【作者】 胡侨丹；

【导师】 严有为；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2007， 博士

【摘要】 MoSi2具有中等密度(6.24g·cm-3)、高的熔点(2030℃)、较低的热膨胀系数(8.1×10-6K-1)和良好的导电、导热等性能,在航空航天、能源化工、冶金机械等领域有着广阔的应用前景。为了进一步提高MoSi2及其复合材料的性能,并弥补燃烧合成(Combustion synthesis)技术难以制备致密结构材料的不足,本文采用电场激发与压力辅助燃烧合成(Field-activated and pressure assisted combustion synthesis, FAPACS)技术,实现了MoSi2和MoSi2-SiC复合材料的原位合成与同步致密一体化。着重研究了各种工艺参数对材料的合成组织和致密性的影响,探讨了FAPACS过程中材料组织结构的演变规律,研究了材料的力学与高温摩擦磨损性能,并对FAPACS过程进行了计算机数值模拟。通过系统研究,本文得出如下主要结论。对于Mo-Si反应体系,通过FAPACS技术合成的材料一般由MoSi2和分布于晶界上的Mo5Si3和少量的SiO2组成。其中,Mo5Si3是Mo和Si在较低温度下发生固相界面扩散反应的产物,而SiO2是Si与反应物粉末中残留的氧发生反应的结果。在FAPACS过程中,烧结温度对合成材料的组织和致密性有着明显的影响。当烧结温度高于Si的熔点(1410℃)时,Si粉熔化,并包覆在Mo颗粒表面,不仅有利于大量而细小的MoSi2合成,而且有利于进一步提高合成材料的致密性。结合Mo-Si体系的反应热力学和动力学的研究表明,高于Si熔点的烧结温度和大于100℃/min的加热速度以及足够的烧结时间是保证体系反应完全且获得单一、致密MoSi2块体材料的基本条件。在烧结温度为1500℃、升温速率为100℃/min、压力为35MPa、保温时间仅为10min的条件下,通过对Mo-Si体系进行电场激发与压力辅助,成功燃烧合成了致密度高(98%)、晶粒细小(5~10μm)的单相MoSi2块体材料。对于Mo-Si-C反应体系,在电场活化与等离子放电的诱发作用下,体系中的Si和C能发生燃烧合成反应,合成在普通条件下难以合成的SiC。原位合成的SiC呈颗粒状,尺寸在100nm ~ 3μm之间,纳米SiC颗粒弥散分布于MoSi2晶粒的内部,而大尺寸的SiC颗粒则分布于MoSi2的晶界,从而原位合成了所需的MoSi2-SiC复合材料。在合成的MoSi2-SiC复合材料组织中,未出现夹杂物SiO2相,说明C的存在有利于消除SiO2。但是,在复合组织中产生了Mo5Si3C三元中间相。Mo5Si3C是一种非稳态的过渡相,通过延长保温时间,使之发生Mo5Si3C + 8Si=5MoSi2 + SiC反应,可以使Mo5Si3C中间相的含量降低甚至得以消除。在烧结温度1500℃、保温时间30min、轴向压力30MPa的条件下,成功地实现了MoSi2-SiC复合材料的原位合成与同步致密一体化,从而为MoSi2-SiC复合材料的制备提供了另一条有效的新途径。在制备的MoSi2-SiC复合材料中,随着SiC含量的增加,复合材料的显微硬度增加,断裂韧性( K1C )明显提高。当SiC的体积百分含量为30%时,MoSi2-SiC复合材料的断裂韧性K1C值可达到5.58 MPa? m,比单一的MoSi2提高了33%。进一步的研究表明,单一MoSi2呈典型的脆性穿晶断裂,而MoSi2-SiC复合材料为沿晶和穿晶的混合型断裂,其增韧机制主要表现为细晶增韧、裂纹偏转、桥联及纳米SiC颗粒的钉扎等作用。系统研究了MoSi2及其复合材料的高温摩擦磨损性能。结果表明,在室温~700℃的温度范围内,单一MoSi2的摩擦系数在0.35~0.53之间,而MoSi2-SiC复合材料的摩擦系数在0.26~0.44之间,且MoSi2-SiC复合材料的耐磨性较单一MoSi2材料的耐磨性提高了36% ~59.6%。对于MoSi2-SiC复合材料,在载荷和滑动速度一定的条件下,随着摩擦磨损温度的增加,材料的摩擦磨损过程分为三个阶段。从室温到300℃材料的摩擦系数降低,磨损率也呈小幅降低;随着温度继续升高至600℃,摩擦系数逐渐增加,而磨损率则先增加并在400℃~600℃时基本保持不变;当摩擦磨损温度达到700℃时,摩擦系数和磨损率均出现回落。材料的高温摩擦磨损机理的研究表明,粘着、氧化和疲劳断裂是造成单一MoSi2材料磨损的主要原因,而氧化、转移和转移层疲劳剥落是MoSi2-SiC复合材料磨损的主要机制。建立了FAPACS过程的数理模型,并对FAPACS过程进行了计算机数值模拟。结果表明,冲头-试样-模具系统中的温度场特性是电场焦耳热、体系化学反应热与模具系统传热效应的综合结果,且由于焦耳热与化学热的叠加作用,试样中心具有最高温度,并沿径向形成温度梯度,从而影响合成材料的组织均匀性。因此,在FAPACS过程中,对试样内部的温度梯度进行合理控制,有利于获得组织均匀、晶粒细小且致密性高的材料。更多还原

【Abstract】 With moderate density of 6.24g·cm-3, high melting point up to 2030℃, relatively low thermal expansion coefficient of 8.1×10-6K-1, and good thermal and electrical properties, MoSi2 exhibits promising potential in fields of aeronautic and astronautic industries, power and chemical engineering and metallurgical machineries. In the present study, Field-activated and pressure-assisted combustion synthesis (FAPACS) was employed to improve mechanical properties of MoSi2 and its composite, in which in situ synthesis and densification of composite could be achieved simultaneously. The effects of processing parameters on microstructure evolution were investigated. Mechanical properties and high temperature wear-resistance capability of the products were analyzed. Process of FAPACS was modeled by means of computer aided simulation.The following conclusions were made with this work:For Mo-Si system, microstructure of the product shows that Mo5Si3 distributes at grain boundaries on matrix of MoSi2, with production of small amount of SiO2. Mo5Si3 is a product of interfacial diffusion reaction between Mo and Si at relatively lower temperature. SiO2 results from oxidation of Si.In FAPACS, sintering temperature plays an important role in microstructure morphology and degree of densification. When it is higher than melting point of Si (1410℃) , the molten Si wraps particles of Mo. It helps to synthesize substantial fine grained MoSi2 with higher density. Analysis with sintering dynamics shows that combination of environment higher than melting point of Si, no less than heating rate of 100℃/min, and sufficient sintering time basic condition ensure obtaining of pure and dense bulk MoSi2.That was realized in experiments with sintering temperature 1500℃, heating rate 100℃/min, pressure 35Mpa and holding time of 10min. As regards system of Mo-Si-C, SiC was obtained from Si and C with spark plasma activated by electrical field, which is difficult to synthesize under conventional environment. SiC particles were scaled over 100nm ~ 3um. Coarse particles distributed at grain boundaries of MoSi2. Whereas the finer ones dispersed within grains.Existence of carbon is favor of elimination of SiO2 since it is free in synthesized MoSi2-SiC composite. However, inter phase of Mo5Si3C produced, which is unstable and transitive. With extension of holding time, reaction of Mo5Si3C + 8Si=5MoSi2 + SiC took place, resulting in decreasing or even elimination of its contention. In the experiments, a simultaneous in situ synthesis and densification of MoSi2-SiC composite was achieved under sintering temperature 1500℃, pressure 30Mpa and holding time of 30min.For mechanical properties of MoSi2-SiC, The micro-hardness and fracture toughness of the synthesized MoSi2-SiC composites improved with increasing SiC contents. When the volume fraction of SiC was 30%, the fracture toughness of the MoSi2-SiC composite, K1C , was up to 5.58 MPa ? m, which is 33% higher than that of MoSi2.Further study shows MoSi2 is characterized by brittle transgranular fracture with cracks within grains. Whilst, MoSi2-SiC composite possesses a hybrid fracture attribute with both intergranular and transgranular cracks around or within grains. Its mechanisms of toughness mainly lie in refinement of grains, deflection and bridging of cracks, and pinning effect of fine SiC particles.The high temperature wear resistances of MoSi2 and its composites were studied. The friction coefficient of MoSi2 is 0.35~0.53 over the range 25~700℃, and 0.26~0.44 for MoSi2-SiC composite. Moreover, the wear resistance of the latter was enhanced by 36% ~59.6% higher than that of the former.For MoSi2-SiC composite, under certain load and speed of sliding, worn process could be staged with increasing temperature as: 1) decrease of friction coefficient with slightly dropping of wear rate over room temperature to 300℃; 2) continuing enhancement of friction coefficient with gradually increasing temperature up to 600℃, and a saturation of increasing of wear rate kept over 400-600℃; and 3) leveling off of friction coefficient and wear rate to temperature of 700℃.The study of wear mechanism shows that adhesions, oxidation and fatigue fracture are coactively cause the worn of MoSi2. However, those are mainly oxidation, shifting and tearing off of friction layer for MoSi2-SiC composite.The FAPACS modeling was made and a numerical simulation was done by means of computer aided finite element method. The results show that FAPACS temperature field was determined by an interaction among Joule heating of electric field, heat released by chemical reaction, and heat transferring characteristics of die-sample system. Due to overlay of heats by Joule effect and chemical reactions, the highest temperature was in the center of the sample and a radius temperature gradient was established. That significantly took effects to uniformity of microstructure and densification degree. Therefore, it is important to adjust temperature gradient within the sample of FAPACS to prepare dense and fine grained bulk materials.更多还原