FR2649995A1 - PROCESS FOR SILICIURATION OF STEEL BY CHEMICAL DEPOSITION IN THE GAS PHASE - Google Patents

Abstract

A process for improving the surface properties of steel parts, in particular their surface hardness, according to which a steel part is prepared in the clean state; the part is heated in an oven under an inert atmosphere up to a predetermined siliciding temperature of between 800 degree (s) C and 1100 degree (s) C; a gaseous mixture of Sin H2n & I1 A2 silane and argon is injected into the furnace where the steel part is kept at this temperature, so as to form a diffusion layer containing 10 to 40% silicon in atomic percentage at the surface of the steel part, the volume proportion of silane in the gas mixture preferably being between 0.1 and 5%.

Description

PROCESS FOR SILICIURATION OF STEELS BY CHEMICAL DEPOSITION IN THE GAS PHASE.

  The present invention relates to a method for improving the surface properties of metal parts by siliciding with

  by means of a chemical deposit in the gas phase.

  The invention relates, more particularly, to a process of siliciding by chemical dipSt in the gas phase for the improvement of Pruprifta Boperfilcielles, note the hardness and the

  corrosion resistance of steel parts.

  From one year to the next. after their ease in Form. metal parts need surface treatment to meet the technical and economic requirements of their environment of use. The surface treatment makes it possible to improve the surface properties of the metal parts, either by a modification of the composition of a surface layer of these parts, or by a depSt of a layer of another material to 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 C.V.D.), 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 depSt method in the gas phase 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 siliciding methods 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 inhomogeneity of the depst ( ion implantation, plasma projection) or excessively thick layers of depSt layers (plasma projection, sintering), etc. It is already known to carry out siliciding using SiC14 by the chemical depSt method in the gas phase 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

  particularly harmful to the anti-corrosion properties of parts.

  The possible addition of hydrogen to the SiC14 stream has been considered 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 there

porosities.

  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 ° C. The document reports on a silicon diffusion process in the metal obtained by pretreating the substrates under a hydrogen atmosphere,

  controlling and minimizing the rate of water vapor in the oven.

  The silicon diffusion layer obtained improves the properties of high tauprattre. It inhibits the formation of coke.

  during oil cracking operations.

  Other work has been done to obtain passivating layers of silicide on the surface of metal parts in particular

  in 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 carrying out siliciding of

  these parts under special conditions.

  The object of the present invention is therefore to produce, under special conditions, a silicided layer of metals by chemical deposition in a gaseous silane phase (SinH2n + 2) to improve the surface properties, in particular the mechanical properties of

parts processed.

  The petd6 of the ventiea nt ptyet ea latttceltey d'tmlioret the prap4xi4U Au exfgiels of steel parts, especially for their

superlIcial hardness.

  According to the invention, a steel part is prepared in the clean state; then the part is heated in an oven Up to a predetermined temperature TO between 800 and 1100 C; then a gas mixture containing an inert gas and a silane SinH2n + 2 is injected into the furnace where the steel part is kept at the temperature TO, 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 TO0 is advantageously greater than 850 ° C. In this case, after having been exposed to the temperature TO, the part is preferably cooled to around 850 C, then quenched, for example, in an oil at room temperature. One can then carry out an income of the part at a temperature of the order of 5500 to 600 C for approximately 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 TO for a period of between 0.5 and 40 hours depending on the value of TO. The flow rate of the gas mixture per unit volume of the oven can vary between 1 and 10. The total pressure of the mixture

  gaseous 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 does not contain any 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 diffusion and can cause poor adhesion of the deposit with the substrate. Pieces of pure iron, superalloy or stainless steel can also be treated by the process of the invention. The addition of small amounts of hydrofoil not less than% by volume in the inert gas, during the heating phase of the part, has the effect of creating a reducing atmosphere which makes it possible to remove the oxide layer which may remain. eventually

  after cleaning the part, for example in an ultrasonic bath.

  The pressure in the oven during space heating 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 TO and a gas mixture is injected consisting of silane having a volume proportion of between 100 ppm and 1%, of prefrem between 0.5 and 1Z, diluted in a gas. inert, for example argon. The siliciding time at temperature TO can be

  MweMtege.e 4eautr - "eatr 2 and 10 hea s.

  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 at the

surface of the room.

  To carry out in practice the siliciding of the part, maintained at the temperature TO, 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 treated 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 the

  siliciding according to the method 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 a coating.

  The invention will be better understood on studying the description

  details of two exemplary embodiments of the invention taken without any limitation being implied and certain results of which are illustrated by the appended 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 1000C in argon having a pressure below 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 around 300 Pascals and at a temperature of 1000 C. The volume proportion of silane is about 0.5%. The total flow rate of the silane / argon mixture em of the order of 0.4 dm3mli ,, e. The reaction is allowed to proceed for 2 hours, then the temperature is lowered to 850 ° C., it is maintained for approximately 30 minutes, the part is quenched and

an income at 550 C for 1 hour.

  The results obtained show a significant increase in surface hardness which varies from 330 H11V (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 1 mm. The samples are cleaned in a liquid medium under ultrasound using Branson (registered trademark) product, so as to

  degrease and oxidize the surface of the samples.

  L- -erti8tillots smnt placed in a tubular oven which is then cbau! Fî - the temperature of s.liciuratlon To above & SrC, so ar-wa at 500 S ZJacal. Loxqaz the temperature TO is reached, the samples are maintained at this temperature while a gas mixture consisting of 2.5% H2, 0.5% Si4, 9% Re and 88% Ar is injected into the oven. total and the total pressure of the

41 3

  gas mixture are respectively 1 dm / minute and 300 Pascals. The

  siliciding time is 2 hours.

  The temperature is then lowered to 850, then the samples are tempered to 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 microsurface.

  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

  TO = 1000 C and TO = 1100 C are shown in the figures.

  As illustrated in FIG. 1, for the siliciding temperature T0 equal to 10OO & C, the hardness rapidly de-rotates from the surface of the silicided layer to a depth of approximately / μm inside this layer. Then the hardness becomes almost

  constant when the depth analyzed exceeds 100 / µm.

  In the case where the siliciding temperature TO is equal to

  1100 C, the hardness profile differs from that obtained for TO = 1000 C.

  As the analyzed depth of the silicide diffusion layer increases, the hardness decreases relatively slowly from the surface of the layer to about 250 µm. Then the hardness suddenly decreases between 250 / um and 300 / um in depth to reach a

  plateau from 300 / µm deep.

  It can be seen that the depth of siliciding is greater when the siliciding temperature TO0 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 n 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 TO = 1000C at 525 lv for T0 = 1100 "C at the surface of the silicon diffusion layer obtained according to Example 2 of the invention. We thus improve

  significantly the hardness of the surface of the samples.

  FIG. 2 shows the profile of the hardness at 25 μm in depth in the silicon diffusion layer as a function of the atomic percentage of silicon at this depth. On the two curves shown corresponding to silicon temperatures respectively equal to 10000C and 11000C, an almost constant hardness is observed for the silicon content of between 0% and 10 atomic%. From 10% silicon in the siliconized layer, the hardness crott very quickly depending on the silicon content dbm the eoe.m. lm deu dau curves oaut practically confused for the silicon content lower than 15%. When the silicon content goes beyond 15 atomic%, the hardness curve corresponding to T - 1000'C experiences less marked growth than the curve corresponding to TO = 1100 C. In FIG. 2, a significant increase in the 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 temperature of

siliciding T0.

  Furthermore, it is known that when the silicon content in the diffusion layer reaches 10 atomic%, a dense layer is obtained.

  and passivating allowing good protection against corrosion.

  In the present case, obtaining a significant increase in hardness simultaneously implies a silicon content in the layer

  _périie-From to IZ and ptemt eouc d 'to & lloraz leas propzité "ani-

corrosion of the steel surface.

  It is further observed that when the concentration of silicon is greater than 40%, the silicon diffusion layer certainly presents

  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% and 40 atomic%. Preferably, this percentage varies between 15% and%. We can therefore emphasize the importance of the process of the invention for improving the surface hardness as well as

  anti-corrosion and anti-wear properties of steel parts.

  Of course, the present invention is not limited to the sole method of chemical deposition in the gas phase by low pressure thermal heating as described above. Other derived methods, such as gas phase pressure depSt

  atmospheric or plasma or laser assisted, can be used.

  Other heating means, for example induction heating of the room, can be used. However, chemical vapor deposition by low pressure thermal heating is more

  easily adaptable to use on an industrial scale.

  The process of the invention can undergo slight modifications to allow the improvement of other surface properties of metals, in particular their abrasion resistance, 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 some

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 adapt the composition of the surface to promote ceramic-etal adhesion and for the case

of ceramics based on silicon.

  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 (11)

  1. Method for improving the surface properties of steel parts, in particular their surface hardness, characterized by Js following steps: A steel part is prepared in a clean state; the part is heated in an oven to a * predetermined temperature (TO) between 800 and 1100 C; a gaseous mixture containing an inert gas and a silane of the formula SinH2n + 2 is injected into the furnace where the steel part is kept at the temperature To, so as to form a diffusion layer containing from 10% to 40% of silicon in atomic percentage at the surface of the steel part, the volume proportion of silane in the gas mixture being less than 10%.   2. Method according to claim 1, characterized in that the part is maintained at the temperature TO 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 heap 4 & 4m 4 / a @U c e i àU Pzata re T is understood between, 2 and 10 hit.   4. prtIde selI l'mDe galconque dfes revandiratiDns previous, characterized in that the volume proportion of   silane is between 0.1% and 5%.   5. Method according to any one of the claims   above, characterized in that the gaseous mixture injected during the temperature maintenance step (To) also 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 the claims   above, characterized in that during the heating step of said room, the oven is filled with argon which may contain hydrogen, the total pressure being less than or equal to the atmospheric pressure.   7. Method according to any one of the claims   above, characterized in that the part to be treated is in   carbon steel with a carbon content of less than 0.5%.   8. Method according to any one of the claims   above, characterized in that an oxygen-rich gas is injected into the furnace after having interrupted the injection of the gaseous mixture, so as to obtain a surface oxidation of the silicided layer. 9. Method according to claim 7 or 8, characterized in that the part is made of steel containing about 0.4% of carbon and that the siliciding temperature TO is greater than 1000 C, so as to form on the surface of the exhibits a silicon diffusion layer of thickness between 100 / μm and 300 / μm and of an an   silicon varying between 15% and 30 atomic%.   10. Method according to any one of the claims   above, characterized in that the silane used is the   monosilane (SiH4) or disilane (Si2H6).   11. Steel part whose surface properties, in particular hardness and resistance to wear, are improved according to the   process defined in any one of the preceding claims