Tribology of Self-assembled monolayers (SAMs)
(a) Tribology of PFPE overcoated SAMs
Perfluoropolyether (PFPE) is the traditional lubricant used in Hard-Disk Drives (HDD). In the present study we have formed ultra-thin layers of PFPE onto different SAMs using dip-coating technique. PFPE layer of few nanometers thickness has shown greater improvement in wear durability when coated onto reactive SAMs such as APTMS (aminopropyltrimethoxysilane) and GPTMS (glycidoxypropyltrimethoxysilane) and marginal improvement when coated onto OTS (octadecyltrichlorosilane) SAM.
Fig: Coefficient of friction vs number of sliding cycles curves for bare Si and Si/OTS SAM surfaces tested at 330MPa and 0.02-0.04 ms-1 sliding velocities and Si/UHMWPE and Si/UHMWPE/PFPE surfaces tested at 370MPa and 0.04-0.08 ms-1 sliding velocities.
Fig: Wear life of bare Si, Si/OTS, Si/UHMWPE and Si/UHMWPE/PFPE samples. Error bars show the minimum and maximum values for three independent tests.
(b) Tribology of Polyimide films on Si surface
In this study we have demonstrated that the addition of CNTs (carbon nanotubes) to the polyimide film on Si surface has increased the nano-mechanical properties of PI film and also the tribological properties of PI film (especially the wear life). This study elucidates the importance of the addition of filler materials to the polymer films on their tribological properties.
(c) Effect of molecular structure of polymer films on Si surface on tribological properties
This study elucidates the effect of the molecular structure of the polymer films on the tribological properties. It has been identified that if the polymer molecules contain the simple, linear molecular structure without any bulky groups show low coefficient of friction and high wear durability.
Table: Water contact angle, coefficient of friction and wear life data of bare Si, Si/APTMS and polymers films.
|Sample||Water contact angle, degrees||Surface roughness#, nm||Thickness, nm||Coefficient of friction||Wear life, number of cycles|
*the lowest and highest wear life data among three tests for Si/APTMS/PE are 3000 and 7100 cycles, respectively.
#RMS roughness measured using AFM over a scan area of 1 µm x 1 µm.
Fig: Variation of frictional force with respect to the normal load applied for bare Si, Si/APTMS, Si/APTMS/PE and Si/APTMS/PS, tested against 4mm diameter Si3N4 ball, at a sliding velocity of 1mm sec-1 using ball-on-plate configuration. The linear trend lines with the corresponding equations are also shown for each graph.
Fig: The dynamic variation of coefficient of friction with respect to number of sliding cycles for bare Si, Si/OTS, Si/APTMS, Si/APTMS/PE and Si/APTMS/PS samples, obtained in ball-on-disk tests against 4 mm Si3N4 ball at a normal load of 5 g and sliding velocity of 0.021 ms-1.
(d) Tribology of hard and soft composite films
(d-1) DLC and UHMWPE as hard/soft composite film on Si for improved tribological performance
This study is to explore the advantages of tribological properties using a composite thin film of ultra high molecular weight polyethylene (UHMWE) on a hard diamond like carbon (DLC) coating deposited on Si. As a result of higher load carrying capacity due to the presence of DLC layer, the wear life of Si/DLC/UHMWPE coated layer is approximately five times greater than that of Si/UHMWPE. Looking at the film thickness effect, UHMWPE film shows maximum wear resistance when the film is of optimum thickness (6.2 μm–12.3 μm) on DLC.
Fig. (a) Coefficient of friction, (b) wear life (logarithmic scale) of bare Si and Si coated with different single and composite films and (c) coefficient of friction versus sliding cycles of some films at a normal load of 40 mN and at a rotational speed of 500 rpm (linear speed is 5.2 cm/s) where UHMWPE thickness is fixed as 28 µm for all coated samples. (A1 = bare Si, A2 = Si/UHMWPE, A3 = Si/UHMWPE/PFPE, A4 = Si/DLC, A5 = Si/DLC/UHMWPE, A6 = Si/DLC/UHMWPE/PFPE)
Fig. Wear life for different UHMWPE thicknesses for Si/DLC/UHMWPE. Data are averages of three repeated tests. For 12.3 µm thick film there was no failure at 300,000 cycles of sliding when the experiments were stopped due to long test duration.
(d-2) Effect of interfacial energy of Si surface on the final tribological properties of Si/UHMWPE films
This paper presents results on the tribological properties of UHMWPE (6μm thickness) films on Si substrates modified by interfacial layers of different surface energies (hydrophobicity). Bare Si, which as the most hydrophilic of all, and Si/OTS, the most hydrophobic interface, showed much over wear resistance when compared with heated Si, Si/APTMS and Si–H interfaces, with Si–H interface giving the best result.
Photo: Ball-on-disc tribometer
Fig: Friction and wear properties of UHMWPE film with different interfaces where the normal load is 4 mN and sliding speed is 500 rpm (0.1 ms-1). (a) Typical friction traces as a function of the number of sliding cycles for all samples. (b) Consolidated wear life data for all samples.
|Table. Water contact angles and wear lives for different interfacial modifications on Si.|
|Interface||Bare Si||Heated Si||Si/APTMS||Si-H||Si/OTS|
|Contact Angle (°)||21||46||52||71||104|
|Wear Life (cycles)||1,000||60,000||100,000||250,000||3,000|