Mitigation of False Brinelling in a Roller Bearing: A Case Study of Four Types of Greases

Four commercially available greases were used to investigate their performance for mitigating false brinelling wear in a roller bearing. Table 1 summarizes the main properties of the greases. All greases have similar density between 0.88 and 0.9 g/cm³. In this study, four types of new and unworked greases were used in the experiments, as this replicates the starting condition in which bearing assemblies would leave their place of manufacture. During the false brinelling, base oil can be released from the grease under the vibrations or small oscillations, that is to say, the grease is worked, as it would be during the transport of bearing assemblies by rail/sea.

The custom false brinelling simulator test rig, illustrated in Fig. 1a, was designed and fabricated for the authors’ previous work [12]. In the current work, the test rig was configured such that the tested cylinder roller bearing experienced lateral vibration while radial load was applied (Fig. 1b). These testing parameters are the main causes of false brinelling wear during sea and rail transportation found in authors’ work [12]. The shaft was press fit into the inner race of a FAG NU1018 cylindrical roller bearing prior to the test. To investigate the impact of grease on mitigating false brinelling damage, cylinder roller bearings were tested under dry and grease lubricated conditions. Figure 1a illustrates the dry test was performed solely. For the grease lubricated conditions, as shown in Fig. 1a, a fresh and unused grease was applied on to the raceway. It is noted that two grease lubricated bearings, placed 90 mm apart, were able to perform the tests at the same time.

The allocation of test rig experiments is summarized in Table 2. All tested greases were worked for 8 days and at the frequency of 24 Hz. These parameters were chosen based on field data, recorded during rail/sea transportation for roller bearings in the authors’ other work [5, 12].

To further understand how different greases mitigate false brinelling damage, pin-on-cylinder reciprocating wear tests were performed, which enabled us to conduct investigations under more controlled conditions. That is to say, the reciprocating wear test is representative of a single contact between a rolling element and the raceway in a roller bearing; allowing friction and wear coefficient of different grease lubricated contacts to be measured, which can be challenging when using full bearing assemblies in the false brinelling test rig.

After completion of false brinelling test rig experiments, the inner races were rinsed to remove grease samples. Surface topography of the wear mark produced by dry and lubricated conditions were obtained with Taylor Hobson Talysurf i5 surface profiler. The evaluation of false brinelling wear under dry and grease lubricated conditions was achieved by measuring the maximum wear depth from the cross-section of each wear mark profile. For each testing condition, three scans were performed cross the wear mark to identify the maximum wear depth.

The experiments were carried out using a tribometer (UMT, Bruker, USA). The tribometer was adapted to be a pin-on-cylinder set-up as shown in Fig. 2. In the tests, a ping with a curved surface of 39.38° was subjected to a normal load while reciprocating against the flat surface of a stationary cylinder. A small amount of fresh and unused grease was applied onto the estimated contact region of the friction pairs prior to the test. The tested cylinders and the pins were made of 52,100 high carbon bearing quality steel with the hardness is similar to the railway bearings [5, 12]. The diameter and width of the cylinder is 34.989 mm and 8.730 mm, respectively. The part of the pin in contact with the bearing had a diameter of 4 mm.

Table 3 specifies the experimental conditions for each type of grease, and each test was repeated two times. The amplitude of the sliding is 250 µm. Each test was conducted for a fixed number of 200,000 cycles so that all experienced the same sliding distance. This means tests performed under low frequency lasted longer than those at the high frequency. Overall, greases were worked between 7 and 27 h. The oscillating velocity is from 29.9 to 119.9 mm/s depending on the frequencies. The applied normal loads were 4 kg and 10 kg, equivalent to 340 MPa and 700 MPa, respectively. Greases were tested under four different frequencies and two different pressures at ambient temperature. Two extra sets of reciprocating wear tests were performed on ZDDP-containing greases, PU/PFPE + E and LiC/Syn + M as Shown in Table 3. For each grease type, the tests were carried out with a normal pressure of 700 MPa, 4 Hz and 200,000 cycles at temperatures of 45 °C and 85 °C, respectively.

It should be noted that the vibration direction subjected to bearings in false brinelling test rig experiments is different from the sliding direction in the reciprocating wear tests, in addition, a higher frequency of 24 Hz was used in the false brinelling experiments. The parameters used in the false brinelling test rig were set accordingly to simulate false brinelling occurring during rail/sea transportation [5, 12]. However, due to the limitations of the tribometer, both the sliding direction and frequencies used in the reciprocating wear tests had be adjusted. Hence, the comparison between two tests cannot be made directly. As mentioned earlier, the reciprocating wear test was used as a supplementary test, which aimed to evaluate the tribological properties of four greases, under controlled laboratory conditions, so that the mitigation of false brinelling under different types of greases can be better understood.

During the tests, the vertical and lateral forces were recorded by the instrument, allowing for the COF to be determined. After completion of each reciprocating wear test, the mean COF of each grease type was calculated when the COF value reached a steady state (between 50,000 and 150,000 cycles). Tested grease samples were collected and tested cylinders were rinsed with n-hexane to remove supernatant grease. Wear marks on the cylinders were located by optical microscopy. SEM/EDS (JEOL 7001, JEOL, Japan) was used to examine the (1) topography of the worn surfaces and (2) the composition of the worn surfaces produced through the four greases. For PU/PFPE + E and LiC/Syn + M, XPS analysis was carried out on the worn surface if the chemical compositions associated with ZDDP-induced tribofilm were identified by SEM/EDS. Therefore, possible microscopic mechanisms of the formation of the tribofilm under low oscillations can be confirmed. A XPS survey spectrum was acquired first, followed by a narrow scan to determine the relative atomic compositions of tribofilms. The charge of the XPS specimen was corrected by carbon binding energy to 284.8 eV. The microstructure of each grease type was imaged using SEM before/after the reciprocating tests. This is to examine potential structural change in thickener in relation to the small oscillations. The sample preparation of grease samples and set-up for SEM examination can be found in the authors’ other work [26].

The ability of oil to be released, or bleed, from grease and enter the contact is important for rolling bearing applications [27]. The base oil released from thickener under fretting wear process can be different from the oil bleed from the static grease (indicated as oil separation by grease manufactures) [19]. This is because the oil separation is measured by putting grease in a cone sieve and the relative amounts of oil which are bled out due to gravity, as opposite to bleeding out under small-amplitude oscillation for false brinelling. Hence, oil bleed during false brinelling is considered as ‘dynamic bleed’ in this study. The ‘dynamic bleed’ was measured and characterized by centrifuge method [26, 28‐30]. Approximately 0.5 g of grease was placed in a filter centrifuge tube, the refrigerated centrifuge was then spun at centrifugal force of 10,000×g at a constant temperature of 35 °C. The base oil recovered from the grease was weighed every 5 h. The weight fraction of base oil was then estimated by weight of the bled oil divided by that of the sampled grease.

False brinelling test rig experiments were performed under dry and grease lubrication conditions. The wear marks on the raceway were inspected and their maximum wear depths were estimated for assessment of false brinelling wear. Figure 3 shows the representative surface topography of observed wear marks, their corresponding cross-sections as well as the measured maximum wear depths. Overall, longer and wider wear marks were produced under unlubricated conditions (Fig. 3a). The average width of the wear marks decreased 80% (0.5 mm for dry and ~ 0.1 mm for grease conditions) in lubricated conditions. It is clear from Fig. 3b that the maximum depth of wear profiles reduced significantly by ~ 92% in Li/M and 97% in Li/Syn compared to the dry condition. No wear mark was observed when the tests were performed using PU/PFPE + E and LiC/Syn + M.

To understand the mitigation of false brinelling under four different greases found in the false brinelling test rig experiments, reciprocating wear tests were performed. A series of different reciprocation frequencies and two normal pressures were applied with all grease lubricated contacts.

Two ZDDP-containing greases (LiC/Syn + M and PU/PFPF + E) were found to be superior for mitigating false brinelling wear. As shown in Fig. 3, no wear mark was visually observed on the raceways. The mechanism of mitigating false brinelling wear during sea/rail transport is due to the formation of a tribofilm by ZDDP on the fretting wear contacts. EDS and XPS analysis confirmed that a small oscillation frequency (4–8 Hz), low pressure (< 1 GPa) and ambient temperature can facilitate the ZDDP tribofilm growth on the worn surfaces. From Table 1 and Fig. 10, it is important to note that both greases contain higher viscous base oil compared to Li/M and Li/Syn, and PU/PFPE + E demonstrated the worst ‘dynamic bleed’ among all tested greases (shown in Fig. 10). It can be inferred that oil bleed behaviour and oil viscosity do not, at least, directly influence the performance of false brinelling wear resistances in ZDDP-containing greases.

In oil lubricated systems, it has been discovered that heat generated by sliding can stimulate the formation of ZDDP films [19, 25], but this has not previously been investigated in grease. Here, heat is referred to the localized and transient temperature rise on the rubbing surfaces and not due to the testing/environment conditions. Accordingly, heat can also be one of the drivers for promoting the tribofilm formation under grease lubricated conditions. Under the tested conditions in this study, it is likely the grease lubricated tests were operating under boundary lubrication regimes [39]. As a result, heat generated at asperity contact would occur and the resulting local shear stress and heat promoted ZDDP tribofilm formation.

It is well-document that ZDDP films can form with a combination of increased temperature (flash temperature) and shear of the lubrication film in oil lubricated system [8, 25, 40]. These factors are described as ‘mechanochemical reactions’, which drive the ZDDP tribofilm formation under the rubbing surface at low environmental temperatures 25 °C to 85 °C [25]. Furthermore, in these oil lubricated systems, the ZDDP tribofilm forms even if there is no asperity contact due to EHL (Elastohydrodynamic Lubrication) regime, on the proviso that the shear stress is sufficiently high [40]. However, the shear of the lubrication film is unlikely to be the contributing factors for the ZDDP tribofilm formation in the current result. This is because the ZDDP-containing greases were tested at the lower range of parameters compared to other grease studies (i.e., frequency > 20 Hz, pressure > 1GPa) [8, 17, 23]. As a result, low frequencies (low sliding speeds, below 0.1 m/s for 2–6 Hz) and the shear stress within the contacts are considered very low (due to low contact pressure). The mechanical driver of tribofilm formation would have occurred as the grease was pushed out of the fretting contact regions during small sliding oscillatory movements; meaning that the contact is most likely operating under a boundary lubrication regime. As a result, asperity contact would occur and the resulting local shear stress and heat promoted ZDDP tribofilm formation.

As XPS analysis clearly shows that a layer of Zn, Fe and S were formed on the top of the worn surfaces. As such, the combination of a high intensity oxygen, with localised iron/zinc sulphide, iron-induced tribofilm has been seen in other studies [31, 34, 36, 41, 42]. Hence, the chemical driver of the ZDDP tribofilm formation can be explained by the mechanism proposed by Ito et al. [42], where the composition of the tribofilm can vary depending on the oxidation state of the initial contact area between the frictional pairs. That is to say, ZDDP molecules start to decompose in the event of sliding, resulting in a layer of zinc-rich, sulfur-free adsorption layer (i.e., FeS₂ and ZnS), where the initial tribofilm formation occurs on the iron oxide. The formation of polythio-phosphate may occur later in the reaction sequence once the temperature in the contact region rises, activating this reaction.

EDS results (Fig. 6b, c) showed the peak and intensity variations in tribofilm-related elements per selected areas. One explanation is that the thickness of the tribofilm in some selected areas are much less than the interaction volume of electron beam and the tested material (~ 2 µm). If that is the case, the relevant tribofilm peaks generated under tested conditions may be present but fall below the detection limit of the SEM/EDS techniques. Under the same principle, the quantitative comparison of the film composition between different grease conditions was not possible. Nevertheless, EDS analysis was able to confirm the growth of the tribofilm formation under the designated conditions. ZDDP-induced tribofilm under the studied conditions is rather patchy on the worn surfaces in this study. The present finding is different from the observations in oil lubrications, where ZDDP reaction layers grow quickly and easily on the rubbing surface, in as little time as 75 min and around 60 µm thick [43, 44]. Additionally, a general observation of tribofilm formation under oil lubricants in the EHL regime occurs when a combination of temperature increase and shear of the lubricant takes place [8, 23, 25].

On the other hand, grease has more complicated lubrication regimes compared to oil due to the presence of thickener in grease. As a result, the fully flooded contact and the replenishment condition are not as straightforward as has been observed in oil [27, 45, 46]. When considering false brinelling, the grease lubricated bearings tend to operate in the starved EHL regime, where there is a limited supply of lubricant to the contact due to greases being squeezed out of the contact zones under small-amplitude oscillations or vibrations [4, 6, 10, 11]. Consequently, the development of ZDDP tribofilm on the worn surface can be slow and uneven. Noticeably, with the less intensive parameters applied in the current work (i.e., low frequency, normal pressure, sliding velocity), thus the formation of tribofilm seems less evident than those reported in the literature. With respect to the false brinelling test rig experiments, an increase in the local temperature due to a higher frequency (24 Hz), higher pressure and longer testing durations seems reasonable, suggesting the relative thick and evenly distributed tribofilm can certainly form on the contacting surfaces, therefore, no distinguishing wear marks were found in the case of PU/PFPE + E and LiC/Syn + M.

The reciprocating wear tests indicated LiC/Syn + M imparted better wear resistance than PU/PFPE + E due to the lowest measured COF (< 0.05) under the studied conditions. The difference in COF measurements can be attributed to thickener types and the polarity of base oils present in the greases, leading to different tribological performance. As reported by Suarez et al. [22, 47], polar base oil (i.e., PFPE + E) impinges on ZDDP molecules accessing the contact surface of the steel, leading to lower adsorption rates than ZDDP blended in a non-polar (or less polar oil, i.e., Syn + M). This results in a thinner layer of tribofilm growth and influencing tribological performance. Suarez et al. [22, 47] also found that the ZDDP-derived tribofilm grew slowly when blending in the polar base oil than non-polar base oil. Furthermore, the present types of thickeners in the greases adds the complexity of ZDDP-induced tribofilm formation on the steel surfaces due to the competitive interactions between ZDDP-thickener and the thickener-steel surface. For example, it was reported that adding ZDDP either in Li grease or polypropylene grease resulted in better performance than in LiC grease under the tested conditions [46, 48, 49]. Considering that the volume of the base oil is generally up to 70% and given that the remainder is thickener and less than 5% additives, the polarity of base oil would have stronger impact on the formation of ZDDP tribofilm, therefore, influencing the variation in tribological performance (shown in COF measurements) of the two greases as seen in reciprocating wear tests.

Compared to PU/PFPE + E and LiC/Syn + M, shallower wear depths were found under Li/Syn and Li/M lubricated conditions after false brinelling tests. The performance of false brinelling reduction observed in Li/Syn and Li/M is associated with the consistency of the grease, thickener structure as well as the polarity of the base oil. From Fig. 3, 60% wear depth reduction was found in Li/Syn compared to Li/M. One explanation is the structure of Li/Syn is less rigid and the thickener loosely connected (NLGI = 1) compared to Li/M (NLGI = 3). As shown in Fig. 9c, the degradation of thickener in Li/Syn was observed after low frequency reciprocating wear tests at 8 Hz, 340 MPa. Hence, the structural integrity of Li/Syn thickener degrades easily, leading to potential accumulation of oil and potential thickener into the fretting contact areas [17]. The oil bleed seems to be one of the contributing factors, as more oil was bled out from Li/Syn (0.5%) compared to Li/M (0.15%) in the first 5 h (Fig. 10). It is known that the polar base oil molecules has higher affinity to adhere to the steel surface compared to non-polar oil. Therefore, the ability of synthetic oil in Li/Syn reacts with the steel metal to form the lubrication film is considerably easier than mineral oil in Li/M. The above results suggested that Li/Syn has a more abundant and less viscous oil, which bleeds readily from the more opening structured thickener compared to Li/M. These variables would have contributed synergistically to reduce the false brinelling wear depth.

It is noted that, for Li/M and Li/ Syn, the mechanism preventing the fretting wear are different to Maruyama et al. [17], where high oil separation and viscosity does not contribute to fretting wear reduction. Instead, it was a layer of residual thickener found in the fretting contact that effectively prevented fretting wear. Nogi et al. [39] theoretically indicated that the concentration of thickener increases surrounding EHD contact. The different findings between above research and the current work could be due to the types of thickeners (thickener geometry) and base oil (polarity and viscosity); polyurea as used in the study by Maruyama et al. [17] is generally platelets/spherical shapes [30], whereas the lithium thickener in the present work is fibrous. In Nogi et al. work [39], grease with high polarity and low viscosity was used to predict the grease film thickness. As suggested by Cyriac et al. [30] and Saatchi [14], the geometry of thickener and the ratio of thickener volume fraction to oil, impact on how thickener transport, structural squeezing and thickener rotation through the tribological contact areas occurs.

From Fig. 4b, the result indicated ZDDP-formed tribofilm offers superiors friction and wear resistance than EP-formed sulfide layer as evidenced by lower COF values of PU/PFPE + E and LiC/Syn + M. Furthermore, no false brinelling wear was observed in the raceways for ZDDP-containing greases but EP-containing grease (Li/M). One explanation can be the ZDDP-formed film is more resistant to wear than EP-formed film, which was demonstrated by Hao et al. through the scratching tests on the thermal-induced films (i.e., 600 °C) by both ZDDP and EP on high-speed steel material [50]. In this study, a more brittle oxide iron layer (Fe₂O₃) was found on the EP-induced film, which was absent in ZDDP-induced film under the studied conditions. Future study could include a greater variety of grease formula, such as a grease with same base oil and same type of thickener, but pairing with different additives (i.e., ZDDP versus EP), so the effect of additives on false brinelling wear mitigation under grease lubricated conditions can be clearer.

In Sect. 2.6, the ‘dynamic bleed’ was introduced based on the assumption that, in the tribological contact, oil release mechanism under small oscillations or vibrations (during transportation) which would be different from the standard bleed rate test, refers as ‘static bleed’. Interestingly, PU/PFPE + E has the worst ‘dynamic oil bleed’ compared to the other greases (Fig. 10), whereas the ‘static oil bleed’ data indicated Li/M has the worse oil bleed (oil separation = 0.6 in Table 1). The difference observed in’ static’ and ‘dynamic’ oil bleed would be due to the geometry of the thickener types as well as the consistency of the greases (amount of base oil). There is less PU thickener (indicated by NLGI = 2, lower consistency) than Li thickener in the grease (NLGI = 3, higher consistency). That means that the degree of dilution for PU thickener is greater in PU/PFPE + E than the Li thickener in Li/M and yet the plate/particle-like PU and short fibres structure (Fig. 9c) do not impede the oil released from the grease in the ‘static bleed’ test. In ‘dynamic bleed’, viscosity of the oil and the geometry of thickener are contributing factors to the lower oil bleed found in PU/PFPF + E (Fig. 10). That said, PU/PFPE + E contains a base oil with higher viscosity and the plate/particle-like PU structure could readily move forming a more closely packed structure under centrifugal force, hence, affecting the flow of oil in the effective thickener structure. It is interesting to discover that although PU/PFPE + E shows the slowest oil bleed in the dynamic condition (Fig. 10), the grease can reduce the false brinelling wear as visualised on the raceway (Fig. 3b). This finding also highlights the effectiveness of ZDDP on false brinelling wear resistance.

Our results show that ZDDP, as one of most commonly used antiwar agents, mitigates false brinelling of roller bearings occurring rail/sea transportation. To broaden the scope of this research, future work can include grease with other AW agents, which will help identify the optimal grease formula to prevent false brinelling of roller bearings. Finally, it is noted, that from this study there are multiple variables that affect the formation of a tribofilm in cases of false brinelling. Whilst this study confirms that the presence of a tribofilm as a major contributor to the mitigation of false brinelling damage, an in-depth investigation into the tribofilm formation mechanisms as well as the thickness of the tribofilm, were beyond the scope of this study. Hence, future investigations on tribofilm formation under formation under false brinelling conditions is warranted.