# Transient Nanoscale Tribofilm Growth: Analytical Prediction and Measurement
## 2 Experimental Methodology  
The current investigation uses a Group III high viscosity base oil with a 0.5 wt% concentration (approximately equating to 500 ppm) of MoDTC. The concentration MoDTC used here guards against interruption in tribofilm growth on account of a limited supply of the additive. MoDTC is a typical engine oil additive for generation of low friction MoS\({}_{2}\) films and provides anti-oxidation properties [33]. The MoS\({}_{2}\) films are generally produced in tandem with ZDDP films for combined wear protection and low frictional performance where direct boundary interactions occur [34]. The base oil used here has been investigated in detail for its shear characteristics as well as for boundary interactions of this additive at microscale tribometry, using a s range of engineering surface materials [35]. The investigation considered the interaction of a reciprocating sliding tribometer, intended to replicate a piston ring to cylinder liner conjunction, with different lubricant-material combinations. These included Nickle-Silicon-carbide, Borided Steel and EN14 steel (for the sliding strip). These materials were tested for frictional losses with the use of base oil containing only a single friction modifier species. The base oil, without friction modifier, was also investigated in isolation using LFM to determine the Eyring shear stress of the lubricant. The substrates used in the current investigation are super-finished EN31 steel specimens with isotropic topography and roughness of approximately 0.05 \(\upmu\)m (Sa). These plates were washed in an ultrasonic bath with petroleum ether (40-60) for a period of 10 min and air dried prior to experimentation. To determine a base line coefficient of friction for the test specimens, friction was measured in the absence of any lubricant for a range of AFM probe tip contact loads. Bruker DNP-10 twin-arm silicon nitride cantilever with a nominal tip radius of 20 nm was used for these measurements. In order to characterise the cantilever a \"blind\" calibration technique was employed in line with that proposed by [PERSON] et al. [36] using a TGF 11 monocrystalline silicon grating. Following the cantilever calibration, the tip geometry was determined by taking a topographical measurement of a TGF 1 silicon wafer surface with calibrated peaks. Off-line deconvolution of the topographical data enables the determination of tip geometry. The measured tip radii were within 1 nm of those specified by the manufacturer, when averaged over 20 peaks from the calibration sample's scanned area. Measurements were taken from 3 areas of all the EN31 steel samples, with a new tip calibrated and used for each test. The steel sample was a 50 mm \(\times\) 20 mm \(\times\) 5 mm plate with two counter-sunk mounting holes. A range of loads were applied to the sample and a 1 \(\times\) 2 \(\upmu\)m\({}^{2}\) area scanned (2 \(\upmu\)m in the fast scan direction) at a sliding speed of 2 \(\upmu\)m/s, whilst simultaneously recording the applied load, friction and the height of the cantilever. The results are shown in Figure 1, indicating a coefficient of friction of 0.27 for the nominally dry tested surface. Figure 1 shows very good agreement between the results of the three tests, indicating that the procedure and sample surface topography are both repeatable and precisely controlled. The coefficient of friction is found as the gradient of the line and is in good agreement with similar tests on EN14 steel [35; 37]. The contact is taken as nominally dry since the tests were conducted in a controlled atmosphere with 50 \(\pm\) 2% relative humidity resulting in a slight offset to the intercept, in agreement with water-surface condensation and meniscus growth models [38; 39]. This is assumed to produce a constant offset to the friction trace, providing a small positive intercept to the graph in Figure 1, but insufficient to cause any hydrodynamic lift at the slow sliding speeds investigated.