Effect of Electrolyte on Micro-arc Oxidation Discharge Mode and Wear Resistance of Titanium Alloy

Micro-arc oxidation technology is an advanced surface treatment technology for in-situ growth of oxide ceramic coating on the substrate surface. It can improve the surface properties such as wear resistance, corrosion resistance and high temperature oxidation resistance without changing the properties of the substrate material. Because different discharge modes produce different structures, the difference of structural defects significantly affects the wear resistance of the coating under high load. In this paper, the effects of different electrolytes on the discharge mode and wear resistance of titanium alloy micro-arc oxidation were studied. Using wire cutting, TC4 titanium alloy was cut into rectangles of 15 mm×15 mm×5 mm as the substrate, and the surface oxide coating and oil were removed by grinding the 600#, 1000# and 1500# water sandpaper in turn, and then ultrasonic cleaning with absolute ethanol and deionized water. For electrolyte, a high concentration aluminate electrolyte and a high concentration aluminate-low concentration phosphate composite electrolyte with a total of 3 electrolyte systems were used. The MAO-10HB micro-arc oxidation complete sets of equipment were used for micro-arc oxidation, the power mode was constant current mode. Relevant electrical parameters were as follows: current density - 7 A/dm2, frequency - 200 Hz, and duty cycle - 30%. The wear rate index comprehensively determined the optimal ratio of each group of electrolyte system. The microscopic morphology of the coating and wear area was observed by a scanning electron microscopy (SEM). An X-ray energy dispersion spectrometer (EDS) was used to analyze the distribution of elements and changes in the surface and wear area of the coating. An X-ray diffractometer (XRD) was used to characterize the structural composition of the coating. The hardness of the dense layer of the coating was measured with a Vickers microhardness tester. The tribological performance of the coating was tested by a friction and wear tester, and the abrasion profile was analyzed by a three-dimensional morphological microscope. B-type discharge was a strong electric field dielectric discharge throughout the coating, forming a dense coating structure, which was the main reason for the dense coating, and could make the electrolyte react with the substrate to form a new phase; A, C-type discharge was only breakdown on coating surface or in the middle of weak electric field during gas discharge. The coating structure was loose. It was the main cause of coating loose. When the titanium alloy was micro-arc oxidized in high concentration aluminate electrolyte, AlO2- ions participated in the coating-forming reaction to form Al2O3, and a secondary reaction occurred under the action of high temperature and high pressure plasma: Al2O3 + TiO2 → Al2TiO5, forming a large number of aluminum titanate hard ceramic phase. In the phosphate electrolyte, micro-arc oxidation was very likely to cause impurity discharge, resulting in micro-arc oxidation reaction B-type discharge was significantly weakened, A-type and C-type discharge increased significantly, resulting in a significant reduction in coating density, seriously reducing the wear resistance of micro-arc oxidation coating. The Al2TiO5/γ-Al2O3/R-TiO2 oxidation ceramic coating with good wear resistance was prepared in the micro-arc oxidation of TC4 titanium alloy (the optimal electrolyte ratio was: NaAlO2-20 g/L, NaOH-1 g/L). The hardness was as high as HV0.981100, and the wear rate was only 7.22 % of the titanium alloy substrate. The main reason for the excellent wear resistance of the coating prepared in the optimal ratio electrolyte and the significant improvement of the wear resistance of the coating is the large number of hard phases of aluminum titanate, and the second reason is the dense coating generated by the micro-arc oxidation reaction led by B-type discharge. The defects are significantly reduced, effectively avoiding the failure of the coating due to the expansion of defects when high load friction is effectively avoided. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.