Mechanism of Ablation of Metals

7/28/2019 Mechanism of Ablation of Metals

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M E C H A N I S M F O R A B L A TI O N O F M E TA L S B Y E X P L O S IO N P L A S ~ SE. G. Popov

T h e i n t e r a c t i o n o f c o n c e n t r a t e d e n e r g y f l u x e s w i t h m e t a l s i s th e o bj e c t o f m u l t i f a c e t e di n v e s t i g a t i o n s w h i c h f e e d c o n t e m p o r a r y e n g i n e e r i n g a n d t e c h n o l og y . V a r i o u s k i n d s o f e l e c t r i c a ld i s c h a r g e s , a s w e l l a s l a s e r a n d e l e c t r o n b e a m s , s e r v e a s i n s t r u m e n t s f o r t hi s . N e w p o s s i -b i l i t i e s a r e o p e n e d b y t h e e f f e c t o n m e t a l s o f t h e d e n s e p l a s m a f o r m e d i n g a s e s w i t h t h e a i do f e x p l o s i v e s . R a p i d e v a p o r a t i o n o f s o l i d s , i o n i z a t i o n , a n d r e l e a s e o f v a p o r a r e c a u s e d b yt h e e m i s s i o n f r o m s u c h p l a s m a s [ 1- 4] . C e r t a i n p r o c e s s e s a s s o c i a t e d w i t h t h e e n t r y o f o b j e c t sf r o m o u t e r s p a c e i n t o p l a n e t a r y a t m o s p h e r e s h a v e b e e n s i m u l a t e d [ 4- 6] . A n u m b e r o f h a r d a n dr e f r a c t o r y s u b s t a n c e s h a v e b e e n s y n t h e si z e d [ 7 ] . M e t a s t a b ! e s t a t e s a n d a l l o y s as w e l l asv e r y h a r d n i t r i d e d s u r f a c e s t h a t a r e o f i n t e r e s t f o r m e t a l l u r g y a n d h e a t t r e a t m e n t h a v e b e e nobtained [8-15].

R e s u l t s f r o m a s t u d y o f th e g a s d y n a m i c a n d t h e r m a l p h e n o m e n a t h a t o c c u r w h e n a n e x p l o -s i o n p l a s m a c o m e s i n t o c o n t a c t w i t h m e t a l s a r e p r e s e n t e d h e r e .

E X P E R I M E N T A L M E T H O DM e t a l s a m p l e s w e r e p r o c e s s e d b y t h e p l a s m a i n a s p e c i a l l y c o n s t r u c t e d m u l t i p l e - u s e e x -p l o s i o n s h o c k t u b e i n w h i c h t h e m a i n i n t e r a c t i o n p a r a m e t e r s w e r e r e c o r d e d [ 1 3- 1 5] . T h e b r i -

s a n t e f f e c t o f t h e e x p l o s i o n w a s e l i m i n a t e d b y u s i n g a s p a t i a l l y d i s t r i b u t e d c y l i n d r i c a lc h a r g e w h i c h p r o d u c e d a s t r o n g s h o c k w a v e w i t h a s t ab l e a m p l i t u d e i n t h e g a s [ 4 ] . T h e a p-p a r a t u s i s i l l u s t r a t e d i n F i g. I . A n e l e c t r i c a l d e t o n a t o r ( I) i n i t i a t e d a c h a r g e o f p e n t o -l i t e ( 2) a n d t h e e x p l o s i o n p r o d u c t s m o v e d i n t o a v e s s e l ( 6 ) , s h o c k - c o m p r e s s i n g a n d h e a t in gt h e g a s l o c a t e d t h e r e ( a r g o n , n i t r o g e n , o r a i r a t a t m o s p h e r i c p r e s s u r e ) . T h e s p e e d o f t h es h o c k w a v e w a s m e a s u r e d b y a s y s t e m o f i o n i z a t i o n p r o b e s ( 3 ) , t h e s i g n a ls f r o m w h i c h w e r e r e -c o r d e d o n a n o s c i l l o s c o p e . T h e p r o b e m o u n t e d n e a r t h e s a m p l e s w a s d e s i g n e d t o m o n i t o r t h ed u r a t i o n o f t h e p l a s m a i n t e r a c t i o n . S u b s e q u e n t l y , a p i e z o c e r a m i c p r o b e ( 4) w h i c h a b s o r b e dp a r t o f t h e e x p l o s i v e l o a d i n g w a s u s e d f o r t h i s p u r p o s e .

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Fig. I. A dia gra m of the appara tus.D n e p r o p e t r o v s k . T r a n s l a t e d f r o m F i z i k a G o r e n i y a i V z r y v a , V o l . 2 0, N o. 6 , p p. 1 2 6 - 1 3 4 ,

N o v e m b e r - D e c e m b e r , 1 9 8 4 . O r i g i n a l a r t i c l e s u b m i t t e d J u l y 6 , 1 9 8 3.

0 0 1 0 - 5 0 8 2 / 8 4 / 2 0 0 6 - 0 7 1 1 5 0 8 . 5 0 9 1 9 8 5 P l e n u m P u b l i s h i n g C o r p o r a t i o n 7 1 1


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Fig. 2. The ionizatio n (a) and pressure (b) in a 0.5-m-lo ng vesselfor a 600-g charge. (The sine wave has a frequency of I00 kHz.) a:I, 2) Spikes in the signals from probes at distance of 50 and i0 cmfrom the packet; c: ionizatio n (upper trace) and pressure (below)in a l-m chamber from a 900-g charge (10-kHz sine wave; the straightlines are calibration signals).

The temperature and pressure of the plasma were varied by changing the mass of the chargefrom 150 to 900 g. The duration of the interacti on was regulated by the length of the cham-ber (0.5 or i m with an attachment) and by an inert shell surroundin g the charge. Chargeswit h masses of 600-900 g, whic h ensured less dam ping of the shock wave, were used with theextende d chamber. Well- studi ed and widel y used mater ials were selected for the studies. Themetals included commercial grade iron, copper, and aluminum. Since in practice iron alloysare used to withstand the destructive action of explosions and other extreme physical effects,this group was large, including the simple carbon steels (0.Skp; St. 3, I0, 20, 25, 35, 45,K6, UB, UII), alloy steels (35Kh, 45Kh, KhlSN9T), and cast irons. The aluminu m alloys wererepresented by silumins with 6-38% Si, AMg6 an alloy with magnesium, and duralumlnum DI6.

Samples of the alloys were cut into slabs and ground to fit a size of 20 x I0 x 3 mm andweigh ed on an analytic balance. Then 5-13 slabs were glued into a packet which was attachedto the botto m of the chamber. Simultane ous plasma processing of several alloys made it pos-sible rigorously to compare the changes occurring in them. After an explosion the packet wasremoved from the apparat us and studied visually. Then the slabs were separated, cleaned ofglue, repeate dly measure d with a micrometer, and weighed. Thin slices were prepared fromsamples potted in epoxy resin for purposes of metall0graphic analysis. The microstructureof the alloy s was determ ined by etching and examin~ing them under an MIM-SM microsco pe and aPMT-3 microhardness gauge.

RESULTSPlasma-Int eraction Regimes. Cylindrical charges produced constant-amplitude shock waves

with velocities of 5-11 km/sec in argon and 5-12 km/sec in nitrogen and air inside a 0.5-m-long vessel. In the 1-m-long vessel the damping of the shock had an effect and near the sam-ples the shock velo city fell to 5-7 km/sec. The plasma paramet ers were determine d from themeasured velocities and the known shock adiabats for gases [4]. The temperature and pressurein the shock wave re flected from the botto m of the vessel wer e (2-6) x 104 ~ and (3-20) 108 Pa, w hi le the plasma density was -60 kg/m s in nitrogen and air and ~90 kg /m 3 in argon.

The 0.8-1.5-m-high column of gas enclosed by the charge and the vessel was compressedinto a plasma slug of t hickness 2-3 cm and diameter 4 era. At this point, to judge from thepress ure traces, the shock compres sion of the gas ceased. After about I0 6 see the pressureentered a plateau coincident with the pressure of the reflected shock and then decayed smooth-ly. In one of the experimen ts a sharpened coppe r dowel was in stalled in place of the packet.A shock with a velocit y of 6 km/sec compress ing a l-m column of air unifor mly melted the lowerportion of the dowel while leaving only the color of oxide tint and soot on the remaining sur-face. A sharp melti ng boundary at a level of 21 mm from the bottom of the vessel shows thatafter ref lecti on the plasma and the colder explosion products remained motionless for some

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d

Fig. 3. The micro struc ture of surface layers: a) copper, xl00~ b) iron,x600; c) silumin, x600; d) St. 45, x600; e) St. 35 x400; f) St. 45, x200.

Fig. 4. The micr ostr uctu re of surface layer of St. 25 after multipleplasma processing, x400.

time and did not mix. This is confirmed by an earlier streak photogr aph of the reflectedshock and streams of explos ion products from a cylindric al c harge [4].

The packet of samples sticking l0 mm above the bottom of the chamber was inside the slugof dense plasma. The ionization and pressur e signals for charges without a shell were ofequal duration, 0. 1-0.3 msec and 0.2-0.4 msec in 0.5-m and l-m chambers, resp ectively. Awater shell of mass 5 kg delayed the drop in ionization by about 0.i msec and in pressure byabout 0.3 msec (Fig. 2a and b). In some experimen ts the entire apparat us was placed undera 1-m-th ick laye r of tamped moist clay. Then the ionizat ion signal was lengthened slightlyto 0.6 msec while the pressure had an ~xtended plateau lasting ~I0 s sec and decayed slowly(-3 msec) (see Fig. 2c). The fluctuations in the ioni zation traces (Fig. 2a and c) were ran-dom and not reproducib le. The signal was smoothed out (Fig. 2c) if the probe electrode wasnot a needle, but the packet itself, isola ted from the botto m of the vessel. The pressure

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t r a c e s w e r e r e p r o d u c i b l e a n d h a d c h a r a c t e r i s t i c r e c t a n g u l a r d i p s ( F i g. 2 b) w h i c h w e r e i n t e r -p r e t e d a s t h e r e s u l t o f r e f l e c t i o n o f w a v e s i n t h e b o d y o f t h e a p p a r a t u s a n d t h e s t e e l b a s eplate-

S t r u c t u r e o f t he S u r f a c e L ay e r . A f t e r a n e x p l o s i o n t h e p a c k e t w a s w a r m t o t h e t o u c h,w i t h t r a c e s o f m e l t i n g o n t h e o u t e r s u r f a c e s , a n d r e s e m b l e d g r a p h i t e i n c o l o r a n d l u s t er .T h e m i d d l e o f t he p a c k e t w a s h e a t e d l i t t l e , a s i s i n d i c a t e d b y t h e p r e s e r v a t i o n o f t h e g l u eb e t w e e n s l a bs . U n d e r a m i c r o s c o p e t h e t r a ce s o f m e l t i n g i n t h e f o r m o f s o l i d i f i e d d e p o s i t sa n d w a v e s o f h e i g h t 1 0 - 5 0 ~ m a r e s u p p l e m e n t e d b y n u m e r o u s p o r e s o f d i a m e t e r 1 - 2 0 l~m i n t h e1 0 - 1 0 0 - ~ m - t h i c k e x t e r i o r l a y e r ( Fi g. 3 ). T h e l ar g e p o r e s c o me t o th e s ur f a c e , a n d in p l a c e sw h o l e p i e c e s o f t h e l a y e r h a v e b r o k e n a w a y f r o m t h e b a s e m a t e r i a l ( Fi g. 3 c a n d f ; F i g . 4 a ) .

T h e m i c r o s t r u c t u r e o f t he l a y e r i n c o p pe r , a l u m i n u m , a n d i t s a l l o y s is f i n e - g r a i n e d( Fi g. 3 a a n d c ). I n t he s i l u m i n s s e c o n d a r y r o u n d g r a i n s o f s i l i c o n o f d i a m e t e r 1 - 5 ~ m w i t ha p o r e i n t h e c e n t e r a r e v i s i b l e a t a m a g n i f i c a t i o n o f W i t h ir o n a l l o y s t h e m i c r o -s t r u c t u r e o f t he l a y e r w a s n o t r e v e a l e d b y s t a n d a r d e t c h i n g . A f t e r r e p e a t e d e t c h i n g w i t h r e -p o l i s h i n g , p e r p e n d i c u l a r s u r f a c e s w e r e b a r e l y v i s i b l e : c o l u m n a r ( in e x p e r i m e n t s w i t h a r g o n)o r f a n - s h a p e d s l a b l i k e f o r m a t i o n s ( i n e x p e r i m e n t s w i t h n i t r o g e n a n d a i r ) . T h e o n l y e l e m e n t so f t h e o r i g i n a l s t r u c t u r e p r e s e r v e d i n t h e l a y e r w e r e p a r t i a l l y d i s s o l v e d l a r g e i n c l u s i o n s o fg r a p h i t e i n g r a y c a s t i r o n o r h y p e r e u t e c t i c s i l i c o n i n t h e s i l u m i n s . T h e s e i n c l u s i o n s w e r es u r r o u n d e d b y a h a r d - e t c h e d b o r d e r w h i c h e x p a n d e d t o 1 0 - 2 0 u m t o w a r d t h e s ur f a c e . T h e m i c r o -h a r d n e s s o f t h e l a y e r i s 2 - 3 t i m e s h i g h e r i n t h e s i l u m i n s a n d 2 - 1 0 t i m e s i n t h e i r o n- -c ar bo na l l o y s , t h a n i n t h e b a s e . ( Th e g r e a t e s t h a r d n e s s H = 1 5 - 2 0 G P a w a s o b t a i n e d w h e n l o w - c a r b o ns t e e l s w e r e p r o c e s s e d b y n i t r o g e n o r a i r p l a s m a s [ 1 5] .)

Und er the outer mel ted layer I (Figs. 3 and 4) lies a ther mal effect zone. In the iron--c a r b o n a l l o y s i t i s s e p a r a t e d b y a l a y e r 2 o f f i n e a c i c u l a r m a r t e n s i t e f o r m e d i n t h e l i m i t so f p r e v i o u s p e a r l i t e g r a i n s . I t s m i c r o h a r d n e s s i s H~ = 8 - 1 2 GP a , o r r o u g h l y 1 . 5 t i m e s h i g h e rt h a n a f t e r t e m p e r i n g o f t he s a m e a l l o y s i n w a t e r . I n t h e s i l u m l n s t h i s z o n e i s d i s t i n g u i s h e df r o m t h e b a s e b y a d a r k - e t c h i n g b o r d e r a r o u n d s i l i c o n c r ys t a l s . I n o t h e r a l l o y s a n d m e t a l so n l y t h e c h a n g e i n t h e g r a i n s h a p e c h a r a c t e r i s t i c o f r e c r y s t a l l i z a t i o n i s ob s e r ve d .

T h e p o r o s i t y o f t h e m e l t i n g z o n e i s l e s s d i s t i n ct i n s i n g l e - p h a s e s y s te m s . I n p a r t i c u -l a r , s t e e l s p r e t e m p e r e d t o m a r t e n s i t e y i e l d f e w e r p o r e s t h a n i n t h e i n i t i a l t w o - p h a s e s t a t e .T h e m e t a l s a n d a l l o y s s t u d i e d h e r e c a n b e o r d e r e d i n t e r m s o f i n c r e a s i n g p o r o s i t y a s f o l l o w s :Cu, AI, D16, AMg6, K hlBN9 T, pre temp ered steels , Fe, cast irons, carbo n steels, silumins.

T h e s t r u c t u r e o f t h e s u r f a c e l a y e r d e p e n d s s t r o n g l y o n t h e t r e a t m e n t r e g i m e . F o r l o wp r e s s u r e s l a y e r i h a s a l m o s t n o p or e s , i t s t h i c k n e s s i s d e t e r m i n e d b y t h e d u r a t i o n o f t h ei o n i z a t i o n , a n d , i n t h e st e e l s , i s r o u g h l y e q u a l t o t h e t h i c k n e s s o f t h e m a r t e n s i t e l a y e r 2( Fi g. 3 d ). A s t h e p r e s s u r e i s r a i s e d t h e p o r o s i t y i n c r e a s e s . I n s t e e l s l a y e r I b e c o m e st h i c k e r t h a n l a y e r 2 w h e n t h e p r e s s u r e i s r e d u c e d s l o w l y a n d t h e l a t t e r a c q u i r e s t h e d a r k -e t c h i n g s t r u c t u r e o f t e m p e r e d m a r t e n s i t e ( F i g. 3 e ). A s h a r p d r o p i n t h e p r e s s u r e ( c h a r g e sw i t h o u t s h e l l s ) y i e l d s m a n y p o r e s w h i c h c o m e up t o t h e s u r f a c e ( Fi g. 3 b ) a n d p a r t i a l o r c o m -p l e t e s e p a r a t i o n o f l a y e r I ( Fi g. 3 c a n d f ). R e t a r d i n g t h e d r o p l e a d s t o a c o n c e n t r a t i o n o ft h e p o r e s n e a r e r t o t h e m e l t i n g b o u n d a r y ( Fi g. 4 a ) .

R e m o v a l o f M e t a l . A f t e r a n e x p l o s i o n t h e p l a n e s u r f a c e o f a p a c k e t b e c o m e s s t e p p e d - I nc o p p e r a n d a l u m i n u m a n d i t s a l l o y s , f a i l u r e o cc u r s . S l a b s o f t h e i r o n a l l o y s a r e n o t v i s i b l yd i f f e r e n t i n h e i g h t . H o w e v e r , m e a s u r e m e n t s w i t h a m i c r o m e t e r s h o w e d t h a t r e m o v a l t a k e s p l a c ee v e n w i t h t h e m , a n d t h e m o r e s o t h e s m a l l e r t h e a m o u n t o f C , C r, a n d o t h e r a l l o y i n g e l e m e n t si n t h e a l l o y . L o s s e s f r o m t h e u p p e r a n d si d e s u r f a c e s a r e t h e s a m e . F o r p r e s s u r e s a b o v e IG P a t h e p a c k e t u s u a l l y f a i l e d , t h e s l a b s w e r e d e f o r m e d , a n d i t b e c a m e m e a n i n g l e s s t o p e r f o r mr e p e a t e d m e a s u r e m e n t s o n th e m . A t l o w p r e s s u r e s t h e a m o u n t r e m o v e d i s so s ma l l t h at t h e m e l t -e d s u r f a c e w a s f o g g e d w i t h n o n u n i f o r m i t i e s . T h e m o s t r e l i a b l e m e a s u r e m e n t s o f t h e a m o u n t r e -m o v e d ~ w e r e o b t a i n e d i n e x p e r i m e n t s w i t h t h e f o l l o w i n g p a r a m e t e r s :

G a s 7 , , K o . G P a x i, m ~ c c p, m m c 6 A l . m m 0 c a, m m 6 Fe , m mAr 45 t , 1 0 , 4 0 ,5 0 , 4 0 , 3 0 , tA r 4 5 t ,1 0 , 4 i , t , 0 , 3 0 , 2 0 , 05 'A i r 30 t , 6 0 , 4 t , 1 9 0 ,2 0,05 0,01

w h e r e T ~ a n d p ~ a r e t h e t e m p e r a t u r e a n d p r e s s u r e o f t h e p l a s m a , a n d T i a n d T p a r e t h e i o n i z a -t i o n a n d p r e s s u r e d u r a t i o n s . I n t h e s e e x p e r i m e n t s t h e a m o u n t r e m o v e d w a s d e t e r m i n e d b y w e i g h -i n g t h e s a m p l e s a n d w a s 2 0 - 3 0 % g r e a t e r t h a n t h e a b o v e v a l u e s .

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I ' I "I

O - 0 , 2 . 0 , 4 z , m mFig. 5. Temper ature profiles in metals,T = 0.4 msec, p: ~ i GPa (Tm~- Tmo ismelting temperature and T t is the limit-ing temperature for tempering on marten -site.

Weigh ing makes it possible to detect removal of matter hidden in pores and nonunif ormi-ties or when the samples are deformed. Initially, data obtained by this method were suspectsince working of the steels with an air or nitrogen plasma at a pressure of 0.3-0.5 GPa wasaccompa nied by an increase (by 1-3 mg) in the mass of the samples. (An explana tion for thisis offered later.) When steels are worked with an argon plasma having T, = 41 103~ Pl =0.9 GPa for times r i = 0.3 msec a nd rp = 0.4 msec, the mas s loss h corre spon ded to the fol-lowin g value s of ~ = A/Sp (S is the wor ked surf ace; p is the density);

MateNal Kh i8NgT Ur8 45 5%~i 30Kh 35 20 08Kp Fe6, ~m 24 26 28 28 29 30 35 58 75Th is k ind o f p roces s ing o f the p re tempere d s t ee l s 20 , 30Kh , 35 , 45, and U8 y ie ld s lo s s es w h icha re rough ly 1 .5 t imes low er . A t the max imum temper a tu re s and p res s u r es ach ie ved w i th a rgonthe packe t s b roke up and the s l abs w ere s ub jec t to nonun i fo rm ac t ion o f the p la s ma f rom a l ls ides and lo s t 2 -5% o f the i r mas s .

M u l t i p l e P l a s m a T r e a t m e n t s . F i g u r e 4 s ho ws t h e s t r u c t u r e o f t h e s u r f a c e l a y e r o f t h es tee l w a l l o f the s ocke t in w h ich the packe t s w ere p lace d . The w a l l w as s ub jec ted to thep l a s m a a n d e x p l o s i o n p r o d u c t s i n 38 s h o t s . I n i t s g e n e r a l f e a t u r e s , t h e s t r u c t u r e o f t h i slayer is the same as for single shots. Figure 4a illustrates the breakaw ay of the outerlayer owing to growth of bubbles in the me lt. This structure is characteristic for the por-tion 0-2 cm from the bottom of the vessel. Here the wall, as well as the samples in the pack-ets, was in contact with the plas ma slug for a long er time. The struct ure sh own in Fig, 4bis typical for a level 3-6 cm from the bottom. During each explosion this portion was brief-ly in contact with the plasma two times (initially with the plasma slug behind the passingshock wave and then with the plasma as it expanded after reflection). The features of thisstructure include the absence of pores in the melt zone and the presence of a layer of tem-p e r e d d a r k - e t c h e d m a r t e n s i t e s e p a r a t ed f r o m i t b y l i g h t -e t c h e d m ar t e n s i t e . T h e m i c r o h a r d n e s sof both of the latter is 7 GPa and that of the melt zone is 6 GPa (as for one-time p rocessingof pretempere d steel).

At a level of 0-2 cm from the bottom the removal of metal is quite distinct and leads toa reduct ion in the thickness of the wall by 0.8-1.0 mm. (A single explosion corresponds to21-26 ~m for a single interaction.) Above 2 cm the thinning of the wall is less than 0.! mmafter 38 explosions.

D I S C U S S I O N O F R E S U L T SThe main c ontribut ion to heating of the samples was from the powerful ra diation produce d

by the plasma which in such experiment s is optica lly thick [1-4]. At the moment the shock isreflected the radiat ion flux at the surface reached q = 4-70 MW/c m 2 in argon and 1-9 MW/cm 2in nitrog en and air. However, at the surface the plasma cooled rapidl y and the radiatio nflux fell over a time

t ~ - e l / q . ~ t0 -7 ~- t0 -6 sec, (1)

715


6/8

w h e r e e ~ I 0 " i 0 *~ J / m s i s t h e e n e r g y p e r u n i t v o l u m e o f t h e p l a s m a a n d Z ~ i 0 s c m i s t h em e a n f r e e p a t h f o r 5 - 1 5 - e V p h o t o n s i n t h e p l a s m a . R a d i a t i o n c h a r a c t e r i z e d b y t h e t r a n s p a r -e n c y t e m p e r a t u r e T 2 o f t h e ga s p a s s e d o u t w a r d [ 16 ]. F r o m t h e d a ta o f [ 4] a n d t h e e x p e r i -m e n t a l c o n d i t i o n s , T 2 ~ 3 x 1 0 ~ ~ f o r a r g o n 2 x 1 0 ~ ~ f o r a i r . A h i g h e r p l a s m a t e m p e r a t u r eT , h a d n o e f f e c t o n t h e p r o c e s s i n g ; a f t e r a s h o r t b u r s t t h e r a d i a t i o n f l u x a t t h e s a m p l e sf e l l t o 4 M W / c m 2 i n a r g o n a n d I M W / c m 2 i n a i r . T h e t e m p e r a t u r e T ,, w h i c h d e t e r m i n e s t h ep l a s m a e n e r g y e , c o u l d a f f e c t t h e d u r a t i o n o f t h e e m i s s i o n f r o m t h e p l a s m a s l u g . S u b s t i t u t -i n g Z ~ 2 c m ( t he t h i c k n e s s o f t h e p l a s m a ) i n t o E q . ( I ) , w e f i n d t ~ 0 . 8 - 4 m s e c i n a r g o n a n dt ~ 2-12 msec in air.

A f t e r c o o l i n g d o w n t o t h e t r a n s p a r e n c y t e m p e r a t u r e T 2 t h e p l a s m a s l u g c o u l d s t i l l e m i ta s a v o l u m e s o u r c e f o r - 1 0 - 2 se c. H o w e v e r , i n t h e e x p e r i m e n t s r a d i a t i v e c o o l i n g o f t h e p l a s -m a , t o j u d g e f r o m t h e i o n i z a t i o n s i g n a l s , e n d s a f t e r 0 . 1 - 0 . 6 m s e c , b o t h i n a r g o n a n d i n a ir .O n e o f t h e r e a s o n s f o r t h i s i s t h e r a p i d e x p a n s i o n o f t h e p l a s m a b e c a u s e t h e e x p l o s i o n p r o d -u c t s e s c a p e f r o m t h e v e s s e l . T h i s i s i n d i c a t e d b y t h e a g r e e m e n t b e t w e e n t h e d u r a t i o n s o fi o n i z a t i o n a n d p r e s s u r e i n t h e e x p e r i m e n t s o n c h a r g e s w i t h o u t s h e ll s . I n th o s e e x p e r i m e n t sw h e r e a n i n e r t s h e l l r e t a r d ed d i s i n t e g r a t i o n , i o n i z a t i o n w a s o b s e r v e d t o l a s t s o m ew h a t l o n g e ri n t he v e s s e l . T h e r e a s o n t h a t i o n i z a t i o n a n d r a d i a t i o n a r e o f l i m i t e d d u r a t i o n i n t h i s c a s em i g h t b e t h e m i x i n g o f t h e p l a s m a w i t h e x p l o s i o n p r o d u c t s . B a s e d o n e s t i m a t e s o f t h e R e y n o l d sn u m b e r , t h e f l o w o f t h e g a s i n t h e v e s s e l i s t u r b u l e n t . A f t e r t h e f l o w s l o w e d d o w n t h e f l u c -t u a t i o n s d i d n o t d i s a p p e a r a t o nc e a n d t h e p l a s m a w a s g r a d u a l l y m i x e d w i t h t h e e x p l o s i o np r o d u c t s . T h e s h a r p d i s o r d e r e d d i p s i n t h e i o n i z a t i o n t r a c e s ( Fi g . 2a a n d c ) a r e p r o b a b l ya l s o a r e f l e c t i o n o f t h i s t u r b u l e n t m i x i n g . E v i d e n c e f o r m i x i n g i s p r o v i d e d b y a l a y e r o fS i C o n t h e s u r f a c e l a y e r t h a t is o b s e r v e d d u r i n g x - r a y s t r u c t u r e a n a l y s i s o f s i l um i n s .

T h e t i m e t o h e a t t h e S u r f a c e o f t h e s a m p l e s t o t h e b o i l i n g p o i n t i s g i v e n b y2 2,' 2= 0.79Tb~ ,q a, (2 )

w h e r e ~ a n d a ar e t h e t h e r m a l c o n d u c t i v i t y a n d t h e r m a l d i f f u s i v i t y o f t h e m e t a l a n dr Rr o ln5] - (3)T b = T b o [ I - - - Z - P o j

i s t h e b o i l i n g t e m p e r a t u r e u n d e r p r e s s u r e . S e t t i n g p , = 0 . 3 G P a , q A r = 4 M W / c m 2, and qb =1 M W / c m 2 a n d u s i n g t h e t h e r m a l p r o p e r t i e s o f t h e m e t a l s [ 1 7] , w e h a v e

a,cm~/sec W~ c~ .~ L, kl /m o~ Tb0. K r b, K TAr, ~seCTb, ~secFe 0,i5 0,51 390 30 73 6500 3 45Cu i, i2 3,89 3i0 2855 7400 37 600A1 0,87 2,09 28i 2600 6800 l i i8~

F o r p , ~ 1 G P a t h e p a r a m e t e r s o f t h e m o l t e n f i l m o n t he s a m p l e s r e a c h e d t h e c r i t i c a l ( v a n d e rW a a l s ) v a l u e s . ( A c c o r d i n g t o c a l c u l a t i o n s [ 18 ], t h e s e v a l u e s f o r t h e m e t a l s a r e: F e -- 9 6 0 0~ 0 . 8 2 5 G P a , 2 0 3 0 k g / m ' ; C u - - 8 3 9 0 ~ 0 . 7 4 6 G P a , 2 3 9 0 k g / m ' , A 1 -- 8 0 0 0 ~ 0 . 4 4 7 G P a , 6 4 0k g / m S ~ ) U n l i k e f o r t h e e q u i v a l e n t l a s e r i r r a d i a t i o n , a n i n c r e a s e o f a b o u t 3 t i m e s i n th eb o i l i n g t e m p e r a t u r e o f th e m e t a l s l e d t o a t e n f o l d d e l a y i n v a p o r i z a t i o n .

T h e r e f o r e , i n a r g o n a t t h e b e g i n n i n g o f t h e i n t e r a c t i o n , a n d i n a i r c l o s e r t o t h e e n d ,t h e m e t a l s a n d a l l o y s b e g a n t o v a p o r i z e r a p i d ly . H o w e v e r , v a p o r i z a t i o n w a s a l m o s t i m m e d i a t e -l y h a l t e d o w i n g t o s c r e e n i n g o f t h e r a d i a t i o n b y t h e v a p o r s [ 2 J . P h o t o n s w i t h e n e r g i e s h i g h -e r t h a n t h e i o n i z a t i o n p o t e n t i a l o f t h e m e t a l a r e a b s o r b e d a f t e r v a p o r i z a t i o n o f a r o u g h l yl - ~ m l a y e r . O n c e i o n i z e d , t h e v a p o r s t r o n g l y a b s o r b s l o w e r - e n e r g y p h o t o n s a s w e l l . ( S c r e e n -i n g o f t h e r a d i a t i o n f r o m a s h o c k w a v e i n a r g o n b y v a p o r h a s b e e n f o u n d [ 3] t o r e d u c e t h e v a -p o r i z a t i o n o f A 1 b y a f a c t o r o f 4 a f t e r - i 0 ~ s e e , w h e n a t o t a l o f 1 . 5 ~ m o f A 1 h a s b e e n v a p o r -i z e d. ) A p p a r e n t l y , a f t e r v a p o r i z a t i o n o f a l a y e r o f m e t a l 1 - 5 ~ m t h i c k th e o n l y h e a t i n g thato c c u r s i s t o m a i n t a i n t h e s u r f a c e a t a t e m p e r a t u r e T b . A r a n d o m b u r s t i n q ( o wi n g , f o r e x -a m p l e, t o f l u c t u a t i o n s i n t h e p l a sm a ) i s a c c o m p a n i e d b y a d d i t i o n a l v a p o r i z a t i o n o f t h e m e t a la n d b y i n c r e a s e d s c r e e n i n g t o c o m p e n s a t e t h e b u r s t . O n t h e o t h e r h a n d , t h e d r o p i n q l e a d st o c o n d e n s a t i o n o f v a p o r , w e a k e n i n g o f t he s c r e e n i n g , a n d e n h a n c e d h e a t i n g .

V a p o r i z a t i o n o f t h e s a m p l e s d u r i n g t h e p l a s m a i n t e r a c t i o n t i m e T i c a n n o t e x p l a i n t h eo b s e r v e d r e m o v a l o f ~ = 0 . 1 - 0 . 5 m m . L e t u s c o n s i d e r t h e p r o c e s s e s i n t h e f i n a l s t a g e o f t h ei n t e r a c t i o n . T e m p e r a t u r e p r o f i l e s i n m e t a l s ( f o r w h i c h T ( 0 , T) = T b ) o f t h e f o r m

71 6


7/8

F ( z , x ) = F ~ I [ e x p - - ~2 a tJ0

( x i s t h e h e a t i n g t i m e a n d z i s t h e d i s t a n c e f r o m t h e s u r f a c e ) a r e s h o w n i n F i g . 5 . A s u b -s t a n t i a l p o r t i o n o f t h e m e l t i s h e a t e d a b o v e t h e b o i l i n g p o i n t T b o f o r a t m o s p h e r i c p r e s s u r e .W hen t h e p r e s s u r e d r o p s s u d d e n l y , i t i s s u p e r h e a t e d . A s a c o n s e q u e n c e , r a p i d v o l u m e v a p o r -i z a t i o n t a k e s p l a c e a n d p a r t o f t h e m e l t i s s p u t t e r e d w h i l e t h e r e m a i n d e r i s r a p i d l y s o l i d i -f l e d o n t h e c o l d b a s e . T h i s m e c h a n i sm e x p l a i n s t h e b r e a k i n g - u p a n d t h e p o r o s i t y o f th e o u t e rl a y e r . T he t h i c k n e s s e s o f t h e s u p e r h e a t e d m e l t a nd r e m o v e d m e t a l a r e a p p r o x i m a t e l y t h e s am ei n t h e e x p e r i m e n t s .

R e t a r d a t i o n o f t h e p r e s s u r e d r o p b y a n i n e r t s h e l l l e a d s t o r e m o v a l o f h e a t f r o m t h es u p e r h e a t e d m e l t i n w a r d a n d o u t w a r d i n t o t h e c o l d e r e x p l o s i o n p r o d u c t s . A t e m p e r a t u r e p r o -f i l e w i t h a p e a k i n s i d e t h e m e l t r e s u l t s , w h e r e b o i l i n g a l s o o c c u r s w h e n t h e p r e s s u r e d r o p s( F i g . 4 a ) . I f t h e p r e s s u r e d r o p s s u f f i c i e n t l y s l o w l y ( e x p e r L m e n t s u ~ L t h t h e a p p a r a t u s b u r i e di n c l a y ) , t h e m e l t s o l i d f i e s w i t h o u t p o r e s ; t h e m a r t e n s i t e u n d e r i t i s t e m p e r e d o w i n g t o t h el o n g c o n t a c t w i t h t h e e x p l o s i o n p r o d u c t s ( Fi g. 3 e) , A t l o w p r e s s u r e s t h e s u p e r h e a t i n g i ss m a l l a n d t h e f r a c t i o n o f t h e m e l t ( T > T m ) i n t h e h e a t i n g z o n e i s l e s s, s o t h a t t h e o u t e rl a y e r h a s n o p o r e s a n d i n t h i c k n e s s a p p r o a c h e s t h e m a r t e n s i t e l a y e r ( F i g . 3 d) . T h e w i d t h o ft h e z o n e f o r t e m p e r i n g f r o m t h e s o l i d s t a t e ( T > T > T t ) , a s ~ a l c u l a t e d f r o m t h e t e m p e r a t u r ep r o f i l e s u s i n g t h e m e a s u r e d v a l u e s o f T i a n d h a n d b o o k d a t a ( a, T t , T ) f o r s p e c i f i c s t e e l s ,i s p r a c t i c a l l y i n d e p e n d e n t o f t h e p r e s s u r e a n d c o i n c i d e s w i t h t h e o b s e r v e d w i d t h o f t h e m a r -t e n s i t e l a y e r .

T h u s , t h e s t r u c t u r e o f t h e s u r f a c e l a y e r i s p r i m a r i l y d e t e r m i n e d b y t h e t i m e s pe n t i nc o n t a c t w i t h t h e p l a s m a , t h e p r e s s u r e , a n d t h e w a y t h e p r e s s u r e d r o p s . T h e t h e r m a l d i f f u s i v -i t y a a l s o h a s a s i g n i f i c a n t e f f e c t o n t h e t h i c k n e s s o f t h e l a y e r a n d o n th e l o s s o f m e t a l .F o r j u s t t h i s r e a s o n a g r e a t a m o u n t o f m a t e r i a l i s r e m o v e d i n t h e m e t a l s t h a t c o n d u c t h e a tb e s t , c o p p e r a n d a l u m i n u m . T h e i r o n a l l o y s , e s p e c i a l l y t h e h i g h - c a r b o n a n d a l l o y e ds t e e l s , h a v e m u c h l o w e r t h e r m a l c o n d u c t i v i t i e s a n d t h e r e f o r e l o s e l e s s . T h e t h e r m a lc o n d u c t i v i t y o f s t e e l s f a l l s w h e n t h e y ar e t e m p e r e d , a n d t h i s a l s o a f f e c t s t h e a m o u n to f l o s s . I m p u r i t y i n c l u s i o n s s e r v e d as v a p o r n u c l e i a n d m a d e b o i l i n g o f t h e s u p e r -h e a t e d m e l t e as i e r . I n p r e h a r d e n e d s t e e l s t h e i m p u r i t i e s a r e d i s s o l v e d s o t h a t p o r o s i t y a n dl o s s a r e l e s s p r o n o u n c e d . B o i l i n g i s d i f f i c u l t i n c o p p e r t ha t h a s b e e n e l e c t i c a l ! y p u r i f i e dd u r i n g p r o d u c t i o n . P r o b a b l y f o r t h i s r e a s o n t h e r e a r e a l m o s t n o p o r e s , a n d t he a m o u n t o f m a -t e r i a l l o s t i s l e s s t h a n w i t h a l u m i n u m a n d i t s a l l o y s .

H e a t i n g t h e m e t a l u n d e r p r e s s u r e t o t e m p e r a t u r e s 2 - 3 t i m e s i t s b o i l i n g t e m p e r a t u r e s h a r p -l y i n c r e a s e d t h e d i f f u s i o n p r o c es s e s . T h e d i f f u s i o n c o e f f i c i e n t d e p e n d s e x p o n e n t i a l l y o n t h et e m p e r a t u r e D = D o e - E / k T . ~ E s t i m a t e s s h o w t h a t f o r c a r b o n a n d n i t r o g e n i n i r on , D i n c r e a s e sb y a f a c t o r o f i 0 ~ - i 0 5 w h e n t h e t e m p e r a t u r e i s c h a n g e d f r o m l 0 s t o 8 1 0 ~ ~ D e s p i t e t h es h o r t i n t e r a c t i o n t i m e, d i f f u s i o n p r o c e s s e s e x t e n d t o a d e p t h z ~ Y . D T ~ I 00 ~ m. S a t u r a t i o no f th e s u r f a c e l a y e r b y n i t r o g e n f r o m t h e p l a s m a a p p a r e n t l y e x p l a i n s t h e s l i g h t i n c r e a s e i nm a s s o f th e s t e e l s a m p l e s ( w h i c h m a s k s t h e r e m o v a l o f m e t a l a t h i g h p r e s s u r e s ) . T h e m a s s i n -c r e a s e c o r r e s p o n d s t o a n a v e r a g e c o n c e n t r a t i o n o f n i t r o g e n i n t he l a y e r o f 1 % , s u f f i c i e n t t oc a u s e t h e h i g h h a r d n e s s o f t h e l a y e r [ i i , 1 5 ] . T h e n i t r i d e A I N a n d t h e c a r b i d e S i C a r e o b -s e r v e d d u r i n g x - r a y a n a l y s i s o f t h e s i l u m in s . I n t h e e x p e r i m e n t s w i t h a r g o n , w h i c h i s p r a c -t i c a l l y i n s o l u b l e i n m e t a l s , t h e a l l o y i s s a t u r a t e d w i t h c a r b o n f r o m t h e e x p l o s i o n p r o d u c t s .T h i s f a v o r e d t h e f i x a t i o n o f a u s t e n i t e a n d t h e e q u a l ( d e s p i t e t h e d i f f e r e n t i n i t i a l c a r b o nc o n t e n t ) h a r d n e s s e s o f t h e l a y e r s in s t e e l [ 1 3 -1 5] . T h e e x p a n d i n g b o r d e r a r o u n d l a r g e i n -c l u s i o n s o f g r a p h i t e i n c a s t i r o n s a n d o f s i l i c o n i n s i l u m i n s h a s a d i f f u s i o n a l o r i g i n . M e a -s u r e m e n t o f t h e w i d t h o f t h i s b o r d e r c o u l d s e r v e a s t h e f o u n d a t i o n o f a n e w m e t h o d f o r f i n d -i n g t h e d i f f u s i o n c o e f f i c i e n t s u n d e r h i g h t e m p e r a t u r e a n d p r e s s u r e c o n d i ti o n s .

T h e f o r m a t i o n o f a d o u b l e m a r t e n s i t e l a y e r d ur i n g m u l t i p l e - s t e p p r o c e s s i n g i s m o s t e a s i -l y i n t e r p r e t e d a s t he r e s u l t o f s u p e r i m p o s i n g t h e t h e r m a l p r o f i l e s f r o m t h e s u c c e s s i v e e x -p l o s i o n s . B u t t r a n s p o r t o f d i s l o c a t i o n s a n d i m p u r i t i e s i n t h e s t r e s s z o n e a h e a d o f t h e h e a t -i n g w a v e c a n n o t b e e x c l u d e d .

7]_7


8/8

CON C],IIS l ONS1 . U n d e r t h e p re s s u re o f an ex p l o s i o n t h e s u r f ace l ay e r o f a m e t a ] i s h ea t ed h y t h e

p l a s m a t o c r i t i c a l t e m p e r a t u r e s , b u t v a p o r i z a t i o n o f t h e m e t a l ~ s l o w o w i n g t o s c r e e n i n g o ft h e s u r f ac e b y v ap o r . 2 . M os t o f t h e r em o v a l o f m e t a l o c cu r s a t t h e en d o f t h e i n t e rac t i on

o w i n g t o v ap o r i z a t i o n an d s p u t t e r i n g o f t h e a l l o y w h en t h e p re s s , r e d ro p s .LITERATURE C]TED

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Mechanism of Ablation of Metals