GB2360790A - Low friction coatings produced by cathodic arc evaporation - Google Patents

Abstract

A substrate is provided with an adherent, ultra low friction coating (ž<SP>H</SP> no more than 0.025) by coating with inorganic Fullerene like particles or particles of MOS<SB>2</SB>. The Fullerene particles may be in the form of nanoparticles. The coating may be formed over a layer of wear resistant nitride. The coating may be formed by subjecting a cathode of inorganic Fullerene to cathodic arc evaporation.

Description

2360790 Low friction coatings The present invention relates to substrates

bearing extremely low friction adherent solid coatings, articles comprising said substrates and methods for forming the coatings.

Many articles are currently produced bearing a coating Of MOS2 as a low friction coating. Such coatings have been shown to be suitable for dry machining of metals and are therefore used on many metal forming and cutting tools and on tools for cutting and forming other materials.

A conventional film Of MOS2 has a friction coefficient in vacuum (j) of as low as 0.01 but in humidity (e.g. normal air atmosphere) this rises to 0.1. The coefficient of friction in humid conditions (g H) can be improved by including titanium. This both helps to increase the hardness of the f ilm and to prevent oxidation by pacifying some of the dangling bonds of the MOS2 and by acting as a getter of oxygen. VLH for a Ti-MOS2 film can be about 0.05.

Although MOS2 can be used as a powder, there is a growing market for MOS2 films in wear applications. Hard nitride coatings such as TiN, CrN, TiC, Ti (C, IST), (A1, TiN) and ZrN are used for wear applications such as drilling, forming, casting and high speed machining. The perf ormance of the nitride coatings can be dramatically improved if a relatively thin (lmm on top of 5 gm) MOS2 f ilm is added on the top of the nitride film.

Most nitride coatings are deposited by reactive are evaporation which uses an arc discharge on a metal electrode to initiate emission of the metallic vapour in the presence 2 of a reactive gas(nitrogen for nitrides, hydrocarbonfor carbides). Very hard and dense coatings can be obtained from cathodic arc evaporation. The method can also be run in a vacuum to produce pure metal coatings.

The subsequent application of a MOS2 coating presently is difficult in that it can only be done by sputtering, so that the components have to be removed from the vacuum chamber and reheated to 400 OC for sputtering deposition of MOS2.

It would be desirable to have lower friction dry coatings available for use in atmospheric humidity. It would further be desirable to have MOS2 or similar coatings that can be produced by a method not requiring high temperatures.

Feldman et al have described a method for the bulk is synthesis of inorganic fullerene like particles from their respective trioxides. This produces a sooty deposit of powder containing the particles on a substrate. This is not an adherent coating and can be wiped of easily. The particles can be collected and used as an additive in liquid lubricants.

It has previously been known (Chhowalla et al) to produce thin films of carbon having a fullerene and nanotube rich structure by cathodic arc evaporation of a graphite cathode in the presence of a localised relatively high pressure atmosphere. The carbon is deposited onto a substrate placed in a substantially lower pressure atmosphere. This is achieved by releasing gas in the immediate vicinity of the cathode, suitably via a passage running through the cathode. The substrate and the cathode are contained in a continuously pumped vacuum chamber. As 3 the plasma of the arc evaporated cathode is rapidly quenched by the expanding gas, clusters of carbon atoms form and are carried to the substrate.

We have now surprisingly found that a similar method may be applied to MOS2 and similar materials to provide a low friction coating on a substrate at low temperatures.

Accordingly, the present invention now provides in a first aspect a substrate having an adherent solid surface coating providing a coefficient of friction LI4 (as hereinaf ter defined) no more than 0.025.

The coefficient of friction. gH is measured by the ballon-disc pure sliding method in which a 440C stainless steel ball of 0.7cm diameter is slid over a 5 cm disc of the coated substrate which has been lap polished to an Ra roughness value of 0.04-0.05 Lm, said sliding being done in an atmosphere of nitrogen at 45% humidity in an ambient temperature of 251C. The mating ball 'is also made from 440C stainless steel, but uncoated, and of comparable Ra roughness to the disc. The applied load is 1ON (using a dead-weight) on to the rotating disc (speed = 50cms-1) giving a maximum Hertzian pressure of 1.1GPa and contact diameter of 100 1m. The friction coefficient is recorded automatically by monitoring the tangential and normal forces.

Preferably, 9 H is no more than 0.015. As is demonstrated below, it is possible to obtain materials having a iH value of as low as 0. 008.

The films produced according to the invention are not loose, sooty deposits as in Feldman et al, but are adherent solid films. 'Adherent' in this context will generally imply' an adherence of not less than 10 N measured using a VTT 4 scratch tester with a 200 gm Rockwell C indenter and maximum load of 10ON with a load rate of 1.7 Ns-1.

Preferably, the adhesion of the coating to the substrate is no less than 15 N, more preferably not less than 20 N.

The coating is preferably of inorganic fullerene like particles (IF particles) or fragments thereof. Most preferably, the particles are MOS2 particles. However other metal chalcogenides or dichalcogenides may be used such as WS2 or corresponding selenides. Similar structures may also be obtained from boron nitrides.

Said coating may be formed over a layer of a wear resistant nitride.

The substrate may be made f rom a very wide range of materials including both metals and plastics. Metal tools bearirig such coatings may be used as knives, milling cutters, drills, taps, punches, saw blades, coining dies, drawing dies, and pultrusion dies. Coated substrates according to the invention may be used in bearings or in the bearing surfaces of joints, e.g. in prostheses. Magnetic tapes may be coated to reduce friction and wear of recording /playback heads. In a further aspect, the invention provides a substrate having an adherent, solid coating comprising IF nanoparticles or fragments thereof. 25 The invention includes in a further aspect, a method for the deposition of a surface coating comprising IF nanoparticles or fragments thereof comprising exposing a substrate to be coated to a plasma produced by subjecting a cathode of IF nanoparticle forming material to cathodic arc evaporation in the presence of localised relatively high gas pressure at the surface of the cathode.

The cathode is preferably of MOS2.

The localised gas pressure at the surface of the cathode is preferably from 0.1 to 50 Torr, e.g. about 10 Torr. The pressure at the substrate is preferably from 0.1 to 10 MTorr, e. 9. about 5 mTorr. The ratio between the pressure at the substrate and at the cathode is suitably from 1:1,000 to 1:4000.

Preferably, the localised high pressure is produced by releasing a stream of gas in the immediate vicinity of the cathode.

The temperature of the substrate during coating deposition is preferably not greater than 200 OC, e.g. from 20 to 200 'C.

The deposition of MOS2 nanoparticle films using an arc method very similar to that used at present commercially to deposit nitrides and diamond like carbon thin films could easily be incorporated into existing thin film technology for coating automotive parts and tool components. This method beneficially allows the deposition of the low friction layer directly onto the components instead of dispersing the nanoparticles into oil, which could lead to adverse clogging of engine parts. The low temperature process also allows it to be used on top of hard or magnetic thin films as a low friction layer.

In the example illustrating the invention which follows, ultra low friction and wear thin films Of MOS2 are created by depositing closed, hollow nanoparticles in the form of a thin film onto a substrate. The films show a very low 6 friction coefficient in dry nitrogen atmosphere but also show ultra low friction and wear even in 45% humidity in,dry' sliding conditions. The low friction is attributed to the presence of curved S-Mo-S planes that prevent oxidation and preserve the layered structure. The friction coefficient in humidity obtained here is an order of magnitude lower than for sputtered MOS2 thin films.

The invention will be further described and illustrated with reference to an example and with reference to the accompanying drawings in which:

Figure 1 is a set of four high resolution transmission electron microscope images of Sonm nanoparticle MOS2 film deposited on Si [100] substrates obtained using JEOL 2000FX TEM operated at 20OkV; and Figure 2 is a graph showing the friction coefficient as a function of the time in 45% humidity (and nitrogen atmosphere) for nanoparticle MOS2 films sputter deposition and in 45% humidity by arc deposition according to the invention.

Exam.ple A MOS2 film was deposited by cathodic arc evaporation using a pure MOS2 cathode and a pure mo anode. The substrate was a 5 cm 440C stainless steel disc of 0.8cm thickness placed 20 cm away from the cathode. The gas pressure at the substrate was kept constant at 1OmTorr.

Localised high pressure was generated by introducing a carrier gas via a lmm hole in the MOS2 target. The carrier gas was nitrogen, but helium has also been used.

7 The arc was ignited (at 75A and 22V) in the localised high-pressure region of nitrogen.

The dynamics involved in the formation of MOS2 nanoparticles are presently unclear but the mechanism is probably similar to the formation of carbon nanoparticles as the technique used here is identical to that described in detail in Chhowalla et al. The nanoparticles Of MOS2 are thought to form immediately above the cathode surface and are carried via expansion from the localised high- pressure region near the arc discharge to the substrate. Films deposited in the absence of localised high-pressure appear mostly amorphous with only local hexagonal structure under high resolution electron microscopy (HREM). However, well formed circular nanoparticles (figure la) and curved S-Mo-S planes (figure 1b) could be readily seen for films deposited in the presence of nitrogen. The key mechanism in the formation of IF nanoparticles is the bending and rearrangement of the basal planes. Displacement of Mo or S via collisions with energetic ions and electrons in the arc plasma can lead to rearrangement of atoms in the plane in order to maintain electrical neutrality, introducing defects in the lamellar lattice. The results from figure 1 indicate for the first time that it is possible to generate well formed hollow fullerene-like 'onion, clusters in the form of a thin film using the localised arc discharge method.

In addition to the structural characterisation, the mechanical properties of the films were also investigated. The films were deposited on 440C stainless steel discs with a diameter of 5cm The substrates were lappolished so that the final Ra roughness value was 0.04 - 0.05 m. The film 8 thickness was kept constant at 1.2 + 0.1 gm for all mechanical and tribological tests reported in this study.

The adhesion (measured using a VTT scratch tester with a 200 gm Rockwell C indenter and maximum load of 10ON with a load rate of 1.7Ns-1), was found to be 25N. The hardness of the films was measured using a Fisher microindentation system with Berkovich (pyramidal) tip.

The hardness, extracted using the plastic displacement obtained from the intercept with the displacement axis of the unloading curve at maximum applied load (SmN), was found to be 10GPa, comparable to values reported for sputtered MOS2. The surface roughness of the deposited film measured using the Dektak IIA profilometer was found to be approximately 30nm.

is The frictional coefficient and wear rate M of the nanoparticle MOS2 films were determined by the ball-on-disc (pure sliding) method, similar to that described elsewhere [Suzuki et al, Westergaard et al]. The discs were the same ones used to measure the hardness and adhesion above. The mating ball was left uncoated and was also made of 440C stainless steel with a diameter of 0.7cm. The results from the different types of films tested are summarised in table 1. The evolution of g as a function of the number of rubbings is plotted in figure 2.

The nanoparticle MOS2 films show very low friction coefficient in dry nitrogen atmosphere (ID = 0.006)..g and w remain ultra low even in 45% humidity. In fact, tH and w are slightly lower for the nanoparticle MOS2 film in 45% humidity than for the sputtered MOS2 film in dry nitrogen.

Examinations of the wear patterns on the uncoated ball show 9 clear differences between the nanoparticle and sputtered films. In the case of the sputtered film, a heavily worn and rough surface with bright and shiny wear tracks along with some dark patches was observed. The wear pattern on the uncoated ball after failure was also examined under a scanning electron microscope (SEM) fitted with energy dispersive x-ray (EDX) detector. EDX analysis revealed that the wear crater contained predominantly iron oxide in all regions. X-ray photo-electron spectroscopy (XPS) analysis of the wear region on the coated disc also confirmed the formation of M003 on the sputtered film (after failure). The test on the nanoparticle film was stopped prior to failure after 2.4 x 10 5 rubbings (three times longer than failure in the sputtered film) due to the limitations of our experimental apparatus. In contrast, substantial wear in the form of bright and dark patches was not observed in the crater of the ball mating with the nanoparticle MOS2 film. Instead, substantial film transfer from the coated disc to the uncoated ball was observed. EDX analysis of the worn region showed that primarily Mo and S were present with only residual peaks arising from C and 0. The XPS analysis of the worn region on the nanoparticle MOS2 film prior to failure revealed a smaller peak of M003 as compared to the sputtered film. In order to study the structural changes in the film after the test, MOS2 debris from the wear crater of the nanoparticle film was collected and observed under HREM. The observations revealed that although the curved hexagonal lattice is still maintained almost no closed circular nanoparticles could be. seen after the test, indicating that under these loading conditions the large fullerenes fragment into smaller irregularly shaped closed crystallites.

We believe that the mechanisms responsible for the low friction coefficient in IF-MoS2 thin films reported here are probably similar to sputtered films but with additional features. The results clearly indicate that it is possible to improve the frictional and wear properties Of MOS2 thin films via the incorporation of fullerene- like nanoparticles. The low friction coefficient measured for the nanoparticle films may simply be a consequence of the highly ordered structure. That is, the incorporation of nanopartic.les through localised high-pressure arc discharge allows the deposition of a highly ordered MOS2 structure which is not (or may not be) accessible by sputtering or other methods. Furthermore, mechanisms such as intercrystallite slip can still occur in fragmented nanoparticles and crystallites consisting of irregularly shaped curved S- Mo-S planes, even though shearing between the layers is unlikely in closed nanoparticles. An additional feature of these films is the presence of curved hexagonal planes that clearly enhance the stability of the material in humid environments and thus aid to preserve the lamellar lattice longer. In the case Of MOS2 powder (or sputtered f ilm), oxidation can occur through unterminated bonds of the hexagonal planes, causing the material to stick, leading to rapid deterioration of its low-shear-strength properties. The curvature of the hexagonal planes appears to buffer the Mo atom from oxidation by reducing the number of exposed dangling bonds at the edges of the planes.

Table 1: The first column gives the types of films tested, the second column indicates the measured friction coefficient and third column gives the wear rate.

Film 9 (in 45% w (mm3/(N-mm)) humidity) Nanoparticle MOS2 0.008 - 0.01 1 X 10-11 Sputtered MOS2 0.1 - 0.3 3 x 109 Hard TiN 0.4 - 0.6 12 References:

Feldman et al, Bulk Synthesis of Inorganic Fullerene-like MS2 (M=Mo, W) from the Respective Trioxides and the Reaction Mechanism, J.Am.Chem.Soc. 1996, 118, 5362-5367.

Chhowalla, M. Aharanov, R. A., Kiely, C. J., Alexandrou, I.

A. and Amaratunga, G. A. J. Generation and deposition of fullerene- and nanotube-rich carbon thin films. Philosophical Magazine Letters 75, 329-335 (1997).

WestergArd, R. and Ax6n, N. Tribologia -Finnish Journal of Tribology 17, 14 (1998).

Suzuki, M. Comparison of tribological characteristics of sputtered MoS2 films coated with different apparatus. Wear 218, 110-118(1998).

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Claims (16)

1 A substrate having an adherent solid surface coating providing a coefficient of friction ptH (as hereinbefore 5 defined) no more than 0. 025.

2. A substrate having a coating as claimed in Claim 1, wherein P is no more than 0.015.

3. A substrate having a coating as claimed in Claim 1, wherein the adhesion of the coating to the substrate is no less than 15 N.

4. A substrate having a coating as claimed in Claim 3, is wherein said adhesion is not less than 20 N.

5. A substrate having a coating as claimed in Claim 1, wherein the coating is of inorganic Fullerene like particles (IF particles) or fragments thereof.

6. A substrate having a coating as claimed in Claim 3, wherein the particles are MOS2 particles.

7. A substrate as claimed in any preceding claim, wherein said coating is formed over a layer of a wear resistant nitride.

8. A method for the deposition of a surface coating comprising IF nanoparticles or fragments thereof 3 0 comprising exposing a substrate to be coated to a plasma 14 produced by subjecting a cathode of IF nanoparticle forming material to cathodic arc evaporation in the presence of localised relatively high gas pressure at the surface of the cathode.

9. A method as claimed in Claim 7, wherein the cathode is Of.MOS2.

10. A method as claimed in Claim 7 or Claim 8, wherein said localised gas pressure at the surface of the cathode is from 0.1 to 50 Torr.

11. A method as claimed in Claim 9, wherein said localised gas pressure at the surface of the cathode is about 10 is Torr.

12. A method as claimed in any one of Claims 7 to 10, wherein the pressure at the substrate is from 0.1 to 10 mTorr.

13. A method as claimed in Claim 11, wherein the pressure at the substrate is about 5 mTorr.

14. A method as claimed in any one of Claims 7 to 12, wherein the ratio between the pressure at the substrate and at the cathode is from 1:1,000 to 1:4000.

15. A method as claimed in any one of Claims 7 to 14, wherein the temperature of the substrate during coating deposition is not greater than 200 OC.

dl-,?, 0 is

16. A substrate having an adherent, solid surface coating comprising IF nanoparticles or fragments thereof.