Elsevier

Tribology International

Volume 152, December 2020, 106546
Tribology International

Effect of chromium doping on high temperature tribological properties of silicon-doped diamond-like carbon films

https://doi.org/10.1016/j.triboint.2020.106546Get rights and content

Highlights

  • Chromium doping reduced the relative fraction of Si–O–C, which plays a role in the friction.

  • Hardness, reduced Young's modulus and the C–C sp3 fraction were found to be reduced as result of increasing chromium doping.

  • Increasing chromium content reduced the thermal stability of the films and promoted the clustering of the films.

  • Specific wear rate was increased for the chromium containing films as result of increasing temperature.

Abstract

Amorphous carbon films were deposited by means of closed-field unbalanced magnetron sputtering (CFUBMS). The silicon content was fixed at 1.3 at. %, while the chromium content was increased by modification of the current applied to the chromium magnetrons, with two doping levels, 0.3 and 2.7 at. %. Both, hardness and thermal stability were found to decrease as result of increasing chromium. Ball-on-disk tests revealed friction coefficients of 0.06 at room temperature with similar specific wear rate in all films (~4 × 10−13 m3 N−1 m−1). Increasing annealing temperatures were found to reduce the coefficient of friction compared to room temperature values, while increasing the specific wear rate for all films.

Introduction

Amorphous carbon coatings (a-C) are a metastable form of carbon with a wide range of applications due to their exceptional properties, such as chemical inertness, nano-smoothness, high hardness, low friction and wear resistance [1]. These films have particular advantages in demanding applications, such as high performance tools, atomic microscope probes or hard disks [2,3]. Their outstanding tribological properties allow their application not only in the automotive sector [[4], [5], [6], [7]], where they provide reductions in fuel consumption and CO2 emissions [2], but also in harsh environmental conditions, such as the ones found in the low earth orbit [8]. Nevertheless, these films have several limitations, such as residual stresses that may lead to adhesion failure [9], thermal stability [10], fracture toughness [11] or the humidity and gaseous environment under which the contact occurs [12,13].

Several dopants, both non-metallic [[14], [15], [16]] and metallic [[17], [18], [19], [20], [21], [22]], have been previously investigated to overcome such drawbacks. Silicon has been reported to have an effect on the residual stresses of the films [23] and reduce the hardness up to a certain silicon content threshold in films without hydrogenated precursors [23,24]. The tribological properties have also been reported to be enhanced by silicon doping, reducing the coefficient of friction with increasing silicon content both at room temperature and high temperatures [25,26] due to the formation of silicon oxides on the sliding surface [27,28].

Among the metallic dopants, chromium is known by its ability to form carbide nanoparticles within the carbon matrix. Chromium (a-C:Cr) doping has also been related to reduction in the residual stresses [29,30] and friction [4,[30], [31], [32]], while increasing the critical load [4,31,32] and fostering cluster formation as the content increases [33]. Different tribological behaviours have been reported under high temperatures for such dopants compared to non-doped films. The wear rate of these films has been reported to increase with increasing chromium content [34], while friction may be reduced with small chromium contents at high temperatures [33]. As for Si–Cr co-doping, Staia et al. [35] reported super-low friction values for temperatures as high as 450 °C although the doping levels were not reported.

In this work, we investigate the properties of chromium, silicon co-doped films deposited using closed-field unbalanced magnetron sputtering (CFUBMS), and the effect of chromium additions on thermal stability of silicon-doped films as well as their mechanical and tribological properties.

Section snippets

Experimental methods

Amorphous carbon films with high sp2 carbon content were deposited at Teer Coatings Ltd. (Worcestershire, UK) using closed-field unbalanced magnetron sputtering equipment, Teer UDP-1250a. Circular AISI M42 high speed steel specimens of 30 mm diameter were used as substrates for the mechanical testing and steel foils were used for the chemical testing. Prior to deposition, all substrates were thoroughly cleaned in an ultrasonic bath with acetone for 10 min and dried using a hot air dryer.

Six

As-deposited characterisation

The silicon and chromium content of each film was modified through variations in the current applied to the targets. The same bias voltage and silicon target current were used during the deposition of the different films and all inherent changes in content were due to the addition of chromium.

XPS measurements of the as-deposited films were employed to measure the bonding states of C and Si through the estimation of the binding energies (BE) of the C 1s and Si 2p peaks. Measurements of the bulk

Conclusions

Diamond-like carbon films were deposited via closed-field unbalanced magnetron sputtering techniques. Both dopants were found to have an effect in the mechanical properties. The hardness was reduced from 16.1 ± 1.0 GPa in the a-C:Si to 10.4 ± 1 GPa in a-C:Si, Cr (2) and the H/E’ ratio was reduced with increasing chromium content. Small silicon additions (<1.3 at. %) reduced the cluster size and had a positive effect in the thermal stability of the films, increasing it, at least by 50 °C, while

CRediT authorship contribution statement

Bruno J. Rodriguez: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Project administration. Tara L. Schiller: Writing - original draft, Supervision, Formal analysis. Daniela Proprentner: Writing - original draft, Supervision, Formal analysis. Marc Walker: Formal analysis, Writing - original draft, Investigation. C.T. John Low: Supervision. Barbara Shollock: Funding acquisition, Supervision. Hailin Sun: Project administration, Funding acquisition. Parnia

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The present work has been supported by the Engineering and Physical Science Research Council (EPRSC) through the Center for Doctoral Training in Diamond Science and Technology (EP/L015315/1). The authors are grateful to Drs. Sue Field, Ben Breeze and Raul Chinchilla for the fruitful discussions.

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    Present address: B.S.: Department of Engineering, King's College London, London, U.K., WC2R 2LS.

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