Effect of specimen size on the tensile strength of WC-Co hard metal

The fracture behaviour of an ultrafine grained WC-Co hard metal was investigated in tensile and bending tests using different specimen sizes and test arrangements in order to study the size effect on the tensile strength, by varying the effectively tested volume over a range of roughly 10 orders of magnitude. Mechanical testing of centimetre sized specimens was performed by means of tensile tests using an hour glass shaped specimen. Millimetre sized specimens were tested in four point and three point bending test set-ups. Micrometre sized specimens, rectangular beams produced via focused ion beam milling, were loaded in situ in a scanning electron microscope utilizing a piezo-electrically controlled cube corner micro-indenter. The resulting fracture surfaces were examined in order to identify crack origins. The main result of the present work is that strength values are found to increase from about 2500 to about 6000 MPa when the size of the effectively loaded volume is varied from about 100 to about 10(-8) mm(3). This kind of behaviour is typical for brittle materials in which strength is defect controlled and can be explained by a size effect according to Weibull theory. In the case of the micrometre sized specimens no defects were found on the fracture surfaces. Estimations of critical defect sizes in these specimens based on linear elastic fracture mechanics give values in the order of magnitude of the submicron sized tungsten carbide particles. It is therefore expected that the high strength values found in these specimens are close to the inherent material strength. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.