ta-C based coatings with improved hardness

Abstract

A substrate is coated with a multi-layer coating, comprising in order: (i) a first functional layer comprising ta-C, (ii) a second functional layer comprising ta-C, (iii) (a) a third functional layer comprising ta-C and a first intermediate layer comprising a carbide of a first element, or (b) a first intermediate layer comprising a carbide of a first element, and a second intermediate layer comprising the first element, wherein the ta-C has a hydrogen content less than 10% and an sp2 content less than 30%; wherein (i) the Young's modulus or (ii) the hardness or (iii) both the Young's modulus and the hardness independently stay the same or increase from layer to layer in (iii) (a) from the first intermediate layer to the first functional layer, or in (iii) (b) from the second intermediate layer to the first functional layer.

Claims

1. A substrate coated with a multi-layer coating, comprising in order from the outside towards the substrate: a first functional ta-C-containing layer of hardness 2000 HV or greater, a second functional ta-C-containing layer of hardness 1200 HV or greater, a first intermediate layer comprising tungsten carbide, a second intermediate layer comprising chromium tungstide, and a further intermediate layer comprising chromium and adjacent the substrate, wherein the ta-C has a hydrogen content less than 10% and an sp.sup.2 content less than 30%; wherein the first functional layer has a hardness at least 200 HV greater than the second functional layer; and wherein (1) the Young's modulus or (2) the hardness or (3) both the Young's modulus and the hardness independently stay the same or increase from layer to layer from the second intermediate layer to the first functional layer.

2. A coated substrate according to claim 1, wherein the coating comprises one or more further functional layers comprising ta-C between the second functional layer and the first intermediate layer.

3. A coated substrate according to claim 1, wherein the ta-C has a hydrogen content of 5% or less and an sp.sup.2 content of 20% or less.

4. A coated substrate according to claim 1, wherein the substrate is steel or a variety of steel.

5. A coated substrate according to claim 1, wherein the Young's modulus increases over a set of three adjacent layers in the coating.

6. A coated substrate according to claim 5, wherein the average increase in the Young's modulus is 10 GPa per layer, or more.

7. A coated substrate according to claim 1, wherein the coating has a hardness of at least 2000 HV.

8. A coated substrate according to claim 1, wherein the coating has a hardness of at least 4000 HV.

9. A coated substrate according to claim 1, wherein the hardness increases over any set of three adjacent layers in the coating.

10. A coated substrate according to claim 9, wherein the average increase in hardness is at least 300 HV per layer.

11. A coated substrate according to claim 10, wherein the average increase in hardness is at least 400 HV per layer.

12. A coated substrate according to claim 1, wherein the hardness of the functional layer adjacent the first intermediate layer is 1600 HV or more.

13. A coated substrate according to claim 1, wherein the coating has a total thickness of 5 microns or less.

14. A coated substrate according to claim 1, wherein the coating has a total thickness of 2 microns or less.

15. A coated substrate according to claim 1, wherein each layer of the coating has a thickness of 1 micron or less.

16. A coated substrate according to claim 1, wherein the substrate is selected from tools, cutting tools, tooling, industrial machines and components therefor.

17. A coated substrate according to claim 1, wherein the substrate is an engine component.

18. A method of making a coated substrate according to claim 1, comprising providing the substrate, and coating onto the substrate, in order: a further intermediate layer comprising chromium, a second intermediate layer comprising chromium tungstide, a first intermediate layer comprising tungsten carbide, a second functional ta-C containing layer of hardness 1200 HV or greater, a first functional ta-C containing layer of hardness 2000 HV or greater, wherein the ta-C has a hydrogen content less than 10% and an sp.sup.2 content less than 30%; wherein the first functional layer has a hardness at least 200 HV greater than the second functional layer; wherein (1) the Young's modulus or (2) the hardness or (3) both the Young's modulus and the hardness independently stay the same or increase from layer to layer from the second intermediate layer to the first functional layer.

19. A method according to claim 18, wherein the functional layers are deposited by PVD.

20. A method according to claim 19, wherein the functional layers are deposited by FCVA.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is now illustrated with reference to the accompanying drawings in which:

(2) FIG. 1 shows is a schematic diagram showing the structure of Comparative Coating 1 (not to scale).

(3) FIG. 2 shows is a schematic diagram showing the structure of Comparative Coating 2 (not to scale).

(4) FIG. 3 shows is a schematic diagram showing the structure of Comparative Coating 3 (not to scale).

(5) FIG. 4 shows is a schematic diagram showing the structure of Comparative Coating 4 (not to scale).

(6) FIG. 5 shows is a schematic diagram showing the structure of Coating A of the invention (not to scale).

(7) FIG. 6 shows is a schematic diagram showing the structure of Coating B of the invention (not to scale).

COMPARATIVE EXAMPLES

(8) Four prior art coatings, all commercially available, were used (named Comparative Coatings 1 to 4) as comparative examples, with structures as set out below:

(9) Comparative Coating 1

(10) A coated piston (10) was obtained from a commercial supplier, with properties (see FIG. 1):

(11) TABLE-US-00002 Layer Thickness DLC (14) 1.413 μm CrC (13) 0.606 μm CrN (~50% N) (12) 0.714 μm Substrate (11) Total Thickness ~2.7 μm
Comparative Coating 2

(12) A coated plunger (20) was obtained from a commercial supplier, with properties (see FIG. 2):

(13) TABLE-US-00003 Layer Thickness DLC (25) 1.187 μm CrC (24) 0.188 μm CrN (~50% N) (23) 0.569 μm CrN (~20% N) (22) 0.219 μm Substrate (21) Total Thickness ~2.2 μm
Comparative Coating 3

(14) A coated tappet (30) was obtained from a commercial supplier, with properties (see FIG. 3):

(15) TABLE-US-00004 Layer Thickness DLC (35) 2.806 μm Si (34) 0.110 μm CrN (~20% N) (33) 0.504 μm Cr (32) 0.234 μm Substrate (31) Total Thickness ~3.7 μm
Comparative Coating 4

(16) A second coated tappet (40) was obtained from a commercial supplier, with properties (see FIG. 4):

(17) TABLE-US-00005 Layer Thickness DLC (44) 1.702 μm WC (43) 0.332 μm CrN (~20% N) (42) 1.817 μm Substrate (41) Total Thickness ~3.8 μm

Examples

(18) Two coatings of the invention were prepared as described below:

(19) Coating A (50, see FIG. 5)

(20) TABLE-US-00006 Layer Thickness Ta-C (55) 1.5 μm WC (54) 0.2 μm CrW (53) 0.5 μm Cr (52) 0.3 μm Substrate (51) Total Thickness ~2.5 μm (determined by CAR2)
Coating B (60, see FIG. 6)

(21) TABLE-US-00007 Layer Thickness Ta-C (65) 0.8 μm WC (64) 0.1 μm CrW (63) 0.1 μm Cr (62) 0.2 μm Substrate (61) Total Thickness ~1.2 μm (determined by CAR2)

(22) In more detail, onto SUS304 HSS substrates a seed layer of Cr was sputtered, followed by subsequent layers in order of CrW, then WC with the thicknesses in the tables above. Onto these intermediate layers was then deposited multi-layer functional ta-C coatings (4 for A and 3 for B) using FCVA apparatus. Depositions parameters and Young's modulus and hardness of the layers were as follows:—

(23) TABLE-US-00008 Coating A Layer information Hardness Young's modulus Bias/ Range Typical Range Typical Layer material* Low High value Low High value ta-C 4 600 V/15% 2700 3100 2800 325 355 344 ta-C 3 800 V/15% 2450 2870 2500 305 330 310 ta-C 2 1000 V/15% 2100 2500 2300 262 310 280 ta-C 1 1000 V/20% 1650 1927 1850 220 250 245 SPT 3 WC 1340 1832 1627 237 280 245 SPT 2 CrW 900 1280 1050 216 260 235 SPT 1 Cr 750 950 850 200 240 220 Sub- SUS304/HSS 450 1056 929 229 274 253 strate *bias refers to FCVA bias, material refers to sputtered layers

(24) TABLE-US-00009 Coating B Layer information Hardness Young's modulus Bias/ Range Typical Range Typical Layer material* Low High value Low High value ta-C 3 400 V/30% 3800 4500 4300 367 471 400 ta-C 2 400 V/10% 2550 2950 2756 310 350 332 ta-C 1 1000 V/20% 1650 1927 1850 220 260 245 SPT 3 WC 1340 1832 1627 237 280 245 SPT 2 CrW 900 1280 1050 216 260 235 SPT 1 Cr 750 950 850 200 240 220 Sub- SUS304/HSS 450 1056 929 229 274 253 strate *bias refers to FCVA bias, material refers to sputtered layers

Example 1—Hardness

(25) The hardness of the coatings of the invention and comparative coatings were determined using a nanoindenter (CSM NHT2) with a maximum load of 8 mN, a load/unload rate of 16 mN/min and a pause of 30 seconds. From the loading/unloading curve which shows force against indentation, the Vickers hardness value (HV) of each of the coatings was determined. These values are provided below:

(26) TABLE-US-00010 Coating Hardness (HV) Coating A 3020 Coating B 4960 Comparative Coating 1 1900 Comparative Coating 2 1890 Comparative Coating 3 1816 Comparative Coating 4 1900

(27) As can be seen, the coatings of the invention had superior hardness values compared to the comparative prior art coatings.

Example 2—Taber Abrasion Test

(28) As an indication of the wear-resistance of the coatings, a Taber abrasion test was conducted on each of the coatings, with the following conditions: Instrument: Taber Linear Abraser TLA 5700 Abradant: CS-17 Wearaser® Test Load: 1.5 kg weight Cycle Speed: 60 cycles/min Stroke Length: 15 mm

(29) The number of cycles that each coating was subjected to and the status of the coating (acceptable or unacceptable) at the end of the indicated number of cycles is provided below.

(30) TABLE-US-00011 No. of Taber Abrasion Coating Cycles Coating Status Coating A 70,000 Acceptable Coating B 70,000 Acceptable Comparative Coating 1 50,000 Acceptable Comparative Coating 2 11,000 Unacceptable Comparative Coating 3 17,000 Unacceptable Comparative Coating 4 70,000 Acceptable

(31) The quality of comparative coatings 2 and 3 was unacceptable after less than 20,000 cycles. Whilst the coating status for comparative coating 1 was recorded as acceptable, this coating was only subjected to 50,000 cycles. By contrast, the coatings of the invention (A and B) and comparative coating 4 were all deemed acceptable, even after 70,000 cycles.

Example 3—Ball Crater Wear Test

(32) As a further test of the wear-resistance of the coatings, each of the coatings were subjected to a wear test using a ball crater machine, as described below. Instrument: Ball Crater Machine (CAT2) Wear Time: 400 seconds Ball Rotation Speed: 300 rpm Ball diameter: 30 mm Sample Slope Angle 25°

(33) The results of testing were:

(34) TABLE-US-00012 Wear/Crater Outer Radius Anti-Wear Coating (μm) Performance Coating A 223 Best Coating B 222 Best Comparative Coating 1 324 Good Comparative Coating 2 336 Good Comparative Coating 3 540 Poor Comparative Coating 4 580 Poorest

Example 4—Raman Spectroscopy

(35) Raman Spectroscopy can be used to provide an indication of the ratio of sp.sup.2 to sp.sup.3 carbon atoms in amorphous carbon coatings. The measured Raman Spectroscopy curve can be compared with a simulated curve for a sample having 100% sp.sup.3 carbon content. The I.sub.D/I.sub.G ratio is therefore indicative of any differences between the observed spectrum and the expected spectrum for a coating with 100% sp.sup.3 carbons—a higher I.sub.D/I.sub.G ratio being indicative of a greater sp.sup.2 carbon content.

(36) TABLE-US-00013 I.sub.D/I.sub.G Ratio (Fit by Coating G Peak Position (cm.sup.−1) BWF + Lorentzian) Coating A 1527.7 0 Coating B 1575.9 0 Comparative Coating 1 1548 0.16 Comparative Coating 2 1546.2 0.12 Comparative Coating 3 1561.2 0.34 Comparative Coating 4 1564.4 0.55

(37) The zero I.sub.D/I.sub.G ratio for the coatings of the invention correlate to a match between the observed curve and the simulated curve for a coating having 100% sp.sup.3 carbon content. Therefore, no sp.sup.3 carbons were detected using this method in the coatings of the invention. By contrast, in the Comparative Coatings, higher I.sub.D/I.sub.G ratios show the higher sp.sup.2 content of the coatings, characteristic of DLC coatings.

Example 5—Scratch Test

(38) A scratch test was performed on each of the coatings to determine their resistance to scratches applied along the coating surface under force. The scratch test was conducted using a moving diamond indenter/stylus with the following parameters: Stylus material: Diamond Spherical stylus tip radius: 200 μm Stylus shape: Rockwell C geometry with an angle of 120° Maximum load: 60N Load speed: 40N/min

(39) The critical loads (i.e. the load at which severe coating deformations were first observed) for each coating are provided in the table below:

(40) TABLE-US-00014 Coating Coating Thickness (μm) Critical Load (N) Coating A 2.5 37 Coating B 1.2 15 Comparative Coating 1 2.7 10 Comparative Coating 3 3.67 18 Comparative Coating 4 3.5 27

(41) As can be seen from the table, the greatest critical load was observed for Coating A. Whilst the critical load for Coating B was comparable to comparative coatings 2 and 3, it is noted that the thickness of Coating B is much less than for the comparative coatings.

(42) Hence, on the basis of these findings, it is expected that the coatings of the invention are improved resistance to scratches compared to conventional coatings of a similar thickness.

Example 6—Tribo Test

(43) In order to determine the wear resistance of the coatings under repeated, high force oscillating movements, a Tribo test was conducted using the Bruker TriboLab System. The Tribo test is a reciprocal “pin-on-disk” sliding test and mimics oscillating wear that may occur within an automobile engine. The Tribo test was carried out using the following parameters: Reciprocal Sliding Frequency: 2 Hz Loading force: 500-1600N Pin (fixed ball) size: ¼ inch (6.35 mm) diameter

(44) The maximum loads that each of the coatings were subjected to and the resultant wear track dimensions (width and depth) are provided below.

(45) TABLE-US-00015 Wear Track Maximum Width Depth Coating Load (N) (mm) (mm) Performance Coating A 1600 0.77 2.35 Lowest track depth wear under high load Coating B 1600 0.93 8.53 Low track depth under high load Comparative 1500 1.28 10.36 High track Coating 1 depth under high load Comparative 800 0.75 3.75 Low track Coating 3 depth under low load Comparative 1400 0.9 9.56 High track Coating 4 depth under high load

(46) Coating A had the lowest wear track (in terms of both width and depth) at the highest load (1600N). Whilst the wear track for Coating B was less than for Coating A, good wear resistance is still exhibited at a load of 1600N. Comparative coatings had significant wear tracks at maximum loads of less than 1600N.

Example 7—Wear and Scuff Test

(47) The resistance of the coatings were tested against small lengths of pipe in place of drill bits, using a mini bench drill.

(48) For the wear test, the pipe was an SUS304 stainless steel pipe with an outer diameter of 6 mm and an inner diameter of 4.5 mm. Any delamination of the coating was recorded.

(49) For the scuff test, the pipe was an aluminium pipe with an outer diameter of 5 mm and an inner diameter of 4 mm. Any delamination of the coating or insertion of the aluminium into the coating was recorded.

(50) The drill was set to rotate the pipe at a speed of 2500 rpm. The loads and drilling times were varied and oil was added evening 5 minutes during testing.

(51) The observations are detailed in the table below.

(52) TABLE-US-00016 Wear Test (with Stainless Scuff Test (with Steel pipe) Aluminium pipe) Coating A Some wear trace but no Some wear trace but no delamination following delamination following testing for 15 min with testing for 5 min with 35 kg load 30 kg load No delamination but some insertion after testing for 5 minutes with 35 kg load. Coating B Some wear trace but no Some wear trace but no delamination following delamination following testing for 15 min with testing for 5 min with 35 kg load 30 kg load No delamination but some insertion after testing for 5 minutes with 35 kg load. Comparative Some wear trace but no Some wear trace but no Coating 3 delamination following delamination following testing for 15 min with testing for 5 min with 30 kg load, but some 10 kg load delamination following No delamination but some testing for 15 min with insertion after testing for 35 kg. 5 minutes with 12 kg load.

(53) As can be seen in the Example above, coatings of the invention can have increased hardness, critical load, wear resistance and scuff resistance compared to the comparative coatings.

Example 8

(54) A further coating of the invention was prepared as described below:

(55) TABLE-US-00017 Coating C Layer Thickness ta-C 0.83 μm ta-C 0.2 μm WC 0.25 μm CrWC 0.95 μm Cr 0.8 μm Substrate - Tool Steel Total ~3.0 μm Thickness (determined by CAR2) Hardness/HV Layer Typical value ta-C 4500 ta-C 2500 WC 1586 CrWC 1090 Cr 850

(56) Coating C was tested and found to show high hardness and passed our internal sandpaper test, with no delamination, both before and after exposure to 500° C. for 2 hours.

Example 9

(57) A further coating of the invention was prepared as described below:

(58) TABLE-US-00018 Coating D Layer Thickness ta-C 0.5 μm ta-C 0.3 μm ta-C 0.2 μm SiC 0.5 μm Substrate - Ceramic Total Thickness ~1.5 μm (determined by CAR2) Hardness/HV Layer Typical value ta-C 3500 ta-C 3000 ta-C 2800 SiC 665

(59) Coating D was tested and passed our internal hatch test both before and after exposure to 500° C. for 2 hours. Coating D was also subjected to a Taber test with a Taber Abraser set to 1 kg, 60 rpm and 17 mm. The coating passed this test with no scratches both before and after exposure to 500° C. for 2 hours.

(60) The invention thus provides hard coatings on substrates and methods for preparing the same.