Fracture toughness of cemented carbides: Testing method and microstructural effects

Abstract

Fracture toughness is one the most important parameters for design applications and performance assessment of WC-Co cemented carbides (hardmetals). Different from hardness, fracture toughness is commonly a property more difficult to evaluate, particularly in brittle materials. A large number of different testing methods have been introduced to evaluate toughness of hardmetals, but in general all of them have either theoretically debatable issues or important experimental difficulties. In this study, three different fracture toughness testing methodologies are investigated: three-point bending on Chevron notched specimen ("reference" baseline), Palmqvist indentation test, and Hertzian indentation method. The work is conducted in several cemented carbide grades with different microstructures, in terms of both WC grain size and Co binder content. It is found that Chevronnotched three-point bending test yields suitable fracture toughness values for a wide range of cemented carbide grades with varying hardness. Concerning indentation methods, the Hertzian one may be particularly recommended, as compared to Palmqvist method, as far as hardness (HV30) drops below 1300. On the other hand, if HV30 is higher than 1300 Palmqvist indentation procedure yields reliable fracture toughness measurements. Experimental findings are finally analyzed and discussed on the basis of two theoretical models proposed in the literature.

Contact damage resistance of TiN-coated hardmetals: Beneficial effects associated with substrate grinding

“…Indeed, such issue has been addressed and discussed by the authors in Ref. [22] of the submitted manuscript. It corresponds to a paper just published by us on testing method (and microstructure) effects on the fracture toughness of cemented carbides (Int.…”

“…Hardmetals with such high Co content are frequently used for application with intermittent loading conditions such as cutting tools for milling due to their improved toughness. The cemented carbide under consideration here has a Vickers hardness and fracture toughness of 14.8 GPa and 11.2 MPa√m respectively [22].…”

A New Method for Evaluating the Indentation Toughness of Hardmetals

“…One can note that both Km and Wm values are higher for the as-sintered WC-Co materials (samples from Seikh et al [9]) as compared with the vacuum-annealed carbide materials (samples from Exner et al [10]) for essentially similar WC-Co compositions, in spite of the fact that un-annealed …”

“…It was not possible to compare indentation toughness before and after vacuum annealing in that work because no crack measurements were conducted on the as-sintered un-annealed samples. However, it was possible to compare the results with the published data of Seikh and colleagues [9], who performed extensive Palmqvist crack measurements at an indentation load of 30 kg. Therefore, indentation toughness was calculated on a few vacuum annealed WC-Co samples at an indentation load of 30 kg and the indentation toughness results were compared with the indentation toughness data taken from the work of Seikh and colleagues [9].…”

“…Recently, Seikh and colleagues published a paper [9] measuring the indentation toughness and K IC values on a large number of straight WC-Co cemented carbide samples using an indentation load of 30 kg for both Vickers hardness and Palmqvist crack measurements. Ten measurements were performed for each WC-Co material for a total of eight different carbide materials.…”