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Reliability of scanning electron microscopy information

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RESEARCH

COUNCIL OF CANADA

OONSElL NATIONAL DE

RECHERCHES DU CANADA

RELIABILITY OF

SCANNING ELECTRON MICROSCOPY

INFORMATION

by

P. E. Grattan-Bellew, E. G. Quinn and P. J. Sereda

4

Repr~nted from

CEMENT AND CONCRETE RESEARCH

Vol. 8, No. 3, May 1978

pp. 333-342

'7 '3

O ; . L $

DBR Paper No. 768

Division of Building Research

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P r i n t e d i n t h e U n i t e d States.

ANALYZED

RELIABILITY

OF SCANNING ELECTRON MICROSCOPY INFORMATION

P.E. Grattan-Bellew, E.G. Quinn and P.J.

Sereda

Materials Section, Division of Building Research

National Research Council of Canada, Ottawa, Canada

(Communicated by

S.

Diamond)

(Received Feb. 10; i n f i n a l form, March 14, 1978)

ABSTRACT

The reliability of information obtained with the scanning electron

microscope is influenced by human and instrumental factors. The

human factor enters, for example, when a microstructure is described

as being typical of the material when in fact it is only a minor

component. Some instrumental factors include charging of the

specimen, problems differentiating between positive and negative

relief and distortion of the shape of the sample in micrographs.

The determination of the amount of a phase present in a composite is

a complex problem involving both preparation of representative

surfaces and appropriate methods of evaluation. A number of

experimental results are provided. Where smearing of surfaces is not

a problem, point counting evaluation on sawn surfaces is shown to

provide reliable results.

La v6racitS des renseignements obtenus 2 l'aide du microscope

glectronique 2 balayage est influencge par les facteurs humains et

instrumentaux. Le facteur humain entre en jeu, par exemple,

lorsqu'une microstructure est dgcrite c o m e Stant typique du mat6riau

lorsqu'elle n'est en rSalit6 qu'une composante secondaire. Certains

facteurs instrumentaux comprennent le chargement du spgcimen, les

problSmes de diffgrenciation entre le relief nggatif et le positif et

la distorsion de la forme de l'gchantillon dans les microphotographies.

La dgtermination de la quantitg d'une phase prssente dans un composg,

occasionne un problSme complexe de prsparation des surfaces reprgsen-

tatives et de mise au point de mGthodes dlSvaluation

appropri6es. Un

nombre de rSsultats expgrimentaux est fourni. Lorsque les taches en

surfaces ne constituent pas un problsme, 116valuation

en comptant les

points aux surfaces sciSes fournit des rSsultats fiables.

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I n t r o d u c t i o n

Understanding of t h e mechanism of h y d r a t i o n of cement h a s been g r e a t l y ad- vanced by t h e u s e of t h e scanning e l e c t r o n microscope (SEM) d u r i n g t h e p a s t decade. However, a c e r t a i n amount of c o n f u s i o n may have a l s o been g e n e r a t e d by t h e p u b l i c a t i o n of n o n - r e p r e s e n t a t i v e micrographs o r t h e m i s i n t e r p r e t a t i o n of them. I n t h i s paper some problems t h a t may l e a d t o t h e m i s i n t e r p r e t a t i o n of r e s u l t s o b t a i n e d w i t h t h e SEM a r e d i s c u s s e d . The major problems a f f e c t i n g t h e r e l i a b i l i t y of t h e d a t a can be d i v i d e d i n t o human and i n s t r u m e n t a l f a c t o r s .

Human F a c t o r s

A problem commonly a f f e c t i n g t h e v i s u a l i n t e r p r e t a t i o n of SEM d a t a i s t h e s e l e c t i o n of n o n - r e p r e s e n t a t i v e micrographs t h a t a r e s u b s e q u e n t l y d e s c r i b e d a s showing t h e t y p i c a l morphology of t h e sample. Sometimes micrographs a r e s e l e c t - ed f o r i n c l u s i o n i n a paper p r i m a r i l y because t h e y show a well-defined morphol- ogy, b u t t h e a u t h o r has f a i l e d t o n o t e t h a t t h e observed f e a t u r e may have com- p r i s e d o n l y a v e r y s m a l l p e r c e n t a g e of t h e sample. The i n c i d e n c e of t h i s s o r t of problem i s perhaps on t h e wane, a s r e s e a r c h e r s g a i n more e x p e r i e n c e i n scan- n i n g e l e c t r o n microscopy.

Another problem t h a t sometimes a r i s e s i s t h e m i s i n t e r p r e t a t i o n of t h e observed morphology. T h i s problem i s e s p e c i a l l y s e r i o u s when, i n t h e absence of energy- d i s p e r s i v e x-ray f a c i l i t i e s , morphology i s used e x c l u s i v e l y t o i d e n t i f y t h e phases observed. There a r e c a s e s on r e c o r d where a c u b i c phase was d e s c r i b e d b u t examination of t h e micrographs showed no c l e a r l y d e f i n e d c u b i c m a t e r i a l t o be p r e s e n t .

I n s t r u m e n t a l F a c t o r s

I n t h i s s e c t i o n , some examples of r e s u l t s t h a t a r e m i s l e a d i n g due t o i n s t u - m e n t a l f a c t o r s a s s o c i a t e d w i t h t h e c o a t i n g a p p a r a t u s o r t h e SEM a r e d i s c u s s e d . The t h i c k n e s s of t h e c o a t i n g f r e q u e n t l y v a r i e s c o n s i d e r a b l y and s m a l l a r e a s where i t i s v i r t u a l l y a b s e n t may o c c u r . A t h i g h e r m a g n i f i c a t i o n s r e g i o n s t h a t a r e p o o r l y coated g i v e r i s e t o c h a r g i n g a r t i f a c t s t h a t may be m i s - t a k e n f o r f e a t u r e s of t h e sample. However, w i t h t h e c o a t i n g t e c h n i q u e s c u r r e n t l y being employed and i n t h e m a g n i f i c a t i o n range commonly used i n cement r e s e a r c h t h i s i s n o t u s u a l l y a s e r i o u s problem.

Holland (1) h a s r e p o r t e d t h e o c c u r r e n c e of a r t i f a c t s when a s p u t t e r c o a t i n g u n i t was used w i t h gold and palladium. I n c o r r e c t o p e r a t i o n of t h e u n i t may c a u s e . d e p o s i t i o n of f i n e g l o b u l e s of gold which a t h i g h m a g n i f i c a t i o n impart a n a r t i f i c i a l s u r f a c e t e x t u r e .

One of t h e SEM-related f a c t o r s t h a t sometimes c a u s e s d i f f i c u l t y i s t h e p o s s i - b i l i t y of i n c o r r e c t l y d i f f e r e n t i a t i n g between p o s i t i v e and n e g a t i v e r e l i e f , es- p e c i a l l y when examining micrographs. I f t h e micrographs a r e i n v e r t e d by m i s t a k e , c r a c k s appear a s r i d g e s , a s can be s e e n i n Fig. 1. T h i s i s l e s s of a problem i n t h e microscope i t s e l f , e s p e c i a l l y i f a y-modulation u n i t i s a v a i l a b l e t o d i f - f e r e n t i a t e between t h e two.

*

Purchase of a 3-D viewing system s u c h a s t h a t of C h a t f i e l d ( 2 ) , may be war- r a n t e d when c o r r e c t i d e n t i f i c a t i o n of p o s i t i v e and n e g a t i v e r e l i e f of t h r e e - deminsional e f f e c t s a r e i m p o r t a n t . The system produces two images, one r e d and one g r e e n , superimposed on a c o l o r t e l e v i s i o n s c r e e n . The combined image is viewed w i t h a p a i r of r e d and green f i l t e r s t e r o g l a s s e s . A l t e r n a t i v e l y , s t e r e o - s c o p i c p a i r s of micrographs can be made w i t h t h e c o n v e n t i o n a l SEM by t i l t i n g t h e sample between exposures, b u t t h i s method i s somewhat t e d i o u s and i s n o t regu- l a r l y used.

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( a ) FIG. 1 E f f e c t of i n v e r s i o n of a micrograph on topographic f e a t u r e s : ( a ) micrograph i n c o r r e c t o r i e n t a t i o n showing c r a c k s i n t h e s u r f a c e of a f i b e r r e i n f o r c e d p l a s t i c ; (b) same micrograph i n v e r t e d , c r a c k s a p p e a r i n g a s r i d g e s . ( a ) FIG. 2

Micrographs of r e p l i c a s of e t c h p i t s i n i c e viewed from d i f f e r e n t

d i r e c t i o n s : ( a ) a p p a r e n t c u b i c morphology; (b) t r u e hexagonal morphology. A r t i f a c t s a f f e c t i n g t h e a p p a r e n t shape of f e a t u r e s observed i n t h e SEM may be g e n e r a t e d by t h e geometry of t h e i n c i d e n t e l e c t r o n beam and t h e t a k e o f f a n g l e of t h e secondary e l e c t r o n d e t e c t o r r e l a t i v e t o t h e sample. T h i s i s i l l u s t r a t e d i n F i g . 2 , which i s a micrograph of a r e p l i c a of e t c h p i t s i n a s u r f a c e of i c e . I n F i g . 2 ( a ) , t h e e t c h p i t s appear c u b i c i n shape b u t Fig. 2 ( b ) , which was ob- t a i n e d by r o t a t i n g t h e sample, shows t h e t r u e hexagonal morphology. F i g u r e 3

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Grattan-Bellew,

E.G.

Q u i n n ,

P.J.

Sereda

electron

beam

T 7 e lee tron

w

beam

0

electron cof lector

0

electron collector (b) FIG. 3

Model showing how some i c e . c r y s t a l s may appear t o have c u b i c morphology when viewed i n t h e SEM:

( a ) a p p a r e n t c u b i c morphology when f a c e ' c ' i s i n e l e c t r o n shadow and n o t v i s i b l e ;

(b) model viewed from a n o t h e r a n g l e ; f a c e ' c ' i s now i l l u m i n a t e d by e l e c t r o n beam and t r u e hexagonal morphology i s s e e n .

F i g . 3 ( a ) , t h e hexagonal f a c e ' c ' i s i n v i s i b l e because t h e e l e c t r o n beam does n o t s t r i k e i t and no e l e c t r o n s a r e r e f l e c t e d from t h i s f a c e t o t h e c o l l e c t o r . A s a r e s u l t , t h e o b s e r v e r s e e s an a p p a r e n t c u b i c form. I n Fig. 3 ( b ) t h e hexa- gonal f a c e i s t i l t e d towards t h e c o l l e c t o r and t h e o b s e r v e r s e e s t h e t r u e hexa- gonalmorphology of t h e e t c h p i t . Shape d i s t o r t i o n of t h e image may a l s o be caused by t i l t of t h e specimen i n t h e microscope i f t h e image i s n o t c o r r e c t e d e l e c t r o n i c a l l y . T h i s d i s t o r t i o n is e v i d e n t w i t h spheres' t h a t appear e l l i p t i c a l i n o u t l i n e a s s e e n i n Fig. 4.

P e r c e n t a g e E s t i m a t i o n

Two f a c t o r s s t r o n g l y i n f l u e n c e t h e e s t i m a t i o n of t h e p e r c e n t a g e of a compon- e n t , e s p e c i a l l y a minor one, i n a composite m a t e r i a l : ( i ) t h e r e p r e s e n t a t i v e - n e s s of t h e s u r f a c e b e i n g examined, and ( i i ) t h e a c c u r a c y of t h e method used. When a composite m a t e r i a l i s f r a c t u r e d , t h e - f r a c t u r e w i l l p a s s p r e f e r e n t i a l l y

through t h e weaker phase; hence more of t h a t phase w i l l be v i s i b l e on t h e f r a c - t u r e d s u r f a c e t h a n i s p r e s e n t i n t h e m a t e r i a l . It i s t h e r e f o r e e v i d e n t t h a t e s t i m a t e s of t h e p e r c e n t a g e s of m a t e r i a l s p r e s e n t i n composites d e r i v e d from i n f o r m a t i o n o b t a i n e d on f r a c t u r e d s u r f a c e s may n o t always be c o r r e c t , even when an a c c u r a t e method of e s t i m a t i o n i s used. However, when t h e purpose of t h e ex- amination i s t o d e t e r m i n e t h e c a u s e of f a i l u r e of t h e composite, examination of a f r a c t u r e d s u r f a c e i s e s s e n t i a l .

Even when a s u r f a c e i s r e p r e s e n t a t i v e of a sample, t h e d e t e r m i n a t i o n of t h e p e r c e n t a g e of a component i s n o t a n e a s y t a s k . Various s t a t i s t i c a l methods f o r d e t e r m i n i n g volume p e r c e n t a g e s from a r e a l measurements a r e a v a i l a b l e ; u n f o r t u n - a t e l y , i n cement and c o n c r e t e r e s e a r c h , v i s u a l e s t i m a t i o n , a most u n r e l i a b l e method, i s g e n e r a l l y used. F i g . 5 i l l u s t r a t e s t h i s problem. I n a t r i a l c a r r i e d o u t i n t h i s l a b o r a t o r y on "dummy" samples c o n t a i n i n g 5% of a w h i t e phase a s shown i n F i g s . 5a and 5b, e s t i m a t e s of t h e amount thought t o be p r e s e n t v a r i e d

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FIG. 4

Micrograph showing l a t e x s p h e r e s i n p o r t l a n d cement p a s t e ; sample con- t a i n e d 12% l a t e x . Latex s p h e r e s appear e l l i p t i c a l i n o u t l i n e because t i l t c o r r e c t i o n n o t a p p l i e d t o image. from 2% f o r F i g . 5a t o 40% f o r F i g . 5b. I n a second t r i a l c a r r i e d o u t a t t h e an- n u a l T r a n s p o r t a t i o n Research Board Meeting i n Washington, i n J a n u a r y 1988, a group of experienced cement c h e m i s t s and micro- s c o p i s t s e s t i m a t e d more a c c u r a t e l y t h e p e r c e n t a g e s i n t h e two f i g u r e s , s u g g e s t - i n g t h a t accuracy of e s t i m a t i o n improves w i t h e x p e r i e n c e . Q u a n t i t a t i v e Determination of P r o p o r t i o n s P r e s e n t AS e a r l y a s 1848, t h e French p e t r o g r a p h e r D e l e s s e ( 3 ) showed m a t h e m a t i c a l l y t h a t i n a random c r o s s - s e c t i o n of a uniform a g g r e g a t e t h e a r e a occupied by each c o n s t i t u e n t i s p r o p o r t i o n a l t o i t s volume i n t h e sample. T h i s concept l e d t o t h e development of q u a n t i t a t i v e microscopy. D i f f e r e n t methods of d e t e r m i n i n g t h e a r e a l amount and hence volume of a c o n s t i t u e n t on a p l a n e s u r f a c e a r e des- c r i b e d i n v a r i o u s handbooks on p e t r o g r a p h i c p r o c e d u r e s , e . g . , Q u a n t i t a t i v e Microscopy ( 4 ) .

The s i m p l e s t method i s probably t h e p o i n t count. The advantage of t h i s method is t h a t a t each p o i n t t h e o n l y d e c i s i o n t h a t has t o be made i s i f t h e p o i n t i s on o r off t h e s e l e c t e d f e a t u r e . I n t h i s s t u d y , known amounts of d i f - f e r e n t phases i n specimens were determined by t h e p o i n t count method u s i n g t h e Cambridge Mark I I A scanning e l e c t r o n microscope. The procedure was a s f o l l o w s :

(b) FIG. 5

Diagrams i n which p e r c e n t a g e s of w h i t e phase 'in b l a c k m a t r i x a r e t o be e s t i m a t e d v i s u a l l y :

( a ) w h i t e p a r t i c l e s of v a r i a b l e shape and s i z e ; (b) w h i t e p a r t i c l e s of more uniform shape and s i z e .

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Q u i n n , P.J.

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The sample was s t e p scanned i n t h e x d i r e c t i o n by r o t a t i n g t h e x - a x i s c o n t r o l by 180' f o r each s t e p . A f t e r each r o t a t i o n i t was noted i f a p o i n t a t t h e c e n t r e of t h e s c r e e n was on o r o f f t h e d e s i r e d phase. When t h e edge of t h e sample was r e a c h e d , t h e sample was moved one o r more s t e p s a t r i g h t a n g l e s t o t h e t r a v e r s e d i r e c t i o n and t h e procedure r e p e a t e d u n t i l t h e d e s i r e d number of frames had been counted. A p o i n t a t t h e c e n t r e of t h e s c r e e n was used because, when n e c e s s a r y , t h e m a g n i f i c a t i o n could be i n c r e a s e d t o a i d i n i d e n t i f i c a t i o n without moving t h e sample.

I n o r d e r t o e s t i m a t e t h e p e r c e n t a g e of a phase i n t h e sample i t was n e c e s s a r y t o determine t h e number of frames t o be counted, u s i n g t h e f o l l o w i n g e q u a t i o n :

L

where P is t h e number of frames r e q u i r e d t o g i v e t h e d e s i r e d a c c u r a c y , o i s t h e v a r i a n c e and Vv i s t h e e s t i m a t e d volume f r a c t i o n of t h e phase t o be determined. An e s t i m a t e of t h e volume f r a c t i o n Vv was f i r s t made by c o u n t i n g u n t i l about 1 0 p o i n t s f e l l on t h e phase t o be determined and was s u b s t i t u t e d i n Eq. ( 1 ) . P was t h e n c a l c u l a t e d .

Table 1 shows t h e number of frames t h a t have t o be counted, i n h y p o t h e t i c a l samples c o n t a i n i n g v a r y i n g amounts of t h e phase being i n v e s t i g a t e d , i n o r d e r t o o b t a i n an accuracy of 10%. I f a n a c c u r a c y of 5% r a t h e r t h a n 10% i s r e q u i r e d , t h e number of frames counted must be i n c r e a s e d from 1900 t o 7490 i n a sample con-

t a i n i n g 5% of t h e phase under i n v e s t i g a t i o n ; i f , f o r Table 1 t h e same sample, a n a c c u r a c y of 20% i s a c c e p t a b l e ,

t h e number of frames counted can be reduced t o 475. R a t i o of Number of Frames Comparatively few i n v e s t i g a t i o n s i n cement and con- Counted t o P e r c e n t a g e of c r e t e r e s e a r c h w a r r a n t t h e e x p e n d i t u r e of e f f o r t Phase i n Sample, f o r a n n e c e s s a r y t o count manually t h e l a r g e number of Accuracy of 10%. frames n e c e s s a r y t o o b t a i n a r e a s o n a b l e l e v e l of P e r c e n t a g e of Number of a c c u r a c y . I n f a v o r a b l e c a s e s , however, when a pol- Phase in Sam le Frames i s h e d s u r f a c e can be prepared w i t h o u t undue smearing

of t h e phase of i n t e r e s t , a n image a n a l y s e r could be - 5 1900 used t o d e t e r m i n e , a u t o m a t i c a l l y , t h e p e r c e n t a g e of 10 900 t h a t phase. The Quantimet Image Analyser* i s a typ- 1 5 700 i c a l a p p a r a t u s , c o n s i s t i n g of a p l u m b i c o n s c a n n e r , a 2 0 400 T.V. d i s p l a y , a system c o n t r o l module, a 2-D d e t e c - t o r and a computation module. The image on t h e T.V.

-

monitor i s l i n e scanned. There a r e 800 p i c t u r e p o i n t s p e r l i n e and hence t h e

system can be compared t o a v e r y a c c u r a t e p o i n t system. The a n a l y s e r can be pro- grammed t o g i v e g r a i n s i z e d i s t r i b u t i o n , p e r c e n t a g e s of t h e d i f f e r e n t phases t h a t a r e d i s t i n g u i s h a b l e , and a r e a s occupied by t h o s e p h a s e s ; t h e r e s u l t s a r e o b t a i n e d i n a m a t t e r of seconds. An a c c u r a c y of about

+

2% h a s been r e p o r t e d (5). Some t y p e of image a n a l y s i s a p p a r a t u s i s undoubtedly t h e b e s t method f o r o b t a i n i n g q u a n t i t a t i v e d a t a from micrographs. For s a t i s f a c t o r y r e s u l t s , however, i t is e s s e n t i a l t h a t r e p r e s e n t a t i v e samples be used and t h a t d i s t o r t i o n of shape does n o t occur because of i n s t r u m e n t a l f a c t o r s .

An image a n a l y s i s system can b e o b t a i n e d f o r about $18,000 b u t i t s c a p a b i l i - t i e s a r e v e r y l i m i t e d . A u s e f u l system w i l l c o s t between $50,000 and $100,000, a p r i c e t h a t i s d i f f i c u l t t o j u s t i f y u n l e s s t h e machine i s i n almost c o n s t a n t u s e .

A l t e r n a t i v e l y , t h e r e a r e methods by which t h e p e r c e n t a g e of a component can be e s t i m a t e d more e x p e d i t i o u s l y , though w i t h l e s s e r a c c u r a c y , t h a n t h e p o i n t

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count procedure. One of t h e s e , t h e g r i d count method, was t e s t e d i n t h i s l a b o r - a t o r y . Ten micrographs were t a k e n , uniformly d i s t r i b u t e d i n a c r o s s - p a t t e r n on t h e sample. A g r i d 1 x 1 cm i n s i z e w i t h 120 s q u a r e s was l a i d over each micro- graph and t h e a r e a occupied by t h e d e s i r e d phase was e s t i m a t e d . The r e s u l t s were g e n e r a l l y u n s a t i s f a c t o r y b u t were probably more a c c u r a t e t h a n t h o s e t h a t could be o b t a i n e d by v i s u a l e s t i m a t i o n . D e t a i l s of t h e s e experiments w i l l be d e s c r i b e d l a t e r .

Procedures f o r Sample P r e p a r a t i o n and E v a l u a t i o n of Methods of Determining P e r c e n t a g e s of Phases P r e s e n t

P r e p a r a t i o n of Sample S u r f a c e s

The e x p e r i m e n t a l r e s u l t s p r e s e n t e d i n t h i s paper show t h a t f r a c t u r e d s u r f a c e s of some composite m a t e r i a l s can p r o v i d e i n c o r r e c t d a t a when t h e a v e r a g e composit- i o n of t h e sample i s r e q u i r e d . S u r f a c e s prepared by c u t t i n g w i t h a diamond saw e l i m i n a t e t h e problem encountered w i t h some f r a c t u r e d s u r f a c e s , b u t w i t h s o f t composites, smearing a t t h e s u r f a c e may p r e v e n t i n d e n t i f i c a t i o n of phases.

There a r e o t h e r , more s o p h i s t i c a t e d , p o s s i b l e methods t h a t could be a p p l i e d t o t h e problem. P o s s i b i l i t i e s i n c l u d e t h e f r e e - a b r a s i v e w i r e s l i c i n g system s u p p l i e d by Geos Corporation* and t h e w a t e r - j e t c u t t i n g a p p a r a t u s s u p p l i e d by Flow Research Inc.** Another p o s s i b i l i t y would be t o b e g i n w i t h a sawn s u r f a c e and machine a f r e s h s u r f a c e w i t h an i o n m i l l i n g a p p a r a t u s . One problem w i t h t h i s a p p a r a t u s i s t h a t i f t h e sample i s h e a t s e n s i t i v e ( a s i s cement p a s t e ) i t

i s e s s e n t i a l t o u s e a c o l d s t a g e . T h i s adds c o n s i d e r a b l y t o t h e c o s t . I n f a c t t h e main problem w i t h t h e s e a l t e r n a t i v e methods suggested i s c o s t . The i o n m i l - l i n g a p p a r a t u s w i t h a c o l d s t a g e a t t a c h m e n t c o s t s about $20,000; t h e o t h e r equip- ment i s even more expensive. I t i s d i f f i c u l t t o j u s t i f y such a n e x p e n d i t u r e un-

l e s s t h e a p p a r a t u s i s i n almost d a i l y u s e .

R e p r e s e n t a t i v e n e s s of S u r f a c e s and Methods of E v a l u a t i o n

I n t h e q u a n t i t a t i v e a n a l y s i s of a composite c o n t a i n i n g a s m a l l p e r c e n t a g e of a weak phase i n a s t r o n g m a t r i x a s u r f a c e prepared by c u t t i n g , e . g . , w i t h a d i a - mond saw, should n o t be i n f l u e n c e d by t h e d i f f e r e n c e s i n s t r e n g t h between t h e two components a s would a f r a c t u r e s u r f a c e . A number of samples were prepared t o d e m o n s t r a t e t h i s e f f e c t .

I n t h e f i r s t system, l a t e x s p h e r e s about 25 um i n d i a m e t e r were mixed w i t h p o r t l a n d cement, which was t h e n h y d r a t e d . A micrograph of t h e sample is shown i n F i g . 4. The r e s u l t s of t h e p e r c e n t a g e s o b t a i n e d by p o i n t c o u n t i n g on sawn and f r a c t u r e d s u r f a c e s a r e shown i n Table 2.

Table 2

Comparison of R e s u l t s of P o i n t and Grid Counts on Sawn and F r a c t u r e d Sur- f a c e s of Samples Containing 1 2 and 5% L a t e x Spheres i n P o r t l a n d Cement ( p . c . 1

Sample S u r f a c e P r e p a r a t i o n P o i n t Count Grid Count

12% l a t e x i n p.c. f r a c t u r e d 19.7 1 3 . 9 12% l a t e x i n p.c. sawn 11.4 7.9

5% l a t e x i n p.c. f r a c t u r e d 1 0 . 8 5 . 1 5% l a t e x i n p. c . sawn 5.0 3 . 3

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I n Table 2 i t is c l e a r t h a t t h e p o i n t count r e s u l t s f o r sawn s u r f a c e s a r e s a t i s f a c t o r y and u n b i a s e d , but t h a t t h e p o i n t count r e s u l t s f o r f r a c t u r e d s u r - f a c e s f o r b o t h 5% and 12% specimens a r e much t o o h i g h . Although some of t h e e r - r o r s can be a t t r i b u t e d t o s u r f a c e roughness of t h e f r a c t u r e d s u r f a c e s , t h i s would n o t account f o r t h e l a r g e e r r o r s t h a t were found. The bond between cement p a s t e and l a t e x was e v i d e n t l y weaker t h a n t h a t between c r y s t a l s i n cement and t h e r e f o r e a f r a c t u r e would tend t o r u n round one l a t e x s p h e r e and on t o a n a d j a c e n t one r a t h e r than c u t d i r e c t l y through t h e sample. A s a r e s u l t , an e x c e s s i v e amount of l a t e x was observed on f r a c t u r e d s u r f a c e s .

The r e s u l t s o b t a i n e d by t h e g r i d count method i n v o l v i n g 1 0 uniformly d i s t r i b - u t e d micrographs were r e a s o n a b l y a c c u r a t e f o r t h e f r a c t u r e s u r f a c e s but q u i t e low f o r t h e sawn s u r f a c e s .

A second system c o n s i s t e d of s t r o n g s i l i c o n c a r b i d e w h i s k e r s i n ( r e l a t i v e l y ) weak p o r t l a n d cement p a s t e . The r e s u l t s a r e shown i n Table 3 and a micrograph of a sample i s provided a s F i g . 6. Here sawn s u r f a c e s were t o o smeared f o r t h e S i c t o be observed. F r a c t u r e d s u r f a c e s , a s e x p e c t e d , y i e l d e d v a l u e s t h a t were lower t h a n t h e t r u e v a l u e s . These were s l i g h t l y lower f o r t h e p o i n t count method, and c o n s i d e r a b l y lower f o r t h e g r i d count method.

Table 3

Comparison of R e s u l t s of P o i n t and Grid Counts on Sawn and F r a c t u r e d Sur- f a c e s of Samples Containing 5 and 10% S i l i c o n Carbide i n P o r t l a n d Cement(p.c.)

Sample S u r f a c e P r e p a r a t i o n P o i n t Count Grid Count

10% S i c i n p . c . f r a c t u r e d 8 . 6 4.5

10% S i c i n p.c. sawn

-

*

-

5% S i c i n p . c . f r a c t u r e d 3 . 6 2 . 3

5% S i c i n p . c . sawn

-

-

*

The s u r f a c e was t o o smeared f o r t h e S i c t o be observed.

Both p r e c e d i n g systems involved reason- a b l y s t r o n g (cement p a s t e ) m a t r i c e s . To s t u d y t h e e f f e c t of a weak m a t r i x a number of samples of gypsum were prepared w i t h v a r i o u s i n c l u s i o n s . Micrographs of two of t h e s e a r e shown i n F i g s . 7 and 8.

I n t h e f i r s t s e t , 19.8% l a t e x was added t o hemihydrate which was t h e n hydrated and prepared f o r examination. The r e s u l t s a r e g i v e n i n Table 4.

I n Table 4 , where b o t h t h e i n c l u s i o n ( l a t e x s p h e r e s ) and t h e m a t r i x (gypsum) a r e s o f t , t h e p o i n t count r e s u l t s a r e r e a - s o n a b l y good f o r b o t h sawn and f r a c t u r e d s u r f a c e s . The g r i d count r e s u l t s s e r i o u s - l y u n d e r e s t i m a t e t h e amount of l a t e x pre- s e n t f o r e i t h e r t y p e of s u r f a c e .

FIG. 6 Another s e t of specimens was made, con-

Micrograph showing S i c w h i s k e r s s i s t i n g of mica i n gypsum; t h e r e s u l t s ob- i n p o r t l a n d cement p a s t e ; sample t a i n e d from examination of them a r e shown c o n t a i n e d 10% S i c . i n Table 5. Here, a s e x p e c t e d , t h e f r a c t -

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FIG. 7 FIG. 8

Latex s p h e r e s i n gypsum. Large, f l a t , mica f l a k e s i n gypsum; sample c o n t a i n e d 20% mica.

Table 4

Comparison of R e s u l t s of P o i n t and Grid Counts on Sawn and F r a c t u r e d S u r f a c e s of Samples Containing Latex Spheres i n Gypsum

Sample S u r f a c e P r e p a r a t i o n P o i n t Count Grid Count 19.8% l a t e x i n gypsum f r a c t u r e d 19.5% 9.0%

19.8% l a t e x i n gypsum sawn 17.5% 11.1%

u r e s u r f a c e s y i e l d e d p o i n t count e v a l u a t i o n s t h a t were t o o h i g h , i n d i c a t i n g t h a t t h e f r a c t u r e had passed p r e f e r e n t i a l l y through t h e mica f l a k e s . Again t h e sawn s u r f a c e s provided good p o i n t count e s t i m a t i o n s . I n t h i s p a r t i c u l a r s e r i e s t h e g r i d count e s t i m a t i o n s were r e a s o n a b l y s a t i s f a c t o r y .

T a b l e 5

Comparison of R e s u l t s of P o i n t and Grid Counts on Sawn and F r a c t u r e d S u r f a c e s of Samples Containing 5 and 20% Mica i n Gypsum

Sample S u r f a c e P r e p a r a t i o n P o i n t Count Grid Count 20% mica i n gypsum f r a c t u r e d 25.8 17.2

20% mica i n gypsum sawn 20.2 21.5

5% mica i n gypsum f r a c t u r e d 8.8 5.4

5% mica i n gypsum sawn 6.6 5.9

Conclusions

During t h e p a s t 12 y e a r s , i n v a l u a b l e i n f o r m a t i o n h a s been o b t a i n e d by t h e ex- a m i n a t i o n of samples i n t h e SEM. However, i n c o r r e c t i n t e r p r e t a t i o n of t h e mor- phology of v a r i o u s f e a t u r e s can a r i s e owing t o a number of c a u s e s .

1. The s u r f a c e t e x t u r e of t h e sample may be a f f e c t e d by improper u s e of a s p u t t e r c o a t i n g u n i t .

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2. I f t h e micrographs a r e i n v e r t e d by m i s t a k e , n e g a t i v e r e l i e f can appear p o s i t i v e and v i c e v e r s a .

3. The sample geometry and i t s r e l a t i o n s h i p t o t h e i n c i d e n t a n g l e of t h e e l e c t r o n beam and t h e t a k e o f f a n g l e of t h e d e t e c t o r can r e s u l t i n m i s - i n t e r p r e t a t i o n of t h e morphology. I n t h e sample c i t e d i n t h i s p a p e r , hexagonal e t c h p i t s appeared t o be c u b i c .

4. D i s t o r t i o n of t h e shape of t h e image of t h e sample w i l l occur i f t h e sample i s t i l t e d and e l e c t r o n i c t i l t c o r r e c t i o n is n o t a p p l i e d t o t h e image.

Q u a n t i t a t i v e e s t i m a t e s of phases p r e s e n t i n composites a r e d i f f i c u l t t o o b t a i n and t h e problem may be compounded i f t h e phases a r e n o t e a s i l y i d e n t i f i e d . When f r a c t u r e d s u r f a c e s of composites c o n t a i n i n g phases of d i f f e r e n t s t r e n g t h s were examined, i t was shown t h a t t h e f r a c t u r e s passed p r e f e r e n t i a l l y through t h e weak- e r phase c a u s i n g t h e amount of t h a t phase t o be o v e r e s t i m a t e d . T h i s o b s e r v a t i o n may be used t o advantage, however, when a sample i s examined t o d e t e r m i n e t h e c a u s e of i t s f a i l u r e . Examination of sawn s u r f a c e s overcomes t h e problem of over- e s t i m a t i n g t h e amount of t h e weaker phase b u t sawing smears t h e s u r f a c e of many samples.

For q u a n t i t a t i v e e s t i m a t e s of t h e p r o p o r t i o n s p r e s e n t i t is c l e a r t h a t p o i n t count e s t i m a t e s a r e much s u p e r i o r t o g r i d count r e s u l t s , t h e l a t t e r y i e l d i n g o n l y v e r y rough e s t i m a t e s p a r t i c u l a r l y when o n l y s m a l l p r o p o r t i o n s of t h e phase of i n - t e r e s t a r e p r e s e n t . For some samples, however, t h i s may be b e t t e r t h a n a v i s u a l e s t i m a t e , and comparatively l i t t l e e x t r a e f f o r t would be needed t o t a k e t h e r e - q u i r e d 1 0 uniformly d i s t r i b u t e d micrographs. V i s u a l e s t i m a t e s tend t o be h i g h l y u n r e l i a b l e u n l e s s made under f a v o r a b l e c'ircumstances by experienced i n v e s t i g a t o r s .

Acknowled~ements

Out thanks a r e extended t o J.F. Young, Department of C i v i l Engineering, Univer- s i t y of I l l i n o i s , and t o K. Mather and B. Mather, USAE Waterways Experiment S t a t - i o n , Vicksburg, M i s s i s s i p p i , f o r t h e i r d i s c u s s i o n of t h i s paper when i t was pre- s e n t e d a t t h e 1977 a n n u a l meeting of t h e T r a n s p o r t a t i o n - R e s e a r c h Board. We a r e a l s o i n d e b t e d t o t h e Mathers f o r s u p p l y i n g i n f o r m a t i o n on t h e w a t e r j e t and f r e e a b r a s i v e m u l t i p l e w i r e c u t t i n g t o o l s .

T h i s paper i s a c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research Council of Canada, and is published w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e D i v i s i o n .

References

1. V.F. Holland, Proc. 9 t h Annual Scanning E l e c t r o n Microscope Symp. (Toronto). Scanning E l e c t r o n Microscopy

1,

7 1 (1976).

2. E . J . C h a t f i e l d and V. N i e l s e n , T u t o r i a l Proc. 9 t h Annual Conf. Microbeam Anal. Soc. (Ottawa) Paper No. 28 (1974).

3. A D e l e s s e , Ann. Mines I V , 379 (1848).

4. R.T. DeHoff and F.N. Rhines, Q u a n t i t a t i v e Microscopy, 442p. McGraw-Hill, New York (1968).

Figure

FIG.  7   FIG.  8

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