Temperature resistant carbon coatings

Abstract

The invention provides a substrate coated with a multi-layer coating, comprising in order: (a) the substrate; (b) a thermally insulating layer (e.g. Si.sub.3N.sub.4); (c) an interfacial layer (e.g. SiC); and (d) one or more layers comprising ta-C; wherein the interfacial layer promotes adhesion of the one or more layers comprising ta-C to the thermally insulating layer; and methods for producing such coatings.

Claims

1. A coated substrate with a multi-layer coating, the coated substrate comprising in order: (a) the substrate; (b) a seed layer comprising SiC; (c) a thermally insulating layer comprising Si.sub.3N.sub.4; (d) an interfacial layer comprising SiC; and (e) one or more layers comprising ta-C; wherein the interfacial layer promotes adhesion of the one or more layers comprising ta-C to the thermally insulating layer.

2. A coated substrate according to claim 1, wherein the one or more (e) layers consist of ta-C.

3. A coated substrate according to claim 1, wherein the thermally insulating layer has a thickness of from 0.2 m to 1 m.

4. A coated substrate according to claim 1, wherein the one or more layers comprising ta-C have a total thickness of from 0.1 m to 1 m.

5. A coated substrate according to claim 1, wherein the coating has a total thickness of from 1 m to 5 m.

6. A coated substrate according to claim 1, wherein the substrate comprises graphite.

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

8. A coated substrate according to claim 1, wherein the coating comprises two or more layers comprising ta-C, optionally wherein the hardness of each layer comprising ta-C increases from the layer comprising ta-C adjacent the interfacial layer to the outermost layer of ta-C.

9. A coated substrate according to claim 1, wherein: (a) the substrate is graphite sealed with an organic sealant; (b) the seed layer comprises SiC and has a thickness of from 0.1 m to 0.2 m; (c) the thermally insulating layer comprises Si.sub.3N.sub.4 and has a thickness of from 0.4 m to 0.6 m; (d) the interfacial layer comprises SiC and has a thickness of from 0.1 m to 0.2 m; and (e) the one or more layers comprising ta-C have a thickness of 0.1 m to 0.2 m.

10. A coated substrate according to claim 1, wherein: (a) the substrate is SiO.sub.2 coated ceramic; (b) the seed layer consists of SiC and has a thickness of from 0.4 m to 0.8 m; (c) the thermally insulating layer consists of Si.sub.3N.sub.4 and has a thickness of from 0.4 m to 0.8 m; (d) the interfacial layer consists of SiC and has a thickness of from 0.4 m to 0.8 m; and (e) the one or more layers consisting of ta-C have a thickness of 0.4 m to 0.8 m.

11. A method of making a coated substrate according to claim 1, comprising depositing onto the substrate a coating comprising in order: (a) a seed layer comprising SiC; (b) a thermally insulating layer comprising Si.sub.3N.sub.4; (c) an interfacial layer comprising SiC; and (d) one or more layers comprising ta-C; wherein the interfacial layer promotes adhesion of the one or more layers comprising ta-C to the thermally insulating layer.

12. A coated substrate according to claim 1, wherein the thermally insulating layer has a thermal conductivity of 10 W/(m,K) or less when measured at a temperature of 25 C.

13. A coated substrate according to claim 1, wherein the interfacial layer has a thickness of from 0.05 m to 0.3 m.

14. A coated substrate according to claim 1, wherein the thermally insulating layer consists of Si.sub.3N.sub.4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows a schematic diagram of the structure of the coating of the invention described in Example 1 (not to scale);

(3) FIGS. 2A and 2B show the loading and unloading curves obtained during hardness testing of the coating of the invention described in Example 1; and

(4) FIG. 3 shows is a schematic diagram showing the structure of the coating of the invention described in Example 3 (not to scale).

EXAMPLES

Example 1

(5) A first example of the coating of the invention (see FIG. 1, 10) was prepared as described below:

(6) TABLE-US-00002 Layer Thickness Ta-C (15) 0.2 m SiC (14) 0.15 m Si.sub.3N.sub.4 (13) 0.5 m SiC (12) 0.15 m Graphite Substrate (11) Total Thickness ~1 m (determined by CAR2)

(7) In summary, onto a sealed graphite substrate, a seed layer of SiC was sputtered, followed by subsequent layers in order of Si.sub.3N.sub.4, then SiC with the thicknesses in the tables above. Onto these intermediate layers was then deposited a ta-C coating using FCVA apparatus.

(8) A commercially available sealant specially formulated for graphite surface (e.g. Resbond Graphite Sealer 931S-1) was sprayed onto the surface of a graphite glass lens mould and the sintered in a vacuum furnace at 1000 C. in order to seal the substrate.

(9) Coating of the sealed graphite substrate was conducted using a large-scale ta-C coating equipment designed and manufactured by Nafeng. Firstly, the sealed graphite substrate was placed into the coating equipment chamber and the chamber was then heated and evacuated. Under vacuum, ion cleaning and ion-etching was conducted on the surface of the substrate in order to enhance adhesion with the coating. The seed layer of SiC was then deposited on the substrate by magnetron sputtering, followed by subsequent layers in order of Si.sub.3N.sub.4, and then SiC with the thicknesses in the tables above. The target material was Si and the reaction gases used were acetylene and nitrogen. After the underlying coating was completed, a layer of ta-C was deposited using FCVA coating technology.

(10) The hardness of the coating was determined by using a nanoindenter (CSM NHT2). The loading and unloading curves before and after being subjected to a temperature of 500 C. are shown in FIGS. 2A and 2B respectively. Before heating, the coating had a hardness value of 2930HV and after heating the hardness value was 3044HV. Hence, the hardness of the coatings is not impaired as a result of being subject to high temperatures.

Example 2

(11) A second example of the coating of the invention was prepared in an analogous manner to the coating of Example 1, with the structure shown in the table below.

(12) TABLE-US-00003 Layer Thickness Ta-C 0.2 m TiC 0.1 m AlTiN 0.5 m Substrate Total Thickness ~0.8 m (determined by CAR2)

Example 3

(13) A thermal printing head was coated (see FIG. 3, 30) in an analogous manner to the coating of Example 1, with the structure shown in the table below:

(14) TABLE-US-00004 Layer Thickness Ta-C (35) 0.6 m SiC (34) 0.6 m Si.sub.3N.sub.4 (33) 0.5 m SiC (32) 0.6 m Substrate - SiO.sub.2-coated ceramic (with optional SiC coating) (31)

(15) The performance of this coating was evaluated following exposure to 500 C. for two hours under atmospheric pressure.

(16) Firstly, a sample of the coating was cut using a cross-hatch cutter (variable-length 1.5 mm.sup.2 cutter). A length of 3M610 tape was then applied to the cut surface of the coating and then peeled off. No coating was peeled off by the tape and hence this coating is well adhered to the substrate.

(17) As an indication of the wear-resistance of the coating, a Taber abrasion test was conducted on the coating, with the following conditions: Instrument: Taber Linear Abraser TLA 5750 Abradant: CS-17 Wearaser Test Load: 1 kg weight Cycle Speed: 60 cycles/min Stroke Length: 25 mm

(18) Following 10,000 cycles, none of the substrate was exposed and no scratches were observed in the coating.

Example 4

(19) A thermal printing head was coated in an analogous manner to the coating of Example 1, with the structure shown in the table below:

(20) TABLE-US-00005 Layer Thickness Ta-C 0.6 m SiC 0.6 m Si.sub.3N.sub.4 0.5 m Substrate - SiO.sub.2-coated ceramic (with optional SiC coating)

(21) The performance of this coating was evaluated following exposure to 500 C. for two hours under atmospheric pressure and the results were comparable to those obtained in Example 3.

Example 5

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

(23) TABLE-US-00006 Layer Thickness Hardness/HV Ta-C 0.3 m 3794 Ta-C 0.4 m 2672 SiC 0.42 m 1406 Si.sub.3N.sub.4 0.36 m 1324 SiC 0.42 m 1406 Substrate - Graphite Total thickness 1.9 m

(24) This coating was subjected to our internal tests. This coating passed our cross hatch test. The coating was also subjected to our internal steel wool test (1 kg, 60 ms.sup.1, 17 mm) and our internal wear test (2 kg, 10 minutes). Again, the coating passed both of these tests.

(25) A further test was carried out according to VDI standard 3198 with a load of 60N. This coating was found to fall into the HF-2 quality class since there was no peel off but some scratches after completion of the test.

Example 6

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

(27) TABLE-US-00007 Layer Thickness Hardness/HV Ta-C 0.3 m 3794 Ta-C 0.4 m 2672 SiC 0.4 m 1406 Substrate - Graphite Total thickness 1.1 m

(28) This coating was subjected to our internal tests. This coating passed our cross hatch test. The coating was also subjected to our internal steel wool test (1 kg, 60 ms.sup.1, 17 mm) and our internal wear test (2 kg, 10 minutes). Again, the coating passed both of these tests.

(29) A further test was carried out according to VDI standard 3198 with a load of 60N. This coating was found to fall into the HF-1 quality class since there was no peel off and no scratches after completion of the test.

(30) As can be seen in the Examples above, coatings of the invention are stable and maintain their hardness upon exposure to high temperatures.