Abstract

Conventional alloys and composites are widely used as coating materials in various contacting interfaces within gas turbine engines. These alloys form in-situ lubricious oxides during sliding at high temperatures, which help to reduce friction and wear. However, the formation of an oxide film on the contact zones requires time and depends on the chemical nature of the materials as well as the contact conditions and environment. In most cases, the running-in period for traditional alloys or composites is relatively long, which ultimately causes an overall increase in wear. Since the lubricious oxide is responsible for low wear and steady-state friction coefficient at high temperatures, it could be beneficial to replace the conventional alloys/composites and use such oxides instead. Based on prior work, ionic potential, and interaction parameters, which emphasis the lubricity at high temperatures, examples of such oxides include CuO, Ta2O5, CoO, NiO, Co-Ni-O.

In this dissertation, CuO, Ta2O5, CoO, NiO, Co-Ni-O oxides were sprayed to produce thick coatings with dense, homogeneous microstructures using Suspension Plasma Spray (SPS) and High Velocity Oxygen Fuel (HVOF). The effects of spray parameters on the composition and microstructure of the coatings were investigated. The CuO and NiO coatings produced by SPS partially reduced to Cu2O, Cu and Ni, respectively. On the other hand, CoO, Ta2O5, and Co-Ni-O coatings remained single phase.

The thermally sprayed coatings were tested on a ball vs flat tribometer with dry sliding reciprocating condition at various temperatures (i.e., 25°C, and 450°C) against an alumina counterface. CuO and CoO were found to have low coefficients of friction at high temperatures compared to other oxides (i.e., Ta2O5, NiO, Co-Ni-O). On the other hand, CoO and Co0.75Ni0.25O were found to be superior in terms of wear resistance at high temperatures.

Scanning electron microscopy (SEM), electron channeling contrast imaging (ECCI), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman analysis, and focused ion beam (FIB) were used to characterize the coatings and the corresponding wear tracks to determine the dominant wear mechanisms. In general, brittle fracture, cracking, and tribofilm delamination were found to be the main wear mechanisms leading to high wear of the oxides at room temperature. In contrast, the formation of a relatively ductile, smeared tribofilm, grain refinement, and amorphous layer closer to the wear track surface contributed to friction and wear reduction at high temperatures.

In addition, a low interaction parameter of the oxides, regardless of the microstructure of the oxide coatings, led to the low friction. Such a correlation was not observed with the high interaction parameter and ionic potential. Furthermore, the high sintering ability or diffusion coefficient of the oxides could play a role in reducing friction and wear at high temperature.