Fabrication and tribological properties of composite coatings produced by lithographic and microbeading methods

The deposition of composite coatings for tribological applications is reviewed in this article. The focus of recent research has been to fabricate hard coatings that incorporate noble metals with the goal of reducing friction in dry contact. In the present case, we consider coating systems intended for applications in temperature regimes where oxidation and thermal structural evolution is limited. Deposition of coatings with immiscible components (such as TiC/Ag) results in a composite-structured coating that under certain conditions (such as vacuum or high temperatures) has demonstrated reduced friction. Another approach is to pattern coatings by artificial methods such as masking, laser drilling, or photolithography. Our recent work has focused on photolithographic methods, as well as a new masking approach which we refer to as microbeading. Evaluation of microreservoir-containing coatings fabricated by the microbeading method was carried out by depositing a TiN film and using graphite as the solid lubricant. Pin-on-disk tests using an alumina counterface showed that substantial reductions in friction coefficients were obtained for the larger bead sizes (5–10 µm). The microbeading coating method was also implemented on cutting tools for machining where indium was used as the solid lubricant. Turning tests were conducted by high-speed machining of hardened 4340 steel, and the TiN–In coated inserts showed flank wear (in liquid lubricated machining conditions) of up to 4 times longer wear life than a TiN coating without indium. However, the role of the microreservoirs was not clear as the In coated TiN sample without microreservoirs also showed a significant performance improvement over TiN alone. Additional studies were made on samples using photolithography (TiN coatings incorporating either 4 or 9 µm reservoirs) and tested with various solid lubricants using a pin-on-disk test. We observed lubricant entrapment (within the reservoirs) of graphite, but also found examples of debris collection, both from the counterface (a steel ball) and hard coating (TiO2 transformed from TiN).