Non-homogeneous structures in cemented carbides

Abstract

Cemented carbides are composite materials traditionally consisting of hard ceramic tungsten carbide (WC) particles, commonly referred to as WC grainsand metallic cobalt (Co) matrix. This is a combination that gives the material aunique set of properties, combining high hardness and toughness that are suitable for demanding applications such as rock drilling or metal cutting. Depending on application the material properties can be tailored by differentcombinations of WC grain size, metallic binder content and additional hard phases.

In many applications it has been found beneficial to have a variation in microstructure between surface and bulk. Today there are a number of production technologies by which a non-homogenous structure, also called functional gradients, can be produced. These gradients can be generated onvarious length scales. 

In this work, functional gradients, comprising macro gradients on the mm scale, have been produced by modifying the green body before sintering. Here, this means that a grain growth inhibitor, titanium carbide (TiC), has been locally added on the surface of a WC-Binder sample, here WC-Binder means WC-Co, WC-Ni or a WC-Fe composite. These samples are primarily pressed green bodies, but TiC has also been added on sintered materials to study the effect ofporosity on gradient formation in a second sintering. By locally adding TiC an alternative way of introducing a grain growth inhibitor is explored. This approach allows for sustainable use of raw materials, by introducing the TiCwhere it is needed. Upon sintering, Ti and C diffuses into the WC-Binder bulk affecting the structure and composition, and a non-homogenous material is created. 

The sintered samples have been manufactured through standard metallurgical procedures and extensively studied using microscopy and microanalysis. Within the scope of this thesis, different parameters, such as; WC raw material, sintering temperature and porosity are investigated in the WC-Co system to learn how they influence the final structure in the sintered composite material. The possibility to extend functional gradient sintering, producing macro gradients, to include alternative binders, Nickel (Ni) and Iron (Fe) is also investigated. 

In these gradient materials, a relation exists between the diffusion of Ti and C from the applied TiC layer and the growth of WC grains in the bulk material. This relation becomes particularly evident under conditions that promote WC grain growth, such as elevated sintering temperatures or the use of small initial WC powder sizes. Also binder chemistry affects the WC grain growth. The developed compositional and structural gradients can be related to the mechanical properties, and a higher hardness is achieved at the surface of the TiC addition compared to the bulk material. A comparable hardness gradient is observed in samples with Ni and Co binders, while those with Fe binders display a less pronounced gradient. 

To complement the experimental work, diffusion modelling has also been applied to better understand and predict the gradient formation for varying sintering temperatures and binder chemistries. The model accurately reflects the trends in Ti gradient formation but careful consideration of the system, for example WC raw material and amount of TiC addition is needed to understand and make use of the model results. 

The results from this thesis can contribute to the understanding and further development of sustainable tool materials for industrial implementation.

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