WO2000015869A1

WO2000015869A1 - Part based on aluminium coated with amorphous hard carbon

Info

Publication number: WO2000015869A1
Application number: PCT/FR1999/002158
Authority: WO
Inventors: Caroline Berhault; Frédéric Schuster
Original assignee: Commissariat A L'energie Atomique
Priority date: 1998-09-11
Filing date: 1999-09-10
Publication date: 2000-03-23

Abstract

The invention concerns a part based on aluminium coated with amorphous hard carbon. Said part comprises a coating with a gradient of hardness containing a base layer in Al2O3 or Al (O, N), intermediate layers based on Si such as SiOx and SiC, or Si (O, N) and Si (C, N), and a top layer of hydrogenated and optionally nitrogenated amorphous hard carbon.

Description

ALUMINUM PART COATED WITH AMORPHOUS HARD CARBON

DESCRIPTION

Technical area

The present invention relates to aluminum or aluminum alloy parts provided with an amorphous hard carbon coating.

Such coatings make it possible to improve the mechanical and tribological properties of aluminum and its alloys, in particular their hardness.

Parts provided with such coatings can be used in a wide variety of fields where the properties of lightness, malleability, power of heat dissipation, and recycling of aluminum are of great interest.

For example, parts of this type can be used in engine blocks and cylinder heads, synchronization parts, piston skirts, oil pump parts, plastic injection molds, suspension and cylinder housings.

State of the art

Aluminum and its alloys are materials used in a wide variety of fields due to the special properties of aluminum.

However, these materials are corrodible and do not have good mechanical and tribological properties; in in particular, they have a low hardness, which makes it necessary to resort to a surface treatment of these parts.

Among the coatings envisaged for parts of this type, amorphous hard carbon (Diamond like Carbon) is of great interest because it has mechanical, electrical and thermal properties similar to that of diamond. This carbon exists only in the form of a thin layer because of high residual stresses in the film which limit its thickness. It has the advantageous properties of having a low coefficient of friction, low wear, great hardness and good abrasion resistance, and of being in the form of a relatively flexible layer.

As amorphous hard carbon coatings are very hard, they are difficult to deposit on soft substrates such as aluminum or aluminum alloy parts. It would therefore be of great interest to produce hardness gradient coatings on such substrates.

The subject of the present invention is precisely parts made of aluminum or aluminum alloy, provided with such a coating, thanks to the presence of one or more intermediate layers between the base part and the coating of amorphous hard carbon, which ensure the adhesion of the amorphous hard carbon film.

Statement of the invention

According to the invention, the part made of aluminum or aluminum alloy comprises a coating, the hardness increases from the surface of the part to the upper surface of the coating, said coating comprising an upper layer of amorphous hard carbon, two intermediate layers based on silicon including a layer of SiO _x with x ranging from 1 to 2 and a layer of SiC, the layer of SiC being adjacent to the upper layer of the coating, and a lower layer of A1 ₂ 0 ₃ between the surface of the part and the layer of SiO _x . In this part, the intermediate layer or layers of the coating make it possible to form a hardness gradient which ensures the adhesion of the amorphous carbon film to the aluminum or aluminum alloy base. According to an alternative embodiment of the invention, the part made of aluminum or aluminum alloy comprises a coating whose hardness increases from the surface of the part to the upper surface of the coating, said coating comprising an upper layer of carbon. hydrogenated and nitrogenous amorphous hard, two intermediate silicon-based layers including a layer of Si (0, N) and a layer of Si (C, N), the layer of Si (C, N) being adjacent to the upper layer of the coating, and a lower layer of A1 (0, N) between the surface of the part and the layer of Si (0, N).

In this variant, the formulas Si (0, N), Si (C, N) and A1 (0, N) relate respectively to the compounds SiO _x , SiC and A1 ₂ 0 ₃ , and mean that, in these compounds, 5 at 50% of the oxygen or carbon atoms can be replaced by nitrogen atoms.

According to the invention, the total thickness of the coating is advantageously from 2 to 4 μm. Such hardness gradient coatings can be obtained by subjecting the aluminum or aluminum alloy substrate to a two-stage surface treatment. In a first step, a diffusion layer is produced by ionic oxynitriding or oxidation processes. Through this, diffusion layers of Al ₂ 0 ₃ and A1 (0, N) are obtained respectively. These layers have the particularity of already being harder than the substrate and allow a smooth transition in terms of hardness.

Following this ion diffusion treatment, a vapor deposition process assisted by cold plasma (PACVD or PVD) is carried out with a composition and hardness gradient coating.

On the substrate side, the coating is of SiO _x type when the diffusion layer is Al ₂ 0 ₃ and Si (N, 0) when it is A1 (0, N).

The intermediate layer is of SiC: H type. The latter promotes the attachment of the upper layer of DLC (hydrogenated amorphous carbon) or DLN (hydrogenated amorphous carbon doped with nitrogen).

This last layer of DLC (or DLN) gives the assembly a high hardness as well as a very low coefficient of friction.

The alumina diffusion layer can be formed on the substrate by ionic oxidation from an oxygen plasma. The diffusion layer in A1 (0, N) can be formed in the same way by oxynitriding by means of a plasma of oxygen and nitrogen.

The intermediate layers are then produced by conventional PACVD techniques using from a gas mixture comprising an organic compound of silicon, oxygen and optionally nitrogen.

The upper layers of amorphous hard carbon can be formed by the PECVD or PACVD technique described in Surface and Coatings Technology 98, 1998, pp. 976-984 [1].

The advantage of the plasma-assisted vapor deposition technique has the advantage of allowing the deposition of an amorphous hard carbon layer on soft substrates by forming a hardness gradient coating, which improves the mechanical and tribological properties of aluminum or aluminum alloy parts. Thus, according to the invention, an amorphous hard carbon coating can be produced on aluminum or aluminum alloy parts, of various dimensions and geometry.

With this deposition technique, the upper layer based on amorphous hard carbon can be deposited from a gas mixture comprising a hydrocarbon and hydrogen.

According to an alternative embodiment, the gas mixture used for this deposition further comprises nitrogen, and a layer of hard hydrogenated and nitrogenous amorphous carbon is thus obtained.

The hydrocarbon used can be any hydrocarbon which can be used to form a carbon deposit. Preferably, methane diluted with hydrogen, and optionally nitrogen, is used.

When the upper layer is a layer of hard hydrogenated carbon obtained from a mixture of hydrocarbon and hydrogen, the layers silicon-based intermediates, can consist of a first layer of SiO _x with x ranging from 1 to 2, which is deposited from a gaseous mixture comprising an organic compound of silicon and oxygen, and by a second layer of SiC which is deposited from a gas mixture comprising an organic compound of silicon, argon and hydrogen.

In the case where the upper layer is a layer of hard hydrogenated and nitrogenous carbon, the intermediate layers based on silicon are a first layer of Si (0, N) which is deposited from a gaseous mixture comprising a compound organic silicon, oxygen and nitrogen, and a second layer of Si (C, N) which is deposited from a gaseous mixture comprising an organic compound of silicon, nitrogen, argon and hydrogen.

The organic silicon compound used for these deposits can be any organic compound capable of reacting in the plasma to form an oxide or a silicon carbide. Preferably, tetramethylsilane (TMS) is used.

Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of illustration and not limitation, with reference to the appended drawing.

Brief description of the drawing

Figure 1 shows schematically in vertical section a reactor suitable for the production of a coating according to the invention on large parts.

Detailed description of the embodiments

In FIG. 1, it can be seen that the deposition reactor comprises an earthed enclosure (1), inside which is a support (3) for the parts to be treated (5), this support being connected to a radio frequency generator (7). The support (3) can be a tray or a rod for hanging the parts to be treated.

The generator (7) comprises a system for adjusting the power and the frequency as well as a device for displaying the self-biasing voltage.

The gases necessary for deposits are introduced into the enclosure by a pipe (9) connected to a gas diffuser (11) located in the enclosure which may be cylindrical. The pipe (9) is connected to a gas supply system (13) which comprises at least 6 lines by which hydrogen, oxygen, nitrogen can be brought to the gas diffuser (11) , hydrogenated argon (volume ratio Ar: H ₂ of 9: 1), the hydrocarbon C _x H _y and the organic silicon compound. The enclosure is also connected to a pumping unit (15) at its lower part to achieve and maintain the desired pressure in one enclosure.

A control console not shown in the figure has pumping, gas flow and total pressure control devices, the pressure measurement being provided by a capacitive gauge. Each gas supply line is equipped an electronically regulated mass flow meter and a shut-off valve which are connected to the control cabinet fitted with a flow display and simultaneous adjustment device. All the steps of the process of the invention can be implemented in this reactor.

Thus, after having introduced the parts to be treated into the enclosure, it is possible first of all to carry out a pickling treatment using a plasma of inert gas, for example argon or a mixture of argon and hydrogen in a volume proportion of 9: 1, using a pressure low enough to allow a powerful ion bombardment, for example a polarization of -700 V. It is then possible to carry out in this enclosure the formation of a first layer d alumina or A1 (0, N) by means of an oxygen or oxygen and nitrogen plasma. Next, the silicon-based layers are deposited, still in the same enclosure, by modifying the gases introduced and the gas flow rates.

In the last step, the hard amorphous carbon film is formed, changing the gas source.

The following examples, given by way of non-limiting illustration, illustrate the process of the invention.

In these examples, methane is used for the deposition of the amorphous hard carbon layer and tetramethylsilane (TS) for the deposition of the intermediate layers based on Si. Example 1

In this example, a layer of amorphous hard carbon (DLC) having very good mechanical properties (friction, wear, hardness and elasticity) is deposited on a piece of aluminum and silicon alloy.

On this aluminum alloy substrate, ion oxidation is carried out in radio frequency plasma at 13.56 MHz. The processing conditions are as follows:

₃ Flow 0 ₂ : 50 cm / min.

Duration: 60 min Temperature: 250 ° C

2 Power density: lW / cm Pressure: 0.01 mbar

Next, the hardness gradient layer is produced by PACVD RF (13.56 MHz).

For SiOx / SiC / DLC. We use a gas mixture

TMS (tetramethyl silane) + oxygen at a pressure of 0.01 mbar and a power density of 0.5 / cm.

The incorporation of oxygen is gradually cut in order to arrive at the SiC: H composition, then, under the same electrical and pressure conditions, a gas mixture H ₂ + CH ₄ is sent to produce the final layer of DLC.

The total duration of this deposit phase is 3 hours. A coating of total thickness of between 2 and 4 μm is thus obtained with an average deposition rate of the order of 1 μm / h. The nature of the deposit is determined by infrared spectrometry. The thicknesses of the layers are determined by observation with a scanning electron microscope of metallographic section of the samples.

A tribological characterization is carried out by a pawn / disc type test. A coefficient of friction of 0.05 and a nanosecurity of 20,000 MPa are recorded.

The thicknesses of the layers are the following A1 ₂ 0 ₃ 0.2 to 0.5 μm SiO _x 0.2 to 0.5 μm SiC 0.3 to 0.5 μm Amorphous hard carbon (DLC): 1 to 3 μm. The hardnesses obtained are as follows: Substrate in aluminum alloy: 2000 to 2500 MPa layer in A1 ₂ 0 ₃ : 8000 MPa layer in SiO _x : 8000 MPa layer of DLC: 18 000 to 20 000 MPa.

Example 2

In this example, a layer of hard hydrogenated and nitrogenous amorphous carbon DLN (Diamond Like Nanocomposite) is formed on an aluminum alloy part. The three intermediate layers providing a hardness gradient in the coating in this case contain nitrogen.

On an aluminum alloy substrate, ion oxynitriding is carried out in radio frequency plasma at 13.56 MHz.

The processing conditions are as follows: Flow rate 0 ₂ : 50 cm / min. Flow N ₂ : 50 cm / min. Duration: 60 min Temperature: 250 C

2 Power density: lW / cm Pressure: 0.01 mbar

Next, the hardness gradient layer is produced by PACVD RF (13.56 MHz). For Si (0, N) / Si (C, N) DLN A gas mixture is used

TMS (tetramethyl silane) + oxygen + nitrogen at a pressure of 0.01 mbar and a power density of 0.5 W / cra ² .

The incorporation of oxygen is gradually cut in order to arrive at the Si (C, N) composition, then, under the same electrical and pressure conditions, a gas mixture H ₂ + CH ₄ is sent to make the final layer of DLN.

The total duration of this deposit phase is 3 hours.

Citation cited

[1]: Surface and Coatings Technology 98, 1998, pp. 976-984.

Claims

1. An aluminum or aluminum alloy part comprising a coating whose hardness increases from the surface of the part to the upper surface of the coating, said coating comprising an upper layer of amorphous hard carbon, two intermediate layers based on silicon including a layer of SiO _x with x ranging from 1 to 2 and a layer of SiC, the layer of SiC being adjacent to the upper layer of the coating, and a lower layer of A1 ₂ 0 ₃ between the surface of the part and the layer of SiO _x .

2. Part made of aluminum or aluminum alloy comprising a coating whose hardness increases from the surface of the part to the upper surface of the coating, said coating comprising an upper layer of amorphous hard hydrogenated and nitrogenous carbon, two intermediate layers based on silicon, including a layer of Si (0, N) and a layer of Si (C, N), the layer of Si (C, N) being adjacent to the upper layer of the coating, and a lower layer of A1 ( 0, N) between the surface of the part and the layer of Si (0, N).

3. Part according to any one of claims 1 and 2, wherein the total thickness of the coating is 2 to 4 microns.