稀土在硬质合金球形粉末烧结体表面富集现象的观察

Enrichment of rare earth on sintering skin of cemented carbide spherical powder

采用合金粉末烧结体表面观察法研究了稀土在WC-Co硬质合金中的作用机理。合金中的稀土分别以混合稀土(以La和Ce为主体成分)-Co预合金粉(简称RE-Co预合金粉)形式和La(NO3)3的丙酮溶液形式在湿磨时直接加入。用扫描电镜和能谱仪对平均粒径小于200μm的2种球形稀土硬质合金粉末(简称合金粉末)烧结体表面进行观察与分析,发现:以RE-Co预合金粉形式加入稀土时,在烧结过程中,合金中的稀土La和Ce以及杂质元素在合金粉末烧结体表面产生了明显的富集,形成了含La,Ce,S,Ca,W,C和O的复杂形式的化合物,在稀土富集物中没有检测到Co。以La(NO3)3形式加入稀土,在合金粉末烧结体表面没有检测到La。说明在合金粉末烧结体表面不存在La的富集或聚集。稀土的添加形式同时也影响合金粉末烧结体表面硬质相WC与粘结相的比例,以RE-Co预合金粉末形式加入稀土,合金粉末烧结体表面粘结相含量较少,因此,稀土的添加形式影响其在硬质合金中的作用机理。以RBCo预合金粉形式加入稀土,合金中的稀土不但具有较好的富集合金中杂质元素的作用,而且还可以阻止烧结体表面富粘结相结构的形成。

Sintering skin (surface) observation and analysis method was used to study the behavior of rare earth in WC-Co cemented carbide powder. The addition forms of rare earth were mischmetal (mainly containing lanthanum and cerium) and cobalt pre-alloyed powder and La(NO3)3 dissolved in acetone which were added during the wet-milling of cemented carbide. The results by SEM and EDAX show that when rare earth is added in the form of mis-chmetal-cobalt pre-alloyed powder( for short, RE-Co powder), during the sintering, lanthanum and cerium as well as the impurity elements in the starting materials of cemented carbide get enriched on the sintering skin of cemented carbide spherical powder ( for short, alloyed powder) with an average diameter of less than 200 μm, forming a third phase containing La, Ce, S, Ca, W, C and O. Cobalt is not detected in the compound. However, when rare earth is added in the form of La(NO3 )3, lanthanum is not detected on the sintering skin, indicating neither enrichment nor gathering of lanthanum on the sintering skin. The addition form of rare earth also affects the amount of the binder phase on the sintering skin. When rare earth is added in the form of RE-Co powder, the amount of the binder phase is smaller than the one when rare earth is added in the form of La(NO3)3. Therefore, the addition form of rare earth determines the behavior of rare earth in cemented carbide. When rare earth is added in the form of RE-Co powder, it can not only enrich the impurity elements in cemented carbide, but also inhibit the formation of the binder phase enrichment structure on the sintering skin of the alloyed powder.