23rd International Colloquium Tribology - January 2022 367 The use of the MTM rig for wear testing Matthew Smeeth PCS Instruments, London, UK Corresponding author: matt.smeeth@pcs-instruments.com Clive Hamer PCS Instruments, London, UK 1. Introduction The wear rate of any lubricated contact is dependent on many factors, including, surface roughness, lubricant composition, environment, operating conditions, temperatures, etc. There are also many different wear mechanisms, often operating in parallel. Since wear in itself is not an intrinsic property of a system, different wear tests can give very different results. Despite their popularity and widespread use, all wear bench test methods all have some shortcomings when used to investigate complex lubricant additive combinations. However sophisticated the test method, they are inevitably unable to directly mimic real lubricated contacts conditions of machine elements. Interpretation of different bench test results can be difficult and misleading conclusions can sometimes be drawn. A new pure sliding wear test is described which can produce measurable wear withing a reasonable period of time. The repeatability and merits of the test method is discussed. 1.1 Background The thickness and distribution of reaction layers formed by ZDDP and other antiwear additives is controlled by several factors including load, temperature, sliding speed and additive concentration. Invariably, the thickness formed will increase at higher temperatures and higher pressures The chemical antiwear film formed between running surfaces is complex in composition and has been examined using a large number of different analytical techniques; however the exact mechanism by which they form is still not clearly understood. Spikes et al (1,2) and others has showed that antiwear additives form a film which is initially an amorphous glassy structure, which then converts to a nanocrystalline structure during prolonged rubbing. The nanocrystalline structure is relatively weak in comparison to the more crystalline structures, since they inherently possess greater obstruction to dislocation movement For this reason, short wear tests must be very carefully controlled, since the running in period at the start of the test (when the antiwear film is initiated) is critical. Cleanliness and complete lack of contamination is also critical, since this can adversely affect the repeatability of the test. Conversely, longer tests are less susceptible to these variations but obviously suffer the (considerable) disadvantage that the duration may be excessively long, to the point where gathering and meaningful amount of reliable test data is impractical. 1.2 Test procedure A series of tests were carried out using the MTM test rig under the following conditions Configuration Pure sliding, reciprocating Lower specimen 52100 steel disc, 760Hv Upper specimen 52100 steel ball, 3mm diameter, 800Hv Load 20N (2.74 GPa max Hertzian pressure) Temperature 100°C Frequency 20 Hz Stroke length 4mm Duration 16 hours Table 1: Test conditions A pure sliding test was chosen since the wear rate was relatively high. This allowed the wear scar formed on the ball to be measured as the primary wear indicator. Although mixed sliding and rolling wear tests have been carried out in previous studies the wear rates are considerably lower and have therefore required additional instrumentation to fully evaluate the wear. Using the reciprocating mechanism allowed a measurable wear track to form on the disc. Over the 16 hour period, 1.15 million contact cycles were run, which produced an easily measurable and clearly defined wear scar for all the fluids tested. 368 23rd International Colloquium Tribology - January 2022 The use of the MTM rig for wear testing Figure 1: Typical wear scar showing clearly defined boundary 1.3 Test fluids A group 2 mineral base oil with various concentrations of antiwear additive was tested. The viscometric properties of the base oil are shown below KV 40V 67.3 cSt KV 100C 8.8 cSt VI 103 Table 2: Base oil properties Different concentrations of the same additive was used in the base oil. 1.4 Results The graph in figure 2 shows the average wear scar dimension, taken as the average of the sliding the orthogonal direction. In all cases the wear scar boundary was clearly defined and repeat measurements were always within 20 microns of each other. The results are presented as the wear scar diameter as a function of P content. Figure 2: Variation of wear scar with blend P content Measurements were also taken of the disc using a while light interferometer, which can be used to calculate the wear volume lost during the test. The discs from 3 tests at different concentrations were measured and showed excellent correlation with the wear scar measured on the ball. As expected, relatively low concentrations of additive showed lower wear with a relatively smooth transition between the high and very low additive concentration regions The initial results indicated that the wear test was highly repeatable. To investigate this further, a series of tests were carried out on a commercial 0w-20 engine oil with the following properties Moly 60 Zinc 710 Phosphorous 633 Calcium 1344 Boron 250 Table 3: Formulated oil composition The test was very repeatable with a mean wear scar of 325 microns and standard deviation of 7 microns, shown in figure 3. 23rd International Colloquium Tribology - January 2022 369 The use of the MTM rig for wear testing Figure 3: Repeat wear scar measurements on formulated oil sample 2. Conclusion The test showed that a highly repeatable wear test could be carried out over a reasonable time scale. The test was proven to be robust using both base oil/ individual additive mixtures and fully formulated blends. The tests were carried out on Ph and Zinc containing oil,s but can be used as base line tests for ashless and other novel antiwear chemistries as a compassion. References [1] Luiz, J.F., Spikes, H. Tribofilm Formation, Friction and Wear-Reducing Properties of Some Phosphorus-Containing Antiwear Additives. Tribology Letters 68, 75 (2020) [2] Spikes, H. The History and Mechanisms of ZDDP. Tribology Letters 17, 469-489 (2004)