Fracture toughness of cemented carbides and its correlations with other material properties

For cemented carbides, also known as hardmetals, fracture toughness has always been viewed as one of the key properties, which resulted in large amount of research in the subject. This study presents fracture toughness results for 30 cemented carbide grades, covering wide range of microstructures and associated properties. Toughness data was generated at room temperature by three-point bend testing of chevron notched samples. In addition to relatively well studied influences of cobalt content and carbides grain size on fracture toughness, current research also examines impacts of cubic carbides content and alloying elements on material toughness. In this work, traditional “hardness - toughness” diagram is complemented by “coercivity - toughness”, which is shown to be more appropriate for illustration of the detrimental effect that cubic carbides have on fracture toughness. The results presented here also indicate that alloying of binder with Cr or Ru does not have any noticeable effect on room temperature toughness and presence of eta-phase in the microstructure is not necessarily harmful for toughness. In addition, this research illustrates a correlation between fracture toughness and thermal conductivity of cemented carbides. Most importantly, by utilising broad experimental data and regression analysis, an attempt is made to formulate a set of empirical equations that would allow fracture toughness estimate from readily available or easily measurable material parameters. It is shown that fracture toughness of cemented carbides can be predicted with good accuracy from coercivity and cubic carbides content. In addition, regression equations for estimate of hardness and the average carbides grain size are proposed.