A semi-deterministic model for MoS₂ formation and removal from MoDTC-containing oils in boundary lubrication

Dichu Xu^1,2, Cayetano Espejo¹, Chun Wang¹, Jiugen Wang², Anne Neville¹, Ardian Morina¹

¹ Institute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK

² College of Mechanical Engineering, Zhejiang University, Hangzhou, 310027, Zheijiang, China

INTRODUCTION: One of the main targets in engine oil development is to increase fuel efficiency by means of reducing friction losses in tribological contacts. In order to minimize friction losses in this regime, a series of additives namely friction modifiers are used. Among these additives molybdenum containing additives, MoDTC shows an excellent performance by forming an efficient low friction tribofilm in boundary lubrication regime.

In this study, several techniques featuring Raman microscopy are used to assess both formation of MoS2 and other molybdenum species during the build-up of the tribofilm from MoDTC and depletion kinetics of the low friction tribofilm when MoDTC is removed from the oil. By means of analytical techniques, distribution and quantity of MoS2 friction-reducing platelets is determined as a function of time and tribological contact conditions.

Supported by experimental data, a deterministic model is proposed to predict the kinetics of tribofilm formation and depletion and the friction performance as a function of temperature. 

METHODS: Tribotests were conducted using a ball-on-disc tribometer under the boundary lubricated condition. Table 1 lists the detailed information on the materials and test conditions.

Table 1: Conditions for MoS₂ tribofilm growth tests in the boundary lubrication regime

Tribotests were performed as a function of time (from 2.5 min to 30 min) at 80 °C and 120 °C, respectively. The wear scars of discs were then analysed with the Raman spectroscopy, in order to understand the growth of MoS₂ tribofilm during the transient process. To investigate the transient process of MoS₂ tribofilm removal, tests were performed in which the MoDTC/ZDDP tribofilm was formed for 1h using the basic oil containing 1%wt ZDDP and 0.5%wt MoDTC. After that, the tribometer was halted to drain the oil and replace it by the ZDDP-containing oil before resuming the tribotests. Raman analyses were then done in-situ at different increments of sliding time.

Figure 1 - Raman map of MoS₂ tribofilm (A_1g  frequency mode) on the sample Disks wear scar during 2.5, 15, 30 min rubbing time of (a-c) 80^oC, (i-iii) 120°C.

Figure 2 - Normalized Raman single spectrums as a function of removal time at (a) 80 °C and (b) 120 °C

The depletion of MoS₂ tribofilm was shown to fit well to an exponential function expressed

                              (1)

where C₂ denotes the removal rate of MoS₂ layers with the values of 0.3 s^-1 and 0.4 s^-1 at 80 °C and 120 °C, respectively.

DISCUSSION: The transient shear stress and surface temperatures can be established on the MoS₂ tribofilm distribution from Raman maps (Figure 1) as well as the elasto-plastic contact model and surface temperature model. Subsequently, based on the MoS₂ tribofilm removal rate yielded from the experiments, the MoS₂ tribofilm formation rate can be determined. The MoS₂ tribofilm formation rate at 80 °C and 120 °C were then calculated.