【作者】 施元；

【导师】 杨士勤；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 硕士

【摘要】 钛合金具有优异的比强度,良好的耐腐蚀性能、生物相容性等性能,但是,钛合金极差的摩擦磨损性能制约了其在工程中的应用。微弧氧化处理是改善钛合金耐摩擦磨损性能的有效方法。本文在Na₂SiO₃-（NaPO₃）_6-Na₂MoO₄溶液中,使用恒定脉冲电流和恒定脉冲电压两种控制模式制备Ti₆Al₄V合金的微弧氧化陶瓷层,分析了微弧氧化处理的电参数特性。使用扫描电子显微镜、X射线衍射、涡流测厚、超显微硬度、动态极化曲线、球-盘摩擦等方法分别研究电参数对陶瓷层形貌、相结构、厚度、显微硬度、耐腐蚀及摩擦学性能的影响,研究了陶瓷层的抗变形能力、拉伸强度以及电绝缘性能。研究结果表明,钛合金微弧氧化过程存在三个阶段:第一阶段合金表面迅速生成一层厚约10μm的陶瓷层;第二阶段微弧放电区域不断扩展,陶瓷层厚度缓慢增长;放电区域扩展到整个表面后为第三阶段,陶瓷层厚度随时间快速增长。钛合金微弧氧化陶瓷层主要由锐钛矿相和金红石相的TiO₂组成,含有少量无定形相和Ti的不饱和氧化物。随着处理时间延长或者脉冲能量升高,金红石相含量增加。溶液中的成分参与成膜的化学反应,陶瓷层中含有Si、P等元素,Si元素在靠近外层处含量较高,而P元素则在内层偏聚;加入负向脉冲后,外层的Si元素含量明显升高。经过微弧氧化处理后,钛合金表面的摩擦学性能大幅提高,微弧氧化陶瓷层摩擦系数相对于基体显著降低。对动态极化曲线分析表明,存在最佳的处理时间和负向脉冲参数,以获得较好的耐腐蚀性能;较小的占空比和正向脉冲电压条件下获得的陶瓷层耐蚀性能更好。其中处理时间为20min和占空比为4%的陶瓷层腐蚀电流相对于基体降低了4个数量级。钛合金微弧氧化陶瓷层具有良好的机械性能,厚度大于30μm的陶瓷层在500kg压力作用下能保持与基体的良好结合。工件承受拉应力变形时,陶瓷层直到断裂破坏时才开始从基体上剥离。在金属表面原位形成陶瓷层后,减少了工件有效承受载荷的面积,拉伸强度降低了约5%。电击穿试验结果表明,钛合金微弧氧化陶瓷层具有良好的电绝缘性能,其绝缘电压可达到800V以上。更多还原

【Abstract】 Titanium alloys have high strength-to-weight, excellent corrosion resistance and biocompatibility. However, the poor tribological behavior has seriously restricted extensive applications of titanium in various industry sectors. As a novel surface modification technique, micro-arc oxidation （MAO） is an effective method for improvement wear resistance of titanium alloys.In this work, MAO technology was applied to fabricate ceramic coatings on Ti6Al4V alloys to improve surface properties of Ti alloy in Na₂SiO_3-（NaPO₃）_6-Na₂MoO₄ solution by constant positive pulse voltage and constant positive pulse current modes, and electrical characteristics of these two different process modes were discussed. Microstructure, phase composition, coating thickness, hardness, corrosion resistance and wear resistance were systematically investigated by scanning electron microscopy （SEM）, X-ray diffraction （XRD）, thickness tester based on eddy principle, dynamic ultra micro hardness tester, electrochemistry tester and ball-on-disk tribometer, respectively. In addition, tensile-strength, anti-deforming capability and electric insulation ability of MAO coatings were also investigated.The results show that there exists three stages during micro-arc oxidation processing: at the initial stage, ceramic coating with a thickness of 10μm quickly formed on the surface of titanium alloy. Then, in the second stage, micro-arc discharge zone spreads on the surface, the thickness of coating increased slowly. And the growth rate of ceramic coating become relatively rapid at the last stage in the duration of treatment time.The MAO coatings of Ti6Al4V alloy are mainly composed of meta-stable anatase, stable rutile phase, as well as a small quantity of amorphous phase and unsaturated oxide. The content of rutile phase increases with increasing the oxidizing time or pulse energy. The elements in the solution such as Si and P can participate in chemical reaction to form the oxide coatings. High P content can be obtained in the inner layer. In contrast, the outer layer has a high Si content which contributes to higher negative pulsed bias.The wear resistance is greatly improved by MAO treatment. The friction coefficient of MAO coatings is significant lower than that of untreated Ti6Al4V alloy. The results of electrochemical corrosion indicate that there exits an optimal treatment time and negative pulse parameters, which give a better corrosion resistance. More important, good corrosion resistance samples can be obtained by low duty cycle and positive pulse voltage. For instance, the corrosion current decreases by about four orders for the sample treated by duty cycle of 4% and processing time of 20min.The MAO coatings possess excellent mechanical properties. A coating with the thickness of more than 30μm shows good adhesion to the substrate even at a pressure of 500kg. When the samples are under the tensile deformation, delamination occurs until the rupture of coatings. The tensile strength of the treated samples decreases by about 5% compared with that of titanium alloy substrates due to the decrease of the bearing area resulting from situ-formed coatings.The MAO coatings of the titanium alloy possess good electric insulation ability during the breakdown test. The insulation voltage can reach to as high as 800V.更多还原