【作者】 吴红艳；

【导师】 徐重；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2008， 博士

【摘要】 Ti-Al系金属间化合物具有高的比强度、比模量、良好的抗氧化性及优良的高温强度等,近来已成为国内外航空、航天、军工等工业部门的热点材料。然而,Ti-Al金属间化合物存在着耐磨性不足的问题,限制了其应用。因此,提高Ti-Al系金属间化合物耐磨性能已成为关键的工程问题之一。本课题针对航空发动机高温运动副零部件的耐磨性问题,采用我国独创且具有自主知识产权的双层辉光等离子表面合金化技术,在Ti₂AlNbO相合金基体表面制备出高硬耐磨的合金层。本研究首先利用“固体与分子经验电子理论”通过计算分析元素加入到基体时的价电子结构,得出选择铬和钨作为欲渗元素,可能有利于提高Ti₂AlNb摩擦性能;而后采用双层辉光等离子表面合金化技术,对Ti₂AlNb合金进行渗Cr、渗W处理以及后续等离子渗碳处理的工艺优化试验。利用光学电镜（OM）、扫描电镜（SEM）、能谱仪（EDS）、X射线衍射（XRD）、透射电镜（TEM）和X射线光电子能谱仪（XPS）进行组织成分检测,对合金渗层分别进行了显微硬度检测和纳米压入分析及摩擦学性能研究。工艺试验结果表明:等离子表面渗铬层的有效厚度约25μm,表面Cr元素含量达到75%,渗层中铬元素含量呈梯度分布,合金渗层中以Cr2Nb,Al₈Cr₅/Al₉Cr₄为主;渗W层的有效厚度为25μm左右,表面W含量达55%,W含量沿深度方向呈梯度分布,渗层中以TixW1-x化合物为主;Cr-C共渗层主要含有Cr₂₃C₆和Cr2Nb,且有部分单质C和Cr;W-C共渗层的组分主要以W2C或W6C2.54为主。渗Cr、渗W层表面硬度均达1100 HV0.1以上,共渗层的表面硬度均高达1650 HV0.1,远高于基体的400 HV0.1左右。纳米压入测量结果表明,各种渗层的弹性模量远高于基体。渗钨层的弹性模量最高在700 GPa左右,其他三种渗层的弹性模量分布在200 GPa 500 GPa之间,而基体约100 GPa。摩擦学试验表明:双辉等离子渗Cr、渗W以及后续渗碳处理均提高了Ti₂AlNbO相合金的耐磨性能。Ti₂AlNb合金经等离子渗Cr处理,室温摩擦系数由1.2降低为0.6,高温摩擦系数由1.0降低为0.5,经过后续渗碳处理,摩擦系数均降低到0.2以下;经过等离子渗Cr处理,室温磨损率降低了一个数量级,高温磨损率降低了60%,再经过后续渗碳处理,磨损率降低为基体的1/10 1/25。经渗W处理,室温摩擦系数由基体的1.2降低为0.5,高温摩擦系数由1.0降低为0.15,但渗W层的摩擦系数在长时间的摩擦中会逐渐上升,而经后续渗碳处理,高温摩擦系数始终维持在0.15左右。Ti₂AlNb合金渗W后室温磨损率降低为基体的1/10,高温磨损率仅为基体的1/5,经过后续渗碳处理,磨损率比基体降低了2个数量级。经过W、C共渗处理,获得了最低的高温摩擦系数和磨损率。以上实验结果证实了“固体与分子经验电子理论”预测选择铬和钨作为欲渗元素,有利于提高Ti₂AlNb合金的摩擦磨损性能。最后,研究了双层辉光等离子渗金属过程中轰击溅射条件下所产生的空位机制,并基于辐照增强扩散理论建立了渗金属过程中的扩散模型。更多还原

【Abstract】 Ti-Al intermetallic compounds have high specific strength, high specific modulus, excellent oxidation resistance, high temperature strength and so on. Recently, they have been regarded as the high temperature materials in aeronautics, astronautics, war industry and otherwise. However, the low hardness and poor wear resistance of Ti-Al intermetallic compounds restrict their practical application. The improvement of the tribological performance has become one of crucial project problems.The paper aims at solving the wear-resistant problem of elevated temperature motion components. The high hardness and wear-resistant alloying layers are prepared on Ti₂lNb O-phase alloys using an advanced technology-the double glow plasma surface alloying technique.The research firstly applied the“Empirical Electron Theory of Solid and Molecule”to calculate the molecule structure when the elements added to substrate. The results of analysis showed that the choice of Cr and W can be beneficial to improve the tribological properties of Ti₂lNb. Subsequently, the optimizing experiment of the plasma chromising, tungstening and the duplex treatment （carburization following plasma alloying） were performed using the double glow plasma surface alloying technique. Optical metallography （OM）, scanning electron microscope （SEM）, energy diffraction spectroscopy （EDS）, X-ray diffraction （XRD）, transmission electron microscope （TEM） and X-ray photoelectron spectroscopy （XPS） were performed to study the phase formation and microstructure. Micro-hardness, nano-indentation and tribological properties were tested and analyzed for different alloying layers, respectively.The optimizing results indicated that the effective thickness of the plasma chromized layer was about 25μm. The surface chromium concentration reached 75%, and the content of Cr decrease in gradient with the increase of the alloying depth. Cr₂Nb, Al₈Cr₅ or Al₉Cr₄ were the major phases of the chromized layer; the thickness of the plasma tungstenzed layer was about 25μm. The surface tungsten concentration reached 55%, and the content of W decreased in gradient with the increase of the alloying depth. Ti_xW_1-x compound was the major phases of the tungstenzed layer; Cr-C duplex-treated layer mainly consisted of Cr₂₃C₆, Cr2Nb, and a few of carbon and Cr; W-C duplex-treated layer is primarily composed of W₂C or W₆C_2.54.The micro-hardness of plasma chromising and tungstening got to above 1100 HV, and the micro-hardness of duplex-treated layer reached 1650 HV. The four layers had higher hardness than 400 HV of the substrate. The results of nano-indenter showed that the elastic modulus of different layers was much higher than that of the substrate. The maximum elastic modulus of the tungstened layer reached about 700 GPa, that of other three layers distributed between 200 GPa to 500 GPa,while that of the substrate was only 100 GPa.Tribological test indicated that the double glow plasma chromising, tungstening and plasma duplex treatment all improved the wear resistance of Ti₂AlNbO-phase alloy. The friction coefficient of plasma chromising on Ti₂AlNb alloy decreased from 1.2 to 0.6 at the room temperature, while at the high temperature it decreased from 1.0 to 0.5. The friction coefficient at the room/high temperature reduced 0.2 below. At the room temperature, the wear rate of plasma chromising was about 10 times lower than that of the substrate, while at the high temperature, it reduced 60%. The wear rate of the duplex treatment lowered tenth to twenty-fifth of the substrate.At the room temperature, the friction coefficient of plasma tungstening decreased from 1.2 to 0.5, while at the high temperature it decreased from 0.5 to 0.15. However, the friction coefficient at the high temperature gradually ascending for long friction, while that of the duplex treatment can maintain about 0.15. At room temperature, the wear rate of plasma tungstening decreased tenth of that of substrate, while at the high temperature, it was only fifth of substrate. The wear rate of the duplex treatment was about 100 times lower than that of the substrate. At the high temperature, the lowest friction coefficient and wear rate was obtained using the carburization following plasma tungstening.The above experimental results confirmed that Cr and W as the alloying elements helped to improve of tribological properties of Ti₂AlNb alloy through the“Empirical Electron Theory of Solid and Molecule”forecasting.Finally, the vacancy mechanisms produced by the bombardment and sputtering under the double glow plasma alloying were studied. The diffusion model was built up for the alloying process based on the radiation enhanced diffusion theory.更多还原