【作者】 魏东博；

【导师】 姚正军；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2013， 博士

【摘要】 20世纪50年代以来，钛合金因其高比强度而成为重要的航空结构材料。这对于减轻飞机、航空发动机重量，提升其性能具有重要的意义。但钛合金存在抗高温氧化性能差的缺点，在500℃以上高温环境服役时，表层氧化膜极易破裂而导致失稳氧化，从而使整个构件失效。某些条件下，氧化甚至会引发“钛火”故障，引发机毁人亡的重大事故。这严重限制了钛合金在航空航天领域的进一步应用。进入21世纪，高性能航空发动机的发展对钛合金的使用温度提出了更高的要求。500℃以上抗高温氧化性能的不足已成为制约钛合金在航空航天领域应用的关键问题。目前，表面高温氧化防护技术的研究已成为钛合金领域的研究热点。双层辉光等离子表面冶金技术（以下简称“双辉技术”）具有一些独特优点，为提升钛合金的抗高温氧化性能提供了一种新的技术手段。本论文基于双辉技术，在钛合金表面设计和制备了一种抗高温氧化合金层，深入研究了合金层的成分、组织和结构，探讨了制备工艺对基体钛合金组织和性能的影响，并对合金层的高温氧化行为和摩擦磨损行为开展了系统的研究。论文首先根据钛的氧化机理和合金元素对钛合金氧化性能的影响机制，结合双辉技术的特点，提出了抗高温氧化合金层成分和结构的设计准则：合金元素的熔点应高于双辉技术的工艺温度，与Ti能形成溶解度较高的固溶体和稳定的化合物，高温下其氧化物能形成连续致密保护性的氧化膜。合金层应包含完整的沉积层、互扩散层和轰击溅射影响区结构。沉积层应为连续致密的结构，高温下能有效阻隔氧与钛的反应。互扩散层应保持适当厚度，高温下能实现冶金结合和阻氧扩散作用的平衡。在保证合金层成分和结构的前提下，应减小轰击溅射影响区的厚度和组织相变。基于上述准则，论文设计了Ti-Cr和Ti-CrNi两种合金层的成分和结构。利用双辉技术，在TC4合金表面成功制备了Ti-Cr和Ti-CrNi合金层，并获得了适宜制备工艺。研究发现：抗高温氧化合金层厚25～35μm，无孔洞，与基体结合良好。随Ni含量的增加，合金层生长模式由“岛状模式”转变为平面逐层生长。随Ni含量增加，合金层表面硬度从998HV0.1逐渐下降至650HV0.1，其中沉积层的硬度略微下降，但互扩散层和轰击溅射影响区的硬度变化不大。Ni的加入降低了沉积层的弹性模量。随Ni的含量增加，沉积层的弹性模量从634GPa下降至95GPa，其弹性回复能力逐渐变弱。互扩散层比沉积层具有更好的弹性回复能力，随Ni含量的增加，互扩散层的弹性模量从1174GPa上升至1290GPa。Ni的加入有效提升了合金层表面沉积层的粘附性。随Ni含量的增加，沉积层的粘附力不断增强。论文研究了合金层制备工艺对基体TC4合金组织和力学性能的影响。经合金层制备工艺后，TC4合金中的α相增多，组织中出现粗大的魏氏组织和篮网状组织。合金层制备工艺对TC4合金的拉伸性能影响不大。合金层制备工艺对TC4合金疲劳性能的影响与应力水平相关。在高应力水平下（≧200MPa），TC4合金的循环寿命减小了20～40%，少量魏氏组织和篮网状组织是导致TC4合金疲劳性能下降的主要原因。论文系统研究了合金层在650～950℃时的高温氧化行为，并对氧化机制进行了深入分析。Ti-Cr合金层氧化时，Cr和基体元素Ti、Al外扩散形成氧化膜；氧的扩散侵入被限制于互扩散层外。但Ti（Cr, Al）₂析出相的脆性减弱了氧化膜与基体的结合力。不断积累的交变应力是导致氧化膜剥落的主要原因。Ti-CrNi合金层氧化时，Ni主要以两种形式分布于氧化膜中。一是以NiO氧化物夹杂于Cr₂O₃膜中，另外是形成独立的连续NiO膜。NiO的分布形式除与合金层中Ni含量相关外，还受到氧化温度的影响。一定量Ni的加入提升了合金层的恒温氧化性能，但过高的Ni含量降低了合金层的恒温氧化性能。Ni加入的基本原则是不应干扰高温氧化时合金层表面连续致密Cr₂O₃膜的形成。一定量的Ni会在Cr₂O₃膜下形成连续的NiO膜，其不仅会增强氧化膜的阻氧能力，提升合金层的抗高温氧化性能；而且NiO膜的高韧性实现了氧化膜与Ti-CrNi析出相层间的过渡，提升了氧化膜的粘附性。当合金层中的Ni含量为40at.%时，Ti-CrNi合金层具有最好的抗高温氧化性能。论文研究了合金层在室温和高温（500℃）下的摩擦机制和磨损性能。室温摩擦时，合金层中沉积层主要发生粘着磨损，互扩散层主要发生磨料磨损。合金层显著提升了TC4合金的室温耐磨性能。高温摩擦时，产生的高温高压加剧了合金层的氧化和变形，合金层的摩擦磨损加剧。高温下，合金层随载荷和滑动速度的增加，磨损体积和比磨损率迅速上升，耐磨性能迅速下降。在500℃，载荷730g，滑动速度10m/min，摩擦副Si₃N₄的实验条件下，合金层摩擦60min后磨损失效。更多还原

【Abstract】 The titanium alloy，which is regarded as an important aeronautical material because of its highstrength weight ratio, is being helpful to reduce the weight of airplanes and aero engines. It is widelyconsidered the titanium alloy is positive to improve the performance of aero engines since the fiftiesof the20th century. However the weakness of the titanium alloy in oxidation resistance at hightemperature cannot be ignored. At the work condition of500℃, the oxide film of titanium alloy isvulnerable and prone to further oxidized, and then the whole component fails.The even more seriousis the fact that“titanium fire” causes the plane crashunder certain conditions. Further applications oftitanium alloy are restricted by the disadvantages mentioned above.Therefore, in the21st century, theoperating temperatures of the titanium alloys are required to be raised to adapt to the development ofhigh performance aero-engine. The key problems in the practical using of titanium alloy are its poorwear resistance at high-temperature above500℃.At present, high-temperaturesurface oxidation resistance of titanium alloy`has been a hot area ofresearch. The double glow plasma surface metallurgy technique, which offerssome unique properties,provides a new technical method to improve the antioxidant performance of titanium alloy. In thisstudy, the chemical composition and structure of high temperature oxidation resistant coatingwasfirstly designed, and Ti-Cr and Ti-CrNi coating was obtained on the surface of TC4alloy by thetechnique. Then the component, organization and the structure of coatings were studied in-depth.Effects of the processing on microstructures and properties were also analyzed, and the hightemperature oxidation behavior and thetribological behavior of coatings were researchedsystematically.Based on the oxidation mechanisim of titanium alloy, the influence of alloying elements ontitanium alloy and the principle of double glow plasma surface metallurgical technology, thedissertation given a design criteria about high temperature oxidation resistant coating. Firstly, themelting point of alloying elements must be higher than the processing temperature of double glowplasma surface metallurgical technology. Secondly, the alloying elements must be able to inform solidsolutions and intermetallics with Ti, which would be translated into compact, continuous andprotective oxidation film. Thirdly, the high temperature oxidation resistant coating must include acomplete structure of settled layer, mutual diffusion layer and sputtering-affected zone. The settledlayer should be compact and continuous, which would prevent the reaction of Ti with O. The thickness of mutual diffusion layer should be controlled in a certain range to realize the balance ofoxygen blocking and metallurgical bonding. On the premise of guaranteeingcontent and construction,the thickness and phase change of sputtering-affected zone should be strictly controlled. Based on theabove principle, the dissertation designed the contentsand constructions of the Ti-Cr and Ti-CrNicoating.The dissertation studied the best preparation conditions of the Ti-Cr and Ti-CrNi coating on thesurface of TC4alloy by the technology. In general, the thickness of coating was about25～35μm,which had a good bond with TC4alloy. And there was no holes detected in the coating. With theincreasing of Ni, the surface hardness of coating gradually decreased. However, the hardness ofmutual diffusion layer and sputtering-affected zonedid not changed with the amount of Ni. With theincreasing of Ni, the elastic resilience of settled layer decreased, and the elastic resilience of mutualdiffusion layer was constant. Ni increased the toughness and adhesion of settled layer, lowered themodulus of elasticity.The dissertation discussed the impact of the preparation processing on the mechanical propertiesof the substrate. The amount of α phase increased in TC4substrate after the processing, the coarsevermicular widmanstatten organizations and basket network organizations were also appeared. Thepreparation process had a little effect on the tensile properties of the TC4substrate. The effect ofpreparation processing on fatigue performance was relevant to the stress level. The cycle lives of TC4alloy was decreased by20to40%in the high stress level （≧200MPa）, the widmanstattenorganizations and basket network organizations were the main reasons to lead the decline of thetensile and fatigue properties of TC4alloy.This dissertation systematically studied the high-temperature oxidation behavior of the Ti-Cr andTi-CrNi coating at different temperatures and the oxidation mechanism was in-depth analysis. Thehigh-temperature oxidation mechanism of the Ti-Cr coating was: the outer diffusion grew of Cr and Ti,Al resulted the oxide films were formed; the intrusion diffusion of oxygen was limited to theinterdiffusion layer. However, the great brittleness of Ti （Cr, Al）2precipitated phase layer weakenedthe bonding strength of the oxide film and substrate. The accumulation of cyclic alternating stress wasthe main reason of the spalling of oxide film. When the Ti-CrNi coating was oxidized, Ni distributedin the oxide film in two ways. The one was NiO oxide mixed with Cr₂O₃film, the other was theformation of an independent continuous NiO film. The distribution form of NiO was not only relatedto the content of Ni in the coatings, but also to the influence of the oxidation temperature. The Niaddition of a certain amount was able to enhance the isothermal oxidation resistance performance of coatings. However, an excessively high content was to reduce the isothermal oxidation resistance ofcoatings. The basic principle of Ni addition wasNi should not interfer the formation of densecontinuous Cr₂O₃film at high-temperature oxidation. A certain amount of Ni could form a continuousNiO film beneath the Cr₂O₃film, it could not only enhance the capacity of oxygen barrier of the oxidefilm, then to enhance the high-temperature protective property; but also enhance the toughness andadhesion of coatings. The NiO film could achieve a transition between the oxide film and theprecipitation of Ti-CrNi phase layer, it bridge the comprehensive stress differenceof thermalexpansion coefficient of coatings. When the content of Ni was40at.%, the coating had the best hightemperature oxidation resistance.A comparative study on the behaviors of friction was conducted at room temperature and hightemperature. At room temperature, the settled layer of the coating mainly was adhesive wear, and theinterdiffusion layer was abrasive wear. The coating significantly improved the wear resistance of TC4alloy at room temperature. At high temperature, the high temperature and pressure exacerbated theoxidation and deformation of the alloy layer, the friction and wear of the coating exacerbated. Thewear volume and specific wear rate of the coating were rising rapidly with the load and sliding speedincreased, and the wear resistance decreased rapidly at high temperature. In the following experimentconditions:500℃, load730g, sliding speed10m/min, Si₃N₄as the friction pair, the coating wasinvalid for wear after60min.更多还原