Tetrahydropyrazolopyridines as antifriction and antiwear agents: Experimental and DFT calculations

Article abstract of DOI:10.1039/d0ra00794c

Some tetrahydropyrazolopyridines (THPP-H) with the methoxy (THPP-OMe) and methyl (THPP-Me) substituents were synthesized by a one-pot multi-component reaction. NMR spectroscopy (¹H and ¹³C) was used to authenticate the synthesis. Acc

Full text of DOI:10.1039/d0ra00794c

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Tetrahydropyrazolopyridines as antifriction and
antiwear agents: experimental and DFT
calculations†
Cite this: RSC Adv., 2020, 10, 10188
a
a
ab
a
a
Kavita,‡ Pratibha Verma,‡ Dinesh K. Verma, Bharat Kumar, Alok K. Singh,
a
a
a
Nivedita Shukla, Vandana Srivastava and Rashmi B. Rastogi
*
Some tetrahydropyrazolopyridines (THPP-H) with the methoxy (THPP-OMe) and methyl (THPP-Me)
1
13
substituents were synthesized by a one-pot multi-component reaction. NMR spectroscopy ( H and C)
was used to authenticate the synthesis. According to the results of tribological tests ASTM D4172, and
ASTM D5183 on a four-ball tester in paraffin oil (PO) at a concentration of 0.25% w/v, their relative tribo-
activity along with a reference additive, zinc dialkyldithiophosphate (ZDDP) could be figured out as
mentioned below-THPP-OMe > THPP-Me > THPP-H > ZDDP. The calculation of frictional power loss
from the coefficient of friction data of the tested additives supports the given order. As is apparent from
AFM and SEM micrographs of the wear scar surface for plain oil with and without different
tetrahydropyrazopyridines, surface evenness endorses the above trend. Proof for strong adsorption of
the synthesized additives is provided by EDX analysis of the steel ball surface after performing the
tribological test, where nitrogen and oxygen are vividly seen as heteroatoms. XPS studies reveal the
composition of the in situ formed tribofilm. The moieties containing carbon bonded to oxygen/nitrogen
as decomposed products of the additive together with oxides of iron in +II or +III oxidation states are
perceptible in the tribofilm, the tribofilm interferes with the proximity of the surfaces keeping them far
apart. Consequently, friction and wear are remarkably reduced. Findings from Density Functional Theory
(DFT) calculations are in full agreement with the results obtained from tribological experiments.
Received 26th January 2020
Accepted 2nd March 2020
DOI: 10.1039/d0ra00794c
rsc.li/rsc-advances
various standards have been put forth to control sulphated ash,
sulphur and phosphorus (SAPS) contents in an additive.
1
. Introduction
14
1
5,16
Friction and wear together have been immanent adversaries to
mechanical systems. Lubrication stands as the only remedial antiwear behavior but their agglomeration poses a great
Nanolubricants
are well appreciated for their dexterous
1
–5
measure to safeguard such systems. Sundry lubricant systems problem for the stability of their dispersions. In contrast, the
with numerous types of additives, therefore, have been fabri- use of conventional organic compounds is everlasting owing to
cated to address this issue. Among them, heterocyclic organic their comparatively higher solubility in base oils. From the
viewpoint of adsorption over the metal/alloy surface, heterocy-
and metal complexes nd paramount signicance. Zinc dia- clic compounds containing heteroatoms specically, nitrogen
lkyldithiophosphates (ZDDP) have been employed, very oen, and oxygen are categorically advocated for their friction/wear
6
7,8
9
compounds, nanomaterials, inorganic lamellar structures
10
1
1,12
as multifaceted antiwear additives.
use is prohibited because of their susceptibility to attenuating atoms and the aromatic ring electrons are basically involved in
However, their incessant lowering disposition. The lone pair of electrons at hetero-
17–20
the efficacy of exhaust emission catalytic converters, thus the process of adsorption.
A comprehensive literature survey
13
enhancing air pollution. From an environmental perspective, reveals that several groups of workers have chosen to investigate
the tribological efficiency of different types of heterocyclic
compounds. Kamano et al. have patented several pyridines,
pyridazines, pyrazines, pyrimidines, pyrroles, pyrazoles, imid-
21
a
Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), azoles as antiwear agents.
Friction and wear-reducing
Varanasi-221005, India. E-mail: rashmi.apc@iitbhu.ac.in
behavior of indole, indazole and benzotriazole in liquid
b
22
Department of Chemistry, Prof. Rajendra Singh (Rajju Bhaiya) Institute of Physical
paraffin oil have been reported by Zhongyi and co-workers.
Sciences for Study & Research, V. B. S. Purvanchal University, Jaunpur-222003, U.
P., India
23
Tribological properties of the benzimidazoles and tetrazole
24
derivatives have been investigated in liquid paraffin oil. The
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra00794c
25
antiwear and antifriction properties of imidazoline derivatives
in water–glycol mixture and triazine in poly-alpha olen base
These authors contributed equally to this work.
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26
oil have been studied. Desanker et al. have tested the perfor- identied as 0.25% w/v. All tribological tests, therefore, were
mance of alkyl-substituted cyclen derivative (commercial Group III conducted at the concentration, 0.25% w/v.
lubricating oil) as friction modier using pin-on-disk tribometry
tests. Several ionic liquids such as di[bis(2-hydroxyethyl)ammo- in the present investigation were the same as reported from our
Tribological testing procedures and surface analysis adopted
27
2
8
5–8
nium]succinate,
1-acetyl-3-hexylbenzotriazolium benzoate/ laboratory and are provided as the ESI (ESI-2†).
29
sorbate,
poly-alkylimidazoliumbis(triuoromethylsulfonyl)
imide, alkylimidazolium diethyl phosphates, bis(imidazo-
lium)/bis(ammonium)-di[bis(salicylato)borate] are reported as
effective antiwear agents. Quinoline derivatives in polyethylene
glycol (PEG) and Schiff bases of 4-aminoantipyrene with
30
31
2
.4. Computational details
32
33
DFT computations were performed with the help of B3LYP. Full
geometry of the additives was optimized with the standard
36
34
B3LYP/321G+* basis set applying Gaussian 03, revision C. 01.
substituted benzaldehyde in paraffin oil have been reported as
excellent tribological additives from our laboratory. There is
always an urge to develop more and more superior antiwear
additives of different types. This instigated us to synthesize
substituted tetrahydropyrazopyridines, having two pyrazoles and
3
3
. Results and discussion
.1. Antiwear studies
a pyridine ring arranged in a specic fashion and evaluate their The antiwear tests of paraffin oil (PO) with or without synthe-
tribological behavior in paraffin oil on a four-ball tribo-tester. The sized tetrahydropyrazolopyridine derivatives were performed
tribo-activity is directly related to the number of adsorption with the help of a four-ball tester. At rst, the concentration of
centres accessible to the metal surface. The adsorption of the additive was optimized. Fig. 1 displays the deviation in the
substituted tetrahydropyrazopyridines on the steel surface is mean wear scar diameter (MWD) against the concentration of
anticipated to occur primarily through the nitrogen atoms, the synthesized additives (0.00 to 1.0% w/v) in base lube at
provided these are competent enough to approach the surface. 392 N load for 60 minutes.
The inherent properties of the substituents at the pyridine ring
It is obvious from Fig. 1 that as compared to plain paraffin
may facilitate adsorption by enhancing electron density at these oil, the MWD decreases for its blends with tetrahydropyr-
nitrogen atoms. For a deep insight into electronic environments azolopyridines at all the investigated concentrations up to
around adsorption centres, Density Functional Theory calcula- 0.25% w/v. Beyond 0.25% w/v, there is a sluggish hike in MWD
tions were also performed. These calculations have been used to in every case up to 1% w/v. Although the additives are tri-
validate the observed tribological results.
boactive at all the tested concentrations, 0.25% w/v is consid-
ered as the optimum concentration for testing. It is important
to point out here that in case of the commercial additive ZDDP,
the MWD is much higher at 0.25% w/v and the minimum value
is obtained at four times higher concentration, 1% w/v.
Fig. 2 exhibits how COF varies with time for paraffin oil and
its admixtures with various tetrahydropyrazolopyridines for the
optimized concentration (0.25% w/v) at load 392 N and 60 min
duration.
As per gure, the highest diminution in the coefficient of
friction values is noted for THPP-OMe followed by THPP-Me
and nally THPP-H.
Fig. 3 displays magnitude of reduction in MWD and COF
data simultaneously for base oil with or without the synthesized
additives. The MWD for base oil (0.733 mm) undergoes about
2. Experimental section
2.1. Chemicals
AR grade chemicals were utilized throughout the investigation.
2.2. Synthesis of tetrahydropyrazolopyridine derivatives
2.0 mmol hydrazine hydrate and 2.1 mmol ethyl acetoacetate in
ethanol (5 mL) were taken in a round bottom ask. The reac-
tants were stirred for half an hour at 30 C and 1.0 mmol
aldehyde and 4.0 mmol ammonium acetate were successively
added. The prepared reaction mixture was reuxed for 4–8 h
and then cooled to normal temperature. The distilled water (10
ꢀ
35
mL) was added, and the mixture was stirred for 30 min. The
resulting product was ltered, washed with distilled water fol-
lowed by acetone and dried under vacuum (Scheme 1).
NMR spectroscopy was used to characterize the synthesized
21, 26 and 35% reduction in the presence of THPP-H, THPP-Me
and THPP-OMe respectively. However, there is only an 11%
reduction in MWD for the standard additive zinc dia-
lkyldithiophosphate (ZDDP). The observed data indicate the
highest antiwear performance for the THPP-OMe derivative.
Similarly, reduction in values of the coefficient of friction (COF)
of base oil (0.0948) is observable in its blends with ZDDP (11%),
THPP-H (13%), THPP-Me (26%) and THPP-OMe (40%) sup-
porting the maximum antifriction performance again for THPP-
OMe additive.
1
13
product. The H and C NMR spectra of the prepared additives
have been provided in the ESI (ESI-1 Fig. S1–S6†). The IUPAC
names, abbreviations, structures and characterization of the
tetrahydropyrazolopyridines are given in (Table 1).
2.3. Tribological analysis
2.3.1. Preparation of admixtures. Tetrahydropyrazolopyr-
Fig. 4 displays the relation between frictional torque and
idine blends in paraffin oil (base oil) with concentrations (0.00, time at succeeding loads beginning from 98 N experimental
0
4
.125, 0.25, 0.50, 0.75 and 1.0% w/v) were made by stirring at load. For the base lube, frictional torque increases slowly up to
0–50 C for 1–2 hour followed by 1 h sonication at room 1078 N load and thereaer increases abruptly due to failure of
ꢀ
temperature. The optimized concentration of the additives was the tribolm.
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Scheme 1 Synthesis of tetrahydropyrazolopyridine derivatives.
Thus, base oil alone can sustain the load only up to 1078 N. It lube. On the other hand, for the synthesized additives a sharp
is noticeable that for ZDDP 0.25% w/v concentration is not increase in frictional torque is noted at much higher loads,
compatible with increasing the load-bearing capacity of base THPP-H (2450 N), THPP-Me (2548 N) and THPP-OMe (3038 N).
Table 1 Molecular structures, IUPAC names, abbreviations and characterizations of the tetrahydropyrazolopyridine derivatives (THPP-H, THPP-
Me, THPP-OMe)
S. no. Structure
IUPAC name and abbreviation
Characterization
ꢀ
Melting point 240–242 C
1
H NMR (500 MHz, DMSO) 7.52–7.50 (2H, d),
7
.15–7.13 (1H, d), 6.94 (2H, s), 4.62 (1H, s), 1.80
13
(6H, s). C NMR (126 MHz, DMSO) d 161.92,
3
,5-Dimethyl-4-phenyl-1,4,7,8-
143.71, 140.46, 128.24, 127.97, 125.93, 104.45,
32.81, 10.88.
0
0
1
tetrahydrodipyrazolo[3,4-b:4 ,3 -e]pyridine,
(
THPP-H)
ꢀ
Melting point 242–244 C
1
H NMR (500 MHz, DMSO) d 7.22–7.21 (2H, d)
7
.06–7.03 (2H, d), 4.93 (1H, s), 2.11 (3H, s), 1.80
13
(6H, s). C NMR (126 MHz, DMSO) d 159.92,
140.93, 137.36, 134.87, 129.34, 127.75, 125.04,
3
,5-Dimethyl-4-(4-methylphenyl)-1,4,7,8-
124.52, 102.20, 30.50, 18.63, 9.88.
0
0
2
tetrahydrodipyrazolo[3,4-b:4 ,3 -e]pyridine,
THPP-Me)
(
ꢀ
Melting point 185–186 C
1
H NMR (500 MHz, DMSO) d 7.04–7.02 (2H, d),
6
.78–6.76 (2H, d), 4.75 (1H, s), 3.68 (3H, s), 2.07
13
(6H, s). C NMR (126 MHz, DMSO) d 160.71,
1
5
56.81, 139.42, 134.80, 128.00, 112.75, 103.91,
4.43, 31.55, 9.96.
3
,5-Dimethyl-4-(4-methoxyphenyl)-1,4,7,8-
0
0
3
tetrahydrodipyrazolo[3,4-b:4 ,3 -e]pyridine,
THPP-OMe)
(
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3.2. Surface examination of the wear scar
The worn surface has been examined by SEM and AFM (contact
mode) aer tribological test ASTM D4172. The SEM images of
worn surface on steel ball in the presence of blank lubricant and
its blends with tetrahydropyrazolopyridines (0.25% w/v) are
shown in Fig. 5. The size of wear scar and its smoothness are
directly related to the relative tribological performance of the
synthesized additives. Thus, severely damaged wear scar surface
for plain paraffin oil while substantially smoothened surface in
case of THPP-OMe may be arguably discussed.
The AFM of the worn surface was performed aer the anti-
wear test. The obtained data for surface roughness variables of
wear scars on steel ball are displayed in Fig. 6 together with
their 3D AFM gures.
Fig. 1 Deviation in mean wear scar diameter with the concentration of
different tetrahydropyrazolopyridine additives in PO at 392 N applied
load for 60 min test.
The roughness parameters, root mean square area (S
q
) and
root mean square line roughness (R ) were found to be very high
q
(
S ¼ 674 nm, R ¼ 670 nm) for the blank oil. A huge reduction
q
q
in these values is marked in the presence of additives THPP-
OMe (S
5
1
q
¼ 46.98 nm, R
9.36 nm, R
¼ 59.18 nm), THPP-H (S
20.13 nm). The above-mentioned values of S
q
¼ 37.41 nm), THPP-Me (S
¼ 124.77 nm, R
and R
q
¼
Thus, the developed tribochemical lm is much stronger to
q
q
q
¼
bear the load when THPP-OMe is applied as an additive.
37
q
q
validate
The frictional power loss (P) for different tetrahydropyr-
azolopyridine additives, collected in Table 2 has been calculated
using the given eqn (1). The details are given in (ESI-3†).
the order of activity obtained from tribological data.
3.3. Energy dispersive X-ray spectroscopy (EDX)
P ¼ 2.07 ꢁ m ꢁ 10ꢂ3 (W)
(1)
The EDX analysis of wear scar in the presence of plain oil and its
admixtures with tetrahydrohydopyrazolopyridines has been
performed aer the antiwear test for elemental characterization
of the tribolm. Fig. 7 shows the EDX spectra of base lube and
its admixture with THPP-OMe. Presence of nitrogen as the
heteroatom can be easily seen in Fig. 7(b), emphasizing its
contribution during the development of tribolm.
where, m ¼ coefficient of friction
kW h ¼ 3.6 MJ
1
(2)
The PO shows highest power consumption (0.7064 MJ) as
compared to the other additives, ZDDP (0.6296 MJ), THPP-H
(0.6183 MJ), THPP-Me (0.5200 MJ) and THPP-OMe (0.4264 3.4. X-ray photoelectron spectroscopy (XPS)
MJ). The highest percentage reduction in P was observed in case
of THPP-OMe additive, 39.64%. Consequently, among all the
tested additives, THPP-OMe additive saves substantial energy.
The deconvoluted XPS spectra of C 1s, N 1s, O 1s and Fe 2p of the
tribolm formed on the wear scar surface in the presence of THPP-
OMe aer following ASTM D4172 standards are displayed in Fig. 8.
Fig. 2 Variation of the coefficient of friction with sliding time in the Fig. 3 Variation of mean wear scar diameter and coefficient of friction
presence of different tetrahydropyrazolopyridine additives (0.25% w/v) in the presence of tetrahydropyrazolopyridine additives in PO: load,
ꢀ
ꢀ
in PO: load, 392 N; sliding speed, 1200 rpm; temperature, 75 C; test 392 N; sliding speed, 1200 rpm; temperature, 75 C; test duration,
duration, 60 min.
60 min; concentration of additives, 0.25% w/v.
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indicating three peaks with binding energies 529.8, 532.1 and
5
33.5 eV which could be correlated with Fe–O/Fe O , C–O, C–O–C
2 3
40–42
moieties respectively.
The N 1s spectrum is illustrated in
Fig. 8(c). The presence of only one peak identied at 399.9 eV could
10
be accorded with –N]C/–N–C– group. The binding energies of Fe
p at 710.8 eV (Fig. 8(d)) and O 1s at 529.8 eV (Fig. 8(b)) together
indicate the formation of iron oxides FeO and Fe conrming
2
2 3
O
43
oxidation of steel surface iron during rubbing process. Thus, it
may be inferred that the additive adsorbed on the steel surface
through heteroatoms has decomposed under tribo conditions to
form a complex tribolm along with the mixture of FeO/Fe O and
2
3
rendered active participation in exceeding lubricating behavior.
3
.5. Tribochemistry and proposed mechanism for
Fig. 4 Alteration of frictional torque with stepwise loading and time
triboaction
for different tetrahydropyrazolopyridine additives: sliding speed,
ꢀ
6
00 rpm; temperature, 75 C; concentration of additives, 0.25% w/v.
Based on tribological tests, it may be established that terta-
hyrdopyrazolopyridine formulations prove to be extremely tri-
The core-level spectrum of C 1s, Fig. 8(a) exhibits two peaks at 284.5, bologically active. The AFM and SEM results corroborate the
85.1 eV binding energies corresponding to C–C/C]C, C–O/C–N experimental data for the relative activity of different additives.
2
8,38,39
bonds respectively.
Fig. 8(b) shows core-level spectrum of O 1s The energy dispersive X-ray analysis provides sufficient
Table 2 Loss of frictional power measured for different additives at the concentration, 0.25% (w/v) in PO
Reduction in
% reduction
S. no.
Additives
Power consumption (MJ)
power consumption
in power consumption
1
2
3
4
5
PO
ZDDP
THPP-H
THPP-Me
THPP-OMe
0.7064
0.6296
0.6183
0.5200
0.4264
____
____
0.0768
0.0881
0.1864
0.2800
10.87
12.47
26.39
39.64
Fig. 5 SEM images (inset: full view of wear scar at 100ꢁ, wear scar surface at 2.00kꢁ magnification) of the worn steel surface lubricated with PO
with or without tetrahydropyrazolopyridine additives (0.25% w/v) for 60 min test at 392 N applied load.
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Fig. 6 3D AFM images of the wear scar surface in the presence of PO with or without different tetrahydropyrazolopyridine additives (0.25% w/v)
for 60 min test duration at 392 N applied load (a) PO, (b) ZDDP, (c) THPP-H, (d) THPP-Me and (e) THPP-OMe.
evidence in support of heteroatoms present on the tribo- An endeavour has been made to correlate the test results with
surface. A thin chemical lm is formed under the tribo-test the molecular structures of the synthesized additives. Fig. 9
condition because of additive decomposition and interaction displays the optimized structures of the synthesized tetrahy-
of the decomposed products with the mating surfaces. The dropyrazolopyridine additives THPP-OMe, THPP-Me, THPP-H.
tenacious lm keeps the proximal surfaces away from each At rst, the additive molecules tend to undergo physical or
other and therefore, friction and wear are greatly reduced.
chemical adsorption on the steel ball surface via their polar
ends followed by their chemical interaction at the surface.
Frontier Molecular Orbital (FMO) theory, may be ensued to give
the mechanism of tribological behavior. The quantum chemical
variables like total energy, the energy of highest occupied and
lowest unoccupied molecular orbitals (EHOMO and ELUMO) and
3.6. DFT calculations
The DFT calculations have been conducted to observe the effect
of various substituents on the inherent framework of tetrahy-
dropyrazolopyridine towards antiwear lubricating properties.
Fig. 7 EDX spectra of worn surface lubricated with (a) PO and (b) THPP-OMe.
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Fig. 8 XPS spectra of tribochemical film formed on worn steel surface lubricated with the THPP-OMe additive (0.25% w/v) at 392 N applied load
for 60 min test duration in PO. (a) C 1s (b) O 1s (c) N 1s and (d) Fe 2p.
their energy gap (DE ¼ ELUMO ꢂ EHOMO) are given in Table 3.
These variables have been used to justify the reactivity of the
additive molecules.
In accordance with the Hartree–Fock theorem, the energies
of frontier molecular orbitals are shown by:
s ¼ 1/g
44
Global soness (s) gives information about the reactivity of
a compound.
Based on the hardness–soness concept, it may be stated
that the highest value for DE corresponds to highest stability
index showing hard or inert behavior in a chemical reaction; on
the other hand, its lowest value designates the lowest stability
index accompanied with so or easily polarized behavior. The
so additive with the smallest energy gap DE is anticipated to
show the maximum capability of adsorption and accordingly, it
would be the most tribologically active additive. It is apparent
from Table 3 that additive (THPP-OMe) is having the lowest
value for g and the highest for s. Based on these parameters, the
ascending order of tribological performance of the synthesized
additives may be expressed as:
I ¼ ꢂEHOMO
;
A ¼ ꢂELUMO
where I and A correspond to the ionization potential and the
electron affinity respectively and are depicted in Table 3.
The absolute electronegativity (c) is given as:
c ¼ (I + A)/2
The global hardness (g) is equal to:
g ¼ (I ꢂ A)/2
THPP-H < THPP-Me < THPP-OMe
When metal (iron) and additive come in close proximity to
each other, electron ows from lower c (additive) to higher c
3.7. Structure–activity relationship
The tribological data achieved for different tetrahydropyr-
azolopyridines may be correlated with their corresponding struc-
tures. Structurally, a tetrahydropyrazolopyridine moiety has
a pyridine ring fused with two pyrazole rings on either side. It may
(
iron) values until chemical potentials become equal.
The global soness (s) is dened as:
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Fig. 9 Frontier Molecular Orbitals (HOMO and LUMO) of different tetrahydropyrazolopyridine additives with their DE values.
Table 3 Quantum chemical parameters of tetrahydropyrazolopyridine antiwear additives by B3LYP/3-21G +*
Total energy
(a.u.)
E
HOMO
E
LUMO
DE
S. no. Additives
(Hartree–Fock) (Hartree–Fock) (Hartree–Fock)
I
A
c
g
s
1
2
3
4
Fe
5
(ref. 45)
ꢂ0.18651
ꢂ0.20469
ꢂ0.20413
ꢂ0.20529
ꢂ0.06420
ꢂ0.02943
ꢂ0.02859
ꢂ0.02951
0.12231
0.17526
0.17554
0.17578
0.18651 0.06420 0.12535 0.06115 16.35322
0.20469 0.02943 0.11706 0.08763 11.41162
0.20413 0.02859 0.11636 0.08777 11.39341
0.20529 0.02951 0.11740 0.08789 11.37786
THPP-OMe
THPP-Me
THPP-H
ꢂ963.67
ꢂ888.86
ꢂ849.75
get adsorbed on the steel ball surface via nitrogens of the pyrazoles
(3) Based on antiwear and load ramp tests at 0.25% w/v
and the pyridine ring. All the nitrogens being present in a delo- concentration, the activity of various additives could be estab-
calized system are engaged in surface adsorption. The relative lished as:
performance of different additives is guided by the nature of
th
ZDDP < THPP-H < THPP-Me < THPP-OMe
substituents attached to the benzene ring at the 4 position of the
pyridine group. As discussed above, the best activity is displayed by
methoxy analogue (THPP-OMe) followed by methyl (THPP-Me) and
(
4) Morphological investigations of the wear scar surface
authenticate the relative tribological activity.
5) The DFT calculations verify the experimentally found
data.
then (THPP-H). The observed order reects that the electron
density on the fused rings has decreased accordingly. The +R effect
of methoxy group and +I effect of the methyl group have been
instrumental in increasing the electron-donating tendency of the
corresponding tetrahydropyrazolopyridines.
(
(6) The EDX analysis of the wear scar surface shows the
presence of heteroatoms, nitrogen and oxygen conrming
adsorption of different additives on the steel surface.
(
7) XPS spectra of the wear scar surface conrm that tri-
4
. Conclusions
bochemical lm is made up of decomposed products of the
additive along with iron oxides.
(
1) The tetrahydropyrazolopyridine additives were synthesized
and characterized by NMR spectroscopy.
2) The studied additives having zero SAPs contents show
(
very high tribological activity at all the tested concentrations.
The maximum activity is observed at a very low concentration,
Conflicts of interest
0.25% w/v.
There is no conict of interest.
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