A Tribological Study of SU-8 Micro-Dot Patterns Printed on Si Surface in a Flat-on-Flat Reciprocating Sliding Test

Tribological properties of optimized SU-8 micro-dot patterns on Silicon (Si) were evaluated using a flat-on-flat tribometer. Sliding tests on the patterns were conducted against a SU-8 spin-coated 2 × 2 mm² Si substrate at varying normal loads at a fixed rotational speed. It was observed that the pitch of the SU-8 pattern on Si substrate had significant influence on the wear durability. Ultra-thin layer of perfluoropolyether was over-coated onto SU-8 micro-dot patterned specimens for enhanced wear durability, and the specimen of the optimized pitch 450 μm has shown a wear life of more than 100,000 cycles at a normal load of 650 mN.     

Friction and wear durability studies on the 3D negative fingerprint and honeycomb textured SU-8 surfaces

The effects of two different textures (a 3D negative fingerprint texture and a honeycomb texture) on the tribological performance of SU-8 polymer surface have been investigated with a ball-on-disc tribometer. Friction and wear behaviors of the textured surfaces are conducted against a 4 mm diameter silicon nitride (Si₃N₄) ball counterface. The coefficient of friction for the negative fingerprint textured surface (μ=∼0.08) is much lower than that of the untextured surface (∼0.2) and the honeycomb textured surface (∼0.41) under a normal load of 100 mN and a rotational speed of 2 rpm. The coefficients of friction of the textured surfaces decrease with increasing normal loads between 100 mN and 300 mN. Above the normal load of 300 mN, the coefficient of friction of the negative fingerprint textured surface increases due to the occurrence of plastic deformation. The honeycomb textured surface has shown the highest coefficient of friction. The wear durability tests are also conducted at a normal load of 100 mN and a rotational speed of 500 rpm on the untextured/textured surfaces on SU-8 in the presence of an overcoat of a nano-lubricant, perfluoropolyether(PFPE). Six samples i.e. the untextured surface (Si/SU-8 and Si/SU-8/PFPE), the 3D negative fingerprint textured surface (Si/SU-8/FP and Si/SU-8/FP/PFPE) and the honeycomb textured surface (Si/SU-8/HC and Si/SU-8/HC/PFPE), each with and without PFPE nano-lubricant, have been investigated for their tribological behaviours. The negative fingerprint pattern on SU-8 with PFPE coating has shown the highest wear life of 60,000 cycles under a normal load of 100 mN. The reasons for excellent tribological performance of 3D fingerprinted SU-8 surface are analyzed using the Hertzian contact area calculation.     

Self-lubricating SU-8 Nanocomposites for Microelectromechanical Systems Applications

SU-8 is an industrially useful photoresist polymer for micro-fabrication because of its unique UV-sensitive curing property. It is also used as a structural material for micro-machines such as micro-electro mechanical systems (MEMS). However, it has poor tribological and mechanical properties which make SU-8 inferior to Si, the mainstay MEMS material today. In this paper, we report the fabrication of SU-8 nanocomposites which are self-lubricating and have better mechanical properties. The liquid lubricant i.e., perfluoropolyether (PFPE) and nanoparticles such as SiO₂, CNTs, and graphite were added into SU-8 for this purpose. These self-lubricating SU-8 + PFPE and SU-8 + PFPE + nanoparticle composites have shown a reduction in the initial coefficient of friction by ~6?C9 times and increased wear life by more than four orders of magnitude. The mechanical properties such as the elastic modulus and the hardness have increased by ~1.4 times. These SU-8 nanocomposites can be used as a self-lubricating structural material for MEMS applications requiring no external lubrication. As well, these nanocomposites can find applications in many tribological components of traditional machines.     

Carbon Nanotube Reinforced Polyimide Thin-film for High Wear Durability

In this paper, the influence of single walled carbon nano tubes (SWCNTs) addition on the tribological properties of the polyimide (PI) films on silicon substrate was studied. PI films, with and without SWCNTs, were spin coated onto the Si surface. Coefficient of friction and wear durability were characterized using a ball-on-disk tribometer by employing a 4 mm diameter Si₃N₄ ball sliding against the film, at a contact pressure of ∼370 MPa, and a sliding velocity of 0.042 ms⁻¹. Water contact angle, AFM topography, and nano-indentation tests were conducted to study the physical and mechanical properties of the films. SWCNTs marginally increased the water contact angle of PI film. The addition of SWCNTs to PI has increased the hardness and elastic modulus of pristine PI films by 60–70%. The coefficient of friction of PI films increased slightly (∼20%) after the addition of SWCNTs, whereas, there was at least two-fold increase in the wear life of the film based on the film failure condition of coefficient of friction higher than 0.3. However, the film did not show any sign of wear even after 100,000 cycles of rotation indicating its robustness. This increase in the wear durability due to the addition of the SWCNTs is believed to be because of the improvement in the load-bearing capacity of the composite film and sliding induced microstructural changes of the composite film.     

Tribological analysis of tip-cantilever made of SU-8, talc and PFPE composite

ABSTRACT

SU-8 polymer with talc particle (30?wt-%) and liquid perfluoropolyether (PFPE) (30?wt-%) fillers was used as a composite to fabricate conical tip-cantilever device. The composite tip demonstrated lower coefficient of friction (～0.22) when compared with a tip made of pure SU-8 (～0.65). Fluorine was detected on the wear track and the tip surface, which resulted from the transfer of PFPE from the tip to the wear track. The counterface made of pure SU-8 remained smooth and unworn when slid against the composite tip even after 1000 cycles of sliding. This composite with improved tribological and mechanical properties can be used for fabricating small component devices such as for micro-electro-mechanical systems (MEMS).     

The Role of Functional End Groups of Perfluoropolyether (Z-dol and Z-03) Lubricants in Augmenting the Tribology of SU-8 Composites

In an earlier work, we demonstrated the development of SU-8 composites using perfluoropolyether (PFPE) as lubricant filler which reduced friction coefficient by ~7 times and enhanced wear life of SU-8 by more than four orders of magnitude. In this work, we have investigated the role of chemical bonding between SU-8 and PFPE molecules using two types of PFPE lubricants (i.e., Fomblin^® Z-dol and Z-03) in improving the tribological properties of the composite. Z-dol has polar (–OH) end groups whereas Z-03 has non-polar (CF₃) end groups. SU-8 with Z-dol (SU-8 + Z-dol) films yielded ~8 times greater wear life than SU-8 with Z-03 (SU-8 + Z-03) films and more by four orders of magnitude than pure SU-8. The nature of the films was analyzed in detail by chemical and physical characterization techniques like X-ray photoelectron spectroscopy, water contact angle and thermo-gravimetric analysis. The results validated the role of polar end functional group of Z-dol in covalent binding with SU-8 upon UV plasma treatment that resulted in improved tribological properties.     

The effect of molecular orientation through uniaxial prestraining in poly(vinyl chloride) and polycarbonate on their friction and wear characteristics

The mechanical and tribological properties of polymers generally depend on the exact details of their molecular arrangements, i.e. both their crystalline morphology and their molecular orientation, these being intimately related. The objective of this paper is to investigate experimentally the effect of uniaxial prestraining of specimens of poly(vinyl chloride) (PVC) and polycarbonate (PC) on their friction and wear characteristics.For this purpose tensile sheet specimens of PVC and PC were uniaxially stretched to varying final extensions, thus obtaining various degrees of molecular orientation and mechanical anisotropy. Short pins were subsequently cut from these deformed sheets and tested at a sliding speed of 27.5 cm s⁻¹ and a moderate normal load of 49 N in an instrumented pin-on-disc friction and wear testing machine with stainless steel as the rotating disc material. Results obtained on the friction coefficient and wear rates for various samples are presented and correlations are made with uniaxial prestrain, mechanical energy to fracture and loss modulus.     

Tribological properties of composite multilayer coating

Abstract

The use of surface coatings is emerging as one of the most important approaches in reducing friction and wear in various tribological applications. Even though single layer coatings have a wide range of applications, the performance of the single layer alone may not always be adequate to meet the desired tribological property requirements. Hence, coatings consisting of multilayers to meet different property requirements in demanding applications are required. In this study, the tribological properties of a graded composite multilayer coating, with a specific layer sequence of MoS₂/Ti–MoS₂/TiBN–TiBN–TiB₂–Ti deposited on tool steel substrate, have been investigated at temperatures of 40 and 400°C respectively. The experimental results from the tests at 40°C have shown that the friction coefficient value ranges between 0·02 and 0·034. It was found that the deposition parameters influenced the friction and durability of the coatings. Higher substrate bias was found to result in higher friction, and the coating deposited at high substrate bias and low N₂ flow showed the lowest durability. The friction coefficient and durability of the coatings were found to be highly dependent on temperature. At high temperature, the friction coefficient increases almost threefold, and the durability decreases significantly.     

Tribological behaviour and wear mechanism of MoS2–Cr coatings sliding against various counterbody

MoS₂–Cr coatings with different Cr contents have been deposited on high speed steel substrates by closed field unbalanced magnetron (CFUBM) sputtering. The tribological properties of the coatings have been tested against different counterbodies under dry conditions using an oscillating friction and wear tester. The coating microstructures, mechanical properties and wear resistance vary according to the Cr metal-content. MoS₂ tribological properties are improved with a Cr metal dopant in the MoS₂ matrix. The optimum Cr content varies with different counterbodies. Showing especially good tribological properties were MoS₂–Cr8% coating sliding against either AISI 1045 steel or AA 6061 aluminum alloy, and MoS₂–Cr5% coating sliding against bronze. Enhanced tribological behavior included low wear depth on coating, low wear width on counterbody, low friction coefficients and long durability.     

Wear durability study on self-lubricating SU-8 composites with perfluoropolyther,multiply-alkylated cyclopentane and base oil as the fillers

Though SU-8 has become a useful material for micro-fabrication of MEMS/NEMS components using the micro-fabrication route, its poor tribological properties limit its wider applications. From our previous study [1], it was observed that adding PFPE lubricant to SU-8 possibly promoted chemical reaction between the molecules and helped in the boundary lubrication enhancing the wear durability of SU-8 by more than four orders of magnitude. For further investigation, another two different lubricants, a base oil and a multiply-alkylated cyclopentane (MAC) oil, were also added to SU-8. Both lubricants are hydrocarbons, chemically inert and have no polar reactive terminal groups unlike PFPE which has –OH polar terminal groups. SU-8+PFPE composite exhibited higher wear life than all SU-8 composites at all wt% of the lubricant content. Proper dispersion and possible chemical bonding of PFPE molecules with SU-8 are responsible for the superior tribological properties of SU-8+PFPE composite when compared with other SU-8 composites.     

Tribological behaviour of uni-directionally aligned silicon nitride against steel

Textured silicon nitride, where the β-Si₃N₄ grains were uni-directionally aligned, was fabricated and the effect of anisotropy in microstructure on tribological properties was investigated, compared with a conventional Si₃N₄. The wear tests were carried out for the tribopair of textured silicon nitride ceramic and steel using a block-on-ring tester without lubrication. For the textured Si₃N₄, tribological properties were evaluated in three directions with respect to the grain alignment; the plane normal to the grain alignment and in the directions parallel and perpendicular to the grain alignment in the side plane. The friction coefficient values of each specimen were of the same level under the same sliding conditions. The values of specific wear rate for the plane normal to the grain alignment were lower than those of the other specimens for all sliding conditions. It is considered that the high wear resistance of this plane was caused by restricted microfracture, such as grain dropping and minimal abrasion by wear debris. Both the friction coefficient and specific wear rate were decreased with increasing sliding speed and normal load because of the formation of lubricative FeO between the sliding surfaces.     

Effects of carbon fillers on the tribological and mechanical properties of an epoxy-based polymer (SU-8)

SU-8, an epoxy-based negative photoresist polymer, is highly suitable for making micro-electro-mechanical systems (MEMS) structures. Despite fabrication advantages, its bulk mechanical and tribological properties are the main limitations for application as MEMS material. Carbon filler materials such as graphene, graphite and multi-walled carbon nanotube (MWCNT) are added to SU-8 for tribological and mechanical property enhancements. SU-8/(5 wt%) graphite composite has performed better for the steady-state coefficient of friction at all loads including for the speed effect. SU-8/(5 wt%) MWCNT has shown excellent wear resistance. At 10 wt% graphite content, SU-8/graphite is superior in tribological performance to other composites tested.     

Impact of Cr3C2/VC addition on the dry sliding friction and wear response of WC–Co cemented carbides

Two grades of WC–10 wt.%Co cemented carbide with or without addition of Cr₃C₂/VC grain growth inhibitor during liquid phase sintering were produced with the goal to investigate their reciprocating sliding friction and wear behaviour against WC–6 wt.%Co cemented carbide under unlubricated conditions. The tribological characteristics were obtained on a Plint TE77 tribometer using distinctive normal contact loads. The generated wear tracks were analyzed by scanning electron microscopy and quantified topographically using surface scanning equipment. The post-mortem obtained wear volumes were compared to the online assessed wear. Correlations between wear volume, wear rate and coefficient of friction on the one hand and sliding distance and microstructural properties on the other hand were determined, revealing a significant influence of Cr₃C₂/VC on the friction characteristics and wear performance.     

Effect of densification cycles on continuous friction behavior of carbon–carbon composites

This paper studies the continuous friction behavior of carbon–carbon composites (C/C) fabricated by the coal tar pitch densification process. In the densification process, different numbers of densification cycle are adopted to investigate the influence on physical properties, mechanical properties, microstructure and friction behavior. Experimental results indicate that open porosity decreases with the number of densification cycle. Both bulk density and flexural strength increase with the number of densification cycle. Apparent reduction in wear rate and average friction coefficient can be detected even if the lower densification cycle is adopted. As the number of densification cycle increases, C/C specimens exhibit the lower wear rate and smoother friction coefficient curves. Morphological observations show that the number of open pores for these materials decreases and they exhibit a denser morphology as the number of densification cycle increases. Furthermore, as the number of densification cycle increases, a smooth, adherent lubricating film is formed on the sliding surface. Therefore, the variation in average friction coefficient of specimens becomes smoother; in the meantime, the wear rate becomes lower.     

Friction and Wear Behavior of Single-Phase Fe2B Bulk under Dry Sliding Condition

The effects of normal load and velocity on the friction and wear behavior of single-phase Fe₂B bulk have been investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Results indicate that the friction coefficient and wear rate both decrease at first and then increase with increasing load and velocity, respectively. Attributed to the formation of a lamellar film on the Fe₂B surface, the lowest friction coefficient and wear rate are obtained at a velocity of 0.2 m/s under a load of 12 N. The dynamic friction coefficients under loads of 4 and 12 N are around 0.8 in the initial steady stage and then decrease to about 0.6, whereas the friction coefficient at 20 N shows no obvious change and remains around 0.82. The lubricating film consisting of Fe₂O₃, B₂O₃, SiO₂, and H₃BO₃ reduces the friction coefficient at 0.2 m/s under a load of 12 N.     

Effect of fiber orientation on friction and wear of fiber reinforced polymeric composites

The friction and wear behavior of composites (i.e. uniaxially oriented graphite fiber-epoxy, Kevlar fiber-epoxy and biaxially oriented glass fiber-MoS₂-polytetrafluoroethylene (PTFE)) was investigated as a function of varying fiber orientations with respect to the sliding direction. In graphite fiber-epoxy composites, both wear and friction coefficients were minimum when the orientation of the fibers was normal to the sliding surface. In Kevlar-epoxy composites when the fibers were oriented normal to the surface and the sliding direction, the wear rate was also minimum but the friction coefficient was the highest. In glass fiber-MoS₂-PTFE composites wear was minimum when the largest fraction of fibers was oriented normal to the sliding surface.     

The effect of Al2O3 on the friction performance of automotive brake friction materials

This study consists of two stages. In the first stage, bronze-based break linings were produced and friction-wear properties of them were investigated. In the second stage, 0.5%, 1%, 2% and 4% alumina (Al₂O₃) powders were added to the bronze-based powders and Al₂O₃ reinforced bronze-based break linings were produced. Friction–wear properties of the Al₂O₃ reinforced samples were aslo investigated and compared to those of plain bronze-based ones. For this purpose, friction coefficient and wear behaviour of the samples were tested on the grey cast iron disc. The hardness and density of the samples were also determined. Microstructures of the samples before and after the sintering and the worn surfaces of the wear specimens were examined using a scanning electron microscope (SEM). The sample compacted at 350 MPa and sintered at 820 °C exhibited the optimum friction–wear behaviour. With increase in friction surface temperature, a reduction in the friction coefficient of the samples was observed. The lowest reduction in the friction coefficient with increasing temperature was for the 2% and 4% Al₂O₃ reinforced samples. The SEM images of the sample indicated that increase in Al₂O₃ content resulted in adhesive wear. With increase in Al₂O₃ content, a reduction in mass loss of the samples was also observed. Overall, the samples reinforced with 2% and 4% Al₂O₃ exhibited the best results.     

Tribological behavior of Al–Si–SiCp composites/automobile brake pad system under dry sliding conditions

Tribological behavior of stir-cast Al–Si/SiC_p composites against automobile brake pad material was studied using Pin-on-Disc tribo-tester. The Al-metal matrix composite (Al-MMC) material was used as disc, whereas the brake pad material forms the pin. It has been found that both wear rate and friction coefficient vary with both applied normal load and sliding speed. With increase in the applied normal load, the wear rate was observed to increase whereas the friction coefficient decreases. However, both the wear rate and friction coefficients were observed to vary proportionally with the sliding speed. During the wear tests, formation of a tribo-layer was observed, presence of which can affect the wear behavior, apart from acting as a source of wear debris. Tribo-layer formed over the worn disc surfaces was found to be heterogeneous in nature. Morphology and topography of worn surfaces and debris were studied using scanning electron microscope (SEM). Chemical composition of different wear products was obtained using electron probe micro analyzer (EPMA) and X-ray diffraction (XRD) techniques. Possible wear mechanisms operative in Al-MMC—brake pad tribo-couple have been discussed.     

High-Temperature Tribological Behavior of Al-20Si-5Fe-2Ni/ZrB2 Composites

Abstract

In this work, Al-20Si-5Fe-2Ni/ZrB₂ composites with 0–20?wt% ZrB₂ were fabricated by spark plasma sintering. The effects of ZrB₂ content on the microstructure, mechanical properties and high-temperature tribological behavior of the composites were investigated. The results indicate that Si, Al₅FeSi, and ZrB₂ particles are uniformly distributed in the aluminum matrix. The density, hardness, and compressive strength increase with increasing ZrB₂ content. The friction coefficient and wear rate are dependent on the ZrB₂ content and test temperature. At a certain temperature, the friction coefficient increases with an increase in ZrB₂ content, whereas the wear rate shows a reverse trend. Due to the improvement in thermal stability and high-temperature softening resistance, the composite shows improved wear resistance and increased transition temperature from mild wear to severe wear.     

Tribological characterisation of a‐C:H coatings at room temperature: Effect of counterbody material

Metal‐free amorphous carbon (a‐C:H) coatings with 15% hydrogen were deposited on bearing steel surfaces. The friction and wear performance of these specimens was characterised in oscillating sliding tests with a ball‐on‐flat geometry. Balls of four ceramic and four metallic materials were investigated in tests at room temperature. Special attention was paid to the effect of moisture by testing in dry, normal, and moist air. The effect of water vapour on the friction and wear of the a‐C:H coatings was quite different for the different counterbody materials. The wear was in all cases very low, with a coefficient of wear below 10⁻⁷ mm³/N m for most cases. The coefficient of friction was also very low, between 0.04 and 0.12 for most of the tests. The smallest wear and friction coefficients were found for oxide ceramics, while during tests against SiC and Si₃N₄ the coating was worn through during the test. The effects of counterbody material and the humidity of the surrounding air are discussed in terms of friction and wear mechanisms.