【作者】 刘含莲；

【导师】 黄传真；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2005， 博士

【摘要】 本文针对高速硬态干式切削对刀具材料的要求,从提高陶瓷刀具材料的综合力学性能出发,首次提出了多元多尺度纳米复合陶瓷刀具材料的设计思想。基于此思想,选择不同的多元多尺度陶瓷颗粒复合Al₂O₃基体,成功制备出四种具有高综合力学性能的新型陶瓷刀具材料。从热压烧结工艺、显微结构及其与力学性能的关系等方面,深入研究了多元多尺度纳米复合陶瓷刀具材料的增韧补强机理,发现晶内/晶间混合型微观结构和穿晶/沿晶混合断裂模式,是陶瓷刀具复合材料强韧性提高的主要原因。对新型陶瓷刀具切削性能的研究表明,多元多尺度纳米复合陶瓷刀具的研制成功为高性能陶瓷刀具的进一步开发与应用奠定了基础。提出了多元多尺度纳米复合陶瓷刀具材料的设计思想。添加相中有一相处于纳米级,通过多元多尺度复合,充分发挥微米增韧、纳米补强及多元互补的优势,提高复合陶瓷刀具材料的综合力学性能。本文设计了多元多尺度纳米复合材料的理想显微结构,即不同微米组元晶间分布、纳米颗粒晶内/晶间混合型分布。从裂纹扩展路径出发,论证了多元多尺度纳米复合刀具材料可形成穿晶/沿晶混合的断裂模式,消耗更多断裂能,从而有利于提高刀具材料的抗弯强度和断裂韧性。根据胶体化学中悬浮液的稳定机制,对不同的纳米陶瓷粉末进行了液相分散研究,通过优化分散剂种类、分散介质、悬浮液的pH值以及分散剂的加入量等参数,结合超声分散及机械搅拌工艺,实现了纳米粉末及混合粉末的均匀分散。研制成功了多元纳米复合陶瓷刀具材料AAS(Al₂O₃/Al₂O_3n/SiC_n),其抗弯强度为796MPa、断裂韧性为5.01MPa·m^1/2、硬度21.32GPa。纳米Al₂O₃与纳米SiC的共同作用,使AAS复合材料在较低的烧结温度和较短的保温时间内即可获得高的致密度和细化的晶粒。由沿晶断裂向穿晶断裂模式的转变是其力学性能提高的主要原因之一。研制成功了新型多尺度纳米复合陶瓷刀具材料ASs（Al₂O₃/SiC_μ/SiC_n）,其力学性能为抗弯强度715MPa、断裂韧性8.2MPa·m^1/2、硬度22.57GPa,与单一添加微米SiC或纳米SiC的复合材料相比,其抗弯强度和断裂韧性都大幅提高。在致密的烧结体中,微米SiC与纳米SiC形成了典型的晶内/晶间混合型结构,裂纹从晶间到晶内再到晶间的路径扩展,消耗了更多的断裂能,形成了沿晶/穿晶混合的更多还原

【Abstract】 From the requirement for ceramic tool materials applied in the high speed and dry machining, a new thought for designing the multi-phase and multi-scale nanocomposites was firstly proposed to improve the comprehensive mechanical properties. Based on this thought, four types of advanced Al₂O₃ matrix ceramic tool materials with high mechanical properties were fabricated successfully. The strengthening and toughening mechanisms of the ceramic tool materials based on multi-phase and multi-scale nanocomposites were investigated from the respects of the correlations among the hot pressing process, the microstructures and mechanical properties. It reveals that the intra/inter granular microstructures and the trans/inter granular fracture modes are the main causes for improving the flexural strength and fracture toughness. The good cutting performance of the developed ceramic cutting tools indicated that the study on the ceramic tool materials based on multi-phase and multi-scale nanocomposites has offered a new path to the further research.The new thought for designing the multi-phase and multi-scale nanocomposites was firstly proposed to improve the comprehensive mechanical properties. Multi-phase and multi-scale particles are added to the matrix, and one of the additives is nano-scale particle, thus the comprehensive mechanical properties can be improved by the synergic effects of micro-scale toughening, nano-scale strengthening and mutual benefit of multi-phases. The ideal microstructure of multi-phase and multi-scale nanocomposites was designed, which was composed of intergranular distributed different micro-scale phases and intra/inter granular nano-scale particles. With this ideal microstructure, the trans/inter granular fracture modes can be formed, which will consume more fracture energy during the crack propagation, therefore both the flexural strength and fracture toughness can be improved.Based on the stabilization mechanisms for suspensions in the colloidal chemistry, the dispersions of different nano-scale ceramic powders in liquid suspension were discussed. With the optimal parameters such as the type of dispersant, dispersingmedium, the pH value and the content of dispersant, the nano-scale powders and mixed powders were well dispersed with the combination of ultrasonic vibration and mechanical agitation.Multi-phase nanocomposites ceramic tool material of AkCtyAhCWSiCnOnarked as AAS) was firstly fabricated successfully, its flexural strength, fracture toughness and Vickers hardness are 796MPa, 5.01MPa-m1/2 and 21.32GPa respectively. The coactions of nano-scale AI2O3 and nano-scale SiC led to the reduction of the sintering temperature and the shorter preserving time, and therefore the composite with high density and fine grains was obtained. The improvement of the mechanical properties is mainly derived from the transition from intergranular to transgranular fracture mode.Multi-scale nanocomposites ceramic tool material of Al2O3/SiCM/SiCn（marked as ASs） was successfully fabricated, its flexural strength, fracture tougliness and Vickers hardness are 715MPa, 8.2MPa-m1/2 and 22.57GPa respectively. The comprehensive mechanical properties are much higher than that of any composite added with single nano-scale SiC or with single micro-scale SiC. The micro-scale SiC particles are located between AI2O3 matrix, and the nano-scale SiC particles are located on the grain boundary or within the matrix grain. Thus the typical intra/inter granular microstructure is formed in the dense compacts, which resulted in the trans/inter granular fracture modes. The zigzag crack path, which is from the grain boundary into the grain and then turning to the boundary, can result in higher consumption of fracture energy and the increase of fracture toughness.An advanced multi-phase and multi-scale nanocomposites ceramic tool material Al2O3/TiCn/TiNn（marked as LTN） was fabricated with the flexural strength, fracture toughness and Vickers hardness of 731MPa, 7.8MPam1/2 and 19.76Pa accordingly.. Because of the addition of micro-scale TiC with high elastic modulus and high harness, it is apt to form the framework structures which inlay each other with AI2O3 and improve the flexural strength of composite. Proper content of nano-scale TiN can lead to the further refinement of matrix grains, the remarkable strengthening and toughening effects are resulted from the crack pinning, deflection and branching.The strengthening and toughening mechanisms for multi-phase and multi-scale nanocomposites ceramic tool materials were thoroughly researched. The main strengthening and toughening mechanisms of the materials include the effects of grain fining and grain boundary strengthening caused by the nano-scale particles, residualstress toughening and the toughening effect caused by particular microstructure. The strengthening and toughening mechanisms for multi-phase and multi-scale nanocomposites ceramic tool materials were emphasized on the toughening effects caused by residual stress and the particular microstructures in the present study.A new mathematics model is firstly established to calculate the residual stress in the composites, the advantage of the new model over the existing simple model consist in considering different-sized particles and the irregular distribution of grains. Two finite element models for analyzing the residual stress were set up. The results analyzed with the ANSYS software clearly showed that the stress characters and values were significantly influenced by the particles’ size, grain distribution and the content of the added particles. Combined with the residual stress and the corresponding crack propagation path, the toughening mechanisms of multi-scale nanocomposites ceramic tool materials were discussed, which were well consistent with the experimental results.AI2O3 nanobelts were firstly observed in the material ASs with HRTEM, the forming of ductile nanobelts is helpful for the improvement of fracture toughness. In addition, the special microstructure such as dislocations and compound twins were also observed in the nanocomposites AAS and ASs with TEM, the complicated dislocation configurations and the forming of twins in larger SiC particles also contributed to the fracture toughness.The investigation for the cutting performance showed that, the novel tool AAS is preferable to LT55 when cutting 40Cr materials. While for novel ceramic "tool ASs and LTN appeared preferable when cutting hardened T10A with middle or a little lower cutting speed, their cutting performances were higher than that of SG4 ceramic tool, which was related to the higher comprehensive mechanical properties.更多还原