【作者】 欧阳辰鑫；

【导师】 朱世根；

【作者基本信息】 东华大学 ， 材料加工工程， 2014， 博士

【摘要】 硬质合金具有高强度、高硬度、高弹性模量和低热膨胀系数等优点,在切削刀具、石油与矿产勘探、精密模具和耐磨零件等领域有着广泛应用,因此受到国内外学术界、产业界的重视,成为新型工具和结构材料的研究热点之一。钨钴类(WC-Co)硬质合金是该领域材料的研究主题,其中金属钴(Co)是合金材料韧性的前提,碳化钨(WC)作为硬质相是材料高硬度的保证。但Co是一种昂贵而稀缺的战略资源,且由于其特殊的物理、化学性能,在催化剂、电池、电子部件等领域的需求量也在增加。此外,由于Co的低熔点、高温时易软化、高化学活性等特点使得合金材料的硬度和耐蚀性受到影响。因此,研制兼具高硬度和高韧性、原材料易得的新型复合材料,成为钨钴类合金的理想替代物,具有重要的战略和经济意义。本文中,以氧化镁(MgO)作为Co的替代材料,采用机械合金化法制备出具有纳米结构的WC-MgO复合粉末以及运用热压烧结工艺获得WC-MgO块体复合材料。研究工作从分析复合粉末的烧结致密化过程开始,通过改善烧结体的显微组织结构来提高其力学性能,然后再考察该材料的腐蚀、磨损等性能。主要工作包括：研究复合粉末的烧结致密化过程；设计针对纳米复合粉末的热压烧结工艺,系统观察烧结过程中材料显微结构变化；考察该复合材料的断裂韧性；研究烧结试样在酸性、碱性环境中的腐蚀和腐蚀磨损行为；探讨烧结试样的抗冲蚀磨损规律；比较添加晶粒长大抑制剂(碳化钒／碳化铬)对烧结试样组织和性能的影响等内容。主要研究结果如下：1.对于具有纳米结构的WC-MgO复合粉末而言,烧结温度和时间对其成型块体的致密化程度有着重要影响。研究了该复合粉体在烧结过程中的致密化速率,运用主烧结曲线理论对该粉体在不同加热速率下的致密度演化过程进行研究。结果表明：适合于WC-4.3wt.%MgO纳米复合粉末的烧结温度区间为1500℃-1800℃。基于全期烧结模型理论,建立了该纳米复合粉体的主烧结曲线,并估算出该粉体的烧结激活能为361.8kJ/mol。经过多组重复实验验证该主烧结曲线的有效性。已建立的主烧结曲线可以预测复合粉体在不同加热速率或不同保温时间条件下的致密度化速率和最终致密度,并且实现根据所需的最终致密度来制定相应的烧结工艺。2.以同样的纳米复合粉末为研究对象,系统地分析不同热压烧结制度对烧结体的致密度和晶粒生长过程的影响,研究添加碳化钒(VC)和碳化铬(Cr3C2)对烧结体的致密化过程、晶粒生长和力学性能的影响。实验结果表明：综合考量材料在烧结过程中的致密化程度与晶粒生长速率,确定了最佳的烧结制度为烧结温度1650℃、保温时间90min、烧结压力39.6MPa的真空条件。在此条件下可获得致密度97.58%TD、维氏硬度15.43GPa、抗弯强度1065.3MPa的块体烧结材料。添加一定量的VC/Cr3C2可以减缓烧结体的致密化速率并有效地抑制晶粒生长过程,促使晶粒生长时所需激活能从426.85kJ/mol增加至492.53kJ/mol,进而细化基体WC的显微组织,提高MgO颗粒分布的均匀性以及颗粒与基体的结合性能,以致提高其宏观力学性能。3.以在最佳烧结制度条件下获得的块体复合材料为研究对象,考察烧结试样的断裂韧性。研究烧结试样的断裂韧性受加载载荷变化的影响,准确地确定该材料断裂韧性值并且分析其增韧机制。研究结果表明：在研究烧结试样的压痕断裂韧性时,得出Niihara力学模型能够较准确的估算此类新材料的压痕断裂韧性,对应硬度加载载荷为49N。结合断裂力学模型分析发现,较小的第二相颗粒粒径与均匀的第二相颗粒分布状态使裂纹在该复合材料中扩展路径加长,发生偏转,产生一定的增韧效果。4.采用浸泡腐蚀和电化学腐蚀方法对不同成分烧结试样在酸性(pH=1)和碱性(pH=13)环境下的腐蚀行为进行研究,探讨添加VC/Cr3C2对WC-MgO复合材料耐蚀性的影响,考察烧结试样在两种腐蚀环境下的磨损规律,研究结果表明：在酸性溶液中,第二相MgO颗粒优先被溶解,而基体WC保持稳定,烧结体呈现出较好的耐蚀性和钝化趋势,钝化膜是由WO3·H2O和WO3·2H2O组成,添加VC/Cr3C2后材料的耐蚀性得到提高。然而,在碱性溶液中,基体WC受到明显的腐蚀,基体与第二相界面被破坏,材料无钝化现象,力学性能受严重影响,且添加VC/Cr3C2加速了材料的腐蚀速度。类似于耐蚀性的差异,烧结试样在酸性环境中的耐磨性较高,且组织细化能够提高其耐磨性；在碱性环境中烧结试样的磨损量较高,且组织细化未能显著的帮助材料减少磨损量。5.以石英砂(SiO2)为冲蚀颗粒,考察不同冲蚀角度下烧结试样的冲蚀磨损规律,建立一种微观动力学模型以深入研究冲蚀条件和靶材的显微组织对磨损机制的影响。研究结果表明：WC-MgO复合材料的冲蚀磨损规律与陶瓷类材料类似,随着冲蚀角度的增加,冲蚀磨损量也增加。靶材受颗粒冲蚀时材料表面发生沿晶断裂和穿晶断裂。添加VC/Cr3C2后,材料在各个角度的磨损量均有减少,且高角度(90。)冲蚀时其抗冲蚀能力的提高程度最为明显。基于牛顿力学定律和材料自身的性能(杨氏模量、屈服强度、拉伸强度和断裂强度)建立了一种用于模拟WC-MgO复合材料冲蚀磨损行为的微观动力学模型,发现晶粒尺寸和晶界强度对抗冲蚀磨损能力有重要影响：只有当晶界强度足够强时,晶粒细化才能发挥其正面有效的作用；当晶界强度很弱时,晶粒细化反而加速了材料的磨损。本文中,作者以纳米WC-MgO复合粉末及其热压成型技术为研究基础,利用现代测试技术,对热压烧结成型的复合材料的物相、显微组织、力学性能、腐蚀行为、腐蚀磨损和冲蚀磨损规律进行深入研究,探讨晶粒长大抑制齐(VC/Cr3C2)对烧结试样的组织和性能的影响,为WC-MgO复合材料的进一步研究和推广奠定了坚实的基础。更多还原

【Abstract】 Cemented carbides with superior strength and hardness, high elastic modulus and low thermal expansion coefficient, have been taking an important role in the industrial areas of cutting tools, milling and drilling, molds and wear resistant parts. Among hard alloys, WC-Co materials embrace the widest applications, in which Co guarantees the good toughness and WC ensures the high hardness. However, Co is rare and expensive strategic resource. Moreover, due to its attracting physical and chemical properties, the demand of Co in catalyst, batteries, electronic components and other aspects is increasing rapidly. In addition, the hardness and corrosion resistance of WC-Co composites are deteriorated because of the low melting point, easily soften phenomenon and high chemical activity of Co. Therefore, it is of great urgency and significance to fabricate new composites with higher hardness and strength using economical raw materials, which could be considered as the alternative materials for WC-Co composites.In this paper, MgO was chosen as the substitute of Co. The nanocomposite WC-MgO powders were produced using mechanical alloying method and the subsequent consolidation of WC-MgO bulk materials was conducted using hot pressing sintering technique. This study was begun with the investigation on the densification process of composite powders. Methods were conducted to improve the microstructures and mechanical properties of sintered materials. Then, the corrosion and wear behavior of sinterined samples were studied. The main aims of this study are: Studying the densification process of composite powders during sintering; Observing the microstructural evolution of bulk materials under different designed sintering regimes; Evaluating the method to determine the hardness and fracture toughness of as-sintered materials; Studying the corrosion and corrosive wear behaviors of compacts in the acidic and alkaline solutions, respectively; Investigating the solid particle erosion behavior of compacts; Comparing the effects of adding grain growth inhibitors on the microstructures properties of as-sintered samples. Significant original results are listed as follows:Firstly, sintering temperature and time largely affect the densification process of the nanocomposites WC-MgO powders. The densification process of composites powders was investigated. Master sintering curve was also established based on the results of densification rate under different heating rate. The experimental results demonstrated that the optimum sintering temperature regime for WC-4.3wt%MgO nanocomposites powder is1500℃-1800℃. The master sintering curve was established based on the combined sintering theory. The apparent activation energy for the powders is361.8kJ/mol. The ability of MSC models to predict and control the sintering process for identical WC-MgO compacts was verified by a series of sintering experiments. The established sintering curve could be used to predict the final density and desification process of this materials under different heating rate and sintering time. In addition, suitable sintering parameters could be decided based on a required final density using this master sintering curve.Secondly, the same nanocomposites powders were used as raw materials powders. The influence of sintering parameters on the density and grain size of as-sintered samples was studied. The effects of adding VC and Cr3C2on the densification process, grain growth and mechanical properties were also studied. The results demonstrated that optimum sintering parameters for this composites were found after carefully investigating the densification process and gran growth during sintering. Hot pressing sintering at the temperature of1650℃with applied pressure of39.6MPa for90min could achieve compacts with a relative density of97.58TD%and desired mechanical properties, i.e. Vickers hardness of15.43GPa and flexural strength of1065.3MPa. The addition of0.25wt.%VC and0.25wt.%Cr3C2decreased the densification rate of sintered samples and retarded the grain growth during sintering. The apparent activation energy for grain growth during isothermal sintering was increased from492.53kJ/mol to426.85kJ/mol. Correspondingly, this addition of VC/Cr3C2decreased the grain size, improved the dispersion statues of second phase, increased the interfacial bonding strength between the matrix and second phase and thus enhanced the mechanical properties of sintered samples.Thirdly, WC-MgO compacts sintered at the optimum sintering parameters were used to investigate their fracture toughness. The influences of applied load on the fracture toughness of sintered compacts were also studied. Then the techniques to accurately determine the fracture toughness and mechanisms responsible for increasing tougheness were discussed. The experimental results revealed that values of fracture toughness determined using the Niihara’s equation under the load of49N was chose to represent the indentation fracture toughness of WC-MgO composites. The improved toughness of this composite can be attributed to the second phase toughening effects. Cracks deflections and extentions that enhance the toughenss were induced by smaller particle size and better degree on dispersion of-second phase.Fourthly, the corrosion behavior of WC-MgO compacts in the acidic (pH=1) and alkaline (pH=13) solutions was investigated using immersion corrosion method and electrochemical tests. The influence of adding VC/Cr3C2on the corrosion resistance was also studied. Then the corrosive wear behavior of samples in the same corrosive environments was observed. The experimental results demonstrated that the MgO phases were preferentially dissolved and WC matrix had a chemical stability in the acidic solution. Samples could become passivate in this solution and the passive layers were made of WO3·H2O and WO3·2H2O. The addition of VC/Cr3C2improved the corrosion resistance in the acidic solution. However, in the alkaline solution, WC matrix was severely attacked and the interfacial bonding between the matrix and second phase was deteriorated. Samples had no the ability to become passivate in this situation. Interestingly, the addition of VC/Cr3C2accelerated the corrosion rate in the alkaline solution. Similar to the corrosion behaviors of the materials, samples revealed a higher wear resistance in the acidic solution. VC/Cr3C2addition also increased the wear resistance in the acidic solution. On the contrary, the wear loss of materials in the alkaline was more pronounced and could be reduced slightly by improving the mechanical properties.Fifthly, SiO2was taken as erodent particles. The solid particle erosion behaviors of sintered samples under different impingement angles were investigated. A micro scale dynamic model (MSDM) was established to study the effect of experimental parameters and microstructure on the erosion resistance. The results revealed that the erosion behavior of WC-MgO composites was similar to that of ceramics based materials. The wear loss caused by solid particle erosion increased with increased impingement angle. The dominant fracture model for this composites is combined by intergranular and transgranular fracture. With the addition of VC/Cr3C2, the erosion resistance at each impingement angle was improved. The grain growth inhibitors particularly benefited the high-angle erosion resistance of the material due to the elevated toughness. A micro scale dynamic model was established based on the Newton’s law and mechanical properties (Young’s modulus, yield modulus, tensile strength and fracture strength) to study erosive behaviors and mechanisms of WC-MgO composites. Modeling results demonstrated that grain size and grain boundary strength play considerably roles in the erosion resistance. Reduced grain size could benefit the erosion resistance only when the grain boundary is reasonably strong. The wear loss caused by erosion became much more pronounced with a weak grain boundary.The experiments and results in this dissertation were based on the synthesis of nanocomposites WC-MgO powders and its hot pressed sintering technique. The microstructures, mechanical properties, corrosion, corrosive wear and erosion wear of the as-sintered samples were investigated in detail. The effects of adding a small amount of VC/Cr3C2on the microstructures and properties of WC-MgO composites were also discussed. This study laid a solid and profound foundation for the understanding and applications of WC-MgO composites.更多还原