# Transient Nanoscale Tribofilm Growth: Analytical Prediction and Measurement
## 3 Experimental Results  
The results of three separate tests are provided in Figures 2 and 3, showing the cumulative tribofilm height relative to the initial scan and the evolution of coefficient of friction with time.  
Figures 2-4 show several distinct trends. Firstly, there is a clear initial drop in friction relative to the nominally dry contact measurement which is attributed to the reduction in adhesion through wetting of the contact under partial (mixed) regime of lubrication in line with that proposed by [PERSON] [40], [PERSON] [41] and [PERSON] [42]. Secondly, following this initial drop relative to dry friction, there is a significant, but unstable further reduction in friction over the first 45 min of scanning. This coincides with an unstable film growth rate. Referring to Figures 2 and 3, for this region of variations an unstable film growth can be observed for tests 1 and 3, but not for test 2 where a steady reduction in friction with a steady film growth is observed. In the case of tests 1 and 3, an increase in friction is observed prior to a stable coefficient of friction being reached after 120 min of scanning. Test 2 displays a slower frictional response rate, also reaching a stable coefficient of friction after approximately 120 min.  
Figure 2: Measured surface height data from three scanned areas on the EN31 steel sample with the base oil containing MoDTC.  
An overlay of the mean measured coefficient of friction and the mean surface height for the three scans can be seen in Figure 4.  
Contrary to the evolving frictional characteristics, there is almost negligible change in the tribofilm height for the initial 30 min scanning duration, followed by an unsteady film growth for the next 30 min interval for tests 1 and 3, whilst in test 2 a slower, but smoother change in the growth of film height is observed. Finally, for all tests, the film growth stabilises after an hour and appears to plateau near the completion of test duration. The film growth in Figure 2, between approximately 120 and 300 min, appears to follow a near linear growth rate which is in agreement with the shear-promoted thermal activation model as the contact conditions and coefficient of friction remain almost unaltered. However, it is essential to modify the shear-induced thermal activation model in order to account for the reduced growth rate nearing the end of these tests and take into account the reduced film formation rate observed at the beginning of the tests, despite relatively high friction.  
Figure 4: Mean coefficient of friction and mean surface height data from three scanned areas on the EN31 steel sample with base oil containing MoDTC.  
Figure 3: Measured coefficient of friction data from three scanned areas on the EN31 steel sample with base oil containing MoDTC.  
Several studies have considered the wear of AFM tips through either scanning electron microscopy [43; 44], or measurement of calibration samples [45] or even by measuring a nominally worn tip using a second fresh tip [29]. These techniques are not possible in the current study due to the application of the lubricant to the sample surface and the fragility of the components. The tip wear reported elsewhere has been rather slight and it is unclear what effects the lubricant and the formed tribofilm would have on the tip wear rate. Therefore, the tip wear is neglected in the current study.