【作者】 吴鹏；

【导师】 郑勇；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2014， 博士

【摘要】 为了进一步提高Ti(C,N)基金属陶瓷的使用性能,本文首先对Ti(C,N)基金属陶瓷的强韧化技术和相关理论进行了研究。在此基础上,研究了基于等离子渗碳制备梯度结构Ti(C,N)基金属陶瓷的技术,探讨了梯度结构金属陶瓷渗碳过程中碳原子的扩散及相关机理。首先对Ti(C,N)基金属陶瓷的真空烧结工艺进行了系统地研究。结果表明:在粘结相为23wt.%Ni的Ti(C,N)-Ni基金属陶瓷中,试样在1450℃下保温60min后,环形相包覆完整且厚度适中,组织分布均匀,从而有较佳的机械性能。而用4.6wt.%Co部分替代Ni的Ti(C,N)-Ni-Co基金属陶瓷,试样在1410℃下保温60min后,材料就可获得较佳的显微组织和性能。研究了化学成分对Ti(C,N)基金属陶瓷组织和性能的影响。实验结果表明:随着Ni加入量的增加,组织逐渐变得均匀,硬质颗粒逐渐球化,粘结相中Ti、W、Mo等合金元素的含量逐渐降低。当用Co部分替代Ni时,可改变硬质颗粒芯部的大小和Rim相的厚度。当Co/(Ni+Co)相对含量为0.2时,材料有较佳的抗弯强度。随Ta C加入量的增加,复杂的(Ti,Mo,W,Ta)(C,N)环形相优先形核部位逐渐从Ti(C,N)颗粒的表面转移到了Ta C颗粒的表面。当Ta C加入量为5wt.%时,晶粒细化、显微组织分布均匀且Rim相厚度适中,材料获得较高的抗弯强度。研究了碳纳米管对金属陶瓷组织和性能的影响。得到如下实验结论:经过等离子体处理后,碳纳米管表面和管束内的非晶碳和杂质大部分被剥离掉,并在碳纳米管表面引入了大量的活性基团,从而使分散性明显提高。随着碳纳米管的增加,粘结相中溶解的Ti、W和Mo合金元素含量减少。过多碳纳米管的添加会出现团聚现象。当碳纳米管含量为0.5wt.%时,晶粒细化、组织分布均匀且粘结相的体积分数较高,获得较佳的综合机械性能。碳纳米管的增韧主要是由裂纹偏转、桥联和拔出等机制共同作用的结果。研究了Si C晶须对金属陶瓷组织和性能的影响。实验结果表明:由于粘结相与Si C晶须之间的润湿性差,随Si C晶须添加量的增加,金属陶瓷材料的孔隙率逐渐增加。Si C晶须的添加不会在金属陶瓷中产生新相。当Si C晶须含量为1.0 wt.%时,材料的抗弯强度和断裂韧性分别提高了约24%和29%。材料的增强是由晶粒细化、Rim相厚度适中和组织分布均匀所致,材料的增韧是由裂纹偏转、晶须桥联和晶须拔出等机制共同作用的结果。采用等离子渗碳技术制备了梯度结构Ti(C,N)基金属陶瓷。随渗碳温度的升高,原子的扩散系数增加,元素的扩散速度加快,导致贫粘结相层厚度的增加。随渗碳时间的增加,表面粘结相含量的降低且由表及里晶体缺陷逐渐减少,使贫粘结相层生长速度不遵循抛物线规律。当Ti(C,N)基金属陶瓷在1200℃下渗碳180min时,材料表层形成了约20μm的富C和Ti的贫粘结相层,表面层晶粒有所细化,而在距表层约20?m~35?m的近表层富Ni。材料的表面硬度和耐磨性得到较大的提高。研究了梯度结构Ti(C,N)基金属陶瓷渗碳过程中碳原子的扩散和相关机理。得到了金属陶瓷表面碳浓度分布的半经验公式,计算所得结果与实验测试结果基本吻合。由于金属陶瓷材料中各组元与碳元素之间的化学亲和力不同,在等离子渗碳过程中,Ti、W和Mo向金属陶瓷的表面迁移,迫使Ni向材料内部迁移。在等离子轰击时,工件表面的大量晶体缺陷使等离子渗碳的扩散系数比普通渗碳要大很多。更多还原

【Abstract】 In order to further improve the performance of Ti(C,N)-based cermets, in this dissertation, the strengthening-toughening technology and related theories of Ti(C,N)-based cermets were studied. Based on above studies, the plasma carburizing fabrication technology of graded structure Ti(C,N)-based cermets was investigated. The diffusion characteristics and related mechanism of carbon atoms during carburization of graded structure Ti(C,N)-based cermets were discussed.In the first part of dissertation, the vacuum sintering process on Ti(C,N)-based cermets have been studied systematically. The results showed that the Ti(C,N)-Ni-based cermets with 23 wt.%Ni sintered at 1450 ℃for 60 minutes had the moderate thickness of rim phase and the homogeneous microstructure, resulting in better mechanical properties. When Ni was partly replaced by 4.6wt.% Co, the Ti(C,N)-Ni-Co-based cermets sintered at 1410 ℃for 60 minutes had better microstructure and properties.The influence of the chemical composition on the microstructure and properties has been studied. The results showed that with Ni addition increasing, the microstructure of the cermets became even gradually, the hard phase got rounded, and the dissolution of Mo、W and Ti in binder phase decreased gradually. When Ni was partly replaced by Co, the core size of hard particle and the thickness of rim phase changed. When the ratio of Co to Ni+Co was 0.2, the cermets had better transverse rupture strength(TRS). The dominant precipitation sites for the complicated(Ti, Mo, W, Ta)(C,N) rim phase changed from the surface of the Ti(C,N) to the surface of Ta C with increasing Ta C addition. The higher TRS for cermets with 5wt.% Ta C addition was mainly attributed to the finer grain size, the homogeneous microstructure and the moderate thickness of rim phase.The influence of the multi-walled carbon nanotubes(MWCNTs) addition on the microstructure and performance of cermets has been studied. The conclusions were obtained as follows: after plasma treatment, most of the amorphous carbons and impurities were peeled from the surface and bundle of MWCNTs, and some reactive groups were successfully introduced on the surface of MWCNTs, resulting in the improvement of dispersibility. The increase of MWCNTs addition decreased the dissolution of Ti, W and Mo in the binder phase. However, too high MWCNTs addition resulted in agglomeration of the grains. When the MWCNTs addition was 0.5wt.%, the cermets had better mechanical properties, resulting from the finer grain size, the homogeneous microstructure and the higher volume fraction of binder phase. The toughening mechanisms were characterized by crack deflection, bridging and pulling-out.The influence of the Si C whisker addition on the microstructure and performance of cermets has been studied. Experimental results showed that the porosities of the cermets increased with increasing Si C whisker addition because of the bad wettability between the binder and Si C whiskers. No new phase in the cermets was formed with the Si C whisker addition. When the Si C whisker addition was 1.0wt.%, the TRS and fracture toughness of the cermets increased by 24% and 29%, respectively. The strengthening mechanisms were attributed to the finer grain size, the moderate thickness of rim phase and the homogeneous microstructure. The toughening mechanisms were characterized by crack deflection, whisker bridging and whisker pulling-out.The graded structure Ti(C,N)-based cermets were prepared by plasma carburizing technology. The diffusion coefficient of atoms increased, and the diffusion rate of the elements increased with increasing carburizing temperature, resulting in the increase of binder-deficient layer. As the carburizing time increased, the surface binder content reduced, and the crystal defects from surface to inside of the cermets decreased, which resulted in the growth rate of the binder-deficient layer deviating from the parabolic law. When the cermets were treated at 1200 ℃for 180 min, a 20μm binder-deficient layer, enriched C and Ti was formed, the particle size became finer. A binder-rich area was formed in the near surface area between about 20?m and 35?m depth. The surface hardness and wear resistance of the cermets improved greatly,The diffusion characteristics and related mechanism of carbon atoms during carburization of graded structure Ti(C,N)-based cermets were investigated. The semi-empirical formula calculating the surface carbon concentration of the cermets was obtained. The calculated results were consistent with experimental results. As the chemical affinities between carbon element and other elements in the cermets were different, Ti, W and Mo moved to the surface of the cermets, Ni was forced to transport inwards. The diffusion coefficient of plasma carburizing technology was much higher than the diffusion coefficient of common carburization, which resulted from more crystal defects on the surface of the cermets.更多还原