Effect of dispersant on nano-PTFE based lubricants on tribo-performance in fretting wear mode

Abstract

Very recently nano-PTFE particles have shown great promise as anti-friction (AF), anti-wear (AW) and extreme-pressure (EP) additives. However, their stability in oil in suspension form was limited and the issue needed immediate attention. Hence, a proper dispersant was required to be tried, which could avoid the sedimentation and agglomeration problem of nano-particles NPs for a longer time. Hence polyisobutylene succinimide (PIBSI) was selected as a dispersant in this work. This paper describes the effect of nano-PTFE-PIBSI additive interaction in a lubricant system. Initially few oils were prepared containing various amounts (1, 5 and 10%) of PIBSI to investigate its influence on the extent of stability and influence on the tribo-properties leading to selection of an optimum amount for the best possible combination of properties. Oils with PIBSI in a fixed amount (1%) and varying amount of nano-PTFE particles (0 to 6 wt%) were then prepared. The oils were characterized for their physical properties such as density, viscosity, pour point and flash point. The friction and wear studies performed in the oscillating wear mode indicated that PIBSI 1% (wt) was an optimum amount. However, the PIBSI-NP combination clearly showed competition as an AF and AW additive, with a film-forming tendency of the counterface, and hence it showed antagonistic behaviour. With an increase in the amount of NPs, the dominance of the NPs increased and finally the combination of PIBSI and NPs (6 wt%) could significantly improve the AF property (35%) as well as the AW property (25%) of the virgin oil besides improving the stability of the suspension significantly. The topography and surface chemistry of the specimens were examined using scanning electron microscopy/atomic force microscopy to establish the nature of the protective surface film formed on the steel surfaces.

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1. Introduction

Additives in lubricating oils to reduce friction and wear of surfaces in dynamic contact is always of topical interest. Wear protection is extremely important to the industry from a mechanical maintenance and economic point of view. In the case of sliding elements, extreme pressure (EP) additives are extensively used in hypoid gears, metal cutting and metal forming operations.

Solid lubricants such as molybdenum disulphide (MoS₂), graphite, polytetrafluoroethylene (PTFE) etc. are used in oils primarily as extreme pressure (EP), anti-wear (AW) or anti-friction (AF) types of additives.^1,2 PTFE has an un-branched chain-like molecular structure and weak van der Waals forces between aligned PTFE molecules which result in low μ and wear due to thin, coherent and uniform transfer film on the rubbing surfaces.^3,4 The use of nano-particles (NPs) as solid lubricants in oils is a comparatively recent concept. When a particle is fragmented into NPs, its reactivity increases. The smaller the particle size is, the higher the surface area is. NPs have a very high surface area to volume ratio, due to which a greater number of atoms can interact with other surfaces. NPs are considered as potential additives in lubricants, mainly as AF, AW or EP types.^5–15

In our earlier studies,^16,17 PTFE based micro-lubricants (MLs) and nano-lubricants (NLs) with varying sizes were prepared in the API group II lubricating base oil. The approximate particle sizes were 50 nm, 150 nm, 400 nm and 12 μm, and the particle concentrations were 4, 8 and 12 wt%. They were characterised for physical and tribological properties. Significantly improved tribo-performance (EP, AW and AF) was reported due to the inclusion of PTFE particles, especially due to NPs. The paper also dealt with the optimum amount of nano-PTFE required for the best performance properties of NP-oils and the detailed mechanism behind the observed improvements. However, these studies were based on the exploration of PTFE NPs in oil, without any dispersant. Owing to the requirement of long standing stability of finished lubricants, it was thought to be worthwhile to incorporate appropriate dispersants to improve the storage stability of nano-PTFE dispersions in oil. Since dispersants are long chain molecules with considerable surface activity, it is pertinent to investigate the impact of these dispersants on the overall tribological performance of a nano-PTFE dispersion apart from the stability.

The tribological benefits obtained due to the nano-sized inorganic fullerene-like molybdenum disulfide (IF-MoS₂) were reported to be lost in the presence of succinimide based dispersants.¹⁸ Aralihalli and Biswas¹⁹ selected a range of dispersants (on the basis of their polar moieties) to suspend nano-MoS₂ particles in an industrial lubricating oil. They reported that the presence of a dispersant led to efficient de-agglomeration of the NPs, which in turn resulted in a reduction in μ from 0.080 to 0.035.

The literature indicates that although PTFE is a very promising solid lubricant mainly for composites and also for oils and greases, no papers are available for exploring nano-PTFE as an additive in oil except for our own work. Such a type of study becomes more important when nano-PTFE is already reported as very promising NPs in high performance polymer composites.²⁰

In view of the above, the current paper reports on the results of NP-oils, prepared with different concentrations of nano-PTFE particles (0 to 6 wt%) along with a selected dispersant and related tribo-data generated in fretting motion. Since the work is a stage ahead of earlier work, some data from a previous paper had to be added for comparison.

2. Materials and methodology

2.1 Selection of material

2.1.1 Selection of nano-PTFE particles. Nano-PTFE powder was selected to formulate the nano-lubricants. The powder (30–50 nm in size) was procured from Shanghai SMEC Trading Co. China. Field emission scanning electron microscopy (FESEM, ZEISS, Supra 55) result of the nano-PTFE particles is shown in Fig. 1.²⁰

Fig. 1 High resolution FESEM micrograph of nano-PTFE particles appearing as 70–100 nm from the micrograph.²⁰ (The figure is reproduced with kind permission from Springer).

2.1.2 Base oil selected. The same base oil as that used in earlier studies (150 N group II base oil) was selected as a base stock for the studies. This parent oil (designated as O_P) was used for preparing various NP-oils based on different concentrations of two types of the selected additives viz. nano-PTFE and a dispersant PIBSI.

2.1.3 Selection of dispersant. Lubricants need to be very stable under diverse temperature conditions and should not precipitate/sediment the solid particles. Dispersants are usually ashless (non-metallic) organic chemicals. They generally keep contaminants and byproducts dispersed in the oil. They help to prevent the formation of deposits during oil usage. Dispersants prevent filter blockage and varnish the metal surface. In this research work, however, they are added with the objective of stabilizing and keeping the NPs in a suspension mode. The instability of suspensions is mainly due to aggregation and sedimentation of particles.

Polyisobutylene succinimide (PIBSI) was used as a dispersant in the present study and its typical properties are shown in Table 1 while the structure is shown in Fig. 2.

Table 1 Properties of selected dispersants

Properties
Kinetic viscosity (KV) @100 °C cSt	250
Flash point, °C PMCC	145
TBN, mg KOH g^¬1	12
Nitrogen, % wt	0.99
Specific gravity @15.6 °C	0.918
Polymeric component, % wt	45
Mol. wt of polymeric component	3317

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Fig. 2 Chemical structure of polyisobutylene succinimide (PIBSI).

2.2 Preparation of NP-oils

NP-oils based on different concentrations of nano-PTFE and a dispersant were developed using the probe sonication technique and the details are discussed in our earlier paper.¹⁶ Details of the nano-PTFE powder along with designations of the prepared NP-oils are shown in Table 2. It was also important to examine if the dispersant also contributes to the tribo-performance of the oils. Hence three oils containing 1, 5 and 10 wt% dispersant were also developed.

Table 2 Details of oils developed in the earlier work¹⁷ and in the present work^a

Sl. no.	Concentration (wt%) and type of additive used in parent oil		Designation of oils	Ref.
	Nano-PTFE	Dispersant
a Oils 8–16 were prepared for the work in this paper.
1	0%	0%	OP	17
2	1%	0%	ON1
3	2%	0%	ON2
4	3%	0%	ON3
5	4%	0%	ON4
6	5%	0%	ON5
7	6%	0%	ON6
8	0%	1%	OD
9	1%	1%	ODN1
10	2%	1%	ODN2
11	3%	1%	ODN3
12	4%	1%	ODN4
13	5%	1%	ODN5
14	6%	1%	ODN6
15	0%	5%	OD5
16	0%	10%	OD10

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2.3 Characterization of oils

The oils thus prepared were characterized in detail for their physical properties as shown in Tables 3 (oils with increasing amount of dispersant) and 4 (fixed amount of dispersant and increasing amount of nano-PTFE), respectively.

Table 3 Physical properties of developed oils with increasing amount of dispersant

	Parameter/instrument/method		OP	OD	OD5	OD10
1	Density (g cm^−3) at 15.5 °C (Anton Paar DMA-4100) ASTM D 1298		0.846	0.847	0.848	0.850
2	Kinematic viscosity (cSt) (Cannon CAV-2000) ASTM D-445	40 °C	29.56	30.68	32.67	35.61
		100 °C	5.177	5.329	5.637	6.03
3	Viscosity index ASTM D-2270		104.31	106.31	111.51	114.7
4	Pour point (°C) (Lawler DR4-11) ASTM D-97		−24	−33	−27	−27
5	Flash point (°C) (Tanaka ACO-T602) ASTM D-92		227	235	236	237

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Table 4 Physical properties of developed oils containing 1 wt% PIBSI and an increasing amount of nano-PTFE

	Parameter/instrument/method		OP	OD	ODN1	ODN2	ODN3	ODN4	ODN5	ODN6
1	Density (g cm^−3) at 15.5 °C (Anton Paar DMA-4100) ASTM D 1298		0.846	0.847	0.851	0.856	0.861	0.866	0.871	0.877
2	Kinematic viscosity (cSt) (Cannon CAV-2000) ASTM D-445	40 °C	29.56	30.68	30.94	31.63	32.36	33.12	34.09	40.25
		100 °C	5.177	5.329	5.368	5.454	5.535	5.624	5.751	6.431
3	Viscosity Index ASTM D-2270		104.3	106.3	106.97	107.63	107.7	108.07	109.19	109.54
4	Pour point (°C) (Lawler DR4-11) ASTM D-97		−24	−33	−33	−33	−30	−30	−27	−27
5	Flash point (°C) (Tanaka ACO-T602) ASTM D-92		227	235	231	231	231	231	237	243

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As seen from Table 3, addition of a dispersant provided a marginal increase in the density, viscosity (at both temperatures) and VI of the oil. The pour point also decreased but not uniformly. The flash point temperature also increased due to inclusion of the dispersant sharply form 227 °C for O_P to 235 °C for O_D, with further marginal increase for the remaining oils.

From Table 4, it was observed that trends were similar due to the addition of NPs. An increase in the number of NPs led to a marginal increase in the density, viscosity (at both temperatures) and VI of the oil. The pour point also decreased almost regularly due to the addition of dispersant. The flash point temperature showed irregular trends.

2.4 Tribo-performance of oils

An optimol SRV-III oscillating friction and wear tester was employed to examine the tribological properties of the developed oils. The friction test was conducted in a reciprocating ball-on-block mode, by oscillating an AISI 52100 steel ball (diameter 10 mm) over an AISI 52100 steel block (ϕ 24 mm × 7.9 mm) where a drop of oil was placed in the beginning of the experiment. The tribo-pair was thoroughly rinsed with petroleum ether prior to and after the test. The operating conditions were: frequency 50 Hz, stroke length 1 mm, duration of the test 60 min, and load 100 N. The total distance fretted was 360 meters. The contact pressure was 2.19 GPa. 0.3 mL of lubricant was used in each test and the average reading of the three tests was considered to plot the graphs. After completion of the test, the steel discs were cleaned with petroleum ether (40–60 °C) prior to the investigations of worn surfaces.

2.5 Worn surface analysis

Atomic force microscopy (AFM, Park Systems XE-70) and SEM were used for the surface morphological study. The AFM images give three-dimensional (3D) topographic features. The topography of the worn steel disc was studied with AFM. AFM measurements were carried out in contact mode with silicon nitride cantilevers.

Energy dispersive X-ray analysis (EDAX) spectra were obtained with a Thermo Electron Corporation model C10017 system to examine the chemical features and elemental compositions of the tribo-film generated on the worn surfaces.

3. Results and discussion

3.1 Stability studies on oil suspensions: visual inspection

The photographs of the parent oil and dispersant added oils with and without PTFE are shown in Fig. 3i and ii. The NP-oils were kept for 45 days after sonication. All the dispersant added nano-PTFE particles remained dispersed in the parent oils and hardly any sedimentation of the NPs was observed even after 63 days, contrary to our earlier studies,^16,17 where the stability of the nano-suspension without dispersant was reported as 15 days. However, minute inspection indicated that after 63 days initiation of sedimentation had started.

Fig. 3 Stable suspensions of O_P, O_D, O_DN1, O_DN2, O_DN3, O_DN4, O_DN5 and O_DN6: (i) observed after 45 days and (ii) after 63 days.

3.2 Stability studies on oil suspensions: DLS (dynamic light scattering)

Two samples O_N6 and O_DN6 were selected as representative samples to study the effect of dispersant on the suspension stability. The particle size distribution of the samples was analysed based on the principle of dynamic light scattering (DLS) with Zetasizer Nano ZS 90, Malvern Instruments UK. The NP-oils sample was first diluted with petroleum ether (40–60 °C) (oil to ether volume ratio: 7 : 3). The sample was injected slowly in the polystyrene cuvette to avoid air bubbles. The cuvette was placed into the Zetasizer and equilibrated at 25 °C for 120 seconds prior to the particle size measurements. Three repeat readings on each sample were taken and the average value was considered.

Table 5 and Fig. 4a and b compare the DLS data on the two oils containing 6% NPs. The first part is for the oil without dispersant while the other part of the table (bold letters) is for the oil with dispersant.

Table 5 Size distribution of the NPs with dispersant by volume for O_DN6 and O_N6 (DLS studies)

Days	ON6					ODN6
	Z-Ave (d.nm)	Peak	Size (d.nm)	% vol	St. dev.	Z-Ave (d.nm)	Peak	Size (d.nm)	% vol	St. dev.
0	112.4	P1	100.6	85.8	28.45	64.05	P1	59.98	100.0	13.74
		P2	5270	14.2	715.2		P2	—	—	—
7	190.1	P1	176.7	70.1	45.75	69.89	P1	63.18	100.0	12.24
		P2	5448	29.9	639.0		P2	—	—	—
14	517.8	P1	472.4	52.0	118.0	71.58	P1	68.22	100.0	13.82
		P2	5341	48.0	688.4		P2	—	—	—
21	843.3	P1	642.1	42.3	155.6	77.58	P1	71.59	100.0	18.69
		P2	5281	57.7	711.8		P2	—	—	—
28	2973	P1	1281	27.3	180.9	99.26	P1	89.30	100.0	18.72
		P2	5599.6	72.7	814.3		P2	—	—	—
35						115.9	P1	111.0	100.0	27.37
							P2
42						158.1	P1	162.1	100.0	34.87
							P2
49						174.4	P1	174.5	100.0	46.38
							P2	—	—	—
56						190.5	P1	189.6	100.0	48.19
							P2	—	—	—
63						294.7	P1	318.4	79.0	90.93
							P2	5443	21.0	640.4

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Fig. 4 Comparison of particle size distribution as a function of time for (a) O_D6 and (b) O_DN6 (DLS studies).

3.2.1 Observations for dispersant-free oil (DFO). It was observed that with an increase in time, the tendency of agglomeration of NPs increased for both the oils viz., the dispersant-stabilized oil (DSO-O_DN6) and dispersant-free oil (DFO-O_N6). The extent, however, differed. For the DSO, it was slow right from the beginning while for the DFO it increased disproportionately faster. When the DFO was freshly prepared, the average particle size (APS) of the major portion (86%) was ≈100 nm while the remaining portion was 5270 nm indicating heavy agglomeration of particles in spite of extensive sonication. The 100 nm size was higher than the original claimed size of the NPs (≈50–70 nm), indicating that it is extremely difficult to de-agglomerate all the NPs if added in a higher percentage. After 7 days, the APS increased to 176 nm from 100 nm and the other portion increased to 5448 nm from 5270 nm indicating re-agglomeration of NPs with time. After 14 days the rate of agglomeration was almost 5 times more. With further increase in time, the portion of smaller particles decreased rapidly while that of the bigger ones increased. After 28 days the agglomeration was so high that the average particle size was 1181 nm (27%) and the bigger ones were 5600 nm (73%). Thus it was concluded that without dispersant, the NPs cannot stay in the suspended form for an extended time.

3.2.2 Observations for dispersant-stabilized oil (DSO). For the DSO, a beneficial effect due to the dispersant was clearly seen. The APS was 64 nm (100% portion) indicating efficient de-agglomeration of the NPs with the help of the dispersant. After 7 days, the APS slightly increased to 70 nm for the 100% portion. After 14, 21 and 28 days, the NPs in oils were still in excellent condition (de-agglomerated) showing an APS of 68, 72 nm and 90 nm respectively. There was no secondary peak indicating excellent performance of the dispersant. After 35, 42, 49 and 56 days, slight agglomeration started. The APS increased slightly to 110, 162, 175 and 190 nm respectively. The secondary peak was still absent indicating a very satisfactory condition of the oil after almost 2 months. After 63 days, however, the oil started showing heavy agglomeration indicating that the efficiency of the dispersant to hold the NPs was limited to approximately 2 months only.

3.3 SRV test for oils with dispersant only

The tribo-performance of O_P was compared with that of O_D, O_D5 and O_D10 (Fig. 5a and b for friction and Fig. 6 for wear).

Fig. 5 (a) Friction coefficient as a function of fretting duration for selected oils (stroke: 1 mm, frequency: 50 Hz, temperature: 50 °C, load: 100 N and duration: 1 h). (b) Comparison of the coefficient of friction as a function of the amount of dispersant.

Fig. 6 Comparison of the wear scar diameter (WSD) as a function of amount of dispersant.

From Fig. 5 and 6 it is clear that inclusion of the dispersant showed a reduction in μ and its fluctuations apart from a reduction in the wear scar diameter (beneficial effects). With an increase in the amount, however, the benefits reduced slowly confirming that 1% was an optimum amount for the best friction and wear performance.

3.4 SRV test for oils with dispersant and nano-PTFE

As observed from Fig. 7, from the μ fluctuations point of view, O_P proved to be worst followed by O_DN1 indicating the initial antagonistic effect of the combination of dispersant and 1 wt% NPs. O_DN2 was the third in the series in this aspect indicating initiation of slow synergism in the performance, which then continuously improved as the percentage of NPs increased. Interestingly, in the case of the magnitude of μ (Fig. 7b) O_D showed a large reduction (32%) in μ with respect to O_P, indicating that the dispersant itself acted as an anti-frictional (AF) additive in the oil. Inclusion of 1% NPs to O_D initially increased μ sharply (from 0.175 to 0.225), which was just 7% lower than that of a parent oil showing complete incompatibility of the two additives. It clearly indicates the competition between the two for beneficial layer formation on the surfaces, which led to the antagonistic effect. With an increase in the number of NPs, however, this effect diminished slowly and the PTFE particles became more dominant in controlling μ to the extent that μ O_DN5 was identical to O_D. Furthermore, O_DN6 showed the lowest μ in the series, at 0.107. It reduced the μ value of the parent oil by 35%.

Fig. 7 (a) Friction coefficient as a function of fretting duration for selected oils (stroke: 1 mm, frequency: 50 Hz, temperature: 50 °C, load: 100 N and duration: 1 h). (b) Friction coefficient for selected oils at 100 N load (part of the data (pink bars) is already published elsewhere)¹⁷. (c) Influence of the concentration of nano-PTFE on the wear scar diameter (WSD) of the oils under 100 N load (part of the data (pink bars) is already published elsewhere)¹⁷.

The dispersant had to be added to increase the stability of the nano-suspension and antagonism arising out of the competition, and layer formation had to be combated with 6 wt% NPs, which finally was most effective in controlling the μ value.

In earlier series the 3 wt% NPs had exhibited the lowest μ value. A further increase in NPs led to a slight reduction in benefits. It was argued that beyond 3% the NPs start agglomerating and the number of NPs available for beneficial film formation reduces in spite of the total amount of PTFE in the oil (6 wt%). For new oils, the dispersant was helpful to accommodate more particles of PTFE as NPs in oils and also to enhance the stability of the nano-suspension for more time. However, this was at the cost of the competitive role of the dispersant as an AF additive for film transfer with PTFE.

O_N3 still remained being the best candidate to exhibit the lowest μ value, though the difference was marginal when compared with new oils viz., O_D and O_DN6.

Fig. 8c shows the variation of the wear scar diameter (WSD) with addition of varying concentrations of NPs in the dispersant added parent oil.

Fig. 8 SEM images of steel discs worn in various oils at 100 N load.

All the developed oils performed better than the virgin oil. It can be clearly seen that the best AW property was shown by O_DN6. The dispersant also acted as a powerful AW additive (19% reduction in wear as compared to the parent oil). The moment NPs were added, antagonism in functioning of the two additives started. It was highest for O_DN1, which slowly diminished with an increase in the number of NPs. As the amount of NPs increased beyond 4 wt%, the AW performance of the oils improved by the dominance of the NPs and a 25% improvement was observed for O_DN6 as compared to 19% for O_D. The friction and wear performance of O_DN5 and O_D was identical. A further increase in NPs (6%) led to a noticeable decrease (7%) in wear as compared to O_D.

The overall performance improvement in the AW property at 100 N load was as follows:

O_DN6 (25%) > O_DN5 (19%) > O_D (19%) > O_DN4 (15%) > O_DN3 (13%) > O_DN2 (9%) > O_DN1 (6%) > O_P.

Comparative trends in performance of new oils and old oils also indicated similarity with friction studies.

3.5 SEM-EDAX and AFM studies on worn surfaces

3.5.1 SEM-EDAX studies on worn disc surfaces. SEM studies were done on the worn surfaces to correlate the surface topography with wear trends. This is a supporting technique and did not reveal quantitative aspects. Fig. 8 shows the micrographs of the worn surfaces of the steel disc lubricated with different NP-oils.

Efforts were made to examine if the trends in the tribo-performance of the oils and topography of the worn surfaces showed some correlation. The SEM technique describes mainly the topography of the surfaces qualitatively and is used in the literature as a supporting technique and not a confirmatory one. The reduction in μ and wear due to the inclusion of dispersant/nano-PTFE in the parent oil was correlated qualitatively with SEM observations. As seen in Fig. 8a and b, even after fretting for one hour in parent oil, the surface was rough along with many thick and deep furrows. On the contrary, the surface fretted in O_D (Fig. 8c and d) was much smoother compared to that with O_P indicating the beneficial film formation which might have reduced the direct contact of asperities leading to a reduction in friction and wear. In Fig. 8e and f, in spite of the presence of NPs, the texture of the film was not as smooth as in the earlier case. The deterioration in the quality of the film indicated antagonistic behaviour between nano-PTFE and the selected dispersant. The texture of the film continued to improve as the PTFE concentration in the oils increased. In the case of O_DN6, the film is distinctly more uniform, coherent and indicative of material transfer.

For a better understanding of the chemical nature of the film, SEM and EDAX studies on the wear tracks on the steel discs were done to have some idea of the composition of the transferred film and the micrographs are collected in Fig. 9. Micrographs are shown in Fig. 9a, b, d and h while their corresponding dot maps are shown in the remaining micrographs.

Fig. 9 SEM micrographs and EDAX dot maps on worn steel discs (load: 100 N, stroke: 1 mm, frequency: 50 Hz, temperature: 50 °C and duration: 1 h): (a) with lubricant O_P; (b and c) with lubricant O_D confirming the presence of dispersant traces in the transferred film; (d–g) SEM micrographs of N, C and F dot maps on the disc with lubricant O_DN1; and (h–k) SEM micrographs of N, C and F dot maps on the disc with lubricant O_DN6 confirming that the transferred film contains both the dispersant and PTFE.

The micrograph in Fig. 9a is for the surface topography of the oil (O_P) lubricated disc which shows some film transfer. EDAX studies show the presence of Fe and not N or F (dot maps for Fe are not included since Fe is present on every disc by default). On the contrary, Fig. 9b and c show the surface to be heavily covered with a third body. The tribo-layer of the dispersant (Fig. 9c) was confirmed from the dot mapping of N (nitrogen). This efficient film transfer protected the contact of the metal surface leading to reduced friction and wear of the lubricant. For O_DN1 and O_DN6 there is uniform distribution of fluorine on the surfaces (Fig. 9g and k, respectively). Also carbon (Fig. 9f and j) is present on the worn surfaces of O_DN1 and O_DN6 respectively. The presence of carbon and fluorine confirms the PTFE film on the wear scar. The presence of the dispersant was confirmed by dot mapping of N (nitrogen) (Fig. 9e and i) for O_DN1 and O_DN6 respectively. This transformed film protected the metal surface from asperity–asperity contacts. Being small in size the NPs can easily enter into the valleys/crevices on the surfaces and they form the desirable film of a solid lubricant. For reduced friction and wear minimal asperity–asperity contact is desirable. A thin coherent film covering the maximum asperity–asperity contact leads to improved tribological performance.

3.5.2 AFM studies on the worn disc surfaces. AFM was used to study the topography of the wear tracks produced on the steel discs after a 60 minute test at 100 N load. The three-dimensional morphology of the tracks produced during fretting in the presence of O_P, O_D, O_DN1 and O_DN6 is shown in Fig. 10. The images were taken in the wear track on the steel disc after sliding.

Fig. 10 AFM images of the worn surfaces of the steel discs (load: 100 N, stroke: 1 mm, frequency: 50 Hz, temperature: 50 °C and duration: 1 h): (a) O_P, (b) O_D, (c) O_DN1 and (d) O_DN6.

The AFM images of the wear tracks for O_D, O_DN1 and O_DN6 (Fig. b–d) are different from that for O_P in the Z-direction. The topography of the protruded portion or roughness of the surface for O_P is in microns while that for the others is in units of nm. This confirmed the significantly rougher characteristics of the first surface related to O_P, which showed the highest wear. Other surfaces showed a pad-like structure and the tribo-film formed by the experimental NP-oils is an indication of a good boundary film. A similar morphology for the additives that form protective tribo-films was studied elsewhere.^21–26

Fig. 11 shows the AFM deflection images and corresponding line profiles for the steel disc surfaces worn in the presence of O_P, O_D, O_DN1 and O_DN6 (Fig. 9a–d). The roughness (R_a) values of the line profiles of O_P, O_D, O_DN1 and O_DN6 are listed in Table 6. The wear pattern was in the order: O_P > O_DN1 > O_D > O_DN6.

Fig. 11 AFM deflection images and corresponding line profiles for the steel surfaces: (a) O_P, (b) O_D, (c) O_DN1 and (d) O_DN6.

Table 6 The surface roughness (R_a) of the line profile of the selected NP-oils

	OP	OD	ODN1	ODN6
Ra (nm)	401	32	37	30

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It can be seen from Table 6 that the surface roughness decreased with the addition of dispersant and nano-PTFE. For O_DN6 it was minimum (30 nm), while for O_P it was maximum (401 nm). O_DN1 showed a higher R_a value (37 nm) than O_D (32 nm) which was in accordance with the wear pattern.

4. Conclusions

In this paper, the effect of nano-PTFE-PIBSI additive interaction in a lubricant system is reported. The NP-oils (using nano-PTFE as an additive) developed in earlier work showed a significant enhancement in performance properties. However, they suffered from a stability issue and the NPs started settling down after 15 days. It was necessary to enhance the stability with the help of a dispersant. Hence 1 wt% PIBSI was added as a dispersant and the results of simultaneous interaction of the two additives in oil are reported in this paper.

We had also reported in the earlier paper that beyond an optimal concentration (3 wt%), the NPs did not enhance the performance and inefficient de-agglomeration of the NPs was offered as a reason for this phenomenon.

Inclusion of PIBSI (1 wt%) in NP-oils enhanced the stability of the nano-suspension significantly. Until 2 months there was no indication of sedimentation as confirmed from visual inspection and DLS (dynamic light scattering) studies. The dispersant itself proved to exhibit very good anti-friction and anti-wear properties. Amongst the selected amounts (1, 5 and 10 wt%), 1% was found to be optimum for the best tribo-properties. The tribo-properties of the oils containing both the additives (dispersant and nano-PTFE), however, were affected adversely due to the dispersant. The performance (both friction and wear) decreased when nano-PTFE was added in a low amount. It was due to the competition of the film transfer on the counterface by both additives. As a net result hardly any good uniform and beneficial film could be transferred. With an increase in the amount of PTFE, however, this antagonism problem diminished to the extent that oil with 6 wt% NPs stabilised by 1 wt% PIBSI showed the lowest friction and wear.

In the case of nano-PTFE without dispersant 3 wt% NPs were observed to be the optimum amount for a significantly enhanced performance. Compared with the new oils with the dispersant and NPs, 6 wt% proved to be the best since more NPs were in the de-agglomerated form, which was not possible without PIBSI. The performance of the NP-oils without dispersant was always significantly better than those with dispersant. Finally it was concluded that the stability of the nano-suspension was achieved by adding PIBSI, but at the cost of some initial loss in the tribo-performance.

The paper should be of interest to researchers and practitioners in the area of formulations of finished oils, lubricants and additives, nanoparticles, polymeric materials and their interaction with metallic surfaces. In depth analysis of the interacting surfaces by scanning electron microscopy/atomic force microscopy showed film transfer on the counterface. However, further studies are needed to fully quantify the structure and thickness of these tribo-films.

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