Abnormal Grain Growth of Tungsten Carbide in Small Amount of Cobalt: Mechanism and Application

Abstract

<P>CONSIDERABLE ABNORMAL GRAINS WERE OBSERVED IN LIQUID PHASE SINTERED CEMENTED WC WITH 0.2~0.9WT% COBALT. MICROSTRUCTURE EXAMINATIONS SHOWED GRAIN BOUNDARIES PARALLEL TO (0001) PLANES AND AGG WAS EXPLAINED BY GRAIN BOUNDARY RE-ENTRANT EDGE (GBRE) ASSISTED 2-D NUCLEATION PROCESS. GRAIN COALESCENCE IN LIQUID COBALT WAS ASSUMED TO BE THE SOURCE FOR THE GRAIN BOUNDARIES.</P><P>THE DENSITY OF ABNORMAL GRAINS COULD BE TAILORED BY THE COMPOSITIONS AND SINTERING CONDITIONS AND THE FRACTURE TOUGHNESS COULD BE IMPROVED BY INTRODUCING ABNORMAL GRAINS, BUT AT SOME EXPENSE OF HARDNESS. THE HIGHEST VALUE OF FRACTURE TOUGHNESS, 7.34 MPAB "M0.5, WAS ACHIEVED FOR WC-0.5WT%CO-0.25WT%VC. </P><P>TWO-MATERIAL POWDER INJECTION MOLDING WAS APPLIED TO SYNTHESIZE DOUBLE LAYERED COMPONENTS WITH GRADED STRUCTURE. A HIGHER DENSITY OF ABNORMAL GRAINS BUT WITH SMALLER ASPECT RATIO WERE FOUND IN PIM SAMPLES. A GRADED STRUCTURE IN TERMS OF DENSITY OF ABNORMAL GRAINS WAS FORMED, ACCOUNTING FOR THE GRADED HARDNESS, BE