Elsevier

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Volume 259, Issues 7–12, July–August 2005, Pages 861-869
Wear

Characterisation and wear response of metal-boride coated WC–Co

https://doi.org/10.1016/j.wear.2005.01.031Get rights and content

Abstract

Wear resistant transition metal boride coatings have received comparatively little investigation. In the present work, a series of CrB2, Mo2B5 and WB coatings were successfully applied to WC–6wt.%Co cutting tool inserts using unbalanced magnetron sputtering technology. The CrB2 coatings were fully crystalline and had a {1 0 1} preferred orientation, while the Mo2B5 coatings although containing some crystallinity were mainly amorphous. The WB coatings on the other hand were produced in the crystalline and the amorphous states (designated WB(XL) and WB(AMOR) respectively). A range of hardness and scratch resistance was observed, the hardest coating being CrB2 (HV ∼2764 at room temperature), while the toughest was the WB(AMOR) coated WC–Co, the latter showing high resistance to both adhesive and cohesive failure. Machining tests carried out on commercial purity titanium (CP-Ti) revealed that the WB(XL) coated WC–Co inserts were the most resistant to rake face wear. Such wear, attributed to a combination of discrete plastic deformation and dissolution-diffusion mechanisms, was minimised by the ability of the WB(XL) coatings to display a combination of good high temperature hardness (strength) and chemical inertness with respect to CP-Ti.

Introduction

In general the transition metal borides are harder than their corresponding, nitrides, silicides or oxides. In fact they are amongst the hardest materials known to man, Vickers microhardness values in the range of 2000–4000 kg/mm2 being typical when in the monolithic or sintered form [1], [2]. While there are numerous borides that are of potential interest for resisting wear, so far, the investigation of such materials as protective coatings has mainly been restricted to ZrB2, TiB2 and Ti(B,N) [3], [4], [5], [6]. Some of these have reported hardness values of 6000 HV or greater [4], [6]. By contrast, the research activity in related coating materials such as cubic-BN and BCN has been quite lively in recent years [7], [8].

The present work stems from a long standing interest in the exploitability of transition metal borides for the protection of metal cutting tools and other devices intended for use in relatively hostile conditions [9], [10]. Such situations require chemical inertness as well as high strength and hardness; properties that many transition metal borides should be able to meet. Although metal cutting tools used for the machining of steels and cast-irons have undergone a revolution, made possible by the introduction of various protective hard coatings, tools for machining titanium and its alloys have been neglected. Conventional coating materials simply do not work in this application and various ceramic based tools are also no match for simple WC–Co cemented carbides [11]. It has therefore been evident for some time that new tool materials or coatings were needed for this application. A hypothesis that was argued in previous work [11], was that tool materials made from transition metal borides might enjoy superior wear resistance, compared to WC–Co, when used to machine Ti and its alloys. However, to prove this is a rather problematic technical goal. It was therefore decided to apply such materials as wear resistant coatings to standard WC–Co metal cutting tools (inserts). This paper describes the viability of producing such tools and the wear resistance and wear mechanisms that resulted when they were used to machine commercial purity titanium.

Section snippets

Magnetron sputtering

A pilot-scale unbalanced magnetron sputter coating facility, built and based at Leeds, was used to produce the test coatings. The magnetron cathodes were fitted with hot pressed 100 mm diameter targets of WB, Mo2B5 and CrB2 obtained from Cerac Inc, USA. A maximum target current of 1 A (direct current) was used for all sputter coating experiments (carried out in a direct current Ar plasma), which in combination with optimisation of cathode cooling, minimised the risk of thermal cracking of the

Microstructure of the coatings

SEM images of the fracture surfaces of the CrB2 and Mo2B5 coated WC–Co inserts are shown in Fig. 1, Fig. 2. All the boride coatings were ∼6–8 μm thick. For the CrB2 coating it was not possible to resolve clear detail of the grain morphology (Fig. 1a) while examination of several areas of the Mo2B5 coating revealed a fracture surface that was mainly smooth and featureless (Fig. 1b). However, there were several areas that contained small (∼1 μm or less) round feature that may have been isolated

Discussion

In the field of metal cutting (machining), conditions at the tool-workpiece interface are very demanding. Of all the workpiece materials, titanium and its alloys are amongst the most difficult to machine. Temperature in excess of 1000 °C are quickly reached on the tool rake face, even at moderate cutting speeds [11], [15]. In an early investigation [11] the principal wear mechanisms of a diverse range of tool materials, used to cut titanium alloys, was comprehensively investigated. Both

Conclusions

  • 1.

    A series of CrB2, Mo2B5 and WB coatings were successfully applied to a number of WC–6wt.% Co cutting tool inserts using unbalanced magnetron sputtering technology. The CrB2 coatings were fully crystalline and had a {1 0 1} preferred orientation, while the Mo2B5 coatings contained some crystallinity but were mainly amorphous. The WB coatings on the other hand were produced in the tetragonal crystalline ({1 0 1 and {1 1 2} preferred orientation) and amorphous conditions (designated WB(XL) and WB(AMOR),

Acknowledgements

The authors wish to thank the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom for financing most of this work under grant GR/M87122/01. Thanks are also due to Sandvik Coromant, of Sweden who supplied the cemented carbide inserts. Prof. Hans Berns of the Institute for Materials, Ruhr-University, Bochum, Germany is thanked for access to the hot microhardness test facility while Prof. T. H. C. Childs, University of Leeds, is acknowledged for enabling access to

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