Influence of surface topography of HF-CVD diamond films on self-mated planar sliding contacts in dry environments

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Abstract

Surface roughness of self-mated CVD diamond coatings have a decisive effect on coefficient of friction (COF) and wear especially under dry ambient conditions. Therefore diamond films with four different grain size domains were deposited onto silicon carbide front faces and tested in unlubricated planar sliding contacts. In accordance to the different growth morphologies, the roughness of the coarsest diamond films is about four times higher in average than that of the nanocrystalline films. The diamond film quality, expressed in terms of sp3-hybridised diamond fraction, decreases from nearly 100% to about 73% with falling grain size due to an increase in grain boundaries. The topography of the respective diamond coatings differs also considerable. While the coarse-grained films exhibit a pronounced wedge shape and a strong two-waviness, the fine-grained diamond films display a good flatness with a maximum profile height of only 280 nm. All results obtained from the dry sliding tests are visualised in a tribo map. The best tribological behaviour in terms of low COF and little wear is achieved by the fine-grained diamond coatings. The worst results reveal the nanocrystalline films with Cauliflower structure, mainly due to the highest number of local film delaminations in this test series. For high-end sealing applications, smooth fine-grained microcrystalline CVD diamond coatings therefore seem to be the most promising approach.

Introduction

The variety of novel applications for chemical vapour deposited (CVD) diamond coatings go along with new research in changing its surface state. Unique surface properties like the well-known extreme hardness, low coefficient of friction and high chemical inertness are amended by low surface energy in the hydrogenated surface state [1], negative electron affinity and high over-voltage electrode behaviour in the boron doped p-type semi-conductive state [2].

Despite of these quite new and very attractive fields of usage for CVD diamond films as electrode material for synthesis and waste-water treatment, there are still potentials in the more classical applications like that of a low friction and wear-resistant coating. These outstanding benefits allow the design of single-sealing systems with only the working fluid acting as lubricant and cooling media, avoiding the need of more complex and particularly more expensive double-working sealings with separate cooling and lubrication systems [3]. In case of a temporary lack of working fluid, diamond-coated ceramic seals are able to sustain the mechanical and thermal strain in the dry running sealing system. Without diamond coating, the unlubricated seal faces generate an enormous amount of friction heat within a few seconds. This often leads to sealing ring deformation, causing direct contact of the very fast rotating seal faces and finally to the breakdown of the complete sealing system due to brittle fractures of the ceramic counterparts.

Nevertheless CVD diamond films also cause friction and suffer from wear in self-mated direct contacts. Beside the influence of environmental conditions on dry sliding behaviour like temperature and pressure [4] as well as the effect of load and sliding velocity [5], the surface grain size of the columnar growing diamond crystallites is known to have a decisive effect on the coefficient of friction (COF) and wear [6]. In the recent past, a lot of experimental work has been done in this field. However most of the published research concerning the effect of CVD diamond surface roughness on friction and wear performance was executed in pin-on-disk or ball-on-disk tests, and with different counterpart materials like alumina or steel balls [7], [8], ceramic or metal carbide balls [9], [10], sapphire styluses [11] or single-crystal diamond tips [12]. Furthermore, the tribological testing parameters in the above mentioned work, with loads ranging from 0.1 to 10 N, sliding velocities in the scale of mm/s and sliding distances ranging from a few millimetres up to a several hundred metres, are completely different from those presented in this paper.

As friction evolution is very sensitive on the tribological system, i.e. the testing parameters and counterpart materials, one cannot directly deduce the tribological behaviour of self-mated CVD diamond films presented here from the results published elsewhere. Due to the permanent contact of the complete friction area between the two mating sealing faces, cooling by ambient atmosphere or friction and wear reducing adsorption effects, i.e. saturation of free dangling bonds by oxygen or water vapour [4], are excluded or at least highly unlikely. The objective of this study is therefore to investigate the influence of CVD diamond surface roughness on sliding friction and wear in self-mated dry planar contacts. The respective diamond films are deposited on silicon carbide sliding rings and are tested in an axial ring-on-ring configuration. The CVD diamond coatings are divided into four different grain size domains, and their friction and wear performances are evaluated from unlubricated sliding tests under ambient conditions. The results are listed in a tribo map in order to find the optimal diamond film for high-end sealing applications.

Section snippets

Substrate material and preconditioning

The CVD diamond films were deposited onto the front faces of solid-sintered silicon carbide sliding rings (SSiC). The geometry of the stationary ring is 26.7 mm × 20.7 mm × 8.0 mm, the dimensions of the rotating counterpart is slightly larger with 28.0 mm × 18.0 mm × 8.0 mm. The contact faces of the specimens were lapped mechanically to a two-dimensional mean surface roughness of about 0.15 μm, with a flatness of less than two Helium light-bands (∼ 600 nm) before diamond deposition. In order to enhance

Morphology and quality of CVD diamond films

The morphologies of the different grain sized diamond films homogeneously deposited onto SiC sliding faces are displayed in Fig. 1, all in the same 2000-fold magnification. The grain size is regarded here as the average crystallite diameter on the growth surface evaluated by linear intersection technique. As is clearly visible, the extra coarse and coarse-grained diamond films in Fig. 1a and b are composed of well-faceted diamond crystallites which have sharp cubooctahedral shape and grain

Conclusion

Results obtained from planar sliding experiments of self-mated CVD diamond coatings in dry ambient atmosphere point out that fine-grained diamond coatings with low surface roughness and good flatness are best for unlubricated sliding. In comparison to coarser diamond films they indeed reveal in some cases longer run-in times, i.e. longer transition stages with high friction coefficients and large fluctuations. But after passing this transitional period, the mean COF is low and steady just as

Acknowledgements

The author acknowledges the financial support of the project by the Deutsche Forschungsgemeinschaft (DFG) and his colleagues at the department of Materials Science, University of Erlangen for considerable discussions.

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