The tribological properties of cold sprayed Ni-WC metal matrix composite (MMC) coatings were investigated under dry sliding conditions from room temperature (RT) up to 400°C, and during thermal cycling to explore their temperature adaptive friction and wear behavior. Characterization of worn surfaces was conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy to determine the chemical and microstructural evolution during friction testing. Data provided insights into tribo-oxide formation mechanisms controlling friction and wear. It was determined that the steady-state coefficient of friction (CoF) decreased from 0.41 at RT to 0.32 at 400˚C, while the wear rate increased from 0.5×10-4 mm3/N·m at RT to 3.7×10-4 mm3/N·m at 400˚C. The friction reduction is attributed primarily to the tribochemical formation of lubricious NiO on both the wear track and transfer film adhered to the counterface. The increase in wear is attributed to a combination of thermal softening of the coating and a change in the wear mechanism from adhesive to more abrasive. In addition, the coating exhibited low friction behavior during thermal cycling by restoring the lubricious NiO phase inside the wear track at high temperature intervals. Therefore, cold sprayed Ni-WC coatings are potential candidates for elevated temperature and thermally self-adaptive sliding wear applications.

This research conducts tribological investigation in three areas. The first area of research is to obtain basic understanding of tribological properties of high performance Inconel alloys. Pin-on-disk testing was conducted through a range of applied normal loads and sliding velocities in an unlubricated condition. Average friction coefficient, friction work, and specific wear rates were calculated from the data and microscopy techniques were used to observe and characterize wear mechanisms. Experimental results show a dependence of average coefficient of friction as a function of frictional work. Also shown is the wear rate dependence on frictional work, predicated by a wear mechanism change. This research gives a tribological baseline for high performance alloys. The second area of research is in the in situ spatial study of friction, complemented by monitoring changes in electrical contact resistance (ECR). Pin-on-disk testing of samples was done under low normal loads and velocities. Friction and electrical contact resistance measurements were taken spatially in the wear track during each friction cycle, giving a spatial evolution of friction and resistance change, in situ. Results show a lowering in the ECR under increased friction cycles, which was closely related to a change in the friction coefficient of the material. Using surface profilometry and X-ray Photoelectron Spectroscopy, we determined that the lowering of resistance is a result of surface modification through wear and development of a friction induced conductive tribo-film. This research provides a simple method for in situ monitoring of friction and solidifies a fundamental relationship between friction and contact resistance. The third area of research is the design of a variable force tribometer, incorporating the fundamental results demonstrated in the first two experiments. The creation of a novel testing apparatus to test materials under dynamic tribological conditions is given in detail. Simple experiments were performed on an Inconel sample and preliminary results show how dynamic normal and tangential forces affect the friction coefficient. These early results utilizing the variable force tribometer will lay the groundwork for more advanced research into the dynamic nature of friction.

Silicon nitride on steel sliding contacts may provide advantageous tribological properties over traditional self-mated pairs, however the friction and wear behavior at high sliding speeds (>1 m/s) is not well understood. Previous studies at low sliding speeds (< 1 m/s) have found that the wear mechanisms change as a function of the operating parameters, e.g. atmosphere, sliding speed, load, and temperature, due to the formation of transition metal oxides such as Fe2O3 and Fe3O4. This study detected transient effects of the dry silicon nitride on steel contact over a range of sliding speeds to understand their relation to tribochemical reactions and the resulting tribological behavior. Two sets of dry silicon nitride on steel experiments were conducted at 1.45 GPa maximum Hertzian pressure. The first set were low sliding speed reciprocating experiments, conducted at an average of 0.06 m/s, conducted at variable operating temperature, ranging from 23 °C to 1000 °C. In the low sliding speed experiments, transitions of the wear mechanism from adhesive wear, to abrasive wear, then to oxidative wear was observed when the operating temperature increased. The second set were high sliding speed experiments, conducted at variable sliding speeds, ranging from 1 m/s to 16 m/s. In the high sliding speed experiments, a transition from adhesive wear to oxidative wear was observed when the sliding speed surpassed 4.5 m/s. The high sliding speed experiments were accompanied by in-situ instrumentation which detected the presence of a tribofilm which correlated to a reduction in friction, and its formation was linked to tribochemical reactions induced by high flash temperatures. Both sets of experiments had a maximum estimated contact temperature of 1000 °C where oxidative wear was prevalent. Although, the low sliding speed experiments underwent severe bulk oxidation and thermal softening effects, while the high sliding speed experiments experienced localized flash heating events with temperatures sufficient to form a semi-coherent tribofilm that was lubricious and significantly improved wear resistance. Therefore, the effects of transition metal oxides in sliding contacts are determined to be significantly influenced on their mechanisms of formation and interrelated to the operating parameters as found for dry sliding silicon nitride on steel contacts.