Effects of phase separation on crack growth in borosilicate glass

J o u r n a l of N o n - C r y s t a l l i n e North-Holland Publishing

S o l i d s 38 & 39 Company

(1980)

503-508

EFFECTS OF PHASE SEPARATION ON CRACK GROWTH IN B O R O S I L I C A T E GLASS Catherine J. Simmons S t e p h e n W. F r e i m a n National Bureau of Standards Washington~ D.C. 20234

The effect of phase separation on crack p r o p a g a t i o n was determined in a s o d a - b o r o s i l i c a t e glass. Cra~k vel~city - K I curves in the velocity range of 1O -~ to i0m/s were shifted to lower K I with incKeasing heattreatment time. A velocity plateau of I0 -~ m/s occurred, independent of heat treatment conditions. It is shown that the c o m p o s i t i o n a l changes in the SiO 2 rich phase formed during heat treatment controls crack growth rates. Results are discussed in terms of glass corrosion mechanisms. INTRODUCTION Phase separation is an important phenomenon in many glasses. The formation and growth of a two-phase m i c r o s t r u c t u r e can influence crack growth in two ways: (i] by changing the chemical composition and, therefore, the corrosion resistance of each phase and, (2) by producing phase boundaries which can affect crack motion. • .~ The b o r o s i l i c a t e glass chosen, (J0{0 Sl02 - 23~o B203 - 70~. Na~0),~ wl-h an i m m i s c i b i l i t y temperature of 755 °C, has been extensively characterized by other m e a s u r e m e n t s 1-3 and shows a large change in chemical durability and m i c r o s t r u c t u r e size with heat-treatment. This glass readily phase separates to form an interconnected m i c r o s t r u c t u r e in which one phase is p r e d o m i n a n t l y B2@ 3 and Na20 (phase I), making it highly susceptible to chemical attac~, while the other (phase II) is rich in Si02, and is, therefore, much more durable. As a result of the speed at which this glass phase separates, a~y forming operation for samples other than rapidly drawn thin fibers will invariably develop an initial m i c r o s t r u c t u r e characteristic of some unknown none q u i l i b r i u m state. Subsequent h e a t - t r e a t m e n t s at given temperatures for different times lead to a composition change of both phases in order to approach the new equilibrium. Selecting a glass whose p h a s e - s e p a r a t i o n behavior is well c h a r a c t e r i z e d allows one to determine, qualitatively, the way in which small changes in composition affect glass durability and crack growth.

The m e c h a n i s m for glass corrosion is generally divided into two stages 4. The first stage consists of a p r o t o n - a l k a l i ion exchange which occurs fairly rapidly to form a dealkalized surface layer. The thickness and porosity of this layer, and the rate at which it forms, is determined by the chemical durability of the glass in a given environment, increasing as glass durability decreases. As the layer becomes thicker, ion diffusion is inhibited and the corrosion process is then controlled by the second stage m e c h a n i s m which is congruent, or matrix, dissolution. It has been suggested that the first stage (ion exchange) m e c h a n i s m controls crack motion at velocities of 1O -g to 10 -4 m/sec and that matrix dissolution can only be seen at velocities apnroachinz the du{ability limit of the glass being studied (i.e., 10 -9 - i0 -IZ m/sec) ~"

503

50~

C.J. Simmons, S.W. Freiman / Crack Growth in Borosilicate Glass

The o b j e c t i v e of th~s w o P k was to d e t e r m i n e the e f f e < t o[ c h m g e s £n c h e m i c a l c o m p o s i t i o n and m i c r o s t r u c t u ? e size on c P a c k p r o p a g a t i o n in a p h a s e - s e p a r a t e d b o r o s i l i e a t e glass. It w i ] l be shown that change'~ in the c o m p o s i t i o n of the p h a s e s as a f u n c t i o n of h e a t t r e a t m e n t , p r o d u c i n g c h a n p . s in c o r r o s i o n r e s i s t a n c e , are the d o m i n a n t f a c t o r in d e t e r m i n i n g crack g r o w t h rates. EXPERIMENTAL

PROCEDURE

The s o d i u m - b o r o s i l i c a t e g l a s s wa~ p r e p a p e d by me] tin% r e a g e n t g r a d e c h e m i c a l s in a p l a t < h u m c r u c i b l e at ]3!0 °C w h i l e stir'Ping. After c a s t i n s , the 8]ass was a n n e a l e d at 550 °C
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C.J. Simmons, S.W. Freiman / Crack Growth in Borosilicate Glass

HEAT TREATMENT T e m p (°C) T i m e (min] 550 390 620 33 G20 2087 650 5:~81

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i)ISCUSSION

The ! a t a were plotted as crack veloeity-vs-stress intensity factor~ KZ, ( F I B . 2 ) £o~' s a m p l e s 1 to 4 as listed in Table I. All samD!es s h a p ~ a common p l a t e a u velocity of 2 x l0 -4 m/s in re~ion lI oven a w i d e r a n z e of s t r e s s - i n t e n s i t y v a l u e s . D i f f e r e n c e s in the m i c r o ;tru, ture size and c h e m i c a l d u p a b i l i t y w o u l d not be , x p e c t e d to inflienc.~ ~he e p a c k v e l o c i t y in the p l a t e a u r e g i o n since c r a c k S r o w t h is c o n t r o l l e d by the P a t e of d i f f u s i o ~ of w a t e E to the c r a c k iiip 7. T%~t,~ o b t a i n e d b e l o w a v e l o c i t y of I0m/s show ~ small but =~,;trmatic s h £ f t to lower KI v a l u e s as a f u n c t i o n of h e a t - t r e a t m e n t t [me fop the t h r e e 620 oC s a m p l e s . S a m p l e ~, h e a t - t r e a t e d at 650 o~ for' 91 h o u r s , shows a l a r g e r shift to the left, ~rldi
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Crack growth data l:or s n d a - b o r o s J l i < : a t e de~rees of phase sepapatlon.

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506

C.J. Simmons, S.W. Freiman / Crack Growth in Borosilicate Glass

In a g l a s s v e r y s i m i l a r in c o m p o s i t i o n to the one s t u d i e d here, U t s u m i and c o w o r k e r s n o t e d a d e c r e a s e in f l e x u r a l s t r e n g t h 9 w i t h h e a t - t r e a t m e n t t i m e and a t t r i b u t e d this b e h a v i o r to i n c r e a s e d m i c r o s t r u c t u r e size. U t s u m ~ and Sakka, r e p o r t i n g on b a l l i n d e n t a t i o n s t r e n g t h m e a s u r e m e n t s I ~ on the same glass, c o n d u c t e d at r e l a t i v e h u m i d i t i e s v a r y i n g f r o m 50-75% did not s h o w a s y s t e m a t i c d e c r e a s e as a f u n c t i o n of h u m i d i t y e x c e p t t h a t a d e c r e a s e was n o t i c e a b l e at 75% RH. S i n c e it is not k n o w n w h i c h of the t h r e e r e g i o n s of c r a c k g r o w t h has the g r e a t e s t i n f l u e n c e on t h e i r r e s u l t s , it is d i f f i c u l t to d i r e c t l y r e l a t e the two s t u d i e s . H o w e v e r , it s h o u l d be n o t e d that t h e y o b s e r v e d a d e c r e a s e in s t r e n g t h as a f u n c t i o n of h e a t - t r e a t m e n t time. The m a g n i t u d e of this d e c r e a s e c o r r e s p o n d s a p p r o x i m a t e l y to t h a t w h i c h w o u l d be p r e d i c t e d b a s e d on our slow c r a c k g r o w t h r e s u l t s . In o r d e r to c o n f i r m that an i n c r e a s i n g a m o u n t of w a t e r v a p o r in the e n v i r o n m e n t w i l l p r o d u c e a c o r r e s p o n d i n g i n c r e a s e in the r a t e of c h e m i c a l a t t a c k at the c r a c k tip, m e a s u r e m e n t s w e r e m a d e on s a m p l e at d i f f e r e n t r e l a t i v e h u m i d i t i e s . The s h i f t of the V - K c u r v e s to the left (Fig. 3) c l e a r l y shows the e x p e c t e d e n v i r o n m e n t a l e f f e c t .

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Chemica~ ~rability m e a s u r e m e n t s on p h a s e - s e p a r a t e d soda-borosili<.ate glasses ' as d e t e r m i n e d by a l k a l i e x t r a c t i o n r a t e s ~ h a v e s h o w n that the d u r a b i l i t y w o r s e n s as a [ u n c t i o n of h e a t - t r e a t m e n t time, a p p r o a c h i n g e q u i l i b r i u m a f t e r a b o u t 25 hrs. at 625 °C and a b o u t G h o u r s at 650 °C~ for the c o m p o s i t i o n s t u d i e d here. A l s o , ~he d u r a b i l i t y a p p e a r s to i m p r o v e as the h e a t - t r e a t m e n t t e m p e r a t u r e is increased. T h i s is e x p e c t e d since the two p h a s e s are m o r e n e a r l y ,~like at h i g h e r temperat~]res, and the i(~ported glass d u r a b i l i t y wa~ h a s e d on the rate of d i s s o l u t i o n of the more s o l u b l e a l k a l i - b o r a t e r i c h p h a s e and not on the d i s s o l u t i o n of the glass as a whole. L o o k i n g at Fig. 2 for a c o m p a r i s o n of the 620 ° and 650 °C h e a t

C.J. Sin~nons, S.W. Freiman / Crack Growth in Borosilicate Glass

507

t r e a t m e n t s we see that the result is contrary to the behavior predicted by the changes in chemical d u r a b i l i t y in ref. 2. If the 650 °C heat treatment leads to h i g h e r durability as shown, one would expect a shift to the right. This d i s a g r e e m e n t arises because cracks must propagate through both phases of an i n t e r c o n n e c t e d structure and it is n e c e s s a r y to consider the effect of changes in the chemical d u r a b i l i t y of the Si0o-rich phase as well. In fact, since the ~lower d i s s o l u t i o n occurs in the silica-rich phase (nhase II), changes in durability in this phase are expected to have a larger. influence on region I crack growth b e h a v i o r than do changes in phase I. In comparing variations in c o m p o s i t i o n of the two phases with heat treatment time and temperature and the r e s u l t i n g variations in chemi~l durability we turn to recent chemical durability measurements w h i c h show a huge decrease in durabilty with small additiom~ o£ alkali to high silica glass, the rate of change decreasing with increasing alkali content. Therefore, even though the alkali content of phase II changes little from 620 ° to 650 °C, the c o r r e s p o n d i n g a l t e r a t i o n in d u r a b i l i t y of this high silica phase is expected to be very large and o v e r w h e l m the r e l a t i v e l y smaller change in durability in the s o d i u m - b o r a t e rich phase. C o n s e q u e n t l y , in this phaseseparated glass system it is clear that the m e c h a n i s m of crack p r o p a g a t i o n in region I is controlled by the changes in c o m p o s i t i o n and durability of the s i l i c a - r i c h phase. In order to follow the changes in chemical durability in phase II of ~he glass studied here, it is n e c e s s a r y to consider the thermal history of the glass prior to, and during, heat treatment. Because the glass was initially poured in the form of a block, it phase separated to some unknown n o n - e q u i l i b r i u m state during cooling. i%ubsequent annealin Z at 850 °C insured the formation of phases w~th c o m p o s i t i o n s a p p r o a c h i n g those typical of a low temperature heat treatment (7550 °C or less). In this state the s o d i u m - b o r a t e phase contains r e l a t i v e l y little silica while the silica phase is very low in alkali content. During the heat treatment at 620 °C, the compos[< ion of the high silica phase changes to a higher alkali content ~nd lower durability as a function of time. The rate of change in :omp~sit~on of nhase !I toward the 620 °C e q u i l i b r i u m is very slow due to the h i g h = v ~ s c o s i t y of that phase I" This chanse is reflected in the crack p r o p a g a t i o n b e h a v i o r of the glass at 620 °C (samples iin fig. 2) which shows a shift in the curves to lower K I values. F o l l o w i n g the 650 °C heat treatment the c o m p o s i t i o n of phase ii has an even higher alkali content and lower chemical durability. This is, again, r e f l e c t e d in the large shift observed for sample 4. CONCLUSIONS The s u b c r i ~ i c a l crack growth b e h a v i o r of a phase senarated sodiumborosilicate glass was studied. A shift in the region I curves tc lower stress intensity values was observed as a function of heattreatment time and temperature. This shift is consistent with dat~ obtained in environments of increasing relative h u m i d i t y and indicates a greater susceptibli [ty to stress corrosion. l~efore reach~n£ an e q u i l i b r i u m state there is a ~l'adua] change in ~he c o m p o s i t i o n of both phases which occurs during heat treatment belc~w ~he immiscibility temperature, causin 8 p r o g r e s s i v e ehanges in the chemical durability of each phase. It is r~ossible to conclude, based on the thermal history of the specimens tested, that: the chemical i u r a b i l i t y of the al}:ali-boron rich
508

C.J. Simmons, S.W. Freiman / Crack Growth in Borosilicate Glass

the silica-rich phase worsens, as a function of increased heattreatment time and ternperature. The c r a c k ~
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