Polydopamine + Sio2 Underlayer For Improving Diamond-Like Carbon Coating Adhesion And Durability

Abstract

A composite comprising: substrate having thereon an intermediate layer and a diamond-like carbon (DLC) top layer on said intermediate layer, with increased adhesion strength to DLC and other hard coatings, and which provides a buffer layer for adjusting the uneven expansion/compression behavior of DLC coatings and substrates.

Claims

1. A composite comprising: substrate having thereon an intermediate layer and a diamond-like carbon (DLC) top layer on said intermediate layer.

2. The composite of claim 1 wherein said intermediate layer is polydopamine (PDA).

3. The composite of claim 1 wherein said intermediate layer is a PDA+SiO.sub.2 nanoparticle composite coating.

4. The composite of claim 2 wherein said substrate is a metal.

5. The composite of claim 2 wherein said substrate is a metallic compound.

6. The composite of claim 2 wherein said substrate is stainless steel.

7. The composite of claim 3 wherein said substrate is a metal.

8. The composite of claim 3 wherein said substrate is a metallic compound.

9. The composite of claim 3 wherein said substrate is stainless steel.

10. The composite of claim 6 wherein an average roughness of PDA coated stainless steel was 50±5 nm.

11. The composite of claim 10 wherein an average roughness of PDA+SiO.sub.2 coated stainless steel was 60±7 nm.

12. The composite of claim 1 further including nanoparticles between said intermediate layer and said DLC top layer.

13. The composite of claim 12 wherein said nanoparticles are from the group comprising: Al2O3, ZrO2, TiO2, N-TiO2-C, Fe3O4, MoS2, WS2, diamond, graphite, Ag, Au, Cu, Ta, and combinations thereof.

14. The composite of claim 1 further including nanoparticles within said intermediate layer and said DLC top layer.

15. The composite of claim 14 wherein said nanoparticles are from the group comprising: Al2O3, ZrO2, TiO2, N-TiO2-C, Fe3O4, MoS2, WS2, diamond, graphite, Ag, Au, Cu, Ta, and combinations thereof.

16. The composite of claim 2 wherein said substrate is a ceramic.

17. The composite of claim 2 wherein said substrate is a polymer.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0015] In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.

[0016] FIG. 1, includes cross-sectional views of a substrate, specifically, FIG. 1(a) is a cross-sectional view of a substrate.

[0017] FIG. 1, includes cross-sectional views of a substrate, specifically, FIG. 1(b) is a cross-sectional view of a substrate coated with PDA and SiO.sub.2 nanoparticle composite coating.

[0018] FIG. 1, includes cross-sectional views of a substrate, specifically, FIG. 1(c) is a cross-sectional view of a substrate with PDA and SiO.sub.2 nanoparticle composite coating, and then a DLC coating (not to scale).

[0019] FIG. 2 shows the average and root mean square surface roughness of DLC coated samples measured from 20 μm×20 μm AFM images.

[0020] FIG. 3 shows a comparison of critical loads Lc1 (initial crack propagation), Lc2 (Initial delamination), and Lc3 (Global domination) among the DLC coated samples with various underlayers.

[0021] FIG. 4 shows a comparison of surface cracks and delamination within the final part of scratch wear tracks between TMS/DLC and PDA+SiO.sub.2/DLC.

[0022] FIG. 5 shows a comparison of the coefficient of friction profiles among DLCs with various underlayers.

[0023] FIG. 6 shows a comparison wear track on DLCs and transfer film on counterface Si.sub.3N.sub.4 balls after 500 cycles.

[0024] FIG. 7 shows a comparison of the cross-sectional areas of wear tracks after 500 cycles among DLCs with various underlayers.

[0025] FIG. 8 shows a comparison of the average area of cracks in the wear tracks after 500 cycles between TMS/DLC and PDA+SiO.sub.2/DLC.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure, or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.

[0027] In certain embodiments, the present invention provides PDA and PDA+SiO.sub.2 underlayers that may be deposited on substrates such as stainless steel (SS). In one preferred embodiment, a digital rocking bath of 1.21 gm/L Tris(hydroxymethyl)aminomethane was added to 60° C. deionized water to make an ideal buffer solution for polymerizing dopamine hydrochloride (DA) into PDA. While the water-based buffer solution temperature was kept at 60° C., the rocking bath was rocked at 20 rpm with a 7° rocking angle. 13.33 mL/L of Colloidal silica dispersion (SiO.sub.2, Nissan Chemicals ST-PS-M) was added to the buffer solution after 5 min following the addition of 2 gm/L of DA, in order to incorporate SiO.sub.2 nanoparticles in the PDA underlayer. The polymerization process continued for another 40 min. This procedure provides a mechanically robust adhesive layer. Trimethylsilane [(CH.sub.3).sub.3SiH] was deposited using a plasma immersion ion deposition (PIID) process on SS. A 300 nm thin DLC was then fabricated on SS, SS/TMS, SS/PDA, and SS/PDA+SiO.sub.2. The structure of the DLC coating is illustrated in FIG. 1A-1C.

[0028] FIG. 1A shows a substrate (11) which may be a metal, metallic compound, ceramic, or polymer and many other materials, e.g., metals such as cast iron, carbon steels, or intermetallic materials, such as 60NiTi, Nitinol, and polymers. In one embodiment, the substrate is stainless steel (SS) with an average surface roughness of 26±2 nm. FIG. 1B shows a cross-sectional views of substrate (11), coated with PDA or PDA+SiO.sub.2 nanoparticle composite coating (12). The average roughness of PDA coated SS was 50±5 nm, and the average roughness of PDA+SiO.sub.2 coated SS was 60±7 nm.

[0029] FIG. 1C shows a preferred embodiment of the present invention. This embodiment concerns a composite 100 comprised of substrate (11) coated with TMS or PDA or PDA+SiO.sub.2 nanoparticle composite underlayer (12) and a DLC top layer (13).

[0030] In other embodiments, the present invention provides a substrate coated with TMS or PDA or PDA+SiO.sub.2 nanoparticle composite underlayer and a DLC top layer.

[0031] In other embodiments, nanoparticles such as Al.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2, N—TiO.sub.2—C, Fe.sub.3O.sub.4, MoS.sub.2, WS.sub.2, diamond, graphite, Ag, Au, Cu, Ta, and combinations thereof may be used within the intermediate layer (12). In yet other embodiments, nanoparticles may be added between the intermediate layer and the DLC top layer

[0032] In yet other embodiments, nanoparticles such as SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2, N—TiO.sub.2—C, Fe.sub.3O.sub.4, MoS.sub.2, WS.sub.2, diamond, graphite, Ag, Au, Cu, Ta, and combinations thereof may be used between the intermediate layer and the DLC top layer (13).

[0033] The average and root mean square surface roughness of DLC-coated samples measured from 20 μm×20 μm images obtained by atomic force microscope (AFM) are shown in FIG. 2. The average surface roughness of PDA/DLC (23) and PDA+SiO.sub.2/DLC (24) were 33.3% and 113.3% higher than the TMS/DLC (22), respectively. Similarly, the root mean square surface roughness of PDA/DLC and PDA+SiO.sub.2/DLC were 35.3% and 129.4% higher than the TMS/DLC, respectively.

[0034] Scratch and coating wear tests were carried out using a Bruker Tribometer (UMT TriboLab, Bruker, USA). The objectives of the scratch tests were to determine three critical loads for a) lateral cracks (Lc1), b) initial delamination (Lc2), and c) global delamination (Lc3). A linearly increased normal load from 0.5 to 18 N was applied in the scratch tests using a 400 μm diameter diamond coated tip. The scratch length was 15 mm, and the speed was 0.15 mm/s. The average critical loads for Lc1, Lc2, and Lc3 are shown in FIG. 3. The highest average Lc1 was 2.52 N for the PDA+SiO.sub.2/DLC (34), which was 4.66 and 1.57 times higher than those for the DLC only (31) and the TMS/DLC (32), respectively. More importantly, the size and number of cracks were significantly smaller and fewer compared to those of the TMS/DLC. Similarly, the highest average Lc2 was 7.81 N for the PDA+SiO.sub.2/DLC (34), which was 6.30 and 1.55 times higher than those for the DLC only (31) and the TMS/DLC (32), respectively. Finally, there was no global delamination for the PDA/DLC (33) and PDA+SiO.sub.2/DLC (34) at 18 N, whereas the Lc3 for the DLC (31) and TMS/DLC (32) were 4.60 N and 12.2 N, respectively.

[0035] FIG. 4 shows the comparison of wear tracks for the TMS/DLC (41) and PDA+SiO.sub.2/DLC (42) at 17.5-18 N load range during the scratch test. Most of the TMS/DLC (41) was delaminated, whereas the PDA+SiO.sub.2/DLC (42) was without any global delamination; however, it suffered from a few microcracks and local delamination.

[0036] Linear reciprocating wear tests were performed for 500 cycles to determine the wear rate and wear mechanism. The average normal load was 2 N, speed was 1 mm/sec, whereas the counterface was 6.35 mm diameter Si.sub.3N.sub.4 balls. The comparison of the COF profiles over time among DLCs with various underlayers is shown in FIG. 5. The COF of all DLC coatings had a transition period from high to low at the beginning. However, the COF of TMS/DLC (52) showed a rapid increase after 123 cycles, whereas the DLC only (51) had a steady low COF for 250 cycles before it increased, most likely facilitated by the transfer film. The COF of PDA/DLC (53) had a slight increase at 105 cycles and then increased sharply at 360 cycles. Remarkably, the COF of PDA+SiO.sub.2/DLC (54) remained low throughout the test after the initial drop. The DLC only (51) coating showed the second-lowest COF due to the coating delaminate early on, as shown in FIG. 6 (61), and the loose particles generated reduced the resistance to the counterface movement. The PDA+SiO.sub.2 (54) coatings had larger and harder textures than the PDA/DLC coating due to the addition of SiO.sub.2 NPs, which resulted in less real area of contact and thus less friction.

[0037] The wear tracks in the DLC coatings also supported the COF profiles, which are shown in FIG. 6. The DLC only (61) coating was almost entirely delaminated and had a clear sign of plowing into the SS substrate. The associated ball (65) was worn and had transfer film and loose DLC particles. The TMS/DLC (62) and PDA/DLC (63) coatings were also severely damaged as a significant portion of the coatings were delaminated with micro cracks. The counterface balls for the TMS/DLC (66) and PDA/DLC (67) had transfer films and loose DLC particles, but none of them was worn. In contrast, the PDA+SiO.sub.2/DLC (64) coating did not have any delamination. The coating was plastically deformed, and most of its roughness peaks were worn out. The associated counterface ball (68) had transfer film and fewer number of loose DLC particles.

[0038] FIG. 7 shows a comparison of the average cross-sectional area of wear tracks for the DLC only (71), TMS/DLC (72), PDA/DLC (73), and PDA+SiO.sub.2/DLC (74). The average cross-sectional area of the PDA+SiO.sub.2/DLC (74) was 3.91 and 2.51 times smaller than that of the DLC only (71) and TMS/DLC (72), respectively.

[0039] Scanning electron microscope images of the wear tracks of the TMS/DLC and PDA+SiO.sub.2/DLC that went through 500 cycles of wear test were analyzed to determine the size of the cracks within the wear tracks. The cracks of the PDA+SiO.sub.2/DLC were significantly smaller than that of the TMS/DLC. As shown in FIG. 8, the average size of the micro-cracks for the PDA+SiO.sub.2/DLC (82) was 23.8 nm.sup.2, which was 40 times smaller than the microcracks of the TMS/DLC of 943.6 nm.sup.2 (81).

[0040] The AFM images of the surfaces of the wear tracks after tested for various cycles were inspected to understand the wear mechanisms better. While the wear mechanisms of the TMS/DLC involved initial detachment, cracking, and delamination, those of the PDA+SiO.sub.2/DLC only went through the worn of roughness peaks and permanent deformation. The behavior of the PDA+SiO.sub.2/DLC indicated its superior strength in preventing initial detachment, cracking, and delamination. The nanoscale roughness and the toughness and adhesiveness of PDA+SiO.sub.2 underlayer played an essential role in achieving these outstanding performances of PDA+SiO.sub.2/DLC coatings.

[0041] While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above-described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure.