This research effort focuses on a new approach to high-temperature self-lubrication, using wear-resistant nanoporous coatings. The key idea is to control and guide the flow of easily-sheared solid lubricant inclusions through nanopore channels within the hard matrix of a composite coating towards its sliding contact surface. It is envisioned that the lubricant flow can be controlled by (i) the original lubricant agglomerate size, (ii) the channel width, and (iii) a thin barrier-layer that allows diffusion only exactly where wear indicates lubricant depletion. The primary advantage of this approach over isotropic or multilayered adaptive composite coatings is the much smaller amount of wear that is required to initiate the supply of additional lubricant to the surface, resulting in less abrasive wear debris and, in turn, greatly enhanced coating life-time.
While specifically exploring silver solid lubricant inclusions within a chromium nitride nanoporous matrix, this new strategy is applicable to a range of coating systems and has the potential to become a true breakthrough technology by providing low friction surfaces in various environments and during multiple temperature cycles ranging from 0 to ~1000°C. Applications include hard wear-resistant lubricious coatings for high-temperature bearings in fuel-efficient jet-engines and gas-turbines, solid-lubrication in cyclic air-vacuum environments for space applications, and oil-free air-foil bearings in gas compressors for fuel cells and hydrogen storage. The research effort will also provide the dissertation experience of one graduate student through completion of the doctoral degree, as well as a mind-broadening experience for several undergraduate students to also be involved in the research laboratories. Finally, a hands-on friction and wear sliding test involving solid lubricant-containing materials will be performed by over 200 mechanical engineering undergraduate students per year in their required junior-level Mechanical Systems Laboratory course, providing them direct exposure to the field of engineered multi-functional adaptive materials.
This project has explored a new method to provide high-temperature lubrication. Lubrication at high temperatures (300-1000 °C) presents a considerable challenge to the tribology community. At elevated temperatures, conventional liquid lubricants oxidize or thermally degrade and the most common lattice-layered solid lubricants like graphite and molybdenum disulfide oxidize quickly above 350 °C. High temperature lubrication will ultimately facilitate the development of high-temperature bearings in fuel-efficient jet-engines and gas-turbines, solid-lubrication in cyclic air-vacuum environments for space applications, and oil-free air-foil bearings in gas compressors for fuel cells and hydrogen storage. The project has successfully demonstrated that the proposed new lubrication method works (within the constraints of a laboratory environment). In particular, it has been demonstrated that solid lubricant flow can be guided through nanopore channels within a composite coating to the sliding contact surface. A key finding of the study is that the temperature at which a self-lubricating coating is deposited onto the substrate that it protects determines the temperature at which the coating most effectively acts as a lubricant. This is because the deposition temperature determines the microstructure and porosity of the coating material, which in turn determines what temperature is needed to activate lubricant flow. This project has provided a conceptually new interesting aspect into the development of high temperature self-lubricating coatings: Previous published work almost exclusively looks at the effect of coating composition on the tribological properties, with the microstructure being a secondary parameter. This research has shown that engineering the microstructure (beyond multilayering and/or optimizing density or uniformity), has great potential to design coatings with desired tribological properties. There are various research-and-development laboratories (including governmental and industrial laboratories) that have used the results of this study to explore new high temperature lubrication for their particular applications. This project has also provided training in very essential engineering skills (thin film deposition, coating development, materials characterization, lubrication) to a total of five graduate and four undergraduate students, many of which have already entered the U.S. workforce in high-tech companies. The project has also facilitated the updating of a Friction and Solid Lubrication experiment which is used as educational tool, impacting over 200 students majoring in Mechanical Engineering.
