WO2023217406A1

WO2023217406A1 - Amorphous carbon coating for reduction of friction and wear in a slide component

Info

Publication number: WO2023217406A1
Application number: PCT/EP2023/000028
Authority: WO
Inventors: Jukka Kolehmainen; Sanna TERVAKANGAS; Noora MANNINEN; Olivier Jarry
Original assignee: Oerlikon Surface Solutions Ag, Pfäffikon
Priority date: 2022-05-09
Filing date: 2023-05-08
Publication date: 2023-11-16

Abstract

According to the present invention a non-hydrogenated amorphous carbon coating such as an a-C and/or ta-C coating on a surface of a sliding part is disclosed, wherein the sliding part is foreseen for use under lubricated conditions and occasionally under dry running conditions. The coating is applied on a ceramic surface, such as for example surfaces comprising silicon carbide (SiC), carbon containing silicon carbide (SiC-C); silicon embedded silicon carbide (Si-SiC), tungsten carbides (WC), and combinations of previously mentioned materials.

Description

Amorphous carbon coating for reduction of friction and wear in a slide component

The present invention relates to an amorphous carbon coating, free of hydrogen, able to provide reduced friction and wear to slide components, used for example in mechanical seals. The main function of the coating is to reduce friction, and consequently reduce temperature, in the interface of sealing faces.

Field and background of the invention

It is well known that diamond like carbon (DLC) coatings can provide reduced friction and wear. DLC coatings comprise a family of amorphous non-hydrogenated carbon based coatings which range typically from a-C (amorphous carbon coatings) which have a large fraction of sp² bonds (typical carbon bonds found in graphite) to ta-C (tetrahedral amorphous carbon) which shows a large fraction of sp³ bonds (typical carbon bonds found in diamond). The incorporation of hydrogen allows to obtain hydrogenated DLC coatings. a-C and ta-C coatings are typically deposited by physical vapor deposition (PVD) where solid graphitic carbon target atoms are evaporated by means of energetic bombardment by ions or neutral species. Hydrogenated carbon coatings are typically obtained by Chemical Vapor Deposition (CVD) methods where the main coating source is a hydrocarbon gas.

The wide range of carbon based coatings allows to obtain a wide range of coating properties, namely: hardness; wear resistance; friction coefficient and surface energies. When it comes to wear and friction behavior the performance of the system is determined not by the coating itself but by the entire tribological system, which includes: (i) coating; (ii) substrate, (iii) combination of materials on main body and counter-body and (iv) environment or lubricants used. It is well reported in literature that the friction behavior of carbon based coatings depends on the level of humidity. Typically hydrogenated amorphous carbon coatings show very low friction coefficient in dry or inert atmospheres and the friction coefficient tends to increase with increase in

CONFIRMATION COPY humidity, which is attributed to adsorbed water vapor on the contacting surfaces leading the higher friction values. Contrary, non-hydrogenated carbon coatings show high friction coefficient in dry or inert atmospheres and low friction coefficients in atmospheres with high humidity. This is attributed to the dissociation and adsorption of water molecules on the DLC film surface and formation of C-OOH and C-H bonds that can passivate the dangling bonds of the coating surface.

Hydrogenated DLC coatings (US9556960B2) as well as crystalline diamond coatings (US11028926B2) are currently patented for application in mechanical seals. The current invention is related with non-hydrogenated amorphous carbon coatings.

Particularly challenging conditions are encountered for slide components, such as mechanical seals, which during operation is exposed, sequentially or alternatingly, to both dry running conditions and humid surface conditions. For example, mechanical seals in a water pump often experience phases of dry running conditions typically occurring during run-in periods upon startup or after any interruptions. Dry running is here characterized by the absence or partial absence of the liquid acting as lubricant and leading to the direct contact ofthe two sliding surfaces. Such periods of dry running, also known as dry sliding, leads to increased shear stress and heat up between the sliding surfaces, and may lead to mechanical failure.

It is therefore an objective of the present invention to at least partially overcome the mentioned challenges for slide components. In particular, it is an objective to provide a coating for slide components for use under lubricated conditions as well as under dry running conditions which allows enhanced performance ofthe slide components, along with a method for the manufacture of such coating, and a method of use, in particular for mechanical seals.

Solution according to the present invention

The solution according to the present invention is to provide a slide component with a amorphous carbon coating which allow to increase wear resistance and specially to reduce friction and consequently reduce temperature in the interface of sliding surfaces. Furthermore, a method is disclosed - preferably involving PVD techniques - for producing amorphous carbon coatings with the mentioned benefits. Additionally, a method for use of the amorphous carbon coatings is disclosed.

The surfaces are mainly intended to be used in lubricated or water or hydrogen containing environments and/or atmospheres, where non-hydrogenated amorphous carbon coatings can provide low friction coefficients. The high hardness of a-C and specially of ta-C coatings also helps to provide enhanced wear resistance. The coating described is suitable for sliding components like mechanical seals.

Description of the present invention

With the coating of a-C and/or ta-C a coating, a method for producing amorphous carbon coatings is provided, which allow to reduce wear and friction coefficient in lubricated sliding surfaces, typically water lubricated, and surfaces used in environments with high humidity level or hydrogen containing atmosphere. The invention is mostly significant for applications like mechanical seals used for example in water pumps.

In the context of slide components such as mechanical seals, the dry running regime is encountered when there is typically not enough liquid to generate a lubricating film separating the sliding surfaces. These conditions encountered in mechanical seals immersed in liquids, e.g. in water pumps, are not equivalent to the application conditions for dry gas seals, where a dedicated dry gas flow is employed to maintain a minute gap between rotating mechanical face seals.

Under lubricated conditions, a continuous lubricating film is formed and maintained across the sliding surfaces. The lubricant is typically water or water based liquids. However, non-water based liquid lubricants like oil may also be applicable.

The advantage of amorphous carbon coatings (ta-C) over hydrogen containing amorphous carbon coatings (a-C:H) was also confirmed in ball-on-disc tests where coated ceramic coupons were immersed in water and tested over 35000 sliding cycles in a reciprocating pin-on-disc test, at contact pressure of 150 MPa and linear speed of 5 mm/s. The friction coefficient (CoF) of ta-C coating was about 0.14, while a-C:H coating showed a CoF of 0.19, as can be depicted on Figure 1 .

The coating material is to be applied mainly on ceramic surfaces comprising silicon carbide (SiC), carbon containing silicon carbide (Si-C); silicon embedded silicon carbide (Si-SiC), tungsten carbide (WC) and combinations of previously mentioned materials. No interlayer materials are applied, meaning that amorphous carbon coating layer is directly applied on the ceramic surface. The amorphous carbon coating is thus in direct contact with the ceramic material of the substrate. The coating should cover at least a part of the ceramic material of the slide component to have a functional benefit.

In some embodiments, the slide component may be composed of several materials, including for example metallic and ceramic materials. In such embodiments, it is preferred that the functional surface, which is in sliding contact with a counter body, is composed of ceramic material. To achieve friction reduction, at least part of the functional surface of ceramic material should be coated.

In order to improve adhesion of the coating, the substrates are cleaned in ultra-sonic bath previous to placing them in the PVD coater. Previously to the coating deposition an etching step is performed where the ceramic surface of the substrate to be coated is bombarded with energetic ions (typically Ar), allowing to remove any surface contamination and therefore improving the coating adhesion. The coating process is performed by PVD, where a graphite target is used as source material for the deposition and evaporated. The resulting carbon species (ions and/or atoms) will condense on the ceramic surface of the substrates. The formation of amorphous carbon coatings (a-C) is typically performed by sputtering and ideally by HiPIMS or S3p technology, which allow to obtain higher hardness in relation to sputtered coatings. ta-C coatings are obtained by arc technology, ideally by filtered arc technology. After the coating deposition a post-treatment step can be applied, in order to remove any defects remaining from coating process. The main coating properties affecting the performance of ta-C coatings are: coating thickness and presence of defects, typically droplets generated in arc process. ta-C coatings were tested in ball-on-disc set-up in both dry running and water lubricated conditions. Although in applications like mechanical seals in water pumps in most cases the system is running with water lubrication, in some circumstances, the amount of water might be very low, regime described as dry running, resulting in increase in friction coefficient and consequently on temperature raise, which might promote damage to elastomeric components in the sealing assembly. Therefore, the behavior in more aggressive dry sliding is relevant to represent dry running conditions. Dry sliding tests and water sliding tests performed in unidirectional ball-on-disc experiments with SiC rings sliding against SiC balls have shown the advantage of ta- C coating on both reduction of CoF (see Figures 2 and 3) and reduction of wear on both SiC rings and SiC ball (see Figures 4 and 5). Uncoated SiC rings (U) were compared with ta-C coated SiC rings. The ta-C coated rings showed different thicknesses, 1 pm (M1P) and 2 pm (M2P), being both samples submitted to a posttreatment process after coating step, which consisted in polishing with SiC2000 paper. The impact of post-treatment was judged by comparing a sample with post-treatment (M1 P) and without post treatment (M1 ), both with a thickness of 1 pm.

Description of the figures:

Figure 1 - Friction coefficient of ta-C and a-C:H coatings sliding in water lubricated reciprocating pin-on-disc test

Figure 2 - Friction coefficient of uncoated SiC rings and ta-C coated SiC rings (M1, M1P,M2P) in dry sliding unidirectional ball-on-disc tests

Figure 3 - Friction coefficient of uncoated SiC rings and ta-C coated SiC rings (M1, M1P,M2P) in water sliding unidirectional ball-on-disc tests

Figure 4 - Wear rate of uncoated SiC rings (U) and ta-C coated SiC rings (M1, M1P,M2P) in dry sliding and water sliding unidirectional ball-on-disc tests Figure 5 - Wear rate of SiC balls sliding against uncoated SiC rings (U) and ta-C coated rings (M1, M1P,M2P) in dry sliding and water sliding unidirectional ball-on-disc tests

Figure 6 - SEM micrographs of uncoated SiC ring and ta-C coated ring without posttreatment (M1) and ta-C coated ring with post-treatment (M1P)

Figure 7 - Tabular overview of performance for test bodies of SiC prepared with different combinations ta-C coating layer thickness, interlayer and surface condition measured as reduced peak height (Rpk). Performance is ranked according to time until failure. The longer the time until failure the higher the performance.

In water sliding conditions all variants of ta-C provide a clear advantage in comparison with uncoated rings, both on reduction of friction coefficient which decreased from 0.2 for uncoated SiC ring to about 0.05 in all ta-C coated rings and on reduction of wear rate on rings and counterpart (ball). The tests in dry sliding condition revealed that the post-treatment of ta-C coatings has a strong impact on coating performance, being observed that the coating without post-treatment showed the highest wear rate and also a high CoF, similar to CoF value of uncoated ring.

Figure 6 shows scanning electron microscopy (SEM) micrographs of uncoated SiC ring and of ta-C coated SiC rings with 1 pm thickness, without post-treatment (M1 ) and with post-treatment (M1 P). The main visible impact of post-treatment is the removal of the droplets, which consist on the particles with a diameter of about 1 pm to 2 pm.

The effect of post-treatment is mainly to remove defects, mainly droplets resulting from arc process. This droplets have a size in range of few micrometers (< 10 pm) and are composed mainly by hard carbon. This particles form asperities with high hardness, which can also be removed during the sliding process, resulting in third body abrasive particles, which cause severe wear on both uncoated and coated surface as observed in the tribological tests reported. In particular, leading to reduced friction at beginning of periods with dry running conditions. Other methods that allow to reduce the amount of droplets also result in an improvement of tribological behavior, namely the use of new filtering methods that allow to reduce the density of droplets growing in coating surface during deposition.

To quantify the post-treated condition and relate to functional behavior and performance of the coating, methods for measuring the roughness of the surface of the amorphous carbon coating may be used. In particular, measure methods employing tactical probes or optical probes may be used. There are several customary roughness parameters defined in norms and standards for surface characterization. Examples include Ra (arithmetical mean roughness value) and Rz (mean roughness depth).

The evaluation of ceramic materials, which typically contain a certain degree of porosity, is particularly challenging. An evaluation based on the bearing area curve, also known as Abbot-Firestone curve, gives further detailed insights. In this procedure, described for example in the norm DN EN ISO 21920-2, the profile height is plotted against material percentage. The peaks and the valleys of the surface profile can be characterized by considering the outer ends of the of the material percentage axis, i.e. close to 0% and close to 100%, where the bearing area curve are at the edges of an S-shaped profile. The peak height (Rpkx) and valley depth (Rvkx) at said outer ends is determined by subtracting the core roughness (Rk), which represent a linear interpolation along the centermost 40% material percentage of the bearing area curve. The reduced peak height (Rpk), respective reduced valley height (Rvk) is further determined by the geometrical simplification of a right-angled triangle having the same area as the roughness peaks in the case of Rpk, and the roughness valleys Rvk in the case of Rvk. The metric reduced peak height (Rpk) thus provide a measure that quantify the highest peaks in the roughness profile. In particular for tribological applications, the magnitude and characteristics of the highest peaks in the roughness profile are often critical between performance and failure. Therefore, the reduced peak height therefore provides useful information not obtainable form e.g. Ra and Rz measures.

It was found that sliding performance under dry running conditions correlated well with the reduced peak height (Rpk). A critical value of Rpk was found to be 0.1 pm. As exemplified in the table in Figure 7, sample 3 which had a 1 .0 pm ta-C layer with Rpk > 0.1 pm could run 100 s until failure. Improving the surface roughness to Rpk < 0.1 pm as in sample 4 of Figure 7 extended the time before failure to 500 s.

Regarding the post-treated coatings it was also observed that the thickness has an impact on coating lifetime, since the main wear mechanism is by abrasive wear and the coating’s thickness allows to increase its lifetime. At the end of 2000 m of sliding test the M1 P coating wear depth was about 2 pm, while the wear depth of M2P coating was about 1 pm indicating that the lifetime of the coating can be adjusted by varying the coating thickness.

A critical minimum thickness for the amorphous carbon layer was found to be 1 pm. A coating thickness at or above 1 pm has the effect that the maximum shear stress during use is moved from the region of the interface between substrate and coating, as would be the case with too low thickness, to being within the coating. Avoiding high shear stresses at the interface between substrate and coating is critical to prevent delamination or failure of the coating. As exemplified by sample 5 in Figure 7, coating thickness in excess of 1 pm, in this particular example 1.8 pm, was observed to give further enhanced performance. The coating thickness can however not be made arbitrarily high due to limitations of intrinsic stress that increases with thickness, among other factors. It is therefore preferred that the coating thickness does not exceed ca 15 pm. An ideal range for coating thickness according to the present innovation is thus between 1 pm and 15 pm. In this description it is disclosed the use of a non-hydrogenated amorphous carbon coating such as an a-C and/or ta-C coating on a surface of a sliding part foreseen for use in lubricated environments occasionally operating under dry running conditions.

In some embodiments, the non-hydrogenated amorphous carbon coating may contain one or more doping elements to help to adjust structure and properties of the coating. Doping elements may be metallic elements or transition metal elements. Nitrogen, boron or fluorine may also be advantageously used as doping elements, due to similar electronic structure as carbon as evidenced by the close position in the periodic table. In particular, the elements N, B, F, Ti, W, Ta, Cr, Zn, Si, Mo, Cu, and Fe may be used as doping elements. The concentration of doping elements may be up to approximately 20 at%, preferably lower than 10 at%. Oxygen may additionally be present as impurity.

The sliding part can be at least part of a mechanical seal in a water pump.

The coating can be applied on a ceramic surface, such as for example surfaces comprising silicon carbide (SiC), carbon containing silicon carbide (Si-C); silicon embedded silicon carbide (Si-SiC), tungsten carbide (WC) and combinations of previously mentioned materials.

It is preferable not to foresee any interlayer between the amorphous carbon coating and the ceramic surface. It was found that an interlayer between the ceramic surface of the substrate and the amorphous carbon coating has negative effects on performance. As exemplified by sample 2 in the table in Figure 7, a 1 pm ta-C coating with a titanium interlayer could run only 10 s until failure. This is much less than the 100 s lifetime of sample 3, where a comparable coating was prepared without interlayer. While not being bound by speculation, the effect may origin from particularly good bonding between the amorphous carbon coating and the ceramic surface of the substrate material. In particular when the ceramic surface of the substrates comprises carbon, a good match can be achieved with the amorphous carbon coating due to the chemical character and potential for chemical bonding. The bonding may be impaired if an interlayer is introduced between the ceramic surface of the substrate and the amorphous carbon coating.

Having no interlayer between the amorphous carbon coating and the ceramic surface may also reduce the risk of chemical attack or corrosion, which may be an issue in particular with metallic interlayers.

A preferred method for the manufacture of amorphous carbon coatings according to the present invention is to apply PVD techniques, in particular cathodic arc deposition, where a plasma is created from carbon containing targets, e.g. graphite target. One important advantage of cathodic arc deposition is that a highly ionized plasma is created. The ions in the plasma may contributes to reduce any native oxide layers on ceramic materials (e.g. SiO2 on SiC substrates) at the beginning of the deposition process. Furthermore, the ions in the deposition plasma may provide increased bonding strength to ceramic surface of the substrate materials, as result of ion bombardment which leads to an ion milling effect. Through this effect, the coating may be better interlocked arid bonded to the substrate.

A preferred cathodic arc deposition method is method is filtered arc technology. As mentioned above, the filter helps to reduce the amount of macroparticles reaching the substrate. Thereby the roughness of the deposited amorphous carbon is reduced, which enables the achievement of required roughness target (Rpk < 0.1 pm) with reduced post-treatment effort. The filtered cathodic arc deposition may furthermore be employed using pulsed deposition, where the arc is operated on the carbon containing target for a set period of time, followed by a pause where the arc is not operated, before the next pulse. The pulsed cathodic arc deposition helps mitigate the challenge of charge buildup for nonconductive substrates, including most ceramic materials.

Claims

1. A slide component with at least one surface of ceramic material for use under lubricated conditions and dry running conditions characterized in that at least a part of the surface of ceramic material of the slide component is coated with a non-hydrogenated amorphous carbon coating.

2. Slide component according to claim 1 , wherein the amorphous carbon coating is applied in direct contact with the surface of ceramic material of the slide component.

3. Slide component according to claim 1 or 2, wherein the ceramic material contains carbon.

4. Slide component according to claim 3, wherein the ceramic material comprise or consist of one or more of the materials selected from the group of: silicon carbide (SiC), silicon embedded silicon carbide (Si-SiC), tungsten carbide (WC).

5. Slide component according to one of the preceding claims, wherein the amorphous carbon coating comprises a-C or ta-C, or consist of a mixture of a-C and ta-C.

6. Slide component according to one of the preceding claims, wherein the nonhydrogenated amorphous carbon coating contains at least one doping element, preferably the doping element is selected from the group of N, B, F, Ti, W, Ta, Cr, Zn, Si, Mo, Cu, and Fe.

7. Slide component according to one of the preceding claims, wherein the thickness of the amorphous carbon coating is or exceeds 1 pm.

8. Slide component according to one of claims 1-6, wherein the surface of the coated slide component has a reduced peak height (Rpk) value of less than 0.1 pm.

9. Slide component according to one of claims 1-6, wherein the thickness of the amorphous carbon coating is or exceeds 1 pm, and the surface of the coated slide component has a reduced peak height (Rpk) value of less than 0.1 pm.

10. Slide component according to one of the preceding claims, where the amorphous carbon coating is ta-C.

11.Slide component according to claim 1 , where the slide component is a part of a mechanical seal.

12. Slide component according to claim 11, where the mechanical seal is part of a pump for liquids, preferably a water pump.

13. Use of a slide component according to claim 1 , where the slide component is exposed to dry running conditions and subsequently exposed to lubricated conditions.

14. Use of a slide component according to claim 13, where the slide component is periodically exposed to dry running conditions and periodically exposed to lubricated conditions.

15. Use of a slide component according to claim 13 or 14, where the thickness of the amorphous carbon coating is or exceeds 1 pm.

16. Use of a slide component according to claim 13 or 14, where the surface of the coated slide component has a reduced peak height (Rpk) value of less than 0.1 pm.

17. Use of a slide component according to claim 13 or 14, where the thickness of the amorphous carbon coating is or exceeds 1 pm, and the surface of the coated slide component has a reduced peak height (Rpk)* value of less than 0.1 pm.

18. Method for coating a slide component, having at least one surface of ceramic material comprising the steps of

- cleaning the slide component

- applying a coating to at least a part of the ceramic surface by PVD methods characterized in that

- the coating is a non-hydrogenated amorphous carbon coating and,

- in the PVD method, a carbon containing target is used as source material for plasma creation

19. Method according to claim 18, wherein the coating is applied directly onto the surface of ceramic material without any interlayer.

20. Method according to claim 18 or 19, where the ceramic material contains carbon.

21. Method according to claim 20, wherein the ceramic material comprise or consist of one or more of the materials selected from the group of: silicon carbide (SiC), silicon embedded silicon carbide (Si-SiC), tungsten carbide (WC).

22. Method according to one of claims 18-21 , wherein the PVD method is a cathodic arc deposition method.

23. Method according to claim 22, where the cathodic arc deposition method includes employing a filter to reduce the exposure of macroparticles to the surface of the ceramic material.

24. Method according to claim 22 or 23, where the cathodic arc deposition method includes pulsed deposition.