The project leading to this application has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 665337.

The invention relates to a method for improving the tribological properties of a metallic component which has a tribological surface, e.g. subjected to sliding friction, wherein this surface is provided with a surface structure.

Furthermore, the invention relates to a metallic component with a surface subjected to sliding friction having a, in particular metallic, surface structure.

The improvement of the tribological properties of a sliding friction loaded tribological component by structuring the corresponding surfaces is already known. Thus, for example, DE 10 2011 009 598 A1 describes a method for producing a highly loaded running surface of a cylinder or a plain bearing, in which a smooth running surface is produced, lubricating pockets are introduced into the running surface and the running surface is nitrided. The lubrication pockets are introduced by means of the ps laser (picosecond) or by means of the fs laser (femtosecond).

The object of the present invention is to improve the tribological properties of a metallic component with a surface subjected to sliding friction.

The object is achieved by the method mentioned above, according to which a self- organized surface structure is produced on the surface of the metallic component subjected to sliding friction with an ultrashort pulse (pulse duration less than 1ps) laser or a short pulse (pulse duration higher than 1 ps) laser.

Further, the object is achieved in the aforementioned metallic component in that the surface structure is a laser-induced self-organized surface structure.

In a first aspect, a superoleophilic metallic component with oil retention properties for friction reduction is disclosed. The superoelophilic metallic component has a surface with at least a surface portion shaped by a short or ultrashort pulse laser beam to comprise one or more types of superoleophilic self-organized surface structures with oil retention properties. The self-organized surface structures are selected from:

- ripples having crests and troughs wherein the distance between two adjacent crests or two adjacent troughs is ranging between 200 nm and 2 pm,

- spikes having a maxima adjacent distance ranging between 500 nm to 40 pm,

- furrows having a minima adjacent distance ranging between 2 pm and 20 pm,

- and any combination thereof.

By selecting the self-organized surface structures from the above ranges, the surface of the metallic component becomes superoleophilic. However, a further technical effect has been demonstrated, in that the surface also acquires oil retention properties in the above mentioned ranges. Thus friction reduction is performed when the metallic component is used as a tribological component.

In addition, the object is achieved in the aforementioned metallic component thus that the surface structure is a laser-induced self-organized surface structure with an ultrashort pulse laser or a short pulse laser by line sweeping the surface with the laser beam. In one example, at least one of the following laser parameters are used:

- Pulse repetition rate: 1 Hz to 1 GHz

- Pulse duration between 30 fs and 800 ps

- Pulse peak fluence between 0.1 J / cm ² and 3.0 J / cm ²

- Spot diameter at least 1 micron

- Effective pulse number (number of pulses per spot) between 2 and 3000

- Line spacing between 0.02-fold and 4-times spot diameter.

By way of illustration, it should be noted that a self-organized surface structure is understood to mean a structure in which the shaping and limiting influences emanate from the elements of the organizing surface itself.

The advantage here is that through the formation, i. e. the induction of formation, of the self-organized structure no forming of special lubricating pockets is required. The processing of the surface can thus be carried out faster, since little or no material removal is required. Thus, the production time of the metallic component per se can be shortened. The surface structure acts by the structure itself friction reducing. In addition, the surface structure has a better wettability with a lubricant (oleophilicity), compared to the polished metallic surface. At the same time the retention capacity of the surface for the lubricant is significantly improved, compared to the polished metallic surface. Besides this the lubricant transport velocity on the surface structure is significantly improvred compared to the polished metallic surface. Friction reduction is realised via a surprising combination of lubricant wettability, lubricant transport and lubricant retention capacity properties. The better lubricant retention capacity also makes it possible to ensure that the surface can be better protected against abrasion due to friction, e.g. when the component is restarted after a standstill.

Preferably, the surface structure is in the form of waves with wave crests and troughs and / or in the form of pits and / or in the form of cones or truncated cones and / or in the form of furrows. It can thus be built up a lubricant back pressure on the tribologically stressed surface, whereby the starved lubrication behavior of the surface can be improved. In particular, when the waves are aligned transversely to the direction of flow of the lubricant (for example, at an angle of 90° to the flow direction), a further improvement of this effect can be achieved.

The above-mentioned effect may be provided according to the further embodiments of the invention that

- The waves are arranged at a distance from each other, which is selected from a range of 200 nm to 2 microns.

- The furrows are arranged at a distance from each other, which is selected from a range of 2 microns to 20 microns.

- The pits and / or the cones / truncated cones are arranged at a distance from each other, which is selected from a range of 500 nm to 40 microns, wherein the pits in particular have a maximum diameter selected from a range of 1 micron to 30 microns and / or wherein the cones or truncated cones in particular have a maximum diameter, which is selected from a range of 1 micron to 5 microns, - And / or that the peaks at the highest point (reversal point) have a wave crest, wherein a distance between adjacent wave crests is selected from a range of 200 nm to 40 microns.

For a given metal type a parametric analysis may be performed to identify and select the optimal set of irradiation parameters [Pulse repetition rate, Pulse duration, Pulse peak fluence, Effective pulse number, Line spacing] that are leading to the creation of the aforementioned types of self-organized structures.

In another aspect, the oleophilic retention properties of a metallic component complemented with engine oil (e.g. Shell Rimula), which has a surface subjected to sliding friction is disclosed. In particular:

- The waves exhibit superoleophilic (the contact angle of a 4pl engine oil droplet placed onto the surface is less than 5 degrees) properties compared to 15+/-4 degrees of the polished mettalic surface. At the same time, the oil retention after centrifuging the sample at 3000 rpm for 120 min is in the range of 30-63% (the percentage is calculated by the formula (m-mo)/mo, where m is the weight of structured sample impregnated with lubricant after centrifugation and mo is the weight of structured sample impregnated with lubricant.

- The furrows exhibit superoleophilic properties(the contact angle of a 4mI engine oil droplet placed onto the surface is less than 5 degrees). At the same time, the oil retention after centrifuging the sample at 3000 rpm for 120 min is 61-82%.

- The pits and / or the cones / truncated cones exhibit superoleophilic properties (the contact angle of a 4pl engine oil droplet placed onto the surface is less than 5 degrees). At the same time, the oil retention after centrifuging the sample at 3000 rpm for 120 min is 79-92%.

In another aspect, the engine oil (e.g. Shell Rimula) transport velocity of a metallic component, which has a surface subjected to sliding friction is disclosed. In particular:

- The waves exhibit oil transport velocity in the range of 0.05 - 0.1 mm/s.

- The furrows exhibit oil transport velocity in the range of 0.10 - 0.20mm/s. - The pits and / or the cones / truncated cones exhibit oil transport velocity in the range of 0.20-0.50 mm/s.

In yet another aspect, a metallic component which has a surface subjected to sliding friction, consisting of patches made by self-organized structures and unstructured areas, is disclosed.

According to a further embodiment of the invention, it can be provided that the furrows are provided with wave crests and / or the wave troughs, wherein the furrows are preferably formed at a distance from each other, which is selected from a range of 2 microns to 10 microns. The additional furrow superstructure further improves the lubricant retention capacity of the structure or the volume of lubricant stored in the structure.

In one example, the afformentioned structures may be produced on a steel surface (for instance grade 1.7131 steel) and/or a steel surface component. However, it is also possible to apply the method to other steel types or other types of metal, in particular Stainless steel, Ti, Aluminum, Copper, Bronze and their alloys.

It may also be provided according to another embodiment of the invention that the surface structure is provided with a coating, whereby the surface structure can be protected better against damage, especially wear.

The coating may be a DLC (diamond-like carbon) layer. The coating is preferably an amorphous carbon layer, in particular selected from a group comprising hydrogen- free amorphous carbon layers (aC), tetrahedral hydrogen-free amorphous carbon layers (ta-C), metal-containing hydrogen-free amorphous carbon layers (aC:Me), hydrogen-containing amorphous carbon films (aC:H), tetrahedral hydrogen- containing amorphous carbon films (ta-C: H) or metal-containing hydrogen-containing amorphous carbon films (a-C:H:Me).

However, it is also possible to apply other PVD to the metallic component, and instead of the surface of the metallic component, to form the surface of this coating as a self-structured surface. For example, such a PVD layer may be a nitridic layer, such as in particular CrN, TiN, TiAIN, CrCN, CrTiAIN. With a CrN layer structured in the sense of the invention, the coefficient of friction of the tribologically stressed surface can be reduced in particular in comparison to a non-structured CrN layer.

According to an embodiment variant of the method, it can be provided that an ultrashort pulse laser or a short-pulse laser with a laser spot is used whose diameter is at least twice as large as the smallest dimensioning of the surface structure to be produced. Thus, the desired structure can be better formed.

According to further embodiments of the method:

The ultrashort pulse laser or the short pulse laser is operated at a pulse repetition rate, which is selected from a range of 1 Hz to 1 GHz; and or

- The surface of the metallic component is scanned with laser pulses with a pulse duration between 30 fs and 800 ps; and or

- The surface of the metallic component is scanned with laser pulses having a pulse peak fluence between 0.1 J / cm ² and 3 J / cm ² ; and or

- The surface of the metallic component is scanned with laser pulses with a spot size of at least 1 micron in diameter; and or

- The surface of the metallic component is scanned with laser pulses with a linear polarization; and or

- The surface of the metallic component is scanned with laser pulses having one of the following polarization states: circular, elliptical, radial, azi-mutal; and or

- The surface of the metallic component is scanned with laser pulses with an effective pulse number between 2 and 3000; and or

- The surface of the metallic component is scanned with laser pulses with a line spacing between 0.02 times and 4 times the spot diameter.

For a better understanding of the invention, this will be explained in more detail with reference to the following figures.

FIG. 1 shows a first embodiment of a surface structure;

FIG. 2 shows a detail of the surface structure according to FIG. 1 in cross section; FIG. 3 shows a second embodiment of a surface structure;

FIG. 4 shows a detail of the surface structure according to FIG. 3 in plan view;

FIG. 5 shows a third embodiment variant of a surface structure; FIG. 6 shows a detail of the surface structure according to FIG. 5 in cross section; FIG. 7 shows a fourth embodiment variant of a surface structure;

FIG. 8 shows a fifth embodiment of a surface structure;

FIG. 9 shows a sixth embodiment of a surface structure;

FIG.10 shows the contact angles of the surface structures with engine oil

FIG. 11 shows the engine oil transport velocity on the surface structures

FIG. 12 shows the engine oil retention properties of the surfaces structures

FIG. 13 shows the course of the friction coefficient over time;

FIG. 14 shows the course of the speed for the measurement of the coefficient of friction of FIG. 13.

By way of introduction, it should be noted that in the differently described embodiments, the same parts are provided with the same reference numerals or the same component designations, wherein the disclosures contained throughout the description can be applied mutatis mutandis to the same parts with the same reference numerals or component names. Also, the positional items chosen in the description, such as up, down, laterally, etc. related to the directly described and illustrated figure and are the position information in a change in position mutatis mutandis to the new location to carry.

The invention relates to a metallic component having a surface that is subjected to sliding friction or generally to a tribologically stressed surface.

The metallic component may, for example, be (for) an application in a loading situation (provided), e.g. a machine element that constrains relative motion to only a desired motion.

In particular, the metallic component is also intended for applications where oil retention and friction reduction is important, such as components of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms.

The component comprises at least a region of the sliding friction or tribologically stressed surface of a metallic material. The component may therefore be provided in this area with only a metallic coating. Preferably, however, the entire component is formed from at least one metallic material. The metallic material may, for example, be a pure metal such as e.g. Titanium, or a metal alloy, e.g. Steel, titanium alloys, or an intermetallic compound, e.g. CrN, TiN, TiAIN, CrCN, CrTiAIN.

The surface subjected to tribological friction or sliding friction is provided with a surface structure at least in regions, in particular in its entirety. The surface structure is generated with an ultrashort pulse laser or a short pulse laser. The term ultrashort pulse laser or short pulse laser is understood to mean the so-called picosecond laser with pulse durations in the picosecond range and femtosecond laser with pulse durations in the femtosecond range. Such lasers are known from the prior art, so that further discussion is unnecessary.

Preferably, the ultrashort pulse laser or the short pulse laser is a femtosecond laser. The structuring is done by the line by line of the surface with the laser. Preferably, the following parameters are used:

- Pulse repetition rate: 1 Hz to 1 GHz. Both waves, furrows and pits and / or the cones / truncated cones can be formed, regardless the repetition rate value, depending on the combination of pulse peak fluence and effective number of pulses used.

- Pulse duration between 30 fs and 800 ps. Both waves, furrows and pits and / or the cones / truncated cones can be formed, regardless the pulse duration value, depending on the combination of pulse peak fluence and effective number of pulses used.

- Pulse peak fluence between 0.1 J / cm ² and 3.0 J / cm ² , in particular between 0.4 J / cm ² and 2 J / cm ² .

- Spot diameter (laser spot) of at least 1 micron diameter, in particular between 10 microns and 500 microns. Both waves, furrows and pits and / or the cones / truncated cones can be formed, regardless the spot diameter value, depending on the combination of pulse peak fluence and effective number of pulses used.

- effective pulse number between 2 and 3000, in particular between 20 and 2000

- Line spacing between 0.02 times and 4 times the spot diameter, in particular between 10 micron and 40 micron.

In particular: - Waves can be formed in the range of peak fluences 0.1-1 J/cm ² and effective pulse number lower than 100 pulses.

- Furrows can be formed either a) in the range of peak fluences 1-3 J/cm ² and effective pulse number between 40-400 pulses or b) in the range of peak fluences 0.1-1 J/cm ² and effective pulse number between 200-10000 pulses.

- Pits and / or the cones / truncated cones can be formed in the range 1.5-3.0 J/cm ² and effective pulse number between 500-10000 pulses.

Furthermore, laser radiation with linear polarization may be used as well as with alternative states of polarization (circular, elliptical, radial, azimuthal).

Furthermore, the surface to be structured, which is subject to sliding friction, may be impacted with the laser radiation not only at right angles (= normal angle of incidence) but also at other angles, which, among other things, affect the shape and size / spacing of the structures.

Preferably, the surface is scanned in the form of lines with the laser. The individual tracks may also overlap. The coverage may, for example, be selected such that the line spacing may be between 0.02 and 4 times the spot size (diameter) of the laser beam on the surface of the metallic component.

In general, an ultrashort pulse laser or the short pulse laser with a laser spot whose diameter is at least twice, preferably between two times to one hundred times, as large as the smallest dimension of the surface structure to be produced is preferably used. With regard to the preferred surface structures and their dimensions, reference is made to the following statements. Flowever, the preferred diameter of the laser spot may also be determined without previous knowledge by means of fewer tests, in which a surface structure is generated in a first step, then their dimensions are measured and then the smallest dimension of the structure is determined from the data obtained. If necessary, this sequence may be repeated for a better result.

Flowever, the smallest dimension is not the dimension that occurs in the overall structure, the entire surface. Rather, the structure has a height or depth, a width, a length, a diameter of individual structural elements that occur repeatedly in the surface structure. The smallest dimension is therefore that dimension which has a structural element repeating in the surface structure. With the method and with the preferred embodiment variants of the method, self- organized metallic surface structures are produced on the surface of the metallic structural part subjected to sliding friction or tribologically stressed.

In principle, the structure geometry can be made variable over the surface of the metallic component with regard to periodicity, depth, structure type, etc.

In the course of the work within the scope of the invention, however, the structures described below have proven to be particularly suitable and therefore preferred.

For example, the laser-induced, self-organized surface structure in the form of waves 1 with wave crests 2 and troughs 3 may be formed, wherein between the wave troughs 3 is formed in each case between adjacent wave crests 2, as these Figs. 1 and 2 show, the SEM image of Surface structure (Fig. 1) and a detail of this surface structure (Fig. 2) show.

For such a surface structure with waves 1 , an ultrashort pulse laser or short pulse laser is operated with the following parameters:

Femtosecond laser (emission wavelength l = 1030 nm)

Pulse duration = 350 fs

Pulse repetition rate 500 kFIz

Pulse peak fluence 0.5 J / cm ²

Spot diameter 39 pm

Polarization linear (horizontal)

effective pulse number 20

Line spacing 20 pm.

The structure was produced on a steel surface (in particular grade 1.7131 steel).

As FIG. 1 shows, the waves 1 do not have to extend continuously over the entire surface, but several wave structures can also be formed next to one another on the surface of the metallic component. The wave crests 2 each have a wave peak 4. The wave peak 4 is the highest point of a wave crest 2, the point in which the rising edge of the wave crest 2 merges into the falling edge of the wave crest 2. According to one embodiment, it can now preferably be provided that a distance 5 between adjacent wave peaks 4 is selected from a range of 200 nm to 10 pm, in particular from a range of 200 nm to 2 pm. The mean distance over ten successive wave crests can be between 0.4 pm and 0.5 pm, as is the case with the structure of FIG. 1

The laser-induced, self-organized surface structure may also have pits 8, as shown in FIGS. 3 and 4, which show an SEM image of the surface structure (FIG. 3) and a detail of this surface structure (FIG. 4).

For such a surface structure with these pits 8, an ultrashort pulsed laser or a short- pulse laser may be operated with the following parameters:

Femtosecond laser (emission wavelength l = 1030 nm)

Pulse duration = 500 fs

Pulse repetition rate 2000 kFIz

Pulse peak fluence 0.74 J / cm ²

Spot diameter 39 pm

Polarization linear (vertical)

effective pulse number 400

Line spacing 30 pm.

The structure was produced on a steel surface (in particular grade 1.7131 steel). According to the preferred embodiment, these pits 8 are arranged at a distance from one another which is selected from a range of 200 nm to 40 pm, in particular from a range of 1 pm to 20 pm.

It may further be provided that the pits 8 have a maximum diameter which is selected from a range of 1 pm to 10 pm, in particular from a range of 1 pm to 3 pm.

The laser-induced, self-organized surface structure may further be formed of cones 10 or truncated cones, as shown in FIGS. 5 and 6, which show an SEM image of the surface structure (FIG. 5) and a detail of this surface structure (FIG.6).

For such a surface structure with cones 10 or truncated cones, an ultrashort pulse laser or a short pulse laser is operated with the following parameters: Femtosecond laser (emission wavelength l = 1030 nm)

Pulse duration = 350 fs

Pulse repetition rate 500 kHz

Pulse peak fluence 0.5 J / cm ²

Spot diameter 39 pm

Polarization linear (horizontal)

effective pulse number 2000

Line spacing 20 pm.

The structure was created on a CrN coating of a metallic component.

According to another embodiment, the cones 10 or truncated cones are arranged at a distance 11 from each other, which is selected from a range of 500 nm to 40 pm, in particular from a range of 1 pm to 20 pm. The distance 11 is measured between the conical tips or between the midpoints of the smaller truncated cone surface.

It can further be provided that the cones 10 or truncated cones have a maximum diameter which is selected from a range of 1 pm to 30 pm, in particular from a range of 2 pm to 20 pm.

The maximum diameter of the cones or truncated cones is measured at half their height.

In the context of the invention, it is also possible to form mixed structures, that is, in particular, waves 1 with pits 8 and / or cones 10, and / or waves 1 with grooves and / or pits 8 with cones 10.

According to a further embodiment variant of the metallic component, it may be provided that the surface structure is or will be provided with a coating. For example, the coating may be deposited on the structured surface using a PVD (Physical Vapor Deposition) method.

The coating may be a DLC layer. The coating is preferably an amorphous carbon layer, in particular selected from a group comprising or consisting of hydrogen-free amorphous carbon layers (aC), tetrahedral hydrogen-free amorphous carbon layers (ta-C), hydrogen-free amorphous carbon layers containing metal (aC: Me), hydrogen- containing amorphous Carbon layers (aC: H), tetrahedral hydrogen-containing amorphous carbon layers (ta-C: H), metal-containing hydrogen-containing amorphous carbon layers (aC: H: Me).

Furthermore, the coating may have a layer thickness which is selected from a range of 1 pm to 50 pm, in particular from a range of 2 pm to 10 pm.

However, it is also possible to apply other PVD to the metallic component, and instead of the surface of the metallic component, to form the surface of this coating as a self-structured surface. For example, such a PVD layer may be a nitridic layer, such as in particular CrN or TiN or TiAIN or CrCN or CrTiAIN.

FIGS. 7 to 9 show further embodiments of surface structures.

Thus, Fig. 7 shows the superimposition of waves 1 with furrows forming a superstructure arranged at right angles to the waves 1. For such a surface structure, an ultrashort pulse laser or short pulse laser is operated with the following parameters:

Femtosecond laser (emission wavelength l = 1030 nm)

Pulse duration = 500 fs

Pulse repetition rate 500 kHz

Pulse peak fluence 2.0 J / cm ²

Spot diameter 39 pm

Polarization linear (horizontal)

effective pulse rate 100

Line spacing 26 pm.

The structure was produced on a steel surface (in particular grade 1.7131 steel).

The grooves may be formed at a distance which is between 2 microns and 20 microns. With regard to the superimposed wave geometry, reference is made to the above explanations.

FIG. 8 shows pits 8 on a steel surface 1.7131. For such a surface structure, an ultrashort pulse laser or short pulse laser may be operated with the following parameters:

Femtosecond laser (emission wavelength l = 1030 nm) Pulse duration = 500 fs

Pulse repetition rate 2000 kHz

Pulse peak fluence 1.5 J / cm ²

Spot diameter 39 pm

Polarization linear (vertical)

effective pulse number 400

Line spacing 30 pm.

FIG. 9 shows cones 10 on a steel surface 1.7131. For such a surface structure, an ultrashort pulse laser may be operated with the following parameters:

Femtosecond laser (emission wavelength l = 1030 nm)

Pulse duration = 500 fs

Pulse repetition rate 100 kFIz

Pulse peak fluence 2 J / cm ²

Spot diameter 39 pm

Polarization linear (horizontal)

effective pulse number 400

Line spacing 30 pm.

The metallic component may be in sliding contact with a further, untreated component or in tribological contact in general. However, it is also possible that the further component on the corresponding surface, which is in contact with the self- structured surface of the metallic component, has also been treated with a method mentioned above and has a self-structured surface, which may be different from that of the metallic component.

To evaluate the properties of the structured surfaces subject to sliding friction, tests were carried out in which two steel plates were pressed against steel bolts on opposite sides of the lateral surface of the bolts. The lateral surfaces of the steel bolts were structured as described above. The steel bolts were set in rotation as part of a cyclic measuring program (acceleration phase and subsequent deceleration phase). By way of example, FIG. 10 shows the result of a steel bolt structured with a wave structure according to FIG. 1 (lower curves). The three uppermost curves show the result of the experiment with non-surface-structured steel bolts. Plotted in FIG. 10 is the friction coefficient (ordinate) over time in [s] (abscissa), or in FIG. 11 on the ordinate the velocity in [m / s] and on the abscissa the time in [s], FIG. 11 shows the velocity profile for measuring the coefficients of friction for the illustration of FIG. 10. The measurements show a significantly reduced friction coefficient of the structured steel bolts compared to the unstructured ones.

The invention also includes a device for forming a self-organized structure on the surface metallic component, comprising an ultrashort pulse laser or a short pulse laser on which at least one of the above-mentioned laser parameters can be adjusted.

The embodiments show possible embodiments of the metallic component or its laser- induced, self-organized surface structure, it being noted at this point that the invention is not limited to the specifically illustrated embodiments thereof, but also various combinations of the individual embodiments are also possible.

For the sake of order, it should finally be pointed out that for a better understanding of the construction of the metallic component or its laser-induced, self-organized surface structure, it is not absolutely necessary to set a true scale.

LIST OF REFERENCE NUMBERS

1 wave

2 wave crest

3 wave trough

4 wave peak

5 distance

6 distance

7 distance

8 pits

9 distance

10 cone

11 distance