EP1954998B1

EP1954998B1 - Induction furnace

Info

Publication number: EP1954998B1
Application number: EP05849749.6A
Authority: EP
Inventors: Ralf Siegfried Goller; Giovanni Paolo Pacchiana
Original assignee: Freni Brembo SpA
Current assignee: Brembo SpA
Priority date: 2005-12-02
Filing date: 2005-12-02
Publication date: 2015-07-15
Anticipated expiration: 2025-12-02
Also published as: EP1954998A1; SI1954998T1; WO2007063562A1

Description

The present invention relates to an induction furnace and in particular to an induction furnace as defined in the preamble of the attached first claim.
It is well-known that composite ceramic materials are used in various applications where high resistance to impact, compression and temperature generated by friction is required, characteristics which cannot be guaranteed by simple ceramic materials due to their intrinsic fragility.
In particular, composite ceramic materials are known for braking applications obtained through infiltrations of silicon into a mixture comprising a plurality of carbon filaments and additives. Said infiltration normally takes place at a temperature in which the silicon is in a molten state.
According to the prior art, preparation of said composite materials can be carried out by:

mixing the plurality of filaments with binding resin, pitch and other additives;
placing the resulting mixture in a mold to obtain a shaped semi-finished product by means of heating and applying pressure;
firing the semi-finished product in a furnace with a temperature such as to cause carbonization or pyrolysis of the resin.

The semi-finished product is then subjected to an infiltration process which comprises further firing with materials to be infiltrated, in this case silicon. Said firing takes place at a temperature such as to cause melting of the silicon and its infiltration into the pore of the semi-finished product.
As it is known, the above-described infiltration step (which in this case is also called silication) is very time consuming and costly. Said step comprises heating of the silicon to between 1400°C and 1700°C to reach a molten state and a decrease in pressure from 900mbar to 300mbar to facilitate the infiltration process. For this purpose, specific furnaces are used which can supply the desired temperatures and can be placed under vacuum. The furnaces generally used are substantially of two types: the so-called discontinuous or batch furnaces and tunnel furnaces.
The batch type furnaces carry out a step to load a quantity of material equal to their capacity, a heating step, a soaking step at a given temperature together with a vacuum for silication and a cooling step. The loading capacity of these furnaces is generally 250 kg of material. The duration of the heating, soaking and cooling cycle varies between 24 and 48 hours in order to allow infiltration which lasts about 10 minutes.
This type of furnace, besides the structural complexity due to the fact that it has to resist external pressure, has the disadvantage that it must be fully loaded and, therefore, is not convenient for production of small quantities or single workpieces, a disadvantage further aggravated by the fact that it is necessary to wait for at least 24 hours to obtain the product.
The tunnel type furnaces, however, foresee loading of the material to be treated in a special loading station, transit in the heating area and unloading from a loading station. Even though they permit a continuous flow of finished material, said furnaces have the disadvantage of a structural complexity even greater than the batch type furnaces: it is sufficient to consider that the material has to transit in an area where a vacuum is present.
On the basis of the above, it can be seen how there is a need for an infiltration furnace, especially for constant and fast production of single workpieces, which is structurally simple and has reasonable cycle times. WO 02075166 A1 discloses an induction crystal-pulling furnace having a crucible and a crucible holder, wherein the crucible holder is a susceptor and is made by carbon/carbon.
The object of the present invention is to satisfy that need by providing an infiltration furnace which is economical, structurally simple, with low cycle times and versatile in use.
Said object is reached by means of an induction furnace as described in the attached claim 1 in its most general form and the preferred embodiments as defined in the dependent claims.
Further features and advantages of the present invention will become more apparent from the following detailed description of an exemplary but non-limiting embodiment thereof, as illustrated in the attached figure 1, in which a frontal view of an furnace according to the present invention, with a cross-section of some parts, is schematically shown.
As can be seen in figure 1, the furnace 1 includes a supporting structure 2 provided with a chamber 3 inside which, by means of an vacuum pump 4 placed outside the chamber 3, a partial vacuum can be formed. A hole 5, made in one of the walls of the supporting structure 2, enables communication between the chamber 3 and the vacuum pump 4.
Preferably, the vacuum pump 4 is such as to bring the working pressure in the chamber 3 from between 800 mbar and 900 mbar to between 0.01 mbar and 500 mbar, and more preferably equal to approximately 2 mbar.
The internal walls of the chamber 3 are provided with a thermally insulating lining. In a particularly preferred embodiment, said lining comprises graphite felt panels 6 coupled to the internal walls of the chamber 3.
The furnace 1, which is particularly an induction furnace, includes an infiltration crucible 7 and a coil 8 placed inside the chamber 3.
The inside of the chamber 3 is also provided with supporting elements 9 for the crucible 7 such as to keep the crucible 7 at a distance from the internal walls of the chamber 3 and in particular from the panels 6 of the internal insulation lining of the chamber 3.
As can be seen in figure 1, the coil 8 comprises inductor windings wound around the infiltration crucible 7. In the embodiment in figure 1, the windings of the coil 8 are in contact with some of the walls of the infiltration crucible 7. In a modification of the embodiment, the windings of the coil 8 could be wound around the infiltration crucible 7 but be placed at a distance from it, for example, they could be adjacent to the panels forming the internal lining 6 of the chamber 3. In a particularly preferred embodiment, the coil 8 is formed of a copper pipe wound spirally around the infiltration crucible 7 and filled with a cooling liquid, such as running water, flowing inside it.
The coil 8 is fed by an alternating current generator connected to the coil 8 and not shown in the figure. Said generator is such as to output an alternating current signal with a frequency preferably in the range of 1kHz - 30kHz.
The infiltration crucible 7 of the induction furnace 1 is made of Carbon/Carbon, and is formed of walls 10, 11, 12 which define an infiltration chamber 13 inside the crucible 7 and such as to house a workpiece 14 in ceramic material to be subjected to infiltration and the material 15 intended to be infiltrated inside the workpiece 14. The walls 10, 11, 12 include an upper plate 10, a lower plate 11 and a lateral annular wall 12 placed between the upper plate 10 and the lower plate 11.
The lateral annular wall 12 is a substantially circular wall and the two upper 10 and lower 11 plates are two disk-shaped plates.
In a particularly advantageous embodiment, the type of Carbon/Carbon of which the infiltration crucible is made, is preferably long fiber Carbon/Carbon fabric, with a 0/45° orientation in the upper 10 and lower 11 plates and unidirectional in the circumference of the lateral annular wall 12. Preferably, the density of the Carbon/Carbon used to make the infiltration crucible 7 is comprised between 1 g/cm³ and 1.9 g/cm³, more preferably equal to approximately 1.7 g/cm³.
In a particularly preferred embodiment, inside the infiltration chamber 13 there are suitable supporting means 18 to hold the workpiece 14 suspended over the lower plate 11.
In a particular embodiment, the workpiece 14 to be subjected to infiltration is a semi-finished workpiece, ring or disk shaped, and previously subjected to a carbonization step (or pyrolysis) such as to determine the formation of porosity in the workpiece 14.
Preferably, the chamber 13 inside the crucible has a volume approximately twice that of the semi-finished workpiece 14 to be subjected to infiltration, so that the space between the workpiece 14 and the internal walls of the chamber 13 can be filled with the material 15 intended to be infiltrated into the porosity of the workpiece 14. Preferably, said material 15 is silicon, for example pure silicon, or an alloy of silicon and aluminum or copper, in grains or powder.
As represented in figure 1, in a particularly advantageous embodiment, the lateral annular wall 12 of the infiltration crucible 7, is provided with an insulating lining 16 to prevent or reduce loss of heat accumulated in the infiltration chamber 13 through the lateral wail 12. Preferably, said insulating lining 16 is an annular layer of lining placed outside the lateral wall 12 and held between the upper plate 10 and the lower plate 11.
The functioning of an induction furnace according to the present invention is briefly described hereunder.
In a first step, the semi-finished workpiece 14 in ceramic material is placed inside the infiltration crucible 7 together with the material 15, for example silicon, to be infiltrated into the porosity of the workpiece 14.
In a successive step, the vacuum pump 4 is activated to bring the pressure inside the chamber 3 of the furnace 1 from a value of approximately in the range of 800 - 900 mbar to a value of approximately in the range of 0.01 - 250 mbar.
In a further step, simultaneous with or subsequent to the vacuum pump 4 activation step, the coil 8 is fed by an alternating current signal with a frequency and a strength such as to heat the walls of the crucible 7 and, in particular, the two plates 10 and 11. Said heating takes place as a result of parasite currents induced inside the two plates 10 and 11 by the electromagnetic field produced by the passage of the alternating current in the coil 8. In this way, the two plates 10 and 11 transmit heat to the inside of the infiltration chamber 13 until a temperature is reached at which the material to be infiltrated 15 turns into a molten state. In the case of silicon, the two plates 10 and 11 are heated until a temperature of approximately between 1400°C and 1700°C is reached inside the infiltration chamber 13. In this temperature range, the silicon melts and infiltrates into the porosity of the semi-finished workpiece 14.
On the basis of the above, it can be seen how the objects of the present invention are fully reached. In particular, an induction furnace according to the invention makes it possible to complete infiltration of a semi-finished product in much shorter times than those of the above-described furnaces known in the art. In particular, it was observed that an furnace according to the present invention is able to take the material contained inside it to the infiltration temperature of 1500°C in approximately 10-15 minutes.
Furthermore, a furnace according to the invention makes it possible to carry out the infiltration process economically even on single workpieces of small dimensions.
Advantageously, the presence of a crucible made in Carbon/Carbon also makes it possible to guarantee excellent resistance of the furnace to high temperatures.
Naturally, in order to satisfy contingent and specific requirements, a person skilled in the art may apply to the above-described furnace according to the invention many modifications and variations, all of which, however, are included within the scope of protection of the invention as defined by the following claims.

Claims

Induction furnace (1) including an infiltration crucible (7) and a coil (8) provided with windings wound around said crucible (7), said crucible (7) being provided with walls (10, 11, 12) defining an infiltration chamber (13) adapted to house a workpiece (14) to be subjected to infiltration and a material (15) to be infiltrated into porosities present in said workpiece (14),
characterized in that
- said infiltration chamber is defined inside said crucible (7);

- said walls of the crucible (7) are made in Carbon/Carbon and can be heated through inductive effect by said coil (8) in order to heat said infiltration chamber (13) to a temperature such as to enable melting of said material (15) and to enable infiltration of said material (15) into said workpiece (14);

- said walls (10, 11, 12) include an upper plate (10), a lower plate (11) and a lateral annular wall (12) placed between said upper plate (10) and said lower plate (11), said lateral annular wall (12) being a substantially circular wall and said upper (10) and lower (11) plates being substantially disk-shaped plates;

- said coil being adapted to heat said walls and in particular said plates, said heating taking place as a result of parasite currents induced inside said upper and lower plates (10,11) by an electromagnetic field produced by a passage of an alternating current in the coil (8).
Induction furnace (1) according to claim 1, wherein said Carbon/Carbon has a density with a value comprised between 1 g/cm³ and 1.9 g/cm³.
Induction furnace (1) according to any of the preceding claims, wherein said Carbon/Carbon is of the long fiber fabric type.
Induction furnace (1) according to claim 5, wherein said Carbon/Carbon is orientated at 0/45° in the upper (10) and lower (11) plates, and unidirectional in the circumference of the lateral annular wall (12).
Induction furnace (1) according to claim 1, further comprising an annular layer of lining (16) placed on the outside of the lateral annular wall (12), said lining layer being interposed between the upper plate (10) and the lower plate (11).
Induction furnace (1) according to claim 1, wherein said windings of the coil (8) are in contact with some of the walls (10, 11) of said crucible (7).
Induction furnace (1) according to claim 1, wherein said coil (8) is made of a copper pipe wound spirally around the infiltration crucible (7) and such as to permit internal passage of a cooling liquid.
Induction furnace (1) according to claim 1, further comprising a supporting structure (2) provided with a vacuum chamber (3) such as to house said crucible (7) and said coil (8).
Induction furnace (1) according to claim 1, wherein said workpiece (14) is ring or disk shaped and wherein said chamber inside the crucible has a volume approximately twice that of said workpiece (14).