EP0112206B1 - Method of coating metallic surfaces with carbides - Google Patents

Abstract

1. A process for forming a chemical coating in a vapour phase of halogenides, of the surface of metallic parts which must have a high hardness, resulting in the obtainment of monophase superficial layers of carbides of metallic elements of the following series : silicon, titanium, vanadium, chromium, zirconium, niobium, hafnium, tantalum and tungsten, this process, which does not comprise a prior ionic nitriding, being constituted by a gaseous cementation treatment of halogenides of at least one of the aforementioned metallic elements, at temperatures between 800 degrees and 1100 degrees C, during periods between 2 and 20 hours, characterised in that the halogenides of said metallic elements are obtained by using a cement maintained at a distance from the surface to be coated and comprising as sole constituents : at least one of said addition metallic elements, either in the form of ferro-alloy, or in the technically pure state in the metallic form ; an addition of ammonium chloride or fluoride in a proportion between 0.2 and 1.5% by weight of the total mass of the cement ; and an addition of powdered carbon in a proportion between 0.1% and 1.5% of the total mass of the cement, the atmosphere employed being constituted by a neutral gas.

Description

The present invention relates to a chemical vapor deposition process for halides, for producing a carbide coating on the surface of metal parts which must have a high hardness. This involves making hard coatings composed of carbides of one or more elements from the following series: silicon - titanium - vanadiumchrome - zirconium - niobium - hafnium - tantalum - tungsten, all belonging to groups IVa, Va and Vla of the periodic table of the elements. In what follows, all the elements of the series mentioned above limitatively will be conventionally designated by: "the metals concerned".

The carbides of the metals concerned are known for their very high hardness and their great chemical stability, which gives them good properties of resistance to friction, wear and corrosion. In the case where the mechanical parts are highly stressed by heat, these carbide coatings can constitute excellent thermal barriers and lead to a significant increase in the longevity of the materials.

• - une réaction d'échange entre l'halogénure du métal d'apport et le fer, le nickel ou le cobalt qui constituent les éléments d'alliage essentiels des pièces mécaniques traitées suivant ce procédé.
• - une réaction de réduction de ces mêmes halogénures par l'hydrogène utilisé assez souvent comme gaz porteur.

There are many methods for coating metal surfaces of mechanical parts made of steels or metal alloys by chemical deposition of a filler metal in the vapor phase; these processes mostly use a halide of the filler metal in the gaseous state, at temperatures between 700 and 1100 ° C., and the metal is supplied according to two distinct reactions:

• - An exchange reaction between the halide of the filler metal and the iron, nickel or cobalt which constitute the essential alloying elements of the mechanical parts treated according to this process.
• - A reduction reaction of these same halides by hydrogen used quite often as a carrier gas.

These reactions are often facilitated by the use of a pressure below atmospheric pressure between 50 and 500 Torr.

The use of reactive atmospheres containing controlled volume percentages of gaseous hydrocarbons or ammonia makes it possible to produce, in the same way, on the surface of metallic materials, coatings of inorganic compounds such as carbides or nitrides.

Known methods for producing these compounds in the vapor phase require expensive equipment and cause difficulties in obtaining homogeneous coatings on parts of complicated shape or inside bores.

This difficulty can be identified using a technique that is now well known. Obtaining metallic vapor is carried out by the action of a halide, for example ammonium chloride or fluoride, on a powdery cement formed by the element to be deposited, in the form of metal or ferro-alloy (chromium or ferro-chromium when it is a chromization, for example).

The advantage of this process lies in obtaining a homogeneous metallic vapor, provided that the cement is in contact or at a given distance, but relatively small and constant, of the parts to be coated.

• - une première phase de nitruration ionique à une température comprise entre 450 et 650° C dans des conditions opératoires permettant de réaliser des couches de diffusion d'azote seules.
• - une deuxième phase de chromisation s'inspirant de la technique précédemment connue, appliquée à des températures comprises entre 850° et 1100°C, mais dans laquelle le cément utilisé est de préférence un ferro-chrome de granulométrie comprise entre 0,5 et 4 mm, de teneur en chrome comprise entre 50 et 75 % et de teneur en carbone comprise entre 1 et 3 %, sans liant alumineux ni magnésien, avec un pourcentage d'halogénure d'ammonium (fluorure ou chlorure en l'occurence) compris entre 0,4 et 1,5%.
• - une troisième phase de traitement thermique approprié.

Two previous industrial property titles (French patent n ° 78-30308, or 2,439,824, of October 25, 1978 and its certificate of addition n ° 80-11950, or 2,483,468 of May 29, 1980) have described improvements made to this technique in the case of chromization: it involves obtaining on metallic materials single-phase layers of chromium carbides or carbonitrides of the Cr ₂ (C, N) type using a sequenced treatment including:

• - A first phase of ionic nitriding at a temperature between 450 and 650 ° C under operating conditions making it possible to produce nitrogen diffusion layers alone.
• - A second chromization phase inspired by the previously known technique, applied at temperatures between 850 ° and 1100 ° C, but in which the cement used is preferably a ferro-chromium with a particle size between 0.5 and 4 mm, chromium content between 50 and 75% and carbon content between 1 and 3%, without aluminous or magnesium binder, with a percentage of ammonium halide (fluoride or chloride in this case) between 0.4 and 1.5%.
• - a third phase of appropriate heat treatment.

The use of this technique, now well known, of a vapor deposition by the action of a metal halide obtained from an ammonium halide and a ferro-alloy of the metal concerned, makes it possible to carry out carbide layers by surface reaction between the metallic element provided by the high temperature vapor and the carbon which backscatter from the interior of the coated material to and up to the surface.

• - la présence de porosités dans le revêtement, liée à une différence entre la vitesse de croissance de la couche et la vitesse de rétrodiffusion du carbone au travers de cette même couche (effet Kirkendhal). Ces porosités sont plus nombreuses au voisinage de l'interface couche-substrat.
• - la formation de couches biphasées constituées, soit par plusieurs types de carbures lorsque le diagramme d'équilibre entre le carbone et l'élément métallique d'apport le permet (exemple: chrome, vanadium, tantale ...), soit par un type de carbures et un alliage de diffusion entre l'élément métallique d'apport et le substrat.
• - la formation, dans le cas de substrats d'aciers et pour les éléments d'apport très avides de carbone (titane par exemple), d'une sous-couche décarburée dont le niveau de résistance, après traitement thermique ultérieur, est plus faible que celui de l'acier à coeur.

Under these conditions, the metallic supply is almost always too great in relation to the amount of carbon backscattered at the surface, this regardless of the chemical composition of the substrate to be coated. This can result in several phenomena:

• - the presence of porosities in the coating, linked to a difference between the speed of growth of the layer and the speed of backscattering of carbon through this same layer (Kirkendhal effect). These porosities are more numerous in the vicinity of the layer-substrate interface.
• - the formation of two-phase layers consisting either of several types of carbides when the equilibrium diagram between the carbon and the metallic filler element allows it (example: chromium, vanadium, tantalum, etc.), or by a type carbides and an alloy of diffusion between the metallic filler element and the substrate.
• - the formation, in the case of steel substrates and for the very greedy carbon input elements (titanium for example), a decarburized undercoat whose resistance level, after subsequent heat treatment, is lower than that of steel at heart.

These three phenomena lead to a significant reduction in the mechanical properties and the adhesion of the coatings.

This type of drawback is also not avoided by the use of powdery mixtures placed in direct contact with the parts to be treated and containing ferroalloys or pure alloying elements, for example chromium or titanium, diluents such as for example alumina to avoid agglomeration of the cement during treatment, ammonium halides and carbon in the form of charcoal or graphite is known; by way of example, the patents SU-A-692,909 and FR-A-2,486,103 will be cited.

The object of the present invention is to avoid all these drawbacks by producing a coating of single-phase carbides, obtained by moderating the contribution of the metallic element of the coating as a function of the chemical composition of the substrate, and by ensuring a additional supply of carbon in the vapor phase.

To this end, the subject of the present invention is a process for the chemical coating, in the vapor phase of halides, of the surface of metallic parts which must have a high hardness, leading to the production of single-phase surface layers of carbides of metallic elements. of the following series: silicon - titanium - vanadium - chromium - zirconiumniobium - hafnium - tantalum and tungsten, this process, which does not include any prior ionic nitriding, consisting of a carburizing treatment by gaseous halides from one to less of the aforementioned metal elements, at temperatures between 800 ° and 1100 ° C, for periods of between 2 and 20 hours, and being characterized in that the halides of the aforementioned metal elements are obtained by using a cement kept at a distance from the surface to be coated and comprising as sole constituents: at least one of the aforementioned filler elements, either in the form of a ferro-alloy, or in the technically t pure in metallic form; an addition of ammonium chloride or fluoride in a proportion of between 0.2% and 1.5% by weight of the total mass of cement; and an addition of pulverulent carbon in a proportion of between 0.1 and 1.5% by weight of the total mass of the cement, the atmosphere used consisting of a neutral gas, such as argon, for example.

The present invention can be applied by using a cement comprising for its metallic part either only one of the aforementioned filler elements, or two of them, such as, for example, chromium and titanium, or chromium and vanadium, or even more.

The invention is preferably implemented with a precise adjustment of the proportion of pulverulent carbon in the cement, always between 0.1 and 1.5% by weight of the total mass of this cement, to the chemical composition of the cement. substrate to be coated, and at the thickness targeted for the layer of carbides to be produced in a given time.

According to a characteristic of the invention, the cementation takes place exclusively in the gaseous phase without contact between the cementation and the surface of the part to be coated and for this purpose, the cementation is kept at distances from the surface of the parts to be treated comprised of preferably between 2 and 15 millimeters by means of wire mesh, the mesh size of which is adapted to the particle size of the cement used.

Claims 4 and 5 relate to other advantageous embodiments of the invention.

The main advantage of the invention is to make it possible to obtain coatings of single-phase metal carbides, free of porosity, and without decarburization of the metal substrate, because the carbon provided by the cement causes the reduction or even the suppression of backscattering. towards the surface of the carbon contained in the part to be coated.

• (a) D'une part, par l'intermédiaire de la phase vapeur au sein de laquelle les fines particules de carbone se trouvent en suspension, ce carbone, en se déposant à la surface de la pièce à revêtir et en la cémentant, freine et limite l'apport du métal concerné;
• (b) D'autre part, à la surface de la pièce à revêtir ou substrat, ce carbone participe in situ à la formation du revêtement de carbures, limitant ou éliminant l'intervention, par rétrodiffusion, du carbone contenu dans les couches plus profondes du substrat.

In fact, carbon from cement works in two ways:

• (a) On the one hand, through the vapor phase within which the fine particles of carbon are in suspension, this carbon, by depositing on the surface of the part to be coated and by cementing it, brakes and limits the contribution of the metal concerned;
• (b) On the other hand, on the surface of the part to be coated or substrate, this carbon participates in situ in the formation of the coating of carbides, limiting or eliminating the intervention, by backscattering, of the carbon contained in the deeper layers. of the substrate.

In order to clearly understand the invention, two embodiments of the method according to the invention will be described below, by way of nonlimiting examples, one for forming a layer of titanium carbide TiC, the other to form a layer of VC vanadium carbide.

First mode - Formation of single-phase TiC titanium carbides.

In this case, the cement intended to form the vapor phase at high temperature consists of titanium metal shavings of maximum dimensions between 5 and 15 millimeters to which are added an ammonium chloride powder in a proportion of between 0.3 and 0.7% and a carbon powder in proportion between 0.7 and 1.3%. The carrier gas used is a neutral gas, for example argon, which is found at atmospheric pressure. The temperature to which the parts are brought is between 900 and 950 ° C. To avoid any phenomenon of localized corrosion, the parts are kept outside the cement and at a distance from the latter comprised between 5 and 15 millimeters. The duration of the treatment is between 2 and 20 hours depending on the thickness of the layer to be produced.

To allow the cementation of the surface to be coated to be kept at a suitable distance (5 to 15 millimeters), the latter is arranged in an assembly produced by welding of wire mesh which surrounds the part to be treated.

• - 10 micromètres dans le cas d'un acier XC 90, .c'est-à-dire : acier fin à 0,9% C.
• - 5 micromètres dans le cas d'un acier Z 160 CDV 12, c'est-à-dire : acier fortement allié, à 1,60 % C. à 12 % Cr, 0.90 % Mo et 0.90 % V.
• - 7 micromètres dans le cas d'un acier 32 CDV 13, c'est-à-dire : acier faiblement allié, à 0,32 % C, à 3,25 % Cr, 1 % Mo et 0,2 % V.

Under these conditions, titanium carbide single-phase coatings are obtained, free of porosities and without decarburization of the metal substrate. The thickness of the coating depends on the treatment time and on the content of carbon and carburogenic elements in the substrate: for example, after a treatment of 3 hours at 940 ° C., the layer of TiC carbide reaches:

• - 10 micrometers in the case of XC 90 steel, i.e.: fine steel at 0.9% C.
• - 5 micrometers in the case of Z 160 CDV 12 steel, that is to say: highly alloyed steel, at 1.60% C. at 12% Cr, 0.90% Mo and 0.90% V.
• - 7 micrometers in the case of a 32 CDV 13 steel, that is to say: low alloy steel, at 0.32% C, 3.25% Cr, 1% Mo and 0.2% V.

The parts thus treated are then extracted from the case hardening device to undergo an appropriate heat treatment, of known type.

Second mode - Formation of single-phase VC vanadium carbides.

In this case, the cement intended to form the vapor phase at high temperature consists of ferrovanadium containing between 80 and 85 by weight of vanadium, the particle size of which is between 0.5 and 5 mm, to which are added a chloride powder. ammonium in proportion between 0.8 and 1.2%, and a carbon powder in proportion between 0.1 and 0.5%. The carrier gas used is a neutral gas, for example argon, at atmospheric pressure. The temperature at which the parts are brought is between 900 ° and 950 ° C. To avoid any phenomenon of localized corrosion, the parts are kept outside the cement and at a distance from the latter between 2 and 10 millimeters. The duration of treatment is between 2 and 20 hours.

The device used to produce the coating is the same as that described in the previous example.

After case hardening, the parts are subjected to an appropriate heat treatment.

• - 20 micromètres dans le cas d'un acier XC 90, c'est-à-dire : acier fin à 0,9 % C.
• -10 micromètres dans le cas d'un acier Z 160 CDV 12, c'est-à-dire : acier fortement allié, à 1,60 % C, 12 % Cr, 0.9 % Mo et 0,9 % V.
• - 12 micromètres dans le cas d'un acier 35 CD 4, c'est-à-dire : acier faiblement allié à 0,35 % C, 1 % Cr et 0,22 % Mo.

Under these conditions, coatings of single-phase vanadium carbides are obtained, free of porosities and without decarburization of the metal substrate. The thickness of the coating depends on the treatment time and on the content of carbon and carburogenic elements in the substrate: for example, after a treatment for 15 hours at 920 ° C., the layer of VC carbide reaches:

• - 20 micrometers in the case of XC 90 steel, that is to say: fine steel at 0.9% C.
• -10 micrometers in the case of a Z 160 CDV 12 steel, that is to say: highly alloyed steel, at 1.60% C, 12% Cr, 0.9% Mo and 0.9% V.
• - 12 micrometers in the case of 35 CD 4 steel, that is to say: low alloy steel at 0.35% C, 1% Cr and 0.22% Mo.

The invention can have many applications, for example in armament material, in nuclear power plant material, to solve spinning problems of light alloys, to improve the hardness of wood slicing tools.

It applies as well, as a substrate, to structural steels, as to tool steels or to stainless steels.

Claims (5)

1. A process for forming a chemical coating in a vapour phase of halogenides, of the surface of metallic parts which must have a high hardness, resulting in the obtainment of monophase superficial layers of carbides of metallic elements of the following series : silicon, titanium, vanadium, chromium, zirconium, niobium, hafnium, tantalum and tungsten, this process, which does not comprise a prior ionic nitriding, being constituted by a gaseous cementation treatment of halogenides of at least one of the aforemetioned metallic elements, at temperatures between 800° and 1100° C, during periods between 2 and 20 hours, characterised in that the halogenides of said metallic elements are obtained by using a cement maintained at a distance from the surface to be coated and comprising as sole constituents : at least one of said addition metallic elements, either in the form of ferro-alloy, or in the technically pure state in the metallic form ; an addition of ammonium chloride or fluoride in a proportion between 0.2 and 1.5 % by weight of the total mass of the cement ; and an addition of powdered carbon in a proportion between 0.1 % and 1.5 % of the total mass of the cement, the atmosphere employed being constituted by a neutral gas.

2. A coating process according to claim 1, characterised in that the cement which gives rise to the metallic halogenides comprises in its metallic part a plurality of said elements, either in the form of a ferro-alloy mixture of different types, or in the form of various metallic elements in the technically pure state.

3. A coating process according to any one of the claims 1 and 2, characterised by the precise adjustment of the proportion of powdered carbon of the cement, both in the chemical composition of the metallic surface to be coated and in the contemplated thickness for the carbide layer to be produced within a given period.

4. A chemical coating process according to claim 1, in which the coating of the surface of the metallic parts to be coated is constituted by titanium carbides, characterised in that the cement which produces the metallic chlorides is constituted by : metallic titanium cuttings of maximum dimensions between 0.3 and 0.7 % by weight of the total mass of the cement ; and an addition of carbon powder in a proportion between 0.7 and 1.3 % by weight of the total mass of the cement ; the distance between the cement and the parts to be coated being between 5 and 15 mm, and the treatment temperature being between 900° and 950° C.

5. A chemical coating process according to claim 1, in which the coating of the surface of the metallic parts to be coated is constituted by vanadium carbides, characterised in that the cement which produces the metallic chlorides is constituted by : ferro-vanadium containing between 80 % and 85 % of vanadium, in a particle size between 0.5 and 5 mm ; an addition of ammonium chloride powder in a proportion between 0.8 and 1.2 % by weight of the total mass of the cement ; and an addition of carbon powder in a proportion between 0.1 % and 0.5 % by weight of the total mass of the cement ; the distance between the cement and the parts to be coated being 2 and 10 mm, and the treatment temperature being between 900° and 950°C.