Method for producing a ceramic layer on a surface formed from an Ni base alloy

Abstract

A method for producing a ceramic layer on a surface formed from a Ni base alloy, includes the following steps: producing on the surface a ceramic layer containing ZrO.sub.2 as a main constituent; producing a gas phase having a temperature in the range from 400 to 900 C., in which a vapor formed from a salt melt with the components alkali chloride, alkali sulphate and ZnCl.sub.2 is contained in a carrier gas formed from an inert gas with an addition from 0.5 to 10% by weight HCl; and bringing the ceramic layer into contact with the gas phase for a period of time that is sufficient for an intermediate layer having a thickness of at least 0.1 m to form between the ceramic layer and the surface.

Claims

1. A method for producing a ceramic layer on a surface formed from a Ni base alloy containing Cr in a quantity from 5 to 25% by weight, comprising: producing on the surface a ceramic layer comprising ZrO.sub.2; producing a gas phase having a temperature in the range from 400 to 900 C., and containing gas phase components formed from a salt melt including components of alkali chloride, alkali sulphate and ZnCl.sub.2, and a carrier gas formed from an inert gas with 0.5 to 10% by weight HCl, the components included in the salt melt being contained essentially in equimolar composition; and bringing the ceramic layer into contact with the gas phase for a period of time that is sufficient for an intermediate layer having a thickness of at least 0.1 m to form between the ceramic layer and the surface, wherein ZrO.sub.2 is contained in the ceramic layer and the gas components are contained in the gas phase, such that the gas phase components contained in the gas phase form a quaternary eutectic with ZrO.sub.2 in the range from 400 to 900 C., and the HCl contained in the gas phase reacts with Cr contained in the Ni base alloy to form chromium chlorides, thereby forming the intermediate layer between the ceramic layer and the surface in the step of bringing the ceramic layer into contact with the gas phase.

2. The method according to claim 1, wherein the Ni base alloy contains Cr in a quantity from 15 to 25% by weight.

3. The method according to claim 1, wherein the ceramic layer is produced by means of physical vapor deposition or by thermal spraying.

4. The method according to claim 1, wherein the ceramic layer further contains Y.sub.2O.sub.3 in order to stabilise the ZrO.sub.2.

5. The method according to claim 1, wherein the ceramic layer further contains Al.sub.2O.sub.3.

6. The method according to claim 5, wherein the ceramic layer contains 30 to 70 mol % of Al.sub.2O.sub.3.

7. The method according to claim 1, wherein N.sub.2 is used as the inert gas.

8. The method according to claim 1, wherein the inert gas contains 1.0 to 4.0% by weight HCl.

9. The method according to claim 1, wherein the components in the salt melt further contain ZnSO.sub.4.

10. The method according to claim 1, wherein the components in the salt melt contain following components: KClK.sub.2SO.sub.4ZnCl.sub.2ZnSO.sub.4.

11. The method according to claim 1, wherein the ceramic layer is brought into contact with the gas phase for the period of time that is sufficient for the intermediate layer having a thickness from 0.5 to 5.0 m to form between the ceramic layer and the surface.

12. The method according to claim 1, wherein the period of time is from 1 to 100 hours.

13. The method according to claim 12, wherein the period of time is from 20 to 75 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic view of an apparatus for solvothermal treatment,

(2) FIG. 2 shows the solubility of ZrO.sub.2 over temperature depending on the composition of the gas phase,

(3) FIG. 3 shows an image recorded by SEM of a cross section through a surface of a Ni base alloy coated with the ceramic layer,

(4) FIG. 4 shows a schematic view of a cross section through an object,

(5) FIG. 5 shows the adhesiveness of the ceramic layer of a conventional test specimen compared with a test specimen according to the invention,

(6) FIG. 6a shows an image recorded by SEM of a surface of a test specimen according to the invention without thermal load change,

(7) FIG. 6b shows the surface according to FIG. 6a after 20 thermal changes,

(8) FIG. 7 shows the thermal cycle stability of a conventional test specimen compared with test specimens according to the invention,

(9) FIG. 8 shows the coefficient of friction of a conventional test specimen compared with a test specimen according to the invention,

(10) FIG. 9 shows the Vickers microhardness of a conventional test specimen compared with further test specimens according to the invention, and

(11) FIG. 10 shows the porosity of a conventional test specimen compared with further test specimens according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(12) The invention will be explained in greater detail hereinafter on the basis of the drawings. With the apparatus shown in FIG. 1 for solvothermal treatment, a nitrogen gas reservoir 1 is connected via a first gas control valve 2 to a gas feed line 3. An HCl gas reservoir 4 is likewise connected to the gas feed line 3 via a second gas control valve 5. The gas feed line 3 leads into a furnace 6. As can be seen from the enlarged detail, a container 7 produced for example from quartz glass and containing a salt melt 8 is received in the furnace 6. The container 7 is advantageously coated with YSZ (not shown here) on its inner face facing the salt melt 8. The salt melt 8 can be formed in equimolar composition from KClK.sub.2SO.sub.4ZnCl.sub.2ZnSO.sub.4, for example. Reference sign 9 denotes a gas inlet or a mouth of the gas feed line 3. Reference sign 10 denotes a gas outlet or an end of a gas discharge line 11.

(13) A test specimen 12 is arranged above the salt melt 8 in the furnace 6. The test specimen may be a steel cylinder that is coated with a Ni base alloy, wherein the Ni base alloy is in turn coated with a ceramic layer made of YSZ applied by means of thermal spraying.

(14) The gas discharge line 11 leads into a first container 13, in which a drying agent is received. The dried waste gas is transferred from the first container 13 via a second gas discharge line 14 into a second container 15, in which a lye is received. The dried and neutralised waste gas is discharged via a waste gas line 16.

(15) FIG. 2 shows the solubility of ZrO.sub.2 depending on the temperature and depending on the composition of the gas phase. The measurement results indicated by squares show the solubility of ZrO.sub.2 depending on the temperature and in the presence of an equimolar salt melt 8 made of KClK.sub.2SO.sub.4ZnCl.sub.2ZnSO.sub.4, wherein N.sub.2 with an addition of 2% by weight HCl has been used as carrier gas (=model system). As can be seen from FIG. 2, ZrO.sub.2 has a maximum solubility at a temperature of the melt of 700 C. By contrast, the measurement result indicated by a triangle shows that ZrO.sub.2 hardly dissolves in the salt melt with an omission of HCl in the carrier gas or an omission of sulphate salts.

(16) To determine the solubility of ZrO.sub.2, test specimen bodies formed from YSZ were each treated in a predefined quantity of the salt melt 8 for 72 hours at the temperature specified in each case. The salt melt 8 was then analysed quantitatively by means of ICPMS.

(17) FIG. 3 shows an image recorded by SEM of a cross section through a test specimen treated in accordance with the invention. Reference sign 17 denotes a conventional Ni base alloy, for example alloy 625. The Ni base alloy contains 20 to 23% by weight Cr and 8 to 10% by weight molybdenum and, as further constituents, tantalum in particular. Reference sign 18 denotes a ceramic layer that is produced from YSZ. An intermediate layer 19 is arranged between the Ni base alloy 17 and the ceramic layer 18 and here has a thickness from approximately 1.0 to 3.0 m. The intermediate layer 19 is the result of the proposed solvothermal treatment of the test specimen. According to initial findings, it basically contains chromium oxides, possibly also proportions of chromium sulphides. If the ceramic layer as further main constituent also contains Al.sub.2O.sub.3 besides ZrO.sub.2, Al.sub.2O.sub.3 is then probably also contained in the intermediate layer 19 besides ZrO.sub.2.

(18) FIG. 4 shows a schematic cross section through an object that forms a substrate 20. The substrate 20 can be produced from steel, for example. The substrate 20 can be coated at least in portions with the Ni base alloy 17, which is in turn covered by the ceramic layer 18. The intermediate layer 19 is formed between the Ni base alloy 17 and the ceramic layer 18 as a result of the solvothermal treatment according to the invention of the test specimen.

(19) As a result of the solvothermal treatment of the ceramic layer and the rearrangement processes caused thereby, the porosity of said layer decreases in the direction of the intermediate layer 19. The table below shows the dependency of the densification rate in the region of the ceramic layer 18 on temperature, HCl content in the gas phase and sulphate proportion in the salt melt 8, wherein the system KClK.sub.2SO.sub.4ZnCl.sub.2ZnSO.sub.4 was used as salt melt and N.sub.2 was used as carrier gas:

(20) TABLE-US-00001 Temperature ( C.) 500 600 700 700 700 700 HCl 2% 2% 2% 4% 8% 2% proportion Sulphate 50% 50% 50% 50% 50% 44% proportion Densifi- <5 m/d <5 m/d 130 m/d <10 m/d <5 m/d 40 m/d cation rate

(21) As can be seen from the table, particularly high recrystallisation takes place in particular at a temperature of 700 C., with an HCl content of 2% by weight and a sulphate proportion of 50 mol %, that is to say an equimolar salt melt. The densification rate or the growth rate of the densification zone in the ceramic layer is particularly high here at 130 m/d.

(22) FIG. 5 shows the results of measurements of the adhesiveness of a ceramic layer made of YSZ which has been applied by means of plasma spraying to the Ni base alloy, alloy 625. In the case of the solvothermal treatment of the ceramic layer, an intermediate layer 19 with a thickness of approximately 1.0 m has formed. As can be seen from the results, the adhesiveness of the solvothermally treated test specimen is approximately twice that of the conventional test specimen.

(23) FIGS. 6a, 6b and 7 show the results of the thermal cycle stability of the aforementioned test specimens. The thermal cycle stability has been determined by means of SEM images of the surfaces. FIG. 6a shows the surface of a solvothermally treated test specimen prior to the start of the thermal load change cycles. FIG. 6b shows the same surface after 20 thermal load change cycles. As can be seen in particular from FIG. 7, flaking of the ceramic layer is observed with untreated test specimens after just 2 thermal load change cycles. With the present tests, the thermal cycle stability was defined as the moment in time at which 20% of the ceramic layer exhibited flaking. As can be seen further from FIG. 7, some of the solvothermally treated test specimens demonstrate a drastically improved thermal shock resistance compared with the untreated test specimen.

(24) FIG. 8 shows the tribological properties of a conventional test specimen and a solvothermally treated test specimen, of which the ceramic layer was again produced from YSZ. The measurement results shown in FIG. 8 were measured by means of a ball-on-disc tribometer in a three ball on disc test. As can be seen from FIG. 8, the test specimens treated in accordance with the invention demonstrate a coefficient of friction that is reduced by a factor of 3.

(25) FIGS. 9 and 10 concern results of tests on further test specimens, in which the ceramic layer was produced in each case from an equimolar mixture of YSZ and Al.sub.2O.sub.3. The ceramic layer was in turn applied by means of plasma spraying to a substrate made of a Ni base alloy, alloy 625. The solvothermal post-treatment was again performed with use of the model system described with reference to FIG. 2 at a temperature of 700 C.

(26) As can be seen from FIG. 9, the solvothermally treated test specimens demonstrate a considerably improved Vickers microhardness. It can be seen from FIG. 10 that the solvothermally treated test specimens additionally have a drastically reduced porosity.

(27) The reduction of porosity occurs with the solvothermally treated test specimens since YSZ and/or Al.sub.2O.sub.3 dissolve as a result of the action of the gas phase and diffuse in the direction of the interface formed by the Ni base alloy. There, recrystallisation of the dissolved ceramic phase takes place, whereby in particular the pore space of the ceramic layer in the region of the interface is filled. The solvothermally treated test specimens thus are not characterised just by the formation of an intermediate layer between the Ni base alloy and the ceramic layer, but also by a porosity within the ceramic layer decreasing from the layer surface of the ceramic layer in the direction of the interface. Conventional layers produced by means of thermal spraying generally have a porosity in the region of 9%. By contrast, solvothermally treated ceramic layers have a drastically reduced porosity in the range from 3 to 5.50. The specified porosities relate here again to results obtained by means of image evaluation on a micrograph.

LIST OF REFERENCE SIGNS

(28) 1 nitrogen gas reservoir 2 first gas control valve 3 gas feed line 4 HCl gas reservoir 5 second gas control valve 6 furnace 7 container 8 salt melt 9 gas inlet 10 gas outlet 11 gas discharge line 12 test specimen 13 first container 14 further gas discharge line 15 second container 16 waste gas line 17 Ni base alloy 18 ceramic layer 19 intermediate layer 20 substrate