Elsevier

Wear

Volumes 368–369, 15 December 2016, Pages 53-59
Wear

Investigation of the self-healing sliding wear characteristics of NiTi-based PVD coatings on tool steel

https://doi.org/10.1016/j.wear.2016.08.004Get rights and content

Highlights

  • Bilayer composite NiTi/TiCN coatings were deposited under various thickness ratio.

  • Decreasing the thickness of NiTi interlayers improve the wear resistance of the composite coatings.

  • Increasing the thickness of NiTi interlayers enhances the adhesion of the composite coatings to the substrates.

Abstract

Excellent damping capacity and superelasticity of the bulk NiTi shape memory alloy (SMA) makes it a suitable material of choice for tools in machining process as well as tribological systems. Although thin film of NiTi SMA has a same damping capacity as NiTi bulk alloys, it has a poor mechanical properties and undesirable tribological performance. This study aims at eliminating these application limitations for NiTi thin films. In order to achieve this goal, NiTi thin films were magnetron sputtered as an interlayer between reactively sputtered hard TiCN coatings and hot work tool steel substrates. The microstructure, composition, crystallographic phases, mechanical and tribological properties of the deposited thin films were analyzed by using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. It was found that under a specific coating architecture, the superelasticity of NiTi inter-layer can be combined with high hardness and wear resistance of TiCN protective layers. The obtained results revealed that the thickness of NiTi interlayers is an important factor controlling mechanical and tribological performance of bilayer composite coating systems.

Introduction

NiTi shape memory alloy (SMA) is one of the most attractive smart materials due to its unique properties such as shape memory effect (SME) and superelasticity (SE) in conjunction with a good corrosion resistance and biocompatibility [1]. As a consequence of superelasticity behavior, deformations following an external loading can be immediately healed upon unloading [2]. This self-healing effect is due to the reversible isothermal phase transformation from austenite to martensite phase (upon loading) and subsequently, from martensite to austenite phase (upon unloading). Superelasticity behavior of NiTi makes it an excellent wear resistant material to be used in tribological systems. According to previous studies, NiTi exhibits a better wear resistance than that of conventional wear resistant materials, such as AISI 304 stainless steel. GCr15 steel, BS11 rail steel, 2024 aluminum, Co45 alloy, and surface nitrided 38CrMoAIA alloys [3]. It is known that NiTi shape memory alloy thin films exhibit the same shape memory effect and superelasticity as NiTi bulk alloys [4]. The most convenient technique to fabricate these thin films is physical vapor deposition (PVD) by magnetron sputtering [5].

In spite of excellent wear resistance of NiTi shape memory alloys in bulk form, thin films of these alloys have not attracted attention as a reliable tribological coating system. The reason is that NiTi thin films possess limited hardness, poor wear resistance, and large coefficient of friction. Application of these thin films is mostly limited to micro-electro-mechanical systems [6] and data storage technology [7]. There are only a few numbers of reports on application capability of these thin films for coating of machining tools [8] as well as deposition of cavitation resistant coatings [9]. Some researchers claim that deposition of NiTi and/or Ni–Ti(–Cu) as an interlayer between hard coatings and substrates results in enhancing mechanical and tribological properties of NiTi thin films [10], [11], [12]. One critical problem associated with this technique is degradation of shape memory effect and superelasticity of NiTi interlayers which could occur after depositing protective hard coating layers. It has not been clarified within the previous studies whether NiTi interlayers could retain their shape memory effect and superelasticity in bilayer coating systems.

The authors of this paper have already developed a hard TiCN coating system [13] and reported that TiCN coating can be used as a suitable protective layer for NiTi thin films [14]. As reported in the reference [14], in order to maintain the shape memory effect and superelasticity of the NiTi interlayers, the thickness of the upper TiCN coating layers must not be more than two times thicker than the thickness of the NiTi interlayers. Such a finding is suggestive of fabricating a noble class of coating system which combines superelasticity of NiTi interlayers with wear resistance capability of hard coating materials. This, in fact, was the motivation of the current study to deposit bilayer composite NiTi based coatings and analyze their tribological performance.

For this paper, bilayer composite NiTi based coating systems are deposited under a proper NiTi/TiCN thickness ratio, ensuring the existence of superelasticity within NiTi interlayers. Sliding wear resistance as well as coating-to-substrate adhesion of these coatings were analyzed along with their microstructure and mechanical properties. It was found that the adhesion strength of the bilayer coatings and their wear resistance can be separately controlled by changing the thickness of NiTi interlayers. During ball-on-disc sliding wear test, alumina (Al2O3) was used as a counter body. Al2O3 is a ceramic material with an excellent insulation properties as well as wear resistance and anti-corrosion performance which make it a reliable material to be used in insulating bearings [15]. Analyzing tribological performance of Al2O3 balls sliding against NiTi/TiCN coatings could simulate performance of these coatings against the insulating bearings such as bearings in three phase motors, traction motor bearings, and Bearings in generators.

Section snippets

Experimental

For this paper, one single-layer NiTi coating as well as three sets of bilayer composite NiTi/TiCN coating systems were deposited with total thickness of 3–4 µm. It was planned to deposit bilayer coating systems under different NiTi to TiCN thickness ratios of about 2, 1 and 0.5 which are displayed as (NiTi/TiCN)2, (NiTi/TiCN)1 and (NiTi/TiCN)0.5, respectively. The deposition of NiTi and bilayer NiTi/TiCN thin films were carried out on plasma nitrided hot work tool steel (X38CrMoV51) substrates

Microstructure and crystallographic phases

Fig. 1 shows the FESEM images of the fractured cross-section of the deposited NiTi and NiTi/TiCN coating systems. Single-layer NiTi thin films illustrated a finely packed fibrous microstructure. Within bilayer coating systems, NiTi interlayers displayed a glass-like featureless microstructure. The protective TiCN coating layers displayed a columnar microstructure which is quite distinguishable with NiTi interlayers.

The XRD diffractogram of the deposited coatings are shown in Fig. 2. The XRD

Conclusion

For this study, bilayer composite NiTi/TiCN coating systems were deposited with total thickness of 3–4 µm. It was found that in such composite coating systems, properties of each of the single layers can be combined to the benefit of sliding wear and coating-to-substrate adhesion strength. Thickness ratio of NiTi to TiCN layer is a critical factor in these bilayer coating systems. Mechanical properties and tribological performance of these coatings can be independently tailored by controlling

Cited by (21)

  • Effect of Si element and YF<inf>3</inf> addition on microstructure and phase transformation of NiTi alloy coatings

    2022, Materials Characterization
    Citation Excerpt :

    Due to the advantages of small thermal deformation and combination with matrix metallurgy, laser cladding technology has become a part of the rapid development of advanced manufacturing technologies [12–14]. The present research shows that NiTi coating is often used as an intermediate layer to improve the wear resistance and impact resistance of the whole coating [15,16]. However, due to the dilution of the substrate material, the intermetallic compounds such as NiTi2 and Ni3Ti are found in the NiTi coating prepared by cladding technology, which affects its phase transformation characteristics and toughness [17,18].

  • Effect of NiTi metallic layer thickness on scratch resistance and wear behavior of high entropy alloy (CrAlNbSiV) nitride coating

    2021, Surface and Coatings Technology
    Citation Excerpt :

    Several researchers have found that superelastic NiTi metallic layer can improve the adhesion strength and the wear resistance of coatings. To optimize the tribological performance, the thickness of NiTi metallic layer is critical. [14–17]. The compliant metallic layer acts as a stress reliever to accommodate stress upon the presence of external loading.

  • Nano-mechanical properties of zirconia-alumina-benzotriazole nano-composite coating deposited on Al2024 by the sol-gel method

    2019, Thin Solid Films
    Citation Excerpt :

    It also plays an important role in reducing the elastic modulus, flexibility, and hardness. As the applied force is increased, benzotriazole reduces the strength of the coatings and enhances the formation of a solid solution [34–37]. Increasing the force from 50 to 60 μN leads to reduced hardness and elastic modulus since a larger load accentuates the effects of defects and movement of dislocations in the zirconia coating as shown by the horizontal part of the nano-indentation curve and maximum penetration depth.

View all citing articles on Scopus
View full text