Molybdenum monoxide layers, and production thereof using PVD

Abstract

The invention relates to a coating comprising at least one molybdenum-containing layer having molybdenum oxide, said molybdenum being essentially molybdenum monoxide. The invention further relates to a PVD process for producing the disclosed coating, in which the layer comprising the molybdenum monoxide is produced using arc evaporation. The invention also relates to a component that has said coating.

Claims

1. Coating comprising: at least one layer containing molybdenum oxide, characterized in that the molybdenum oxide consists essentially of a molybdenum monoxide, at least one molybdenum nitride layer, and at least one Mo—N—O layer including molybdenum-nitrogen compounds and molybdenum-oxide compounds.

2. Coating according to claim 1, characterized in that the XRD spectrum of the molybdenum monoxide layer essentially exhibits neither MoO.sub.2 peaks nor MoO.sub.3 peaks.

3. Coating according to claim 1, characterized in that the Mo—N—O layer lies between a molybdenum nitride layer and a molybdenum monoxide layer.

4. Coating according to claim 1, characterized in that the atomic concentration ratio in the Mo—N—O layer is described by the following formula, Mo.sub.z(N.sub.dO.sub.e), wherein: e: is the atomic oxygen concentration in the Mo—N—O layer d: is the atomic nitrogen concentration in the Mo—N—O layer z: is the atomic molybdenum concentration in the Mo—N—O layer and z≧e.

5. Coating according to claim 1, characterized in that the Mo—N—O layer is a graded layer, wherein the nitrogen concentration decreases from the boundary surface close to the substrate towards the boundary surface close to the surface of the Mo—N—O layer whilst the oxygen concentration increases.

6. Coating according to claim 1, characterized in that the Mo—N—O layer is a graded layer, wherein the oxygen concentration decreases from the boundary surface close to the substrate towards the boundary surface close to the surface of the Mo—N—O layer whilst the nitrogen concentration increases.

7. Coating according to claim 5 or 6, characterized in that the coating comprises at least one coating according to claim 5 and at least one coating according to claim 6.

8. Coating according to claim 1, characterized in that the coating has a cover layer with MoO.sub.3.

9. Coating according to claim 1, characterized in that at least one bonding layer and/or at least one functional layer is deposited between a base body and the layer containing Mo.

10. Coating according claim 1, characterized in that at least one layer containing Mo comprising (Mo.sub.1-x, Me.sub.x) A.sub.aB.sub.bC.sub.c, wherein 0≦x≦0.99 and Me is a metal from the group of W, Ti, Al, Cr, Si, Zr, Ta, Nb, Ag, Cu and V or the combination of two or more metals of this group and A is nitrogen (N) with 0.5≦a≦1 B is carbon (C) with 0≦b≦0.5 C is oxygen (O) with 0≦c≦0.5 and wherein a, b and c indicate in at % between the elements N, C and O with a+b+c=1, and x≧c.

11. Coating according to claim 1, characterized in that at least one layer containing Mo comprising (Mo.sub.1-y, Me.sub.y) A.sub.uB.sub.vC.sub.w, wherein 0≦y≦0.99 and Me is a metal from the group of W, Ti, Al, Cr, Si, Zr, Ta, Nb, Ag, Cu and V or the combination of two or more metals of this group and A is nitrogen (N) with 0.5≦u≦1 B is carbon (C) with 0≦v≦0.5 C is oxygen (O) with 0.5≦w≦1 and wherein u, v and w indicate in at % between the elements N, C and O with u+v+w=1, and y≧w.

12. Coating according to claims 10 and 11, characterized in that the coating comprises at least one first layer according to claim 10 and a second layer according to claim 11.

13. Component with a coating, characterized in that the coating is a coating according to claim 1.

14. Coating according to claim 1, wherein the molybdenum monoxide is contained in the coating.

15. Coating according to claim 10, wherein 0≦x≦0.5.

16. Coating according to claim 11, wherein 0≦y≦0.5.

17. Coating comprising: at least one molybdenum nitride layer, at least one Mo—N—O layer formed on the at least one molybdenum nitride layer, the at least one Mo—N—O layer including molybdenum-nitrogen compounds and molybdenum-oxide compounds, and at least one molybdenum oxide formed on the at least one Mo—N—O layer, wherein the molybdenum oxide consists essentially of a molybdenum monoxide.

Description

DETAILED DESCRIPTION OF THE INVENTION ON THE BASIS OF EXAMPLES

(1) The following examples serve to present the invention in detail, they are now however to be considered a limitation of its scope.

Example 1

(2) It is generally advantageous for piston rings and shaft seal rings to exhibit good mechanical stability and simultaneously good lubricating properties and/or smoothness. In the present example, a piston ring of stainless steel was coated with Mo—N by means of spark evaporation. For this, the piston ring is placed in the coating chamber of a coating facility. The coating chamber is evacuated and, in order to clean/activate the surface, the surface is heated and etched before the coating procedure. After letting in nitrogen into the coating chamber, a spark is ignited in the nitrogen atmosphere on a molybdenum target serving as cathode, which results in the evaporation of molybdenum and/or already reacted molybdenum nitride. The deposition of the material onto the piston surface results in the formation of a 2 μm thick Mo—N layer. The coating process enables the coating thickness to be adapted depending on the application.

(3) The inventive coating of pistons with Mo—N by means of spark evaporation has led the inventors to the observation that the friction during the operation is considerably reduced and also the service life of the piston ring is considerably increased. The inventors ascribe the reduced friction among others to the spatterings or droplets but into the layer by the spark evaporation. These spatterings or droplets are numerically so few that they do not essentially impair the stability and thus the hardness of the Mo—N layer yet are present in such numbers that the molybdenum thus available further improves, thanks to its lubricating properties, the already good sliding properties of the Mo—N layer which is smooth due to the production process.

(4) A particular advantage of spark evaporation is the possibility of coating functional surfaces of precision components since the layer thickness can be deposited exactly according to the required precision reap. the requirements in the tribological system.

(5) If for the above mentioned coating method pulsed technology is used, the growth of different phases of Mo—N can be very well regulated by adjusting different pulse parameters when applying a pulsed bias voltage and/or when operating a pulsed spark evaporation source. This makes it possible, by a specific adjustment of the coating parameters and especially of the pulse parameters, to promote the growth of a particular phase and to produce Mo—N layers with particular properties, adapted to the use of the coated component, R is also possibly, by varying the coating parameters during the coating process, to build for example also in an alternating fashion multi-layer Mo—N layers with different phases. In particular, a layer system can thus be achieved in which cubic Mo—N alternates with hexagonal Mo—N. Such a layer system, which comprises at least two layers, then includes at least one cubic and thus hard layer and an outer hexagonal and thus softer layer.

(6) According to a further embodiment of the present invention, a two-layer coating system can be deposited onto the function surface resp. onto the substrate 1, e.g. for particularly heavy-duty pistons in combustion engines or also in order to be able to dispense with external intermediates: the first layer is then a Mo—N layer and the second layer is a molybdenum monoxide layer 9, as represented in FIG. 7a.

(7) Here too, as already described further above, an additional bonding layer can be provided, as shown in FIG. 7b. The bonding layer 3 is provided between the substrate 1 and the first layer 5 in the layer system. The bonding layer can be both a pure metal layer (such as for example Cr, Mo, Ti, Si) or also a composite (such as for example Me-N or Me-NC), wherein Me can also be a combination of metals.

(8) The transition between the first layer 5 and the second layer 9 can be designed as a gradient in such a manner that with increasing distance from the surface of the substrate, the Mo—N concentration diminishes and simultaneously the Mo—O concentration increases. This results in a very good adhesive strength within the layer system. This applies accordingly for all layer transitions of the layer systems mentioned in this description. The gradient can in particular comprise the entire layer and/or the entire layers.

(9) A further improvement of the toughness in the layer system can be achieved by the deposition of several nano-layer coatings or by producing nano-composite structures, for example by embedding the harder phase in the form of nano-grains in a matrix from the softer phase.

(10) One speaks of X-layer, in the frame of the present description, when the layer contains predominantly X, wherein X can be an elementary material or a compound.

(11) In the frame of the present description, a coating has been disclosed which comprises at least a layer containing molybdenum and which is characterized in that the molybdenum is essentially molybdenum monoxide.

(12) The inventive coating can contain for example a molybdenum oxide layer comprising essentially molybdenum monoxide. A MoO layer can be distinguished from MoO.sub.2 and MoO.sub.3 layers in that for example the XRD spectrum of the molybdenum monoxide layer (MoO) exhibits essentially neither MoO.sub.2 peaks nor MoO.sub.3 peaks.

(13) A preferred embodiment of the inventive coating can for example additionally also comprise a molybdenum nitride layer.

(14) The inventive coating can also contain a Mo—N—O layer comprising molybdenum-nitrogen compounds and molybdenum-oxide compounds, wherein essentially the molybdenum atoms build a compound with maximum one oxygen atom and wherein molybdenum monoxide is preferably comprised in the coating.

(15) A preferred embodiment of the inventive coating with at least one Mo—N—O layer is characterized in that the Mo—N—O layer lies between a molybdenum nitride layer and a molybdenum monoxide layer.

(16) The atomic concentration ratio in the Mo—N—O layer can be described by the following formula: Mo.sub.z(N.sub.dO.sub.e), wherein:

(17) e: is the atomic oxygen concentration in the Mo—N—O layer

(18) d: is the atomic nitrogen concentration in the Mo—N—O layer

(19) z: is the atomic molybdenum concentration in the Mo—N—O layer and z≧y.

(20) The Mo—N—O layer can also be a graded layer, wherein the oxygen concentration decreases from the boundary surface close to the substrate towards the boundary surface close to the surface of the Mo—N—O layer whilst the nitrogen concentration increases.

(21) An inventive coating can also be deposited as a multi-layer coating system, wherein several MoN and MoO layers are deposited in alternating fashion and can also contain several Mo—N—O layers that lie for example between the MoN and MoO layers and that can be deposited as graded layers with adapted variations of the nitrogen and oxygen concentrations to improve the adhesive strength within the layer.

(22) The inventive coating can also additionally include a cover layer with MoO.sub.3 if the MoO.sub.3 produces significant advantages for the specific application.

(23) According to the invention, the coating can also include one or several bonding layer(s) and/or functional layer(s) between the base body and the layer containing Mo.

(24) An also preferred variant embodiment of the invention is a coating that comprises at least one layer containing Mo and having in particular very high mechanical stability and containing (Mo.sub.1-x, Me.sub.x) A.sub.aB.sub.bC.sub.c, wherein 0≦x≦0.99, preferably 0≦x≦0.5 and Me is a metal from the group of W, Ti, Al, Cr, Si, Zr, Ta, Nb, Ag, Cu and V or the combination of two or more metals of this group and

(25) A is nitrogen (N) with 0.5≦a≦1

(26) B is carbon (C) with 0≦b≦0.5

(27) C is oxygen (O) with 0≦c≦0.5

(28) and wherein a, b and c indicate in at % between the elements N, C and O with a+b+c=1 and additional further elements can be provided but are preferably not provided, and x≧c.

(29) A preferred variant embodiment of the invention is also a coating that comprises at least one layer containing Mo and having in particular very good lubricating properties and sliding characteristics and containing (Mo.sub.1-y, Me.sub.y) A.sub.uB.sub.vC.sub.w, wherein 0≦y≦0.99, preferably 0≦y≦0.5 and Me is a metal from the group of W, Ti, Al, Cr, Si, Zr, Ta, Nb, Ag, Cu and V or the combination of two or more metals of this group and

(30) A is nitrogen (N) with 0.5≦u≦1

(31) B is carbon (C) with 0≦v≦0.5

(32) C is oxygen (O) with 0.5≦w≦1

(33) and wherein u, v and w indicate in at % between the elements N, C and O with u+v+w=1 and additional further elements can be provided but are preferably not provided, and y≧w.

(34) Preferably, a coating could also be structured according to the invention to comprise: at least a first MoN-containing layer and thus exhibiting in particular very good mechanical stability (according to the (Mo.sub.1-x, Me.sub.x) A.sub.aB.sub.bC.sub.c layer described above), and at least a second layer exhibiting in particular very good lubricating and sliding properties (according to the (Mo.sub.1-y, Me.sub.y) A.sub.uB.sub.vC.sub.w layer described above).

(35) A preferred variant of the method for producing the inventive coating is a PVD process or combined PVD/CVD process, wherein the molybdenum monoxide is deposited in the molybdenum-containing layer by means of spark evaporation.

(36) A further preferred variant of the method for producing the inventive coating is a PVD process, wherein the molybdenum-containing layers are produced by means of spark evaporation of at least one target containing molybdenum in a reactive atmosphere.

(37) As material source for the layers containing molybdenum, both molybdenum targets as well as alloyed molybdenum-containing targets are used that are produced for example by metallurgical smelting or powder-metallurgically.

(38) Components can be coated depending on the application with the most suitable variant of the inventive coating.

(39) In particular, machining tools and forming tools that are coated according to the invention for the production of components of metal or metal alloys, in order to reduce or preferably completely prevent smearings of the machined metals or metal alloys on the coated surface of the tool.

ILLUSTRATIONS

(40) FIG. 1: sketch of the layer morphology of a MoN layer deposited by means of spark evaporation with CrN bonding layer in SEM rupture cross-section.

(41) FIG. 2: sketch of the layer morphology of a Mo—N/Mo—N—O/Mo—O layer with CrN bonding layer in SEM rupture cross-section, wherein the molybdenum oxide phases MoO.sub.2 and MoO.sub.3 can be evidenced in the layer by XRD.

(42) FIG. 3: sketch of the layer morphology of a Mo—N/Mo—N—O/Mo—O layer produced according to the invention by means of spark evaporation without bonding layer, in SEM rupture cross-section, wherein no molybdenum oxide phases MoO.sub.2 and MoO.sub.3 can be evidenced in the layer by XRD, although more than 10 atomic percent oxygen can be evidenced in the layer by EDX. The layer was produced by the adjunction, interrupted at time intervals, of an oxygen flow of 50 sccm (at 4 Pa set pressure) with an on-time of 40 s and an off-time of 3 minutes.

(43) FIG. 4: Typical XRD spectrum of a MoN layer, whose Bragg reflexes are in conformity with the MoN reference ISDN 00-025-1367.

(44) FIG. 5: Typical XRD spectrum of a MoO.sub.3 layer, whose Bragg reflexes are in conformity with the MoO.sub.3 reference ISDN 00-001-0706.

(45) FIG. 6: Typical XRD spectrum of a molybdenum monoxide layer.

(46) FIGS. 7a-7c: Layers in coating systems according to the invention.

REFERENCE LIST

(47) 1: substrate 3: bonding layer and/or functional layer 5: first layer 7: transition layer 9: second layer