Effective solution for the tribological problems of Ti-6Al-4V: Combination of laser surface texturing and solid lubricant film

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Abstract

Titanium alloys are characterized by poor tribological properties, and the traditional use of titanium alloys has been restricted to nontribological applications. Surface texturing has been recognized as an effective means of surface engineering to improve tribological properties of sliding surfaces. In this study, the patterns of micro-dimple with different dimples density were fabricated on the surface of Ti-6Al-4V by using laser. The effect of dimples density on the friction behavior of the titanium alloy was investigated under dry friction and coated MoS2. The results showed that the textured surface with higher dimples density had lower friction coefficients only at low load and speed under dry friction. When combining the solid lubricant with dimples, some textured samples showed excellent tribological performance for all applied loads. The optimum surface pattern was found, and the mechanisms for friction reduction and anti-wear were discussed.

Highlights

►The regular dimples were prepared on the surface of Ti-6Al-4V by laser ablation. ►The textured surface showed lower friction coefficients and wear at dry friction. ►The combination of texture with MoS2 exhibited better tribological performance.

Introduction

Titanium and Ti-alloys exhibit continuously increasing interest in some sectors such as aerospace, medical and offshore engineering, because of their high strength-to-weight ratio, excellent corrosion resistance and biocompatibility [1], [2]. Among these applications, titanium alloys are heavily used in aeronautical components which undergo sliding and fretting, such as loaded parts and turbine blades [3]. However, its poor tribological behaviors such as high and unstable friction coefficient, severe adhesive wear and a strong tendency to seize limit their applications [4], [5]. The principal reason for the high wear rates is that titanium, a very reactive metal, exhibits a low resistance to plastic shearing and the low protection exerted by the surface oxides against itself and other metals [6]. Surface engineering methods have proven to be the most promising way to improve the surface performance of titanium, such as wear and corrosion resistance.

At present, many surface engineering methods such as ion implantation [7], [8], thermal oxidation [9], anodizing [10] and deposition coating [5], [11], [12] have been developed for wear protection of titanium alloys. However, ion implantation and thermal oxidation treatments are still time-consuming and high energy consuming. Anodizing of titanium only produces a very thin coating, and is commonly applied for decorative and bonding purposes [13]. Deposition coatings with a high degree of hardness are difficult to be directly prepared on soft substrates such as pure titanium and Ti6Al4V alloy. Therefore, it is also necessary to seek a simple and easy method to improve the tribological properties of titanium and titanium alloys.

Surface texturing as a surface engineering method used for fabricating an artificial topography on the surface of a material is well known for many years. Many researches found substantial improvement of tribological performance when applied surface texturing under different conditions [14], [15], [16], [17]. Various texturing techniques, such as mechanical machining, ion beam texturing, laser texturing and chemical etching, are employed for preparing the micrometer-sized features. Among these techniques, laser surface texturing (LST) seems to be the most advanced of all known methods of surface texturing for tribological applications [18]. In recent years, the studies show that by a combination of surface texturing and the application of a solid lubricant, a desired reduction in wear and friction can be achieved. Rapoport and Moshkovich [19], [20] reported that the tribological properties were significantly improved through the storage of MoS2 on the textured steel surfaces. Voevodin [21], [22] investigated an altogether new concept of three dimensional design of coating architecture with an objective of attaining an improvement in tribological properties of hard coatings. The lubricant was filled in dimples to form lubricant pockets. This combination was proofed to reduce friction and to improve the durability of the coatings.

Applying texture on the surface is an effective way to improve the tribological performance of contact surfaces. At the same time, there are little studies on the tribological properties of textured surface of titanium alloys. For this purpose, we prepared texturing dimples on Ti-6Al-4V surface by laser micro-machining, then prepared textured surface coated by burnishing MoS2. Furthermore, we investigated the effect of texture parameters on reducing the friction and wear.

Section snippets

Materials and laser texturing

The alloy blocks (ø25 × 8 mm) of Ti-6Al-4V (the atomic composition: 5.5–6.8% Al, 3.5–4.5% V, 4.2% Fe, 0.2% O, 0.15% Si, 0.1% C and balance Ti) were used for texturing. Before laser texturing, the samples were ground using 4000-grit abrasive paper until they reached a roughness (Ra) below 0.1 μm, and cleaned in acetone for 5 min using an ultrasonic washer. A commercial pulsed Q switch Nd:YAG laser (Bright Solution, Italy) with a wavelength of 1064 nm was used for the surface engineering operation.

Tribological properties under dry contact

Fig. 3 shows the curves of the friction coefficient of the untextured and textured surfaces as a function of sliding distance under the dry contact. It can be seen that the friction coefficients are extremely different for the textured and untextured surfaces. Under a normal load of 1 N, friction coefficient was initially low (about 0.2) for the untextured surface, then it quickly rose up to 0.8 with severe fluctuation (Fig. 3a). In the case of textured surface, the friction coefficients were

Conclusions

  • 1.

    A regular dimple texture was produced using laser micromachining in Ti-6Al-4V surfaces. The effect of dimples density on the tribological properties of an LST surface under different conditions was analyzed.

  • 2.

    Under the dry friction, the textured surface with higher dimples density led to a lower friction coefficient compared with an untextured surface only at low load and sliding speed. It could be ascribed to the effective capture of wear debris for the surface with high dimples density.

  • 3.

    The LST

Acknowledgments

This work was supported by the National Basic Research Program of China (973 Program) (grant no. 2011CB706603) and the National Natural Science Foundation of China (51175493).

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