Rickard Gåhlin

ME-C:H coatings in motor vehicles

Abstract The trends in the automotive industry are towards higher performance, improved reliability and tolerances, less lubricants and more environmental friendly products. In addition, there is a strong demand for decreased fuel consumption. All this is a tough challenge for the automotive designers who will need new ways of thinking and occasionally unconventional solutions. A solution is to improve the wear resistance and lower the friction of the tribological system by deposition of a low friction diamond like carbon (DLC) coating on critical components. Me-C:H is a DLC type of coating which has been tested on a large number of automotive components during the last 10 years. Today, parts coated in serial production are mainly to be found in diesel injection systems. However, the use of Me-C:H coatings in vehicles increases rapidly. Nearly 30 million coated parts are delivered to the automotive industry every year with an annual increase of approximately 50%. In this paper, an overview of results from laboratory testing as well as real applications are presented and discussed. Emphasis will be put on the aspects of design to optimise the tribological system; e.g. what is required of the substrate to ensure a successful use of the coated component. The majority of the results are positive; friction has decreased drastically and lifetime, wear resistance and resistance to seizure has increased significantly. The high performance of Me-C:H coatings and the high potential of an optimised tribological system predict a further rapid growth of these coatings in the automotive industry.

The particle size effect in abrasion studied by controlled abrasive surfaces

The effect of abrasive particle size on the wear rate is well known. For small particles the wear rate increases with increasing particle size. Above some critical size the wear rate becomes almost independent of further size increases. Several theories have been presented to explain the size effect. We have examined a theory based on the particle shape, specifically the bluntness of the abrading tips. A micro mechanical etching technique has been used to produce relatively large surfaces covered by silicon abrasive tips of extremely well defined shape, size and lateral distribution. Different particle sizes were simulated by changing the packing density. Further, were both sharp and blunt pyramidal tips produced. Wear rates were obtained by a pin on abrasive disc configuration with tin as abraded material. By using a soft material as tin as abraded material the silicon tips retained their initial shape the entire test. The results show that the wear rates are similar for ideally sharp tips of different packing densities. Hence, sharp tips exhibit no size effect. For blunt tips, however, a size effect is present. The difference between the wear rates obtained for the different packing densities is significant. These results confirm the shape theory to be valid under the test conditions used.

Properties of TiN and CrN coatings deposited at low temperature using reactive arc-evaporation

Abstract PVD TiN and CrN coatings were deposited at both low (≈200°C) and standard temperatures (≈400 °C) using reactive arc- evaporation. Variations in microstructure (obtained by TEM and XRD), morphology (studied using SEM) and chemical composition (determined by both EDS and GD-OES) were correlated to mechanical (residual stress state, microhardness) and tribological properties (scratch response, abrasive wear resistance). The results show a significant increase in hardness and residual compressive stress for the low-temperature TiN coating as compared with the standard-temperature coating. This was attributed to a drastic decrease in grain size and an increased compressive microstrain for the low-temperature coating. Moreover, the critical load and abrasive wear resistance of the low- temperature TiN coating had decreased as compared with the standard-temperature coating, although the hardness was significantly higher. This was due to the poor quality of the low-temperature TiN coating in combination with the high residual compressive stress. It is further shown that mechanical and tribological properties of the low-temperature CrN coating were comparable with the standard-temperature CrN coating, even though there was a difference in microstructure and chemistry.

A novel method to map and quantify wear on a micro-scale

The present work first introduces and then demonstrates the possibilities of a novel high resolution topographical difference method. The method comprises two techniques to measure the actual local wear volume and to map the distribution of wear. Both techniques are based on comparing the topography of the same surface region before and after testing. The combination of mapping and measuring the wear makes it a valuable method both for wear testing and for fundamental studies on wear. The method has no principal restriction to a specific topographical technique. In the present investigation, however, only the atomic force microscope (AFM) has been used. To investigate and demonstrate the techniques, four tests have been performed. Three tests involved changing the initial topography of ground steel specimens by scratching or polishing. The fourth demonstration involved the wear of hydraulic motor cam rollers. These techniques promise to bring evaluation of wear to a new level of sensitivity and detail. The extremely high resolution may improve tribological testing of real machine elements by reducing the need for excessively accelerated tests or extremely long test times.

The crater grinder method as a means for coating wear evaluation — an update

Abstract The crater grinder test has been developed in order to obtain the abrasive resistance of thin coatings. It is capable of determining the specific abrasive wear rate of the coating and the substrate individually, even for very thin coatings. This paper reports on further development and evaluation of the test. The equation for calculation of the specific wear rate has been reformulated and the influence of several test parameters has been evaluated. As a consequence the test procedure has been revised. The amended test procedure yields more accurate values of the specific wear rate as compared to the earlier version. Moreover, a standard test procedure is suggested to ensure reliable and reproducible results.

The effects of compressive stresses on the abrasion of diamond coatings

Abstract The effects of high biaxial compressive stresses on the abrasion of diamond coatings by diamond particles were evaluated using an atomic force microscope (AFM). The specimens subjected to testing were diamond coatings deposited on cemented carbide substrates, receiving high biaxial stresses due to thermal expansion mismatch, and freestanding stress-free diamond coatings. It is shown that the highly stressed coatings obtain a smoother worn surface and a significantly lower wear rate as compared to the stress-free coatings. Thus, biaxial stresses increase the inherently high wear resistance of diamond coatings. The results agree with a previously proposed model for the effect of high biaxial compressive stress on the abrasion of brittle coatings.

Ion transport in porous Sn oxide films: Cyclic voltammograms interpreted in terms of a fractal dimension

Sn oxide films, made by reactive r.f. magnetron sputtering, were studied in a Li+-conducting electrolyte. Cyclic voltammograms taken at different sweep rates were interpreted in terms of a unique structural parameter related to the fractal dimension of a self-affine surface relief.

Designed abrasive diamond surfaces

With the aim to explore their abrasive and grinding properties, flat diamond surfaces with protruding pyramidal abrasive tips have been designed and manufactured. The manufacturing process is a replica technique based on hot filament chemical vapour deposition of diamond onto a silicon wafer, in which the shape, size and packing pattern of the pyramids have been defined by photolithography and etching. After the diamond deposition, the silicon master is removed by etching and the thin diamond film is cemented to a supporting steel disk. Three different pyramid sizes, each with two different packing densities, were fabricated on the same silicon wafer. One of these structures was selected for further evaluation. The resulting shape and quality of the diamond surface was evaluated by scanning electron microscopy (SEM) and Raman spectroscopy. The abrasive properties and durability were evaluated in a pin-on-disc test followed by SEM studies of the abrasive structures and the abraded surfaces. The manufacturing process proved successful in producing well-defined abrasive diamond structures, showing practically constant tip shape and size. The diamond structures suffered negligible damage when abrading brass and tin, exhibited limited fracture when abrading steel, and rather extensive fracture when abrading aluminium oxide. The fracture was mainly due to an unintentional limited diamond thickness in the outermost parts of the pyramids. The combination of extreme mechanical properties of diamond and possibilities to design exceptionally well-defined abrasive structures promise very interesting possibilities for development of novel grinding tools and standardised abrasive wear tests.

Wear volume and wear distribution of hydraulic motor cam rollers studied by a novel atomic force microscope technique

Abstract The wear volume and wear distribution of chromium steel cam rollers in a high-torque hydraulic motor has been investigated. The cam roller is a part of a novel silicon nitride/chromium steel journal bearing system. An atomic force microscope (AFM) was used to obtain topographical images of the cam roller surfaces before and after use in a full-scale test of the hydraulic motor. The surfaces were marked with small Vickers indentations to make it possible to reposition the AFM to the same locations. To measure the microscopical wear and produce high resolution maps of the local distribution of wear, a recently developed method was utilised. The method is based on two techniques to treat digital topographical images. To map the distribution of wear, the image of the unworn surface is substracted by the image of the worn surface. To measure the wear volume, the bearing histogram is used to calculate a volume relative to a fixed depth. The calculated volume of the unworn surface is then subtracted by the volume of the worn surface. The ceramic/metal system displayed an extremely low wear rate. corresponding to a typical total mean wear depth of about 30 nm. The surface topography showed very limited changes with the minute wear mainly localised to the uppermost part of the surface ridges. The adopted method thus allowed a unique high resolution mapping and volumetric measurement of the initial stages of wear (1 mg lost out of 600 g) on a real machine element. This high resolution analysis is promising for improving tribological testing of real machine elements with long expected wear lives, by reducing the need for excessively accelerated tests or extremely long and costly test durations.

The influence of tip shape in abrasion studied by controlled multiasperity surfaces

The influence of abrasive tip angle, tip blunting and tip packing density on the wear rate has been studied using controlled multiasperity surfaces of silicon. The surfaces were manufactured by micromechanical etching techniques and controlled with respect to tip angle, size, radius and intertip distance. In this work, pyramidal abrasive structures were produced with three tip packing densities, two tip radii and different tip angles. Wear tests were performed in a pin-on-abrasive disk configuration with tin as abraded material. Both the abrading silicon tips and the worn tin surfaces were studied by scanning electron microscopy (SEM). The results reveal an apparently linear dependence of tip angle on the measured wear rate for both sharp and blunt tips. For sharp tips, the angle dependence of the abrasion rate is approximately linear and independent of the packing density of the tips. Blunt tips, however, give lower wear rates and also lower sensitivity to variation in the tip angle than sharp tips. Further, the sensitivity of tip angle falls with increased packing density of the tips. This paper demonstrates a novel method for fundamental studies of the effects of the abrasive tip shape on the resulting wear rates. The present study is more closely related to real abrasive and grinding processes than the frequent single tip experiments.