【作者】 成红梅；

【导师】 黄传真；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 陶瓷刀具材料具有高的强度、较强的红硬性和耐磨性,是一类极具发展前途的刀具材料,但是由于断裂韧度较低,限制了其广泛应用。纳米复合陶瓷刀具材料的出现有望从根本上解决陶瓷刀具材料断裂韧度低的问题。材料的宏观力学性能主要取决于微观组织,对材料进行宏、微观尺度耦合模拟,研究微观组织和宏观力学性能之间的关系,对纳米复合陶瓷刀具材料的研发具有重要的理论指导意义。本文研究了烧结过程中陶瓷刀具材料晶粒生长理论和Monte Carlo模拟方法,建立了无缺陷两相纳米复合陶瓷刀具材料微观组织演变的Monte Carlo Potts模型,基于微软公司开发的Visual C++6.0平台,利用C++编程语言,开发了模拟程序,对微观组织演变过程进行了模拟和试验验证。模拟研究了纳米第二相的粒径及含量对纳米复合陶瓷刀具材料微观组织演变的影响。结果表明,纳米颗粒对基体晶粒的生长具有阻碍作用,能够细化基体晶粒。在含量相同的情况下,尺寸大的纳米颗粒钉扎作用弱；在尺寸相同的情况下,纳米颗粒含量越高,其钉扎作用越强。同时模拟得到了晶内／晶间型的微观组织。纳米颗粒的位置取决于其尺寸和含量。粒径较小的颗粒容易进入基体晶粒内部,而粒径大的纳米颗粒则主要位于基体晶粒边界上。当纳米相含量较低时,由于总的钉扎力较小,因而纳米颗粒更易进入基体晶粒内部。模拟研究了不同晶界能比率对纳米复合陶瓷刀具材料微观组织演变的影响。结果表明,晶界能与界面能的竞争会影响纳米复合陶瓷刀具材料的微观组织和晶粒生长过程。当界面能高于晶界能时,纳米颗粒可能进入基体晶粒,形成晶内／晶间型的纳米复合陶瓷刀具材料,并且纳米颗粒对基体晶粒的生长产生较强的钉扎作用；当界面能低于晶界能时,纳米颗粒则主要位于晶界上,形成晶间型的纳米复合陶瓷刀具材料。模拟研究了初始基体晶粒尺寸对纳米复合陶瓷刀具材料微观组织演变的影响。结果表明,初始基体晶粒尺寸影响纳米复合陶瓷刀具材料的微观组织类型及晶粒生长过程。当纳米晶粒的尺寸及含量一定时,初始基体晶粒尺寸越大,最终复合材料的晶粒尺寸越大,但是晶粒生长速率越低,进入基体晶粒内部的纳米颗粒越少,越难得到晶内型的微观组织。建立了烧结温度和晶粒生长速度之间的关系模型、烧结压力和晶粒生长速度之间的关系模型、模拟时间和保温时间之间的关系模型,并将这些模型耦合到模拟程序中,建立了考虑烧结工艺参数的纳米复合陶瓷刀具材料微观组织模拟模型,实现了不同烧结温度、烧结压力和保温时间下的模拟。结果表明,随着模拟时间的增长,平均晶粒半径呈抛物线规律不断增大；随着烧结温度、烧结压力的升高,平均晶粒半径不断增大,烧结温度对晶粒生长的影响大于烧结压力对晶粒生长的影响。建立了含有气孔的纳米复合陶瓷刀具材料微观组织演变Monte Carlo Potts模型,开发了模拟程序,模拟研究了纳米颗粒和气孔对微观组织演变、晶粒生长和致密化过程的影响。模拟结果表明,纳米颗粒阻碍晶粒生长；气孔对基体相晶粒长大具有明显的阻碍作用,致密度较低时,晶粒生长缓慢,而致密度较高时,晶粒的生长速度明显加快；陶瓷刀具材料的致密度随模拟时间的增加而增大,晶粒生长抑制致密化。将Monte Carlo算法与有限元法耦合,建立了纳米复合陶瓷刀具材料微观尺度有限元模型,对残余热应力及外力作用下的应力场进行了模拟,实现了宏微观尺度耦合模拟。模拟研究了Al2O3/SiC纳米复合陶瓷刀具材料的残余热应力场。结果表明,纳米SiC颗粒内主要是残余压应力；基体材料内不仅存在拉应力区,而且存在不同程度和不同范围的压应力区和剪应力区。残余热应力大小和分布形式与第二相颗粒的含量、颗粒尺寸及基体相初始粒径密切相关。最大残余拉应力随着第二相粒径和体积含量的增加而增大,而最大残余压应力则随着第二相体积含量的增加呈现先上升后下降的趋势。模拟研究了Al2O3/SiC纳米复合陶瓷刀具材料在单轴拉力作用下的应力场。结果表明,残余热应力的存在使材料内部产生较大的残余压应力区,有利于裂纹尖端闭合,对增韧有一定作用。结合残余应力的有限元模拟结果,分析了纳米复合陶瓷刀具材料的残余应力增韧机理。材料内部的残余压应力对裂纹尖端具有闭合作用,纳米相周围产生的局部拉应力能够诱发穿晶断裂,同时促使裂纹分叉和偏转,从而提高陶瓷刀具材料的断裂韧度。为减小拉应力范围,降低拉应力值,可对第二相的含量和粒径以及基体相初始粒径应加以控制,保证残余压应力较大而拉应力较小。对Al2O3/SiC纳米复合陶瓷刀具材料进行了烧结试验,实验获得的微观组织特征与模拟的微观组织特征吻合较好,证明了模拟模型和模拟结果的正确性。更多还原

【Abstract】 Ceramic tool material has been one of the most important cutting tools because of its high hardness, favorable high-temperature hardness and good wear resistance. However, the fracture toughness of ceramics is low which limits the application of ceramic tools. The appearance of nanocomposite ceramic tool materials can substantially solve the problem of low fracture toughness of ceramic materials. The macro mechanical properties of materials are governed by their microstructures, it is very meaningful to simulate the micro structure of nanocomposite ceramic tool materials by coupling macro-scale and micro-scale simulation methods and study the relationship between microstructure and mechanical properties.In this paper, the grain growth theory for ceramic tool materials during fabrication and Monte Carlo simulation algorithm have been analyzed. The Monte Carlo Potts model for microstructure evolution in two-phase nanocomposite ceramic tool materials has been established. Based on Microsoft Visual C++6.0 compiler, the simulation program has been developed using C++language and the microstructure evolution has been simulated and verified by experiments.The effect of nanoparticles with various size and area fraction on microstructure evolution of nanocomposite ceramic tools has been simulated. It is shown that the nanoparticles have inhibition effect on matrix grain growth and refine the ceramic matrix grain. Large-sized nanoparticles have weaker pinning effect than the small one for a specific area fraction, and nanoparticles with high area fraction have stronger pinning effect on matrix for a given grain size. The microstructure of nanocomposite ceramic obtained from simulation is intra/intergranular-type, and the positions of nanoparticles depend on the size and area fraction of nano-phase. The small-sized nanoparticles are easier to enter into the matrix grains, whereas the large one prefers to locate on the grain boundaries. When the area fraction is lower, the total pinning force is weaker, as a result the nanoparticle is easier to be entrapped inside the matrix grains.The effect of grain boundary energy ratio on microstructure evolution of nanocomposite ceramic tools has been simulated. It is shown that the competition between the boundary energy and the interfacial energy affects the microstructure and grain growth of nanocomposite ceramic tool materials. When the interfacial energy is higher than the grain boundary energy, the nanoparticles may enter into the matrix grains and form an intra/intergranular-type nanocomposite ceramic material and the nanoparticles have a stronger pinning effect on matrix grain growth. When the interfacial energy is lower than the grain boundary energy, the nanoparticles mainly locate at the grain boundaries and form an intergranular-type nanocomposite ceramic material.The effect of initial matrix grain size on microstructure evolution of nanocomposite ceramic tools has been simulated. It is shown that the initial matrix grain size affects the microstructure type and grain growth process. For a given grain size and area fraction of the nano-phase, the larger the initial matrix grains, the larger the final grain size is, but the lower the grain growth rate is. There are less nanoparticles entering into the matrix grains, and it is more difficult to obtain an intragranular-type microstructure.The relationship between fabrication temperature and grain growth velocity, the relationship between fabrication pressure and grain growth velocity, and the relationship between simulation time and real duration time have been establishied. These three relationships have been incorporated into simulation program. The microstructure of nanocomposite ceramic tool materials has been simulated allowing for the fabrication parameters. It is shown that the average grain radius increases with an increment in simulation time, fabrication temperature and fabrication pressure. But the effect of fabrication temperature on grain growth is bigger than that of fabrication pressure.The Monte Carlo Potts model for microstructure evolution in nanocomposite ceramic tool material containing pores has been set up and programmed. The effect of nanoparticles and pores on the microstructure evolution, grain growth and densification has been simulated. It is shown that the nanoparticles inhibit grain growth, and the pores affect grain growth and densification. The inhibition by pores is very obviously, the grain growth is slower at a lower density and vice versa. The densification of nanocomposite ceramic tool material increases with an increment in simulation time, and the grain growth restrains densification.The microscale finite element simulation model for nanocomposite ceramic tool materials has been established by coupling Monte Carlo algorithm with finite element method. The residual thermal stress and the stress generated by applied load has been simulated, and the coupling simulation at macro-scale and micro-scale is accomplished.The residual thermal stress of Al2O3/SiC nanocomposite ceramic tool materials has been simulated. The results show that there are mainly residual compressive stresses in nano SiC particles, and there are not only residual tensile stresses but also compressive stresses and shearing stresses in matrix. The magnitude and distribution of residual thermal stress is closely related to the content and size of nano-phase and the size of matrix-phase. The maximum residual tensile stress increases with an increment in the size and content of nano-phase, and the maximum residual compressive stress increases at first and then decreases with the content of nano-phase.The stress field of Al2O3/SiC nanocomposite ceramic tool materials has been simulated under uniaxial pressure. The results show that the residual thermal stress can cause bigger compressive stress field which is helpful to close the crack tip and result in toughening effect.The toughening mechanism by residual thermal stress in nanocomposite ceramic tool materials has been analyzed by using the simulation results of finite element method. The internal residual compressive stress can close crack tip, and the partial tensile stress around nano particles can induce intragranular failure, make crack branch and deflect, to improve the fracture toughness of nanocomposite ceramic tool materials. In order to reduce the distribution scope and magnitude of tensile stress, the nano-phase content and size as well as the initial matrix grain size should be controlled.A kind of nanocomposite ceramic tool material Al2O3/SiC has been developed to verify the simulated results. The characteristics of the micro structure of the developed material Al2O3/SiC are in well agreement with those of the simulated microstructure. Therefore the established models and the simulation results in the dissertation are convinced.更多还原