The primary focus of our research group is the development and characterization of protective coatings, primarily made by cold spray but also physical vapor deposition. We utilize specialized techniques to characterize the mechanical and tribological properties of coatings and materials. These techniques are implemented, in parallel with chemical and microstructural characterization, to provide new understanding of the process-structure-property-performance relationships for the coating. In many instances, the information provided by our studies is only obtainable with our specialized techniques and is necessary to optimize the materials design for better material performance.
Tribology of MoS2 Coatings
We have studied the tribology of MoS2 containing coatings since 2005. This has included pure coatings, metal-doped nanocomposites and “chameleon” nanocompsosites made by PVD based techniques. More recently, we have studied cold sprayed composite coatings containing MoS2 . While the solid lubricating properties of MoS2 are well established, the specific mechanisms are still debated. We have added new understanding in this area by using specialized methods such as in situ tribometry, which allows us to directly image third bodies (e.g. wear debris, transfer films) that form in a tribological contact. For the “chameleon” coatings (J9), we were able to make specific recommendations of changes to coating chemistry based on observations made by in situ tribometry. For metal-doped nanocomposites, we used an in situ tribometer and a nanoindentation-based microtribology technique to compare the effect of length scales on the lubrication mechanisms (J4). Current research on cold sprayed composites with MoS2 have focused on the fretting wear and sliding wear behavior of these coatings (J1, J2). To summarize, we have for the past 10 years led a significant research effort to develop new understanding of MoS2 coatings, focusing specifically on lubrication mechanisms and effects of scale, load and environment. We are currently developing new methods for fabrication of MoS2 coating by cold spray and studying their tribology.
Micro- and nanomechanical testing for cold sprayed coatings and splats
We have developed two new test methods for characterization of mechanical properties of cold sprayed materials. First, a new method was developed to determine the adhesion strength of cold sprayed splats to substrates or previously sprayed materials (J15). This technique measures adhesion strengths beyond those measurable by a traditional bond pull test. Use of this technique is helping to identify and optimize the most important processing conditions to obtain highest quality coatings. A second technique was developed for characterization of the cohesive strength between cold sprayed particles in a coating (J12). Comparing the different length scales of indentation led to the development of a hardness loss parameter related to both coating porosity and cohesive strength. Variation of this parameter with coating process conditions helped determine the optimal spray conditions to ensure bulk- like mechanical behavior of the cold sprayed material. These methods are also being used in parallel with tribology measurements to understand the relationships between wear resistance and the structure and mechanical properties of the coating.
Composite Cold Spray Coatings and Their Tribological Properties
We have recently studied the tribology of Al-Al2O3 composites fabricated by cold spray. This material system is a popular one for the cold spray process as it is used as a repair technology for Mg gearboxes used in the aerospace industry. With a combined approach of in situ tribometry and ex situ analysis by focused ion beam (FIB) cross-section, we were able to elucidate the structure-property relationships leading to enhanced wear resistance in these composites. Observation of third bodies demonstrated the formation of mechanically mixed layers (MMLs) that exhibited higher hardness than the as sprayed coatings. The formation of these MMLs occurred more readily and were harder when the Al2O3 particles were spherical rather than angular (J16). To explain this effect, high pressure torsion tests were conducted that emulate the mechanical conditions in a tribological contact. Observation of the microstructures of the deformed materials, combined with finite element analysis (FEA) revealed that spherical particles provide a stress state in their vicinity that resists crack opening. This reduces wear and led to more cohesive, homogeneous MMLs. The understanding generated by this work is currently being used for other material systems fabricated by cold spray such as Ni-WC and Ti-TiC coatings.