PDA/PTFE Solid Lubricant Coating for 60NiTi Applications

The intermetallic alloy 60NiTi has a unique combination of high hardness and low elastic modulus, which makes it highly resistant to dents. Additionally, 60NiTi is extremely corrosion resistant and chemically inert. These properties make 60NiTi a desirable material for challenging mechanical component applications with high contact stresses and in harsh environments. However, lubrication issues have hindered the use of 60NiTi because it has poor tribological performance if it is not properly lubricated. The mechanical properties of hardened 60NiTi and its microconstituents were studied by nanoindentation. This study showed that the bulk properties of 60NiTi are driven by the properties of the NiTi + Ni4Ti3 region. The large Ni3Ti precipitates were found to have significantly higher hardness. This discovery inspired new theories for the wear behavior of 60NiTi. Despite its high hardness and extraordinary hardness-to-elasticity ratio, 60NiTi has poor tribological performance in unlubricated sliding. Since oil-based lubrication cannot be used in many applications, there is need for a suitable solid lubricant coating that can be applied to 60NiTi. A polytetrafluoroethylene (PTFE)-based solid lubricant coating that uses a polydopamine (PDA) adhesive underlayer was developed and evaluated for use on 60NiTi. PDA/PTFE coating was evaluated on 60NiTi substrate by linear-reciprocating wear tests against a Si3N4 ball. The coating reduced friction and protected the substrate surface from adhesive and abrasive wear during accelerated testing. The durability of the coating was drastically improved by grinding and polishing the substrate surface to produce a valley-dominant surface. The valleys of the substrate surface provided mechanical interlocking of the PDA/PTFE coating. Nanoindenter scratch tests and scanning electron microscopy imaging of the scratches provided a detailed understanding of the microstructural behavior and failure mechanisms of the coating. Additionally, incorporating graphite particles in the PTFE layer was shown to further enhance the coating performance. The coating was found to compact under repetitious sliding under normal loads to form a less-porous structure with enhanced cohesion. The valley-dominant substrate surface reduced tensile stresses in the coating, and the graphite particles added to the PTFE layer enhanced the coating cohesion. The combined effect was better resistance to coating tears that could lead to global coating delamination. Lastly, methods were developed to apply and test the PDA/PTFE coating as a ball bearing lubricant. A custom bearing tester was designed and built to study the effectiveness of the coating in ball bearing applications. The balls and races of hybrid R8 bearings were coated with PDA/PTFE or PDA/PTFE+GPs, and the bearing tests showed promising results. This work advanced the understanding of the unique mechanical properties of 60NiTi and developed PDA/PTFE solid lubricant coatings that are suitable solid lubricants for 60NiTi. Additionally, the microstructural behavior and failure mechanisms of PDA/PTFE-based coatings are better understood. The results are promising solutions for lubricating 60NiTi in challenging mechanical component applications