Dental tool or instrument comprising a working part having varying mechanical properties

Abstract

The present disclosure concerns a dental tool or instrument (100) comprising a working part (110) made of a nitinol alloy. The nitinol alloy of the working part (110) is transitionable from an R-phase alloy state when heated above a transition temperature. The working part (110) comprises at least a first portion (111) having a first transition temperature (TT1) and at least a second portion (112) having a second transition temperature (TT2). The first transition temperature (TT1) is a temperature lower than a working temperature such that, at working temperature, the first portion (111) is in the austenitic phase. The second transition temperature (TT2) is a temperature greater than the working temperature such that, at working temperature, the second portion (112) is in the R- phase. The present disclosure further concerns a method of manufacturing the dental tool or instrument.

Description

Dental tool or instrument comprising a working part having varying mechanical propertiesTechnical domain

[0001] The present disclosure concerns a dental tool or instrument comprising a working part destined to be engaged in the root canal and comprising portions having different physical characteristics based on local specific mechanical properties and specific geometry. The present disclosure concerns a method of manufacturing the dental tool or instrument.Related art

[0002] The introduction of NiTi in the fabrication of dental tools and instruments has resulted in improving the efficiency, quality of the dental tools and instruments and patients' experience and satisfaction. Indeed, dental tools and instruments made of a Niti alloy such as nitinol have low elastic modulus and show superelasticity. The effective strain range of NiTi is much larger than that of stainless steels. Dental tools and instruments made of a Niti alloy have improved fatigue resistance.

[0003] Endodontic instruments, including files, reamers, and broaches comprise have a cutting section or so-called working part configured to form, trim, and clean the inside walls of the root canal in order to prepare it for receiving the treatment materials and then a filling material, in order to exclude any oxygen that could enable bacteria to develop in the tooth and particularly in the vicinity of the root.

[0004] There are a variety of factors which dictate the required physical characteristics of such instruments. These include the desired malleability and / or elasticity of the instrument, as well as the sharpness of its cutting edges (which relates to the hardness as well as the structure of thematerial) coupled with certain dimensional and design limitations for the different root canals.

[0005] Titanium based alloys and Ni / Ti materials, such as nitinol alloys, have been introduced for use in the manufacture of endodontic instruments. For example, European patent number EP0402293B1 relates to endodontic canal instruments made of titanium or titanium alloys. The use of materials such as titanium or Ni / Ti have certain advantages in the flexibility of the material. However, endodontic instruments of such materials may have as a drawback the lack of necessary stiffness, particularly in small sized (diameter) instruments, sufficient to provide guidance in the root canals. Furthermore, the sharpness of the cutting edges in such instruments is compromised due to the lower hardness of the material.

[0006] Document US6431863B1 discloses an endodontic instrument having a working shaft portion made of a nickel-titanium alloy wherein the shaft portion has a modulus of elasticity that varies along its length to such that the tip of the working shaft portion has a greater hardness than the rest of the working shaft portion. The flexibility / stiffness of the instrument is controlled by selected heat treatment of specific areas of the working shaft.

[0007] There is a need for a dental tool or instrument which has a certain malleability and elasticity that enables the dental tool or instrument be used with increased efficiency. For example, an endodontic instrument that can better adapt to the longitudinal profile of the root canal.Summary

[0008] The present disclosure concerns a dental tool or instrument comprising a working part being made of a nitinol alloy. The nitinol alloy of the working part is transitionable from an R-phase alloy state to an austenitic phase alloy state when heated above a transition temperature.The working part comprises at least a first portion having a first transition temperature and at least a second portion having a second transition temperature. The first transition temperature is a temperature lower than a working temperature of the working part such that, at working temperature, the first portion is in the austenitic phase. The second transition temperature is a temperature greater than working temperature such that, at working temperature, the second portion is in the R-phase.

[0009] The present disclosure further concerns a method of manufacturing the dental tool or instrument.

[0010] In addition to having a high resistance to fracture and cyclic fatigue, the dental tool or instrument disclosed herein provides mechanical properties varying along the length of the working part. More particularly, the dental tool or instrument has a transition temperature between the R- phase and austenitic phase that varies, and thus nitinol microstructures that varies, along the length of the working part. The dental tool or instrument has enhanced safety and performance capabilities.Short description of the drawings

[0011] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:Fig. 1 illustrates an endodontic instrument comprising a working part, according to an embodiment;Fig. 2 illustrates the endodontic instrument, according to another embodiment;Fig. 3 represents the endodontic instrument according to another embodiment;Fig. 4 shows a detail of the working part of the endodontic instrument of Fig. 3;Fig. 5 shows the working part of the endodontic instrument of Fig. 2, according to an embodiment;Fig. 6 illustrates a variant of the endodontic instrument of Fig. 5; andFig. 7 illustrates another variant of the endodontic instrument.Examples of embodiments

[0012] Fig. 1 illustrates an endodontic instrument 100 (or another type of taper instrument) for cleaning a root canal. The endodontic instrument 100 comprises a working part 110 that generally extends along a longitudinal axis 101, between an apical end 103 and a shaft 104 of the endodontic instrument 100. The working part 110 is destined to be engaged in the root canal to be treated. The endodontic instrument 100 can further comprise a shank (not shown) secured to the shaft 104 and destined to be mounted on a rotating support to cause the endodontic instrument 100 to rotate about its longitudinal axis 101. Instead of a shank, the endodontic instrument 100 may comprises a handle such as to manually rotate the endodontic instrument 100. The working part 110 can comprise an apical segment 120, substantially straight relative to the longitudinal axis 101, and an intermediate segment 130, extending between the apical segment 120 and the shaft 104. The apical segment 120 and the intermediate segment 130 are arranged sequentially from the apical end 103 to the shaft 104.

[0013] The working part 110 is made of a nickel titanium alloy, also known as nitinol, containing nickel and titanium in roughly equal atomic percentages. Nitinol is a shape memory alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated. Nitinol can be malleable and superelastic at different temperatures.

[0014] The nitinol alloy can comprise a medical grade nitinol, for example such as defined in the ASTM standard 2063:2018 containingbetween 54.5%wt and 57%wt Ni. The nitinol alloy has less than 0.1 %wt of another element than Ni and Ti.

[0015] Nitinol can be characterized by an interpenetrating simple cubic crystallographic structure referred to as austenite phase. Nitinol can further spontaneously transform to a more complicated rhombohedral crystallographic structure known as R-phase. The passage between the R- phase and the austenite phase occurs at a transition temperature (austenite finish temperature, Af). The nitinol is in the R-phase and malleable below the transition temperature and is in the austenite phase and superelastic above the transition temperature. A superelastic alloy can deform reversibly to very high strains.

[0016] In an embodiment, the working part 110 comprises at least a first portion 111 having a first transition temperature Tn and at least a second portion 112 having a second transition temperature TT2. The first transition temperature Tn is a temperature lower than a working temperature such that, at working temperature the first portion 111 is in the austenitic phase. In the case of an endodontic instrument where the working part 110 is used in a root canal, the working temperature can correspond to body temperature. Here, body temperature means normal human bodytemperature, typically between 36.5°C and 37.5°C. In fact, body temperature further corresponds to an operating temperature of the working part 110. The second transition temperature TT2 is a temperature greater than body temperature such that, at body temperature the second portion 112 is in the R-phase.

[0017] When using the endodontic instrument 100, the working part 110 is within a root canal and is heated at the body temperature. Once within the root canal, the first portion 111 is above the first transition temperature Tn and is in the austenitic phase. The first portion 111 is thus superelastic. The second portion 112 is below the second transition temperature TT2 and is in the R-phase. The second portion 112 is thus malleable.

[0018] Once the working part 110 is within the root canal, the austenite phase in the first portion 111 allows for the first portion 111 to be substantially static relatively to the second portion 112 and can have an improved cutting efficiency. The R-Phase allows the second portion 112 to deform with low elastic feedback (malleable) under radial constraint (no spring effect). The second portion 112 has an enhanced malleability relative to the first portion 111 and can act as a pivot for the first portion 111.

[0019] In the case of a working part 110 that extends straightforwardly (such as the endodontic instrument 100 shown in Fig. 1), the elastic first or second portion 111, 112 minimizes angular deflection under torsional constraint. The other malleable second or first 112, 111 provides a "prebending feature" or guiding capability only where it is required along the working part 110. This is particularly advantageous for instrument having a small wire diameter.

[0020] In a possible configuration shown in Fig. 1, the working part 110 comprises one second portion 112 extending along the apical segment 120 and one first portion 111 extending along the intermediate segment 130. When the working part 110 is within a root canal and is heated at the body temperature, the apical segment 120 is in the R-Phase and can deform with low elastic feedback (malleable) under radial constraint. The intermediate segment 130 is in the austenite phase and has enhanced elasticity relative to the apical segment 120.

[0021] In another possible configuration (not shown), the working part 110 comprises one first portion 111 extending along the apical segment 120 and one second portion 112 extending along the intermediate segment 130.

[0022] In yet another possible configuration shown in Fig. 2, the working part 110 comprises a plurality of first portions 111 and a plurality of second portions 112 arranged in alternance. In the example of Fig. 1,the working part 110 comprises two second portions 112 arranged between three first portions 111. The apical segment 120 corresponds to a first portion 111. The alternance of the first and second portions 111, 112 can be reversed relative to the one shown in Fig. 2. In that case, the apical segment 120 corresponds to a second portion 112.

[0023] In one aspect, the first transition temperature Tn can be below 35°C, for example, equal or below a temperature between 35°C and 20°C. The second transition temperature TT2 can be equal or above 39°C, for example above a temperature between 39°C and 45°C. Alternatively, the first and / or second transition temperature Tn, TT2 is in the vicinity of the body temperature, for example between 32.5°C and 35°C such that the first and / or second portion 111, 112 exhibits the shape memory effect, i.e., the first and / or second portion 111, 112 is in the R-phase and is malleable at room temperature and in the austenitic phase and superelastic at body temperature. Here, room temperature means a temperature between 20°C and 25°C.

[0024] Fig. 3 represents the endodontic instrument 100 according to another embodiment, where the intermediate segment 130 comprises a first curved part 131 and a second curved part 132. The endodontic instrument 100 is represented comprising a shank 105 secured to the shaft 104.

[0025] Fig. 4 shows a detail of the working part 110 of the endodontic instrument 100 of Fig. 3. The first curved part 131 has a first peak A1 relative to the longitudinal axis 101 and the second curved part 132 has a second peak A2 relative to the longitudinal axis 101. The first and second peaks A1, A2 respectively correspond to the maximal amplitude of the first and second curved parts 131, 132, i.e., the greatest distance of the first and second curved parts 131, 132, relative to the longitudinal axis 101. The first curved part 131 can be curved along the same plane as the second curved part 132, but in an opposite direction.

[0026] Also shown in Fig. 4, the apical segment 120 may have a tapered section from the intermediate segment 130 toward the apical end 103. An apical envelope 125 of the apical segment 120 forms an apical taper angle a relative to the longitudinal axis 101.

[0027] The first peak A1 and the second peak A2 are respectively located at a first distance D1 and a second distance D2 from the apical end 103 (towards the shaft 104). The first peak A1 can be smaller than the second peak A2, such that the intermediate segment 130 has a tapered shape from the shaft 104 toward the apical end 103. As shown in Fig. 4, an intermediate envelope 135 of the intermediate segment 130, tangent to the first and second peaks A1, A2, forms an intermediate taper angle 0 relative to the longitudinal axis 101.

[0028] In an aspect, the apical taper angle a can be smaller than the intermediate taper angle 0. The small apical taper angle a corresponds to the tapering of a narrower endodontic instrument typically used in earlier stages (when entering the canal). The larger intermediate taper angle 0 of the intermediate envelope 135 corresponds to the tapering of a wider endodontic instrument as used in the final stage or stages of a root canal procedure.

[0029] As described in international patent application number WO2022IB61697 by the present applicant, the working part 110 with the intermediate envelope 135 may act as a whip that brushes against the walls of the canal and removes the soft portions that can line the surface thereof. The function of the working part 110 is that of cleaning the surface of the canal walls, regardless of the geometry of the canal. The shape of the working part 110 allows for better removal of the debris in the canal.

[0030] In an embodiment shown in Fig. 5, the working part 110 can comprise one second portion 112 between the first peak A1 and the apical end 103, one second portion 112 between the first peak A1 and the secondpeak A2, and one second portion 112 between the second peak A2 and the shaft 104. More generally, at least one second portion 112 can be arranged between the first peak A1 and the apical end 103. At least one second portion 112 can be arranged between the first peak A1 and the second peak A2. At least one second portion 112 can be arranged between the second peak A2 and the apical end 103. The apical segment 120 corresponding to a first portion 111.

[0031] In the configuration of Fig. 5, the working part 110 has an improved cleaning efficiency, also in root canals having oval or irregular cross sections. Indeed, during the rotation of the endodontic instrument 100 (with continuous and / or reciprocating movements), inertial centrifugal force is generated on the eccentric parts of the working part 110 that are not on the instrument longitudinal axis 101 (such as the first and second curved parts 131, 132). In this configuration, the eccentric parts can freely move radially due to the R-Phase non-elastic first portions 111.

[0032] The malleable first or second portion 111, 112 can acts as prebending to facilitate the entrance of the working part 110 into the root canal, while insuring strong cutting efficiency in the portion of the first and second curved parts 131, 132 (due to spring effect pushing the curved parts 131, 132 against the root canal walls).

[0033] Again, the alternance of the first and second portions 111, 112 can be reversed relative to the one shown in Fig. 5. In that case, the apical segment 120 corresponds to a second portion 112.

[0034] Fig. 6 illustrates a variant of the endodontic instrument 100 of Fig. 5, where a R-Phase non-elastic first portion 111 extends along the apical segment 120 and a second portion 112 extends along the intermediate segment 130. Alternatively, the second portion 112 can extend along the apical segment 120 and the first portion 111 can extend along the intermediate segment 130.

[0035] Fig. 7 illustrates another variant of the endodontic instrument 100, where the working part 110 comprises four curved parts 131-134, each having a peak A1-A4. The peaks A1-A4 are defined relative to the longitudinal axis 101 and correspond to the maximal amplitude of the curved part 131-134 relative to the longitudinal axis 101. Other geometries are also possible. For example, the working part 110 can comprise three or more than four curved parts.

[0036] Similarly to the configuration of Fig. 5, the first and second portions 111 , 112 can be between the peaks A1 -A4, such that a R-Phase non-elastic portion or an austenitic phase superelastic portion is present between the peaks A1-A4. Alternatively, a R-Phase non-elastic first portion 111 can extend along the apical segment 120 and a second portion 112 can extend along the intermediate segment 130. Furthermore, the second portion 112 can extend along the apical segment 120 and the first portion 111 can extend along the intermediate segment 130.

[0037] It is understood that the present invention is not limited to the exemplary embodiments described above and other examples of implementations are also possible within the scope of the patent claims.

[0038] For example, the endodontic instruments can comprise enlargers, compactors, accessory instruments, shaping and cleaning. The present invention can further concern a dental tool or instrument made of a nitinol alloy, wherein the nitinol alloy of the working part 110 is transitionable from an R-phase to an austenitic phase alloy state alloy state when heated above a transition temperature. The dental tool or instrument can comprise an orthodontic tool or instrument, an orthodontic wire, a matrix band or retainer. The working part 110 can comprise the totality or a portion of the endodontic instrument or the dental tool or instrument.

[0039] In an embodiment, a method of forming the endodontic instrument 100 comprises:providing a nitinol alloy having first or second transition temperature Tn, 2; performing a local heat treatment locally on at least a portion of the working part 110 such that the working area 110 comprises said at least a first portion 111 having a first transition temperature Tn and said at least a second portion 112 having a second transition temperature Tn.

[0040] In one aspect, the local heat treatment comprises locally heating the working part 110 such as to generate the first or second portion 111, 112.

[0041] The local heat treatment can comprise scanning a laser beam along a surface area corresponding to the first or second portion 111, 112. Scanning the laser beam can be carried out by using a two-dimensional laser scanning unit. Scanning the laser beam can be carried out in air atmosphere or another gas composition. The laser beam can be scanned along a longitudinal axis of between 0.5 mm and 5 mm.

[0042] In one aspect, the local heat treatment can be performed with a laser pulse duration between 1 ms and 20 s and at a local heating temperature THL between 300°C and 600°C when generating the first portion 111 and is between 350°C and 800°C when generating the second portion 112.

[0043] In one particular example, the laser pulse can irradiate at a power of about 120 W. The laser beam scans a region of about 103mm. The laser irradiation time is between 0.5 s to 1 s. The laser irradiated region reaches a temperature between 500°C and 700°C. The local heat treatment can be performed such as to obtain an austenite finish temperature Af smaller than 20°C.

[0044] Alternatively, the local heat treatment can be performed by using local induction heating or another rapid local heat treatment process.

[0045] The local heat treatment should be able to locally heat the first or second portion 111, 112 without impacting the rest of the working area 110. Indeed, the nitinol alloy easily propagates the temperature, and the local heat treatment should be performed for a very short duration.

[0046] The method can further comprise performing an initial heat treatment on the whole working part 110 such as to generate the first portion 111 on the whole working part 110, or generate the second portion 112 on the whole working part 110.

[0047] The initial heat treatment comprises placing at least the working part 110 in an oven and heating the working part 110 at an initial heating temperature THI between 300°C and 600°C when generating the first portion 111, and is between 350°C and 800°C when generating the second portion 112. Note that the whole dental tool or instrument can be placed in the oven. The initial heat treatment can be carried out in air atmosphere or in an inert gas such as nitrogen (N), or another gas composition, depending on the desired surface treatment (for example the formation of a titanium oxide).

[0048] The initial heat treatment can be performed during an initial time period PHI between 20 min and 120 min. In a particular example, the initial heat treatment is performed at 400°C during 1 h. This initial heat treatment yields an austenite finish temperature Af of about 40°C.

[0049] The initial heat treatment can be configured such that the whole working part 110 is characterized by the first transition temperature Tn (the whole working area 110 corresponds to the first portion 111). One or a plurality of the second portion having the second transition temperature TT2 can then be added locally along the working part 110. Alternatively, the first heat treatment can be configured such that the whole working area 110 is characterized by the second transition temperature TT2 (the whole working area 110 corresponds to the second portion 112). One or a plurality of the first portion having the first transition temperature Tn canthen be added locally along the working part 110 by using the local heat treatment.

[0050] Alternatively, the first heat treatment can be performed in a mold 30 (see Fig. 6) heated at the first heating temperature THI . Here, only the working part 100 can be inserted in the mold 30. In the case the working part 110 has the profile shown in Figs. 3 to 5, the first heat treatment can be performed in a mold 30 having a shape corresponding the profile. Molding the working part 110 into a molded shape having at least one curved part 131, 132 can be achieved by applying a slight pressure to the working part 110 which is only sufficient to induce the working part 110 to follow a contour of the mold 30.

[0051] The initial and local heating temperatures THI, THL depend on the desired first and second transition temperatures Tn, TT2. For instance, the initial or local heat treatment can be configured to transform the nitinol microstructure by the appearance of Ni rich phase Ni4Ti3 when the nitinol is at working temperature (such as body temperature). The Ni rich phase Ni4Ti3 has the effect of reducing the amount of Ni in the nitinol matrix and favoring the appearance of the R-phase as well as increasing the transition temperature Tn, TT2. Then, the second or first heat treatment can be configured to dissolve the Ni4Ti3 phase, allowing the Ni go back into the nitinol matrix. The increased Ni content in the nitinol matrix decreases the transition temperature TT2, Tn and the R-Phase is replaced by the austenite phase.

[0052] In a configuration not illustrated, the initial heat treatment can be performed on the whole working part 100 by using a mold heated at the initial heating temperature THI. Here, only the working part 100 is inserted in the mold. Moreover, the mold can comprise at least one mold section configured to perform the local heat treatment on the working part 110. The mold section is configured to apply the local heating temperature THL to the working portion 110. The working part 100 can beput in the mold and the initial and local heat treatments can then be performed sequentially.Numeros de reference employes sur les figures11 shank12 proximal end13 distal end100 endodontic instrument101 instrument longitudinal axis103 apical end104 shaft105 shank, handle110 working part111 first portion112 second portion120 apical portion130 intermediate portion131 first curved part132 second curved part135 intermediate envelope20 cutting surfaces a apical taper angleP intermediate taper angleA1 first peakA2 second peakD1 first distanceD2 second distanceTHI first transition temperatureTH2 second transition temperatureTHI initial heating temperatureTHL local heating temperature

Claims

Claims1. A dental tool or instrument (100) comprising a working part(110) made of a nitinol alloy; wherein the nitinol alloy of the working part (110) is transitionable from an R-phase to an austenitic phase alloy state alloy state when heated above a transition temperature; wherein the working part (110) comprises at least a first portion(111) having a first transition temperature (Tn) and at least a second portion (112) having a second transition temperature (TT2); wherein the first transition temperature (Tn) is a temperature lower than a working temperature of the working part (110) such that, at working temperature, the first portion (111) is in the austenitic phase; and wherein the second transition temperature (TT2) is a temperature greater than the working temperature such that at working temperature the second portion (112) is in the R-phase.

2. The dental tool or instrument comprising an endodontic instrument (100) wherein the working part (110) is destined to be engaged in a root canal, the working part (110) and generally extending along a longitudinal axis (101), between an apical end (103) and a shaft (104); and wherein the working temperature is a body temperature between 36.5°C and 37.5°C.

3. The dental tool or instrument according to claim 2, wherein the working part (110) comprises an apical portion (120), substantially straight relative to the longitudinal axis (101) and an intermediate segment (130) extending between the apical segment (120) and the shaft (104); and wherein the working part (110) comprises one first or second portion (111, 112) extending along the apical segment (120) and respectively one second or first portion (112, 111) extending along the intermediate segment (130).

4. The dental tool or instrument according to claim 2 or 3, wherein the working part (110) comprises an apical portion (120), substantially straight relative to the longitudinal axis (101) and an intermediate segment (130) extending between the apical segment (120) and the shaft (104); and wherein the working part (110) comprises a plurality of first portions (111) and a plurality of second portions (112) arranged in alternance.

5. The dental tool or instrument according to claim 4, wherein the intermediate segment (130) comprises a first curved part (131) having a first peak (A1) relative to the longitudinal axis (101), and a second curved part (132) having a second peak (A2) relative to the longitudinal axis (101) and larger than the first peak (A1); and wherein the working part (110) comprises one first or second portion (111, 112) extending along the apical segment (120) and one second or respectively first portion (112, 111) extending along the intermediate segment (130).

6. The dental tool or instrument according to claim 4, wherein the intermediate segment (130) comprises a first curved part (131) having a first peak (A1) relative to the longitudinal axis (101), and a second curved part (132) having a second peak (A2) relative to the longitudinal axis (101) and larger than the first peak (A1); and wherein at least one of the first or second portions (111, 112) is arranged between the first peak (A1) and the apical end (103), between the first and second peaks (A1, A2), and between the second peak (A2) and the shaft (104).

7. The dental tool or instrument according to claim 5 or 6, wherein an apical envelope (125) of the apical segment (120) forms an apical taper angle (a) relative to the longitudinal axis (101), and an intermediate envelope (135) of the intermediate segment (130), tangent to the first and second peaks (A1, A2), forms an intermediate angle (0) relativeto the longitudinal axis (101), the apical angle (a) being smaller than the intermediate angle (|3).

8. The dental tool or instrument according to any one of claims 1 to 7, wherein the first transition temperature (Tn) is equal or below 35°C, for example between 32.5°C and 35°C and wherein the second transition temperature (Tn) is above 39°C.

9. The dental tool or instrument according to any one of claims 1 to 7, wherein the first and / or second transition temperature (Tn, Tn) is above between 32.5°C and 35°C.

10. The dental tool or instrument according to any one of claims 1 to 9, comprising an orthodontic tool or instrument, an orthodontic wire, a matrix band or retainer.

11. A method of manufacturing a dental tool or instrument according to any one of claims 1 to 10, the method comprising: providing a nitinol alloy having first or second transition temperature (Tn, Tn); performing a local heat treatment locally on at least a portion of the working part (110) such that the working part (110) comprises said at least a first portion (111) having a first transition temperature (Tn) and said at least a second portion (112) having a second transition temperature (Tn).

12. The method according to claim 11, wherein the local heat treatment comprises locally heating the working part (110) such as to generate the first or second portion (111, 112).

13. The method according to claim 12, wherein the local heat treatment comprises scanning a laser beam along a surface area corresponding to the first or second portion (111, 112).

14. The method according to claim 13, wherein the local heat treatment is performed with a laser pulse duration between 1 ms and 20 s and at a local heating temperature (THL) between 300°C and 600°C when generating the first portion (111) and is between 350°C and 800°C when generating the second portion (112).

15. The method according to claim 11 or 12, comprising performing an initial heat treatment on the whole working part (110) such as to generate the first portion (111) on the whole working part (110), or generate the second portion (112) on the whole working part (110).

16. The method according to claim 13, wherein the initial heat treatment comprises placing at least the working part (110) an oven and heating the working part (110) at an initial heating temperature (THI) between 300°C and 600°C when generating the first portion (111) and is between 350°C and 800°C when generating the second portion (112).

17. The method according to claim 16, wherein the initial heat treatment is performed during an initial time period (PHI) between 20 min and 120 min.

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