Geometric and Crystallographic Characterization of WC Surfaces and Grain Boundaries in WC-Co Composites

Electron backscattered diffraction has been used to determine the orientation of WC crystals in a WC-Co composite and atomic force microscopy has been used to measure the shapes of planar sections of the same crystals. A stereological analysis has been used to determine that { } 1010 prism facets and the {0001} basal planes are the WC surfaces that are most frequently in contact with Co. Further, the WC habit is an approximately equiaxed trigonal prism bound by three prism facets and two basal facets. An analysis of ~15,600 grain boundaries shows that certain interfaces occur with a frequency that is much higher than would be expected in a random distribution and that the grain boundary habit planes also have { } 1010 and {0001} orientations. Eleven percent of all the observed WC-WC interfaces are 90° twist boundaries about [ ] 1010 . Two types of boundaries with a 30° rotation about [0001], a twist and an asymmetric tilt, comprise 3% of the population. Version 03.27.03, in press, Interface Science Kim and Rohrer Interfaces in WC-Co Composites p. 2 Introduction WC-Co composites have a combination of hardness and toughness that make them useful for machining, mining, construction, and other applications where it is necessary to shape, cut, or crush hard materials. [1, 2] A typical WC-Co composite will contain 70 to 90 volume percent (v/o) of WC crystals in Co matrix. In these composites, there are Co-Co boundaries, WC-WC boundaries, and WC-Co boundaries. The focus of this paper is on the crystallographic distribution of WC-Co and WC-WC boundaries. Here, we employ a recently proposed stereological technique that combines orientation data, measured by electron back scattered diffraction, and geometric data measured by atomic force microscopy (AFM), to determine the habit planes of surfaces and grain boundaries in WC-Co composites [3]. Based on the observation of deeply etched samples, the WC habit has been identified both as a trigonal prism, with three prismatic and two basal facets, or a truncated trigonal prism, where some of the corners or edges of the trigonal prism are replaced by new facets [2, 4, 5]. Therefore, this composite system is a good practical test for the new stereology [3]. To what is already known about the WC habit, the current analysis adds quantitative measurements of the relative surface areas of the WC-Co habit planes. Much less is known about the habit planes of WC-WC grain boundaries [6-8]. Based on the inhomogeneous distribution of observed dihedral angles between contiguous WC grains, it has been proposed that certain grain boundaries are not Kim and Rohrer Interfaces in WC-Co Composites p. 3 penetrated by Co and, therefore, occur with a higher frequency [2]. By the measurement of grain boundary misorientations, and the stereological analysis of the grain boundary traces on the plane of observation, we demonstrate that there are preferred misorientations and habit planes. Since these boundaries correspond to special geometric arrangements, we surmise that they are also low energy configurations not spontaneously wet by Co and this explains their high frequency of occurrence in the composite structures. Method