PLASTICITY AND FRACTURE OF INORGANIC GLASSES AT HIGH SPEED GRINDING

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PLASTICITY AND FRACTURE OF INORGANIC GLASSES AT HIGH SPEED GRINDING

M. Schinker, W. Döll

To cite this version:

M. Schinker, W. Döll. PLASTICITY AND FRACTURE OF INORGANIC GLASSES AT HIGH SPEED GRINDING. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-603-C9-606.

�10.1051/jphyscol:19829120�. �jpa-00222426�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n°12,Tome 43, dSoembre 1982 page C9-603

PLASTICITY AND FRACTURE OF INORGANIC GLASSES AT HIGH SPEED GRINDING M.G. S c h i n k e r and W. Doll

Fraunhofer-Institut fur Werkstoffmechanik, Rosastr. 9, GFR 7800 Freiburg, F.R.G.

Résumé•-Pour différents verres optiques il a été procédé à une simulation de meulage à haute vitesse, un test de rayure par un diamant unique à une vitesse allant jusqu'à 100m/s. La plasticité et la fracture et le comportement des verres pour une seule rayure ont été étudiés directement au microscope et analysés consécutive- ment par microscopie électronique à balayage. La création des sur- faces de fracture perpendiculaires a été restreinte à basse vitesse jusqu'à plusieurs mètres par seconde dépendant du type de verre.

A plus haute vitesse il est observé que la plasticité des rainurages a lieu et indique une très haute température. Le comportement plastique des verres montre un changement dépendant du verre et de la vitesse qui est associé à un changement du quasi-isotherme vers 1'adiabatique dans la zone de contact à plus haute température.

A b s t r a c t . - For d i f f e r e n t optical glasses the high-speed g r i n d i n g process has been simulated b y s c r a t c h i n g t h e surfaces w i t h a single point diamond tool in the speed range up to 1oo m/s and the p l a s t i c i t y and f r a c t u r e behavior o f the glasses in the only once scratched grooves have been investigated b y d i r e c t microscopic observation and b y subsequent SEM-analysis. The generation o f median cracks p e r p e n d i c u l a r to t h e surface is found to be r e s t r i c t e d to low speeds i n t h e range of metres per second, dependent on the t y p e of glass. A t h i g h e r speeds plastic g r o o v i n g takes place and indication to v e r y h i g h temperatures are observed in the g r o o v e s . The plastic behavior o f t h e glasses shows a glass and speed d e p e n d - ent change which has been associated to a change from quasi-isothermal y i e l d i n g to adiabatic flow at h i g h e r temperatures in t h e scratched zone.

1. I n t r o d u c t i o n . - G r i n d i n g o f glass w i t h diamond tools is the main technique f o r p r o - d u c i n g lenses, p l a t e s , p r i s m s , ophthalmic glasses e t c . Improvements i n t h i s technique are expected b y a b e t t e r u n d e r s t a n d i n g of t h e complex g r i n d i n g mechanisms w h i c h are influenced b y numerous parameters as g r i n d i n g speed, feed r a t e , d e p t h of c u t , form o f the diamong, t y p e o f glass, i n t e r a c t i o n o f adjacent s c r a t c h e s , environment, temperature e t c . T h e r e f o r e a separation of t h e parameters is the common proceeding in l i t e r a t u r e and most w o r k has been done in indentation tests and in s c r a t c h experiments o f glass s u r f a c e s , especially at v e r y slow speeds ( 1 ) .

In this paper r e s u l t s will be presented w h i c h have been obtained on d i f f e r e n t optical glasses b y s c r a t c h i n g experiments at technical relevant speeds up to loo m / s .

2. Experimental r e s u l t s . - The basis o f o u r w o r k is f i r s t t h e d e f i n i t e p r o d u c t i o n o f single scratches on glass surfaces and secondly t h e d i r e c t microscopic observation o f t h e c r a c k i n i t i a t i o n and propagation under the v e r y fast moving diamond t i p . T h e used experimental p r o c e d u r e and f i r s t results o f t h e h i g h speed microscopy o f the single diamond s c r a t c h i n g experiments have been r e p o r t e d p r e v i o u s l y ( 2 - 5 ) .

Some main r e s u l t s o f the d i r e c t microscopic observation o f the h i g h speed s c r a t c h i n g at 1o m/s to loo m/s speed w h i c h , i s common f o r most glasses, are t h e f o l l o w i n g : Within t h e f i r s t microseconds after t h e contact o f t h e diamond t i p w i t h the glass surface s h o r t lat- eral c r a c k s are initiated and propagate s y n c h r o n o u s l y w i t h the moving diamond; some o f these c r a c k s are pushed b y t h e stress field in f r o n t of t h e moving i n d e n t e r . By d i r e c t and subsequent microscopic observations a c r i t i c a l d e p t h of c u t C has been found at which a s i g n i f i c a n t change in t h e removal o f t h e glass from plastic to b r i t t l e c h i p p i n g Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19829120

C9-604 JOURNAL DE PHYSIQUE

occurs. T h i s change i s s t r o n g l y dependent on t h e t y p e o f glass a n d o n t h e s c r a t c h i n g speed. Evidence o f median c r a c k s was f o u n d n e i t h e r d u r i n g h i g h speed observations n o r a f t e r w a r d s b y SEM i n t h e grooves o r in t h e cross-sections o f etched grooves. O n t h e

o t h e r h a n d median c r a c k s a t t h e bottom o f s c r a t c h e d g r o o v e s a r e w e l l k n o w n (6,7) f r o m experiments p e r f o r m e d a t slow speeds.

In o r d e r t o g e t f u r t h e r informations o n t h e process a t s c r a t c h i n g detailed inspections b y SEM w e r e p e r f o r m e d on o n l y once s c r a t c h e d surfaces o f a l a r g e numer o f o p t i c a l glasses. T h e p l a s t i c b e h a v i o r o f t h e glass in t h e abrasion zone has been f o u n d t o d r a s - t i c a l l y change f r o m quasi-isothermal y i e l d i n g u n d e r t h e high p r e s s u r e o f t h e i n d e n t e r t o adiabatic flow a t h i g h temperature, d u e t o t h e v e r y low thermal c o n d u c t i v i t y o f t h e glass. T h i s change i n t h e p l a s t i c b e h a v i o r c a n c l e a r l y b e seen i n t h e SEM-graphs o f p l a s t i c grooves in Fig. 1 t o l o ( w h e r e v i s t h e s c r a t c h i n g speed, C t h e local d e p t h o f c u t , a n d S t h e t h i c k n e s s o f t h e c h i p p i n g s ) . In t h e low m e l t i n g lead o x i d e glasses RS 520 a n d SF 5 b r i t t l e c h i p p i n g o c c u r s along smooth g r o o v e s up t o 1 m l s ( F i g . I ) , t h e n p a r t l y d u c t i l e (Fig. 8) a n d medium viscous c h i p p i n g a t 8 m / s (Fig. 3) c a n b e observed;

e x t r e m e l y low viscous f i b r e s a n d d r o p s ( F i g . 9) have been t h r o w n o u t o f t h e a b r a s i o n zone a t l o o m/s. T h e h i g h m e l t i n g lanthanum o x i d e glass 9 0 0 4 0 3 shows a d i s t i n c t change from b r i t t l e t o d u c t i l e i n a n a r r o w speed r a n g e a t a few meters p e r second ( F i g . 2 a n d 5 ) . T h e g e n e r a t i o n o f median c r a c k s a n d large-scale l a t e r a l c r a c k s along t h e s c r a t c h e d grooves i s r e s t r i c t e d t o h i g h viscous y i e l d i n g u n d e r t h e m o v i n g i n d e n t e r as shown f o r t h e glass B K 7 a t 1 m / s i n Fig. 4 a n d schematically r e p r e s e n t e d in Fig. 11 a n d 12. T h e r m a l l y i n d u c e d c r a c k s h a v e been o b s e r v e d a t h i g h e r speeds a t w h i c h t h e temperature o f t h e glass i n t h e abrasion zone o r wear zone increases r a p i d l y d u e t o adiabatic e f f e c t s (Fig. 1 0 a n d schematically Fig. 14 a n d 15).

A special t y p e o f y i e l d i n g a n d f r a c t u r e , t h e so-called saw t o o t h t y p e , has b e e n f o u n d in t h e wear zone diamondlglass s u r f a c e in a r e l a t i v e wide speed range a t scratching o f a special g r o u p o f glasses, as soda lime glass, B K 7, p h o t o t r o p i c glass, a n d f u s e d silica. Examples a r e shown in F i g . 6 and 7. T h e formation mechanism o f t h e saw t o o t h p a t t e r n i s assumed t o b e d u e t o d u c t i l e f r a c t u r e o f t h e p r e v i o u s l y g l u e d glass a t t h e i n t e r f a c e diamondlglass s u r f a c e as can be d e r i v e d from t h e schematic d r a w i n g in F i g . 13.

T h e glass dependent change from low speed quasi-isothermal y i e l d i n g a t low tempera- t u r e s t o h i g h speed adiabatic flow a t h i g h temperatures in t h e wear zone diamondlglass seems t o b e c o n t r o l l e d b y t h e t e m p e r a t u r e dependence o f t h e glass v i s c o s i t y . T h u s ex- periments a t l o o m / s speed h a v e shown t h a t f o r t h e e x t r e m e l y h i g h m e l t i n g glass f u s e d silica ( F i g . 16) t h e r e i s o n l y a v e r y t h i n l a y e r o f softened glass, t h e r e a r e also median c r a c k s along t h e g r o o v e d u e t o a zone o f d e n s i f i e d glass. O n t h e o t h e r h a n d t h e r e a r e g r e a t d i f f e r e n c e s in t h e t h i c k n e s s o f t h e c h i p p i n g s ( F i g . 2 a n d 3) a n d t h e r e f o r e i n t h e t h i c k n e s s o f t h e softened zone u n d e r t h e i n d e n t e r , t h u s i n d i c a t i n g t h a t t h e thermal c o n d u c t i v i t y o f t h e glass i s a n o t h e r main parameter c o n t r o l l i n g t h e p l a s t i c i t y in t h e g r i n d i n g process a t h i g h e r speeds.

3. Acknowledgement.

-

4. References. -

( 1 ) BUSCH, D.M., Thesis ( 1 9681, Techn. U n i v e r s i t a t , Hannover/Germany

( 2 ) SCHINKER, M, DOLL, W., Soc. Photo-Opt. I n s t r . E n g r s . SPlE 189 (1978) 784 (3) SCHINKER, M. e t al., B e r i c h t W178, FhG-IWM, Rosastr. 9, 7800-iburg

( 4 ) STAHN, D., SCHINKER, M., DOLL, W., SOMMER, E., Glastechn. Ber.

53

(1980) 259

( 5 ) SCHINKER, M., DOLL, W., B e r . W3/82, FhG-IWM, Rosastr. 9, 7 8 0 0 F r e i b u r g ( 6 ) ERNSBERGER, F.M., J. Amer. Ceram. Soc.

51

(7) DICK, E., Glastechn. B e r .

5

Glass RS 520

Fig.l:Smooth p l a s t i c groove w i t h b r i t t l e Fig.6:Saw t o o t h p a t t e r n along a p l a s t i c c h i o ~ i n a s groove w i t h d e n s i f i c a t i o n zone

l a s s 900403

= 1 m/s

= 3,6 pm

L = 2,.

.

Fig.2: B r i t t l e c h i p p i n g s beside a groove Fig.7: D e t a i l s o f saw t o o t h p a t t e r n i n a ~ l a s t i c aroove

beside a groove

Fig.4: Median crack along t h e d e n s i f i c a - Fig.9: Glass f i b e r s and drops thrown o u t t i o n zone below a scratched groove o f a p l a s t i c groove

l a s s 900403

= 2 rn/s

= 9,5 pm

Fig.5:Softened g l a s s i n a v e r y deep Fig.lO:Drops,bubbles and t h e r m a l l y in.duced

p l a s t i c groove cracks i n a groove

JOURNAL DE PIiYSIQUL

ISOTHERMAL FLOW

E SCALE DENSlFlCATlON ^{'-\ SMALL} - SCALE DENSIFICATION

F i g . 11

Abrasion zone d u r i n g low speed s c r a t c h i n g o f glass

F i g . 14

Abrasion zone d u r i n g h i g h speed s c r a t c h i n g o f glass

DROP FlBER

THERMALLY INDUCED CRACKS

MEDIAN CRACK

F i g . 12

Abrasion zone a f t e r low speed s c r a t c h i n g o f glass

F i g . 15

Abrasion zone a f t e r h i g h speed s c r a t c h i n g o f glass

F i g . 13

Saw t o o t h mechanism d u r i n g s c r a t c h i n g o f glass

F i g . 16

Temperature versus v i s c o s i t y of d i f f e r e n t glasses