Single crystal boules of molybdenum disilicide for performing static fatigue test

Berkowitz, J.

Rossetti, M.

English

NASA Technical Reports Server

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QUARTERLY REPORT 
THE PEST PHENOMENON IN INTERMETALLICS 
NASW-1403 
JOAN BERKOWITZ MICHAEL ROSSETTI 
ARTHUR D. LITTLE, INC . 
ACORN PARK 
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COS W M O d  A 1 1 1 1 3 V d  
CAMBRIDGE, MASSACHUSETTS 
GPO PRICE $ 
C #6 82 5 3 
CFSTl PRICE(S) s 
July 28, 1966 Microfiche (M F) $ 5 0  
ff 653 July 65 
https://ntrs.nasa.gov/search.jsp?R=19660025514 2020-03-16T20:12:57+00:00Z
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Introduction and Summary 
Under NASW-1167 w e  established t h a t  s i n g l e  c r y s t a l s  cif molybdenum 
d i s i l i c i d e  are subject t o  delayed f a i l u r e  o r  s ta t ic  f a t i g u e  when subjected 
t o  a bending stress at  5 O O O C  i n  air.(') 
a f t e r  f a i l u r e  are covered with the  powdery yellow oxide, a mixture of 
Si02 and Moo3, t h a t  has come t o  be assoc ia ted  with pes t  f a i l u r e  i n  poly- 
c r y s t a l s .  (2) 
The f r a c t u r e  sur faces  observed 
Previous r e s u l t s  were in te rpre ted  on the  b a s i s  of t h e  Charles-Hillig 
theory f o r  s ta t ic  f a t i g u e  i n  b r i t t l e  materials. Reasonable values of t h e  
parameters involved,the a c t i v a t i o n  volume f o r  pes t  f a i l u r e  and t h e  in t e r -  
f a c i a l  energy between MoSi2 and the p e s t  oxide, were obtained. 
t h e  d a t a  w e r e  of low prec is ion  due t o  t h e  l a r g e  v a r i a t i o n  i n  t h e  base 
l e v e l  s t r eng ths  of t h e  s i n g l e  c r y s t a l  specimens t e s t ed .  
s t r eng ths  of n ine  s i n g l e  c r y s t a l  samples of MoSi2 i n  4-point bending a t  
51OOC i n  vacuum varied from 4,230 p s i  t o  147,500 ps i .  
of t h e  n ine  values,  53,000 2 29,000 p s i ,  was  used i n  analyzing t h e  de- 
layed f a i l u r e  r e s u l t s  obtained i n  a i r .  
However, 
Base level 
A weighted average 
I n  order t o  c o r r e l a t e  delayed f a i l u r e  measurements i n  the  pes t  range 
with classical chemical k i n e t i c  measurements, s ta t ic  f a t i g u e  curves must 
be obtained as a func t ion  of temperature. It seemed c l e a r  t h a t  v a r i a t i o n s  
i n  base l e v e l  s t r eng th  of t h e  kind described above could completely mask 
the  temperature a f f e c t s  w e  are looking f o r .  
f i r s t  quar te r  of t h i s  year t o  preparing enough s i n g l e  c r y s t a l  molybdenum 
d i s i l i c i d e  of well-controlled base l e v e l  s t r eng th  f o r  a l l  of t h e  planned 
experiments. 
-7- 1.  I.-*- a*-. - ----- * * - - A-A ---- i n  nhtaininp a number of s i n n l e  crystal boules of 
molybdenum d i s i l i c i d e  s u f f i c i e n t  f o r  a t  least 50 s ta t ic  f a t i g u e  measure- 
ments. The base level s t r eng th  measured on four (4) samples c u t  a t  
random w a s  found t o  be 51,000 2 3500 ps i .  
W e  t he re fo re  devoted t h e  
By c a r e f u l  a t t e n t i o n  t o  t h e  materials preparation parameters, 
Work has continued on simultaneous weight-change-oxygen consumption 
measurements t o  obta in  t h e  k ine t i c  d a t a  t h a t  w e  w i l l  hopefully be a b l e  
t o  c o r r e l a t e  with s t a t i c  f a t i g u e  measurements. 
1 
EXPERIMENTAL 
Material Preparation 
As i n  previous work, s i n g l e  c r y s t a l  rods of molybdenum d i s i l i c i d e  
were prepared by zone melting of s in te red  compacts. 
w a s  200 mesh molybdenum d i s i l i c i d e  of 99.5% p u r i t y  purchased from Sylvania. 
This w a s  aixed with 4% bg weight of excess s i l i c o n  t o  compensate f o r  
evaporation during synthesis.  
The s t a r t i n g  material 
Improvements were made i n  several aspec ts  of t he  materials prepara- 
The powder is  now s in t e red  t o  about 85% of t h e o r e t i c a l  t i o n  technique. 
dens i ty  i n  a boron n i t r i d e  d i e  i n  cont ras t  t o  60-70% i n  previous work. 
The d i e  is heated t o  130OOC f o r  3 hours i n  a flowing argon atmosphere. 
The s in t e red  rod, about 7 inches long and 3/8 inch i n  diameter, is then 
zone melted i n  a flowing 95% argon--5% hydrogen atmosphere, by passing 
a slowly moving induction c o i l  up along i t s  length.  I n  order t o  avoid 
thermal cracking, a s p l i t  molybdenum rad ia t ion  sh ie ld  which is inser ted  
i n t o  a ceramic tube moves along the  length  of t h e  s i n g l e  c r y s t a l  behind 
t h e  induction c o i l .  Further improvement was ef fec ted  by seeding the  c r y s t a l s  
of molybdenum d i s i l i c i d e  with a s i n g l e  c r y s t a l  seed which is f r e e  from macro- 
cracks. 
B a r  specimens 1 / 2  - 3 /4  inch x 1 / 8  x 1/16 were c u t  from t h e  zone 
melted rods f o r  t h e  bend tests. 
a 320 g r i t  diamond wheel, then lapped and polished on 600 g r i t  s i l i c o n  
carb ide  and 6 micron diamond, respectively.  The polished specimens were 
then examined f o r  macrocracks under an o p t i c a l  microscope. 
The bar specimens w e r e  ground t o  s i z e  on 
Base Level Strength T e s t  
The rectangular bar specimens discussed above w e r e  t e s t ed  i n  bending 
under four-point loading. 
less steel j i g  with aluminum oxide (Lucalox) kn i f e  edges and 1/8 inch 
between the  upper kn i f e  edges. The j i g  w a s  mounted i n  an Ins t ron  and 
enclosed i n  a tungsten r e s i s t ance  furnace. 
The bend test apparatus c o n s i s t s  of a 304 stain- 
Bend s t r eng ths  were obtained on four bar specimens obtained from a 
s i n g l e  c r y s t a l  rod, designated 81052. 
at 5OOOC and a pressure  of 2 x 
cm/min. Results are summarized i n  Table I. 
The s t r eng th  measurements w e r e  made 
The crosshead speed w a s  0.05 t o r r .  
2 
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TABLE I 
Base Level Strengths of Single Crys ta l  MoSi2 
Sample No. 
1052-M-1 
105 2-M-2 
1052-M-3 
1052-T-1 
0 
47,000 
50,200 
56,700 
50,000 
Plans f o r  t h e  N e x t  Quarter 
Continuation of t h e  present work w i l l  be i n  t h e  d i r e c t i o n  of obtain- 
ing f a t i g u e  curves based on the recently formed s i n g l e  c r y s t a l s  of molyb- 
denum d i s i l i c i d e .  
f o r  delayed f r a c t u r e  and those required f o r  t h e  base level s t r eng ths  w i l l  
be obtained from t h e  same s i n g l e  c r y s t a l  boule. 
inves t iga ted  w i l l  be within and above t h e  pes t ing  range. 
For a spec i f ied  temperature, both t h e  bars  required 
The temperatures t o  be 
(1) F ina l  Report, The Pes t  Phenomenon i n  In t e rme ta l l i c s ,  NASW-1167, 
May, 1966. A. D. L i t t l e ,  Inc. 
R. W. Bartlett and P. R. Gage, ASD-TDR-63-753, P a r t  11, July  1964. (2) 
3 


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