COATED MICROGRAIN CARBIDES FOR WEAR RESISTANCE

MICROGRAIN CARBIDES ARE SMALL GRAIN SIZE (1-5 MICRON DIAMETER) PARTICLES OF TUNGSTEN CARBIDE (WC) THAT HAVE BEEN "CEMENTED" TOGETHER WITH A PREDOMINANTLY COBALT MATRIX, USING LIQUID PHASE SINTERING. THEY ARE AN EXCELLENT MATERIAL FOR CARBIDE CUTTING TOOLS AND OTHER WEAR- AND/OR CORROSION-RESISTANT PARTS, HAVING BETTER MECHANICAL PROPERTIES DUE TO THEIR FINER GRAIN SIZE THAN TYPICAL CARBIDE TOOLS (20-50 MICRONS). HOWEVER, SOME OF THEIR POTENTIAL ADVANTAGE IS LOST BECAUSE OF THE DIFFICULTY OF HANDLING SUCH FINE POWDERS AND MIXING THEM WITH COBALT AND NICKEL POWDERS TO FORM THE BLEND WHICH IS THEN PREFORMED AND SINTERED. PROBLEMS THAT ARISE DURING HANDLING AND BLENDING INCLUDE INCREASED IMPURITY CONTENT, NONHOMOGENEOUS DISTRIBUTION OF MATRIX POWDERS, AND WC-WC PARTICLE CONTACT, ALL OF WHICH CAUSE LOWERED MECHANICAL PROPERTIES IN THE FINISHED PART. THE RESULT IS THAT HIGHER COBALT CONTENTS MUST BE INCORPORATED TO ACHIEVE THE DESIRED FRACTURE TOUGHNESS AND TRANSVERSE RUPTURE STRENGTH, WITH A CONSEQUENT DECREASE IN TOOL HARDNESS LIFE. PHASE I WILL DEMONSTRATE THE FEASIBILITY OF COATING EACH INDIVIDUAL WC GRAIN WITH THE PROPER THICKNESS OF COBALT MATRIX MATERIAL SO THAT, AFTER CONSOLIDATION, THE PROBLEMS RESULTING FROM POWDER BLENDING (ESPECIALLY WC-WC PARTICLE CONTACT AND CARBIDE FORMATION) WILL BE MINIMIZED OR ELIMINATED. THIS WILL NOT ONLY IMPROVE MECHANICAL PROPERTIES AND PERFORMANCE OVER CONVENTIONAL MICROGRAIN CARBIDES, BUT ALSO PERMIT FASTER CONSOLIDATION TIMES WITH LOWER IMPURITY CONTENTS AND LESS DELETERIOUS INTERACTION BETWEEN THE MATRIX AND CARBIDE. A LARGE REDUCTION IN THE AMOUNT OF COBALT REQUIRED IS ALSO EXPECTED.