Arc-deposited Ai—Cr—O coatings having enchanced coating properties

Abstract

The present invention relates to a method for coating AI-Cr-0 coatings with the help of a PVD-coating process. The PVD-coating process is performed with the help of Al and Cr comprising targets which are doped with Si. The doping of Si prevents the forming of oxide islands on the target during the reactive coating process.

Claims

1. Method for producing PVD-oxide-coatings with at least one layer consisting essentially of Al, Cr, Si and O, the method comprising at least the following steps: a) providing a PVD-coating chamber, b) loading in the PVD-coating chamber substrates having at least one surface to be coated, and c) performing a reactive PVD coating process using a process gas containing a reactive gas that reacts with metal ions produced from one or more targets for depositing the at least one layer consisting essentially of Al, Cr, Si and O on the substrate surface, characterized in that, the one or more targets used for performing the reactive PVD coating process in step c) have an element composition in atomic percent given by the formula: Al.sub.1xyCr.sub.xSi.sub.y with 0.05y0.10 and 0.20x0.25 and the reactive gas is oxygen thereby producing a coating with the at least one layer consisting essentially of Al, Cr, Si and O, wherein, if oxygen is not taken into account, in the at least one layer the silicon concentration is less than the silicon concentration in the one or more targets, and the PVD coating process is an arc evaporation process.

2. Method according to claim 1 characterized in that the process gas comprises essentially only oxygen.

3. Method according to claim 1 characterized in that y=0.05 and x=0.25.

4. Method according to claim 1 characterized in that wherein, if oxygen is not taken into account, in the at least one layer the silicon concentration is equal or less than half of the silicon concentration in the one or more targets.

5. Method according to claim 1, wherein the coating forms an oxidation barrier.

6. Method according to claim 1, wherein the coating forms a chemical barrier.

7. Method according to claim 1, wherein the coating is used as a solid lubricant for tribological applications performed at temperatures higher than 200 C.

Description

(1) For a better understanding of the present invention, some further details will be described using the FIGS. 1 to 4:

(2) FIG. 1: Photos of two surfaces corresponding to two different targets operated by reactive cathodic arc-evaporation a) Photo of the surface of an Al.sub.70Cr.sub.30 target which was operated for 1.5 h in a pure oxygen atmosphere at a flow of 800 sccm oxygen. b) Photo of the surface of an Al.sub.70Cr.sub.25Si.sub.5 target which was operated for 1.5 h in a pure oxygen atmosphere at a flow of 800 sccm oxygen. c) Image magnification of the target surface showed in FIG. 1a. d) Image magnification of the target surface showed in FIG. 1b.

(3) FIG. 2: XRD spectra of the surfaces of the both targets showed in FIG. 1 a) Al.sub.70Cr.sub.30 target b) Al.sub.70Cr.sub.25Si.sub.5 target

(4) FIG. 3: SEM-micrographs of the fracture morphology of two coatings deposited by reactive cathodic arc evaporation in a pure oxygen atmosphere a) from a Al.sub.70Cr.sub.30 target at an oxygen flow of 800 sccm b) from a Al.sub.70Cr.sub.25Si.sub.5 target at an oxygen flow of 800 sccm

(5) FIG. 4: XRD spectra of the coating deposited from a Al.sub.70Cr.sub.25Si.sub.5 target at an oxygen flow of 800 sccm for which the fracture morphology is shown in the FIG. 3b

(6) FIG. 5: XRD spectra of the coating at different Si concentrations.

(7) In FIG. 1a, the presence of many black dots at the surface of the Al.sub.70Cr.sub.30 target can be observed, these black dots are oxide islands containing some amount of corundum structured Al.sub.2O.sub.3 (as identified by XRD). While in FIG. 1b, it can be observed that the surface of the Al.sub.70Cr.sub.25Si.sub.5 target is free of black dots. The surfaces of the both targets Al.sub.70Cr.sub.30 and Al.sub.70Cr.sub.25Si.sub.5 were analyzed by X-ray diffraction analysis in order to identify the phases present at the target surface for both target materials. The XRD spectra obtained from the target surfaces are shown in FIG. 2. The analysis of the target surface of the Al.sub.70Cr.sub.30 (FIG. 2a) is consistent with previous investigations and shows besides the formation of Al and Cr phases also the formation of Al.sub.8Cr.sub.5 and Al.sub.4Cr phases. The analysis of the Al.sub.70Cr.sub.25Si.sub.5 target (FIG. 2b) shows similarly as in FIG. 2a the formation of Al and Cr phases as well as Al.sub.8Cr.sub.5 and Al.sub.4Cr phases, but in this case, the Al.sub.8Cr.sub.5 and Al.sub.4Cr peaks are shifted to higher diffraction angles. This may be explained by the incorporation of Si in these phases and additionally the possible presence of a CrSi phase can be observed.

(8) An embodiment of the present invention relates to a reactive cathodic arc-evaporation coating method for producing AlCrO using AlCr targets (as source coating material) which are doped with silicon. The AlCrSi targets having preferably following element composition in atomic percent:
Al.sub.aCr.sub.1abSi.sub.c with 90>=a>=60, 40>=1ab>=10, 20>=c>=1

(9) Thus it is possible to reduce or prevent the growth of oxide islands by the evaporation of the targets in pure oxygen atmosphere or in gas mixtures containing oxygen, inclusively using high oxygen flows.

(10) Within the description of the present invention flowing flows and pressures will be considered as low, middle or high flows:

(11) Low oxygen flows: about 100 to 250 sccm (200 sccm0.3 Pa in coating chamber)

(12) Middle oxygen flows: about 250 to 500 sccm

(13) High oxygen flows: about 800 to 1000 sccm (>=2.3 Pa in coating chamber)

(14) Doping the target with e.g. 5 at. % Si changes the Al/Cr ratio compared to the Al70Cr30 target from 2.3 to 2.8 which in turn would be comparable to an Al(74)Cr(26) target composition for an un-doped target. Based on previous investigations (Ramm et al 2007) one would expect that the metallic target composition would be reproduced in the metallic composition of the synthesized ternary oxide. This is not the case. The Al/Cr ratio in the synthesized coating is shifted to higher Al ratios for both target compositions. In Table 1, the compositions of the Al/Cr ratios for the synthesized AlCrO coatings are displayed.

(15) TABLE-US-00001 TABLE 1 Element composition of two different coatings produced by reactive cathodic arc-evaporation from respectively Al.sub.70Cr.sub.30 and Al.sub.70Cr.sub.25Si.sub.5 targets by EDX and ERDA Coating Coating element composition element composition measured by EDX [at. %] measured by ERDA [at. %] Target Al Cr O Al Cr O Al.sub.70Cr.sub.30 29.5 10.9 58.6 Al.sub.70Cr.sub.25Si.sub.5 30.92 11.11 57.97 30.3 9.7 59.4 31.38 10.89 57.73 31.97 10.93 57.10

(16) The compositions were measured by two independent analysis methods: Energy Dispersive X-ray Spectroscopy (EDX) and Elastic Recoil Detection Analysis (ERDA). The modified Al/Cr ratio which results from doping with Si, however, is reflected to some degree in the coating composition. It was, however, completely unexpected that no Si could be detected in the coating which was synthesized from the target with the composition of Al70Cr25Si5. This effect could be explained by a volatilization of the Si in combination with oxygen. In the publication of Shyklaev et al Initial reactive sticking coefficient of O.sub.2 on Si(111)-77 at elevated temperatures, Surface Science 351 (1996) 64-74, reactions are described which indicate this effect. However, the conditions which are described in this publication are somewhat different from the conditions under which the oxide synthesis was performed for this work. Therefore, the explanation of the fact that no Si can be found in the oxide coating is an assumption only. Surprisingly is the fact that no or nearly no Si is incorporated in the coating.

(17) The present invention allows the utilization of AlCr targets with silicon doping with the advantage that no oxide islands are formed at the target surface and the synthesis of pure AlCr oxides without essential Si doping of the coating.

(18) In FIGS. 3a and b, the morphology of the synthesized oxide coatings obtained for different target compositions is compared by cross sectional scanning electron microscopy (X-SEM). The morphology of the oxide layer obtained from the Al.sub.70Cr.sub.30 target (a) shows distinctive columnar structure. Based on the existing knowledge, this is a typical behaviour for AlCrO coating materials produced by reactive arc evaporation: increasing oxygen flow results in a pronounced change of the morphology from dense structure (obtained by using low oxygen flows) to columnar growth (obtained by using higher oxygen flows). FIG. 3b was prepared from the coating obtained with the same high oxygen flow (800 sccm) and under identical process conditions, with the exception that the Al.sub.70Cr.sub.25Si.sub.5 target was utilized. The micrograph shows a completely different morphology characterized by a very dense structure. Facing the fact that the coating does not contain Si, this is a completely unexpected result. This dense layer growth, however, makes arc evaporated AlCr oxides suitable for oxidation and corrosion resistant coatings for which diffusion processes must be inhibited and for which the columnar structure would be too leaky. Additional experiments with Si doping of A-Cr targets showed that an addition of Si between 1 and 20 at. % result in similar densifications of the A-Cr oxide coatings, with a preference of Si doping in the range of 2 to 10 at. %.

(19) Although no or negligible (compared to target composition) Si can be found in the synthesized oxide coating, Si doping of the target results in a completely modified morphology of the oxide coating which is characterized by a dense structure without columnar growth and despite the high oxygen flows utilized for the synthesis.

(20) The XRD analysis of the layer synthesized from the Al.sub.70Cr.sub.25Si.sub.5 target at a oxygen flow of 800 scm (FIG. 4) showed a distinctive peak near 2theta=46. This peak is attributed to the cubic phase of A-CrO in accordance with a publication of Khatibi et al Phase transformations in face centered cubic (Al0.23Cr0.68)2O3 thin films, Surface & Coating technology 206 (2012) 3216-3222. Although, electron diffraction indicates also additions of the AlCrO solid solutions in corundum structure, for the high oxygen flow, the cubic structure is more pronounced. Oxygen flow and the Al/Cr ratio can be, however, adjusted to leverage the amount of cubic to corundum phase in the A-CrO. The XRD analysis shows additional peaks. The peak with the highest intensity near 69 is attributed to the silicon substrate. The additional peak with high intensity near 67 is characteristic for Al.sub.2O.sub.3 in corundum structure or alpha alumina. Therefore, doping the target with Si supports the growth of cubic AlCrO phases in the coating and may additionally also produce pure corundum phases.

(21) Recommended applications of the coating produced according to the present invention are: Corrosion resistant coatings Oxidation barriers Chemical barriers Running in layers for high temperature tribological applications Fuel cell applications Solid lubricant for high temperature tribology

(22) A further very interesting aspect of the present invention is that by using Si doped AlCr targets as coating material source for the deposition of AlCrO coatings in an oxygen comprising environment by means of reactive cathodic arc evaporation PVD processes, the formation of the cubic phase of the AlCrO in the coating when the Si concentration in the AlCrSi target is about 5 at. % cannot be detected by X-ray examinations as it is shown in the FIG. 5.

(23) Furthermore, a considerable reduction of the formation of oxide islands at the target surface was also observed when the Si concentration in the AlCrSi targets was about 5 at %

(24) Particular details of the present invention are mentioned in the following claims 1 to 14.

(25) This description discloses a method for producing PVD-oxide-coatings with at least one layer consisting essentially of Al, Cr, Si and O, the method comprising at least the following steps: a) providing a PVD-coating chamber b) loading in such PVD-coating chamber substrates having at least one surface to be coated c) performing a reactive PVD coating process wherein the process gas contains a reactive gas with reacts with metal ions produced from one or more targets for depositing the at least one layer consisting essentially of Al, Cr, Si and O on the substrate surface, characterized in that, the one or more targets used for performing the reactive PVD coating process in step c) have an element composition in atomic percent given by the formula: Al.sub.1xyCr.sub.xSi.sub.y with 0.05y0.10 and 0.20x0.25 and the reactive gas is oxygen thereby producing a coating with at least one layer consisting essentially of Al, Cr, Si and O, wherein, if oxygen is not taken into account, in the at least one layer the silicon concentration is less than the silicon concentration in the one or more targets.

(26) The PVD coating process is for example an arc evaporation process.

(27) According to one embodiment of the processing gas comprises essentially only oxygen. It is possible and preferable to choose y=0.05 and x=0.25.

(28) The silicon concentration may be equal or less than half of the silicon concentration in the one or more targets

(29) The method may be used to produce a coating system. A substrate can be coated with the coating system

(30) The coating system can be used for improving the corrosion resistance.

(31) The coating system can be used as oxidation barrier, and/or chemical barrier, and/or running in layer for high temperature tribological applications, for example above 200 C., and/or fuel cells, and/or solid lubricant for tribological applications performed at temperatures higher than 200 C.

(32) The coating system as described above may be applied on a substrate to be used in an application requiring one or more of the above described characteristics.