【作者】 张鹏飞；

【导师】 王亚明；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2008， 硕士

【摘要】 本文采用三种电解液体系(Al-P2、Al-P3-B和Al-P3-Si)中在TA15合金(Ti6Al2Zr1Mo1V)表面制备了微弧氧化陶瓷涂层。利用XRD、SEM和EDS等分析方法和极化曲线、交流阻抗谱、盐雾试验和电偶试验等腐蚀测试方法研究了涂层的组织结构和耐腐蚀性能。采用不对称拉伸疲劳试验研究了微弧氧化涂层对TA15合金疲劳寿命的影响规律。结果表明,涂层可以分为内外两层,内层致密,外层疏松多微孔。随着氧化时间的延长,涂层厚度逐渐增加,内层变得更加致密,但由于涂层向局部基体过度生长,导致涂层与基体的结合面逐渐变的凹凸不平。涂层结构取决于电解液组成,Al-P2和Al-P3-B涂层以Al2TiO5为主,Al-P3-Si涂层以金红石型TiO2为主,还含有少量的非晶相。与TA15合金相比,Al-P2、Al-P3-B及Al-P3-Si涂层的自腐蚀电位分别提高了0.636V、0.58V和0.692V。随氧化时间延长,Al-P3-Si涂层厚度增加,自腐蚀电位提高,自腐蚀电流增大。涂层的阻抗值大于TA15合金取决于致密的内层,随氧化时间的增加,Al-P3-Si涂层内层阻抗值增大,分别为1.16×106?·cm2 (10min)、1.2×106?·cm2 (30min)、3.8×106?·cm2 (50min)。在浓度为5%的NaCl盐雾条件下腐蚀720h后,TA15合金与Al-P3-Si涂层(30min)的质量损失率分别为1.5625mg/(h·m2)和0.43403mg/(h·m2),涂层的质量损失率明显低于TA15合金。在浓度为3.5%的NaCl溶液中进行电偶试验15天后,钢/TA5合金及钢/Al-P3-Si涂层(30min)偶联时的平均电偶电流密度分别为1.5683(μA/cm2)和0.2467(μA/cm2),后者明显低于前者,涂层良好的绝缘性能,降低了电偶腐蚀敏感度。微弧氧化30min的Al-P3-Si涂层试样,随疲劳应力由550MPa增加至750MPa,疲劳寿命由9.834×104周次减小至3.005×104周次。疲劳应力水平σmax=750MPa时,基体的疲劳寿命为2.08×106周次,而涂层试样氧化10min时的疲劳寿命为2.57×104周次,氧化30min时为3×104周次,涂层试样的疲劳寿命低于TA15合金。TA15合金先喷砂再微弧氧化可提高涂层试样的疲劳寿命,σmax=750MPa时前者的疲劳寿命是后者的2.7倍。TA15合金表面涂层的生长降低了基体的疲劳寿命,主要原因在于涂层向局部基体过度生长区域引起的应力集中和基体近界面处的残余拉应力的作用。火花在局部基体处持续放电,该处基体过度氧化,形成深入基体的缺陷区,它是疲劳裂纹源萌生的主要区域。将涂层试样等效为表面缺口试样,缺口半径R≈30μm,可以计算出缺口处的复合应力集中系数K′t≈2.4,疲劳缺口敏感度q≈0.63,涂层试样的疲劳极限σ-1n≈290 MPa。更多还原

【Abstract】 Ceramic coatings were fabricated on Ti6Al2Zr1Mo1V (TA15) alloy by microarc oxidation (MAO) in three differenct electrolytes (Al-P2, Al-P3-B, and Al-P3-Si). XRD and SEM techniques were employed to investigate the microstructure, phase and chemical composition of the coatings. The corrosion properties were determined using polarization curve, electrochemical impedance spectroscopy, salt fog corrosion and galvanic corrosion measearment technologies. Fatigue properties of coated specimens were examined by tensile fatigue testing at room temperature.MAO coating is compact in the inner layer, puff in the outer layer, with many micropores (pore size 1~10μm) on the surface. With the increasing oxidation time, the coating thickness increases accompanied with the more compact inner layer. While, the over growth of the coating into the substrate at the local sites leads to the formation of more rough interface between coating and substrate. The microstructure of coatings depends mainly on the type of electrolyte. Al-P2 and Al-P3-B coatings are mainly composed of Al2TiO5. The Al-P3-Si coating mainly consists of rutile and anatase TiO2, with a small amount of amorphous phase.Compared with TA15 alloy, the self-corrosion potential rises by 0.636V, 0.58V, and 0.692V for Al-P2, Al-P3-B and Al-P3-Si coatings, respectively. With increasing oxidation time, the self-corrosion potential of coatings rises accompanying the increasing current density. The electrochemical impedance of Al-P3-Si coating, determined mainly by the compact inner layer, rises with the increasing of oxidation time and registers as 1.161×106?·cm2, 1.201×106?·cm2 and 3.806×106 ?·cm2 for 10min, 20min and 30min, respectively.After corrosion in 5% NaCl slat fog for 720 hours, visible etching pits are observed on TA15 alloy surface, while perfect on coatings. The mass loss rate of TA15 alloy and Al-P3-Si coating (30min) is 1.5625mg/h·m2 and 0.4 mg/h·m2, respectively. When coupled with steel in 3.5%NaCl for 15 days, the current density of TA15 alloy and Al-P3-Si coating (30min) is 1.5683μA/cm2 and 0.2467μA/cm2, respectively. The dense inner layer of the coatings can hinder the movement of electrons, which leads to the lower susceptive of coupled current corrosion.The fatigue life of the Al-P3-Si coated specimen with oxidation time of 30minutes decreases from 9.834×104 cycles to 3.005×104 cycles when subjected to the fatigue stress ranging from 550MPa to 750MPa. At 750 MPa, the fatigue life of coated specimen is lower than that of TA15 alloy. The uncoated specimen could withstand a maximum cyclic stress of 750 MPa for 2.08×106 cycles, while the coated specimen fails after 2.57×104 and 3×104 cycles for 10 and 30 minutes oxidized coating, respectively. The duplex process of sand blasting treatment before MAO on the TA15 alloy surface enables to improve the fatigue life of coated specimen. In contrast to the unblasted specimen, the fatigue life of the blasted specimen increases by 170%.The growth of coating on TA15 alloy surface causes the decline of fatigue life of the underlined substrate, which can be attributed to the stress concentration at the over-growth area of coating into the substrate and the tensile residual stress existed in the substrate adjacent to the coating. As a result of persistent sparks discharging, the over-growth area of coating along the interface forms, which is considered as the potential site of crack initiation. The specimen with over-grown area of coating can be simulated as notched specimen. In that case, the notch’s radius is R≈30μm, mechanical parameters of coated sample are drawn with theoretical stress concentration factor Kt≈2.7, fatigue strength reduction factor Kf≈2.1 and fatigue notch sensitive factor q≈0.63, and finally the fatigue limit is calculated asσ-1n≈290MPa.更多还原