纳米及亚微米复合粉添加对超粗晶硬质合金晶粒长大的影响及机制

Influence of Nano and Submicron Composite Powder Additives on Grain Growth of Ultra-coarse Cemented Carbides and Its Mechanism

在粗颗粒WC/Co混合粉末中分别添加平均粒径为100、250、400nm的WC-8Co复合粉,经球磨混合压坯后在不同温度进行Ar气保护烧结。针对烧结块体的形貌、晶粒尺寸及其分布进行了研究,并分析了复合粉添加对不同烧结阶段WC晶粒长大的影响机理。研究发现,在WC/Co混合粉中加入纳米和亚微米复合粉末均可制备得到超粗晶硬质合金,且添加纳米复合粉烧结的试样平均晶粒尺寸达到9.3μm。烧结初期,纳米和亚微米复合粉通过增加混合粉末的表面能而有效促进WC晶粒长大;当达到液相烧结温度时,添加纳米复合粉的烧结块体中,由于小晶粒具有更大的溶解驱动力,促使小晶粒溶解并在周围大晶粒表面析出,进一步增大烧结块体的晶粒尺寸;添加亚微米复合粉的块体中,小晶粒WC呈集中分布,使其溶解驱动力较小,且析出主要发生在周围细小晶粒之间,达到溶解析出动态平衡,从而使烧结块体的平均晶粒尺寸增长缓慢。

The synthesized WC-8 Co composite powders with the particle sizes of 100, 250 and 400 nm were added to the coarse WC/Co powders. The mixture was sintered at different temperatures in an Ar atmosphere. The morphology, grain size and distribution of the sintered bulks were studied. The WC grain growth mechanism of the samples with composite powder additives at different sintering stages was analyzed. The result shows that the ultra-coarse cemented carbide can be prepared by WC/Co mixed powders to which the nano or submicron particle size composite powder additives are added. In particular, the cemented carbide with a grain size of 9.3 μm can be prepared by using the mixed powder with the nanocomposite powder additives as raw materials. At the initial sintering stage, the addition of the nano and submicron composite powders obviously increases the surface energy of the mixed powders, which is beneficial for promoting the growth of WC grains effectively. During the liquid sintering, the sintered bulks with the nanocomposite powder have a larger dissolution driving force, which causes the small grains to dissolve and precipitate on the surrounding large grains, further increasing the grain size of the sintered bulks. In the bulks where the submicron composite powder is added, the small grains WC are concentrated and the dissolution driving force is small, and the precipitation mainly occurs between the surrounding small grains, and the dissolved precipitate reaches a dynamic equilibrium, so that the average grain size of sintered bulk grow s slowly.