COATED SUBSTRATE

Abstract

The invention to which this application relates is improvements to the provision of Molybdenum and/or Tungsten containing coatings of the type which can be used to improve certain characteristics of the surface of a substrate to which the coating is applied. In one embodiment the coating also includes Ti to provide the advantages of high adhesion, high humidity and wear resistance of the coating and TiB.sub.2 to promote the formation of a relatively uniform, dense, coating, so strengthening the coating which is formed and improving the high temperature performance of the coatings.

Claims

1. A coating applied to a surface of a substrate wherein said coating comprises: MS.sub.x, and/or MSe.sub.x, where M is Molybdenum (Mo), and Tungsten (W) and x=0.8 to 2 and wherein the coating includes repeating layers of two forms, a first form including MS.sub.x and/or MSe.sub.x and Ti; and a second form including MS.sub.x and/or MSe.sub.x, Ti and TiB2 and within this form of layer, layers of the TiB2 material are sandwiched between layers of MS.sub.x, and/or MSe.sub.x and Ti.

2. A coating according to claim 1 wherein x=2.

3. A coating according to claim 1 wherein the coating also includes carbon.

4. A coating according to claim 1 wherein the coating includes at least one of Zirconium, Nickel, Cobalt, lead, Chromium, Gold, Silver, Carbon, Niobium, Tantalum, PTFE.

5. A coating according to claim 1 wherein the coating includes at least one of CaF.sub.2, BaF.sub.2, CeF.sub.3, LaF.sub.3; PbO, NiO, Cu.sub.2O, ZrO.sub.2, Ag.sub.2O, Al.sub.2O.sub.3, Sb.sub.2O.sub.3; TiB.sub.2, ZrB.sub.2, CrB.sub.2, Boron Carbide, Boron Nitride, Silicon Nitride, Titanium Nitride, Titanium Carbide, Silicon Carbide and/or Tungsten Carbide.

6. A coating according to claim 1 wherein the coating includes an MoS.sub.2 layer, a metal material layer and a metal boride layer.

7. A coating according to claim 1 wherein the coating is formed of MoS.sub.2 a metal material layer and a metal boride which are co-deposited to form an intimate mixture.

8. A coating according to claim 1 wherein the coating is formed from a plurality of Molybdenum disulphide/metal composite and TiB.sub.2 nano-scale multilayers which are applied to form the coating of required thickness whilst substantially maintaining the wear rate of the coating which is formed.

9. A coating according to claim 8 wherein at a coating thickness of up to 4.0 m the wear rate of the coating is at least 110.sup.17 m.sup.3 N.sup.1 m.sup.1.

10. A coating according to claim 1 wherein the working temperature of the coating is >450 C.

11. A coating according to claim 1 wherein the coating is nano-crystalline and the basal planes of the nano-crystals are substantially parallel to the substrate surface to which the coating is applied.

12. An article with a coating as described in claim 1 applied to a surface thereof.

13. An article according to claim 12 wherein the coating is used to form a solid lubricant surface coating for the article which is used in any, or any combination of, moulding; forming; dry drilling and cutting; sheet-drawing; heavy duty sliding bearings, gears, piston rings, cylinder liners, valve stems, and/or rollers.

14. (canceled)

15. (canceled)

16. (canceled)

17. (canceled)

18. (canceled)

19. A method for forming a coating on a substrate surface, said method comprising the steps of: applying a layer of Ti to the surface of the substrate applying a ramping layer of material during which material, in addition to Ti, starts to be applied to the Ti layer and wherein the method includes the steps of applying repeating layers of two forms, a first form including MS.sub.x, and/or MSe.sub.x where M is Molybdenum or Tungsten and Ti; and a second form including MS.sub.x, and/or MSe.sub.x, Ti and TiB2 and within this form of layer, layers of the TiB2 material are sandwiched between layers of MS.sub.x, and/or MSe.sub.x and Ti.

20. A method according to claim 19 wherein MS.sub.x is applied in the form of MoS.sub.2.

21. A method according to claim 20 wherein the sequence of operation is to operate one or more magnetrons which are associated with a target which includes Boron to sputter material therefrom before the magnetrons associated with the MoS2 targets are operated.

22. A method according to claim 21 wherein once the operation of all of the magnetrons has commenced at least the magnetrons with the MoS2 targets are continuously operated until the coating is completed.

23. A method according to claim 1 wherein at least one magnetron is provided in association with a target including Titanium and that magnetron, once operation of the same has commenced, is operated continuously until the coating is completed.

Description

[0035] Specific embodiments of the invention are now described with reference to the accompanying drawings; wherein

[0036] FIG. 1 illustrates the surface topography of a coating formed in accordance with one embodiment of the invention;

[0037] FIG. 2 illustrates a SEM cross-section of a coating formed in accordance with one embodiment of the invention;

[0038] FIG. 3 illustrates the diffraction pattern of a coating formed in accordance with one embodiment of the invention;

[0039] FIG. 4 provides Rockwell indentation results of (a) 1.3 (b) 2.1 and (c) 3.8 m thickness of a coating formed in accordance with the invention;

[0040] FIG. 5 illustrates scratch testing results of coating formed in accordance with the invention;

[0041] FIG. 6 provides a taper cross-section of wear tracks of coatings of (a) 1.3, (b) 2.1 and (c) 3.8 m, following ball-on-disc tests at 80 N for 720 m

[0042] FIG. 7 illustrates the friction coefficient of a coating in accordance with the invention sliding against WC+Co and steel ball in ball-on-disc tests in ambient condition under different normal loads;

[0043] FIG. 8 illustrates apparatus for use in the application of coatings in accordance with one embodiment of the invention;

[0044] FIG. 9 illustrates a TEM cross sectional image of a coating formed in accordance with one embodiment of the invention; and

[0045] FIG. 10 illustrates TEM image of part of a coating formed in accordance with the invention.

[0046] It is preferred that the coating which is formed in accordance with the invention and which, in this embodiment, comprises MoS2, Ti and TiB.sub.2, is deposited by sputter depositing material from targets provided with respective magnetrons using apparatus as illustrated in FIG. 8.

[0047] The magnetrons 2 are provided in a chamber 4 on which a carrier 6 on which the substrates 8 to be coated are mounted for movement, most typically by rotation of the carrier about axis 7, or, alternatively, by linear movement, is located.

[0048] In one embodiment, the magnetrons 2 are provided in a closed field configuration and, if necessary, one or more magnet arrays can be provided in order to allow the closed field to be formed so as to trap the electrons therein and hence improve and intensify the magnetic field 10 in which the carrier is located, with the sputtering of the materials performed using unbalanced magnetron sputtering techniques.

[0049] In one embodiment, the apparatus includes first and second magnetrons 2,2, each with an MoS.sub.2 target 12, a magnetron 2 with a target 14 of Ti and a further magnetron 2 with a target 16 of TiB.sub.2 material. In the embodiment described herein the pressure in the chamber 4 before deposition was around 5610.sup.6 Torr and argon gas was introduced into the chamber and provided at a sputtering pressure of approximately 3.010.sup.3 Torr. The distance between the targets and the substrates to be coated wit he coating and which are held on the carrier is 150 mm and a pulsed DC power supply is used to provide a negative bias, in this case about 350 V bias on the substrates, during a cleaning step in which the substrates are cleaned and at a bias in the region of 45 V during the selective sputter deposition of material from the targets during deposition.

[0050] In the first step of the coating application method a thin layer of Ti is applied to the surface of the substrates to act as an adhesion layer for the subsequent coating material to the substrate 8. Subsequently, the coating is formed in accordance with the invention with the carrier being rotated about axis 7 so as to move the substrates in front of each of the targets 12,14,12,16 in turn. A suitable speed of rotation is 4.0 rpm with the temperature of the substrates during the deposition process maintained at a temperature lower than 150 C.

[0051] During the method, a current is applied to the Ti and TiB.sub.2 target magnetrons to cause the sputtering of the material therefrom onto the substrates and the current is varied as shown in Table 1 in order to optimize the tribological properties of the deposited coatings. The sputtering current of the MoS.sub.2 target magnetrons is set at 0.6 A, and the coating thickness is controlled to be at about 1.30 micrometres in the optimization stage.

TABLE-US-00001 TABLE 1 sputtering current applied to the magnetrons with Ti and TiB.sub.2 targets Ti 0 0.3 0.5 0.7 0.8 TiB.sub.2 0.4 0.6 0.8 1.0 1.2

[0052] In the method described herein the substrates used are an M42 steel test piece to allow for analysis of the tribological properties of the coating applied thereto and a Si wafer was coated for the purposes of analysis of the structure of the coating applied thereto. The adhesion of the coatings to the respective substrates was measured using a Teer ST3001 scratch/wear apparatus using a Rockwell diamond tip sliding against the coating with a load increasing by 100 N min.sup.1 from 10 to 100 N at a sliding speed of 10 mm min.sup.1. The scratch critical load Lc of the coating was determined by visual observation in an optical microscope and defined as the point at which coating delamination was first observed either in, or at the edge of, the scratch track. A standard Rockwell-C indentation test under a load of 150 kg was also used as an indication of adhesion (HF).

[0053] The plastic hardness of the coatings was determined using a Fischerscope H100 tester with a Vickers indentor from the load penetration curves. Five indents were made on each sample and the data from the same was averaged. The loading/unloading rate was 10 m N s.sup.1, with a maximum applied load of 50 mN.

[0054] The tribological behaviour of the coatings was determined using a Teer POD-2 pin-on-disc tester. The test used a WCCo ball of 5 mm diameter, which was loaded on top of the coated flat samples at normal loads of 20 N, 40 N or 80 N. The flat sample was rotating at a controlled speed while the ball remained at a fixed position, so that the linear sliding speed between the counterbodies was set at 200 mm s.sup.1. The tests were conducted for 60 min. at an ambient condition with the relative humidity level of 3040%. The wear volume was determined by producing a ball crater on the wear track.

[0055] The surface and cross-sectional morphology and structure of the coatings were analysed using a JEOL 7000 FEG Scanning Electron Microscope (SEM) and a Philips CM20. The coatings were therefore deposited at different conditions by varying the sputtering current on the Titanium and Boron target magnetrons. As shown in Table 2, coatings with a coefficient of friction of 0.02-0.06 and a wear rate of 110.sup.17 m.sup.3 N.sup.1 m.sup.1 in ball-on-disc tests conducted in ambient air, were achieved even with the coating thickness increased up to 4.0 m. Furthermore, the thicker the coating that is applied then the longer the wear life as shown by the prolonged ball-on-disc test results of Table 2, which show that the wear life of the coatings in ball-on-disc test under 80 N normal load, are increased from about 2 hrs to about 12 hrs with the thickness of the coatings increased from 1.3 m to 3.8 m.

TABLE-US-00002 TABLE 2 Testing results on coatings with different thicknesses Thickness Friction Wear rate Wear life m Lc N HF coefficient m/Nm hr 1.3 100 2 0.03 8 1.sup.18 2 2.1 100 3 0.04 1 10.sup.17 6 3.8 100 4 0.04 2 10.sup.17 12 Note: the friction and wear properties are the pin-on-disc results at a normal load of 80N.

[0056] SEM observation of the surface as shown in FIG. 1 and the fractured cross-section as shown in FIG. 2 of the coating revealed that the coating is very dense without any pores or columnar feature. The diffraction pattern of the coating in FIG. 3 by TEM shows that the coating is in an amorphous structure.

[0057] The detected hardness of the coatings was found to be in the region of 900 kg/mm.sup.2 and the indentation tests show that the HF level increased as the coatings become thicker, but could still be less than HF 4 for up to 4.0 m thick coatings as shown in Table 2 and in FIG. 4. The scratch tests show that the total failure load of all the coatings with different thickness is more than 100 N, although some edge chipping or isolated flakes could be seen at the beginning or in the middle of the scratch trace shown in FIG. 5.

[0058] It is therefore shown that the high wear resistance of the coating, in the region of 110.sup.17 m.sup.3 N.sup.1 m.sup.1, is maintained even when the coating thickness is increased up to 4.0 micrometres as illustrated in Table 2 and FIG. 6.

[0059] This is significant for the practical usages of the coating, as the lifetime of the coated substrate increases accordingly.

[0060] Thus, the provision of the Ti material continues to provide the advantages of high adhesion, high humidity and wear resistance while the addition of TiB.sub.2 promotes the formation of a relatively uniform, dense, coating, so strengthening the coating which is formed and improving the high temperature performance of the coatings.

[0061] FIG. 7 illustrates a typical friction coefficient curve of the coating in accordance with the invention sliding against a WC+Co ball in a ball-on-disc test performed in ambient air at different normal loads. The friction coefficient is lower than 0.05 even at a low load and so it is found that the higher the normal loads, the lower the friction coefficient of the coatings.

[0062] The TEM images in FIGS. 9 and 10 show a cross section through the coating 20 formed in accordance with the invention, with FIG. 9 including the substrate 8 on which the coating is formed. FIG. 10 is focused on the main part of the coating formed in accordance with one embodiment of the invention and shows that the coating, in this embodiment is built up by the repeating of two bands, i.e. the dark band 22 (MoS.sub.2 and Ti); and the bright band 24 (MoS.sub.2 and Ti and TiBZ). It can be seen in the detailed portion 26 of FIG. 10 that within the bright band, bright stripes 28 (TiB.sub.2 layers) are sandwiched between the dark stripes (MoS.sub.2 and Ti).

[0063] The thickness of the dark band (MoS.sub.2 and Ti) band indicates that the dark band in FIG. 9 is composed of four basal-oriented (MoS.sub.2 and Ti) monolayers. With the inclusion of TiB.sub.2 in the bright band, the growth of the (MoS.sub.2 and Ti) layer is interrupted hence effectively two basal-oriented (MoS.sub.2 and Ti) monolayers are deposited. That is, the bright band in FIG. 9 is formed with two nano-scale-basal-oriented (MoS.sub.2 and Ti) monolayers sandwiched between nano-layers of TiB.sub.2.

[0064] Thus the coating achieved in accordance with the invention achieves a dense, amorphous structure shown by SEM, XRD and TEM analyses with high load bearing capacity and excellent tribological properties of the coatings shown by ball-on-disc tests at ambient conditions and so the thickness of the coatings can be increased to 4.0 m while the specific wear rate of the coatings, 110.sup.17 m.sup.3 N.sup.1 m.sup.1, is maintained at the same level as that of a 1.0 m thick coating.

[0065] This is significant for the practical applications of solid lubricant coatings as the same can be applied to substrates for use in cutting and forming industries and in addition for substrates for use in heavy duty, long endurance and high reliability requirements and for substrates for use as components with high roughness.