The use of and demand for bio-lubricants is slowly increasing, primarily as a result of newly introduced legislation. Mineral oil based lubricants are now prohibited in certain areas, such as in lakes and forests, by several countries, including Belgium, Germany, Austria and Switzerland. The European Union’s Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) is thought to be the most important legislation to aid the development of bio-lubricants. REACH has been implemented in an attempt to clean up the chemical industry and reduce the use of toxic substances. Additionally to the environmental reasons, the increase in cost of crude oil, along with concerns around the security of supply, gives a long term financial incentive to switch to bio-lubricants. It is critical therefore, that the performance of bio-lubricants can match mineral based lubricants.
The aim of this research was to assess the feasibility of using bio-lubricants with materials found in a typical oil circuit in four stroke internal combustion engine. This was done by assessing the tribological properties of bio-lubricants through the use of various experimental and analytical methods, including reciprocating wear, elastomer relaxation, advanced microscopy, and chemical analysis.
Current and novel automotive surface treatments were used in multi-layers. This was done to analyse the interaction of the bio-lubricants with these treatments and assess if any performance gains could be made if they were used in automotive contacts. The treatments used were: diamond like carbon (DLC), a calcium based chemical dip, shot blasting using a molybdenum disulphate doping media and nano fullerene.
The surface treatments used did not give any performance advantages in comparison to a super finished steel surface. A reduction in wear and coefficient of friction was found when bio-lubricants were used with DLC coatings, compared with mineral based lubricants. The calcium based chemical dip proved to be as effective, in terms of wear protection, as DLC and works well with bio-lubricants. There was no apparent tribological benefit to using multiple layers of surface treatments.
Wear and friction data was acquired to assess tribological performance. Potential bio-lubricant base stock candidates jojoba, soybean and palm kernel oil were used, with a mineral base stock for comparison. An assessment of lubricant performance was also made through calculations of spreading parameter and Hanson Solubility Parameters.
Stress relaxation tests with EPDM, nitrile rubber and fluorocarbon rubber, with the bio-base stock candidates revealed that soybean and palm kernel oil are compatible with EPDM, which is in disagreement with industry chemical resistance data that presents a cautious overview of compatibility. Jojoba and mineral oil cause relatively high levels of swell with EPDM. Tests with nitrile rubber found no difference in the compatibility of bio-base stocks compared with mineral base stock. All base stock candidates were found to be compatible with fluorocarbon rubber. The use of Hanson Solubility Parameters to predict material compatibility was found to work for EPDM.
Overall, this work has shown that there may be some performance benefits to using bio-lubricants over conventional mineral based lubricants, in internal combustion engines. Bio-lubricants can offer lower wear rates and coefficient of friction, particularly with the novel multi-layer surface treatments used in this work. Bio-lubricants are at least as compatible with elastomers as mineral based lubricants.
