超微粒超硬合金における諸現象

Abstract

Submicro-grained WC-Co cemented carbide is very important tool material for solid endmills, PCB drills, etc. On manufacturing the alloy, addition of grain growth inhibitors is inevitable and many relating reports have been published. However, there are some problems left behind such as the sintering process, the mechanism of grain growth inhibition, etc. In this report, a summarization of author's investigations relating to those problems is introduced.Submicro-grained WC-12mass%Co alloys with mono-doped 0.5mass%VC or 0.9mass%Cr3C2 were used as specimens which were heated up in vacuum to certain temperatures and immediately cooled. Some of them were fully sintered at 1380°C for 1hr. They were examined by density measurement, TG-DTA, SEM, HRTEM, etc.Major results are as follows. (1) DTA revealed that addition of VC or Cr3C2 decreased a liquidus temperature by about 30 to 40°C. However, their addition retarded shrinkage from 1080°C for VC and 900°C for Cr3C2, and also retarded diffusion of Co onto WC grains and facetting of the grains up to about 1200°C, accompanied by delay in densification. Both alloys reached almost full density at 1320°C as well as non-doped alloy. (2) VC remarkably retarded grain growth from 1200°C, and Cr3C2 retarded it at 1380°C but not so remarkably as VC. (3) In VC-doped alloy, HRTEM revealed steps on WC grains consisted of WC(1010) and WC(0001), and segregation of V mainly on WC(0001)/Co interface. V was also detected at WC/WC grain boundary accompanied with Co, but not in the binder phase. Another V was observed substituting Co binder phase which was assumed to be (V, W)C. (4) In Cr3C2-doped alloy, such steps on WC grains were rarely observed. Segregation of Cr on WC grains was not always detected, but it was detected at WC/WC grain boundary with Co as well as in binder phase.Some discussion on the mechanism of grain growth inhibition was presented basing on the results obtained. However, further and precise investigations are needed in order to comprehensively understand it.