Comparative study of wear-resistant DLC and fullerene-like CNx coatings produced by pulsed laser and filtered cathodic arc depositions

doi:10.1016/j.surfcoat.2004.06.021

Surface and Coatings Technology

Volume 197, Issue 1, 1 July 2005, Pages 116-125

https://doi.org/10.1016/j.surfcoat.2004.06.021 Get rights and content

Abstract

Unhydrogenated amorphous diamond-like carbon (DLC) and fullerene-like carbon nitride (CN_x, N/C ratio ∼0.2) are rival surface protection coatings with exceptional mechanical and tribological properties. A comparative study was made on their performance under sliding wear. Identical surface preparation procedures, coating architectures, thickness, and substrates were used to facilitate the comparison. Coatings were produced using pulsed laser deposition (PLD) and filtered cathodic arc deposition (FCAD). Samples of laser-DLC, laser-CN_x, arc-DLC, and arc-CN_x coatings were prepared and an extensive comparison of properties was completed: composition, structure, hardness, elastic modulus, stress, friction, and wear. The tribological properties were determined using steel and SiC balls in humid and dry environments. The comparison indicated that PLD and FCAD produce coatings with very similar compositions, structures, and properties. Independent of the growth technique, DLC coatings had hardnesses of 52–57 GPa and elastic moduli in the range of 490–560 GPa. The CN_x coatings offered a reasonably high hardness of 28–30 GPa, while their elastic modulus was as low as 160 GPa. There was a clear difference in the tribological behavior of DLC and CN_x, which depends on humidity. In humid air, DLC coatings had a coefficient of friction (c.o.f.) of 0.1, a very low wear rate, and formed a graphitic transfer film in friction contact. In the same tests, CN_x coatings had a c.o.f. of 0.3–0.4, a higher wear rate, and did not form a transfer film. In this environment, the wear tracks on CN_x coatings were polished by abrasive wear. In dry nitrogen, DLC coatings had a c.o.f. of approximately 0.15 and a higher wear rate, while CN_x coatings had a c.o.f. of 0.03–0.04 and a lower wear rate with formation of a graphitic-like transfer film. The observed difference in mechanical response and tribological performance can be used to optimize the selection between DLC and CN_x coatings, depending on application requirements.

Introduction

Diamond-like carbon (DLC) coatings provide exceptional mechanical properties and tribological performance as highlighted in recent reviews [1], [2], [3], [4], [5]. These reports indicate a strong dependence of DLC properties on the preparation method, hydrogen concentration, and deposition configuration. High-quality unhydrogenated DLC coatings are typically produced by either pulsed laser deposition (PLD) [6], [7] or filtered cathodic arc deposition (FCAD) [8], [9], [10], [11], [12]. They have predominantly sp³ interatomic bonding, hardness of 50–60 GPa, elastic modulus of 400–600 GPa, and coefficients of friction (c.o.f.) about 0.05–0.1 in unlubricated sliding against steel under ambient conditions [1], [2], [3], [4], [5]. The unhydrogenated DLC coatings, however, were less effective at elevated temperatures and/or in high vacuum environments due to the onset of graphitization by sp³–sp² bond relaxation and associated increase in c.o.f. and wear [13], [14], [15], [16].

Recently, another carbon-based material has emerged, providing competition to DLC for wear protection. Amorphous carbon nitride coatings are already replacing DLC for surface protection of hard disk drives. At the same time, fullerene CN_x was reported to have a unique resilient behavior, providing hardness and flexibility [17]. This material was originally produced by magnetron sputtering [18] and has about 20 at.% N concentration, predominantly sp² interatomic bonding, a high degree of graphite plane bending and cross-linking, hardness around 10–20 GPa, elastic modulus of 40–120 GPa, and c.o.f. about 0.3 at ambient conditions [17], [19], [20]. Fullerene-like CN_x typically has better thermal stability than DLC, and the degree of stability depends on the preparation temperature and type of nitrogen bonding in the coating [21].

Most recently, PLD was demonstrated to produce fullerene-like CN_x coatings by laser ablation of graphite in a nitrogen environment. In comparison to magnetron sputtering, the coatings had less order in the graphitic planes, a considerably higher hardness of 30 GPa, and elastic modulus of about 200 GPa [22], [23]. These mechanical properties are closer to that of unhydrogenated DLC grown by PLD and FCAD. Cathodic arc evaporation of graphite, including FCAD technique, was also reported to produce CN_x coatings by either introducing nitrogen to the deposition chamber [24], [25], [26], [27] or using a nitrogen ion beam-assisted FCAD [28].

With these developments, it is important to understand what benefits CN_x coatings may offer for friction reduction and wear protection as compared to DLC coatings. The question is somewhat difficult to answer because properties of both DLC and CN_x are strongly influenced by the preparation method. Even for the same method, there are property deviations due to different configurations from one deposition system to another. Nevertheless, the existing experience with DLC and CN_x applications indicates that these materials behave differently, depending on the operating conditions. For example, it is widely known that DLC provides low friction and reduced wear in humid air and at moderate temperatures (typically below 300 °C). At the same time, CN_x recently replaced DLC as an overcoat for some magnetic disk applications because it provides a combination of good wear resistance and active bonding to surface lubricants.

This paper provides direct comparison between DLC and CN_x coatings produced by PLD and FCAD in the same deposition system. Coatings produced by PLD are designed as laser-DLC and laser-CN_x, and coatings produced by FCAD are designated as arc-DLC and arc-CN_x. The cross-comparison of their chemical, mechanical, and tribological properties is used to discuss differences between DLC and CN_x coatings independent of the preparation method. Special attention was paid to the differentiation of the tribological responses, depending on the environment and counterpart sliding material.

The paper also investigates the interchangeability of PLD and FCAD in growing both DLC and fullerene-like CN_x. Over the years, PLD was shown as a versatile tool in manufacturing tribological materials ranging from single-phase solid lubricants to complex hard and environmently adaptive “chameleon” composites, which self-adjust their surface chemistry and structure to maintain low friction and wear in variable environments [29], [30], [31]. PLD is still awaiting a large-scale engineering coating application, which is becoming closer to reality as the price of laser output energy continues to drop and more efforts are being made for PLD technology scale-up [32], [33], [34]. At the near term, FCAD might be used to duplicate PLD coatings if the techniques would be proven to provide similar quality tribological materials.

Section snippets

Coating deposition

Mirror-polished 440C steel coupons of 25.4-mm diameter were used as tribological substrates and Si<100>wafers were coated for film stress measurements. Substrates were ultrasonically cleaned in an acetone bath and placed in a vacuum chamber, which was evacuated to a base pressure of 4×10⁻⁶ Pa. Prior to coating deposition, a 30-min sputter cleaning with a 1 kV glow discharge in 6.7 Pa of Ar was performed. The substrate temperature was set to 100 °C for DLC and 300 °C for CN_x growth using

Chemical composition and bonding

XPS analyses of DLC films found only carbon with the C 1s binding energy peak at 284.9 eV for laser-DLC and at 284.8 eV for arc-DLC. Fig. 2a provides Raman spectra of laser-DLC and arc-DLC. Both spectra are similar and correspond to high-quality DLC as evidenced from a single broad peak positioned at about 1560 cm⁻¹. The spectra are best fitted with two Gauss functions (not shown in Fig. 2): one located at 1565 cm⁻¹ (known as the G-peak) and another at 1381 cm⁻¹ (known as the D-peak). The

Conclusions

The composition, structure and tribological and mechanical properties of laser- and arc-DLC and laser- and arc-CN_x were very similar. It is clear that PLD and FCAD can be used interchangeably for the growth of DLC and CN_x. This opens the door to transition PLD coating developments for commercial applications by material duplication and scale-up using FCAD.

Comparison of DLC and CN_x coatings shows that DLC coatings are super hard and stiff, while CN_x coatings offer a combination of reasonably

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