ASTM to ballot scuffing test method developed by PetroChina
Scuffing, from the Swedish word skuffa, which means to push, is defined as a form of wear occurring in inadequately-lubricated tribosystems, a system that contains one or more tribo-elements, including all mechanical, chemical, and environmental factors relevant to tribological behavior. 1 While often used interchangeably with scoring, scuffing “originates from the adhesive bond creation between the mating surfaces, which are then sheared,” whereas “scoring results from mechanical abrasion of the surface by the very hard wear particles under conditions of a very high load.” 2
Like scuffing, scoring is one of the most dangerous modes of gear wear. Scuffing often happens abruptly and can result in an immediate increase in friction on components, such as gears and bearings. This can cause system inefficiencies or, at the extreme, catastrophic failure.
It is tricky to study scuffing in a repeatable manner, because, other than abrasive wear, it happens randomly. Scuffing has typically been attributed to improperly lubricated tribosystems, additive depletion, especially in oils exceeding the drain recommendations, and, in particular, the lubricant critical temperature, where engines operate “hot”.
The Mercedes-Benz MBN 10474 (Execution of Scuffing Limit Tests with Oscillating-Friction-Wear Tribometer) has been included in the Mercedes-Benz oil specifications since 2013 and examines scuffing resistance in a tribometer using a myriad of test conditions. 3 It maps the resistance against scuffing in a wide range of operating conditions. MBN 10474 operates outside of an engine using test pieces machined from real engines. The German OEM-specific test can also be extended to other metallurgically different piston ring and cylinder liner components.
PetroChina Lubricant Company, the lubricant arm of China’s state-owned China National Petroleum Corporation (PetroChina), has developed a new test procedure to determine the scuffing temperature limit of lubricating oils under the direction of project leader, Jin Zhiliang. The new test method also uses the SRV test machine to measure the temperature limit for operating a lubricant under prescribed conditions. It can also determine the ability of non-extreme pressure lubricating oils to resist scuffing. Furthermore, the friction coefficient of the oils can be monitored to assess their tribological performance and retention over a range of temperatures.
Until now, tribological scuffing tests and extreme pressure tests have been generated by different tribometric test methods, such as FZG, Four Ball, SRV, Timken, etc., using load or high Hertzian contact stresses. Hertzian contact stress refers to the localised stresses that develop as two curved surfaces come in contact and deform slightly under the imposed loads.
The PetroChina test method operates under constant load, frequency and stress, using a temperature step increase.
“This represents a new technical approach,” said Dr. Mathias Woydt, managing partner of MATRILUB Materials and a technical contact for SRV-based test methods at Committee D02 (Petroleum Products, Liquid Fuels, and Lubricants) of ASTM International.
The SRV test machine analyses friction and wear of lubricating oils, greases and additives and generates data compliant with standard-setting organisations such as ASTM International, Deutsches Institut für Normung e.V. (DIN) and the International Organization for Standardization (ISO) as well as the Standards Administration of China (SAC). SRV stands for Schwingung (Oscillating), Reibung (Friction), Verschleiß (Wear). The SRV test machine assesses physical interactions under loaded contact in either rotational or linear oscillatory motion.
The PetroChina test method oscillates a test roller at constant frequency and stroke amplitude and under constant load against a test disk in high-carbon content, lamellar grey cast iron wetted by the lubricant to be tested. A longitudinal roller moves at a 90-degree angle to the sliding direction and the temperature is increased in steps of 10°C during the test. The time increment is increased from seven to 15 minutes during the entire length of the test. A piezo-electric device mounted in the test disk assembly measures friction force. Peak values of the coefficient of friction are determined and recorded as a function of temperature. Scuffing is indicated by a sudden increase in the coefficient of friction at a given temperature. The method has been determined to be highly repeatable. The ASTM D02 research report reveals a repeatability r of 12.9 Kelvin (K) and a reproducibility R of 17.3 K, which means scuffing occurs within 1-2 temperature steps of 10 K.
The procedure is appropriate for high-speed vibrational or start-stop motions over extended periods—while under initial high Hertzian line contact pressures. The ease of execution by base oil and additive manufacturers and formulators is significant, as it does not require components from engines, said Woydt. This makes it a simpler test to implement than the Mercedes-Benz MBN 10474. PetroChina has found the test method useful as a screening test for engine oils, gear lubricants and turbine oils—where temperatures may reach up to 300°C.
PetroChina’s scuffing(T) test method indicates that the initial contact pressure of the roller-on-disk geometry is better retained throughout the test than the current ASTM D6425 (Standard Test Method for Measuring Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils Using SRV Test Machine) with their ball-on-disk geometry.
The scuffing test is one of several SRV-based test methods for internal combustion engines developed by PetroChina, including T/CSAE109 (2019 Standard Test Method for Determining the Friction and Wear Properties of Diesel Engine Oil)—which is for sooted oils and uses the SRV cylinder liner-piston ring method and the Q/SY1495 (2012 Standard Test Method for determining the friction-reducing properties of gasoline engine oils by using the SRV Test Machine).
As the SRV-based tests operate outside of engines, the tests are quicker and cheaper to run, compared to engine tests, said Woydt. The test methods also demonstrate it is possible to execute meaningful tribometric tests outside of engines, he said, especially when test pieces manufactured from engines are used in the test method
PetroChina submitted an initial application to ASTM Committee D02 for the new test method on July 28, 2022. This was preceded by initial validation of the improved test conditions in a national round-robin test with six oil samples in late 2021, as well as the establishment of precision statements through an additional international round-robin test in the fall of 2022 with the same six oil samples.
The test method remains under development within ASTM Committee D02, which is completing critical statistical analysis of the round-robin test data and the revision of the existing draft. A negative vote received during a ballot within ASTM Subcommittee D02.L0 (Industrial Lubricants and Engineering Sciences of High Performance Fluids and Solids) was withdrawn in December 2022. The procedure now proceeds for balloting within the main ASTM Committee D02 as work item WK82495 this year.
1 ASTM G40-17: Standard Terminology Relating to Wear and Erosion, ASTM International, June 30, 2021
2 New Scuffing Test Methods for the Determination of the Scuffing Resistance of Coated Gears, Remigiusz Michalczewski, Marek Kalbarczyk, Michal Michalak, Witold Piekoszewski, Marian Szczerek, Waldemar Tuszynski and Jan Wulczynski, Tribology, May 22, 2013.
3 MBN 10474, Execution of Scuffing Limit Tests with Oscillating-Friction-Wear Tribometer, Mercedes Benz AG, procedural instruction, April 2016