Fracture at small scale of WC grains of hardmetals

Abstract

Hardmetals are used in different industrial sectors in demanding applications such as cutting and mining tools because of their good combination of properties such as hardness, fracture toughness and high resistance to wear. These are a direct consequence of their composite structure of interpenetrating networks of the ceramic phase, usually tungsten carbide, and a metallic binder, usually cobalt.

In order to improve these material properties and prevent from failures, a deeper knowledge of mechanisms controlling strength and toughness are needed. These mechanisms depend mainly on their microstructural characteristics. For a proper description of crack propagation, it is important to understand the mechanisms related to tungsten carbide intergranular and intragranular fracture. Moreover, finit element micromechanical models have been developed for analysing crack propagation under fatigue conditions. Micro-sample testing appear to be a promising technique for providing reliable information of mechanical properties of different phases of the material.

The purpose of this final Bachelor's project is to perform micromechanical tests in tungsten carbide grains of a hardmetal in order to obtain results that will allow to understand these mechanisms of crack propagation and determine fracture toughness values.

For the determination of these values, micro-pillars and micro-cantilevers where prepared with focused ion beam technique in basal and prismatic orientation of tungsten carbide grains. The samples were tested by the nanoindenter based techniques of pillar spitting and cantilever bending.

Different results have been obtained depending on the location of the applied load because of the low nanoindenter precision, in micro-pillars, and depending on the dimension of the notch in cantilevers. Obtained values of fracture toughness have been compared between different techniques used in this project and with other values obtained by calculation of fracture toughness with nanoindentation technique.