Influence of nano particles on the performance parameters of lube oil – a review

Anoop Pratap Singh; Ravi Kumar Dwivedi; Amit Suhane

doi:10.1088/2053-1591/ac2add

List of abbreviations

3D	Three-Dimensional
AFM	Atomic force microscopy
AISI	American Iron and Steel Institute
API	American Petroleum Institute
COF	Coefficient of Friction
DIA	Diameter
EDS or EDX or EDAX	Energy Dispersive x-Ray Spectroscopy
FE-SEM	Field Emission Scanning Electron Microscope
HRSEM	High-Resolution Scanning Electron Microscope
HRTEM	High-Resolution Transmission Electron Microscopy
LEN	Length
LSM	Laser Scanning Microscopy
Max.	Maximum
Min.	Minimum
MWCNTs	Multi-Walled Carbon Nanotubes
nm	Nano Meter
NPs	Nanoparticles
OM	Optical Microscope
PAO	Polyalphaolefin
PTFE	Polytetrafluoroethylene
Ref.	Reference
SAE	Society of Automotive Engineers
SAXS	Small-angle x-ray scattering
SEM	Scanning Electron Microscope
STM	Scanning Tunnelling Microscopy
SWCNTs	Single-Walled Carbon Nanotubes
TEM	Transmission Electron Microscopy
THK	Thickness
UV	Ultraviolet
XPS	X-ray Photoelectron Spectroscopy
XRD	X-ray powder diffraction
μm	Micro Meter

1. Introduction

In general terms, efficiency is the ratio of output to input. There are two available methods to improve the efficiency of a mechanical system that come to mind, the first is to reduce input, and the second is to minimize losses. The system's output also varies in the first case. Still, the system response becomes unaltered or better in the second case. Thus, friction is a significant element on the way to reduce energy loss. In typical applications, friction cannot be eliminated but reduced to some level where energy loss is minimal. So, there is a need for an efficient lubrication system. Nowadays, lubricating oil contains 90% hydrocarbons and the rest of the additives, which play an essential role in its behaviour. Organic phosphorous and sulfide compounds have played positive roles as friction modifiers and wear resisters [1].

Nano lubricant is a homogenous mixture of base oil, nanoparticles, and dispersants or surfactants. In recent years, nanoparticles played an essential role in the field of additives. Generally, the size of the particles is less than 100 nm. Many researchers say that nanoparticles within lubricating oil act as friction modifiers and positively impact thermal requirements [2].

**Figure 1.** Lubrication mechanism.
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The lubrication mechanism derives from the physical and chemical interactions between molecules of lubricants, material surfaces, and the environment. Based on nanoparticle interaction within the tribological surfaces, four mechanisms are derived [1], [3], [4]. First, nanoparticles of the spherical-shaped act as the tiny balls in between the tribological surfaces, which changes the sliding action into the rolling action results in the reduction of COF [3], [5–7]. Second, sometimes nanoparticles form a protective layer over the mating [8–12]. Third, nanoparticles fill in the asperities, and friction cracks result in a smoother surface, termed the mending effect [13]. Forth, Surface roughness cuts down due to nanoparticles' abrasive action, termed the polishing effect [14]. Figure 1 shows a pictorial representation of the lubrication mechanism.

Nanoparticles could be used as an add-on in diesel and biodiesel to improve fuel efficiency, engine performance, exhaust emission, combustion [15–18]. They were also effective in the area of heat transfer [19], [20].

After analysing the number of articles in nano lubricants, many questions arise in mind, such as how can the quantity of nanoparticles be optimized to enhance the tribological performance? What are the properties of nanoparticles relevant for lubrication? Can we measure their effects? During the literature review of several hundreds of publications, it is observed that each study is based on unique conditions, such as base oil, additive concentration, nanomaterials, and their surface functionalities, workpiece material, test parameters, lubrication regime, etc many others. There is no single standard condition that can be used for a fair comparison. The researchers took a nanoparticle, two or more nanoparticles, or nanocomposites for their study. Therefore, the performance parameters were analysed for better comparison. Statistical analysis is based on experimental results collected from about 80 papers on nano lubricant additives listed in various tables. For the analysis, the minimum (Min.) value of COF, the maximum (Max.) % reduction in COF, and the maximum % reduction in wear are taken as the performance parameters of the nanoparticles in the lubricating oil. The study of individual parameters does not clarify the picture regarding the performance of nanoparticles in lubricating oil. Comparison of all three parameters at the same time is required for each nanoparticle. Therefore, this review summarizes the results and shows a new way of selecting nanoparticles according to performance parameters. The effect of chemical composition, particle size, and nanoparticle morphology on lube oil has been analysed. The impact of nanoparticles on lubricating oil with a base oil and the optimum value of nanoparticles with their best performance characteristics are also shown in this review. This comprehensive review provides detailed information about nano lubricants in a precise manner.

2. Impact of nanoparticles chemical composition on the performance of lube oil

Nanoparticles of every composition and section have been well studied. It is separated by metals, metal oxides, sulfides, carbon and its derivatives, rare earth compounds, nanocomposites, etc The type of nanoparticles is directly related to the particles' mechanical properties, which ultimately reflects their effect on the performance of lubricating oil. Metal particles such as Ag, Au, Cu, etc, are soft and show mending effects. Cu is the most common nanoparticle that has been studied with many combinations. Metal oxides are hard particles that act as small balls of rollers between tribological surfaces. The hard particles are also capable of showing a polishing effect. Nanoparticles have been divided into segments shown in table 1.

Table 1. Nanoparticles categorization according to its chemical composition.

Types	Nanoparticles
Metals	Ag, Bi, Co, Cu, Fe, Ni, Pd, Sn
Metal oxide	Al₂O₃, CuO, Fe₃O₄, TiO₂, ZnO, ZrO₂
Carbon and its derivatives	Graphene, Diamond, SWCNT, MWNTs
Sulfides	WS₂, MoS₂, CuS
Rare earth compounds	LaF₃, CeO₂
Nanocomposites	Al₂O₃/SiO₂, Al₂O₃/TiO₂, Cu/graphene oxide, WS₂/MoS₂
Others	SiO_2, PTFE, BN, Serpentine

Figure 2 shows the statics presenting the distribution of the nanoparticle composition. According to the analysis, the metal oxide nanoparticles cover the maximum. Metal oxides are followed by metals, sulfides, nanocomposites, other carbon and its derivatives, and rare earth compounds.

Figure 3 shows the effect of different chemical compositions on the performance parameters. According to the analysis, when the lowest value of COF is the point of concentration in the performance parameters. Metallic nanoparticles showed the best performance, followed by sulfides, carbon and its derivatives, nanocomposites, others section, rare earth compounds, and metal oxides. Sulfides perform best when it comes to the maximum reduction in COF as a matter of concentration. And carbon and its derivatives, others section, metals, metal oxide nanocomposites, and rare earth compounds are in order. For the maximum reduction in wear, the compositions are sulfides, other segments, metals, metal oxides, carbon, and its derivatives, nanocomposites, and rare earth compounds. The sulfide segment performs exceptionally well in all three performance parameters.

2.1. Metals

Metallic nanoparticles have unique physical and chemical properties as lubricant additives [21–29]. Nanoparticles like Ag, Bi, Co, Cu, Fe, Ni, Pd (palladium), Sn, etc, come under this section. Within the metal segment of nanoparticles, Cu is the most concentrated Nanoparticles (NPs) due to its low melting point, small particle size, and desired ductility [11], [13], [27–36]. Copper nanoparticles show superior antiwear and anti-frictional properties. In Reference [32], Cu nanoparticles with 0.3% by weight give the best results with a mineral base oil and show a good stability period of 8 months. In another case, Cu nanoparticles do not offer any significant improvement in synthetic esters. Still, they can improve the antiwear property of mineral oil by up to 64% with a 0.3% concentration [22].

Nanoparticles such as Ag have been shown to getter friction reducers due to their ductile nature [37], [38]. There is an improvement noted on lubricant characteristics of mineral oil by adding Bi particles [39]. The suspension of Ni particles shows a decrease in the average COF and wear for Polyalphaolefin (PAO) 6 [40], [41]. Metallic nanoparticles are not only known as friction modifiers but also to improve load-bearing capabilities. The load-bearing capabilities of base oil are enhanced by adding Pd [42], [43]. At the comparison node between Fe, Cu, Co, the COF and wear values show a reduction of up to 53% and 50%, respectively, for Fe+Cu [21]. Both Fe and Sn NPs show a reduction in friction, wear, and frictional heat compared to MACs oil [44].

Table 2 shows details about metallic additives for nano lubricants. The name of the NPs and the particle size and morphological characteristics are mentioned within this table. This accurate aggregated description also gives information about base oils and also about surfactants or dispersants. The table also lists the technique used to characterize nanoparticles, nano lubricants, and worm surfaces. The importance of any nanoparticle is checked only by tribological testing methods. And as friction and wear are superficial phenomena. Therefore, information about the surface material of the tribological member and the tribological test is an essential component. So, this table lists the information about the tribological test methods and the surface material of the test components. The stability of nanoparticles is also mentioned in nano lubricants. Finally, the minimum value of COF obtained in the tribological test is also mentioned. The optimum quantity of nanoparticles is mentioned at which nano lubricants show their highest efficiency. This optimum value is displayed as % of nanoparticle/unit volume. The maximum reduction in COF and wear value is also listed. The particular case is laid on the table. The average value of COF and wear is taken for this analysis. A negative value of COF and a decrease in wear indicate an increase in COF and wear values.

Table 2. Metallic nanoparticles as lubricant additives.

S. no.	Nanoparticle	Nano particles size	Nano particles shape	Base oil	Surfactants or dispersants	Characterization equipment	Tribological test methods	Material of tribo surfaces	Stability	Min. value of COF	The optimum value of NPs	Max. % reduction in COF	Max. % reduction in wear	References
1	Ag	6–7 nm	Spherical	Multialkylated cyclopentanes (MACs) lubricant	—	Scanning Electron Microscope (SEM), Energy Dispersive x-Ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM)	Optimal SRV Oscillating Friction and Wear Tester	Society of Automotive Engineers (SAE) 52100 Steel	—	0.1	2%	9.09	16.67	[37]
2	Bismuth (Bi)	7–65 nm	Spherical	BS900 (heavy base oil) and BS6500 (light base oil)	—	TEM, Small-angle x-ray scattering (SAXS)	Four-Ball Tester	American Iron and Steel Institute (AISI) 52100 Steel Balls	—	0.0475	300 mg l⁻¹	34.48	14.62	[45]
3	Cu	Diameter (DIA): 5 nm	—	Lubricant Chevron Taro 30 DP 40 and Teboil Ward Chevron	Oleic acid	Atomic force microscopy (AFM), XPS	Pin on Disk	Disk: Gray Cast Iron (GG 25, Lamellar Graphite) Pin: Chromium-Plated Steel 100Cr6	More stability quoted as compare to not modified surface	0.1075	0.3%	49.41	22.22	[46]
		20–130 nm	Spherical	500SN	Polyisobutylenebutadimide	SEM, Scanning Tunnelling Microscopy (STM), TEM, x-ray powder diffraction (XRD)	Pin on Disc	Disks: 20CrMnTi Steel Pins: H62 Bronze	—	0.033	0.1%	25	—	[13]
		20 nm	Globular	Mineral oil SN 650	—	SEM, EDS, XPS	Disc on Disc (Test Rig)	1045 steel	—	0.059	0.6%	26.9	57.2	[31]
		—	Spherical	CAL-1 (5W- 40), CAL-2 (5W-40) and CAL-3 (5W-20)	DDP(di-n-octodecyldithiophosphate)	TEM, Three-Dimensional (3D) Optical Profiler	Four Ball Tester	GCr15 Steel Balls	—	0.049	1.6% in CAL - 3	38.8	24	[47]
		25 nm	Spherical	Avocado oil	—	3D Profilomete, SEM, EDS	Pin on Disc	Pin: Aluminum Alloy 6061 Disc: EN 31 Steel	10 days stability reported	0.04	1%	50	75	[48]
		10–30 nm	Oblong	Paraffinic mineral and Synthetic ester	—	SEM	Pin On Disc And Four-Ball Tester	Ball and Disk: AISI 52100 Pin: AISI 1020	8 months stability reported	0.01	0.3% with mineral oil	60	20	[22]
4	Ni	20 nm	Spherical	PAO6	—	SEM, EDS	Block on Ring Configuration and Four-Ball Tester	Rings: AISI D3 Steel Blocks: AISI 1045 Steel Balls: AISI 52100 Steel	—	0.015	0.5%	30	45	[40]
		7.5–28.5 nm	Spherical	PAO6	Oleylamine and oleic acid	TEM, XRD, SEM, XPS	Four-Ball Tester	GCr15 Steel Balls	—	0.0675	0.05%	−18.52	18.64	[41]
5	Pd (palladium NPs)	2 nm	—	Tetrahydrofurane (THF)	Tetrabutylammonium acetate (TBA)	XRD, Optical Micrographs	Ball on Disk Tribometer	AISI 52100 Steel	Not Required	0.05	1%	9.09	94	[42]
6	Fe, Cu, and Co	50–80 nm	—	SAE 10	Converse emulsions of water in lubricant solution (CEWLS)	SEM, Energy Dispersive X-Ray Spectroscopy (EDX)	Four Ball Tribotester	100 Cr6 Bearing Steel	—	—	0.5% Fe	39	19.67	[21]
											0.5% Cu	49	47.54
											0.5% Co	20	4.918
											Fe+Co (0.25% + 0.25%)	36	32.79
											Fe+Cu (0.25% + 0.25%)	53	50.82
											0.5% FeCu	55	29.51
											0.5% FeCo	50	26.23
7	Sn and Fe	Sn: 30–60 nm Fe: 20–70 nm	Spherical	Multialkylated cyclopentanes (MACs)	—	SEM	Four-Ball Tester	AISI 52100	—	0.0625	1% Sn	37.5	38.75	[49]
										0.07	1% Fe	30	51.25

2.2. Metal oxide

Lubrication mechanisms of Oxides are analogous to those of metallic NPs, including the formation adsorption film, rolling effect, and sintering. Some common metal oxides are Al₂O₃, ZnO, Fe₃O₄, ZrO₂, Co₃O₄, etc, which positively affect oil performance [50–52].

Al₂O₃ are hard particles but positively respond to the lube oil's frictional and wear characteristics [52–55]. There are studies available on the anti-frictional and antiwear behaviour of [56–58]. CuO NPs at 0.1% exhibit better antiwear performance compared with other concentrations [59]. The effect of CuO particles with TiO₂, ZnO, Al₂O_3, and nano diamond has been investigated [6], [60–62]. Some oxides like magnetic Fe₃O₄ can reduce COF by forming a protective film between the tribo [63], [64].

Several studies reported on the performance of [65–71]. When TiO₂ nanoparticles were added to engine oil, the co-efficient of friction was reduced by 86% with 0.3% concentration by weight of the oil compared to the oil without TiO₂ nanoparticles for load 4 kg [70]. Almost identical performance has been reported in [65], a 0.3% (TiO₂) concentration in engine oil with a similar reduction in COF of 86%. The optimum concentration of ZrO2 is 0.5% for the reduction in friction coefficient up to 27.34% under the thrust-ring test [72].

The essential information about the metal oxides nanoparticles as lubricant additives is mentioned in table 3.

Table 3. Metal oxides nanoparticles as lubricant additives.

S. No.	Nanoparticle	Nano Particles Size	Nano Particles Shape	Base oil	Surfactants or dispersants	Characterization equipment	Tribological Test Methods	Material of Tribo Surfaces	Stability	Min. value of COF	The optimum value of NPs	Max. % reduction in COF	Max. % reduction in wear	Reference
1	Al₂O₃	78 nm	Spherical	Lube Oil with VI 95	KH-560	SEM, EDS	Four-Ball Tester and Thrust Ring Friction Tester	GCr15 Steel Balls	Chemically modified nano composite shows good dispersion stability	0.055	0.1%	19.12	41.75	[53]
2	CuO	50 nm	Spherical	Castor Oil (CO) Paraffin oil (PO)	Sodium dodecyl sulfate (SDS)	SEM, AFM	Four Ball Tester	AISI 52100 Steel	Visual inspection, upto 72 h	0.05	0.1%	34.6	28.3	[58]
		5 nm	Spherical	PAO oil	Oleic acid	XRD, TEM, optical microscopy	HFRR Tribometer	AISI 52100 steel	Stability reported up to 30 days	0.104	0.1%	16.13	21.82	[59]
3	Fe₃O₄	10 nm	Spherical	Liquid paraffin	Oleic acid	TEM, XRD	Ball on Flat Contact Tester	0.45% Carbon Steel and 440C Stainless Steel	—	0.15	2 g l⁻¹	25	64.7	[63]
4	Magnesium borate	10 nm	Amorphous	500 SN base oil	Sorbitol monostearate - 1%	SEM, XPS, TEM, XRD	Four-Ball Tester and Block on Ring Tribometer	Ring: Quenched CrWMn Steel Block: 45 Steel	—	0.035	0.757%	22.22	36	[8]
5	TiO₂	10–25 nm	Spherical	Engine oil Servo 4T Synth 10W-30	—	Ultraviolet (UV) spectrometer	Pin on Disc	Pin: Aluminium alloy (LM 25) (Al-Si7Mg)	Dispersion analysis using UV spectrometer	0.015	0.3%	86.49	92	[70]
		100–200 nm	Hollow sheet	American Petroleum Institute (API)-1509 base oil	Oleic acid	TEM, IR, Polarized Microscopy (PM), XPS	Four-Ball Tribometer And Ball On Disk Tribometer	AISI K 19,195 Steel	Improves after surface modification	0.07	0.5%	26.32	28.69565	[65]
6	ZnO	20 nm	Spherical	Polyalphaolefin (PAO6)	Octacare (OL100 and OL300)	SEM, EDS	Four-Ball Tester	AISI 52100	—	—	0.3%	—	35.5	[73]
7	ZrO₂	<50 nm	Amorphous	20# machine oil	KH-560	TEM, EDS, XRD	Four-Ball Tester and A Thrust-Ring Tester	Balls: AISI-52100 Ring Specimen: 45 Steel	—	0.01661	0.5%	27.34	—	[72]
8	Al₂O₃ and TiO₂	Al₂O₃: 8–12 nm TiO₂: 10 nm	Spherical	Engine oil (5W-30)	Oleic acid	Field Emission Scanning Electron Microscope (FE-SEM), EDS, XPS, XRD, 3D Surface Profile	Test Rig	Piston ring/cylinder liner interface in an engine according to ASTM G181	DLS method, stability period mentioned is 14 days	0.038	0.25% Al₂O₃	35	21	[54]
										0.37	0.25% TiO₂	46	29
9	CuO and Al₂O₃	<50 nm	CuO: Spherical Al₂O₃: Tubular	GL-4 (SAE75W-85) and Poly-alpha olefin 8 (PAO8)	—	SEM, Energy Dispersive X-Ray Spectroscopy (EDAX)	Optical SRV 4 Tester and Four-Ball Tribotester	AISI 52100 Steel	—	0.13	2% Al₂O₃ + PAO8	16.13	−25	[60]
										0.115	2% CuO + PAO8	25.81	11.76
										0.13	0.5% Al₂O₃ + GL4	0	0.735
										0.125	0.5% CuO + GL4	3.85	13.24
10	CuO, Tio₂, Nano-Diamond	CuO: 5nm TiO₂: 80 nm Nano-Diamond: 10nm	Spherical	API-SF (VI100) engine oil and a Base oil (VI107)	Glycol	TEM, Optical Microscope (OM), SEM, and EDX	Reciprocating Sliding Tribotester	Chromium-Coated Steel Ball and Plate	—	0.085	0.1% CuO + API-SF	18.4	16.67	[6]
										0.095	0.1% CuO + Base oil	5.8	78.79
11	CuO and ZnO with 0.5%	ZnO: 11.71 nm CuO: 4.35nm	ZnO: Hexagonal CuO: Monoclinic	Soybean oil Sunflower oil Mineral oil Synthetic oil (polyalphaoleifin)	—	SEM, XRD, EDS	High-Frequency Reciprocating Test Rig (HFRR)	A rigid steel ball (570–750 HV)	—	0.113	Mineral+CuO	−8.65	−2.597	[62]
										0.099	Mineral+ZnO	4.81	5.009
										0.084	PAO + CuO	22.22	7.677
										0.096	PAO + ZnO	11.11	3.459
										0.061	Sunflower+CuO	−19.61	−5.867
										0.06	Sunflower + ZnO	−17.65	−7.733
										0.057	Soybean + CuO	−7.55	−13.12
										0.062	Soybean+ZnO	−16.98	−18.18

2.3. Sulfides

Sulfides are an essential and long-used nano additive. Lube oil with sulfides nanoparticles shows more antifriction and antiwear properties. Some common examples of sulfides nanoparticles are MoS₂, WS₂, and CuS.

MoS₂ and WS₂ are widely accepted for their chemical stability and fullerene-like structure. Hollow polyhedral structures called fullerene-like NPs (IF-NPs) provide stability to the oil particles [74]. The 0.25% MoS₂ microfluid results in an almost 40% reduction in friction and a 6% reduction in wear compared with the base oil in fully flooded EHL contacts [75]. In another example, MoS₂ can reduce COF by up to 70% and show superior antiwear properties with a concentration of 1% [76].

Table 4 presents examples of sulfides used as nano additives along with a description of nano lubricants.

Table 4. Sulfides nanoparticles as lubricant additives.

S. No.	Nanoparticle	Nano Particles Size	Nano Particles Shape	Base oil	Surfactants or dispersants	Characterization equipment	Tribological Test Methods	Material of Tribo Surfaces	Stability	Min. value of COF	The optimum value of NPs	Max. % reduction in COF	Max. % reduction in wear	Reference
1	CuS	20 nm	Spherical	Liquid Paraffin	Oleic acid	SEM, TEM	Four ball Tester	—	—	0.0652	0.5%	—	—	[77]
2	IF-MoS₂	Crystalline: 80 nm Poorly crystalline: 20–50 nm	Spherical	Polyalphaolefin (PAO6)	—	High-Resolution Transmission Electron Microscopy (HRTEM), SEM, XPS	Pin On Flat Tribometer	AISI 52100 Steel	—	0.025	with 1% of poorly crystalline	85.71	58.33	[78]
3	MoS₂	0.5–2 μm	Spherical	500 SN base oil	—	TEM, SEM, HRTEM	Oscillating Friction and Wear Tester	GCr15 Bearing Steel	—	0.085	0.1 %	29.17	62.07	[9]
		150 and 350 nm	Spherical	Blend of PAO 4 and PAO 40	—	HRTEM, EDX, XPS	HFRR (High-Frequency Reciprocating Rig)	AISI 52100 (100Cr6) Steel	—	0.06	1%	70	91.11	[76]
		250 nm	Flower-like	Polyolester and naphthenic oil	No dispersant	XRD, FTIR, TEM	Pin on Disk	Pin: AISI-52100 Steel Disk: Grey Cast Iron	Visibly stable for approximately four days for the polyolester oil and over eight days for the naphthenic oil	0.03	1% with naphthenic oil	86.36	—	[79]
		50–100 nm	Spherical	Dioctyl sebacate (DOS)	—	XRD, SEM, EDS, TEM, Laser Scanning Microscopy (LSM), XPS	Ball on Disc Tribometer	GCr15 Steel Ball	—	0.06	0.5%	45.45	40	[80]
		DIA: 100 nm Length (LEN): 20 μm	Nanotubes	Polyalphaolephin (PAO) oil	—	SEM, HRTEM, EDS, Ex-situ optical micrographs	Ball on Disc Tribometer	Ball and Disc: AISI 52100/DIN 100Cr6 Steel	—	0.0495	2%	28.78	—	[81]
		DIA: 100–500 nm	Multi-Wall Nanotubes (MWNTs)	Polyalphaolephin (PAO) oil	—	TEM, Optical micrographs, SEM	Ball on Disc Tribometer	Ball and Disc: AISI 52100/DIN 100Cr6 Steel	—	0.07	5%	56.25	88.89	[82]
		<2 μm	Spherical	Mineral base oil N-150	Trichlorooctadecylsilane	XRD, SEM, EDX	Four-Ball Tester and Pin on Roller Tribotester	Ball: GCr15 Steel Pin: EN-31 Steel	Lubricant blends stable upto 28 days	0.0226	0.25%	37.95	10.07	[75]
		62–84 nm	Amorphous Ball-Like	Pentaerythrityl tetracaprylate/caprate ester	Caproic, lauric, stearic, and oleic acid.	SEM, Metallograph, FE-SEM, EDX, XRD	Four-Ball Tester	—	—	0.103	0.05% and lauric acid	8.04	13.34	[83]
4	WS₂	50 to 350 nm	Fullerene Like	SN 90, SN 150, bright stock	—	XPS	Test rig	AISI 2510, Brass, Bearing Steel	—	0.03	5%	48.28	22	[84]
		—	Tubes and Spherical form with coatings	Polyalphaolefin-4 (PAO-4)	—	TEM, Optical Microscope, High-Resolution Scanning Electron Microscope (HRSEM), EDS	Ball on Flat Test Rig	Plate: Stainless Steel (AISI 316) Balls: AISI 50100	—	0.075	1%	55.88	70.21	[85]

2.4. Nanocomposites

Nanocomposites are the field of taking advantage of two or more nanoparticles at the same time. In nanocomposites, two or more nanoparticles are combined into a base oil. Sometimes researchers also synthesize nanocomposites. The nanocomposite is an area where two dislike nanomaterials and can increase lube oil quality like Cu/SiO₂ Nanocomposite. Many researchers say that some nanocomposite shows better performance than the performance of individual nanoparticles in lube [86], [87]. The trends also suggest that research on nanocomposites has increased over the years. Many nanocomposites are evaluated for their performance in lubricating oils, which are Al₂O₃/SiO₂, Al₂O₃/TiO₂, Cu/Graphene oxide, WS₂/MoS_2, etc.

The lubricating oil with Al₂O₃/TiO₂ nanoparticle of 75 nm average diameter and KH-560 shows good dispersion stability. It can reduce friction and wear with an optimum concentration of 0.1% [87], [88]. The performance of lubricating oil using Al₂O₃/SiO₂ nanoparticles as additives was improved compared to pure Al₂O₃ or SiO₂ particles [89], [90]. A solid/liquid dual lubrication system was seen by mixing WS₂-MoS₂ films with two specific oils, including MAC and CPSO. The tribological performance of the WS₂–MoS₂/MACS system was much better than that of WS₂-MoS₂/CPSO [86]. Nano-Cu/graphene oxide exhibits better performance than single GO nanoparticles [91]. Cu/SiO₂ nanocomposite as a lubricant additive can effectively improve the friction-reducing and anti- wear ability load-carrying capacity of distilled water [91]. ZnAl₂O₄ NPs as a lubricant additive can significantly improve antifriction and antiwear characteristics of lube oil, with the optimum concentration of 0.1 wt% [92].

Table 5 shows examples of Nanocomposites used as nano additives along with a description of nano lubricants.

Table 5. Composite nanoparticles as lubricant additives.

S. No.	Nanoparticle	Nano Particles Size	Nano Particles Shape	Base oil	Surfactants or dispersants	Characterization equipment	Tribological Test Methods	Material of Tribo Surfaces	Stability	Min. value of COF	The optimum value of NPs	Max. % reduction in COF	Max. % reduction in wear	Reference
1	Al₂O₃/ TiO₂	Al₂O₃:8–12 nm TiO₂: 10 nm	Spherical	Synthetic oil (5W 30)	Oleic acid	FE-SEM, EDS, and 3D Surface Profiler	Test rig	Piston Ring: 320 VH Cylinder Liner: 413VH	—	0.038	0.1% (0.05%+0.05%)	47.61	17	[87]
		75 nm	Spherical	Lube Oil (VI 95)	KH-560 (3-glycidoxypropyltrimethoxysilane)	SEM, EDS	Four-Ball Tester and Thrust Ring Friction Tester	Steel Ball GCr15	Chemically modified nano composite shows good dispersion stability	0.052	0.1% (Four ball tester)	20.51	44	[88]
										0.048	0.1% (Thrust-ring friction test)	22.15	—
2	Al₂O₃/ SiO₂	Al₂O₃: 13 nm SiO₂: 30 nm	—	Polyalkylene Glycol (PAG 46)	—	FE-SEM	Tribology Test Rig	Aluminium Al 2024	Visually sedimentation observed after 14 days	0.06525	0.06%	4.78	12.96	[90]
		70 nm	Elliptical shaped	lubricant oil	KH 560	TEM, SEM, EDS	Four-Ball and Thrust Ring Tester	—	Checked for 3 months	0.028	0.5% (Four ball test)	20	22.16	[89]
										0.032	0.5% (Thrust ring test)	50	—
3	WS₂/ MoS₂	Thickness (THK): 100 nm	Feather-Like Morphology	MACs (multialkylated cyclopentanes) and CPSO (chlorinated-phenyl with methyl-terminated silicone oil)	—	SEM, TEM, EDS	Ball on Disk	AISI 440C Steel Ball	—	0.06	MACs-WS₂/MoS₂	45.45	97.02	[86]
										0.08	FCPSO-WS₂/MoS₂	42.86	−1982.46
4	BN/calcium borate	BN: 100–200 nm Coating THK: 12 nm	Spherical BN NPs coated with calcium borate	Mineral-base oil (saturated cycloparaffin and paraffin hydrocarbon)	No surfactant	TEM, XRD, XPS	Four-Ball Tester	Steel Balls GCr15	—	0.094	1%	23.6	25.2	[93]
5	Copper nanoparticle/Graphene oxide	5–10 nm	Nano sphere and Nano sheets	Liquid paraffin oil	Stearic acid	XRD, TEM, SEM	Four Ball Tester	Steel Balls GCr15	10 days quoted	0.065	0.05%	27	52.7	[94]
6	Cu/ SiO₂	Cu: 20 nm Silica layer THK: 2 nm	Spherical	Distilled water	3-mercaptopropyl- trimethoxysilane (MPTS)	XRD, TEM, SEM, EDS	Four-Ball Tester	GCr15 Bearing Steel (SAE- 52100)	—	0.14	2%	51.72	38.46	[91]
7	Mg/Al/Ce ternary layered double hydroxides (LDHs)	190.1 nm	Hexagonal layered structure	Diesel engine oil (CD 15W-40)	Succinic acid and lauric acid	XRD, SEM, FT-IR Spectroscopy, EDS	Four-Ball Tester and Air Compressor Test	GCr15 Steel (AISI 52100)	—	0.063	0.5g/100 ml	44.74	30.2	[95]
8	La-doped Mg/Al layered double hydroxide (LDH)	185.6 nm	Hexagonal Laminate Structure	Diesel engine oil (CD 15W-40)	Sodium dodecyl sulfate (SDS)	XRD, SEM, FT-IR spectroscopy, EDS	Four-Ball Tester and Air Compressor Test	GCr15 Steel (AISI 52100)	—	0.082	0.5g/100 ml	26.13	16.5	[96]
9	lanthanum-doped TiO₂	20 nm	Spherical	Rapeseed oil	Oleic acid	XRD, SEM and FTIR Spectrometry, XPS	Four-Ball Tester	GCr15 Steel	—	0.051	0.25 %	5.56	7.27	[97]
10	Zinc aluminate (ZnAl₂O₄)	95 nm	Spherical	Lubricant oil	Oleic acid	XRD, SEM, IR Spectrum, EDS	Four-Ball Tester and Thrust Ring Tester	—	—	0.0643	0.1%	33.37	31.15	[92]

2.5. Others

Nanoparticles such as SiO₂, PTFE (polytetrafluoroethylene), BN, Serpentine, etc, fall under this other section. More than half of the research under this section focuses on SiO₂ nanoparticles. SiO₂ shows some significant improvements in friction and wear capabilities of lubricating oil. SiO₂ can improve anti-frictional and antiwear properties by 77.7 and 74.1% in sunflower oil with 1.25% of SiO₂ [98].

The nano-BN oils significantly improve the performance of the base oil [99], [100]. The concentration of 0.1% BN particles shows better performance as compared to SE 15W-40 [101]. PTFE NPs show excellent potential to improve the EP performance of lube oil [102], [103]. 1.5 wt% of serpentine into oil is the most efficient in reducing friction and wear [104].

Many studies have focused on SiO₂ NPs because of their friction and wear-reducing ability in lubricating oil [98], [105–111]. It is observed that small nano-SiO₂ nanoparticles exhibited optimal friction-reduction performance under low frequency. In contrast, for large nano-SiO₂, it occurred under high frequency [108]. SiO₂ with surface modifier KH-550 has the potential to improve the performance of lubricating oil [105]. In [110], a significant decrease in COF upon the increase of SiO₂ nanoparticles (0 to 0.6 wt%) has been reported. In another case, when comparing the performance between SiO₂, graphite, Cu, CuO, and WS₂. SiO₂ in 15W40 lubricating oil showed the highest reduction in COF and wear by 25.55% (at 588 N load) and 59.91% (at 588 N load) [111].

Table 6 shows nanoparticles from other sections used as nano additives along with a description of nano lubricants.

Table 6. Others section of nanoparticles as lubricant additives.

S. No.	Nanoparticle	Nano Particles Size	Nano Particles Shape	Base oil	Surfactants or dispersants	Characterization equipment	Tribological Test Methods	Material of Tribo Surfaces	Stability	Min. value of COF	The optimum value of NPs	Max. % reduction in COF	Max. % reduction in wear	Reference
1	BN	DIA: 120 nm THK: 30 nm	Disk Shape	SE 15W-40	Oleic acid	XRD, TEM, AFM, SEM	Friction Tester	No. 45 stainless steel	—	0.015	0.1%	76.92	—	[101]
2	Hexagonal boron nitride (hBN)	70 nm, 0.5 μm, 1.5 μm, and 5.0 μm	Hexagonal	Avocado oil	—	SEM	Pin on Disc	Pin: Oxygen-Free Electronic Copper (C101) Disk: 2024 Aluminium	—	0.0144	5 %	64	72	[99]
		5 μm	Hexagonal	API Group III 150N	Oleic acid, PIBSI, Lubrizol 6412TM and Oloa 11000	SEM, EDAX, and Raman spectroscopy	Four-Ball Tester	AISI E 52100	Stability Checked up to 3 weeks	0.1	with 5% of Oloa 11000 and No Nano Particle	37.5	10.54	[100]
3	Polytetrafluoroethylene (PTFE)	90–100 nm	Spherical	150 N API Group II oil	—	TEM, SEM	Four-Ball Tester and SRV Friction Tester	AISI E 52100 Steel	Stable for 7 days	0.14	3%	26.32	28	[102]
		200–300 nm	Spherical, Oval, and Cylindrical	Group I, II, III, and IV(PA04) according to API	—	TEM, XRD, FTIR, Raman, and XPS Spectroscopy	Four-Ball Tester and SRV^® III Tester	AISI E 52100 Steel	Particle size analysis (based on DLS, Dynamic light scattering) principle and UV–Vis Spectroscopy 80% up to two weeks.	0.11	3% with Group II	35.29	61.87	[103]
4	Serpentine ultrafine powders	0.1–5 μm	Irregular	Diesel engine oil (grade: 50 CC)	Boric acid ester and Span 60	SEM, XRD, EDS	Sliding Friction Tribotester Test Rig	1045 Steel Contact	Good dispersion stability	0.04	1.5%	58.1	89	[104]
5	SiO₂	15–20 nm	—	ST5W/30 gas mobile oil (GMO)	KH 570	TEM, IR, XPS, TG, EDS, FTIR	RFT-III Reciprocating Tribotester and Four-Ball Tester	Reciprocating Column: 45# Steel Balls: GCr15 Steel	Stable after 5 months	0.026	0.3%	42.86	28.86	[105]
		58 to 684 nm	Spherical	Liquid paraffin	Oleic acid	SEM, EDS, AFM	Ball on Ring	Ball: Bearing Steel GCr15 Ring: AISI 52100 Steel	Visual Check 30 days	0.065	0.2%	25.71	30.77	[106]
		102 ± 33 nm	Spherical	Rust and oxidation lubricant (ISO 32 and 68)	Silane coupling agents, APTEOS and TMSDETA	SEM, EDX, Static Multiple Light Scattering (MLS)	Four ball Tester	AISI 52100 Steel	Agglomeration starts within 8 days	0.06	0.5%	42.86	−13.64	[107]
		40 and 90 nm	Spherical	PAG (Polyalkylene Glycol)	—	SEM, EDS, Optical Interferometer, Rheometer	Optimal-SRV Oscillating Friction and Wear Tester on Slider-On-Disc Mode	—	—	0.1125	0.2%	4.26	37.5	[108]
6	MoS₂ and SiO₂	MoS₂: 90 nm SiO₂: 30 nm	MoS₂: Nanosheets SiO₂: Spherical	EOT5# Engine oil	No dispersant	FE-SEM	Ball on Flat Tribometer	Magnesium Alloy/Steel Contacts	—	0.045	0.7% SiO₂	43.75	7.5	[109]
										0.055	0.7% MoS₂	31.25	20
7	SiO₂ and TiO₂	SiO₂: 20–30 nm TiO₂: 25–35 nm	Spherical	Sunflower Oil	–	FE-SEMFE-SEM, EDS	Block on Ring Sliding Tester	Blocks: AISI 304 Steel Rings: AISI 52100 Steel	—	0.0144	1.25% SiO₂	77.7	74.1	[98]
										0.0032	0.75% TiO₂	93.7	70.1

2.6. Carbon and its derivatives

Carbon shows a variety of properties from soft to hard when comparing graphite to diamond. Carbon and its derivatives are studied as nano additives for their different sizes and shapes. Graphene, Diamond, SWCNT (Single Wall Carbon Nano Tube), MWNTs (Multi-Wall Carbon Nano Tube) are important carbon derivatives. The tube form of nanoparticles is mainly studied for carbon. According to this analysis, graphene, diamond, carbon nanotubes are uniformly covered and improve oil capacities. Nano diamond in ethylene glycol exhibits anti-frictional properties. It has a more negligible wear effect in stainless Steel - stainless steel aboriginal contact [112]. The COF of graphite mixed with oil decreased by 28%, and the antiviral properties of the mixture improved by 32% compared to pure 10W40 oil [113].

It is of interest to study the tribological properties of graphene in recent years [114–116]. Graphene additives can reduce COF up to 78% with 0.5 wt%. In the same case, the scar diameter was significantly reduced compared to pure PAO2 lubricants under a load of 120 N and a rotation speed of 250 rpm [114]. The addition of Gr resulted in a significant reduction in friction and wear values compared to the reference oil (5W-30) [115]. Diamond is another derivative of carbon that is intensively used as a friction modifier [112], [117–119]. Nano-diamond ethylene glycol dispersions exhibit friction and wear lowering effects in stainless Steel–stainless steel tribological contact [112].

Table 7 presents examples of Carbon and its derivatives used as nano additives along with a description of nano lubricants.

Table 7. Carbon and its derivatives nanoparticles as lubricant additives.

S. No.	Nanoparticle	Nano Particles Size	Nano Particles Shape	Base oil	Surfactants or dispersants	Characterization equipment	Tribological Test Methods	Material of Tribo Surfaces	Stability	Min. value of COF	The optimum value of NPs	Max. % reduction in COF	Max. % reduction in wear	Reference
1	Carbon nanotubes	DIA: 10–20 nm LEN: 3–15	Nanotubes	Castor oil	Oleic acid	SEM	Four-Ball Tester	—	—	0.035	0.02 %	24	6.6	[120]
2	Diamond	15 nm	Diamond	Ethylene glycol	—	SEM, EDS, Optical Profilometer	Pin on Disk Tribometer	Disk: Stainless Steel (AISI440B) Ball: Stainless Steel (AISI420)	No significant sedimentation was observed during six months of storage	0.11	1.1%	31.25	62.96	[112]
		<30 nm	Diamond	Mineral oil and PAO oil	—	Nanosight LM-10 instrument, EPSON Perfection Scanner	Tribology Test Rig and Four-Ball Tester	GCr15 Steel and Conveyor Belt	—	0.08	0.01% with Mineral Oil	63	52	[119]
3	Fullerene NPs	10 nm	Fullerene Shaped	Mineral oil	—	SEM, AFM	Disc on Disc	Grey Cast Iron (GC200)	—	0.006	0.1%	76	—	[3]
4	Graphene	10 μm	Sheets	Polyalphaolefin-2 (PAO2) oil	No surfactant	SEM, EDS, 3D Optical Micrograph	Four Ball Tester	AISI-52100 Steel	Sedimentation starts within two weeks	0.025	0.05%	78	14.47	[114]
		DIA: 5–10 μm THK: 3–10 nm	Nano-Sheets	Castrol EDGE professional A5 (5W-30)	Oleic acid (2%)	SEM, TEM, XRD	Bench of Ring/ Liner Assembly	Piston Ring/Liner Reciprocating	11 days	0.035	0.4 wt %	29–35	22–29	[115]
5	Oxidized graphite flakes	—	Flakes	10W40 oil	—	XPS, Micro- Raman Spectroscopy	Ball on Disk Tribometer	Balls: 100Cr6 Steel Disc: 316 LN Steel	Dispersion stability improved	0.118	0.05%	28	32	[113]
6	Onion-like carbon (OLC), single/multiwall carbon nanotubes (SWNT/MWNT), or nano graphene platelets (NGPs)	DND: 4–5 nm MWCNT: DIA 8–15 nm, LEN 10–50 μm SWCNT: DIA 1–2 nm, LEN 5–30 μm Gr: 10–20 nm THK and 14 μm LEN	Onion-Like Carbon, Multi-Walled Carbon Nanotubes (MWCNT), SWCNT, Platelets	Polyalphaolefin (PAO6)	Molybdenum dialkyldithiophosphate (MoDDP)	Optical micrographs	Block on Ring	Block: SAE 01 tool Steel Ring: Stainless Steel	—	0.002	0.015% ND	69.17	66.86	[118]
										0.013	0.5% MWCNT	53.33	25.59
										0.034	0.5% CWCNT	47.5	18.95
										0.02	0.5% Nanographene	48.33	33.99
7	Nanodiamond and SiO₂	Nanodiamond: 110 nm SiO₂: 92 nm	Spherical	Liquid paraffin	Oleic acid	SEM, IR, EDS	Ball on Ring Wear Tester	Bearing Steel GCr15	—	0.07	0.2% ND	34.88	36.81	[121]
										0.065	0.2% SiO₂	39.53	38.2
8	Graphene and MoS₂	2 nm	Nanosheets	Hydraulic oil	–	SEM, AFM, Micrographs	Ball on Disk	Bearing Steels and Brass	—	0.18	1% Graphene	28	26.67	[116]
										0.05	1% MoS₂	80	86.67

2.7. Rare earth compounds

LaF₃, CeO_2, and CeBO₃ are some common examples of rare earth compounds. LaF₃ nanoparticles as a lubricant additive can effectively reduce friction, improve antiwear capacity, and increase the load-carrying capacity of fluoro silicone oil under optimal concentrations of 0.08 wt% [122].

The stearic acid-modified cerium borate nanoparticles exhibited a spherical morphology with an average of 8 nm and were dispersively stable in rapeseed oil. Moreover, SA/CeBO₃ nanospheres markedly enhanced tribological performances of rapeseed oil [123]. When lubricated by ZrO₂ additive oil, the Ta-Si/steel contact COF becomes the lowest [124].

Table 8 shows examples of rare earth compounds used as nano additives along with a description of nano lubricants.

Table 8. Rare earth compounds nanoparticles as lubricant additives.

S. No.	Nanoparticle	Nano ParticlesSize	Nano Particles Shape	Base oil	Surfactants or dispersants	Characterization equipment	Tribological Test Methods	Material of Tribo Surfaces	Stability	Min. value of COF	The optimum value of NPs	Max. % reduction in COF	Max. % reduction in wear	Reference
1	LaF₃	10–30 nm	Hexagonal	Fluoro silicone oil	3- (heptafluoroisopropoxy)propyltriethoxysilan	TEM, XRD, FT-IR, and TGA, SEM	Four-Ball Tester	GCr15 Bearing Steel (SAE- 52100)	—	0.055	0.08 %	31.25	42	[122]
		10–30 nm	Hexagonal	Liquid paraffin	Tributyl phosphate	XRD, TEM, SEM, XPS, FTIR	Four-Ball Tester	GCr15 bearing Steel (SAE-52100)	—	0.0975	0.4%	13.33	14.52	[125]
2	SA/ CeBO₃	DIA: 8 nm	Spherical	Rapeseed oil	Stearic acid	SEM, EDS, XRD, XPS	Four-Ball Tester	Steel Balls GCr15	—	0.055	1.5%	25.42	7.143	[123]
3	Cerium oxide (CeO₂) and zirconium dioxide (ZrO₂)	CeO₂: 500 nm ZrO₂: 300 nm	—	Poly alpha-olefin (PAO4)	–	SEM, EDS	Cylinder-on-Disk Tribotester	Roller: Carbon Chrome Steel (SUJ2) with Tetrahedral Amorphous Carbon (ta-C) Coating Disk: carbon Steel (S55C)	—	0.05	0.2% ZrO₂	40	−43.17	[124]
										0.088	0.2% CeO₂	−10	77.05

3. Impact of particle size

The diameters in spherical and tubular shapes, the thicknesses in the sheets are characteristic dimensions. Typically, the size of nanoparticles is less than 100 nm. Due to the small size of nanoparticles can enter in between the contact of tribological members. The high surface-to-volume ratio also enables NPs to react with the surroundings. Hence this section elaborates the effect of nanoparticle size on the performance of lube oil.

Since the size of the particles governs the mechanical and physical properties of the nanoparticles, it is essential to use nanoparticles of optimum size. In [126], CaCO₃ nanoparticles show better performance at higher frequencies with larger and lower frequencies with smaller dimensions. In another case, the IF-MoS₂ [76] nanoparticle also exhibits similar properties. It refers to the ability of the particle to enter the point of contact. The probability of NPs entering the contact area increases with the decrease in the size of the particles [127].

The performance of SiO₂ nanoparticles was evaluated for various sizes ranges from 58 to 684 nm and finally quoted that 58 nm size nanoparticles show better tribological properties. On the other hand, CuO nanoparticles are also examined for 2.5, 4.4, and 8.7 nm and found better results on 2.5 nm.

The performance of SiO₂ nanoparticles was evaluated for different sizes from 58 to 684 nm and finally quoted that 58 nm size nanoparticles showed better tribological properties [106]. Similarly, CuO nanoparticles were also examined for 2.5, 4.4, and 8.7 nm sizes, and better results were found at 2.5 nm [128].

The nanoparticle size has been divided into four sections for better understanding. They are <30 nm, 31–60 nm, 61–100 nm, and >100 nm. Figure 4 shows the statistics for the size distribution of nanoparticles. The analysis indicates that the dominant concentration of researchers in the past has been under the size of <30 nm.

Medium-sized segments (30–60 nm) of nanoparticles may be part of future studies in nano lubrication due to the potential of NPs in this field is high fewer studies reported in this section.

Figure 5 shows the effect of nanoparticle size on lubricating oil performance. The analysis shows that the size segment <30 nm shows the best results. When the minimum value of COF is the point of concentration in the performance parameter followed by 61–100 nm, 31–60 nm, and >100 nm size segments. Again, the size segment <30 nm shows the best results for the maximum reduction concentration in COF followed by >100 nm, 61–100 nm, and 31–60 nm. The order of size segments is 61–100 nm, <30 nm, >100 nm, and 31–60 nm when maximum reduction in wear is the point of concentration. The nanoparticles with less than 60 nm particle size show equilibrium type reduction in COF and wear. This means that the reduction in COF and wear is almost equal. But the difference between COF and wear reduction seems to widen with increasing size.

4. Impact of the morphology of nanoparticles

The shape of nanoparticles is again an important feature that affects the performance of lubricating oil [129]. The lubrication mechanism explains the importance of particle shape in nano lubrication. It talks about the pressure-carrying capacity of the nanoparticles. Generally, the shapes of nanoparticles are classified into five sections: spherical, granular, nanosheet, nanotube, and onion.

The spherical shape shows their importance because of the low surface energy. The onion-shaped nanoparticle has spherical shapes with a laminar structure inside. Suppose the shape of the onion is stable. In that case, it behaves like a spherical otherwise, it acts as a sheet form of nanoparticles. The most significant advantage of the onion shape is that it has tangled bonds and a spherical shape.

The superior tribological properties are exhibited by the onion, leaf, and spherical particles. The spherical-shaped particles show high load-bearing capacity as well as EP properties due to ball-bearing effects. In spherical particles, the sliding motion turns into rolling motion which eventually reduces the friction [88], [130]. The spherical-shaped NPs also reduce the contact area with a point contact. A line and area contacts are found on nanotubes and nanoplates, respectively.

In [131], There is evidence where MnO₂ nanoparticles improve oil performance by first rolling action and then converted into a protective film on the surface. On the other example, the layered structure of MnO₂ nanoparticles can reduce the value of the friction coefficient by forming a tribo film on the surface [132], [133]. The 2D structure of MnO₂ is a potential nanoparticle as a friction modifier [134], [135].

Figure 6 shows the statics of the nanoparticle's morphology. Since spherical particles can reduce the value of friction and wear. Therefore, the concentration of analysis is more focused on spherical-shaped nanoparticles. Next to a spherical shape, granular, nanosheet, nanotube, and onion-shaped particles are arranged in the order of evaluation.

Figure 7 shows the effect of nanoparticle shape on the performance parameters. Onion-sized particles perform best when the minimum value of COF is the focal point of the performance parameters. And other particles come in the form of nanosheets, nanotubes, spherical and granular in order. Nanosheets perform best when the maximum percentage reduction in COF is a matter of concentration. Then come onions, nanotubes, spherical and granular, respectively. Onion performs best when it comes to maximum percent wear reduction followed by nanotube, granular, nanosheet, and spherical ordering.

The cumulative study of performance parameters for a broad section, such as chemical characterization of the shape of nanoparticles, does not provide factual information about individual nanoparticles. Any application may require more details on the nanoparticle. Single performance parameter explains less about nanoparticles. So, three performance parameters for different nanoparticles were considered in this review, which provides a clear picture of the performance of nanoparticles in lubricating oil. Here author represents a new way of showing the performance comparison of nanoparticles. Single paper results have been taken to compare the performance of nanoparticles in the case of more than one paper on the same nanoparticles.

Figure 8 shows the performance parameters compared to the various nanoparticles listed above. Some nanoparticles do not have red or green streaks, which means that the author does not include this part in his study.

This analysis reveals that fullerene nanoparticles achieved a minimum value of coefficient 0.006 with a reduction in COF of 76% [3]. The minimum value of COF in Cu is 0.01, and the maximum reduction in COF and wear is 60% and 20%, respectively [22]. In hBN, the minimum COF achieved is 0.0144, with the maximum reduction in COF being 64% and wear reduction being 72% [99]. TiO₂ can achieve a minimum value of 0.015 (COF), reducing COF and wear to 86.49% and 92.4%, respectively [70]. Fullerene, Cu, BN, and TiO₂ perform very well compared to other nanoparticles. In these nanoparticles, the value of COF is desirably low, and the decrease in COF and wear is high, which is a favorable condition for any nanoparticles.

5. Future scope

At the time of analysis, it is observed that the performance parameters change by 10%–15% upon small (in mg) changes of the nanoparticles. The performance of the lube oil in two consecutive additions of nanoparticles is unknown. Therefore, there is a need to develop a method to characterize the performance parameter of nano lubricants between two successive additions.

Various researchers have well covered the composition of nanoparticles. It has shown their performance well under multiple conditions and materials. The compatibility of nanoparticles with surface material remains untouched so far. This is a critical research gap for future research in the field of nano lubrication.

Medium-sized segments (31–60 nm) of nanoparticles may be part of future studies because this section can improve the characteristics of the oil. The authors use a fixed number to represent the size, which means all the nanoparticles' dimensions. Researchers can also focus on the size range. As is known, small-sized nanoparticles can efficiently provide rectification effects, and large-sized nanoparticles can easily show rolling effects. So, the combination of both can improve the quality of nano lubricants, which will be the subject of future studies.

Studies about multiple-shaped comparisons are lagging. It may perform unfavorably, but its analysis should be needed in the future. Take an example of a nanotube and a spherical-shaped particle. Spherical shapes of nanoparticles can help reduce COF and improve nanotube wear.

6. Conclusions

The analysis shows that the literature concerning the potential of nanoparticles to improve lubrication performance is still active. Evidence is available where the nanoparticles' chemical composition, size, and shape are essential in lubricating oil performance. Nanoparticles having multiple chemical compositions, sizes and shapes have been evaluated under different environmental conditions with various base oils. And there is no doubt that nanoparticles have led to extraordinary improvements in the efficiency of lubricants. It can be said with confidence that many nanoparticles have also been evaluated for their effectiveness. Still, now a significant concentration is on environmentally friendly lubricants. Also, the areas mentioned in the future scope are subjects of attention.

Some more conclusions from this review of the literature are listed here.

The performance of sulfide nanoparticles is awe-inspiring in terms of performance characteristics.
Basic information about nanoparticle size, shape, base oil, and surfactant or dispenser provides a clear picture of nano lubricants.
Performance of Size <30 nm The performance of nanoparticles is astonishing.
Nanoparticles such as spherical and globular play exceptionally well in lubricating oil performance.

Data availability statement

The data that support the findings of this study are available upon reasonable request from the authors.

Influence of nano particles on the performance parameters of lube oil – a review

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1. Introduction