Process for treating a piece of tantalum or of a tantalum alloy

Abstract

A process for treating a piece of tantalum or of a tantalum alloy, which consists in: placing the piece in a furnace and heating the furnace under vacuum at least at 1 400 C.; forming a carbon multilayer in the peripheral part of the piece, by injecting, in the heated furnace, a gas carbon source at a pressure 10 mbar, the multilayer comprising at least one layer C1 of tantalum carbide, which is located at the surface of the piece, and two layers C2 and C3 comprising a carbon content lower than the carbon content of the layer C1; stopping the formation of the multilayer by cooling the piece; placing around the piece a device capable of trapping carbon, oxygen and nitrogen to protect the piece from carbon and oxygen and nitrogen traces present in the furnace; causing the diffusion of carbon present in the layer C1 towards the layers C2 and C3, by heating the furnace under vacuum, the piece being held in the protecting device; and stopping the diffusion of carbon in the piece by cooling the piece under vacuum before the carbon present in the multilayer reaches the center part of the piece. Thus, a piece the surface of which is free from TaC, the center part of which is free from carbon and the part of which located between the surface and the center part comprises tantalum and carbon is obtained.

Claims

1. A process for treating a piece of tantalum or of a tantalum alloy, the piece having a peripheral part and a centre part, the process comprising the steps of: a) placing the piece in a furnace and heating the furnace under vacuum at a temperature at least equal to 1 400 C.; b) forming a carbon multilayer in the peripheral part of the piece, by injecting, in the heated furnace, a gas carbon source at a pressure at most equal to 10 mbar, the carbon multilayer comprising at least one layer C1 of tantalum carbide, which is located at a surface of the piece, and two underlying layers C2 and C3 each comprising a carbon content which is different and lower than a carbon content of the layer C1; c) stopping the formation of the carbon multilayer by cooling the piece; d) placing around the piece a protecting device for trapping carbon, oxygen and nitrogen to protect the piece from carbon as well as possible oxygen and nitrogen traces present in the furnace; e) causing a diffusion of all or part of carbon present in the layer C1 towards the layers C2 and C3, by heating the furnace under vacuum, the piece being held in the protecting device; and f) stopping the diffusion of carbon in the piece by cooling the piece under vacuum before carbon present in the carbon multilayer reaches the centre part of the piece; whereby a piece the surface of which is free from tantalum as TaC, the centre part of which is free from carbon and a part of which is located between the surface and the centre part comprises tantalum and carbon is obtained.

2. The process of claim 1, wherein step d) comprises: placing the piece in a closed cavity of the protecting device, the closed cavity having walls made of a material attracting carbon, oxygen and nitrogen; and draining the cavity using an inert gas.

3. The process of claim 1, wherein step a) comprises: introducing the piece into the furnace; putting the furnace under vacuum; and heating the furnace until a working temperature between 1 500 and 1 700 C. is reached.

4. The process of claim 1, wherein step b) comprises injecting the gas carbon source in the furnace at a flow rate between 1 and 100 L/h and an injection pressure lower than or equal to 10 mbar.

5. The process of claim 4, wherein the injection of the gas carbon source in step b) is made at an injection pressure of 5 mbar for a flow rate of 20 L/h and in a furnace heated at a temperature of 1 600 C.

6. The process of claim 1, wherein step e) comprises heating the furnace at a temperature of 1 600 C. and at a pressure of 10.sup.2 mbar.

7. The process of claim 1, wherein the gas carbon source used in step b) is ethylene.

8. The process of claim 2, wherein the material attracting carbon, oxygen and nitrogen is tantalum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a is a schematic cross-section view of a portion of a tantalum piece obtained at the end of step b) of the process object of the invention according to a particular embodiment (1 hour of carburising at 1 600 C.) and showing the carbide layers created at the surface of the piece.

(2) FIG. 1b represents a picture obtained by scanning electron microscopy (SEM) of the piece illustrated in FIG. 1a.

(3) FIGS. 2a and 2b respectively represent the carburising duration as a function of time at different temperatures for the growth of TaC layers (FIG. 2a) and the growth of Ta.sub.2C layers (FIG. 2b).

(4) FIG. 3a represents a schematic cross-section view of a portion of a tantalum piece obtained according to a particular embodiment of the process object of the invention (1 hour of carburising at 1 600 C., cooling and 1 hour of heating under vacuum at 1 600 C.) and showing the carbide layers created at the surface of the piece.

(5) FIG. 3b represents a picture obtained by SEM of the piece illustrated in FIG. 3a.

(6) FIG. 4a represents a schematic cross-section view of a portion of a tantalum piece obtained according to a particular embodiment of the process object of the invention (with 1 hour of carburising at 1 600 C. in step b) and 6 hours of heating under vacuum at 1 600 C. in step e)).

(7) FIGS. 4b and 4c respectively represent a picture obtained by SEM of the piece illustrated in FIG. 4a at two different magnifications. It is to be noted that, in FIG. 4c, the piece has undergone a chemical attack in order to reveal the presence and location of Ta.sub.2C precipitates (black spots).

(8) FIG. 5a represents a schematic cross-section view of a portion of a tantalum piece obtained according to a particular embodiment of the process object of the invention (with 2 hours of carburising at 1 600 C. in step b) and 30 minutes of heating under vacuum at 1 600 C. in step e)).

(9) FIG. 5b represents a picture obtained by SEM of the piece illustrated in FIG. 6a.

(10) FIG. 6a represents a schematic cross-section view of a portion of a tantalum piece obtained according to a particular embodiment of the process object of the invention (with 2 hours of carburising at 1 600 C. in step b) and 6 hours of heating under vacuum at 1 600 C. in step e)).

(11) FIG. 6b represents a picture obtained by SEM of the piece illustrated in FIG. 6a.

(12) It is to be noted that in the figures above, the centre part of the piece is never represented.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

(13) The process object of the invention enables the carburising of a piece of tantalum or of a tantalum alloy to be controlled, while choosing the nature and crystal structure of the surface layer of the piece. Indeed, it helps in the choosing of obtaining, at the surface of the piece, a surface layer of Ta.sub.2C type tantalum carbide with an underlying layer of TaC type tantalum carbide or of carbon saturated tantalum with Ta.sub.2C at the grain boundaries, a mixed surface layer consisting of carbon saturated tantalum with Ta.sub.2C at the grain boundaries, or even a surface layer of carbon saturated tantalum with an underlying layer of Ta.sub.2C type tantalum carbide, while controlling the thickness of this surface layer.

(14) As mentioned previously, the heating duration in step b) of the process object of the invention depends on the carbon amount desired to be fed to the piece. The heating duration in step e) is in turn a function of the nature of the layer desired to be obtained at the surface, as well as on the thickness desired for it. By varying these parameters, single layer structures (a surface layer of Ta.sub.2C, C sat. Ta+Ta.sub.2C or C sat. Ta on a core of tantalum or of a tantalum alloy) or even multilayer structures (Ta.sub.2C/TaC/Ta.sub.2C/C sat. Ta+Ta.sub.2C layers; C sat. Ta+Ta.sub.2C/Ta.sub.2C/C sat. Ta+Ta.sub.2C layers; etc., on a core of tantalum or of a tantalum alloy) can be obtained. Obtaining these different structures enables the hardness and/or corrosion resistance of the piece to be enhanced, in order to make it compatible with its final use.

(15) To illustrate the invention, a preferred embodiment of the process object of the invention will now be described.

(16) A piece of tantalum, for example a crucible having a diameter of 100 mm, for a thickness of 1.5 mm and a height of 150 mm is used.

(17) The piece to be treated is installed in the enclosure of a furnace, for example a furnace with the brand BMI bearing the reference BMICRO.

(18) Then, the furnace enclosure is put under vacuum until a pressure of 10.sup.20.01 mbar is reached.

(19) After the pressure is stabilised, the enclosure is heated with a ramp of 30 C./min, until 1 600 C.1% is reached.

(20) The carburising of the piece is then conducted by injecting in the enclosure a fuel gas under a low pressure (pressure lower than about ten millibars) for a determined duration. In this example, ethylene (C.sub.2H.sub.6) is injected into the enclosure under a pressure of 51 mbar and under a controlled flow rate of 20 L/h for 1 hour.

(21) A cooling of the piece is then conducted, for example by means of nitrogen injected into the furnace enclosure under a pressure of 1 bar for a duration of 90 minutes.

(22) In the peripheral part of the tantalum piece, a carbon multilayer 1 comprising a surface layer C1 of TaC type tantalum carbide, an underlying layer C2 of Ta.sub.2C type tantalum carbide and an underlying layer C3 of carbon saturated tantalum with Ta.sub.2C precipitates at the grain boundaries (FIGS. 1a and 1b) are thereby obtained.

(23) The thickness of the carbon multilayer 1 (and thus the total carbon amount fed in the piece) depends on the time the tantalum piece is held under the flow of fuel gas (FIGS. 2a and 2b). Indeed, in a known manner, the growth of the layers of tantalum carbides follows the parabolic law W={square root over (kt)} where W is the thickness of the carbide layer (in m), t the holding time (in minutes) and k the growth coefficient (m.sup.2.Math.min.sup.1). Analogously, the same formula is applied for the formation of the carbon multilayer 1. The formation speed of the carbon multilayer also depends on the carburising temperature. This formation speed exponentially increases with temperature.

(24) The piece thus treated is then moved away from any carbon source, as well as possible pollutants. This step is necessary if the pollution phenomena of the tantalum should be avoided during the diffusion step and the carbon amount present in the piece should be controlled. Indeed, the tantalum is a very reactive element when hot towards atoms as carbon, oxygen and nitrogen and these elements can for example be found as molecules adsorbed on the walls of the furnace enclosure.

(25) For this, according to a preferred embodiment of the process according to the invention, the piece is placed in a cavity (for example formed by depositing a bell on a support, the bell and the support being both of tantalum) which is placed in the furnace enclosure. This enables pollutant elements (I, N.sub.2, etc.), as well as possible carbon atoms present on the walls of the furnace enclosure, to be trapped, before they come in contact with the piece. This also enables gas exchanges to be reduced between the furnace enclosure and the piece to be treated, which turns out to be favourable in the carbon diffusion process.

(26) A double pumping of the furnace enclosure can possibly be conducted by performing an intermediate nitrogen draining (pressure of 10.sup.2+/0.01 mbar) in order to discharge any pollutant.

(27) Then, the piece is heated. The heating under vacuum in step e) will enable carbon present in the layer C1 of the carbon multilayer 1 to diffuse to the layers C2 and C3 of the multilayer.

(28) The heating holding time of the set formed by the piece and the protecting device depends on three parameters:

(29) the type of structure desired to be obtained at the end of the process;

(30) the thickness of the multilayer formed during the carburising step;

(31) the thickness of the piece.

(32) The set formed by the piece and the protecting device (cavity) is heated at 30 C./minute until the wanted treatment temperature is reached. It is chosen here to use the same temperature as that used for carburising, that is 1 600 C.+/1%.

(33) At the end of step b) (after carburising), the tantalum piece included at the surface a carbon multilayer 1 having a surface layer C1 of TaC, an underlying layer C2 of Ta.sub.2C and an underlying layer C3 of carbon saturated tantalum with Ta.sub.2C precipitates at the grain boundaries. During heating in step e), carbon diffuses from the surface layer C1 of TaC (the richest carbon layer) to the layer C2 of Ta.sub.2C, and from the layer C2 of Ta.sub.2C to the layer C3 of C sat. Ta+Ta.sub.2C. This cascade carbon diffusion causes a decrease in the thickness of the TaC layer in favour of the Ta.sub.2C layer. It is then possible to make totally disappear the TaC layer in favour of a single Ta.sub.2C layer at the surface of the piece. If heating is continued, the Ta.sub.2C layer is also decomposed, therefore disappeared completely. Accordingly, there remains at the surface only carbon saturated tantalum having Ta.sub.2C precipitates at the grain boundaries.

(34) Different structures possibly obtained by varying the heating duration in step b) and/or in step e) are illustrated in the following figures.

(35) As mentioned above, after heating the tantalum piece for 1 h at 1 600 C. in step b), a carbon multilayer 1 having a layer C1 of TaC, a layer C2 of Ta.sub.2C and a layer C3 of C sat. Ta+Ta.sub.2C is obtained (FIGS. 1a and 1b).

(36) If it then undergoes the other steps of the process object of the invention, including 1 h of heating under vacuum at 1 600 C. in step e) after having isolated it from any carbon source, a surface layer 2 of Ta.sub.2C is obtained on an underlying layer 3 of C sat. Ta+Ta.sub.2C (FIGS. 3a and 3b).

(37) If, on the contrary, the piece provided with the carbon multilayer undergoes heating under vacuum of 6 h at 1 600 C. in step e), a surface layer 2 of carbon saturated tantalum with Ta.sub.2C precipitates at the grain boundaries is obtained (FIGS. 4a, 4b and 4c, the precipitates being visible in black colour in FIG. 4c). Here, it can be assumed that carbon diffusion is such that the layers C1, C2 and C3 of the carbon multilayer have transformed into the surface layer 2.

(38) According to another example, if the piece has undergone carburising by heating under vacuum at 1 600 C. for 2 h in step b) and heating under vacuum at 1 600 C. for 30 minutes in step e), a piece having a surface layer 2 of Ta.sub.2C, a first sub-layer 3 of TaC, a second sub-layer 4 of Ta.sub.2C and a third sub-layer 5 of C sat. Ta+Ta.sub.2C is obtained (FIGS. 5a and 5b).

(39) If, on the contrary, it undergoes carburising by heating under vacuum at 1 600 C. for 2 h in step b) and heating under vacuum at 1 600 C. for 6 h in step e), a surface layer 2 of C sat. Ta, a first sub-layer 3 of Ta.sub.2C and a second sub-layer 4 of C sat. Ta+Ta.sub.2C are obtained (FIGS. 6a and 6b).

REFERENCES CITED

(40) [1] U.S. Pat. No. 5,916,377

(41) [2] U.S. Pat. No. 5,383,981