WO2018172859A1 - Surface treatment method and product thereof - Google Patents

Abstract

Method for treating a surface by means of exposure to a nitrogen plasma jet at high pressure and high velocity. The method of the invention makes it possible to achieve a very high surface hardness, particularly in titanium articles, and can advantageously be applied to implantable devices, because the substrate is deep hardened, without depositing heterogeneous layers which could separate. The method of the invention is fast and can be applied to parts with complex geometry, with cavities, such as, for example, orthopaedic screws.

Description

 Surface treatment method and product of such process

Technical area

The present invention relates to a method of high-pressure plasma jet treatment of metal articles, particularly, but not exclusively, titanium or titanium alloy. The inventive method improves the surface micro-hardness, reduces the coefficient of friction of the surfaces, and can modify the surface condition and the roughness and reduces the variations of these parameters, so that the method of the invention can also be used to standardize metal items.

The treated surfaces have a characteristic color, so that the method can also be used for the marking and coding of the treated products.

State of the art

[0003] Titanium and its alloys are used in a large number of applications of the art, because of their lightness, toughness, and resistance to corrosion. However, these materials do not have a very high hardness. For example, titanium can be used to produce lightweight and robust screws. A disadvantage of this approach is that tightening and loosening by conventional tools can irreparably damage the surfaces to receive and resume the tightening torque. A related limitation is the strong adhesion of the tight metal elements together, if on the one hand it contributes to the strength of the junction, disassembly of parts is more difficult, especially when the application of lubricant is not or no more possible. This situation often leads to the application of excessive force to the screw footprint, and to their destruction. [0005] These same problems are also present for others

metallic materials, for example stainless steel and aluminum.

The biocompatibility of titanium and many of its alloys is known. These materials are widely used to produce implantable devices, such as prostheses for orthopedics, osteosynthesis plates, orthopedic screws, etc. In these applications, the need for a stronger alloy, which can withstand easier handling, tightening and loosening, is particularly acute.

Titanium alloys include titanium-aluminum-niobium (TAN) and titanium-aluminum-vanadium (TAV). These elements improve the mechanical performance of titanium, however, failures remain frequent.

[0008] Anodic oxidation processes are known which make it possible to create a titanium oxide layer T1 O2 that is thicker than the natural oxide layer. Anodizing gives rise to color layers by interference effect: The colors thus obtained are very stable and are determined by the thickness of the oxide layer, and therefore by the applied voltage. However, it is difficult to obtain very saturated hues by this route. Anodizing slightly improves the superficial micro-hardness of titanium. Nevertheless, the anodized layers are fragile and can separate from the metal substrate during clamping and loosening procedures. It is also observed that the hardness of the anodized titanium layers is very influenced by the environmental characteristics, such as temperature and humidity, so that several copies of the same product may have strong deviations in surface hardness.

Anodizing is often followed by a finishing microballing step. This operation is intended to clean the product and improve its surface condition. However, it is usually manual, which further increases quality deviations. Galvanic processes, including titanium anodizing, use harmful chemicals and are generally perceived as dangerous to workers and the environment and generate toxic waste. It is also known that titanium nitride (TiN) can be deposited in layers, for example by plasma spraying or sputtering, and that these layers are very hard and biocompatible.

It is also known to use spraying methods for depositing TiN layers on cutting tools, in order to improve their performance. Although effective, these heterogeneous layers can separate from the substrate. They are avoided in the field of medical devices

implantable, because of this risk, which can create debris

dangerous.

There is a known method of laser for hardening the surface of titanium and its alloys (UK Patent No. 2183692, IPC C22F 1/18, published the

20.06.2002), comprising a laser surface treatment in an air environment. The disadvantages are the complexity of the equipment, the low productivity and a low depth of hardening.

There is a process for the surface treatment of titanium and its titanium alloys (UK Patent No. 2318077, IPC S23S 8/6, published on February 27, 2008), comprising a heat treatment at 950 ° C. in an atmosphere. active gas consisting of 10% (by weight) nitrogen and 90% (by weight), under argon.

The patent application W01 1 161251 describes a device capable of generating a plasma jet from a compressed gas. US5326362 also describes a surface hardening process of a titanium implant, including a hip prosthesis, by heat treatment in a nitrogen atmosphere. This process, like those mentioned above, is based on the diffusion of nitrogen at high temperature, and is very slow. Even with a treatment duration of several days, only very thin nitride layers, of a micrometer or fraction of a micrometer, are obtained.

Brief summary of the invention

An object of the present invention is to provide a component surface hardening process of titanium, or titanium alloy, by a plasma jet. This method has proved very effective, requiring only a few seconds of treatment, and allows the creation of thick nitride layers, with a gradual chemical transition in the substrate.

Another object is a product made of titanium or titanium alloy with a homogeneous protective, decorative layer, and with

higher hardness and friction characteristics.

Another object of the present invention is to provide a production system for treating metal articles with the method raised in an efficient and economical way, to obtain a product

efficient and highly uniform. These objects are achieved by the objects of claims

in the corresponding categories.

Description of figures

The accompanying figures show non-limiting examples of the invention and allow a better understanding:

• Figure 1 is a simplified diagram, not to scale, of a plasma nozzle facing a target;

Figure 2 is a diagram of the measured nitrogen concentration in titanium products treated according to the invention; FIGS. 3 and 4 are two photomicrographs of the surface of the products treated according to the invention;

• Figure 5 schematically illustrates a processing facility, and "Figure 6 shows a detail.

• Figure 7 is a diagram of the nitrogen concentration in

 depth function characterizing the products of the invention;

FIGS. 8 and 9 show a variant of the invention with

 characteristics to control the thickness of the nitrided layer and its final characteristics, thus increasing the toughness of the final product.

Example (s) of embodiment of the invention

The invention comprises a plasma jet treatment at atmospheric pressure of nitrogen. By "atmospheric pressure plasma" is meant, in the context of this invention, a plasma jet in an environment in which the pressure approaches that of the atmosphere. It should not be believed, however, that the invention is limited to a limited range of pressures. The method of the invention, in particular, may include supersonic plasma jets that strike the target at high speed in a confined reactor. The reactor is not in a state of static equilibrium and the pressure inside it can vary considerably. To fix ideas, it is conceivable that the average pressure in the active region of the reactor is somewhat higher than atmospheric pressure, possibly decreasing gradually to the reactor openings. Locally, however, the total pressure, including the dynamic pressure generated when the plasma jet strikes the target, may be much higher. It is estimated that the total pressure at the target is

approximately between 5 and 20 bar, ie plasma speeds of between 600 and 1400 m / s, and the thermal power of the plasma is greater than 50 W / mm ² , able to reach and exceed 100 and 200 W / mm ² .

The jet is generated by an electrically energized nozzle, and directed to the workpiece, as shown in Figure 1. The illustrated device is similar to that described by W01 1 161251, and the sources produced by Swissnanocoat SA . Its structure will be recalled here briefly. A hollow metal cathode 65 encloses a discharge chamber 107 and, at one end, a coaxial anode 60. The pressurized gas enters the injector device configured to generate a vortex 105 in the axis of the device, for example by virtue of the combined action of the axial injector 104 and the tangential injector 103. The anode 104 is connected to a positive voltage source relative to the cathode, which can be connected to the earth. The swirling motion inside the anode gives rise to an extended electric discharge, and produces a plasma jet 120 at high temperature and velocity. Importantly, the outlet port 68 is configured to produce a turbulent plasma stream.

The mouth 68 may have an abrupt termination, with a cylindrical channel terminating directly on the outer surface of the cathode 65, or a diverging diffuser, approximately conical, the major base towards the surface of the object to be treated. Its internal diameter d is preferably between 1 mm and 3 mm. We can also consider, if the dimensions of the object to be treated justify it, diameters

mouthpiece between 0.1 mm and 50 mm.

This form of plasma source is characterized by the axial extension of the discharge, and by relatively high values of the voltage drop, for example between 800V and 2000V, with an anode current of between 1 and 2A. These values of tension can be obtained by an elongated discharge chamber 107, in which the ratio between the length L and the inner diameter D is greater than 1 1. Particularly satisfactory results have been obtained with nozzles in

which L / D ratio is between 16 and 20, or higher. The electrical power absorbed by each nozzle 20 is greater than 2.5 kW. Although particularly effective for the execution of the inventive method, it is not essential yet. The method of the invention can also be realized by different sources, for example conventional low voltage DC sources, or AC or RF sources. Several discharge gases can be used in the method of the invention. In the case of treatment aimed at improving the

mechanical performance of an orthopedic screw made of titanium, TAN, or another titanium-based alloy, nitrogen will preferably be chosen. It is also possible to envisage the use of a nitrogen-hydrogen mixture (forming gas), for example NH 5, NH 6, NH 8 or NH 10. For a maximum effectiveness of the treatment, the article to be treated is positioned at a short distance from of the nozzle 20, and the size of the orifice 68 is determined because of the size of the target 30 and its anfractuosities. FIG. 6 shows that when treating a screw head, the nozzle 68 is preferably dimensioned so as to send a turbulent plasma jet over the entire head of the screw 30, and also inside the slot 38. The jet velocity at the outlet of the nozzle is typically greater than 500 m / s. It can reach and exceed 1000 m / s. The shape and the section of the nozzle can be adapted according to the surface to be treated and the parameters of the plasma. The diameter of the orifice 68 is preferably between 1 and 8 mm, it will be for example 2-4 mm for the treatment of an orthopedic screw. The distance a between the nozzle and the target is typically 10mm.

The nitrogen mixture with hydrogen, argon or other inert gas also modifies the surface state of the part, and can be considered when trying to give roughness to the part, or we want a surface appearance mast.

Returning now to FIG. 1, the jet of plasma 120 at high speed and concentration causes a rapid surface heating of the target 30. Preferably it is positioned on a support 35 which makes it possible to hold the target firmly in place, and to cool it down. The support can be made of metal, for example copper, and the cooling is preferably provided by a circuit 1 15 in which circulates water, or other refrigerant, temperature controlled.

The inventors have determined that rapid surface heating at high temperatures is beneficial for the process of the invention. The power of the source is chosen so as to obtain, for example, a surface temperature of 1100 ° C. after 3s of treatment. As mentioned above, the density and the velocity of the plasma on the surface of the target are very high. The heat exchange coefficient is

consequence, and these temperature profiles can be

generally reached by a source of a power of a few kW.

The cooling circuit 1 15 limits the rise in temperature along the treatment, for example we seek a

maximum temperature of 1500 ° C after 20s.

At the same time, the inventors have determined that the oxidation state and cleanliness of the surface of the target 30 is a determining factor for the success of the treatment. Preferably, the process comprises a preparation step which precedes the nitriding phase mentioned above. All common chemical and mechanical cleaning processes, for example with solvents, detergents, ultrasounds, etc. are compatible with the method of the invention and can be used in combination.

In the case of titanium articles, a thin oxide layer, such as for example that derived from a brief exposure to the atmosphere, is not detrimental, but deeper alterations, as well as contaminations of the surface by solvents or moisture are very harmful. Therefore, a suitable cleaning and drying treatment is recommended, followed immediately by exposure to the nitrogen plasma jet.

Excellent results were obtained when the nitriding was preceded by a cleaning step by an inert plasma jet, for example an argon plasma jet. The characteristics of speed and Thermal distribution of the plasma preparation jet may be substantially the same as that of the nitrogen plasma jet. The plasma jet of preparation can be generated by the same nozzle as the jet of nitrogen plasma, or by a nozzle provided for this purpose. Exposure to argon plasma at low pressure has also been shown to be effective. Preferably, the cleaning treatment and the nitriding treatment are carried out under a protective atmosphere, for example nitrogen and / or argon, in order to avoid any contact with the atmosphere.

FIG. 5 diagrammatically illustrates a processing device that can be used in the context of the invention. The workpieces 30, fixed on their supports, are moved by a transport device 195 in a tunnel reactor 190. The workpieces 30 pass successively opposite one or more atmospheric pressure plasma sources 20, for example an Ar source for cleaning and removing the oxide layer, and one or more N sources for deep nitriding of the surface.

The reactor 190 can be opened to the atmosphere, through the slots 198, so as to evacuate the gas blown by the sources 20. The openings 198 are dimensioned so as to give rise to an overpressure between the inside of the reactor and the atmospheric pressure sufficient to exclude any contamination of oxygen, water vapor, and other impurities.

To more effectively protect the surfaces to be treated, it is possible to provide means for filling the reactor 190 with nitrogen, or inert gas, and for evacuating it prior to treatment. For this purpose, the treatment system of the invention may include one or more vacuum pumps, a gas supply system at the desired pressure, as well as pipes and valves adapted to isolate the reactor from the atmosphere, the communicating with the suction port of the pump and / or with the gas supply lines. During the plasma treatment, the gases blown by the sources 20 can be discharged directly towards

the atmosphere, as described above, or through the vacuum pump. The jets of plasmas give rise to a temperature and pressure gradient inside the reactor 190, the treatment zone being

pressurized, hot and oxygen free. The circulation of gases

the inside of the reactor 190 provides preheating of the parts before the treatment. Preferably, the liquid flowing in the circuit of

Cooling 1 (see Fig. 1) has a relatively high temperature, eg 40 ° C, to aid preheating and drying. In general, the temperature of the outgoing gases is much lower than 1000 ° C., and there is no formation of NO _x when these are mixed with the oxygen of the atmosphere.

Preferably, the processing device of FIGS. 5 and 6 operates in "batch" mode, in the sense that the parts to be loaded are

previously positioned in sufficient number on the transport system, and then presented one after the other to the plasma sources 20 to be processed. The invention also includes continuous flow embodiments and variations in which the articles are stationary, while the plasma nozzles 20 move to treat one after the other.

[0041] Preferably, the transport system 195 and the sources 20 are arranged so that each source processes a single piece 30 at a time, each piece 30 being presented to a single source 20 at a time, for a maximum bombardment intensity. .

The processing device of FIGS. 5 and 6 admits

many variants, still within the scope of the invention. The transport system 195, for example, can assume all shapes known in the art, including ribbons, carousels, etc. The transport device could move the pieces along a linear path, or curve. Treatment plants without a solid enclosure could also be envisaged, in which the treatment region is kept free of oxygen by a dynamic flow of protective gas. Compared with processes that are based on simple heating in a nitrogen atmosphere, in the process of the invention nitrogen is used to the generation of plasma which, therefore, is not limited to heating the room, but brings a high concentration of ion, radicals, and excited nitrogen atoms. Compared with conventional low pressure plasma treatments, the process of the invention is characterized by

much higher concentration (by a factor of 10 ⁵ ) in excited and / or ionized nitrogen atoms. The highly turbulent flow of the plasma on the article 30 makes it possible to effectively treat all the exposed surfaces, including the vertical walls of the cavities 38. It is also observed that the nitride layers are formed at a speed much greater than that obtained when the article is heated to comparable temperatures by other means, for example in an oven, by an arc, or with a plasma of another chemical species.

Without wishing to be limited by theory, the high temperature is also advantageous because higher than those to which the formation of T12N is favored. It is considered that the formation of T12N in conventional methods limits diffusion depth. Since the formation temperature of T12N is about 1100 ° C, rapid heating to a higher temperature limits the concentration of this chemical species in the surface layers. Preferably, the treatment method comprises the following phases:

• Preferably, the treatment process involves the phases

 following:

• pretreatment with a plasma jet of Ar ⁺

· Reactor filling with protective atmosphere of N2

 • application of a plasma jet at atmospheric pressure, or

 higher, with a velocity of v> 500 m / s, preferably v> 1000 m / s;

 • heating by the plasma jet at a temperature of 1300 ° C-1500 ° C, so as to increase the formation of TiN and limit the formation of

T12N to the deepest possible layers.

• treatment time: 3-60s • cooling by the optimized sample holder.

FIG. 2 illustrates the Ti and N concentration profiles after 3s (curve 140) and 20s (curve 130) of treatment. The double scale of the abscissa indicates the energy of the probe beam and the depth

corresponding, in micrometers.

It is clearly seen that the N / Ti ratio approaches the stoichiometric ratio 50/50 for a considerable thickness, with a gradual transition zone with a chemical gradient, up to the pure titanium of the substrate (ratio N / Ti = 0). / 100). The duration of the treatment determines the thickness of the nitrided layer.

The superficial micro hardness of a pure titanium sample was measured by nanoindentation, with a durometer CSM instrument, with the results summarized in the following table:

Untreated measure 3s 20s

HIT (O & P) average 2060.518 9665.076 10415.305

[MPa] deviation 90.475 330.938 1781.671

HVIT (O & P) average 190,826 895,091 964,571

[Vickers] diversion 8.379 30.648 165.002

EIT (O & P) average 73,713 129,213 109,465

[GPa] deviation 4.916 3.712 10.545

Table 1 [0049] A very significant increase in surface hardness is observed, and also a smaller deviation than that obtained, for example, by conventional anodizing processes, so that the method of the invention also allows standardization of hardness, at the same time as its increase. HIT surface hardnesses greater than 10 GPa, typically about 12 GPa, were obtained 5 times higher than those of titanium. In addition, the thickness of the TiN layer is greater than 5 μηη, for example 10 μηη. Figures 3 and 4 show the surface condition after 3,

respectively 20s of treatment.

The nitride layer is homogeneous, without continuity solution or steep transitions, the concentration of nitrogen decreasing

progressively inward, below the fully nitrided zone, eliminating any risk of delamination. T12N is formed below the TiN layer at an estimated depth of 10-50 μηη.

An advantage of the method of the invention is that the implementation is fast, and that it can also treat the vertical walls inside the cavities, including the notches in the screw heads.

The layers thus produced have the yellow color proper to TiN, and this color is resistant to the usual mechanical and galvanic treatments and localized in the areas affected by the plasma. It is possible to envisage applications for marking and decorating parts made of titanium or titanium alloy. Furthermore, a particularity of the inventive method is that the yellow layer extends inside the cavities and crevices of the product, in particular in a screw head, the vertical walls of the notch, as well as the bottom of the notch , are covered by a uniform yellow layer. By the process of the invention it is also possible to produce products whose part of the surface is yellow-gold, resulting from the nitriding process, and another has a different color, obtained by another route, for example by anodizing. For this purpose the parts will be prepared in order to protect from the plasma jet the surfaces that will have to be

successively anodized. The nitrided surface is not affected by the galvanic bath and it is not necessary to protect it. The pieces thus obtained have two or more surface colors obtained by two different treatments, and the color combinations can be used in a coding. The method of the invention can be advantageously also applied to threaded rods of orthopedic screws, to increase the hardness and tribological performance.

Furthermore, the method is not limited to screws, but is also applicable to osteosynthesis plates that the screws must fix on the bone, to take advantage of the tribological characteristics and higher hardness of the TiN at these locations.

It is also possible, by the method of the invention, implantable titanium devices with hardened articulation and friction surfaces and with a reduced coefficient of friction, for example total hip, shoulder prosthesis, etc.

The process of the invention is not limited to a nitrogen plasma but may use other inert or reactive gases. Among the inert gases argon may be mentioned. These gases do not chemically bond to the substrate, but the bombardment with an inert gas is useful for preparing the surface, as already mentioned, and can modify the roughness of the treated object.

Among the reactive gases, oxygen can be used to create surface layers of titanium oxide or oxynitride TiO _x N _y , for example to achieve color effects. It is also possible to envisage the use of nitrogen mixtures and carbon compounds to obtain TiCN titanium carbonitride layers. For example, a gaseous mixture comprising nitrogen and hydrocarbons could be used.

SNC technology can be applied to titanium parts to manufacture a new product, characterized by the concentration profile shown in Figure 7. The product comprises a homogeneous surface layer 41 of TiN, TiCn, or TiO _x N _y , with a thickness greater than 5 μηη, for example 10 μηη, followed by a transition zone 44 with a

gradually decreasing concentration of TiN, TiCn, or TiO _x N _y , of a thickness greater than 5 μηη followed by substrate 46 of titanium, TAN, or another titanium-based alloy, essentially free of nitrogen. Referring now to Figure 8, there is described a reactor for treating a titanium article, in the example a screw, including a surgical screw, with a thread on the outside of the head for orthopedic applications, with advanced control of the temperature time profile. The inventors have found that the application of high-power atmospheric plasma in the form of brief pulses, typically lasting between 1 ms and 2 s, leads to a better toughness of the final product.

The duration, or width, of plasma application pulses determines the depth of penetration of heat into the material. The idle time between the pulses allows a fast cooling, an exact control of the temperature is possible by acting on the width of the pulses or, alternatively, on the duty cycle.

The pulse number determines the amount of nitrogen dissolved in the surface layers of the treated article. The number can be determined beforehand by thermal and diffusion models, or by laboratory tests. Typically, the treatment of a screw head requires several tens of pulses.

Preferably, the total power (all plasma sources combined) plasma jets is greater than 3 kW, better still, greater than 5 kW. The power during the pulse is preferably sufficient to heat the surface of the titanium to the melting point (1500 ° C) in 0.01 seconds.

During the pulse, the surface layers of the article are melted to a depth of 0.01-2 micrometers. The melting depth is determined by the duration of the pulses. The molecules, atoms and nitrogen ions react vigorously with the melted surface, and it can be estimated that the surface metal reaches a saturation level of nitrogen after a determined number of pulses, typically around ten. Between the pulses, the molten layers are rapidly cooled, in particular because of the thermal conduction to the deep, cold layers, and solidify it. These layers largely determine the hardness and microhardness of the article after the treatment, and may be partially in an amorphous state. Optimization of treatment is paramount here. The thickness of the cured layer is determined by the duration of the treatment, and the width of the pulses. Overheating should be avoided as it could weaken this surface layer.

A too long treatment period is also to be avoided because it can lead to a growth of crystalline grains, which is also a source of fragility. In general, the penetration of heat is 1000 times faster than the diffusion of nitrogen.

The inventors have found that, during a plasma treatment lasting 15s, the metal is melted more solidified to a thickness of 3 microns. Changes in crystal structure extend up to 1 mm and are detectable by increasing grain size.

The pulsed plasma method defined above allows a reduction and a better control of the depth to which the heat penetrates, and reduces the embrittlement, by acting on the width and the duty cycle of the pulses.

This technique is favorable to pulsed laser nitriding. The laser treatment is generally localized, depending on the diameter of the beam which can have a size of 0.01 -1 mm. The energy density is extremely high, which involves evaporation and extensive material transport, with crater formation. The surface thus obtained is rough and, in general, fragile.

The heating that can be obtained by the pulsed plasma technique of the invention is more homogeneous and may interest at the same time complex surfaces, for example the round, threaded surface of an orthopedic screw head. In addition, the nitrogen is in an active state and its density is higher. This leads to faster nitriding, and a smoother and tougher surface.

The nitrogen atmosphere is sufficient to protect against oxidation. Nitrogen is rapidly absorbed by the melted layers on the surface and diffuses inwards. The inventors have obtained yellow TiN layers of a depth of more than 500 nm with applications of a few seconds of plasma. It is believed, without this theoretical assessment limiting the invention, that the presence of nitrogen has a positive role during solidification by increasing the percentage of vitrified material. The heat treatments pushed can change the

crystalline microstructure, increasing the size of the grains and weakening the material in depth. The application of heat by short pulses reduces the thickness of this weakened zone.

This undesirable effect can also be reduced by a heat treatment before and after the application of the plasma, for example preheating at 500 ° C. and post-heating at 500-600 ° C., followed by

gradual cooling. These parameters can be optimized according to the nature of the article, its size, its geometry, the power of the plasma and so on. The application of ultrasound source is advantageous during the solidification. The crystalline structure results in finer and the percentage of amorphous material higher.

9 illustrates a variant with an ultrasonic source for the ultrasonic refining of crystalline grains during solidification. Satisfactory results (grain size reduction) were obtained from a 750W to 20 kHz active source during plasma treatment and cooling.

The treatment reactor of the invention comprises

preferably a plurality of atmospheric plasma sources generating convergent plasma jets on the surface to be treated. In this case, the head of the screw.

The example shown comprises three plasma sources generating three convergent and symmetrical plasma jets, the article to be treated being placed in the intersection point. The number of jets is not a limiting feature, however, and the invention could include a single jet, to treat small objects, or four, five or more, for larger articles.

The treatment chamber also has orifices for the evacuation of gases. They are preferably, as in this example, in the opposite position to the plasma sources, but their number and position are not critical.

The heat shield allows to isolate the barrel of the screw of the direct action of the plasma, and avoids that this critical area for the strength and strength of the screw is weakened. According to another aspect of the invention, the screw has a special profile which makes it possible to minimize this undesirable effect.

The description specifically refers to the curing of titanium articles, however, the present invention is not so limited and can be applied favorably to hardening of titanium alloy articles, or articles made of a metal capable of hardening by nitriding. In this case the method and system of the invention gives rise to a product having a homogeneous surface layer of MN, MCN, or MOxNy, with 'M' denoting the metal capable of nitriding hardening. The special case M = Ti refers, of course, to the hardening of titanium mentioned above.

Among the materials capable of hardening by nitriding are considered: titanium and all its alloys, ferrous metals and steels, including strongly bonded steels, nickel-based alloys, and aluminum and its alloys . The surface of articles treated by the process of the invention has unique characteristics that contribute to its superior mechanical characteristics. In particular, it comprises a homogeneous amorphous or partially amorphous layer of variable thickness, but

preferably between 1 and 5 μηη, and a gradient of decreasing nitrogen concentration while progressing inwards.

The table above summarizes the differences between the product of the process of the invention and those of known curing processes, for example Glow-discharge nitriding.

 Table 2

Although the invention has been described with reference to a titanium screw, this is not the only application possible. The claimed process makes it possible to improve the hardness and the erosion resistance of any article made of titanium or a material capable of hardening by

nitriding. It can be used in particular to treat turbine wheels and turbine blades of a metal capable of nitriding hardening, in particular to improve the erosion resistance, as well as the hardness, without compromising toughness.

An advantage of the inventive method is that it does not use toxic substances and that it does not generate polluting waste. Reference numbers

[0087]

20 source of atmospheric plasma

30 target, screw, item under treatment

35 support with cooling

38 slot

 39 nitrided layer

 41 zone saturated with nitrogen

 44 transition zone

 46 substrate

60 anode

 65 cathode

 68 outlet port

 103 axial inlet of the compressed gas

104 tangential entry of compressed gas 105 vortex

 107 discharge chamber

 1 10 cooling of the source

 1 15 cooling the target

 120 plasma jet

130 nitrogen concentration after 20s

140 nitrogen concentration after 3s

170 source of ultrasound

 190 reactor

 195 conveyor, transport system 196 heat shield

 198 opening, slot

Claims

claims A method of surface treating an article made of titanium, titanium alloy, or a metal capable of nitriding hardening, comprising exposure to a plasma jet of nitrogen, oxygen, or a mixture or nitrogen / hydrogen, or a nitrogen / oxygen mixture at atmospheric pressure or higher, until a layer of MN is obtained , MCN, or MOxNy of determined thickness, with 'M' denoting titanium or metal capable of nitriding hardening, followed by a transition layer in which the titanium concentration increases gradually to reach the titanium concentration of the substrate of the article. 2. Method according to the preceding claim, wherein the thickness of the MN layer, MCN, or MO _x N _y is greater than 1 μηη, preferably greater than 5 μηη. 3. Method according to one of the preceding claims, wherein the exposure to the nitrogen plasma jet is preceded by a preparation comprising an exposure to a plasma jet of inert gas, for example argon. 4. Method according to one of the preceding claims, wherein the thickness of the transition layer is greater than 1 μηη, preferably greater than 5 μηη. 5. Method according to one of the preceding claims, wherein the surface of the article is heated by the plasma and reaches a temperature above 1100 ° C in a time less than or equal to 5s, preferably 3s. The method according to one of the preceding claims, wherein the power of the plasma jet at the surface of the article is greater than 50 W-mm ² , preferably greater than 100 W-mm ² , more preferably greater than 200 W -mm ² . 7. Method according to one of the preceding claims, wherein the dynamic pressure of the plasma jet on the surface of the article is greater than 1 bar, preferably greater than 10 bar, more preferably greater than 20 bar. 8. Method according to one of the preceding claims, wherein the plasma jet has a speed greater than 500 m / s, preferably greater than 1000 m / s. 9. Article made of titanium, titanium alloy, or a metal capable of nitriding hardening with a layer of MN, MCN, or MO _x N _y , with 'M' denoting titanium or metal capable of hardening by nitriding, with a surface thickness greater than 1 μηη, preferably greater than 5 μηη, followed by a transition layer in which the concentration of titanium increases gradually until the titanium concentration of the substrate of the article, whose thickness is greater than 1 μηη, preferably greater than 5 μηη. 10. The article of the preceding claim, having a surface nano-hardness HIT greater than 8 GPa. 1 1. The article of one of claims 9-10 being one of: a screw orthopedic, an osteosynthesis plate, an implantable device, an articulated prosthesis. 12. The article of one of claims 9-1 1 having a yellow-gold surface and having a HIT surface nano-hardness in the range 8- 10 GPa. The article of one of claims 9-12 having a yellow-gold surface and a second surface of a different color. A system for treating titanium, titanium alloy, or nitriding hardening metal parts comprising: at least one source (20) generating a high speed plasma jet (120); nitrogen, oxygen, or a nitrogen / hydrogen mixture, or a nitrogen / oxygen mixture at atmospheric pressure or higher, a transport system (195) for carrying a plurality of targets (30) on carriers (35) provided with a cooling system, facing said at least one source and expose it to the plasma jet, until obtaining a layer of MN, MCN, or MO _x N _y , with 'M designating titanium or nitriding hardenable metal of a determined thickness, followed by a transition layer in which the titanium concentration gradually increases to the titanium concentration of the substrate of the article. 15. System according to the preceding claim, wherein the thickness of the MN layer, MCN, or MO _x N _y is greater than 1 μηη, preferably greater than 5 μηη. 16. System according to one of claims 14 to 15, wherein the exposure to the jet of nitrogen plasma is preceded by a preparation comprising exposure to a plasma jet of inert gas, for example argon. 17. System according to one of claims 14 to 16, wherein the thickness of the transition layer is greater than 1 μηη, preferably greater than 5 μηη. 18. System according to one of claims 14 to 17, wherein the surface of the article is heated by the plasma and reaches a temperature greater than 1100 ° C in a time less than or equal to 5s, preferably 3s. 19. System according to one of claims 14 to 18, wherein the plasma jet has a speed greater than 500 m / s, preferably greater than 1000 m / s. 20. System according to one of claims 14 to 19, wherein the source (20) comprises a substantially cylindrical discharge chamber, contained in a hollow cathode (65), with an anode (104) axially positioned at one end of the discharge chamber, and a mouth (68) for emission of plasma at an opposite end of the discharge chamber opposite the anode (104), one or more carrier gas supply ports (103, 104) shaped to generate a vortex (105) within the discharge chamber, wherein the ratio of a length ( L) and inner diameter (D) of the discharge chamber is greater than 1 1, preferably greater than 16, more preferably greater than 20. 21. System according to the preceding claim, wherein the anode current is less than 2A, the voltage drop between anode and cathode is between 800V and 2000V, the power preferably greater than 2.5 kW. 22. System according to the preceding claim, wherein a diameter of the mouth (68) is between 0.1 mm and 50 mm, preferably between 1 mm and 3 mm, and has a cylindrical or conical shape. 23. System according to the preceding claim, wherein the mouth (68) has a conical shape with the larger opening to the object to be treated. 24. System according to one of claims 14 to 22, wherein the plasma jets (120) produced by the sources (20) are directed inside a treatment reactor (190) comprising at least one passage (198) for discharging the gases blown by the sources (20), the passage (198) being connected to the suction port of a pump, or giving directly in atmosphere. 25. System according to one of claims 14 to 23, wherein the transport system (195) and the sources (20) are arranged so that each piece (30) is presented to a single source (20). at a time, and each source (20) processes a single piece (30) at a time. 26. System according to one of claims 14 to 24 having a temperature gradient which provides a preheating of the parts (30) before exposure to the plasma jets. The system of claim 14, comprising the at least one source (20) arranged to generate a pulse plasma jet, characterized by active time intervals of limited duration during which the power of the plasma jet is nominal, separate by inactive intervals during which the power of the plasma jet is zero, or substantially reduced. 28. System according to the preceding claim, wherein the width of the active intervals is between 0.0001 s and 5 s, preferably less than 100 ms, preferably between 1 and 2 ms, and / or the nominal power of a source is included between 3kW and 5kW. 29. System according to one of claims 27 or 28, comprising a controller arranged to determine the width, and / or the frequency, and / or the duty cycle of the active intervals.