Tribological studies of some SAPS-free Schiff bases derived from 4-aminoantipyrine and aromatic aldehydes and their synergistic interaction with borate ester

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Tribological studies of some SAPS-free Schiﬀ bases

derived from 4-aminoantipyrine and aromatic

aldehydes and their synergistic interaction with

borate ester†

Cite this: J. Mater. Chem. A, 2014, 2,

10424

a

Vinay Jaiswal, Kalyani, Rashmi B. Rastogi and Rajesh Kumar^b

*

Tribological performance of sulfur, phosphorous and metal-free Schiﬀ bases of 4-aminoantipyrine with

benzaldehyde (AAPB), salicylaldehyde (AAPS), p-chlorobenzaldehyde (AAPC) and p-methoxybenzaldehyde

(AAPM) and their synergistic formulations with borate ester (BE) in paraﬃn oil has been evaluated using a

four-ball tester at optimized concentrations of Schiﬀ bases (1% w/v) and their synergistic formulations with

BE (0.5% w/v for each) by varying loads for 30 min duration and varying test durations at 392 N load.

Synergistic formulations eﬀectively enhance antiwear properties of oil and possess high load carrying

capacity in comparison to the conventional zinc dibutyldithiophosphate (ZDDP)/BE/Schiﬀ bases alone. The

best eﬃciency is shown by AAPM, followed by AAPC, AAPS and then AAPB. The same order is observed in

their corresponding synergistic formulations. AFM and SEM micrographs of the wear scar lubricated with

synergistic formulations show a drastic decrease in surface roughness in comparison to borate ester/

ZDDP alone. EDX analysis of the worn surface in the presence of AAPM + BE exhibits nitrogen, boron and

oxygen indicating adsorption of the additive on the surface. XPS of the triboﬁlm shows B₂O₃, BN, Fe₂O₃,

Fe₃O₄and adsorbed carbon in the form of –C–C– and –C(O)O– moieties. Quantum chemical

calculations (DFT) for interactions of Schiﬀ bases with surfaces show good agreement with experimental

results.

Received 10th March 2014

Accepted 22nd April 2014

DOI: 10.1039/c4ta01191k

www.rsc.org/MaterialsA

surface.³³A literature survey reveals that the tribological appli-

cations of zinc, molybdenum and lanthanum complexes of

dithiohydrazodicarbonamides,^17,18dialkyldithiophosphates,^19,20

1. Introduction

A wide variety of lubricant formulations have been used to reduce

interfacial friction and enhance wear-resistance of the moving

surfaces. The additives containing sulfur,^1–3phosphorous,^4–7

halogen,⁸nitrogen^9–13or boron^14–16and metal complexes^17–27are

being used in lubricating oils as multifunctional additives like

antiwear, extreme pressure, antioxidant and corrosion inhibi-

tion. The lubricant is believed to perform these functions by

forming a stable interfacial tribochemical lm between two

proximate surfaces, thereby reducing wear and friction and

increasing the life of surfaces in contact.^28–32The extent to which a

lubricant composition enhances antiwear and antifriction prop-

ertiesof baseoildepends on a numberoffactors like thestructure

of the additives and their concentration, interface chemistry,

sliding speed, applied load, contact temperature and nally tri-

bochemical characteristics of the lubricating lm formed at the

dithiocarbamates,^21,22and tricresylphosphates²³are extensively

recognized. Among the variety of additives, zinc dia-

lkydithiophosphates (ZDDP) are arguably the most successful

multifunctional lubricant additives ever invented.^23–27However,

the excessive use of additives containing phosphorous, sulfur,

halogen and metals has been limited due to the several negative

impacts they have caused to the environment as well as to the

engines.³⁴The phosphorous and sulfur contents adversely aﬀect

the catalytic eﬃciency of exhaust emission catalytic converters.³⁵

In addition to environmental pollution, the health hazards

include eye irritation, allergic contact dermatitis and mutage-

nicity.^36,37Nowadays there are several norms to strictly limit the

SAPS (sulfated ash, phosphorous and sulfur) contents of addi-

tives.^38,39Therefore, concerted eﬀorts have been made in this

direction to develop new antiwear additives to replace ZDDP and

reduce the SAPS content of additives without compromising the

performance of the base oil.

^aDepartment of Chemistry, Indian Institute of Technology (Banaras Hindu University),

Varanasi-221005, India. E-mail: rashmi.apc@iitbhu.ac.in; Fax: +91 542 2368428

^bDepartment of Mechanical Engineering, Indian Institute of Technology (Banaras

Hindu University), Varanasi-221005, India

Modern lubricant formulations, in addition to their high

level performance need to be environment friendly also.

Recently, Schiﬀ base derivatives and borate esters have

attracted much more attention.^40–42Schiﬀ bases being

† Electronic supplementary information (ESI) available. See DOI:

10.1039/c4ta01191k

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biologically active materials are frequently used as antitumor,

The lubricating base oil, neutral liquid paraﬃn oil (Quali-

antibacterial, antifungal and anticancer drugs. On the other gens Fine Chemicals, Mumbai, India) having specic gravity

ꢀ

hand, boron containing compounds have also been used as 0.82 at 25 C, kinematic viscosity, at 40 C and 100 C, 30 and

eﬀective antiwear additives, antioxidants, friction modiers 5.5 cSt respectively, viscosity index 122, cloud point À2 ^ꢀC, pour

and corrosion inhibitors.^14–16,43Their tribological behavior is point À8 ^ꢀC, ash point 180 ^ꢀC and re point 200 ^ꢀC, was used

comparable to that of ZDDP, because like ZDDP, these without further purication. Commercial borate ester was

compounds also form a glassy lm on the metal surface. obtained as a gi under the trade name of Vanlube 289 which

Thus boron containing compounds appear to be an attractive contains borate ester as the main component. This is a yellow

alternative to the conventional additives, showing almost liquid (color ASTM D1500 L1.5) with boron content 1.00%,

similar antiwear performance to that of ZDDP but without density 0.99 mg mm^À3at 15.6 C and viscosity 458.00 cSt and

ꢀ

adding sulfur and phosphorous to the base oil. This has been 22.30 cSt at 40 C and 100 C, respectively.

explored well in our earlier work on thiosemicarbazones

and their synergistic mixtures with borate esters as

eﬃcient low/zero SAPS antiwear additives in paraﬃnic base

oil.^40–42

In continuation to that, the present communication reports

the synthesis of a series of SAPS-free Schiﬀ bases derived from

the condensation of 4-aminoantipyrine with benzaldehyde,

2.2 Synthesis and characterization of Schiﬀ base lubricant

additives

The Schiﬀ bases have been synthesized by the earlier reported

methods.^44,45An anhydrous ethanolic solution (30 ml) of

4-amino-1,5-dimethyl-2-phenylpyrazol-3-one (0.3 mol) was

added dropwise to a round bottom ask containing ethanolic

solution (30 ml) of substituted benzaldehyde (0.3 mol) equipped

with the magnetic stirrer and condenser. The reaction mixture

was reuxed for 4–5 h (Scheme 1). The progress of the reaction

was monitored by TLC. Aer cooling, the obtained yellow color

precipitate was ltered, washed several times with ethanol and

recrystallized with ethanol, and then dried in vacuo. The purity

of compounds has been conrmed by TLC using n-hexane and

ethyl acetate (6 : 4).

salicylaldehyde,

p-methoxybenzaldehyde

and

p-chlor-

obenzaldehyde. The base 4-aminoantipyrine (4-amino-1,5-

dimethyl-2-phenyl-1H-pyrazol-3(2H)-one) has been chosen as it

is free from S, P, and metal and contains a phenyl ring and

pyrazolone moiety having a number of triboactive centers like

N, O and methyl groups which assist in lubrication through

adsorption. The tribological behavior of Schiﬀ bases has been

evaluated in the absence and presence of borate ester (Vanlube

289) in paraﬃn oil using a four-ball tester. These metal, sulfur

and phosphorous free formulations are expected to provide

excellent tribological and environment friendly compatibilities

when used as an additive in paraﬃn oil. Being ashless, these

antiwear lubricant additives have potential to nd applications

in various automotive industries as sludge formation is

reduced to improve the machine eﬃciency. A comparison of

the observed tribological behavior of Schiﬀ bases and their

synergistic formulations with that of ZDDP shows that these

formulations oﬀer an environment friendly alternative to the

conventional high SAPS containing antiwear lubricant addi-

tives ZDDP. The characterization of the tribochemical lm

formed on the interacting metallic surfaces has been done and

their surface topography has been studied with the help of

X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive

X-ray spectroscopy (EDX), Scanning Electron Microscopy (SEM)

and contact mode Atomic Force Microscopy (AFM). Quantum

chemical calculations of studied Schiﬀ bases have been also

performed using the Gaussian 03 program to correlate their

experimentally obtained tribological behavior with the theo-

retical one.

The ¹H NMR and ¹³C NMR spectra of these Schiﬀ base

additives have been mentioned in the ESI [S1].†

2.3 Tribological characterization

2.3.1 Sample preparation. Paraﬃn oil blends of Schiﬀ

bases, having concentrations 0.00, 0.25, 0.5, 0.75 and 1.0% (w/v)

with 1.00, 0.75, 0.50, 0.25 and 0% (w/v) of borate ester respec-

tively, were made by stirring for 1–2 h on the magnetic stirrer.

The entire antiwear and load carrying tests were carried out at

an optimized concentration i.e., 1.0% w/v Schiﬀ bases, 1% w/v

synergistic formulations (0.5% Schiﬀ base + 0.5% BE) and

compared with those of 1.0% w/v zinc dibutyldithiophosphate

(ZDDP) and commercial borate ester (BE) in paraﬃn oil.

2.3.2 Specimen. The balls of 12.7 mm diameter made up of

AISI 52100 alloy steel having hardness 59–61 HRc were used for

the tests. Before and aer each test, balls were cleaned with

n-hexane and thoroughly air-dried.

2.3.3 Antiwear testing. The friction and wear properties of

these compounds and their synergistic formulations as

2. Experimental section

2.1 Chemicals

The starting materials 4-aminoantipyrine (99%, Merck), benz-

aldehyde (98%, Merck), salicylaldehyde (99%, Merck), 4-chlor-

obenzaldehyde (99%, Merck) and 4-methoxybenzaldehyde

(99%, Merck) were used without further purication. All the

solvents used throughout the experiment were of analytical

grade and were used without further purication.

Scheme

1 Synthesis of Schiﬀ bases of 4-aminoantipyrine with

diﬀerent substituted aldehydes.

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antiwear additives in base oil were evaluated using a Four-Ball eﬃciency of a particular compound depends on its concentra-

Lubricant Tester (Stanhope-Seta, London Street, Chertsey, UK) tion in base lube, it is important to optimize the additive

at 1475 rpm (equivalent to a sliding speed of 567 mm s^À1) using concentration before conducting the tribological tests. The

diﬀerent loads for diﬀerent time durations according to ASTM mean wear scar diameter is an indication of the extent of wear

D4172. The wear scar diameter on the lower three balls was when sliding contact occurs. Fig. 1a shows the variations of the

measured aer running for 15, 30, 45, 60, 75 and 90 min, mean wear scar diameter (MWD) of steel balls lubricated with

respectively at 392 N load and at various loads 294, 392, 490, 588 the diﬀerent combinations of synthesized Schiﬀ bases and

and 686 N for 30 min test duration. In order to get more reliable commercial borate ester in paraﬃn oil by keeping the total

values, each tribological test was repeated thrice. An optical additive concentration as 1% w/v at 392 N load for 60 min test

microscope was used to measure the wear scar diameter of three duration. It is evident from Fig. 1 that all of the additives when

stationary balls, then the mean value was calculated and cited mixed in paraﬃn oil in diﬀerent concentrations reduce the wear

here as the mean wear scar diameter (MWD). The details of scar diameter on the steel surface. The value of the mean wear

experimentation and various tribological parameters have been scar diameter has been found to be high in the case of the

mentioned in the ESI [S2].†

surface lubricated with borate ester/Schiﬀ base additives alone

at 1% w/v. By adding a small amount (0.25% w/v) of the Schiﬀ

base additive to borate ester, a signicant decrease in the MWD

value is observed. There is a maximum decrease in MWD when

the Schiﬀ base and BE both are 0.5% w/v. On further increasing

2.4 Surface characterization

Scanning electron microscope (SEM) images of the worn surface

areas of the steel balls were taken using a ZEISS SUPRA 40

electron microscope. The elemental compositions of the tribo-

lm formed on the worn surface of the steel balls were deter-

mined by using an energy dispersive X-ray spectroscope (EDX).

A contact mode atomic force microscope (model no. BT 02218,

Nanosurf Easyscan 2 Basic AFM, Switzerland) was used to

investigate roughness of the worn surfaces with an Si₃N₄

cantilever (Nanosensor, CONTR type) having a spring constant

of ꢁ0.1 N m^À1and tip radii more than 10 nm. X-ray photo-

electron spectroscopy (AMICUS, Kratos Analytical, Shimadzu,

U.K.) was used for analyzing the chemical composition of the

tribolm formed on the worn surface. Aer testing the syner-

gistic formulation of the Schiﬀ base with borate ester (0.5 + 0.5

wt%) under a load of 392 N for 90 min duration, one of the three

lower balls was ultrasonically cleaned in hexane for about 5 min

and allowed to dry in the atmosphere. The XPS of wear scar on

this ball was recorded. The radiation source Mg Ka line with a

pass energy of 29.35 eV and the binding energy of C 1s (284.6 eV)

was used as a reference.

2.5 Computational details

Density Functional Theory (DFT) is found to be a suitable

method for theoretical calculations of electron densities at

various centers of a molecule. All the calculations were per-

formed using B3LYP⁴⁶that uses Becke's three-parameter func-

tional (B3) and includes a mixture of Hartree–Fock with DFT

exchange terms associated with the gradient corrected

exchange–correlation functional of Lee, Yang and Parr (LYP).⁴⁷

It has comparatively lesser convergence problems than those

found in pure DFT methods. Thus, B3LYP has been used in this

paper to carry out quantum calculations. Full geometry opti-

mizations of all additives were carried out with the standard

B3LYP/6-31G++(d,p) basis set⁴⁸using Gaussian 03, D.01.⁴⁹

Fig. 1 (a) Eﬀect of change in the concentration of diﬀerent additive

formulations on the mean wear scar diameter in paraﬃn oil at 392 N

applied load for 60 min duration. (b) Variation of the mean wear scar

diameter in the absence and presence of diﬀerent concentrations of

AAPM with borate ester in paraﬃn oil at 392 N applied load and 60 min

3. Results and discussion

The potentiality of the synthesized Schiﬀ base compounds and

their synergistic formulations as antiwear additives in paraﬃn

oil were evaluated using a four-ball tester. Since the antiwear duration.

10426 | J. Mater. Chem. A, 2014, 2, 10424–10434

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the concentration of the Schiﬀ base up to 0.75% w/v and keeping eﬃciency of Schiﬀ bases alone in base lube has been found to

BE 0.25%, the value of MWD increases abruptly. The increase in be poorer than those of conventional high SAPS containing

the concentration of the Schiﬀ base to 1% w/v (absence of BE) ZDDP and commercial BE. It is interesting to note that the

further raises the MWD. Thus, the best wear resistance behavior mixtures of the synthesized additives with BE signicantly

is shown when Schiﬀ bases and borate ester both are present at improved the antiwear behavior of base lube showing a

0.5% w/v concentration of each. Out of all the tested Schiﬀ bases, decrease in the value of the MWD to a greater extent.

the minimum MWD value is obtained in the case of synergistic

The antiwear behavior of paraﬃn oil as a function of time in

formulation containing AAPM. To show synergy between Schiﬀ the presence of diﬀerent additives can be discussed on the basis

bases and BE, a bar diagram has been constructed, Fig. 1b, of Fig. 2. Aer 15 min of time duration there is a slight increase

taking together BE and AAPM as the best antiwear Schiﬀ base in the MWD but aer 30 min there is an abrupt increase in the

additive. The diagram evidently exhibits synergy between AAPM MWD. A substantial increase in the MWD is observed aer

and BE as MWD shows appreciable decrease in its value when 60 min of time duration which progressively continues up to

0.5% of each are taken instead of 1% alone of the individuals. 90 min time duration. Almost similar nature of curves is

Thus, entire tribological tests have been conducted with an observed in the presence of admixtures of paraﬃn oil with

optimized concentration of synergistic formulation of Schiﬀ Schiﬀ bases/BE/ZDDP/mixtures of Schiﬀ bases with BE. As

bases along with borate ester (0.5 + 0.5% w/v) and compared with apparent from the gure beyond 30 min time duration, the

those of 1% w/v of the individual additives.

Herein, the concentration dependence of the wear reducing these additives as compared to paraﬃn oil alone.

ability of the additives can be explained on the basis of their Furthermore, the MWD of p-methoxyphenyl, p-chlorophenyl,

slope of these curves has drastically reduced in the presence of

aﬃnity towards additive–additive interactions along with o-hydroxyphenyl and phenyl Schiﬀ base derivatives is reduced

interactions of the additives with the metal surface. More active by 36, 29, 23 and 19%, respectively, compared to paraﬃn base

additives are prone to reduce wear even at comparatively lower lube. However, there is an appreciable enhancement in the

concentrations by forming protective tribochemical lms on reduction of MWD to 51, 49, 48 and 47%, respectively for their

sliding interfaces.

corresponding mixtures with commercial borate ester. Thus,

there is a signicant improvement in the antiwear eﬃciency of

synergistic mixtures as compared to Schiﬀ bases alone. The

overall antiwear behavior of diﬀerent additives on the basis of

MWD values may be arranged in the following order:

3.1 Antiwear behavior

A series of friction and wear tests have been conducted to

evaluate the tribological properties of synthesized Schiﬀ bases

and their mixtures with commercial borate ester in paraﬃn oil.

Fig. 2 illustrates the variation of the mean wear scar diameter

with time duration 15, 30, 45, 60, 75 and 90 min at 392 N applied

load in the absence and presence of 1% concentration of Schiﬀ

base additives/borate ester/ZDDP and mixtures of Schiﬀ bases

with borate ester having 0.5% of each. From Fig. 2, it is evident

that the MWD in paraﬃn oil decreases, in general, in the

presence of Schiﬀ bases than in their absence. Thus the tested

Schiﬀ bases act as antiwear additives. However, the antiwear

Schiff bases + BE > ZDDP > BE > Schiff bases > parafﬁn oil

The observed antiwear behavior with respect to time can be

discussed on the basis of tribolm formation. Initially, there is

no tribolm on the interacting surfaces. As the time increases

the additive molecules get uniformly adsorbed over the matting

surfaces during sliding under operating conditions and may

react with the metal surface to form a tribochemical lm. The

formation of a tribolm is time dependent; therefore, some

time is required to form a durable tribolm on sliding

surfaces.⁵⁰During the rst 15 min test, the tribolm formation

on the interacting surfaces could not get initiated, consequently

large values of MWD are observed in every case. Later on,

increase in the MWD value becomes lesser in the case of Schiﬀ

base additives aer 30 min of the test run, however, in the case

of corresponding synergistic mixtures it is aer 15 min only i.e.

earlier than the Schiﬀ base. A subsequent lesser increase in the

value of MWD, thereaer, suggests the existence of some tri-

bolm on the surface.

The antiwear action of Schiﬀ bases may be ascribed to their

adsorption through a number of active centers like N, O, phenyl

and methyl groups. To understand the eﬀect of substituents on

additives towards their adsorption on surfaces in contact,

diﬀerent derivatives of Schiﬀ bases like p-methoxyphenyl,

p-chlorophenyl and o-hydroxyphenyl have been tested. On the

basis of +I and +R eﬀects, these groups may be arranged in the

following order:

Fig. 2 Variation of the mean wear scar diameter with time in paraﬃn

oil containing (1% w/v) zinc dibutyldithiophosphate (ZDDP), borate

ester, Schiﬀ bases and synergistic formulations at 392 N applied load.

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trend can be explained on the basis of formation and break-

down of the tribolm occurring almost simultaneously in the

beginning. As the test time increases the tribolm formed on

the sliding surfaces prevents metal–metal contact and lowers

frictional force. The time required for the formation of the

persistent tribolm may be diﬀerent for diﬀerent additive

compositions. Further, the friction coeﬃcient increases

abruptly aer 75 min run in every case. This may be due to the

wear assisted surface damage and the generation of wear debris

produced with time under operating conditions at the

interface.⁵¹

The friction coeﬃcient data support the order of eﬃciency

for diﬀerent additives and their mixtures with BE as discussed

before, accordingly the highest coeﬃcient of friction for every

run is observed for base lube and the lowest for the synergistic

mixtures of AAPM with BE. Thus, both antiwear and friction

reducing properties of blends of Schiﬀ base additives with BE in

base lube are considerably better than those of the reference

and commercial additives ZDDP and BE, respectively. Variation

in these properties in the case of diﬀerent Schiﬀ bases can be

correlated well with the composition of the tribolm formed on

the surface due to diﬀerent substituents present around the

additive skeleton.

To estimate wear more realistically it is important to

examine the variation of the mean wear volume with time

instead of variation of the mean wear scar diameter with time.⁵²

The mean wear volume in the absence and presence of diﬀerent

additives at 392 N load for paraﬃn oil was plotted as a function

of time and a linear regression model was tted on the points

including origin to nd out the overall wear rate, Fig. 4. The

overall wear rate was found to be very high in the absence of

additives. Among all the AAPs–Schiﬀ bases the following order

has emerged for the overall wear rate

–OCH₃> –OH > –Cl

Accordingly increase in the electron density on the ring is

expected to occur in the above order. Consequently adsorption

of the additive and hence its antiwear action should also follow

the above order. However, the antiwear behavior of the

p-chlorophenyl derivative has been found to be superior to the

o-hydroxyphenyl derivative. This may be due to the ÀI and +R

nature of the –Cl group which results in an increase in the

electron density on the chloro group instead of the ring causing

more adsorption through the chloro group.

The similar order of tribological behavior has been observed

in the case of corresponding synergistic formulation of Schiﬀ

bases with borate ester. However, the extent of wear reduction

in the case of a synergistic formulation has been found to be

much larger than the initial Schiﬀ base alone. This can be

explained on the basis of formation of the donor–acceptor

complex where N-containing additives donate their lone pair of

electrons to the electron decient boron atom of commercial

borate ester. Thus under sliding conditions, the donor–acceptor

complex further facilitates its adsorption on the steel–steel

interface by increasing the surface coverage in order to form a

tribochemical lm showing pronounced synergistic action.

These synergistic formulations in paraﬃn show excellent wear

reducing behavior even much better than that of ZDDP and BE

alone.

Fig. 3 illustrates the variation of the friction coeﬃcient with

time in the absence and presence of 1% BE/ZDDP/Schiﬀ base

additives and their mixtures with BE in paraﬃn oil at 392 N

applied load. It can be seen that the blends of diﬀerent Schiﬀ

base additives with BE show lower friction coeﬃcient values

than in the case of ZDDP or BE. In every case, the friction

coeﬃcient increases progressively from 15 to 30 min duration

due to the static friction caused by direct asperity–asperity

contact. From 30 to 45 min run usually the friction coeﬃcient

decreases in most of the cases, on the other hand, between

45 and 60 min run an irregular trend is observed. This irregular

AAPM > AAPC > AAPS > AAPB

The same order of the overall wear rate has been observed for

their corresponding mixtures with BE. This again is in confor-

mity with the conclusion drawn earlier that owing to the

formation of a stronger, adherent tribochemical lm, syner-

gistic formulations eﬃciently smoothen the surface irregulari-

ties thus reducing wear to a great extent.

The running-in wear rate is always higher than the steady-

state wear rate as it involves morphological modications of the

surfaces (Fig. S1 and S2†). The comparison of running-in,

steady-state and overall wear rates of diﬀerent additives and

their synergistic formulations with BE has been displayed in

Fig. 5, which clearly reects that the addition of AAPs–Schiﬀ

bases to the base lube signicantly reduces the running-in,

steady-state and overall wear rate of base lube. However, this

reduction in wear rates was found to be poorer than the surface

lubricated with BE and ZDDP. Among all of the studied Schiﬀ

bases, AAPM most signicantly reduces the running-in and

overall wear rates of base lube. It is noteworthy that the value of

its steady-state wear rate has been found to be much lower than

ZDDP and BE alone.

Fig. 3 Variation of the friction coeﬃcient with time in paraﬃn oil

containing (1% w/v) zinc dibutyldithiophosphate (ZDDP), borate ester,

Schiﬀ bases and synergistic formulations at 392 N applied load.

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Fig. 4 Determination of the overall wear rate by varying the mean wear volume with time in paraﬃn oil containing (1% w/v) zinc dibutyldi-

thiophosphate (ZDDP), borate ester, Schiﬀ bases and synergistic formulations at 392 N applied load.

base additives and their mixtures with borate ester (1% w/v).

Fig. 6 illustrates the plots of MWD as a function of applied load

at 30 min test duration.

At an initial load (294 N), MWD is very large in the absence of

additives but in the presence of Schiﬀ base additives it is fairly

reduced. It reduces further in the presence of ZDDP and borate

ester and attains the minimum value for the mixtures of Schiﬀ

base additives with BE. At 392 N load, MWD increases consid-

erably in every case but this increase attains the minimum value

in the presence of synergistic mixtures. This may be due to the

fact that the thin lm of the lubricant and additive adsorbed on

the interacting surfaces resists much increase in MWD on

increasing the applied load. At 490 N load the base oil, borate

ester and ZDDP show abrupt increase in MWD; however, this

increase is very small in the presence of the Schiﬀ base additives

and their mixtures with BE. Thus, the tribochemical lm

Fig. 5 Wear-rates for paraﬃn oil in the presence and absence of

diﬀerent additives and their synergistic formulations (1%w/v) at 392 N

applied load for 90 min test duration.

The steady-state wear rate in the presence of the studied

synergistic formulations, in general, reduces drastically. Since

the life of engineering components is estimated on the basis of

the steady-state wear rate, for a better antiwear additive it is

important to achieve steady-state as early as possible and it

must be stable for longer duration.⁵⁸It is interesting to note that

the steady-state wear rate has been found to be the lowest in the

case of the synergistic mixture of AAPM + BE, however, its value

for AAPM alone was also found to be comparable with those of

the other studied synergistic formulations.

3.2 Eﬀects of load

In order to investigate the eﬀect of the applied load on the mean

Fig. 6 Variation of the mean wear scar diameter with applied load in

wear scar diameter, the tests have been carried out at diﬀerent

loads 294, 392, 490, 588 and 686 N for 30 min test duration for

paraﬃn oil containing (1% w/v) zinc dibutyldithiophosphate (ZDDP),

borate ester, Schiﬀ bases and synergistic formulations for 30 min test

paraﬃn oil in the presence and absence of synthesized Schiﬀ duration.

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Paper

formed in situ is further capable of carrying a higher load.

Beyond 490 N load the tribolm fails to sustain the load in the

case of paraﬃn oil alone and in the presence of borate ester

while in the case of ZDDP/AAPs, the lm fails beyond 588 N

load. On further increasing the applied load up to 686 N, blends

of AAPs with BE could successfully bear the load without surface

destruction. Among all the synthesized additives, the

p-methoxyphenyl derivative and its blend with BE in base lube

show tremendous load bearing ability with a relatively much

smaller wear scar diameter. Moreover, it can be inferred from

Fig. 6 that the synergistic formulations of Schiﬀ base additives

played an important role in remarkably improving the load

carrying ability of the base lube.

A correlation between the friction coeﬃcient and applied

load at 1% concentration of BE/ZDDP/Schiﬀ bases and blends

of Schiﬀ base additives with BE (0.5% of each) for a 30 min test

run is presented in Fig. 7. The increase in the friction coeﬃcient

with load is much higher in the case of paraﬃn oil in the

absence of additives whereas in the presence of additives it

progressively increases with load. The eﬃciency of Schiﬀ base

additives towards their antifriction behavior lies in the same

order as obtained from MWD vs. load plots.

Fig. 8 SEM micrographs of the worn steel surface lubricated with

paraﬃn oil in the presence and absence of diﬀerent additives (1% w/v)

for 90 min test duration at 392 N applied load: (a) paraﬃn oil, (b) AAPM,

(c) ZDDP and (d) AAPM + BE.

3.3 Surface characterization

3.3.1 Surface morphology. The surface morphology of the

wear scar has been studied by scanning electron microscopy

(SEM) and atomic force microscopy (AFM). Fig. 8 shows the SEM

images of the worn surface of steel balls in the presence and

absence of 1% ZDDP, AAPM and mixture of AAPM with borate

ester (0.5% of each) at 392 N applied load for 90 min duration.

The worn surfaces in the presence of additives are smoother in

comparison to the surface lubricated with paraﬃn alone. The

observed smoothness of micrographs in the presence of

diﬀerent additives follows the same order as discussed above on

the basis of their tribological behavior.

observed due to the adhesive wear, on the other hand, the

surface lubricated with its admixture having AAPM (Fig. 8b)

shows less deeper grooves along the sliding direction. In the

presence of ZDDP better smoothening of the surface is observed

(Fig. 8c). There is a tremendous increase in the smoothness of

the surface when synergistic formulation of AAPM with BE

(Fig. 8d) is used which may be due to the formation of a strong

and adherent tribochemical lm on the sliding contacts. At a

higher load (588 N for 30 min duration) also, the extent of

smoothening of surfaces in contact has been found to be in the

same order as discussed above.

In the case of the steel surface lubricated with base oil

(Fig. 8a) huge surface destruction with much deeper grooves is

The surface irregularities of the worn steel surfaces observed

aer antiwear and load carrying tests in paraﬃn oil were

examined by contact mode atomic force microscopy in the

presence and absence of synergistic formulations, ZDDP and

borate ester at 392 N load for 90 min test duration. The

roughness parameters, root mean square (S_q) and peak-valley

height (S_y), obtained through the soware Nanosurf basic Scan

2 are mentioned in Fig. 9. From Fig. 9 it is apparent that the

synergistic admixtures of Schiﬀ base additives with borate ester

in base lube exhibit drastic reduction in area roughness and

average asperity height of the lubricated surfaces than the base

oil or base oil with borate ester/ZDDP alone. This pronounced

reduction of surface roughness in the case of synergistic

formulations is supposed to be due to a very low running-in

wear rate and early stabilization of the steady-state wear rate

which is caused by the interaction between the N-atoms of

Schiﬀ bases (Lewis base) and the boron of borate ester (Lewis

acid) resulting in the formation of a durable tribolm under

lubricating conditions. This behavior can be exemplied by the

values of area roughness 409 nm (S_q) for the paraﬃn oil, 19 nm

Fig. 7 Variation of the friction coeﬃcient with applied load in paraﬃn

oil containing (1% w/v) zinc dibutyldithiophosphate (ZDDP), borate

ester, Schiﬀ bases and synergistic formulations for 30 min test

duration.

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Journal of Materials Chemistry A

boron, nitrogen, iron and oxygen on the wear track whereas all

of these elements except boron were found in the case of surface

lubricated with AAPM alone. Herein, the presence of boron on

the worn surface lubricated with AAPM + BE reects strong

additive–additive interactions resulting in the formation of a

donor–acceptor complex through which their adsorption is

facilitated. This brings about its tribochemical reaction with the

metal surface to form stable protective layers reducing friction

and wear. The increase in the atomic concentration of boron

and nitrogen on the increasing applied load (588 N: 30 min)

suggests that the formation of the tribolm is more favourable

at a higher load. It is evident from Table 2 that the atomic

concentration of diﬀerent elements increases appreciably with

increase in the load in the case of Schiﬀ base AAPM + BE while

its reverse is observed in the case of ZDDP. In contrast this may

be due to the strengthening of the tribolm in the former case

while its rupture in the latter one.

Fig. 9 Surface roughness parameters obtained from digital processing

software of Nanosurf-basic Scan 2 for diﬀerent additives at 392 N load

for 90 min test duration.

The XPS analysis of the worn surface is subjected to ascertain

the chemical compositions of the tribolm formed under

sliding conditions and to study the mechanism of the observed

synergistic behavior of the p-methoxyphenyl derivative with

borate ester in paraﬃn oil. The XPS spectra of C 1s, B 1s, N 1s, O

1s and Fe 2p of the worn surface lubricated with AAPM + BE at

392 N load for 90 min test duration are shown in Fig. S5.†

The spectrum of C 1s on the worn surface exhibits peaks at

285.9 and 287.3 eV corresponding to –C(O)O– and C–C/C–H

moieties respectively, which suggest that the decomposed

products of borate ester were adsorbed on the steel surface.⁵²

Formation of BN on the worn surface has been conrmed by

combining the binding energies of the prominent signal of B 1s

appearing at about 189.7 eV with the binding energy of N 1s at

about 398.7 eV.^53,54Appearance of an additional weak peak

around 194.0 eV in the spectrum of B 1s indicates the presence of

some amount of boron in the form of B₂O₃.⁵⁵Furthermore,

combining the binding energies of O 1s with B 1s, the peak at

532.9 eV also conrms the presence of B₂O₃on the worn

surfaces.^55,56It is interesting to note that for FeB or Fe₂B, peaks

are not observed in the spectrum showing that boron does not

reactwiththemetalsurface. OxidationofirontoFe₂O₃and/Fe₃O₄

during the rubbing process has been conrmed by combining

the binding energies of Fe 2p at 711.5 eV with O 1s at 530.8 eV.⁵⁷

(S_q) for the blend of AAPM with borate ester and 74 nm (S_q) for

the conventional additive ZDDP. Similar observations are also

evident from their 3D-AFM and corresponding line prole

graphs (Fig. S3†).

In order to investigate the suitability of these additives and

their blends with BE towards smoothening of surfaces under

extreme conditions i.e., high load and high temperatures, the

AFM-images have also been taken at 588 N applied load for

30 min test duration (Fig. S4†). The observed trend of the

additives towards the reduction of surface roughness under

extreme conditions is the same as above in the case of 392 N

load for 90 min duration.

3.3.2 Tribochemistry of synergistic lubricant formulation.

The energy dispersive X-ray analysis has been performed to

determine the elemental compositions of the worn surfaces

lubricated with and without ZDDP, AAPM and AAPM + BE in

paraﬃn oil at 392 N load for 90 min test duration. The observed

data are summarized in Table 1. It is apparent from the table

that the wear track lubricated with paraﬃn oil alone does not

show the presence of any hetero atom except oxygen which may

be due to the oxide formation. On the other hand, in the case of

surface lubricated with ZDDP, the presence of hetero-atoms

zinc, phosphorous, sulfur and nitrogen is evident. The

elemental composition of the wear scar surface lubricated with

the mixture of AAPM with borate ester show the presence of

3.4 Proposed mechanism

The above surface analysis and tribochemical investigation of

worn surfaces under diﬀerent test conditions demonstrate that

Table 1 EDX analysis data of the worn steel surface lubricated with

paraﬃn oil in the presence and absence of additives (1% w/v) for Table 2 EDX analysis data of the worn steel surface lubricated with

90 min test duration at 392 N applied load

diﬀerent additives (1% w/v) for 30 min test duration at 588 N applied

load

Atomic %

Lubricants

C

O

B

N

Fe

Zn

S

P

Lubricants

C

O

B

N

Fe

Zn

S

P

Paraﬃn oil 16.74 21.14

—

62.12

—

ZDDP

AAPM

13.64 15.42

06.57 11.66

62.46 05.02 01.42 02.04 ZDDP

17.89 19.57

06.04 07.08

AAPM + BE 06.11 06.70 02.81 06.81 77.84

—

61.60 00.49 00.14 00.30

04.53 76.62

—

AAPM

07.36 79.30

—

AAPM + BE 08.71 09.23 01.84 02.89 77.26

—

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Fig. 10 Optimized structures of Schiﬀ bases calculated with B3LYP/6-31G++(d,p) basis set: (a) AAPB, (b) AAPS, (c) AAPC and (d) AAPM.

the N-containing Schiﬀ bases act as ligands (Lewis base) and spectra. Besides this, adsorbed nitrogen, boron oxide and iron

may initially interact with electron decient boron of the borate oxide were also responsible for improving the tribological

ester (Lewis acid) in order to form adducts.⁵⁸These adducts get behavior of base oil to some extent.

easily adsorbed on the metal surface through their active

centers (especially B, N, –CH]N– moiety and phenyl group) due

to their greater surface coverage area than their individual ones.

3.5 Quantum chemical calculation

Under the rubbing process, as the time and/or load increases, Quantum chemical calculations based on density functional

additives bring about chemical reactions among themselves or theory (DFT) were performed for the Schiﬀ base additives

with the metal surface to form a tribochemical lm. As sug- AAPM, AAPC, AAPS and AAPB. An attempt has been made to

gested by Cavdar et al.⁵⁰the formation of the tribolm is time establish the relationship between the molecular structures of

dependent and it will take time to form a stable tribochemical these compounds with the experimentally observed wear-rates.

lm on the surface of tribo-pairs. As the temperature increases The full geometry optimized structures of the studied Schiﬀ

(due to increase in load or due to increase in frictional heat), bases AAPM, AAPC, AAPS and AAPB are shown in Fig. 10. The

these adducts may decompose to form a boron nitride lm calculated quantum chemical parameters like total energy,

under sliding conditions on the metal surface. EDX analysis E_HOMO, E_LUMOand orbital energy gap (DE) between HOMO and

also conrmed that the formation of a tribolm is more favor- LUMO of the additive molecules are presented in Table 3. The

able at relatively higher temperature i.e. extreme conditions. interaction mechanism between lubricants or lubricant addi-

Herein, it can be supposed that boron nitride produced due to tives and metal surface usually include an adsorption

the tribochemical reaction plays a major role in the lubrication phenomenon. The strength of adsorption of lubricants on the

process and its lubricity is related to its layered structure. In metallic surface depends on the electrostatic attraction of the

general, for the formation of boron nitride, much higher polar head of the additive molecules with the metal surface.

temperature will be needed but during the rubbing process it Adsorption of the molecular additive on the metal surface is not

seems to be easily formed.^58,59The occurrence of boron nitride only a precursor of the surface chemical reaction but also

on the metallic surface has been also conrmed from the XPS provides an important contribution to tribological performance

Table 3 Calculated quantum chemical parameters of Schiﬀ base antiwear lubricant additives calculated with B3LYP/6-31G++(d,p) basis set

Additives

Total energy (a.u.)

E_HOMO(Hartree–Fock)

E_LUMO(Hartree–Fock)

DE (Hartree–Fock)

Wear rate (10^À4Â mm³h^À1

)

58

Fe₅

À0.18651

À0.18772

À0.20064

À0.19474

À0.19859

À0.06420

À0.05623

À0.06803

À0.05929

À0.06310

0.12231

0.13149

0.13261

0.13545

0.13549

AAPM

AAPC

AAPS

AAPB

À1050.14

À1395.21

À1010.83

À0935.61

11.73

18.77

28.37

30.79

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Journal of Materials Chemistry A

of lubricants. According to the frontier molecular orbital (FMO) Vanderbilt Company Inc. 30 Wineld Street, Norwalk, U.S. for

theory, the energies of the highest occupied molecular orbital providing borate ester (Vanlube 289) additive as a gi.

(E_HOMO) and the lowest unoccupied molecular orbital (E_LUMO

)

are the important indices for predicting the reactivity of any

chemical species. The diﬀerence between E_LUMOand E_HOMOi.e.,

the energy gap (DE), is the important stability index that has

been found to have excellent correlation with antiwear eﬃ-

ciencies in a tribological reaction.^60,61The larger value of DE

implies high stability or inertness of a moiety in a reaction.⁶²

The DE has also been associated with the hardness and soness

of the involved additives. The small energy gap (DE) between

interacting HOMO and LUMO orbitals of the molecule is

indicative of its so nature i.e. it can be easily polarized whereas

a large energy gap corresponds to its hard behavior. Therefore,

it can be stated that the antiwear eﬃciency of the additives

would increase when the values of DE decreases. On the basis of

DE value for the studied Schiﬀ bases, the order of eﬃciency is

given below:

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10434 | J. Mater. Chem. A, 2014, 2, 10424–10434

Article Doi

Tribological studies of some SAPS-free Schiff bases derived from 4-aminoantipyrine and aromatic aldehydes and their synergistic interaction with borate ester

DOI: 10.1039/c4ta01191k

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Article abstract of DOI:10.1039/c4ta01191k

Full text of DOI:10.1039/c4ta01191k

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