EP3023521A1

EP3023521A1 - Metallic substrate with ceramic coating and method for obtaining it

Info

Publication number: EP3023521A1
Application number: EP13792712.5A
Authority: EP
Inventors: Juan Carlos Merino Senovilla; José Antonio MAROTO SOTO; Ana RIVAS SALMÓN; Daniel MORIÑIGO SOTELO; Raúl ARRABAL DURÁN; Endzhe Matykina; Angel PARDO GUTIÉRREZ DEL CID; Ma. Concepción MERINO CASALS
Original assignee: Fundacion Cidaut
Current assignee: Fundacion Cidaut
Priority date: 2013-07-19
Filing date: 2013-07-19
Publication date: 2016-05-25
Also published as: US20160153112A1; WO2015007924A1

Abstract

Metallic substrate with a ceramic coating obtained by plasma electrolytic oxidation, resistant to degradation by tribocorrosion of light metals and their alloys, in a liquid and/or semi-solid state with a thickness from 10 to 300 µm and resistant to immersion in said light metals and their alloys without compositional modification of the substrate. Method for obtaining a metallic substrate with a ceramic coating by plasma electrolytic oxidation in which said substrate comprises a core and an outer layer of a metal different from that of the core or of an intermetallic compound, said outer layer being obtained by thermal spraying, laser, solid or liquid diffusion, hot galvanising or cementing. This allows obtaining a ceramic coating applicable to any metallic substrate and resistant to degradation by tribocorrosion.

Description

OBJECT OF THE INVENTION

The present invention lies in the field of electrolytic coatings.
The invention relates to a metallic substrate with a ceramic coating obtained by plasma electrolytic oxidation, resistant to degradation by tribocorrosion of light metals and their alloys, in a liquid and/or semisolid state with a thickness from 10 to 300 µm and resistant to immersion in said light metals and their alloys without compositional modification of the substrate.
The invention also relates to a method for obtaining a metallic substrate with a ceramic coating by plasma electrolytic oxidation wherein said substrate comprises a metallic core and an outer layer of a metal different from that of the core or an intermetallic compound, said outer layer being obtained by thermal spraying, laser, diffusion, hot galvanising or cementing.

BACKGROUND OF THE INVENTION

There exist numerous applications in the casting industry, specifically for light alloys such as those of aluminium, in which the material in a molten or semisolid state is in contact with a support metal that suffers tribocorrosion, this is, a degradation due to a combination of corrosion and wear, that renders it unusable in a short or medium term.
Examples of applications in which a metal suffers the effect of molten aluminium are rotors, refractory oven walls, casings of immersion heater resistors, pistons and injection chambers of metals and manufacturing of compounds with a metallic or polymer matrix.
Consequently, there is a need to provide the metallic materials with a coating that withstands the aforementioned degradation due to tribocorrosion.
The use is known of ceramic materials in relation to molten metals, such as in heaters and thermocouple sleeves. These materials are very expensive and are highly inefficient in transferring heat due to the air present between the heating and measuring elements and the tube, as well as to their very low heat conductivity. However, their greatest drawback is the highly fragile nature and lack of tenacity of ceramic materials.
Ceramic coatings are known that are attached to the part to protect which partially resist degradation by the use of relatively large thickness and for a relatively short period; in addition, de to the technique used in their manufacture they cannot be used in parts with complex shapes, so that their use in areas with orifices, notches, lips, etc. is entirely excluded and they are recommended only for exposed surfaces and relatively small dimensions. Another of their drawbacks is the difficulty in controlling the homogeneity of the layer thickness throughout their extension, exhibiting high porosity and thickness variation.
Patent EP1231299 discloses a coating on a nonferrous substrate in which a series of functional compounds are introduced on a porous starting layer.
The drawback of this coating is that it does not allow using ferrous substrates, which are common in the industry, as said ferrous substrates have outstanding mechanical performance and a low cost, instead using as substrate alloys of light metals (Al, Mg,...) which cannot be used in accordance with the invention of said patent, as the core of the coating would degrade when subjected to temperatures very near its melting point. In addition, this process is directly intended to achieve an open surface porosity that ensures the possibility of introducing lubricating elements. This porosity, in some cases greater than 30%, would be deleterious for use with molten light metals, as it would enhance the corrosion process by allowing the molten metal to penetrate to the core.
Patent US20120177837 discloses a coating on a metallic substrate made from ceramic powder mixed with metal, this mixture being applied on said substrate.
The drawback of this coating is that it is intended for manufacturing an immersion resistor that is submerged in a static metal bath and has a very simple shape. The technique used to coat the metallic substrate with a ceramic powder having a complex formulation is quite costly due to its complexity, and in addition it cannot be used for complex shapes. It would not be possible to use it for a great number of components and shapes that are currently used in the casting industry, so that its application is limited exclusively to that for which it was designed, namely an immersion heater.
As shown above, the form in which the coating is obtained affects its properties, this is, the method used to obtain the coating is related to the properties that it will have.
To overcome the aforementioned drawbacks of the state of the art, the following invention is proposed for a metallic substrate with a ceramic coating and the method for obtaining it.

DESCRIPTION OF THE INVENTION

The present invention is established and characterised in the independent claims, while the dependent claims describe additional characteristics thereof.
The subject matter of the invention is a metallic substrate with a ceramic coating, wherein the metallic substrate is of any type and the coating is resistant to degradation by tribocorrosion of light metals and their alloys, in liquid and/or semi-solid state, applicable to any part of any size, however small, and to any place of the part, however internal. The technical problem to solve is obtaining said metallic substrate with a ceramic coating having ideal properties regarding tribocorrosion of light metals and their alloys, in liquid and/or semi-solid state, that allows implementing it in any type of part.
In view of the above, the present invention relates to a metallic substrate with a ceramic coating obtained by plasma electrolytic oxidation, resistant to degradation by tribocorrosion of light metals and their alloys, in a liquid and/or semi-solid state with a thickness from 10 to 300 µm and resistant to immersion in said light metals and their alloys without compositional modification of the substrate. Particularly, it is verified that a specific time value for the resistance to immersion of the coating 81) is 1000 hours.
Optionally, the metallic substrate is a single metal, which can be titanium or zirconium.
In another option the metallic substrate comprises a metallic core and an outer layer of a metal different from that of the core or an intermetallic compound, wherein the core can be made of steel, nickel, titanium, zirconium, refractory metals (Cr, Co, Nb, Mo, W,...) or any of the alloys thereof, and the outer layer is made of an intermetallic compound of titanium and aluminium. The outer layer of a metal different from that of the core is normally obtained by thermal spraying, laser, hot galvanising or cementing, while the outer layer of an intermetallic compound is normally obtained by spraying or solid or liquid diffusion.
The ceramic coating on any substrate has been verified to present a hardness from 100 to 2000 HV, an adherence from 2 to 30 MPa, and a mean roughness, Ra, from 1 to 5 µm. Optionally, these parameters can be improved by physical operations such as polishing, machining, etc. as the applied coating withstands physical operations applied to it.
The coating provides the assembly with the substrate with the advantages of tenacity, hot mechanical strength, resistance to thermal shock and the conductivity of a metallic material.
Another advantage is that the substrate with the coating can replace ceramic materials, which are fragile and expensive, in direct contact with the molten metal that withstands tribocorrosion.
A further advantage is that the substrate is reusable, and the ceramic coating can be generated on it as many times as needed, preventing wasting a large amount of metallic material and making the process recyclable and low in cost.
The invention also relates to a method for obtaining a metallic substrate with a ceramic coating by plasma electrolytic oxidation wherein said substrate comprises a core and an outer layer of a metal different from that of the core or an intermetallic compound, said outer layer being obtained by thermal spraying, laser, solid or liquid diffusion, hot galvanising or cementing.
Preferably, the thermal spraying, laser, hot galvanising and cementing is performed on a core when the coating is obtained as an outer layer of a metal different from that of the core, while spraying and solid or liquid diffusion is performed on a core when the coating is obtained as an outer layer of an intermetallic compound.
One advantage of the method is that is allows obtaining a coating with a great resistance, particularly on steels, as it includes an intermediate metallic substrate or an intermetallic layer using known techniques that are therefore simple and relatively inexpensive.

DESCRIPTION OF THE DRAWINGS

This specification is supplemented with a set of drawings illustrating the preferred embodiment, which are never intended to limit the invention.

Figure 1 represents a SEM image of the coating after being subjected to a performance test in molten aluminium in static conditions, showing the coating layer on a metallic substrate which has not suffered any attack.
Figure 2 represents a SEM image of the coating after being subjected to a performance test for forced tribocorrosion in molten aluminium, showing the molten aluminium layer that bathed the coating and the coating layer on a metallic substrate which has not suffered any attack.
Figure 3 represents an assembly used for the forced tribocorrosion test in which a motor turns a shaft from which hangs the sample while it is introduced in a crucible filled with light metal in liquid or semisolid state, the shaft turning in any one of the two senses, clockwise or anticlockwise.
Figure 4 represents an assembly used for the forced tribocorrosion test in which a motor turns a shaft with a carrousel from which hang the samples while they are introduced in a crucible filled with light metal in liquid or semisolid state, the shaft alternatively turning in one sense and then the opposite sense, in order to increase the wear induced by the process.

PREFERRED EMBODIMENT OF THE INVENTION

To obtain the metallic substrate (2) with a ceramic coating (1) that is the subject matter of the invention, a surface preparation is performed for the metallic substrate (2) as follows:

Stripping in an acidic solution adapted to the base material being treated;
Working area delimited by resin;
Electrical contact with a copper or aluminium wire.

The plasma electrolytic oxidation (PEO) coating (1) is performed as follows:

a) Apparatus: power supply, control and data acquisition card;
b) Electrochemical cell: reactor with stirring in thermostatic sleeve, cathode AISI 316L ;
c) Aqueous alkaline electrolyte, specific for each of the metallic substrates (2) to be coated at a temperature of up to 60 °C;
d) Treatment (0-10000 s): pulsed signal frequency of 50-4000 Hz, current density 5-800 mA/cm.

The voltage records show values from 10 to 1000 V.
This allows obtaining a thickness for the coating (1) from 10 to 300 µm, according to measurements as per ISO 2360.
The coating (1) is subjected to multiple tests for resistance to tribocorrosion of light metals in liquid and semi-solid state, both in stationary conditions in which it is simply submerged in the metal and in dynamic conditions, using tests of 1000h in light alloy at different temperatures, preferably 590°C, 650°C and 750°C. Figure 1 shows an enlarged view of a coating (1) on its substrate (2) after being subjected to a performance test in molten aluminium in static conditions.
The dynamic conditions are obtained by stirring in axial mode, figure 3, and in carrousel, figure 4, showing that the coating (1) remains intact at the end of the test period, figure 2.
In the forced tribocorrosion test with stirring in axial mode, figure 3, a motor (4) turns a shaft (5) from which hangs the sample (6) while it is introduced in a crucible (7) filled with light metal (8) in liquid or semisolid state, the shaft (5) turning in any one of the two senses, clockwise or anticlockwise.
In the forced tribocorrosion test with stirring in carrousel mode, figure 4, the motor (4) turns a shaft (5) with a carrousel (9) from which hang the samples (6) while they are introduced in a crucible(7) filled with light metal (8) in liquid or semisolid state, the shaft (5) alternatively turning in one sense and then the opposite sense, in order to increase the wear induced by the process.
Multiple tests were carried out to characterize the material, changing the time of permanence inside the molten metal, increasing it (24 hours, 48 hours, 96 hours...) to show the resistance of the material without compositional variation of the substrate for at least 1000 hours.
Figure 2 represents an enlarge view of a coating (1) on its substrate (2) after being subjected to a performance test for forced tribocorrosion in molten aluminium, showing the molten aluminium layer (3) that bathed the coating (1), revealing that the coating (1) did not suffer any attack after 100 hours of testing.
The substrate (2) on which the coating (1) is obtained is made of a single metal or a core and an outer layer of a different metal than that of the core or an intermetallic compound, with similar test results for any of these configurations.
The coating (1) has a hardness from 100 to 2000 HV, depending to a great extent on the material of the substrate (2), as obtained from 10 measurements with a load of 10 g and a penetration time of 20 s.
An adherence from 2 to 30 MPa is obtained from 3 measurements with a dolly 10 mm in diameter, epoxy adhesive and a uniform traction tension increase of 1 MPa/s, as per ISO 4624.
The mean roughness, R_a, from 1 to 5 µm, and the maximum roughness, under 20 µm, are each obtained from 3 measurements with a roughness tester and 0.25 mm Gaussian filter, at a distance of 4 mm.
The coefficient of friction from 0.2 to 1" is obtained from 3 measurements using a ball-on-disk tribometer with a load of 2 N, distance 1000 m, 200 rpm (0.08 m/s), turning radius 4 mm, as per ASTM G99-04; as a countersample: WC ball with 6 mm diameter.

Claims

Metallic substrate with a ceramic coating obtained by plasma electrolytic oxidation, resistant to degradation by tribocorrosion of light metals and their alloys, in a liquid and/or semi-solid state, characterised in that said coating (1) has a thickness from 10 to 300 µm and is resistant to immersion in said light metals and their alloys without compositional modification of the substrate (2).
Metallic substrate with a ceramic coating according to claim 1, wherein the metallic substrate (2) is a single metal.
Metallic substrate with a ceramic coating according to claim 1, wherein the metal is titanium or zirconium.
Metallic substrate with a ceramic coating according to claim 1, wherein the metallic substrate (2) comprises a metallic core and an outer layer of a metal different from that of the core, said layer being obtained by thermal spraying, hot galvanising or cementing.
Metallic substrate with a ceramic coating according to claim 1, wherein the metallic substrate (2) comprises a metallic core and an outer layer of an intermetallic compound, said layer being obtained by spraying or by solid or liquid diffusion.
Metallic substrate with a ceramic coating according to claims 4 or 5, wherein the core is of steel, nickel, titanium, zirconium, refractory materials or any of the alloys thereof.
Metallic substrate with a ceramic coating according to claim 4, wherein the outer layer is of a light metal selected from among aluminium, magnesium, titanium and zirconium.
Metallic substrate with a ceramic coating according to claim 5, wherein the outer layer is of an intermetallic compound of titanium and aluminium.
Metallic substrate with a ceramic coating according to any of claims 1 to 8, wherein the coating (1) has a hardness from 100 to 2000 HV.
Metallic substrate with a ceramic coating according to any of claims 1 to 8, wherein the coating (1) has an adherence from 2 to 30 MPa.
Metallic substrate with a ceramic coating according to any of claims 1 to 8, wherein the coating (1) has a mean roughness, R_a, from 1 to 5 µm.
Method for obtaining a metallic substrate (2) with a ceramic coating (1) by plasma electrolytic oxidation characterised in that said substrate (2) comprises a core and an outer layer of a metal different from that of the core or of an intermetallic compound, said outer layer being obtained by thermal spraying, laser, solid or liquid diffusion, hot galvanising or cementing.
Method according to claim 12, wherein the thermal spraying, laser, hot galvanising and cementing is performed on a core when the coating (1) is obtained as an outer layer of a metal different from that of the core.
Method according to claim 12, wherein the spraying and solid or liquid diffusion is performed on a core when the coating (1) is obtained as an outer layer of an intermetallic compound.