Titanium nitride/vanadium nitride alloy coatings: mechanical properties and adhesion characteristics

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Abstract

This study examines the mechanical properties and adhesion characteristics of thin coatings of TiN, VN and various compositions in the Ti–V–N system deposited on stainless steel. Nano-indentation was used to determine the Young's modulus and hardness of the coatings. Tensile tests have been used to introduce controlled strains in the coatings, through the stainless steel substrate, to characterise the strength, fracture toughness and adhesion behaviour. The external stresses applied result in multiple cracking and localised delamination of the coatings, which have been followed in situ using optical microscopy. The Young's modulus shows a simple linear variation with composition within experimental error while the hardness and fracture properties show a clear maximum for the ternary alloy containing 23% VN. The Young's modulus behaviour can be explained by linear mixing while the maximum in the other properties may arise from optimum pinning of dislocations at Vanadium sites in the ternary alloys.

Introduction

Transition metal nitride coatings on metals show considerable promise in applications where severe contact-induced loading is common, such as cutting tools, due to their high hardness and wear resistance [1], [2], [3], [4]. The load-bearing capacity of these extremely hard coatings provides superior protection to the underlying substrate from external contact stresses and spurious impacts with foreign bodies. In a companion paper [5], we examined the deposition, structure and intrinsic residual stresses of TiN, VN and a range of Ti1−xVxN alloy compositions. We demonstrated that the composition Ti0.77V0.23N produced the highest indentation hardness and hence yield stress. This composition also showed the highest intrinsic residual stress, which we explain in terms of its reduced flow during deposition. The microstructure of the alloys, analysed by transmission electron microscopy and electron diffraction, indicates that they are homogeneously mixed with the rocksalt structure. At higher Vanadium contents, a (200) preferred orientation develops as a result of the low surface energy of the VN (100) surface.

The questions addressed here are: How strong and tough are the Ti1−xVxN coatings? And how well do they remain adhered to the surface on which they are deposited when subjected to external stresses? These questions are central to the potential uses of such coatings. The advantages of using hard protective coatings on metallic tools and engineering components are well known [6]. However, the mechanical improvement and resistance to contact damage, wear and corrosion offered by the deposited coating is strongly dependent on its toughness and adhesion properties to the metallic substrate [7], [8], [9]. Clearly, if defects exist and if cracking occurs at the coating–substrate interface after the deposition process or if such damaging processes are sustained during use, catastrophic failure of the system can occur leading to delamination of the coating exposing the underlying surface. Even the case of minor cracking or small-scale delamination of the functional coating may have significant repercussions particularly where corrosion is an issue and ultimately lead to premature failure.

Accordingly, we examine the mechanical properties and adhesion characteristics of the same coatings examined in the companion paper but here deposited on stainless steel. Tensile tests are used to reproducibly subject the coating–substrate systems to large strains while viewing the damage evolution in situ using optical microscopy. The study focuses on five different coatings: pure TiN, pure VN and three Ti1−xVxN alloy compositions each with a nominal thickness of 70 nm. Along with the adhesion tests, nano-indentation experiments were performed to ascertain the hardness and Young's modulus of the entire suite of coatings [5].

Section snippets

Coating deposition

A dual source pulsed cathodic arc system [10] was used to deposit the TiN/VN alloy films at ambient temperature using two 10-mm diameter cathodes, one each of V and Ti, separated by 20-mm centre-to-centre distance. A 0.7-mm diameter tungsten trigger wire was located centrally in each cathode 2 mm above the cathode surface. Each cathode was alternately triggered to deliver a preset number of pulses in each cycle, designed to obtain the desired composition. A pulse length of 0.5 ms and a

Results and discussion

A summary of the hardness, Young's modulus and the in situ tensile test fracture data of each coating examined are given in Table 1. Irrespective of the substrate used, the nano-indentation measurements of hardness and modulus were found to be essentially equivalent. The hardness and Young's modulus as a function of the fraction x in the Ti1−xVxN alloy are shown in Fig. 1. The Young's modulus displays essentially a linear decline with increasing percentage of VN whereas the hardness data show a

Conclusions

In this study we have presented hardness, Young's modulus, strength and fracture toughness data for five coating systems ranging from pure TiN through the various ternary alloy Ti1−xVxN compositions to pure VN. The adhesion characteristics of the coatings have also been examined. There is a pronounced maximum in the hardness, fracture strength and fracture toughness at compositions in the region of Ti0.77V0.23N. A simple linear rule of mixtures does not apply to any of these properties in this

Acknowledgements

We would like to thank Ted Roach of ANSTO for polishing the stainless steel coupons. This work was supported by the Australian Research Council.

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