THICK, LOW STRESS TETRAHEDRAL AMORPHOUS CARBON COATINGS

Abstract

A method of continuously depositing a coating on a substrate comprises (a) depositing a first layer of ta-C on a substrate via a CVA process, said first layer having a first hardness and a first thickness of 100 mm or greater; (b) adjusting the parameters of the CVA process and depositing a second layer of ta-C on a substrate via a CVA process, said second layer having a second hardness and a second thickness of 10 mm or less, and (c) repeating the above steps to provide a coating comprising at least 5 such first layers and at least 4 such second layers, wherein the first thickness is greater than the second thickness and the first hardness is greater than the second hardness.

Claims

1-17. (canceled)

18. A method of depositing a coating on a substrate, the method comprising: (a) depositing a first layer of ta-C on a substrate via a CVA process, said first layer having a first hardness and a thickness of 100 nm or greater; (b) depositing a second layer of ta-C on a substrate via a CVA process, said second layer having a second hardness and a thickness of 10 nm or less, and (c) repeating step (a) at least once, wherein the hardnesses of the first layers are greater than the hardnesses of the second layers.

19. A method according claim 18, comprising continuously depositing the coating on the substrate and repeating the steps to provide a coating comprising at least 5 such first layers and at least 4 such second layers.

20. A method according to claim 18, comprising carrying out the deposition using a substantially continuous CVA process for the first and second layers using the same CVA source, preferably a FCVA source.

21. A method according to claim 18, comprising (a) depositing the first layer using a CVA process at a first bias voltage; (b) depositing the second layer using a CVA process at a second bias voltage more negative than the first.

22. A method according to claim 18, wherein the hardnesses of the first layers are greater than 1500 HV.

23. A method according to claim 18, wherein the hardnesses of the second layers are 1500 HV or less.

24. A method according to claim 18, wherein the hardnesses of the first layers are less than 7000 HV.

25. A method according to claim 18, wherein the thickness of the second layers is 5 nm or less.

26. A method according to claim 18, wherein the thickness of the first layers is 200 nm or greater.

27. A method according to claim 18, wherein the ratio of the thicknesses of the first to second layers is from 1000:1-50:1.

28. A method according to claim 18, wherein the substrate is a stainless steel substrate.

29. A method according to claim 18, which further comprises the step of depositing a seed layer onto the substrate prior to depositing the first layer in step a).

30. A method according to claim 29 wherein the seed layer is formed from Ti, Cr, Ni, W, Si or combinations, alloys, carbides or nitrides thereof.

31. A method according to claim 30 wherein the seed layer is chromium or titanium.

32. A method according to claim 29 wherein the seed layer has a thickness of 0.05 μm to 0.5 μm.

33. A method according to claim 18, wherein the total thickness of the coating is from 1-100 microns.

34. A method according to claim 18, wherein the total thickness of the coating is 5 microns or greater.

35. A substrate comprising a coating deposited according to the method of claim 18.

36. A substrate according to claim 35, wherein the hardness of the coating is 2000 HV or greater.

37. A substrate according to claim 35, wherein the substrate is an engine component, e.g. a piston ring, a piston pin, a cam shaft, a lift valve or an injection nozzle.

Description

[0057] The invention is now illustrated with reference to the accompanying drawings, in which:—

[0058] FIG. 1 shows hardness result for a coating made in accordance with WO 2009/151404;

[0059] FIG. 2 shows hardness result for a coating made in accordance with the invention and set out in Example 2;

[0060] FIG. 3 shows an SEM of the coating made in accordance with WO 2009/151404 after temperature testing;

[0061] FIG. 4 shows an SEM of the coating of Example 2 after temperature testing;

[0062] FIG. 5 shows an SEM of a further coating of the invention with thickness approx. 30 microns;

[0063] FIG. 6 (a, b) shows the coating of FIG. 5 prior to temperature testing;

[0064] FIG. 7(a, b, c, d) shows the coating of FIG. 5 after temperature testing;

[0065] FIG. 8 (a, b) shows the coating of example 4 after temperature testing;

[0066] FIG. 9 shows an SEM of a further coating of the invention with thickness approx. 13 microns; and

[0067] FIG. 10 shows the loading/unloading curve for the coating of FIG. 9 as used to calculate its hardness.

EXAMPLE 1—COMPARATIVE EXAMPLE

[0068] Following the teaching of WO 2009/151403, example 3, a ta-C coating was prepared on a HSS piston ring having the following parameters:

[0069] Seed layer Ti, approx. 250 nm

[0070] FCVA layers approx. 350 nm

[0071] Sputtered layers approx. 10 nm

[0072] Total coating thickness approx. 20 microns

[0073] Nanohardness was tested at a maximum loading of 8 mN, loading speed 16 mM/min, holding time 30 s. The results are shown in FIG. 1 and gave a calculated hardness of 2211 HV. Critical load was not determined, though in all embodiments of example 3 from WO 2009/151403 the critical load values varied in the range approx. 18-21N.

[0074] Temperature resistance was measured by holding the piston ring at 400° C. for 2 hours, the cooled ring after the test being shown in FIG. 3, with minor delamination/surface damage being seen, indicating a fail of this test.

EXAMPLE 2

[0075] A coating of the invention was prepared to match the coating of the prior art as per example 1, again with ta-C coated onto a HSS piston ring, the coating having the following parameters:

TABLE-US-00002 Seed layer Ti, approx. 250 nm FCVA hard layers approx. 500 nm FCVA soft layers approx. 1-2 nm Total coating thickness approx. 20 microns

[0076] The coating was deposited with alternate first (hard) and second (soft) layers, via FCVA, the soft layers deposited with negative bias of approximately −900V, with a 40% duty cycle using a 44 KHz pulsed DC power supply. Biasing for the first layers was standard for hard ta-C layer deposition, being approximately −200V, duty cycle 30%.

[0077] Nanohardness was tested at a maximum loading of 8 mN, loading speed 16 mM/min, holding time 30 s. The results are shown in FIG. 2 and gave a calculated hardness of 3122 HV. Critical load was not determined. Temperature resistance was similarly measured by holding the piston ring at 400° C. for 2 hours, the cooled ring after the test being shown in FIG. 3, with absence of delamination and surface damage indicating a pass of this test.

EXAMPLE 3

[0078] A HSS Shuang Huan piston ring was coated with approx. 60 repeating layers of approx. 500 nm hard ta-C (bias approximately −200V) and approx. 1-2 nm layers of soft ta-C (bias approx. −1000V). The finished coating thickness was measured by SEM cross-sectional analysis (and one such cross section is shown in FIG. 5); three separate measurements gave thicknesses of 29.71 microns, 29.73 microns and 29.9 microns, hence a smooth, substantially uniform coating with thickness approx. 30 microns was achieved.

[0079] The coated piston ring was subjected to temperature testing, with samples analyzed prior to heating (FIG. 6a, b) and after 2 hours heating at 400° C. (FIGS. 7a, b, c and d). The coating is seen to have passed this heating test.

[0080] The coating was subjected to a first scratch test with a max load of 60N, and passed. The coating was subjected to a second scratch test with a max load of 100N and demonstrated critical load failure at approx. 80N.

EXAMPLE 4

[0081] A HSS diesel engine piston ring was coated with approx. 50 layers of hard ta-C (substrate bias approx. −300V) with soft ta-C layers of approx. 1-2 nm (bias −1000V) giving a coating thickness of approx. 25 microns measured by SEM cross-sectioning.

[0082] The coating was subjected to the same temperature test as in Example 3, with the coating post-testing shown in FIG. 8 (a, b). After heating and cooling there was no delamination of the coating from the piston ring indicating the coating had good thermal stability and good adhesion onto the substrate, and passed the test.

[0083] Coating hardness was tested with a maximum load of 8 mN with a load/unload rate of 16 mN/min with 30 s holding time. The hardness was calculated as approx. 2500 HV.

[0084] Wear evaluation was carried out using a ball crater machine, with a ball rotation speed of 300 rpm, ball diameter 30 mm, sample slope angle of 25° and lapping slurry (with 0.1 micron diameter diamond powder) applied during testing. After 30 minutes of the wear test the crater depth was 3.56 microns and the crater diameter was 743 microns. After 120 minutes of the wear test the crater depth was 7.25 microns and the crater diameter 1019 microns.

[0085] The coating was subjected to a scratch test with an Anton Paar revertest scratch tester having a diamond indenter/stylus with a spherical tip of radius 200 microns and a Rockwell C geometry with an angle of 120°, load speed of 40N/min. With a 60N scratch load there was no cracking in the coating, indicating the critical load of the coating is in excess of 60N.

[0086] The coating was subjected to a separate anti-wear performance evaluation, measuring wear of the coated piston ring in response to reciprocation of the piston ring on a cylinder liner. Using a load force between the piston ring and the liner of 500N, a reciprocating frequency of 4 Hz, with engine lubricant/oil applied between the interface of the piston ring and the cylinder liner, the test was continued for 230,400 cycles (3 cm per cycle), amounting to approx. 16 hours of test duration. After the test, the coating was investigated by microscopy and a stylus profiler. The wear track depth was measured as less that 0.2 microns, indicating very low wear for this test.

[0087] The coating was separately subjected to a co-efficient of friction test using a Tribo-tester (Bruker® Tribolab system, Bruker Corporation). Friction between the piston ring and a cylinder liner was measured with and without lubricant oil (Castrol® engine oil, 5w-30) with a normal force between the ring and the liner of 10N and a pin with velocity 0.84 mm/sec (0.03 Hz, 14 mm stroke). With lubrication the coefficient of friction (COF) was measured as 0.04-0.14 and without oil the COF was measured as 0.15-0.18.

EXAMPLE 5

[0088] A HSS piston ring was coated according to example 4, though with reduced number of coating layers, yielding a coating of approx. 13 microns thickness.

[0089] SEM cross-sectional analysis showed a dense, uniform coating (see FIG. 9).

[0090] Coating hardness was measured using a nanoindenter with a maximum load of 8 mN, a load/unload rate of 16 mN/min with 30 s holding time. The loading/unloading curve is shown in FIG. 10, giving a hardness calculated as 2850 HV.

[0091] The coating was subjected to a Taber abrasion test (TLA 5700) with abradant CS-17, a test load of 1.5 kg and a cycle speed of 60 cycles/min with a stroke length of 50 mm. After 70,000 cycles the coating remained intact, and hence passed the test.

[0092] Accordingly, the present invention provides methods for making a ta-C coating and substrates coated therewith.