CERAMIC MATERIALS IN POWDER FORM AND METHOD FOR THEIR PREPARATION

MATERIAUX CERAMIQUES EN POUDRE

English Abstract

The invention relates to a ceramic material in powder form comprising
particles having an average particle size of 0.1 to 30 µm and each formed
of an agglomerate of grains with each grain comprising a nanocrystal of a
ceramic material of formula (I): Si3-xAlxOyNz, wherein 0 <= x <= 3, 0 <= y <=
6 and 0 <= z <= 4, with the proviso that when x is 0 or 3, y cannot be 0. The
ceramic material in powder form according to the invention is suitable for use
in the production of ceramic bodies by powder metallurgy, as well as in the
formation of heat-resistant coatings by thermal deposition. The ceramic bodies
and coatings obtained have improved resistance to thermal shocks.

French Abstract

L'invention concerne des matériaux céramiques en poudre, comprenant des particules possédant une taille moyenne comprise entre 0,1 et 30 µm et formées chacune d'un agglomérat de grains comportant chacun un nanocristal de matériau céramique de formule (I): Si¿3-x?Al¿x?O¿y?N¿z?, dans laquelle 0<=x<=3, 0<=y<=6 et 0<=z<=4, à condition que lorsque x vaut 0 ou 3, y ne puisse valoir 0. Le matériau céramique en poudre de l'invention convient à la production de corps en céramique par métallurgie des poudres, ainsi qu'à la formation par dépôt thermique de revêtements résistant à la chaleur. Les corps et les revêtements en céramique obtenus présentent une résistance accrue aux chocs thermiques.

Claims

-7-

Claims

1. A ceramic material in powder form comprising particles
having an average particle size of 0.1 to 30 µm and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of a
ceramic material of the formula:

Si3-x Al x O y N z~(I')

wherein 0 < x < 3, 0 < y <= 6 and 0 < z <= 4, and wherein ceramic
materials
of formula (I) in which x = y and z - 4 - x are excluded.

2. A ceramic material in powder form according to claim 1,
wherein x is 0.2, y is 0.3 and z is 3.7.

3. A ceramic material in powder form according to claim 1,
wherein x is 1.5, y is 2.5 and z is 1.5.

4. A ceramic material in powder form according to claim 1,
further including at least one additive comprising at least one element
selected from the group consisting of B, C, Mg, Sc, Ti, V, Cr, Mn, Fe, Co,
Ni, Cu, Zn, Se, Rb, Sr, Y, Zr, No, Mo, Rh, Cd, Te, Ba, La, Ce, Pr, Nd, Sm,
Eu, Gd, Tb, Dy, Hb, Er, Tm, Yb, Lu, Hf, Ta, Os, Ir and Tl.

5. A ceramic material in powder form according to claim 4,
wherein said additive is boron or carbon.

6. A ceramic material in powder form according to claim 1,
wherein said average particle size ranges from 0.1 to 5 µm.

7. A process for producing a ceramic material in powder form
comprising particles having an average particle size of 0.1 to 30 µm and


-8-

each formed of an agglomerate of grains with each grain comprising a
nanocrystal of a ceramic material of the formula

Si3-x Al x O y N z (I)

wherein 0 <= x <= 3, 0 <= y <= 6 and 0 < z <= 4,
with the proviso that when x is 0
or 3, y cannot be 0, the process comprising the steps of:
a) providing at least two reagents comprising as a whole at least
three elements selected from the group consisting of silicon, aluminum,
oxygen and nitrogen; and
b)subjecting said reagents to high-energy ball milling to cause
solid state reaction therebetween and formation of particles having an
average particle size of 0.1 to 30 µm, each particle being formed of an
agglomerate of grains with each grain comprising a nanocrystal of a
ceramic material of the formula (I).


8. A process according to claim 7, wherein said reagents are
selected from the group consisting of silicon, aluminum silicide and oxides,
nitrides and oxynitrides of silicon and aluminum.

9. A process according to claim 8, wherein said reagents are
selected from the group consisting of Si, SiO2, Si3N4, Al, Al2O3 and AlN.

10. A process according to claim 7, wherein step (b) is carried out
in a vibratory ball mill operated at a frequency of 8 to 25 Hz.

11. A process according to claim 10 wherein said vibratory ball
mill is operated at a frequency of about 17 Hz.

12. A process according to claim 7, wherein step (b) is carried out
in a rotary ball mill operated at a speed of 150 to 1500 r.p.m.




-9-

13. A process according to claim 12, wherein said rotary ball mill
is operated at a speed of about 1000 r.p.m.

14. A process according to claim 7, wherein step (b) is carried out
under an inert gas atmosphere.

15. A process according to claim 14, wherein said inert gas
atmosphere comprises argon or helium.

16. A process according to claim 7, wherein step (b) is carried out
under a reactive gas atmosphere.

17. A process according to claim 16, wherein said reactive gas
atmosphere comprises hydrogen, nitrogen, ammonia, carbon, monoxide,
carbon dioxide, silicon tetrahydride, silicon tetrachloride or water vapor.

18. A process according to claim 17, wherein step (b) is carried out
in the presence of a liquid or a greasy substance.

19. A process according to claim 18, wherein said liquid is
selected from the group consisting of butane, acetone, methanol, ethanol,
isopropanol, toluene and water.

20. A process according to claim 18, wherein said greasy
substance is stearic acid.

21. A process according to claim 7, further including the step of
admixing during step (b) at least one additive comprising at least one
element selected from the group consisting of B, C, Mg, Sc, Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Zu, Se, Rb, Sr, Y Zr, Nb, Mo, Rh, Cd, Te, Ba, La, Ce, Pr,
Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Os, Ir and Tl.




-10-

22. A process according to claim 21, wherein said additive is
boron.

23. A process according to claim 21, wherein said additive is
carbon.

Description

CA 02441576 2003-09-19
WO 02/057182 PCT/CA02/00070
-1-
CERAMIC MATERIALS IN POWDER FORM
Technical Field
The present invention pertains to improvements in the field of
ceramic materials. More particularly, the invention relates to ceramic
materials in powder form for use in the formation of ceramic bodies and
coatings having improved mechanical properties.
Background Art
All current methods of producing dense silicon and/or
aluminum-based ceramic bodies are variations of a process wherein a
mixture of powders is compacted and sintered at high temperature.
Densification is usually achieved through the use of sintering aids which
lead to the formation of a liquid phase between powder particles, thus
ensuring densification. However, this usually weakens the material, since
the liquid phase forms upon solidification a vitreous or crystalline phase
having poor mechanical properties.
The current methods also produce coarse grains having typical
dimensions of several microns, caused by the necessity to react raw
materials and densify the ceramics at high temperatures. These coarse grains
are detrimental to mechanical properties such as resistance to thermal
shocks.
Disclosure of the Invention
It is therefore an object of the present invention to overcome
the above drawback and to provide a ceramic material in powder form


CA 02441576 2003-09-19
WO 02/057182 PCT/CA02/00070
-2-
suitable for use in forming ceramic bodies and coatings having improved
mechanical properties and, more particularly, improved resistance to thermal
shocks.
According to one aspect of the invention, there is provided a
ceramic material in powder form comprising particles having an average
particle size of 0.1 to 30 ~m and each formed of an agglomerate of grains
with each grain comprising a nanocrystal of a ceramic material of the
formula:
Si3_XAIXOyNZ (1)
wherein 0 5 x <_ 3, 0 <_ y 5 6 and 0 < z <_ 4, with the proviso that when x is
0
or 3, y cannot be 0.
The term "nanocrystal" as used herein refers to a crystal
having a size of 100 nanometers or less. The nanocrystalline microstructure
considerably favors densification, even without sintering aids, when the
ceramic material in powder form according to the invention is compacted
and sintered to produce dense ceramic bodies. Nanocrystalline powders also
minimize grain growth.
The present invention also provides, in another aspect thereof,
a process for producing a ceramic material in powder form as defined above.
The process of the invention comprises the steps of
a) providing at least two reagents comprising as a whole at least
three elements selected from the group consisting of silicon, aluminum,
oxygen and nitrogen; and


CA 02441576 2003-09-19
WO 02/057182 PCT/CA02/00070
-3-
b) subjecting the reagents to high-energy ball milling to cause
solid state reaction therebetween and formation of particles having an
average particle size of 0.1 to 30 ~,m, each particle being formed of an
agglomerate of grains with each grain comprising a nanocrystal of a
ceramic material of formula (I) defined above.
The expression "high-energy ball milling" as used herein
refers to a ball milling process capable of forming the aforesaid particles
comprising nanocrystalline grains of the ceramic material of formula (I),
within a period of time of about 40 hours.
Modes for Carr' n~ out the Invention
Examples of ceramic material of the formule (I) include
Si2.$Alo.z~o.3N3.~ and Sil.sAh.sOa.sNi.s~
Examples of suitable reagents which may be used in the
process according to the invention include silicon, aluminum, aluminum
silicide as well as oxides, nitrides and oxynitrides of silicon and/or
aluminum. Si, Si02, Si3N4, Al, A1203 and A1N are particularly preferred.
According to a preferred embodiment, step (b) is carried out in
a vibratory ball mill operated at a frequency of 8 to 25 Hz, preferably about
17 Hz. It is also possible to conduct step (b) in a rotary ball mill operated
at
a speed of 150 to 1500 r.p.m., preferably about 1000 r.p.m.
According to another preferred embodiment, step (b) is carried
out under an inert gas atmosphere such as a gas atmosphere comprising
argon or helium, or under a reactive gas atmosphere such as a gas
atmosphere comprising hydrogen, nitrogen, ammonia, carbon monoxide,


CA 02441576 2003-09-19
WO 02/057182 PCT/CA02/00070
-4-
carbon dioxide, silicon tetrahydride, silicon tetrachloride or water vapor. An
atmosphere of argon, helium or hydrogen is preferred. It is also possible to
carry our step (b) in the presence of a liquid such as a hydrocarbon (e.g.
butane), acetone, methanol, ethanol, isopropanol, toluene or water, or a
greasy substance such as stearic acid, to prevent the particles from adhering
to one another.
Additives can be added during step (b) in order to improve the
mechanical properties (e.g. flexural strength and hardness) of the ceramic
bodies and/or coatings eventually made from the ceramics powder of the
invention, to reduce their wettability by molten metals or alloys andlor to
reduce their chemical reactivity (e.g. oxidation) with the environment.
Examples of suitable additives include those comprising at least one element
selected from the group consisting of B, C, Mg, Sc, Ti, V, Cr, Mn, Fe, Co,
Ni, Cu, Zn, Se, Rb, Sr, Y, Zr, Nb, Mo, Rh, Cd, Te, Ba, La, Ce, Pr, Nd, Sm,
Eu, Gd, Tb, Icy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Os, Ir and Tl. Boron and
carbon are preferred.
The ceramic material in powder form according to the
invention can be used to produce dense bodies by powder metallurgy. The
expression "powder metallurgy" as used herein refers to a technique in
which the bulk powders are transformed into preforms of a desired shape by
compaction or shaping followed by a sintering step. Compaction refers to
techniques where pressure is applied to the powder, as, for example, cold
uniaxial pressing, cold isostatic pressing or hot isostatic pressing. Shaping
refers to techniques executed without the application of external pressure
such as powder filling or slurry casting. The dense bodies thus obtained can
be used as structural parts, electronic substrates and other ceramic products.


CA 02441576 2003-09-19
WO 02/057182 PCT/CA02/00070
-5-
The ceramic material in powder form according to the
invention can also be used in thermal deposition applications. The
expression "thermal deposition" as used herein refers to a technique in
which powder particles are injected in a torch and sprayed on a substrate to
form thereon a heat-resistant coating. The particles acquire a high velocity
and are partially or totally melted during the flight path. The coating is
built
by the solidification of the droplets on the substrate surface. Examples of
such techniques include plasma spray, arc spray and high velocity oxy-fuel.
The following non-limiting examples illustrate the invention.
EXAMPLE 1
A Si2,8A1o.20o.3N3.7 powder was produced by ball milling 3.71 g
of Si3N4 and 0.298 of A1203 in a tungsten carbide crucible with a ball-to-
powder mass ratio of ~.1 using a SPEX X000 (trademark) vibratory ball mill
operated at a frequency of about 17 Hz. The operation was performed under
a controlled argon atmosphere. The crucible was closed and sealed with a
rubber O-ring. After 20 hours of high-energy ball milling, a Sy,8A10,20o.3N3.7
nanocrystalline structure was formed. The particle size varied between 0.1
and 5 ~m and the crystallite size, measured by X-ray diffraction, was about
30 nm. The ceramic powder thus obtained was sintered without sintering
aids to produce a dense body having improved resistance to thermal shocks.
EXAMPLE 2
A Sil,sAll.sOa.sNi.s powder was produced by ball milling
0.5238 of Si02, O.OSg of Si and 0.42~g of A1N in a silicon nitride vial with
two 1/2 inch silicon nitride balls. The vial was sealed and placed in a SPEX
X000 vibratory ball mill operated at 17 Hz for 20 hours. The ceramic powder


CA 02441576 2003-09-19
WO 02/057182 PCT/CA02/00070
-6-
thus obtained was sintered without using any additive to produce a dense
body.
EXAMPLE 3
A Si2,8Alo,a0o.3N3.7 powder was produced by ball milling
1:855g of Si3N4 and 0.145g of A1a03 in a silicon nitride vial with two 1/a
inch
silicon nitride balls. O.OSg of boron was added to the mixture prior to
milling. The vial was sealed and placed in a SPEX 8000 vibratory ball mill
operated at 17 Hz for 20 hours. The ceramic powder thus obtained was
sintered to produce a dense body that showed improved thermal shock
resistance.