Characterisation of Surface Breaking Cracks in Alumina Using Scanning Acoustic Microscopy

Abstract

Images o f c r a c k s i n p o l y c r y s t a l l i n e a luminas were s t u d i e d by a c o u s t i c microscopy a t 750 MHz i n spec imens w i t h v a r i o u s s u r f a c e f i n i s h e s . The advan tages of a c o u s t i c microscopy o v e r o p t i c a l microscopy are demonst ra ted f o r d e t e c t i n g c r a c k s , and f o r d e t e r m i n i n g t h e c r a c k p a t h r e l a t i v e t o t h e m i c r o s t r u c t u r e . I . I n t r o d u c t i o n The i n c r e a s i n g u s e o f ce ramics i n l o a d b e a r i n g a p p l i c a t i o n s h a s c r e a t e d a concomi tan t r equ i r emen t f o r methods o f d e t e c t i n g s u r f a c e c r a c k s which are a major s o u r c e of mechanica l weakness. Such d e f e c t s are u s u a l l y d e t e c t a b l e by o p t i c a l microscopy b u t t h e s u r f a c e f i n i s h r e q u i r e d for examinat ion i s far s u p e r i o r t o t h a t needed f o r t h e component i n i ts s e r v i c e envi ronment . Fur thermore , w i th p a r t l y t r a n s l u c e n t ceramics such a s a lumina , o p t i c a l imaging o f s u r f a c e d e f e c t s i s d i f f i c u l t and i t is o f t e n n e c e s s a r y t o r e s o r t t o c o a t i n g t h e s u r f a c e o r t o s c a n n i n g e l e c t r o n microscopy i n o r d e r t o r e s o l v e c r a c k s and d i f f e r e n t i a t e between them and m i c r o s t r u c t u r a l f e a t u r e s . C l e a r l y , a l t e r n a t i v e methods f o r d e t e c t i n g d e f e c t s merit e v a l 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 microscopy ( S A M ) i s a t t r a c t i n g i n c r e a s i n g a t t e n t i o n as a p o t e n t i a l non-des t ruc t ive -eva lua t ion (NDE) t echn ique f o r t h i s a p p l i c a t i o n . The t h e o r y and mechanics o f t h e t e c h n i q u e a r e wel l documented ( 1 , 2 ) . B r i e f l y , when imaging t h e s u r f a c e wi th a scann ing a c o u s t i c microscope a t i g h t l y focussed u l t r a s o n i c beam is raster scanned on t h e s u r f a c e of t h e specimen. Any s u r f a c e ( o r n e a r s u r f a c e ) e las t ic d i s c o n t i n u i t y c a u s e s a modu la t ion of t h e phase and ampl i tude of t h e a c o u s t i c s i g n a l which is r e f l e c t e d back from t h e spec imen. It is t h i s modula t ion which p r o v i d e s t h e c o n t r a s t i n t h e SAM. I n p a r t i c u l a r , the v a r i a t i o n cf t r a n s d u c e r o u t p u t a s a f u n c t i o n of d e f o c u s d i s t a n c e i n t h e a c o u s t i c microscope the (*) Formerly with Department of Electronic & Electrical Engineering, University College London, Torrington Place, London WClE 73E, UK 736 1985 ULTRASONICS SYMPOSIUM V(z) e f f e c t may be used t o enhance the d e t e c t a b i l i t y o f p a r t l y embedded mic roc racks and d i s c o n t i n u i t i e s (3,4). I n t h e p r e s e n t work i t was dec ided t o i n v e s t i g a t e p o t e n t i a l a p p l i c a t i o n s o f SAM f o r examining p o l y c r y s t a l l i n e a luminas s i n c e t h e y are t h e most wide ly used c e r a m i c s i n d u s t r i a l l y , and t h e y pose special problems i n c r a c k d e t e c t i o n .