PROPERTIES AND SHAPING OF LIGHTWEIGHT CERAMICS BASED ON PHOSPHATE-BONDED HOLLOW SILICA MICROSPHERES

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PROPERTIES AND SHAPING OF LIGHTWEIGHT CERAMICS BASED ON PHOSPHATE-BONDED

HOLLOW SILICA MICROSPHERES

G. de With, H. Verweij

To cite this version:

G. de With, H. Verweij. PROPERTIES AND SHAPING OF LIGHTWEIGHT CERAMICS BASED ON PHOSPHATE-BONDED HOLLOW SILICA MICROSPHERES. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-359-C1-363. �10.1051/jphyscol:1986153�. �jpa-00225583�

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PROPERTIES AND SHAPING OF LIGHTWEIGHT CERAMICS BASED ON PHOSPHATE-BONDED HOLLOW SILICA MICROSPHERES

G. DE WITH and H. VERWEIJ

P h i l i p s R e s e a r c h L a b o r a t o r i e s , P.O. Box 80.000, NL-5600 J A E i n d h o v e n , T h e N e t h e r l a n d s

Les v a l e u r s du module de Young's, de l a r e s i s t a n c e , de l a t 6 n a c i t 6 c o n d u c t i v i t 6 thermique de c6ramiques l e g h r e s h base de microsph6res de s i l i c e , agglom6rees p a r un phosphate, sont donn6es en f o n c t i o n des c o n d i t i o n s de t r a i t e m e n t . E l l e s sont compar6es aux donn6es correspon- dantes d ' a u t r e s c6ramiques composite 16ghres. Le formage p a r pressage ou par usinage au t o u r e s t d 6 c r i t briGvement.

A b s t r a c t - The values f o r t h e Young's modulus, s t r e n g t h , f r a c t u r e toughness and t h e r m a l c o n d u c t i v i t y o f l i g h t w e i g h t ceramics based on ~ h o s ~ h a t e - b o n d e d h o l l o w s i l i c a microspheres a r e r e p o r t e d as a f u n c t i o n o f t h e p r o c e s s i n g con- d i t i o n s . Thev a r e c o m ~ a r e d w i t h t h e r e l e v a n t data f o r o t h e r l i g h t w e i g h t ceramic composites. ~ h k shaping by e i t h e r p r e s s i n g o r l a t h e machining i s b r i e f l y discussed.

I - INTRODUCTION

Hollow microspheres a r e a r e l a t i v e l y new m a t e r i a l . They a r e a v a i l a b l e i n t h e form o f glasses, e.g. b o r o s i l i c a t e g l a s s o r v i t r e o u s s i l i c a , and a l s o i n c r y s t a l - l i n e form e.g. alumina o r z i r c o n i a . The h o l l o w g l a s s spheres can be bonded e i t h e r by s i n t e r i n g o r by u s i n g a bonding agent such as phosphate. I n t h e case o f g l a s s spheres, compacts have been r e p o r t e d which were prepared b y s i n t e r i n g ( I ) and, i n t h e case o f v i t r e o u s s i l i c a microspheres, t h e phosphate bonding process y i e l d s homogeneous, e a s i l y manageable ceramics (3). Some t h e o r e t i c a l work has been done on t h e p r e d i c t i o n o f t h e Young's modulus (2,3) and f r a c t u r e toughness ( 2 ) o f these composites. I n t h i s paper we present a f u r t h e r c h a r a c t e r i s a t i o n and some measurements o f t h e mechanical p r o p e r t i e s o f phosphate-bonded ceramics based on h o l l o w s i l i c a g l a s s microspheres. Furthermore we p r e s e n t some examples o f machined and pressed p l a t e s o f complex shape.

11- EXPERIMENTAL

Hollow s i l i c a microspheres were chosen as raw m a t e r i a l (Emerson and Cuming Inc., eccospheres S I ) . The diameter d i s t r i b u t i o n was determined u s i n g a d i g i t a l p l a n i m e t e r . The ceramics were made by t h e phosphate bonding process as described ( 3 ) . The c o n c e n t r a t i o n o f t h e mono-aluminium phosphate (MAP), t h e f i r i n g t i m e and t h e f i r i n g temperature were v a r i e d . D e n s i t i e s , q, were measured on l a r g e f l a t specimens, o f which t h e o u t e r p a r t had been removed by machining, u s i n g weight and dimensional data. Mercury i n t r u s i o n p o r o s i m e t r y (MIP) was used t o s t u d y t h e p o r e s t r u c t u r e o f t h e composites ( C a r l o Erba macro-pore u n i t 120). The t h e r m a l conducti- v i t y was measured u s i n g a t h e r m a l comparator (TC-1000, L a f a y e t t e I n s t r u m e n t s Com- pany). Young's modulus, E, was determined by d e f l e c t i o n measurements on 3-point bending specimens. The f r a c t u r e toughness, KI~, as w e l l a s s t r e n g t h , Sf, were

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986153

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C1- 360 JOURNAL DE PHYSIQUE

determined i n a s i m i l a r way u s i n g g e n e r a l l y 3 specimens o f s i z e 3*9*45 mm3. A s m a l l n o t c h o f r e l a t i v e depth 0.15 was sawn i n t h e K,I specimens w i t h a diamond saw.

The n o t c h was sharpened by c u t t i n g w i t h a r a s o r k n i f e . The compliance was calcu- l a t e d a c c o r d i n g t o t h e formula d e r i v e d by Brown and Srawley ( 4 ) . The f r a c t u r e s u r - faces were examined by scanning e l e c t r o n microscopy (SEMI.

111

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RESULTS

From t h e diameter a n a l y s i s i t was r e v e a l e d t h a t t h e average diameter o f t h e spheres was 49 ym w i t h a r a t h e r l a r g e s t a n d a r d d e v i a t i o n o f about 21 pm. T h i s f i g u r e i s somewhat lower t h a n t h e 80 ym ( w e i g h t b a s i s ) i n d i c a t e d b y t h e manufac- t u r e r . The s i z e d i s t r i b u t i o n appeared t o be bi-modal and r a t h e r wide, i n agreement w i t h t h e manufacturer's data. A t y p i c a l m i c r o s t r u c t u r e o f t h e formed ceramic i s shown i n f i g . l a . From t h e MIP measurements i t was c l e a r t h a t t h e average pore diameter and w i d t h o f t h e pore s i z e d i s t r i b u t i o n i n t h e v a r i o u s composites were r e l a t i v e l y c o n s t a n t a t about 10 ym and 1 pm r e s p e c t i v e l y and independent o f t h e orocessing c o n d i t i o n s .

F i g . 1. F r a c t u r e s u r f a c e o f t h e h o l l o w s i l i c a microsphere composite a s - f i r e d a t 6 0 0 " ~ f o r 3 hours u s i n g 4 wt ?6 MAP ( l e f t ) and a f t e r 3 hours a t 1000°C ( r i g h t ) .

The t h e r m a l c o n d u c t i v i t i e s o f t h e composites are 0.09 W/m.K and 0.1 W/m.K a t d e n s i t i e s o f 180 kg/m3 ( 8 X qr) and 240 kg/m3 (11 X qr) r e s p e c t i v e l y . For q u a r t z foam ceramic (7% qr) a v a l u e o f 0.07 W/m.K was r e p o r t e d ( 5 ) . Good agreement i s t h u s observed. For comparison, t h e t h e r m a l c o n d u c t i v i t y o f h o l l o w spheres under s e l f - l o a d i n g c o n d i t i o n s i n N2 under atmospheric p r e s s u r e was r e p o r t e d t o be 0.026 W/m.K ( 6 ) w h i l e t h e manufacturer c l a i m s 0.05 W/m.K f o r ' l o o s e l y packed m a t e r i a l ' .

The r e s u l t s o f t h e mechanical measurements a r e c o l l e c t e d i n f i g . 2. I n f i g . 2a t h e i n f l u e n c e o f t h e f i r i n g temperature i s depicted. The i n f l u e n c e o f t h i s proces- s i n g parameter on d e n s i t y , Young's modulus, s t r e n g t h and f r a c t u r e toughness i s o n l y s l i g h t . A more o r l e s s c o n s t a n t d e n s i t y o f about 0.18 g/cm3, a Young's modulus o f about 0.4 GPa and a f r a c t u r e s t r e n g t h o f about 0.8 MPa a r e found. On t h e o t h e r hand t h e f r a c t u r e toughness r i s e s from about 16 k ~ a . m l / ~ a t t h e lower f i r i n g tempera- t u r e s t o about 35 k ~ a . m l / z a t t h e h i g h e s t temperature o f 700°C. T h i s r i s e i n K I ~ i s accompanied by a s l i g h t i n c r e a s e i n d e n s i t y .

The i n f l u e n c e o f t h e MAP c o n c e n t r a t i o n i s g i v e n i n f i g . 2b. The values o f a l l mechanical p r o p e r t i e s i n c r e a s e w i t h t h e MAP c o n c e n t r a t i o n as does t h e d e n s i t y . T h i s i n c r e a s e i s t h u s n o t unexpected. A t t h e h i g h e s t d e n s i t y (240 kg/m3) an Sf value o f 2.0 MPa, a K l C value o f 44 k ~ a . m l / ~ and a Young's modulus o f 0.8 GPa were obtained.

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x . K,, ( M P ~ .m1/'1 x : E (GPal - _4.80E-2

1 . 8 - x . E IGPa) - - 4.80E-2 0 : S t r e n g t h IMPaI

o : S t r e n g t h (MPa)

1.5 - - 4.00E-2

1 5 - - 4.00E-2

1 . 2 - - 3.20E-2

- ^3.20E-2

E/S K:,

0 . 9 - - ^2.40E-2

- ^2.40E-2

- ^1.60E-2

0 . 6 - _1.6OE-2

8.OOE-3

0 . 3 8.00E-3

250 250

? (kg/m3)

150 ₂₀₀

-

₃₀₀ ₄₀₀ ₅₀₀ ₆₀₀ ₇₀₀ _BOO ₀ ₈ ₁₆ ₂₄

32 40

Temp. ('C1 / 3hrs KCP c o n c e n t r a t ]on (%1

150

0 5 10 15 20 25

Time (hrs) a t 600°C

F i g . 2a. Dependence on f i r i n g tempera- F i g . 2b. Dependence on MAP concentra- t u r e a t constant t i m e (3 hours) t i o n i n w t L a t c o n s t a n t and MAP c o n c e n t r a t i o n ( 4 wt %I. f i r i n g temperature (600°C)

and t i m e (3 hours).

2 1 5 60E-2

F i g . 2c. Dependence on f i r i n g t i m e a t constant f i r i n g temperature (600°C) and MAP c o n c e n t r a t i o n ( 4 w t L).

F i n a l l y i n fig. 2c t h e i n f l u e n c e o f f i r i n g t i m e i s shown. Although t h e 4 8OE-2 d e n s i t y remains approximately con-

s t a n t , t h e f r a c t u r e toughness and 1.8

s t r e n g t h decrease w i t h i n c r e a s i n g - _OOE-2 t i m e from t h e i r s t a r t i n g values o f 30 k ~ a . m ' / ~ and 1.6 Mpa t o 20 kpa.m1/Z and 0.4 Mpa r e s p e c t i v e l y . 20E-2

Young's modulus shows a much s m a l l e r

E/S K,, decrease from about 0.6 GPA t o 0.3

o 9 . 2 4 0 ~ - 2 GPa. As was shown w i t h X-ray tech-

niques, t h e s i l i c a microspheres devi- t r i f y d u r i n g t h e l o n g e r f i r i n g times.

Y . uic iMPa mif21

- * E ;Gpa) -

0 : S t r e n g t h iMPal

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C1-362 JOURNAL DE PHYSIQUE

I V

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DISCUSSION

I n s p e c t i o n o f t h e f r a c t u r e surfaces o f t h e v a r i o u s composites r e v e a l s t h a t t h e f r a c t u r e mode i s o f mixed n a t u r e . The m a j o r i t y o f t h e bonded s p o t s i n t h e ceramic f r a c t u r e through t h e spheres themselves, b u t a s m a l l p r o p o r t i o n o f them f r a c t u r e a t t h e necks. I n g e n e r a l i t appeared t o be i m p o s s i b l e t o l o c a t e t h e f r a c t u r e o r i g i n s i n t h e Sf specimens.

From t h e s t r e n g t h and f r a c t u r e toughness d a t a an e s t i m a t e o f t h e c r i t i c a l flaw s i z e can be made. The main problem i s now t o e s t i m a t e t h e compliance f a c t o r Y s i n c e t h e shape o f t h e f r a c t u r e o r i g i n ( c r i t i c a l f l a w ) i s unknown. The u s u a l assumption o f a f l a w w i t h s e m i - c i r c u l a r surface ( 7 ) y i e l d s Y = 1.26. Other assumptions l e a d t o values between 1 and 2. C a l c u l a t i o n o f t h e f l a w s i z e w i t h Y = 1.5 g i v e s values o f s e v e r a l hundreds o f micrometers. Since t h e p o r e diameters were about 10 pm a s i n g l e pore channel cannot be r e s p o n s i b l e f o r f a i l u r e . Processing d e f e c t s such as a i r bubbles o r inhomogeneous f i l l i n g o f t h e a v a i l a b l e space by t h e microspheres a r e p r o b a b l y t h e f r a c t u r e o r i g i n s . I f t h i s i s t r u e i t means t h a t t h e s t r e n g t h o f t h e composites can s t i l l be i n c r e a s e d f i v e f o l d o r s i x f o l d by i n t r o d u c i n g a b e t t e r p r o c e s s i n g r o u t e t h a t y i e l d s more homogeneous ceramics. I n one o r two cases t h e f r a c t u r e o r i g i n c o u l d indeed be i d e n t i f i e d as a l a r g e pore.

I n g e n e r a l t h e s t r e n g t h f o l l o w s t h e f r a c t u r e toughness behaviour q u i t e c l o s e - l y . There a r e two apparent exceptions. F i r s t l y , i n f i g . 2a a t 700°C t h e value f o r Sf i s much lower t h a n can be expected from KI,. I n t h i s m a t e r i a l on average somewhat l a r g e r pores appeared t o be present. Secondly, i n f i g . 2c a somewhat steeper decrease i n Sf t h a n i n K,I i s shown. The i n c r e a s i n g c r y s t a l l i s a t i o n o f t h e s i l i c a (and t h e MAP) i s p r o b a b l y r e s p o n s i b l e f o r t h i s .

I t i s i n t e r e s t i n g t o compare t h e present r e s u l t s w i t h t h e r e c e n t f i n d i n g s o f Green (1) on s i n t e r e d g l a s s microspheres. He o b t a i n e d m a t e r i a l s i n t h e r e i a t i v e d e n s i t y range o f 0.08 t o 0.25 and examined t h e E, K l C and S f behaviour a3 func- t i o n s o f d e n s i t y . Non-linear behaviour o f E as w e l l as K I ~ and Sf was observed: a r a t h e r r a p i d l y r i s i n g v a l u e o f t h e p r o p e r t y i n q u e s t i o n w i t h d e n s i t y a t t h e lower d e n s i t i e s and a much slower i n c r e a s e a t t h e h i g h e r d e n s i t i e s . T h i s behaviour was a l s o t h e o r e t i c a l l y d e s c r i b e d (2). The curves f o r h i g h e r d e n s i t i e s c o u l d be i n t e r - p r e t e d i n terms o f e x i s t i n g micromechanical models, b u t f o r t h e lower d e n s i t i e s a new t h e o r y was developed which f i t t e d t h e K,I behaviour w e l l and t h e E behaviour s a t i s f a c t o r i l y (2). F o r foam g l a s s w i t h r e l a t i v e d e n s i t i e s o f about 0.08 t h e values o f t h e measured p r o p e r t i e s can be found on t h e curve e x t r a p o l a t e d from t h e h i g h d e n s i t y p a r t o f t h e g l a s s composites curve. Other l i g h t - w e i g h t ceramics such as t h e space s h u t t l e t i l e m a t e r i a l have l o w e r values o f f r a c t u r e toughness and s t r e n g t h a t t h e same d e n s i t y b u t much h i g h e r values t h a n t h e g l a s s composites. The values f o r our phosphate-bonded s i l i c a spheres a r e s i m i l a r t o t h e v a l u e s f o r t h e s t r o n g d i r e c - t i o n . o f these ( a n i s o t r o p i c ) t i l e s b u t w i t h t h e advantage o f i s o t r o p y . Since we ob- t a i n e d o n l y a l i m i t e d d e n s i t y range, a c r i t i c a l comparison o f t h e e x p e r i m e n t a l r e s u l t s f o r o u r composites w i t h t h e t h e o r e t i c a l p r e d i c t i o n s i s r a t h e r d i f f i c u l t .

I n f i g . 3 v a r i o u s shapes t h a t a r e f a b r i c a t e d from t h i s ceramic a r e depicted.

The l a r g e p l a t e s (diameter 16 cm, t h i c k n e s s e i t h e r 1.0 cm o r 2.7 cm) were made i n o r d e r t o be a b l e t o c u t t e s t specimens f o r mechanical measurements. The r i b b e d p l a t e s were made e i t h e r b y p r e s s i n g a g r a n u l a t e d mass d i r e c t l y i n a mould (weight t y p i c a l l y 10 grams) o r by normal l a t h e machining (weight t y p i c a l l y 13 grams). T y p i - c a l machining c o n d i t i o n s were: c u t t e r diameter 6 mm, speed 350 mm/min, 5000 r e v o l u - tions/min, feed 1 mm. By b o t h techniques q u i t e d e t a i l e d s t r u c t u r e s w i t h h e i g h t t o w i d t h r a t i o s r a n g i n g up t o 5 were produced e a s i l y ,

The r i b b e d p l a t e s were i n t e n d e d t o be loudspeaker membranes. For t h e pressed p l a t e s a f l a t frequency response curve w i t h no c l e a r l y d e t e c t a b l e resonances was o b t a i n e d w h i l e f o r t h e machined p l a t e s a s t r o n g second harmonic was p r e s e n t b u t a t t h e r a t h e r h i g h frequency o f 3.1 kHz. The l a t t e r f a c t i s due t o t h e presence o f t h e r i b s which i n c r e a s e t h e resonance frequency by a f a c t o r o f 1.9 as compared w i t h a f l a t p l a t e w i t h t h e same weight. I n b o t h cases t h e b a s i c resonance was a t about 470 t o 490 Hz b u t t h i s resonance i s completely suppressed by t h e f i x a t i o n o f t h e p l a t e t o t h e d r i v e r c o i l . Another p o s s i b l e a p p l i c a t i o n o f t h e m a t e r i a l i s as flame-

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r e t a r d a n t c o r e m a t e r i a l i n sandwich c o n s t r u c t i o n s i n which t h i s c o r e a l s o c o n t r i - b u t e s t o t h e s t r e n g t h and p o s s i b l y t o n o i s e damping. Fig. I b shows t h e m i c r o s t r u c - t u r e o f t h e m a t e r i a l a f t e r 3 h o u r s a t 1000°C. Although o b v i o u s l y degraded, t h e c h a r a c t e r i s t i c s o f t h e m i c r o s t r u c t u r e a r e still p r e s e n t . Moreover, no c h l o r i n e o r f l u o r i n e is produced d u r i n g t h i s p r o c e s s . Various o t h e r a p p l i c a t i o n s o f t h e mate- r i a l c a n be envisaged.

V

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CONCLUSIONS

Phosphate-bonded hollow s i l i c a microsphere composites can be produced with a low d e n s i t y (180 kg/m3) and r e l a t i v e l y high v a l u e s o f Young's modulus (0.6 GPa), s t r e n g t h (1 MPa) and f r a c t u r e t o u g h n e s s (30 kpa.ml/*). S u b s t a n t i a l l y h i g h e r v a l u e s , i n p a r t i c u l a r f o r t h e s t r e n g t h , c a n be o b t a i n e d a t somewhat h i g h e r d e n s i - t i e s . A r a t h e r low v a l u e o f t h e r m a l c o n d u c t i v i t y o f about 0.1 W/m.K is found. To- g e t h e r with t h e r a t t i e r high compressive ( c r u s h i n g ) s t r e n g t h o f about 1 5 MPa, a s r e p o r t e d e a r l i e r ( 3 ) , and t h e good m a c h i n a b i l i t y , a s demonstrated i n t h i s paper, t h e s e f a c t o r s make t h e m a t e r i a l a promising l i g h t w e i g h t c o n s t r u c t i o n m a t e r i a l . VI

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ACKNOWLEDGEMENTS

The a u t h o r s would l i k e t o thank W. Dekker and P. E l b e r s e ( U t r e c h t S t a t e Uni- v e r s i t y , t h e N e t h e r l a n d s ) f o r t h e MIP measurements and J.E.D. P a r r e n and A.J.

Zwartjens f o r t h e i r c a r e f u l t e c h n i c a l a s s i s t a n c e . REFERENCES

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/2/ Green D.J. and Hoogland, R.G., J. Am. Ceram. Soc., 6 8 (1985) 395.

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/4/ Brown W.F. and Srawley, J.E., ASTM-STP-410 (ASTM, P h i l a d e l p h i a , 1966).

/5/ Podobeda L.P. and Tkacheva, I .I., I n o r g a n i c M a t e r i a l s 16 (1980) 1661.

/6/ Tien C.L. and Cunnington, G.R., Cryogenics 16 (1976) 583.

/7/ Bansal, G.K., J. Am. Ceram. Soc. $9 (1976)