Microstructural and Mechanical characterization of WC-Co cemented carbides
Tutor / director / evaluatorLlanes Pitarch, Luis Miguel
Document typeMaster thesis (pre-Bologna period)
Rights accessRestricted access - author's decision
WC-Co cemented carbides are ceramic-metal composite materials made of carbides embedded in a metal phase that acts as a binder. They exhibit an exceptional combination of strength, toughness and wear resistance as a result of the extremely different properties of their two constitutive phases. Consequently, cemented carbides have been positioned as suitable options when selecting materials for tribomechanical applications, and their implementation continues to gain a place in the market, particularly as structural components and forming tools. Mechanical properties of hardmetals, especially hardness and toughness, strongly depend on the nature, content and size of each microstructural constituent. The aim of this project is to carry out a microstructural and mechanical characterization of two different WC-Co grades with medium and coarse grain sizes and medium binder content, as well as to conduct a detailed study of crack-microstructure interactions at the crack tip during unstable crack propagation. Microstructural characterization includes grain size, binder content, contiguity and mean free path. Grain size distribution has been measured using two different techniques; by traditional linear interception and by image analysis. It is found that image analysis way give similar results in terms of mean carbide size and grain size distribution to those found with linear interception. An intensive study on the grain size distribution as a function of the number of measured grains has been also carried out. The effect of the microstructure on the mechanical properties is discussed on the basis of a fracture mechanics approach. Fracture of cemented carbides is governed by unstable propagation of pre-existing flaws; hence, Weibull statistics are implemented as an effective technique to evaluate the rupture strength and the reliability of the studied grades. After rupture, a detailed fractographic analysis is conducted in order to identify the nature, size and geometry of critical flaws. A detailed study of crack-microstructure interactions at the crack tip is assessed by cross-sectioning and imaging the crack tip using the SEM/FIB tomography technique. This investigation led to the conclusion that the microstructure of each grade has a determinant role since it affects directly the toughness and indirectly the size and distribution of intrinsic defects. On the other hand, fracture process at the crack tip is governed by the nucleation and growth of micro voids in the binder until it fractures by void coalescence.