WEAR E L S E V I E R Wear 209 (1997) 1-7

Studies to evaluate the extreme pressure and anti-wear activity of some thiuram disulphides, xanthogens and dithiocarbamates

Saroj Kumar i Narendra N. Roy * Departnlent of Chemistr3. Regional Institute of Technology. 2amshedpur 831-014. India

Received 17 January 1996; accepted 20 November 1996

Abstract

Some bis-(aryl/alkyl)thiuram disulphides, bis-(aryl)xanthogens and (aryl/alkyl)dithiocarbamates were synthesized and their extreme pressure (EP) and anti-wear (AW) properties evalua~.ed using a Seta-Shell four-ball lubricant testing machine. Bis-(phenyl)xanthogen, bis- (ethanol)thiuram disulphide, bis-(diethanol )thiuram disulphide, bis-( naphthyl ) thiuram disulphide and N,N-die;hanol dithiocarbamate were found to be efficient EP additives showing zt welding load in the range 315-500 kgf. The coefficient of friction and mean specific pressure were evaluated and their variation with load in th~ case of different additives exl~lained. No additive was found to possess efficient anti-wear property. © 1997 Elsevier Science S.A.

Ke)words: Wear scar; Extreme pressure: Anti-wear: Friction coefficient: Thiaram disulphides

1. Introduction

The use of an appropriate load-carrying additive in a lubri- cant increases its mechanical efficiency and diminishes the wear of the machine surfaces. In addition, friction and surface damage of sliding parts are reduced and the load carrying capacity is increased.

Load-carrying additives are divided into two main types: anti-wear (AW) and extreme pressure (EP) additives.

The AW additive is normally most effective under mixed lubrication conditions. These are believed to function by resisting penetration of the oil film more effectively than the base oil itself, thereby reducing the amount of intermetallic contact and wear. These additives are employed in an exten- sive range of lubricants, e.g. automotive crankcase oils for gasoline and diesel-powered engines, automotive transmis- sion fluids, hydraulic fluids, turbine oils, gearbox lubricants, aviation lubricants etc.

As the load is increased, the temperature of the contact points increases until, at a certain value, the bulk oil film collapses. In this region, when present, the EP additivc rcacts with the metal surface to form an inorganic surface coating which can prevent welding of the surfaces. The low shear strength and high melting point of surface films enable them

* Corresponding author. Fax: + 91 657 407598. i Present address: Department of Chemistry. K.G. Arts and Science Col-

lege. Raigarh 496-001 (MP), India.

0043-1648/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved Pll S0043-1648 ( 96 ) 07487 - X

to carry ~ high load. EP additives are mainly used in industrial lubrication, e.g. metal cutting and racial forming operations, heavy gear lubrication, in the rear axles of automotive vehi- cles etc.

A large number of chlorine, sulphur and phosphorus com- pounds having AW and EP properties have been synthesized and used [ 1-25 I. in the present work, a number of bis-(aryl / alkyl ) thiuram disulphides, bis-(aryl) xanthogens and (aryl / alkyl) dithiocarbamates were synthesized and evaluated as EP and AW additives. The results for EP and AW additives were compared with those for the standard additives dibenzyl disulphide and tricresyl phosphate respectively.

2. Experimental details

2.1. Additives

Bis-(aryl /alkyl) thiuram disulphides were prepared by first treating amines with CS2 in the presence of aqueous KOH and then oxidizing the thiocarbamide derivative with sodium nitrite, methyl alcohol and hydrochloric acid [261.

The reaction can be represented as

cS2 IOI RNH 2 - - R N H - C S S K - - R N H - C S - S - S - C S - NHR

KOa

where R denotes naphthyl, ethanol and diethanol groups.

S. Kumar. N.N. Roy / Wear 209 (1997) I-7

B~s-(aryl)xanthogens were prepared following a similar mute [26] and the reaction can be represented as

CS2 [o1 R - O H - - R - O - C S S K - - R - O - C S - S - S - C S - OR

go.

where R denotes phenyl and naphthyl groups. The dithioearbamates were prepared by the interaction of

ammonium hydroxide, carbon disulphide and the appropriate aryl or alkyl amine [7]. The reaction can be represented as

R--NH 2 + CS 2 NH4OH~

R _ N H _ ~ _ S _ N H 4 dil. acid~ R--NH--~--SH

S S where R denotes phenyl, naphthyl, ethanol, diethanol and benzyi groups.

2.2. Materials

A

,i L1 L2

Load ( k g f )

Fig. I. Wear-load curve.

The steel balls used were made of 0.42% C, 0.58% Mn and 0.48% Cr of hardness 765 VHN and had a diameter of 12.7 mm. The base lubricant oil used was light liquid paraffin oil having the following physical characteristics: viscosity 126 cp at 28 °(2; flash point 185 °C; fire point 2000(2; specific gravity 0.81 at 28 °C; cloud point - 2 °(2: pour point - 8 °C.

Calculated amounts of the synthesized additives were dis- solved in the minimum quantity of benzene, and mixed with paraffin oil so as to give 0.5% w/v solution. Some additives did not dissolve and hence were in the suspended form. The mixture was heated to 60 °C while stirring continuously for about 30 min to ensure homogeneity.

2.3. Extreme pressure lubricant testing

The Seta-Sbell four-ball EP lubricant testing machine [ 27 ] was used for evaluation of the EP additives. The scars on the lower three balls were measured at the end of the test to assess wear and lubricant failure. Different parameters were calcu- lated by the standard methods [ 27] or by using the relation- ships below.

2.3.1. Flashtemperatureparameter(FTP) This is a single number used to express the critical tem-

perature above which a given lubricant will fail under given conditions. It is calculated using the following relationship:

F r P ffi ( W/d l ' 4 )trout

where W is the load in kg and d is the mean scar diameter in ram. The FTP is the maximum value of the ratios WId TM

obtained for the various runs [20].

2.3.2. Pressure wear index (PWI) This is a criterion used to express the anti-wear property

as well as anti-seizure property of an oil. Referring to a typical wear-load curve, as shown in Fig. I~ the PWI is calculated using the formula

where/,2 and d2 are the load and scar diameter corresponding to point C. Lm and dl are the load and scar diameter corre- sponding to point B [20].

2.4. Anti-wear lubricant testing

Anti-wear properties were measured with the Shell four- ball machine using balls of 0.5 inch diameter having the same composition as for EP testing. Each test was run at a bulk oil temperature of 50 °(2, at a load of 15 kg, and a speed of ! 500 rev rain- ~. The duration of each test was either 30, 45 o~- 60 min [3]. The test results are reported as the mean wea~- scar diameter in millimetres.

The test conditions were chosen so that bulk seizure of the metal surfaces did not occur. The results were compared with a standard AW additive, lricresyl phosphate, since phospho- rus compounds are good AW additives.

3. Results and discussion

To evaluate the EP properties of the various additives syn- thesized, the wear scar diameter at various loads, initial sei- zure load (ISL), 2.5 s seizure delay load, final welding load (WL), coefficient of friction under different loads, mean specific pressure at the contact surfaces, flash temperature parameter (FTP) and the pressure wear index (PWI) were determined. These are recorded in Table 1.

To evaluate the AW properties, scar diameters for different runs for different durations were determined. These are recorded in Table 2.

S. Kumar, N.N. Roy /Wear 209 (1997) 1-7 3

+

~ ~ • " , ~ . . . . g ~

~ " " ~

..1

=

P,,- e l . ,~ ~ e , i ~ - - w~ r+.

~ ~ o -

.~ .~ .~ ~ ~ .r, -~ ~ ~ ~ .~ ~ 4 - 4 - + + + + + + + + +

i i i i i i i . . . .

.o

+o

~a

N ' - -

3.1. Log load vs. log scar d iameter

Fig. 2 shows curves plotted with reference to the Hertz line. It indicates the relationship between load and mean scar diameter for plain paraffin oil with and without the additives. It is apparent from Fig. 2 that the value of scar diameter increases slowly at first to the initial seizure load, beyond which it increases sharply up to a second discontinuity owing to lubricant failure, and finally it increases slowly up to ti,,e welding load indicating complete failure of the lubricant. The slow increase in scar diameter beyond the second disconti- nuity may he due to additive-metal surface interaction or to a reduction in stress or to both.

For example, the scar diameter-load diagram of bis- (phenyl) xanthogen shows that the curve deviates at its initial seizure load (90 kgf), a second discontinuity occurs at 100 kgf and finally welding occurs at a load of 500 kgf.

Table ! shows that all the additives in the paraffin base oil exhibit higher ISL values, 2.5 s seizure delay loads and weld- ing loads compared with the plain paraffin oil. This indicates that all the additives tested have improved the quality of the base oil. However, bis-(ethanol)thiuram disulphide, bis- (diethanol) thiuram disulphide, bis-( phenyl ) xanthogen and N,N-diethanol dithiocarbamate appear to have much higher EP activity compared with the standard reference additive, dibenzyl disulphide. In the case of the bis-(phenyl)- xanthogen, bis-(ethanol)thiuram disulphide and bis-(di- ethanoi)thiuram disulphide, the welding load is higher by about 100% and 60% respectively than the value for the standard reference additive.

3.2. Load vs. coefficient o f f r ic t ion

The coefficient of friction is plotted against applied load in Fig. 3. In each friction curve, a peak occurs after the ISL values. This indicates seizure between contacting metal sur- faces followed by partial recovery. The graphs show small values of the coefficient of friction/L at the lower load ranges. This may be due to the physisorbed or chcmisorbed film of lubricant on the ball surface which carries the load, and hence the coefficient of friction values are lower. However, at higher loads, owing to the high temperature this thin film fails and so we observe a sharp increase in friction. The decline after the steep rise shown by some curves indicates the formation of a new film by the interaction of the additive with the exposed surface. The 'zig-zag' pattern of the graph in certain cases may be due to the formation of relatively smooth surfaces at certain loads; these worn surfaces make partial elastohydi'odynamic lubrication (EHL) possible [28]. At welding loads, owing to the very high temperature, this thin film also fails.

The curves show that in the case of bis-(phenyi)- xanthogen, bis-(diethanol)thiuram disulphide and bis- (ethanol)thiuram disulphide, the coefficient of friction increases up to certain load and then changes slowly, which is a characteristic of an efficient additive [ 12]. A probable

4 S. Kumar, N.N. RoylWear209(1997) 1-7

Table 2 Mean wear scar diameters al~r 30, 45, and (30 min runs for different additives

Specimen Lubricaut Mean wear scar diameter (nun)

30 min 45 rain 60 min

I 2 3 4 5 6 7 8 9

I0 I1 12

Paraffin oil without additive Paraffin oil +bis-(napthyl) thiuram Paraffin oil + bis-(ethanol) thiuram disulphide Paraffin oil + bis-(diethanol) thioram disulphide Paraffin oil + bis-(phenyl) xanthogen Paraffin oil + bis-( naphihyl ) xanthogen Paraffin oil + phenyl dithiocarbamate Paraffin oil + naphthyl dithiocarbamatc Paraffin oil + N-ethanol dithiocarbamate Paraffin oil + N,N-diethanol dithiocarbamate Paraffin oil + benzyl dithiocarbamate Paraffin oil + tricresyl phosphate

0.70 0.74 0.75 0.75 0.95 1.10 0.50 0.55 0.60 !.i2 1.25 1.35 0.55 0.63 0.70 0.85 1.00 1.20 0.98 1.14 1.30 1.00 1.05 1.15 0.30 0.45 0.58 0.80 0.92 1.07 0.47 0.57 0.65 0.25 0.28 0.30

4 . 0 0

3 .00

2 . 0 0

| J 1.00

0 . 9 0 .~ 0 . 8 0 • o 0 .70

~g 0 .60 g "a 0 .50

O.40

0 .30

H e r l z l ine

0 .20

i I I I I I I ! i I i I i l i i I I I i l 56 170 I 901 . 2 I t 4 0 1 i781 224 2001 3 5 5 1 4 5 O 1

63 80 O0 26 b8 200 250 315 400 500

Log foocl, kgf

Fig. 2. Log load vs. log wear scar diameter: • plain paraffin oil: A bis-(naphthyl)thiuram disulphide; 0 bis-(ethanol)thiuram disulphide; × bis-(di- ethanol)thiuram disulphide; & bis-(phenyl)xanthogen; o bis-(naphthyl)xanthogen; "~ phenyl dithiocarbamate; * naphthyl dithiocarbamate; 0 N-ethanol dithiocarbamate; t-'l N,N-diethanol dithiocarbamate; • benzyl dithiocarbamate; ~7 standard reference additive.

reason for this may be the slow decomposition of additives 3.3. L o a d vs. mean specific pressure at cer tain t empera ture and pressure, w h e n the addi t ive inter- acts wi th the meta l surface to fo rm an inorganic meta l sul- Fig. 4 indicates the relationship be tween the m e a n specific phide f i lm which funct ions up to the we ld ing load [7 ] . pressure Pm and the load. The shapes o f the cu rves are similar,

S, Kumar, N.N. Roy/Wear 200 (1997) I-7 5

0.32

0.30

0.28

0.26

0.24

0.22

0.20

0 . 1 8

0.16

0 . 1 4

0.12

0.10

o.oe

0.06

0.04

o.02

I I I J I I I I I I I I I I I I 4OO 45O 25O ZSO 140 158 315 356 ,ol.o .2 2;.

80 I00 126 Lood (kgf)

Fig. 3. Load v s. coefficient of friction: • plain paraffin oil; A bis- ( naphthyl ) thiuram disulphide; O b; ~- ( ethanol ) thiuram disulphide; × Dis- ( diethanot ) thiurara disulphide; • bis-(phenyl)xanthogen; [:] N.N-diethanol dithioearbamate; ~7 standard reference additive.

E E

i u

g

3 0 0 -

280 L_

240 i 220

200

180

160

140

120

I00

8O

60 ~ i 4oj

2o

I I I I I I I I I I I I I 140 158 178 200 224 250 280 31,5 355 400 450

63 80 tOO 126 Lood (kgf}

Fig. 4. Load vs. mean specific pressure: • plain paraffin oil; A bis-( naphthyl )thiuram disulphide; (3 bis-( ethanol )thiuram disulphide; ×bis-( diethanol )thiuram disulphide; • bis-(phenyl)xanthogen; [] N,N-diethanol dithiocarbamate; V standard reference additive.

the m e a n specific pressure at the contac t points decreases sharply until the 2.5 s seizure delay load, thereafter it remains more o r less constant.

In the case o f the reference addit ive the m e a n specific pressure d ropped to 46 .80-56 . I 0 kg m m - 2, whereas for bis- ( pheny l )xan thogen i t. decreased to 52 .00 kg m m - 2 before

S. Kumar, N.N. Roy~Wear209(1997) 1-7

1.30

1,20

1.10

~ 0.90

| o-'e r ! °"~I 0.50 1 0.40

0 . ~

0.20

0.10

Time (rain)

Fig, 5. Time vs. wear scar diameter. • plain paraffin oil; A bis-

(naphthyl)thiuram disulphide: O bis-(ethanol)thiuram disulphide: x bis- (dietlumol)thiuram disulphide: • bis-(phenyl)xanthogen: o bis- (naphthyl)xanthogen; ~" phenyl dithlocarbamate; - naphthyl dithiocatba- mate; <> N-ethanol dlthiecarbanuue: [ ] N,N-diethanol dithiocarbamate; • benzyl dithiocarbanuae; V standard reference additive.

rising again to 76.41 kg mm -2, for bis-(ethanol)thiuram disulphide it decreased to 36 kg mm -2 before rising again to 57.62 kg mm -2, and for bis-(diethanoi)thiuram disulphide it decreased to 40.88 kg mm -2 before rising again to 81.25 kg mm -2.

The additive-metal interaction films fail at pressures above 76.41 k g m m -2, 81.25 k g m m -2, and 57.62 k g m m -2 for bis-(phenyl)xanthogen, bis-(diethanol)thiuram disulphide and bis-(ethanoi)thioram disulphide respectively (Table I ), whereas this value is only 50.78 kg mm -2 for the reference additive. Thus the. determination oftbe mean specific pressure for these additives also strengthens our belief in their excel- lent functioning as EP additives compared with the standard reference addiu re.

3.4. Anti-wearproperties

Mean wear scar diameters in millimetres for different addi- tives after 30, 45 and 60 min test periods are given in Table 2 and Fig. 5. Since phosphorus compounds are the best AW additives, the results were compared with a standard AW additive, tricresyl phosphate.

The results show that no additive has better performance than the standard additive. This is in support of the view [5] that sulphur compounds are not good AW additives. How- ever, among the additives studied, N-ethanol dithiocarbamate has better performance, giving scar diameters of 0.30 ram, 0.45 mm and 0.58 mm, after 30, 45, and 60 rain respectively. Bis-(ethanol)thiuram disulphide also shows good AW properties.

4. Mechanism

According to Allum and Ford [2] and Forbes [5] the ;oad carrying capacity provided by the lubricant film formed by

the sulphurized additives depends on the ease of cleavage of C-S bonds in the molecule, resulting in the formation of inorganic iron sulphide on the rubbing surface.

Several other authors, however, have pointed out that the formation of organo-inorganic films is chiefly responsible for providing the EP characteristics [ 1,5,29]. According to Forbes also, there is no evidence that pure iron sulphides are the load-canying layers and these layers are more likely to be a complex mixture of iron-sulphor and possibly carbon oxygen compounds [5].

In the disulphide, xanthogen and dithiocarbamate classes of compound considered in the present work, the S-S bond is the weakest of all the bonds present in the molecule and so it will break first [30]. As the load approaches EF regions, C-$ bond cleavage may take place giving an iron sulphide layer. In addition, R-N and R-O bonds may also break to give COS in xanthogens [31] and NHCS (possibly) in thiuram disulphides. Iron will react chemically with S to give an inorganic iron sulphide layer. A complex organic film may be formed by R and COS/NHCS. Thiuram disulphides and xanthogens appear to be better EP additives than dithiocar- bamates. This may be due to the weak S-S bond and avail- ability of more S atoms per molecule.

& i~nrnbm~

The three best of the additives studied gave positive results when tested for their durability after 12 months. The same durability was found with an increase in scar diameter but with almost the same coefficient of friction.The initial seizure load and the welding load were the same as measured 12 months before. For example, with his-(phenyl) xanthogen the sear diameter was 1.75 mm and the coefficient of friction was O. 189 at 450 kgf. When tested at the ~ m e load after 12 months the scar diameter was found to be 2.1 mm and/~ was 0.190.

6. Relative advantages of these additives

6.1. Cost

As the chemicals used in the synthesis of these compounds are very common and cheap also, so the cost will be low. For example, for bis-(phenyl)xanthogen, one requires phenol, CS2, KOH, NaNO2, CH~OH and HC!. For other ,additives also, only different amines are required, and these amines are easily available and cheap.

6.2. Ease of synthesis

The synthesis of the additive is very simple and no sophis- ticated apparatus is required. For example, for disulphides the appropriate amine, carbon disulphide and aqueous KOH are heated under reflux for about 8 h and then the solution is oxidized with the help of sodium nitrite, methyl alcohol and

S. gumar, N.N. Roy/Wear 209 (1997) I-7

hydrochloric acid. The compound is precipitated and is then purified by recrystallizafion.

6.3. Eff icacy at l ower dosage levels

It is evident from Table I that a small concentration of the compound exhibits good performance and hence its efficacy at lower dosage levels is obvious.

7. Conclusion

Bis-(phenyl)xanthogen.bis - (e thanol) th iuramdisulphide , his-(diethanol) thiuram disulphide are potentially very good extreme-pressure lubricant additives. However, other tests need to he done, for example copper corrosion rust and oxi- datiou tests, and compatibil i ty with other additive compo- nents o f a lubricant package, before progressing to field trials o f these additives.

[ 18] V.V. Rumyantscva and P.S. iklov, Nefiepereab, Neflekim, 5 (1989) 14 (in Russian); Chem. Abstr., I I I (1989) 81006a.

1191 A. Bhattacharya. T. Singh, V.K. Verma and K. Nakayama, Wear. 136 (2) (199C) 347.

{ 20] T. Singh, R. Singh, V.K. Verma and K. Nakayama, Tribol. Int., 23 ( I ) (1990) 41.

[21] A. Bhanacharya, 1". Singh, V.K. Verma and N. Prasad, Indian J. TechnoL, 29 (10) ( 1991 ) 478.

[ 22 ] A. Bhanachatya and V.K. Verma, Lubr. Sci., 4 (4) (1992) 241. 123 ] S. Kumar and N.N. Roy, Indian J. Eng. Mater. $ci., I (1994) 292. [ 24l A. Bhattacharya. T. Singh, V.K. Verma and N. Prasad, Tribal Int., 28

(3) (1995) 189. [25l S. Kulnar and N.N. Roy, ili T.S. Mruthyanjaya (ed.), Advances in

Mechanical Engineering, Narosa, New Delhi CD, 1996 pp. 1805. [26] L. Neelkanlan. J. Ors. Chem., 23 (1958) 938. [27] Manual of Seta-S~ell Four.ball £P Lubricant Testing Machine,

Stanhope Seta lad. London. 1965. [ 281 C.V. AgrawaL J. Singh and V. Singh. J. Inst. Eng. India, Meclt Eng.

Div., 53 (1973) ,05. [29] O.N. Anand. V.P. Mallick and K.D. Neemla, Tribol. Int., 19 (3)

(1986) 128. [ 30l D.D. Ebbing and M.S. Wrighton, General Chemisto,. p. 340, 3td can.,

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B iograph ies

N.N. Roy worked as Professor o f Chemistry in the Regional Institute o f Technology, Jamshedpur from Decem- ber 1987 to July 1994 and as Head o f Department f rom December 1987 to December 1992. Prior to this be has been working in this department since September 1960. He worked in Tribhuvan University, Kathmandu, Nepal from April 1973 to December 1976 on deputation by the Government o f India. He received his MSc in Chemistry from Patna University in 1956 and his PhD in Physical Chemistry from the University o f Washington, Seattle, W A in 1970. He is Professor Emer- itus and is working as Principal Investigator in the UGC Research Scheme. His current interests include activity eval- uation o f lubricant additives, environment pollution studies, monitoring and removal o f pollutants, soil acidification.Saroj Kumar obtained his MSc degree in Physical Chemistry f rom the RegioJl al Institute o f Technology, Jamshedpur under Ran- chi University, Ranchi in December 1988. He worked as Research Officer in the same institute f rom April 1990 to October 1993 working on lubricant additives. At present be is Assistant Professor in the Department o f Chemistry, K. Govt. Arts and Science (autonomous post-graduate) Col- lege, Raigarh (MP) India and is working for his PhD degree on the extreme pressure activity evaluation o f sulphur con- raining compounds.