Mechanisms of WC plastic deformation in cemented carbide

Controlled crack growth in a WC-Co hardmetal has been observed in-situ in the SEM using an ultra-stiff test rig to allow controlled application and removal of loads. Cracks were observed to propagate in jumps of between 20 µm and 70 µm with each jump occurring in less than 200 ms. High resolution SEM images and EBSD mapping has been used to characterise the effects of microstructure and crystallography on the crack path and the degree of plastic deformation in both binder and carbide phases. The fracture in the hardmetal grade examined is dominated by transgranular fracture of the carbide phase with increased plastic deformation in grains fracturing on {10-10} planes observed by EBSD. At variance with earlier work, plastic deformation in the Co binder phase ahead of the crack tip has been observed over distances of several times the Co mean free path.

In situ observation of crack growth in a WC-Co hardmetal and characterisation of crack growth morphologies by EBSD.

Microstructural effects on the R-curve behavior of WC-Co cemented carbides

Exceptional fracture toughness levels exhibited by WC–Co cemented carbides (hardmetals) are due mainly to toughening derived from plastic stretching of crack-bridging ductile enclaves. This takes place due to the development of a multiligament zone at the wake of cracks growing in a stable manner. As a result, hardmetals exhibit crack growth resistance (R-curve) behavior. In this work, the toughening mechanics and mechanisms of these materials are investigated by combining experimental and analytical approaches. Focused Ion Beam technique (FIB) and Field-Emission Scanning Electron Microscopy (FESEM) are implemented to obtain serial sectioning and imaging of crack–microstructure interaction in cracks arrested after stable extension under monotonic loading. The micrographs obtained provide experimental proof of the developing multiligament zone, including failure micromechanisms within individual bridging ligaments. Analytical assessment of the multiligament zone is then conducted on the basis of experimental information attained from FIB/FESEM images, and a model for the description of R-curve behavior of hardmetals is proposed. It was found that, due to the large stresses supported by the highly constrained and strongly bonded bridging ligaments, WC–Co cemented carbides exhibit quite steep but short R-curve behavior. Relevant strength and reliability attributes exhibited by hardmetals may then be rationalized on the basis of such toughening scenario.

/pdf/fib-fesem-experimental-and-analytical-assessment-of-r-curve-39i4udpj88.pdf

FIB/FESEM experimental and analytical assessment of R-curve behavior of WC–Co cemented carbides

The evolution of damage induced as contact load is increased has been investigated on single- and multilayered coated cemented carbides by means of spherical indentation. The main objective of the study was to assess the effect of the intermediate wear-resistant carbonitride layer on the contact damage resistance of industrial milling (multilayered) inserts. This was approached by evaluating systems consisting of a single carbonitride layer of different chemical nature: novel Zr(C,N) and conventional Ti(C,N) chemical vapor deposition (CVD) coatings. Deformation and damage phenomena were characterized using a wide range of advanced techniques: confocal laser scanning microscopy, scanning electron microscopy, focused ion beam and X-ray synchrotron. Zr(C,N) coated systems are found to exhibit a higher mechanical integrity than Ti(C,N) counterparts. Main reasons behind are the relatively different thermal residual stresses generated during CVD cooling, as a result of the dissimilar coefficient of thermal expansion between the coating and the substrate, as well as the intrinsic cohesive strength of the studied coatings. Such different mechanical response was also discerned to affect the interaction between cracking and layer assemblage in multilayer coated specimens. It then supports the beneficial effect of using Zr(C,N) as the intermediate wear-resistant layer toward enhanced performance of industrial milling inserts.

pdf/contact-damage-investigation-of-cvd-carbonitride-hard-1kv6xabmrf.pdf

Contact damage investigation of CVD carbonitride hard coatings deposited on cemented carbides

In most of the applications where hardmetals are implemented, mechanical contact plays an important role from a design viewpoint, either as direct key parameter (e.g. wear resistance) or as indirect relevant factor (e.g. residual strength associated with contact-induced damage). In this work, the contact response of three cemented carbides WC–Co with different microstructural features is evaluated. The study is performed using spherical indenters (Hertzian contact) and it is focused on: 1) determining the critical loading parameters affiliated to the emergence and evolution of damage; and 2) investigating the relevance of microstructure on the levels of residual strength attained. It is found that strength retention is improved as contact damage mode goes from brittle to quasi-plastic, a transition directly dependent on microstructure. Hence, microstructural design searching for higher damage tolerance (i.e. deformation prevailing over fracture as damage mode) is concluded to be an optimal approach for the development of hardmetals with improved reliability. In practical terms, this is achieved by enhancing toughness through either higher binder content and/or microstructural coarsening, as far as the field application requirements are satisfied.

Contact damage and residual strength in hardmetals

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Contact damage and residual strength in hardmetals