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Antiwear and extreme pressure additives for lubricants
Antiwear and extreme pressure additives for lubricants E. S. F o r b e s *
The lubrication r e g i o n s c o v e r e d by the t e r m s a n t i - w e a r (a.w.) and e x t r e m e p r e s s u r e (e.p.) a r e defined. The functions of a wide range of t h e s e l o a d - c a r r y i n g additives a r e p r e s e n t e d t o g e t h e r with the c u r r e n t thoughts on t h e i r m e c h a n i s m s of action. The g e n e r a l additive types, namely; o r g a n o - s u l p h u r (e.g. disulphides), o r g a n o - p h o s p h o r u s (e.g. m e t a l diaklyl dithiophosphates, p h o s p h a t e s , etc) and o r g a n o - c h l o r o (e.g. c h l o r i n a t e d paraffins) a r e d i s c u s s e d and broadly c l a s s i fied a s a.w. or e.p. a d d i t i v e s . The effect of b a s e oil s t r u c t u r e on additive p e r f o r m a n c e is shown to be i m p o r t a n t . Typical applications r e q u i r i n g a.w. and e.p. additives a r e given.
T h e r e a r e few p e t r o l e u m p r o d u c t s c u r r e n t l y sold which do not contain at l e a s t one additive component, and the vast m a j o r i t y of p r o d u c t s contain m o r e than one additive. T h e s e a d d i t i v e s g e n e r a l l y have the g r e a t e s t use in l u b r i c a n t s and can be of many d i f f e r e n t types, e.g. v i s c o s i t y index i m p r o v e r s , p o l y m e r i c d i s p e r s a n t s , m e t a l d e t e r g e n t s , c o r r o s i o n inhibitors, a n t i - f o a m i n g a d d i t i v e s and l o a d - c a r r y i n g additives. I intend to d i s c u s s the last c l a s s of a d d i t i v e s which a r e called by a wide range of different n a m e s such a s a n t i - s e i z u r e additives, a n t i - f r i c t i o n additives, lubricity additives, o i l i n e s s additives, and boundary lubrication additives. L o a d - c a r r y i n g a d d i t i v e s a r e divided into two main types; a n t i w e a r (a.w.) and e x t r e m e p r e s s u r e (e.p.) additives. I shall d i s c u s s t h e i r w e a r - r e d u c i n g r a t h e r than f r i c t i o n - m o d i f y i n g properties.
ADDITIVE TYPES A. w. additives The most commontypes of a.w. additives are those containing phosphorus. Examples of typical phosphorus additives are shown in Table i. Table I Typical phosphorus additives
RO\/S]
Ro/ Metal dialkyl dithiophosphate
DEFINITION OF TERMS
RO\ /OR /R RO \o T r i c r e s y l phosphate (R = c r e s y l ) i.e. ,¢; / ~ . cH3 \
The t e r m s a n t i w e a r and e x t r e m e p r e s s u r e define the conditions to which a lubricant is s t r e s s e d in high load e n v i r o n m e n t s . The a.w. additive is n o r m a l l y m o s t effective under mixed lubrication conditions. Under t h e s e conditions of m o d e r a t e l y loaded sliding contacts, it is g e n e r a l l y a c c e p t e d that an oil film e x i s t s between the s u r f a c e s , but i n t e r m i t t e n t p e n e t r a t i o n of this film by s u r f a c e a s p e r i t i e s o c c u r s . The a.w. additive probably functions by r e a c t i n g with the m e t a l a s p e r i t i e s to f o r m f i l m s which aid the oil film to r e d u c e i n t e r m e t a l l i c contact and w e a r . As the load is i n c r e a s e d , the t e m p e r a t u r e of the contact i n c r e a s e s until, at a c e r t a i n value, the bulk oil film c o l l a p s e s . A c a t a s t r o p h i c i n c r e a s e in wear, a c c o m p a n i e d by a rapid i n c r e a s e in t e m p e r a t u r e o c c u r s , leading to welding of the s u r f a c e s in the a b s e n c e of an e.p. additive. However, when p r e s e n t , the e.p. additive r e a c t s with the m e t a l s u r f a c e to f o r m an inorganic s u r f a c e coating which can p r e v e n t welding of the s u r f a c e s and halt the c a t a s t r o p h i c w e a r p r o c e s s . It is probably r e a s o n a b l e to a s s u m e that the m a j o r d i f f e r e n c e between the a.w. and e.p. r e g i o n s of l u b r i c a t i o n is in the t e m p e r a t u r e s r e a c h e d in the sliding contact. * The B r i t i s h P e t r o l e u m Company Limited, BP R e s e a r c h Centre, C h e r t s e y Road, S u n b u r y - o n - T h a m e s , Middlesex
\--,
/0
RO~ /OH
RO K
RO//P%
RO/P~XH
Dialkyl phosphite
Phosphate
Metal dialkyl d i t h i o p h o s p h a t e s a r e also e x t r e m e l y efficient antioxidants and have been i n c o r p o r a t e d into c r a n k c a s e oil f o r m u l a t i o n s for o v e r 30 y e a r s . Organic c a r b o x y l i c a c i d s RCO2H and e s t e r s RCO2R 1 w h e r e R 1 is alkyl, e.g. s t e a r i c , oleic acids and t h e i r e s t e r s a l s o have good a.w. p r o p e r t i e s ; they a r e a l s o known to be good c o r r o s i o n i n h i b i t o r s . Organic a m i n e s R1NR2R 3 w h e r e R 1 is alkyl R2, R 3 --- H, alkyl e t c . h a v e m o d e r a t e l y good a.w. p r o p e r t i e s dependent on t h e i r s t r u c t u r e ; they a r e a l s o often u s e d as s a l t s of c a r b o x y l i c or p h o s p h o r u s a c i d s . A.w. additives a r e e m p l o y e d in e x t e n s i v e r a n g e of l u b r i c a n t s , e.g. automotive c r a n k c a s e oils for gasoline and d i e s e l p o w e r e d engines, automatic t r a n s m i s s i o n fluids, h y d r a u l i c fluids turbine oils, gearbox l u b r i c a n t s , aviation l u b r i c a n t s .
TRIBOLOGY August 1970
145
ExLreme p r e s s u r e
additives
t h e m o s t c o m m o n t y p e of e.p° a d d i t i v e s a r e t h o s e containings u l p h u r a n d / o r c h l o r i n e . T y p i c a l s u l p h u r - c o n t a i n i n g e.p. ~dditives are: O CH 3 (C H 2 ) x - - C H = C H - C H - - ( C H 2 ) x C - - O R
Sn
T h e remainder of the article is devoted to a discussion of the a°w. and e.p. properties of the following classes of additive: I
Metal dithiophosphates
2
Organo-phosphorus additives excepting class 1
3
Org~ano- sulphur additives
4
Organo- chlorine additives
T h e effects of other factors such as base oil type, atmosphere, additive interference, are briefly discussed° Additives of the carboxylic-acid and amine type have been studied ex~ensively from a fundamental viewpoint~ but are not disc u s s e d in t h i s a r t i c l e s i n c e t h e i r c o m m e r c i a l use a s coy/. a n d eop. a d d i t i v e s i s s m a l l c o m p a r e d to the a d d i t i v e c l a s s e s mentioned above.
CH 3 (CH2)x--CH=CH--CH--(Ct! 2 ) x C - O R © S n l p h u r i z e d fatty e s t e r
<~-CH2~--S--CH2 ~ Dibenzyl disulphide
METAL DITHIOPHOSPHATES
~ulphurized t e r p e n e s and s u l p h u r i z e d o l e f i n s a r e o t h e r ~xamples. T h e m o s t c o m m o n c h l o r i n e - c o n t a i n i n g a d d i t i v e s Lre the c h l o r i n a t e d p a r a f f i n s s o l d u n d e r t r a d e n a m e C e r e thlor and t h e c h l o r i n a t e d a r o m a t i c s sold u n d e r t r a d e n a m e %rochlor. C h l o r i n a t e d p a r a f f i n s c a n c o n t a i n up to 70% C1. Fhese e.p. a d d i t i v e s a r e m a i n l y u s e d in i n d u s t r i a l l a b r i c a :ion, e.g. m e t a l c u t t i n g and m e t a l f o r m i n g o p e r a t i o n s , heavy ,~ear l u b r i c a t i o n . H o w e v e r , a v e r y i m p o r t a n t a p p l i c a t i o n of .hese m a t e r i a l s i s in t h e r e a r a x l e s of a u t o m o t i v e v e h i c l e s °
Metal dialkyi and diaryl dithiophosphates have been used for many years as antioxidant anti-corrosion/antiwear additives in a wide range of lubricants. Many attempts to determine their mechanism of action as load-carrying additives have been made3-12; examples of the structures of some of the dithiophosphates are given in Table 2. Before discussing s o m e aspects of this work, it should be stated that, even now~ the detailed mechanism of action of metal dithiophosphates a s load-carrying additives i s s t i l l not understood.. Table 2 Structure of metal diLhiophosphates
%SSESSMENT OF
LOAD-CARRYINGADDITIVES
:deally all a.w. and e.p. additives should be tested in the ~artieular application for which they are developed. The :we main objections to this are t h a t full-scale testing of all :xperimental additives is too expensive and too time~onsuming,
CHaCHCH2CH--©,"
]
CH3
~ence, ever since additives were first put into lubricants, ~imulated accelerated rig tests to assess the performance )f new additives have been developed. Their main purpose ms been to function as screening tests for weeding out poor ~dditives without going to the extent of full scale tests. Many different types of rig test exist today, and a number of reviews on their performance, have been published b 2 Fypical rig tests currently used in the assessment md cop. additives are: I 2 3 4 5 6 7 8
DerLain a n a l y t i c a l t e c h n i q u e s h a v e a l s o b e e n found to be very u s e f u l in s t u d y i n g t h e l o a d - c a r r y i n g a c t i o n of a d d i t i v e s E x t e n s i v e u s e h a s b e e n m a d e of e l e c t r o n p r o b e m i c r o a n a l y s i s (eop.m.a.) an an aid to d e t e r m i n i n g t h e n a t u r e of the film Ln the l u b r i c a t e d c o n t a c t . M o r e d e t a i l s on t h e a p p l i c a t i o n of :his t e c h n i q u e a r e g i v e n l a t e r in t h e a r t i c l e . O t h e r u s e f u l : e c h n i q u e s a r e X - r a y d i f f r a c t i o n , e l e c t r o n m i c r o s c o p y , low ~nergy e l e c t r o n d i f f r a c t i o n (1.e°e.d.) and u s e of r a d i o a c t i v e LSOtOpes of l o a d - c a r r y i n g e l e m e n t s .
:46
TRIBOLOGY
A u g u s t 1970
CH3
j 2
CH 3
CH3 CH--0~ CH 3
Shell four-ball test Falex test Almen-Wieland test SAE machine test Timken test R y d e r g e a r test IAE g e a r t e s t P i n - o n - d i s c test
F h e s e r i g t e s t s c o v e r a wide r a n g e of the p a r a m e t e r s which lave a s~gnifieant e f f e c t on a d d i t i v e p e r f o r m a n c e ; n a m e l y : e m p e r a t u r e , s l i d i n g s p e e d , m e t a l type, g e o m e t r y of c o n t a c t , {nd d u r a t i o n of t e s t .
jS
"x'~//'
Metal di-4-methyl pentyl-2 dithiophosphate ( M = an, Ni~ Cd. i°b etc}
of a.w.
the performances of additives in both the a.wo and e.po L-egions can be determined in the majority of rig tests, and :he one selected is largely up m the particular choice of the nvestigator. Detailed descriptions of the rig tests are given n the literaturel, 2. The most widely used rig test is probably the four-ball machine which can be operated under both Lw. and e.p.test conditions. Other rig tests used in the ~uthor's studies are the Falex, Almen-Wieland and pin-oniisc machines.
~-,
CH
O
\ S ~ M 3 2
CH3
Metal di-iso-propyl dithiophosphate (M a s above) COH19
O
fil
Zinc di nonylphenyl dit~/ophosphate
A considerable amount of the m o r e important w o r k on m e ~ (mainly an) dithiophosphates w a s published in the late 1950's and early 1960'sS-So T h e efforts of the investigators w e r e concentrated on analysing the effect of change in the organic structure of the molecule on performance, and on the use of radioactive ,isotopes for studying element contents on metal surfaces lubricated by oils containing these dithiophospb~teso T h e results of this w o r k showed that: I
Metal dithiophosphates d e c o m p o s e d a~nd reacted with the m e t a l s u r f a c e to f o r m f i l m s c o n t a i n i n g m e t a l , p h o s p h o r u s and s u l p h u r , but the s u l p h u r c o n t e n t w a s m u c h lower, on a reel r a t i o b a s i s ° t h a n t h e p h o s p h o r u s a n d metal content. T h e rate of formation and thickness of film w a s dependent on temperature~ additive concent1~a ~ tion, time eta,
2
Generally, d e c r e a s e d t h e r m a l stability of the d i t h i o p h o s phate c o r r e l a t e d with i m p r o v e d l o a d - c a r r y i n g activity.
More r e c e n t work in t h i s field has been that of Rowe and Dickert 9, Jayne and Elliot lo, and the author and c o l l e a g u e s 11, 12. Rowe and D i c k e r t ' s work involved a s y s t e m a tic study of the effect of both changes in alkyl group and m e t a l a t o m on dithiophosphate p e r f o r m a n c e using a p i n - o n disc a p p a r a t u s . T h i s work showed that the w e a r r a t e of the pin i n c r e a s e d a s the t h e r m a l stability i n c r e a s e d for a s e r i e s of d i i s o p r o p y l d i t h i o p h o s p h a t e s of d i f f e r e n t m e t a l s , e.g. Ag (I), P b (II), Cd (H), Zn (II) and Cu (l.I). They a l s o showed that the w e a r r a t e i n c r e a s e d a s the t h e r m a l stability i n c r e a s e d due to change in the nature of the alkyl group. At t h i s stage, it should be e m p h a s i z e d that in my opinion m e t a l d i t h i o p h o s p h a t e s p r i m a r i l y function a s a.w. additives, with m o d e r a t e e.p. activity due to the p r e s e n c e of sulphur in the molecule. E x t e n s i v e t e s t i n g on a wide r a n g e of m e t a l dialkyl d i t h i o p h o s p h a t e s of varying s t r u c t u r e has been c a r r i e d out at the BP R e s e a r c h C e n t r e in o r d e r to f u r t h e r elucidate t h e i r m e c h a n i s m of action. Typical a.w. r e s u l t s for d i f f e r e n t m e t a l d i - 4 - m e t h y l p e n t y l - 2 dithiophosphates a r e given in Fig 1 and the m a r k e d effect of m e t a l type is c l e a r l y shown. E.p. r e s u l t s for the s a m e compounds a r e given in Fig 2, and s o m e v e r y i n t e r e s t i n g o b s e r v a t i o n s can be made: 1
2
3.0 2-8
2.6 "E 2-4 E ~
2.2
o_ = 2.0! -o ~
1.8 1.6
t-
o ~
1'4
12 Lc
A d i r e c t r e v e r s a l in the o r d e r of p e r f o r m a n c e of t h e s e a d d i t i v e s is o b s e r v e d in going f r o m the a.w. to the e.p. region.
1.0
A.w. activity i n c r e a s e s with d e c r e a s i n g t h e r m a l stability of the dithiophosphate whilst e.p. activity d e c r e a s e s .
0"8 50
In an effort to explain t h e i r m e c h a n i s m f u r t h e r , e l e c t r o n p r o b e m i c r o a n a l y s e s of the w e a r s c a r s f o r m e d u n d e r a.w. and e.p. conditions w e r e c a r r i e d out 12. The r e s u l t s c o n f i r m ed that f i l m s containing the metal, p h o s p h o r u s and sulphur w e r e f o r m e d on the s u r f a c e . They also showed that the p r e s ence of l a r g e quantities of p h o s p h o r u s and to a l e s s e r extent, the metal, was e s s e n t i a l for good a.w. p e r f o r m a n c e ; the p r e s ence of sulphur on the s u r f a c e was of l e s s e r i m p o r t a n c e .
0"80
o Bi I1"[ x Sn rr 0 Sb m A Pb]I
070 E E
•
Zn O Bismuth
o 0.60
E 0 =6
Tin
x
0-50 Lead ¢0
04,0
-
Zinc
Cadmium I
30
I
45 Time [ m i n i
,
,
•
!
.
150 Load [ k g ]
.
.
.
~
m
200
F i g 2 F o u r - b a l l e x t r e m e p r e s s u r e r e s u l t s f o r a s e r i e s of m e t a l d i - ( 4 - m e t h y l pentyl-2) dithiophosphates at a c o n c e n t r a t i o n of 4 m i U t a t o m s of p h o s p h o r u s p e r 100 g of blend
The r e c e n t l y published work by both Jayne and Elliot lo and F u r e y 13 c o n f i r m a c o r r e l a t i o n between the t h e r m a l stability of the additive and l o a d - c a r r y i n g p e r f o r m a n c e , when t h e r m a l stability is a l t e r e d by changing the organic, r a t h e r than the metal, part of the dithiophosphate m o l e c u l e . Jayne and Elliot o b s e r v e d little d i f f e r e n c e in dithiophosphate p e r f o r m a n c e in both f o u r - b a l l e.p. and a.w. t e s t s , but did e s t a b l i s h that the lower the t h e r m a l stability, the b e t t e r the OK load in the Timken t e s t . F u r e y o b s e r v e d a s i m i l a r t r e n d for Zn s a l t s of varying t h e r m a l stability in cam and tappet t e s t s and we have r e c e n t l y o b s e r v e d a s i m i l a r t r e n d using p i n - o n - d i s c tests. Summing up, t h e r e is little doubt that the nature of the m e t a l atom and of the o r g a n i c h y d r o c a r b y l portion of the molecule, h a s a m a r k e d effect on both the a.w. and e.p. activity of m e t a l d i t h i o p h o s p h a t e s . T h e r e i s also c o n s i d e r a b l e e v i d e n c e to show that the m o r e t h e r m a l l y unstable the dithiophosphate is, the b e t t e r its l o a d - c a r r y i n g p r o p e r t i e s .
V"
0"30
,
However, the e.p. r e s u l t s showed that the w e a r s c a r s for each additive had e l e m e n t c o n t e n t s and d i s t r i b u t i o n s s i m i l a r to t h o s e obtained in the a.w. region, d e s p i t e the r e v e r s a l in p e r f o r m a n c e of t h e s e a d d i t i v e s between the two r e g i o n s . T h e s e r e s u l t s t h e r e f o r e highlight the point that good p e r f o r m a n c e by an additive in the a.w. region is not n e c e s s a r i l y a c c o m panied by good p e r f o r m a n c e in the e.p. region. In fact, the r e s u l t s of t h i s work show that s i m i l a r s u r f a c e f i l m s a r e f o r m e d in both a.w. and e.p. r e g i o n s by each additive. The c h e m i c a l s t r u c t u r e of the dithiophosphate then d e t e r m i n e s the region in which the film f o r m e d has good l o a d - c a r r y i n g properties.
v Fe I11 • Ni IT + Cdrr
r~ A g I Load 15 kg
I
100
I
60
F i g 1 F o u r - b a l l a n t i w e a r r e s u l t s for a s e r i e s of m e t a l d i (4-methyl pentyl-2) dithiophosphates at a c o n c e n t r a tion of 4 m i l l i a t o m s of p h o s p h o r u s p e r 100 g of blend
Although much effort h a s gone into trying to d e t e r m i n e m e c h a n i s m s of bulk t h e r m a l d e c o m p o s i t i o n of t h e s e compounds, no c o n c l u s i v e p i c t u r e oI t h e i r m e c h a n i s m of l o a d - c a r r y i n g action h a s e m e r g e d . F u r t h e r c l a r i f i c a t i o n r e q u i r e s detailed s t u d i e s of the a d s o r p t i o n / r e a c t i o n of m e t a l d i t h i o p h o s p h a t e s with m e t a l s u r f a c e s . The main u s e for a d d i t i v e s of t h i s type is always likely to be in the a.w. field r a t h e r than in the very highly loaded e.p. region.
TRIBOLOGY August 1970
147
0RGANO-PHOSPHORUS
ADDITIVES
[t has been generally accepted that compounds containing phosphorus are automatically good a.w. additives with poor e.p. activity. A study of the literature in this field confirms [hat phosphorus additives of the tricresyl phosphate~ dialkyl phosphat% dialkyl phosphite type have no e,p. activity. However~ recently published work ~4 has shown that the presence of phosphorus in a molecule does not 'guarantee ~ good a.w. activity, but that the activity is dependent on the environment of the phosphorus atom in the molecule. Since the grea~ majority of published work on phosphorus compounds no~ containing sulphur refers to tricresyl phosphat% this section will be split up into two parts discussing (a) organs-phosphorus compounds in general and (b) tricresyl phosphate (t.copo) in particular.
General organo-phosphorus additives F o u r - b a i l a,wo and e.po t e s t s (m c e r t a i n c a s e s ) w e r e c a r r i e d out on a wide r a n g e of p h o s p h o r u s compounds ~4 a s listed. the Table 3. The r e s u l t s showed that all the p h o s p h o r u s additives which degraded~ e i t h e r t h e r m a l l y o r hydrolytieaily~ to f o r m a c i d s of low pK a (pKa is a m e a s u r e of the streng%h of an acid and the l o w e r i t s value~ the s t r o n g e r the acid} had
Ta~le 30rgano-phosphorus
additives
CH 3
CH3
These results were also supported by pin-on-disc and Falex test results. No studies on the films formed on the surfaces by ~hese additives (with the exception of t,e.p,) has yet been reported° However, a speculative suggestion is that these phosphorus additives absorb on the surface, degrade under condition of i n c r e a s i n g s e v e r i t y to f o r m p o l a r s p e c i e s which r e a c t with the iron to f o r m i r o n o r g a n o / i n o r g a n o p h o s p h o r u s eompounds~ The cow. activity of the p h o s p h o r u s a d d i t i v e s i s probably c o n t r o l l e d by the ra~e of f o r m a t i o n o~ t h i s !ayer~ the s t r e n g t h of a t t a c h m e n t of the l a y e r and by i t s r a t e of attrition°
T r i c r e s y l p h o s p h a t e (t.Copo) C o n s i d e r a b l e work .has been published on the m e c h a n i s m ei load-carrying action of t.cop.15-1< T.copo has been used extensively in automotive crankcase ferrnu!ations~ hydraulic and turbine oils and aviation lubricants and is probably second only to Zn dialkyl dithiophosphate in its popu!arH:y as an a.w, additive. Although confusion still exists on the exae~ mode of action of t.c.p, as an aoW. additiv% some excellent papers published over the past five to six years trove helped tc correct some original misleading thoughts on its mechanism of action. It was initially thought that toc, po decomposed and reacted with iron surfaces to form an iron iron phosphide eutectie and that the properties of this curestie w e r e r e s p o n s i b l e f o r its a . w . p r o p e r t i e s . Godfreyt6~ B i e b e r et ai 17 and B a r c r o f t and D a n i e P ~ have all published w o r k which l e a d s ~o a c l e a r e r undersflanding of the m e c h a n i s m of action of toc.p, Goldblatt and Appeldoorr~ i s have a l s o r e c e n t l y p u b l i s h e d work in which the e f f e c t s of a t m o s p h e r e and m o i s t u r e on p e r f o r m a n c e w e r e studied~ A c r i t i c a l study of t h e s e p a p e r s coupled with the a u t h o r ' s wor~ in t h i s field l e a d s to the following conclusions,
Tricresyl p h o s p h a t e
C ~H 90
excellent a.w. properties. However, additives, e.g. phosphinic and phosphonic derivatives which degraded to form weak a c i d s had a.w. p r o p e r t i e s ~.ittle d i f f e r e n t to the b a s e oil indicating that the p h o s p h o r u s a~om p e r s e did no~: bouarantee good a.wo activity, A n o t h e r i n t e r e s t i n g f a c t o r e m e r g i n g f r o m these results was that certain phosphorus additives~ eog phosphoramidates and amine phosphates had considerably better initial seizure loads in the e.p,test than tricresyl phosphate°
O
C 4H 9° / / " ~ N H C 6H ~3
l
The polar i m p u r i t i e s in t,Cop. {these i m p u r i t i e s a r e typ[cally dialkyl o r d i a r y [ p h o s p h a t e acids; a r e b e t t e r l o a d - c a r r y i n g a d d i t i v e s than the n e u t r a l e s t e r .
2
The neutral te.p. ester can degrade hydrolytically and/ o r t h e r m a l l y u n d e r a.w. conditions ~o f o r m iron o r g a n o / inorgano phosphate f i l m s which have e x c e l l e n t ioad~ c a r r y i n g p r o p e r t i e s . T h i s d e c o m p o s i t i o n ~s f o r m f i r s t the i r o n organo and finally i r o n - i n o r g a n o phosphate films is elegantly demonstrated in the work of Barcroft and Daniel,
Dibutyl N-hexyl phosphoramidate
C4H90
/JO
C4HpO)~OH Dibutyl phosphate
Hs~R
p C 4 H 90/ Amine C 4H
~O phosphate
90\/0
C4H90/ ~C6H13 Dibutyl hexyl phosphonate
C~H13~p/~ O
¢sH~/ K~OC4H9 Dthexyl butyl p h o s p h i n a t e 148
Organophosphorus additives of the ~.C.po phosphoramidate~ dialkyl phosphate, dialkyl phosphite type are used in appiica~ tions where good a.w. protection and anti-scuffing is required. They have the advantage of being less corroslve~ part~cu~ larly ~o yellow metals~ than metal dithiophosphates but have the d i s a d v a n t a g e of having s i m i l a r ~.p. p r o p e r t i e s ~o base oilso They also tend to be m o r e e x p e n s i v e than typical dithiophosphates.
TRIBOLOGY August 1970
ORGANO-SULPHUR
ADDITIVES
OrganO-sulphur materials were probably amongst the first well known load-carrying additives. Flowers of sulphur have been added to lubricating oils to increase their loadcarrying properties for many years and even today, sulphur-~ ized oils are probably used in certain industrial applications, The most commonly used sulphur additives today are sulphurized fats, sulphurized olefinso and sulphurized terpeneso
Organs-sulphur additives are accepted as the most important additive class for e.p. lubrication and a detailed study of
the m e c h a n i s m of t h e i r l o a d - c a r r y i n g action h a s been c a r r i e d out by the author and c o l l e a g u e s 19, 20,2 ~ T h i s work has been r e c e n t l y s u m m e d up in a r e v i e w p a p e r 22. The e.p. p r o p e r t i e s of a range of o r g a n o - s u l p h u r compounds (Table 4) w e r e a s s e s s e d using p r i m a r i l y the f o u r - b a l l e . p . t e s t but F a l e x and A i m e n - W i e l a n d t e s t s on s e l e c t e d additives w e r e c a r r i e d out.
o ,,
OioLlyl disulphide Dibenzy[ disutphide Di-ter~-buty( disu!,phide v Di-D-bUtyt disu(phide n Diphenyl disu!.phide • Dibenzy[ rnonosu~,phide x Di-_n-buty[ monosu[phide
3"2 f/~Bi~se 3'0 2"8
Table 4 0 r g a n o - s u l p h u r
additives
E
e
/(~>-s-s-, E
Diphenyl disulphide
2"6 2"h
2"2
b
Dibenzyl disulphide
2'0 1"8
c
CH3(CH2)n--S--S--(CH2)nCH3 Di-n-alkyldisulphide
~u
1'6 1.4 1"2
Dibenzyl monosulphide
1"0
CH31 ~ H3 CH3--C--S--S--C--CH3 i
i
CH 3
CH 3
D i - t e r t butyl disulphide
i
The c h e m i c a l s t r u c t u r e of the sulphur compound had a m a r k e d effect on p e r f o r m a n c e
2
Disulphides w e r e b e t t e r than m o n o s u l p h i d e s and the o r d e r of i n c r e a s i n g e.p. activity was; diphenyl < d i - n butyl < d i - t e r t b u t y l < dihenzyl < diallyl (Fig 3)
3
E.p. p e r f o r m a n c e of a r a n g e of dialkyl d i s u l p h i d e s d e c r e a s e d with i n c r e a s e in alkyl chain length, whilst inc r e a s i n g alkyl chain length had no effect on the e.p. p e r f o r m a n c e of d i - t e r t - a l k y l d i s u l p h i d e s (Fig 4).
A m e c h a n i s m was then put f o r w a r d which s a t i s f i e d all the above conclusions. This was that the e.p. activity of sulphur a d d i t i v e s is dependent on the e a s e of cleavage of the c a r b o n sulphur bond in the m o l e c u l e so that an inorganic i r o n - s u l phur l a y e r is f o r m e d . Thus d i s u l p h i d e s would be expected to be b e t t e r than monosulphides, s i n c e in the latter, two c a r b o n sulphur bonds have to be broken to r e l e a s e the sulphur, whilst only one c a r b o n - s u l p h u r bond i s broken in the d i s u l phide. This m e c h a n i s m also explains the o r d e r of p e r f o r m a n c e of the d i f f e r e n t disulphides, s i n c e the c a r b o n - s u l p h u r bond is diallyl and dibenzyl d i s u l p h i d e s would be m o s t easily broken due to the stability of the allyl o r benzyl r a d i c a l f o r m e d . Also i n c r e a s e d branching on the c a r b o n atom n e a r est to the sulphur would weaken the c a r b o n - s u l p h u r bond due to i n c r e a s e d s t e r i c hindrance, i.e. b r a n c h e d groups a r e m o r e bulky thus r e q u i r i n g m o r e s p a c e and hence inducing a s t r a i n on the c a r b o n - s u l p h u r bond. The effect of d e c r e a s i n g activity with i n c r e a s i n g alkyl chain length i s explicable on the b a s i s of l e s s sulphur p e r unit a r e a being available to r e a c t with the s u r f a c e a s the b u l k i n e s s of the disulphide i n c r e a s e s . The a.w. p r o p e r t i e s of t h e s e o r g u n o - s u i p h u r compounds w e r e also d e t e r m i n e d using the f o u r - b a l l a.w. t e s t 2o. O r g a n o sulphur compounds a r e known to be poor a.w. additives and the r e s u l t s of the study c o n f i r m e d t h i s . The b e s t r e s u l t s
Local {kg]
300
400
F i g 3 F o u r - b a l l w e a r against load for a s e r i e s of disulphides and m o n o s u l p h i d e s at a c o n c e n t r a t i o n of 37.04 m i l l i a t o m s sulphur p e r 100 g p a r a f f i n
The e . p . t e s t r e s u l t s 19 for a s e r i e s of m o n o - and disulphides, sulphoxides etc c l e a r l y showed that: 1
200
100
3"2 3-0 2.8 "E
E2.6
~, ~ 2.4 E .o_ 2-2
~ 2.0 ~ 1.8
:~ 1-6 1"4
-z/,
Oi-o-dodecy( disulphide Oi-o-octy[ disulphide Di-n-buty[ disulphide Diethy( disulphide Di-tert-buty[ disulphide Di-tert-dodecy[ disutphide
o
1.2
o
;-0
rl |
100
.
L
,
,
I
~
,
200 Load [ kg ]
]
.
,
,
•
300
400
F i g 4 F o u r - b a i l w e a r against load for a s e r i e s of dialkyl disulphides at a c o n c e n t r a t i o n of 18.52 m i l l i m o l e s p e r 100 g p a r a f f i n TRIBOLOGY August 1970
149
obtained with these additives were only equivalent to a medicore a.w. activity by phosphorus additive standards (Fig 5L The results showed that: i
2
0"80
A.w, properties of disulphides increased along the series di-n-butyl < diallyl < dibenzyl < diphenyl and monosulphides were poorer than the corresponding disulphides (Fig 5) Increasing {he chain increased their a.wo with branched alkyl their corresponding
length of di-n-alkyl effectiveness whilst groups had inferior n-alkyl derivatives
disulphides disulphides properties to (Fig 6).
Thus it was immediately obvious that factors governing the e.p. and a.w. performance of disulphides were different, as already observed for the metal dithiophosphates. These results were explicable on the basis of the a.w. activity of organo-sulphur compounds being dependent on the ease of cleavage of the sulphur-sulphur bond to form an iron mercaptide layer, and by the physical properties of this layer once formed. This hypothesis can explain the results as follows: The sulphur-sulphur bond is known to be the weakest part of the disulphide molecule and is thus likely to break first. Furthermore, diphenyl disulphide would be expected to be the best additive [n this respect, since the resulting phenyl mercaptyl radical would be highly resonance stabiiizedo
a D i - n - buty~ disulphide o D{a[[y[ disulphide x Dibenzy[ disutphide e Diphenyt disulphide _
_
_ ......
0"70
T ~ O-gO
.~ 0-50 c#
0'40F J
Disulphides should be better thmn monosulphides since the latter do not contain a weak sulphursulphur bond.
I
Good phosphcrus-addilive
i
0'30
The improvement in a.w. activity with increased chain length is explicable from the physical properties of the surface layer. Levine and Zisman 23 observed a similar effect on friction reducing properties of n-alkyl earboxylic acids and attributed this to an adlineation of the molecules in the surface to form a solid-type film. They stated that the intermolecular cohesional energy required to effect this adlineation increased with increasing aLkyl chain length. Thus the overall mechanism of load-carrying action of disulphides, and probably organo-sulphur compounds in general~ can be proposed as follows. Firstly, the disulphide is sorbed on the iron surface and cleavage of the sulphur-sulphur bond o c c u r s s o that an zron mercaptide l a y e r i s f o r m e d althougb ~he exact order of this reaction sequence is no~ known with certainty~ As loads approaching e.po condRions are reached, much higher temperatures are found in the contact zone, and cleavage of the carbon-sulphur bond to form an inorganic sulphur-containing-layer occurs,
30
45 Time I.rnin]
~0
Fig 5 Four-ball wear against time for a series of disulphides at a c o n c e n t r a t i o n of 1 8 . 5 2 m i l l i m o t e s p e r 100 g of p a r a f f i n
0"90
o Oi-D-buty ~, disulphide x Oi-0-ocly[ disulphide o Di~-dodecy~ disulphide Di-n- hexadecy[ disu~,phide Di-ter_j-hexodecy! disulphide
0"80
O Di-D-octo.decy[ disulphide
7
i
=
&n electron probe m i c r o analysis study 21 of the s u r f a c e f i l m s f o r m e d u n d e r e.p. a n d a . w . Conditions w a s c a r r i e d out Ln o r d e r to s u b s t a n t i a t e t h e a b o v e h y p o t h e s i s . T h e e . p , m o a . r e s u l t s f o r a s e r i e s of d i s u l p h i d e s in t h e e.po r e g i o n s h o w e d ~hat: 1
S u l p h u r w a s p r e s e n t in a l l t h e w e a r s c a r s and t h e o b s e r v e d o r d e r of i n c r e a s i n g s u l p h u r c o n t e n t w a s :
di-n-lauryl dia!lyl
< diphenyl < di-n-butyl
< di-tert~butyl
<
The sulphur conten~ increased with applied load in the eop~ region and sulphur was present to a depth of several microns. Under a.w. conditions, little sulphur was observed in the wear scars. ~hus these results fully support the suggested mechanisms ~f action of organo-sulphur compounds. We are currently carrying out detailed studies of adsorption/reaction of these additives with metal surfaces with a view to proving this hypothesis. One interesting aspect of this work on organo-sulphur additives was that the trend of results for a range of disulphides in rig ~eszs, gave a strong indication of the condition being
TRiBOLOGY
~ 0"60 N
This order followed exactly their order of e.p. activity, The determined sulphur contents ranged from 6% for di-n-lauryl disulphide to over 30°/°for diallyl disulphide.
150
"~ 0-70
August
1970
°
g 0.50 r
~
~
i
0.3o <
30
Fig 6
45 Time [ rr4nj
Four-bali wear against time for a series of dialkyi disulphides at 18.52 mi!limo!es per I00 g paraffin.
~
a s s e s s e d by the rig. F a l e x and A l m e n - W i e l a n d e.p. and a.w. t e s t s w e r e c a r r i e d out on diphenyl, d i - t e r t - b u t y l and dibenzyl d i s u l p h i d e s and r e s u l t s a r e shown in T a b l e s 5 and 6. F r o m the o r d e r of p e r f o r m a n c e it can be s e e n that the F a l e x e.p. and a.w. t e s t r a t e s the d i s u l p h i d e s in the s a m e o r d e r a s the f o u r - b a l l e.p. and a.w. t e s t , but in both e.p. and a.w. v e r s i o n s , the A l m e n - W i e l a n d t e s t a p p e a r s to a s s e s s a.w. p r o p e r t i e s only of l u b r i c a n t s . Thus a c l e a r e r u n d e r s t a n d i n g is obtained of why so many conflicting r e s u l t s a r e often o b s e r v e d for a s e r i e s of l o a d - c a r r y i n g a d d i t i v e s in d i f f e r e n t t e s t r i g s .
Table 5 F a l e x t e s t r e s u l t s (All additives w e r e t e s t e d at 18.52 m i l l i m o l e s / 1 0 0 g liquid paraffin)
A v e r y significant f a c t o r c o n c e r n i n g o r g a n o - c h l o r i n e a d d i t i v e s is the m a r k e d s y n e r g i s t i c e.p. effect o b s e r v e d when m i x t u r e s of o r g a n o - s u l p h u r and o r g a n o - c h l o r i n e additives a r e used. This fact has b e e n shown as e a r l y as 1946 by P r u t t o n et a126 in an e x c e l l e n t p a p e r in which e f f o r t s to e x plain t h e s e r e s u l t s w e r e given. The main explanation put f o r w a r d was that iron sulphide f i l m s w e r e f o r m e d f i r s t but once f o r m e d , the active c h l o r i n e additives r e a c t e d much f a s t e r with the i r o n sulphide film than with the i r o n s u r f a c e . Thus the i r o n c h l o r i d e film was f o r m e d much m o r e rapidly than in d i r e c t r e a c t i o n between the c h l o r i n e additive and the i r o n s u r f a c e . M i x t u r e s of sulphur and c h l o r i n e - c o n t a i n i n g additives a r e frequently u s e d in heavy duty e.p. applications. OTHER FACTORS
Additive
E.p. t e s t
Wear test
f a i l u r e load [lb]
weight l o s s [g]
Dibenzyl disulphide
4500
0. 0187
Di - t e r t -butyl disulphide
3950
0.0510
750
0. 0216
Diphenyl disulphide
Table 6 A l m e n - W i e l a n d t e s t r e s u l t s (all additives w e r e t e s t ed at 18.52 m i l l i m o l e s / 1 0 0 g liquid paraffin) E.p. t e s t
Wear t e s t
failure l o a d / frictional f o r c e [kg]
weight loss [g]
Dibenzyl disulphide
450/170; 400/140
0. 2720
D i - t e r t - b u t y l disulphide
400/128; 350/111
0. 5026
Diphenyl disulphide
650/170; 650/158
0. 1792
Additive
Why a r e o r g a n o - s u l p h u r compounds good e.p. and poor a.w. additives ? The poor a.w. activity is probably r e l a t e d to the fact that o r g a n o - s u l p h u r compounds a r e not so r e a c t i v e to m e t a l s u r f a c e s at low t e m p e r a t u r e s a s a r e p h o s p h o r u s a d d i tives. Hence a strongly attached, stable film is not readily f o r m e d . On the o t h e r hand, under e.p. conditions the i n c r e a s e d load and t e m p e r a t u r e c a u s e s breakdown of the a d d i tive so that a p r o t e c t i v e i n o r g a n o - s u l p h u r containing film is f o r m e d . The r a t e of f o r m a t i o n and a t t r i t i o n of this film and its i n h e r e n t p r o p e r t i e s a r e r e s p o n s i b l e for the good e.p. activity of sulphur additives. ORGANO-C HLORINE ADDITIVES Typical o r g a n o - c h l o r i n e additives a r e the c h l o r i n a t e d p a r a f fins and c h l o r i n a t e d a r o m a t i c s . T h e s e additives a r e widely u s e d in m e t a l - c u t t i n g and m e t a l - f o r m i n g l u b r i c a n t s and also in automotive r e a r axle oil f o r m u l a t i o n s . O r g a n o - c h l o r i n e additives a r e , in many ways, s i m i l a r to o r g a n o - s u l p h u r a d d i t i v e s in that they have only m o d e r a t e a.w. p r o p e r t i e s but have good e.p. p r o p e r t i e s . Davey 24 has c a r r i e d out a study of the e.p. p r o p e r t i e s of o r g a n o - c h l o r i n e compounds of d i f f e r e n t c h e m i c a l s t r u c t u r e s using the f o u r - b a l l m a c h i n e . His r e s u l t s showed that the m o r e labile the C-C1 bond, the b e t t e r the e.p. p r o p e r t i e s of the c h l o r i n e - c o n t a i n i n g additive. Thus, the e.p. p r o p e r t i e s of t h e s e o r g a n o - c h l o r i n e additives a r e probably dependent on the e a s e of f o r m a t i o n of an inorganic i r o n c h l o r i d e film on the metal s u r f a c e . Little work has been p u b l i s h e d on the a.w. p r o p e r t i e s of t h e s e additives, but l i m i t e d s t u d i e s have shown that the a.w. p r o p e r t i e s of c h l o r i n e additives i m p r o v e s with i n c r e a s i n g lability of the C-C1 bond. It has been s u g g e s t e d that, in the c a s e of c h l o r i n a t e d p a r a f fins, the film f o r m e d f r o m the additive is not a c h l o r i n e - c o n taining one but r a t h e r a p o l y m e r i c c a r b o x y l i c acid m a t e r i a l 25.
Base oil The b a s e oil used in any application can have a m a r k e d effect on the p e r f o r m a n c e of both e.p. and a.w. additives. P r o p e r t i e s such as v i s c o s i t y , p a r a f f i n i c , naphthenic or a r o m a t i c content, sulphur content a r e all i m p o r t a n t . C e r t a i n b a s e oils also have naturally o c c u r r i n g s u r f a c e active s p e c i e s which can compete with the l o a d - c a r r y i n g additive for the m e t a l surface. As a g e n e r a l i z a t i o n , it can safely be said that the t h i c k e r the b a s e oil, the b e t t e r the e.p. or a.w. p e r f o r m a n c e of a blend due to the i m p r o v e d p r o p e r t i e s of the b a s e oil itself. The effect of c h e m i c a l s t r u c t u r e of the b a s e oil on additive p e r f o r m a n c e is c e r t a i n l y not yet c l e a r l y u n d e r s t o o d although w o r k e r s such as Rounds27, 2s, Appeldoorn 29, Vinogradov 3° have done c o n s i d e r a b l e work in this field. G r o s z e k 31 has also shown that the c o m p o n e n t s of b a s e oils can have e x c e l lent a.w. activity in t h e i r own right. Work c a r r i e d out by the author i n d i c a t e s that v e r y effective additives i.e. diallyl disulphide as an e.p. additive or an amine phosphate a s an a.w. additive give good p e r f o r m a n c e i r r e s p e c t i v e of b a s e oil type, However, the p e r f o r m a n c e of the m o r e m o d e r a t e a.w. and e.p. additives has been found to be a f f e c t e d by b a s e oil c h e m i c a l s t r u c t u r e . This i s a field in which c o n s i d e r a b l y m o r e r e s e a r c h could be fruitfully c a r r i e d out.
Atmosphere Variation in a t m o s p h e r e and m o i s t u r e content can have a profound effect on the efficiency of l o a d - c a r r y i n g additives. The effect of change in a t m o s p h e r e and of variation in h u m i dity on the p e r f o r m a n c e of t.c.p, has r e c e n t l y b e e n d i s c u s s e d by Goldblatt and Appeldoorn 18. Also, s i m i l a r s t u d i e s using sulphur compounds have been c a r r i e d out to a l i m i t e d extent by Togoyuchi and Takai 32 and Vinogradov 3°,33. It was d i f ficult to g e n e r a l i z e the e f f e c t s of t h e s e p a r a m e t e r s but the p r e s e n c e of 02 in the s y s t e m i s r e c o g n i s e d to be b e n e f i c i a l whilst e x c e s s i v e m o i s t u r e is d e t r i m e n t a l . Additive i n t e r a c t i o n I n t e r a c t i o n of d i f f e r e n t additives, in f u l l y - f o r m u l a t e d oils is now b e c o m i n g a field worthy of fundamental study. T h e r e is an i n c r e a s i n g t r e n d to put a l a r g e n u m b e r , and a higher conc e n t r a t i o n , of additives in m o d e r n l u b r i c a n t s and hence i n t e r action e f f e c t s b e t w e e n additives a r e b e c o m i n g m o r e i m p o r t ant. A.w. and e.p. additives a r e usually highly p o l a r m a t e r i a l s with a high affinity for m e t a l s u r f a c e s , on which f i l m s m u s t be f o r m e d , b e f o r e the additives b e c o m e effective. On the o t h e r hand, l u b r i c a n t s can also contain r u s t i n h i b i t o r s and m e t a l - c o n t a i n i n g d e t e r g e n t s which also have an affinity f o r m e t a l s u r f a c e s and hence c o m p e t i t i v e a d s o r p t i o n and r e action can occur. Very little s y s t e m a t i c work has b e e n publ i s h e d in this field. However, t h e r e a r e indications that g r e a t e r antagonistic e f f e c t s a r e o b s e r v e d with the l e s s p o l a r additives of the sulphur and chlorine type r a t h e r than with the highly polar, s u r f a c e active p h o s p h o r u s a d d i t i v e s . CONCLUSIONS
This a r t i c l e highlights the complexity of the m e c h a n i s m of action of a.w. and e.p. additives and i n d i c a t e s w h e r e a d v a n c e s TRIBOLOGY August 1970
151
have been made in recent years and where further work is n e c e s s a r y . The m a r k e d effect of the c h e m i c a l s t r u c t u r e of an additive on p e r f o r m a n c e , e v e n i n s i d e the s a m e c l a s s of compounds, is c l e a r l y i l l u s t r a t e d . Additive p e r f o r m a n c e i s affected by a wide r a n g e of p a r a m e t e r s s u c h as t e m p e r a ~ r e , a t m o s p h e r e , and b a s e oil type.
15
B a r c r o f t , Fo T. and Daniel S.G. ' T h e action of neutra~ o r g a n i c p h o s p h a t e s as e.p. a d d i t i v e s ' T r a n s a c t i o n s of the A m e r i c a n Society of M e c h a n i c a l E n g i n e e r s , Vol 87 (1965) p 751
18
Godfrey, D. ' T h e l u b r i c a t i o n m e c h a n i s m of t r i c r e s y i p h o s p h a t e on s t e e l ' T r a n s a c t i o n s of the A m e r i c a n Society of L u b r i c a t i o n E n g i n e e r s Vo! 8~ No 1 (1965)
17
B i e b e r ° H . E . , K l a u s , E . E . a n d T e w k e s b u r y , Eogo ~Astudy of t r i c r e s y l p h o s p h a t e as an additive in boundary l u b r i ~ cation , T r a n s a c t i o n s of the A m e r i c a n Society of Lubri-cation E n g i n e e r s , Vol 1i (1968) p 155
t8
Goldblatt, I. L, and Appeldoorn, J. Ko ~The a n t i w e a r be--~ h a v i o u r of [.C°po in d i f f e r e n t a t m o s p h e r e s and d i f f e r e n t b a s e s t o e k s ' , Presented at A m e r i c a n Society of L u b r i c a tion Engineers/American Society of Meclnanical E n g i n e e r s Meeting, Houston (Oct !969)
19
A l l u m , K. G. and F o r d , J. F o "The i n f l u e n c e of c h e m i c a l s t r u c t u r e on the l o a d - c a r r y i n g p r o p e r t i e s of c e r t a i n o r g a n o - s u l p h u r compounds' J o u r n a l ~f the Insti#a~e of P e t r o l e u m , VoI 51, (1965) p t45
20
A l i u m , K . Goand F o r b e s , E o S . ' T h e l o a d - c a r r y i n g p r o p e r t i e s of o r g a n i c sulphur compounds, t~Aluence of c h e m i c a l s t r u c t u r e on the a n t i w e a r p r o p e r t i e s ~f o r g a n i c d i s u l p h i d e s ' J o u r n a l of ~he I n s t i t u t e of P e t r o l e u m , Vol 53 (1967) p 173
21
Altum, K. G. and F o r b e s , E. So ' T h e l o a d - c a r r y i n g m e c h a n i s m of o r g a n m sulphur c o m p o u n d s - - a p p l i c a t i o n of e l e c t r o n p r o b e m i c r o a n a l y s i s ' T r a n s a c t i o n s of the American Society of Lubrication Erzgineers. Vol 11
ACKNOWLEDGEMENT This article is based on a lecture that was presented at a course entitled 'Chemical aspects of tribology' on 10-12 April 1970, organized by The Institution of Mechanical Engineers, Permission to publish this article has been given by The British Petroleum Company Ltd.
REFERENCES 1
T o u r r e t , R. ' T h e u s e of s i m u l a t i o n - t y p e l u b r i c a n t t e s t r i g s ' I n s t i t u t e of P e t r o l e u m G e a r and L u b r i c a t i o n Cont e r e n c e , B r i g h t o n (1964)
2
Towle, A., G l o v e r , I. and W a l k e r , R. i . H . 'Hypoid oil r e q u i r e m e n t s and d e v e l o p m e n t s for the E u r o p e a n m a r k e t ' I n s t i t u t e of P e t r o l e u m G e a r and L u b r i c a t i o n C o n f e r e n c e , B r i g h t o n (1964)
3
F u r e y , M. J. and Kunc, J . F . 'A r a d i o t r a c e r a p p r o a c h to the study of engine v a l v e t r a i n l u b r i c a t i o n , J o u r n a l of L u b r i c a t i o n E n g i n e e r i n g , Vol 14 (1958) p302
4
B e n n e t t , P. A. and Kabel, R. H. ' T h e a n t i - s c u f f p e r f o r m a n c e of m e t a l o r g a n o d i t h i o p h o s p h a t e a d d i t i v e s ' J o u r n a l of L u b r i c a t i o n E n g i n e e r i n g , Vol 19 (1963) p365
5
L o o s e r , F. H. and T w i s s , S . B . ' A n a l y s e s of f i l m s f o r m e d by radioactive e.pi additives',American Society of Lubrication Engineers meetirN, Cleveland (April 1958)
6
Loeser, E.H.,Wiquist,R.C.and Twiss, SoB. 'Cam and tappet lubrication Ill. Radioactive study of phosphorus in e.p. films'~ Transactions of the American Society of Lubrication Engineers, Vol I, No 2 (1958)
7
Looser, E° H°~Wiquist, R.C.and Twiss,S.B. 'Cam and tappet lubrication IVo Radioactive study of sulphur in e°p. film '~ Transactions of the American Society of Lubrication' Engineers, Vol I~ No 2 (1958)
8
Larson, R. ~Performance of zinc dithiophosphates as iube oil additives' Scientific Lubrication, Vol 10, No 8~ (!958) p12
9
Rowe, C. N. and D i c k e r t , J. Jo ' T h e r m a l d e c o m p o s i t i o n and a n t i w e a r function of m e t a l , O, O - d i a l k y l p h o s p h o r o di~hioates' A m e r i c a n C h e m i c a l Society P r e p r i n t s , Vol 10, No 2 (1965) D71
I0
J a y n e , C. J. J. and Elliot, J . S . ' T h e l o a d - c a r r y i n g properties of m e t a l p h o s p h o r o d i t h i o a t e s ' A m e r i c a n C h e m i cal Society P r e p r i n t s Vol 14, No 4 (1969) A139
11
Allure, K. G. and F o r b e s , E . S . ' T h e l o a d - c a r r y i n g p r o p e r t i e s of m e t a l dialkyl dithiophosphateso The effect of c h e m i c a l s t r u c t u r e ' I n s t i t u t i o n of M e c h a n i c a l E n g i n e e r s T r i b o l o g y Convention, G o t h e n b u r g (1969) P a p e r 2
t2
13
14
152
(1968) p 162 22
F o r b e s , N.S. ~The l o a d - c a r r y i n g action of organo~ s u l p h u r compounds. A r e v i e w ' , P r e s e n t e d at L u b r i c a t i o n S y m p o s i u m , P a i s l e y College of Technolog2¢" (Oct 1969). P u b l i s h e d in W e a r , Vet I5, No 2 (1970) p 87-96
23
Levine, O. and Z i z m a n , W.A. J o u r n a l of P h y s i c a l C h e m i s t ~ r Vet 61 (1957) pp 1089, 1t88
24
Davey~ Wo ' T h e e.p. p r o p e r t i e s of s o m e c h l o r i n a t e d compounds as a s s e s s e d b y the four b a i l m a c h i n e ' J o u r n a l of the I n s t i t u t e of P e t r o l e u m , VoI 31. {i945) p 45
25
J e m m e t t , A . E . ' T h e c h e m i c a l b e h a v i o u r of c h l o r i n a t e d additives under medium range temperature e~nditions' Institution of M e c h a n i c a l E n g i n e e r s Tribo!og7 C o n v e n tion. G o t h e n b u r g (1969) R e p o r t i m p e r 2
26
P r u t t o n , C. R., T u r n b u i i , D. and Dlouhy, Go ' O r g a n i c chlorine and sulphur compounds in e.p. lubrication" J o u r n a l of the I n s t i t u t e of P e t r o l e u m , Vet 32 (1946) p 96
27
Rounds F.G. ' E n v i r o n m e n t a l f a c t o r s affecting s u r f a c e coating f o r m a t i o n with l u b r i c a t i n g oil a d d i t i v e s ' T r a n s a c t i o n s of the A m e r i c a n Society of L u b r i c a t i o n E n g i n e e r s , Vol 8, No 21 (1985}
28
Rounds, Fo G. 'Some effects of n o n - h y d r o c a r b o n b a s e oil c o n s t i t u e n t s on the e f f e c t i v e n e s s of antiWear a n d cop. modifying a d d i t i v e s ' T r a n s a c t i o n s of the A m e r i c a n Society of L u b r i c a t i o n E n g i n e e r s , Vol 11~No 19 (1968)
29
Appeldoorn, J. K. and Taoo F. Fo ;The tubriei~" c h a r a c t e r i s t i c s of h e a v y a r o m a t i c s ' W e a r . Vol 12 [19~8) p 1!7
30
F o r b e s , E . S . , A l l u m , K . G . and S i l v e r , HoB. ' T h e Ioade a r r y i n g p r o p e r t i e s of m e t a l dialkyI dithiophosphateso A p p l i c a t i o n of e l e c t r o n p r o b e m i c r o a n a l y s i s ' I n s t i t u t i o n of M e c h a n i c a l E n g i n e e r s , Tribotogy Convention, G o t h e n b u r g (1969) P a p e r 6
Vinogradov, G. V., Arkarova~ V. V. and P e t r o v . A° Ao ° A n ~ w e a r and a n t i - f r i c t i o n p r o p e r t i e s of h y d r o c a r b o n s u n d e r heavy l o a d s ' W e a r , Vol 4 (!961) p 274
3t
G r o s z e k , A. Jo r W e a r - r e d u e i n g p r o p e r t i e s of m i n e r a l s oils and t h e i r f r a c t i o n s '~ i n s t i t u t i o n of M e c h a n i c a l E n g i n e e r s T r i b o l o g y Convention, P i t l o c h r y (t988) P a p e r 19
F u r e y , M . J . ' F r i c t i o n , w e a r and l u b r i c a t i o n ' J o u r n a l of I n d u s t r i a l and E n g i n e e r i n g C h e m i s t r y , Vol 62, No 3 ( M a r c h 1969) p 12
32
F o r b e s , E . S. and S i l v e r , H.B. ' T h e effect of c h e m i c a l s t r u c t u r e on the l o a d - c a r r y i n g p r o p e r t i e s of o r g a n o p h o s p h o r u s c o m p o u n d s ' A m e r i c a n C h e m i c a l Society P r e p r i n t s Vol 14, No 4 (1969) A102. P u b l i s h e d by The I n s t i t u t e of P e t r o l e u m , Vol 58 (1970) p 90
Toguyuchi, M. and Takai~ Y. ~E.p. l u b r i c a t i o n p r o p e r t i e s of s o m e sulphur compounds in m i n e r a l oil and the e f f e c t of d i s s o l v e d oxygen' B u l l e t i n of the J a p a n P e t r o l e u m I n s t i t u t e . Vol 3, (1961) p 63
33
Vinograd.ov, G., Pavlovskayao N . a n d Podolsky, Yo "Investigation of the l u b r i c a t i o n p r o c e s s u n d e r heavy fric [ion c o n d i t i o n s ' . Wear° Vol 6. (1963) p 202
TRIBOLOGY
August 1970
The lubrication r e g i o n s c o v e r e d by the t e r m s a n t i - w e a r (a.w.) and e x t r e m e p r e s s u r e (e.p.) a r e defined. The functions of a wide range of t h e s e l o a d - c a r r y i n g additives a r e p r e s e n t e d t o g e t h e r with the c u r r e n t thoughts on t h e i r m e c h a n i s m s of action. The g e n e r a l additive types, namely; o r g a n o - s u l p h u r (e.g. disulphides), o r g a n o - p h o s p h o r u s (e.g. m e t a l diaklyl dithiophosphates, p h o s p h a t e s , etc) and o r g a n o - c h l o r o (e.g. c h l o r i n a t e d paraffins) a r e d i s c u s s e d and broadly c l a s s i fied a s a.w. or e.p. a d d i t i v e s . The effect of b a s e oil s t r u c t u r e on additive p e r f o r m a n c e is shown to be i m p o r t a n t . Typical applications r e q u i r i n g a.w. and e.p. additives a r e given.
T h e r e a r e few p e t r o l e u m p r o d u c t s c u r r e n t l y sold which do not contain at l e a s t one additive component, and the vast m a j o r i t y of p r o d u c t s contain m o r e than one additive. T h e s e a d d i t i v e s g e n e r a l l y have the g r e a t e s t use in l u b r i c a n t s and can be of many d i f f e r e n t types, e.g. v i s c o s i t y index i m p r o v e r s , p o l y m e r i c d i s p e r s a n t s , m e t a l d e t e r g e n t s , c o r r o s i o n inhibitors, a n t i - f o a m i n g a d d i t i v e s and l o a d - c a r r y i n g additives. I intend to d i s c u s s the last c l a s s of a d d i t i v e s which a r e called by a wide range of different n a m e s such a s a n t i - s e i z u r e additives, a n t i - f r i c t i o n additives, lubricity additives, o i l i n e s s additives, and boundary lubrication additives. L o a d - c a r r y i n g a d d i t i v e s a r e divided into two main types; a n t i w e a r (a.w.) and e x t r e m e p r e s s u r e (e.p.) additives. I shall d i s c u s s t h e i r w e a r - r e d u c i n g r a t h e r than f r i c t i o n - m o d i f y i n g properties.
ADDITIVE TYPES A. w. additives The most commontypes of a.w. additives are those containing phosphorus. Examples of typical phosphorus additives are shown in Table i. Table I Typical phosphorus additives
RO\/S]
Ro/ Metal dialkyl dithiophosphate
DEFINITION OF TERMS
RO\ /OR /R RO \o T r i c r e s y l phosphate (R = c r e s y l ) i.e. ,¢; / ~ . cH3 \
The t e r m s a n t i w e a r and e x t r e m e p r e s s u r e define the conditions to which a lubricant is s t r e s s e d in high load e n v i r o n m e n t s . The a.w. additive is n o r m a l l y m o s t effective under mixed lubrication conditions. Under t h e s e conditions of m o d e r a t e l y loaded sliding contacts, it is g e n e r a l l y a c c e p t e d that an oil film e x i s t s between the s u r f a c e s , but i n t e r m i t t e n t p e n e t r a t i o n of this film by s u r f a c e a s p e r i t i e s o c c u r s . The a.w. additive probably functions by r e a c t i n g with the m e t a l a s p e r i t i e s to f o r m f i l m s which aid the oil film to r e d u c e i n t e r m e t a l l i c contact and w e a r . As the load is i n c r e a s e d , the t e m p e r a t u r e of the contact i n c r e a s e s until, at a c e r t a i n value, the bulk oil film c o l l a p s e s . A c a t a s t r o p h i c i n c r e a s e in wear, a c c o m p a n i e d by a rapid i n c r e a s e in t e m p e r a t u r e o c c u r s , leading to welding of the s u r f a c e s in the a b s e n c e of an e.p. additive. However, when p r e s e n t , the e.p. additive r e a c t s with the m e t a l s u r f a c e to f o r m an inorganic s u r f a c e coating which can p r e v e n t welding of the s u r f a c e s and halt the c a t a s t r o p h i c w e a r p r o c e s s . It is probably r e a s o n a b l e to a s s u m e that the m a j o r d i f f e r e n c e between the a.w. and e.p. r e g i o n s of l u b r i c a t i o n is in the t e m p e r a t u r e s r e a c h e d in the sliding contact. * The B r i t i s h P e t r o l e u m Company Limited, BP R e s e a r c h Centre, C h e r t s e y Road, S u n b u r y - o n - T h a m e s , Middlesex
\--,
/0
RO~ /OH
RO K
RO//P%
RO/P~XH
Dialkyl phosphite
Phosphate
Metal dialkyl d i t h i o p h o s p h a t e s a r e also e x t r e m e l y efficient antioxidants and have been i n c o r p o r a t e d into c r a n k c a s e oil f o r m u l a t i o n s for o v e r 30 y e a r s . Organic c a r b o x y l i c a c i d s RCO2H and e s t e r s RCO2R 1 w h e r e R 1 is alkyl, e.g. s t e a r i c , oleic acids and t h e i r e s t e r s a l s o have good a.w. p r o p e r t i e s ; they a r e a l s o known to be good c o r r o s i o n i n h i b i t o r s . Organic a m i n e s R1NR2R 3 w h e r e R 1 is alkyl R2, R 3 --- H, alkyl e t c . h a v e m o d e r a t e l y good a.w. p r o p e r t i e s dependent on t h e i r s t r u c t u r e ; they a r e a l s o often u s e d as s a l t s of c a r b o x y l i c or p h o s p h o r u s a c i d s . A.w. additives a r e e m p l o y e d in e x t e n s i v e r a n g e of l u b r i c a n t s , e.g. automotive c r a n k c a s e oils for gasoline and d i e s e l p o w e r e d engines, automatic t r a n s m i s s i o n fluids, h y d r a u l i c fluids turbine oils, gearbox l u b r i c a n t s , aviation l u b r i c a n t s .
TRIBOLOGY August 1970
145
ExLreme p r e s s u r e
additives
t h e m o s t c o m m o n t y p e of e.p° a d d i t i v e s a r e t h o s e containings u l p h u r a n d / o r c h l o r i n e . T y p i c a l s u l p h u r - c o n t a i n i n g e.p. ~dditives are: O CH 3 (C H 2 ) x - - C H = C H - C H - - ( C H 2 ) x C - - O R
Sn
T h e remainder of the article is devoted to a discussion of the a°w. and e.p. properties of the following classes of additive: I
Metal dithiophosphates
2
Organo-phosphorus additives excepting class 1
3
Org~ano- sulphur additives
4
Organo- chlorine additives
T h e effects of other factors such as base oil type, atmosphere, additive interference, are briefly discussed° Additives of the carboxylic-acid and amine type have been studied ex~ensively from a fundamental viewpoint~ but are not disc u s s e d in t h i s a r t i c l e s i n c e t h e i r c o m m e r c i a l use a s coy/. a n d eop. a d d i t i v e s i s s m a l l c o m p a r e d to the a d d i t i v e c l a s s e s mentioned above.
CH 3 (CH2)x--CH=CH--CH--(Ct! 2 ) x C - O R © S n l p h u r i z e d fatty e s t e r
<~-CH2~--S--CH2 ~ Dibenzyl disulphide
METAL DITHIOPHOSPHATES
~ulphurized t e r p e n e s and s u l p h u r i z e d o l e f i n s a r e o t h e r ~xamples. T h e m o s t c o m m o n c h l o r i n e - c o n t a i n i n g a d d i t i v e s Lre the c h l o r i n a t e d p a r a f f i n s s o l d u n d e r t r a d e n a m e C e r e thlor and t h e c h l o r i n a t e d a r o m a t i c s sold u n d e r t r a d e n a m e %rochlor. C h l o r i n a t e d p a r a f f i n s c a n c o n t a i n up to 70% C1. Fhese e.p. a d d i t i v e s a r e m a i n l y u s e d in i n d u s t r i a l l a b r i c a :ion, e.g. m e t a l c u t t i n g and m e t a l f o r m i n g o p e r a t i o n s , heavy ,~ear l u b r i c a t i o n . H o w e v e r , a v e r y i m p o r t a n t a p p l i c a t i o n of .hese m a t e r i a l s i s in t h e r e a r a x l e s of a u t o m o t i v e v e h i c l e s °
Metal dialkyi and diaryl dithiophosphates have been used for many years as antioxidant anti-corrosion/antiwear additives in a wide range of lubricants. Many attempts to determine their mechanism of action as load-carrying additives have been made3-12; examples of the structures of some of the dithiophosphates are given in Table 2. Before discussing s o m e aspects of this work, it should be stated that, even now~ the detailed mechanism of action of metal dithiophosphates a s load-carrying additives i s s t i l l not understood.. Table 2 Structure of metal diLhiophosphates
%SSESSMENT OF
LOAD-CARRYINGADDITIVES
:deally all a.w. and e.p. additives should be tested in the ~artieular application for which they are developed. The :we main objections to this are t h a t full-scale testing of all :xperimental additives is too expensive and too time~onsuming,
CHaCHCH2CH--©,"
]
CH3
~ence, ever since additives were first put into lubricants, ~imulated accelerated rig tests to assess the performance )f new additives have been developed. Their main purpose ms been to function as screening tests for weeding out poor ~dditives without going to the extent of full scale tests. Many different types of rig test exist today, and a number of reviews on their performance, have been published b 2 Fypical rig tests currently used in the assessment md cop. additives are: I 2 3 4 5 6 7 8
DerLain a n a l y t i c a l t e c h n i q u e s h a v e a l s o b e e n found to be very u s e f u l in s t u d y i n g t h e l o a d - c a r r y i n g a c t i o n of a d d i t i v e s E x t e n s i v e u s e h a s b e e n m a d e of e l e c t r o n p r o b e m i c r o a n a l y s i s (eop.m.a.) an an aid to d e t e r m i n i n g t h e n a t u r e of the film Ln the l u b r i c a t e d c o n t a c t . M o r e d e t a i l s on t h e a p p l i c a t i o n of :his t e c h n i q u e a r e g i v e n l a t e r in t h e a r t i c l e . O t h e r u s e f u l : e c h n i q u e s a r e X - r a y d i f f r a c t i o n , e l e c t r o n m i c r o s c o p y , low ~nergy e l e c t r o n d i f f r a c t i o n (1.e°e.d.) and u s e of r a d i o a c t i v e LSOtOpes of l o a d - c a r r y i n g e l e m e n t s .
:46
TRIBOLOGY
A u g u s t 1970
CH3
j 2
CH 3
CH3 CH--0~ CH 3
Shell four-ball test Falex test Almen-Wieland test SAE machine test Timken test R y d e r g e a r test IAE g e a r t e s t P i n - o n - d i s c test
F h e s e r i g t e s t s c o v e r a wide r a n g e of the p a r a m e t e r s which lave a s~gnifieant e f f e c t on a d d i t i v e p e r f o r m a n c e ; n a m e l y : e m p e r a t u r e , s l i d i n g s p e e d , m e t a l type, g e o m e t r y of c o n t a c t , {nd d u r a t i o n of t e s t .
jS
"x'~//'
Metal di-4-methyl pentyl-2 dithiophosphate ( M = an, Ni~ Cd. i°b etc}
of a.w.
the performances of additives in both the a.wo and e.po L-egions can be determined in the majority of rig tests, and :he one selected is largely up m the particular choice of the nvestigator. Detailed descriptions of the rig tests are given n the literaturel, 2. The most widely used rig test is probably the four-ball machine which can be operated under both Lw. and e.p.test conditions. Other rig tests used in the ~uthor's studies are the Falex, Almen-Wieland and pin-oniisc machines.
~-,
CH
O
\ S ~ M 3 2
CH3
Metal di-iso-propyl dithiophosphate (M a s above) COH19
O
fil
Zinc di nonylphenyl dit~/ophosphate
A considerable amount of the m o r e important w o r k on m e ~ (mainly an) dithiophosphates w a s published in the late 1950's and early 1960'sS-So T h e efforts of the investigators w e r e concentrated on analysing the effect of change in the organic structure of the molecule on performance, and on the use of radioactive ,isotopes for studying element contents on metal surfaces lubricated by oils containing these dithiophospb~teso T h e results of this w o r k showed that: I
Metal dithiophosphates d e c o m p o s e d a~nd reacted with the m e t a l s u r f a c e to f o r m f i l m s c o n t a i n i n g m e t a l , p h o s p h o r u s and s u l p h u r , but the s u l p h u r c o n t e n t w a s m u c h lower, on a reel r a t i o b a s i s ° t h a n t h e p h o s p h o r u s a n d metal content. T h e rate of formation and thickness of film w a s dependent on temperature~ additive concent1~a ~ tion, time eta,
2
Generally, d e c r e a s e d t h e r m a l stability of the d i t h i o p h o s phate c o r r e l a t e d with i m p r o v e d l o a d - c a r r y i n g activity.
More r e c e n t work in t h i s field has been that of Rowe and Dickert 9, Jayne and Elliot lo, and the author and c o l l e a g u e s 11, 12. Rowe and D i c k e r t ' s work involved a s y s t e m a tic study of the effect of both changes in alkyl group and m e t a l a t o m on dithiophosphate p e r f o r m a n c e using a p i n - o n disc a p p a r a t u s . T h i s work showed that the w e a r r a t e of the pin i n c r e a s e d a s the t h e r m a l stability i n c r e a s e d for a s e r i e s of d i i s o p r o p y l d i t h i o p h o s p h a t e s of d i f f e r e n t m e t a l s , e.g. Ag (I), P b (II), Cd (H), Zn (II) and Cu (l.I). They a l s o showed that the w e a r r a t e i n c r e a s e d a s the t h e r m a l stability i n c r e a s e d due to change in the nature of the alkyl group. At t h i s stage, it should be e m p h a s i z e d that in my opinion m e t a l d i t h i o p h o s p h a t e s p r i m a r i l y function a s a.w. additives, with m o d e r a t e e.p. activity due to the p r e s e n c e of sulphur in the molecule. E x t e n s i v e t e s t i n g on a wide r a n g e of m e t a l dialkyl d i t h i o p h o s p h a t e s of varying s t r u c t u r e has been c a r r i e d out at the BP R e s e a r c h C e n t r e in o r d e r to f u r t h e r elucidate t h e i r m e c h a n i s m of action. Typical a.w. r e s u l t s for d i f f e r e n t m e t a l d i - 4 - m e t h y l p e n t y l - 2 dithiophosphates a r e given in Fig 1 and the m a r k e d effect of m e t a l type is c l e a r l y shown. E.p. r e s u l t s for the s a m e compounds a r e given in Fig 2, and s o m e v e r y i n t e r e s t i n g o b s e r v a t i o n s can be made: 1
2
3.0 2-8
2.6 "E 2-4 E ~
2.2
o_ = 2.0! -o ~
1.8 1.6
t-
o ~
1'4
12 Lc
A d i r e c t r e v e r s a l in the o r d e r of p e r f o r m a n c e of t h e s e a d d i t i v e s is o b s e r v e d in going f r o m the a.w. to the e.p. region.
1.0
A.w. activity i n c r e a s e s with d e c r e a s i n g t h e r m a l stability of the dithiophosphate whilst e.p. activity d e c r e a s e s .
0"8 50
In an effort to explain t h e i r m e c h a n i s m f u r t h e r , e l e c t r o n p r o b e m i c r o a n a l y s e s of the w e a r s c a r s f o r m e d u n d e r a.w. and e.p. conditions w e r e c a r r i e d out 12. The r e s u l t s c o n f i r m ed that f i l m s containing the metal, p h o s p h o r u s and sulphur w e r e f o r m e d on the s u r f a c e . They also showed that the p r e s ence of l a r g e quantities of p h o s p h o r u s and to a l e s s e r extent, the metal, was e s s e n t i a l for good a.w. p e r f o r m a n c e ; the p r e s ence of sulphur on the s u r f a c e was of l e s s e r i m p o r t a n c e .
0"80
o Bi I1"[ x Sn rr 0 Sb m A Pb]I
070 E E
•
Zn O Bismuth
o 0.60
E 0 =6
Tin
x
0-50 Lead ¢0
04,0
-
Zinc
Cadmium I
30
I
45 Time [ m i n i
,
,
•
!
.
150 Load [ k g ]
.
.
.
~
m
200
F i g 2 F o u r - b a l l e x t r e m e p r e s s u r e r e s u l t s f o r a s e r i e s of m e t a l d i - ( 4 - m e t h y l pentyl-2) dithiophosphates at a c o n c e n t r a t i o n of 4 m i U t a t o m s of p h o s p h o r u s p e r 100 g of blend
The r e c e n t l y published work by both Jayne and Elliot lo and F u r e y 13 c o n f i r m a c o r r e l a t i o n between the t h e r m a l stability of the additive and l o a d - c a r r y i n g p e r f o r m a n c e , when t h e r m a l stability is a l t e r e d by changing the organic, r a t h e r than the metal, part of the dithiophosphate m o l e c u l e . Jayne and Elliot o b s e r v e d little d i f f e r e n c e in dithiophosphate p e r f o r m a n c e in both f o u r - b a l l e.p. and a.w. t e s t s , but did e s t a b l i s h that the lower the t h e r m a l stability, the b e t t e r the OK load in the Timken t e s t . F u r e y o b s e r v e d a s i m i l a r t r e n d for Zn s a l t s of varying t h e r m a l stability in cam and tappet t e s t s and we have r e c e n t l y o b s e r v e d a s i m i l a r t r e n d using p i n - o n - d i s c tests. Summing up, t h e r e is little doubt that the nature of the m e t a l atom and of the o r g a n i c h y d r o c a r b y l portion of the molecule, h a s a m a r k e d effect on both the a.w. and e.p. activity of m e t a l d i t h i o p h o s p h a t e s . T h e r e i s also c o n s i d e r a b l e e v i d e n c e to show that the m o r e t h e r m a l l y unstable the dithiophosphate is, the b e t t e r its l o a d - c a r r y i n g p r o p e r t i e s .
V"
0"30
,
However, the e.p. r e s u l t s showed that the w e a r s c a r s for each additive had e l e m e n t c o n t e n t s and d i s t r i b u t i o n s s i m i l a r to t h o s e obtained in the a.w. region, d e s p i t e the r e v e r s a l in p e r f o r m a n c e of t h e s e a d d i t i v e s between the two r e g i o n s . T h e s e r e s u l t s t h e r e f o r e highlight the point that good p e r f o r m a n c e by an additive in the a.w. region is not n e c e s s a r i l y a c c o m panied by good p e r f o r m a n c e in the e.p. region. In fact, the r e s u l t s of t h i s work show that s i m i l a r s u r f a c e f i l m s a r e f o r m e d in both a.w. and e.p. r e g i o n s by each additive. The c h e m i c a l s t r u c t u r e of the dithiophosphate then d e t e r m i n e s the region in which the film f o r m e d has good l o a d - c a r r y i n g properties.
v Fe I11 • Ni IT + Cdrr
r~ A g I Load 15 kg
I
100
I
60
F i g 1 F o u r - b a l l a n t i w e a r r e s u l t s for a s e r i e s of m e t a l d i (4-methyl pentyl-2) dithiophosphates at a c o n c e n t r a tion of 4 m i l l i a t o m s of p h o s p h o r u s p e r 100 g of blend
Although much effort h a s gone into trying to d e t e r m i n e m e c h a n i s m s of bulk t h e r m a l d e c o m p o s i t i o n of t h e s e compounds, no c o n c l u s i v e p i c t u r e oI t h e i r m e c h a n i s m of l o a d - c a r r y i n g action h a s e m e r g e d . F u r t h e r c l a r i f i c a t i o n r e q u i r e s detailed s t u d i e s of the a d s o r p t i o n / r e a c t i o n of m e t a l d i t h i o p h o s p h a t e s with m e t a l s u r f a c e s . The main u s e for a d d i t i v e s of t h i s type is always likely to be in the a.w. field r a t h e r than in the very highly loaded e.p. region.
TRIBOLOGY August 1970
147
0RGANO-PHOSPHORUS
ADDITIVES
[t has been generally accepted that compounds containing phosphorus are automatically good a.w. additives with poor e.p. activity. A study of the literature in this field confirms [hat phosphorus additives of the tricresyl phosphate~ dialkyl phosphat% dialkyl phosphite type have no e,p. activity. However~ recently published work ~4 has shown that the presence of phosphorus in a molecule does not 'guarantee ~ good a.w. activity, but that the activity is dependent on the environment of the phosphorus atom in the molecule. Since the grea~ majority of published work on phosphorus compounds no~ containing sulphur refers to tricresyl phosphat% this section will be split up into two parts discussing (a) organs-phosphorus compounds in general and (b) tricresyl phosphate (t.copo) in particular.
General organo-phosphorus additives F o u r - b a i l a,wo and e.po t e s t s (m c e r t a i n c a s e s ) w e r e c a r r i e d out on a wide r a n g e of p h o s p h o r u s compounds ~4 a s listed. the Table 3. The r e s u l t s showed that all the p h o s p h o r u s additives which degraded~ e i t h e r t h e r m a l l y o r hydrolytieaily~ to f o r m a c i d s of low pK a (pKa is a m e a s u r e of the streng%h of an acid and the l o w e r i t s value~ the s t r o n g e r the acid} had
Ta~le 30rgano-phosphorus
additives
CH 3
CH3
These results were also supported by pin-on-disc and Falex test results. No studies on the films formed on the surfaces by ~hese additives (with the exception of t,e.p,) has yet been reported° However, a speculative suggestion is that these phosphorus additives absorb on the surface, degrade under condition of i n c r e a s i n g s e v e r i t y to f o r m p o l a r s p e c i e s which r e a c t with the iron to f o r m i r o n o r g a n o / i n o r g a n o p h o s p h o r u s eompounds~ The cow. activity of the p h o s p h o r u s a d d i t i v e s i s probably c o n t r o l l e d by the ra~e of f o r m a t i o n o~ t h i s !ayer~ the s t r e n g t h of a t t a c h m e n t of the l a y e r and by i t s r a t e of attrition°
T r i c r e s y l p h o s p h a t e (t.Copo) C o n s i d e r a b l e work .has been published on the m e c h a n i s m ei load-carrying action of t.cop.15-1< T.copo has been used extensively in automotive crankcase ferrnu!ations~ hydraulic and turbine oils and aviation lubricants and is probably second only to Zn dialkyl dithiophosphate in its popu!arH:y as an a.w, additive. Although confusion still exists on the exae~ mode of action of t.c.p, as an aoW. additiv% some excellent papers published over the past five to six years trove helped tc correct some original misleading thoughts on its mechanism of action. It was initially thought that toc, po decomposed and reacted with iron surfaces to form an iron iron phosphide eutectie and that the properties of this curestie w e r e r e s p o n s i b l e f o r its a . w . p r o p e r t i e s . Godfreyt6~ B i e b e r et ai 17 and B a r c r o f t and D a n i e P ~ have all published w o r k which l e a d s ~o a c l e a r e r undersflanding of the m e c h a n i s m of action of toc.p, Goldblatt and Appeldoorr~ i s have a l s o r e c e n t l y p u b l i s h e d work in which the e f f e c t s of a t m o s p h e r e and m o i s t u r e on p e r f o r m a n c e w e r e studied~ A c r i t i c a l study of t h e s e p a p e r s coupled with the a u t h o r ' s wor~ in t h i s field l e a d s to the following conclusions,
Tricresyl p h o s p h a t e
C ~H 90
excellent a.w. properties. However, additives, e.g. phosphinic and phosphonic derivatives which degraded to form weak a c i d s had a.w. p r o p e r t i e s ~.ittle d i f f e r e n t to the b a s e oil indicating that the p h o s p h o r u s a~om p e r s e did no~: bouarantee good a.wo activity, A n o t h e r i n t e r e s t i n g f a c t o r e m e r g i n g f r o m these results was that certain phosphorus additives~ eog phosphoramidates and amine phosphates had considerably better initial seizure loads in the e.p,test than tricresyl phosphate°
O
C 4H 9° / / " ~ N H C 6H ~3
l
The polar i m p u r i t i e s in t,Cop. {these i m p u r i t i e s a r e typ[cally dialkyl o r d i a r y [ p h o s p h a t e acids; a r e b e t t e r l o a d - c a r r y i n g a d d i t i v e s than the n e u t r a l e s t e r .
2
The neutral te.p. ester can degrade hydrolytically and/ o r t h e r m a l l y u n d e r a.w. conditions ~o f o r m iron o r g a n o / inorgano phosphate f i l m s which have e x c e l l e n t ioad~ c a r r y i n g p r o p e r t i e s . T h i s d e c o m p o s i t i o n ~s f o r m f i r s t the i r o n organo and finally i r o n - i n o r g a n o phosphate films is elegantly demonstrated in the work of Barcroft and Daniel,
Dibutyl N-hexyl phosphoramidate
C4H90
/JO
C4HpO)~OH Dibutyl phosphate
Hs~R
p C 4 H 90/ Amine C 4H
~O phosphate
90\/0
C4H90/ ~C6H13 Dibutyl hexyl phosphonate
C~H13~p/~ O
¢sH~/ K~OC4H9 Dthexyl butyl p h o s p h i n a t e 148
Organophosphorus additives of the ~.C.po phosphoramidate~ dialkyl phosphate, dialkyl phosphite type are used in appiica~ tions where good a.w. protection and anti-scuffing is required. They have the advantage of being less corroslve~ part~cu~ larly ~o yellow metals~ than metal dithiophosphates but have the d i s a d v a n t a g e of having s i m i l a r ~.p. p r o p e r t i e s ~o base oilso They also tend to be m o r e e x p e n s i v e than typical dithiophosphates.
TRIBOLOGY August 1970
ORGANO-SULPHUR
ADDITIVES
OrganO-sulphur materials were probably amongst the first well known load-carrying additives. Flowers of sulphur have been added to lubricating oils to increase their loadcarrying properties for many years and even today, sulphur-~ ized oils are probably used in certain industrial applications, The most commonly used sulphur additives today are sulphurized fats, sulphurized olefinso and sulphurized terpeneso
Organs-sulphur additives are accepted as the most important additive class for e.p. lubrication and a detailed study of
the m e c h a n i s m of t h e i r l o a d - c a r r y i n g action h a s been c a r r i e d out by the author and c o l l e a g u e s 19, 20,2 ~ T h i s work has been r e c e n t l y s u m m e d up in a r e v i e w p a p e r 22. The e.p. p r o p e r t i e s of a range of o r g a n o - s u l p h u r compounds (Table 4) w e r e a s s e s s e d using p r i m a r i l y the f o u r - b a l l e . p . t e s t but F a l e x and A i m e n - W i e l a n d t e s t s on s e l e c t e d additives w e r e c a r r i e d out.
o ,,
OioLlyl disulphide Dibenzy[ disutphide Di-ter~-buty( disu!,phide v Di-D-bUtyt disu(phide n Diphenyl disu!.phide • Dibenzy[ rnonosu~,phide x Di-_n-buty[ monosu[phide
3"2 f/~Bi~se 3'0 2"8
Table 4 0 r g a n o - s u l p h u r
additives
E
e
/(~>-s-s-, E
Diphenyl disulphide
2"6 2"h
2"2
b
Dibenzyl disulphide
2'0 1"8
c
CH3(CH2)n--S--S--(CH2)nCH3 Di-n-alkyldisulphide
~u
1'6 1.4 1"2
Dibenzyl monosulphide
1"0
CH31 ~ H3 CH3--C--S--S--C--CH3 i
i
CH 3
CH 3
D i - t e r t butyl disulphide
i
The c h e m i c a l s t r u c t u r e of the sulphur compound had a m a r k e d effect on p e r f o r m a n c e
2
Disulphides w e r e b e t t e r than m o n o s u l p h i d e s and the o r d e r of i n c r e a s i n g e.p. activity was; diphenyl < d i - n butyl < d i - t e r t b u t y l < dihenzyl < diallyl (Fig 3)
3
E.p. p e r f o r m a n c e of a r a n g e of dialkyl d i s u l p h i d e s d e c r e a s e d with i n c r e a s e in alkyl chain length, whilst inc r e a s i n g alkyl chain length had no effect on the e.p. p e r f o r m a n c e of d i - t e r t - a l k y l d i s u l p h i d e s (Fig 4).
A m e c h a n i s m was then put f o r w a r d which s a t i s f i e d all the above conclusions. This was that the e.p. activity of sulphur a d d i t i v e s is dependent on the e a s e of cleavage of the c a r b o n sulphur bond in the m o l e c u l e so that an inorganic i r o n - s u l phur l a y e r is f o r m e d . Thus d i s u l p h i d e s would be expected to be b e t t e r than monosulphides, s i n c e in the latter, two c a r b o n sulphur bonds have to be broken to r e l e a s e the sulphur, whilst only one c a r b o n - s u l p h u r bond i s broken in the d i s u l phide. This m e c h a n i s m also explains the o r d e r of p e r f o r m a n c e of the d i f f e r e n t disulphides, s i n c e the c a r b o n - s u l p h u r bond is diallyl and dibenzyl d i s u l p h i d e s would be m o s t easily broken due to the stability of the allyl o r benzyl r a d i c a l f o r m e d . Also i n c r e a s e d branching on the c a r b o n atom n e a r est to the sulphur would weaken the c a r b o n - s u l p h u r bond due to i n c r e a s e d s t e r i c hindrance, i.e. b r a n c h e d groups a r e m o r e bulky thus r e q u i r i n g m o r e s p a c e and hence inducing a s t r a i n on the c a r b o n - s u l p h u r bond. The effect of d e c r e a s i n g activity with i n c r e a s i n g alkyl chain length i s explicable on the b a s i s of l e s s sulphur p e r unit a r e a being available to r e a c t with the s u r f a c e a s the b u l k i n e s s of the disulphide i n c r e a s e s . The a.w. p r o p e r t i e s of t h e s e o r g u n o - s u i p h u r compounds w e r e also d e t e r m i n e d using the f o u r - b a l l a.w. t e s t 2o. O r g a n o sulphur compounds a r e known to be poor a.w. additives and the r e s u l t s of the study c o n f i r m e d t h i s . The b e s t r e s u l t s
Local {kg]
300
400
F i g 3 F o u r - b a l l w e a r against load for a s e r i e s of disulphides and m o n o s u l p h i d e s at a c o n c e n t r a t i o n of 37.04 m i l l i a t o m s sulphur p e r 100 g p a r a f f i n
The e . p . t e s t r e s u l t s 19 for a s e r i e s of m o n o - and disulphides, sulphoxides etc c l e a r l y showed that: 1
200
100
3"2 3-0 2.8 "E
E2.6
~, ~ 2.4 E .o_ 2-2
~ 2.0 ~ 1.8
:~ 1-6 1"4
-z/,
Oi-o-dodecy( disulphide Oi-o-octy[ disulphide Di-n-buty[ disulphide Diethy( disulphide Di-tert-buty[ disulphide Di-tert-dodecy[ disutphide
o
1.2
o
;-0
rl |
100
.
L
,
,
I
~
,
200 Load [ kg ]
]
.
,
,
•
300
400
F i g 4 F o u r - b a i l w e a r against load for a s e r i e s of dialkyl disulphides at a c o n c e n t r a t i o n of 18.52 m i l l i m o l e s p e r 100 g p a r a f f i n TRIBOLOGY August 1970
149
obtained with these additives were only equivalent to a medicore a.w. activity by phosphorus additive standards (Fig 5L The results showed that: i
2
0"80
A.w, properties of disulphides increased along the series di-n-butyl < diallyl < dibenzyl < diphenyl and monosulphides were poorer than the corresponding disulphides (Fig 5) Increasing {he chain increased their a.wo with branched alkyl their corresponding
length of di-n-alkyl effectiveness whilst groups had inferior n-alkyl derivatives
disulphides disulphides properties to (Fig 6).
Thus it was immediately obvious that factors governing the e.p. and a.w. performance of disulphides were different, as already observed for the metal dithiophosphates. These results were explicable on the basis of the a.w. activity of organo-sulphur compounds being dependent on the ease of cleavage of the sulphur-sulphur bond to form an iron mercaptide layer, and by the physical properties of this layer once formed. This hypothesis can explain the results as follows: The sulphur-sulphur bond is known to be the weakest part of the disulphide molecule and is thus likely to break first. Furthermore, diphenyl disulphide would be expected to be the best additive [n this respect, since the resulting phenyl mercaptyl radical would be highly resonance stabiiizedo
a D i - n - buty~ disulphide o D{a[[y[ disulphide x Dibenzy[ disutphide e Diphenyt disulphide _
_
_ ......
0"70
T ~ O-gO
.~ 0-50 c#
0'40F J
Disulphides should be better thmn monosulphides since the latter do not contain a weak sulphursulphur bond.
I
Good phosphcrus-addilive
i
0'30
The improvement in a.w. activity with increased chain length is explicable from the physical properties of the surface layer. Levine and Zisman 23 observed a similar effect on friction reducing properties of n-alkyl earboxylic acids and attributed this to an adlineation of the molecules in the surface to form a solid-type film. They stated that the intermolecular cohesional energy required to effect this adlineation increased with increasing aLkyl chain length. Thus the overall mechanism of load-carrying action of disulphides, and probably organo-sulphur compounds in general~ can be proposed as follows. Firstly, the disulphide is sorbed on the iron surface and cleavage of the sulphur-sulphur bond o c c u r s s o that an zron mercaptide l a y e r i s f o r m e d althougb ~he exact order of this reaction sequence is no~ known with certainty~ As loads approaching e.po condRions are reached, much higher temperatures are found in the contact zone, and cleavage of the carbon-sulphur bond to form an inorganic sulphur-containing-layer occurs,
30
45 Time I.rnin]
~0
Fig 5 Four-ball wear against time for a series of disulphides at a c o n c e n t r a t i o n of 1 8 . 5 2 m i l l i m o t e s p e r 100 g of p a r a f f i n
0"90
o Oi-D-buty ~, disulphide x Oi-0-ocly[ disulphide o Di~-dodecy~ disulphide Di-n- hexadecy[ disu~,phide Di-ter_j-hexodecy! disulphide
0"80
O Di-D-octo.decy[ disulphide
7
i
=
&n electron probe m i c r o analysis study 21 of the s u r f a c e f i l m s f o r m e d u n d e r e.p. a n d a . w . Conditions w a s c a r r i e d out Ln o r d e r to s u b s t a n t i a t e t h e a b o v e h y p o t h e s i s . T h e e . p , m o a . r e s u l t s f o r a s e r i e s of d i s u l p h i d e s in t h e e.po r e g i o n s h o w e d ~hat: 1
S u l p h u r w a s p r e s e n t in a l l t h e w e a r s c a r s and t h e o b s e r v e d o r d e r of i n c r e a s i n g s u l p h u r c o n t e n t w a s :
di-n-lauryl dia!lyl
< diphenyl < di-n-butyl
< di-tert~butyl
<
The sulphur conten~ increased with applied load in the eop~ region and sulphur was present to a depth of several microns. Under a.w. conditions, little sulphur was observed in the wear scars. ~hus these results fully support the suggested mechanisms ~f action of organo-sulphur compounds. We are currently carrying out detailed studies of adsorption/reaction of these additives with metal surfaces with a view to proving this hypothesis. One interesting aspect of this work on organo-sulphur additives was that the trend of results for a range of disulphides in rig ~eszs, gave a strong indication of the condition being
TRiBOLOGY
~ 0"60 N
This order followed exactly their order of e.p. activity, The determined sulphur contents ranged from 6% for di-n-lauryl disulphide to over 30°/°for diallyl disulphide.
150
"~ 0-70
August
1970
°
g 0.50 r
~
~
i
0.3o <
30
Fig 6
45 Time [ rr4nj
Four-bali wear against time for a series of dialkyi disulphides at 18.52 mi!limo!es per I00 g paraffin.
~
a s s e s s e d by the rig. F a l e x and A l m e n - W i e l a n d e.p. and a.w. t e s t s w e r e c a r r i e d out on diphenyl, d i - t e r t - b u t y l and dibenzyl d i s u l p h i d e s and r e s u l t s a r e shown in T a b l e s 5 and 6. F r o m the o r d e r of p e r f o r m a n c e it can be s e e n that the F a l e x e.p. and a.w. t e s t r a t e s the d i s u l p h i d e s in the s a m e o r d e r a s the f o u r - b a l l e.p. and a.w. t e s t , but in both e.p. and a.w. v e r s i o n s , the A l m e n - W i e l a n d t e s t a p p e a r s to a s s e s s a.w. p r o p e r t i e s only of l u b r i c a n t s . Thus a c l e a r e r u n d e r s t a n d i n g is obtained of why so many conflicting r e s u l t s a r e often o b s e r v e d for a s e r i e s of l o a d - c a r r y i n g a d d i t i v e s in d i f f e r e n t t e s t r i g s .
Table 5 F a l e x t e s t r e s u l t s (All additives w e r e t e s t e d at 18.52 m i l l i m o l e s / 1 0 0 g liquid paraffin)
A v e r y significant f a c t o r c o n c e r n i n g o r g a n o - c h l o r i n e a d d i t i v e s is the m a r k e d s y n e r g i s t i c e.p. effect o b s e r v e d when m i x t u r e s of o r g a n o - s u l p h u r and o r g a n o - c h l o r i n e additives a r e used. This fact has b e e n shown as e a r l y as 1946 by P r u t t o n et a126 in an e x c e l l e n t p a p e r in which e f f o r t s to e x plain t h e s e r e s u l t s w e r e given. The main explanation put f o r w a r d was that iron sulphide f i l m s w e r e f o r m e d f i r s t but once f o r m e d , the active c h l o r i n e additives r e a c t e d much f a s t e r with the i r o n sulphide film than with the i r o n s u r f a c e . Thus the i r o n c h l o r i d e film was f o r m e d much m o r e rapidly than in d i r e c t r e a c t i o n between the c h l o r i n e additive and the i r o n s u r f a c e . M i x t u r e s of sulphur and c h l o r i n e - c o n t a i n i n g additives a r e frequently u s e d in heavy duty e.p. applications. OTHER FACTORS
Additive
E.p. t e s t
Wear test
f a i l u r e load [lb]
weight l o s s [g]
Dibenzyl disulphide
4500
0. 0187
Di - t e r t -butyl disulphide
3950
0.0510
750
0. 0216
Diphenyl disulphide
Table 6 A l m e n - W i e l a n d t e s t r e s u l t s (all additives w e r e t e s t ed at 18.52 m i l l i m o l e s / 1 0 0 g liquid paraffin) E.p. t e s t
Wear t e s t
failure l o a d / frictional f o r c e [kg]
weight loss [g]
Dibenzyl disulphide
450/170; 400/140
0. 2720
D i - t e r t - b u t y l disulphide
400/128; 350/111
0. 5026
Diphenyl disulphide
650/170; 650/158
0. 1792
Additive
Why a r e o r g a n o - s u l p h u r compounds good e.p. and poor a.w. additives ? The poor a.w. activity is probably r e l a t e d to the fact that o r g a n o - s u l p h u r compounds a r e not so r e a c t i v e to m e t a l s u r f a c e s at low t e m p e r a t u r e s a s a r e p h o s p h o r u s a d d i tives. Hence a strongly attached, stable film is not readily f o r m e d . On the o t h e r hand, under e.p. conditions the i n c r e a s e d load and t e m p e r a t u r e c a u s e s breakdown of the a d d i tive so that a p r o t e c t i v e i n o r g a n o - s u l p h u r containing film is f o r m e d . The r a t e of f o r m a t i o n and a t t r i t i o n of this film and its i n h e r e n t p r o p e r t i e s a r e r e s p o n s i b l e for the good e.p. activity of sulphur additives. ORGANO-C HLORINE ADDITIVES Typical o r g a n o - c h l o r i n e additives a r e the c h l o r i n a t e d p a r a f fins and c h l o r i n a t e d a r o m a t i c s . T h e s e additives a r e widely u s e d in m e t a l - c u t t i n g and m e t a l - f o r m i n g l u b r i c a n t s and also in automotive r e a r axle oil f o r m u l a t i o n s . O r g a n o - c h l o r i n e additives a r e , in many ways, s i m i l a r to o r g a n o - s u l p h u r a d d i t i v e s in that they have only m o d e r a t e a.w. p r o p e r t i e s but have good e.p. p r o p e r t i e s . Davey 24 has c a r r i e d out a study of the e.p. p r o p e r t i e s of o r g a n o - c h l o r i n e compounds of d i f f e r e n t c h e m i c a l s t r u c t u r e s using the f o u r - b a l l m a c h i n e . His r e s u l t s showed that the m o r e labile the C-C1 bond, the b e t t e r the e.p. p r o p e r t i e s of the c h l o r i n e - c o n t a i n i n g additive. Thus, the e.p. p r o p e r t i e s of t h e s e o r g a n o - c h l o r i n e additives a r e probably dependent on the e a s e of f o r m a t i o n of an inorganic i r o n c h l o r i d e film on the metal s u r f a c e . Little work has been p u b l i s h e d on the a.w. p r o p e r t i e s of t h e s e additives, but l i m i t e d s t u d i e s have shown that the a.w. p r o p e r t i e s of c h l o r i n e additives i m p r o v e s with i n c r e a s i n g lability of the C-C1 bond. It has been s u g g e s t e d that, in the c a s e of c h l o r i n a t e d p a r a f fins, the film f o r m e d f r o m the additive is not a c h l o r i n e - c o n taining one but r a t h e r a p o l y m e r i c c a r b o x y l i c acid m a t e r i a l 25.
Base oil The b a s e oil used in any application can have a m a r k e d effect on the p e r f o r m a n c e of both e.p. and a.w. additives. P r o p e r t i e s such as v i s c o s i t y , p a r a f f i n i c , naphthenic or a r o m a t i c content, sulphur content a r e all i m p o r t a n t . C e r t a i n b a s e oils also have naturally o c c u r r i n g s u r f a c e active s p e c i e s which can compete with the l o a d - c a r r y i n g additive for the m e t a l surface. As a g e n e r a l i z a t i o n , it can safely be said that the t h i c k e r the b a s e oil, the b e t t e r the e.p. or a.w. p e r f o r m a n c e of a blend due to the i m p r o v e d p r o p e r t i e s of the b a s e oil itself. The effect of c h e m i c a l s t r u c t u r e of the b a s e oil on additive p e r f o r m a n c e is c e r t a i n l y not yet c l e a r l y u n d e r s t o o d although w o r k e r s such as Rounds27, 2s, Appeldoorn 29, Vinogradov 3° have done c o n s i d e r a b l e work in this field. G r o s z e k 31 has also shown that the c o m p o n e n t s of b a s e oils can have e x c e l lent a.w. activity in t h e i r own right. Work c a r r i e d out by the author i n d i c a t e s that v e r y effective additives i.e. diallyl disulphide as an e.p. additive or an amine phosphate a s an a.w. additive give good p e r f o r m a n c e i r r e s p e c t i v e of b a s e oil type, However, the p e r f o r m a n c e of the m o r e m o d e r a t e a.w. and e.p. additives has been found to be a f f e c t e d by b a s e oil c h e m i c a l s t r u c t u r e . This i s a field in which c o n s i d e r a b l y m o r e r e s e a r c h could be fruitfully c a r r i e d out.
Atmosphere Variation in a t m o s p h e r e and m o i s t u r e content can have a profound effect on the efficiency of l o a d - c a r r y i n g additives. The effect of change in a t m o s p h e r e and of variation in h u m i dity on the p e r f o r m a n c e of t.c.p, has r e c e n t l y b e e n d i s c u s s e d by Goldblatt and Appeldoorn 18. Also, s i m i l a r s t u d i e s using sulphur compounds have been c a r r i e d out to a l i m i t e d extent by Togoyuchi and Takai 32 and Vinogradov 3°,33. It was d i f ficult to g e n e r a l i z e the e f f e c t s of t h e s e p a r a m e t e r s but the p r e s e n c e of 02 in the s y s t e m i s r e c o g n i s e d to be b e n e f i c i a l whilst e x c e s s i v e m o i s t u r e is d e t r i m e n t a l . Additive i n t e r a c t i o n I n t e r a c t i o n of d i f f e r e n t additives, in f u l l y - f o r m u l a t e d oils is now b e c o m i n g a field worthy of fundamental study. T h e r e is an i n c r e a s i n g t r e n d to put a l a r g e n u m b e r , and a higher conc e n t r a t i o n , of additives in m o d e r n l u b r i c a n t s and hence i n t e r action e f f e c t s b e t w e e n additives a r e b e c o m i n g m o r e i m p o r t ant. A.w. and e.p. additives a r e usually highly p o l a r m a t e r i a l s with a high affinity for m e t a l s u r f a c e s , on which f i l m s m u s t be f o r m e d , b e f o r e the additives b e c o m e effective. On the o t h e r hand, l u b r i c a n t s can also contain r u s t i n h i b i t o r s and m e t a l - c o n t a i n i n g d e t e r g e n t s which also have an affinity f o r m e t a l s u r f a c e s and hence c o m p e t i t i v e a d s o r p t i o n and r e action can occur. Very little s y s t e m a t i c work has b e e n publ i s h e d in this field. However, t h e r e a r e indications that g r e a t e r antagonistic e f f e c t s a r e o b s e r v e d with the l e s s p o l a r additives of the sulphur and chlorine type r a t h e r than with the highly polar, s u r f a c e active p h o s p h o r u s a d d i t i v e s . CONCLUSIONS
This a r t i c l e highlights the complexity of the m e c h a n i s m of action of a.w. and e.p. additives and i n d i c a t e s w h e r e a d v a n c e s TRIBOLOGY August 1970
151
have been made in recent years and where further work is n e c e s s a r y . The m a r k e d effect of the c h e m i c a l s t r u c t u r e of an additive on p e r f o r m a n c e , e v e n i n s i d e the s a m e c l a s s of compounds, is c l e a r l y i l l u s t r a t e d . Additive p e r f o r m a n c e i s affected by a wide r a n g e of p a r a m e t e r s s u c h as t e m p e r a ~ r e , a t m o s p h e r e , and b a s e oil type.
15
B a r c r o f t , Fo T. and Daniel S.G. ' T h e action of neutra~ o r g a n i c p h o s p h a t e s as e.p. a d d i t i v e s ' T r a n s a c t i o n s of the A m e r i c a n Society of M e c h a n i c a l E n g i n e e r s , Vol 87 (1965) p 751
18
Godfrey, D. ' T h e l u b r i c a t i o n m e c h a n i s m of t r i c r e s y i p h o s p h a t e on s t e e l ' T r a n s a c t i o n s of the A m e r i c a n Society of L u b r i c a t i o n E n g i n e e r s Vo! 8~ No 1 (1965)
17
B i e b e r ° H . E . , K l a u s , E . E . a n d T e w k e s b u r y , Eogo ~Astudy of t r i c r e s y l p h o s p h a t e as an additive in boundary l u b r i ~ cation , T r a n s a c t i o n s of the A m e r i c a n Society of Lubri-cation E n g i n e e r s , Vol 1i (1968) p 155
t8
Goldblatt, I. L, and Appeldoorn, J. Ko ~The a n t i w e a r be--~ h a v i o u r of [.C°po in d i f f e r e n t a t m o s p h e r e s and d i f f e r e n t b a s e s t o e k s ' , Presented at A m e r i c a n Society of L u b r i c a tion Engineers/American Society of Meclnanical E n g i n e e r s Meeting, Houston (Oct !969)
19
A l l u m , K. G. and F o r d , J. F o "The i n f l u e n c e of c h e m i c a l s t r u c t u r e on the l o a d - c a r r y i n g p r o p e r t i e s of c e r t a i n o r g a n o - s u l p h u r compounds' J o u r n a l ~f the Insti#a~e of P e t r o l e u m , VoI 51, (1965) p t45
20
A l i u m , K . Goand F o r b e s , E o S . ' T h e l o a d - c a r r y i n g p r o p e r t i e s of o r g a n i c sulphur compounds, t~Aluence of c h e m i c a l s t r u c t u r e on the a n t i w e a r p r o p e r t i e s ~f o r g a n i c d i s u l p h i d e s ' J o u r n a l of ~he I n s t i t u t e of P e t r o l e u m , Vol 53 (1967) p 173
21
Altum, K. G. and F o r b e s , E. So ' T h e l o a d - c a r r y i n g m e c h a n i s m of o r g a n m sulphur c o m p o u n d s - - a p p l i c a t i o n of e l e c t r o n p r o b e m i c r o a n a l y s i s ' T r a n s a c t i o n s of the American Society of Lubrication Erzgineers. Vol 11
ACKNOWLEDGEMENT This article is based on a lecture that was presented at a course entitled 'Chemical aspects of tribology' on 10-12 April 1970, organized by The Institution of Mechanical Engineers, Permission to publish this article has been given by The British Petroleum Company Ltd.
REFERENCES 1
T o u r r e t , R. ' T h e u s e of s i m u l a t i o n - t y p e l u b r i c a n t t e s t r i g s ' I n s t i t u t e of P e t r o l e u m G e a r and L u b r i c a t i o n Cont e r e n c e , B r i g h t o n (1964)
2
Towle, A., G l o v e r , I. and W a l k e r , R. i . H . 'Hypoid oil r e q u i r e m e n t s and d e v e l o p m e n t s for the E u r o p e a n m a r k e t ' I n s t i t u t e of P e t r o l e u m G e a r and L u b r i c a t i o n C o n f e r e n c e , B r i g h t o n (1964)
3
F u r e y , M. J. and Kunc, J . F . 'A r a d i o t r a c e r a p p r o a c h to the study of engine v a l v e t r a i n l u b r i c a t i o n , J o u r n a l of L u b r i c a t i o n E n g i n e e r i n g , Vol 14 (1958) p302
4
B e n n e t t , P. A. and Kabel, R. H. ' T h e a n t i - s c u f f p e r f o r m a n c e of m e t a l o r g a n o d i t h i o p h o s p h a t e a d d i t i v e s ' J o u r n a l of L u b r i c a t i o n E n g i n e e r i n g , Vol 19 (1963) p365
5
L o o s e r , F. H. and T w i s s , S . B . ' A n a l y s e s of f i l m s f o r m e d by radioactive e.pi additives',American Society of Lubrication Engineers meetirN, Cleveland (April 1958)
6
Loeser, E.H.,Wiquist,R.C.and Twiss, SoB. 'Cam and tappet lubrication Ill. Radioactive study of phosphorus in e.p. films'~ Transactions of the American Society of Lubrication Engineers, Vol I, No 2 (1958)
7
Looser, E° H°~Wiquist, R.C.and Twiss,S.B. 'Cam and tappet lubrication IVo Radioactive study of sulphur in e°p. film '~ Transactions of the American Society of Lubrication' Engineers, Vol I~ No 2 (1958)
8
Larson, R. ~Performance of zinc dithiophosphates as iube oil additives' Scientific Lubrication, Vol 10, No 8~ (!958) p12
9
Rowe, C. N. and D i c k e r t , J. Jo ' T h e r m a l d e c o m p o s i t i o n and a n t i w e a r function of m e t a l , O, O - d i a l k y l p h o s p h o r o di~hioates' A m e r i c a n C h e m i c a l Society P r e p r i n t s , Vol 10, No 2 (1965) D71
I0
J a y n e , C. J. J. and Elliot, J . S . ' T h e l o a d - c a r r y i n g properties of m e t a l p h o s p h o r o d i t h i o a t e s ' A m e r i c a n C h e m i cal Society P r e p r i n t s Vol 14, No 4 (1969) A139
11
Allure, K. G. and F o r b e s , E . S . ' T h e l o a d - c a r r y i n g p r o p e r t i e s of m e t a l dialkyl dithiophosphateso The effect of c h e m i c a l s t r u c t u r e ' I n s t i t u t i o n of M e c h a n i c a l E n g i n e e r s T r i b o l o g y Convention, G o t h e n b u r g (1969) P a p e r 2
t2
13
14
152
(1968) p 162 22
F o r b e s , N.S. ~The l o a d - c a r r y i n g action of organo~ s u l p h u r compounds. A r e v i e w ' , P r e s e n t e d at L u b r i c a t i o n S y m p o s i u m , P a i s l e y College of Technolog2¢" (Oct 1969). P u b l i s h e d in W e a r , Vet I5, No 2 (1970) p 87-96
23
Levine, O. and Z i z m a n , W.A. J o u r n a l of P h y s i c a l C h e m i s t ~ r Vet 61 (1957) pp 1089, 1t88
24
Davey~ Wo ' T h e e.p. p r o p e r t i e s of s o m e c h l o r i n a t e d compounds as a s s e s s e d b y the four b a i l m a c h i n e ' J o u r n a l of the I n s t i t u t e of P e t r o l e u m , VoI 31. {i945) p 45
25
J e m m e t t , A . E . ' T h e c h e m i c a l b e h a v i o u r of c h l o r i n a t e d additives under medium range temperature e~nditions' Institution of M e c h a n i c a l E n g i n e e r s Tribo!og7 C o n v e n tion. G o t h e n b u r g (1969) R e p o r t i m p e r 2
26
P r u t t o n , C. R., T u r n b u i i , D. and Dlouhy, Go ' O r g a n i c chlorine and sulphur compounds in e.p. lubrication" J o u r n a l of the I n s t i t u t e of P e t r o l e u m , Vet 32 (1946) p 96
27
Rounds F.G. ' E n v i r o n m e n t a l f a c t o r s affecting s u r f a c e coating f o r m a t i o n with l u b r i c a t i n g oil a d d i t i v e s ' T r a n s a c t i o n s of the A m e r i c a n Society of L u b r i c a t i o n E n g i n e e r s , Vol 8, No 21 (1985}
28
Rounds, Fo G. 'Some effects of n o n - h y d r o c a r b o n b a s e oil c o n s t i t u e n t s on the e f f e c t i v e n e s s of antiWear a n d cop. modifying a d d i t i v e s ' T r a n s a c t i o n s of the A m e r i c a n Society of L u b r i c a t i o n E n g i n e e r s , Vol 11~No 19 (1968)
29
Appeldoorn, J. K. and Taoo F. Fo ;The tubriei~" c h a r a c t e r i s t i c s of h e a v y a r o m a t i c s ' W e a r . Vol 12 [19~8) p 1!7
30
F o r b e s , E . S . , A l l u m , K . G . and S i l v e r , HoB. ' T h e Ioade a r r y i n g p r o p e r t i e s of m e t a l dialkyI dithiophosphateso A p p l i c a t i o n of e l e c t r o n p r o b e m i c r o a n a l y s i s ' I n s t i t u t i o n of M e c h a n i c a l E n g i n e e r s , Tribotogy Convention, G o t h e n b u r g (1969) P a p e r 6
Vinogradov, G. V., Arkarova~ V. V. and P e t r o v . A° Ao ° A n ~ w e a r and a n t i - f r i c t i o n p r o p e r t i e s of h y d r o c a r b o n s u n d e r heavy l o a d s ' W e a r , Vol 4 (!961) p 274
3t
G r o s z e k , A. Jo r W e a r - r e d u e i n g p r o p e r t i e s of m i n e r a l s oils and t h e i r f r a c t i o n s '~ i n s t i t u t i o n of M e c h a n i c a l E n g i n e e r s T r i b o l o g y Convention, P i t l o c h r y (t988) P a p e r 19
F u r e y , M . J . ' F r i c t i o n , w e a r and l u b r i c a t i o n ' J o u r n a l of I n d u s t r i a l and E n g i n e e r i n g C h e m i s t r y , Vol 62, No 3 ( M a r c h 1969) p 12
32
F o r b e s , E . S. and S i l v e r , H.B. ' T h e effect of c h e m i c a l s t r u c t u r e on the l o a d - c a r r y i n g p r o p e r t i e s of o r g a n o p h o s p h o r u s c o m p o u n d s ' A m e r i c a n C h e m i c a l Society P r e p r i n t s Vol 14, No 4 (1969) A102. P u b l i s h e d by The I n s t i t u t e of P e t r o l e u m , Vol 58 (1970) p 90
Toguyuchi, M. and Takai~ Y. ~E.p. l u b r i c a t i o n p r o p e r t i e s of s o m e sulphur compounds in m i n e r a l oil and the e f f e c t of d i s s o l v e d oxygen' B u l l e t i n of the J a p a n P e t r o l e u m I n s t i t u t e . Vol 3, (1961) p 63
33
Vinograd.ov, G., Pavlovskayao N . a n d Podolsky, Yo "Investigation of the l u b r i c a t i o n p r o c e s s u n d e r heavy fric [ion c o n d i t i o n s ' . Wear° Vol 6. (1963) p 202
TRIBOLOGY
August 1970