【作者】 鲍瑞；

【导师】 易健宏；

【作者基本信息】 中南大学 ， 材料科学与工程， 2013， 博士

【摘要】 本研究采用多模腔微波高温烧结炉成功制备了WC-8Co硬质合金,通过和真空烧结对比,提出了微波制备硬质合金的工艺特点和性能特征；以微波加热的整体快速加热和选择性加热特点为依据,分析了微波烧结硬质合金的加热特性以及其致密化规律；对微波烧结过程中出现的表层脱碳现象及产生原因进行了系统分析,提出了解决表层脱碳的方法和依据；对微波加热条件下合金中WC晶粒粗化进行了动力学分析,首次提出了局部液相烧结机理(微波加热过程中出现无数随机分布的热点,热点中心的WC颗粒优先发生转动和重排、优先发生共晶反应产生液相、优先发生WC在粘结相Co中的溶解-再析出,以及优先发生WC晶粒合并长大),探讨了硬质合金在微波加热过程中的致密化和合金化过程以及显微组织演变的规律,最后初步分析了微波与硬质合金压坯的相互作用。通过研究本课题取得了以下重要结论：(1)微波烧结具有真空烧结硬质合金方法不可比拟的优势。在微波加热烧结硬质合金时,试样温度能够瞬时响应微波的输入功率,无热惯性,有利于烧结工艺的控制；而且微波加热脱脂和烧结周期都比真空烧结大幅缩短,从而可以大幅减少能耗,提高能源利用率。此外,微波烧结制备的硬质合金具有一致的硬度分布、良好的磁学性能和优异的抗化学腐蚀性能。(2)压坯密度对微波加热合金升温速率的影响很小,而对硬质合金单组元压坯进行微波加热后发现,WC压坯具有最高的升温速率,Co压坯在1100-1150℃时升温曲线出现波动,WC-8Co压坯的升温速率介于两者之间。致密合金坯的升温速率在高温阶段显著滞后于压坯的升温速率,而且经过微波高温处理后的致密合金坯发生严重变形。另外,对不同粉末粒度硬质合金压坯在微波条件下的致密化和组织演化过程进行了研究。(3)金相显微组织和XRD分析表明在微波烧结WC-8Co硬质合金过程中发生了表层脱碳现象,对原始WC粉末进行微波高温(1400℃)处理后发现有W衍射峰存在。微波加热环境(装料器皿和烧结气氛)对制备的硬质合金有重要影响,石墨器皿使合金中的孔隙难以通过表面排除；纯氮气和氮、氢混合气体对合金的表层脱碳有促进作用；气氛中存在微量的甲烷就能造成合金表层严重渗碳；纯氩气不会造成合金表面脱碳,但纯氩气气氛容易在高温阶段发生热失控；压坯在空气中暴露后,会加剧合金表层的脱碳程度。(4)通过补碳措施在一定程度上抑制表层脱碳现象的发生。在混料时添加炭黑的方法可以改善合金的脱碳现象,有利于合金力学性能的提高,超细WC-8Co (0.15μm)合金总碳量调整为6.08wt%时,硬度和抗弯强度分别达到93.2HRA和3200MPa；提高环境碳势可以避免合金表层脱碳,细晶WC-8Co (0.8μm)合金的抗弯强度可以提高到2053MPa；当氮气中含有5vo1%甲烷时有利于避免脱碳,普通晶粒WC-13Co(2.0μm)抗弯强度可以达到2850MPa；当添加0.8wt%Ni粉宽化剂时普通晶粒WC-8Co (2.0μm)合金的抗弯强度提高了25%。(5)对普通晶粒WC-8Co硬质合金的微波烧结致密化过程和WC晶粒粗化动力学进行了分析,发现烧结温度为1300℃时合金的收缩速率显著增加；当烧结温度低于1280℃时,显微组织中存在大量未发育完全的粉末细微颗粒,超过1300℃时,粉末颗粒已经完全消失,晶粒完全呈现出棱柱形状,继续增加烧结温度,WC晶粒尺寸开始增加；通过Arrhenius测量计算,得到微波烧结WC-8Co硬质合金WC晶粒的粗化表观活化能为84.48kcal/mol,明显低于真空烧结。通过微波烧结WC-8Co硬质合金和真空烧结合金的显微组织对比,提出微波致密化和合金化机理,即局部液相烧结机理,微波制备试样的显微组织中存在细晶粒包围粗晶粒,粗细晶粒相间分布(WC晶粒分布存在局部不均匀,整体均匀)的特点；局部液相烧结理论可以很好的解释了微波烧结合金的加热特性和显微组织特点,同样也可以解释微波熔渗W-Cu合金中出现的棒状晶粒和烧结90W-7Ni-3Fe合金时出现的粗细晶粒相互包围分布的现象。更多还原

【Abstract】 In this work, WC-8Co alloys were successfully fabricated by using multi-mode microwave high-temperature sintering furnace. The evolutionary mechanisms of microstructures were studied at different sintering conditions and atmospheres. And the advantages and properties of microwave sintered samples were discussed compared with the conventional prepared ones. Based on the microwave properties of volumetric heating and high heating rate, the densification process and microstructure properties during the microwave sintering were researched. Besides, surface layer decarburization phenomena and mechanisms were studied. Densification mechanisms were discussed after analyzing the kinetic of WC grain coarsening. Local liquid phase sintering mechanism was proposed firstly (the densification and alloying occurred at hot spots preferentially). Finally, the temperature model was built according to the interaction of microwave and WC-Co compactions. The following important conclusions were made:(1) Be compared with the conventional heating method, the microwave sintering processing has no thermal inertia and the sintered compactions can respond the microwave power instantaneously. The time of microwave dewaxing and sintering are greatly shortened. The microwave sintered samples had the finer WC grains and more homogenous Co phase distribution. The whole microwave processing embodied the properties of energy saving and environmentally friendly.In addition, microwave sintered WC-Co alloys had the uniform hardness distribution and magnetism performance and better resistance to chemical and physical corrosions.(2) In multi-mode microwave furnace, the compact density has little effect on the heating rate. For pure materials of WC-Co composites, the WC green compacts had the highest heating rate. Moreover, the heating curve of the cobalt compacts showed the fluction at1100-1150℃. The absorbing ability of WC-8Co compacts was between the Co and WC compacts.The heating rate of the alloy at higher temperature was lower than compact. Besides, the WC-8Co alloy reprocessed in the microwave radiation was deformed.(3) W3Co3C3phase peaks appeared in the XRD pattern of WC-8Co compact, and W phase of WC powder after microwave radiation. WC grain coarsening was not significant in the microwave sintered fine and coarse grain cemented carbides. The sintering atmosphere had an important effect on the microstructures of microwave-sintered samples. The graphite vessel could prevent the pore exclusion.Pure N2and N2+H2atmosphere could accelerate the surface decarburization, but pure Ar atmosphere did not cause the decarburization. However, the heating property of Ar gas is not as stable as N2. A little CH4will lead to the precipitation of graphite phase. The decarburization became seriously after the compact samples exposed in the air.(4) Adding carbon black at the ball mill stage could reduce the decarburization, and improve the mechanical properties. Hardness and TRS of ultrafine microwave sintered WC-8Co samples were93.2HRA and3200MPa respectively when total carbon content was6.08wt%. The maximum TRS value was2640MPa of coarse WC-6Co samples when total carbon content was5.95wt%. The decarburization phenomenon can be reduced by enhancing the carbon potential in the atmosphere, and the TRS of ultrafine alloy could reach2053MPa. Moreover, a little CH4also can reduce the decarburization, and the TRS could reach2850MPa when CH4content was5vol%. In addition, the Ni additive could enhance the TRS when the content was0.8wt%.(5) By researching the densification of normal grain WC-8Co compact, it was found that the shrinkage of the sample was slow when temperature below1280℃, and increased sharply at1300℃until1420℃. Besides, the microstructures changed with the densification. Below1280℃, a lot of small powder still existed; all WC grains became truncated trigonal prism at1300℃and coarsened at higher temperatures. After calculated, apparent activation energy was obtained about84.48kcal/Mol, which was much lower than conventional preparation. The microstructure of microwave sintered WC-8Co alloy had the special WC distribution with small grains enclosed in grown grains, which was not observable in conventional prepared ones. The local liquid phase sintering mechanism could explain this unique microstructure. What’s more, the W-Cu and W-Ni-Fe alloy fabricated in the microwave radiation also can be found the same microstructure characteristics as in the WC-Co alloy. The heating rate was important to influence the microstructure of grain distribution. The faster the heating rate, the more significant the difference between the growth and small grain became.更多还原