are needed to characterise the electrical response of tribofilms under the regime of mixed/boundary lubrication and hydrodynamic fluid films.
Methodologies for electrified tribometry While there are a multitude of measurable electrical quantities, the goal of this study is to, first, establish tribological test methodologies to understand a lubricant´s response on arcing in relation to tribological quantities and, second, to have test results suitable for technical documentation. Particularly, these methodologies are based on the widely used and recognised 4-ball tests as per ASTM D2266 (greases) and D4172 (oils), which operate under 1,200 rpm. In the following, the methodology for a self-explanatory and descriptive electrified tribotest will be detailed for the frictional response and impact on wear resistance as well as tribofilm response of lubes during amperage step-up and step-down.
The methodology is based on the 4-ball apparatus, because it is widely used and well known in laboratories around the world. The aim of this procedure by using 4-ball testing is to illuminate the response of arcing on the frictional profile and its tentative recovery during amperage step-down. The wear scar diameters are measured after custom 4-ball tests were performed on an electrified MFT-5000 tribometer (Rtec Instruments, San Jose, CA, USA). The tests are 105 minutes, so the wear scars are expected to be larger than those achieved during the standard 60-minute test. The custom test involves seven steps. It starts with 15 minutes without current. Then, the current (DC) is increased by 1 A every 15 minutes until 3 A is reached, and then the current is decreased by 1 A every 15 minutes. If one would use a constant amperage ramp rate, it would not be possible to determine if a stable equilibrium in electrical contact resistance and coefficient of friction occurs under a given amperage. During the test, the coefficient of friction (CoF) and electrical contact resistance (ECR) are tracked as a function of time (Figure 1).
2.1. Amperage step-up and step-down of oils Both oils in Figure 2 displayed no response to amperage and the morphology of wear scars (analysed from optical microscopy) revealed no pronounced abrasive wear mechanisms. It seems that, under the amperage cycles, the wear scar diameters increased slightly, but the values stayed within the repeatability limits of D4172.
Figure 2: Evolution of the coefficient of friction in a DC amperage during a step test using electrified ASTM D4172 mod. (RT, FN= 392 N, 1.5 V DC).
2.2 Amperage step-up and step-down of greases The electrical contact resistance (ECR) and coefficient of friction (CoF) from the step up/step down test with four greases are shown in Figure 3. The four greases exhibited different ECR responses to DC amperage, whereas the evolution in friction was unaffected by amperage. The changes in ECR are reversible which indicates reversible changes to the physico-chemical of the tribofilms. The ECR signal also indicated that these changes occurred spontaneously at the start of current flow, i.e., ECR increased as soon as the applied current was stepped up to 1 A.
Figure 3: Evolution of the coefficient of friction in a DC amperage step test using electrified ASTM D2266 mod. (100°C, FN= 392 N, 1.5 V DC, DC; t= 105 minutes).
Figure 1: Representative results from an electrified ASTM D4172 test where the amperage is stepped up and then back down while the current and friction coefficient (CoF) are measured.
Figure 4 summarises the wear scar diameters (WSD), measured using optical microscopy, and wear volumes, measured using white light interferometry, of test balls from the amperage step-up and step-down tests. Comparing the diameters from the amperage tests to unelectrified tests, the wear scar diameters increased for two greases, one grease was
10 LUBE MAGAZINE NO.190 DECEMBER 2025
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