Further improving the mechanical and tribological properties of low content Ti-doped DLC film by W incorporating

Highlights

• W and Ti atoms were introduced successfully into the film together by co-sputtering W/Ti twin-target.

• Introduction of W atom significantly enhanced the hardness and retained the low internal stress.

• The friction and wear resistance could be further improved significantly by W incorporating.

Abstract

W/Ti-doped diamond-like carbon (DLC) films were fabricated on Si substrates by co-sputtering W and Ti targets in methane and argon mixture atmosphere. The composition and the microstructure, internal stress, mechanical and tribological properties of the films were measured by X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), Raman spectra, BGS 6341 type film stress tester, nano-indentor and reciprocating ball-on-disc tester, respectively. The results indicated all films showed the amorphous structural characteristics and a constant thickness value of ∼600 nm (195 ± 15 nm Ti interlayer was designed in order to minimize the influence of Ti interlayer), and the Ti content increased from 0 to 34.5% as the W target current increased from 0 A to 14 A. As the W concentration increased from 0 to 2.6 at.%, the hardness increased to 12.7 GPa and the internal stress maintained at a low value and almost no change. Meanwhile, the friction coefficient and wear rate showed the lowest value (0.023 and 1.2 × 10⁻⁸ mm³/N m, respectively). With the W content increased to 34.5 at.% further, the internal stress, friction coefficient and wear rate of film are dramatically increased.

Introduction

Diamond-like carbon (DLC) film as an effective protected coating, due to its excellent physicochemical properties like high hardness, low friction coefficient and wear, is promising candidate for reducing the wear loss during manufacturing operations including machining and tribological systems such as the plungers running against the cylinder in fuel injection system of automotives [1], [2], [3], [4], [5]. Unfortunately, poor adhesive strength with metallic substrates is the major drawback, which severely limits the successful application of DLC films. Overall, two main reasons are expected to explain it. On the one hand, high residual stress ranging from 1 to 10 GPa lead to spall off from metallic substrates, at the same time, limit the film thickness [6], [7], [8]. This one could be effectively solved by the doping elements such as Ti, Cr, W and so on [9], [10], [11]; On the other hand, weak adhesive strength at the boundary between the film and substrate, which could be solved by the introduction of the addition Me, Me_XC, Me_XN interlayer [12], [13], [14]. Above mentioned these attempts to enhance the adhesion have proved to be very successful, but most works are based on the cost of sacrificing the mechanical properties [15], [16]. Fortunately, our previous studies indicated that the introduction of low content Ti not only significantly reduced the residual stress but also retained the high hardness [17], [18].

However, our works has only shown that the internal stress has been effectively reduced without changing the hardness by incorporating low content Ti atoms. On the basis of it, if other elements were also introduced into the low content Ti-doped DLC film, so that it can effectively enhance the hardness. Namely, if the incorporated elements could be introduced successfully into the DLC films together and carried out “their duties”, so more excellent properties could be obtained. Silva et al. [19] found that the addition of W atom led to a significant hardening of the DLC coating from 10 GPa to 18 GPa. Yue and co-workers [20] investigated the W-doped DLC film, and found that the W content increased to 10.73 at.%, the hardness is increased to ∼20 GPa, they all suggested that the enhancement of the hardness is probably due to the formation of hard phase WC disperses in the carbon matrix. Besides, many other researches also indicated that W is considered an effective doping element especially in improving the high temperature tribological properties of film [21], [22].

In this study, W/Ti-doped DLC films with the different W content were deposited on the Si substrate by co-sputtering W and Ti targets in methane and argon mixture atmosphere. Our concept is W and Ti atom will not react to generate the intermetallic compounds, that may produce synergies between them in the film. The purpose of this work is to further improve the performance of low content Ti-doped DLC film by W incorporating, so that it can fully meet the needs of industrial applications. Although a lot of research works have been carried out about W-doped DLC films and Ti-doped DLC films, there is few works about the effect of W on the mechanical and tribological of low content Ti-doped DLC film. Therefore, mechanical properties and tribological performances of the W/Ti-doped films were investigated systematically based on the composition and microstructure analysis.