Environmentally friendly lubricants and low friction coatings a route towards sustainable products and production processes

The life cycle assessments (LCA) has been performed following the ISO 14040-series of guidelines in order to analyse the environmental impact of biodegradable lubricants in combination with low friction coatings in comparison to mineral oils and uncoated materials. Life cycle assessment is a technique for assessing the environmental aspects and potential impacts associated with a product, by the compilation of an inventory of relevant inputs and outputs (energy, raw materials, air and water emissions, solid waste) of a product system. The potential environmental impacts associated with those inputs and outputs are evaluated using the Ecoindicator 99 and the results of the inventory analysis and impact assessment phases are interpreted to the objectives of the study. This analysis has been carried out in three stages: - A comparison between the production of mineral oils, vegetable and synthetic lubricants. - Study of the impact of coating processes. - A complete life cycle analysis of the whole tribosystem applied to specified components. The results are that the environmental impact of coating is negligible compared to the impact of the lubricant. Sensitivity analysis was performed: Taking into account the assumptions of this study, the Lubricoat system will have the same environmental impact as the conventional system if the EAL lifetime is 63% of the mineral lubricant lifetime. If EAL lifetime is >63% of mineral oil lifetime then the Lubricoat system has better environmental properties than the conventional system. DLC manufacturing The major impact is due to the electricity consumption during the coating process. (Creation and sustaining plasma!) Lubricant manufacturing In the Mineral lubricant manufacturing process the main environmental impact is the depletion of fossil fuel resources caused by the use of crude oil The main environmental impacts of the EAL can be distributed between the transport of raw materials needed in the EAL manufacturing (26 % of the total impact), the TMP of the esterification process (30%) and the Refined rapeseed oil (31%) General conclusion: if a subjective normalisation and weighting step is applied the production of EAL is more environmentally friendly than production of mineral oil based lubricants.

Results of a tribological screening of different biodegradable oils (sunfloweroil, saturated and unsaturated ester) in combination with different DLC- coating types (different dopants). Biodegradable oil based lubricants are prone to thermal degradation, hence frictional heating during accidental contacts of the moving parts or start-up, shut-down contacts limit the lifetime of such environmentally adapted lubricants (EAL). The tests evaluate the potential of DLC types to solve this degradation problem and mainly investigates the difference of oil additive interaction with pure steel or DLC-coatings. The results of this screening can be used to make a first material choices (oil/additive/coating-combination) for the development of sustainable tribological systems in hydraulics (automotive) and centers and rests in grinding machines and others. Several tests have been performed to simulated different contact conditions: Disc on disc tests simulate the contact present in the center/workpiece of grinding machines and ball on disc tests the contact in a guide/workpiece and in a piston/liner system. In both tests it was found that the studied coatings reduce the friction in a combined coating/EAL system, with the best results by an undoped DLC coating. Additives in the lubricant did not show any effect in these test conditions. The wear of the coatings is evidently reduced by applying a lubricant. Early failure of the doped coatings was due to delamination. The steel counter body however is protected by the coating on the ball. Additives in the oil can improve the wear resistance, but the most beneficial type of additive depends on the dopants used in the coating. Block-on-ring tests simulated the rest system in grinding machines. The tests showed a reduction by factor 10 of the wear. The combination of a fully formulated EAL lubricant with an optimised DLC show even lower coefficient of friction and better wear resistance than the coatings and oils used in the screening tests. FZG tests were performed to study the effect of using a coating on oxidation stability of the EAL.DLC/DLC systems showed lower friction coefficient than steel/steel or Steel/DLC combinations, hence there is lower frictional heat generation. A test on a piston pump, which is one of conventional and very frequent types of pumps used in hydraulic applications, has been performed. The results of oil analyses are all favourable for the DLC system, including viscosity (more stable, and not showing the oxidation trend) and TAN is strongly better for DLC than steel system. Contamination of the oil was reduced and the combination EAL/DLC showed very little running in effect. Doimak and Volvo will use this result for exploitation on grinding machines and in hydraulic components.

Anticipated result based on the further studies of working range of the result from the project. Volvo will primarily use the results from the project in their construction equipment sold to customers. Technically the results will be used in coming designs of components in hydraulic systems in construction equipment. Producing such complex systems requires a network of suppliers and Volvo is involved with most of the suppliers in this business. The generated knowledge will result in new demands on the suppliers. Pumps and valves for example, where the results of the project has high potential, are bought from the two German producers Rexroth and Vickers. LFC results will also be used in future designs of engines and transmissions. Since the structure of this business is similar customers and suppliers will experience the outcome of this project. Today suppliers of LFCs are mostly SMEs, thus there will be a large technology transfer to this type of companies in the LFC area.

A low friction DLC coating has been developed for use in tribological applications, in combination with biodegradable lubricants. The development aims at improving both the performance and production cost of the DLC coating. DLC coatings deposited by means of a PACVD process show beneficial properties such as high hardness; low coefficient of friction and high wear resistance. They are primarily used in tribological (e.g. wear protection of machine components) and anti-sticking (e.g. release layers for plastic injection moulds) applications. The deposition process is scaled up to commercially viable dimensions (reactor volume of several m3). However, the adhesion of this coating to the substrate material is not sufficient for severe tribological applications, occurring e.g. in the automotive industry. Consequently, there is a strong need to improve the adhesion of the DLC coating. This goal has been achieved by optimising the coating deposition process. A hybrid PACVD/PVD process is able to deposit DLC coatings with an extremely good adhesion to virtually any kind of substrate material. The drawback of this technology is the process complexity, which has an impact on the cost-effectiveness. The upscaling and optimisation of the hybrid deposition process results in a commercially feasible technique for deposition of high performance DLC coatings. This innovative adjustment of the deposition process will extend the application area of the deposited DLC coatings and increase the market potential by 15%.

In this project, a new testing approach for the characterisation and comparison of "coating-oil-additive" was used. Due to large number of testing combinations, a fast and efficient testing method was needed. We have applied the Striebeck curve testing procedure to determine the lubrication regimes (Lambda) under various working parameters (load, velocity) as a function of pin and disk surface conditions (roughness), which consequently determines the life of the system. The direct result of these tests was: - Coefficient of friction (COF) curve as function of "striebeck" parameter, which define the conditions at which each lubrication regime appears; - The value of COF at each lubrication regime, which enables the relative ranking of different "coating-oil-additive" systems; - Prediction of efficiency of each tested system and ability of the system to provide satisfactory performance at each lubrication regime (though the focus was on boundary lubrication regime); - The minimum possible COF (in the EHL regime) and the value of COF in the boundary lubrication regime; - The "Striebeck" parameter value at which the additives loose/gain their function.

A new biodegradable lubricant(s) especially developed for mobile hydraulic systems equipped with low friction DLC coatings has been developed. New lubricant provides a good lubrication of classic hydraulic systems and systems with DLC coatings as well. It is ready biodegradable and non-toxic. Its lifetime is be comparable with mineral oil. Therefore one unsaturated ester and one complex-saturated ester with appropriate additives have satisfied all technical demands. All base stocks (base oils and additives) are current state of the art in the field of biodegradable lubricants. Of course some structural changes are expected to conform lubricant with DLC coatings. New lubricant is primarily intended for mobile hydraulic systems with DLC coatings in tractors, forestry equipment, construction equipment, agricultural equipment, where problems with soil and water pollution with oil can occur. The market potential is also non DLC hydraulic systems because its very high oxidation stability and biodegradability. New lubricant(s) is also foreseen for lubrication of some parts of grinding machines coated with DLC. The current biodegradable hydraulic oil market size in EU is round 67.000T/year. The potential of a new lubricant is strongly focussed on first fill of project's end users. So far the estimated price of the product will be in the range of other market biodegradable synthetic oils. Total fraction of bio lubes in EU is still very little. Draw backs are high price and shorter change interval. The environmentally (biodegradability, non-toxicity) and technical (DLC compatibility) aspects are strongly emphasised by development of new lubricant.

A modular vacuum doser has been designed and manufactured to dose a reproducible film of lubricant on a previously well characterised and clean substrate. The sample can be subsequently transferred to an analysis chamber. The lubricant can be deposited from atmospheric to 10-6 mbar pressure. Thermal Desorption Spectroscopy can be performed in situ (previously to transfer) and simultaneously to deposition (if pressure is 10-6 mbar). The sample transfer system allows easy and fast transport of the sample under vacuum and clean conditions to an attached analysis system of X-ray Photoelectron Spectroscopy (Electron Spectroscopy for Chemical Analysis). The coating-lubricant interaction can be studied practically in situ. These in-situ measurements can be compared with other ex-situ (atmospheric pressure) analysis such as Attenuated Total Reflection Fourier Transform Infrared Spectroscopy.

The main result of this activity is the definition of the specifications of coatings and lubricants to be used in critical components to be introduced in grinding machines. As a consequence of this it is expected that the availability of the machines will increase up to a 98%. This will represent a competitive advantage to achieve new orders and customers (valuated as 350 k euros/year). A second effect of this result will be the reduction of the period time to probe the requirements of the machines (from 15 to 12 months). On the other hand Doimak expect to reduce the environmental impact of the machines by means of the use of ecological lubricants.

Deliverables

Share this page

Download