A! NASA Technical Memorandum 88978 . Nondestructive Evaluation of.
Structural Ceramics. (NASA-TH-88978). LGhDESTEUCTIFE EVALUAT-EOP.
N87- ...
https://ntrs.nasa.gov/search.jsp?R=19870008676 2017-12-13T04:56:30+00:00Z
A!
NASA Technical Memorandum 88978
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Nondestructive Evaluation of Structural Ceramics (NASA-TH-88978) L G h D E S T E U C T I F E EVALUAT-EOP GP STRUCTURAL C E P A M I C S ( N A S A ) i3 p CSCL 14D
N87-18109
G3/38
Stanley J. Klima Lewis Research Center Cleveland, Ohio George Y. Baaklini Cleveland State University Cleveland, Ohio and Phillip B. Abel Lewis Research Center Cleveland, Ohio
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Prepared for the Twenty-fourth Automotive Technology Development Contractors’ Coordination Meeting sponsored by the U.S. Department of Energy Dearborn, Michigan, October 27-30, 1986
Unclas 43530
NONDESTRUCTIVE EVALUATION OF STRUCTURAL CERAMICS
Stanley J . Klima N a t i o n a l Aeronautics and Space A d m i n i s t r a t i o n Lewis Research Center Cleveland, Ohio 44135 George Y. B a a k l i n i Cleveland S t a t e U n i v e r s i t y Cleveland, Ohio 44115 and P h i l l i p 8. Abel N a t l o n a l Aeronautics and Space A d m i n l s t r a t i o n Lewis Research Center Cleveland, Ohio 44135 SUMMARY
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A review i s presented on research and development o f techniques f o r n o n d e s t r u c t i v e e v a l u a t i o n and c h a r a c t e r i z a t i o n o f advanced ceramics f o r heat engine a p p l i c a t i o n s . H i g h l i g h t e d I n t h i s review a r e Lewis Research Center e f f o r t s i n microfocus radiography, scannlng l a s e r a c o u s t i c microscopy (SLAM), scanning a c o u s t i c microscopy (SAM), scanning e l e c t r o n a c o u s t i c microscopy (SEAM), and photoacoustic microscopy (PAM). The techniques were evaluated by a p p l y i n g them t o research samples o f green and s i n t e r e d s i l i c o n n i t r i d e and s i l i c o n c a r b i d e i n t h e form o f modulus-of-rupture bars c o n t a i n i n g seeded v o i d s . P r o b a b i l i t i e s o f d e t e c t i o n of v o i d s were determined f o r diameters as small as 20 f o r microfocus radiography, SLAM, and SAM. Strengths and l i m i t a t i o n s o f t h e techniques f o r ceramic a p p l i c a t i o n s a r e i d e n t l f i e d . The a p p l i c a t i o n o f u l t r a s o n i c s f o r c h a r a c t e r i z i n g ceramic m i c r o s t r u c t u r e s i s a l s o discussed. INTRODUCTION
The need f o r q u a n t i t a t i v e assessment of t h e r e l i a b i l i t y o f f l a w d e t e c t i o n techniques a r i s e s from t h e a p p l i c a t i o n of f r a c t u r e mechanics p r i n c i p l e s t o t h e design o f c r i t i c a l p a r t s . The f r a c t u r e mechanics concept assumes f l a w s a r e present i n a l l m a t e r i a l s and q u a n t i t a t i v e l y descrlbes t h e l r e f f e c t on s t r u c t u r a l i n t e g r i t y . Thus, a c r i t i c a l crack s i z e t h a t would r e s u l t i n u n s t a b l e crack growth can be d e f i n e d f o r any combination of m a t e r i a l and l o a d i n g condit i o n s ( r e f . 1 ) . For simple components such as pressure vessels, f r a c t u r e c o n t r o l can be achieved by p r o o f t e s t i n g t o a pressure which exceeds t h e o p e r a t i n g s t r e s s . I f t h e vessel does n o t f r a c t u r e , the absence o f c r l t i c a l f l a w s i s assured. For complex s t r u c t u r e s however, p r o o f t e s t i n g l o g i c may n o t be a p p l i c a b l e because i n many cases I t i s n o t p o s s i b l e t o d u p l i c a t e p r e c i s e l y t h e n a t u r e o f o p e r a t i n g stresses. Therefore, s t r u c t u r a l i n t e g r i t y o f complex components must be e s t a b l i s h e d by a d i f f e r e n t approach, t h e most p r a c t i c a l o f which may be n o n d e s t r u c t i v e e v a l u a t i o n (NDE). The r o l e o f NDE would be t o screen out p a r t s c o n t a i n i n g flaws equal t o o r g r e a t e r than t h e c r i t i c a l crack s i z e b e f o r e they a r e p u t i n t o s e r v i c e ( r e f . 2 ) . I n some a p p l i c a t i o n s i t may
be necessary t o ensure t h a t f l a w s s i g n i f i c a n t l y s m a l l e r than c r i t i c a l s i z e a r e absent if s u b c r i t i c a l crack growth due t o such t h i n g s as s t r e s s - c o r r o s i o n o r f a t i g u e i s t o be avoided. The use o f f r a c t u r e mechanics concepts i n ceramic component design places a premium on t h e a b i l i t y o f n o n d e s t r u c t i v e i n s p e c t i o n t o d e t e c t small d e f e c t s ( r e f s . 3 and 4 ) , and on t h e need t o determine t h e p r a c t i c a l r e l i a b i l i t y o f an NDE procedure when t h a t procedure i s considered f o r d e t e c t i o n of flaws of a s p e c i f i c type and s i z e ( r e f . 5 ) . Such i n f o r m a t i o n i s e s s e n t i a l If t h e des i g n e r i s t o r e l y on NDE t o assure t h a t components a r e f r e e o f flaws t h a t exceed a s p e c i f i e d s i z e i n t h e m a t e r i a l chosen f o r a p a r t i c u l a r s e r v i c e e n v i ronment. Ifthe design i s such t h a t t h e c r i t i c a l crack s i z e i s g r e a t e r than t h e s m a l l e s t flaw t h a t can be r e l i a b l y detected ( o r t h e l a r g e s t f l a w t h a t w i l l be missed a s i g n i f i c a n t p r o p o r t i o n of t h e t i m e ) then t h e I n s p e c t i o n process can be used. The d i f f e r e n c e between t h e s m a l l e s t d e t e c t a b l e f l a w and t h e c r i t i c a l crack s i z e can be regarded as a measure o f t h e margin o f safety. The r e l i a b i l i t y o f v a r i o u s conventional n o n d e s t r u c t i v e techniques f o r d e t e c t i o n o f cracks i n m e t a l l i c a i r c r a f t engine components was r e p o r t e d i n r e f e r e n c e 6. The o n l y p r e v i o u s l y r e p o r t e d work on NDE d e t e c t i o n r e l i a b i l i t y f o r f l a w s i n heat engine ceramics i s presented i n references 7 t o 10. This paper examines some o f t h e f a c t o r s t h a t i n f l u e n c e t h e d e t e c t a b i l i t y o f minute f l a w s i n ceramics, and focuses a t t e n t i o n on requirements f o r a s s u r i n g adequate d e t e c t i o n s e n s i t i v i t y and r e l i a b i l i t y w i t h r a d i o g r a p h i c and a c o u s t i c microscropy techniques. The paper a l s o discusses t h e a p p l i c a b i l i t y o f thermoa c o u s t i c techniques f o r ceramic a p p l i c a t i o n s and a program f o r c h a r a c t e r i z i n g ceramic m i c r o s t r u c t u r e s through u l t r a s o n i c measurements. RELIABILITY OF FLAW DETECTION Spec imen P r epar a t ion F i g u r e 1 shows a f l o w c h a r t which describes t h e p r e p a r a t i o n o f specimens c o n t a i n i n g seeded voids. The S I C s t a r t i n g powder contained s i n t e r i n g a i d s and b i n d e r m a t e r i a l (boron and carbonaceous r e s i n s ) . The Si3N4 s t a r t i n g powder contained 6 percent Y2O3 and 6 percent Si02 t o promote d e n s i f i c a t i o n d u r i n g s i n t e r i n g . For surface voids, a l l o f t h e powder necessary t o make up t h e specimen thickness was poured i n t o t h e d i e and t h e microspheres were pressed i n t o t h e t o p surface. For i n t e r n a l voids t h e amount of powder p u t i n t h e d i e b e f o r e and a f t e r placement o f t h e microspheres was c o n t r o l l e d t o achieve t h e d e s i r e d v o i d depth i n t h e f i n i s h e d specimen. The d i e pressed bars were vacuum sealed i n l a t e x t u b i n g and c o l d lsopressed t o approximately 60 p e r c e n t o f f u l l d e n s i t y . A f t e r removing t h e l a t e x t h e compacts were heated i n vacuum t o vaporize t h e p l a s t i c microspheres. The a s - s i n t e r e d d e n s i t y of s i l i c o n c a r b i d e ranged f r o m 94 t o 97 percent o f f u l l t h e o r e t i c a l d e n s i t y w h i l e f o r s i l i c o n n i t r i d e i t was more than 98 percent o f t h e o r e t i c a l . A s - f i r e d t e s t bars measured n o m i n a l l y 28 mm long, 7 mm wide, and 2 t o 4 mm t h i c k , as r e q u i r e d f o r NDE r e l i a b i l i t y determinations. A f t e r NDE, i n t e r n a l v o i d s were exposed by diamond g r i n d i n g and measured by o p t i c a l and e l e c t r o n microscopy. These data a r e l i s t e d i n t a b l e I . The dimensions o f voids i n green compacts were assumed t o be e q u i v a l e n t t o t h e seeded spheres. A complete d e s c r i p t i o n o f specimen f a b r i c a t i o n and v o i d c h a r a c t e r i z a t i o n i s g i v e n i n r e f e r e n c e 10.
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NDE Techniques and S t a t i s t i c a l A n a l y s i s
The r e l i a b i l i t y o f mlcrofocus radiography and scanning l a s e r a c o u s t i c microscopy (SLAM) f o r d e t e c t i o n o f seeded voids was evaluated over a wide range o f v o i d diameters and depths. P r o b a b i l i t y o f d e t e c t i o n d a t a was obt a l n e d f o r scanning a c o u s t i c microscopy (SAM) a t a s i n g l e v o i d diameter and depth. B r i e f descriptions of t h e NDE techniques and t h e procedure used t o s t a t i s t i c a l l y analyze f l a w d e t e c t i o n d a t a f o l l o w . D e t a i l e d d e s c r i p t i o n s a r e g i v e n i n references 7 t o 11. Microfocus x-ray. - The x-ray s y s t e m ( f i g . 2) was operated i n t h e 30 t o 60 kV range w i t h a molybdenum anode which produced photon energy l e v e l s between 1 7 and 20 keV. Higher photon energy l e v e l s were found t o r e s u l t i n reduced f i l m c o n t r a s t which i s undesirable. Radiographic f i l m images a t a m a g n i f i c a t i o n o f 2.5 were made by t h e p r o j e c t i o n method and examined w i t h t h e a i d o f a 7X o p t i c a l m a g n i f i e r .
SLAM. - The SLAM technique i s i l l u s t r a t e d I n f l g u r e 3 . 100 HHz u l t r a sonic waves t r a n s m i t t e d through t h e specimen a r e modulated by m a t e r i a l s u r f a c e and i n t e r n a l c h a r a c t e r i s t i c s . The r e l a t i v e i n t e n s i t y and phase o f t h e waves reaching t h e r e f l e c t i v e f i l m on t h e cover s l i p a r e d e t e c t e d by a l a s e r beam r a s t e r scanned over an area approximately 2 rm square. The beam i s r e f l e c t e d i n t o a photodetector and converted t o an e l e c t r o n i c s i g n a l . Thus, an image o f t h e a c o u s t l c p e r t u r b a t i o n s caused by f e a t u r e s such as cracks, voids, d e n s i t y v a r i a t i o n s , etc., a r e d i s p l a y e d i n r e a l t i m e on a video m o n i t o r a t a m a g n i f i c a t i o n o f n e a r l y 1OOX. SAM. - The SAM technique uses a s i n g l e transducer t o generate and r e c e i v e u l t r a s o n i c energy ( f i g . 4 ) . Good s e n s i t i v i t y and r e s o l u t i o n a r e achleved by f o c u s s i n g moderately h i g h frequency u l t r a s o n i c energy ( 3 0 t o 100 MHz) on a small spot, r a s t e r scanning t h e lens w i t h respect t o t h e sample, and time-gate sampling t h e r e f l e c t e d u l t r a s o n i c pulse amplitude. Any f e a t u r e s t h a t produce an a c o u s t i c impedance mismatch w i t h i n t h e sample o r a change i n a c o u s t i c impedance a t t h e specimen s u r f a c e can cause d e t e c t i b l e v a r i a t i o n s i n t h e d i g i t i z e d and s t o r e d s p a t i a l map of r e f l e c t e d s i g n a l amplitude. I f a f l a w i s l a r g e r than t h e f o c a l spot, some degree of f l a w s i z i n g can be achieved. Flaws s m a l l e r than t h e f o c a l spot can be detected b u t i f two o r more a r e l o c a t e d a wavelength o r l e s s a p a r t they cannot be resolved. S t a t i s t i c a l Analysis Due t o u n c e r t a i n t i e s associated w i t h equipment, o p e r a t o r , f l a w c h a r a c t e r i s t i c s , etc., t h e d e t e c t i o n o f flaws i n engineering m a t e r i a l s and components i s p r o b a b i l i s t i c i n n a t u r e and must be evaluated u s i n g a s t a t i s t i c a l approach. Since an attempt a t d e t e c t i o n o f a g i v e n f l a w can have o n l y two p o s s i b l e outcomes ( i t I s e i t h e r detected o r missed) t h e p r o b a b i l i t y o f detect i o n (POD) can be described by t h e binomial d i s t r i b u t i o n . The f l a w s were grouped i n t o s i z e i n t e r v a l s because f a b r i c a t i o n v a r i a b l e s precluded making l a r g e numbers o f voids i n a wide range o f d i s c r e t e s i z e s . The p r o b a b i l i t y o f d e t e c t i o n was c a l c u l a t e d f o r each i n t e r v a l by t h e ' o p t i m i z e d p r o b a b i l i t y 1 I method described i n r e f e r e n c e 5. A F o r t r a n computer program t h a t c a l c u l a t e s and p l o t s NDE r e l i a b i l i t y d a t a i s l i s t e d I n r e f e r e n c e 7 (NASA TM v e r s i o n )
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P r o b a b i l i t y o f D e t e c t i o n Results NDE r e l i a b i l i t y data f o r t h e microfocus x-ray technique 1 s presented i n f i g u r e s 5 and 6 i n t h e f o r m of p l o t s o f p r o b a b i l i t y o f d e t e c t i o n versus v o i d s i z e expressed as percent of t o t a l specimen t h i c k n e s s . As shown i n t a b l e I, t h e i n t e r n a l void diameters ranged from 50 t o 5 2 8 pm i n t h e green specimens and 20 t o 477 pm i n the f u l l y d e n s i f i e d bars.
~ l of lthe POD curves a r e c h a r a c t e r i z e d by a sharp drop i n p r o b a b i l i t y o f d e t e c t i o n i n a r e g i o n below approximately 2 . 5 percent thickness s e n s i t i v i t y . f i g u r e 5(a) shows t h a t t h e d e t e c t i o n s e n s i t i v i t y o f i n t e r n a l voids i n green s i l i c o n n i t r i d e i s n e a r l y t h e same as i n green s i l i c o n carbide, approximately 2 . 5 percent a t a POD o f 90 percent. The curves i n f i g u r e s 5(b) and ( c ) a r e more r e v e a l i n g , showing t h a t surface voids a r e more e a s i l y detected than i n t e r n a l voids f o r b o t h m a t e r i a l s . Analysis o f a l i m i t e d number o f i n t e r n a l voids i n green compacts revealed they were p a r t i a l l y f i l l e d w i t h loose powder, which has t h e e f f e c t o f reducing c o n t r a s t on t h e x-ray f i l m . This c o u l d account f o r the lower apparent s e n s i t i v i t y . I n n a t u r a l l y o c c u r r i n g voids t h i s may be l e s s o f a problem because they a r e n o t u s u a l l y produced by v a p o r i z i n g a s u p p o r t i n g m a t e r i a l such as t h e seeded spheres used i n t h i s i n v e s t i g a t i o n . F o r green m a t e r i a l s , t h e r e f o r e , t h e POD data presented here f o r surface voids would probably apply t o n a t u r a l l y o c c u r r i n g i n t e r n a l voids regardless o f t h e f a b r i c a t i o n technique used t o make t h e specimens o r p a r t s . For s i n t e r e d m a t e r i a l s , t h e data i n f i g u r e 6 show many i n t e r e s t i n g feat u r e s . For example f i g u r e 6(a) i n d i c a t e s u n u s u a l l y good s e n s i t i v i t y t o i n t e r n a l voids i n s i l i c o n n i t r i d e . However, chemical a n a l y s i s o f v o i d w a l l s , reported i n reference 10, proved t h a t d e t e c t i o n o f some i n t e r n a l voids i n s i l i c o n n i t r i d e was i n f l u e n c e d by a d e p o s i t o f y t t r i u m , a s t r o n g absorber o f Thus, some o f t h e voids appeared t o have a h i g h d e n s i t y s h e l l on t h e x-rays. radiograph, r e s u l t i n g i n enhanced d e t e c t a b i l i t y . This c o n d i t i o n c o u l d occur i n any s i n t e r e d m a t e r i a l t h a t u t i l i z e s s i n t e r i n g a i d s composed o f heavy e l e ments. The enhanced d e t e c t a b i l i t y phenomenon was n o t observed w i t h voids seeded i n s i l i c o n c a r b i d e ( f i g . 6 ( b ) ) , which d i d n o t c o n t a i n s i m i l a r s i n t e r i n g aids. NDE r e l i a b i l i t y data f o r SLAM i s presented i n f i g u r e s 7 and 8. F i g u r e 7 which contains data f o r surface connected voids only, shows t h e e f f e c t o f surface c o n d i t i o n and specimen thickness on d e t e c t a b i l i t y o f flaws. From t h e POD curves f o r a s - f i r e d specimens, i t i s e v i d e n t t h a t 0.90 p r o b a b i l i t y o f d e t e c t i o n a t 0.95 confidence l e v e l was n o t a t t a i n e d f o r any t h i c k n e s s up t o 4 mm. However, when t h e same specimens were l i g h t l y p o l i s h e d t o a surface f i n i s h o f nominally 2 pm, t h e POD was s i g n i f i c a n t l y improved. POD o f 0.90 was achieved for a l l specimen thicknesses. Thus, i t appears t h a t a s - f i r e d surfaces cause a background n o i s e l e v e l h i g h e r than can be t o l e r a t e d f o r n o n d e s t r u c t i v e d e t e c t i o n of minute d e f e c t s by SLAM. The measured surface f i n i s h o f a s - f l r e d samples was 8 pm ( p e a k - t o - v a l l e y ) b u t i t should be noted t h a t topography was t h e o n l y surface c h a r a c t e r i s t i c t h a t was measured. I t i s p o s s i b l e t h a t other f a c t o r s such as f i n e near-surface p o r o s i t y , a c t i n g as u l t r a s o n i c wave s c a t t e r e r s , c o u l d have c o n t r i b u t e d t o t h e g e n e r a l l y low POD f o r as-f ired samples
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F i g u r e 8 shows a p l o t which summarizes SLAM POD data obtained f o r i n t e r n a l voids i n specimens w i t h diamond-ground surfaces ( r e f . 9). The boundaries 4
of the bar graphs i n d i c a t e t h e minimum v o i d s i z e s and maximum depths ( f r o m t h e laser-scanned surface) a t which 0.90/0.95 POD/confidence-level was achieved. A s expected, t h e r e i s a s i g n i f i c a n t e f f e c t of v o i d s i z e and depth. However, t h e p l o t a l s o shows t h a t 0 . 9 0 1 0 . 9 5 i s obtained a t g r e a t e r depths and s m a l l e r v o i d s i z e s i n s i l i c o n n i t r i d e than i n s i l i c o n carbide. This i s probably due t o d i f f e r e n c e s i n m i c r o s t r u c t u r e between t h e two m a t e r i a l s . The g r a i n s i z e i n s i l i c o n c a r b i d e was an order of magnltude g r e a t e r than I n s i l l c o n n i t r i d e . The b u l k p o r o s i t y i n S i l l c o n c a r b i d e was a l s o h i g h e r t h a n i n s i l l c o n n i t r i d e . Both of these c o n d i t i o n s would have the e f f e c t o f i n c r e a s i n g u l t r a s o n i c s c a t t e r and reducing t h e POD o f voids I n s i l i c o n c a r b i d e r e l a t i v e t o s i l i c o n nitride. Also shown i n f i g u r e 8 i s a s i n g l e data p o i n t r e p r e s e n t i n g p r e l i m i n a r y r e s u l t s obtained w i t h t h e SAM technique. Voids a t o n l y a s i n g l e diameter ( n o m i n a l l y 20 pm) s i t u a t e d 1 mn f r o m t h e scanned s u r f a c e were a v a i l a b l e a t t h e t i m e o f t h e SAM e v a l u a t i o n . A l l o f 38 voids were detected i n s i l i c o n n i t r i d e samples, y i e l d i n g a POD b e t t e r t h a n 0.9010.95 f o r t h i s very small v o i d s i z e a t a depth o f about 50 times the diameter. The method needs t o be e v a l uated f o r a g r e a t e r range of depths, m a t e r i a l s , and m i c r o s t r u c t u r e s b u t promi s e s t o be an extremely useful technique f o r scanning c r i t i c a l areas o f heat engine components. THERMOACOUSTIC TECHNIQUES
Thermoacoustic techniques can be a p p l i e d t o ceramics i n e i t h e r a noncont a c t i n g mode s i m i l a r t o x-ray methods, o r a c o n t a c t i n g mode as i n u l t r a s o n i c s . The technique e s s e n t i a l l y measures r e l a t i v e d i f f e r e n c e s i n s u r f a c e and nears u r f a c e thermal p r o p e r t i e s of t h e m a t e r i a l being evaluated. The a b s o r p t i o n o f i n t e n s i t y modulated electromagnetic r a d i a t i o n focused a t any p o i n t on t h e sample g l v e s r i s e t o l o c a l i z e d c y c l i c h e a t i n g and c o o l i n g which i n t u r n genera t e s e l a s t i c waves a t t h e modulation frequency. The amplitude and phase o f these waves can be measured a t another p o i n t on t h e specimen surface by a p i e z o e l e c t r i c c r y s t a l i n c o n t a c t w i t h t h e specimen, or i n t h e surrounding medium by a noncontacting method using a s e n s i t i v e microphone o r a l a s e r . An overview o f t h e t h e o r e t i c a l and experimental aspects o f t h e thermoacoustic techniques i s presented i n r e f e r e n c e 12. ___. SEAM. - A schematic diagram of t h e scanning e l e c t r o n a c o u s t i c microscopy technique (SEAM), which u t i l l t e s a scanning e l e c t r o n beam h e a t i n g source and a c o n t a c t p i e z o - e l e c t r i c a c o u s t i c sensor housed i n a scanning e l e c t r o n microscrope enclosure, i s shown i n f i g u r e 9. The thermal d i f f u s i v i t y o f t h e volume o f m a t e r i a l c o n t a i n i n g t h e f l a w d i f f e r s f r o m t h a t of t h e m a t r i x m a t e r i a l . Thus, t h e e l a s t i c waves produced as t h e I n t e n s i t y modulated e l e c t r o n beam passes over t h e f l a w d i f f e r I n amplitude and/or phase from t h e surrounding m a t e r i a l . Modulation frequencies can range from 100 Hz t o 10 kHz, t h e lower f r e q u e n c i e s a c h i e v i n g g r e a t e r p e n e t r a t i o n . A thermoacoustic image o f a raster-scanned area can be produced, an example of which i s shown i n f i g u r e 10. The SEM backscatter image shows a p o r t l o n o f a crack t h a t o r i g i n a t e s a t t h e edge o f a s i l i c o n c a r b i d e modulus o f r u p t u r e bar, as w e l l as dust p a r t i c l e s on t h e specimen surface. The SEAM image c l e a r l y shows t h e subsurface e x t e n s i o n o f t h e crack as w e l l as t h e p o r t i o n seen on t h e SEM image. Note, however, t h a t t h e surface dust d i d not r e g i s t e r on t h e SEAM Image.
5
Preliminary i n v e s t i g a t i o n of t h e SEAM technique i n t h e l a b o r a t o r y u s i n g s i l i c o n carbide and s i l i c o n n i t r i d e samples c o n t a i n i n g d e f e c t s a r t i f i c i a l l y induced a t predetermined l o c a t i o n s r e s u l t e d i n t h e f o l l o w i n g observations: (I) surface features such as p i t s and nodules were detected and imaged; ( 2 ) cracks produced n a t u r a l l y by processing, and t i g h t cracks produced by knoop i n d e n t a t i o n s were imaged; ( 3 ) seeded subsurface voids up t o 100 pm below t h e i n t e r r o g a t i n g surface were imaged, w h i l e voids a t a depth o f 300 pm were missed. R e l i a b i l i t y of d e t e c t i o n data were n o t obtained f o r t h e SEAM t e c h n i que. Calculations show t h a t r e s o l u t i o n l e s s than 10 pm appears t o be a t t a i n a b l e . The scan r a t e was q u i t e f a s t , w i t h a scan t i m e o f 1 2 5 sec f o r a 256 l i n e image of an area 4 by 5 mm. Although t h e SEAM technique has good s e n s i t i v i t y and r e s o l u t i o n c a p a b i l i t y , t h e need t o perform t h e scan i n a vacuum i s a disadvantage. I n a d d i t i o n , s i n c e s t r u c t u r a l ceramics a r e r e l a t i v e l y poor e l e c t r i c a l conductors, a c o a t i n g i s necessary t o a t t a i n t h e s t a t e d s e n s i t i v i t y and r e s o l u t i o n . Therefore, t h e SEAM technique i s b e s t s u i t e d f o r l a b o r a t o r y m a t e r i a l e v a l u a t i o n and does n o t appear promising f o r p r a c t i c a l I n s p e c t i o n o f heat engine components.
PAM. - A block diagram o f a photoacoustic microscopy (PAM) system i s shown i n f i g u r e 11. The sample t o be evaluated i s placed i n s i d e an i s o l a t i o n c e l l c o n t a i n i n g a s e n s i t i v e microphone and a minimal volume o f gas, u s u a l l y a i r . The sample i s i l l u m i n a t e d through a c l e a r window by a chopped l a s e r beam focused onto the sample surface. L i g h t absorbed by t h e sample i s converted i n p a r t i n t o heat, r e s u l t i n g i n p e r i o d i c heat f l o w from t h e sample t o t h e surrounding gas. The r e s u l t a n t pressure f l u c t u a t i o n s produced by a c y c l i c heati n g and c o o l i n g o f t h e specimen and t h e gas i n t h e immediate v i c i n i t y o f t h e specimen a r e t r a n s m i t t e d by t h e b u l k gas medium t o t h e microphone. The magnitude and phase o f t h e a c o u s t i c s i g n a l i s d i r e c t l y r e l a t e d t o t h e thermal p r o p e r t i e s o f t h e sample and t h e surrounding medium, t h e chopping frequency, and t h e c e l l design. The a c o u s t i c frequency c o i n c i d e s w i t h t h e chopping frequency. The depth o f m a t e r i a l t h a t can be evaluated depends on t h e wavel e n g t h o f t h e i n c i d e n t l i g h t , t h e a b s o r p t i o n c o e f f i c i e n t o f t h e sample, and t h e thermal d i f f u s i o n l e n g t h . Generally, t h e depth o f m a t e r i a l t h a t can be evaluated i s o f t h e order o f one o r two thermal d i f f u s i o n lengths. Thus, l i k e t h e SEAM technique PAM i s l i m i t e d t o specimen s u r f a c e and near-surface evaluation. Extensive i n v e s t i g a t i o n o f t h e PAM technique r e s u l t e d i n t h e t h e f o l l o w i n g observations obtained w i t h s i n t e r e d s i l i c o n n i t r i d e samples: ( 1 ) s u r f a c e connected pores and i n c l u s i o n s 25 pm and l a r g e r were detected; ( 2 ) subsurface voids and i n c l u s i o n s 35 pm and l a r g e r were detected up t o 70 pm below t h e i n t e r r o g a t i n g surface, b u t voids 200 pm diameter and 200 pm deep were missed; ( 3 ) background n o i s e l e v e l was high, i n d i c a t i n g extreme s e n s i t i v i t y t o m a t e r i a l v a r i a t i o n s o t h e r than d i s c r e t e flaws; ( 4 ) t h e method was t i m e consuming, r e q u i r i n g over 4 h r t o scan an area 1 cm on a s i d e w i t h a r e s o l u t i o n o f 25 pm. When applied t o s i l i c o n n i t r i d e samples i n t h e green s t a t e t h e technique was s e n s i t i v e t o surface i m p e r f e c t i o n s and t e x t u r e v a r i a t i o n s b u t d i d n o t d e t e c t near surface anomalies, i n c l u d i n g s i d e d r i l l e d holes up t o 250 pm diameter. Background n o i s e l e v e l produced w i t h t h e green samples was very high, making d e t e c t i o n of even r e l a t i v e l y l a r g e f l a w s d i f f i c u l t . O p t i c a l examination of the r e g i o n scanned by t h e l a s e r revealed an a l t e r e d surface a p p a r e n t l y caused by emission of m a t e r i a l from t h e specimen surface. Some o f 6
t h e e m i t t e d m a t e r i a l coated t h e c e l l window, c u t t i n g down on t r a n s m i s s i o n o f l i g h t . Subsequent s i n t e r i n g o f these samples produced a l a r g e number o f surface cracks i n t h e l a s e r scanned regions t h a t d i d n o t occur i n t h e unscanned zones. Because of t h e apparent damage done t o t h e specimen s u r f a c e t h e PAM technique does n o t appear t o be an a t t r a c t i v e a l t e r n a t i v e f o r examina t i o n of green ceramics, I n s p i t e of t h e f a c t t h a t i t can be used i n t h e noncontacting mode. ULTRASONIC MICROSTRUCTURAL CHARACTERIZATION
I t was p r e v i o u s l y shown t h a t 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 can be used t o measure b u l k d e n s i t y v a r i a t i o n s ( r e f s . 13 and 14) and t o assess m a t e r i a l s t r e n g t h t o t h e extent t h a t i t i s c o n t r o l l e d by micros t r u c t u r e . This a p p l i c a t i o n o f u l t r a s o n i c s i s being explored i n g r e a t e r depth a t NASA Lewis as o u t l i n e d by t h e f l o w c h a r t I n f i g u r e 12. S i n t e r e d s i l i c o n c a r b i d e MOR bars w i t h c o n t r o l l e d d e n s i t i e s and m i c r o s t r u c t u r e s have been prepared in-house by v a r y i n g t h e powder g r i n d i n g , pressing, and s i n t e r i n g c o n d i t i o n s , and by h o t i s o s t a t i c pressing. Surface f l n i s h I s another v a r i a b l e t h a t w i l l be studied. A f t e r i n s p e c t i o n by e s t a b l i s h e d NDE methods, t h e bars w i l l be c h a r a c t e r i z e d by s p e c i a l l y developed h i g h frequency u l t r a s o n i c techniques. A v a i l a b l e frequencies range from 20 MHz t o as h i g h as 1 GHz. A f t e r u l t r a s o n i c c h a r a c t e r i z a t i o n t h e specimens w i l l be f r a c t u r e d I n f o u r - p o i n t bending. C o r r e l a t i o n s between m i c r o s t r u c t u r e , b u l k d e n s i t y , s u r f a c e condit l o n , and s t r e n g t h w i l l then be determined. CONCLUDING REMARKS
R e l i a b i l i t y o f v o i d d e t e c t i o n I n ceramics by microfocus radiography was a f f e c t e d by various m a t e r i a l and process r e l a t e d parameters. I t was observed t h a t photon energy l e v e l s l e s s than 20 keV produced b e t t e r r a d i o g r a p h i c cont r a s t and hence b e t t e r v o i d d e t e c t a b i l l t y than h i g h e r energy l e v e l s . Migrat i o n of y t t r i u m t o t h e r e g i o n surrounding v o i d w a l l s enhanced t h e d e t e c t a b i l i t y o f voids i n many, b u t n o t a l l s i n t e r e d s i l i c o n n i t r i d e specimens. The s e n s i t i v i t y o f x-rays t o voids i n green s i l i c o n c a r b i d e and s i l i c o n n i t r i d e compacts was reduced by t h e presence o f l o o s e powder i n s i d e t h e c a v i t y . R e l i a b i l i t y o f v o i d d e t e c t i o n by scanning l a s e r a c o u s t i c microscopy was a f f e c t e d by a d i f f e r e n t s e t o f parameters. The l a r g e r g r a i n s i z e and h i g h e r r e l a t i v e porosity i n sintered s i l i c o n carbide resulted i n s i g n i f i c a n t l y reduced d e t e c t i o n c a p a b i l i t y than was observed f o r s i l i c o n n i t r i d e . Void d e t e c t i o n i n b o t h m a t e r i a l s was g r e a t l y improved when t h e a s - s i n t e r e d s u r f a c e was removed by surface g r i n d i n g o r p o l i s h i n g . Scanning a c o u s t i c microscopy u t i l i z i n g u l t r a s o n i c frequencies between 30 and 100 MHz promises t o be u s e f u l f o r scanning c r i t i c a l areas on ceramic heat engine components. P r e l i m i n a r y data i n d i c a t e t h a t voids 20 pm diameter and 1 mm deep can be r e l i a b l y d e t e c t e d . These voids were s i g n i f i c a n t l y smaller and deeper t h a n t h e s m a l l e s t v o i d s d e t e c t e d w i t h equal r e l l a b i l i t y by SLAM and r a d i o g r a p h i c methods. Thermoacoustic imaging techniques were i n v e s t i g a t e d t o determine a p p l l c a b i l i t y t o simple ceramic t e s t bars. Scanning e l e c t r o n a c o u s t i c microscopy proved t o be u s e f u l f o r d e t e c t i n g t i g h t s u r f a c e cracks and p i t s and near s u r f a c e voids l e s s than 100 pm i n diameter and 100 pm deep w i t h good 1
r e s o l u t i o n . However, t h e need t o conduct t h e examination i n a SEM enclosure and t o c o a t t h e specimen w i t h a c o n d u c t i v e m a t e r i a l l i m i t s t h e SEAM method t o t h e l a b o r a t o r y . The photoacoustic microscopy method was evaluated on b o t h green and s i n t e r e d specimens. The PAM method was capable o f d e t e c t i n g s u r f a c e p i t s down t o 25 pm and near-surface voids down t o 35 pm diameter. AS w i t h t h e SEAM technique, d e t e c t i o n was l i m i t e d t o w i t h i n 100 pm o f t h e surface. The PAM method was n o t u s e f u l f o r green ceramic compacts because l a s e r scanning apparently caused s u r f a c e c r a c k i n g t o occur when t h e specimens were subsequently s i n t e r e d . I n e i t h e r case ( s i n t e r e d o r green m a t e r i a l s ) t h e method i s t i m e consuming, which makes i t u n a t t r a c t i v e f o r scanning l a r g e areas REFERENCES
1 . Kanninen, M.F.; and Popelar, C.H.: U n i v e r s i t y Press, 1985.
Advanced F r a c t u r e Mechanics.
Oxford
F r a c t u r e Toughness and NDT Requirements f o r A i r c r a f t 2 . Packman, P.F.: Design. Nondestr. Test. ( G u i l d f o r d , Engl.), v o l . 6, no. 6, Dec. 1973, pp. 314-324. 3. Bradt, R.C.; Hasselman, D.P.H.; and Lange, F.F.; eds.: o f Ceramics. Plenum Press, v o l s . 1-2, 1974, v o l s . 3-4,
F r a c t u r e Mechanics 1978.
F a i l u r e Causing Defects I n Ceramics; What NDE Should 4. Rice, R.W., e t a l . : Find. NRL-MR-4075, Naval Research Lab., Oct. 1979. ( A v a i l . NTIS, AD-A078234). R e l i a b i l i t y of Flaw D e t e c t i o n by N o n d e s t r u c t i v e 5 . Packman, P.F., e t a l . : Inspection. Metals Handbook, Vol. 11, 8 t h ed. H.E. Boyer, ed., American S o c i e t y f o r Metals, Metals Park, OH, 1976, pp. 414-424. 6. Rummel, W.D., e t a l . : R e l i a b i l i t y o f N o n d e s t r u c t i v e I n s p e c t i o n (NDI) o f A i r c r a f t Engine Components. SAALC/MM-8151, San Antonio Air L o g i s t i c s Command, Jan. 1984. ( A v a i l . NTIS, AD-A15532D). R e l i a b i l i t y o f Void D e t e c t i o n I n S t r u c t u r a l Ceramics 7. Roth, D.J., e t a l . : by use o f Scanning Laser Acoustic Microscopy. Hater. Eval., v o l . 44, no. 6, May 1986, pp. 762-769, 761. 8
K i s e r , J.D.; and Roth, D.J.: Radiographic D e t e c t a b i l i t y B a a k l i n i , G.Y.; L i m i t s f o r Seeded Voids i n S i n t e r e d S i l i c o n Carbide and S i l i c o n N i t r i d e . Adv. Ceram. Mater., v o l . 1, no. 1, Jan. 1986, pp. 43-49.
9
Roth, D.J.; and B a a k l i n i , G.Y.: R e l i a b i l i t y o f Scanning Laser Acoustic Microscopy f o r D e t e c t i n g I n t e r n a l Voids i n S t r u c t u r a l Ceramics. Adv. Ceram. Mater., v o l . 1, no. 3, J u l y , 1986, pp. 252-258.
Probability o f Detection o f I n t e r n a l 10. B a a k l i n i , G.Y.; and Roth, D.J.: Voids i n S t r u c t u r a l Ceramics Using Microfocus Radiography. 3. Mater. Res., v o l . 1, no. 3, May-June 1986, pp. 457-467.
8
11. Nikoonahad, M.: R e f l e c t i o n Acoustic Microscopy f o r I n d u s t r i a l NDE. Research Techniques i n Nondestructive Testing, v o l . 7, Academic Press, 1984, pp. 217-257. 1 2 . Rosencwaig, A.: Photoacoustics and Photoacoustic Spectroscopy. W i l e y a Sons, 1980.
13. Klima, S.J.: NDE of Advanced Ceramics. Apr., 1986, pp. 571 -576. 1 4 . Klima, S.J., e t a l . : S t r u c t u r a l Ceramics.
Mater. Eval.,
John
v o l . 44, no. 5,
U l t r a s o n i c V e l o c i t y f o r E s t i m a t i n g D e n s i t y of NASA TM-82765. 1981.
9
Material
Original sphere diameter,
Number of spheres . seeded
Dimensions a f t e r s i n t e r i n g Axis, pm Mean
S i 3N4
S iC
Standard deviation
80 115 200 321 528
69 39
20 37
4
31
133
28 21
233 307
17 16 14
50 80 115 200 321
50 47 68 19 39
32 59 77 165 297
10
5
3 6 10 29 19
Diameter, pm Mean
Standard deviation
6 5
25 68 139 267 386
18 15
58 100 131 194 307
3 8 8 11 15
8
- 100 MESH a-Sic POWDER
I
-100 MESH Si3N4 POWDER
MICROSPHERES PLACED ON POWDER
I MICROSPHERE
+
I 1
POSITIONS
I PHOTOGRAPHICALLY RECORDED I I
-
MICROSPHERES COVERED WITH WWDER AND PRESSED AT 120 MPa TO FORM GREEN TEST BAR
VACUUM TO BURN OUT MICROS PHERES
[
Si3biq S iNTERED 2140 OC 2hr 5 MPa N2 PRESSURE
Sic SiNIEKtD 2200 OC 0.5 hr 0.1 MPa A r PRESSURE
Figure 1. - Fabrication of silicon nitride and silicon carbide test specimens w i t h seeded internal voids.
11
r FOCAL SPOT SIZE I
t
-1-
SOURCE-TOOBJECT DISTANCE
T
7 - '!
I
I
' AND/OR
FILM DISTANCE
SECONDARY RAD IATION S
-
Figure 2. Schematic configuration of microfocus projection radiography, where D is thickness of defect, T is thickness of sample, p1 i s attenuation coefficient of the object, and v2 is attenuation coefficient of the defect.
-,,,q,k
RECEIVER DYNAMIC RIPPLE
SCANNING LASER
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I
I
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Figure 3. - Scanning laser acoustic microscopy (SLAM) of ceramic test bars containing seeded internal defects.
12
MECHANICAL XY-SCAN SPECIMEN
PIEZOELECTRIC CRYSTAL 7
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- Scanning acoustic microscope (SAM).
I I I I2
INTERNAL VOIDS IN Si3N4 INTERNAL VOIDS IN Sic
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(a) Green Sic and Si3N4-
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SURFACE VOIDS INTERNAL VOIDS
SURFACE VOIDS
1 INTERNAL I VOIDS
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(b) Green Sic.
-
(c) Green Si3Nq.
Figure 5. Lower bound probability-ofdetection of surface and internal voids in green isopressed SIC and Si3N4 bars by microfocus x-ray. Thickness sensitivity in percent equals 100 tvold dimension in x-ray beam direction)l(thickness of specimen in same direction). Probability of detection calculated at 0.95 confidence level.
13
---
--- INTERNAL VOIDS
INTERNAL VOIDS DIAM < 5 0 ~ m INTERNAL VOIDS DIAM > 50 Mm SURFACE VOIDS
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INTERNAL VOIDS IN Si3N4 INTERNAL VOIDS > 50 vm I N SIC
r---
-
a I
2
3
4
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(c) Sintered Sic and Si3N4
(b) Sintered Sic.
(a) Sintered Si3N4.
1
-
Figure 6. Lower bound probability-of-detection of surface and internal voids in sintered Sic and Si3N4 bars by microfocus x-ray. Thickness sensitivity in percent equals 100 (void dimension in x-ray beam'direction)l(thickness of specimen in same direction). Probability of detection calculated at 0.95 confidence level.
1.0
0.90 PROB-
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I
I
40 80 120 160 200 SURFACE VOID DIAMETER, Pm
(b) Specimen thickness, 3 mm.
I
I
0
AS-FIRED
I I
I
40
I
l
I
;
* ,\;,
I I l l
80
I l l
I
120
160 200
(c) Specimen thickness. 4 mm.
Figure 7. -Effect of specimen thickness and surface condition on probability-ofdetection of surface voids by SLAM in sintered silicon nitride. Effect of thickness is evident only for specimens with as-fired Surfaces. Probability of detection calculated at 0.95 confidence level.
14
SINTERED SILICON CARBIDE
VOID DIAMETER, pm
Figure 8 - Effect of void depth, void diameter, Eind iiiiiirix iiiateiial on proDability-of-de:ection of internal voids by scanning laser acoustic microscopy (SLAM) and scanning acoustic microscopy (SAM). Probability of detection was 0.90 at 0.95 confidence level.
15
SEM ENCLOSURE
J.1 SCANNING Cc fC SPECIMEN SURFACE 7\ \
\
ELECTRON BEAM
MODULATED
$4 . . ATlMHz
- 5 MICRON WAVELENGTH
THERMAL
-7
4-
WAVES
WAVELENGTH
l '
PIEZO-ACOUSTIC SENSOR
TO IMAGE INTENSITY MODULATOR
Figure 9. - Schematic representation of a thermo-acoustic device utilizing an electron beam source and a n ultrasonic sensor housed in a scanning electron microscope enclosure.
16
ORIGINAL PAGE IS OF POOR QUALITY
(b) SEAM image.
(a) SEM backscatter image.
-
Figure 10. Images of a spall crack near the edge of a silicon carbide MOR bar obtained by a conventional SEM technique and by scanning electron acoustic microscopy (SEAM). SEAM reveals subsurface cracking not s h w n by SEM. Dust visible in the SEM image was ignored by SEAM.
SAMPLE 1, CHOPPER
,
I DEVICE
PHONE
PHOTODIODE
I I
GAS CELL’
-
Figure 11. Block diagram of a photoacoustic microscopy (PAM) system utilizing a laser heating source and a gas filled specimen isolation cell.
17
I
5 GROUPS OF &iC
MOR EARS WITH: DIFFERENT DENSITIES DIFFERENT MICROSTRUCTURES
SINTEREO-HIPPED 10 BARSlGROUP
10 BARS/GROUP
I
10 BARS/GROUP I
X-RAY SLAM ULTRASONICS: INSTRUMENTATION HIGH-FREOUENCY PROCEDURE DEVELOPMENT VELOCITY ATTENUATION 4-POINT BEND TEST FRACTURE TOUGHNESS
I
-
.FRACTOGRAPHY MICROSTRUCTURE
Figure 12 Flow chart of program to study applicability of NDE for characterizing ceramic microstructure, surface finish, and effects on strength.
3. Recipient's Catalog No.
2. Government Accession No.
1. Report No.
NASA TM-88978 5. Report Date
4. Title and Subtitle
Nondes t ructi ve Eva1 uati on of Structural Ceramics 6. Performing Organization Code
533-05-01 7. Author(s)
I
Stanley J. Klima, George Y . Baaklini, and Phillip 6. Abel
8. Performing Organization Report No.
E-3446
10. Work Unit No.
9.Performing Organization Name and Address
National Aeronautics and Space Admlnistration Lewis Research Center Cleveland, Ohio 44135 12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration Washington, D.C. 20546
11. Contract or Grant No.
: I
Technical Memorandum
14. Sponsoring Agency Code
I
15. Supplementary Notes
Prepared for the Twenty-fourth Automotive Technology Development Contractors' Coordination Meeting, sponsored by the U.S. Department of Energy, Dearborn, Michigan, October 27-30, 1986. Stanley J. Klima and Phillip 6. Abel, NASA Lewis Research Center; George Y . Baaklini, Cleveland State University, Cleveland, Ohio 441 15. 16. Abstract
A review is presented on research and development of techniques for nondestructive evaluation and characterization of advanced ceramics for heat engine appllcations. Highlighted in this review are Lewis Research Center efforts i n microfocus radiography, scanning laser acoustic microscopy (SLAM), scanning acoustic microscopy (SAM), scanning electron acoustic microscopy (SEAM), and photoacoustlc microscopy (PAM). The techniques were evaluated by applying them to research samples of green and sintered silicon nitride and silicon carbide in the form of modulus-of-rupture bars containing seeded voids. Probabilities of detection o f voids were determined for diameters as small as 20 pm for microfocus radiography, SLAM, and SAM. Strengths and limitatlons of the techniques for ceramic applications are identified. Application of ultrasonics for characterizing ceramic microstructures is also discussed.
118. Distribution Statement
17. Key Words (Suggested by Author@))
Wondestruct ive testing; Nondestructive evaluation; Ultrasonics; Acoustic microscopy; Sonic testing; Radiography; Microfocus x-ray; NDE; NDI; NDT; Re1 iabil i ty; Probability of detection; Haterials 19. Security Classif. (of this report)
Unclassified
Unclassified - unlimited STAR Category 38
20. Security Classif. (of this page)
21. No. of pages
Uncl ass i f 1 ed
*For sale by the National Technical Information Service, Springfield, Virginia 221 61
22. Price'
18
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