【作者】 陈东方；

【导师】 杨士勤；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 硕士

【摘要】 随着航天、航空工业中各种飞行器飞行速度不断提高,服役环境越来越恶劣,飞行器表面气动加热和热冲击问题十分突出。飞行器用材料要求具有足够高的强度和刚度,通常作为结构材料的铝合金难以承受如此高的热载荷,而限制了铝合金的广泛应用。因此必须对其表面进行热防护处理。微弧氧化技术在金属表面原位生长陶瓷膜,可以制备高结合强度、优良热稳定性和耐热冲击性能的膜层。本文针对微弧氧化膜层的上述特点,制备优良热阻隔性能的膜层。本文首先选定了硅酸钠体系做为制备热阻隔膜层的电解液体系。研究了不同反正向电流比j-/j+对微弧氧化电特性、生长速率及能耗的影响。结果显示:反向电流对放电过程产生调制作用,柔化了放电过程,提高了膜层的生长速率,降低了膜层的单位能耗。通过向电解液中添加氟锆酸钾的方式,大大提高了膜层生长速率,较Na₂SiO₃-KOH体系单位能耗大大降低,随着K2ZrF6浓度的提高,膜层生长速率提高,单位能耗降低。通过SEM及附带的能普和XRD测试分析了膜层的形貌、成分和结构。在Na₂SiO₃-KOH体系中,反向电流对微弧放电产生了柔化作用,对膜层的形貌、结构、成分产生了影响,使膜层的致密度提高。当j-/j+=0.5时,膜层中大缺陷消失,孔洞尺寸小,分布均匀,膜层质量最好,并使得膜层的晶体峰明显减少,非晶相增多。Na₂SiO₃-K₂ZrF₆-KOH体系中,反向电流的引入,改善了j-/j+=0时放电通道残留的孔洞尺寸,表面形成的“圆饼”尺寸小。截面形貌显示,与Na₂SiO₃-KOH体系中制备的膜层相比,膜层质量较好,且反向电流引入之后,膜层中的晶体相含量提高。各浓度下膜层的形貌和结构相差不大,膜层由内向外,Si和Zr元素含量逐渐升高。经微弧氧化处理之后,铝合金的拉伸性能基本上没有受到影响;耐磨性能提高,有效地保护了铝合金基体。热冲击测试显示,膜层与基体结合良好。对比分析可知,反向电流的引入提高了膜层与基体的结合力。LY12铝合金经微弧氧化处理之后,抗高温氧化性能提高。火焰烧蚀测试结果显示,微弧氧化膜层提高了LY12铝合金基体的抗火焰烧蚀能力。Na₂SiO₃-KOH体系中不同j-/j+对膜层隔热性能的研究表明,j-/j⁺=0.5时膜层的隔热性能最好,15 min之后温度为180℃,比基体15 min之后的212℃低32℃。电解液中K₂ZrF₆的引入提高膜层的隔热性能。当K₂ZrF₆浓度为7 g/L厚度为90μm的膜层隔热性能最好,15 min后比基体温度低45. 5℃。更多还原

【Abstract】 With the increasing need of high flight speed in space industry and aircraft industry, the components frenquently work in the harsh environment and temperature-rise and thermal shock on aerocrafts surface are quite severe, thus enough strength and rigidity of the utilized materials are necessary. Aluminum alloys, frequently as structural material of aerocrafts, can not bear such high heat effects and widely limited in the high-speed applications. Consquently thermal protection process is required. The micro-arc oxidation （MAO） technique, by which a compound ceramic coating grows in situ on Al, Ti, Mg and many other valve-metals directly, has been developed rapidly in recent years. The MAO coatings exhibit excellent combination of extremely high adhesion, thermal stability and thermal shock. In view of the features above, Coatings with excellent thermal resistance performance are expected to fabricate using MAO technics.Silicate system is chosen to prepare heat resistance coatings. The influence of different current ratio j-/j⁺ （j- reprents the reverse current desity and j⁺ reprents the positive current desity） on electric characteristics, growth rate and power consumption has been studied. The results show that the reverse current results in a“softer”effect on micro-discharges. The growth rate has been improved and the power consumption is reduced. By introducing K₂ZrF₆ to the solution, the growth rate is promoted greatly with lower power consumption.Micro-structure, element distribution and phase composition has been analyzed using SEM, EDS and XRD respectively. In Na₂SiO₃-KOH solution, the reverse current, which affects micro-structure and phase composition by changing discharge characteristics, improves the density of MAO coatings with fewer large-scale defects. Micro-discharges may be interrupted suddenly when the reverse current is applied, so melt materials cool very rapidly without crystallizing. This may reduce the number of crystal peaks. In Na₂SiO₃- K₂ZrF₆-KOH solution, the discharge channel and pancake become small. The MAO coatings are of good quality prepared in Na₂SiO₃- K₂ZrF₆-KOH solution compared with that produced in Na₂SiO₃-KOH solution. The reverse current also improves the quality of MAO coatings and increases the concentration of crystal phase. The content of Si and Zr gradually increase towards the external surface of MAO coatings.Tensile strength of aluminum substrates rarely changes and anti-wear performance has been enhanced after MAO process. This may protect LY12 aluminium alloy from severe wear. The thermal shock experiment shows high adhesion between substrate and coatings. Coatings prepared in Na₂SiO₃-K₂ZrF₆-KOH solution have higher adhesion, due to the effect of the reverse current. In the high temperature oxidation experiment the weight of the control samples increases quickly at the beginning and then slowly with increasing oxidation time. In contrast, the weight of the samples with MAO coatings decreases and then keeps unchangeable. Flame ablation experiment shows the MAO coatings may effectively protect substrates from flame ablation, j-/j⁺ of 0.5 leads to the best heat insulation performance of coatings fabricated in Na₂SiO₃-KOH solution. By introducing K₂ZrF₆ to the solution, the heat insulation performance is improved. The 90μm coatings produced with the K₂ZrF₆ concentration of 7 g/L has the best heat insulation properties, i.e. the temperature on the surface is lower 45.5℃than that of untreated samples after 15 min exposure to 500℃.更多还原