Prashant Mittal and Nitya Nand Gosvami, Indian Institute of Technology, Delhi.
Tribofilms
Tribofilms are thin tribochemical films formed under high contact pressure and temperature at the interface between two sliding surfaces under lubricated conditions. These films play an important role in reducing friction and wear of the sliding surfaces. In an automotive engine, under normal operation, the lubricant film is generally present between the cylinder bore and piston ring. The thickness of the lubricant film varies in different lubrication regimes depending on the piston’s location within the cylinder bore. The most severe lubrication condition occurs when the piston is at the top dead center or in the bottom dead center of its cycle where the highest temperature and contact pressures occur; this regime is known as the boundary lubrication regime.
Understanding the tribological behavior and interaction of lubricant additives requires insights into tribofilm growth mechanisms at the micro and nanoscale, which ultimately contribute to the macroscale tribological behavior. Such studies can enable development of better lubricant formulations for further improvement in energy efficiency of tribological components.
Aluminum-based alloys are increasingly used in the automotive sector for key components in the drive towards light weighting for improved fuel efficiency and reduced exhaust emissions. Aluminum-silicon (AlSi) alloys have found their way into critical engine components including pistons, cylinder liners, and engine blocks. On the lubricant side, one of the most common anti-wear additives found in passenger vehicle lubricants is zinc dialkyl-dithiophosphates (ZDDPs) [1], which reduces wear by the formation of a sulfur and phosphorous containing anti-wear tribochemical film over the rubbing surfaces. This investigation explores the in-situ formation of tribofilms on an AlSi alloy (ADC12) substrate while immersed in a base oil containing ZDDP additive.
AFM to study Tribofilms
In macroscale tests significant plastic deformation and mechanical mixing of wear particles with the substrate takes place at the testing conditions of very high loads in the mN or even Newton range. This phenomenon can interfere with the formation and properties of tribofilm as compared to AFM-based measurements that occur at the much smaller µN load regime where such complexities can be eliminated or at least dramatically reduced [2,3]. Another benefit of using AFM for friction and wear measurements is the ability to perform highly localized measurements with nanometer spatial resolution, while also conducting simultaneous imaging of the sliding interface. Furthermore, the AFM technique can quantify the coefficient of friction values in localized regions of the substrate using lateral force mode, a measurement not possible with the macroscale test such as pin on a disc or mini traction machine (MTM) measurements where only average coefficients of friction values are reported. Thus, AFM is a unique tool to analyze tribofilm formation that allows recording a set of images separately from the sliding action, which enables visualization of the evolution of the sliding interface [3].
Experimental setup
A commercial AFM (FlexAFM, Nanosurf, Switzerland) was used to monitor the in-situ growth of tribofilm and the friction force simultaneously as a function of sliding time. Commercially available silicon cantilevers (Tap190AL-G, Budget sensors, Bulgaria), modified by attaching wear-resistant alumina microspheres with a diameter of 20-30 µm near the end of the cantilever, were used to measure the friction force and topographic evolution within the sliding zone [3]. To quantify the normal forces, the cantilever normal spring constant and force sensitivity were calibrated with the Sader method [4] and slope of the linear region of the normal force vs. displacement curve, respectively. The lateral spring constant was calibrated by the method described in Green et al. [5]. Lateral force sensitivity was calculated 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 calculated using the Hertzian contact model [6]. The topography images and friction data were analyzed with the help of Gwyddion and MATLAB software respectively.
![Figure 1: (a) AFM experimental setup showing various components. (b) schematic diagram illustrating the in-situ AFM experimental setup [7]](/images/applications/anti-wear-tribofilm/Figure-1-experimental-setup.png)
The experimental setup and schematic diagram of the AFM instrument are shown in Figure 1(a-b) [7]. The liquid cell, made of polyether ether ketone (PEEK), with the Viton Oring was mounted over the sample, placed on a heating stage, and was filled with ZDDP containing base oil. The cantilever was mounted on the cantilever holder as shown in Figure 1(a). The experiments were performed at elevated temperature (110 °C).
The sliding tests to study the tribofilm were carried out at a normal load of 10µN in contact mode. The AFM scan size was 12 x 5 µm2, and sliding speed was fixed to 80 ms per line scan, which corresponds to a sliding velocity of ~150 µm/s. Sliding tests were performed for almost 1.5 hours to observe tribofilm formation on the surface of the Al-Si alloy (ADC12) substrate.
Topographic imaging of the whole surface was performed after the experiments at much lower loads (<100nN) and a larger length scale of 20 x 20 µm2, to avoid further modification of the surrounding region of the substrate and capture the whole area of the sliding test in addition to area outside the sliding test.
Coefficient of friction measurements were simultaneously performed using lateral force mode, where the cantilever was scanned in contact mode in a direction perpendicular to the cantilever axis while recording the normal and torsional motion of the cantilever.
Results
Figure 2(a) shows the AFM image that was captured after sliding experiments in the area confined by the dotted rectangular region, revealing tribofilm growth on both the aluminum and silicon phases. The AFM image shows the change in surface morphology in both the Al matrix and the Si phase within the sliding zone, where higher loads were applied for 500 sliding cycles. The topographic height profile of the ZDDP tribofilm formed in different localized regions is shown in Figure 2(b), which shows that the elevation of Si phase from the base of the matrix is about 60-70 nm. The growth of the tribofilm on the Al matrix appears rough with discrete patches in comparison to the Si phase, where tribofilm appears denser (smaller separation between the individual patches of the tribofilm) and thicker.
![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.](/images/applications/anti-wear-tribofilm/Figure-3-3D-image-ZDDP-tribofilm.png)
The tribofilm thickness on the Si phase and on the Al matrix is around 80 ± 10 nm and 50 ± 10 nm respectively [8]. The RMS roughness values of tribofilms on Si phase and on Al matrix are 13 nm and 16 nm respectively. Lower roughness of the tribofilm on Si phase is due to smaller separation between the tribofilm patches (Figure 2(b)). A 3D large scale 20 x 20 µm2 topographic image containing the sliding region in the middle of the scan (enclosed in a yellow rectangle dotted box) of the ADC12 surface after the complete sliding test is shown in Figure 3.
Figure 4 shows the variation of the coefficient of friction (µ) with sliding time while the probe is sliding simultaneously over a region with Si phase (black data points) and Al matrix (red data points) of ADC12 substrate [8]. The coefficient of friction on the Si phase is higher than that on the Al matrix. We observe that when there is direct contact between the tip and substrate at the start of the experiment, the coefficient of friction is higher on both the Al and Si phase regions. However, it decreases with sliding time with the nucleation of tribofilm over the surface, which separates the sliding surfaces and provides lubrication. As the growth of the tribofilm progresses with time, the coefficient of friction again increases, likely due to the tribofilm forming rough, island-like structures over the surface, which may offer more resistance to the sliding probe across the tribofilm surface.
![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].](/images/applications/anti-wear-tribofilm/Figure-4-variation-friction-coefficient.png)
Conclusion
Tribological investigations of lubricated ADC12 substrate in the presence of ZDDP have been performed using an in-situ AFM technique. The coefficient of friction and morphological evolution of the tribofilm was monitored in highly localized regions, i.e., individual phases including the Al matrix and Si phase of ADC12 alloy. It was found that ZDDP tribofilms grow with sliding time at high temperature (110°C) and a given contact pressure over different regions including the Al matrix and Si phase of the ADC12 substrate surface. The coefficient of friction was monitored simultaneously during tribofilm growth and in the absence of any noticeable wear, the coefficient of friction in the Si region is higher than that of the Al matrix. The study demonstrates the unique capabilities of an AFM to monitor the evolution of sliding interface and simultaneous measurements of tribological properties in presence of 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.
