稀土在硬质合金球形粉末烧结体表面的富集现象

Enrichment of Rare Earth on Sinter 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烧结体表面进行了观察与分析。研究结果表明:当以混合稀土 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在合金粉末烧结体表面产生明显的富集,并与主要来自于烧结炉内气氛中的杂质元素形成了含La,Ce,S,Ca,W,C和O的复杂化合物;当以La(NO3)3形式加入稀土时,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,当以混合稀土 Co预合金粉末形式加入稀土时,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,当以混合稀土 Co预合金粉形式加入稀土时,合金中的稀土不但具有较强的富集杂质元素的作用,而且还可以阻止合金粉末烧结体表面富粘结相结构的形成。

Sinter skin 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(NO_3)_3 dissolved in acetone, which were added during the wet-milling of cemented carbide respectively. The results by SEM and EDX show that when rare earth is added in the form of mischmetal-cobalt pre-alloyed powder, during the sintering process, lanthanum and cerium as well as the impurity elements mainly from the sintering atmosphere get enriched on the sinter skin of cemented carbide spherical powder with an average diameter of less than 200 μm, forming a third phase containing La, Ce, S, Ca, W, C and O. However, when rare earth is added in the form of La(NO_3)_3, lanthanum has not been detected on thesinterskin.Theadditionformofrareearthalsoaffectstheamountofthebinderphaseonthesinterskin.Whenrareearthisaddedintheformofmischmetalcobaltprealloyedpowder,theamountofthebinderphaseissmallerthantheonewhenrareearthisaddedintheformofLa(NO3)3.Therefore,theadditionformofrareearthdeterminesthebehaviorofrareearthincementedcarbide.Whenrareearthisaddedintheformofmischmetalcobaltprealloyedpowder,itcannotonlyenrichtheimpurityelements,butalsoinhibittheformationofthebinderphaseenrichmentstructureonthesinterskin.