Silicon Nitride

Silicon Nitride cylindrical void c h a n n e l s parallel to the c axis. Solu-

tion, o r e v a p o r a t i o n , of t h e α p h a s e p e r m i t s t h e re-
Silicon nitride ( S i 3 N 4 ) h a s been k n o w n as a chemical constructive oL-β p h a s e t r a n s f o r m a t i o n t o occur.
c o m p o u n d since the n i n e t e e n t h century. T h e develop- T r a n s f o r m a t i o n of t h e β p h a s e t o the α p h a s e seems
m e n t of a r a n g e of c e r a m i c m a t e r i a l s b a s e d o n silicon n o t t o be possible. Single-crystal densities are
nitride c o m m e n c e d in t h e 1960s, as a result of a search 3185 k g m
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a n d 3196 k g m
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for α a n d β phases,
for new m a t e r i a l s consisting of elements of low a t o m i c respectively. T h e s t r o n g , p r e p o n d e r a n t l y ( ~ 7 0 % )
n u m b e r with useful properties. In the case of silicon c o v a l e n t i n t e r a t o m i c b o n d i n g gives a high m o d u l u s of
nitride, t h e p r o p e r t i e s of m o s t interest are t h e reten- 320 G P a at 25 °C a n d a low t h e r m a l e x p a n s i o n coeffi-
tion of m e c h a n i c a l s t r e n g t h a n d creep resistance at cient of a p p r o x i m a t e l y 3 M K
1
over 0 - 1 0 0 0 °C. N o
high t e m p e r a t u r e a n d , for a brittle m a t e r i a l , excep- reliable values exist for silicon or n i t r o g e n self-diffu-
tionally g o o d resistance t o t h e r m a l shock. Interest in sion coefficients, b u t it is certain t h a t b o t h a r e small.
silicon nitride ceramics h a s intensified in recent years, Silicon nitride d o e s n o t melt u n d e r n o r m a l pressures,
since the realization t h a t they are suitable substitutes b u t d e c o m p o s e s at 2155 Κ u n d e r 1 a t m n i t r o g e n into
for h i g h - t e m p e r a t u r e m e t a l alloys in a r a n g e of gas n i t r o g e n a n d liquid silicon. Dissociation rates b e c o m e
t u r b i n e engines. Such engines w o u l d h a v e the capacity a p p r e c i a b l e at a p p r o x i m a t e l y 1 5 0 0 ° C , a n d a b o v e this
t o o p e r a t e with higher gas inlet t e m p e r a t u r e s t h a n are t e m p e r a t u r e silicon nitride is effectively u n s t a b l e
p e r m i t t e d in c o n v e n t i o n a l m e t a l engines, giving as- u n d e r v a c u u m , o r in t h e presence of silicon v a p o r o r
sociated increases in efficiency a n d higher p o w e r - t o - n i t r o g e n sinks. P u r e silicon nitride s h o w s n o signifi-
weight ratios. C o n s i d e r a b l e research effort h a s been c a n t electrical c o n d u c t i o n .
e x p e n d e d in this a r e a with very e n c o u r a g i n g achieve-
m e n t s . In parallel, effort h a s been directed t o w a r d s t h e 2. Ceramic Forms
i n c o r p o r a t i o n of silicon nitride c o m p o n e n t s in diesel
engines to give i m p r o v e d t h e r m a l efficiency. At the Silicon nitride c o m p o n e n t s m a y be p r o d u c e d in dense
s a m e time, these m a t e r i a l s h a v e established for t h e m - form from a p o w d e r t h r o u g h the c o n v e n t i o n a l
selves a wide r a n g e of c o m m e r c i a l a p p l i c a t i o n s of a sintering or h o t - p r e s s i n g r o u t e s . An interesting a n d
less d e m a n d i n g n a t u r e so t h a t their use as ceramics i m p o r t a n t feature of the material, however, is t h a t
3
in a n engineering c o n t e x t is b e c o m i n g generally lower density ( 2 3 0 0 - 2 7 0 0 k g m ~ ) c o m p o n e n t s m a y
accepted. be p r o d u c e d directly by the n i t r i d a t i o n of c o m p a c t e d
silicon p o w d e r — t h e r e a c t i o n - b o n d i n g (or reaction-
1. Formation of Properties sintering) r o u t e . T h i s h a s t h e a d d i t i o n a l merit t h a t
only slight overall v o l u m e c h a n g e occurs d u r i n g the
F o r m a t i o n of silicon nitride is n o r m a l l y by t h e direct fabrication process, so t h a t complex, accurately di-
reaction of silicon p o w d e r with n i t r o g e n a t t e m p e r - m e n s i o n e d s h a p e s c a n be p r o d u c e d in a single stage
a t u r e s a b o v e a p p r o x i m a t e l y 1200 °C: from a s h a p e d billet of silicon p o w d e r . H o t - p r e s s i n g is
n o w a well-established technique, yielding theoretical
3Si(s) + 2 N 2 ( g ) - > S i 3 N 4 ( s ) (1)
density m a t e r i a l of high s t r e n g t h (greater t h a n
A convenient, b u t infrequently used, alternative r o u t e 1000 M P a in four-point b e n d at 25 °C). Pressureless
is the c a r b o t h e r m a l r e d u c t i o n of silica: sintering h a s been t h e last t e c h n i q u e t o be developed
because of the t e n d e n c y of silicon nitride to d e c o m -
3 S i 0 2 ( s ) + 6C(s) + 2 N 2 ( g )
pose o n h e a t i n g t o very high t e m p e r a t u r e s a n d sin-
- + S i 3N 4( s ) + 6 C O ( g ) (2) tered m a t e r i a l h a s only recently r e a c h e d t h e stage of
c o m m e r c i a l p r o d u c t i o n . P y r o l y t i c silicon nitride m a y
T h e silicon n i t r i d a t i o n r e a c t i o n is strongly e x o t h e r m i c
- 1 be p r e p a r e d by t h e d e c o m p o s i t i o n of a m i x t u r e of
(— 724 kJ m o l ) a n d this c a n lead t o difficulties with
SiCl4 and N H 3 on a hot substrate:
t h e precise c o n t r o l of t e m p e r a t u r e .
T h e r e are t w o silicon nitride crystal structures, α 3 S i C l 4( g ) + 4 N H 3 ( g ) - S i 3 N 4 ( s ) + 12HCl(g) (3)
a n d β. B o t h structures c a n be r e g a r d e d as consisting of
Theoretically dense crystalline material is o b t a i n a b l e ,
interleaved c o r r u g a t e d sheets of 8- a n d 12-membered
b u t it t e n d s t o c o n t a i n w e a k e n i n g residual stresses a n d
rings of silicon a n d nitrogen a t o m s . In the α structure,
c o m m e r c i a l d e v e l o p m e n t seems t o be unlikely in t h e
each a l t e r n a t e sheet is inverted a n d offset slightly with
n e a r future.
respect to the u n d e r l y i n g sheet t o g e n e r a t e a cellular
structure. E a c h n i t r o g e n a t o m is b o n d e d to three
3. Reaction-Bonded Silicon Nitride
silicon a t o m s in a p p r o x i m a t e l y t r i g o n a l p l a n a r config-
u r a t i o n a n d each silicon a t o m is b o n d e d t e t r a h e d r a l l y Silicon u n d e r g o e s a 2 1 . 7 % m o l a r v o l u m e e x p a n s i o n
t o four n i t r o g e n a t o m s . T h e α s t r u c t u r e c o n t a i n s a o n c o n v e r s i o n t o the nitride. This e x p a n s i o n is
degree of strain a n d it is believed t h a t a stabilizing a c c o m m o d a t e d entirely within t h e original void s p a c e
factor, such as the i n c o r p o r a t i o n of traces of oxygen in the silicon p o w d e r c o m p a c t , so t h a t the b u l k vol-
or Si(II), is required. In t h e β s t r u c t u r e , the sheets u m e is m a i n t a i n e d with a c o r r e s p o n d i n g r e d u c t i o n in
are stacked in a regular m a n n e r to give c o n t i n u o u s void fraction. A silicon pQwder c o m p a c t of density

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 Silicon Nitride

3 3
1500 k g r r T ( 3 5 % void) t h u s nitrides t o 2500 k g m " p a r t i c u l a r l y a l u m i n u m . Early a p p l i c a t i o n s were as
( 2 1 % void) silicon nitride. Because of the difficulty of t h e r m o c o u p l e s h e a t h s for m o l t e n steel testing a n d in
ensuring the c o m p l e t e reaction of high-density silicon c o m p o n e n t s used in p i p i n g a n d ladling m o l t e n a l u m i -
p o w d e r c o m p a c t s it is difficult t o a t t a i n very high n u m , the ease with which accurately d i m e n s i o n e d
nitride densities. T h e present m a x i m u m is a p p r o x i - c o m p o n e n t s could be formed w i t h o u t recourse to
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mately 2850 k g m o n thin ( 1 - 2 m m ) section c o m - extensive d i a m o n d m a c h i n i n g being an i m p o r t a n t
p o n e n t s ; usually densities a r e in t h e range factor in its favor. O t h e r a p p l i c a t i o n s h a v e been in t h e
3
2 3 0 0 - 2 7 0 0 k g m ~ . T h e highest strength m a t e r i a l is m e t a l a n d ceramic processing industries w h e r e s u p -
o b t a i n e d by using a n i t r o g e n d e m a n d t e c h n i q u e , in p o r t s are subjected t o c o n t i n u o u s cycling over t e m p e r -
which the nitrogen supply r a t e a n d the t e m p e r a t u r e a t u r e s p a n s of 1000 °C o r m o r e . E x a m p l e s are in m e t a l
p r o g r a m s are interlinked t o minimize i n h o m o g e n e o u s brazing, surface h a r d e n i n g , s p a r k - p l u g b o d y a n d den-
heating of the silicon p o w d e r . Reaction c o m m e n c e s at tal porcelain firing, inert-gas arc-welding t o r c h noz-
a p p r o x i m a t e l y 1 2 0 0 ° C a n d t h e t e m p e r a t u r e is care- zles a n d electric h e a t e r element s u p p o r t s in a e r o s p a c e
fully p r o g r a m m e d to a p p r o x i m a t e l y 1450 °C t o applications. These are unobtrusive but important
achieve c o m p l e t e reaction. T o t a l reaction times of a p p l i c a t i o n s for t h e material.
150-200 h are n o r m a l l y required. N i t r o g e n - h y d r o g e n C o n s i d e r a b l e interest h a s been sustained in reac-
or N - H - H e gas m i x t u r e s are used with a d v a n t a g e t o t i o n - b o n d e d S i 3 N 4 since t h e late 1960s by t h e possibil-
give faster a n d m o r e easily controlled n i t r i d a t i o n ity t h a t it can be used for h o t - z o n e c o m p o n e n t s in a
rates, a n d higher s t r e n g t h material. r a n g e of h i g h - t e m p e r a t u r e gas t u r b i n e a n d diesel en-
A great m a n y studies h a v e been m a d e of t h e nitrid- gines. Engines of varying p o w e r levels, including those
a t i o n reaction m e c h a n i s m . M u c h of the p r o d u c t ( p r e - suitable for t r u c k s a n d a u t o m o b i l e s , are being de-
d o m i n a n t l y α-phase material) is believed t o be formed veloped with p l a n n e d gas inlet t e m p e r a t u r e s between
by the c o m b i n a t i o n (at suitable g r o w t h sites) of ni- 925 °C a n d 1 3 7 0 ° C . T h e versatility of the reaction-
t r o g e n a n d silicon v a p o r o r silicon-containing v a p o r b o n d e d form of silicon nitride lends itself well to the
species such as silicon m o n o x i d e . T h e f o r m a t i o n of t h e c o n s t r u c t i o n of such c o m p o n e n t s as thin-walled c o m -
β p h a s e is favored kinetically by the presence of b u s t i o n c h a m b e r s , s h r o u d rings, s t a t o r a n d r o t o r
liquids such as eutectic c o m p o s i t i o n s in s i l i c o n - m e t a l vanes, a n d , in t h e diesel engine, piston c r o w n s , sealing
(impurity) systems or, a b o v e 1407 °C, liquid silicon rings, liners a n d p r e c o m b u s t i o n c h a m b e r s . W o r k h a s
itself. (Iron, a l u m i n u m , m a g n e s i u m a n d calcium a r e also been carried o u t o n diesel t u r b o c h a r g e r rotors.
c o m m o n impurities at t h e 1 - 0 . 0 1 % level in c o m m e r - T h e s e engineering p r o g r a m s a r e well a d v a n c e d a n d
cial silicon powders.) T h e function of h y d r o g e n as a m a n y test-bed a n d test-vehicle r u n n i n g h o u r s have
reaction accelerator is linked to its ability to speed the been logged. T h e p r o d u c t i o n of gas t u r b i n e a u t o -
elimination (as silicon m o n o x i d e ) of protective films of m o b i l e engines using r e a c t i o n - b o n d e d S i 3 N 4 h o t - z o n e
silica ( S i 0 2 ) o n t h e silicon particle surfaces. T h e resul- c o m p o n e n t s is at a n a d v a n c e d stage of p l a n n i n g , b u t
ting m i c r o s t r u c t u r e of r e a c t i o n - b o n d e d S i 3 N 4 is s o m e - the c o m m e r c i a l viability of this engine m u s t d e p e n d ,
w h a t complex, consisting of a m i x t u r e of fibrous a n d however, o n the d e v e l o p m e n t of techniques for the
equiaxed m a t e r i a l of i n h o m o g e n e o u s density. T h e m a s s p r o d u c t i o n of reliable low-cost c o m p o n e n t s a n d
v o l u m e , n a t u r e a n d d i s t r i b u t i o n of the void space is of this forms an i m p o r t a n t research area.
great i m p o r t a n c e for strength, with t h e highest A conservative e s t i m a t e of t h e a n n u a l c o m m e r c i a l
strengths being o b t a i n e d with high-density m a t e r i a l p r o d u c t i o n of r e a c t i o n - b o n d e d S i 3 N 4 w o u l d be of the
h a v i n g an even d i s t r i b u t i o n of fine porosity. T h e r e is o r d e r of 25 t, a significant p r o p o r t i o n of this being
s o m e evidence t h a t f o r m a t i o n c o n d i t i o n s favoring t h e p r o d u c e d in the U K . Silicon nitride is also p r o d u c e d
p r o d u c t i o n of the α p h a s e also lead t o a higher in p o w d e r form for a p p l i c a t i o n s as diverse as a re-
strength material. S t r e n g t h s at r o o m t e m p e r a t u r e , as fractory insulation m a t e r i a l a n d a n aircraft b r a k e - p a d
measured in four-point bend, are typically filler.
2 5 0 - 3 0 0 M P a for m a t e r i a l of density of a p p r o x i -
- 3
mately 2700 k g m . T h i s s t r e n g t h is n o r m a l l y m a i n - 4. Hot-Pressed Silicon Nitride
tained t o 1400 °C in air, with s o m e i m p r o v e m e n t in Fully dense silicon nitride was first p r o d u c e d in 1961
the s h o r t t e r m d u e t o oxide infilling of surface flaws. by hot-pressing silicon nitride p o w d e r s c o n t a i n i n g a
Silicon nitride is p r o t e c t e d against s p o n t a n e o u s o x i d a - densification aid. O x i d e s such as M g O or Y 2 0 3 at the
tion by films of crystalline o r a m o r p h o u s silica. In t h e 5-15 m o l . % level are c o m m o n l y used for this p u r p o s e ,
p u r e material, these p r o v i d e a n a d e q u a t e b a r r i e r u p t o b u t m u c h e x p e r i m e n t a l w o r k h a s been carried o u t
a p p r o x i m a t e l y 1400 °C. Metallic-ion c o n t a m i n a t i o n with a wide r a n g e of oxides a n d nitrides. T h e additive
of the silica reduces its effectiveness in this respect. reacts at the pressing t e m p e r a t u r e (1650-1800 °C) with
R e a c t i o n - b o n d e d S i 3 N 4 d e p e n d s for its successful t h e surface silica film o n the silicon nitride particles
c o m m e r c i a l application primarily o n its resistance t o (naturally present at t h e 1.2-12 m o l . % level d e p e n d -
thermal shock (a function of t h e low t h e r m a l ex- ing o n particle size a n d p r e t r e a t m e n t ) to form liquid
pansion coefficient of silicon nitride) a n d its n o n - p h a s e s in which silicon nitride h a s appreciable solu-
wetting c h a r a c t e r in c o n t a c t with m o l t e n metals, bility. Pressure-assisted ( 7 - 3 5 M P a ) solution a n d

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reprecipitation occurs, and full density can be obtained 5. Sintered Silicon Nitride
within m i n u t e s . H o t pressing is n o r m a l l y c o n t i n u e d
T h e sintering of p u r e S i 3 N 4 t o full density has, so
for 1 h or m o r e to e n s u r e c o m p l e t e t r a n s f o r m a t i o n of
far, been found t o be impossible d u e t o the high-
the a - S i 3 N 4 t o ß - S i 3 N 4 . O n cooling, the liquid solidi-
t e m p e r a t u r e instability of t h e m a t e r i a l . C o n s i d e r a b l e
fies t o form s e c o n d a r y glassy or crystalline p h a s e s
p r o g r e s s h a s been m a d e since t h e early 1980s, h o w -
which m a y c o n t a i n s t r u c t u r a l nitrogen. T h e s e c o n d a r y
ever, using l i q u i d - p h a s e sintering with additives de-
p h a s e is l o c a t e d as a thin ( ~ 5 n m ) film at grain
veloped for h o t - p r e s s e d S i 3 N 4 p r o d u c t i o n . H i g h e r
b o u n d a r i e s a n d as p o c k e t s at triple p o i n t s . C r y s t a l -
t e m p e r a t u r e s (1825-2080 °C) a n d longer times (up t o
lization of this p h a s e is desirable, b u t is difficult t o
5 h) a r e required. T h e best studied additives h a v e been
achieve fully at t h e grain b o u n d a r i e s . T h e a- to ß-
M g O a n d Y 2 0 3 , singly, in c o m b i n a t i o n a n d t o g e t h e r
p h a s e t r a n s f o r m a t i o n results in a fibrous grain m o r -
with A 1 2 0 3 . P r e c a u t i o n s m u s t be t a k e n against losses
p h o l o g y which increases fracture t o u g h n e s s t o
1 /2 of silicon nitride by d e c o m p o s i t i o n o r by t h e e v a p o -
6MPam for greater t h a n 7 0 % a - S i 3 N 4 in t h e
r a t i o n of S i O , a n d losses of t h e sintering aid itself.
s t a r t i n g silicon nitride p o w d e r . T o a very large extent,
W e i g h t loss c a n b e s u p p r e s s e d by sintering in a high
the m e c h a n i c a l p r o p e r t i e s of hot-pressed S i 3 N 4 o t h e r -
p r e s s u r e of n i t r o g e n ( 1 - 8 M P a ) a n d / o r using a silicon
wise d e p e n d o n the c o m p o s i t i o n , crystallinity a n d
nitride p o w d e r bed, which m a y also c o n t a i n the
d i s t r i b u t i o n of the s e c o n d - p h a s e material. S t r e n g t h s in
sintering aid t o g e t h e r with b o r o n nitride p o w d e r t o
four-point b e n d of 8 0 0 - 1 0 0 0 M P a are regularly o b -
r e t a r d sintering of t h e bed. T h e s t r e n g t h s of sintered
tained at 25 °C. (Strengths of 1400 M P a h a v e been
S i 3 N 4 so far a t t a i n e d (to 1000 M P a in bend) a p p r o a c h
r e p o r t e d for A 1 2 0 3 / Y 2 0 3 h o t - p r e s s e d m a t e r i a l with a
t h o s e for t h e best h o t - p r e s s e d S i 3 N 4 a n d there is n o
high degree of s e c o n d - p h a s e crystallinity.) H i g h -
d o u b t t h a t i m p r o v e m e n t s will c o n t i n u e t o be m a d e .
t e m p e r a t u r e s t r e n g t h a n d creep resistance strongly
d e p e n d o n the n a t u r e of t h e second p h a s e . S t r e n g t h s of T h e sintering process is i m p o r t a n t in its o w n right
400 M P a in b e n d at 1400 °C c a n be achieved, b u t as a m e a n s of o b t a i n i n g s h a p e d silicon nitride c o m -
softening of the g r a i n - b o u n d a r y p h a s e leads t o r a p i d p o n e n t s with t h e p r o p e r t i e s of h o t - p r e s s e d S i 3 N 4 , b u t
fall off in s t r e n g t h a b o v e this t e m p e r a t u r e . at a fraction of t h e cost. A further i m p o r t a n t r e a s o n for
the interest in t h e process lies in t h e p o s t s i n t e r i n g of
P u r e silicon nitride is well p r o t e c t e d against o x i d a -
reaction-bonded S i 3N 4. The attraction here depends
tion by a silica film. In t h e case of the best-studied
o n t h e fact t h a t r e a c t i o n - b o n d e d S i 3 N 4 c a n be p r o -
magnesia-pressed material, g r a i n - b o u n d a r y c a t i o n s
d u c e d readily t o m u c h higher densities t h a n c a n be
play an i m p o r t a n t p a r t in the o x i d a t i o n process a n d
achieved by c o m p a c t i n g sinterable silicon nitride
the o u t w a r d diffusion of these c a t i o n s i n t o the surface
p o w d e r . L i n e a r s h r i n k a g e s at t h e sintering stage are
silica layer a p p e a r s t o be r a t e controlling. P a r a b o l i c
p r o p o r t i o n a t e l y reduced, from a p p r o x i m a t e l y 1 5 % t o
rate c o n s t a n t s d e p e n d linearly o n the M g O c o n t e n t
a b o u t 5 % , a n d t h e final d i m e n s i o n a l a c c u r a c y is
a n d are several o r d e r s of m a g n i t u d e higher t h a n t h o s e
higher. T h e c o m p o n e n t therefore requires less-
for p u r e silicon nitride. N o n e t h e l e s s , t h e use of h o t -
expensive d i a m o n d m a c h i n i n g for precise d i m e n s i o n s
pressed S i 3 N 4 at t e m p e r a t u r e s a b o v e 1000 °C for sev-
a n d c o m m e r c i a l l y will be m o r e attractive. Sintering
eral t h o u s a n d h o u r s c a n be c o n t e m p l a t e d . I m p r o v e d
aids c a n b e infiltrated o r diffused i n t o t h e r e a c t i o n -
o x i d a t i o n resistance is found for yttria-pressed m a -
b o n d e d S i 3 N 4 c o m p o n e n t , o r b l e n d e d with t h e start-
terial, p r o v i d e d t h a t the overall system c o m p o s i t i o n
ing silicon p o w d e r before n i t r i d a t i o n . T h e a d v a n c e s in
lies within the S i 3 N 4 - S i 2 N 2 0 - Y 2 S i 2 0 7 c o m p a t i b i l i t y
sintering t e c h n o l o g y t h u s m a k e possible t h e m a s s
triangle, t h u s excluding t h e presence at t h e grain
p r o d u c t i o n of c o m p l e x engineering c o m p o n e n t s with
b o u n d a r i e s of the readily oxidized S i - Y - N - O p h a s e s .
p r o p e r t i e s similar t o t h o s e of h o t - p r e s s e d S i 3 N 4 ; it is
T h e small n u m b e r of a p p l i c a t i o n s for h o t - p r e s s e d t o be expected t h a t sintered S i 3 N 4 a n d p o s t s i n t e r i n g
S i 3 N 4 d e p e n d s largely o n its h a r d n e s s a n d strength, of r e a c t i o n - b o n d e d S i 3 N 4 will be d e v e l o p e d t o t h e
giving g o o d resistance t o w e a r a n d a b r a s i o n . T u b e - exclusion of h o t - p r e s s e d S i 3 N 4 , except possibly for t h e
d r a w i n g plugs a n d dies, a n d m e t a l - c u t t i n g inserts h a v e m o s t d e m a n d i n g of a p p l i c a t i o n s .
been successfully developed o n a m o d e s t scale. O t h e r
possible a p p l i c a t i o n s are as ball a n d roller b e a r i n g s See also: Nitrides
for use u n d e r abrasive c o n d i t i o n s , o r w h e r e lubric-
a t i o n m a y n o t be a d e q u a t e , as in seabed drilling
e q u i p m e n t . As in the case of r e a c t i o n - b o n d e d S i 3 N 4 , Bibliography
c o n s i d e r a b l e interest in h o t - p r e s s e d S i 3 N 4 h a s been
b r o u g h t a b o u t by c e r a m i c gas t u r b i n e d e v e l o p m e n t Bunk W, Böhmer M (eds.) 1981 Keramische Komponenten
p r o g r a m s . It is a p r i m a r y c a n d i d a t e m a t e r i a l for axial- für Fahrzeug-Gasturbinen—//. Springer, Berlin
Burke J J, Lenoe E M, Katz R Ν (eds.) 1978 Ceramics for
flow r o t o r h u b s , w h e r e high tensile centrifugal stresses High Performance Applications—II. Brook Hill Publi-
(400 M P a ) are developled at t e m p e r a t u r e s in t h e r a n g e shing, Chestnut Hill, MA
5 0 0 - 8 0 0 °C. O n e design for a r o t o r system consists of Lange F F 1980 Silicon nitride polyphase systems: Fabric-
r e a c t i o n - b o n d e d S i 3 N 4 blades b o n d e d t o a h o t - ation microstructure and properties. Int. Metall. Rev. 25:
pressed S i 3 N 4 h u b — t h e d u o d e n s i t y concept. 1-20

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 Silicon Nitride Fibers

Lenoe E M, Katz R Ν, Burke J J (eds. ) Reliability, Ceramics with a n increasing p r o b a b i l i t y of p r o d u c i n g stable

for High Performance Applications, Vol. III. Plenum S i - C b o n d s leading t o a mixed n i t r i d e - c a r b i d e fiber,
Press, New York o r t o t h e presence of residual chlorine. T h e poly-
Moulson A J 1979 Reaction-bonded silicon nitride: Its silazanes a n d p o l y m e r s c o n t a i n i n g S i - Η b o n d s suffer
formation and properties. J. Mater. Sei. 14: 1017-51 from t h e d i s a d v a n t a g e of m a r k e d reactivity t o w a r d s
Popper Ρ 1982 Sintering of silicon nitride: A review. In: Riley
water, a n d t h e i n c o r p o r a t i o n of oxygen a n d the pre-
F L (ed.) 1982 Progress in Nitrogen Ceramics. Nijhoff, The
Hague, The Netherlands, pp. 187-210 sence of S i - O b o n d s in t h e c e r a m i c fiber; for this
Riley F L (ed.) 1977 Nitrogen Ceramics. Noordhoff, Leiden, r e a s o n , d r y - b o x h a n d l i n g is s t a n d a r d practice.
The Netherlands T h e use of t h e organofibers as a s t a r t i n g p o i n t to
c e r a m i c fiber p r o d u c t i o n places further restrictions o n
F L Riley t h e p r e c u r s o r p o l y m e r p r o p e r t i e s ; this n o r m a l l y re-
[ U n i v e r s i t y of Leeds, Leeds, U K ] sults in a n u m b e r of c o m p r o m i s e s . T h e ideal fiber is
b a s e d o n a linear, s y m m e t r i c a l p o l y m e r chain, with
strong intermolecular bonding resulting from
the presence of p o l a r g r o u p s , a n d of m o d e r a t e l y
Silicon Nitride Fibers high ( 5 0 0 0 - 2 0 000) m o l a r m a s s t o give g o o d spinning
T h e f o r m a t i o n of silicon nitride ( S i 3 N 4 ) by t h e p y r o - characteristics.
lysis of o r g a n o m e t a l l i c p o l y m e r s c o n t a i n i n g silicon
h a s been of interest for s o m e years as a potentially
2. Production
c o n v e n i e n t r o u t e t o t h e p r o d u c t i o n of c o m p o n e n t s of
c o m p l e x s h a p e . Because m a n y p r e c u r s o r o r g a n o - A linear c h a i n p o l y m e r softens a n d deforms o n heat-
metallic p o l y m e r s c a n be readily s p u n i n t o c o n t i n u o u s ing so t h a t , after s p i n n i n g , it is necessary t o carry o u t a
filament fibers using c o n v e n t i o n a l textile processes, a p r e l i m i n a r y crosslinking ("curing") step in o r d e r t o
r o u t e also exists t o t h e p r o d u c t i o n of silicon nitride b r i d g e p o l y m e r c h a i n s a n d t o p r e v e n t melting a n d
fibers. If these fibers c a n be m a d e with a n a d e q u a t e excessive d e p o l y m e r i z a t i o n a n d e v a p o r a t i o n of the
degree of c o n t r o l of s t r u c t u r e a n d surface finish, it is fiber d u r i n g t h e later h i g h - t e m p e r a t u r e pyrolysis
expected t h a t , like c a r b o n fiber, they will h a v e high stage. C r o s s l i n k i n g requires active sites within the
m o d u l u s a n d high s t r e n g t h . Such fibers a r e of c o n - p o l y m e r chain. Identification of t h e best crosslinking
siderable interest as possible reinforcing m a t e r i a l s in m e t h o d will d e p e n d o n t h e p o l y m e r chemistry b u t
p o l y m e r matrices a n d for h i g h - t e m p e r a t u r e appli- t e c h n i q u e s e x p l o r e d include c o n t r o l l e d i n t e r m e d i a t e
c a t i o n s in c e r a m i c m a t r i x systems w h e r e better resis- t e m p e r a t u r e ( ~ 5 0 - 2 0 0 °C) o x i d a t i o n , e x p o s u r e t o
t a n c e to o x i d a t i o n t h a n t h a t s h o w n by c a r b o n fibers ultraviolet ( < 300 n m ) light, t o high-energy (2 MeV)
m a y be expected. electrons a n d t o γ rays. C r o s s l i n k i n g using reactive
molecules (e.g., H S i C l 3 ) h a s been useful in special
cases. A p r o p e r l y crosslinked fiber is b o t h insoluble
1. Precursor Systems
a n d infusible.
T h e p r i m a r y r e q u i r e m e n t for t h e p o l y m e r i c p r e c u r s o r After crosslinking, a h i g h - t e m p e r a t u r e heat treat-
is t h a t t h e m a i n c h a i n c o n t a i n s silicon. A n u m b e r of m e n t of the fiber c a n be carried o u t t o b r i n g a b o u t the
systems h a v e been e x a m i n e d : s t r u c t u r a l r e a r r a n g e m e n t r e q u i r e d t o g e n e r a t e a three-
d i m e n s i o n a l n e t w o r k of S i - N b o n d s a n d t o eliminate
(a) polysilazanes - f - S i - N ^ , with a n S i - N chain;
u n r e q u i r e d a t o m i c species. T e m p e r a t u r e s in the r a n g e
(b) p o l y c a r b o s i l a n e s - f - S i - C H 2 ^ , in which CH2 1000-1300 °C a r e n o r m a l l y sufficient t o p r o d u c e a
g r o u p s s e p a r a t e the silicon a t o m s ; a n d silicon nitride fiber of r e a s o n a b l e purity. Polysilazanes
c a n be pyrolyzed u n d e r d r y a r g o n ; n i t r o g e n or a m -
(c) polysilanes -f-Si-Si-f^,.
m o n i a a t m o s p h e r e s a r e n o r m a l l y used for fibers based
Of these, t h e first t w o types h a v e received t h e m o s t o n t h e p o l y c a r b o s i l a n e s . A n i m p o r t a n t factor in the
a t t e n t i o n . T h e polysilazanes c o n t a i n t h e necessary pyrolysis process is t h e r a t e of t e m p e r a t u r e rise. This
n i t r o g e n in t h e c h a i n c o n v e n i e n t l y a l r e a d y b o n d e d t o m u s t be slow a n d , typically, a r a t e of a r o u n d 3 ° C
1
the silicon. T h e p o l y c a r b o s i l a n e s c a n be nitrided a t m i n ~ is used t o allow t i m e for ejection of pyrolysis
t e m p e r a t u r e s b e t w e e n 500 °C a n d 800 °C in a p r o d u c t s w i t h o u t excessive d a m a g e t o the fiber. M a s s
n i t r o g e n - c o n t a i n i n g a t m o s p h e r e (e.g., a m m o n i a ) t o loss starts at a r o u n d 300 °C a n d is c o m p l e t e d by a b o u t
effect n i t r o g e n insertion with d e v e l o p m e n t of S i - N 800 °C.
b o n d s . A n i m p o r t a n t factor for t h e p r o d u c t i o n a n d As with o t h e r forms of ceramic, g o o d m e c h a n i c a l
s u b s e q u e n t h e a t - t r e a t m e n t steps of t h e o r g a n o p o l y - p r o p e r t i e s c a n only be o b t a i n e d with m a t e r i a l s of
m e r is t h e n a t u r e of the r e m a i n i n g side g r o u p s re- g o o d m i c r o s t r u c t u r a l quality. Defects of a n y k i n d lead
q u i r e d t o c o m p l e t e t h e silicon a n d n i t r o g e n valencies t o s t r e n g t h d e g r a d a t i o n . P o r e s , m i c r o c r a c k s a n d sur-
of four a n d three, respectively. H y d r o g e n w o u l d , face d a m a g e a r e p a r t i c u l a r l y undesirable; this applies
theoretically, be ideal; in practice, alkyl ( C H 3 ) , aryl equally t o the p r e c u r s o r fiber because of the virtual
( C 6 H 5 ) , a n d h a l o g e n (CI) g r o u p s m a y also be linked impossibility of a n n e a l i n g o u t such defects d u r i n g

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