The surface topography of an ion milled WC-Co sample

Abstract

A tungsten carbide-cobalt (WC-30 wt% Co) thin foil was prepared for transmission electron microscopy (TEM) by electron-spark cutting a thin slice out of bulk material, polishing mechanically both sides of the slice on diamond powder (down to 0.25/~m particle size) and ion milling both sides in a Gatan 600 Dual Ion Mill. The milling process consisted of bombarding simultaneously both sides of the sample with Ar ion beams of 5 keV, impinging on the surfaces at a nominal angle of 15 ° to the sample surface while the sample rotated at approximately 2 rpm. The sample obtained was a 3.2 mm disc with an outer rim approximately 100 ktm thick and a central hole surrounded by a narrow electron-transparent region. Two microcracks radiated from the hole, relieving the residual tensile stresses in the cobalt [11. The sample was held in the ion mill between two plates each with a central hole of 2.3 mm diameter. The surface of the sample which was not covered by these plates was eroded by the Ar beams. The intensity of the beams may be assumed to have had a Ganssian distribution around a central axis. The centre of the beam impinged on the sample at the centre of the resulting hole and its intensity decreased with increasing distance from the hole. Therefore, by examining the topography of the ion milled surfaces at various distances from the hole, i.e. in areas that received various doses of Ar ions, it was possible to acquire information on how the resulting topography developed. The surface topography was examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Random areas at various distances from the outer rim of the sample were analysed by both techniques. A Jeol JSM 840 operating at 20 kV was utilized for the SEM studies and an AFM using a PSI autoprobe. The autoprobe was used to produce topographs of the sample, to measure the dimensions of surface features and to measure the average roughness of the sample surface. The Si3N4 AFM tip had a square pyramidal geometry with an apex nominal radius of 40 nm. Fig. 1 is a three-dimensional AFM micrograph of