Tribofilms and Anti-Wear Tribofilm Formation

Tribofilms are thin tribochemical films produced as a result of high contact pressure and temperature at the interface under lubricated conditions between two sliding surfaces. Such films play a crucial role in limiting the wear of the sliding surfaces and friction.

Under normal operating conditions in automotive engines, the lubricant film is usually between the cylinder bore and piston ring. However, the thickness of the lubricant film can vary for differing lubrication regimes contingent on the location of the piston in the cylinder bore.

The most serious lubrication condition arises when the piston is either in the bottom dead center or at the top dead center of its cycle where the highest temperature and contact pressures occur; this regime is referred to as the boundary lubrication regime.

Comprehending the interaction of lubricant additives and tribological behavior demands an understanding of the tribofilm growth mechanisms at the micro and nanoscale, which eventually contribute to the macroscale tribological behavior.

This can facilitate the development of improved lubricant formulations for enhancing the energy efficiency of tribological components further.

In the automotive sector, aluminum-based alloys are increasingly used for crucial components in the drive towards light-weighting for reducing exhaust emissions and enhancing fuel efficacy.

Aluminum-silicon (AlSi) alloys have ended up being incorporated into crucial engine components, such as cylinder liners, engine blocks and pistons.

One of the most frequently found anti-wear additives used in passenger vehicle lubricants on the lubricant side is zinc dialkyl-dithiophosphates (ZDDPs),¹ which limits wear in a composition of a sulfur and phosphorous-containing anti-wear tribochemical film over the contact surfaces.

This article investigates the in-situ development of tribofilms on an AlSi alloy (ADC12) substrate while submerged in a base oil containing a ZDDP additive.

Using AFM to Study Tribofilms

A considerable amount of plastic deformation and mechanical mixing of wear particles with the substrate occurs under very high load testing conditions in the mN or even Newton range during macroscale tests.

This can impede the effective properties and hinder the formation of tribofilm as compared to AFM-based measurements that occur at the far lesser µN load regime where such complexities can be dramatically reduced or even eradicated.^2,3

A further advantage that AFM offers when measuring friction and wear is the capability to perform highly localized measurements with nanometer spatial resolution while simultaneously carrying out imaging of the sliding interface.

Moreover, using the AFM method, evaluations of the coefficient of friction values in localized regions of the substrate can be conducted by applying lateral force mode, a measurement that cannot be achieved with the macroscale test such as pin on a disc or mini traction machine (MTM) measurements where only average coefficients of friction values are observed.

Therefore, AFM is a special tool for the analysis of tribofilm formation that facilitates the recording of a set of images independent of the sliding action, which allows the evolution of the sliding interface to be visualized.³

Experimental Setup

To simultaneously track the in-situ growth of tribofilm and the friction force as a function of sliding time, a commercial AFM (FlexAFM, Nanosurf, Switzerland) was used.

Commercial grade silicon cantilevers (Tap190AL-G, Budget sensors, Bulgaria), altered by affixing wear-resistant alumina microspheres with a diameter of 20-30 µm near the end of the cantilever, were applied for measuring the friction force and topographic evolution within the sliding zone.³

The normal spring constant and force sensitivity of the cantilever was calibrated using the Sader method to quantify the normal forces,⁴ and slope of the linear region of the normal force vs. displacement curve, respectively.

The lateral spring constant was calibrated using the method described in Green et al.⁵

Lateral force sensitivity was determined from the slope of the initial linear region (stick regime) of the lateral force vs. sliding distance curve.³ The Hertzian contact model was applied to calculate the contact diameter and contact pressures.⁶

Gwyddion and MATLAB software were employed respectively to evaluate the topography images and friction data.

The experimental setup and schematic diagram of the AFM instrument are presented in Figure 1(a-b).⁷ The liquid cell, comprised of polyether ether ketone (PEEK), with the Viton Oring, was placed over the sample, mounted on a heating stage and filled with ZDDP containing base oil.

The cantilever was attached to the cantilever holder, as displayed in Figure 1(a). The experiments were conducted at an elevated temperature (110 °C).

Figure 1. (a) AFM experimental setup showing various components. (b) schematic diagram illustrating the in-situ AFM experimental setup"⁷ Image Credit: Nanosurf AG

The sliding tests to investigate the tribofilm were performed at a normal load of 10 µN in contact mode. The AFM scan size was 12 x 5 µm², and the sliding speed was set to 80 ms per line scan, which is equivalent to a sliding velocity of ~150 µm/s.

In order to see the tribofilm formation on the surface of the Al-Si alloy (ADC12) substrate, sliding tests were carried out for almost 1.5 hours.

The entire surface was subjected to topographic imaging after the experiments at reduced loads (<100nN) and a larger length scale of 20 x 20 µm², to prevent further alteration of the surrounding region of the substrate and capture the entire area of the sliding test in addition to the area beyond the sliding test.

Coefficient friction measurements were performed simultaneously through the application of lateral force mode, where the cantilever was scanned in contact mode perpendicular to the cantilever axis while registering the normal and torsional motion of the cantilever.

Results

Figure 2(a) presents the AFM image that was acquired after sliding experiments in the area restricted by the dotted rectangular region, uncovering tribofilm growth on both the aluminum and silicon phases.

Figure 2. (a) AFM topographic image acquired after scanning the alumina probe simultaneously over Al matrix and Si phase for 500 cycles in the dotted rectangular region (experimental scan area) where tribofilm growth has occured. (b) Topographic height profile along Al matrix and Si phase revealing thickness of the tribofilm on Al matrix and Si phase." Image Credit: Nanosurf AG

The AFM image demonstrates the changes in surface morphology where higher loads were applied for 500 sliding cycles in both the Al matrix and the Si phase within the sliding zone.

The topographic height profile of the ZDDP tribofilm that formed in various localized regions is displayed in Figure 2(b), which demonstrates that the elevation of Si phase from the base of the matrix was about 60-70 nm.

The growth of the tribofilm on the Al matrix seems to be rough with distinct patches in contrast to the Si phase, where tribofilm appears denser (smaller separation between the individual patches of the tribofilm) and thicker.

The tribofilm thickness on the Si phase and on the Al matrix was approximately 80 ± 10 nm and 50 ± 10 nm, respectively.⁸ The RMS roughness values of tribofilms on Si phase and on Al matrix were 13 nm and 16 nm, respectively.

The lower roughness of the tribofilm on Si phase was as a result of a smaller separation between the tribofilm patches (Figure 2(b)).

A 3D large scale 20 x 20 µm² topographic image incorporating the sliding region in the center of the scan (encased in a yellow rectangle dotted box) of the ADC12 surface after the sliding test was complete is displayed in Figure 3.

Figure 3. 3D image of ZDDP tribofilm on an ADC12 alloy substrate.⁸ Yellow rectangle indicates experimental region which was selected for the tribofilm growth. Image Credit: Nanosurf AG

Figure 4 exhibits the coefficient variation of friction (µ) with sliding time while the probe was simultaneously sliding over a region with Si phase (black data points) and Al matrix (red data points) of ADC12 substrate.⁸

Figure 4. Variation of the coefficient of friction (µ) with sliding time (min) when sliding tests were performed on both Al matrix (red circle symbols) and Si phase (black square symbols) simultaneously of an ADC12 alloy surface."⁸ Image Credit: Nanosurf AG

The coefficient of friction on the Si phase was greater than that on the Al matrix. It was also observed that when there was direct contact between the substrate and tip at the beginning of the experiment, the coefficient of friction was greater on both the Al and Si phase regions.

However, with sliding time, there was a decrease in the nucleation of tribofilm over the surface, which divides the sliding surfaces and offers lubrication.

As the growth of the tribofilm advanced over time, there was an increase in the coefficient of friction, likely as a result of the tribofilm forming rough, island-like structures across the surface, which may provide more resistance to the sliding probe across the surface of the tribofilm.

Conclusion

Tribological investigations of lubricated ADC12 substrate in the presence of ZDDP have been conducted via the application of an in-situ AFM technique. In extremely localized regions, i.e., individual phases including the Al matrix and Si phase of ADC12 alloy, the coefficient of friction and morphological evolution of the tribofilm was monitored.

It was discovered that ZDDP tribofilms expand with sliding time at high temperatures (110 °C) and a specified contact pressure over various regions, including the Al matrix and Si phase of the ADC12 substrate surface.

The coefficient of friction was monitored synchronously throughout tribofilm growth, and in the absence of any discernable wear, the coefficient of friction in the Si region was greater than that of the Al matrix.

The study determines the novel capabilities of an AFM to simultaneously track the evolution of the sliding interface while measuring tribological properties in the presence of an industrial lubricant environment on engineering alloy surfaces.

References

1. Spikes H. The history and mechanisms of ZDDP. Tribol Lett 2004;17:469–89.
2. Gosvami NN, Bares JA, Mangolini F, Konicek AR, Yablon DG, Carpick RW. Mechanisms of anti-wear tribofilm growth revealed in situ by single asperity sliding contacts. Science (80) 2015;348:102–6.
3. Gosvami NN, Ma J, Carpick RW. An In Situ Method for Simultaneous Friction Measurements and Imaging of Interfacial Tribochemical Film Growth in Lubricated Contacts. Tribol Lett 2018;66:1–10.
4. Sader JE, Larson I, Mulvaney P, White LR. Method for the calibration of atomic force microscope cantilevers. Rev Sci Instrum 1995;66:3789–98.
5. Green CP, Lioe H, Cleveland JP, Proksch R, Mulvaney P, Sader JE. Normal and torsional spring constants of atomic force microscope cantilevers. Rev Sci Instrum 2004;75:1988–96.
6. Hertz H. On the elastic contact of elastic bodies. J Reine Angew Math 1881;92:156–71.
7. Mittal P, Rai H, Gosvami NN. Microscopic Tribology of ADC12 Alloy Under Lubricant Containing ZDDP and MoDTC Using In-Situ AFM. Tribol Lett 2021;69:1–10.
8. Mittal P, Maithani Y, Pratap J, Gosvami NN. In situ microscopic study of tribology and growth of ZDDP anti-wear tribofilms on an Al –Si alloy. Tribol Int 2020;151:106419.

This information has been sourced, reviewed and adapted from materials provided by Nanosurf AG.

For more information on this source, please visit Nanosurf AG.