【作者】 乔志军；

【导师】 李家俊；

【作者基本信息】 天津大学 ， 材料学， 2007， 博士

【摘要】 爆轰法合成的纳米金刚石(nanodiamond)不但具有金刚石所固有的硬度极高、化学稳定性和导热性好等优异性能,而且还具有纳米材料的奇异特性,作为铜基复合材料的弥散增强相具有很大的应用潜力。本文采用纳米金刚石作为弥散强化相,用粉末冶金法制备了综合性能较好的铜基复合材料(ND/Cu),重点研究了纳米金刚石的石墨化转变、ND/Cu复合材料的性能以及相应的强化机制。对纳米金刚石在不同温度下(900～1400°C)进行退火处理,研究了纳米金刚石的结构变化和石墨化转变机理。结果表明,纳米金刚石石墨化转变的起始温度为1100～1200°C,石墨化转变温度与纳米颗粒的大小相关,颗粒尺寸越大,发生石墨化转变的温度越高。在1400°C退火60min后纳米金刚石转变为形态各异的洋葱碳,转变过程中可以形成内核为纳米金刚石外层由富勒烯外壳包裹的过渡相巴基金刚石(Bucky-diamond)。纳米金刚石向洋葱碳转变的机制为:首先在纳米金刚石表面形成具有六元环结构的石墨碎片;石墨碎片连接并弯曲,在金刚石表面形成封闭的碳壳;转变由表层向心部逐渐进行,直到转变完全。采用机械球磨工艺制备纳米金刚石和铜粉的复合粉末,复合粉末经冷压—烧结工艺制得ND/Cu复合材料。研究了成型压力、烧结温度、烧结时间等工艺参数对复合材料微观结构和性能的影响,获得了本实验条件下最佳的粉末冶金工艺参数。采用该工艺制备了不同含量的ND/Cu复合材料并研究了纳米金刚石含量对材料性能的影响。结果表明,纳米金刚石含量低于1.0wt.%时,复合材料的强度、高温稳定性和耐磨性能显著提高;含量大于1.0wt.%,由于纳米金刚石出现较多的团聚现象,复合材料的强度等性能下降。采用退火处理的方法对纳米金刚石进行表面改性,研究了表面改性后纳米金刚石的分散性和ND/Cu复合材料的性能。结果表明,纳米金刚石在惰性气体中1000°C退火60min,由于表面结构和化学组成发生变化,纳米颗粒的团聚现象减少。因此复合材料的电导率和强度等性能指标明显提高,其中1.0wt.%ND/Cu复合材料的综合性能较好:电导率>80%IACS,硬度>HV120,抗拉强度>200MPa,抗软化温度为>800°C。用环-块式摩擦方法探讨了纳米金刚含量对复合材料摩擦磨损性能的影响及纳米金刚石的减摩机理。结果表明:干摩擦条件下的磨损过程是粘着磨损、氧化磨损、磨粒磨损和疲劳磨损多种磨损机制同时作用的结果。纳米金刚石在改善复合材料耐磨性方面表现出良好的自润滑性,具有减小摩擦系数和增强基体耐磨性的双重作用。采用高分辨透射电子显微镜(HRTEM)分析了ND/Cu复合材料的微观组织和界面特征,并探讨了纳米金刚石在复合材料中的强化机制。结果表明,铜基体上分布着单分散的纳米金刚石颗粒,颗粒分布比较均匀。ND/Cu复合材料的主要强化机制为纳米金刚石阻碍位错运动的Orowan机制。分散性良好的纳米金刚石与铜基体之间结合紧密,界面处无溶解和扩散现象,形成增强相与基体直接结合的界面微观结构。更多还原

【Abstract】 Nanodiamond (ND) powders synthesized by explosive detonation possess not only excellent characteristics of bulk diamond such as superhardness, chemical stability and high thermal conductivity, but also special properties of nanomaterials. Therefore, ND powders as a kind of promising reinforcement have great application potentials in the field of copper matrix composite. In the paper, ND reinforced copper matrix composite (ND/Cu) was prepared by powder metallurgy technique. Graphitization of ND and the composite properties ( tensile strength, softening temperature, wear resistance and corresponding strengthening mechanism ) were mainly investigated.The microstructural transformation and graphitized mechanism of ND were discussed after the nanoparticles were annealed at 900～1400°C. The onion-like carbons began to form in the range of 1100～1200°C and the graphitized temperature of diamond nanoparticles changed with the crystalline degree corresponding to nanoparticle size. Annealed at 1400°C for 60min all the diamond nanoparticles could transform into onion-like carbons, and there existed an intermediary phase–Bucky diamond with a diamond core encased in fullerene-like shells. The transformation process of onion-like carbons from ND included: formation of graphite fragments with hexagonal carbon rings, connection and curvature of graphite sheets around the surface of nanoparticles, closure of graphite layers from the particle surface towards the center. Composite powders (ND+Cu) after ball milling were cold pressed and sintered. We have obtained the optimized parameters of powder metallurgy by analyzing the effect of initial pressure, sintering temperature and time on composite microstructure and properties. ND/Cu composite was prepared by the optimized process and the effect of ND content on composite properties was discussed. The results showed that low ND content (less than 1.0wt.%) could obviously improve the tensile strength, thermal stability and wear resistance of composite. When the ND content exceeded 1.0wt.%, the tensile strength of composite decreased owing to the aggregating of ND.The effect of surface modification on ND dispersion and composite properties was investigated. After annealed at 1000°C for 60min, ND aggregates were diminished due to the modification of surface structure and surface chemistry. The electrical conductivity and strength of composite were increased by surface modification of ND. The 1.0wt.% ND/Cu composite possessed preferable properties of electrical conductivity>80%IACS, hardness>HV120, tensile strength>200MPa and softening temperature>800°C.Tribological behavior of ND/Cu composite and the mechanism of ND on reducing the friction were studied by ring-on-block wear method. The results indicated that the wear process of composite was comprehensive effect of adhesion wear, oxidation wear, abrasive wear and fatigue wear. On improving the wear resistance of ND/Cu composite, ND particles exhibited self-lubrication characteristic by reducing friction coefficient and increasing the matrix strength.The microstructure and interface characterization were carried out by high- resolution transmission electron microscope and scanning electron microscope. The results demonstrated that the ND particles were homogeneously dispersed in the copper matrix. Dispersion strengthening of Orowan mechanism was the main factor of ND reinforced copper matrix composite. ND and copper matrix combined closely to form the directly integrating interface and there was no dissolution and diffusion in the interfacial area.更多还原