The influence of Si on the microstructure and sintering behavior of ultrafine WC

Abstract

The microstructure of sintered nanoscale tungsten carbide powders with 1 wt % Si addition was found to be populated by an abnormally large number of elongated grains. Interrupted sintering experiments were conducted to clarify the origins of the excessive abnormal grain growth seen in the microstructure. It was observed that rapid coarsening occurred at high temperatures owing to the formation of a liquid phase. However, the grain shape evolution during this coarsening period was found to be a consequence of excessive stacking faults and micro twins on the basal planes probably generated by reaction of WC with Si. Analyses of the microstructures and the isothermal and non isothermal coarsening behaviors suggested that the platelet morphology evolved by defect-assisted nucleation and growth on faceted grains. Based on experimental evidence from samples interrupted at low temperatures and crystal growth theories, we discuss the possible mechanisms that eventually led to the rampant platelet-type morphology. Further, the influence of such rapid grain growth on the shrinkage rate during sintering is also discussed. In comparison with the cyclic coarsening-densification process of sintering in pure nanoscale WC, the addition of Si leads to only two distinct sintering stages: either densification dominated or coarsening dominated. Concurrent densification and coarsening cannot be sustained particularly in the presence of a liquid phase that significantly enhances coarsening.

Keywords:

• nanoscale WC-Si
• abnormal grain growth
• spark plasma sintering
• faceting
• platelet

Acknowledgements

One of the authors, AKNK wishes to record his deep sense of gratitude to Prof. Kurokawa, for his constant encouragement and help during the course of this work. Profs. N. Sakaguchi and A. Yamauchi are also gratefully acknowledged for many useful discussions. The anonymous reviewer is also thanked for pointing out certain valuable references that have been included in this text.