THEORY

AND I. M.

PRACTICE Fedorchenko

OF and

SINTERING V.

V.

Skorokhod

P o w d e r bodies a r e objects which a r e in a state of t h e r m o d y n a m i c nonequilibrium and have a l a r g e e x c e s s of f r e e e n e r g y - - the moving f o r c e of the s i n t e r i n g p r o c e s s . The e x c e s s of f r e e e n e r g y is a c o n s e quence of s e v e r a l f a c t o r s which a r e c h a r a c t e r i s t i c of bodies c o m p a c t e d f r o m p o w d e r s . Among the m o r e i m p o r t a n t of t h e s e f a c t o r s a r e the p r e s e n c e of a n e t w o r k of t w o - and t h r e e - d i m e n s i o n a l m a c r o d e f e e t s , i m p e r f e c t i n t e r p a r t i c l e c o n t a c t s and p o r e s , an extensive s y s t e m of g r a i n boundaries, c r y s t a l l i n e lattice m i c r o d i s t o r t i o n s , c o n c e n t r a t i o n heterogeneity, etc. The d i v e r s i t y of p o s s i b l e m a c r o - and m i c r o d e f e e t s in an u n s i n t e r e d p o r o u s body is r e s p o n s i b l e f o r the multiplicity of t h e i r "healing" m e c h a n i s m s . F r o m the viewpoint of sintering, the m o s t significant m o l e c u l a r - k i n e t i c p r o c e s s e s a r e c h e m i c a l r e a c t i o n s at i n t e r f a c e s and b o u n d a r i e s , s u r f a c e and vohune s e l f diffusion, and t h e r m a l l y a c t i v a t e d dislocation p r o c e s s e s . Although the full quantitative t h e o r y of s i n t e r i n g p r o c e s s e s is f a r f r o m complete, the qualitative c o n cepts have a l r e a d y b e e n e x h a u s t i v e l y f o r m u l a t e d . An i m p o r t a n t p a r t in t h e i r development has b e e n played by the r e s e a r c h e s of the Soviet s c i e n t i s t s M. Yu. Bal' shin, V. A. I v e n s e n , G. A. Meerson, B. Ya. Pines, I. M. F e d o r c h e n k o , Ya. L F r e n k e l ' , and o t h e r s , which have a l r e a d y b e e n a c c o r d e d adequate p r o m i n e n c e in l i t e r a t u r e r e v i e w s and m o n o g r a p h s [1, 2, 3]o In r e c e n t y e a r s , a l a r g e n u m b e r of i m p o r t a n t e x p e r i m e n t a l and t h e o r e t i c a l investigations o f s i n t e r i n g have b e e n c a r r i e d out, as a r e s u l t of which it is now e a s i e r to u n d e r s t a n d the m e c h a n i s m of s i n t e r i n g and to d e s c r i b e quantitatively the s i n t e r i n g phenomenon. Substantial advances in this field have b e e n m a d e in the evolution of the phenomenological d e s c r i p t i o n of the s i n t e r i n g p r o c e s s as a d i f f u s i o n - v i s c o u s flow and in the study of the role played by s u r f a c e phenom.ena in the s i n t e r i n g p r o c e s s and of the t y p e s of c r y s t a l l i n e s t r u c t u r e defects in m e t a l powders, as well as of t h e i r r o l e in the s i n t e r i n g p r o c e s s . The f u l l e r understanding of the m e c h a n i s m of the sintering p r o c e s s has m a d e p o s s i b l e m a j o r a d v a n c e s in investigations aiming at the development of the t h e o r y and p r a c t i c a l methods of a c t i v a t e d sintering. Phenomenologieal

Description

of the

Sintering

Process

Following the investigations by F r e n k e l ' [4], who laid the foundations of the phenomenological d e s c r i p t i o n of the s i n t e r i n g p r o c e s s as a g e n e r a l i z e d v i s c o u s flow, f u r t h e r s t e p s in this field w e r e t a k e n by Mackenzie [5] and Skorokhod [6]. The l a t t e r - - unlike F r e n k e l ' , who studied the p r o b l e m of filling of a p o r e c o n s i d e r e d on its own - - e x a m i n e d the densification p r o c e s s of the p o r o u s body a s a whole, which was m a d e p o s s i b l e by the introduction of a second (volume) v i s c o s i t y coefficient of the p o r o u s c o m p r e s s i b l e body. This a p p r o a c h was f i r s t adopted by Mackenzie f o r a body with u n i f o r m l y d i s t r i b u t e d s p h e r i c a l p o r e s and then by Skorokhod [6] f o r a m o r e g e n e r a l model of the p o r o u s body, namely, a s t a t i s t i c a l m i x t u r e of m a t e r i a l and voids. The c o r r e s p o n d i n g differential densification equations obtained by these two authors a r e [5, 6]:

a)

dQ

3~ (1 - - Q)

--~-

2~1or

(1) ,

w h e r e p is the r e l a t i v e density, ~ the s u r f a c e tension, ~ 0 the v i s c o s i t y coefficient of the m a t e r i a l , and r the m e a n p o r e radius; dO

b) ~

3

-- 2

~0 ( 3 - - ~)(1 - - 0)~

(1 - - 2(0 ~lo'ro

'

(2)

w h e r e r 0 is the m e a n p o r e radius, taken to be equal to the m e a n p a r t i c l e radius, and 0 the r e l a t i v e p o r o s i t y . Institute of M a t e r i a l s Science, A c a d e m y of Sciences of the UkrSSR. T r a n s l a t e d f r o m P o r o s h k o v a y a Metallurgiya, No. 10(58), pp. 29-50, October, 1967.

790

Equation (2) is more accurate in describing the densification of porous bodies with porosities ranging between 0.i and 0.5. Mackenzie's model and Eq. (1) are more applicable to bodies of low porosities and to the late stages of sintering. The phenomenologieal approach exhaustively describes the sintering of amorphous bodies. For porous bodies compacted from metal powders, the phenomenologieal theory of sintering is capable merely of taking into account the so-called "geometrical" factor, which controls the rate densifieation, and to reduce the problem of sintering kinetics (in particular, densification kinetics) to a study of the dependence of the viscosity coefficient 40 on temperature, acting stresses, and time, i.e., the specific problems in metalphysics. A somewhat different direction in the phenomenologieal study of sintering, linked with a search for kinetic equations satisfactorily describing the sintering process, was taken earlier by Bal'shin and Ivensen [2]. Investigation

of the

Sintering

Mechanism

At present it may be taken as established that the densification of a body undergoing sintering results from both diffusion-viscous flow (diffusion creep) and the usual processes of volume diffusion. The difference between the mechanisms of pure diffusion and diffusion-viscous flows is that, in the former case, there exist macroscopical vacancy streams which emerge on to the outer surface of the porous body, whereas in the latter case there are only local vacancy streams between substructure elements. These streams begin and end on internal surfaces, where vacancies may be formed and annihilated. This difference was pointed out by Alexander, Kuczynski, and Dawson [7]. It is now known, as a result of work by Geguzin and Lifshits [8], under what conditions either porevolume contraction mechanism predominates in a real crystalline body. The diffusion mechanism is predominant when r < L, i.e., when the pore radius is smaller than the linear size of coherent regions. Thus, the diffusion mechanism plays an important part at the late stages of sintering, and is particularly conspicuous during the annealing of single crystals in which, in one way or other, porosity has been established [9]. In the majority of cases encountered in practice, however, densification during sintering is effected by diffusion-viscous flow. The t h e o r y of the d i f f u s i o n - v i s c o u s flow of nonideal c r y s t a l s was developed by N a b a r r o [10], H e r r i n g [11], and, in the m o s t s y s t e m a t i c f o r m , L i f s h i t s [12]. Fundamentally, this theory s t a t e s that the v i s c o s i t y of a r e a l c r y s t a l l i n e body is a function of the l e a s t l i n e a r dimension of c r y s t a l s u b s t r u c t u r e e l e m e n t s , i~ grains or mosaic blocks: 1 ~1

D6 a kTL ~ '

w h e r e ~ is the coefficient of v i s c o s i t y , D the coefficient of self-diffusion, 6 the atomic d i a m e t e r , and L the l i n e a r dimension of the region in which the lattice is c o h e r e n t . Conclusions of c o n s i d e r a b l e i m p o r t a n c e to the p r a c t i c e of s i n t e r i n g follow f r o m this theory. Since s h r i n k a g e during s i n t e r i n g (i.e., t h e i n t e n s i t y of the s i n t e r i n g p r o c e s s ) i n c r e a s e s with d e c r e a s i n g v i s c o s i t y , then to activate the s i n t e r i n g p r o c e s s it is n e c e s s a r y to a i m at high values of the coefficient of diffusion and at low v a l u e s of the l i n e a r s i z e s of the regions in which the c r y s t a l lattice is coherent. The f u l l e r understanding of the s i n t e r i n g m e c h a n i s m has given r i s e in r e c e n t y e a r s to an expansion of r e s e a r c h into m e t h o d s of a c t i v a t e d sintering. Investigations with the a i m of activating the s i n t e r i n g p r o c e s s a r e being conducted in s e v e r a l d i r e c t i o n s . Activation can be attained by e x e r t i n g an additional c h e m ical o r p h y s i c a l influence on the b a s i c p r o c e s s e s d e t e r m i n i n g the k i n e t i c s of sintering. Such an influence m a y be e x t e r n a l r e l a t i v e to the body undergoing sintering (sintering in special g a s eous m e d i a r e s p o n s i b l e f o r c e r t a i n c h e m i c a l r e a c t i o n s ; application of p r e s s u r e ; s i n t e r i n g in an u l t r a s o n i c o r an a l t e r n a t i n g e l e c t r o m a g n e t i c field; cyclic v a r i a t i o n of t e m p e r a t u r e o r the g a s e o u s m e d i u m , etc.) o r i n t e r n a l ( e s t a b l i s h m e n t of a defective c r y s t a l l i n e s t r u c t u r e ) . F o r convenience, t h e s e methods of activated s i n t e r i n g m a y be divided into t h r e e m a i n g r o u p s : 1) activation of s i n t e r i n g resulting f r o m an intensification of p r o c e s s e s of bulk flow of m a t e r i a l ; 2) activation produced by v a r i o u s additions influencing the p r o c e s s e s taking p l a c e in the s u r f a c e l a y e r s of p a r t i c l e s during s o l i d - p h a s e sintering; 3) activation produced by additions f o r m i n g a liquid p h a s e during sintering. L e t us now c o n s i d e r t h e s e t h r e e c a s e s .

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Material

Flow

Processes

The volume of flow p r o c e s s e s taking place during sintering may be activated in two ways, namely, (1) by i n c r e a s i n g the coefficient of volume self-diffusion, or, m o r e p r e c i s e l y , the coefficient of volume diffusion of vacancies o r (2) by substantially d e c r e a s i n g the c h a r a c t e r i s t i c l i n e a r dimension of coherent regions, i.e., in fact, inc r e a s i n g the density of dislocations forming the mosaic block walls.

i "~

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S~176

of Bulk

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One of the "natural" ways of intensifying the densification p r o c e s s during sintering involves the use of active powders, which are produced under noneqnilibrium conditions, by e l e c t r o l y s i s , d i s s o c i a tion o f carbonyls, l o w - t e m p e r a t u r e reduction of oxides, dissociation of metal f o r m a t e s and oxalates, etc. It has been d e m o n s t r a t e d by V. V. Skorokhod and A. F. Khrienko that powders p r e p a r e d by different methods are c h a r a c t e r i z e d b y different block sizes and d e g r e e s of c r y s t a l l i n e lattice distortions; consequently, during heating,the kinetics of block growth and r e m o v a l of c r y s t a l l i n e lattice d i s t o r tions is different f o r each method of powder p r e p a r a t i o n . In turn, this c i r c u m s t a n c e affects the c o u r s e of the sintering p r o c e s s .

~,,j-~. 300 400 500

Annealing temp.,~ Fig. 1. Kinetics of block growth (D) and r e m o v a l of c r y s t a l l i n e distortions (Aa/a) for nickel powd e r s p r e p a r e d by different methods [19]: 1) e l e c t r o l y t i c nickel; 2) r e duced nickel; 3) carbonyl nickel.

F i g u r e i shows, a f t e r Skorokhod and Khrienko, the t e m p e r a t u r e dependence of the block size D and c r y s t a l l i n e lattice d i s t o r t i o n s Aa/a for various - - carbonyl, reduced, and e l e c t r o l y t i c - - nickel powders. The data w e r e obtained by the x - r a y diffraction method with the aid of a URS-50I x - r a y apparatus, by p e r f o r m ing a harmonic analysis of i n t e r f e r e n c e line p r o f i l e s . The nickel powders in the f o r m of compacts w e r e annealed in hydrogen f o r 1 h at various t e m p e r a t u r e s . As can be seen f r o m this figure, with r i s e in annealing t e m p e r a t u r e , the distortions steadily d e c r e a s e and the block size i n c r e a s e s . E l e c t r o l y s i s nickel has the g r e a t e s t c r y s t a l l i n e lattice distortions, which cannot be eliminated even by annealing at 550~ In the p u r e r carbonyl and reduced nickel powders, the distortions c o m p l e t e l y d i s a p p e a r already at a t e m p e r a t u r e of 500~ The beginning of p r e - r e c r y s t a l l i z a t i o n block growth f o r e l e c t r o l y t i c , reduced, and carbonyl nickel powders o c c u r s at 500-550, 450-500, and about 400~ r e s p e c t i v e l y . This difference in the kinetics of block growth and r e m o v a l of c r y s t a l l i n e lattice dist o r t i o n s is r e f l e c t e d in the b e h a v i o r of compacts f r o m these powders during sintering. F i g u r e s 2 and 3 show the shrinkage kinetics of compacts f r o m carbonyl and e l e c t r o l y t i c nickel powd e r s during sintering [13]. The densification of carbonyl nickel compacts is almost one o r d e r of magnitude g r e a t e r than that of compacts f r o m e l e c t r o l y t i c powder. The growth of i n t e r p a r t i c l e contacts, d e t e r m i n e d by the e l e c t r i c a l r e s i s t i v i t y method, also p r o c e e d s much m o r e intensely in the case of carbonyl nickel. Similar r e s u l t s w e r e also obtained by Minaev and Kolerov [14] in t h e i r investigation into the influence of the reduction t e m p e r a t u r e of nickel powder o v e r the range 350-550~ on block size and the d e g r e e of c r y s t a l l i n e lattice distortion. It was found that powders obtained at a l o w e r t e m p e r a t u r e w e r e c h a r a c t e r i z e d

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a3 ~[i 9

0

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I

I

40

80

i

I

k?O 150 200 240 440 430 C.min

Fig. 2

..4-.*J 40 80 120 ~60

200240 ~4gCmin

Fig. 3

Fig. 2. Kinetics of l i n e a r shrinkage of compacts f r o m e l e c t r o l y t i c nickel. T e m p e r a t u r e , ~ 1)400; 2)500; 3)600; 4 ) 7 0 0 . Fig. 3. Kinetics of l i n e a r shrinkage of compacts f r o m carbonyl nickel. Temperature, ~ 1) 200; 2) 400; 3) 500; 4) 600; 5) 700.

792

~vV#. ~5

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4 5

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iod~

Fig. 4. T e m p e r a t u r e dependence of s h r i n k age of c o m p a c t s f r o m i r o n p o w d e r with o r without additions of nickel o r nickel c o m pounds: 1) i r o n without addition; additions: 2) 0.8% nickel powder; 3) 1.6% nickel p o w der; 4) 1.6% Ni as Ni203; 5) 1.6% Ni as Ni (NO3)2 with reduction annealing; 6) 0.8% Ni as NiC204; 7) 1.6% Ni as NiC204.

by s m a l l e r block s i z e s and g r e a t e r values of c r y s t a l l i n e l a t tice d i s t o r t i o n s , dislocation density, and specific s u r f a c e . Such p o w d e r s exhibited a h i g h e r activity during s i n t e r i n g and a g r e a t e r r e s i s t a n c e to p l a s t i c d e f o r m a t i o n during the p r e s s i n g of c o m p a c t s . In the s a m e investigation it was o b s e r v e d that m o r e active p o w d e r s , obtained at l o w e r t e m p e r a tures, gave higher d e n s i t i e s a f t e r s i n t e r i n g . The active b e h a v i o r of fine m e t a l p o w d e r s is due to the f a c t that they contain a l a r g e quantity of nonequilibrium point and line defects [13, 15], as well as special stacking faults. Nonequilibrium v a c a n c i e s a r e not capable of e x e r t ing a prolonged influence on densification r a t e , b e c a u s e t h e i r r e l a x a t i o n t i m e i s l e s s than 10 -2 sec [16]. However, in the p r e s e n c e of a continuous s o u r c e of e x c e s s v a c a n c i e s , d e n s i ficatien p r o c e s s e s during s i n t e r i n g undergo a m a r k e d a c tivation. As was d e m o n s t r a t e d by Skorokhod [17], dislocations surrounding stacking faults m a y p r o v i d e such v a c a n c y s o u r c e s in active p o w d e r s . The high initial dislocation density in active m e t a l p o w d e r s , which amounts to, a c c o r d i n g to x - r a y diffraction data, 101~ ll c m -2 [18, 19], is also a powerful f a c t o r capable of v i g o r o u s l y intensifying the e a r l y s t a g e s of the densification p r o c e s s ,

An activation of s i n t e r i n g due to a high initial density of dislocations m a y be achieved by subjecting the powder to w o r k - h a r d e n i n g , f o r instance, by grinding in a ball m i l l . Such a powder can only be used, however, in molding p r o c e s s e s o t h e r than conventional compaction. G e n e r a l l y speaking, activation of the densification p r o c e s s as a r e s u l t of d i s t o r t i o n s in the s t a r t i n g c r y s t a l l i n e s t r u c t u r e is only effective at e a r l y s t a g e s of sintering b e c a u s e of the t h e r m a l instability and rapid exhaustion of i m p e r f e c t i o n s , p a r t i c u l a r l y at high sintering t e m peratures. Aksenov and K r y u k o v [20] m a d e an a t t e m p t to evaluate the influence of the internal e n e r g y r e s e r v e on the s h r i n k a g e of c o p p e r powder c o m p a c t s during sintering. Changes in the e n e r g y state of s t a r t i n g p o w d e r s w e r e s e c u r e d by employing different oxide reduction t e m p e r a t u r e s (300 and 600~ and using p o w d e r s with different p a r t i c l e s i z e s ( - - 0.090 + 0.075, - - 0.075 + 0~ - - 0.040). The e x p e r i m e n t a l r e s u l t s obtained d e m o n s t r a t e d that higher absolute shrinkage values a r e attained for s p e c i m e n s c o m p a c t e d f r o m p o w d e r s p r e p a r e d by reduction at l o w e r t e m p e r a t u r e s and f r o m p o w d e r s of f i n e r f r a c t i o n s . The above authors b a s e t h e i r r e a s o n i n g on the concept that the e n e r g y expended on the f o r m a t i o n of the m e t a l l i c contact m a y be r e p r e s e n t e d as a function of the s u r f a c e e n e r g y of the powder, the e n e r g y of c r y s t a l l i n e lattice d i s t o r t i o n s and defects, the e n e r g y of d e f o r m a t i o n built up during p r e s s i n g , and the t h e r m a l e n e r g y supplied e x t e r n a l l y during sintering. Using methods d e s c r i b e d in the l i t e r a t u r e , the authors calculated the e n e r g y of b l o c k b o u n d a r i e s and s e c o n d - t y p e distortions, as well as the s u r f a c e e n e r g y of p a r ticles, added t o g e t h e r t h e s e e n e r g i e s , and c o m p a r e d the values obtained with the extent of s h r i n k a g e of c o m p a c t s during s i n t e r i n g and its m a x i m u m r a t e . It follows f r o m t h e i r data that the absolute magnitude of s h r i n k a g e is p r o p o r t i o n a l to the value r e f e r r e d to by the authors as the total internal e n e r g y of the powder. A t t e m p t s to evaluate the activity of p o w d e r s f r o m the r e s e r v e of internal e x c e s s e n e r g y a r e useful. In a s t r i c t solution of this p r o b l e m , however, c o n s i d e r a b l e difficulties m u s t be encountered, b e c a u s e the e n e r g y expended on the f o r m a t i o n of the m e t a l l i c contact is not a s i m p l e sum of the v a r i o u s t y p e s of e n e r g y a c c u m u l a t e d in p o w d e r p a r t i c l e s . In principle, a l o s s of f r e e e n e r g y should be equal to the w o r k of d i s s i p a tive f o r c e s , but, f r o m the viewpoint of densifieation, the "efficiency" of this w o r k m a y v a r y . Thus, f o r i n stance, when the extent of reduction during p r e s s i n g is i n c r e a s e d , the value of i n t e r n a l energy will also inc r e a s e ; however, with i n c r e a s i n g c o m p a c t density, shrinkage during s i n t e r i n g will no l o n g e r i n c r e a s e , but instead will d e c r e a s e . In view of this, f u r t h e r study will be n e c e s s a r y b e f o r e it b e c o m e s p o s s i b l e to e v a l uate the activity of p o w d e r s f r o m t h e i r f r e e e n e r g y r e s e r v e .

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Fig. 5. T e m p e r a t u r e dependence of s h r i n k a g e of c o m p a c t s f r o m i r o n powder with o r without additions of cobalt o r cobalt compounds: 1) i r o n without addition; additions: 2) 1.6% cobalt powder; 3) 1.6% Co as Co203; 4) 1.6% Co as Co(NO3) 2 with reduction annealing; 5) 2% i r o n as Fe203. Fig. 6. T e m p e r a t u r e dependence of shrinkage of c o m p a c t s f r o m i r o n p o w d e r with o r without additions of m a n g a n e s e o r m a n g a n e s e compounds: 1) i r o n without addition; additions: 2) 2% i r o n as Fe203; 3) 1.55% Mn powder; 4) 3.1% Mn powder; 5) 3.1% 1Vin as MnO; 6) 3.10% Mn as Mn(NO3) 2 with reduction annealing; 7) 3.1% Mn as f e r r o m a n g a n e s e . Activation

of Surface

Material

Transport

Processes

In a n u m b e r of investigations, F e d o r c h e n k o has noted the i m p o r t a n t p a r t played by s u r f a c e s e l f - d i f f u sion p r o c e s s e s during sintering, p a r t i c u l a r l y in its initial stages [21-23]. Geguzin has a t t e m p t e d to a s s e s s the r e l a t i v e p a r t played by s u r f a c e p r o c e s s e s with the aid of the nondimensional n u m b e r s ~/ and ~/' f o r the c a s e s of s u r f a c e s e l f - d i f f u s i o n and t r a n s p o r t through the g a s e o u s p h a s e [24]. When 7 o r 7' is g r e a t e r than unity, the r o l e of s u r f a c e self-diffusion o r t r a n s p o r t through the g a s e o u s p h a s e b e c o m e s d e c i s i v e . Thus, to intensify the s u r f a c e t r a n s p o r t of m a t e r i a l during effective values of the coefficient of s u r f a c e self-diffusion or the be achieved e i t h e r by employing a cyclic h e t e r o g e n e o u s r e a c t i o n active g a s e o u s r e a g e n t f o r m i n g volatile compounds (for instance,

sintering, it is n e c e s s a r y to i n c r e a s e the v a p o r p r e s s u r e of the m e t a l . This m a y (oxidation-reduction) o r by introducing an halides) with the m e t a l .

In the f o r m e r c a s e , a m e t a l l a y e r with a nonequilibrium, d i s t o r t e d c r y s t a l l i n e lattice is e s t a b l i s h e d on the p o r e s u r f a c e . Surface self-diffusion is effected not only by a m i g r a t i o n of a t o m s on the s u r f a c e alone, but also e m b r a c e s , in the c a s e of r e a l c r y s t a l s , diffusion in a thin d i s t u r b e d l a y e r n e a r the s u r f a c e [24]. F o r bodies in a s t a t e of n e a r - e q u i l i b r i u m , the t h i c k n e s s of this l a y e r is equivalent to a few atomic spacings (~ 10 -7 cm), but cyclic oxidation and reduction m a y i n c r e a s e the t h i c k n e s s of the loosened up l a y e r to 10 -2 am or more. Thus, the effective s u r f a c e diffusion flux m a y be i n c r e a s e d by two o r t h r e e o r d e r s of magnitude, which g r e a t l y intensifies the p r o c e s s of f o r m a t i o n and growth of i n t e r p a r t i c l e m e t a l l i c contacts, p o r e s p h e r o i d i z a tion, and o t h e r p r o c e s s e s beneficial f r o m the point of view of s e c u r i n g high final p r o p e r t i e s . F o r this r e a son, the p r e s e n c e on the p a r t i c l e s u r f a c e of oxide f i l m s undergoing reduction a c t i v a t e s the sintering p r o c e s s , as c o n f i r m e d , f o r instance, by the r e s u l t s quoted in [25]. When a halide i s added to the gaseous m e d i u m , the halogen f o r m s a volatile compound with the m e t a l being s i n t e r e d . The p r e s e n c e of such compounds r a i s e s the effective v a p o r p r e s s u r e of the m e t a l by s e v e r a l o r d e r s of magnitude and substantially i n c r e a s e s the role of t r a n s p o r t through the g a s e o u s p h a s e . A m a r k e d influence on the sintering p r o c e s s m a y be e x e r t e d by such nonequilibrium defects as, f o r instance, e x c e s s v a c a n c i e s , which a r e m o r e o r l e s s continuously g e n e r a t e d during h i g h - t e m p e r a t u r e s i n t e r ing. L a r g e quantities of e x c e s s v a c a n c i e s a r e f o r m e d during heterodiffusion as a r e s u l t of a d i f f e r e n c e of

794

5~

. . . .

45

35

25 ]5

Addition~ayer - -

505

I5

25

35

~5

55

55 75 85 95

1~5 lisp

Iron pla~

Fig. 7. Distribution of nickel content in layer of addition material and in porous iron plate after sintering for 2 h at 1200~ Loose powder layer of: i) earbonyl nickel; 2) Ni203; 3) NiC204. p a r t i a l diffusion coefficients [26, gives r i s e to intensive f o r m a t i o n sional growth of c o m p a c t s during capable of s t r o n g l y activating the

27]. With l a r g e amounts of mutually diffusing components, this difference of diffusion p o r o s i t y , inhibition of shrinkage, and s o m e t i m e s even d i m e n s i n t e r i n g [28, 29]. However, s m a l l quantities of c e r t a i n additions a r e densification p r o c e s s .

At the p r e s e n t time, t h e r e a r e two distinct views on the r e q u i r e m e n t s to be s a t i s f i e d by a m e t a l l i c addition intended f o r the activation of s h r i n k a g e during sintering. One of the a u t h o r s with a c o - w o r k e r [30] o b s e r v e d that activation of densification o c c u r s in the c a s e of p r e d o m i n a n t diffusion of addition m e t a l a t o m s into the m a i n m e t a l . F o r instance, a nickel l a y e r d e p o s i t e d on iron powder s t r o n g l y i n c r e a s e d shrinkage. Vacek [31, 32] and E u d i e r [33], on the other hand, a r e of the opinion that the addition should be v i r t u ally insoluble in the m a i n metal, while strongly dissolving it. In such a case, the p r e d o m i n a n t diffusion flux will be d i r e c t e d f r o m the m a i n m e t a l into the addition m e t a l . The s y s t e m s W - - Ni and Fe - - A u comply with this rule. F o r t h e s e s y t e m s , the s i n t e r i n g t e m p e r a t u r e is l o w e r e d by hundreds of d e g r e e s . The m e c h a n i s m of influence of s m a l l additions which a r e s p a r i n g l y soluble in the m a i n m e t a l while v i g o r o u s l y dissolving it has not yet been elucidated. Some authors [34] c o n s i d e r that nickel envelops tungsten p a r t i c l e s and the sintering p r o c e s s in the s a m e way as the d i s s o l u t i o n - p r e c i p i t a t i o n p r o c e s s ; tungsten in this c a s e diffuses through a a n i c k e l - b a s e solid solution. Another view [35] is that, during the diffusion of tungsten into nickel nickel into tungsten and along its s u r f a c e , specific point defects, p o s s i b l y e x c e s s v a c a n c i e s , f o r m tungsten and a d s o r b the d i s s o l v e d nickel a t o m s .

is effected l a y e r of and of in the

Paniehkina [36] c a r r i e d out an investigation into the mutual diffusion of tungsten and nickel during the s i n t e r i n g of tungsten c o m p a c t s to which nickel had been added in the f o r m of powder p a r t i c l e s o r as a s u r face f i l m obtained by reducing an applied l a y e r of nickel oxides. L o c a l a n a l y s i s of these constituents by m e a n s of an e l e c t r o n m i c r o p r o b e d i s c l o s e d that the n i c k e l - b a s e p h a s e was a s a t u r a t e d solid solution of tungsten in nickel and that no p e n e t r a t i o n of nickel into tungsten had taken place. It was also e s t a b l i s h e d that, i r r e s p e c t i v e of the method of supply of nickel (in the f o r m of p o w d e r p a r t i c l e s o r a thin l a y e r ) to the p a r t i c l e s u r f a c e , a thin f i l m of a solid solution of tungsten in nickel extended along the tungsten grain bounda r i e s ; it w a s thus d i s c o v e r e d that nickel v i g o r o u s l y diffuses along the tungsten p a r t i c l e s u r f a c e , while tungsten equally v i g o r o u s l y d i s s o l v e s in the nickel. It was concluded that the activation of the s i n t e r i n g p r o c e s s in this c a s e is due to i n c r e a s e d d e f e c t i v e n e s s of the s t r u c t u r e in the s u r f a c e l a y e r s of tungsten, resulting f r o m the diffusion of the l a t t e r into the nickel film.

795

t

%

50

1oo/o

Addition layer

3o

5u

70 9u #

Iron plate

Fig. 8. Distribution of cobalt content in addition l a y e r and in p o r ous i r o n plate a f t e r sintering f o r 2 h at 1200~ Loose powder layer of: 1) m e t a l l i c cobalt; 2) Co203. F e d o r c h e n k o and Ivanova m a d e an attempt to d e t e r m i n e , on the e x a m p l e of sintering of i r o n powder c o m p a c t s , the influence e x e r t e d by the n a t u r e of addition m e t a l and type of c h e m i c a l compound on the s h r i n k age k i n e t i c s and p h y s i e o m e c h a n i c a l p r o p e r t i e s of s i n t e r e d c o m p a c t s [37]. The additions employed w e r e p o w d e r s of nickel, cobalt, iron, and m a n g a n e s e , as well as t h e i r oxides and c e r t a i n orgardc compounds (oxa l a t e s and formates)o The oxides w e r e introduced e i t h e r by m e c h a n i c a l l y mixing the p o w d e r s o r by applying an oxide f i l m to the s u r f a c e of p a r t i c l e s of the m a i n m e t a l . F o r this p u r p o s e , a solution of nickel, c o balt, o r m a n g a n e s e n i t r a t e w a s added to i r o n powder, and the r e s u l t i n g m i x t u r e was d r i e d at a t e m p e r a t u r e of 80-100~ Next, the oxides w e r e reduced by annealing the p o w d e r s in hydrogen f o r 2 h at 800~ The r e s u l t s of this investigation d e m o n s t r a t e d that the magnitude of shrinkage depends to a l a r g e e x tent on the type of m e t a l contained in the addition. The g r e a t e s t i n c r e a s e of shrinkage is s e c u r e d b y n i c k e l compounds (Fig. 4). In t h e i r p r e s e n c e , shrinkage ranges, depending on the type of compound, f r o m 4 to 8% a f t e r s i n t e r i n g at 1000~ and f r o m 9 to 15.5% a f t e r sintering at 1200~ A s m a l l e r effect is produced by cobalt (Fig. 5), whose compounds give a shrinkage of the o r d e r of 1.0-5.0% at 1000~ and 3-11% at 1200~ In the p r e s e n c e of m a n g a n e s e (Fig. 6), shrinkage is as a rule s m a l l e r than the s h r i n k a g e of p u r e iron. The s h r i n k a g e values r e c o r d e d w e r e 1.5-4.5% a f t e r sintering at 1000~ and 5.5-11.5% at 1200~ In Fig. 5, the s h r i n k a g e c u r v e f o r f e r r i c oxide additions shows that, at sintering t e m p e r a t u r e s up to and including 1100~ this compound e x e r t s no activating influence. I n c r e a s i n g the amount of m e t a l added does not significantly affect the extent of shrinkage. Thus, doubling the amount of nickel (from 0.8 to 1.6%) i n c r e a s e d shrinkage by only 0.5-2 abs.%~ In the c a s e of m a n g a n e s e , additions of 1.55 and 3.1% p r o d u c e d p r a c t i c a l l y the s a m e shrinkage~ To d e t e r m i n e the dominant d i r e c t i o n of diffusion fluxes, a l a y e r - b y - l a y e r s p e c t r a l a n a l y s i s was p e r f o r m e d on s i n t e r e d s p e c i m e n s consisting of a p o r o u s i r o n plate with a l a y e r , applied by loose pouring, of p o w d e r s of nickel, cobalt, o r t h e i r compounds. The r e s u l t s of this a n a l y s i s showed that, in the c a s e of nickel, the diffusion of nickel a t o m s f r o m the loose powder l a y e r containing nickel into the p o r o u s i r o n p l a t e w a s p r e d o m i n a n t (Fig. 7). Nickel was found to p e n e t r a t e into the plate to a depth of up to 80-120 ~, w h e r e a s i r o n e n t e r e d into the nickel powder l a y e r to a depth of only 20-40 # (curves 1-3). In the e a s e of cobalt (Fig. 8), the diffusion of i r o n a t o m s f r o m the i r o n plate into the cobalt p o w d e r l a y e r was p r e d o m i n a n t . The depth of p e n e t r a t i o n of iron a t o m s into the cobalt p o w d e r l a y e r was m o r e than 200 # and that of cobalt a t o m s into the i r o n plate only 100 p. The r a t e of diffusion of the addition m e t a l into the p o r o u s i r o n plate was found to depend on the type of addition. The highest r a t e of diffusion w a s exhibited by nickel introduced in the f o r m of nickel oxalate.

796

TABLE 1. C h e m i c a l Compositions and Melting Points of Alloys Alloy

Melting point, *C

Chemical comp., %

Mn--Si Cu--Si Si--Ca*

1050 980 1080 800 1050

Fe--89,3 P--10,5 Ni--56 Si--41 Mn--89 Si--10 Cu--80,02 Si--16 Fe--2,2 Si--54,5

Mn--Ni Fe--S Al--bin Cu--AI Fe--Sb

1020 990 880 860 1002

Fe--P Ni--Si

Ca--33,3 Mn--64.2 Fe--70 A1--62 Cu--76,5 Fe--50

A1--1,8 Ni--35.2 S--29 Mn--38,8 A1--23.4 Sb---50

In this case, nickel p e n e t r a t e d during s i n t e r i n g f r o m the toose p o w d e r l a y e r into the backing plate m e t a l to a depth of up to 120 #. During the s a m e sintering time, the depth of p e n e t r a tion of nickel was 100 # in the c a s e of nickel oxide and 80 # in the c a s e of c a r b o n y l nickel powder. When cobalt additions w e r e employed, no m a r k e d d i f f e r e n c e s in diffusion r a t e s w e r e o b s e r v e d f o r m e t a l l i c cobalt powder and cobalt oxide, although in the l a t t e r c a s e the activity of diffusion p r o c e s s e s was s o m e what l e s s e n e d .

E x p e r i m e n t s with the addition of the s a m e m e t a l in the p u r e f o r m o r in the f o r m of v a r i o u s compounds show that the g r e a t e s t activating effect is achieved when heating g i v e s r i s e , as a r e s u l t of d e c o m p o s i t i o n o r reduction, to the a p p e a r a n c e of active a t o m s of the addition metal, which have a high diffusion * The Si - - Ca alloy w a s c o n t a m i n a t e d mobility. Intensified s h r i n k a g e o r strengthening is o b s e r v e d with i r o n and a l u m i n u m . as a r e s u l t of sintering at t e m p e r a t u r e s which enable reduction p r o c e s s e s to p r o c e e d v i g o r o u s l y . The use of additions in the f o r m of p o w d e r s of the m e t a l s t h e m s e l v e s o r of p r e a n n e a l e d p o w d e r s p r o d u c e s a negligible activating effect on the s i n t e r i n g p r o c e s s . This is p r e s u m a b l y due to the high r a t e s of diffusion and the l a r g e n u m b e r s of active a t o m s in the c a s e of e m p l o y m e n t of oxides and o x a l a t e s . A c o n f i r m a t i o n of this explanation is p r o vided by r e s u l t s of l a y e r - b y - l a y e r s p e c t r a l a n a l y s i s of t w o - l a y e r specimens~ In the c a s e of nickel oxalates and oxides, the depth of diffusion of nickel a t o m s into an i r o n backing plate is m u c h g r e a t e r than f o r c a r bonyl nickel~ When the p a r t i a l coefficients of diffusion a r e equal, as is the c a s e when iron oxides a r e introduced into i r o n powder, the activating effect is p r a c t i c a l l y absent. On the b a s i s of t h e s e data, it is p o s s i b l e to f o r m u l a t e s o m e g e n e r a l laws governing the activation p r o c e s s in s o l i d - p h a s e sintering.

The d e g r e e of influence of activating additions on the s o l i d - p h a s e sintering p r o c e s s is d e t e r m i n e d by a n u m b e r of f a c t o r s , which m a y act in opposite d i r e c t i o n s and affect differently the shrinkage, p h y s i c o m e c h a n i c a l , and o t h e r p r o p e r t i e s of c o m p a c t s undergoing sintering. A decisive influence is e x e r t e d by the mobility of the s u r f a c e a t o m s , which p r o m o t e s shrinkage and i n c r e a s e s the s t r e n g t h of the m a t e r i a l . F o r this reason, all f a c t o r s l o w e r i n g the activity of the s u r f a c e a t o m s m a y be expected to affect unfavorably the s i n t e r i n g p r o c e s s and to hinder the attainment of the m a x i m u m level of p r o p e r t i e s . Mutual alloying of the s u r f a c e l a y e r s of p a r t i c l e s m a y have a beneficial effect on the s t r e n g t h of the m a t e r i a l being s i n t e r e d , but, on the o t h e r hand, m a y reduce s h r i n k a g e by strengthening contacts; in view of this, the decision r e g a r d i n g alloying m u s t be taken in the light of the actual r e q u i r e m e n t s to be satisfied by the m a t e r i a l being s i n t e r e d . The inhibiting influence of heterodiffusion on s h r i n k a g e m a y also be a consequence of the fact that heterodiffusion helps to e l i m i n a t e d i s t o r t i o n s of the c r y s t a l l i n e lattice of active p o w d e r s [29], t h e r e b y d e c r e a s i n g the activity of the l a t t e r . Of considerable importance, too, is the predominant direction of diffusion fluxes. When the partial coefficients of diffusion of the main and addition metals are unequal and there is a predominant flow of atoms from the addition metal into the main metal, the shrinkage process may be expected to be intensified, because the resulting diffusion porosity in a thin surface layer of the addition metal will tend to facilitate the diffusion creep observed during sintering. Conversely, a densifieation and strengthening of the addition layer by the introduction of atoms of the main metal must hinder the shrinkage process. These experimental

data lead to the following conclusions.

io The activating effect of additions is governed by the diffusional mobility of atoms of the addition metal, direction of the diffusion flows between the addition metal and the main metal, and the ability of the metals to form mutual solid solutions. 2. In solid-phase sintering, the activating effect due to the addition of metal oxides which will readily be reduced during sintering is greater than that produced by the addition of powders of the metals themselves.

797

Fig. 9. S t r u c t u r e of s u r f a c e l a y e r of i r o n powder c o m p a c t d u r ing p e n e t r a t i o n of a drop of molten alloy: a) F e - - P; b) Mn - - Si; e) C u - - S i ; d) S i - - C a ; e) A 1 - - M n . 3. By subjecting p o w d e r s to p r i o r annealing and employing alloying additions strengthening the contact s u r f a c e between p a r ticles, it is p o s s i b l e to r e g u l a t e the magnitude of shrinkage of p a r t s being s i n t e r e d . 4. The use of additions in the powder c h a r g e is a potentially useful method of activating the s i n t e r i n g p r o c e s s and i m p r o v i n g the p h y s i c o m e c h a n i c a l p r o p e r t i e s of s i n t e r e d m a t e r i a l s .

2l

Activation 0

2 4 F Amount of addition wt. %

8

Fig. 10. Influence of c o n c e n t r a t i o n on shrinkage of iron c o m p a c t s with v a r i o u s additions: 1) F e - S(s);

2) F e - - P ; 5) N i - - S i ;

3) F e - - S b ; 4) Mn--Ni; 6) F e - - S ; 7) M n - - S i ;

during

Sintering

with

a Liquid

Phase

I. M. F e d o r c h e n k o and A. V. P e r e p e l k i n studied the s i n t e r ing of i r o n powder c o m p a c t s with the u s e of activating p o w d e r s c o n s i s t i n g of b i n a r y alloys of s e v e r a l m e t a l s and m e t a l l o i d s f o r m ing r e l a t i v e l y l o w - m e l t i n g e u t e c t i c s in the t e m p e r a t u r e r a n g e 800-1090~ The following e l e m e n t s w e r e t r i e d : A1, Ca, Cu, Mn, Ni, P, S, Sb, a n d S i (see T a b l e 1).

Special e x p e r i m e n t s w e r e c a r r i e d out to study the c h a r a c t e r of the r e a c t i o n between a liquid p h a s e and a p o r o u s iron powder skeleton. A lump of an addition alloy was p l a c e d on a p o r o u s iron powder p l a t e (initial p o r o s i t y 20%), a f t e r which the plate w a s heated in a v a c u u m to ll00~ (above the m e l t i n g point of the alloy) and held at that t e m p e r a t u r e f o r 30 min. A f t e r cooling, the extent to which the molten alloy drop had s p r e a d on the backing plate w a s examined and a t r a n s v e r s e m i e r o s e e t i o n of the plate w a s p r e p a r e d . 8) S i - - C a ; 9) C u ~ S i ; 1 0 ) C u - - A 1 ; 11) A1 - - Mn. Sintering at 2 h at 1100~

A study of the m i c r o s t r u c t u r e s e x a m i n e d d e m o n s t r a t e d that the r e a c t i o n of a liquid p h a s e with i r o n during s i n t e r i n g depends on its composition, A liquid p h a s e initially i m p r e g n a t e d with i r o n (Fe - - P alloy) is able to p r e s e r v e b e t t e r its c h e m i c a l c o m p o s i t i o n and fluidity and p e n e t r a t e d e e p e r along g r a i n bounda r i e s (Fig. 9a).

798

A liquid phase initially f r e e f r o m iron (Mn m Si and Cu - - Si alloys) actively d i s s o l v e s the s u r f a c e l a y e r s of iron p a r t i c l e s ; in such a case, the amount of the liquid phase i n c r e a s e s , wide veins of the liquid phase f o r m between the grains, and contacts between p a r t i c l e s a r e broken (Fig. 9b and e)o A1 - - Mn and Si - - Ca alloys p e n e t r a t e to a s m a l l e r depth into a p o r o u s skeleton and f o r m a surface t r a n s i t i o n l a y e r . An A1 - - Mn alloy f o r m s a drop which fails to spread f r e e l y on the s u r f a c e (Fig. 9e). It is apparent that the s u r f a c e tension and v i s c o s i t y of these alloys are higher than those of the r e m a i n i n g m a t e r i a l s . The c h a r a c t e r of the r e a c t i o n between a liquid phase and a p o r o u s skeleton m a y be expected to affect the shrinkage p r o c e s s of c o m p a c t s during sintering. Sinfering is also accompanied by the diffusion of elements f r o m the liquid into the solid phase, as confirmed by the i n c r e a s e in the m i c r o h a r d n e s s of the f e r r i t i c m a t r i x of c o m p a c t s a f t e r sintering. F i g u r e 10 shows c u r v e s of the concentration dependence of shrinkage with various additions at a sint e r i n g t e m p e r a t u r e of 1100~ It will be seen that shrinkage is effectively p r o m o t e d only by such additions as F e m P (0.25-8.0%), Fe - - S (0.25-1.5%) introduced by t r e a t i n g the starting iron p o w d e r with an aqueous solution of hydrogen sulfide, Fe - - Sb (0.25-4%), Mn m Ni (0.25-4%), Mn - - Si (0.25-2.0%), and Cu ~ Si (0.25-1.0%)o In the p r e s e n c e of l a r g e r quantities of these additions, with the exception of Fe - - P alloys, the c o m p a c t volume begins to increase~ tn the case of such additions as A1 m Mn, Cu - - A1, and Si ~ Ca, however, even small amounts a r e sufficient to produce an i n c r e a s e in the volume of compacts, which continues to grow as the amount of alloy added is raised. A c o n s i d e r a b l e influence on the shrinkage p r o c e s s during sintering is exerted by the method of i n t r o duction of an addition. When the addition m a t e r i a l c o v e r s the p a r t i c l e s u r f a c e in the s t a r t i n g powder in the form of athin~uniform l a y e r and is p r e s e n t on the contact s u r f a c e s between p a r t i c l e s , its influence on s h r i n k age is much g r e a t e r than when the same amount of the addition m a t e r i a l is introduced in the f o r m of powder, whose p a r t i c l e s a r e uniformly distributed throughout the main m a t e r i a l in the f o r m of d i s c r e t e inclusions. This statement is c l e a r l y c o n f i r m e d by the Fe - - S(s) and Fe - - S c u r v e s in Fig. 10, w h e r e the Fe - - S (s) c u r v e r e p r e s e n t s c o m p a c t s containing FeS in the f o r m of a s u r f a c e l a y e r on the starting i r o n powder p a r ticles, while the Fe ~ S c u r v e i l l u s t r a t e s the shrinkage of c o m p a c t s made f r o m a c h a r g e with an addition of FeS powder. A n a l y s i s of the phenomena o c c u r r i n g during the sintering of c o m p a c t s with additions r e a c t ing with the principal metal during sintering and f o r m i n g a liquid phase leads to the conclusion that the densifieation p r o c e s s in this c a s e is governed by p r o c e s s e s of mutual dissolution of the liquid phase and the principal metal, as well as by the v i s c o s i t y and s u r f a c e e n e r g y c h a r a c t e r i s t i c s of the resulting liquid phase. F o r the c a s e of additions capable of producing a liquid phase during sintering and r e a c t i n g with the principal metal of the compact, the following basic laws governing the sintering p r o c e s s m a y be f o r m u l a t e d : 1 . When the additions introduced into the m a t e r i a l of c o m p a c t s to be sintered c o n s i s t of eutectic bina r y alloys with melting points below the sintering t e m p e r a t u r e and contain elements which actively dissolve in the c o m p a c t m a t e r i a l during sintering, e i t h e r an intensification of shrinkage o r a substantial i n c r e a s e in the c o m p a c t volume will take place as a r e s u l t of the simultaneous o c c u r r e n c e of s e v e r a l mutually independent p r o c e s s e s which either p r o m o t e o r oppose shrinkage. 2. The g r e a t e s t activation of the sintering p r o c e s s t a k e s p l a c e when the addition f o r m i n g the liquid phase is of the s a m e metal as the c o m p a c t and contains suitable additions lowering its melting point or, alternatively, c o m p r i s e s eutectic alloys of other m e t a l s and is i m p r e g n a t e d with the main metal from which the c o m p a c t is made. F r o m the point of view of chemical composition, the liquid phase should be close to t h e r m o d y n a m i c equilibrium with the principal metal, but should have a slight deficiency of the l a t t e r to e n sure good wetting and little dissolution of the metal being infiltrated. 3. High fluidity and low values of s u r f a c e e n e r g y of the liquid phase activate shrinkage during sintering. 4. An addition f o r m i n g the liquid phase during sintering is p a r t i c u l a r l y effective in activating s i n t e r ing if it coats the p a r t i c l e s u r f a c e of the s t a r t i n g powder in the f o r m of a thin s u r f a c e l a y e r and r e m a i n s on the contact s u r f a c e s between p a r t i c l e s a f t e r compaction. 5. F o r the additions t e s t e d in this investigation, the optimum amount is 0.25-1%; g r e a t e r amounts of these additions m a y lead to an i n c r e a s e in the volume of sintered c o m p a c t s . 6. F o r the c a s e s investigated, d i f f e r e n c e s in the atomic radii of the main and l o w - m e l t i n g addition m e t a l s and the p r e s e n c e of ~ - - ~/-transformation do not significantly affect the shrinkage of c o m p a c t s d u r ing sintering.

799

Unsolved

Problems

in the

Theory

of Sintering

F r o m what has been said above it follows that the m a i n foundations of the t h e o r y of s i n t e r i n g have a l r e a d y been laid; a v a s t m a j o r i t y of the known e x p e r i m e n t a l f a c t s fit into the existing s y s t e m of t h e o r e t i c a l c o n s i d e r a t i o n s . However, although the m a j o r i t y of the phenomena o c c u r r i n g during s i n t e r i n g can be e x plained qualitatively, quantitative evaluation of the kinetics of s i n t e r i n g p r o c e s s e s is so f a r p o s s i b l e only on a r e l a t i v e l y s m a l l s c a l e . This is m a i n l y due to the f a c t that the m e c h a n i s m of d e f o r m a t i o n of r e a l c r y s t a l line bodies i s not yet u n d e r s t o o d in all details. Among the i m p o r t a n t p r o b l e m s in the t h e o r y of s i n t e r i n g f o r which a quantitative t h e o r y is urgently r e q u i r e d is that of activity. By " a c t i v e sintering" is understood s i n t e r i n g at r e l a t i v e l y low t e m p e r a t u r e s , which is a c c o m p a n i e d by p r o n o u n c e d densification and is c h a r a c t e r i z e d by low values of activation energy. The r a t e s of densification in active s i n t e r i n g s o m e t i m e s exceed by m a n y o r d e r s of magnitude the r a t e s p r e dicted by the t h e o r y of d i f f u s i o n - v i s c o u s flow, while the e n e r g y of activation f o r the densifieation p r o c e s s is as little as 0.2-0.4 of the e n e r g y of activation f o r v o l u m e self-diffusion. Thus, a c c o r d i n g to r e c e n t data, the e n e r g y of activation is not m o r e than 24 k c a l / m o l e f o r the s i n t e r i n g of e l e c t r o l y t i c i r o n p o w d e r in the a - r e gion [38], 12-15 k c a l / m o l e f o r c o p p e r and s i l v e r p o w d e r s [39], and 18 k c a l / m o l e f o r nickel; the e n e r g y of activation f o r densifieation is about 38 k c a l / m o l e f o r m o l y b d e n u m [40], 75 k c a l / m o l e f o r tungsten [41], and 68 k c a l / m o l e f o r tungsten with s m a l l nicket additions [34]. Thus, the e n e r g y of activation o r densification during active s i n t e r i n g is found to be m u c h s m a l l e r than the e n e r g y of activation f o r boundary as well as volume self-diffusion [27]. Active sintering is exhibited a l s o b y c r y s t a l l i n e oxides, the coefficients of self-diffusion of the cations and anions being in this c a s e hundreds of t i m e s s m a l l e r than the values a r r i v e d at on the b a s i s of the o b s e r v e d densification r a t e s . This p r o b l e m , which was i n v e s t i g a t e d by Kuczynski [42], cannot be explained r a t i o n a l l y with the aid of the e x p e r i m e n t a l data available. Active s i n t e r i n g cannot be a t t r i b u t e d to the p r e s e n c e of e x c e s s v a c a n c i e s , b e c a u s e of t h e i r s h o r t life. A widely held c u r r e n t view - - advanced and substantiated by Geguzin [15] - - is that active s i n t e r i n g is linked with boundary s e l f - d i f f u s i o n o v e r m a e r o d e f e c t s which a r e p r e s e n t in r e a l c r y s t a l s o r a r e g e n e r a t e d during the condensation of e x c e s s v a c a n c i e s . This view is c e r t a i n l y plausible qualitatively, but it is i m p o s s i b l e to evaluate densifieation k i n e t i c s quantitatively on the b a s i s of the c o n c e p t s p r o p o s e d , b e c a u s e not all the p a r a m e t e r s r e q u i r e d f o r calculation a r e known; in p a r t i c u l a r , the t h i c k n e s s of the boundary l a y e r in which ~ c e e l e r a t e d diffusion m a y o c c u r is not known. A p a r t f r o m this, G e g u z i n ' s c o n s i d e r a t i o n s , which fit well into the f r a m e w o r k of the p u r e diffusion theory, a r e not c o m p a t i b l e with the t h e o r y of d i f f u s i o n - v i s c o u s flow. Admittedly, L i f s h i t s [12] h a s e x a m i n e d the c a s e of p r e f e r e n t i a l diffusion of v a c a n c i e s along block b o u n d a r i e s , but the l a c k of quantitative data m a k e s it i m p o s s i b l e to a s s e s s w h e t h e r such a situation can in fact a r i s e in p r a c t i c e . F u r t h e r m o r e , as noted above, the e n e r g y of activation in active s i n t e r i n g is frequently found to be m u c h l o w e r than the e n e r g y of activation f o r boundary self-diffusion. Thus, the detailed nature of a c t i v i t y cannot yet be said to be fully understood, although in p r i n c i p l e it is c l e a r that the p r i n c i p a l role in it i s played by c r y s t a l l i n e s t r u c t u r e d e f e c t s and t h e i r mutual r e a c t i o n s and t r a n s f o r m a t i o n s . In view of this, a n a l y s i s of the influence of c r y s t a l l i n e s t r u c t u r e defects on the r a t e of d e f o r m a t i o n and sintering p r o c e s s e s is one of the outstanding t a s k s in the evolution of the t h e o r y of sintering. It m u s t be noted that the m a j o r i t y of authors studying the t h e o r y of s i n t e r i n g have u n d e r e s t i m a t e d the role of dislocations as defects of p a r a mount i m p o r t a n c e in d e f o r m a t i o n p r o c e s s e s . Recently, L e n e l [43] once again introduced the concept of a dislocation m e c h a n i s m of d e f o r m a t i o n in densification and e x p r e s s e d the view that the m e c h a n i s m of d e f o r m a t i o n c o r r e s p o n d s to W e e r t m a n ' s c r e e p [44]. B e a r i n g in mind that W e e r t m a n ' s c r e e p can only take p l a c e above the yield point, it is i m p o s s i b l e to a g r e e with L e n e l ' s view. However, the n u m e r o u s m a n i f e s t a t i o n s of dislocation d i s p l a c e m e n t u n d e r the influence of s m a l l s t r e s s e s and t h e r m a l fluctuations inevitably lead to the conclusion that the role of d i s l o c a tions in active s i n t e r i n g p r o c e s s e s m a y be substantial. F u r t h e r t h e o r e t i c a l and e x p e r i m e n t a l r e s e a r c h e s in this field will be n e c e s s a r y . Of p a r t i c u l a r i n t e r e s t a r e investigations aiming at r e a l i z i n g the m a x i m u m p o s s i b l e elimination of r e s i d u a l p o r o s i t y by s i n t e r i n g without the application of p r e s s u r e . In this c a s e , the p r i n c i p a l p a r t is played by p u r e diffusion p r o c e s s e s , and consequently, development of the diffusion t h e o r y of s i n t e r i n g taking into account m a c r o d e f e c t s is of g r e a t i n t e r e s t . Many i m p o r t a n t r e s u l t s in this connection have b e e n obtained by Geguzin [24] and E u d i e r [33], but a c o m p l e t e quantitative t h e o r y of the p r o c e s s of r e s i d u a l p o r o s i t y r e m o v a l has not yet b e e n f o r m u l a t e d .

800

Fig. 11. Continuous furnace with r o l l e r h e a r t h (Lindberg Co.).

Fig. 12. Continuous furnace with grid c o n v e y e r (Lindberg C o . ) .

Fig. 13. Continuous c o n v e y e r type furnace for t e m p e r a t u r e of 700-800~ (Heavy-Duty Co.). In r e c e n t y e a r s , much w o r k has been done on developing an i m p o r t a n t new b r a n c h of p o w d e r m e t a l l u r g y of p r a c t i c a l significance, n a m e l y , f i b e r m e t a l l u r g y . However, the t h e o r y of s i n t e r i n g of p o r o u s f i b e r m a t e r i a l s is still in a r u d i m e n t a r y state. F o r the d e v e l o p m e n t of a t h e o r y of s i n t e r i n g of f i b e r s it is i m p e r a t i v e that all the t h e o r e t i c a l and e x p e r i m e n t a l investigations which have a l r e a d y been p e r f o r m e d f o r powd e r m a t e r i a l s should now be g r a d u a l l y r e p e a t e d f o r f i b e r m a t e r i a l s . It m u s t be mentioned that for m e t a l f i b e r s , in c o n t r a s t to p o w d e r s , studies of s i n t e r i n g on m o d e l s a r e of c o n s i d e r a b l e and d i r e c t value. On the o t h e r hand, f o r the p h e n o m e n o l o g i c a l d e s c r i p t i o n of the s i n t e r i n g of f i b e r m a t e r i a l s , it is n e c e s s a r y to i n v e s t i g a t e t h e o r e t i c a l l y and e x p e r i m e n t a l l y the dependence of t h e i r p h y s i o c m e c h a n i c a l p r o p e r t i e s on p o r o s i t y .

801

Fig. 14

Fig. 15

Fig. 14. Endogas g e n e r a t o r of up to 214 m S / h output (Lindberg Co.). Fig. 15. Endogas g e n e r a t o r of up to 428 m 3 / h output (Lindberg Co.). Finally, it is i m p o s s i b l e not to mention the g r e a t i m p o r t a n c e of t h e o r e t i c a l r e s e a r c h e s into the s i n t e r ing of c h e m i c a l l y active m e t a l s , p a r t i c u l a r l y m e t a l s with difficultly reducible oxides, and t h e i r alloys. This p r o b l e m f o r m s p a r t of a w i d e r r e s e a r c h field, namely, that of d e t e r m i n i n g the i n t e r r e l a t i o n s h i p between s u r f a c e c h e m i c a l r e a c t i o n s and m e t a l s i n t e r i n g p r o c e s s e s . T h e o r e t i c a l investigation in this field has v i r tually not y e t begun, and even methods of a p p r o a c h to the p r o b l e m have not yet been developed, although m a n y e x p e r i m e n t a l data a r e available and c o n s i d e r a b l e industrial e x p e r i e n c e has been gained in the p r o duction of c h e m i c a l l y active m e t a l s and alloys. Undoubtedly, t h e r e a r e also o t h e r i m p o r t a n t a s p e c t s of the t h e o r y of sintering which will r e q u i r e elucidation, but even the e x a m p l e s quoted h e r e a r e sufficient to indicate how m u c h w o r k r e m a i n s to be done in this field. Modern

Methods

of Sintering

and

Equipment

The t r e n d in the design of sintering f u r n a c e s is toward i n c r e a s e d f u r n a c e productivity, p r o c e s s auto.mation, and stabilization of o p e r a t i n g conditions. F i g u r e s 11-13 show m o d e r n continuous sintering f u r n a c e s built in A m e r i c a [45]. T h e s e f u r n a c e s a r e p r o v i d e d with a r o l l e r h e a r t h o r a c o n v e y e r belt and a r e intended f o r s i n t e r i n g at t e m p e r a t u r e s of 800-1000~ The c o n t r o l l e d g a s a t m o s p h e r e s c o m m o n l y u s e d nowadays include hydrogen, c r a c k e d ammonia, c o n v e r t e d n a t u r a l gas, and e n d o t h e r m i e g e n e r a t o r gas whose c a r b u r i z i n g potential can be v a r i e d . The use of c o n t r o l l e d a t m o s p h e r e s f o r s i n t e r i n g on an industrial scale r e q u i r e s special equipment f o r the production of gas a t m o s p h e r e s . In F i g s . 14 and 15 a r e shown two A m e r i c a n endogas g e n e r a t o r s of high capacity, which a r e u s e d in p o w d e r m e t a l l u r g i c a l p r o c e s s e s [45]. Oxygen and w a t e r a r e oxidizing i m p u r i t i e s and m u s t be r e m o v e d f r o m g a s a t m o s p h e r e s ; this is p a r t i c u l a r l y i m p o r t a n t in the s i n t e r i n g of alloys whose m e t a l l i c c o m p o n e n t s can f o r m difficultly reducible oxides. I n d u s t r i a l p u r i f i c a t i o n of g a s e o u s m e d i a can s e c u r e a dew point of between - - 50 and - - 60~ Under t h e s e conditions it is p o s s i b l e to s i n t e r alloys containing c h r o m i u m , m a n g a n e s e , silicon, and titanium. T h e r e h a s been a tendency of late to simplify the sintering p r o c e s s , i n c r e a s e its productivity, and d e c r e a s e its cost. One of the m o s t r a d i c a l w a y s of simplifying the s i n t e r i n g technique is to d i s p e n s e with the use of p r o t e c t i v e g a s e o u s m e d i a . Two p r o c e s s e s d e s e r v e p a r t i c u l a r attention in this connection, namely, s h o r t t e r m s i n t e r i n g in a i r b y m e a n s of induction heating and s i n t e r i n g in molten g l a s s . S h o r t - t i m e s i n t e r i n g in

802

Fig. 16. H i g h - t e m p e r a t u r e continuous furnace (Heavy-Duty Co.).

Fig. 17. H i g h - T e m p e r a t u r e furnace with manual p u s h e r (Lindberg Co.). a i r with the aid of h i g h - f r e q u e n c y induction heating m a y be employed f o r bushings, piston rings, etc. [46, 47]. The s i n t e r i n g p r o c e s s u n d e r t h e s e conditions l a s t 30-40 sec, during which no significant oxidation can take p l a c e . The s i n t e r i n g of sufficiently dense powder c o m p o n e n t s in molten g l a s s baths o f f e r s full p r o t e c t i o n against oxidation, even in the c a s e of alloys containing constituents which v i g o r o u s l y r e a c t with oxygen (aluminum, c h r o m i u m ) [48]. Under a l a y e r of molten g l a s s it is p o s s i b l e to s i n t e r p a r t s which n o r m a l l y r e quire a vacuum f o r this p u r p o s e . Sintering in g l a s s s e c u r e s a high p r o d u c t i v i t y and is v e r y economical b e c a u s e of the excellent t h e r m a l p r o p e r t i e s and low cost of g l a s s and the absence of special gas a t m o s p h e r e s . P a r t s s i n t e r e d in g l a s s m u s t be subjected to v i b r a t o r y t r e a t m e n t or sand blasting to r e m o v e the g l a s s l a y e r adhering to t h e m . T h e r e is e n o r m o u s i n t e r e s t in the use of the sintering operation as a fundamental p a r t of the m o d e r n f a b r i c a t i o n technique f o r the p r e p a r a t i o n of c e r t a i n i m p o r t a n t - - i n p a r t i c u l a r , r e f r a c t o r y - - m e t a l s and alloys. F o r the sintering of m a s s i v e tungsten and molybdenum blanks, h i g h - t e m p e r a t u r e f u r n a c e s with a l a r g e heating s p a c e m u s t be built. Such f u r n a c e s m a y o p e r a t e in a continuous m a n n e r , b e c a u s e the sintering of tungsten and m o l y b d e n u m is p e r f o r m e d in hydrogen. Two continuous h i g h - t e m p e r a t u r e f u r n a c e s with m o l y b denum heating e l e m e n t s and an o p e r a t i n g t e m p e r a t u r e of 1800~ m a d e by the H e a v y - D u t y Co. and the L i n d b e r g Co. in A m e r i c a , a r e shown in F i g s . 16 and 17.

803

The modern techniques employed f or the sintering of r e f r a c t o r y , highly active metals is c h a r a c t e r ized by an extensive use of vacuum as a sintering medium. 'Vacuum must be r e s o r t e d to in the sintering of semiproducts and pa r t s from tantalum, niobium, titanium, zirconium, and other metals which have a high affinity f o r hydrogen, oxygen, and nitrogen. However, vacuum is also beneficial in the sintering of metals which do not exhibit a high gas absorption capacity. Vacuum sintering has the following advantages: Firstly, vacuum promotes a m or e rapid and fuller removal of volatile contaminants from contact surfaces; se c ondly, vacuum strongly intensifies the p r o c e s s e s of t ransport through the gaseous phase and, consequently, acceler ates the p r o c e s s e s of formation and growth of metallic contacts at the early stages of sintering; thirdly, the absence of gas in por e s s e c ur es g r e a t e r densification at the l a t e r stages of sinteringo Finally, the use of vacuum occasionally enables the required results to be attained by suppressing certain undesirable reactions involving the participation of the gaseous phase. F o r instance, vacuum sintering enables cobalt to be successfully replaced by nickel as the binder metal in hard-alloy part s without a significant increase in t hei r brittleness. Modern vacuum furnaces have a large heating capacity, operate at high t e m p e r a t u r e s (1500-2200~ which are attained either by induction heating or by the direct passage of current, and are provided with automatic devices f o r charging, clischarging, and cooling the compacts. In view of the growing use of r e f r a c t o r y metals as constructional heat-resisting materials, it is nec e s s a r y to explore new methods f or the manufacture of p a r t s and semiproducts. Among such new methods, of p a r t i c u l a r in t e r es t are isost~fic high-temperature pressing and plasma spraying. Isostatic pressing is p er f o r med in special autoclaves in which working gas p r e s s u r e s of up to 700 atm and t e m p e r a t u r e s of up to 1600~ can be attained. The blank to be subjected to isostatic hot pressing is enclosed in a molybdenum sheet container, which is then evacuated. Under these conditions, it is possible to obtain tungsten stock with a relative density of 95-98%. Using such stock, crucibles, nozzles, and other part s intended for hight e m p e r a t u r e service may be produced by machining. Plasma spraying combines in a single production p r o c e s s the preparation of a r e f r a c t o r y metal powder and the formation of parts by a technique amounting to l a y e r - b y , l a y e r sintering with the additional action of inertia f o r c e s . This method is suitable for the formation of both simple and multilayer parts from r e f r a c t o r y metals. One field to be explored in the search for ways of improving sintering equipment and techniques is the possibility of employing furnaces in which the muffle is arranged in a vertical position. In a furnace ,f this type, the charge is heated more uniformly, and the protective gas is able to reach it more effectively. However, owing to the weight of the compacts, which are stacked vertically one on another, vertical shrinkage becomes much g r e a t e r than diametrical shrinkage. In such furnaces it is possible to p e r f o r m sintering with the application of additional p r e s s u r e to the compacts. Experiments in which bushings were sintered in this type of furnace revealed no warpage or b a r r e l - t y p e distortion. The chief problems in p r e s e n t - d a y sintering technology are, firstly, to secure adequate strength through short-time sintering (measured in seconds) and, secondly, to reduce to a minimum or completely eliminate residual porosity in powder m a t e ri al s by sinteringo In addition, among the m ore important technical problems is that of developing a p r o c e s s for the preparation of various metal and alloy powders with predetermined sintering c h a r a c t e r i s t i c s . The fact that today we are producing tungsten and molybdenum powders which have excellent sintering c h a r a c t e r i s t i c s and that we have m a s t e r e d the technique of p r e p a r a tion of a carbonyl nickel powder which exhibits v e r y high activity during sintering is an indication that this problem can be successfully solved with the means we now have at our disposal. LITERATURE 19

2~ 3o 4.

5. 6. 7.

M. Yu. Bal'shin, Powder Metallography [in Russian], Moscow (1948). Problems of Powder Metallurgy [in Russian], Kiev, Acad. Sci. UkrSSR P r e s s (1961). I. M. Fedorchenko and R. A. Andrievsk~i, Principles of Powder Metallurgy [in Russian], Kiev, Acad. Sci. UkrSSR P r e s s (1961)~ Ya~ I. Frenkel', Zh~ t~ksperim, i Teoro Fiz., 16_, No. 1 (1946). J. K. Mackenzie, Proc. Phys. Soc., 63(13}, No. 1 (1950). V~ V. Skorokhod, Poroshkovaya Met., 1_, No. 2. B~ H. Alexander, G. C. Kuczynski, and M. H. Dawson, Physics of Powder Metallurgy, ed. M. Kingston

(1951).

804

CITED

8. 9. I0o Ii. 12. 13. 14.

15. 16. 17. 18. 19~ 20.

21o 22. 23. 24. 25. 26. 27. 28~ 29~ 30~ 31o 32~ 33. 34~ 35~ 36~ 37~ 38. 39. 40. 41. 42. 43~ 44~ 45~ 46. 47~ 48. 49. 50.

Ya. E. Geguzin and I. M. Lifshits, Fiz. Tverdogo Tela, 4, No. 5 (1962). Ya. E. Geguzin, Macroseopical Defects in Metals [in Russian], Moscow (1962). F.R.N. Nabarro, Repo Conf. Strength Solids, Phys. SOCo, London (1948)o C. Herring, J. Appl. Phys., 21, 437 (1950). I.M. Lifshits, Zh. ]~ksperim. i Teoro Fiz., 4_~4(4),1349 (1963). V.V. Skorokhod and G. O. Ranneva, Poroshkovaya Met., No. 3, 25 (1963). E.M. Minaev and O. K. Kolerov, Collection of Proceedings of the Eighth All-Union Scientific-Research Conference on Advanced Methods of Production of Parts from Powders [in Russian], Minsk (1966). Ya. E. Geguzin, Fiz. Metal. i Metalloved., 6_ No. 4, 650 (1958). Ya~ E. Geguzin, Fiz~ Metal. i Metalloved., 9, 842 (1960)~ Vo V. Skorokhod, Poroshkovaya Met., No. 1 (1964). L . P . Litvinenko, Dissertation [in Russian], Kiev (1966). V. Vo Skorokhod and A. F. Khrienko, Poroshkovaya Met., No. 5 (1965). G . I . Aksenov and V. I. Kryukov, Collection of Proceedings of the Eighth All-Union Scientific-Research Conference on Advanced Methods of Production of P a r t s from Powders [in Russian], Minsk (1966). I.M. Fedorchenko, Izv. Akado Nauk SSSR, Otdel. Tekhn. Nauk, No. 3 (1951). I.M. Fedorehenko, IZVo Akad. Nauk SSSR, Otdel. Tekhn. Nauk, No. 2 (1952). I.M. Fedorchenko, IZVo Akad. Nauk SSSR, Otdel. Tekhn~ Nauk, No. 3 (1953). Ya. E. Geguzin, Microscopical Defects in Metals [in Russian], Moscow (1962). I.M. Fedorchenko, G. G. Gnesin, and R. Ao Andrievskii, Supplementary Collection: Powder Metallurgy [in Russian], Yaroslavl' (1957). B. Yao Pines, Essays on Metal Physics [in Russian], Khar'kov (1963). Gertsriken and I. Yao Dekhtyar, Solid-Phase Diffusion in Metals and Alloys [in Russian], Moscow (1963). Ao I. Raichenko and I. Mo Fedorchenko, Problems of Powder Metallurgy, No. 6 [in Russian], Kiev (1958). Ya. E. Geguzin, Fizo Metal~ i Metalloved., 2, No. 3, 406 (1956). R . A . Andrievskii and I. M. Fedorchenko, Izv~ Akad. Nauk SSSR, Met. i Toplivo, No. 3 (1961). J. Vacek, Planseeberiehte Pulvermetallurgie, _7, No. 1-2 (1959). J. Vacek, Hutnicke Listy, 10__,No~ 8, 469 (1955). M, Eudier, Powder Metallurgy, London (1963), p. 17. J. Brophy, Lo Shepard, and S. Wulff, Powder Metallurgy, London (1962). Co Agte and J. Vacek, Tungsten and Molybdenum, Prague (1954). V.V. Panichldna, 2oroshkovaya Met., No. 2 (1967). I . M . Fedorchenko and I. I. Ivanova, Poroshkovaya Met. (!966)o S . L . Forse, Modern Development in Powder Metallurgy, 2, Proceedings of the 1965 International Powder Metallurgy Conference, edo H. Hausner, New York (1966). I. Mo Fedorchenko and R. A. Andrievskii, Principles of Powder Metallurgy [in Russian], Kiev (1961). J. S~ Smith, J. Less-Coma~aon Met., 8, No. 3 (1965)o N . C . Kothari, J. Less-Common Met., 5, No. 2 (1963). G . C . Kuczynski, Modern Development in Powder Metallurgy, 1 (cf. [38]). F. Lenet, Modern Development in Powder Metallurgy, l ( c f . [38]). J. Weertman, J. Appl. Phys., 26, No~ 10, 1213 (1955)o Catalogs of the Heavy-Duty Co. and the Lindberg Co. N.V. Nikitina and V. V. Vologdin, Avtomobil. Prom., No. 2, 39 (1963). I~ M. Fedorchenko, B~ Io Chaika, andVoV. Vologdin, Poroshkovaya Met., No. 12 (1965). I . D . Radornysel'skii and N. I. Shcherban', Poroshkovaya Met~ No. 12 (1965). Catalog of the Autoclave Engineers Inc. A . N . Nikolaev and T. A. Mikryukova, Collection of Proceedings of the Eighth All-Union ScientificResearch Conference on Advanced Methods of Production of P a r t s from Powders [in Russian], Minsk (1966).

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