【作者】 郑东海；

【导师】 李元元；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2013， 博士

【摘要】 WC-Co硬质合金具有优良的综合性能，在现代刀具材料、耐磨材料等方面得到广泛应用。WC-Co合金中的WC相具有极高的硬度和优异的耐蚀性和耐高温性，金属类粘结剂（Co）的加入虽然可以提高合金的强韧性，但同时会使合金的硬度下降和耐腐蚀性变差，而且在高温下粘结相会发生软化等特性还限制了其在高温环境下的应用。利用放电等离子烧结等新技术可以制备出致密的无粘结相纯WC材料，但其偏低的韧性是此类材料获得广泛应用的最大障碍。为了提高无粘结相WC材料的韧性，传统陶瓷材料的增韧手段对此具有很重要的借鉴价值。本文以无粘结相WC材料为研究对象，研究了该材料在不同SPS烧结工艺下的致密化行为、相组成、显微组织以及力学性能；针对该种材料偏低的断裂韧性，借鉴传统陶瓷材料的增韧手段，引入相变ZrO₂颗粒、Al₂O₃颗粒对其进行了增韧研究，还首次成功地运用“原位生成”技术制备了由原位自生β-Si₃N₄晶须增韧的无粘结相WC材料，并且扩展了“两步烧结法”的理论内容及应用范围，将其成功地应用于SPS烧结WC-Si₃N₄材料中，将β-Si₃N₄晶须生成过程与WC晶粒快速生长过程分离开来；另外还研究了VC与Cr₃C₂晶粒生长抑制剂对上述材料体系的影响。通过研究分析，主要的研究结果如下：（1）采用SPS制备了无粘结相的纯WC材料、WC-x wt.%ZrO₂（x=1，2，3，6，8，10）复合材料、WC-x vol.%Al₂O₃（x=0，6.8，12.4，16.7，43.6）复合材料以及WC-xwt.%Si₃N₄（x=1，3，6，8，10，12，15）复合材料。ZrO₂/Al₂O₃/Si₃N₄（93wt.%Si₃N₄-6wt.%Y₂O₃-1wt.%Al₂O₃）的加入均可以促进WC材料的致密化过程。（2）WC-ZrO₂材料经过SPS烧结后，试样里的ZrO₂颗粒仍以四方结构为主，均匀弥散分布在WC晶粒之间；随ZrO₂含量从0增加到10wt.%，WC-ZrO₂材料的硬度从²4GPa下降至¹8GPa，而断裂韧性则从⁶.0MPa·m^1/2提高至¹0.60MPa·m^1/2，主要增韧机制为四方ZrO₂颗粒的相变增韧。对于WC-8wt.%ZrO₂材料，在1500¹700°C烧结温度范围内，随温度升高，WC晶粒与ZrO₂颗粒都逐渐长大，而VC和Cr₃C₂添加剂的加入可以完全抑制住WC晶粒与ZrO₂颗粒的生长；材料组织越细，则材料的硬度越高，但断裂韧性对材料组织的粗细度变化不敏感；采用含有VC和Cr₃C₂添加剂的细小WC粉末（⁰.2μm）作为原料，经过在1600°C保温5min烧结得到的WCVCr-8ZrO₂试样其硬度和断裂韧性分别为22.20GPa和11.40MPa·m^1/2。对于WC-ZrO₂复合材料，在维氏硬度测试（10kg）中，当ZrO₂含量较低时（≤3wt.%），材料所对应的裂纹系统为半圆型系统，而随着ZrO₂含量的增多（≥6wt.%），其所对应的裂纹系统会向巴氏裂纹系统发生转变。（3）Al₂O₃颗粒的加入对WC材料有一定的增韧作用，但效果并不显著，WC-Al₂O₃材料的断裂韧性最高仅为6.49MPa·m^1/2；增韧机制主要为Al₂O₃颗粒对裂纹的桥接作用以及材料断裂模式从沿晶断裂转变为部分穿晶断裂。（4）对于WC-Si₃N₄材料，在100°C/min升温速率下，β-Si₃N₄在1700°C以上会发生“快速熟化”现象——α→β-Si₃N₄快速转变、β-Si₃N₄晶须快速长成，与此同时WC基体晶粒也迅速发生长大；在1550¹600°C进行长时间保温，由于β-Si₃N₄熟化过程进行得比WC晶粒生长过程快，因而可以将WC晶粒快速长大过程与β-Si₃N₄晶须长成过程分离开来，从而实现抑制WC晶粒快速生长的目的；采用两步烧结方式（先升温到1700°C随即冷却至1600°C保温30min）可以制备出WC晶粒相对细小而且生成了大量β-Si₃N₄晶须的WC-10wt.%Si₃N₄材料。致密的WC-10wt.%Si₃N₄材料其硬度与WC基体晶粒平均尺寸成反相关，其断裂韧性主要随β-Si₃N₄晶须数量的增多而增大，β-Si₃N₄晶须对WC材料的增韧机制主要包括晶须拔出和裂纹桥接。采用两步烧结的WC-x wt.%Si₃N₄（x=1¹5）复合材料，随着Si₃N₄含量的增多，α→β-Si₃N₄相转变率先增大后减小，材料的断裂韧性也呈现出同样的规律，当Si₃N₄含量为10wt.%时相转变率达到最大，为¹00%，其断裂韧性值也达到最大，为10.94MPa·m^1/2，与此同时材料的硬度为17.65GPa。VC与Cr₃C₂添加剂的存在对α→β-Si₃N₄相转变过程的影响并不大，却对WC晶粒的生长具有显著的抑制作用，因而可以在相同的烧结条件下提高WC-Si₃N₄材料的硬度；含VC与Cr₃C₂添加剂的WC-10wt.%Si₃N₄材料经过在1900°C不保温烧结后，其硬度和断裂韧性分别为17.43GPa和10.07MPa·m^1/2。更多还原

【Abstract】 WC-Co cemented carbides have been widely used as cutting tools and wear-resistantcomponents due to a singular combination of mechanical properties. The addition of Cometallic binder phase to the WC which possesses extremely high hardness and excellentcorrosion-resistance as well as high temperature performance, increases the strength andtoughness of the composite considerably. However, the metallic binders in cemented carbidesare deleterious on hardness and corrosion-resistance, and also be an obstacle for theapplication at elevated temperature where it does soften. Pure WC can be densified by somenewly developing sintering techniques such as spark plasma sintering （SPS）, but the lowtoughness of which limits the application of it. To improve the toughness of WC withoutmetallic binder phase, attention should be attracted to the toughening methods used intraditional ceramics.In this study binderless WC were sintered at different SPS conditions with focus on thedensification behavior, phase constitution, microstructure and mechanical properties of thematerial. In order to elevate the fracture toughness of the binderless WC,transformation-toughening by partially stabilized ZrO₂（PSZ） and particle toughening byAl₂O₃were adopted, as well as whisker toughening by in-situ grown elongated β-Si₃N₄grains, which have not been investigated to date. Moreover,“two-step sintering” wasextended and successfully applied to preparation of WC-Si₃N₄composites with largeelongated β-Si₃N₄grains in a fine WC matrix. In addition, effects of VC and Cr₃C₂additionson the material systems mentioned above were also studied. The main research results are asfollows.（1） Pure WC, WC-x wt.%ZrO₂（x=1，2，3，6，8，10） composites, WC-x vol.%Al₂O₃（x=0，6.8，12.4，16.7，43.6） composites, and WC-x wt.%Si₃N₄（x=1，3，6，8，10，12，15） composites are fabricated by SPS. The addition of ZrO₂/Al₂O₃/Si₃N₄（93wt.%Si₃N₄-6wt.%Y₂O₃-1wt.%Al₂O₃） to WC facilitates sintering of the composites.（2） After SPSed, most of the tetragonal ZrO₂remain until room temperature, and theZrO₂-particles are homogeneously dispersed in the WC matrix. As the ZrO₂content increasesfrom0to10wt.%, the hardness of the WC-ZrO₂composites decreases from²4GPa to¹8 GPa, and the fracture toughness of the WC-ZrO₂composites increases from⁶MPa m^1/2to¹0.6MPa m^1/2. The transformation toughening is believed to be the most importanttoughening mechanism. For the WC-8wt.%ZrO₂composites sintered at1500¹700°C, theaverage size of WC and ZrO₂grains increases with sintering temperature, however, theaddition of VC and Cr₃C₂significantly suppresses the growth of WC and ZrO₂grains. Themicrostructure coarsening at elevated temperatures causes degradation in hardness, whereasthe fracture toughness seems insensitive to the coarseness of microstructure. TheWCVCr-8ZrO₂specimen sintered at1600°C for holding5min using fine WC powder （⁰.2μm） with VC and Cr₃C₂possesses hardness and fracture toughness of22.20GPa and11.40MPa·m^1/2respectively. For the WC-ZrO₂composites, the Palmqvist cracks are typicallyobserved in the high ZrO₂-content grade composites （≥6wt.%）, while the median cracks,corresponding to the half-penny cracks system, are involved for the low ZrO₂-content grade（≤3wt.%）.（3） The toughening effect exerted by Al₂O₃particles is not significant, and the fracturetoughness of the WC-Al₂O₃composites reaches6.49MPa·m^1/2only. The tougheningmechanisms include the crack-bridging by Al₂O₃particles and the local change in fracturemode from intergranular to transgranular.（4） For the WC-Si₃N₄composites sintered with a heating rate of100°C/min, fasttransformation of Si₃N₄from α to β and the β-Si₃N₄grain fast-growth resulted from“dynamic ripening” happen at above1700°C, accompanied with WC-grain fast-growth. Byexploiting the difference in kinetics between WC grain-growth and β-Si₃N₄grain-growththrough sintering at1550¹600°C for holding a long time, the separation of β-Si₃N₄grain-growth from WC-grain fast-growth and the suppression of WC-grain fast-growth areachieved. The WC-10wt.%Si₃N₄specimen after heated to1700°C and treated at1600°C for30min （two-step sintering） obtains large elongated β-Si₃N₄grains in fine WC matrix. For thedense WC-10wt.%Si₃N₄specimen, the hardness increases against the WC-grain size, andthe fracture toughness increases with the amount of elongated β-Si₃N₄grains. The majortoughening mechanisms are found to be elongated Si₃N₄grain-pullout and crack-bridging byelongated Si₃N₄grain. For the two-step sintered WC-x wt.%Si₃N₄（x=1¹5） composites,the α→β-Si₃N₄transition rate and the fracture toughness increase with the Si₃N₄content, and then decrease as the Si₃N₄content exceeds10wt.%. As the Si₃N₄content reaches10wt.%,the α→β-Si₃N₄transition rate reaches¹00%, and the hardness and fracture toughness of thespecimen reaches17.65GPa and10.94MPa·m^1/2respectively. Addition of VC and Cr₃C₂donot hinder the transformation of α to β-Si₃N₄or the growth of elongated β-Si₃N₄grains, butshows an inhibitory effect against WC grain growth, which results in higher hardness for theWC-Si₃N₄composites. The WC-10wt.%Si₃N₄specimen containing VC and Cr₃C₂sinteredat1900°C without holding possesses hardness and fracture toughness of17.43GPa and10.07MPa·m^1/2.更多还原