Elsevier

Wear

Volumes 330–331, May–June 2015, Pages 3-22
Wear

Topographical orientation effects on friction and wear in sliding DLC and steel contacts, part 1: Experimental

https://doi.org/10.1016/j.wear.2015.02.014Get rights and content

Abstract

The effect of surface roughness and topographical orientation on friction and wear has been investigated for diamond like carbon (DLC) coated and uncoated steel surfaces with three levels of surface roughness in the range of 0.004–0.11 μm Ra value and with topographical orientations at 0°, 45° and 90° angles from grinding marks. In this first part we report the experimental observations that form the basis for future computational modelling of the tribological effects and mechanisms. The surfaces were characterised by the scanning electron microscopy (SEM) and focused ion beam (FIB) method and mechanical properties were measured. In the topographical characterisation measurements included the fractal signatures, the texture aspect ratio signatures and the texture direction signatures were measured and calculated by the variance orientation transform (VOT) method. The friction and wear were measured and observed in scratch testing, micro tribological testing and linear reciprocating testing in three directions of topographical orientation, as well as in rotational pin-on-disc testing. The topographical orientation had considerable effect on both friction and wear in DLC vs DLC contacts while the effect was minor and sometimes not even observable in steel vs steel contacts. A surface strengthening effect which is higher for smooth DLC surfaces and micro-cracking and micro-delamination on asperity tips at low loads for rougher surfaces is reported. The 45° orientation resulted in higher friction and considerably higher ball wear in linear reciprocating pin-on-plate testing of DLC surfaces compared with the 0° and 90° orientations.

Introduction

The influence of tribology in our society is remarkable. Recent studies have shown that about 20% of all energy consumed worldwide is used to overcome friction [5], [11], [12], [13]. The influence of wear has not been analysed in such detail, but a common understanding is that it may well have an impact of similar magnitude on product lifetime, value of spare parts, maintenance costs, breakdown and loss of production in our society, especially in industry and transportation.

Wear has over the last fifty years been studied intensively and the main wear mechanisms have been described [1], [23], [27], [28]. Surface roughness and surface topography are crucial parameters influencing wear but despite this there have been only a small number of studies on the basic mechanisms on how these factors influence wear.

Over the last decade fundamental studies using e.g. atomic force microscopy have shed some light on the relationship between surface roughness and friction [2]. Extremely low friction of DLC coatings, down to μ=0.001, could be attributed in part to very smooth coating surfaces that eliminate asperity interlocking effects [3].

Our understanding of how surface roughness affects wear is based mainly on observations gathered from tribological wear experiments. In dry sliding wear, the severity of the surface roughness effect depends on the combination of materials used in the frictional pair. The surface roughness effect is most pronounced in abrasive wear where a hard material abrades a softer material. Generally, as the surface roughness of the hard material increases the wear mechanism changes from polishing or burnishing to ploughing or even microcutting, with accompanying increases in wear rates [17], [27]. In dry contacts between ductile materials of similar hardness (e.g. steel on steel) this effect is less significant as the initial roughness is rapidly altered as soon as the sliding commences. In lubricated contacts the contact conditions are milder with regards to wear and the lubrication mechanism is often dominating the tribological performance which is affected or disturbed by the surface topography on macro, micro and nano scale [15], [18], [35].

When one or both mating surfaces are coated, the effect of surface roughness on wear is more complex [9]. Apart from the coating and counterbody surface finish, the surface roughness of the substrate should be taken into account both in dry and lubricated tribological contacts [19], [20], [24], [29]. Experiments have shown an increasing wear rate of the DLC coatings with an increase in the substrate roughness [14]. At high substrate roughness, a transition from adhesive wear to chip-flake formation and coating fragmentation was observed [14]. Another contributing factor is the contact load. For example, in low load conditions wear of plasma-deposited diamond coatings increased significantly with surface roughness no matter whether sliding was in vacuum, dry nitrogen or in air [22].

In wear studies, surface topography is typically characterised by standard 2D roughness parameters such as Ra and Rq. These parameters tend to work well with isotropic surfaces but are not able to provide full information about surface anisotropy and surface roughness at different scales of measurement [16], [27]. This limitation of standard parameters is crucial since most real engineering surfaces are anisotropic and multi-scale objects. To address the effects of surface topographic orientation on wear, new techniques have been developed [25], [31], [32], [33].

One of the techniques, the variance orientation transform (VOT) method, has a unique ability to accurately characterise surface roughness and anisotropy in all possible directions at individual scales [32]. The VOT method is so sensitive that, based on the surface topography changes, it can detect minute differences between wear particles generated under different operating conditions [32]. In this paper, the topography of DLC-coated and uncoated steel surfaces is characterised by the VOT method after wear experiments. One of the objectives is to explore whether there is a correlation between surface topography characteristics (i.e., roughness orientation) and wear of the DLC-coated and steel samples.

The combination of experimental tribotesting and computational modelling and simulation will be used to explore the influence of surface roughness and surface topographic orientation on friction and wear in dry sliding conditions with steel vs steel surfaces and DLC coated steel vs DLC coated steel surfaces. In this paper we report the experimental results, and the second part, including the computational modelling, is under preparation and will be published separately.

Section snippets

Methodology

This study investigates the effects of surface roughness and topographical orientation on wear both experimentally and by computational modelling and simulations. The presentation and methodology is based the so called PSPP (process–structure–properties–performance) approach to material investigation presented more in detail by Holmberg et al. [6], [7], [8]. It includes a description of the materials and coating deposition processing, detailed characterisation on microstructural and

Materials and characterisation

The samples used in this investigation are steel discs with three different surface roughnesses both uncoated, and coated with a diamond-like carbon (DLC) coating. The flat steel samples were manufactured of bearing steel (AISI52100) and heat treated to 7 GPa hardness. Bearing balls of the same material, both coated and uncoated, were used as the counter sample. The diameter of the discs was 40 mm and the diameter of the balls 10 mm. On the steel discs and balls the coatings were deposited by

Scratch testing

Scratch testing procedures were based on the European Standard EN 1071-3. A linearly increasing, progressive normal load was applied to the sample with a Rockwell C diamond tip. The minimum and maximum normal forces applied in the tests were 0.03 N and 30 N, respectively. The initial normal force was 0.03 N. The scratch length was 8 mm and the scratch speed was 10 mm/min. The scratch testing was carried out in three directions, as shown in Fig. 2. On the smooth surface the roughness marks were not

Indentation and scratch testing

Indentation was used both for material characterisation, as presented in Table 3, and for measuring material performance under loading for model validation purposes, as shown in Table 5. The indentation depth was in the range of 3.91–4.20 μm for the DLC coated surfaces and in the range of 4.20–4.47 μm for the steel surfaces at a load of 3 N. The indentation depth for the DLC surface compared to the steel surface was 2% lower for the smooth surface and 13% lower for the rough surfaces. For the

Topographical analysis

The values of surface roughness Ra are in a good agreement with the mean values of the fractal signature FS90. For the smooth surfaces they are lower than those obtained for the average and rough surfaces.

It can be seen from Table 4 that the texture aspect ratio signature StrS values obtained for the STE-S and DLC-S surfaces are larger, at most scales, than those calculated for the rougher four surfaces. This indicates that the smooth surfaces exhibit lower changes of roughness with direction

Conclusions

A set of experimental tribotesting of uncoated and DLC coated steel surfaces with three levels of surface roughnesses in the range of 0.004–0.11 μm Ra value and with specified topographical orientations in three directions, 0°, 45° and 90° from grinding marks, was carried out. The following conclusions were made:

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    The variance orientation transformation (VOT) method gives a detailed quantitative representation of the real surface topography as surface roughness at fractal scale, roughness

Acknowledgements

In this study the DLC coating deposition was carried out by City University of Hong Kong, material characterisation by Saarland University in Germany, topographical characterisation by Curtin University in Australia, nanoindentation and micro tribology testing by National Physical Laboratory in UK and characterisation, indentation, scratch testing, linear reciprocating pin-on-plate and rotational pin-on-disc testing by VTT Technical Research Centre of Finland. The authors acknowledge the

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