determined by standards ASTM D2596 and 2783. The methods were adapted by the Institute of Petroleum. DIN and AFNOR but are essentially all similar and intend to determine a lubricant’s extreme pressure behaviour.
researchers and industrial users. The former uses a convenient test method to compare or understand mechanisms, the latter needs a method to assure quality. This ambiguous approach to lubricant testing exists until today and requires a constant education for the users and producers of lubricants.
A variation of the EP method, focusing on anti-wear properties, followed. Resulting wear can be measured easily in the Four Ball configuration: 3 more or less circular wear scars are produced by the test and measured by a low power microscope (ASTM D2266 and D4172). The low cost of test specimens and relative ease of measurement have made Four Ball test methods very popular for quality control and lubricant development. It is an efficient screening method with sufficient precision and sensitivity to distinguish between good and bad lubricants and to be used as a development screening tool.
Standard tests require standard equipment
Friction and wear are system properties. This means that the result of a test will not only depend on the product and the test conditions, but also on the test instrument that is used. So not only the procedure, but also the test machine itself need to be standardised, and the repeatability and reproducibility of the test are crucial information to conclude on the significant superiority of one product over another. An ASTM method is not complete without the results of a statistically relevant round-robin test, determining precision. Although ASTM does not endorse commercial manufacturers of test equipment, it is necessary to provide the information what manufacturers have participated in the ASTM round robin. This information can be found in the footnote on equipment, in most standards. Besides using the correct ASTM test equipment, it is equally crucial to keep the test machine calibrated and maintained to have repeatable and reliable test results.
TRIBOLOGY IS BORN: the 1960s and beyond
Although people had been testing the properties of lubricants for many years, the term ‘tribology’ was only coined by the famous Jost Report for the UK House of Commons in August 1966. It stated the importance of friction and wear as damage mechanisms and led to an increase in funding and research in the field. Tribology became a multidisciplinary field of technology, with chemists, physicists, engineers and technicians all contributing to further understanding of the phenomena. A big boom in Tribology research followed, new journals focusing only on friction and wear (e.g. ‘WEAR’ or ‘Tribology International’). Ironically, only very few new standard methods were developed in this period, among the most important are the Block on Ring (ASTM D2714) and Thrust Washer (ASTM D3702) methods from Dow Corning and the Cameron-Plint (ASTM G133) for engine oils. This illustrates the discrepancy between the needs of
NEW DEMANDS – NEW TEST CHALLENGES: the 1990s and the new millennium From the 1990s onwards, a change in focus has appeared. While extreme pressures properties are still encountered in a number of lubricated systems (gears, tools, etc), a lot of components are designed to run mostly under modest contact pressures and in moderate wear regimes. But in these regimes, the operational parameters are constantly challenged, machinery is required to run faster, at higher temperatures, for longer periods of time without maintenance. This has led to an emphasis on friction reduction and on wear testing, without standards following this trend.
Friction reduction: there aren’t many efficient standards that focus on the coefficient of friction, perhaps with the exception of ASTM D5183 in the Four Ball configuration. Naturally, the friction force can be measured in many standard test machines, but is usually not measured precise enough to evaluate the small but significant effects of, for instance, friction modifiers. This is caused by the strong mechanical construction of typical standard test machines, that require stiffness and high loading capacity at the expense of a sensitive friction measurement.
Our answer to this emerging challenge is to use a new generation of force sensors in combination with a tribometer. Although the highest precision in friction forces can be reached with Atomic Force microscopes or Nanoscratch/ Nanoindentation equipment, these tools are often not suitable to evaluate real engineering surface with roughness, and especially in combination with a lubricant. The intermediate microtribometers, employed nowadays in our labs, are capable of distinguishing effects of friction modifiers in a statistically relevant way. It is now a matter of uptake of these procedures by industry who can give a push towards standardisation of the methods and introduction of specifications on friction performance of lubricants.
LUBE MAGAZINE NO.123 OCTOBER 2014
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