EP0409687A1 - Method of silicifying metallic parts by chemical vapour deposition - Google Patents

Abstract

Method of improving the surface properties of steel articles, especially their surface hardness, according to which a steel article is prepared in the clean state; the article is heated in an oven under inert atmosphere to a predetermined silicidising temperature of between 800 DEG C and 1100 DEG C; a gaseous mixture of silane (SinH2n+2) and of argon is injected into the oven where the steel article maintained at this temperature is located, so as to form a diffusion layer containing 10 to 40% of silicon as atomic percentage at the surface of the steel article, the proportion by volume of silane in the gaseous mixture being preferably between 0.1 and 5%.

Description

The present invention relates to a method for improving the surface properties of metal parts by siliciding by means of chemical gas deposition.

The invention relates, more particularly, to a method of siliciding by chemical gas deposition for improving the surface properties, in particular the hardness and the corrosion resistance of steel parts.

In general, after their shaping, the metal parts need a surface treatment to meet the technical and economic requirements of their environment of use. Surface treatment makes it possible to improve the surface properties of metal parts, either by modifying the composition of a surface layer of these parts, or by depositing a layer of another material on the surface of these parts. .

Among many known methods for surface treatment, the present invention relates more particularly to the chemical gas deposition method (sometimes called "Chemical Vapor Deposition" or CVD), in particular for the siliciding of steel parts.

In the literature, one can find several methods of developing siliconized surface layers: ion implantation, sintering of silicon powders and the chosen metal, plasma spraying, electrolysis in molten salt bath and chemical deposition. in the gas phase.

The chemical gas deposition method for metal siliciding has many advantages. It makes it possible in particular to obtain a uniform deposit layer on the surface of the parts to be treated with relatively low treatment temperatures, and does not require a high vacuum. The other methods of siliciding known in the art have on the contrary a certain number of drawbacks, such as the high cost of production (ion implantation), the difficulties of implementation (electrolysis, ion implantation), the non-uniformity of the deposit ( ion implantation, projection of plasma) or too great thicknesses of deposition layers (projection of plasma, sintering), etc.

It is already known to carry out siliciding using SiCl₄ by the chemical gas deposition method with the aim of increasing the magnetic permeability and reducing the magnetostriction, and above all reducing the magnetic losses, of the steel sheets of transformers. . During siliciding, a silicon diffusion layer is formed on the surface of steel parts. The volatile chloride creates porosities in the silicided layer which are harmful in particular to the anti-corrosion properties of the parts. The possible addition of hydrogen to the flow of SiCl₄ has been envisaged to eliminate the possibility of the formation of iron chloride. But chemical reactions produce gaseous hydrochloric acid which can also attack the silicon diffusion layer by forming porosities there.

Also known, from patent application EP 0 226 130 (AIR PRODUCTS), is the siliciding of metals by chemical deposition in the gas phase using silane (SiH₄) in a hydrogen flow at temperatures in the region of 600 to 700 °. vs. The document describes a process of diffusion of silicon in the metal obtained by making a pretreatment of the substrates under a hydrogen atmosphere, controlling and minimizing the rate of water vapor in the oven. The silicon diffusion layer obtained improves the anti-corrosion properties at high temperature and inhibits the formation of coke during cracking operations of hydrocarbons.

Other work has been carried out to obtain passivating layers of silicide on the surface of metal parts, in particular iron, titanium and nickel, using silane.

The Applicant has now discovered, and this constitutes the inventive idea on which the present invention is based, that it was possible to obtain a substantial modification of the surface mechanical properties of steel parts by siliciding these parts in special conditions.

The object of the present invention is therefore to produce, under special conditions, a silicided layer of metals by chemical deposition in the gaseous phase of silane (Si _n H _{2n + 2} ) to improve the surface properties, in particular the mechanical properties of the parts. processed.

The method of the invention makes it possible in particular to improve the surface properties of steel parts, in particular for their surface hardness.

According to the invention, a steel part is prepared in the clean state; then the part is heated in an oven to a predetermined temperature T₀ between 800 ° and 1100 ° C; then a gaseous mixture containing an inert gas and a silane Si _n H _{2n + 2} is injected into the furnace where the steel part is kept at temperature T₀, so as to form a diffusion layer containing from 10% to 40% of silicon in atomic percentage on the surface of the steel part. The volume proportion of silane in the gas mixture is preferably less than 10%.

The siliciding temperature T₀ is advantageously greater than 850 ° C. In this case, after having been exposed to the temperature T₀, the part is preferably cooled to around 850 ° C., then quenched for example in an oil at room temperature. We can then perform a tempering of the part at a temperature of on the order of 550 ° to 600 ° C for about 30 to 60 minutes. The stages after siliciding can be modified according to the type of steel constituting the part to be treated.

Preferably, the steel part is maintained at the siliciding temperature T₀ for a period of between 0.5 and 40 hours depending on the value of T₀. The flow rate of the gas mixture per unit volume of the oven can vary between 1 and 10. The total pressure of the gas mixture is preferably less than 1000 Pascals.

The volume proportion of silane in the gas mixture is between 10 ppm and 5%, and is strongly influenced by the siliciding temperature T₀. Advantageously, this proportion can be chosen between 0.1% and 5%.

The gas mixture injected, during the step of maintaining the temperature T₀ at which the siliciding of the part takes place, preferably contains no other gaseous constituents than silane and argon. However, it may be advantageous in certain cases to add hydrogen to the gas mixture in volume proportions of less than 20% and / or of helium in volume proportions of more than 1%. Helium can also be used as the sole diluent for silane.

During the step of heating the steel part, the furnace is preferably filled with an inert gas, in particular argon. It is also possible to inject a small volume percentage of hydrogen of less than 20% in this inert gas. The total gas pressure is preferably less than 1000 Pascals, but can also reach atmospheric pressure.

The method of the invention is particularly suitable for treating carbon steel parts having a carbon content of less than 0.5%. The improvements concerning in particular the hardness and the corrosion resistance of the parts are particularly remarkable for this type of steel.

The preparation in the clean state of the part before its introduction into the oven is of great importance insofar as the part will be heated under an inert atmosphere, since the possible presence of oxides on the surface of the part to be treated constitutes a barrier to diffusion and can cause poor adhesion of the deposit with the substrate. Parts of pure iron, superalloy or stainless steel can also be treated by the method of the invention.

The possible addition of small quantities of hydrogen of less than 20% by volume in the inert gas, during the heating phase of the part, has the effect of creating a reducing atmosphere making it possible to remove the oxide layer which could possibly remain after cleaning the part, for example in an ultrasonic bath.

The pressure in the oven during the heating of the room can go up to atmospheric pressure. Advantageously, this pressure is controlled below 1000 Pascals. Under these conditions, the total flow rate of the gas or gases per unit volume of the oven during the heating phase is preferably chosen between 1 and 10.

During the siliciding phase, the part is maintained at the temperature T₀ and a gas mixture consisting of silane is injected having a volume proportion of between 100 ppm and 1%, preferably between 0.5 and 1%, diluted in a inert gas, for example argon. The siliciding time at temperature T₀ can advantageously be controlled between 2 and 10 hours.

The quality of the silicon diffusion layer obtained on the surface of the part after siliciding depends essentially on the composition and the surface condition of the substrate, and on the kinetics of siliciding, the main parameters of which are the temperature of the part. and the amount of silane present on the surface of the part.

To carry out in practice the siliciding of the part, maintained at the temperature T₀, the gas mixture containing the silane is therefore introduced inside the furnace, so as to bring about the contact between the gaseous atmosphere and the surface of the part in steel. It is then estimated that the following phenomena occur:
- adsorption of gaseous species, including silane, on the surface of the part;
- chemical reaction on the surface of the part consisting in part of the decomposition of the silane into silicon and hydrogen;
- diffusion of silicon in the steel part forming a silicon diffusion layer;
- desorption and diffusion of the volatile products formed.

It is interesting to note that the hydrogen released during the decomposition of silane allows a reduction of the oxygen possibly present in the environment thus avoiding the possibility of formation of an oxide layer on the surface of the part to be treat during siliciding.

The total pressure of the gas mixture during siliciding is preferably kept below 500 Pascals. Advantageously, monosilane (SiH₄) or disilane (Si₂H₆) is used for siliciding according to the process of the invention.

It is possible to carry out a surface oxidation of the silicided layer of the part by introducing an oxygen-rich gas after the siliciding phase and before carrying out any quenching of the part which can be followed by tempering.

• la figure 1 représente des profils de dureté des pièces siliciurées en fonction de la profondeur analysée; et
• la figure 2 représente des profils de dureté en fonction du pourcentage atomique de silicium contenu dans la couche de diffusion analysée.

The invention will be better understood on studying the detailed description of two exemplary embodiments of the invention taken without any limitation being implied and certain results of which are illustrated by the attached drawings, in which:

• FIG. 1 represents hardness profiles of the silicided parts as a function of the depth analyzed; and
• FIG. 2 represents hardness profiles as a function of the atomic percentage of silicon contained in the diffusion layer analyzed.

EXAMPLE 1

A piece of carbon steel type 42 CD 4 (0.41% C; 0.31% Si; 0.64% Mn; 0.94% Cr, 0.21% Mo) is formed. The surface of the part is degreased and deoxidized in an ultrasonic bath with acids and solvents. The part is then placed in a horizontal oven with hot walls which is then heated to 1000 ° C in argon having a pressure of less than 1000 Pascals. As soon as the oven has reached this temperature, the silane (SiH₄) diluted in argon (Ar) is injected into the oven maintained at a pressure of approximately 300 Pascals and at a temperature of 1000 ° C. The volume proportion of silane is approximately 0.5%. The total flow rate of the silane / argon mixture is of the order of 0.4 dm³ / minute. The reaction is allowed to proceed for 2 hours, then the temperature is lowered to 850 ° C, maintained for about 30 minutes, the part is quenched and tempered at 550 ° C for 1 hour.

The results obtained show a significant increase in surface hardness which varies from 330 HV (in VICKERS hardness) before treatment to around 500 HV after treatment, hardness tests being carried out with 200 g of filler.

EXAMPLE 2

42 CD 4 carbon steel samples are prepared in the form of pellets with a thickness of 2 mm and a diameter of 10 mm. The samples are cleaned in a liquid medium under ultrasound using Branson (registered trademark) product, so as to degrease and deoxidize the surface of the samples.

The samples are then placed in a tubular oven which is then heated to the siliciding temperature T₀ above 850 ° C., under argon at 500 Pascals. When the temperature T₀ is reached, the samples are maintained at this temperature while a gas mixture consisting of 2.5% K₂, 0.5% SiH₄, 9% He and 88% Ar is injected into the oven. total and the total pressure of the gas mixture are respectively 1 dm³ / minute and 300 Pascals. The siliciding time is equal to 2 hours.

The temperature is then lowered to 850 °, then the samples are tempered at 550 ° for half an hour. The samples thus treated are then analyzed using the usual observation devices, such as the scanning electron microscope, roughness meter, the X-ray analyzer, the AUGER spectroscope and the microdurometer.

Figures 1 and 2 show hardness profiles respectively as a function of the depth of the silicon layer of the sample and the atomic percentage of silicon (% Si) contained in the layer. Only the hardness profiles obtained according to Example 2 and corresponding to two siliciding temperatures T₀ = 1000 ° C and T₀ = 1100 ° C are shown in the figures.

As illustrated in FIG. 1, for the siliciding temperature T₀ equal to 1000 ° C., the hardness decreases rapidly from the surface of the silicided layer to a depth of approximately 100 μm inside this layer. Then the hardness becomes almost constant when the analyzed depth exceeds 100 µm.

In the case where the siliciding temperature T₀ is equal to 1100 ° C., the hardness profile differs from that obtained for T₀ = 1000 ° C. As the analyzed depth of the silicide diffusion layer increases, the hardness decreases relatively slowly from the surface of the layer to around 250 µm. Then the hardness decreases suddenly between 250 µm and 300 µm deep to reach a plateau from 300 µm deep.

It can be seen that the siliciding depth is greater when the siliciding temperature T₀ is greater, which can be interpreted by the increasing speed of diffusion of the species as a function of the temperature. It can be considered that the siliciding depth corresponds to the thickness of the silicon diffusion layer in the iron. The hardness profile as a function of the depth shows that this silicon diffusion layer has a thickness of the order of 100 μm for the siliciding temperature T₀ = 1000 ° C., and of the order of 250 μm for the temperature of siliciding T₀ = 1100 ° C.

The starting steel 42 CD 4 has an average hardness of the order of 330 HV (in VICKERS hardness). After siliciding, the hardness of the samples can reach around 475 HV for T₀ = 1000 ° C to 525 Hv for T₀ = 1100 ° C at the surface of the silicon diffusion layer obtained according to Example 2 of the invention. This significantly improves the hardness of the surface of the samples.

FIG. 2 shows the profile of the hardness at 25 μm deep in the silicon diffusion layer as a function of the atomic percentage of silicon at this depth. On the two curves shown corresponding to siliciding temperatures respectively equal to 1000 ° C and 1100 ° C, a hardness is observed almost constant for the silicon content between 0% and 10 atomic%. From 10% of Silicon in the siliconized layer, the hardness increases very quickly depending on the silicon content in the layer. The two two curves are almost identical for the silicon content of less than 15%. When the silicon content goes beyond 15 atomic%, the hardness curve corresponding to T₀ = 1000 ° C experiences less marked growth than the curve corresponding to T₀ = 1100 ° C. In FIG. 2, a significant increase in hardness is observed for a percentage of silicon greater than 10%. By comparing the two hardness curves obtained at T₀ = 1000 ° C and T₀ = 1100 ° C, we can consider, in a first approximation, that the hardness for an atomic silicon value of less than 15% is independent of the siliciding temperature T₀.

Furthermore, it is known that when the silicon content in the diffusion layer reaches 10 atomic%, a dense and passivating layer is obtained allowing good protection against corrosion. In the present case, obtaining a significant increase in hardness simultaneously implies a silicon content in the layer greater than 10% and therefore makes it possible to improve the anti-corrosion properties of the steel surface.

It is further observed that when the concentration of silicon is greater than 40%, the silicon diffusion layer certainly has a very high hardness but becomes brittle and the layer cracks.

In general, the method of the invention aims to obtain a percentage of silicon in the diffusion layer of between 10% and 40 atomic%. Preferably, this percentage varies between 15% and 30%.

We can therefore emphasize the importance of the process of the invention for improving the surface hardness as well as the anti-corrosion and anti-wear properties of steel parts.

Of course, the present invention is not limited to the only method of chemical deposition in the gas phase by low pressure thermal heating as described above. Other derived methods, such as chemical pressure gas deposition atmospheric or plasma or laser assisted, can be used. Other heating means, for example induction heating of the room, can be used. However, chemical gas deposition by low pressure thermal heating is more easily adaptable for use on an industrial scale.

The process of the invention may undergo slight modifications to allow the improvement of other surface properties of metals, in particular their resistance to abrasion, the obtaining of a layer of high magnetic permeability, the creation of an interface. silicided on the metal allowing easy attachment of ceramic parts or deposits, obtaining an inert layer for certain chemical reactions.

The process could be further improved by multilayer deposits having complementary properties, such as for example an anti-corrosion silicide layer and a lubricating iron sulfide layer. It is also possible to produce, using the process of the invention, composite layers of silicide and iron nitride to increase the hardness or to adapt the composition of the surface to promote ceramic-metal adhesion and for the case of silicon-based ceramics.

The metal parts obtained by the present process can be used in particular in the mechanical industries and the steel transformation industries for the improvement of surface properties with respect to corrosion, hardness, resistance to abrasion, passivation, adhesion, magnetism, etc.

Claims (6)

1. A method of improving the surface properties of steel parts, in particular their surface hardness, comprising the following steps: a steel part is prepared in a clean state; the part is heated in an oven to a predetermined temperature (T₀) below 1100 ° C; a gaseous mixture containing an inert gas and a silane of formula Si _n H _{2n + 2} with a silane content of less than 10% is injected into the furnace where the steel part is kept at temperature T₀, characterized in that operates at a temperature above 800 ° C to form a diffusion layer containing 10% to 40% silicon in atomic percentage on the surface of the steel part. 2. Method according to claim 1, characterized in that the part is maintained at temperature T₀ for a period of between 0.5 and 40 hours, that the flow rate of the gaseous mixture per unit volume of the oven is between 1 and 10 and that the total pressure of the gas mixture is less than 1000 Pascals. 3. Method according to claim 1, characterized in that the duration of maintaining the part at the temperature T₀ is between 2 and 10 hours. 4. Method according to any one of the preceding claims, characterized in that the volume content of silane is greater than 0.1%. 5. Method according to any one of claims 1 to 4, characterized in that the gas mixture injected during the step of maintaining the temperature (T₀) further contains hydrogen having a volume proportion of less than 20% and / or helium having a volume proportion greater than 1%. 6. Method according to any one of claims 1 to 5, applied to the treatment of carbon steel having a carbon content of less than 0.5%, characterized in that the part is made of steel containing about 0.4% of carbon and the siliciding temperature T₀ is greater than 1000 ° C, so as to form on the surface of the part a silicon diffusion layer with a thickness between 100 µm and 300 µm and with silicon content varying between 15% and 30 atomic%.

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