Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Shaping or treating of multilayered – impregnated – or...
Reexamination Certificate
1999-05-12
2001-04-10
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Shaping or treating of multilayered, impregnated, or...
C264S643000, C264S647000, C264S670000, C264S655000, C264S662000, C264S678000, C164S098000, C419S006000
Reexamination Certificate
active
06214284
ABSTRACT:
The invention concerns a process for manufacturing a part made of high-temperature resistant ceramic material, based on aluminum nitride AlN, by reaction-sintering.
Aluminum nitride is a ceramic material which has in this context a relatively low density (2.8 g/cm
3
), and good strength at high temperatures (1800 K) in an oxidizing and corrosive atmosphere. This particular material also possesses other valuable properties such as good thermal conductivity (15 to 18 W/m.K) and a high thermal coefficient of expansion (9×10
−6
/° C.) which enables it to be assembled to metals, in particular by the diffusion-brazing technique. All these properties make the use of aluminum nitride advantageous for various applications, in particular in the field of heat exchanges, motor vehicle parts, electrical insulators, electronic components and stopcock seals.
Known processes for producing a ceramic material from aluminum and boron nitride lead to products having at least some of the following defects:
a heterogeneous structure with coarse grains and high porosity so that the properties cannot be reproduced;
the absence of a monophase structure;
service temperature limited to 600° C.;
mechanical strength below 30 MPa.
The reaction-sintering technique, which consists of producing a ceramic part directly by chemical reaction between the reagents in the state of particles distributed in a homogeneous manner in the metal-ceramic composite of the preform, enables this part to be obtained at a temperature lower than that required for the sintering of pre-existing particles of ceramic material. However, the reaction-sintering process is usually accompanied by considerable shrinkage in volume which makes it necessary to machine the part to its final dimensions after conversion, using costly diamond tools which are the only ones capable of working ceramics. Moreover, reaction-sintering processes using aluminum as the reagent to provide an aluminum ceramic compound allows appreciable quantities of residual free aluminum to remain which harm the temperature resistance of the material.
The object of the invention is to overcome these disadvantages.
The process according to the invention comprises the following steps:
a) preparation of a homogeneous mixture of boron nitride BN, aluminum and a binder, in the liquid or paste-like state, able to solidify by chemical reaction and/or by solvent loss through drying, the boron nitride and aluminum being in the form of powders;
b) the forming of the said mixture by casting in a mould followed by pressing and heating to a temperature not exceeding approximately 70° C. to harden the binder and to obtain a solid and manipulable unfired part;
c) elimination of the binder by heating to a temperature of the order of 300° C.;
d) impregnation of the pores of the unfired part by immersion in a bath of aluminum or of an aluminum alloy in the molten state, removal of the impregnated preform from the said bath and cooling;
e) machining the preform of the Al—BN composite to the final dimensions of the part to be obtained;
f) reaction sintering at a temperature of between 900 and 1000° C. to form a ceramic based on aluminum nitride;
g) heat treatment to a temperature of between 1100 and 1250° C. to cause the residual aluminum to migrate out of the pores of the part, and mechanical elimination of the aluminum which has collected on the surface after any cooling.
One of the original features of the invention results from the succession of steps a) to c), and in particular impregnation with liquid aluminum or aluminum alloy of an unfired ceramic already containing aluminum, which leads to a blank consisting of a BN/aluminum alloy composite able to undergo reaction-sintering without appreciable shrinkage, and of which the coherence resulting from the absence of porosity enables it to be machined with the aid of conventional tools without diamond used for aluminum alloys, in particular those made of high quality high speed steel, to obtain a part of which the dimensions will be preserved after reaction-sintering (step e).
Step f) leads to the formation of a ceramic based on aluminum nitride AlN, and one or more aluminum borides such as AlB
2
, AlB
10
, AlB
12
and AlB
25
. The increase in volume which accompanies these reactions compensates for the shrinkage due to sintering the ceramic material and enables parts to be obtained having a very low degree of porosity.
A porous network nevertheless remains which enables residual aluminum, melted during the heat treatment of step g), to flow towards the surface of the part, from whence it may be removed mechanically in a very simple manner, for example by brushing.
The ceramic parts prepared by the present invention also have the property of being capable of being assembled by diffusion-brazing. Assembly may be made either with two ceramic parts, or with two parts made of Al—BN, or with a ceramic part and a part made of an Al—BN composite. Assembly by brazing may be carried out using a braze having a suitable composition and by working at a temperature greater by 20° C. than the melting point of the braze, consisting of an Al—Si alloy. Assembly by diffusion-brazing is followed by a reaction-sintering treatment.
Complementary or alternative optional features of the invention are described below:
in step a), 30 to 60 parts by weight of BN are mixed with 70 to 40 parts by weight of aluminum, and preferably approximately 40 parts by weight of BN with 60 parts by weight of aluminum.
step a) is subdivided into
a1) preparation of a first mixture formed of BN and aluminum powders and a first binder fraction;
a2) compaction of the first mixture by pressing under a pressure of 30 to 140 bar into an ingot and heating to harden the binder;
a3) elimination of the binder;
a4) impregnation of the pores of the ingot with aluminum in the molten state, to obtain a composite ingot of Al—BN;
a5) machining of the Al—EN composite ingot into turnings, mixing it with the EN powder and grinding it;
a6) mixing of the resulting powder with a second binder fraction.
The mass proportions of the constituents are close to 90% EN for 10% aluminum in step al), 60% turnings of the first Al—BN composite for 40% EN powder in step a5) and 75% powders for 25% binder in steps a1) and a6).
The binder is an aqueous solution of an organic polymer, preferably polyvinyl alcohol.
The forming of step b) and/or the compacting of stage a2) are carried out under a pressure close to 45 bar.
The impregnation of step c) and/or of stage a3) is carried out by immersion in the impregnating bath, first of all under vacuum, preferably under a residual pressure of the order of 10 Pa, and then under a pressure of between 40 and 60 bar, preferably 50 bar.
The impregnation of step c) is carried out with an alloy of aluminum and silicon having a melting point lower than that of aluminum, preferably a eutectic alloy, brought to a temperature of between 740 and 850° C., preferably approximately 800° C.
Step f) is carried out in a neutral atmosphere, preferably under nitrogen.
The heat treatment of step g) is carried out in air at atmospheric pressure.
Two components are prepared separately by carrying out at least steps a) to e), these two components are assembled by diffusion-brazing and the assembly obtained is subjected, where appropriate, to the steps of the process which have been omitted for at least one of the components.
The two components are obtained following step e) and assembly by diffusion-brazing is followed by reaction-sintering carried out within the range of 950 to 1250° C., preferably at approximately 950° C. for approximately 1 hour and then at approximately 1250° C. for approximately two hours.
The two components are obtained following reaction-sintering at a temperature of between 950 and 1100° C., preferably at approximately 1000° C., and assembly by diffusion-brazing is followed by post-sintering carried out at between 1000 and 1250° C., preferably at approximately 1100° C.
The subject of the invention is also a part produced by reaction-sintering such as can be o
Avran Patrick
Grishaev Vladimir
Soudarev Anatoli
Connolly Bove Lodge & Hutz LLP.
Derrington James
Onera
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