MA reduces wear of self-lubricating materials

M A r e d u c e s w e a r of self-lubricating materials Refining the microstructure of solid high temperature lubricants has been suggested as a way of improving their p e r f o r m a n c e . B. Gunther, H.-D. Kunze and G. Vetl of the Fraunhofer Institute of Applied Materials Research in Bremen, Germany, together with K. Takahashi of Toyota Motor Corp in Aichi, Japan, have been investigating mechanical alloying as a means of achieving this improvement.

FIGURE 1: Microstructure of mechanically alloyed CuZnWS2-composite powders after (a) 10 hour milling (b) 13 hour milling (c) and 15 hour milling.

FIGURE 2: TEM micrograph of a powder particle milled for 13 hours. Note the lattice planes of WS2.

20 MPR November 1993

lide b e a r i n g s g e n e r a l l y use oil lubrication to reduce friction. When service t e m p e r a t u r e s a r e b e y o n d t h e t h e r m a l s t a b i l i t y of o r g a n i c l u b r i c a n t s , however, solid high t e m p e r a t u r e l u b r i c a n t s like graphite, m o l y b d e n u m d i s u l p h i d e or h e x a g o n a l b o r o n n i t r i d e a r e used. Besides factors like surface roughness, service t e m p e r a t u r e a n d h a r d n e s s of m a t r i x , t h e m i c r o s t r u c t u r e of t h e friction m a t e r i a l h a s a significant influence on t h e p e r f o r m a n c e of t h e tribologic system. There a r e i n d i c a t i o n s in t h e l i t e r a t u r e t h a t a r e f i n e m e n t o f microstructure may lead to a further i m p r o v e m e n t in t r i b o l o g i c a l b e h a v i o u r of solid l u b r i c a n t s . Mechanical alloying (MA) h a s p r o v e n to be a powerful tool in d e s i g n i n g u l t r a f i n e m i c r o s t r u c t u r e s , so t h i s s t u d y u s e d it to d i s p e r s e t h e solid l u b r i c a n t s g r a p h i t e a n d t u n g s t e n d i s u l p h i d e in a b r a s s m a t r i x . As s t a r t i n g m a t e r i a l s b r a s s p o w d e r C u 6 0 Z n 4 0 (wt%) w i t h a p a r t i c l e size > 40 ]~m, WS2 p o w d e r a n d c r y s t a l l i n e flaky g r a p h i t e w i t h a m e d i u m p a r t i c l e size of 10 p m were used. In e a c h alloy system two c o m p o s i t i o n s , 20 vol% a n d 40 vol% solid lubricant, were e x a m i n e d . Mechanical alloying was p e r f o r m e d in steel vials u n d e r an a r g o n a t m o s p h e r e using a Retsch PM4 p l a n e t a r y ball mill. Milling t i m e s were b e t w e e n 5 h o u r s a n d 25 hours. Milling ball to p o w d e r w e i g h t r a t i o w a s 8:1 in every milling. The p r o c e s s e d p o w d e r s were e x a m i n e d by m e t a l l o g r a p h y a n d t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y using a Philips EM300. The p o w d e r s c o n t a i n i n g WS2 were cons o l i d a t e d via h o t p r e s s i n g a t 620°C with a p r e s s u r e of 280 N / m m 2. Powders m i l l e d with g r a p h i t e were cold c o m p a c t e d by die p r e s s i n g w i t h a p r e s s u r e of 700 N / m m 2 a n d s i n t e r e d for one h o u r a t 810°C u n d e r a n A r atmosphere. The d e v e l o p m e n t of t h e m i c r o s t r u c t u r e of a c o m p o s i t e of b r a s s c o n t a i n i n g 20 vol% WS2 w i t h i n c r e a s i n g milling t i m e is shown in Figure 1. Obviously, t h e l a m e l l a r micros t r u c t u r e is r e t a i n e d a n d refined d o w n to m i c r o m e t r e size. However, a f t e r 15 h o u r s milling this c h a r a c t e r i s t i c m i c r o s t r u c t u r e h a s d i s a p p e a r e d . X-ray m e a s u r e m e n t s of t h i s p o w d e r s h o w t h a t WS2 h a s r e a c t e d w i t h t h e zinc of t h e b r a s s to form ZnS. High r e s o l u t i o n m i c r o s c o p y (Figure 2) of t h e

S

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powder after 13 hours milling, however, shows t h a t WS2 becomes widely dispersed in the b r a s s m a t r i x . E v a l u a t i o n of t h e lattice distances a n d SAD p a t t e r n confirms t h a t WS2 r e m a i n s chemically u n c h a n g e d after this milling process. This m e a n s t h a t d u r i n g 2 hours milling the chemical reaction has been activated by the energy i n p u t of MA. In the b r a s s / g r a p h i t e system, a change from lamellar to dispersed m i c r o s t r u c t u r e occurs after short milling times (Figure 3). Evidently, the graphite is milled into the brass p h a s e a n d c a n n o t act as a l u b r i c a n t between the powder particles a n y more. Thus, a n i n c r e a s i n g a m o u n t of metallic brass contacts occur d u r i n g milling, which have the t e n d e n c y to weld to each other. This leads to a c o a r s e n i n g of the powders. After 25 hours milling (Figure 3c) there is no m i c r o s t r u c t u r e visible in the light optical micrograph of the powder particles. Transmission electron microscopy (TEM) (Figure 4) reveals t h a t graphite is dispersed h o m o g e n o u s l y in the brass matrix. SAD p a t t e r n s of the composite powder show t h a t graphite r e m a i n s in its h e x a g o n a l lattice d u r i n g milling into the fine dispersion. In order to quantify the d e v e l o p m e n t of the m i c r o s t r u c t u r e d u r i n g the MA process, a dispersion p a r a m e t e r , D, has been introduced, which is defined by D = (S+V)/N

SPECIAL

FEATURE

FIGURE 3: Microstructure of mechanically alloyed brass-graphite (20 vol%) after (a) 5 hours, (b) 9 hours and (c) 25 hours milling time.

(pm 2)

S = cross sectional area of powder particle, V = volume fraction of solid l u b r i c a n t (0.2 or 0.4), N = n u m b e r of brass grains in the powder particle. Figure 5 shows this dispersion p a r a m e t e r as a f u n c t i o n of the a p p l i e d milling time. The decrease of the dispersion p a r a m e t e r , which is a m e a s u r e for the fineness of m i c r o s t r u c t u r e , is steeper in the b r a s s / graphite composites t h a n in the brass/WS2 powders. The curve of b r a s s / 2 0 vol% WS2 e n d s at 13 h o u r s m i l l i n g time. Longer milling times cause the chemical decomposition reaction of WS 2 with zinc. The dispersion p a r a m e t e r was also t a k e n as the criterion for choosing powders with comp a r a b l e m i c r o s t r u c t u r e s for the p r e p a r a t i o n of the wear test specimen. Wear tests were performed on powders with equivalent dispersion parameter (5 pm 2 a n d 1 pm 2) e.g., a 9 h o u r milled b r a s s / 2 0 vol% graphite powder c o m p a r e s with a 13 h o u r milled b r a s s / 2 0 vol% WS2 in its m i c r o s t r u c t u r a l dimensions. The consolidation methods, as described earlier, were selected in o r d e r to p r e v e n t e x t e n s i v e m i c r o s t r u c t u r a l changes of the materials. The m i c r o s t r u c t u r e of a s i n t e r e d brass-

FIGURE 4: TEM micrograph of mechanically alloyed brass-graphite powder containing 20 vol% graphite after 25 hours milling. graphite composite with 40 vol% graphite is shown in Figure 6. The m i c r o g r a p h shows t h a t some c o a r s e n i n g of the m i c r o s t r u c t u r e in the powder particles takes place. However, the l a m e l l a r feature of the s t r u c t u r e essentially remains. C o n s o l i d a t e d t e s t p i e c e s w e r e fine g r o u n d to final d i m e n s i o n s a n d tested with

MPR November

1993 21

FIGURE 5: Dispersion of the brass phase in brass/graphite and brasslWS2 composites as a function of milling time.

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='~ 0.40~ FIGURE 7: Wear depth and friction coefficient of brass-solid-lubricant composites processed to different equivalent degrees of dispersion.

22 MPR November 1993

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a S t e y r t r i b o m e t e r (SSPG 11) in block on r i n g g e o m e t r y w i t h w h e e l d i a m e t e r of 40 mm, a c c o r d i n g to t h e F r e n c h s t a n d a r d NF T 51 107. Friction coefficients (p) and wear d e p t h s were d e t e r m i n e d a t a sliding s p e e d of 1 m / s u n d e r a l o a d of 24.5 N. W e a r d e p t h w a s m e a s u r e d a f t e r a sliding d i s t a n c e of 3 km. All t e s t s were p e r f o r m e d in t h e unlubricated condition and consolidated powder b l e n d s were u s e d as reference m a t e r i a l . The r e s u l t s of t h e t r i b o m e t e r t e s t s a r e s u m m a r i z e d in Figure 7. Generally, low p-values a r e c o n n e c t e d to high w e a r losses a n d vice versa. In s o m e of t h e s p e c i m e n s t h e friction coefficient is as low as t h a t of t h e p u r e solid l u b r i c a n t s (p = 0.15), i n d i c a t i n g t h a t t h e sliding surface is covered by a t h i n layer of solid lubricant. A m i n i m u m w e a r loss is o b s e r v e d in t h e 13 h o u r s MA b r a s s / 2 0 vol% WS2 composites. The d e v e l o p m e n t of t h e m i c r o s t r u c t u r e of t h e c o m p o s i t e p o w d e r s is d e t e r m i n e d by t h e l u b r i c a t i n g b e h a v i o u r of WS2 a n d g r a p h i t e a n d t h e i r a b i l i t y to w i t h s t a n d t h e r a t h e r h a r s h m i l l i n g c o n d i t i o n s of t h e p l a n e t a r y ball mill. The WS2 a n d g r a p h i t e act in t h e b e g i n n i n g of t h e milling p r o c e s s like a milling a g e n t t h a t p r e v e n t s e x t e n s i v e w e l d i n g o f t h e m a t e r i a l to t h e m i l l i n g e q u i p m e n t . With i n c r e a s i n g milling t i m e t h e solid l u b r i c a n t is i n c o r p o r a t e d into t h e p o w d e r p a r t i c l e s a n d no l o n g e r acts as a milling agent. Thus, t h e w e l d i n g b e t w e e n p o w d e r p a r t i c l e s b e c o m e s i n c r e a s i n g l y effective. This b e h a v i o u r is m o r e p r o n o u n c e d in t h e b r a s s / g r a p h i t e system. It is also a n i n d i c a t i o n t h a t g r a p h i t e is less effective as a l u b r i c a n t t h a n WS2. On t h e o t h e r hand, it is p o s s i b l e to p r o d u c e very fine d i s p e r s i o n s of g r a p h i t e in b r a s s by a p p l y i n g r a t h e r s h o r t m i l l i n g times. The use of WS2 is strictly confined by its t e n d e n c y to r e a c t w i t h zinc d u r i n g e x t e n d e d m i l l i n g p e r i o d s w h i c h d e s t r o y its s o l i d l u b r i c a t i n g effect. The different m a t e r i a l s p r o d u c e d in t h i s s t u d y do n o t e x h i b i t an a b s o l u t e o p t i m u m p r o p e r t y w i t h r e s p e c t to b o t h f r i c t i o n coefficient a n d w e a r loss. However, t h e r e a p p e a r to be s o m e q u i t e p r o m i s i n g combin a t i o n s of ~ a n d w e a r in b r a s s / 2 0 vol% g r a p h i t e (9 h o u r mill), b r a s s / 2 0 vol% WS2 (25 h o u r mill) a n d b r a s s / 4 0 vol% g r a p h i t e (25 h o u r mill). They offer a c o m b i n a t i o n of a c c e p t a b l e friction coefficient a n d a t t r a c tively low w e a r losses. On t h e b a s i s of t h e s e results further developments with high s t r e n g t h m a t r i x m a t e r i a l s a r e underway. I

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Reproduced with permission by the Japan Society of Powder and Powder Metallurgy.