite, graphite, and organic substances had been formed on the titanium alloy surface by the addition of serpentine into the oil. The self-repairing composite layer had a high hardness and an elastic modulus close to that of metal, exhibiting the dual characteristics of metal and ceramic. It had good plasticity and toughness, which was conducive to the adsorption and spreading of lubricating oil on the surface of titanium alloy and hinders direct contact between steel and titanium alloy surfaces. This effectively suppressed adhesive wear and reduces friction, significantly improving the tribological properties of titanium alloy under oil lubrication conditions.