23rd International Colloquium Tribology - January 2022 303 Influence of Black Oxide Coating on Micropitting and ZDDP Tribofilm Formation Mao Ueda Technology Centre Research & development Team, Shell Lubricants Japan K.K., Japan Tribology Group, Department of Mechanical Engineering, Imperial College London, London, United Kingdom Corresponding author: mao.ueda@shell-lubes.co.jp Hugh Spikes Tribology Group, Department of Mechanical Engineering, Imperial College London, London, United Kingdom Amir Kadiric Tribology Group, Department of Mechanical Engineering, Imperial College London, London, United Kingdom 1. Introduction Micropitting is a common type of surface fatigue damage caused by stress fluctuations that occur due to asperity interactions as the contacting bodies move over each other. These asperity stress cycles result in initiation of numerous tiny surface fatigue cracks which then propagate until small fragments of material detach from the surface. Recently, Brizmer et al. [1] reported that Black Oxide (BO) coating can reduce micropitting damage when it is applied on the rougher of the two bodies in rolling-sliding contact. They suggested that the mechanism by which BO reduces micropitting is related to enhanced running-in of the BO-coated surfaces that is at least partly brought about by BO suppressing the formation of ZDDP antiwear tribofilms. The consequences of the low hardness of the BO coatings in relation to running-in are now relatively well-established [1]; however the potential enhancement of the running-in process, and hence reduced micropitting, via chemical effects of the BO coating in suppressing the antiwear film buildup has not been directly observed and warrants further investigation. This research aims to establish the effect of BO on micropitting and, more importantly, clarify the mechanisms by which BO influences micropitting performance. 2. Test methods 2.1 Test conditions and procedures All micropitting tests were conducted using the same method as described in our study [2]. A mini-traction machine (MTM) ball-on-disc tribometer with a spacer layer imaging attachment (SLIM) was employed both to generate micropitting on the ball specimen and to monitor ZDDP tribofilm formation. BO coating was applied to both the ball and the disc, and four tribopair combinations of a steel ball/ steel disc, a steel ball/ BO-coated steel disc, a BO-coated steel ball/ steel disc, and a BO-coated steel ball/ BO-coated steel disc were studied. 2.2 Test lubricants The solution of primary-secondary mixed ZDDP in a PAO base oil at a concentration of 0.08 wt.% P was studied. The base oil was PAO 10 having viscosity 62.8 mm 2 / s at 40 °C and 9.9 mm 2 / s at 100 °C. 3. Results 3.1 Evolution of surface damage on balls Fig. 1 shows representative optical micrographs of the wear tracks on the balls of the four tribopairs after 0.1, 1, 4 and 8 million. The tribopairs of steel/ steel and BO/ steel formed cracks on the balls after 0.1 million cycles. As the test progressed, the number of cracks significantly increased. After 4 million cycles, small pits associated with these cracks became apparent. After 8 million cycles, these micropits covered large areas of the wear tracks. By contrast, cracks and micropits were not observed on the balls of the tribopairs of steel/ BO and BO/ BO at any time throughout the 8 million cycle tests. These results confirm that a BO coating on the rough counterface prevents micropitting. 304 23rd International Colloquium Tribology - January 2022 Influence of Black Oxide Coating on Micropitting and ZDDP Tribofilm Formation Figure 1: Optical micrographs of wear tracks on balls at different number of loading cycles from mcropitting tests with steel-steel, steel/ BO, BO/ steel and BO/ BO tribopairs Fig. 2 shows the corresponding depth of the rubbed ball tracks measured using a stylus profilometer. The tribopairs of steel/ steel and steel/ BO gave low levels of material loss after each cycle. By contrast, the triopairs of BO/ steel and BO/ BO generated high amount of material loss from the BO-coated balls after 1000 cycle. This material loss occurred mainly in the first 1000 cycles and after that, the rate of loss of material from the balls of BO/ steel and BO/ BO contacts was similar to that of steel/ steel and steel/ BO contacts respectively. Considering that BO coating has approximately 1 µm thickness, this suggests that most BO coating on the balls was removed in the first 1000 cycles. This implies that since blue-coloured ZDDP tribofilms were observed in all tribopairs (Fig. 1), ZDDP tribofilms grew on the steel substrates after removal of BO coating. Figure 2: Measured depth of ball wear track of the tribopairs with BO coating 3.2 Evolution of surface roughness of counterface discs Fig. 3 shows the evolution of the disc roughness during tests. In all cases the disc roughness reduced in the initial stages of the test and then remained relatively stable - behaviour indicative of running-in. The tribopairs of steel/ steel and BO/ steel gave disc roughness reduction from the initial Ra value of 0.44 µm to approximately 0.30 µm after 1000 cycles and staying constant at this level for the rest of the test. By contrast, the tribopairs of steel/ BO and BO/ BO reduced disc roughness from 0.42 µm to approximately 0.18 µm after the first 1000 cycles, and then reached approximately 0.13 µm at the end of the tests. This result shows that steel discs coated with BO experience a very considerable roughness reduction during the first 1000 cycles. Figure 3: Disc roughness evolution of the tribopairs with BO coating 3.3 Friction behaviour Fig. 4 shows the evolution of friction coefficient in tests conducted with the four tribopair combinations. The friction coefficient obtained from the tribopairs of steel/ steel and BO/ steel was initially ca. 0.1, and gradually decreased to reach 0.08 at the end of the test. By contrast, the friction coefficient obtained from steel/ BO and BO/ BO contacts immediately decreased to approximately 0.05 after the start of tests, and then stabilized at this value through to the end of the tests. Since high surface roughness generates high friction in mixed lubrication conditions, this result suggests that when BO coating was applied to rough countersurface steel discs, their roughness decreased almost immediately after the tests started, resulting in an increase in lambda ratio and thus a significant drop in friction coefficient. 23rd International Colloquium Tribology - January 2022 305 Influence of Black Oxide Coating on Micropitting and ZDDP Tribofilm Formation Figure 4: Friction behaviour in tests of the tribopairs with BO coating 4. Conclusion This study provides new understanding of the impact of BO reaction coatings on micropitting and suggests the relevant mechanisms by which BO coatings mitigate micropitting. Key conclusions are as follows. This study has been published in 2021 [3]. • Micropitting of balls is completely prevented by applying a BO coating to rough counterface steel discs. • BO coatings on rough steel discs are very rapidly removed from the surface asperities shortly after the onset of rubbing, and this results in an almost immediate and very significant reduction of disc surface roughness. This reduction in roughness produces an overall reduction in friction coefficient. • Because BO coating is removed from the asperities at the beginning of the tests, a ZDDP tribofilm then forms mainly on the steel surfaces. This tribofilm suppresses further surface roughness reduction of BO-coated discs. • The immediate reduction in roughness of the rough counterface removes the high asperity stresses that would otherwise initiate and propagate the microcracks needed to produce micropitting. • The key to this behaviour is the low hardness of BO coating, which is only one quarter that of the steel substrate. Hence, it is easily worn during rubbing to reduce surface roughness. Reference [1] Brizmer, V., et al. (2017). Tribology Transactions, 60(3), 557-574. [2] Ueda, M., et al. (2019). Tribology International, 138, 342-352. [3] Ueda, M., et al. (2021). Tribology Transactions, 1-21.