Temperature effects on aluminum alloy, nickel, and electrolytic chromium surface stability of astronomical mirro

Lorentsyan, G. M.

English

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.' ' c. 
National Aeronautics and Space Administration 
Goddard Space Flight Center 
Contract No.NAS-5-12487 
. 
ST -MR -0A - 10700 
INVESTIGATION OF THERMAL STABILITY 
OF THE MICROSURFACE OF ASTRONOMICAL MIRRORS 
MADE OF ALLOY A M ~ ~ L  WITH CHROMIUM AND NICKEL COATINGS 
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G. M. Lorentsyan 
( A ~ S S R  ) 
N 6 8 - 2 1 0 8 7  (ACCESSION NUMBER) (THRU) 
k/ 1 
5 
P -
> LJ / 
(CODE) 
(NASA CR OR TMX OR AD NUMBER) 
15 APRIL 1968 
https://ntrs.nasa.gov/search.jsp?R=19680011618 2020-03-12T08:34:04+00:00Z
ST -MR -0A - lOj'30 
INVESTIGATION OF THERMAL STABILITY 
OF THE MICROSURFACE OF ASTRONOMICAL MIRRORS 
MADE OF ALLOY A Mg6L WITH CHROMIUM AND NICKEL COATINGS 
("1 
Soobshch. Byurakanskoy Observa tor i i  
Vypusk 30,  str. 66 - 71, '  
Izd-vo V. A .  Ambartsumyana 
Akadernii Nauk Armyanskoy SSR, 1967. 
by G. M .  Lorents ian  
An i n v e s t i g a t i o n  w a s  made of t h e  e f f e c t  o f  temperature  v a r i a t i o n s  and 
o f  t h e  d i f f e r e n c e  i n  t h e  l i n e a r  expansion c o e f f i c i e n t  of a l l o y  A M ~ L ~ ,  of 
e l e c t r o l y t i c  chromium and of chemical n i c k e l  on t h e  q u a l i t y  of  microsurfaces  
of mi r ro r s .  These mi r ro r s  were made of A M ~ L ~  a l l o y  wi th  chromium and n i c k e l  
coa t ings  of  va r ious  th i cknesses .  The experiment w a s  c a r r i e d  out  a t  tempera- 
t u r e s  of - 9 5 O ,  +20°, +60° and +lOO°C.  
It w a s  shown t h a t  mi r ro r s  coated wi th  10 t o  5 0 ~  of chromium d i d  not  a l t e r  
t h e i r  microsurface q u a l i t y .  I n  t h e  cases  of coa t ings  t h i c k e r  than  50p , cracks 
appeared on t h e  mi r ro r  su r face .  Temperature v a r i a t i o n s  have no e f f e c t  on t h e  
q u a l i t y  of n i c k e l  coa t ings .  
g r e a t  i n t e r n a l  s t r a ins  i n  t h e  chromium coat ings .  The g r e a t e r  t h e  d i f f e rence  
between t h e  c o e f f i c i e n t  a of t h e  coa t ing  substances and t h e  mi r ro r  i t s e l f ,  
t h e  g r e a t e r  t h e  inne r  s t r a i n s  t h a t  o r i g i n a t e  i n  t h e  coa t ing .  
The o r i g i n  of t h e  cracks can be expla ined  by 
* 
* *  
I n  o rde r  t o  determine t h e  e f f e c t s  of high and low tempera tures ,  o f  t h e  
d i f f e r e n c e s  i n  t h e  expansion c o e f f i c i e n t s  of  aluminum a l l o y ,  e l e c t r o l y t i c  
chromium and chemical n i c k e l  on t h e  q u a l i t y  of mi r ro r s '  microsur face ,  we  con- 
ducted a series of t es t s  on pol i shed  mi r ro r s  made of a l l o y  A M ~ ~ L  with chromium 
and n i c k e l  coa t ings .  
+60° and +lOO°C.  
laboratory- type 111-005 thermosta t .  The p resc r ibed  temperature  w a s  automatic- 
a l l y  obta ined  wi th  t h e  a i d  of a TK-6 contac t  thermometer achieving a p rec i -  
s i o n  t o  51'. The low temperature t e s t s  of samples were conducted i n  a herme- 
t i c a l l y  sea l ed  box wi th  s o l i d  carbon dioxide ( d r y  i c e ) .  The samples were d i s -  
posed between carbon d ioxide  slugs. The temperature was measured by means of 
a thermometer with measurement range from +20° t o  + l O O ° C .  A t  t h e  l a t t e r  t e m -  
These t e s t s  were conducted a t  temperatures  of -95 ' ,  +20°, 
Increased  temperature  (+60° and + l O O ° C )  w a s  obtained i n  t h e  
( * )  ISSLEDOVANIYE NA TERMOSTOYKOST' MIZROPOVERKHNOSTI ASTRONOMICHESKIKH 
ZERKAL, IZGOTOVLENNYKH I Z  SPLAVA A M ~ ~ L  S KHROMOVYN I MIKELEVYM POKRYTIYEMI 
h 
L 
p c r a t u r e  t h e  t i m e  l a g  was of 6 hours ,  a t  bO°C it  w a s  of 8 hours and a t  - 9 5 O  - 
20 hours .  
took p l a c e  g radua l ly ,  s t a r t i n g  from room temperature .  
temperature  a l s o  w a s  gradual .  
The hea t ing  of p a r t s  i n  thermosta t  t o  t h e  requi red  temperature  
The cool ing  t o  room 
Compiled i n  Table 1 a r e  the  g r e a t e s t  and t h e  smallest th i cknesses  of  
t e s t e d  samples '  coa t ing ,  immediately a f t e r  a p p l i c a t i o n :  EL, and a l s o  a f te r  
p o l i s h i n g  and f i n i s h :  b . - 
ZOATING 
- _ _ ~  
ZHROMIUM 
a 
b 
NICKEL 
a 
b 
T A B L E  1 
No. of  Samples _ _  
1 
47 - 40 
10 - 1 5  
30 - 35 
i o  - 1 5  
2 
~- .. 
55 - $ 5  
30 - 20 
40 - 45 
1 5  - 20 
3 
85 - 100 
30 - 50 
50 - 70 
35 - 40 
~ 
4 
100 - 120 
50 - 80 
5 
220 - 240 
120 - 140 
"he q u a l i t y  o f  t h e  microsurface w a s  checked by v i s u a l  means and a l s o  by 
photographing on t h e  un ive r sa l  microscope "NEOPHOT" by Zeiss wi th  a 200 x 
magn i f i ca t ion  p r i o r  and a f t e r  t e s t s .  
The resul ts  o f  t h e  tes ts  fo r  chromium coa t ings ,  completed i n  t h e  
chrome p l a t i n g  regime, are compiled i n  Table 2 .  
The r e s u l t s  o f  t hese  experiments show t h a t  f o r  t h e  i n d i c a t e d  cyc le  of 
thermal  tes ts  t h e  microsurface of m i r r o r s  wi th  chromium coa t ings  from 10 t o  
50 p remained unchanged. For chromium c o a t i n  5hickness  from 50 t o  1201.1 cracks 
have formed on mi r ro r  su r face  a f t e r  hea t ing .  Consequently, t h e r e  e x i s t s  a 
t h r e s h o l d  th i ckness  th ickness  of chromium l a y e r  over A M ~ G L  a l l o y ,  app l i cab le  
as t h e  o p e r a t i o n a l  r e f l e c t i n g  su r face  of t h e  astronomical  mi r ro r .  Microcracks 
appear  on t h e  s u r f a c e  of t h e  mir ror  when t h i s  t h re sho ld  i s  exceeded. The fo r -  
mation of microcracks on t h e  chromium coat ing  can be expla ined  as fol lows.  
As i s  w e l l  known, i n n e r  s t r a i n s  appear i n  t h e  e l e c t r o l y t i c  chromium l a y e r ,  
a t t a i n i n g  high va lues  (3700 - 6000 kg/cm2) and depending upon t h e  na tu re  o f  
of coa t ing ,  t h e  method of i t s  app l i ca t ion  and t h e  s t a t e  o f  t h e  packing block.  
I n  t h e  case  i n  ques t ion  t h e  coat ing regimes,  t h e  na tu re  and t h e  s t a t e  of t h e  
packing block are ident . ica1 i n  a l l  t h i cknesses .  I n  chromium l a y e r s  of t h i ck -  
ness  from 100 t o  1201-1 no microcracks were observed immediately a f t e r  coa t ing ,  
b u t  above t h a t ,  say a t  220 ll, webs of c racks  were no t i ceab le .  These cracks 
were formed i n  t h e  e l e c t r o l y s i s  process  on account of t h e  excess  of  i nne r  
s t r a i n  of chromium layer ' sbreaking  p o i n t .  
3 
V 
0 
0 
0 
rl + 
4 
The magnitude of t h e  r e s idua l  i n t e r n a l  stress of chromium l a y e r  i s  
inf luenced  by t h e  concomitant ex is tence  of temperature  v a r i a t i o n  and d i f f e r -  
ence i n  t h e  c o e f f i c i e n t s  of l i n e a r  expansion g of adhering m a t e r i a l s .  
more s i g n i f i c a n t  t h e  d i f f e r e n c e  i n  t h e  va lues  of c o e f f i c i e n t  2, t h e  g r e a t e r  
t h e  i n t e r n a l  s t r e s s e s  occurr ing  a t  temperature  v a r i a t i o n .  J u s t  how g r e a t  i s  
t h e  d i f f e r e n c e  i n  t h e  va lues  of the  l i n e a r  expansion c o e f f i c i e n t  of e l e c t r o -  
l y t i c  chromium and A M ~ ~ L  a l l o y  i s  w e l l  known: f o r  t h e  e l e c t r o l y t i c  chromium 
a = 6.25 1 deg, ,and for A M ~ ~ L  a l l o y  a = 23 .8 .  1 deg. 
The 
It fol lows from t h e  r e s u l t s  of t h e  experiment t h a t  i n  order  t o  o b t a i n  on 
mi r ro r s  a microsurface of  good q u a l i t y ,  when t h e s e  mi r ro r s  are made of alumi- 
num-magnesium a l l o y  wi th  chromium coa t ing ,  a th i ckness  of chromium coa t ing  
g r e a t e r  than  85 t o  100~ i s  inadmissible .  
The r e s u l t s  of t es t s  show t h a t  f o r  small th i cknesses  of  chromium coa t ings  
( t o  50 p) t h e  va lue  of t h e  i n t e r n a l  stress does not  exceed t h e  breaking s t r e n g t h  
of  chromium l a y e r  and t h i s  i s  why no d e s t r u c t i o n  of  t h e  coa t ing  i s  observed. 
But ,  as t h e  th i ckness  o f  t h e  coat ing inc reases  ( 5 0 ~  ar:d above) t h e  temperature  
g r a d i e n t  induces i n s i d e  t h e  coat ing stresses exceeding t h e  breaking  s t r e n g t h  
of e l e c t r o l y t i c  chromium, which l e a d s ,  i n  t h e  f i n a l  count ,  t o  t h e  appearance 
of a web of microcracks over  t h e  su r face  o f  t h e  coa t ing .  
The microsurface of chemical n i c k e l  coa t ing  over  an alumino-magnesium 
base d i d  not  vary e i t h e r  f o r  extreme coa t ing  th i cknesses  of 65 t o  75p ( 4 0  t o  
50 1-1 aft.er t r e a t m e n t ) ,  or for th innes t  coa t ings  of 30 t o  35 1.1 ( i . e .  10 t o  15p 
after t r e a t m e n t ) ,  as temperature  f l u c t u a t e d  w i t h i n  t h e  l i m i t s  from + l O O ° C  t o  
-95OC.  Consequently, t h e  obta in ing  of a q u a l i t y  microsurface on n i c k e l  coa t ing  
a p p l i e d  chemical ly ,  i s  p o s s i b l e  fo r  t h e  given temperature  range ( -90 t o  + looo)  
and th i ckness  (up t o  65 - 75 u). 
I n  conclnsion it should be s t r e s s e d  t h a t  t h e  thermal  t e s t s  of  t h e  i n v e s t i -  
ga t ed  samples were conducted by us wi th  t h e  cooperat ion of s e n i o r  engineer  
G .  G .  Lavrent 'yeva. 
Contract  No.NAS-5-12487 Trans l a t ed  by ANDRE L. BRICHANT 
VOLT TECHNICAL CORPORATION 
1145 - 19th  St.NW 
WASHINGTON D . C . 20036 
Telephone: 223-6700 (XX-36 & 37) 
on 14 A p r i l  1968 


Investigation of Thermal Stability of the Microsurface of Astronomical Mirrors Made of Alloy Amg6l with Chromium and Nickel Coatings

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