PSI - Issue 5

Pavol Hvizdoš et al. / Procedia Structural Integrity 5 (2017) 1385–1392 Pavol Hvizdoš et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Modern hardmetals are composite materials made up by very hard, usually some sort of ceramic, particles placed inside or bond together by metal based matrix, often some solid solution. A typical example is WC-Co cermet, developed in 1920s, used for cutting, machining, and other types of toolmaking. Here, WC particles provide very high hardness and excellent abrasion resistance. Soft cobalt binder brings certain degree of ductility, sufficient fracture toughness, and controlled stiffness, (Exner (1979), Roebuck and Almond (1988)). In recent years there is an effort to replace the tungsten based materials with other alternatives, potentially lighter and more environmentally friendly (Rabezzana et al (2001)). A potential candidate is Ti (C, N) which is a solid solution of the TiC-TiN system, reported in Pastor (1988), Clark and Roebuck (1992), Zhang (1993), or Ettmayer et al. (1995). It has good lifetime, slow wear and good chemical stability at high temperatures. Its main disadvantage is lower strength. In order to improve it, other binary carbides (e.g. NbC, TaC, Mo 2 C) are usually added (Daoush et al. (2009), Chicardi et al. (2014a), and Zhang (2013)). In particular, TaC is often used to enhance stability at higher temperatures, hardness, thermal shock and creep resistance. However, sintering of TiTa(C,N) in presence of cobalt usually leads to formation of intermetallic compounds within the Co binder. These are hard and stiff, and so they make the material brittle. Further addition of carbon into the mixture can prevent the formation of such undesirable intermetallic compounds within the binder and in this way it offers a possibility of enhanced properties of the final material. There are many ways how to prepare the complex TiTaCN-Co material (Córdoba et al. (2009), Chicardi et al. (2014a)). Mechanochemistry is an interesting method of chemical synthesis in solid state materials. It usually uses high energy milling (Bayer et al. (2005), Fernández-Bertran (1999) , Baláž et al. (2006)). It is potentially cheap and easily scalable technique where dissolution or melting of the reactants is not required. The aim of the work was to prepare a series of alternative (Ti, Ta) (C, N)/Co-C hardmetal systems by means of high energy milling, to characterize them from the point of view of microstructure, hardness, modulus of elasticity, wear properties at room and elevated temperatures, and to assess the benefits of carbon (graphite) addition. Ti (99% purity, < 325 mesh, Strem Chemicals), Ta (99.6% purity, < 325 mesh, Alfa-Aesar), graphite (< 270 mesh, Fe ≤ 0.4%, Merck) and Co powders (99.8% purity, < 100 mesh, Strem Chemicals), together with N 2 (H 2 O and O 2 ≤ 3 ppm, Air Liquide), were used as raw materials to develop the (Ti,Ta)(C,N)-Co. First, the ceramic phase, i.e., the Ti – Ta carbonitride solid solution with nominal composition Ti 0.95 Ta 0.05 C 0.5 N 0.5 , was synthesized from the starting materials by the mechanochemical MSR (mechanically induced self-sustaining reaction) process using a planetary mill (Pulverisette 4, Fritsch) that allowed for operation at a constant gas pressure and for the detection of self propagating reactions during milling (Gotor et al. (2013)). This composition was chosen according to a previous study that showed that the presence of Ta significantly improves the oxidation resistance of cermets (Chicardi et al. (2014b)), which is of crucial importance for high-temperature applications. Concretely, 46.5 g of elemental Ti, Ta, and graphite powder mixture with an atomic Ti:Ta:C ratio of 0.95:0.05:0.5 were placed together with thirteen tempered steel balls (d = 20 mm, m = 32.6 g) in a 300 ml tempered steel vial (67Rc) and ball milled under 6 atm of N 2 at a spinning rate of 400 rpm, for both the rotation of the supporting disc and the superimposed rotation in the direction opposite to the vial. After detecting ignition, milling was prolonged for 5 min to ensure full conversion (Chicardi et al. (2012)). Subsequently, the carbonitride solid solution phase was mixed by dry milling (Pulverisette 7 planetary mill, Fritsch) with 20 wt.% or 30 wt.% of Co and different small amounts of C in the form of graphite from 0 wt.% to 2.2 wt.% to obtain the powdered cermets. They were introduced together with seven tempered steel balls (d = 15 mm, m = 13.7 g) in a 45 ml tempered steel vial (67Rc) and ball milled under 6 atm of high-purity helium gas (H 2 O<3 ppm, O 2 <2 ppm and C n H m <0.5 ppm, Air Liquide) at a spinning rate of 600 rpm. The mixture was milled for 30 min, which was the minimum time necessary to produce the optimal homogenization of the powdered cermets, as required to achieve an optimal densification after sintering (Chicardi et al. (2012)). 2. Experimental Materials