Patent Application
20130130951
Various embodiments of present disclosure are directed to self-lubricating polymer materials providing desirable, improved, or significantly enhanced tribological and mechanical properties. In multiple embodiments, such materials include one or more types of photosensitive or non-photosensitive polymers (e.g., SU-8, KMPR, or JSR THB 151N photoresist; polymethylmethacrylate (PMMA), polydimethylsiloxane (PDMS); or another material) that are combined or mixed (e.g., homogeneously blended) with a liquid lubricant (e.g., a perfluoropolyether (PFPE) and/or other lubricant); and optionally one or more types of nanoparticles or nanostructures (e.g., Carbon nanotubes (CNTs), SiO2 nanoparticles, Graphite nanoparticles, and/or other types of nanoparticles). Particular embodiments in accordance with the present disclosure are directed to low friction, mechanically robust materials that can (1) be patterned to form microstructures or nanostructures in association with the fabrication of microelectromechanical (MEMS) or nanoelectromechanical (NEMS) components or devices; (2) provide a solid lubricant for sliding/moving/semi-moving/physically couplable non-biological or biological structures or interfaces; and/or (3) form a portion or surface of a lubricating tape (“Lub-tape”) that can be selectively or preferentially applied to a variety of structures or interfaces, e.g., to reduce friction.
MEMS are miniaturized devices consisting of one or more micro machined components or structures. These devices are usually fabricated through the integration of several components with diversified functions such as mechanical, electrical, magnetic, biological etc. . . . functions. In the past two decades, many MEMS devices have been successfully put into applications, although the majority of MEMS devices are still under research or investigation. MEMS device research and development is rapidly emerging, and now extends to nearly every industrial field including automotive, bio-engineering/biomedical, telecommunications, electronics, space, military and gaming industries, etc. . . . [Marinis 2009].
In general, despite ongoing research and development activities, the reliability of MEMS devices needs significant improvement. One of the major issues limiting the reliability of MEMS devices is tribology, specifically friction, adhesion and wear. If two non-lubricated MEMS surfaces come into contact, they show high adhesion force, high friction and eventually lead to wear (loss of material). Hence, if these tribological issues are not addressed properly, they limit the operating life of MEMS devices and eventually their reliability [Kim et al 2007].
As the size of the MEMS devices is measured in micro-meters (or sub-millimeters), the surface-to-volume ratio is very high when compared to the gravity and inertial forces, and hence surface forces such as van der Waals, capillary, electrostatic, and chemical forces play an important role in device performance. Hence, in these devices, the tribological and the interfacial forces are comparable to or higher than the forces causing device motion. Hence, there is a great need for the solutions which address tribological issues in MEMS devices.
The mainstay structural material from which MEMS devices are made has been Si, particularly as a result of the mature Si material fabrication capabilities (arising from integrated circuit/microchip industry) at the micro-meter scale. However, Si does not have good tribological properties and it shows high friction, adhesion force and wear, respectively, when sliding against itself or any other material. Therefore, thorough research was conducted over the past two decades in an effort to improve the tribological properties of Si. Because of the small length scales of MEMS devices, macro scale oil based lubrication methods are not applicable and hence researchers have developed certain specific types of nano-meter to sub-micrometer thick films (mostly solid lubricants) which reduce friction, adhesion force and wear, respectively. These thin films include self-assembled monolayers (SAMs), polymer coatings, vapor deposited organic layers, fluorine based organic layers, solid coatings etc [Patton et al 2007 & 2008; Knieling et al 2007; Henck 1997; Satyanarayana and Sinha 2005; Eapen et al 2005; Ma et al 2007; Sidorenko et al 2002 (a); Satyanarayana et al 2009; Asay et al 2008 (a) & (b); Lee et al 2005; Prasad et al 2009 and Scharf et al 2006]. Though Si has several advantages, it has certain inherent drawbacks such as brittleness, high friction and adhesion force, non-biocompatibility etc. . . . Therefore, recently, SU-8 has been replacing Si for certain applications [Abgrall et al 2007].
SU-8 polymer is an epoxy-type UV-sensitive negative photoresist material which is used as a photoresist as well as a structural material in the fabrication of MEMS. SU-8 has several advantages such as ease of fabrication, hydrophobic nature, high thermal stability, good chemical resistance and biocompatibility which makes it an attractive material from which to fabricate MEMS devices. Despite its advantages, SU-8 has some disadvantages such as poor mechanical and tribological properties, respectively, and high internal stresses which limit its use as a structural material for fabricating reliable MEMS structures with complex designs and multiple functions.
Jiguet et al [2006 (a) & (b)] have studied the effect of the addition of silica nanoparticles (diameter: 13 nm and concentration: 5 wt %) to SU-8, and the effect of thermal treatment on the friction and the wear properties of SU-8. Sliding tests were conducted against steel and POM (polyoxymethylene) balls. The SU-8 nanocomposites reduced wear rates and friction coefficients marginally when compared to the un-reinforced SU-8. The improvement was very minimal when compared to the stringent property requirements of
MEMS devices in terms of their reliability, longer operating life time and accuracy. This study has also deduced that the coefficient of friction of the composites depends on the counterface material and the elastic properties of the SU-8 material. This study has also shown that heat treatment can considerably reduce the wear rate of reinforced and un-reinforced materials.
Singh et al [2011 (a)] have developed a two-step SU-8 surface modification method (i.e., modification of an outer surface of an SU-8 structure) which has improved the tribological properties of SU-8 film surfaces by several fold. The two-step surface modification consists of first treating an SU-8 film surface with oxygen plasma, followed by the application of a nanolubricant such as PFPE. By the application of the two-step surface modification method to SU-8 thin (thickness: 500 nm) and thick films (thickness: 50 μm) on Si surfaces, the initial coefficient of friction has been reduced by ˜4-7 times, the steady-state coefficient of friction has been reduced by ˜2.5-3.5 times and the wear durability has been increased by >1000 times. The authors have done the tribological tests under the loading conditions of a normal load of 150 g and a rotational speed of 200 rpm. The two-step surface modification method has slightly reduced the elastic modulus and hardness of pristine SU-8 thick films, as observed in nanoindentation tests.
Singh et al [2011 (b)] have also developed a two-step SU-8 chemical modification method (i.e. modification of an outer surface of an SU-8 structure) which has improved the tribological properties of SU-8 film surfaces by several fold. The two-step chemical modification method consists of first chemically treating an SU-8 film using ethanolamine-sodium phosphate buffer, followed by the coating of PFPE nanolubricant. By the application of the two-step chemical modification method to SU-8 thin (thickness: 500 nm) and thick films (thickness: 50 μm) on Si surfaces, the steady-state coefficient of friction has been reduced by ˜4-5 times and the wear durability has been increased by >1000 times. The authors have done the tribological tests under the loading conditions of a normal load of 150 g and a rotational speed of 200 rpm. The authors have attributed the significant reduction in the friction coefficients to the lubrication effect of PFPE nanolubricant, while the exceptional increase in their wear life was attributed to the bonding between the —OH functional groups of ethanolamine treated SU-8 thin/thick films and the —OH functional groups of PFPE.
Voigt et al [2007] have demonstrated the miscibility of epoxy resin surface-modified SiO2 nanoparticles (average particle diameter: 20 nm) into epoxy photomaterial to create a photo-patternable material with improved lithographic, optical and mechanical properties. The addition of the epoxy resin surface-modified SiO2 nanoparticles to epoxy resin has increased the Young's modulus as observed in the nanoindentation tests and it was observed that the Young's modulus has increased with the content of the SiO2 particles. For the highest silica content added, the Young's modulus has increased from 5.8 GPa to 8.7 GPa.
Chiamori et al [2008] have investigated the mechanical properties of SU-8 composite materials added with diamondoids and SWCNTs (single-walled carbon nanotubes) (average diameter: 0.8 nm and concentration: 1 wt % and 5 wt %). Uniaxial tensile tests were conducted on nanocomposite samples using a MTS Bionix 200 tensile tester and the effective elastic modulus was extracted from the force-displacement curves and geometry of the samples. SU-8 has shown an elastic modulus of 1.6 GPa whereas the diamondoid and SWCNTs added SU-8 has shown elastic modulus values of 1.9 GPa and 1.3 GPa, respectively. In their study, the addition of the nanoparticles did not show any significant improvement in the mechanical properties of SU-8 (SWCNTs have in fact slightly reduced the elastic modulus of SU-8).
Mionic et al [2010] have fabricated SU-8+MWCNT (multiwalled carbon nanotubes) (concentration: 5 wt %) composites and measured their mechanical properties using nanoindentation. They have also studied the influence of SU-8 solvent on the structural homogeneity and mechanical properties of the composites. They have observed that the solvent type and the functionalization of MWCNTs affect the Young's modulus of the composites. In their study, a highest increase of the Young's modulus of 104% in respect to the parent material was observed when acetone was used as the solvent.
From the above studies it is clear that researchers have developed different strategies in their attempts to improve either tribological properties or mechanical properties of SU-8. That is, the above studies have demonstrated independent improvement in tribological properties of SU-8, without improvement in mechanical properties of SU-8; or independent improvement in the mechanical properties of SU-8, without improvement in tribological properties of SU-8.
Liquid lubricants have been added to certain polymer nanolayers [Julthongpiput et al 2002 (a) and (b), Sidorenko et al 2002 (b) and Ahn et al 2003] and also to other polymers such as high density polyethylene (HDPE), polyolefin and ultra-high-molecular weight polyethylene (UHMWPE) [Puukilainen et al 2005, 2006, 2007]. In research works conducted by Tsukruk and co-workers [Julthongpiput et al 2002 (a) and (b), Sidorenko et al 2002 (b) and Ahn et al 2003], paraffin oil (C15H32—C24H50) was added into a polymer nanolayer of poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS), and the enrichment of paraffin oil into the polymer nanolayers improved wear durability.
Whereas, in the research works conducted by Puukilainen and co-workers [Puukilainen et al 2005, 2006, 2007], they have blended PFPE into polymers such as HDPE, polyolefin and UHMWPE and the blending of PFPE into these polymers has improved hydrophobic property and the tribological properties (i.e., lowered the coefficient of friction and increased abrasion resistance).
A need clearly exists for a manner of simultaneously improving the tribological and mechanical properties of SU-8 or other materials that can be used to produce MEMS components or devices.
In accordance with an aspect of the present disclosure, a hybrid polymer composition, material, or structure (also referred to herein as a hybrid polymer nanocomposite) includes (a) at least one polymer such as SU-8, KMPR, JSR THB 151N, PMMA, or PDMS; (b) at least one liquid lubricant such as PFPE, mineral oil, base oil, multiply alkylated cyclopentane (MAC) lubricant, or a plant-based oil such as a vegetable oil; and optionally (c) at least one inorganic filler material such as CNTs, SiO2, graphite, and/or other particles, where the at least one polymer, the at least one liquid lubricant, and the at least one inorganic filler material are distributed relative to each other within the composition in a substantially homogeneous manner. The PFPE and/or other lubricant(s) are included for the purpose of improving tribological properties, and the inorganic filler material(s), if present, such as CNTs, SiO2 and graphite particles are included to improve mechanical properties. Compared to SU-8 by itself or in isolation, such SU-8 hybrid polymer compositions have reduced the coefficient of friction of SU-8 by ˜6 times, improved the wear life by >2000 times, improved the elastic modulus and the hardness by ˜1.4 times, respectively.
In accordance with related aspects of the present disclosure, a process for producing or manufacturing a hybrid polymer composition includes combining the polymer(s), the liquid lubricant(s), and the optional nanomaterial(s) to form a homogeneous or substantially homogeneous mixture (e.g., which can exist in a substantially liquid form prior to one or more curing or partial curing procedures); and a process for producing or manufacturing a device, component, or element that includes a hybrid polymer nanocomposite involves producing a set of partially or fully cured hybrid polymer nanocomposite layers, and forming micrometer and/or nanometer scale features, structures, or patterns in such layers.
In accordance with a further aspect of the present disclosure, a polymer—liquid lubricant layer, coating, or film, such as an SU-8+PFPE coating, can be formed using a coating or deposition technique such as spin-coating or simple dip-coating, onto different substrates such as Si, glass, steel, stainless steel or aluminum. On all these substrate materials, SU-8+PFPE coating has improved the tribological properties by several orders.
In accordance with yet another aspect of the present disclosure, a film and/or laminate structure, such as a self-contained, isolatable/isolated, or stand-alone film, that includes a hybrid polymer film (e.g., an SU-8 nanocomposite film) can be used to form a lubricating interface or tape for mechanical components where improved tribological properties are desired. Such a film or laminate structure can be selectively disposed or positioned (e.g., adhered) as a counterface material relative to particular structural components, elements, or features.
Various embodiments in accordance with the present disclosure are directed to a homogeneous, substantially homogeneous, or generally homogeneous composition that includes the following:
Compositions in accordance with various embodiments of the present disclosure can include approximately 70% -99.5% base polymer(s) (e.g., approximately 80% -99.9%, or about 95% base polymer); approximately 0.1% -29.5% liquid lubricant(s) (e.g., about 1% -20%, or about 2% -8%, or about 5% liquid lubricant); and possibly/optionally approximately 0.4% - 29.9% nanomaterial(s) (e.g., about 1% - 20%, or about 5% - 15%, or about 10% nanomaterials), where such percentages can be weight percentages, and where such percentages are selectable or adjustable to yield a 100% composition in a manner readily understood by one of ordinary skill in the relevant art.
A composition in accordance with the present disclosure exhibits enhanced tribological and mechanical properties compared to the composition's polymer(s) considered in isolation. Depending upon embodiment details, compositions in accordance with the present disclosure can reduce a coefficient of friction by at least approximately 50%; increase wear life by at least approximately 100%, and in various embodiments at least approximately an order of magnitude; increase elastic modulus by at least approximately 5% -10%; and/or increase hardness by at least approximately 5% -10% relative to the composition's polymer(s) considered by themselves or in isolation. The selection of a given percentage of base polymer(s), liquid lubricant(s), and/or optional nanomaterial(s) within a composition in accordance with the present disclosure, and correspondingly the composition's resulting tribological and/or mechanical properties, can be based upon tribological, mechanical, spatial/volumetric, and/or other requirements or specifications associated with a surface or environment in which a composition in accordance with the present disclosure is intended to be used.
Compositions in accordance with the present disclosure can exist in various forms, such as bulk materials, coatings, films, laminates, inserts, or overlays. Furthermore, compositions in accordance with the present disclosure can provide or form assemblies, structures, devices, components, elements, or interfaces having enhanced or dramatically enhanced tribological and/or mechanical properties relative to a very wide range of non-biological applications (e.g., involving sliding, moving, or physically couplable mechanical surfaces, interfaces, or components) and biological applications (e.g., involving medical devices or instruments such as catheters, stents, tubes, needles, syringes, knives, etc. . . . for which reduction or minimization of friction and adhesion between the medical device or instrument and bodily tissue is desired or required).
Some specific non-limiting examples of MEMS applications in which compositions in accordance with the present disclosure can be used or incorporated to simultaneously enhance the tribological and mechanical properties of MEMS structures, components, or devices include RF MEMS switches, MEMS micro engines/motors, MEMS stages, MEMS watches, micro-gears, micro-shutters, micro-minors etc. An individual of ordinary skill in the relevant art will understand that compositions in accordance with embodiments of the present disclosure can also be used to enhance tribological and/or mechanical properties, and in various embodiments simultaneously enhance both tribological and mechanical properties in MEMS, NEMS, and/or other types of structures, components, devices, or media.
In some embodiments, compositions in accordance with the present disclosure can be carried by, incorporated into or onto, or form a portion of a load/unload ramp material or structure in data storage devices such as hard disk drives. More particularly, in recent years, a dynamic ramp load/unload technique has replaced the conventional contact-based start/stop technique in desktop and server had disk drives. This ramp load/unload technique has led to a new tribological interface between a ramp structure and a suspension element. Friction between the ramp structure and the suspension element plays an important role in hard disk drive reliability. For adequate or maximum hard disk drive reliability, this new tribological interface requires extremely low friction and minimum or essentially no wear particle generation. Hard disk drive ramp materials or structures can include one or more compositions in accordance with embodiments of the present disclosure (e.g., an SU-8 based composition in accordance with the present disclosure can be used as a hard disk drive ramp material), thereby meeting or exceeding such friction and wear particle generation requirements.
Depending upon embodiment details, the order of formation of the first and second homogeneous mixtures can be reversed. Additionally, mixing or blending can involve manual and/or automatic mixing techniques. Moreover, liquid lubricant(s) and nanoparticles can be added to the base polymer in separate process portions as indicated above, or simultaneously or substantially simultaneously. Furthermore, mixing, blending, or stifling can be accompanied by the application of heat for a given or predetermined period of time. For instance, forming a homogeneous mixture of an SU-8 base polymer and a PFPE lubricant can involve magnetic stifling at approximately 70 degrees C. for about 1-2 hours. A stirring technique, duration, and/or temperature can be selected in a manner that avoids under-stirring (which can lead to inhomogeneity, the presence of air pockets in a coated layer, or improper bonding between the composition and a substrate) and over-stirring (which can lead to partial solvent evaporation, internal curing, non-uniform composition distribution or dispersion over a substrate, and/or fibrous structure formation). During a mixing process, materials properties or characteristics can be monitored manually (e.g., visually) or automatically (e.g., by measuring optical properties) on a periodic, regular, or essentially continuous basis. Following the preparation or manufacture of a composition in accordance with the present disclosure, the composition can be applied to a substrate (e.g., via spin or dip coating) within a recommended or predetermined time period, or essentially immediately to avoid internal curing.
In accordance with a particular aspect of the present disclosure, a hybrid nanocomposite material or composition that includes SU-8 (also referred to herein as an “SU-8 based hybrid nanocomposite” or “SU-8 nanocomposite”) is provided which includes SU-8, PFPE lubricant, and nanoparticles corresponding to SiO2, CNTs and/or Graphite. SU-8 based hybrid nanocomposites in accordance with embodiments of the present disclosure have simultaneously reduced the coefficient of friction of SU-8 by ˜6 times, improved wear life by >2000 times, and improved the elastic modulus and hardness by ˜1.4 times, respectively. SU-8 nanocomposites in accordance with embodiments of the disclosure can be used as a self-lubricating structural material for MEMS or other types of structures, components, devices, or media where contacting surfaces are present, and hence self-lubricating SU-8 nanocomposites in accordance with embodiments of the disclosure can minimize or eliminate the need for external, extrinsic, or non-native lubrication. It will be appreciated by a person of ordinary skill in the relevant art that several components, processes, material applications or alternatives thereof in accordance with aspects of the present disclosure can be desirably combined into other or different components, processes, and/or material applications.
MEMS or other types of components where improved tribological properties are desired or required can be made using SU-8 nanocomposites in accordance with embodiments of the present disclosure, for instance, where no or essentially no need for external lubrication exists. Depending on the specific objectives or requirements of a MEMS or other application under consideration, a selected nanostructure or nanoparticle filler material can be combined, mixed, or blended with or added to SU-8 along with PFPE such that improved, significantly improved, or dramatically enhanced (1) lubricity as well as (2) mechanical and possibly other desired or required properties can be synergistically provided or achieved.
In accordance with another particular aspect of the present disclosure, SU-8+liquid lubricant (e.g., SU-8+PFPE) based nanocomposite films can be carried by or fabricated on different types of carriers, support members, or substrates. For instance, SU-8 nanocomposite films were fabricated on Si, glass, steel, stainless steel, and aluminum by way of dip-coating, and their tribological properties were evaluated. The SU-8+PFPE composite film on all the above substrates have shown significant improvement in tribological properties when compared to the bare substrate.
As SU-8+PFPE based films can be applied onto essentially any type of surface with simple dip coating, such a composite material can be used for essentially any type of mechanical component where improved tribological properties are desired or needed. Some specific non-limiting examples include gears, cams, piston, pivots, seals, couplings, engine, guides, bearings, shafts etc.
In addition or as an alternative to the foregoing, embodiments of the present disclosure can accommodate or include base or reference polymer substitution, variation, or modification. For instance, SU-8 material is available in different grades, and particular desired or required application objectives or requirements can influence or determine the respective grade of SU-8 to be used. For instance, SU-8 2050 has been used for spin coating experiments, and SU-8 2005 has been used for dip coating experiments. Alternatively, other SU-8 based materials, or another type of photoresist material such as KMPR (MicroChem Corp., Newton, Mass.) or JSR THB 151N (JSR Micro, Inc., Sunnyvale, Calif.), can be used to achieve essentially identical, similar, or analogous results.
In representative embodiments, PFPE Z-dol 4000 can be added to SU-8 as a liquid lubricant filler material to improve tribological properties. Other alternatives to PFPE Z-dol 4000, such as multiply alkylated cyclopentane (MAC), ionic liquid, pennzane, low cost base oils etc. . . . , can be used and substantially identical, similar, or analogous improvements in tribological properties can be achieved.
In various embodiments, one or more of SiO2, CNT and graphite particles can optionally be added to SU-8 as a structural or solid filler material for improving mechanical properties. A substantially identical, similar, or analogous improvement can be obtained by the addition of other types of filler materials (which can be organic or inorganic filler materials) such as MoS2, W52, TiO2, graphene, C60, UHMWPE, PTFE, PEEK etc. . . . The extent of improvement will depend on the type of filler material(s) and eventually the type of interaction(s) between SU-8 and the added filler material(s).
Spin coating and/or dip coating can be used to produce coatings, layers, or films in accordance with embodiments of the present disclosure on different types of substrates. Other techniques for producing coatings in accordance with embodiments of the disclosure, such as brushing, sol-gel, spraying, sputtering etc. . . . , can be used to obtain SU-8+PFPE nanocomposite coatings or layers having substantially identical, similar, or analogous properties.
In accordance with another aspect of the present disclosure, a coating, layer, or film (e.g., an SU-8 nanocomposite film) can be selectively or preferentially positioned relative to or upon a target surface or interface in order to improve the tribological and/or mechanical properties of the target surface or interface. Herein, the term “Lub tape” refers to various embodiments of (a) a coating, layer, or film (e.g., a substantially homogeneous hybrid polymer nanocomposite) that can be selectively positioned relative to a target surface, interface, or structure; and possibly (b) a corresponding support film, layer, medium, or material that carries the film. The layer provides at least one of enhanced tribological and mechanical properties relative to its support medium. For instance, the layer can provide Lub-tape with a first side having target or desired tribological and/or mechanical properties; and the support medium can provide the Lub-tape with a second side having different tribological and/or mechanical properties (e.g., a higher coefficient of friction, a lower elastic modulus, and/or lower hardness) than the Lub-tape's first side.
Lub-tape can be applied or adhered (e.g., selectively or preferentially adhered) to or between surfaces where a reduction in friction and/or an increase in wear durability are desired, intended, or needed. The Lub-tape' s high(er) friction surface can be applied, adhered, attached, or otherwise secured to a target surface of interest, for instance, using a common adhesive material, backing, or glue (such as epoxy glue), while the Lub-tape's low friction surface faces against a target counterface material. Lub-tape can be applied or adhered to a surface prior to or at the beginning of mechanical wear processes associated with the surface on which the Lub-tape resides, or following a time period during which mechanical wear has already occurred.
More particularly, Lub-tape can significantly or dramatically reduce the friction of device parts that exhibit unnecessarily or undesirably high friction, such as device parts which are already deployed or which are in use or in application, and which exhibit increased friction (e.g., which have lost an initial low friction property) because of wear and tear. In such cases, SU-8 nanocomposite Lub-tape can be adhered to particular surfaces to extend or enhance their operating life. In addition to the use of Lub-Tape in structures or devices where the restoration of improved tribology is useful or needed (e.g., for components already in application, which have lost their low-friction and low-wear properties), Lub-Tabe can also be used in applications in which friction between two fresh sliding surfaces is undesirable and can or should be reduced or minimized.
In several embodiments, an overall thickness of an SU-8+PFPE based Lub-tape film is approximately 150 microns. If this thickness is not sufficient to withstand higher loads and/or higher speeds, e.g., in macro-machinery applications, films with greater thickness (e.g., exceeding 150 microns, up to approximately 2 mm or more) can be produced. Using an appropriate SU-8 grade, a thickness of up to 300 μm can easily be obtained in one spin-coating process while up to 2 mm thicknesses can be obtained by multilayer coatings using multiple spin-coatings [Conradie and Moore 2002].
Experiments or tests to determine tribological and mechanical properties of various SU-8 based films, including nanocomposite films, in accordance with embodiments of the present disclosure are provided as follows:
In this experiment, SU-8 based films were spin coated onto Si and then they were subsequently characterized for their tribological properties and mechanical properties.
Si wafers were cut into ˜2 cm×2 cm pieces and were thoroughly cleaned with soapy water, distilled water and isopropyl alcohol (IPA), respectively, and finally dried with N2 gas. The cleaned Si wafers were then subjected to heating at 150° C. for about 3-4 min to remove any adsorbed moisture content and then subsequently treated with the oxygen for about 15-20 min using plasma cleaner PDC-32G (Harrick plasma, N.Y., USA). The purpose of the oxygen plasma treatment was to remove the contaminants and to generate hydroxyl groups on the surface which eventually enhance the adhesion between the substrate and the coating. The cleaned Si wafers were then subjected to the spin coating immediately by using SCS P6700 spin coater (Speciality Coating Systems, Indiana, USA). The SU-8 used for the sample preparation was SU-8 2050 (Microchem Ltd, USA). For the preparation of the pristine SU-8 films, SU-8 was spin coated onto Si at an initial speed of 500 rpm for duration of 5 seconds, followed by an increase in the spinning speed to 3000 rpm for duration of 30 seconds which results in SU-8 films with a thickness of ˜150 microns. The spin coated SU-8 films were then subjected to the pre-baking at a temperature of 65° C. for 4 minutes followed by 95° C. for 9 minutes. The pre-baked SU-8 films were then exposed to UV (ultra-violet) rays (wavelength: 365 nm and power: 210 mJ/cm2) for a duration of 1 min by using Black-Ray B-100SP UV lamp (UVP, LLC, upland, CA, USA). A post exposure bake was carried out at a temperature of 65° C. for 1 minute followed by 95° C. for 7 min, after UV exposure. The samples were then stored in the desiccators before any further characterization. The conditions of the spin coating, pre-baking, UV curing, post-baking are approximately same for both pristine SU-8 films and SU-8 nanocomposite films.
For the preparation of the SU-8+PFPE nanocomposites, a 5 wt % of PFPE was added to SU-8 2050 and then the composite material was thoroughly mixed by using magnetic stirring for about 2 hrs and then the composite solution was used for spin coating. Whereas for the preparation of SU-8+PFPE+NP (NP stands for nanoparticle), 5wt % of PFPE and 5 wt % of NP was added to SU-8 2050 along with few drops of SU-8 thinner and the whole mixture was thoroughly mixed using magnetic stirring for about 2 hr and then the mixture was used for spin coating.
Friction and wear tests were carried out using UMT-2 (Universal Micro Tribometer,
CETR, USA) in ball-on-disk mode where the coefficient of friction was measured with respect to the number of sliding cycles. A Si3N4 ball of 4 mm diameter with a surface roughness of 5 nm was used as the counterface. The tests were conducted at different normal loads (up to 300 g) and at different rotational speeds (up to 2000 rpm), respectively. All experiments were performed in air at room temperature (23° C.) and at a relative humidity of approximately 60%. From the sliding tests, an initial coefficient of friction (μi ) was noted after 16 s of sliding and the steady-state coefficient of friction (μs) was noted after the stabilization of the coefficient of friction and reported the same in the present document. The wear life for the tested conditions was taken as the number of sliding cycles after which the coefficient of friction exceeded 0.3 or a visible wear scar was observed on the substrate with fluctuating friction values, whichever occurred earlier. For each modification, 3 tests were repeated and average data are reported. An optical microscope was used to image the worn surfaces after the sliding tests to assess the damage to the film surfaces and the ball, respectively.
The elastic modulus and the hardness of the SU-8 based films were measured using a MTS Nano Indenter XP with a continuous stiffness measurement (CSM) technique. A triangular pyramid Berkovich diamond indenter was employed for nanoindentation tests. The depth of indentation was set to 3,000 nm. The CSM technique has the load and the displacement resolutions as 50 nN and <0.01 nm, respectively. A total of 10 indents were done on each sample and the tests were repeated on 3 different samples and an average value is reported.
The SU-8 films formed under the conditions provided in the experimental section have shown a thickness of ˜100 μm. Both the pristine and the SU-8 nanocomposite films have shown same thickness values.
indicates data missing or illegible when filed
Table 1 shows the initial coefficient of friction (μi), steady-state coefficient of friction (μs) and wear life of pristine SU-8 based films tested at different normal loads and rotational speeds. The tribological properties obtained at a normal load of 300 g and a rotational speed of 500 rpm will be mainly discussed in this section and the data at other loads can be referred from Table 1. The pristine SU-8 film has shown high friction properties (μi: 0.87 and μs: 0.76) and low wear life (n<500) when tested at a normal load of 30 g and a rotational speed of 200 rpm. The high coefficient of friction and low wear life of SU-8 clearly indicate the necessity for the works involving the methods to improve the tribological properties of SU-8. SU-8 molecules have poor lubricious nature and also low elastic modulus of SU-8 (E: ˜4-5 GPa) usually shows high contact area when pressed by the Si3N4 ball under the applied load and hence shows high friction. Under the loading conditions of the normal load of 300 g and the rotational speed of 500 rpm, SU-8+PFPE has shown very low coefficients of friction (μi: 0.04 and μs: 0.14) and high wear life (n>500,000). This is a dramatic improvement when compared with the pristine SU-8.
Table 2 shows the elastic modulus and the hardness data of SU-8 based films. Pristine SU-8 film shows an elastic modulus of ˜3.8 GPa and a hardness of ˜0.27 GPa and the addition of PFPE does not have a significant effect on the mechanical properties of SU-8 (for SU-8+PFPE, E: 4.0 GPa and H: 0.32 GPa) except a slight increase. Therefore, the nanocomposite SU-8+PFPE shows significant improvement in tribological properties (the μi has been reduced by ˜22 times, the μs has been reduced by ˜5 times and the wear life has been improved by >1000 times) with a marginal improvement in the mechanical properties when compared to pristine SU-8. SU-8+NP sample has shown a μi of 0.8, μs of 0.68 and the wear life of <1000 cycles and also E of 3.9 GPa and H of 0.17 GPa, respectively. This composite of SU-8 with the nanoparticle addition did not improve the tribological properties and have shown the similar elastic modulus as that of SU-8 and also have slightly reduced the hardness. Therefore, we further continued with the fabrication of hybrid SU-8 nanocomposites with the addition of PFPE and the nanoparticles.
The weight percent added in the nanoparticle composites was approximately 5 wt %. SU-8+PFPE+SiO2 has shown μi of 0.04, μs of 0.09 and wear life of >500,000 cycles and E of 4.5 GPa and H of 0.40 GPa. These properties are of significant improvement when compared to pristine SU-8. When compared to SU-8+PFPE, SU-8+PFPE+SiO2 has shown similar tribological performance and improved mechanical properties. SU-8+PFPE+CNTs has shown μi of 0.06, μs of 0.13 and wear life of >500,000 cycles and E of 4.0 GPa and H of 0.28 GPa. SU-8+PFPE+Graphite has shown μi of 0.05, μs of 0.25 and wear life of >500,000 cycles and E of 5.0 GPa and H of 0.16 GPa. Among the three hybrid SU-8 nanocomposites, all three have shown similar improvements in their tribological performance and SU-8+PFPE+Graphite has shown highest elastic modulus while SU-8+PFPE+SiO2 has shown highest hardness. Overall, these hybrid composites have shown improved tribological performance and mechanical properties, both at the same time. The selection of right composite entirely depends on the application i.e. high hardness/modulus requirement.
In view of the foregoing, it can be expected that in some embodiments, a simultaneous optimization of elastic modulus and hardness, while retaining significantly or greatly reduced coefficient of friction, can be achieved by way of a composition that includes SiO2 for purpose of hardness enhancement, plus Graphite for purpose of elastic modulus enhancement. Such a composition can include or be based upon a polymer such as SU-8; a liquid lubricant such as PFPE; SiO2; and Graphite.
In this experiment, SU8+PFPE based films were dip-coated onto different substrates such as Si, steel, stainless steel, aluminum and glass to establish its ability to work as a solid lubricant over the machine parts and then subsequently characterized for their tribological properties.
Substrates (Si, steel, stainless steel, aluminum and glass) of 2 cm×2 cm were cut and thoroughly cleaned with soapy water, distilled water and IPA, respectively. For dip-coating, we have to use the SU-8 with low viscosity and hence we have used SU-8 2005. For the preparation of dip-coated samples, the cleaned substrates were heated at 150° C. for few minutes to remove any adsorbed moisture and subsequently subjected to oxygen-plasma treatment for 15-20 min. The purpose of oxygen plasma treatment was to remove any contaminants and to generate functional groups which eventually enhance the adhesion between the substrate and the coating. A concentration of 5 wt % PFPE was added to SU-8 to prepare the composite films. The oxygen plasma treated samples were then subjected to dip-coating with SU-8 2005+PFPE. Dip-coating was done with a custom-built dip-coating machine using a dipping time of 60 s and a withdrawal speed of 2.1 mm/s. The dip-coated samples were then pre-baked at 95° C. for 4 min and then subjected to UV treatment (wavelength: 365 nm and power: 210 mJ/cm2) for 30 s and finally post-baked for 4 min at 95° C. The dip-coated samples were stored in desiccators before any further characterization.
Friction and wear tests were carried out using UMT-2 in the ball-on-disk mode. The Si3N4 ball of 4 mm diameter with the surface roughness of 5 nm was used as the counterface. The tests were conducted at a normal load of 300 g and a rotational speed of 500 rpm. For each modification, 3 tests were repeated and average data are reported. An optical microscope was used to image the worn surfaces after the sliding tests to assess the damage to the film surfaces and the ball, respectively.
The thickness of the SU-8+PFPE composite coating on the substrates was ˜75-100 microns (which was measured using WYKO NT1100 optical profiler).
indicates data missing or illegible when filed
Table 3 shows the initial coefficient of friction (μi), steady-state coefficient of friction (μs) and wear life of bare substrates without any lubricant coating and SU-8+PFPE coated substrates tested at a normal load of 300 g and a rotational speed of 500 rpm, respectively, and the corresponding coefficient of friction versus number of sliding cycles curves are shown in
In this experiment, SU-8 film based “Lub-tape” were spin coated onto OHP polyethylene sheets and aluminum foil and then subsequently characterized for their tribological properties. For these feasibility studies regarding Lub-tapes, only one composite i.e. SU-8+PFPE was used and only the tribological characterization was done.
OHP sheet wafers were cut into ˜3 cm×3 cm pieces and were thoroughly cleaned with soapy water, distilled water and isopropyl alcohol (IPA), respectively, and finally dried with N2 gas. The cleaned OHP sheet wafers were then subjected to the spin-coating immediately. The SU-8 used for the sample preparation was SU-8 2050. For the preparation of the “Lub-tape” films, SU-8 was spin-coated onto OHP sheet wafer at an initial speed of 500 rpm for duration of 5 seconds, followed by an increase in the spinning speed to 3000 rpm for a duration of 60 seconds which results SU-8 films with a thickness of ˜75 microns. Thicknesses of the films were measured using WYKO NT1100 optical profiler (Vecco Instruments Inc, USA). The spin-coated SU-8 films were then subjected to the pre-baking at a temperature of 65° C. for 4 minutes followed by at 95° C. for 9 minutes. The pre-baked SU-8 films were then again subjected to spin-coating of SU8+PFPE flim over the pre-baked SU8 film at an initial speed of 500 rpm for duration of 5 seconds, followed by an increase in the spinning speed to 3000 rpm for duration of 60 seconds which results in the SU-8+PFPE film with a thickness of ˜75 microns. The spin coated SU-8+PFPE films over SU8 films on the same wafer were then subjected to the pre-baking at a temperature of 65° C. for 4 minutes followed by 95° C. for 9 minutes. Then the spin-coated SU8+PFPE film along with the bare SU8 film was subjected to UV (ultra-violet) rays (wavelength: 365 nm and power: 210 mJ/cm2) for a duration of 30 seconds. A post- exposure bake was carried out at a temperature of 65° C. for 1 minute and followed by 95° C. for 7 min, after UV exposure. Finally, the “Lub-tape” film was peeled off from the OHP substrate with mild heating. The peeled off Lub-tapes were stored in the desiccators before any further characterization.
Friction and wear tests were carried out using the UMT-2 in ball-on-disk mode. The Si3N4 ball of 4 mm diameter with the surface roughness of 5 nm was used as the counterface. The tests were conducted at different normal loads (up to 300 g) and at different rotational speeds (up to 2000 rpm), respectively. For each modification, 3 tests were repeated and average data are reported. An optical microscope was used to image the worn surfaces after the sliding tests to assess the damage to the film surfaces and the ball, respectively.
The SU-8 based “Lub—Tape” films formed under the conditions provided in the experimental section have shown a thickness of ˜150 μm.
Table 4 shows the initial coefficient of friction (μi), the steady-state coefficient of friction (μs) and the wear life data for Lub-tape films (conducted at both sides i.e. high friction and low friction sides, respectively) with different PFPE concentrations and at different loads and rotational speeds, respectively. The tribological properties obtained at a normal load of 300 g and a rotational speed of 1000 rpm will be discussed in this section and for other results, Table 4 can be referred. The high friction pristine SU-8 side has shown high friction properties (μi: 0.62 and μs: 0.4) and low wear life (n<2000) when tested at a normal load of 300 g and at rotational speeds of 1000 rpm and 2000 rpm, respectively. Under the same loading conditions, SU-8+PFPE side has shown very low coefficients of friction (μi: 0.08 and μs: 0.12) and high wear life (n>500,000). Therefore, the SU-8+PFPE film present on the other side of the tape shows significant improvement in tribological properties (the μi has been reduced by ˜8 times, the μs has been reduced by ˜3.5 times and the wear life has been improved by >250 times) when compared with the properties of high friction side (pristine SU-8). The high friction side of the Tape can be used to adhere to the surface of interest and apparently the SU8+PFPE layer will be exposed against the counterface material and can reduce the friction and increase the wear life. The PFPE concentration in the composites was increased from 5 wt % to 10 wt % and then the tribological behavior was evaluated. An increase of PFPE concentration from 5 wt % to 10 wt % has shown an improvement in tribological properties by 30-40%. Therefore, the concentration of the PFPE has to be optimized depending on the application requirements and the operating conditions of the Lub-tape once it is placed in the application.
Experiment 4: Evaluation of Other Liquid lubricants
In this experiment, SU-8 based films were formed using SU8+MAC and SU-8+Base oil, in a manner identical, essentially identical, or analogous to that described above for SU-8+PFPE.
The tribological behavior of SU-8 based films made using different liquid lubricant formulations can be ascertained from Table 5, and graphically seen from
B., Melosh, N. A. and Pruitt, B. L. “Suspension of nanoparticles in SU-8: Processing and characterization of nanocomposite polymers”, Microelectronics Journal 39 (2008) 228-236.
Grubbs, R. K. “Growth, structure, and tribological behavior of atomic layer-deposited tungsten disulphide solid lubricant coatings with applications to MEMS”, Acta Materialia 54 (2006) 4731-4743.
Aspects of particular embodiments of the present disclosure address at least one aspect, problem, limitation, and/or disadvantage associated with existing compositions, coatings, films, structures, or laminates. While features, aspects, and/or advantages associated with certain embodiments have been described in the disclosure, other embodiments may also exhibit such features, aspects, and/or advantages, and not all embodiments need necessarily exhibit such features, aspects, and/or advantages to fall within the scope of the disclosure. It will be appreciated by a person of ordinary skill in the art that several of the above-disclosed systems, components, processes, or alternatives thereof, may be desirably combined into other different systems, components, processes, and/or applications. In addition, various modifications, alterations, and/or improvements may be made to various embodiments that are disclosed by a person of ordinary skill in the art within the scope and spirit of the present disclosure. Such modifications, alterations, and/or improvements are encompassed by the following claims.
This application claims priority to and is a non-provisional conversion of U.S. Provisional Patent Application Serial No. 61/563,522, entitled “SU-8 Nano-Composites with Improved Tribological and Mechanical Properties,” filed on Nov. 23, 2011, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61563522 | Nov 2011 | US |
