Atmospheric Plasma Spraying of Al₂O₃-Graphite Coating and Its Tribological Properties

The feeding powders were prepared using AlO(OH) and graphite through hydroxylation, physical mixing, and spray granulation, which were well agglomerated. Al₂O₃/C composite coating and Al₂O₃ coating were prepared using atmospheric plasma spraying (APS) technique. The microstructure and physical phase composition of the coatings were studied systematically using scanning electron microscopy (SEM) and energy spectroscopy (EDS). The friction behavior of Al₂O₃/C composite coatings and Al₂O₃ coatings sliding against 316L stainless steel balls under different loads was investigated using the UMT-3 friction wear tester. During the spraying process, the AlO(OH) phase in the AlO(OH)/C composite powder was dehydrated and transformed into Al₂O₃ phase, which was finally deposited on the substrate as the Al₂O₃/C composite coating, with uniform composition distribution, fewer defects and good melt state. Due to the dehydration of AlO(OH) and the oxidation of part of the graphite by the high-temperature flame flow, gaseous carbon oxides were produced, the porosity of the Al₂O₃/C coating was higher than the porosity of the Al₂O₃ coating. After dry sliding friction test, the results showed that the friction coefficient of Al₂O₃/C coating was slightly lower than that of Al₂O₃ coating when the friction load was small (3 N and 5 N), but the wear rate of composite coating was slightly larger than that of alumina coating. However, the difference of friction coefficient was obvious when the load was increased (10 N and 20 N), a maximum 32% reduction occurred in composite coatings, at the same time, the wear rate increased by about 36%. The friction surfaces of the composite coatings and stainless steel balls were characterized using Raman spectroscopy, carbon transfer films were found on the friction surfaces of both the Al₂O₃/C coatings and the steel balls, and no cracks appeared on the friction surfaces of the coatings. Due to the larger porosity and lower hardness of the Al₂O₃/C composite coating, the wear rate of the Al₂O₃ coating was slightly lower than that of the Al₂O₃/C composite coating at different loads, however, when the friction load was changed from 3 N to 20 N, the wear rate of the Al₂O₃ coating was increased by about 62 times, whereas the wear rate of the Al₂O₃/C composite coating increased by a factor of only about 12 under the same conditions. Besides, the wear rate of the Al₂O₃ coating was basically close to that of the Al₂O₃/C composite coating after friction with 10 N and 20 N loads. The wear rate of the 316L stainless steel ball decreased gradually with the increase in load. When the load was 20 N, the wear rate of stainless steel balls rubbed with Al₂O₃/C composite coating was lower than the wear rate of stainless steel balls rubbed with Al₂O₃ coating. In terms of wear mechanisms, the hardness of Al₂O₃ coating was much larger than that of 316L stainless steel ball, so that its wear mechanism was dominated by adhesive wear, accompanied by abrasive wear, and Al₂O₃/C composite coating was dominated by abrasive wear due to the presence of a large number of hard particles of aluminum oxide being pulled out of the friction surface.