METHOD FOR DEPOSITING AN ALUMINUM OXIDE COATING

Abstract

A method for depositing on a metal substrate a continuous coating of aluminum oxide by induction heating-assisted pressurized, temperature-controlled chemical deposition, the method including a solvothermal synthesis procedure based on an aluminum oxide precursor dissolved in a water-co-solvent mixture heated by induction to a temperature of between 400 C. and 700 C. and a pressure of between 1 MPa and 25 MPa.

Claims

1. A method for depositing on a metal substrate a continuous coating of aluminum oxide by means of induction heating-assisted pressurized, temperature-controlled chemical deposition, said method comprising a solvothermal synthesis step based on an aluminum oxide precursor dissolved in a water-co-solvent mixture heated by induction to a temperature of between 400 C. and 700 C. and a pressure of between 1 MPa and 25 MPa.

2. The method according to claim 1, comprising the following steps: aadding the metal substrate to be coated to an induction heating-assisted pressurized, temperature-controlled chemical deposition reactor; badding the aluminum oxide precursor previously dissolved in water and the co-solvent to the reactor; cimplementing the solvothermal synthesis based on said aluminum oxide precursor by means of induction heating at a temperature of between 400 C. and 700 C. and a pressure of between 1 MPa and 25 MPa of the aluminum oxide precursor dissolved in the water-co-solvent mixture; drecovering the substrate coated with a continuous coating of aluminum oxide.

3. The method according to claim 1, wherein the metal substrate is made of titanium alloy.

4. The method according to claim 1 to 3, wherein the co-solvent is selected from alcohols, nitrogen, carbon dioxide, argon and mixtures thereof.

5. The method according to claim 1, which is a semi-continuous method.

6. The method according to claim 1, wherein the aluminum oxide of the continuous coating is a metastable aluminum oxide, an alpha aluminum oxide or a mixture of these oxides.

7. The method according to claim 1, wherein the deposition speed is of between 100 and 500 nm/min.

8. The method according to claim 2, wherein the pressure of step c) is of between 6 MPa and 10 MPa.

9. The method according to claim 3, wherein the metal substrate is made of an alloy based on titanium aluminide.

10. The method according to claim 4, wherein the co-solvent is nitrogen.

11. The method according to claim 6, wherein the aluminum oxide of the continuous coating is an alpha aluminum oxide.

12. The method according to claim 7, wherein the deposition speed is of 300 nm/min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] FIG. 1 shows the partial diagram of a device according to the invention, without the metal assembly or the seals.

[0084] FIG. 2 schematically and partially shows a sectional view of a device according to the invention with the metal assembly and the seals.

[0085] The device 100 allows to deposit an aluminum oxide coating on a metal substrate 104. The device 100 comprises a cylindrical chamber 102 delimited by walls forming a closed volume V. The chamber 102 is adapted to receive a pressurized and heated fluid by means of a set of polymer seals 200, in particular located at the sapphire window 112 and the junction between the metal assembly 202 and the chamber 102, a metal assembly 202, in particular cylindrical, rigidly screwed together by metal columns 204, in particular 6 in number distributed equidistant from each other, in which circulates a fluid controlled in temperature at 20 C. by a cryostat 206, in particular the fluid being located above and below the cylindrical chamber, more particularly on either side of the sapphire window 112 and the inlet 120 and outlet 124. The metal assembly 202, the metal columns 204 and the screws being in particular made of 306L steel.

[0086] The device 100 also comprises an inlet 120 located in the lower portion of the chamber 102 to be able to introduce into the volume V a fluid which will be the co-solvent. It also comprises an inlet 116 located in the upper portion of the chamber 102 to be able to add water and precursor materials previously dissolved in water to this same volume V. The inlets 116 and 120 can be equipped with a pump 118 and 122.

[0087] An outlet 124 is also present in the device 100 to purge the volume V, and thus allow semi-continuous operation of the deposition device 100. The outlet 124 can be equipped with a pressure regulator 126.

[0088] A support 106 is positioned in the chamber 120 to support the metal substrate 104 on which the coating is deposited. Preferably, the support 106 is positioned in the chamber 120 so that the metal substrate 104 is held in the center of the inductor forming the induction heater 109. Preferably, the support 106 has a shape allowing to support the metal substrate 104 with a minimum of contact points in order to coat the largest possible surface of the metal substrate with the deposited aluminum oxide coating and to limit disturbances in the convection flow of the induction. The support 106 is composed of a material transparent to electromagnetic radiation. It is for example composed of a thermally and electrically non-conductive material such as alumina.

[0089] An induction heater 109 consisting of an induction generator 108 and an induction loop 110 surrounds the chamber 102. The induction heater allows to heat the metal substrate 104 while limiting the heating of the precursor materials present in the volume V.

[0090] In order not to disturb the induction heating of the metal substrate 104, the walls of the chamber 102 are transparent to electromagnetic radiation. They are for example made of ceramic. The ceramics used can be boron nitride, aluminum nitride, alumina or silicon nitride, more particularly silicon nitride. These examples of dense and non-porous ceramics allow the walls of the chamber 102 to have excellent mechanical strength and thus to withstand the pressures present in the volume V.

[0091] A sapphire window 112 is arranged on the upper portion of the chamber 102 and allows the temperature of the metal substrate 104 to be controlled using a bichromatic pyrometer 114 arranged outside the chamber 102.

EXAMPLE

[0092] The reactor used in the examples and as described above with reference to FIGS. 1 and 2 and consists of a cylindrical ceramic chamber made of silicon nitride Si3N4 with an internal volume of approximately 300 mL which contains the pressurized fluid and the metal substrate held by an alumina support. This ceramic being insensitive to magnetic fields, it is surrounded by an induction loop, itself connected to an induction generator with a maximum power of 7 kW. This allows to preferentially heat the metal substrate located at the center of the pressurized fluid. This ceramic is maintained pressurized by a set of polymer seals (Peek, Viton, EPDM and/or Kalrez) and a cylindrical metal assembly rigidly screwed together by 6 metal columns equidistant from each other made of 316 L steel. To avoid excessive deformation due to the increase in the temperature of this metal assembly, a fluid (ethylene glycol) controlled in temperature at 20 C. by means of a cryostat circulates therein. This holding assembly offers the possibility of pressurizing a fluid up to 25 MPa. Another improvement concerns the addition of tappings on the upper metal portion allowing the injection of fluids from the top while maintaining the location of a sapphire window. The latter allows to control the temperature of the metal substrate using a bichromatic pyrometer while keeping the injection of fluids and the precursor from the top of the reactor. The fluids and precursors are injected at a controlled flow rate using an HPLC pump for water and an Isco pump for the co-solvent (here nitrogen). The pressure is maintained by an outlet pressure regulator. This reactor operates in semi-continuous mode with a fixed substrate and fluids in continuous circulation.

[0093] A parallelepiped metal substrate of gamma-based titanium-aluminium with dimensions of 1.51.50.5 cm is therefore introduced into this reactor.

[0094] Deposition of a continuous coating of alpha aluminum oxide with water and nitrogen (as co-solvent) flow rates of 1.3 mL/min and 2 mL/min respectively, a pressure of 10 MPa and temperatures of 510 C., 630 C. and 700 C. were carried out on this substrate using aluminum nitrate Al(NO.sub.3).sub.3 as precursor material.

[0095] Using a grazing incidence X-ray diffractometer at an angle of 1 (or GIXRD for Grazing incidence angle XRD) on the coated substrate and analyzing the diffractogram obtained using EVA software, it is observed that at a temperature of 510 C., the deposition contains a mixture of metastable aluminum oxide, the kappa phase, and, above all, alpha aluminum oxide.

[0096] At a temperature of 630 C., the diffraction lines of an alpha aluminum oxide are more easily observed at 25.51 and 43.23, still with this metastable phase which is kappa aluminum oxide.

[0097] At 700 C., it is difficult to note the presence of alpha aluminum oxide. The diffraction lines of a new metastable phase, theta aluminum oxide, are clearly distinguished.

[0098] By observation with a scanning electron microscope of the cross-section produced by metallographic preparation of the coated substrate, it can be seen that the morphology of the depositions produced in a water/nitrogen mixture is homogeneous and consists of aggregates of hexagonal grains of a few hundred nanometers.

[0099] It is also observed that the cross-section of this coating made at 600 C. provides information on the morphology of the deposition in depth and also on its thickness. The deposition appears to have two types of structures going from the substrate to the outside, a relatively dense one over 2 m (the closest to the substrate) and a relatively porous one over 60 m. Furthermore, the average thickness is approximately 6111 m which allows to establish a deposition speed of approximately 300 nm.Math.min.sup.1. These two areas consist of aluminum oxides.

[0100] In conclusion, this induction heating-assisted pressurized and temperature-controlled chemical deposition method allowed to develop alpha and mixed aluminum oxide coatings at temperatures significantly lower than 850 C. at a pressure of between 1 MPa to 25 MPa on TiAl metal substrates of complex geometries.