On the formation of very large WC crystals during sintering of ultrafine WC-Co alloys

During liquid phase sintering of very fine-grained WC powders (average particle size in the range 100-200 nm) very large WC crystals form with sizes of >20 mum. Such grains typically exhibit the shape of plates. On tracing back the formation of these crystals during sintering it was observed that already on heating to 1150 C small platelets were present in the still porous sintering body. At 1250degreesC, i.e. before eutectic liquid formed, they had grown to a plate length of up to 7 mum with aspect ratios higher than 1:10. This early rapid WC crystal growth was observed preferentially in the outer areas of the sintering body where the sample is in closer contact with the sintering atmosphere. SEM imaging and TEM studies reveal the presence of twin boundaries in the plate-like WC crystals, rendering the possibility of a defect-assisted growth of the WC. Intermediate formation of eta-carbides (M12C, M6C) and their subsequent transformation might be an explanation for this early occurrence of extraordinarily large grains. (C) 2002 Published by Elsevier Science Ltd.