Studying Anti-Wear Tribofilm Formation

Tribofilms are thin tribo-chemical films that form when exposed to high contact pressure levels and elevated temperatures at the interface between two sliding surfaces under lubricated conditions. These films have a key function in reducing friction and wear between sliding surfaces. During the standard operation of an automotive engine the lubricant film is typically applied between the cylinder bore and piston ring.

The thickness of the lubricant film may vary from one lubrication regime to the next, depending on the piston’s location within the cylinder bore. The most extreme lubrication conditions, known as the boundary lubrication regime, occur when the piston is positioned at the top dead center or in the bottom dead center of its cycle where the highest temperature and greatest contact pressures occur.

To understand the tribological behavior and interaction of lubricant additives, understanding tribofilm growth mechanisms at both the micro and nanoscale is necessary, which, in essence, contributes to the macroscale tribological behavior.

Such studies can facilitate the development of improved lubricant formulations for enhancing the energy efficiency of tribological components. Aluminum-based alloys are used increasingly in the automotive industry for crucial application components in the transition towards light weighting for better fuel efficiencies and reducing exhaust emissions.

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

Aluminum silicon (Al-Si) alloys have made their way into key engine components, including pistons, cylinder liners, and engine blocks.

On the lubricant side, one of the most frequently used anti-wear additives applied in passenger vehicle lubricants is zinc dialkyl dithiophosphate (ZDDPs), which decreases wear by forming sulfur and phosphorous-containing anti-wear tribochemical film over the rubbing surfaces.¹ This article describes the in-situ formation of tribofilms on an Al-Si alloy (ADC12) substrate while submerged in a base oil containing ZDDP additive.

AFM to Study Tribofilms

In macroscale tests, considerable amounts of plastic deformation and mechanical mixing of wear particles with the substrate occurs at the testing conditions of extremely high loads in the mN or even Newton range. This phenomenon can impact the formation and properties of tribofilm in contrast to AFM-based measurements that pass at the much smaller μN load regime, where such issues can be canceled out or reduced significantly.^2,3

Another advantage of the application of AFM for friction and wear measurements is the capability to conduct highly localized measurements with nanometer spatial resolution, while also performing simultaneous imaging of the sliding interface.

Moreover, the AFM technique can determine the coefficient of friction values in localized regions of the substrate utilizing lateral force mode, a measurement not practically possible using the macroscale test such as pin on a disc or mini traction machine (MTM) measurements where only average coefficients of friction values are determined.

Therefore, AFM is a novel tool that can be used to analyze tribofilm formation that facilitates the recording of a set of images separately from the sliding action, which allows visualization of the evolution of the sliding interface.³

Experimental Setup

To simultaneously monitor the in situ growth of tribofilm and the friction force as a function of sliding time, a commercial AFM (FlexAFM, Nanosurf, Switzerland) was used. Silicon cantilevers available commercially (Tap190AL-G, Budget sensors, Bulgaria), altered by fixing wear-resistant alumina microspheres with a diameter of 20-30 μm close to the end of the cantilever, were applied to measure the friction force and topographic evolution within the sliding zone.³

To determine the normal forces, calibration of the cantilever normal spring constant and force sensitivity was performed in line with the Sader method and slope of the linear region of the normal force vs. displacement curve, accordingly.⁴ Calibration of the lateral spring constant was carried out using the method described in Green et al.⁵

Calculation of the lateral force sensitivity was determined from the slope of the initial linear region (stick regime) of the lateral force vs. sliding distance curve.³ Contact diameter and contact pressures were quantified by applying the Hertzian contact model.⁶ Analysis of the topography images and friction data was conducted with the help of Gwyddion and MATLAB software.

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

The cantilever was fixed on the cantilever holder, as displayed in Figure 1(a). The experiments were performed at elevated temperatures (110 °C). The sliding tests to study the tribofilm were carried out at a normal load of 10 μN in contact mode.

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 size of the AFM scan was 12 x 5 μm², and sliding speed was set at 80 ms per line scan, which relates to a sliding velocity of ~150 μm/s. Sliding tests were carried out for almost 1.5 hours to track the formation of the tribofilm on the surface of the Al-Si alloy (ADC12)  substrate.

Topographic imaging of the entire surface was conducted once the experiments were complete at lower loads (<100 nN) and a greater length scale of 20 x 20 μm², to prevent any further alteration of the surrounding region of the substrate and capture the entire area of the sliding test as well as the area beyond the sliding test.

Coefficient of friction measurements was performed simultaneously with the application of lateral force mode, where scanning of the cantilever was performed in contact mode in a direction perpendicular to the cantilever axis while recording the cantilever’s standard and torsional motion.

Results

Figure 2(a) indicates that the AFM image captured once sliding experiments were conducted in the area confined by the dotted rectangular region, shows tribofilm growth on both the aluminum and silicon phases. The AFM image exhibits the transformation in surface morphology in both the Al matrix and the Si phase within the sliding zone, where increasingly higher loads were applied for 500 sliding cycles.

The ZDDP tribofilm’s topographic height profile across various localized regions is displayed in Figure 2(b), which indicates that the elevation of the Si phase from the base of the matrix is around 60-70 nm. The growth of the tribofilm on the Al matrix seems to be rough with discrete patches compared to the Si phase, where tribofilm is seemingly denser (smaller separation between the individual patches of the tribofilm) and thicker.

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

Lower roughness of the tribofilm on the Si phase results from a smaller separation between the tribofilm patches (Figure 2(b)). A 3D large-scale 20 x 20 μm² topographic image, which includes the sliding region in the middle of the scan (encased in a yellow rectangle dotted box) of the ADC12 surface after the complete sliding test 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 variation of the coefficient of friction (μ) with sliding time while the sliding of the probe occurs simultaneously across 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 is more significant than on the Al matrix. It was observed that when direct contact is made between the tip and substrate early in the experiment, the coefficient of friction is greater on both the Al and Si phase regions.

However, a reduction is seen relative to sliding time with the nucleation of tribofilm across the surface, which isolates the sliding surfaces and offers lubrication.

Over time, the growth of the tribofilm causes an increase in the coefficient of friction, likely due to the tribofilm forming rough, island-like structures across the surface, which may give more resistance to the sliding probe across the tribofilm surface.

Conclusion

Tribological studies of lubricated ADC12 substrate in the presence of ZDDP have been conducted using an in-situ AFM technique. The coefficient of friction and morphological evolution of the tribofilm was tracked in highly localized regions, i.e., single phases, including the Al matrix and Si phase of ADC12 alloy.

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

During tribofilm growth, the coefficient of friction was simultaneously monitored. In the absence of any discernable wear, the coefficient of friction in the Si region is greater than that of the Al matrix.

The study highlights the novel capacity of AFM to monitor the evolution of sliding interface and simultaneous measurements of tribological properties alongside an industrial lubricant environment on engineered alloy surfaces.

References and Further Reading

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.

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