Al-Cr-O-BASED COATINGS WITH HIGHER THERMAL STABILITY AND PRODUCING METHOD THEREOF

Abstract

The present invention relates to a method for producing an Al—Cr—O-based coatings comprising at least one Al—Cr—O-based or Al—O-based film on a workpiece surface, wherein the method comprises following steps: a) placing at least one workpiece having a surface to be coated in the interior of a vacuum chamber, and b) depositing a film A comprising aluminum and chromium on the workpiece surface to be coated, wherein the ratio of aluminum to chromium in the film in atomic percentage has a first value corresponding to Al/Cr≤2.3 and wherein the method further comprises following steps: c) forming volatile compounds of Cr—O, e.g. CrO3 and/or CrO2, thereby causing that at least part of the chromium contained in the film A left the film in form of Cr—O volatile compounds, d) executing the step c) during a period of time, within which the chromium content in the film A is reduced until attaining a second value of ratio of aluminum to chromium in the film in atomic percentage, thereby the film A being transformed in a film B containing a reduced content of chromium, which corresponds to an aluminum and chromium in a proportion corresponding to a ratio of Al/Cr≥3.5 or corresponding to a Cr content in atomic percentage of 0% or of approximately 0%.

1 . A method for producing an Al—C—O-based coating on a workpiece surface, the method comprising:
 a) placing at least one workpiece having a surface to be coated in an interior of a vacuum chamber; 
 b) depositing a film A comprising aluminum and chromium on the workpiece surface to be coated, wherein a ratio of aluminum to chromium in the film A in atomic percentage has a first value corresponding to Al/Cr≤2.3; and 
 c) forming volatile compounds of Cr—O, thereby causing at least part of the chromium contained in the film A to leave the film A in a form of Cr—O volatile compounds, 
 
       wherein step c) is executed during a period of time, within which the chromium content in the film A is reduced until attaining a second value of the ratio of aluminum to chromium in the film A in atomic percentage corresponding to Al/Cr≥3.5, 
       and wherein the film A is transformed into a film B having a reduced content of chromium, corresponding to 0% of chromium or corresponding to said second value of ratio Al/Cr resulting in higher thermal stability than the film A having the ratio of Al/Cr corresponding to said first value. 
     
     
         2 . The method according to  claim 1 , wherein an oxygen plasma is generated within the vacuum chamber and the film A deposited in step b) is exposed to the oxygen plasma until Cr diffuses out of the film A and reacts with oxygen available at a surface of the film A, thereby forming the volatile compounds of Cr—O in step c). 
     
     
         3 . The method, according to  claim 1 , wherein, the film A deposited in step b) is subjected to annealing in an oxygen-comprising atmosphere at a temperature of above 900° C. until Cr diffuses out of the film A and reacts with oxygen available at a surface of the film A, thereby forming the volatile compounds of Cr—O in step c). 
     
     
         4 . The method according to  claim 1 , wherein conditions for forming the volatile compounds of Cr—O in step c) are adjusted in such a manner that essentially only the volatile compound CrO 2  is formed. 
     
     
         5 . The method according to  claim 1 , wherein the film A deposited in step b) is an Al—Cr—O film. 
     
     
         6 . The method according to  claim 5 , wherein a period of time during which step c) is executed, is selected to be lone enough that at least 90% of the Cr contained in the film A deposited in step b) diffuses out of the film A, and the film B being formed comprises at least 90% of alpha-alumina with corundum structure. 
     
     
         7 . The method according to  claim 6 , wherein the period of time during which the step c) is executed, is selected to be long enough that all of the Cr contained in the film A deposited in step b) diffuses out of the film, and the film B is formed comprising only alpha-alumina with corundum structure. 
     
     
         8 . The method according to  claim 5 , wherein a period of time during which step c) is executed, is selected to be long enough that so much Cr contained in the film A deposited in step b) diffuses out of the film that the film B is produced comprising essentially only Al—Cr—O solid solution with corundum structure, thereby having the so produced Al—Cr—O solid solution film B a second value of the ratio of aluminum to chromium in atomic percentage corresponding to Al/Cr≥4. 
     
     
         9 . The method according to  claim 3 , wherein the annealing temperature is selected to be above 1000° C. 
     
     
         10 . The method according to  claim 3 , wherein the annealing temperature is selected to be above 1100° C. 
     
     
         11 . method according to  claim 3 , wherein the annealing temperature is selected to be above 1200° C. 
     
     
         12 . A workpiece with a surface coated by using a method according to  claim 6 , thereby comprising an alpha-alumina film with corundum structure, which exhibits pores that have been formed during diffusion of Cr out of the film. 
     
     
         13 . A workpiece with a surface coated by using a method according to  claim 1 , wherein the surface material of the workpiece that is coated is polycrystalline alumina, sapphire corundum, mullite or compounds comprising or consisting of a mixture of:
 alumina and yttrium stabilized zirconium oxide, and/or   alumina and silicon nitride, and/or   alumina and silicon carbide, and/or   alumina and yttrium oxide, and/or   alumina and erbium oxide.   
     
     
         14 . The workpiece according to  claim 12 , wherein the workpiece is a turbine engine component. 
     
     
         15 . A workpiece with a surface coated by using a method according to  claim 7 , thereby comprising an alpha-alumina film with corundum structure, which exhibits pores that have been formed during diffusion of Cr out of the film. 
     
     
         16 . The workpiece according to  claim 13 , wherein the workpiece turbine engine component. 
     
     
         17 . The workpiece according to  claim 15 , wherein the workpiece is a turbine engine component.