Film Thickness and Friction of ZDDP Tribofilms

ZDDP films were formed and their film thickness and frictional properties studied using a Mini Traction Machine with Spacer Layer Imaging (MTM–SLIM) [10, 35]. In the MTM a lubricated steel ball and disc are rubbed together in mixed sliding-rolling at controlled load and temperature while friction is monitored.

The Spacer Layer Imaging Method (SLIM) allows measurement of sub-micron thick tribofilms on rubbed steel balls in situ in the MTM. The principles of SLIM, which are based on optical interferometry, are shown in Fig. 1. During a measurement, rotation of the steel test ball is halted and it is uploaded against a coated glass mapper disc. White light is shone into the resulting contact where some is reflected by a semi-reflective coating on the underside of the glass while the remainder passes through a silica spacer layer and any antiwear film on the ball before being reflected back from the steel substrate. The two beams undergo optical interference depending on their optical path difference and thus on the antiwear film thickness. The result is an interference image as shown in Fig. 1, which is captured as a RGB pixel map by a high resolution camera. A calibration chart of RGB colour versus optical path difference obtained using ultrathin film interferometry is then used to convert this RGB pixel map to a ZDDP tribofilm thickness map [37]. A refractive index of 1.6 was assumed for the ZDDP tribofilm. The advantage of SLIM is that tribofilm thickness is measured in situ without rinsing or cooling the MTM ball or removing it from the test rig.

In the current study AISI 52100 steel balls and discs were used. These were cleaned ultrasonically in toluene and then Analar isopropanol for 10 min prior to use. Tests lasted 3 h and involved slow speed rubbing to build ZDDP tribofilms, with periodic interference image capture using SLIM and Stribeck friction curve measurement to monitor tribofilm build up and friction.

The test sequence is shown schematically in Fig. 2. An initial SLIM interference image was taken before any rubbing commenced to determine the spacer layer thickness. Each rubbing step then involved rubbing the ball and disc at 50% SRR and a low entrainment speed of 50 mm/s for a set time. This low entrainment speed produced a theoretical initial EHD film thickness of 5.5 nm and thus mixed lubrication conditions. After this slow speed rubbing the ball was uploaded against the glass disc and a SLIM interference image was captured. Finally a Stribeck friction curve was measured in which friction was measured over a range of entrainment speed at 50% SRR to complete the rubbing step. Figure 5 shows typical sequences of SLIM images while Fig. 9 shows series of Stribeck curves for three of the ZDDP solutions.

Test conditions are summarised in Table 3.

Scanning white light interference (SWLI) microscopy was first used successfully in tribology research by Tasan et al. to map wear on rubbed surfaces with high resolution [38]. Benedet et al. investigated the use of SWLI as a measurement technique for ZDDP antiwear film thickness and noted that internal reflections within the transparent tribofilm could lead to misleading measurements [39]. They showed that ZDDP tribofilms could, however, be made reflective using a thin layer of gold. This approach, coupled with the use of ethylenediamine tetraacetic acid (EDTA) solution to locally remove ZDDP film, enabled the thickness of film to be mapped using SWLI.

In the current study, after an MTM test, the ball and disc specimens were rinsed with heptane. The specimens were then dried and a drop of 0.05 M EDTA solution was placed on part of the wear track on both ball and disc and wiped off after 60 s. The aim was to check if any significant amount of wear was present underneath the antiwear film, and also to provide a reference plane on each side of the analysed area of interest [17]. Then the rubbed area of interest was coated with ca 25 nm of gold both to avoid spurious internal reflection within any antiwear film and to provide constant reflection properties inside and outside the rubbed track. An SWLI microscope (Wyko NT9100) was then employed in vertical scanning interferometry (VSI) mode to capture a topography image across the whole width of wear track as shown in Fig. 3. Height and roughness values of the film were obtained from the 50 × 50 µm² region close to the centre of the track and marked as a blue square in Fig. 3.

Atomic force microscopy (AFM) is arguably one of the most powerful techniques to study the topography [17, 24‐27] and also, as demonstrated by Gosvami et al., to monitor the formation of antiwear films [40].

MTM discs which had been previously cleaned for the purpose of SWLI measurement were used for AFM measurement, studying a region of the rubbed track that had not been coated with gold. For this study, a confocal AFM-Raman alpha300 RA produced by WiTec was employed. As for SWLI study, half of the wear track width was treated with EDTA solution to provide a reference plane close to the centre of the track as shown in Fig. 4. AFM topography images were then taken from a 150 × 150 mm2 region but, like SWLI, film thickness and roughness values were determined only from a 50 × 50 µm² area, as shown in Fig. 4.

To help understand the effect of ZDDP tribofilms on lambda ratio and thus friction, the elastohydrodynamic (EHD) film-forming properties of the test fluids in the absence of such films was required. EHD film thickness was measured using a PCS ultrathin film measurement system [41]. A 19.5 mm diameter steel ball on coated glass disc contact in nominally pure rolling was studied over a range of entrainment speeds at a load of 20 N and a temperature of 100 °C. In practice, the concentration levels of ZDDP used (ca 1% wt.) had negligible effect on fluid viscosity and film thickness so the data used in this paper are those of the base fluid blend.

Figure 10a shows an SWLI image of a gold-coated part of the rubbed track on an MTM disc obtained after a test with Z2. Cross-sections of the height across the rubbed track on a rubbed ball and disc after a 3 h rubbing test with Z2 are shown in Fig. 11b. One is from a non gold-coated part of the disc and suggests a negative height, i.e. wear. However the other two profiles are from the disc and ball after coating with a thin gold film. It can now be seen that there is a similar film on both surfaces, of thickness approximately 110 nm, with some regions reaching over 200 nm thickness. As shown by Benedet et al. [39], it is evident that the rubbed track has to be covered with a thin, reflective coating to avoid spurious SWLI measurements. From Fig. 10b it can also be seen that the antiwear film is thinner close to the edge of unrubbed surface. Initially the thickness of antiwear film was measured on both ball and disc. The aim of it was to check if the film was similar on both rubbed surfaces. In fact the thicknesses were very similar and further study was thus made only on the disc, which was preferred for comparison with the AFM technique since large areas could not be scanned reliably on the ball using AFM because its curvature.

Mean film thickness from analysed disc specimens was calculated based on averaging four 50 × 50 mm² areas around the wear track close to centre, as shown in Fig. 3. From each area, each pixel was analysed and results extracted as a histogram as shown in Fig. 11. Most of the film has thickness 30–200 nm. The few negative results in Fig. 11 probably represent individual pixels where the incorrect optical order was chosen by the SWLI software, combined with an imperfect zeroing of the baseline, rather than wear. The points below 0 nm were therefore excluded when calculating mean film thickness. It is also likely that an incorrect order may have led to the few very high values seen in Fig. 11. Values above 230 nm were therefore also excluded from calculation of mean film thickness although in practice since there are so few such points, they had only slight effect on the calculated mean. However, uncertainty concerning these points meant that it was difficult to estimate the maximum film thickness using SWLI.

AFM was used to scan several central regions around the disc and mean and maximum film thickness values were calculated from 50 × 50 µm² areas, as shown in Fig. 4. The values from different regions were then averaged, as shown in Fig. 12. For Z1 and Z2 the standard deviation was low (ca 6%) indicating that the antiwear films were evenly distributed along the wear track. Z3 formed thinner antiwear films with large variation in thickness between analysed regions ranging from 20 to 100 nm.

Figure 13 compares the mean film thicknesses measured using the three experimental techniques for all ZDDPs.

For the ZDDPs that formed stable, thick antiwear films, Z1, Z2 and ZM, it is evident that SLIM measures considerably thicker mean film thickness than AFM and SWLI. As discussed by Topolovec [17], this is believed originate from the fact that SLIM involves a coated glass disc being loaded against the tribofilm and measures the separation between glass disc and steel ball substrate rather than the actual tribofilm thickness. Because the ZDDP tribofilm consists of pads separated by deep, narrow valleys it is unlikely that the glass disc can fully conform to the tribofilm surface within the valleys, as illustrated schematically in Fig. 14. The non-conformed valley regions will thus augment the separation due to the tribofilm itself, making the latter’s thickness appear greater than it actually is and quite close to the maximum film thickness value. This observation is supported by the fact that the mean values obtained using SLIM and maximum film thickness of averaged areas using AFM were similar, as can be seen by comparing Figs. 12 and 13. Topolovec et al. also found that maximum film thickness of ZDDP films was similar when measured using SLIM and AFM but did not calculate the mean values [17].

In the case of Z3 the three methods give quite different film thickness measurements. This probably originates from the very patchy, uneven nature of the film at the end of the tests, which is evident from Fig. 7. This means that the measured film thickness was strongly dependent on the regions chosen for analysis. With SWLI and AFM several measurements were taken around the track and averaged but these showed considerable variations, while for SLIM only one measurement was made at the end of each test. For ZA, SLIM and SWLI give similar mean film thickness while for AFM the film is considerably thinner. Again this probably arises from large variations in film thickness across the rubbed track.

In general, SWLI gives a film thickness around 20 nm greater than AFM. This may be an optical effect in which the beam reflected from the coated surface of the antiwear film rebounds from the edges of antiwear film pads resulting in the seemingly longer path of the reflected beam. It is unlikely to represent the thickness of the gold film applied in SWLI since this also coats the reference surface outside the rubbed track. Figure 15 compares film thickness histograms of a Z2 tribofilm obtained using AFM and SWLI and shows that the SWLI distribution is quite different from that measured using AFM and that perhaps some of the valleys are filled with gold.

Based on the above it is clear that the three methods of measuring ZDDP films have different strengths and weaknesses. The very great advantage of SLIM is that it enables tribofilm formation to be monitored throughout a test, providing a series of thickness values as the test progresses. This enables film growth rate (and also removal rate) to be studied very easily and conveniently: to obtain similar information using AFM or SWLI a series of tests of different duration must be carried out, as described in Sect. 6.1. Unfortunately, as this study confirms, the actual mean film thickness values (and also the roughness values) obtained using SLIM are quite inaccurate – the trends seen are correct but the actual values of mean film thickness are overestimates, though they do approximate to maximum thickness values. SWLI can only be carried out at the end of a test since it requires a gold coating to be applied. It can be easily applied to both ball and disc surfaces and is experimentally straightforward but does appear to produce some artefacts where analysis predicts a negative and possible a falsely high film thickness. Gold coating also skews the height distribution, possibly by filling some of the valleys. AFM is also generally applied at the end of a test though it could in principle be used in fully immersed mode during tests as was done by Gosvami et al. [40]. It is quite difficult to apply to an MTM ball due to the latter’s curvature, but reasonably straightforward on the MTM disc. It can only analyse a relatively small area that may not span the whole track width, necessitating removal of part of the film to determine film thickness in the track centre. Based on this this study it is believed that measurements using AFM are the most reliable.

The film-forming behaviour found in this study using different types of ZDDP is broadly similar to previous studies, with aryl ZDDP forming thinner films more slowly than alkyl ones [13, 16, 17]. Previous work has generally found that secondary alkyl ZDDPs form films faster than primary ones. In the current study one secondary ZDDP, Z2, formed a film at about the same rate as the primary and mixed one while another secondary one, Z3, formed a film extremely rapidly. This is probably because, as shown in Table 2, Z3 consists almost entirely of secondary ZDDP molecules while Z2, although described by the supplier as secondary, actually consists of a 2:1 blend of secondary and primary ZDDPs. Thus the true concentration of secondary ZDDP is only 0.53 wt% P. Z2 also contains a higher proportion of basic ZDDP molecules. Topolovec also noted differences in film formation between secondary ZDDPs but focussed on the final film thickness formed [17]. The current study suggests that the rate of film formation is more sensitive to alkyl ZDDP structure than the maximum tribofilm thickness formed.

As discussed in the Sect. 1 section, some researchers have identified the presence of a viscous surface layer, possibly consisting of alkyl phosphate precipitates, on top of the generally-accepted solid-like ZDDP tribofilm [28‐30], although other work has shown no such film [31]. It has been suggested that this film is removed during solvent washing and therefore is not normally seen in ex-situ studies [30]. In SLIM the rubbed ball surface is neither rinsed nor cooled so any viscous ZDDP film forms should be present when the ball is loaded against the coated glass disc. It is well known that the normal approach of a ball on a flat coated with oil can result in the rate of formation of a local entrapment or “dimple” as the rate of increase of the pressure and thus viscosity rise within the centre of the contact exceeds the rate at which fluid can flow radially from the contact centre [44‐46]. This phenomenon depends of the rate of normal approach and the viscosity and pressure viscosity coefficient of the fluid. If a highly viscous film were present on top the ZDDP tribofilm, this should form such an entrapment. However no entrapment was seen in the current work using SLIM when the ball was uploaded against the coated glass disc, as indicated in the images in Fig. 5. To test what viscosity of surface film would produce an entrapment on a rough ZDDP film, a ball with a rinsed ZDDP tribofilm was coated with thin films of base oils of various viscosities and uploaded against the glass disc using the standard SLIM load and approach rate. It was found that an entrapment was formed only for films of viscosity greater than 150 mPas, as shown in Fig. 16. This implies that if a viscous film is present on top of the solid ZDDP tribofilm it must have viscosity less than ca 200 mPas. This is much less than the value of 5.10⁴ to 3.10⁵ Pas estimated by Bec et al. [30].

First AFM measurements were made of the roughness of the MTM tribofilm surfaces during tests. Since AFM can only be applied at the end of a test a series of tests of different duration were carried out. Results are shown in Table 4 for Z1 and Z2. It is important to note that the same area of 50 × 50 mm² was used for all determinations since it is well known that roughness increases as the size of the area analysed increases, due to the incorporation of longer and longer wavelength features [47].

The EHD film-forming properties of the fluids studied were then determined by measuring the BB film thickness using optical interferometry at the test temperature of 100 °C, as shown in Fig. 17. This shows classical EHD behaviour, with film thickness dependent on entrainment speed according to a best fit of;

where h_c is the central film thickness (in nm) and U the entrainment speed (in mm/s).

Equation 1 applies to a glass flat (elastic modulus 70 Ga) on steel ball (elastic modulus 210 GPa) contact in pure rolling at a load of 20 N. In the MTM contact used in the current study, both ball and flat were steel and the applied load was 31 N. The radius of the ball was the same in both film thickness and MTM experiments. To allow for the difference in elastic modulus and load, Eq. 1 was adjusted using the dependence of EHD film thickness on load and elastic modulus expressing in the Chittenden equation [48]. In practice, EHD film thickness is only weakly dependent on load and elastic modulus so this gave a very slight adjustment relationship to;

In practice EHD film thicknesses were measured in pure rolling while MTM tests were carried out at 50% slide/roll ratio (SRR). However it is well known that EHD film thickness varies only very slightly with SRR up to 50% [49].

From surface roughness and EHD film thickness, the lambda ratio (defined as ratio of central film thickness to composite surface roughness) can be calculated, where composite surface roughness, R_qc is given bywhere Rq1 and Rq2 are the roughnesses of the disc and ball respectively. In this study the roughness of the ball was taken to be the same as that measured from the disc so that;

The lambda ratio is thus given by;

Equation 5 can be used to convert plots of friction coefficient versus entrainment speed as shown in Figs. 8 and 9 to plots of friction coefficient versus lambda ratio. This is shown for Z1 in Fig. 18. It is evident that all the plots now coincide quite closely, indicating that a single Stribeck curve describes the transition from boundary to mixed lubrication at all stages of Z1 tribofilm growth. This strongly suggests that the shift in friction curve seen with ZDDP originates from the influence of the tribofilm on roughness in the contact inlet.

In principle the collapse onto a single master Stribeck curve seen in Fig. 16 should be the same for all ZDDP tribofilms except that the bounding values of boundary friction coefficient at very low lambda ratio may differ between ZDDPs. This can be taken into account if we consider friction in mixed lubrication to arise from the sum of boundary and EHD components so that;where F_B is the load supported by asperity contacts operating in the boundary lubrication regime and F_EHD is the load supported by EHD fluid film pressure.

Equation 6 can be written as where µ_EHD and µ_B are now the friction coefficients in boundary and full film EHD conditions, µ is the overall friction coefficient and W the applied load.

If X is the fraction of applied load supported by boundary film then;

A similar equation was described by Beltzer and Jahanmir [50].

Equation 8 can be rearranged in terms of the fraction of load supported by boundary lubrication

so that any plot of the variation of friction coefficient, µ, versus entrainment speed can be converted to one of X versus entrainment speed (or lambda ratio) so long as µ_EHD and µ_B are known.

The value of µ_EHD is the asymptote value of friction coefficient measured at high speed for the smooth surface (rubbing time = 0) and represents the EHD friction coefficient at 100 °C, 50% SRR and 31 N load for the base blend. This can be seen in Fig. 8 to have the value 0.024. The value of µ_B is the asymptote of the friction coefficient after rubbing at low speeds from Fig. 8. For Z1 this varies only slightly with rubbing time but for Z2 is reduced quite markedly after prolonged running. Values from Fig. 8 used in the current analysis are shown in Table 5.

Figure 19 shows plots of this fraction X versus lambda ratio for Z1 and Z2. It is evident that both ZDDPs collapse onto a single master curve at all rubbing times when plotted in this way. This is further proof that the rise in mixed friction with ZDDPs results from the effect of ZDDP tribofilm roughness on fluid entrainment. It can be seen from Fig. 19 that under the load conditions used, the contact operates in full boundary lubrication to a lambda ratio of ca 0.1 and reaches full EHD lubrication at a lambda value of ca 5. This is in reasonable agreement with both experimental and modelling predictions of the effect of roughness on friction in point contact [51, 52].