NASA Technical Memorandum 100177

Flaw Imaging and Ultrasonic Techniques for Characterizing Sintered Silicon Carbide

(NASA-TR-700177) FLAW I P I A G I N G AND N88-12106 ULTRASONIC TECHNIQUES FOB CHARACTERIZING SINTERED S I L I C O N CARBIDE [NASA) 21 p Avail: NTIS HC A03/HF A07 CSCL 14D Uncla s

G3/38 010650'1

George Y. Baaklini Cleveland State University .

Cleveland, Ohio

and

Phillip B. Abel Lewis Research Center Cleveland, Ohio

Prepared for the Conference on Nondestructive Testing of High-Performance Ceramics cosponsored by the American Ceramic Society and the American Society for Nondestructive Testing Boston, Massachusetts, August 25-27, 1987

https://ntrs.nasa.gov/search.jsp?R=19880002724 2018-06-29T05:12:58+00:00Z

FLAN IMAGING AND ULTRASONIC TECHNIQUES FOR CHARACTERIZING SINTERED SILICON CARBIDE

George Y. Baaklini Cleveland State University Cleveland, Ohio 44115

and

Phillip B. Abel National Aeronautics and Space Administration Lewi s Research Center 441 35

ABSTRACT

This investigation assessed the capabilities o f projection microfocus x-radiography, ultrasonic velocity and attenuation, and reflection scanning acoustic microscopy for characterizing silicon carbide specimens. Silicon carbide batches covered a range of densities and different microstructural characteristics. Room-temperature, four-point flexural strength tests were conducted. Fractography was used to identify types, sizes, and locations o f fracture origins. Fracture toughness values were calculated from frac- ture strength and flaw characterization data. Detection capabilities of radiography and acoustic microscopy for fracture-causing flaws were evalu- ated. Applicability of ultrasonics for verifying material strength and toughness was examined.

INTRODUCTION

Fracture mechanics plays an important role in the development of and the failure prediction for ceramic materials, but its application to structural reliability is limited in general by cost-prohibitive proof testing and by related shortcomings in duplicating a complex state of stresses (ref. 1). Hence, there exists the need for nondestructive testing which ensures that materials are free of detrimental defects, and which helps in ranking mate- rials according to their predicted strength and toughness. It is therefore of prime interest to critically examine available nondestructive techniques for the evaluation of ceramic materials.

Substantial advances in nondestructive characterization and defect detec- tion have been made through numerous flaw imaging techniques, including nuclear magnetic resonance (ref. 2 > , microfocus radiography (refs. 3 and 4>, scanning laser acoustic microscopy (ref. 5 > , scanning acoustic

Batch S i n t e r i n g Hot i s o s t a t i c p r e s s i n g number

Temper- Time, Argon Temper- Time, Argon a t u r e , h r pres- a t u r e , h r pres-

"C su re , "C su re , MPa MPa

1 2200 0.5 0.1 ---- --- --- 2200 1.5 2100 0 . 5 138

4 2300 1 .o --- 2HP

1 .o 138 1 .o 138

4HP 2300 1 .o 5HP 2100 .75

---

microscopy (ref. 6>, and scanning photoacoustic microscopy (refs. 7 and 8 ) . Most of these techniques were applied to materials with artificially seeded defects that simulate naturally occurring, fracture-causing flaws, either to assess flaw detectability or to establish the reliability of detection. This paper will examine the application of scanning acoustic microscopy and microfocus radiography to detection of naturally occurring, fracture-causing flaws in silicon carbide (SIC) ceramics. This will entail establishing or verifying the resolution of these systems needed for ceramic materials, and assessing whether further investigation of these techniques is merited.

D e n s i $ y * a g/cm

3.12 3.14 3.05 3.10 2.92

Flexural strength and fracture toughness dependency on ceramic microstruc- ture is well developed and documented in the literature (refs. 9 to 11). Recent studies (refs. 12 to 14) have shown that attenuation depends on the microstructure of Sic. Experimental correlations between ultrasonic attenu- ation and fracture toughness were established in metallic materials (ref. 15). A survey of the literature does not reveal any attempt to relate ultrasonic attenuation with fracture mechanics parameters in Sic ceramic systems. This paper will use destructive testing to determine four-point flexural strength and will use corresponding fractography analy- sis to calculate fracture toughness from the applied stress and from the size, shape, and location of fracture-causing flaws. This paper will assess, for the carbide systems under consideration, the influence of micro- structure on attenuation, flexural strength, and fracture toughness, and will investigate the attenuation toughnessjstrength relationship.

EXPERIMENTAL PROCEDURE

Specimen Preparation

Twenty-five modulus of rupture (MOR) bars were prepared from -100-mesh Cr-SiC powder containing boron and carbonaceous resin binders. Compaction of the green specimen included dry pressing the powder by using a double- action, tungsten-lined die, vacuum sealing the green bars in thin-wall latex tubing, and cold isopressing the bars at 420 MPa. These bars repre- sent five different batches (table 1) which were sintered or sintered and hot isostatically pressed in order to tailor their density and microstruc- ture. All bars were machined, the four long edges beveled, and further polished to a 0.07-pm rms surface finish. Nominal test bar dimensions were 2.72 by 5.58 by 31.71 mm.

TABLE I. - S I N T E R I N G AND HOT I S O S T A T I C PRESSING CONDITIONS

2

M i c r o s t r u c t u r a l C h a r a c t e r i z a t i o n

Mean Dore s i z e , shape, and o r i e n t a t i o n were de te rm ined from photom

Mean po re s i ze ,a9b pm

C i r c l ee EII ipse’.++

Diameter Ma jo r M ino r

1.60 1.61 1.59

1.63 1.75 1.50

3.82 4.00 3.64 3.44 3.60 3.27 2.29 2.38 2.19

c r o -

G r a i n shape

Equiaxed and e l ongated

Equiaxed and e l ongated

E longa ted E longa ted Equiaxed

g raphs o f p o l i s h e d r e p r e s e n t a t i v e samples from each b a t c h by a p p l y i n g two-d imens iona l F o u r i e r t r a n s f o r m t h e o r y ( r e f . 1 6 ) . Mean g r a i n s i z e was d e t e r m i n e d from pho tomic rog raphs o f p o l i s h e d and e t c h e d r e p r e s e n t a t i v e sam- p l e s o f each b a t c h by u s i n g an i n t e r a c t i v e image a n a l y s i s system, where g r a i n b o u n d a r i e s c o u l d be t r a c e d a t a d i g i t i z e r t a b l e t . Tab le I1 l i s t s mean p o r e s i z e s , mean g r a i n s i z e s , and t h e i r c o r r e s p o n d i n g shapes for a l l f i v e ba tches ( r e f . 1 3 ) .

TABLE 11. - CHARACTERIZATION OF MICROSTRUCTURE

Ba tch Densi j jy , number g/cm

1 3.12

2HP 3.14

Mean g r a i n s i z e , a pm

C i r c l ec

Diameter Major M ino r

5.76 7.94 4.56

6.75 9.61 5.15

EI 1 i psed

4 4HP 5HP

3.05 11.56 19.39 7.73 3.10 11.18 18.08 7.82 2.92 3.36 4.40 2.78

a+0.2 p. b i o p r e f e r r e d o r i e n t a t i o n . CAssumi n g a1 1 g r a i ns a r e equi axed. dAssumi ng a1 1 g r a i ns a r e e l ongated. eCal c u l a t e d average f rom fReal measurements o f f t h e mean shape.

f .

R a d i o g r a p h i c E v a l u a t i o n

A l l specimens were f i l m r a d i o g r a p h e d i n o r d e r t o d e t e c t p o t e n t i a l f r a c t u r e - c a u s i n g f l a w s . mode ( 5 X m a g n i f i c a t i o n ) and i n t h e 30 t o 60 kV range w i t h a beam c u r r e n t range o f 0.25 t o 0.32 mA. The system had a molybdenum anode and a 10-pm f o c a l s p o t . A l l r a d i o g r a p h s were manua l l y developed, and were examined w i t h t h e a i d o f a 7 X o p t i c a l measur ing m a g n i f i e r under v a r i a b l e - i n t e n s i t y b a c k l i g h t i n g (1000 t o 9000 l m / m 2 > i n subdued room l i g h t i n g . r a d i o g r a p h e d i n two modes, t h e (W,L> mode, where x - rays were t r a n s m i t t e d t h r o u g h t h e t h i c k n e s s , and t h e (T ,L>, mode where x- rays were t r a n s m i t t e d t h r o u g h t h e w i d t h , wh ich when combined form a th ree -d imens iona l r a d i o - g r a p h i c l o c a t i o n o f f l a w s .

Scann ing A c o u s t i c M ic roscopy E v a l u a t i o n

The m i c r o f o c u s s y s t e m used was o p e r a t e d i n t h e p r o j e c t i o n

T e s t b a r s were

The scann ing a c o u s t i c m ic roscope used was o f t h e p u l s e - r e f l e c t i o n t y p e , o p e r a t i n g w i t h a reduced-aper tu re l e n s a t a nominal 50-MHz c e n t e r f r e - quency. b a t h b o t h genera ted and d e t e c t e d u l t r a s o n i c p u l s e s . t h e sample i n a r a s t e r p a t t e r n as t h e r e f l e c t e d s i g n a l a m p l i t u d e was d i g i t - a l l y s t o r e d as a f u n c t i o n o f p o s i t i o n . p u l s e g e n e r a t i o n and s i g n a l d i g i t i z a t i o n a l l o w e d imag ing o f e i t h e r s u r f a c e or s u b s u r f a c e f l a w s wh ich c o u l d cause f r a c t u r e . A schemat ic d iag ram o f t h e lens-spec imen c o n f i g u r a t i o n i s shown i n f i g u r e 1 .

An a c o u s t i c l e n s p o s i t i o n e d above t h e sample i n a f l u i d c o u p l i n g S tepper motors d rove

An a d j u s t a b l e d e l a y t i m e between

U l t r a s o n i c E v a l u a t i o n

V e l o c i t y and a t t e n u a t on measurements were de te rm ined f o r a l l samples t h r o u g h t h e t h i c k n e s s a t t h r e e d i f f e r e n t l o c a t i o n s i n t h e h i g h e s t s t r e s s e d a r e a o f t h e t e s t spec men, as i n d i c a t e d i n f i g u r e 2 . The pu lse-echo tech - n i q u e , w i t h a 100-MHz broadband l o n g i t u d i n a l - w a v e t r a n s d u c e r , was used t o measure t h e c r o s s c o r r e l a t i o n v e l o c i t y ( r e f . 17) and t h e a t t e n u a t i o n c o e f f i - c i e n t ( r e f . 18) from t h e f i r s t and second back s u r f a c e r e f l e c t i o n s . The f r o n t s u r f a c e r e f l e c t i o n s o b t a i n e d w i t h and w i t h o u t t h e b a r i n p l a c e were used t o c a l c u l a t e t h e r e f l e c t i o n c o e f f i c i e n t o f t h e b u f f e r r o d - c o u p l a n t - sample i n t e r f a c e . T h i s f requency-dependent r e f l e c t i o n c o e f f i c i e n t was i n c o r p o r a t e d f o r p r e c i s i o n a t t e n u a t i o n measurements ( r e f s . 13 and 18) .

T e s t i n g and F r a c t o g r a p h y

F o u r - p o i n t f l e x u r a l s t r e n g t h t e s t s w e r e conducted a t a c rosshead speed o f 0 .51 mm/min, w i t h i n n e r and o u t e r spans o f 9.53 and 19.05 mm, respec - t i v e l y . Tes ts were conducted i n a i r a t room tempera tu re . F r a c t u r e su r - f a c e s were examined i n a scann ing e l e c t r o n mic roscope (SEM) to d e t e r m i n e t h e l o c a t i o n , s i z e , and t y p e o f f a i l u r e - i n i t i a t i n g f l a w s . F r a c t u r e s t r e n g t h s a t t h e t e n s i l e s u r f a c e ( u t ) were c a l c u l a t e d by u s i n g

ut = (MC>/I ( 1 )

where I i s t h e moment o f i n e r t i a , C i s t h e d i s t a n c e from t h e n e u t r a l a x i s t o t h e s u r f a c e o f t h e sample, and M i s t h e a p p l i e d f o u r - p o i n t bend- i n g moment, F r a c t u r e s t r e n q t h s a t t h e f r a c t u r e o r i g i n s (o f> were computed b y u s i n g e q u a t i o n ( 1 ) w i t h C as t h e d i s t a n c e from t h e n e u t r a l a x i s t o t h e c e n t e r of t h e f r a c t u r e o r i g i n . F r a c t u r e toughness (KIc) v a l u e s were o b t a i n e d from t h e r e l a t i o n s h i p between f r a c t u r e s t r e n g t h and f l a w shape, s i z e , and l o c a t i o n ( r e f s . 19 t o 2 3 ) . From r e f e r e n c e 21, KIC v a l u e s were c a l c u l a t e d by u s i n g t h e f o l l o w i n g r e l a t i o n

- - 2 %

F O f - Y ( 2 )

where Of i s t h e f r a c t u r e s t r e s s a t t h e o r i g i n , a i s t h e f l a w d e p t h for a s u r f a c e f l a w and h a l f t h e d i a m e t e r f o r a s p h e r i c a l s u b s u r f a c e f l a w ( e q u a l t o t h e m i n o r a x i s f o r an e l l i p s o i d ) , Z i s t h e f l a w shape pa ramete r , and Y i s a g e o m e t r i c a l pa ramete r equa l t o 2 . 0 and 1.77 ( r e f . 19) f o r s u r f a c e and s u b s u r f a c e f r a c t u r e o r i g i n s , r e s p e c t i v e l y , wh ich a r e much s m a l l e r t h a n t h e specimen d imens ion . The f l a w shape pa ramete r Z i s t h e p r o d u c t o f two independen t , d i m e n s i o n l e s s pa ramete rs , Ze and Zd ( r e f s . 20 and 2 1 ) . The parameter v a r i e s w i t h t h e f l a w shape i n t h e f r a c t u r e p l a n e and can be o b t a i n e d from f i g u r e 2 i n r e f e r e n c e 20. The pa ramete r zd v a r i e s w i t h t h e r a t i o of t h e l e n g t h o f t h e r a d i a l c r a c k e x t e n d i n g from t h e f l a w p e r i p h e r y i n t h e f r a c t u r e p l a n e t o t h e d i a m e t e r o f t h e p o r e or i n c l u s i o n ( r e f . 2 1 ) .

Ze

4

RESULTS

FLAW- D 3

7 SPECIMEN

F r a c t o g r a p h y

Batch number

1

2HP

4

4HP

5HP

aSee ske tch .

Out o f 25 specimens t e s t e d , 17 f r a c t u r e o r i g i n s were i d e n t i f i a b l e . The t y p e , shape, s i z e , and l o c a t i o n o f f r a c t u r e o r i g i n s a r e t a b u l a t e d i n t a b l e 111. The 17 f r a c t u r e o r i g i n s i d e n t i f i e d were 5 b u l k v o i d s , 2 s u r f a c e v o i d s , 1 i r o n - r i c h i n c l u s i o n , 1 l a r g e g r a i n , 3 b u l k agg lomera tes , and 5 s u r - f a c e agg lomera tes . SEM m ic rog raphs of t y p i c a l f r a c t u r e o r i g i n s a r e p r e - sen ted i n f i g u r e 3.

Specimen TY Pe Shape D,a Aa o r 2A, 2C, number Pm Pm Clm

3 U n i d e n t i f i e d ___--_-________ ___ --_ --- 5 Agglomerate C i r c u l a r 6 45 45 8 Void I r r e g u l a r 14 69 69

12 Agglomerate E l l i p t i c a l 60 40 70 13 Void E l 1 i p t i c a l 40 80 104

2 Void Semi-el 1 i p t i c a l 0 19 67 5 Agglomerate E l l i p t i c a l 12 65 127 8 Vo id E l 1 i p t i c a l 120 58 92

11 U n i d e n t i f j e d ___--__________ ___ --_ -_- 23 Agglomerate Semi -e l 1 i p t i c a l 0 46 92

4 Void E l l i p t i c a l 50 69 127 14 Void Semi-el 1 i p t i c a l 0 58 1 1 7 15 Uni d e n t i f i e d -__--__-----___ _ _ _ ___ -_- 16 U n i d e n t i f i e d ---- _------___- ___ ___ ___ 18 Large G r a i n T rapezo ida l 7 34 58

1 U n i d e n t i f i e d ___---_________ ___ --- --- 5 Uni d e n t i fi ed _______________ _ _ _ _-_ __- 8 Fe i n c l u s i o n I r r e g u l a r 50 140 140 9 Uni d e n t i fi ed ___-------_--_- -__ --- ---

24 U n i d e n t i f i e d _______________ ___ _-- --- 35 Agglomerate S e m i - e l l i p t i c a l 0 50 150 36 Agglomerate I r r e g u l a r 0 35 55 37 Agglomerate Semi c i r c u l a r 0 30 200 46 Void E l l i p t i c a l 120 100 120 59 Agglomerate C i r c u l a r 0 60 60

TABLE 111. - ASSESSMENT OF FRACTURE O R I G I N S

D e t e c t i o n of F r a c t u r e O r i g i n s

A c o u s t i c m ic roscopy . - The r e d u c e d - a p e r t u r e f o c u s i n g l e n s gave a maximum p o i n t - t o - p o i n t r e s o l u t i o n a t t h e sample s u r f a c e on t h e o r d e r o f 100 pm. Large s u r f a c e - b r e a k i n g v o i d s j u s t be low t h e l e n s r e s o l u t i o n c o u l d be d e t e c t e d , though n o t a c o u s t i c a l l y r e s o l v e d .

I n t h e sample volume o f i n t e r e s t , w i t h i n a f e w hundred m ic romete rs o f t h e sample t e n s i l e s u r f a c e , a s t r o n g su r face r e f l e c t i o n masked any s u b s u r f a c e d e f e c t s i g n a l p r e s e n t . The t a i l of t h e s u r f a c e r e f l e c t i o n s i g n a l i n t e r - f e r e d up t o a p p r o x i m a t e l y 90 ns a f t e r t h e s t a r t o f t h e i n p u t p u l s e . On t h e b a s i s o f t h e sound v e l o c i t y i n t h e s e b a r s , d e f e c t s w i t h i n abou t 500 pm from t h e scanned ( t e n s i l e ) s u r f a c e were masked.

Scanning from t h e compress ion s i d e , w i t h t h e b u l k sample volume o f i n t e r e s t between t h e a c o u s t i c p u l s e sou rce and t h e sample t e n s i l e s u r f a c e , e l i m i - n a t e s t h e s u r f a c e mask ing and p l a c e s t h e volume under e v a l u a t i o n away from t h e scanned s u r f a c e . A d d i t i o n a l m a t e r i a l may be i n t r o d u c e d between t h e a c o u s t i c l e n s and t h e sample volume o f i n t e r e s t , however, depend ing upon t h e sample t h i c k n e s s . I n h o m o g e n e i t i e s i n t h e sample i n t e r i o r can mask f e a t u r e s or c o m p l i c a t e t h e i n t e r p r e t a t i o n of images from deep w i t h i n t h e specimen. F i g u r e 4 shows a c o u s t i c m ic rog raphs of Sic specimens where s u r f a c e and sub- s u r f a c e f l a w s a r e imaged. Subsur face f l a w s as deep as 2 mm i n t o t h e s p e c i - men were d e t e c t e d . For dep ths g r e a t e r t h a n 2 mm, s i d e w a l l i n t e r f e r e n c e and m a t e r i a l v a r i a t i o n s became more pronounced ( f i g . 4 ( d ) > . The S ic b a r s used i n t h i s s t u d y had an average t h i c k n e s s of 2 .72 mm. Hence, t h e s e b a r s p roved too t h i c k f o r r e l i a b l e c r i t i c a l d e f e c t d e t e c t i o n i n t h e immedia te p r o x i m i t y o f t h e s u r f a c e s .

M i c r o f o c u s r a d i o g r a p h y . - R a d i o g r a p h i c d e t e c t a b i l i t y r e s u l t s o f f r a c t u r e o r i g i n s a r e t a b u l a t e d i n t a b l e IV. D i m e n s i o n s o f t h e f r a c t u r e - c a u s i n g f l a w s , as w e l l as what t h e y r e p r e s e n t i n te rms o f d i m e n s i o n a l s e n s i t i v i t y i n t h e w i d t h and t h e t h i c k n e s s d i r e c t i o n s , a r e a l s o t a b u l a t e d . Seven o u t o f seventeen i d e n t i f i e d f r a c t u r e o r i g i n s were r a d i o g r a p h i c a l l y d e t e c t e d . A l l f i v e b u l k v o i d s , one o u t o f two s u r f a c e v o i d s , and t h e i r o n - r i c h i n c l u - s i o n were d e t e c t e d . A l l su r face and subsur face agg lomera tes , and t h e l a r g e g r a i n were missed. F i g u r e 5 shows a b l a c k and w h i t e p r i n t o f t h e r a d i o - g r a p h i c f i l m imag ing t h e f r a c t u r e o r i g i n ( b u l k v o i d ) i n one o f t h e s i n t e r e d b a r s . I n g e n e r a l t h e f l a w s i z e as i t appeared on t h e r a d i o g r a p h was l e s s t h a n or equa l t o i t s r e a l s i z e as d e t e r m i n e d o p t i c a l l y ( t a b l e I V ) .

F l e x u r a l S t r e n g t h and F r a c t u r e Toughness

F l e x u r a l t e s t r e s u l t s ( a t ) a r e t a b u l a t e d i n t a b l e V. Because f r a c t u r e o r i - g i n s can g e n e r a l l y be d e t e r m i n e d 60 p e r c e n t o f t h e t i m e ( r e f . 4 ) , ut used i n s t e a d o f Of i n t h e a n a l y s i s o f ce ramic s t r e n g t h . There i s no rea - son t o b e l i e v e , on t h e b a s i s o f an f - t e s t a t t h e 95 -pe rcen t c o n f i d e n c e l e v e l ( r e f . 24 ) , t h a t t h e f i v e ba tches d i f f e r i n s t r e n g t h v a r i a b i l i t y . Fur - t h e r , t h e r e i s o n l y r e a s o n t o b e l i e v e t h a t t h e mean s t r e n g t h f o r b a t c h l exceeds t h a t o f b a t c h 4HP; o t h e r w i s e n o d i f f e r e n c e s i n mean s t r e n g t h s o f a l l o t h e r ba tches a r e n o t e d . T h i s i s s u b s t a n t i a t e d b y a t - t e s t a t t h e 95-percent c o n f i d e n c e l e v e l ( r e f . 2 4 ) .

i s

F r a c t u r e toughness r e s u l t s a r e t a b u l a t e d i n t a b l e V. I n t h i s paper r a d i a l c r a c k s were comparable t o p o r e / a g g l o m e r a t e d i a m e t e r , hence Zd was t a k e n t o be equa l t o 1.0. F u r t h e r , no l i g a m e n t f a i l u r e was n o t e d t o o c c u r p r i o r t o c a t a s t r o p h i c f a i l u r e caused by a p o r e / a g g l o m e r a t e l o c a t e d benea th t h e s u r f a c e . Where f r a c t u r e o r i g i n s were l o c a t e d up t o 15 pm be low t h e s u r f a c e , t hese o r i g i n s were t r e a t e d as s u r f a c e f l a w s , because t h e minimum s u r f a c e f l a w s i z e needed f o r l i g a m e n t f a i l u r e t o o c c u r ( r e f . 20) was a p p r o a c h i n g

I 6

TABLE I V . - RADIOGRAPHIC DETECTION OF FRACTURE O R I G I N S

[Specimen d imensions measured a l o n g t h i c k n e s s , T , w i d t h , W , and l e n g t h , L. Flaw d imensions XT, XW, and XL measured a l o n g T , W , and L, r e s p e c t i v e l y . ]

TABLE V. - FLEXURAL STRENGTH AND FRACTURE TOUGHNESS RESULTS

Specimen number

Batch number

F r a c t u r e F r a c t u r e ( a t ) a v g . Y / Z F r a c t u r e s t r e n g t h s t r e n g t h s t a n d a r d toughness,

s u r f a c e , KIC,

MN . m-312

o f t e n s i l e a t o r i g i n , d e v i a t i o n , MPa

a 3; M h 1 3

5 8

12 13

2HP

356 --- 338248 --- --- 389 385 1.27 3.23 269 265 1.27 2.80 382 369 1.42 3.31 326 317 1.31 3.71

~~ ~

4

4H P

3 5 8

12 13

5HP

356 --- 338248 --- --- 389 385 1.27 3.23 269 265 1.27 2.80 382 369 1.42 3.31 326 317 1.31 3.71

5 8

1 1 23

21 1 267 322 305 402 28 1 28 1

---

4 14 15 16 18

1 5 8 9

24

2 I 319 1 319 I 319k52 I 1.60 I 2.23

265 256 304 304 342 366 296 28 1

25 1 334 209 306 27 1

--- ---

--- --- 20 1 --- ---

3.59 1 ;::; 1 3.25

3 1 5k40 1.44 1.27 ---- ---- 1.60

297225 3.19 1 :::: 1 2.40 35

37 288 288 1 1.25 4.10 36 1 %: 1 ::: 46 319 1.26 3.75 59 I 264 I $2: I I 1.26 I 2.57

3.2650.26

2.8820 .65

3.22?0.39

t h e average p o r o s i t y v a l u e f o r t h e s p e c i f i c b a t c h . On t h e b a s i s o f an f- and t - t e s t a t t h e 95 -pe rcen t con f idence l e v e l , a l l f o u r ba tches d i d n o t d f e r i n toughness v a r i a b i l i t y or i n mean toughness .

f-

U1 t r a s o n i c V e l o c i t y and A t t e n u a t i o n

Average v e l o c i t y based on t h r e e measurements a t d i f f e r e n t l o c a t i o n s i n t h e MOR b a r ( f i g . 2) was p l o t t e d as a f u n c t i o n of b u l k d e n s i t y f o r a l l 25 b a r s i n f i g u r e 6. The d a t a show t h a t v e l o c i t y i s an i n c r e a s i n g f u n c t i o n o f d e n s i t y . The average v e l o c i t i e s a r e 1.184, 1 .188, 1.169, 1.189, and 1.099 cm/ps f o r ba tches 1 , 2HP, 4 , 4HP, and 5HP, r e s p e c t i v e l y . By compar- i n g t h e s e r e s u l t s t o c o r r e s p o n d i n g average b u l k d e n s i t i e s , for a 1 - p e r c e n t change i n d e n s i t y t h e r e i s a p p r o x i m a t e l y a 1 -pe rcen t change i n v e l o c i t y .

A t t e n u a t i o n c o e f f i c i e n t r e s u l t s a r e p l o t t e d as a f u n c t i o n o f f r e q u e n c y f o r a l l f i v e ba tches i n f i g u r e 7 . P l o t t e d d a t a a r e t h e measured a t t e n u a t i o n a t or v e r y near t h e l o c a t i o n where f r a c t u r e took p l a c e . Dashed l i n e s r e p r e - s e n t t h e boundary o f t h e s c a t t e r i n t h e f r e q u e n c y reg ime shown. They a l s o r e p r e s e n t t h e e x a c t d a t a for two o u t of t h e f i v e specimens wh ich compr i sed each b a t c h . A t t e n u a t i o n c o e f f i c i e n t i n t h e f requency reg ime shown does d i f - f e r e n t i a t e s u b s t a n t i a l l y between ba tches , on t h e b a s i s o f t h e i r d e n s i t y and m i c r o s t r u c t u r a l c h a r a c t e r i s t i c s , as was demons t ra ted i n p r e v i o u s work

a

( r e f s . 12 t o 1 4 ) . F u r t h e r , f i g u r e 7 shows for e v e r y b a t c h t h e average f l e x - u r a l s t r e n g t h and t h e average f r a c t u r e toughness f o r t h e pu rpose o f compar i - son w i t h c o r r e s p o n d i n g a t t e n u a t i o n measurements.

DISCUSSION

Radiography and A c o u s t i c M i c r o s c o p y

The r a d i o g r a p h i c s e n s i t i v i t y t o t h e v o i d s d e t e c t e d ranged from 1 .2 t o 5.3 p e r c e n t . B u t t h r e e of t h e s e v o i d s were n o t d e t e c t e d t h r o u g h w i d t h w i t h s e n s i t i v i t i e s o f 1.7, 1 .9, and 2 . 3 p e r c e n t . I n t h e case o f t h e 2 .3 p e r c e n t s e n s i t i v i t y , f i l m d e v e l o p i n g was t h e reason. I n t h e o t h e r two cases , i t i s b e l i e v e d t h a t m i s s i n g t h e s e v o i d s was due t o m a t e r i a l v a r i a t i o n s i n t h e x - ray p a t h and to t h e v o i d morpho logy i t s e l f , where a v o i d appears l i k e an agg lomera te , as shown i n f i g u r e 3 ( c > f o r t h e 1 . 9 p e r c e n t case. r e s u l t s agree w i t h p r e v i o u s work, where t h e s e n s i t i v i t y o f t h e m i c r o f o c u s x - ray system was e s t a b l i s h e d b y u s i n g seeded s u r f a c e s and i n t e r n a l v o i d s ( r e f . 3 ) . The s u r f a c e v o i d missed by x - rays r e p r e s e n t e d a 0.7- and a 1 .2 -pe rcen t s e n s i t i v i t y i n t h e t h i c k n e s s and w i d t h d i r e c t i o n s , respec - t i v e l y . b i l i t y o f t h e x - ray system ( r e f . 3 ) .

These

These r a d i o g r a p h i c s e n s i t i v i t i e s a r e beyond t h e r e l i a b l e d e t e c t a -

On t h e b a s i s o f t h e r e s u l t s h e r e i n , we b e l i e v e t h a t i n o r d e r t o image f r a c t u r e - c a u s i n g f l a w s a t or near t h e s u r f a c e of MOR ce ramic b a r s , a f u l l - a p e r t u r e l e n s and h i g h e r f r e q u e n c i e s a r e needed. l e n s , t h e r a y s i n c i d e n t on t h e t e n s i l e s u r f a c e a t t h e R a y l e i g h a n g l e s h o u l d p roduce s u r f a c e a c o u s t i c waves t r a v e l i n g on t h e s u r f a c e o f t h e specimen, l e a k i n g back i n t o t h e w a t e r , and b e i n g c o l l e c t e d b y t h e l e n s ( r e f . 25 ) . Vo ids and c r a c k s normal t o t h e specimen su r face l o c a t e d w i t h i n one R a y l e i g h wave leng th o f t h e s u r f a c e w i l l s c a t t e r t hese R a y l e i g h waves and a f f e c t t h e m ic roscope s i g n a l ( r e f s . 25 and 2 6 ) . O p e r a t i o n a t h i g h e r f r e q u e n c i e s (100 t o 200 MHz) would improve t h e r e s o l u t i o n . I n t h e case of s u b s u r f a c e f l aws l o c a t e d away from t h e R a y l e i g h wave p a t h , a s h o r t e r i n p u t p u l s e i s needed t o reduce t h e i n t e r f e r e n c e between t h e image p u l s e and t h e s u r f a c e p u l se.

W i t h a w i d e - a p e r t u r e

Rad iography p r o v e d u s e f u l i n d e t e c t i n g h i g h - d e n s i t y i n c l u s i o n s and i s o l a t e d v o i d s as f r a c t u r e o r i g i n s i n Sic MOR b a r s , b u t failed t o detect surface and s u b s u r f a c e l o w - d e n s i t y agg lomera tes and l a r g e g r a i n s as f r a c t u r e - c a u s i n g d e f e c t s . T h i s was expec ted , because l o w - d e n s i t y agg lomera tes a r e v o i d l i k e r e g i o n s f i l l e d w i t h m a t e r i a l wh ich reduces d r a s t i c a l l y t h e d i f f e r e n t i a l i n x - r a y a b s o r p t i o n between t h e agg lomera te and t h e m a t r i x . La rge g r a i n s d o n o t p r e s e n t enough d i f f e r e n c e i n d e n s i t y from t h e m a t r i x i n o r d e r t o be de- t e c t e d . A c o u s t i c m ic roscopy , u s i n g a w ide-ang le a c o u s t i c l e n s , has po ten - t i a l f o r d e t e c t i n g l a r g e g r a i n s and l o w - d e n s i t y agg lomera tes l o c a t e d a t or near t h e s u r f a c e ( r e f . 2 5 ) . T h i s d e f i n i t e l y m e r i t s f u r t h e r i n v e s t i g a t i o n .

The Ro le o f U l t r a s o n i c V e l o c i t y and A t t e n u a t i o n

F o r t h e c a r b i d e system i n v e s t i g a t e d h e r e i n , a more d i r e c t c o r r e l a t i o n was f o u n d between v e l o c i t y and d e n s i t y ( f i g . 6) t h a n between f l e x u r a l s t r e n g t h / f r a c t u r e toughness and d e n s i t y ( t a b l e s I and V ) . b a t c h 1 (3 .12 g/cm3) t o b a t c h 5HP (2 .92 g/cm3>, where average f r a c t u r e

For example, compar ing

9

s t r e s s and f r a c t u r e toughness a r e s t a t i s t i c a l l y t h e same fo r b o t h , v e l o c i - t i e s were d i f f e r e n t (1 .184 and 1.099 c m / p s , r e s p e c t i v e l y ) . V e l o c i t y was n o t s e n s i t i v e t o changes i n mean g r a i n and p o r e s i z e ; t h a t i s , no subs tan- t i a l d i f f e r e n c e i n v e l o c i t y was d e t e c t e d between b a t c h 4HP (1 .189 c m / p s ) and b a t c h 1 (1 .184 c m / p s ) , a l t h o u g h b a t c h 4HP had o v e r t w i c e t h e mean g r a i n and p o r e s i z e o f b a t c h 1 .

A t t e n u a t i o n a t or near t h e l o c a t i o n where t h e f r a c t u r e o c c u r r e d agreed we1 w i t h p r e v i o u s a t t e n u a t i o n measurements from p r e v i o u s work ( r e f . 1 3 ) . Fu r - t h e r , a t t e n u a t i o n dependency on n e t average p o r o s i t y can be seen by compar i n g b a t c h 4 and b a t c h 4HP a t t e n u a t i o n d a t a , where a 1 .6 -pe rcen t d i f f e r e n c e i n d e n s i t y ( a l s o a 1 -pe rcen t d i f f e r e n c e i n v e l o c i t y ) r e f l e c t s a 56 -pe rcen t d i f f e r e n c e i n a t t e n u a t i o n a t a f r e q u e n c y of 100 MHz. The a t t e n u a t i o n d a t a f o r ba tches S H P , 4, and 1 demons t ra te t h a t t h e dominant a t t e n u a t i o n mechan- i s m s a r e average p o r e s i z e and n e t average p o r o s i t y . There i s no c l e a r e v i - dence o f a g r a i n boundary s c a t t e r i n g mechanism i n any of t h e f i v e ba tches .

For ba tches 1 and 4HP, v e l o c i t i e s and a t t e n u a t i o n c o e f f i c i e n t s d i d n o t d i f - f e r g r e a t l y , whereas t h e average s t r e n g t h for b a t c h 1 exceeded t h a t o f b a t c h 4HP, o n t h e b a s i s of t - t e s t d a t a a t t h e 95 -pe rcen t c o n f i d e n c e l e v e l . T h i s d i f f e r e n c e i s p r o b a b l y due t o d i f f e r e n t f l a w p o p u l a t i o n s t h a t e x i s t e d i n t h e s e two b a t c h e s . F r o m r a d i o g r a p h y , 4HP had many h i g h - d e n s i t y i n c l u - s i o n s , w h i l e b a t c h 1 had m a i n l y v o i d and agg lomera te t y p e s of f l a w s .

F l e x u r a l s t r e s s and f r a c t u r e toughness d a t a demonst ra ted g r e a t s c a t t e r w i t h - i n b a t c h e s , wh ich made i t d i f f i c u l t t o d i s c e r n common m i c r o s t r u c t u r a l f a c - tors t h a t a f f e c t e d b o t h a t t e n u a t i o n and f r a c t u r e mechanics pa ramete rs . A l t h o u g h p o r o s i t y and g r a i n s i z e dependence of s t r e n g t h and toughness a r e expec ted , t h e p rob lem s t i l l r ema ins t h a t one dominant f l a w can mask t h e e f f e c t o f b u l k p o r o s i t y and m i c r o s t r u c t u r e on s t r e n g t h and toughness . t h e r , u n c e r t a i n t i e s a s s o c i a t e d w i t h t h e shape and g e o m e t r i c f a c t o r s ( r e f . 1 ) used i n t h e toughness c a l c u l a t i o n s m i g h t have masked t h e presence o f t h i s dependency. H e r e i n , a t t e n u a t i o n - t o u g h n e s s b e h a v i o r i s l i m i t e d by sho r t comings i n KIC d e t e r m i n a t i o n , and a d e f i n i t i v e s ta temen t canno t y e t be made. A d d i t i o n a l r e s e a r c h i s m e r i t e d for f u l l a p p r e c i a t i o n of t h e a t t e n - u a t i o n measurements i n d i f f e r e n t f r e q u e n c y reg imes, where d i f f e r e n t s c a t t e r - e r s may be t h e common dominant a t t e n u a t i o n and f r a c t u r e mechanisms.

Fur -

CONCLUSIONS

P r o j e c t i o n m i c r o f o c u s x - rad iog raphy , u l t r a s o n i c v e l o c i t y and a t t e n u a t i o n , and r e f l e c t i o n scann ing a c o u s t i c m ic roscopy were assessed f o r c h a r a c t e r i z - i n g s i l i c o n c a r b i d e specimens. Rad iography p roved u s e f u l i n d e t e c t i n g h i g h - d e n s i t y i n c l u s i o n s and i s o l a t e d v o i d s , b u t f a i l e d i n d e t e c t i n g s u r f a c e and subsur face agg lomera tes and l a r g e g r a i n s as f r a c t u r e o r i g i n s . U l t r a s o n i c v e l o c i t y dependency on d e n s i t y was e v i d e n t . d e n s i t y and mean p o r e s i z e was c l e a r l y demonst ra ted . t i o n as a f u n c t i o n o f toughness was l i m i t e d by sho r t comings i n KIC d e t e r - m i n a t i o n . f o c u s i n g l e n s t o d e t e c t n e a r - s u r f a c e f l a w s was l i m i t e d , i n d i c a t i n g t h e need f o r a f u l l - a p e r t u r e l e n s f o r n e a r - s u r f a c e f a i l u r e - c a u s i n g f l a w s .

A t t e n u a t i o n dependency on U n d e r s t a n d i n g a t t e n u a -

The a p p l i c a b i l i t y o f a c o u s t i c m ic roscopy u s i n g a reduced-aper tu re

I 10

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11

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14. E . R . Generaz io , D.J. Roth, and G . Y . B a a k l i n i , " Imag ing S u b t l e M i c r o - s t r u c t u r a l V a r i a t i o n s i n Ceramics W i t h P r e c i s i o n U l t r a s o n i c V e l o c i t y and A t t e n u a t i o n Measurements," NASA TM-100129 (1987) .

15. A . Vary , " C o r r e l a t i o n s Among U l t r a s o n i c P r o p a g a t i o n F a c t o r s and Frac- t u r e Toughness P r o p e r t i e s o f M e t a l l i c M a t e r i a l s , " M a t e r . E v a l . , 36 171 55-64 (1978) .

16. E . R . Generaz io , " D e t e r m i n a t i o n o f G r a i n S i z e D i s t r i b u t i o n F u n c t i o n U s i n g Two-Dimensional F o u r i e r T rans fo rms o f Tone P u l s e Encoded Images," NASA TM-88790 ( 1 986 ) .

17. D .R . H u l l , H . E . Kau tz , and A . Vary , "Measurement o f U l t r a s o n i c V e l o c i t y U s i n g Phase-Slope and C r o s s - C o r r e l a t i o n Methods," M a t e r . E v a l . , 43 1111 1455-1 460 ( 1 985) .

18. E . R . Generaza io , "The R o l e o f t h e R e f l e c t i o n C o e f f i c i e n t i n P r e c i s i o n Measurement o f Ultrasonic Attenuation," Mater. Eval., 43 181 995-1004 ( 1 985 ) .

19. W.F. Brown, J r . , and J .E. S raw ley , P lane S t r a i n Crack Toughness T e s t i n g o f H i g h S t r e n g t h M e t a l l i c M a t e r i a l s . ASTM-STP-410, ASTM, P h i l a d e l p h i a , PA, 1967, pp . 1-15.

20. A.G. Evans, and G . Tap in , " E f f e c t s o f M i c r o s t r u c t u r e o n t h e S t r e s s t o Propaga te I n h e r e n t F laws," P r o c . B r . Cerarn. S O C . , No. 20, 275-297 (1972) .

21. G.K. Bansa l , and W.H. Duckwor th , " F r a c t u r e S t r e s s as R e l a t e d t o F law and F r a c t u r e Mirror S izes , " J. Am. Ceram. SOC., 6 0 [7-81 304-310 (1977) .

26. I . R . Smi th , R . A . Harvey , and D.J. F a t h e r s , "An A c o u s t i c M ic roscope f o r I n d u s t r i a l A p p l i c a t i o n s , " I E E E T rans . Son ics U l t r a s o n i c s , 32 [ 2 1 274-288 (1985) .

12

,- TRANSDUCER

LENS ROD I /

1

I I

2 .72 MM

7- L

THICKNESS. T

P/2 P/2 -9.53 nn - I I

I

WATER LENS COUPLING

I

1 I

LENGTH. L -4

.SCANNED SURFACE (COMPRESSION OR TENSILE SURFACE)

.,-SAMPLE VOLUME OF INTEREST

FIGURE 1. - SCHEMATIC DIAGRAM OF LENS-SPECIMEN CONFIGURATION FOR SCANNING ACOUSTIC MICROSCOPY. FOCAL PLANE REFLECTION.

RAY PATHS FOR SURFACE REFLECTION AND ABERRATION SHOWN I N ADDITION TO

I--- 19.05 MPI -_I P/2 P/2

FIGURE 2 . - PULSE-ECHO ULTRASONICS OF MOR BARS. NOTED BY T. AND P IS APPLIED LOAD.

1, I r B A C K SURFACE (BS)

L A O N T .I SURFACE (FS)

QUARTZ /

BUFFER ROD' 7

L F S REFLECTION WITHOUT BAR

LOCATION OF TRANSDUCER FOR ULTRASONIC MEASUREMENTS IS DE-

13

ORIGINAL PAGE IS OF POOR QUALITY

FIGURE 3. - SCANNING ELECTRON MICROGRAPHS OF FRACTURE ORIGINS I N SINTERED SIC MOR BARS. (A) BULK VOID, (B ) SURFACE AGGLOMERATE, (C) BULK VOID (AGGLOMERATE-LIKE). AND (D) LARGE GRAIN.

14

ORIGMAL PAGE IS OF POOR QUALITY

FIGURE 3. - CONCLUMD.

15

FIGURE 4. - ACOUSTIC MICROGRAPHS OF SIC SPECIMENS (NUMBERS 15 AND 4 OF BATCH 4 ) AT DIFFERENT FOCUS DEPTHS. ( A ) NUNBER 15. SURFACE: (6) NUMBER 15. 6 5 0 ptl; (C) NUMBER 15. 1200 p ~ ; AND (D) NUMBER 4. 2500 U M .

ORJGINAL PAGE IS OE POOR QUALITY

16

ORIGINAL PAGE Is OF POOR QUALITY

FIGURE 4. - CONCLUDED.

17

FIGURE 5. - MICROFOCUS RADIOGRAPH ( W , L ) OF SINTERED SIC BAR (NUMBER 4 OF BATCH 4 ) . ARROW POINTS TO DETECTED FRACTURE

ORIGIN (BULK VOIDY.

1.20 Y)

1.15 I

V 0 2 W 2-

u 2

l.1°

Q t- 1 3

1 .OS

-

000 - BATCH VELOCITY,

NUMBER CM/~S

1.184

1.169

0 5HP 1.099

0 0

50 0

I

5.25 r

BATCH DENSITY, MEAN PORE FLEXURAL NUMBER c/cn3 DIAMETER, STRENGTH.

1 3.12 1 .60 338f48 2HP 3.14 1.63 319k52 4 3.05 3.82 315f40

273k51 4HP 3.10 3.44 5HP 2.92 2.38 297k25

I1M MPA

r $ 4.50 L

g 3.75 w 3.00

s 2.25

2 1.50

LL U W

L

5 p .75 s z

0 60 80 100 120 140 160

FRACTURE TOUGHNESS.

fiN . ,-3/2 3.26f0.37 2.88f0.65 3.22k0.39 _ _ _ _ _ _ _ _ _

3.20t0.73

r 2 L

c z w u, LL U W 0 V

L

t-

z W t-

0 5

s

.50

.75

. 00

-25

.50

.75

0 60 80 100 120 140 160

FREQUENCY, MHz

FIGURE 7. - COMPARISON OF ATTENUATION, FLEXURAL STRENGTH, AND FRACTURE TOUGHNESS.

OFJGINAL PAGE IS OF POOR QUALITY

18

NASA Nat~anal Aeronauttcs and Report Documentation Page 1. Report No.

NASA TM-100177 2. Government Accession No.

7. Author@)

George Y. B a a k l i n i and P h i l l i p B. Abel

17. Key Words (Suggested by Author(@) Nondestruct ive t e s t i ng/eval u a t i on; U l t rason ics ; Radiography; Ve loc i t y ; At tenuat ion; Ceramics; Fractography; Frac ture toughness; F lexu ra l s t rength ;

9. Performing Organization Name and Address

Nat iona l Aeronautics and Space Admin is t ra t ion Lewis Research Center Cleveland, Ohio 44135-3191

18. Distribution Statement

Unc lass i f ied - Un l im i ted Subject Category 38

2. Sponsoring Agency Name and Address

Nat ional Aeronautics and Space Admin is t ra t ion Washington, D.C. 20546-0001

9. Security Classif. (of this report) 20. Security Classif. (of this page)

3. Recipient's Catalog No.

5. Report Date

21. No of pages 22. Price'

6. Performing Organization Code

Unc lass i f i ed

8. Performing Organization Report No.

E-3753

Unc lass i f i ed 19 A02

10. Work Unit No.

533-05-1 1

11. Contract or Grant No.

13. Type of Report and Period Covered

Tec hn i ca 1 Memorandum 14. Sponsoring Agency Code

5. Supplementary Notes

Prepared f o r t h e Conference on Nondestruct ive Test ing o f High-Performance Ceramics, cosponsored by t h e American Ceramic Society and t h e American Soc ie ty f o r Nondestruct ive Test ing, Boston, Massachusetts, August 25-27, 1987. George Y. Baak l i n i , Cleveland Sta te Un ive rs i t y , Cleveland, Ohio 44115; P h i l l i p B. Abel, NASA Lewis Research Center.

Th is i n v e s t i g a t i o n assessed the c a p a b i l i t i e s o f p r o j e c t i o n microfocus x-radiography, u l t r a s o n i c v e l o c i t y and a t tenuat ion , and r e f l e c t i o n scanning acous t ic microscopy f o r cha rac te r i z ing s i l i c o n carb ide specimens. S i l i c o n carb ide batches covered a range o f dens i t i es and d i f f e r e n t m i c r o s t r u c t u r a l c h a r a c t e r i s t i c s . Room-temperature, four -po in t f l e x u r a l s t reng th t e s t s were conducted. Fractography was used t o i d e n t i f y t ypes , s i z e s , and l o c a t i o n s o f f r a c t u r e o r i g i n s . Frac ture toughness values were ca l cu la ted from f r a c t u r e s t reng th and f l a w cha rac te r i za t i on data. Detect ion c a p a b i l i t i e s o f radiography and acous t ic microscopy f o r f racture-causing f laws were evaluated. A p p l i c a b i l - i t y of u l t r a s o n i c s f o r v e r i f y i n g ma te r ia l s t rength and toughness was examined.

6. Abstract