| Depending on the good corrosion resistance, high specific strength and low density, titanium alloys are chosen as a kind material that used in oilfields, especially for oil pipes. Titanium alloys also shows a good corrosion resistance for H2 S, CO2, Cl- corrosive medium. However, low bearing capacity and weak tribological properties, would influence the safety and durability of the oil casing during working. Based on plenty of surface treatment technologies to ameliorate the corrosion and wear resistance is a critical step. Microarc oxidation(MAO) has received widespread research for its contribution to improve wear resistance by fabricating ceramic coatings on titanium alloys. However, the quality of ceramic layer at middle and later period of MAO is poor, which would decrease the practical applications of the technology. Therefore, in this paper ultrasonic outfield assist was brought into MAO to solve this problem. Moreover, the ion adsorption capacity of ceramic layer in electrolyte is related to ion kinds. Thus, this dissertation mainly researched TC4 alloy in two different electrolytes system-NaAlO2 system and(NaPO3)6 system to fabricate ceramic layer. By comparing the change curve of voltage with time, thickness, surface morphology and phase composition of ceramic coatings fabricated in above systems with and without ultrasonic assist MAO. Furthermore, the corrosion and wear resistance of ceramic coatings were also studied. The main achievements were listed as follows:(1) With ultrasonic assist MAO, the cavitation effect of ultrasonic caused discharge breakdown became easily, the number of discharge channel on the coating surface increased, thus the number of microporous has also increased. Moreover, acoustic streaming can strengthen the stirring of the electrolytes, so improve the effective ion concentrations for plasma chemical oxidation reaction, thermochemical oxidation reaction and electrochemical oxidation reaction that the thickness of micro-arc oxidation coatings was increased. With ultrasonic assist MAO treated 15 min, in NaAlO2 and(NaPO3)6 system, the thickness increased by 34 % and 15 %, respectively.(2) Under different electrolytes by MAO can be prepared different phases composition of ceramic coatings, with the extension of treatment time, the intensity of the main phase increased. Besides, with bring the ultrasonic, the phases composition of ceramic coatings didn’t changed. In NaAl O2 system, the main phase was Al2TiO5. In(NaPO3)6 system, the main phase were TiO2(rutile and anatase) and certain Al PO4 phase.(3) The saturated simulated oilfield fluid was used as the electrochemical etching solution, the corrosion behavior of ceramic coatings fabricated in different electrolytes with and without ultrasonic assist MAO were studied, respectively. It displayed that in different electrolytes system, ultrasound have different effects on the corrosion resistance of coating. In NaAlO2 system, when the treatment time is short, ultrasound have less impact on the corrosion resistance of the coating; while ultrasound duration is long, the corrosion current increased an order of magnitude, so the corrosion resistance of the coating be significantly improved. In(NaPO3)6 system, ultrasound has little effect on the corrosion resistance of the coating, and corrosion current of each of the coating at the same order of magnitude.(4) Under the load of 10 N, wear resistance of the coatings formed in two different electrolytes system against Si3N4 ceramic ball was studied, respectively. It turns out that the wear resistance of coatings was decreased with a certain extent, but the wear mechanism didn’t changed. In NaAl O2 system, the coatings all were worn out, but the weight losses of coatings is more serious and wear scar is wider with the addition of ultrasound. Abrasive wear is the main wear mechanism, while fatigue wear secondary. In(NaPO3)6 system, the coatings all were not worn out, it showed a good wear resistance. The weight losses of coatings is more serious even appeared the obvious coating crack and spall with the addition ultrasound. Wear mechanism take fatigue wear as the primary and abrasive wear as the second. |
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