【作者】 刘炳强；

【导师】 黄传真；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2007， 博士

【摘要】 传统的SiC晶须增韧氧化铝陶瓷刀具在高速切削时，其中的SiC晶须易与淬硬钢产生化学反应，而且SiC与氧化铝间的热膨胀失配较严重；同时在制备晶须增韧陶瓷刀具时，存在晶须分散困难、制备成本高和危害人体健康等问题，因此本文采用原位合成复相粉末，研制成功了原位生长TiC晶须增韧氧化铝陶瓷刀具材料，系统研究了碳热还原法中TiC晶须的生长机理、原位生长TiC晶须增韧氧化铝陶瓷刀具材料复相粉末制备工艺及机理、热压烧结工艺、力学性能、显微结构、增韧机理、切削性能及刀具磨损机理。研究了碳热还原法中TiC晶须的生长机理，提出了熔滴表面输运机制和SLLS晶须生长机理，研究了SLLS晶须生长机理中的晶须生长方式和影响晶须产量与长径比的主要因素。结果表明，在碳热还原法生长TiC晶须的过程中，Ni熔滴是引导晶须生长的触媒相，NaCl熔滴是传质相，NaCl熔滴通过对反应物颗粒和Ni熔滴的粘附及NaCl熔滴自身的蒸发，不断向Ni熔滴输运反应物颗粒，维持晶须的连续生长。在SLLS晶须生长机理中，存在着反应生长、熟化生长和连接生长三种晶须生长方式。采用搅拌混料方式、添加足以粘附所有原材料颗粒的传质相、提高触媒相的含量、提高升温速度以及根据传质相的蒸发速度和触媒相对原材料颗粒的溶解速度控制生长容器内外的气氛交换速度等，都能够提高晶须产量。调节触媒相与传质相在含量上的配比则可以改变晶须的长径比。SLLS机理采用触媒液滴引导从固相原材料中生长晶须，因此不仅具有VLS机理适合大部分晶须生长并对晶须生长状态具有较好可控性的优点，而且具有SLS机理适合于规模化生产的优点，是继VLS机理后又一种重要的晶须生长机理。研究了原位生长TiC晶须增韧氧化铝陶瓷刀具材料复相粉末的制备工艺和机理。结果表明，碳热还原法是制备复相粉末的理想工艺，氧化铝基体与TiC晶须前驱体粉料之间具有良好的化学相容性。湿磨混料可使晶须在基体粉末中分散生长，最佳生长温度是1480℃。当碳含量为标准化学计量比的97％左右时，刀具材料具有较好的综合力学性能。采用高温区保温的晶须生长工艺有利于提高化合碳含量，降低游离碳含量，提高刀具材料的力学性能，因此最佳的晶须生长工艺是二步保温法，即在1480℃左右保温60min，然后继续在1700℃左右保温30min。Ni和NaCl的最佳含量（摩尔比）为TiO₂∶Ni∶NaCl=1∶0.05∶0.5。振实装料和氧化铝相变可以抑制基体晶粒长大和团聚，改善刀具材料的力学性能。研制成功了原位生长TiC晶须增韧氧化铝陶瓷刀具材料ATW30。在理想的烧结工艺下，ATW30的抗弯强度可达800MPa，断裂韧度可达7.63MPa·m^1／2，硬度可达19.5GPa。提出将陶瓷刀具材料中晶须与基体间的界面结合强度划分为热力学结合强度和机械结合强度两类，由于在足够高的烧结温度下才能形成晶须与基体间较高的界面热力学结合强度，因此在以热力学结合强度为主的晶须增韧陶瓷刀具材料ATW30中，只有达到一定的烧结温度，才能够保证刀具材料具有较高的断裂韧度。在原位生长晶须增韧陶瓷刀具材料中发现了大量晶内型晶须，并在包裹晶内型晶须周围的基体晶粒中发现了位错群。大量晶内型晶须及位错群有利于提高刀具材料的致密度和力学性能。建立了反映了断裂韧度和裂纹扩展面积实效比、弹性模量及断裂表面能之间关系的裂纹扩展面积实效比增韧模型。结果表明，影响刀具材料ATW30弹性模量和断裂表面能的主要因素是材料组分。晶须-基体界面的剥离和沿晶断裂均能提高刀具材料ATW30的裂纹扩展面积实效比和断裂韧度。采用实效比增韧模型预测了陶瓷刀具材料ATW30的断裂韧度，其预测值(8.85MPa·m^1／2)与实测值(7.63MPa·m^1／2)基本吻合，证明了该模型的正确性。裂纹扩展面积实效比增韧模型不涉及刀具材料的具体增韧机理，具有更广泛的普适性，对研制新型陶瓷刀具材料具有重要的理论指导意义。研究了陶瓷刀具材料ATW30切削淬硬40Cr、45^#钢和T10A时的切削性能和刀具磨损机理，并与陶瓷刀具SG4进行了对比。结果表明，ATW30的抗磨损能力明显优于SG4。ATW30在低速切削淬硬40Cr时的抗磨损能力优于高速切削，其中低速切削时的主要磨损机理是磨粒磨损，高速切削时的主要磨损机理是磨粒磨损和粘结磨损。ATW30在高速切削淬硬45^#钢和T10A时的抗磨损能力优于低速切削，主要磨损机理是磨粒磨损，这表明ATW30与45^#钢及T10A的亲和力较小，具有良好的化学稳定性。更多还原

【Abstract】 SiC whiskers are generally used in whisker toughening Al₂O₃ ceramic tool materials. However, the compound SiC can react with the element Fe when machining hardened steels in high cutting speed and its thermal expansion coefficient is too low to be an ideal toughening additive to the Al₂O₃. Moreover, the application of whisker in ceramic tool materials is still limited by such disadvantages as difficult dispersion, a high cost, healthy hazard, etc. Therefore, a TiC whisker toughening Al₂O₃ matrix ceramic tool material is successfully fabricated by an in situ growth technology. The growth mechanisms of TiC whisker, the fabrication for mixture powder of in situ growth whisker toughening Al₂O₃ matrix ceramic tool material, as well as the fabrication, mechanical properties, microstructure, toughening mechanisms, cutting performance and tool wear mechanisms of the in situ growth whisker toughening Al₂O₃ matrix ceramic tool are investigated.The growth mechanisms of TiC whisker synthesized by a carbothermal reduction technology are investigated. A flux globe surface transport mechanism and a SLLS （solid-liquid-liquid-solid） whisker growth mechanism are proposed. The growth patterns and the factors affecting the yield and aspect ratio of the whisker in the SLLS mechanism are investigated. It is shown that the Ni is the accelerant phase to induce the growth of TiC whiskers and the NaCl is the transport phase in the carbothermal reduction technology. Based on the surface wettability and evaporation of NaCl flux globes, the raw materials can be transported as solid particles directly to the Ni droplet to sustain the growth of TiC whiskers. There are three whisker growth patterns in the SLLS mechanism such as reaction, Ostwald ripening and coalescence growth. The yield of whiskers in the SLLS mechanism can increase by the means of mixing the raw materials in a blender, increasing the transport phase content to adhere the majority of raw material particles, increasing the accelerant phase content, elevating the heating rate and adjusting the exchanging rate of the protective atmosphere between inside and outside of the reaction container according to the evaporating rate of the transport phase and the dissolving rate of the accelerant phase to raw material particles. The aspect ratio of whiskers can be controlled by such a method as to adjust the content ratio of the accelerant phase to the transport phase in the precursor. Because that the growth of whiskers is reduced by accelerant droplets from solid raw materials, the SLLS mechanism has the advantages of both the VLS mechanism and the SLS mechanism such as wide application in growth of varied whiskers, controllable growth state and commercial foreground, will be an important whisker growth mechanism just as the VLS mechanism.The fabrication for mixture powder of in situ growth TiC whisker toughening Al₂O₃ matrix ceramic tool material is investigated. The results show that the carbothermal reduction technology can be used to prepare the mixture powder, the chemical compatibility between the Al₂O₃ matrix and the precursor for TiC whiskers is ideal. Ball milling in ethonal medium can be used to prepare the precursor to synthesize homogeneou mixture powder in which the TiC whiskers is well dispersed. The ideal growth temperature for TiC whiskers in Al₂O₃ matrix powder is 1480℃. The mechanical properties of the Al₂O₃ ceramic tool material toughened by in situ growth of TiC whiskers can be improved when the carbon content in the precursor decreases to be 97% of the stoichiometric content. Synthesizing at a higher temperature can decrease the element carbon content in the mixture powder by increasing the compound carbon content in the TiC whiskers, which can also improve the mechanical properties of the tool material, therefore the ideal fabrication technology of the mixture powder is to be firstly heated to 1480℃and held for 60min duration, subsequently heated to 1700℃and held for 30min duration. The ideal content （molar ratio） of Ni and NaCl in the precursor is that TiO₂:Ni:NaCl=1:0.05:0.5. A vibration operation to full the precursor powder into react container and the phase transformation fromγ-Al₂O₃ toα-Al₂O₃ can both be used to control the growth and aggregation of the Al₂O₃ matrix grains, improve the mechanical properties of the ceramic tool material.An in situ growth TiC whisker toughening Al₂O₃ ceramic tool material ATW30 is successfully fabricated. The flexure strength, fracture toughness and Vickers hardness of ATW30 are 800MPa, 7.63MPa·m^1/2 and 19.5GPa, respectively. A new concept is suggested that the interface bonding strength of whisker-matrix can be divided into two kinds such as the thermal bonding strength and the mechanical bonding strength. The fracture toughness of ATW30 can be improved by fabrication at a higher temperature because that the whisker-matrix interface bonding strength in ATW30 is mainly formed by the thermal bonding strength that can be strengthened at a higher temperature. Large amount of intragranular TiC whiskers and dislocation configuration are observed in ATW30, which can improve the density and mechanical properties of the material.The model for the ratio of practical area to effective area of crack propagation is established in which the relationship among the ratio of practical area to effective area of crack propagation, elastic modulus, fracture surface energy and fracture toughness is investigated. The results show that the main factor affecting the elastic modulus and fracture surface energy of ATW30 is the composition. The ratio of practical area to effective area of crack propagation of ATW30 is increased by the intergranular fracture and the debond on the whisker-matrix interface. The verification of the model on the ceramic tool material ATW30 shows that the calculated fracture toughness value (8.85MPa·m^1/2) is in consistent with the measured value (7.63MPa·m^1/2). Because that the concrete toughening mechanism need not to be related, the model has more universal applicability and important theoretical guidance significance to the study on advanced ceramic tool materials.The cutting performance and tool wear mechanisms of the ceramic tool material ATW30 when machining hardened steels such as 40Cr, 45^# and T10A are investigated and compared to the ceramic tool material SG4. The results show that the wear resistance ability of ATW30 is superior to SG4. The wear resistance ability of ATW30 when machining hardened 40Cr in low cutting speed is superior to that in high cutting speed. The main wear mechanism is abrasive wear in low cutting speed whereas abrasive wear and adhesive wear in high cutting speed. The wear resistance ability of ATW30 when machining hardened 45^# and T10A in high cutting speed is superior to that in low cutting speed. The wear mechanism is abrasive wear, indicating that the ceramic tool material ATW30 has excellent chemical stability and very low affinity when machining such hardened steels as 45^# and T10A.更多还原