Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Reexamination Certificate
2001-01-08
2003-07-08
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
C501S096300, C219S553000
Reexamination Certificate
active
06589898
ABSTRACT:
TECHNICAL FIELD
The invention relates to materials intended for use in an oxidative medium at high temperatures, including the manufacture of high-temperature electric heaters, parts, sensors and tools operating at temperatures of up to 1900° C. and higher.
BACKGROUND ART
Known in the art are refractory composite materials produced by powder metallurgy techniques, with a matrix from molybdenum disilicide (MoSi
2
), reinforced with SiC fibers. The total silicon carbide concentration in this case does not exceed 40 vol. %. In order to preserve high properties of the silicon-carbide fiber, the temperature of the diffusion interaction between molybdenum disilicide and silicon carbide is limited to 1400° C.
A disadvantage of the resulting material is a high porosity and liability to crack formations especially after temperature cycling. Furthermore, it is necessary to use costly equipment for hot pressing at 1375° C. during 1 to 1.5 hours at a pressure of 28 to 240 MPa. High mechanical properties of the material are preserved only at temperatures not over 1400° C. (M. J. Maloney, R. J. Hecht, Development of continuous-fiber-reinforced MoSi
2
-base composites, Materials Science and Engineering, v. A153, 1992, pp. 19-31).
The prior art most relevant to the proposed invention is the high-temperature composite material produced by powder metallurgy techniques and containing from 15 to 45 vol. % of silicon carbide in a molybdenum disilicide matrix. Such material has a low porosity. (R. M. Aikin, Jr., Strengthening of discontinuously reinforced MoSi
2
composites at high temperatures, Materials Science and Engineering, vol. A155, 1992, pp. 121-133).
The main disadvantages of the material are its insufficiently high stability in temperature cycling (repeated heating to working temperatures and cooling down after the operation), especially in the case of abrupt temperature changes (thermal shocks); and insufficiently high level of heat resistance. The labor inputs and expenses involved in making products of intricate configurations and large size increase, because the known materials containing molybdenum disilicide and silicon carbide are produced by powder metallurgy techniques which require preparing starting fine-grained powders and fibers, mixing thereof, as well as using expensive and technically complicated hot pressing at 1300-1800° C. for 1-10 hours in vacuum or in a protective atmosphere under a pressure of up to 310 MPa.
ESSENCE OF THE INVENTION
It is an object of the invention to provide materials with a high heat resistance, resistance to thermal shocks and thermostability, this being ensured by introducing silicides of different composition and in different amounts into the material, by obtaining a material with different ratios of the main phases (suicides of high-melting metals, silicon carbide and carbon) with different structure (mutual disposition of the phases, their size and form, crystallographic orientation, etc.) and, hence, with different combination of the indicated useful properties.
Said object is accomplished by a composite material comprising molybdenum disilicide and silicon carbide, and further comprising W
5
Si
3
and Mo
5
Si
3
and/or (Mo, W)
5
Si
3
and/or (Mo, W)
5
Si
3
C, as well as WSi
2
and/or (Mo, W)Si
2
with the following ratio of the components (in vol. %):
W
5
Si
3
and Mo
5
Si
3
and/or (Mo, W)
5
Si
3
15-85%
and/or (Mo, W)
5
Si
3
C and/or Mo
5
Si
3
C
silicon carbide
2-85%
tungsten and/or molybdenum disilicides
WSi
2
and MoSi
2
and/or (Mo, W)Si
2
0.8-55%
the ratio of molybdenum and tungsten in the total mass of the high-melting metals in the material ranging within (in wt. %):
Mo
7-80%
W
20-93%.
The composite material can also comprise rhenium in an amount of 0.5-20 atomic % of the total content of the molybdenum and tungsten substituted by it in the material.
Besides, the composite material can further comprise inclusions of graphite and/or carbon fibers which partially substitute silicon carbide, in an amount of 5-80 vol. % of the volume not occupied by the silicides of high-melting metals.
Furthermore, the composite material can be made multilayered, the inner layers thereof consisting of graphite and/or of layers of pyro-compacted carbon fabric or other dense carbon or silicon carbide material.
Moreover, the composite material can contain in the silicide phases at least one element of the group consisting of tantalum, niobium, titanium, zirconium, and hafnium, with the following ratio of these components with respect to the total mass of molybdenum and tungsten which they substitute, in wt. %: Ta, 0.1-18; Nb, 0.1-8; Ti, 0.05-10; Zr, 0.05-8; Hf, 0.1-16.
Furthermore, the composite material can contain pores occupying 15-78% of the volume of the material.
The composite material can also contain in its composition at least one of the elements which actively bind oxygen: boron, germanium, aluminum, magnesium, barium, strontium, calcium, sodium, potassium, yttrium, scandium, and rare-earth elements (lanthanoids) in an amount of 0.1-2 wt. %.
The essence of the invention is also in that an electric high-temperature heater is made from a composite material according to the invention, in which heater different sections can be made from different variants of the compositions or structures of the composite material; said electric high-temperature heater can be fully made from said material or with only the working portion of the electric heater or the most high-temperature part of the current leads being made from said material.
The essence of the invention is also in that that the structural part operating at a high temperature can be fully made from the composite material of the invention, different portions of said part being made from different variants of the compositions or structures thereof; said part can be fully made from said material or only the most high-temperature portion of said part can made from said material.
It is established experimentally, that relatively close values of the thermal expansion factors of the phases entering into the composition of the composite material, (3-)×10
.6
/deg, throughout the temperature interval of their existence in the solid form, the appearance of noticeable plasticity in the silicide phases at temperatures above 1100° C., make it possible to obviate formation of cracks both when preparing the composite material and during temperature-cycling thereof, if said phases are used in the ratios indicated in the claims. All these phases are chemically compatible with each other at temperatures below 1850° C., mutual solubility variations with temperature for the main components are insignificant, and this also contributes to the heat resistance and stability of the materials of the present invention during temperature-cycling thereof.
The use of compositions MeSi
2
—Me
5
Si
3
of eutectic type at temperatures above 1900° C. makes it possible to treat a wide range of carbon and silicon carbide materials with silicide melts. These melts wet adequately both carbon and silicon carbide materials, penetrating under the effect of capillary forces into all voids therein: pores, cracks, hair-seams, etc. As a result, the porosity of the obtained materials, as a rule, does not exceed 10 vol. %, usually being 3-5%.
If porosity is useful, for instance, from the standpoint of increasing the electrical resistance or reducing the thermal conductivity of the material, it can be specially provided within the controlled limits indicated in the claims.
When the proposed composite material is prepared from carbon-containing starting materials, displacement reactions are used (Me=Mo, W; MeSi
2
=(Mo, W)Si
2
; Me
5
Si
3
+(Mo, W)
5
Si
3
; Me
5
Si
3
C=(Mo, W)
5
Si
3
C):
5 MeSi
2
+7 C→Me
5
Si
3
+7 SiC (1)
5 MeSi
2
+8 C→Me
5
Si
3
C+7 SiC (2)
Similarly, for a combination of the disilicide phases WSi
2
, MoSi
2
, (Mo, W)Si
2
, the term “MeSi
2
” may be used.
This makes it possible, owing to diffusion interaction of the melt of silicides with car
Gnesin Boris Abramovich
Gurzhiyants Pavel Artemovich
Group Karl
Institut Fiziki Tverdogo Tela Rossiiskoi Akademii
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