Influence of WC replacement by TiC and (Ta,Nb)C on the oxidation resistance of Co-based cemented carbides

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Abstract

The effect of partial replacement of WC by cubic refractory carbides (TiC and (Ta,Nb)C) on the oxidation resistance of WC–Co cemented carbides at 600 and 800 °C is investigated by using TGA, DTA, in-situ synchrotron XRD and SEM/EDX.

At 600 °C, the oxidation kinetics obeys a linear function and the increasing content of (Ta,Nb)C causes a reduction of the oxidation rate, which is significantly further decreased by alloying with TiC. In contrast, at 800 °C, the oxidation kinetics follows a parabolic law. In addition, results reveal that an improvement of oxidation resistance at 800 °C is only attained by TiC addition. The oxide scale consists mainly of Co3O4, WO3 and CoWO4. Inclusions of TiO2 within the scale were observed for TiC contents above 14 mol%. Thus, the main factor affecting the oxidation resistance is the content of WC after the replacement by the mixed carbides and the passivation effect of the Ti, Ta and Nb oxides.

Introduction

Cemented carbides are multiphase materials composed of refractory metal carbides embedded in a metallic binder. Due to their high hardness and superior wear resistance, cemented carbides are used in tools for cutting and machining of ferrous and non-ferrous alloys, wood and rocks. The most common refractory metal carbides used in cemented carbides are WC, TiC and (Ta,Nb)C. Co is the dominating binder metal [1], [2], [3]. The performance and service life of cemented carbide cutting tools depend on the mechanical, structural and physicochemical properties as well as on the operating conditions and the characteristics of the environment in which they are used. The degradation of mechanical properties at high cutting temperatures is mainly due to a loss in mechanical strength, especially the hardness. In metal cutting operations under dry conditions at temperatures above 550 °C, it is expected that the tool life is influenced by the oxidation resistance of the cemented carbides [4].

A detailed survey of the oxidation performance of hard materials and cemented carbides of the type TiC–WC–Co–Ni–Cr in the temperature range of 600–1200 °C was conducted by Kieffer and Kölbl in the 1950s [5]. Most of the recent investigations on the high-temperature oxidation of cemented carbides have dealt with WC–Co cemented carbides [6], [7], [8] and with the effect of binder composition and binder content in cemented carbides containing only WC as the hard phase [9]. In addition, the effect of ultra-fine grain sized WC hard phases was studied [4]. Monteverde and Bellosi [10] investigated the oxidation behavior of Ti(C,N)–WC ceramics and cermets and, more recently, Tanaka et al. [11] studied the effect of the TiC content on the oxidation behavior of sintered WC–TiC–TaC alloys without binder phase at 700 °C. They show that an increasing TiC content (above 38 mol%) in the carbide composition leads to a decrease in weight gain as a result of oxidation and hence, to a decrease in thickness of the oxide layer. This is associated with a change of the scale components from WO3 to TiO2 for TiC-containing alloys.

In this work, we investigate the effect of a partial replacement of WC with cubic refractory carbides (e.g. TiC, TaC and NbC) on the high-temperature oxidation behavior of WC–Co based cemented carbides. Both 600 and 800 °C were chosen for a systematic investigation of cemented carbide oxidation because intensive oxidation of WC starts above 500 °C [9] and temperatures of 800 °C and higher are often registered at the cutting tool tip during dry machining operations. The primary objectives of this study concern the microstructure analysis of the oxide scale, the determination of the oxidation kinetics at both temperatures and their dependence on the type and amount of cubic carbide replacements in the cemented carbides.

Section snippets

Cemented carbide alloys

The oxidation of cemented carbides with five different compositions was investigated. The green compacts were produced by state-of-the-art powder sintering techniques. Commercial powders of WC (initial grain size of 0.8 μm), TiC (1.2 μm), (Ta,Nb)C (1.0 μm) and Co (1.0 μm) were employed as raw materials. In order to study the influence of WC replacement, the selected compositions differ systematically by the amount of WC partially replaced by TiC and/or (Ta,Nb)C (with a Ta/Nb mol%-ratio of 2:1) (

Morphology and phase composition

As-sintered cemented carbides show the typical microstructure consisting of WC particles (white faceted single crystals) and TiC, (Ta,Nb)C or (Ti,W,Ta,Nb)C (cubic carbides with an irregular morphology) homogeneously embedded in the Co-binder phase (Fig. 1). With increasing amount of cubic carbides in the starting formulation, the final microstructure shows the formation of clusters of mixed cubic carbides. A quantitative grain size analysis by the intercept method showed an average WC grain

Expected oxidation reactions according to cemented carbide composition

Thermodynamic data predict the formation of stable WO3, WO2, CoO, Co2O3, CO3O4 and CoWO4 as oxide products for the system WC–Co [15]. The addition of TiC promotes the formation of TiO2 and the addition of TaC the formation of oxides of the type Ta2O5 [16].

The main component in all cemented carbides is WC, which oxidizes to WO3 as revealed by XRD analysis. The formation of such oxide takes place according to the following equations [7], [9]:WC+5/2O2WO3+CO2,or eitherWC+2O2WO3+CO

The oxidation of

Conclusions

Oxidation kinetics are temperature-dependent: at 600 °C, oxidation kinetics follow a linear law whereas at 800 °C oxidation kinetics obey a parabolic law, indicating a diffusion-controlled oxidation process. The linear law is a result of a phase-boundary process, which takes place when there are several defects in the layer, such as cracks and porosity.

The onset of WC–Co oxidation is mainly determined by the binder itself rather than by the added cubic carbide.

Porosity is mainly attributed to the

Acknowledgements

Dr. F. Stein is kindly acknowledged for the DTA measurements and helpful discussions. We would also like to thank Mr. T. Schildheuer for the thermogravimetric measurements as well as Mr. G. Bialkowski and Mr. D. Cardinali for sample preparation. The authors especially thank Prof. P. Ettmayer for helpful discussions.

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Present address: Corus Research, Development & Technology, P.O. Box 10.000, 1970 CA IJmuiden, The Netherlands.

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