WO2006130118A1

WO2006130118A1 - Ultra-high-molecular-weight polyolefin-based coatings

Info

Publication number: WO2006130118A1
Application number: PCT/SG2006/000140
Authority: WO
Inventors: Sujeet Kumar Sinha; Satyanarayana Nalam; Boon Hong Ong; Seh Chun Lim
Original assignee: National University Of Singapore
Priority date: 2005-06-03
Filing date: 2006-06-02
Publication date: 2006-12-07

Abstract

There is disclosed a composite coating comprising an ultra high molecular weight polyolefin layer and a halogenated polymer layer deposited on the polyolefin layer.

Description

ULTRA-HIGH-MOLECUIIAR-WEIGHT POLYOLEFIN-BASED COATINGS

Technical Field

The present invention generally relates to wear resistant coatings on substrates such as silicon, and methods of applying the coatings on the substrates.

Background

Substrates, such as silicon substrates, are generally used as base materials for making components of micro- machines and micro-electromechanical systems (MEMS) . Silicon substrates generally have poor tribological properties (i.e., high surface friction and low lubrication) , thereby resulting in shorter life cycles of the micro-machine and MEMS components due to wearing of the components over prolonged use. Additionally, silicon substrates also have high stiction, which restricts motion of the components during interaction.

One known method of improving the tribological properties and reducing stiction of the silicon substrates is to coat the substrates with a lubricant. However, conventional lubricants are undesirable for such a purpose as the viscous forces of the lubricants can be quite large relative to the frictional forces involved in running these components. Furthermore, the presence of such lubricants may introduce contaminants onto the surfaces of the components.

Other methods mainly involve coating a thin organic lubricant layer, or molecular lubricant layer, which lowers the surface energy and thereby reduces capillary forces and adhesion between the components.

U.S. Pat. No. 5,512,374, discloses applying a perfluoropolyether (PFPE) film onto certain regions/parts of a DMD (deformable mirror device or digital micro-mirror device) which are prone to adherence with each other without any anti-stiction coating.

U.S. Pat. No. 6,624,944, discloses coating a halogenated material, such as perfluorinated material, onto certain parts of optical devices such as a spatial light modulator or an infrared detector or receiver, to reduce stiction.

U.S. Pat. No. 6,841,079, discloses chemically modifying Si substrates having Si-H bonds using various fluorinated chemicals, and the applications of these modifications are primarily to reduce stiction.

A problem with many of the above-mentioned coatings is that they do not reduce both stiction and wear of the components over prolonged use. Ultra-thin organic molecular layers have been proposed and tested as wear resistant lubricants for Si and especially Si based MEMS systems. These lubricant layers can be formed by two methods: (1) the Langmuir-

Blodgett method and (2) the Self-assembly method as disclosed in A.ϋlman, "Introduction to ultra thin Organic

Films", academic,. San Diego, CA.

The first method cannot be used for three-dimensional surfaces and is mainly concerned with flat surfaces such as magnetic recording media. Furthermore, these L-B films are only physically bonded with the substrate by van der

Waal forces, which are weaker in comparison to chemical bonds .

Much attention has been given recently to SAMs (SeIf- assembled monolayers) because of their easy preparation and their excellent properties such as low thickness, stable chemical and physical properties and good covalent bonding with the substrate. Moreover, the properties of the SAMs can be widely varied by changing the type and length of the molecules, terminal group and the degree of cross linking within the layer, which makes them more attractive than the L-B films.

Among the many SAMs that can be covalently bonded with Si surface, Alkyl silane SAMs have been extensively studied and proposed as the lubricants for MEMS. They can reduce the coefficient of friction, stiction and wear when they are deposited onto the Si substrate. However, despite the low coefficient of friction, the wear resistance achieved by these monomolecular layers is not sufficient to provide long wear life to the high velocity moving MEMS components. These monolayers do not demonstrate high wear durability. Once wearing initiates, the molecules are easily removed from the contact area and there is no replenishment in these layers. Also, the worn particles often act as third body and thus accelerating the wear of the film.

Multilayer or composite layers consisting of either polymers or dual self-assembled monolayers have shown some improvement over single molecular monolayers on Si surfaces. However, there has yet to be discovered a coating which provides both good tribological properties and low stiction to substrates.

Accordingly, there is a need to provide a wear resistant coating that can overcome or at least ameliorate one or more of the disadvantages described above. There also exists a need for an efficient, cost-effective method of depositing such a coating onto a substrate.

Summary

According to a first aspect, there is provided a composite coating comprising: an ultra high molecular weight polyolefin layer; and a halogenated polymer layer deposited on said polyolefin layer.

According to a second aspect, there is provided a coated substrate comprising: a substrate; an ultra high molecular weight polyolefin layer disposed on said substrate; and a halogenated polymer layer deposited on said polyolefin layer. In one embodiment, there is provided a coated silicon substrate comprising: a silicon substrate; an ultra high molecular weight polyolefin layer disposed on said silicon substrate; and a halogenated polymer layer deposited on said polyolefin layer.

According to a third aspect, there is provided a coating method comprising the steps of:

(a) depositing an ultra high molecular weight polyolefin layer on a substrate; and

(b) depositing a halogenated polymer layer on said polyolefin layer.

According to a fourth aspect, there is provided a coated substrate comprising: a silicon substrate; and an ultra high molecular weight polyolefin layer deposited on the silicon substrate.

According to a fifth aspect, there is provided the use of a composite coating comprising: an ultra high molecular weight polyolefin layer; and a halogenated polymer layer deposited on said polyolefin layer, wherein the coating is used to coat a substrate used in a micro-machine. Definitions

The following words and terms used herein shall have the meaning indicated: The term ^Λultra-high-molecular-weighf refers to polymers having a nominal weight average molecular weight of at least 1 x 10⁵, typically higher than 3 x 10⁶. For example, an ^Λultra-high-molecular-weight polyethylene' or "UHMWPE" refers to a polyethylene that typically has a nominal weight average molecular weight of several million, that is greater than three million and usually from three to six million. However, other linear polyethylenes of weight-average molecular weight greater than 500,000 and preferably above one million are encompassed with the scope of this term and include polymers which have been defined by ASTM 4020-81 as those linear polyethylenes which have a relative viscosity of 2.3 or greater at a solution concentration of 0.05% in decahydronapthalene . Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements. As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to β should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of a composite coating and a method of its manufacture will now be disclosed.

The composite coating comprises an ultra high molecular weight polyolefin layer and a halogenated polymer layer deposited on said polyolefin layer. The composite coating is used to coat a substrate.

The ultra high molecular weight polyolefins The ultra-high-molecular-weight polyolefin may have a weight-average molecular weight of about 1 x 10⁵ or more, about 5 x 10⁵ or more, about 1 x 10⁶ or more or about 1.5 x 10⁶ or more, or about 1 x 10⁶ to about 15 x 10⁶, or about 5 x 10⁶ to about 15 x 10⁶, or about 10 x 10⁶ to about 15 x 10⁶, or about 3 x 10⁶ to about 10 x 10⁶.

Advantageously, the high weight-average molecular weight of the polyolefin results in a layer that has high strength and high modulus. The polyolefin layer may comprise branched or straight chain polymers selected from the group consisting of crystalline homopolymers or copolymers of ethylene, propylene, butlylene. Preferable among them is ultra-high- molecular-weight polyethylene composed mainly of ethylene. For example, the polyethylene may include copolymers of at least about 85 weight percent ethylene with up to about 15 weight percent of one or more C3 to ClO α-olefins such as 1-butene, 1-hexene and 1-octene etc. The thickness of the polyolefin layer may be selected from the group consisting of about 0.5μm to about 2μm, about 0.5μm to about 1.5μm, about 0.5μm to about lμm, about lμm to about 2μm, about 1.5μm to about 2μm, about 0. Iμm to about 0.5μm, and about lμm to about 2μm. It should be noted that the thickness of the polyolefin layer is dependent on the nature of its application. For example, in the coating of components in micro-systems, a lower thickness of about O.lμm to about 0.5μm is desired, and in the coating of bearings, a thickness of about lμm to about 2μm is desired.

The halogenated polymers

The halogenated polymers may comprise halogens from the group consisting of fluorine, chlorine and bromine.

In one embodiment, the halogenated polymer is a perfluorinated polymer. In one embodiment, the perfluorinated polymer comprises an ether group. A preferred perfluorinated polymer is perfluoro-polyether (PFPE) .

In one embodiment, the PFPE has polar terminal groups. In one embodiment, the polar terminal group is a - OH group or a -COOH group. In one embodiment, the PFPE may be represented by the structural formula:

In one embodiment, the ratio of p/q is in the range of 1/3 to 3/4. In one embodiment, the ratio of p/q is about 2/3.

Exemplary perfluoro-polyethers are described in US Patent Nos. 3,665,041 and 6,509,509 and European Patent No. 0148482, all of which are incorporated herein in their entirety for reference.

The thickness of the halogenated polymer layer may be selected from the group consisting of about 2nm to about 3nm; about 2nm to about 2.7nm; about 2nm to about 2.5nm; about 2nm to about 2.3nm; about 2.3nm to about 3nm; about

2.5nm to about 3nm; and about 2.7nm to about 3nm.

The substrate The substrate may be comprised of silicon or metals. The metals may be pure metals or alloys, such as nickel- chromium-iron alloys, and group IHb metals (ie such as aluminium) and group IVa metals (ie such as titanium) of the Periodic Table of Elements. One exemplary nickel-chromium-iron alloys are INCONEL™ alloys of Special Metals Corporation of Huntington, West Virginia, United States of America.

In one embodiment, the substrate is comprised of silicon, quartz, INCONEL™, steel, polymer, aluminum, titanium, diamond-like carbon and mixtures thereof. In embodiments where the silicon substrate is used, the silicon substrate may be subjected to a cleaning step.

In one embodiment, the silicon substrate may be subjected to a treatment step, which renders the surface of said substrate hydrophilic. The treatment step may comprises the step of treating the substrate with a hydrophilic agent to render the surface of said substrate generally hydrophilic. In one embodiment, the treatment comprises the step of dipping the substrate into a cleaning solution comprising a sufficient amount of peroxide and aqueous acid to substantially remove impurities from the substrate. For example, one cleaning solution may be a "piranha solution" comprising about 30 vol.% H₂O₂ and about 70 vol.% H₂SO₄ and is applied to the substrate under elevated temperature, (i.e., 70°C) for a period of time.

Formation of the coatings

The coating may be formed by the steps of: (a) depositing the ultra high molecular weight polyolefin layer on the substrate; and

(b) depositing the halogenated polymer layer on said polyolefin layer.

The depositing step (a) may comprise: (c) dip-coating the substrate into a bath of a polyolefin solution comprising said polyolefin dissolved in a solvent.

The depositing step (b) may comprise:

(d) dip-coating the polyolefin-coated substrate into a bath of halogenated polymer solution comprising said halogenated polymers polyolefin dissolved in a solvent. The dip-coating of steps (c) and (d) may be carried out at constant dipping and withdrawal speed of about 1.8 to about 2.5 mm/s.

In one embodiment, the dip-coating of steps (c) and (d) may be carried out for a period of time of about 30 seconds to about 90 seconds. The dip-coating of steps (c) and (d) may be undertaken for a period of time of about 20 seconds to about 50 seconds or about 30 seconds to about 40 seconds. The coating method may comprise, respectively after said dip-coating steps (c) and (d) , comprise the step of:

(e) heating the coatings to substantially dry said coatings. The heating step may be undertaken at a temperature of more than 90°C, more preferably more than 100°C for a period of time of more than 15 hours, or more than 20 hours.

In addition to drying the coating, the heating step

(e) can also remove low molecular weight constituents present in the formed coatings . The polyolefin solution may be prepared by the steps of:

(f) dissolving powdered polyolefin in an organic solvent. Suitable organic solvents for dissolving said polyolefins include Decalin (decahydronaphthalene) , paraffin oil, xylene, toluene and octane.

The dissolving step (f) may comprise the step of: (fl) heating the polyolefin solution to completely dissolve the polyolefin in the organic solvent. The heating step may be undertaken at a temperature of about 150⁰C or more.

The concentration of the polyolefin in the polyolefin solution is in the range of about 1 wt.% to about 5 wt.%. It will be appreciated that the concentration is dependent on the thickness of the coating desired. The thickness of the polyolefin layer increases as the concentration of the polyolefin in the solution increases.

The halogenated polymer solution may be prepared by the steps of: (g) dissolving halogenated polymer in a fluorinated solvents. Suitable fluorinated solvents include (per) fluoropolyethers (Galden™, Fomblin™, Krytox™. , Demnum™) , dihydrofluoropolyethers (H-Galden™) , fluorinated and perfluorinated ethers (Fluorinert™) . The dissolving step (g) may comprise the step of:

(gl) dissolving the halogenated polymer in the fluorinated solvent. The dissolving may be carried out at ambient conditions

The concentration of the halogenated polymer in the halogenated polymer solution is in the range about 0.1 wt% to about 0.3 wt%. It will be appreciated that the concentration is dependent on the thickness of the coating desired. The thickness of the halogenated polymer layer increases as the concentration of the halogenated polymer in the solution increases.

Brief Description Of Drawings

The accompanying drawings illustrate disclosed embodiments and serve to explain the principles of the disclosed embodiments. It is to be understood, however, that the drawings are designed for purposes of illustration only and not as a definition of the limits of the invention. FIG.l is a graph of the water contact angles of Si, Si/UHMWPE and Si/UHMWPE/PFPE modified substrates formed in Example 1.

FIG. 2 is a graph of the coefficients of friction of Si, Si/UHMWPE and Si/UHMWPE/PFPE modified substrates formed in Example 1.

FIG. 3 is a graph of the wear life of Si, Si/UHMWPE and Si/UHMWPE/PFPE modified substrates formed in Example 1, and of a Si substrate_, coated with Octadecyltrichlorosilane (OTS) .

FIG. 4a shows a Scanning Electron Microscopy (SEM) image of a wear track on a Si/UHMWPE substrate formed in Example 1, under the conditions of up to 21,570 sliding cycles at a contact pressure of 370MPa.

FIG. 4b shows an EDS (Energy Dispersive X-ray Spectroscopy) spectrum of a Si substrate formed in Example 1.

FIG. 5a shows an SEM image of a wear track on a Si/UHMWPE substrate formed in Example 1, under the conditions of up to 2,033 sliding cycles at a contact pressure of 370MPa.

FIG. 5b shows an SEM image of a wear track on a Si/UHMWPE substrate formed in Example 1, under the conditions of up to 10,103 sliding cycles at a contact pressure of 370MPa.

FIG. 6 shows an SEM image of a wear track on a Si/UHMWPE/PFPE substrate formed in Example 1, under the conditions of up to 100,000 sliding cycles at a contact pressure of 370MPa.

FIG. 7 shows the relationship between the coefficient of friction and the sliding cycles for Si and Si/OTS SAM surfaces tested at 330MPa and 2-4 x 10^~2 ins^"1 sliding velocity, and Si/UHMWPE and Si/UHMWPE/PFPE surfaces tested at 370MPa and 4-8 x 10^~2 ms^"1 sliding velocity.

FIG. 8 shows a SEM image of a Si/UHMWPE substrate formed in Example 1.

FIG. 9 shows the relationship between the coefficient of friction and the number of sliding cycles for Inconel™, Inconel™/UHMWPE and Inconel™/UHMWPE/PFPE substrates formed in Example 2, at 330MPa and 2-4cm/sec sliding velocity.

FIG. 10 shows the wear life of Inconel™, Inconel™/UHMWPE and Inconel™/UHMWPE/PFPE substrates formed in Example 2, at 330 MPa and 2-4cm/sec sliding velocity.

FIG. 11a shows an optical micrograph of a wear track of Inconel™ substrate tested at 1,000 cycles.

FIG. lib shows an optical micrograph of Inconel™/UHMWPE substrate tested at 10,000 cycles. FIG. lie shows an optical micrograph of Inconel™/ϋHMWPE/PFPE substrate tested at 10,000 cycles.

Detailed Disclosure of Preferred Embodiment

Non-limiting disclosed embodiments will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

EXAMPLE 1 UHMWPE/PFPE Coated Silicon Substrate

A silicon substrate in the form of a single crystal wafer having a surface roughness of 0.3 to 0.5 nra

(measured with atomic force microscope over a scan area of

500nm X 500nm) was used in this example. The single crystal wafer was obtained from Engage Electronics Pte

Ltd, a company incorporated in Singapore.

Prior to coating with an ultra high molecular weight polyethylene (UHMWPE) polymer, the substrate was pre- treated by being thoroughly cleaning to remove any physisorbed contaminants, and then subjected to ^""piranha treatment" using a "piranha solution" as disclosed in US Patent No. 5,403,665, which is incorporated herein in its entirety to remove any chemisorbed contaminants and to increase the hydrophilicity of the substrate.

The cleaning of the substrate involved the following steps : 1. Rinsing and ultrasonic cleaning in commercial soap water for no less than 15 min.

2. Rinsing and ultrasonic cleaning in distilled water for no less than 15 min.

3. Rinsing and ultrasonic cleaning in Acetone for no less than 15 min.

4. Drying the substrate with N₂ gas.

The "piranha treatment" involved the following steps: 1. Preparing a Piranha solution containing 30 vol.% H₂O₂ and 70 vol.% H₂SO₄, and heating the solution to 70°C. 2. Dipping the cleaned substrate into the solution and for Ih while maintaining the temperature at 70°C.

3. Rinsing the substrate thoroughly with distilled water followed by acetone.

4. Drying the substrate with N₂ gas. The pre-treated substrate was then coated with the UHMWPE polymer. The UHMWPE polymer was obtained from Ticona Pte Ltd, a company incorporated in Singapore. The weight average molecular weight of the polymer is in the range of 3xlO⁶ g/mol to 1OxIO⁶ g/mol. The coating was effected by dip-coating the Si substrate into a first coating solution of UHMWPE polymer.

The first coating solution was prepared by dissolving powdered UHMWPE polymer into decahydromaphthalene

(decalin) at 150⁰C under magnetic stirring until the powder completely dissolved. The concentration of the first coating solution was 5 wt . % UHMWPE.

The dip-coating process was carried out by constantly dipping the substrate into the first coating solution for 30 seconds and withdrawing the substrate from the first coating solution at a speed of 2.1 mm/s.

After dip-coating, the substrate is dried in air and then heated at 100°C for 20 hours in an air furnace to remove any decalin remaining on the substrate.

The UHMWPE-coated substrate was further coated with a perfluoropolyether (PFPE) polymer. The coating was effected by dip-coating the UHMWPE-coated substrate into a second coating solution of PFPE polymer. The second coating solution was prepared by mixing liquid PFPE polymer into hydrofluoropolyether (H-Galden™) at room temperature (~21°C) under magnetic stirring until the two liquids are completely mixed to form a homogeneous second coating solution. The concentration of the second coating solution was 0.2 wt. % PFPE.

The PFPE used in this Example had a chemical structure as shown below (where p/q ratio is 2/3) :

H—0— —0—H

The PFPE molecules have terminal OH groups at their ends.

The PFPE polymer was obtained from Solvay Solexis Pte.^' Ltd, a company incorporated in Singapore.

The dip-coating process was carried out by constantly dipping the UHMWPE-coated substrate into the second coating solution for 1 min and withdrawing the substrate from the second coating solution at a speed of 2.1 mm/s.

After dip-coating, the substrate was dried in air at room temperature and stored in a desiccator for further characterization studies as described further below. UHMWPE Coated Silicon Substrate

An UHMWPE-coated silicon substrate was prepared in accordance with the above steps, but with the omission of the dip-coating process involving the PFPE polymer coating solution (i.e., second coating solution). The UHMWPE- coated substrate was stored in a desiccator for further characterization studies as described further below.

Silicon Substrate

Uncoated silicon substrate in the form of a single crystal wafer having a surface roughness of 0.3 to 0.5 nm was provided and stored in a desiccator for further characterization studies as described further below.

Characterization Studies

The uncoated Si substrate, the Si/UHMWPE substrate and the Si/UHMWPE/PFPE substrate are characterized by subjecting the substrates to the followings tests.

1. Contact Angle Results

The static contact angles for ultra-pure water on the surfaces of the three substrates were measured using a VCA Optima Contact Angle System (AST Products, Inc. USA) . A water droplet of 0.5 μL was used for the contact angle measurement.

The measurements were repeated six times for each of the three substrates, and an average value was taken. The variations in water contact angle values at various locations of the substrates were within the range of ± 2°. The measurement error was within the range of ± 1°. The results of the contact angle measurements are shown in FIG. 1. Referring to FIG.l, the water contact angles of Si, Si/UHMWPE and Si/UHMWPE/PFPE substrates are 28°, 127°, 134° respectively. The results show that the UHMWPE coating imparts increased hydrophobicity to the substrate as can be seen from the increase in water contact angle from 28° for uncoated Si substrate to 127° for Si/UHMWPE substrate. The PFPE coating further increases the hydrophobicity of the substrate as can be seen from the increase in water contact angle to 134° for the Si/UHMWPE/PFPE substrate. It is believed that the increase in water contact angle in the Si/UHMWPE/PFPE substrate is due to the presence of CF₂ groups on the surface after coating with PFPE.

Accordingly, the UHMWPE and the PFPE coatings result in a coated substrate surface that is highly hydrophobic and hence reduces the stiction of the surface.

2. Tr±bolog-±cal Results

Friction and wear tests were carried out using a Universal Micro Tribometer (CETR, USA) , in a ball-on-disk mode. A Si₃N₄ ball of 4mm diameter was used as the counterface in the tests. The rotating speed of the spindle was 200 rpm to result in a sliding speed of 0.042 ms-1 and a track diameter of 2mm. The roughness of the ball was 20nm, as provided by the supplier. The ball was cleaned ultrasonically with acetone before the test.

Every ball was viewed under a optical microscope to ensure that it was free of contaminants or manufacturing defects. For each new test, a new ball was employed. The normal load used was 7g, which gave a contact pressure of approximately 370MPa. All experiments were performed in air at room temperature (23 ⁰C) and at a relative humidity of approximately 60%-70%. It should be noted that although typical contact pressures in MEMS are in the order of 1 MPa to a few MPa, the contact pressure was set at 370 MPa for this Example to shorten the duration of the wear tests. The initial coefficient of friction was reported after 4 seconds of sliding (i.e. ~12 cycles of disk rotation), after stabilization of the sliding process. The tribometer constantly measured normal load and friction forces and gauges the coefficient of friction as a real-time ratio.

The wear life is defined as the number of cycles after which coefficient of friction exceeded a value of 0.3 or a visible wear scar was observed on the substrate, whichever happened earlier. The wear life data were obtained from at least five different samples utilizing at least two different tracks on each sample and an average of the three best results was reported.

FIG.2. shows the coefficient of friction data of the Si, Si/UHMWPE and Si/UHMWPE/PFPE substrates measured using the tribometer. The Si substrate has a coefficient of friction of 0.6. The Si/UHMWPE substrate has a coefficient of friction of 0.09. The UHMWPE polymer coating therefore reduces the coefficient of friction. The coefficient of friction is further reduced to 0.08 in the Si/UHMWPE/PFPE substrate.

These readings are consistent with the contact angle readings in that the surface energy of the silicon substrate decreases with the depositing of the UHMWPE and PFPE coatings and hence result in a lower coefficient of friction.

The molecules of the PFPE are also more flexible and hence offer little resistance to the shearing action and hence exhibits lower friction. FIG.3 shows the wear life of Si, Si/UHMWPE and Si/UHMWPE/PFPE substrates. The uncoated Si substrate failed within a few cycles of sliding, with the initial coefficient of friction being 0.6. A clear visible wear track appears soon after commencement of the test. The Si/UHMWPE substrate exhibited a longer wear life of about -12,000 cycles. It has also been observed that even though the coefficient of friction exceeds 0.3 after ~12,000 cycles, there is no noticeable wear to the polymer and the Si surface.

It should be noted that the Si/UHMWPE substrate has a higher wear life when compared to the wear life of -1600 cycles (at a contact pressure of 330MPa and a sliding speed of 2-4 x 10^~2 m s^"1) for conventional OTS lubricant used for MEMS as disclosed in US Patent 5,403,665 entitled "Method of applying a monolayer lubricant to micromachines", which is incorporated herein in its entirety for reference.

The Si/UHMWPE/PFPE substrate exhibited a wear life of 100,000 when the experiment was stopped as the coefficient of fraction had not exceeded 0.3, even after a considerable time had lapsed.

FIG.4 shows a SEM micrograph of a wear track of the Si/UHMWPE substrate when tested at -21,000 cycles of sliding. The wear track shows the compressed state of the film resulting from the flattening of the polymer asperities protruding on the surface, and did not show the formation of any wear debris along the wear track.

SEM/EDS analysis was carried out to identify any wear of the Si surface and the results are shown in FIG. 4. The EDS results showed that there was no Si peak inside the wear track. There was only a strong C peak inside the wear track, which is thought to have originated from the polymer. For a comparison, the strong Si peak on the piranha treated Si wafer obtained with EDS is also shown in the same figure. Thus within the accuracy of the EDS, we can qualitatively conclude that the wear of the Si surface has not started. Therefore, it was observed that there was an increase in the friction of the UHMWPE polymer film as the sliding progressed, even though there was not much wear to the polymer film and the substrate onto which it was coated.

FIG. 5 (a) shows the SEM image of a Si/UHMWPE substrate after -2,000 cycles of sliding and FIG. 5 (b) shows the SEM image of a Si/UHMWPE substrate after -10,000 cycles of sliding. The images show that the amount of material transfer, from the substrate to the ball and the contact area between the ball and the polymer, increased as the number of sliding cycles increased. Since the material transfer and the contact area increase, the friction force also increases. Therefore, progressive removal of the polymer makes the sliding difficult and hence results in increasing friction without appreciable wear. Since many applications need smoother sliding without an increase in friction for an extended number of sliding cycles, it is important to reduce the friction of the UHMWPE film even beyond 12,000 cycles.

PFPE is selected as a second layer to coat onto the UHMWPE. The change in the frictional properties after coating the PFPE onto the UHMWPE has been explained above. There is a greater increase in the wear life, after the coating of PFPE onto the UHMWPE film. The friction was less than 0.3 at 100,000 cycles (the test was stopped after 100,000 cycles). EDS analysis of the wear track, as shown in FIG. 6, has shown the absence of the Si peak inside the wear track and hence it can be reasonably concluded that the wear of the Si surface has not been started even after 100,000 cycles of sliding at a contact pressure of 370MPa. FIG.7 shows the coefficients of friction in relation to the number of cycles for Si, Si/OTS, Si/UHMWPE and Si/UHMWPE/PFPE substrates at the contact pressures and sliding velocities shown in the Figure. It is clear from FIG. 7 that the combination of either only UHMWPE polymer film or PFPE overcoated UHMWPE films is very superior in reducing the friction and wear of the Si surface, when compared to the conventional OTS SAM.

The role of the PFPE in reducing friction and wear is very hard to understand. Since the UHMWPE polymer surface did not contain any reactive chemical groups, the chemical bonding/interactions between the UHMWPE and PFPE can be ruled out. It is speculated that the PFPE is trapped into the large spaces between the polymer molecules present on the surface of the UHMWPE film, which are evident from the surface morphology as shown in FIG. 8. Therefore, as the sliding progresses, the compression of the film or flattening of the polymer segments starts. The trapped PFPE swells out of the polymer and lubricate the sliding, and hence the friction is not increased as the sliding progresses, unlike in the case of the Si/UHMWPE samples. Therefore, the coefficient of friction was very low even after 100,000 cycles of sliding. The wear track after 100,000 cycles of sliding in case of Si/UHMWPE/PFPE is smoother when compared to that of Si/UHMWPE, after 21,000 cycles of sliding. This is because of the appropriate lubrication provided by the PFPE which delays the material transfer to the ball and is expected to swell as the sliding progresses and smoothens the sliding surface. The shear stress is reduced as a result of these changes.

EXAMPLE 2

UHMWPE/PFPE Coated Inconel Substrate An Inconel™ 718 alloy (obtained from Special Metals Corporation incorporated in the United States of America) was used as the substrate in this example.

Table 1 below describes the composition of the Inconel™ 718 alloy.

Table 1

Nickel (plus Cobalt ) 50 . 00-55 . 00

Chromium 17 . 00-21 . 00

Iron Balance* Niobium (plus Tantalum) 4.75-5.50

Molybdenum 2.80-3.30

Titanium 0.65-1.15

Aluminum 0.20-0.80

Cobalt 1.00 max. Carbon 0.08 max.

Manganese 0.35 max .

Silicon 0.35 max.

Phosphorus 0 . 015 max .

Sulfur 0 . 015 max . Boron 0 . 006 max .

Copper 0 . 30 max .

*Reference to the ^balance ' of a composition does not guarantee this is exclusively of the elemen t mentioned but tha t i t predominates and others are present only in minimal quanti ties . The Inconel™, Inconel™/UHMWPE and

Inconel™/UHMWPE/PFPE substrates were prepared as in Example 1 , except that the "piranha treatment" was omitted as the treatment would have resulted in a rust-like film forming on the surface of the substrate .

Tribological Results

Friction and wear tests were carried out using a Universal Micro Tribometer (obtained from the Center For Tribology, Inc, USA), in a ball-on-disk mode. All parameters were the same as those in Example 1, except that the rotating speed of the spindle was lOOrpm thereby- resulting in a sliding speed of 0.021 ms^"1 and a track diameter of 2mm, and that the normal load used was 5g which gave a contact pressure of approximately 330MPa.

The Coefficient of friction of the Inconel™ substrate surface before any modification is 0.15. The coefficient of friction was reduced to 0.08 after coating with the UHMWPE polymer. FIG. 9 shows the relationship between the coefficient of friction with respect to the number of sliding cycles during the wear test for three substrates. For the Inconel™ substrate, the wear test showed that the coefficient of friction exceeded 0.3 within 400 cycles of sliding whereas for the Inconel™/UHMWPE substrate, the coefficient of friction exceeded 0.3 after about 6,000 cycles. After the PFPE polymer is coated onto the Inconel™/ϋHMWPE substrate to form the Inconel™/UHMWPE/PFPE substrate, the coefficient of friction did not exceed 0.3 until 10,000 cycles (i.e., the maximum number of cycles tested so far) .

FIG. 10 shows the wear life of Inconel™, Inconel™/UHMWPE and Inconel™/UHMWPE/PFPE substrates at 330 MPa and 4cm/s sliding velocity. The uncoated Inconel™ substrate failed within 400 cycles of sliding. The Inconel™/UHMWPE substrate exhibited a longer wear life of about 6,000 cycles. The Inconel™/UHMWPE/PFPE substrate exhibited a wear life of at least 10,000 cycles. It should be noted that the experiment for the Inconel®/UHMWPE/PFPE substrate was stopped prematurely as the coefficient of fraction had not exceeded 0.3, even after a considerable time has lapsed. Accordingly, it can be concluded that, the UHMWPE and

PFPE polymer coatings impart improved tribological properties to the Inconel™ surface, as can be seen from the extended wear life of the Inconel™ substrate after coating with the UHMWPE and PFPE polymers.

Optical micrographs of the wear tracks and ball after 1,000 cycles (for the Inconel™ substrate), and after 10,000 cycles (for the Inconel™/UHMWPE and Inconel™/UHMWPE/PFPE substrates) are shown in FIGS. 11 (a) , (b) and (c) . It can be observed from these micrographs that the UHMWPE and PFPE polymer coatings enhances the lubricity of the Inconel™ substrate.

Applications

Advantageously, the disclosed composite coatings are capable of reducing both stiction and wear of components over prolonged use. The disclosed coatings are therefore ideal for use in MEMS devices and systems. The disclosed coatings may be used as wear resistant coatings. Advantageously, such wear resistant coatings overcome, or at least ameliorate, the disadvantages of known composite coatings, particularly when coated on silicon substrates. The disclosed method also provides an efficient, cost-effective method for depositing the disclosed composite coating onto a substrate.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims .

Claims

1. A composite coating comprising: an ultra high molecular weight polyolefin layer; and a halogenated polymer layer deposited on said polyolefin layer.

2. A coating as claimed in claim 1, wherein the ultra- high-molecular-weight polyolefin has a weight-average molecular weight selected from the group consisting of: about 1 x 10⁵ or more, about 5 x 10⁵ or more, about 1 x 10⁶ or more or about 1.5 x 10⁶ or more, or about 1 x 10⁶ to about 15 x 10⁶, or about 5 x 10⁶ to about 15 x 10⁶, or about 10 x 10⁶ to about 15 x 10⁶, or about 3 x 10⁶ to about 10 x 10⁶.

3. A coating as claimed in claim 1, wherein the polyolefin layer comprises branched or straight chain polymers selected from the group consisting of crystalline homopolymers or copolymers of ethylene, propylene, butlylene.

4. A coating as claimed in claim 1, wherein the major portion of the ultra-high-molecular-weight polyolefin layer is polyethylene.

5. A coating as claimed in claim 1, wherein the thickness of the polyolefin layer is selected from the group consisting of about 0.5μm to about 2μm, about 0.5μm to about 1.5μm, about 0.5μm to about Iμm, about lμm to about 2μm, about 1.5μm to about 2μm, about 0. lμm to about 0.5μm, and about lμm to about 2μm.

6. A coating as claimed in claim 1, wherein the halogenated polymers comprise halogens selected from the group consisting of fluorine, chlorine and bromine.

7. A coating as claimed in claim 1, wherein the halogenated polymer is a perfluorinated polymer.

8. A coating as claimed in claim 7, wherein the perfluorinated polymer comprises an ether group.

9. A coating as claimed in claim 8, wherein the perfluorinated polymer is perfluoro-polyether (PFPE) .

10. A coating as claimed in claim 9, wherein the PFPE comprises -OH terminal groups.

11. A coating as claimed in claim 10, wherein the PFPE is represented by the structural formula:

12. A coating as claimed in claim 11, wherein the ratio of p/q is in the range of 1/3 to 3/4.

13. A coating as claimed in claim 1, wherein the thickness of the halogenated polymer layer is selected from the group consisting of about 2nm to about 3nm; about 2nm to about 2.7nm; about 2nm to about 2.5nm; about 2nm to about 2.3nm; about 2.3nm to about 3nm; about 2.5nm to about 3nm; and about 2.7nm to about 3nm.

14. A coated substrate coated with the coating of claim 1.

15. A coated substrate as claimed in claim 14, wherein the substrate is comprised of at least one of silicon and metal.

16. A coated substrate as claimed in claim 15, wherein the metal is selected from the group consisting of nickel-chromium-iron alloys and group HIb metals and group IVa metals of the Periodic Table of Elements.

17. A coated substrate as claims in claim 14, wherein the substrate is selected from the group consisting of silicon, quartz, INCONEL™, steel, polymer, aluminum, titanium, diamond-like carbon and mixtures thereof.

18. A coating method comprising the steps of: (a) depositing an ultra high molecular weight polyolefin layer on a substrate; and

(b) depositing a halogenated polymer layer on said polyolefin layer.

19. A method as claimed in claim 18, wherein the depositing step (a) comprises the step of:

(c) dip-coating the substrate into a bath of a polyolefin solution comprising said polyolefin dissolved in a solvent.

20. A method as claimed in claim 18, wherein the depositing step (b) comprises the step of: (d) dip-coating the polyolefin-coated substrate into a bath of halogenated polymer solution comprising said halogenated polymers polyolefin dissolved in a solvent.

21. A method as claimed in claim 19 or claim 20, wherein the dip-coating step is are carried out at constant dipping and withdrawal speeds of 1.8mm/s to about 2.5mm/s .

22. A method as claimed in claim 19 or claim 20, wherein, after said dip-coating step, the method comprises the step of:

(e) heating the coating to substantially dry said coating.

23. A method as claimed in claim 19 or claim 20, wherein the heating step (e) is undertaken at a temperature of more than 90°C or more than 100⁰C.

24. A method as claimed in claim 18 comprising, before step (a) , the step of:

(f) treating the substrate with a hydrophilic agent to render the surface of said substrate generally hydrophilic.

25. A coated substrate comprising: a silicon substrate; and an ultra high molecular weight polyolefin layer deposited on the silicon substrate.

26. The use of a composite coating comprising: an ultra high molecular weight polyolefin layer; and a halogenated polymer layer deposited on said polyolefin layer, wherein the coating is used to coat a substrate used micro-machine.