Surface tension and the sintering force in copper
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W H E N a m a s s of m e t a l p o w d e r is s i n t e r e d at e l e v a t e d t e m p e r a t u r e , it c o n t r a c t s s p o n t a n e o u s l y , a s though s u b j e c t e d to a h y d r o s t a t i c c o m p r e s s i v e f o r c e . If the p o w d e r is h o m o g e n e o u s l y s t a c k e d and if it i s n e i t h e r p r e c o m p a c t e d n o r s u b j e c t e d to any e x t e r n a l l y a p p l i e d f o r c e d u r i n g s i n t e r i n g , the c o n t r a c t i o n is both i s o t r o p i c and n e a r l y h o m o g e n e o u s throughout the m a s s . It h a s been p r e s u m e d , but n e v e r d e m o n s t r a t e d q u a n t i t a t i v e l y , that the c o n t r a c t i n g f o r c e o r i g i n a t e s in the s u r f a c e t e n s i o n r e s i d e n t upon the m e t a l - v a p o r i n t e r f a c e of the spongy m a s s . 1 The e x p e r i m e n t a l d e v e l o p m e n t , r e p o r t e d h e r e with, was d e s i g n e d to d e m o n s t r a t e both that s u r f a c e t e n s i o n is the s o u r c e of the s i n t e r i n g f o r c e and that the s u r f a c e t e n s i o n of a m e t a l can be m e a s u r e d through the a g e n c y of s i n t e r i n g . This is a c c o m p l i s h e d by m e a s u r ing the s i n t e r i n g f o r c e and r e l a t i n g it to the s u r f a c e t e n sion through the m e a s u r e d c u r v a t u r e of the m e t a l - v a p o r i n t e r f a c e . The s u b j e c t m e t a l of this s t u d y h a s been copper. In o r d e r to m e a s u r e the f o r c e of s i n t e r i n g , it is n e c e s s a r y to o p p o s e it with an e x a c t l y c o u n t e r a c t i n g m e c h a n i c a l f o r c e . Since the f o r c e of s i n t e r i n g b e h a v e s , in the m a s s , a s a h y d r o s t a t i c c o m p r e s s i o n , it is not f e a s i b l e to b a l a n c e it t o t a l l y , b e c a u s e e x p e r i m e n t a l It is p o s s i b l e , h o w e v e r , to b a l a n c e one c o m p o n e n t of the h y d r o s t a t i c f o r c e with an u n i a x i a l t e n s i l e load. In the p r e s e n t r e s e a r c h it h a s b e e n d e m o n s t r a t e d that t h i s can be done, without d i s t u r b i n g the g e o m e t r i c evolution of s i n t e r i n g , o r the o t h e r c o m p o n e n t s of the h y d r o s t a t i c c o m p r e s s i o n . A c c o r d i n g l y , the t e r m " s i n t e r i n g f o r c e " , a s u s e d in this p a p e r , r e f e r s to the f o r c e necessary to just stop the sintering contraction along one axis o f the sinter body. This definition is c o n s i s tent with the z e r o - c r e e p m e t h o d o r i g i n a l l y s u g g e s t e d by Gibbs ~ and used, s u b s e q u e n t l y , to d e t e r m i n e s u r f a c e t e n s i o n in g e o m e t r i c a l l y s i m p l e s y s t e m s involving w i r e s and f o i l s . The m e a s u r e m e n t of the a v e r a g e m e a n c u r v a t u r e of s u r f a c e is r e a d i l y a c c o m p l i s h e d b y using a well e s t a b l i s h e d r e l a t i o n s
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