Sujeet K. Sinha

Scratching maps for polymers

Abstract The scratching technique has gained interest in recent times due to its varied applications to a number of engineering materials, especially for the evaluation of surface scratch resistance of plastics. Scratching provides a convenient and reliable means to investigate the mechanical properties of organic polymers under various contact conditions. The scratch hardness method is widely adopted to provide a first-order evaluation of the relative scratch resistance of materials for comparison purposes. The method also allows the identification and the assessment of the surface deformation processes and maps defining the scratch deformation modes as a function of contact conditions may be generated. These scratching maps may present experimental results in terms of the deformation mechanism, the scratch hardness and the friction coefficient. This paper primarily provides a review of the application of scratching maps for polymers. Results for the scratch hardness and the deformation mechanisms for a poly(methylmethacrylate) (PMMA), a poly(tetrafluoroethylene) (PTFE) and an ultra-high molecular weight poly(ethylene) (UHMWPE) are presented. The PTFE system is also described following the effects of γ-irradiation; radiation produces a marked reduction in toughness. The scratches were produced on the polymer surfaces by cones and spheres of various size under a number of contact conditions (e.g. applied normal load, strain, scratch velocity, etc.). SEM imaging and laser profilometry are used for the study of the deformation mechanisms and the measurements of the scratch profiles. It is shown that polymers exhibit a wide range of scratch deformation characteristics and that the deformation mechanism is determined by the most efficient energy dissipation process for the particular external constraints.

The hardnesses of poly(methylmethacrylate)

The paper describes an experimental study of the normal and scratch hardnesses of a poly(methylmethacrylate). The deformations have been introduced using hard steel cones of a range of included cone angles. The influence of the state of interfacial lubrication is examined and rationalized. The observed time dependence of the two types of computed hardness data is compared and the nature of the correlations between these data is evaluated. It is observed that when the imposed strains are modest, say less than 0.2, the scratch hardness and normal hardness deformations produce self consistent data using first order and rather indiscriminate analyses for both types of deformations. At higher levels of imposed strain, a more critical appraisal of the nature of the deformation produced in the two cases is necessary in order to provide mutually consistent hardness values and hence unequivocal rheological characteristics for this polymer.

Wear of polymers

Abstract This paper reviews some of the recent progress which has been made in the area of the sliding wear of polymers. Wear mechanisms are classified under three broad approaches which reflect primarily the way this subject has been historically studied. It is demonstrated here that the wear of polymers is influenced by the contact conditions, the bulk mechanical properties of the polymer and the properties of the ‘third body’, which generally appears in the form of transfer film or degraded polymer particles between two sliding surfaces. Further, this paper establishes a link between the different contact and material parameters and shows how they are important in elucidating the generic wear mechanisms for polymers. The effects of environment and lubrication upon polymer wear are briefly explained in terms of the chemical interactions between the liquid phase and the polymer. The capabilities and limitations of current predictive wear models for polymeric contacts are also highlighted.

A finite element analysis of the squeeze flow of an elasto-viscoplastic paste material

Abstract A finite element analysis of squeeze flow has been implemented for a material that exhibits elasto-viscoplasticity. The formulation is based upon the assumption that linear elastic deformation occurs prior to yield and that the yield surface is strain rate hardening as defined by an associated viscoplastic flow rule. Both no-slip and lubricated wall boundary conditions are considered. The numerical simulation results are compared with experimental measurements involving a model elasto-viscoplastic material for which the material parameters were derived from tensile and ram extrusion measurements. Satisfactory agreement was obtained for the compressive forces as a function of displacement, the radial displacement fields and the wall normal and shear stress distributions.

Tribology of PFPE overcoated self-assembled monolayers deposited on Si surface

Friction and wear characteristics of monomolecular layers of self-assembled monolayers (SAMs) with and without perfluoropolyether (PFPE) overcoat were studied using ball-on-disk experiments. Ultra-thin layer of PFPE was dip-coated onto two different SAMs, one with non-reactive terminal group (octadecyltrichlorosilane (OTS)) and the other with reactive terminal group (3-aminopropyltrimethoxysilane (APTMS)), which were formed on Si substrate by self-assembly. The effects of PFPE overcoating on physical and chemical properties were evaluated using contact angle measurement and x-ray photoelectron spectroscopy. For a comparison, PFPE was also coated onto the Si surface and their properties were evaluated. All PFPE modified surfaces were baked at 150uC for 2 h, to investigate the effect of thermal treatment on tribological properties. PFPE coating has shown higher water contact angles irrespective of the SAM surface. Coating of PFPE on both SAM surfaces has lowered their coefficients of friction. PFPE overcoating has shown remarkable increase in the wear resistance when it was coated on reactive APTMS SAM and little increase on OTS SAM. Thermal treatment after PFPE coating onto SAMs further reduced the coefficient of friction to a smaller extent. Moreover, thermal treatment has shown an additional increase in wear-life by approximately 30% in the case of PFPE coated APTMS SAM surface and a decrease in the wear-life in the case of PFPE coated OTS SAM. The reasons for these observed phenomena are explained in terms of the amounts of PFPE bonded or mobile, surface energies of SAMs, uniformity and molecular packing of SAM surfaces. (Some figures in this article are in colour only in the electronic version)

Scratch deformation of methanol plasticized poly(methylmethacrylate) surfaces

The results of a series of scratch measurements are reported for a number of methyl alcohol (methanol) plasticized poly (rnethylmethacrylate) (PMMA) surfaces to investigate the deformation mechanism and solvent diffusion characteristics when a PMMA is exposed, to various extents, in a methanol environment. The study confirms that the solvent diffusion in this system occurs by the Case II diffusion process. The scratch deformation map described indicates that there is a major change in the deformation mechanism as the exposure time proceeds and this is related first to a sample plasticization and then to a restructuring process of the PMMA—methanol system due to the greater mobility and relaxation of the polymer molecules as they imbibe the solvent.

Wear durability studies of ultra-thin perfluoropolyether lubricant on magnetic hard disks

Abstract This paper presents wear and friction studies on ultra-thin (~2 nm) film of perfluoropolyether (PFPE) coated on glass substrate magnetic hard disks. The lubricant was coated on the disk by the dip-coating method and the tribological tests were carried out by sliding a 3 mm diameter glass ball slider (normal load=20 mN) on the rotating disk surface. Lube thickness and lube wear profile were measured using an ellipsometer whereas the worn disk surface was studied using a surface reflectivity analyzer. The sliding speed and the lube bonding conditions were varied during the test. From the results, it is concluded that about 80% bonding of the lube to the disk surface leads to an increase in the wear durability of the lubricant by a factor of 2 when compared to the as-lubed condition. Lube bonding has an effect on increasing the coefficient of friction. Initially, increasing sliding speed increases both friction and wear but for very high sliding speed these values tend to decrease. The glass ball surface showed wear due to asperity interactions as well as lube transfer from the disk to the glass surface.

Tribological characterization of a biocompatible thin film of UHMWPE on Ti6Al4V and the effects of PFPE as top lubricating layer

Ultra-high molecular weight polyethylene (UHMWPE) thin film was coated onto Ti6Al4V alloy specimens using dip coating method. Tribological performance of this coating (thickness of 19.6 ± 2.0 μm) was evaluated using 4 mm diameter Si(3)N(4) ball counterface in a ball-on-disk tribometer. Tests were carried out for different normal loads (0.5, 1.0, 2.0 and 4.0 N) and rotational speeds of the disk (200 and 400 rpm). UHMWPE coating formed in this study exhibits high hydrophobicity with water contact angle of 135.5 ± 3.3° and meets the requirements of cytotoxicity test using the ISO 10993-5 elution method. This coating shows low coefficient of friction (0.15) and high wear durability (>96,000 cycles) for the tested conditions. PFPE overcoat on UHMWPE has further increased the wear durability of UHMWPE coating as evaluated at even higher rotational speed of 1000 rpm.

Characterisation of the Scratch Deformation Mechanisms for Poly(methylmethacrylate) using Surface Optical Reflectivity

A novel technique using surface local optical reflectivity measurements for characterising the modes of surface mechanical damage of polymers is described. The technique utilises the sensed laser reflectivity of polymer surfaces, after damage, as a means of measuring the relative extent of brittle and plastic deformation modes for polymers. The polymeric surface used in this paper is a commercial poly(methylmethacrylate). The surface damage is produced by scratching with rigid conical indenters under different contact conditions of strain and load. The results show that the local optical reflectivity of a surface depends upon the topography of the surfaces, which varies for different modes of deformation. The reflectivity of a brittle fractured surface is significantly lower in comparison with that for a plastically deformed surface.

Tribo-functionalizing Si and SU8 materials by surface modification for application in MEMS/NEMS actuator-based devices

Micro/nano-electro-mechanical-systems (MEMS/NEMS) are miniaturized devices built at micro/nanoscales. At these scales, the surface/interfacial forces are extremely strong and they adversely affect the smooth operation and the useful operating lifetimes of such devices. When these forces manifest in severe forms, they lead to material removal and thereby reduce the wear durability of the devices. In this paper, we present a simple, yet robust, two-step surface modification method to significantly enhance the tribological performance of MEMS/NEMS materials. The two-step method involves oxygen plasma treatment of polymeric films and the application of a nanolubricant, namely perfluoropolyether. We apply the two-step method to the two most important MEMS/NEMS structural materials, namely silicon and SU8 polymer. On applying surface modification to these materials, their initial coefficient of friction reduces by ~4?7 times and the steady-state coefficient of friction reduces by ~2.5?3.5 times. Simultaneously, the wear durability of both the materials increases by >1000 times. The two-step method is time effective as each of the steps takes the time duration of approximately 1?min. It is also cost effective as the oxygen plasma treatment is a part of the MEMS/NEMS fabrication process. The two-step method can be readily and easily integrated into MEMS/NEMS fabrication processes. It is anticipated that this method will work for any kind of structural material from which MEMS/NEMS are or can be made.