Vickers Indentation Fracture Toughness of Near-Nano and Nanostructured WC-Co Cemented Carbides

Tamara Aleksandrov Fabijanić,Mateja Šnajdar Musa,Danko Ćorić,Matija Sakoman

doi:10.3390/met7040143

Tamara Aleksandrov Fabijanić, Mateja Šnajdar Musa + Show 2 more

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https://doi.org/10.3390/met7040143

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Journal: Metals	Publication Date: Apr 19, 2017
Citations: 19	License type: CC BY 4.0

Affiliation: University of Zagreb

Abstract

In this paper, the fracture toughness KIc of near-nano and nanostructured WC-Co cemented carbides by Vickers indentation fracture toughness (VIF) was investigated. The aim was to research the type of cracking occurring in near-nano and nano-grained WC-Co cemented carbides with respect to the Co content and, consequently, to evaluate the appropriateness of different models for the fracture toughness calculation. The mixtures with different binder content—4, 6, and 9 wt. % Co—were consolidated by sintering in a hydrogen atmosphere. Vickers indentation using a test force of 294 N was used for the determination of fracture toughness. The type of crack that occurred as a consequence of the applied load on the corners of the Vickers indentations was analysed with optical microscopy before and after repolishing the samples. Different crack models, Palmqvist and radial-median, were applied for the calculation of KIc. Instrumented indentation testing was used to determine the modulus of elasticity of the consolidated samples. From the research it was found that near-nano and nanostructured cemented carbides with 9 and 6 wt. % Co do not exhibit median cracking and the indenter cracks remain radial in nature, while near-nano and nanostructured cemented carbides with 4 wt. % Co exhibit both radial and median cracking. Accordingly, it was concluded that the critical amount of the binder phase in near-nano and nanostructured WC-Co at which the crack changes its geometry from Palmqvist to radial-median is around 4 wt. % Co. Comparing different models it was found that KIc values are not consistent and differ for each method used. Models from Exner crack resistance for the Palmqvist crack showed good agreement. Radial-median crack models showed significant KIc deviations for the same testing conditions for all samples.

Highlights

Near-nano and nanostructured cemented carbides are characterized by unique combinations of high hardness and good fracture toughness
That near-nano and nanostructured cemented carbides with 9 wt. % Co and 6 wt. % Co do not exhibit median cracking and the indenter cracks remain radial in nature, while near-nano and nanostructured indentations for WC-9 wt. % Co and WC-6 wt. % Co (Figure 5a–d), which is not the case for the WCcemented carbides with 4 wt. % Co exhibit both radial and median cracking
Concluded that near-nano and nanostructured cemented carbides with 9 wt. % Co and 6 wt. % Co do not exhibit median cracking and the indenter cracks remain radial in nature, while near-nano and nanostructured cemented carbides with 4 wt. % Co exhibit both radial and median cracking

Summary

Introduction

Near-nano and nanostructured cemented carbides are characterized by unique combinations of high hardness and good fracture toughness. Improvements of hardness and fracture toughness of cemented carbides can be achieved with a decrease of the grain size to the nanoscale [1]. Scientists have developed different methods of measuring the fracture toughness where they usually determine one of two sizes: KIc , the critical stress intensity factor, or GIc , thecritical strain energy release rate, of the fracture surface energy [4,5].

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