Andreas LESON, et al.

Laser-Arc Coating vs Friction

Laser-arc deposited tetrahedral amorphous carbon coating eliminates friction
Jun 08, 2015

Diamond-like coatings save fuel

Dr. Volker Weihnacht, Prof. Andreas Leson and Dr. Hans-Joachim Scheibe (left to right) successfully developed a laser arc method of depositing friction-reducing, wear-resistant coatings on components. Credit: Dirk Mahler/Fraunhofer

Coating engine components with hard carbon reduces friction to almost zero – a development that could save billions of liters of fuel worldwide every year. Now researchers have developed a new laser method to apply the coating on the production line

Scientists already know how to coat components with diamond-like carbon to minimize friction. But now Fraunhofer researchers have developed a laser arc method with which layers of carbon almost as hard as diamond can be applied on an industrial scale at high coating rates and with high thicknesses. By applying carbon coatings to engine components such as piston rings and pins, fuel consumption can be reduced. "Systematic application of our new method could save more than 100 billion liters of fuel each year over the next ten years," says Prof. Andreas Leson from the Fraunhofer Institute for Material and Beam Technology IWS in Dresden, referencing a study that was published in the journal Tribology International in 2012.

Carbon-based coatings are already used in volume production. But now the team of IWS researchers led by Prof. Leson, Dr. Hans-Joachim Scheibe and Dr. Volker Weihnacht has succeeded in producing hydrogen-free ta-C coatings on an industrial scale at a consistent level of quality. These tetrahedral amorphous carbon coatings are significantly harder and thus more resistant to wear than conventional diamond-like coatings. "Unfortunately, you can't just scrape off diamond dust and press it onto the component. So we had to look for a different method," says Dr. Scheibe, who has spent over 30 years investigating carbon's friction-reducing properties.

A pulsed laser controls the light arc

In a similar style to old-fashioned film projectors, the laser arc method generates an arc between an anode and a cathode (the carbon) in a vacuum. The arc is initiated by a laser pulse on the carbon target. This produces a plasma consisting of carbon ions, which is deposited as a coating on the workpiece in the vacuum. To run this process on an industrial scale, a pulsed laser is vertically scanned across a rotating graphite cylinder as a means of controlling the arc. The cylinder is converted evenly into plasma thanks to the scanning motion and rotation. To ensure a consistently smooth coating, a magnetic field guides the plasma and filters out any particles of dirt.

The laser arc method can be used to deposit very thick ta-C coatings of up to 20 micrometers at high coating rates. "High coating thicknesses are crucial for certain applications – especially in the auto industry, where components are exposed to enormous loads over long periods of time," says Dr. Weihnacht.

The automotive and motorcycle manufacturer BMW is working intensively on the industrial-scale implementation of ta-C engine components in its various vehicle models with the aim of reducing their fuel consumption. Prof. Leson sees this as the first major step in using the laser arc method to save resources. And as a motorcycle aficionado himself, he also sees another positive effect stemming from this development: "The fact that our research is helping to make motorcycling more environmentally friendly eases my conscience every time I go for a ride," he says, unable to suppress a smile.

Andreas Leson, Hans-Joachim Scheibe and Volker Weihnacht received the 2015 Joseph von Fraunhofer Prize for the development of the laser arc method and the application of ta-C coatings in volume production.


The invention concerns the production of anti-wear layers disposed on surfaces of internal combustion engine components which are exposed to frictional wear. According to the method, anti-wear layers are formed on the particular surface by means of electrical arc discharge under vacuum conditions. The anti-wear layers are formed from at least approximately hydrogen-free, tetrahedral amorphous (ta-C) carbon consisting of a mixture of sp2 and sp3 hybridized carbon, and have a microhardness of at least 3500 HV and an arithmetic mean surface measure Ra of 0.1 [mu]m, without mechanical, physical and/or chemical surface treatment.

A process for producing a wear protection layer and prepared by the process wear protection layer

The invention relates to a method for the production of wear-resistant coatings which have been formed on surfaces of components of internal combustion engines which are subjected to frictional wear by means of electric arc discharge under vacuum conditions on the respective surface, and produced with the method of wear-resistant coatings.

The wear-resistant coatings are made wasserstrofffreien, tetrahedral amorphous from sp <2> and sp <3> hybridized carbon (ta-C) formed.

It has been shown that these types of layers in which a maximum hydrogen content of 1 atomic%, preferably not more than 0.5 atomic% still want to allow, have particularly favorable wear and lubricating properties.

It was therefore suggested that these layers are on a variety of surfaces of components subject to wear of internal combustion engines, as for example piston rings, tappets, cams of camshafts or piston pin to use.

This layers have a high hardness and therefore increased wear resistance.

The attainable friction at a sliding action on the surface of a not so coated base or GE genkörpers are low, so that this will affect the economic operation and the C02 footprint advantageous effect.

Such layers can with different PVD vacuum coating process using a graphite cathode are made.

It has been found that an especially high deposition rate can be achieved in the process, can be reached in which electric arcs between an anode and a cathode formed from graphite.

In these methods, but it is disadvantageous that in the coating larger particles or even so-called droplets are formed which are deposited in the layer and the surface properties are thereby adversely affected, so that the surface must be leveled by a post.
This prepares in layers, which have a particularly high hardness, but significant problems that cure especially in micro- above 5000 HV detrimental because an extremely high time expenditure is required to obtain sufficiently smooth surfaces of these layers for a favorable sliding behavior to.

Is particularly suitable for the formation of such hard layers called the Laser-Arc process in which an electric arc in a vacuum by means of a pulsed laser beam ignited and with the on

Arc obtained plasma of ionized particle flow in this, the ionized particles can be deposited out to a substrate and a coating, suitable ..

This type of layers may, however also in a per se known method, in which an electrical arc discharge is in the vacuum for generating the plasma used without the Are arc discharge is initiated by a laser beam applied.

Here, the arc in known manner either be ignited only by a sufficiently high voltage between an anode and a cathode connected as a target and, secondly, there is the possibility that ignition means of electrically conductive ignition elements due
Initiate short-circuit.

These known methods, however, have the disadvantage that their plasma is relatively rich in droplets and particles.

However, possibilities have been proposed to counter this drawback, to perform a so-called "filtering" of the plasma for storage of particles.

Several options for this are available from BF Coli and DM Sanders in "Design of Vacuum arenes based Sources"; Surface and Coatings Technology ", No. 81 (1996) 42-51 described.
This is based on these known solutions, the fact that using magnetic fields, the ionized light components of a plasma can be deflected and, because of their unfavorable Droppings charge / mass ratio, serious deflectable particles can be separated from each other substantially larger.

However, these filter assemblies have some significant disadvantages:

The design of these systems is very complex and expensive.

The diameter of the magnetic filter and thus the diameter of the coating surface is limited due to the strong magnetic fields needed and of the necessary electrical power to about 150 mm.

The coating rate of the process is reduced to approximately 15-20% compared to that without use of the magnetic filter.

For processes in which electric arcs are used for coating, is in EN 2006 10 009 160 AI to use an absorber electrode and proposed by magnets to separate larger particles from a plasma and thus prevent these settle in the coating and characterized the surface geometry is adversely affected.

However, no layers are known, which are formed from hydrogen-free tetrahedral amorphous from sp <2> and sp <3> hybridized carbon (ta-C), which have increased hardness, while very good sliding friction at the same time, taking on component surfaces with a increased deposition rate by means of electric arc discharges have been deposited without subsequent surface treatment, which leads to leveling of surveys and reduce the roughness, must be carried out.

It is therefore an object of the invention to provide a wear-resistant coating for stressed on sliding surfaces of components of internal combustion engines are available, having an improved wear resistance and simultaneously improved sliding properties, the production can be effected with reduced effort and increased deposition rate by means of electric arc discharges.

According to the invention this object is achieved with the features of claim 1.
The claim 7 relates to the method produced wear-resistant coatings.

Advantageous refinements and developments of the invention can be used with features that are in the subordinate claims referred realized.

With the inventive method wear protective layers on surfaces of internal combustion engines, which are exposed to frictional wear, produced.

Here, a plasma by pulsed laser irradiation sequentially ignited electric arc discharges in vacuum conditions, in which the electric arc discharge between an anode and a cathode are operated as graphite formed.

There are ionized parts of the plasma as a layer consisting of at least approximately hydrogen-free, tetrahedral amorphous (ta-C), consisting of a mixture of sp <2> and sp <3> hybrisiertem carbon (ta-C according to VDI

Be in 2840) formed and deposited on a surface of at least one component.

Positively charged ions of the plasma are moved by means of an absorber electrode toward the at least one component.

In this case, the same voltage is applied to at least approximately the anode and the absorber electrode.

While the electrical arc discharges can be operated to flow through the absorber electrode, an electric current of at least 1.5-fold greater, preferably at least two times greater than the electrical generic current flowing through the anode, is.
There is no mechanical and / or chemical post-processing of the coated surface of the at least one component, which results in a smoothing of the surface is carried out.
A subsequent smoothing of the surface of the formed wear protection layer is not particularly influencing the current flowing through the anode electrode and the absorber respectively different electric current through the loading required.
Advantageously, the plasma is formed within a laser arc chamber and a vacuum chamber in which the at least one component is arranged, directed versa.

The laser arc chamber can be flanged to the vacuum chamber.

In the laser arc chamber, a vacuum is also respected.

By means of the absorber electrode should be as deflected positively charged ions of the plasma, that it, starting from the cathode do not directly
Way impinge on the surface of the at least one component and electrons move from the plasma toward the absorber electrode, so that they move not possible or with a small number in the direction to be coated component surface.

With at least one arranged arc discharge or sputter source, which is arranged in the vacuum chamber, a thin adhesive layer on the at least one component may be deposited.
Advantageously, an absorber electrode used with a plurality of strips.

Larger droplets or droplets can be discharged between the Streif s, so that it does not impinge on the surface of the at least one component.

A movement direction of droplets inducing reflection can be largely avoided.
The wear-resistant coating according to the invention is produced on surfaces of internal combustion engines that are subjected to frictional wear, trained.
They have been trained by means of electric arc under vacuum conditions on the respective surface, and are made of at least approximately hydrogen-free, tetrahedral amorphous (TAC) consisting constituted <3> hybrisiertem carbon from a mixture of sp <2> and SP.

Preferably, they can be formed with the aforementioned laser-arenes process.

The wear-resistant coating has a microhardness of 3500 HV and an arithmetic mean roughness Ravon 0.1 µ?? on.

Here no subsequent mechanical, physical and / or chemical surface treatment is necessary in order to comply with this roughness can.

The wear-resistant coating according to the invention can advantageously have produced µ?? also an average roughness Rzvon maximum 1.0.

The total averaged roughness Rzentspricht the arithmetic mean of all readings Einzelrau- deep.

The wear-resistant coating according to the invention produced should a reduced peak height Rpkvon µ?? maximum of 0.35, preferably a maximum of 0.25 µ??.

This value is in particular from the standpoint of reduced friction, and therefore are beneficial.

Both the arithmetic average roughness Ra, as well as the other two roughness R2undPkkönnen be determined with the known profile method.

Here, a probe tip that is preferably made of diamond, and a small tip radius should have, be used.

It if the wear protective layer has a microhardness of 3500 HV, preferably 4000 HV, more preferably at least 5000 HV, preferably of at least 5700 HV and most preferably from 6000 HV and preferably a roughness Rakleiner 0.08 µ?? is particularly favorable, more preferably less than 0.05 m_aufweist, whereby the wear resistance and the durability can be further improved or increased.

For the hardness measurement may be achieved in a device "FISCHERSCOPE H100C XYP" from Helmut Fischer GmbH & Co. KG be used.

The test load should be chosen so that the penetration depth of the indenter is a maximum of 1/10 of the film thickness.

With an inventively produced wear-resistant coating on the piston pin, in a tribological system consisting of counter-body connecting rod with sleeve (brass) and aluminum pistons with a vibration friction wear tribometer with piston pin module from Optimol Instruments Prüftechnik GmbH a coefficient of friction less than 0.03, preferably less than 0.025 can be achieved.

This is the case with oil-lubricated tests in a temperature range between 100 ° C and 130 ° C, as is typical for components of internal combustion engines, too.

The friction coefficient changes only slightly over the lifetime of the coated component, whereby a reduction in the coefficient of friction could be detected after a short running time.

The wear rate can be reduced by a factor of 3 compared to conventional DLC coatings.
The proportion of SP should be <3> hybridized carbon preferably well above 40%, above 50%.
In addition, should the anti-wear layer no other chemical elements, such as metals or halogens or Phosphorus be included.

This also applies to chemical compounds.

This only an inert gas such as argon and no particular hydrocarbon compound should be included in the laser chamber and Are in the vacuum chamber, if necessary.

The layer thicknesses should be at least 0.5 µ??, preferably at least 2 µ?? or more.

Advantageous may have been formed on the surface to be coated at least one adhesive and / or intermediate layer, on which then a wear-resistant layer of the invention is formed.
For this purpose, for example, be chosen µ?? a chromium layer having a thickness of at least 0.1.

In the preparation may advantageously a cylindrical cathode (target) can be used made of graphite, which rotates during the process about its longitudinal axis, so that the respective base points of the electrical arc discharges extend over the entire surface of the cathode, thereby achieving a geleich- excessive removal of carbon can be.

Ignition of GE pulses powered electrical arc discharges with a laser beam, the laser beam can be also operated in a pulsed accordingly and deflected so that it impinges at various predetermined positions on the surface of the cathode and there as a result of the energy input an electrical see arc discharge at each laser pulse, can be ignited.

The electrical voltage between an anode and the cathode is controlled so that the arc will go out again in each case after a predetermined time to subsequently burn at a different position, a new arc discharge.

The layer thickness of a wear-resistant coating can be formed by means of the number of unused electric arc discharges, are affected with a known size of a surface to be coated.

Other process parameters should, however, be kept as constant as possible.

With the parameters of electrical current and voltage with the electric arc discharges are operated whose duration, the pulse rate and one on a component to be coated (substrate) during the arc pulse applied bias voltage can also affect the trainees layer are taken.

This concerns in particular the layer structure and in particular the proportions of sp <2> and sp <3> hybridized

So electric currents above 1000 A, are preferably used above 1500 A and a pulse frequency of between 300 Hz and 600 Hz can be selected.

There may be a bias voltage at - 50 V to - 200 V, preferably 100 V is applied to the component to be coated in the field.

Thus, an electric current of 500 A ± 100 A can flow through the absorber electrode (preferably ± 50 A) through the anode and of 1100 ± 100 A A (preferably ± 50 A), when electrical arc discharges are ignited and operated.

Operation of electric arc discharges can be performed with a pulse duration in the range 250 to 600 µ $ µ5.

A termination of the electrical arc discharges can be achieved by reducing the electrical voltage at least at the anode.

An ignition of electric arc discharges according to a pulsed carried out irradiation of the surface of the cathode with a directed laser beam to the surface may take place at an increased electrical voltage, which is reduced after the ignition of the respective electric arc discharge.

In the coating apparatus should be used, with the larger particles are prevented from impinging on the surface to be coated.

This can each alone, be an absorber electrode.

In this case, a structure may be used as it is 10 2006 009 160 AI known from DE, the disclosure of which reference in its entirety is incorporated herein.

In this case, at least one permanent magnetic element is inserted, which is aligned parallel to the rotation axis of the cathode and parallel to the surface of a cathode.

In addition, while an absorber electrode is available with which an electric field is formed, is passed through the electric arc formed by the discharge plasma.

With both the / the Permaentmagnetelement (s), as well as with the absorber electrode larger particles contained in the plasma can thus be influenced in their movement, particularly in its direction of movement that they do not impinge on the surface to be coated or there at an angle, the one Installation avoids into the layer.

Additionally, at least one aperture between the surface to be coated of a part and the cathode may be arranged through which the plasma, the carbon ions, which can be used for layer formation, containing is performed in the direction of the surface to be coated.
The absorber electrode can be arranged in the direction of movement of the plasma by a diaphragm and / or anode.

A Permant magnet element may for example be arranged in the shade of a diaphragm or a diaphragm element.

The invention will be described in closer detail.

In which:

Figure 1 shows in schematic form the structure of a device for the Education is inventive wear-resistant coatings suitable.

Figure 1 showed a vacuum coating system with a vacuum chamber 1 in fixed rotational device to be coated components 14 and thus both with double and triple rotation can be coated.

In the vacuum chamber 1 are known or arc discharge Sputter 2 or a combination of both for plasma etching or the deposition of a thin adhesive layer present.

In the vacuum chamber 1, a laser arc chamber is flanged 3 with rotating graphite roll as KathodelO and a film pull 11 to protect the laser entrance window before steaming.

In direction of the vacuum chamber 1, a filter module 4, with service door and internal structure of the absorber-anode arrangement 5, 6, and side-mounted permanent magnet arrangement 7 is present.

Furthermore, a scanner and focusing with laser entrance window 8 for linear guidance of the laser beam 9 over the full length of the cathode laser arc chamber 3 available.
By the reference numeral 12, the path consisting of the means of electric arc discharge between the cathode 10 and the anode 6 selected from the generated plasma larger particles towards the absorber electrode 5 is illustrated with an arrow.

The electric arc discharges are ignited by means of the deflectable laser beam 9 on the surface of the cathode 10, which consists of 99.9% graphite.

Here, the cathode 10 is rotated about a rotation axis that is aligned perpendicular to the plane of the drawing and the laser beam 9 is deflected along this axis of rotation.
Thereby, a uniform removal of cathode material and at the same time a large area to be coated in the vacuum chamber 1 can be used.

The absorber electrode 5 is electrically connected to a positive potential.

It is provided with a plurality of electrically conductive strip-shaped elements which are arranged at a distance from each other, are formed.

Between the strip-like elements column formed by the larger particles can be performed.
The Bezugszeichenl3 illustrates the path of the deflected carbon ions of the plasma to the rotation device with the components to be coated 14 with an arrow.

When the formation of the wear-resistant coatings on the surfaces of the components 14 to be coated preferably with threefold rotation.

After evacuation of the vacuum chamber 1 and performing the surface cleaning and activation a Cr adhesion layer having a thickness of about 0.1 micron is deposited by sputtering.

Following the deposition of ta-C layer is to a thickness of about 1 pm.

Due to the selected parameters of pulsed laser arc source, electric arc current 1600 A, pulse duration 350 ps at a frequency of 520 Hz in combination with the adapted to the laser arc source substrate-bias parameters in a high voltage range of - 800 V with a pulse length of 350 ps and a low voltage of - 100 V at a pulse length of 200 ps, ??very hard and smooth ta-C coatings with high adhesion to the component surface (Rc 1) are deposited.

There is a distribution of the electric current, wherein 1100 A flowing through the absorber electrode 5 and the anode 6 through 500 A.

The anode 6 is arranged closer to the cathode 10, as the facing in the direction of the cathode 10 the base of the absorber electrode 5.

The specific means of profilometer roughness, are as follows: Raim means 0.09 pm, Rzim means 1.0 pm and 0.28 pm Rpkim means.

The determined by Fischerscope microhardness of the wear-resistant coating amounts to 7040 HV or the specific means LAWave modulus is 740 GPa.

In vibration-friction-wear tribometer with piston pin module from Optimol Instruments Prüftechnik GmbH a friction coefficient of 0.022 with an inlet oil OW30 the company Castrol, as lubricant was determined.

The wear rate was reduced to approximately 30% compared to conventional DLC coatings.

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