Effect of slide to roll ratio on the micropitting behaviour in rolling-sliding contacts lubricated with ZDDP-containing lubricants
Introduction
Micropitting behaviour is a common phenomenon in rolling-sliding tribological systems. There has been a lot of research done on the impact of surface roughness, load, temperature, hardness and slide to roll ratio on micropitting [[1], [2], [3], [4], [5], [6]]. The asperity contact stress between contact surfaces is one of the main reason to promote micropits [1,2]. It is therefore generally considered that micropits can be prevented or lessened by reducing the roughness of the surface or increasing the lambda ratio (through increasing lubricant film thickness or reducing surface roughness). Moreover, Olver and his colleagues [2,6] defined micropitting as rolling contact fatigue caused by roughness. However, the benefit of reducing roughness could be affected by the chemistry of different lubricants [6]. Olver and his colleagues [2,6] found that the micropitting level was lessened by increasing the slide to roll ratio (SRR) but the wear rate did not change significantly.
Additive chemistry has also been proved to affect the micropitting level of the contacting surfaces. Antiwear additives like ZDDP could promote the development of micropits because the additive prevents the removal of the initial roughness of the surface by wear in the running-in period [[3], [4], [5], [6]]. Friction modifiers such as molybdenum dialkyl-dithiocarbamate (MoDTC) can prevent the formation of micropits as a result of the reduction of friction coefficient which then reduces the local tensile stress [7].
Although the effect of SRR and additive chemistry on micropitting has been extensively studied separately, the combined effect of interactions between SRR and additive chemistry on microptting has scarcely been studied. In the current study, the aim is to explore the effect of SRR on micropitting behaviour in the rolling-sliding process when using ZDDP antiwear(AW) additive.
Section snippets
Experimental
The tribological tests using two different oils containing ZDDP additive were firstly done in a micropitting rig to study the friction, wear and micropitting performance. The tested specimen after tribological tests was examined by surface analysis technique: an optical microscope and SEM were used to study the micropitting behavior of the surface, and XPS was applied to study the related tribochemistry behaviour on the surface.
Tribological results
Friction and wear results under 0.5%, 2% and 5% SRR are shown in Fig. 3. Optical microscope and SEM images of rollers after tribological tests are shown in Fig. 4 and Fig. 5, respectively. The colours observed on the optical microscope images (Fig. 4) are due to the presence of transparent surface films of oxides, and tribofilm traces of lubricant or solvent [7].
Fig. 3 shows that increasing SRR from 2 to 5% results in increased wear and slight reduction of friction. Ester based oil always
Conclusions
In this paper, the combined effect of SRR and lubricant chemistry on micropitting has been studied. The main conclusions are:
- (1)
The wear on roller increased with higher SRR, while friction did not change much.
- (2)
Less micropits formed on the roller surface with higher SRR. This can be related to higher wear as micropits can be wiped off from the roller surface due to wear.
- (3)
Micropitting formation and wear were affected by the polarity of base oil. Polar Ester oil can compete with ZDDP additive which
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2020, WearCitation Excerpt :Al-Mayali et al. [12] performed series of twin disc tests under the mixed-lubrication regime to study the micropitting initiation, and it was found that initial micro pits form nearly at one million cycles of the slower disc. The effect of slide-to-roll ratio (SRR) on the incidence of micropitting has been investigated by Cen et al. [13]. They observed lesser micropitting damage at higher SRR due to wiped off of asperities at higher SRR.