An investigation of particle size analyses as applied to WC-Co

Abstract

The WC particle size has been shown1 to be the parameter that most strongly affects the tensile and bending strength of sintered WC-Co when the Co behaves in a ductile manner. A plane model of rigid, hexagonal inclusions in an elastic, perfectly plastic matrix has been used2 to extend earlier work1 to include a discussion of the effects of nonuniform particle sizes on the strength of WC-CO.~ WC particles have been shown by Pfau and Rix4 and others to be triangular prisms. An estimated ratio of side height to length of the prisms, H/L, from their photographs of extracted WC particles is +. In high-volume-fraction WC alloys, the triangular prisms are not as well developed as in alloys with less WC, and some particles with H/L 4 are also seen. Many authors have pointed out that there is a range of particle sizes in WC-CO.~ However, the bulk of the particles may be closer to a single size than the rather broad distribution that has been suggested. The relation used by Fishmeister and Exner3 to obtain WC particle size distribution is essentially the one derived by a number of authors5-’ for the special case of spherical particles or voids. To investigate the applicability of this relation for the WC particle shape found in WC-Co, many small pieces of balsa wood, all of one size (H/L = fr), were embedded in a water-base putty. The ratio of 4 is used here instead of $ because of the occurrence of particles with this ratio of dimensions and also to emphasize the shortcomings of this analysis for triangular prisms by using a slightly more extreme ratio than +. After photographing the surfaces of a number of sections through the model, an analysis as outlined by Rockstiegel’ was done. Figure 1 is a result of this analysis. The distinct broadening of the particle size distribution and the apparent maximum number of particles at the smaller dimension of the particles in the model should be noticed. There is undoubtedly a range of WC particles sizes in WC-Co, but this rather poor performance of the Rockstiegel analysis in discerning single-size triangular prisms raises serious doubts concerning the validity of the results of this analysis for obtaining the actual particle size distribution for alloys with particles of this