The aim of the present research was to investigate influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. First the correlation ...between different surface preparation techniques and roughness parameters was investigated. For this purpose 100Cr6 steel plate samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling. Different surface preparation techniques resulted in different
R
a values from 0.02 to 7
μm. After this, correlation between surface roughness parameters and friction and wear was investigated. For this reason dry and lubricated pin-on-disc tests, using different contact conditions, were carried out, where Al
2O
3 ball was used as counter-body. It was observed that parameters
R
ku,
R
sk,
R
pk and
R
vk tend to have influence on coefficient of friction.
Tailoring the properties of tribological interfaces is difficult, but it can be a very efficient way to reduce wear and friction and so increase service life – providing it can be done successfully. ...However, when considering the interface properties it is necessary not only to take into account the solid–solid contacts, we also need to consider the solid–liquid interfaces, which can have a very significant impact on the micro- and macro-contacts, and which cannot be neglected. In this investigation we show that it is essential to consider the wetting, or slip, properties of the non-fully wetted surfaces when designing the lubrication, and that this is also to a large extent predictable and relatively simple to implement. We discuss the differences between several types of contacts, using steel and DLC as model materials for fully wetted and non-fully wetted surfaces, respectively. We studied steel/steel, steel/DLC and DLC/DLC pairs, and we present the experimental evidence for how the Stribeck curve changes if one or two surfaces in the contact are non-fully wetted. Firstly, in the EHL regime the friction of non-fully wetted surfaces decreases significantly; in our case by around 15% and by more than 20%, respectively, while at minimum friction (the EHL
→
ML transition) the reduction in friction is lower, but still about 6%, and as high as 9%. Secondly, the film thickness of the non-fully wetted contacts is reduced, which causes a shift in the minimum friction (the Stribeck curve) to higher
vη/
F values. This suggests a lower bearing capacity for these contacts and the potential danger of a transition in the lubrication regime and, as a result, increased wear. However, we have derived several “ready-to-use” diagrams that are generally valid for any oil and surface properties and which allow us to define the film thickness, the lambda parameter and the regime transition for different wetting/slip properties. These diagrams can be used directly to improve the prediction and design of the lubrication quality and the regimes of the macro-scale contacts by employing a currently used, conventional methodology and equations. Moreover, based on the experimental results and the “ready-to-use” diagrams derived in this investigation for non-fully wetted contacts, the slip at the solid–liquid interface, i.e., the DLC-PAO oil, was estimated independently from the data of two Stribeck curves to be about 20%.
In this paper, we report on the tribological performance and mechanisms of the boundary-lubricated contacts of steel against diamond-like carbon (DLC) coatings, i.e., steel/DLC, using the same ...materials, oils, additives and conditions as we have previously reported for DLC/DLC contacts. We present and compare the behaviour of two non-doped, two metal-doped (Ti, WC) and one non-metal-doped (Si) DLC coatings in contact with steel surfaces in reciprocating sliding, lubricated with a paraffinic mineral base oil with and without anti-wear and extreme-pressure additives. Friction behaviour similar to that of the steel/steel surfaces is observed in all the steel/coating contacts, irrespective of the coating type. However, the doped and non-doped coatings showed different wear behaviours. The non-doped DLC coatings provided the lowest wear with the base oils, due to the absence of adhesion between the steel and the DLC coating. In contrast, additivation of the oil was necessary for the adhesion prevention and successful operation of the steel/doped-DLC contacts.
Surface texturing by pulsed Nd:YAG laser Vilhena, L.M.; Sedlaček, M.; Podgornik, B. ...
Tribology international,
10/2009, Letnik:
42, Številka:
10
Journal Article, Conference Proceeding
Recenzirano
Introducing specific textures on a tribological surface can contribute to friction reduction in sliding contacts. In the present paper, a pulsed Nd:YAG laser emitting at 1064
nm, was used against ...100Cr6 steel samples in order to produce well-defined surface micro-pores, which can act as lubricant reservoirs, micro-hydrodynamic bearings as well as traps for wear debris. Due to the high flexibility of the laser system, structural features such as shape, size, density and depth can be varied easily by changing the laser parameters. To optimize the parameters of the laser surface texturing process, an investigation was performed using different pulse numbers, various pulse energies and two different modes (single- and multi-mode). The microtextures were characterized with optical microscopy, scanning electron microscopy (SEM) and by topography techniques. The relationship between the laser processing parameters and qualitative and quantitative profile of the micro-pores was studied. Tribological testing of laser textured surfaces was performed in a low frequency–long displacement reciprocating sliding wear tester under boundary lubrication and results compared to un-textured case. Tribological comparison of textured, textured and lapped, and untextured surfaces shows only minimal influence of texturing for contact conditions investigated.
Diamond-like carbon (DLC) coatings, which can nowadays be applied to many highly loaded mechanical components, sometimes need to operate under lubricated conditions. It is reasonable to expect that ...in steel/DLC contacts, at least the steel counter body will behave according to conventional lubrication mechanisms and will interact with lubricants and additives in the contact. However, in DLC/DLC contacts, such mechanisms are still unclear. For example, the “inertness” of DLC coatings raises several questions about whether they are able to provide real boundary “lubrication” or whether they are just a “passive” member in these contacts. On the other hand, biodegradable oils, in particular vegetable base oils, possess a good lubricating ability, often much better than mineral or conventional synthetic oils as a result of the large amount of un-saturated and polar components that can promote the lubricity of DLC coatings. Accordingly, in this study, we present the results of experiments under severe boundary-lubrication conditions during reciprocating sliding. We look at the effect of the type of mating surfaces – steel/DLC, DLC/DLC and steel/steel – and the type of oil on the tribological performance of DLC coatings. We compare the wear and friction behaviours of two types of DLC coatings, i.e., a “pure” non-doped a-C:H DLC coating (denoted as a-DLC) and a WC-containing multilayer coating (denoted as W-DLC) tested with a mineral oil and a biodegradable vegetable oil. These oils, which have very different chemical compositions, were used as base oils and also with mild AW and strong EP additives. Among other things, the results confirm the following: (1) coating/coating lubricated contacts can resemble metal-lubrication mechanisms; (2) additives reduce wear in coating/coating contacts by up to 80%; (3) better wear and friction performance are obtained with oils that contain large amounts of polar and un-saturated molecules; (4) a coating/coating combination generally results in less wear than a steel/coating combination.
► Hard PVD coatings can be successfully used in blanking/piercing applications, even on softer tool steels. ► Preparation of the substrate material and good coating to substrate adhesion are crucial. ...► Even with low friction coatings stamping force exceeds critical value under dry friction conditions and leads to tool failure. ► Oxidation and temperature resistant hard coatings give improved wear resistance of stamping tools, but elimination of lubricants in blanking and piercing processes is still not feasible.
The aim of the present investigation was to examine the possibility of reducing lubrication and replacing expensive tungsten carbide material in blanking/piercing through introduction of hard tool coatings. Results show that hard PVD coatings can be successfully used in blanking/piercing applications, even on softer tool steels, thus leading to reduced friction and wear as well as to lower costs of the tool. However, preparation of the substrate material and good coating to substrate adhesion are crucial. On the other hand, even with the use of low friction coating (DLC) stamping force exceeds critical value under dry friction conditions and leads to tool failure. Therefore, at present oxidation and temperature resistant hard coatings can give improved wear resistance of stamping tools, but elimination of lubricants in blanking and piercing processes is still not feasible.
In this work we present the wear and friction behaviour of boundary-lubricated, hydrogenated, amorphous, diamond-like carbon coatings (a-C:H), in self-mated a-C:H/a-C:H contacts, at three different ...testing temperatures: 20, 80, 150 °C. We present results from Auger electron spectroscopy, X-ray photoelectron spectroscopy and Raman analyses relating to the chemical and structural changes in the diamond-like carbon coatings during sliding in the presence of mineral oil, with and without additives. We show, that chemical reactions between the a-C:H coatings and the oil additives take place, which are dependent on the temperature, on the presence of additives and the type of additives used. At high temperatures the extreme pressure additive interacts with the diamond-like carbon surface and forms a tribochemical layer with a four-times lower sulphur/phosphorous ratio than the additive formulation. In the absence of additives, however, graphitisation of the coating occurs under these conditions, which results in high-wear and low-friction behaviour. Another result from this study is that a-C:H coatings can oxidise during room-temperature experiments, suggesting that some interactions and adsorptions are also possible at lower temperatures.
In contrast to non-doped diamond-like-carbon (DLC) coatings, reliable chemical evidence of the reactions between metal-doped DLC coatings and oil additives under tribological conditions using ...state-of-the-art surface-sensitive chemical analyses is still scarce. In this study we have investigated the reactivity of metal-doped (Ti, WC) DLC coatings with the extreme-pressure (EP) dialkyl dithiophosphate additive — without the presence of a steel counter body in the contact that befogs the actual coating reactions. Static “reactivity” experiments without any tribological or mechanical effects were also performed to provide a further insight into the lubrication mechanisms. The results confirmed the chemical reactions between the EP additive and all the DLC coatings, as well as their oxidation during the tribological contacts. We measured an about 10-times higher chemical activity (a 25-fold P/S ratio increase) for the Ti-doped DLC compared to the WC-doped or non-doped DLC, which also agrees with it having the lowest amount of wear in this study. We suggest that the Ti-DLC boundary lubrication is achieved via binding sites at the O vacancies present in the Ti-doped DLC coating. The data also clearly show, in contrast to most of literature reports, that even though small, some direct chemical activity between the W-DLC and the dialkyl dithiophosphate EP additive is also possible without any iron catalytic effect. However, the chemical changes were significantly smaller, also allowing coating graphitization, which might be one of the reasons for the 50% higher wear of the WC-doped compared to the Ti-doped DLC.
In the last ten years significant progress has been made on development and understanding of diamond-like carbon coatings (DLC), which enabled them to become one of the most promising types of ...protective coatings for various mechanical applications. Their main advantages are low friction, good anti-wear properties, and adhesive protection. However, due to their low surface energy their reactivity with conventional oils and additives is limited, which makes achieving effective boundary lubrication a complex task. For a qualitative step-change that would improve the performance and allow effective optimizing and tailoring of these boundary-lubricated tribological systems, it is necessary to understand the mechanisms of why, how, under which conditions, and with which materials and lubricants, the actual boundary lubrication is possible—if at all. The current results on boundary lubrication of DLC coatings are not many, and are due to the different types of coatings, lubricants and additives used in these studies, often difficult to compare and sometimes even contradictive. However, a recent great demand in different industries to apply the DLC coatings to lubricated systems requires a better understanding of these phenomena and overall performance. Therefore, if we wish to see a more effective continuation of the research and a better understanding of the scattered results, an overview of today’s state-of-the-art of lubricated DLC contacts is needed. In this paper we analyse the behaviors and suggested mechanisms from already-published studies and we summarize the present understanding of the boundary lubrication of DLC coatings. We focus on the DLC-lubricant interaction, thus we analyse only self-mated DLC/DLC contacts in order to avoid the inevitable effects from interactions with other counter materials such as steel.
The operating conditions under which chemical reactions between diamond-like-carbon (DLC) coatings and oil additives occur and the main driving forces, i.e., the “activation criteria” for these ...chemical reactions, have not yet been defined. In order to clarify the difference between the “test” temperature and “real” contact temperature, and to determine the effect of the real contact temperature for these reactions, we have calculated the contact temperatures using two well-known models and compare them with results of tribological experiments and some state-of-the-art analyses of worn surfaces. The results suggest that the actual surface temperatures are 100−130
°C higher than the test temperatures. A contact temperature of about 250−260
°C was found to be the required key activation criterion for chemical reactions between the dialkyl dithiophosphate extreme-pressure (EP) additive and the DLC coating. Gradual formation of a tribochemical protective film from phosphates and organic sulphur/sulphates suggests a lower chemical reactivity and slower formation of the “optimal” tribochemical protective layer on DLC coatings than on steels. No tribological effect of anti-wear (AW) or EP additives could be found on the DLC coatings when the surface temperatures were below 120−140
°C. The temperature-induced graphitisation of the DLC that occurred in the contacts with the base oils, however, require about 250
°C of contact temperature. Lower surface temperatures or the use of additives suppressed this graphitisation.
