Patent Description:
The use of molybdenum coatings or molybdenum comprising coatings is well known. The use of molybdenum nitride as wear reduction coating on surfaces of components is in particular well known.

For example, Mahle I (<CIT>) discloses at least one layer of MoN on a hardened surface of a piston ring having a coating thickness of about <NUM>. Moreover, Mahle II (<CIT>) and Mahle III (<CIT>) both are teaching providing nitride coatings, such as CrN and MoN among others, on components, such as rings, to reduce the wear and friction, as well as to increasing the hardness of the surface of these components.

<CIT> discloses a piston ring coated with a MoN/CrN multilayer by cathodic arc deposition.

On the other hand,<NPL>) describes an arc PVD process that can be used to produce different nitride coatings on steel substrates.

The behavior of the interface between the substrate surface of the component and the molybdenum nitride coating is however still today unsatisfactory and insufficient for meeting the current industrial requirements during application of the so coated components.

This has been especially observed when the substrate material to be coated is not hard enough, it means in this context that the substrate material has a hardness for example between <NUM>-<NUM> HRC, in any case not higher than <NUM> HRC.

<FIG> shows a picture of a coated surface after Rockwell indentation HRC, where the hardness of the substrate is between <NUM> and <NUM> HRC and the substrate is coated with MoN. Ring-shaped fracture of the MoN coating can be clearly observed surrounding the HRC Rockwell indentation.

Essentially this problem can be avoided by using substrates having a higher hardness, such as e.g. substrates made of cemented carbide. However the components used in many automotive applications are made of materials which have Rockwell hardness lower than <NUM> HRC.

The objective of the present invention is to modify the molybdenum nitride coating and/or the substrate surface to be coated in order to improve the contact between substrate surface and MoN coating when the substrate exhibit a hardness of <NUM> HRC or lower and the coated substrate is subjected to load or especially to interrupted load.

In particular it is intended that the inventive solution makes possible that no ring-shaped fracture lines are produced during Rockwell indentation by conduction of HRC Rockwell tests in substrates coated with MoN based coatings when the substrate hardness is lower or equal to <NUM> HRC.

The objective of the present invention is attained by providing a component according to claim <NUM>, having a layer or a layer system between the substrate surface and the coating comprising at least one MoN layer characterized in that said at least one MoN layer consists of molybdenum nitride comprising a mixture of phases, the mixture comprising:.

The inventors found that surprisingly it is possible to avoid ring-shaped fractures lines by subjecting the substrate to be coated with the MoN coating to a nitriding process, wherein the substrate surface hardness is increased before depositing the MoN coating. Afterwards the MoN coating is applied on the component surface which was previously hardened by nitriding as mentioned above.

The MoN coatings in the context of the present invention were deposited by using a reactive PVD process.

In particular, reactive arc PVD process are demonstrated to be suitable for depositing MoN coatings on component surfaces hardened as suggested above.

It is a special advantage of the used method that the nitriding processs of the substrate and the coating process can be done in a same vacuum chamber of a coating machine.

In this manner it was a good adhesion between the nitriding layer and the coating without formation of a white layer was guaranteed.

The MoN coating can be deposited exhibiting a hexagonal phase or a cubic phase or a mixture of hexagonal and cubic phases.

<FIG> shows a picture of a surface coated according to the above mentioned inventive solution that was tested afterwards according to the standard HRC Rockwell test. In this picture it is clear to see that no ring-shaped fracture lines surrounding the Rockwell indentation can be observed.

The inventors found also surprisingly that it is possible to avoid ring-shaped fractures lines by subjecting coating the substrate surface of the component with a modified MoN coating. In this case it was not necessary to modify previously the component surface to be coated, that means that no previous nitriding step is needed.

Concretely, the inventors proposes to apply a multilayer structured MoN/CrN coating, which comprises alternate deposited individual layers of MoN and individual layers of CrN. Especially good results i.e. Rockwell indentations without ring-shaped fracture lines or further adhesive failures were in particular observed by using this second inventive solution when the thickness of the multilayer coating was about <NUM> to <NUM>.

In this case the MoN layers can be also deposited exhibiting a hexagonal phase or a cubic phase or a mixture of hexagonal and cubic phases.

As mentioned above, using this second inventive solution thicknesses of the multilayer structured MoN/CrN coating of about <NUM> to <NUM> were required for obtaining pictures after HRC Rockwell test, which show no ring-shaped fracture lines and also no signs of further adhesive failures. However, it might be that such kind of MoN/CrN multilayer coatings but having a thickness less than <NUM> are suitable for a specific application because the load in a real application might be not as high as in the HRC Rockwell test. In this case a thickness of for example <NUM> might be enough for a MoN/CrN multilayer coating applied on components used in real automotive applications.

The MoN and CrN individual layers can be deposited for example by using a reactive PVD process. According to a preferred embodiment of the present second inventive solution, a reactive arc PVD process is used for depositing the MoN and the CrN individual layers of the multilayer coating.

Since as described in the first inventive solution the component surface to be coated with a MoN coating must be previously nitrided and standard nitriding processes are conducted at temperatures of <NUM> or higher, only components made of materials which can resist such temperatures can be treated in this manner.

In this regard, the second inventive solution (using a MoN/CrN multilayer coating) involves the advantage that the coating process can be conducted at temperatures lower than <NUM>, e.g. of <NUM>. Since as already mentioned no nitriding step is necessary, components made of temperature sensitive materials can be treated, like for example piston pins.

It was observed that in some cases a further improved contact in the context of the present invention was attained by providing an adhesion layer made of CrN. This adhesion layer was deposited for example before depositing the MoN coating according to the first inventive solution or before depositing the modified MoN coating according to the second inventive solution, respectively. The CrN adhesion layer is preferably provided having a layer thickness of at least <NUM>. The thickness of the CrN adhesion layer preferably between <NUM> and <NUM>.

In particular components of the type piston pins, cam followers, piston rings and nozzle needles were treated with the first and second inventive solutions in the context of the experiments concerning the present invention.

It was in particular very surprisingly that cam follower treated according to the first inventive solution did not show any damage after the nitriding step. The components that can be treated according to the present invention are however not limited by this description.

The surfaces of the components treated according to the present invention (according to the first as well as the second inventive solution) exhibit additionally very good tribological properties, in particular concerning increment of wear resistance.

Examples: Different automotive and precision components were treated according to the present invention and stupendous improvements concerning increment of wear resistance were attained.

Following the present invention will be explained in more detail using the example of cam followers. Some cam followers were treated according to the state of the art and others were treated according to some preferred embodiments of the present invention. Subsequently, all treated cam followers were subjected to different analysis and tests.

A qualified reference sample made of <NUM>19MnCrV8 with hardness <NUM> HRC before coating (following referred to as QRS) test piece having a steel surface and at least one cam follower was subjected to the following treatments:.

<FIG> shows a picture of a cross section corresponding to a portion of a multilayer MoN/CrN coating film deposited on a cam follower according to the present invention.

During deposition of the multilayer the MoN individual layers only the Mo target was arc-vaporized, likewise during deposition of the CrN individual layers only the Cr target was arc-vaporized. Between the MoN individual layer and the CrN individual layers, intermediate layers were deposited. During deposition of the intermediate layers both Mo target and Cr target were simultaneously arc-vaporized. In this manner intermediate layers of MoCrN (i.e. intermediate layers comprising Cr, Mo and N) were formed.

The individual MoN layers comprised in the multilayer MoN/CrN coating film deposited in this manner comprised a mixture of hexagonal phase and at least one cubic phase of molybdenum nitride.

The treated QRS were tested by using the HRC Rockwell test and the cam followers were tested by using an engine valve test at different rotation speeds during up to <NUM> hours.

The inventors observed that the all surfaces of the cam followers treated with the Treatments A according to the state of the art presented strong wear after the discontinuous mechanical load test and actually in all cases the coating was after <NUM> hours completely removed and the cam followers themselves showed deep wear grooves.

Contrariwise, all surfaces of the cam followers treated with the Treatments B, C and D according to the present invention presented surprisingly practically no wear after <NUM> hours in the engine test. The highest wear observed in these cases was of <NUM>. This means that in the worst case only <NUM> from the overall coating thickness was removed.

The treated surfaces of the QRS were tested by using In HRC Rockwell test as mentioned above. All surfaces of the QRS treated with the Treatments A according to the state of the art presented ring-shaped fractures after HRC Rockwell test. Contrariwise, all surfaces of the QRS treated with the Treatments B, C and D according to the present invention presented surprisingly no ring-shaped fractures after HRC Rockwell test.

Concretely the present invention relates to:.

According to a preferred embodiment of the present invention, if no multilayer MoN/CrN coating film is comprised between the substrate surface and the at least one MoN layer, said at least one MoN layer has a thickness not less than <NUM>. In such a case, the at least one MoN layer can be deposited in such a manner that it consists of molybdenum nitride comprising at least largely the hexagonal phase δ-MoN or only the hexagonal phase δ-MoN. However, if it is beneficial for the use of the component, it is also possible in such a case to deposit the at least one MoN layer consisting of molybdenum nitride, wherein it comprises the mixture of phases, as required by the present invention, the mixture comprising:.

Preferably, the overall thickness of said at least one MoN layer is not less than <NUM> and not less than <NUM>, more preferably not less than <NUM>,<NUM> and not greater than <NUM>.

According to a further preferred embodiment of the present invention, if no hardened, nitrogen-containing substrate surface layer is comprised between the substrate surface and the at least one MoN layer, said multilayer MoN/CrN coating film has a thickness not less than <NUM>. In this case the above mentioned at least one MoN layer having a thickness not less than <NUM> can be also comprised between two multilayer MoN/CrN coating films, wherein each multilayer MoN/CrN coating film has preferably a film thickness not less than <NUM>.

Preferably the sum of the thicknesses of both multilayer MoN/CrN coating films and said at least one MoN layer is not less than <NUM> and not greater than <NUM>, preferably not less than <NUM>,<NUM> and not greater than <NUM>.

According to a further preferred embodiment of the present invention, at least between one individual MoN layer and one individual CrN layer an individual intermediate layer comprising Mo, Cr and N is comprised.

According to a further preferred embodiment of the present invention, the individual intermediate layer has a thickness not less than <NUM> and not greater than thickness of the individual CrN layer or the thickness of the individual MoN layer that is next to said individual intermediate layer.

According to a further present embodiment of the present invention, at least the majority of the individual MoN layers have an individual layer thickness not less than <NUM> and preferably not greater than <NUM>.

According to a further preferred embodiment of the present invention, at least the majority of the individual CrN layers have an individual layer thickness not less than <NUM> and preferably not greater than <NUM>.

According to a further preferred embodiment of the present invention, the overall thickness of said multilayer MoN/CrN coating film is not less than <NUM> and not greater than <NUM>, preferably not less than <NUM>,<NUM> and not greater than <NUM>.

According to the present invention, the individual MoN layer consists of molybdenum nitride comprising a mixture of phases, the mixture comprising:.

The treatments according to the present invention are particularly advantageously, when the component is an automotive component or a precision component, and more in particular if the substrate surface of the component has a hardness equal or lower than <NUM> HRC.

A preferred method for producing a component according to at least one of the embodiments of the present invention, comprises the deposition of said at least one MoN layer or of at least one of the individual MoN layers by means of a reactive PVD process.

Claim 1:
Component comprising a substrate surface coated with a coating comprising at least one MoN layer having a thickness not less than <NUM>,
wherein between the substrate surface and the at least one MoN layer:
i) a substrate surface hardened layer is comprised, which is a hardened, nitrogen-containing substrate surface layer that is the result of a nitriding treatment carried out at the substrate surface and has a thickness not less than <NUM>, preferably not less than <NUM> and not greater than <NUM>,
and/or
ii) a layer system composed of more than <NUM> MoN layers and more than <NUM> CrN layers is comprised, wherein the MoN and CrN layers forming the layer system are individual layers deposited alternate one on each other forming a multilayer MoN/CrN coating film
characterized in that said at least one MoN layer consists of molybdenum nitride comprising a mixture of phases, the mixture comprising:
- the hexagonal phase δ-MoN and the cubic phase γ-Mο<NUM>N, or
- the hexagonal phase δ-MoN and the oversaturated cubic phase ζ-MoN, or
- the hexagonal phase δ-MoN and the cubic phase γ-Mο<NUM>N and the oversaturated cubic phase ζ-MoN.