EP2049704A1 - Method for depositing non-oxide ceramic coatings - Google Patents

Abstract

The invention relates to a method for depositing a non-oxide ceramic-type coating based on chrome carbides, nitrides or carbonitrides, by means of DLI-CVD carried out at a low temperature and atmospheric pressure on a metallic substrate, said method essentially comprising the following steps: a) a solution is prepared, containing a molecular compound which is a precursor of the metal to be deposited, belongs to the bis(arene) family, and has a decomposition temperature of between 300°C and 550°C, said compound being dissolved in a solvent which is depleted of oxygen atoms; b) said solution is introduced in the form of an aerosol into a heated evaporator (3) at a temperature between the boiling temperature of the solvent and the decomposition temperature of the precursor; and c) the precursor and the vapourised solvent are driven by a neutral gas flow from the evaporator (3) towards a CVD reactor (10) having cold walls and comprising a susceptor (13) carrying the substrate to be covered and heated to a temperature higher than the decomposition temperature of the precursor but to a maximum temperature of 550°C, the evaporator (3) and the CVD reactor (10) being subjected to the atmospheric pressure.

Description

 METHOD FOR DEPOSITING NON-OXIDE CERAMIC COATINGS

The present invention relates to the field of coatings for the protection of metal parts working under severe conditions against wear, corrosion or high temperature oxidation.

It relates to a process for the dry deposition of non-oxide ceramic coatings such as carbides, nitrides or carbonitrides of metal elements, in particular based on chromium, making it possible to obtain monolayer coatings or nanostructured layer stacks. It also relates to a method of manufacturing resistant metal parts for the mechanical industry.

The current needs of the mechanical industries are growing in terms of resistant materials under severe conditions of use. Many tools, such as cutting, forming, milling, stamping tools, molds, bearings or other, include metal parts that are subjected during their use to significant wear, corrosion, or high temperature oxidation. For example, wear problems lead to machine downtime and significant parts repair and replacement costs. To prolong the life of these metal parts - made of steels or alloys - they can be coated with a thin layer, typically of a few microns, of a non-oxide ceramic compound of the carbide, nitride or carbonitride type of a metallic element. which improves their mechanical properties as well as their resistance to wear and corrosion. Chromium is the most commonly used metal element, but other transition metals with similar properties are also used. Nanostructured coatings are now of great interest because they are able to protect tools operating in extreme wear and harsh environments.

To be effective, the deposit must be uniform over the entire surface, and despite the large size of the parts, it must be free of oxide and made by techniques that do not affect the functional properties of the treated tools. In particular, the temperature conditions during the deposition must not lead to deformation of the parts, nor to a structural transformation. In addition, a process is sought whose implementation conditions are easily industrialized and economically inexpensive. In this context, the possibility of making protective deposits at atmospheric pressure is a goal of This is important because it makes it possible to contemplate on-line treatments on parades on large pieces. Finally, we are looking for the least polluting process possible. The processes of wet deposition (bath immersion treatment or electroplating) are therefore prohibited because of the effluent discharges loaded with toxic compounds they generate. They will soon be banned in Europe.

Obtaining non-oxide ceramic coatings by dry deposition techniques is well known. Among these, the techniques of chemical deposition (CVD, for Chemical Vapor Deposition) and physical (PVD, for Physical Vapor Deposition) are mastered and already used in production.

For example, the chemical vapor deposition of nitrides, carbides or carbonitrides of metallic elements is known from a cementum consisting of a metal powder in contact with a volatile reducing compound at high temperature. With regard to chromium-based deposits, the metal powder is confined in the presence of a halide (NH ₄ F or HF) and heated to high temperature (950-1050 ^° C.). Thus, chromium carbide deposits (with the addition of hydrocarbon gases), chromium nitride (NH ₄ F decomposing into H ₂ and N ₂ , nitriding mixture at high temperature) or carbonitrides (NH ₄ F + hydrocarbons) are obtained. ). This process can operate at atmospheric pressure but the deposits are obtained only at high temperature because of the metal source of the halide type used. The so-called "conventional" CVD processes, using directly as a source of chromium, the vapors of the corresponding halide, for their part, operate under dynamic vacuum and at high temperature.

These two highly thermally activated CVD processes allow the treatment of complex surfaces but their main disadvantages are (i) the use of thermally robust, toxic, corrosive and limited volatility halide precursors, and (ii) high deposition temperatures which, on metal substrates, will cause dimensional deformations, changes in microstructure, changes in crystalline structure, resulting in the degradation or loss of the specific functional properties of the tool.

To reduce the deposition temperatures below 550 ^° C., considered as the critical holding temperature of steels and alloys, organometallic molecular precursors have been used (MOCVD process, for Metal Organic CVD). However, given the low volatility and thermal instability of these compounds which are often powders, it is necessary to operate under reduced pressure. In addition, the heating prolonged precursor in the sublimation zone, even at low temperature, can degrade the reagent before it arrives in the form of vapor at the deposition zone, thus causing problems of reproducibility in terms of flow rate, initial reactive gas composition and therefore deposit quality.

A particular method of MOCVD deposition uses a plasma torch to thermally decompose organometallic chromium precursors. The use of a plasma torch, however, does not allow a deposit on a thermally sensitive metal, nor the uniform coating on large parts unless a system of scanning the plasma nozzle is provided, requiring servo complex technique. A parade treatment on large parts is therefore not feasible by this technique.

Given the above-mentioned limitations of CVD deposition processes for protective metallurgical coatings, physical vapor deposition (PVD) processes have been developed and proposed as alternatives to CVD processes. They are generally considered a clean deposit technique, without generation or use of unstable, dangerous and / or corrosive chemicals. They operate at a low temperature, which makes it possible to preserve the basic specificities of the metal substrates to be treated. Numerous PVD processes for producing carbide, nitride and carbonitride coatings are described in the literature. Despite investments in expensive equipment, the size of the deposition frames is limited, the rate of growth is relatively low and the uniformity of the deposits, even with target or rotating substrate techniques, is obtained only for modest size. In all cases, high vacuum techniques are used, requiring sophisticated control and regulation devices. These PVD processes resulting in low productivity are currently reserved for the processing of batches of small pieces of high added value.

It thus appears that it has not been possible so far to make deposits of non-oxide ceramics at low temperature and at atmospheric pressure, applicable for the treatment of large surfaces in line parade. The present invention aims to meet this need. It relates to a process combining the principle of chemical vapor deposition and the liquid injection of a precursor of the metal compound to be deposited, called DLI-CVD for Direct Liquid Injection - Chemical Vapor Deposition.

The principle of the DLI-MOCVD technique is to introduce into a chemical vapor deposition chamber a liquid precursor of the element to be deposited by periodic injection of droplets of said precursor which are entrained by a carrier gas to the deposit enclosure. A DLI-MOCVD device has been developed for the deposition of thin oxide films on microelectronic plates, but has never been used for deposition of non-oxide ceramics at atmospheric pressure. This is because atmospheric pressure work, which has a great advantage for low cost industrial production of large parts, imposes special reaction conditions which have not been defined so far. Furthermore, the operating conditions are not easily transferable to the deposition of non-oxide layers. Indeed, in the case of microelectronic layers, carbon contamination must be avoided. Since this is the result of the large amount of hydrocarbon solvent in the reactor, it is more easily controlled when the decomposition is carried out in the presence of oxygen or an oxidizing gas (N ₂ O, H ₂ O, etc.). . Moreover, for coatings intended for microelectronics, oxygenated solvents are commonly used. A large part of the carbon is then burnt off while an oxide film is deposited, which is not compatible with the purpose of the present invention.

The present invention overcomes these disadvantages by a process which can be carried out at temperatures below 550 ^° C. and at atmospheric pressure, for depositing non-oxide ceramic layers on steel or alloy supports. The method according to the invention thus makes it possible to envisage the deposition under industrial conditions of protective layers of non-oxide ceramics on large metal plates.

More specifically, the subject of the present invention is a process for depositing a non-oxide ceramic type coating based on chromium carbides, nitrides or carbonitrides or metal elements whose chemical properties are similar to those of chromium, by deposition. chemical vapor phase composition on a metal substrate comprising essentially the steps of: a) providing a solution containing a metal precursor molecular compound to be deposited belonging to the bis (arene) family and having a decomposition temperature between 300 ⁰ C and 550 ⁰ C, said compound being dissolved in a solvent free of oxygen atom, b) introducing said precursor solution in aerosol form in an evaporator heated to a temperature between the boiling point of the solvent and the decomposition temperature of the precursor, c) entrain the precursor and the solvent vaporized by a stream of neutral gas from the evaporator to a cold-walled CVD reactor comprising a susceptor supporting the substrate to be coated, heated to a temperature above the decomposition temperature of the precursor and at most 550 ⁰ C, the evaporator and the CVD reactor being subjected to atmospheric pressure.

The liquid direct injection technique is based on the pulsed introduction of a molecular solution of a metal precursor, fractionated into micro droplets to form an aerosol which is vaporized flash in an evaporator heated to at least the temperature boiling of the chosen solvent and well below the decomposition temperature of the precursor and the solvent used. A gas stream arriving at the nose of the injector causes the precursor and solvent vapors of the evaporator to the deposition zone consisting of a susceptor heated by induction and supporting the substrate to be coated. Nitrogen gas can be introduced at the same time into the system, preferably between the evaporator and the reactor, to obtain nitrides or carbonitrides.

According to an advantageous characteristic of the process according to the invention, the evaporator is heated to a temperature at least 50 ^° C., and preferably at least 100 ^° C., below the decomposition temperature of the precursor compound. Thus the aerosol is vaporized flash and avoids premature deposition on the walls of the evaporator.

The injection parameters of the precursor solution are preferably set using a computer program. They are adjusted so as to obtain a mist of fine and numerous droplets, an essential condition for flash evaporation at atmospheric pressure. The micro-droplet fractionation of the solution can be carried out for example by means of a modified diesel automobile injector, set with a short opening time and a high injection frequency. According to an advantageous characteristic of the process according to the invention, the aerosol is obtained by pulsed injection with an opening time of less than 1 ms and a frequency greater than 4 Hz. The liquid injection thus constitutes a high-speed source of precursor solution, allowing a good coating deposition efficiency.

The good conduct of the process according to the invention, in particular its good hydrodynamic quality, requires high gas flow rates. The flow rate of the neutral gas is advantageously between 4 cm / s and 10 cm / s. Thus, for a reactor of 50 mm diameter for example, the gas flow rate is adjusted to more than 5000 cmVmn. In the configuration where the evaporator is located coaxially above the CVD reactor, the flow benefits in addition to the gravitational force.

The neutral gas employed as the carrier gas should preferably be preheated, at least at the temperature of the evaporator, to obtain an effective vaporization which is difficult to realize under atmospheric pressure, which explains that the DLI-CVD techniques used until now all operate under reduced pressure. Thus, according to another characteristic of the process according to the invention, the neutral gas is heated to a temperature at least equal to that of the evaporator before entering.

The carrier gas used is neutral in that it is not likely to oxidize the reactants in the presence. Nitrogen will preferably be chosen as carrier gas for its low cost, but helium or argon, benefiting from a better thermal conductivity, can also be used although more expensive. The stream of neutral gas arriving on the nose of the injector then drives the precursor and solvent vapors from the evaporator to the deposition zone.

The metal element to be deposited is typically chromium, but it can also be any other metal whose chemistry and metallurgy are related to those of chromium. Those skilled in the art know the elements for which the properties of hardness and chemical inertia required in metallurgy are obtained. A metal is chosen which is capable of forming a bis (arene) compound in which the metal is at the zero oxidation state. Thus, according to a preferred characteristic of the method according to the invention, the metallic element can be chosen from Cr, Nb, V, Mo, Mn, Hf.

According to one embodiment of the method according to the invention, the precursor of the metal element to be deposited is a compound having no oxygen atom. It can be chosen from the family of metal bis (arenes) of general formula (Ar) (Ar ') M where M is the metal element with zero oxidation state, and Ar, Ar' each represent an aromatic hydrocarbon ring of benzene or benzene type substituted with alkyl groups.

As the stability of the metal-ligand bond increases with the number of substituents of the benzene nucleus, it will be preferred in a particular embodiment of the process according to the invention, a precursor chosen from the compounds in which Ar and Ar 'represent two weakly aromatic ligands. substituted identical.

More preferably, the precursor may be chosen from the family of bis (arene) chromium, preferably from bis (benzenechrome) or BBC, of formula Cr (C ₆ H ₆ ) ₂ , bis (cumene) chromium of formula Cr (C ₆ H ₅ iPr) ₂ , bis (methylbenzene) chromium of formula Cr (C ₆ H ₅ Me) ₂ , and bis (ethylbenzene) chromium of formula Cr (C ₆ H ₅ Et) ₂ . Only the BBC is in the form of a powder. The other precursors mentioned are liquid and could be directly injected without solvent. The BBC will be chosen preferably for its commercial availability, the knowledge of its reactivity in conventional vacuum MOCVD and its relatively low decomposition temperature (350 ^° C.).

It is noted that the precursors having a decomposition temperature greater than 600 ⁰ C are discarded. Similarly precursors which have a too low decomposition temperature such as tetra (alkyl) metal of general formula M (R) ₄ are excluded.

The use of this type of precursor would lead to deposits on the walls of the evaporator under the required temperature and pressure conditions. Diluted in a hydrocarbon solvent, they could catalyze the pyrolysis of the solvent and lead to a very high contamination of carbon deposits, or even the deposition of carbon-only compounds.

The solvent of the precursor compound plays an important role in the successful completion of the process according to the invention. His choice must meet a set of chemical and physical criteria. Firstly, the boiling point of the solvent must be lower than the temperature of the evaporator to allow flash evaporation in the evaporator, typically heated to at least 150 ^° C. The solvent chosen must also have a low viscosity. to facilitate the entrainment of the injectable solution by liquid way to the evaporator. It must not contain oxygen to avoid the risk of oxidation of the deposits by cracking the solvent used in the deposition zone. It must of course be chemically inert vis-à-vis the precursor in solution and liquid under normal conditions. Thus, according to the invention, the solvent is preferably chosen from liquid hydrocarbons having a boiling point below 150 ^° C., of general formula C _x H _y .

Conveniently, the vapor pressure will be sufficiently low at atmospheric pressure to allow storage of the injectable solution without evaporation of the solvent under normal conditions, in order to avoid the risk of reprecipitation of the precursor if the initial concentration reached saturation, resulting in the clogging of the injectors. The light hydrocarbon solvents are therefore discarded. Aromatic solvents such as toluene, cyclohexane, or saturated linear hydrocarbons for which x> 8 (eg n-octane) are preferred, provided that the other stated conditions are also met.

Furthermore, according to an advantageous characteristic of the invention, the precursor compound and the solvent are chosen so that the saturation concentration of the precursor compound in the solvent is greater than or equal to 0.01M in order to obtain an acceptable precursor flow rate. and much higher than in conventional saturator sublimation method. In the case of the BBC precursor, toluene or cyclohexane will be preferred as the solvent, because their chemical resemblance to the aromatic ligands of the precursor and their relatively low boiling point. The saturation concentration in BBC is satisfactory (0.05 mol / l) and its chemical integrity is preserved during its dissolution: the characteristic vibration bands of BBC in solution in toluene or cyclohexane in the UV-Vis range. are stored in time if the solution is stored under an inert atmosphere.

When it is desired to deposit a nitride or carbonitride, a source of reactive nitrogen must be introduced into the deposition chamber. Nitrogen could be provided by the precursor compound, but it is preferable that it be provided by a gas, such as ammonia or hydrazine, because it is thus easy to alternate layers of different composition by feeding or not the reactor nitrogen, while maintaining the same precursor. This operation is very simple and instantaneous, while changing the precursor solution requires delicate adjustments and time. According to an interesting embodiment, in step b), in addition to the components already mentioned, a reactive gas comprising a nitrogen compound is introduced at the outlet of the evaporator so that in step c), it is driven with the precursor by the flow of neutral gas to the CVD reactor.

The carbon and / or nitrogen content of the coatings and their crystallographic structure can be modulated, depending on the presence and nature of the reactive gas phase (ammonia, hydrazine, etc.). It is thus possible, with a single equipment, to obtain carbide, carbonitride or nitride deposits of a metal element.

Another object of the present invention is a process for obtaining multilayer coatings consisting of a stack of layers deposited successively, as described above, whose individual thickness may be from a few nanometers to a hundred nanometers. The nanostructuring of these multilayer metallurgical coatings gives them remarkable properties (protection, resistance to wear, hardness, etc.), which can be adjusted by controlling the period and modifying the nature of the constituent layers. The different phases of the Cr-NC ternary system can be deposited either individually to take advantage of their specific properties, or alternatively to form multilayer complex coatings with original properties and enhanced by nanostructuring. Those skilled in the art are well aware of the interest of these materials, which it has been possible to obtain so far only by heavy and expensive techniques (PVD only). Thus, according to an advantageous embodiment of the process according to the invention, steps a) to c) are repeated several times to obtain a nanostructured multilayer coating of different non-oxide ceramics. The multilayer stack may consist of layers of the same metal element from a single precursor solution, varying only the composition of the gas phase during deposition (ie whether to supply the reactive gas). In this case, advantageously, for each repetition of steps b) and c), the same precursor solution is used, and the input flow rate of the reactive gas comprising a nitrogen compound is modified or canceled.

The multilayer stack may also consist of layers as described above, with a different metal element for each, thanks to the alternating injection of two precursor solutions based on different metallic elements. In this case, alternatively or in addition to the modification of the supply of reactive gas, in step a), at least two solutions are provided, each containing a precursor molecular compound of a different metal whose decomposition is between 300 ⁰ C and 550 ⁰ C, said compounds being dissolved in a solvent free of oxygen atom, and in step b) is introduced alternately said solutions of precursors in the form of aerosols in the evaporator heated to a temperature between the boiling point of the solvent and the decomposition temperature of the lowest precursor. Preferably, the solvent is the same for all precursor solutions.

The method according to the invention as just described makes it possible to perform a chemical vapor deposition at low temperature and under atmospheric pressure, to obtain protective ceramic coatings based on chromium or another transition metal. as monolayer or multilayer. This method is particularly well suited to the coating of metal parts for use in the mechanical industry under severe conditions, using a hard protective layer resistant to wear, corrosion and oxidation. It can advantageously be implemented, with the appropriate adaptations, for the treatment of large plates parade.

More generally, a method of manufacturing a metal workpiece intended to work under severe conditions is claimed, comprising a chemical vapor deposition operation of at least one layer of a carbide-based non-oxide ceramic hard coating. chromium nitride or carbonitride or a metal element whose chemical properties are close to those of chromium, wherein said chemical vapor deposition is carried out by a process as described above.

Also claimed is a method of manufacturing a metal workpiece for severe working, including a chemical deposition operation in phase vapor of several layers of non-oxide ceramics type based on carbides, nitrides or carbonitrides of chromium or a metal element whose chemical properties are close to those of chromium, said layers forming a nanostructured hard coating, in which said chemical deposition in vapor phase is achieved by a multilayer deposition process described above.

The method described above can be applied to the deposition of a monolayer or non-oxide multilayer coating on a metal substrate. As already mentioned above, it is particularly useful for protection against wear and / or corrosion of steel or alloy steel parts.

The following examples will make it possible to better understand the advantages of the process according to the invention and to illustrate particular aspects thereof.

EXAMPLE 1

DLI-MOCVD deposition device

The DLI-MOCVD device used for the deposition of the ceramic layers which will be described in detail below consists mainly of a cold walled vertical CVD reactor coupled to a commercially available pulsed injection system. It makes it possible to obtain non-oxide ceramic coatings such as carbides, nitrides and carbonitrides of chromium, as well as nanostructured multilayer stacks of these same coatings.

In the configuration in which it is represented in FIG. 1, it comprises, centrally, an evaporator 3 opening into a deposition chamber 10 consisting of a vertical CVD reactor with cold walls. A pressurized storage tank 1 of the precursor solution at ambient temperature feeds an injector 2 whose opening towards the evaporator 3 is computer controlled. A modified diesel automobile injector is conveniently used. A carrier gas supply line 4 arrives on the nose of the injector 2. A slide valve 5 makes it possible to isolate the evaporator 3 from the deposition chamber 10. The connections 6 and 7 above the slide valve 5 allow for one the arrival of the reactive gas and for the other the pumping of the evaporator 3 during the purge cycles or cleaning of the latter. The flange 14 on which the connections 6 and 7 are made, as well as the slide valve 5 at the inlet of the reactor 10 are heated to a temperature close to that of the evaporator 3, that is to say at least 150 ^° C. The gas stream arriving on the nose of the injector 2 drives the vapors of precursor and solvent of the evaporator 3 to the deposition chamber 10 which comprises the susceptor 13 on which is placed the support to be covered. A baffle 8 makes it possible to stop any non-vaporized droplets at the outlet of the evaporator 3 and a grid 9 pierced with holes distributes the gas flow uniformly, the assembly being connected to the deposition chamber 10. This grid 9 allows a good distribution of the gas flow at the inlet of the deposition chamber 10, which contributes to obtaining a good surface finish of the coatings and a uniform thickness. The evaporator 3 is located co-axially above the CVD reactor 10. The whole is provided with a primary pumping unit 11 for purging the system before any implementation. A system for trapping and recycling solvent vapors and by-products of the CVD reaction can be added.

EXAMPLE 2

Obtaining chromium carbide ceramic coatings from a solution of bis (benzene chromium) in toluene

• The precursor solution

The organometallic chromium precursor of bisarene: Cr (C ₆ H ₆ ) ₂ or BBC type, dissolved in degassed toluene and dehydrated on silica gel is used. The decomposition temperature of the BBC is 35 ^° C. The saturation concentration of the BBC in toluene determined by UV-Vis spectrophotometry is 5.1 × 10 ² M, a solution is prepared at a lower concentration (here equal to 3.10 ² mol / 1), to avoid reprecipitation of the precursor which might clog the DLL system injector

The pressurized solution at 2.5 bars (relative) is injected at a rate of 1.4 ml / min at a frequency of 10 Hz and an opening time of 0.5 ms in the evaporator heated to 200 ^° C., previously purged with nitrogen and at atmospheric pressure.

• Deposition The substrates are 1 cm diameter SS304L steel pellets and Si (100) wafers. They are placed on a passive graphite susceptor made of SiC or stainless steel. After a purge phase, they are heated by induction at 500 ^° C. under a flow of carrier gas (in this case nitrogen) of 5000 cm 3 / min for a reactor of 50 mm diameter (ie 4.2 cm / s), itself even preheated to the temperature of the evaporator (200 ⁰ C). The deposition temperature is controlled by a thermocouple housed inside the susceptor. The CVD reactor is at atmospheric pressure. Deposition is carried out as indicated above until a thickness layer of about ten nanometers to a few microns is obtained according to the duration of the deposition. Under the conditions indicated, the growth rate is 0.75 μm / h. For the same deposition time, the growth rate can be multiplied by increasing the frequency of the injector and therefore the precursor flow, without modifying the other parameters (opening time of the injector and gas flow). Figure 2 shows the growth rate of the coating as a function of the mole fraction of injected precursor.

For small thicknesses, a metallic gray metallic deposit is obtained. For higher thicknesses, the roughness of the surface increases, leading to a dull gray surface appearance.

• Characterization of coatings

These coatings are adherent on mirror polished 304L stainless steel and on Si. The sectional examination shows a dense morphology. The surface consists of nodules of a hundred nanometers contiguous to each other.

X-ray diffraction analyzes were performed. The X-ray diffraction pattern in grazing incidence of the nanocrystalline chromium carbide coating obtained on an Si substrate is shown in FIG. 3. It shows a practically amorphous structure, considered as nanocrystalline because the X-ray diagrams have 3 very broad, low intensity and centered peaks. on the lines of chromium carbides. Microprobe analyzes (EPMA) reveal a chemical composition close to Cr ₇ C ,.

For a 1 μm thick deposit, the hardnesses measured by nanoindentation are close to 14 GPa with a Young's modulus of 250 GPa. The macroscopic internal stresses are slightly compressive (-1 GPa) on stainless steel, allowing a good adhesion of the film on a metal substrate.

Although stainless steel is already recognized for its excellent corrosion behavior, preliminary corrosion tests in salt media have shown a significant improvement in the corrosion resistance of 304L stainless steel thanks to the chromium carbide coating in question. . Preliminary polarization and electrochemical impedance spectroscopy tests were performed in 0.1M NaCl solution. The polarization test shows a behavior of steel coated with 1 μm of amorphous chromium carbide similar to that of bare stainless steel (passivation stage then bites). The cathodic current density is greatly diminished by almost two decades. The pitting potential is almost the same as that of bare stainless steel, except that the intensity of the passivation current is Significantly decreased (by almost two decades). The observation of the surfaces after polarization shows that the pitting density is very limited for the coated steel. Bode diagrams from impedance spectroscopy tests around the corrosion potential indicate that stabilization of the low frequency theta phase for amorphous Cr ₇ C coated steel is characteristic of capacitive behavior. The resistance of polarization, reflects the resistance of the tested samples in corrosion: in the case of the coated stainless steel, it is much superior to that of the stainless steel noncovered (2.10 ⁷ W.cm ² against 4.10 ⁶ W.cm ² ).

EXAMPLE 3

Obtaining chromium carbide ceramic coatings from a solution of bis (benzene chromium) in cyclohexane

The procedure is the same as that used in Example 2, except that the BBC chromium precursor is dissolved in cyclohexane at a rate of 1.4 × 10 ^-2 mol. The growth rate is 0.4 μm / h, (lower than previously due to the lower precursor concentration in the solvent).

A dull metallic gray deposit is obtained. X-ray diffraction analyzes show a practically amorphous structure, considered as nanocrystalline because the X-ray plots have 5 very broad peaks, of low intensity but this time centered on the lines of a metastable chromium carbide CrC _{1 x} . (Figure 4). Castaing microprobe (EPMA) analyzes reveal a chemical composition close to Cr ₇ C ₃ . These coatings are adherent on mirror polished 304L stainless steel and on Si.

EXAMPLE 4

Obtaining CrN chromium nitride ceramic coatings

The procedure is the same as that followed in Example 2. The solution for injection contains the BBC chromium precursor dissolved in toluene. By tapping 6 of the flange 14 connecting the evaporator to the CVD reactor arrives high purity ammonia (99.99%).

Three coatings of different compositions were made, with the following NH / BBC molar ratios, such as the sum NH flow, + flow rate N ₂ = 5000 cmVmn: NH / BBC amount of ammonia brought 160 150 cmVmn

300 300 cmVmn

600 560 cmVmn

A dark gray deposit with metallic luster is obtained for thicknesses of a few hundred nanometers. For larger thicknesses, the surface becomes rougher and diffuses light, giving the coated Si or steel samples a milky appearance. For thicknesses up to 2 μm, samples coated with CrN, however, retain a metallic luster.

The X-ray diffraction analyzes are shown in Figure 5 for the three coatings. They show a nanocrystalline structure of CrN type, that is to say cubic faces centered, as evidenced by the good agreement with the positions and intensities of the peaks reported in the literature and represented by the points. Deposit crystallinity appears to increase with the NH / BBC molar ratio.

The chemical composition determined by electron microprobe EPMA (Electron probe microanalysis) is generally Cr _{0> 5} oNo, 5o-χO _x with x not exceeding 10 at% oxygen in the deposit. Despite the precautions taken on the tightness of the DLI-MOCVD frame used, the incorporation of a few atomic percent of oxygen is inevitable, given the high affinity of chromium and especially CrN for oxygen.

The coatings obtained on 304L stainless steel are adherent and dense as on Si. Compressive stresses of 2 GPa have been measured and this state of macroscopic internal stresses seems to become more and more compressive with the increase of the thickness of the deposits. They exhibit remarkable mechanical properties and corrosion resistance: the hardness obtained by nanoindentation is 23.1 ± 1.5 GPa for films of low thickness (less than 300 nm). Nanohardness falls to 15 GPa for CrN coatings of thickness 1.7 μm. The Young's modulus, on the other hand, remains relatively high (280 GPa), always reflecting the ceramic behavior of the CrN deposits.

Corrosion tests identical to those of Example 2 were carried out on a 304L stainless steel sample coated with a CrN coating with a thickness of 1.7 μm. The cathodic current density is reduced by more than two decades (the cathodic current density of the CrN-coated steel is 1, 3.10 ⁶ A / cm ² for -1 V / ECS, whereas it is 5.10 ⁴ A / cm ² at the same potential for bare steel). The corresponding Bode diagrams also show a rather capacitive character for the steel coated with CrN. The polarization resistance is 2.10 ⁷ W. cm ² for the sample coated with CrN, and only 4.10 ⁶ W. cm ² for bare stainless steel. This shows the improvement of the corrosion resistance of stainless steel by the presence of CrN coating.

EXAMPLE 5

Obtaining CrN / CrC multilayer ceramic coatings

The deposition procedure is the same as that described in Example 2 except that the ammonia feed line is periodically open and closed. No purge is necessary for the alternation of different types of layer, the speed of the gas flow being important in the conditions chosen. However, purges can be applied to increase the quality of the interfaces. These purges can be programmed, just like the operation of the injectors. The periodicity of the ammonia supply is adjusted according to the respective growth rates for the two types of materials.

• 100nm CrN / CrC multilayer coating

A first coating (Rl) was made by depositing ten alternating layers of CrC (injection parameters: 10 Hz and 0.3 ms) and CrN (injection parameters: 10 Hz and 0.5 ms) on a substrate nitrided silicon. Ammonia is supplied for 14,328 seconds at 300 cc / min, with interruptions of 4,786 seconds.

• 50nm CrN / CrC multilayer coating

A second coating (R2) was made by depositing twenty alternating layers of CrC and CrN on nitrided silicon substrate. Ammonia is supplied for 7 164 seconds at 300 cc / min, with interruptions of 2393 seconds.

The SIMS (Secondary Ion Mass Spectrometry) profiles of the two coatings were established for the fragments ⁵² Cr, ³ Cs ²⁹ Si, ³ Cs ²¹² C and ³ Cs ²¹⁴ N. FIG. ten alternating layers of CrC and CrN with a target thickness of 100 nm each (coating R1).

Figure 7 shows the stack of twenty alternating layers of CrC and CrN with a target thickness of 50 nm each (coating R2).

The multilayer coatings obtained are adherent on mirror polished 304L stainless steel and dense in section. They have a relatively smooth surface state. They have remarkable mechanical properties and corrosion resistance: the measurements in nanoindentation on the multilayer coating R1 of 100 nm period show a hardness of 19 GPa. For the multilayer coating R2 of 50 nm period, the hardness is 25 GPa. It therefore appears that the nanostructuration has a beneficial effect on the hardness properties. Bode diagrams show that the polarization resistance of coated steel is greater (ie, 2.10-7 W. cm ² ) than in the case of uncoated steel. These results reflect a good behavior in corrosion.

Claims

A method for depositing a coating of non-oxide ceramic type based on carbides, nitrides or carbonitrides of chromium or metal elements whose chemical properties are similar to those of chromium, by chemical vapor deposition on a metal substrate characterized in that it essentially comprises the steps of: a) providing a solution containing a metal precursor molecular compound to be deposited belonging to the bis (arene) family and having a decomposition temperature of between 300 ^° C. C and 550 ⁰ C, said compound being dissolved in a solvent free of oxygen atom, b) introducing said precursor solution in aerosol form in an evaporator heated to a temperature between the boiling point of the solvent and the decomposition temperature of the precursor, c) driving the precursor and the solvent vaporized by a flow of neutral gas from the evaporator to a reactor cold-walled CVD reactor comprising a susceptor supporting the substrate to be coated, heated to a temperature above the decomposition temperature of the precursor and at most 550 ^° C., the evaporator and the CVD reactor being subjected to atmospheric pressure. 2- Method according to claim 1 characterized in that the evaporator is heated to a temperature of at least 50 ⁰ C, preferably at least 100 ⁰ C, at the decomposition temperature of the precursor compound. 3- A method according to claim 1 or 2 characterized in that the aerosol is obtained by pulsed injection with an opening time of less than 1 ms and a frequency greater than 4 Hz. 4- Method according to one of claims 1 to 3 characterized in that the neutral gas is heated to a temperature at least equal to that of the evaporator before entering. 5. Method according to one of claims 1 to 4 characterized in that the flow rate of the neutral gas in the evaporator is between 4 cm / s and 10 cm / s. 6. Method according to one of the preceding claims characterized in that the metal element is selected from Cr, Nb, V, Mo, Mn, Hf. 7- Method according to one of the preceding claims characterized in that the precursor is a compound having no oxygen atom, selected from the family of bis (arenes) metal of general formula (Ar) (Ar ') M where M is the zero oxidation state metal element and Ar, Ar 'each represents a benzene hydrocarbon ring or benzene substituted by alkyl groups. 8- Method according to the preceding claim characterized in that the precursor is chosen from the compounds in which Ar and Ar 'represent two identical weakly substituted aromatic ligands. 9- Method according to the preceding claim characterized in that the precursor is selected from the family of bis (arene) chromium, preferably from bis (benzenechrome), bis (cumene) chromium, bis (methylbenzene) chromium and bis (ethylbenzene) chromium. 10- Process according to any one of the preceding claims, characterized in that the solvent is chosen from liquid hydrocarbons having a boiling point of less than 150 ^° C., of general formula C _x H _y . 11- Method according to one of the preceding claims characterized in that the precursor compound and the solvent are chosen so that the saturation concentration of the precursor compound in the solvent is greater than or equal to 0.01M. 12- Process according to any one of claims 1 to 9, 11 or 12, characterized in that in step b) is further introduced a reactive gas comprising a nitrogen compound at the outlet of the evaporator so that in step c), it is driven with the precursor by the flow of neutral gas to the CVD reactor. 13- Method according to any one of the preceding claims characterized in that steps a) to c) are repeated several times to obtain a nanostructured multilayer coating of different non-oxide ceramics. 14- The method of claim 13 characterized in that for each repetition of steps b) and c), using the same precursor solution and modifying or canceling the inlet flow of the reactive gas comprising a nitrogen compound. 15- The method of claim 13 or 14 characterized in that in step a), at least two solutions each containing a precursor molecular compound of a different metal belonging to the family of bis (arene) and whose decomposition temperature is between 300 ^° C. and 550 ^° C., said compounds being dissolved in a solvent devoid of an oxygen atom, and in step b), said aerosol precursor solutions are alternately introduced into the evaporator heated to a temperature between the boiling point of the solvent and the decomposition temperature of the lowest precursor. 16- A method of manufacturing a metal part intended to work under severe conditions, comprising a chemical vapor deposition operation of at least one layer of a carbide, nitride or carbonitride-based non-oxide ceramic hard coating; chromium or a metal element whose chemical properties are similar to those of chromium characterized in that said chemical vapor deposition is carried out by a process according to one of claims 1 to 11. 17- A method of manufacturing a metal part intended to work under severe conditions, comprising a chemical vapor phase deposition of several layers of non-oxide ceramic type based on carbides, nitrides or carbonitrides of chromium or a metal element whose chemical properties are close to those of chromium, said layers forming a nanostructured hard coating characterized in that said chemical vapor deposition is carried out by a method according to one of claims 12 to 15.