Tribological evaluation of Al₂O₃/GO/ZnO tripartite hybrid based nanofluid for grinding Ti-6Al-4V alloy with minimum quantity lubrication

Abstract

Machining-induced damages encountered during the grinding of titanium alloys are a major setback for processing different components from these materials. Recent studies have shown that nanofluid (NF)-based minimum quantity lubrication (MQL) systems improved the machining lubrication and the titanium alloys’ machinability. In this work, the tribological characteristics of a palm oil-based tripartite hybrid NF (ZnO/Al₂O₃/Graphene Oxide, GO) are studied. The novel usage of the developed lubricants in MQL systems was examined during the grinding of Ti6-Al-4V (TC4) alloy. The NF was produced by mixing three weight percent mixtures (i.e., 0.1, 0.5, and 1 wt.%) of the nanoparticles in palm oil. A comprehensive tribological and physical investigation was conducted on different percentage compositions of the developed NF to determine the optimum mix ratio of the lubricant. The findings indicate that increasing the NF concentration caused an increment in the dynamic viscosity and frictional coefficient of the NFs. The tripartite hybrid NF exhibited superior tribological and physicochemical properties compared with the pure palm and monotype-based NFs. Moreover, the dynamic viscosity of the tripartite-hybrid-based NFs increased by 12%, 5%, and 11.5% for the Al₂O₃, GO, and ZnO hybrid NFs, respectively. In addition, the machining results indicate that the tripartite hybrid NF lowered the surface roughness, specific grinding, grinding force ratio, tangential, and normal grinding forces by 42%, 40%, 16.5%, 41.5%, and 30%, respectively. Hence, the tripartite hybrid NFs remarkably enhanced the tribology and machining performance of the eco-friendly lubricant.

Graphical abstract

Keywords

hybrid nanolubricant / tribology / grinding / surface quality / Ti-6Al-4V / minimum quantity lubrication

1 Introduction

Machining processes are characterized by generation of an immense amount of heat around the contact zone. The traditional coolants prominent in the manufacturing industry have adverse environmental effects, produce greenhouse gasses, and are characterized by poor machining performances [1]. Moreover, during machining with traditional lubricants, the thermally induced surface and sub surface damages on the workpiece material increase [2]. The immense thermal energy generated during machining is the main cause of poor surface quality of the work material, which also reduces the life span of cutting tools. To decrease the adverse effects of greenhouse gases, campaigns toward the reduction of the amount of lubricants used in machining processes have been intense. Scientists have explained that a proper lubrication system that can effectively remove the heat from the contact zones can help decrease these unwanted machining problems. Hence, the utilization of the environmentally friendly cutting oils and bio-enhanced nanolubricants has been promoted due to their enhanced tribological behaviors and eco-friendliness [3].

Ti-6Al-4V alloy (also referred to TC4) is a choice material for various advanced engineering applications due to its light weight, superior strength/toughness, high heat resistance, and corrosion resistance. TC4 is typically used in biomedical applications, maritime industries, and aerodynamic applications [4]. Similarly, TC4 alloys are classified under difficult-to-cut materials due to their low elastic modulus and heat conduction. Traditional lubricants often used during machining of TC4 alloys are expensive and cause several environmental and health hazards [5]. Hence, an alternate lubrication system for machining these materials must be found [6,7].

Minimum quality lubrication (MQL) involves spraying atomized oil droplets at high ejection pressure into the machining region. Studies have shown that the MQL system is a prime candidate for achieving this eco-friendliness in the machining of TC4 alloys [8‒10]. Many researchers have reported that the use of MQL systems in machining these types of alloy materials can obtain improved surface quality, lower machining forces, and enhanced tool lifespan [11‒13]. Several scientific reports have indicated that nanoenhanced lubricants possess higher thermal conductivity and excellent heat removal properties. Recently, the utilization of different nanoparticles (NPs) to produce bioenhanced lubricants for use in machining processes has increased. Previous research showed that water- or oil-based nano enhanced lubricants can effectively evacuate the heat generated in the contact zone [14].

Researchers have revealed that nanoenhanced biolubricants formed using multiple NPs exhibited superior tribological and thermophysical characteristics [15‒18]. Hybrid nanoenhanced lubricants helped improve the machining performance in different processes such as turning [19,20], grinding [21‒25], and milling [17,25,26]. Moreover, studies have indicated that hybrid nanofluids (NFs) increased the heat transfer characteristics in nanoenhanced lubricants [27]. Kalita et al. [28] reported that a hybrid NF containing MoS₂/Al₂O₃ when used in the MQL system produced better tribological performance compared with traditional lubrication techniques. Many researchers have used different combinations of NPs to form hybrid NFs. The composition and mixture solely depended on the properties of each NP and the corresponding physical characteristics that needed to be compensated for by each NP in the developed hybrid mixture.

Hybrid-based NFs have been studied extensively by many researchers. Examples include SWCNT/ZnO [29], Al₂O₃/ZnO [30], SiO₂/MWCNT [31], Al₂O₃/MWCNT [9]), Al₂O₃/SiO₂ [32], graphene/MWCNT [33], and Al₂O₃/GNP [34]. The findings from these works have indicated that hybrid NFs have superior thermophysical and tribological properties compared with the monotype/single NFs. Furthermore, despite the low heat conduction capacity of metal oxide-based NPs, they are favored for the producing NFs due to their low densities, higher oxidation resistances, and enhanced stability. Moreover, ZnO NPs is often selected for NF production due to its availability and low cost compared with other materials [35]. Lee et al. [36] and Mao et al. [37] explained the superb lubrication performance of oil- and water-based Al₂O₃ NFs in grinding operations, respectively. However, other researchers have shown that the lubrication performance of Al₂O₃ NF can be improved considerably by adding other types of oxide NPs to the fluid. Notably, NPs such as CuO, SiO₂, MWCNT, MoS₂, SiC, and graphene oxide (GO) have been used to improve the thermophysical and tribological properties of Al₂O₃-based NFs [23,32,38,39]. The thermal conductivity and lubricity of the hybrid NFs can be increased with a higher volume fraction of the NPs [40,41].

Mixed or hybrid NFs are now the main perspective for numerous researchers because they are currently thoroughly studied as potential alternatives to the traditional lubrication systems due to their superior antifriction capacity. Zhang et al. [42] explored the outcome of utilizing hybrid NFs in MQL systems for grinding Ni-based alloys. The evaluation was performed in terms of grinding forces, grinding force ratio, and surface roughness. The MoS₂/CNT mixed NPs achieved superior lubrication performance compared with the monotype NPs. Similar results were reported by authors [24,43]. Additionally, Hernández Battez al. [44] studied the rheological performance of hybrid NFs (i.e., MWCNT and TiO₂ NPs) manufactured with SAE50 oil. Compared with the pure base oil, the value of viscosity was lower in hybrid NFs at NF concentrations of less than 0.5 wt.%. However, the value of viscosity was higher when the NF concentration was raised from 0.5 to 1 wt.%.

Li [45] studied the tribological behavior of hybrid NPs (i.e., CeO₂ and TiO₂) with a combination ratio of 1:3. The concentration of 0.6 wt.% produced the best tribological performance. Moreover, Chu et al. [46] reported a substantial increase in thermal conductivity and dispersion capacity in MWCNT and Al₂O₃ water-based NF. Li et al. [47] conducted rheological analysis on different NF concentrations of hybrid lubricant (i.e., Acculube Lube2000 with graphene nanoplatelets/Al₂O₃). The hybrid NF could reduce the specific grinding energy by 91.78% and 80.25% compared with commercial lubricants and dry grinding, respectively.

Despite numerous investigations on the lubrication performances of hybrid NFs, work exploring the effect of tripartite-based hybrid NFs remain limited. Studies have shown that the thermophysical performance of multiple NPs mixed in a fluid can be considerably enhanced compared with those of the monotype-based NFs [16,17,48‒50]. The use of multiple NPs to produce the NF can compensate for deficient mechanical, physical, and tribological characteristics in the NF by the different NPs. Hence, an optimum NF combination and concentration for the tripartite-hybrid-based NF lubricants needs to be obtained. Previous works have indicated that the hybrid-based NF can be combined at a mixing ratio of 30:70 in the case of Al₂O₃-based NPs . Moreover, several other mix ratios have produced optimum lubrication performance. Apart from the mix composition, the NF’s tribological characteristics depend on the shape, hardness, size, absorptiveness, hydrophilicity, and chemical interaction of each NP.

This work presents the production and novel uses of tripartite hybrid nanolubricant, which has been developed using GO, Al₂O₃, and ZnO NPs mixed in palm oil. The combination of these NPs was developed based on different explanations provided in previous kinds of literature. The NF was developed according to an initial hypothesis that explains how the individual NPs complement one another’s tribological characteristics and enhance the tribology of the resultant NF. Similarly, studies have shown that these three NPs often employ the creation of slithering NF-based tribofilms for effective lubrication [51,52]. Preliminary rheological tests were performed on the manufactured lubricants to ascertain their tribological behaviors. Additionally, MQL-based grinding experiments were conducted to investigate the lubrication performance of the developed NFs. The grinding experiment was conducted on a TC4 workpiece material using a cubic boron nitride (CBN) grinding wheel. The lubrication performance from the grinding operations was evaluated based on the grinding forces, grinding force ratio, specific grinding energy, and surface quality. A graphical summary illustrating the structure of this work is presented in Fig.1.

Fig.1 Structure of the experimental study.

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Fig.2 illustrates the flow and structure of this paper. This section explains the foundation laid by previous researchers on eco-friendly lubrication systems. Previous studies have established the mass exodus from using traditional lubrication systems in machining to using eco-friendly lubricants. Similarly, the recent utilization of NFs and MQL systems as lubricants is gaining more popularity due to their improved machining efficiency. Furthermore, with the advent of using multiple NPs in manufacturing NFs, previous works have shown that the hybrid NFs outperformed the monotype NF as lubricants in machining systems. This section also presents a brief overview of the recent investigations conducted on the grinding of Ti-6Al-4V alloy. Tab.1 discusses the variables and results from different lubrication and grinding techniques utilized in machining the alloy material. Section 2 presents the material and methods employed and characterizes the NPs material and work material used. Section 2 also discusses the process involved in manufacturing the NFs, with extensive tribological investigations on the fluids. Section 3 details the grinding experiments conducted using the selected hybrid NFs. Section 4 reveals the results and notable observations from the experimental study. Section 5 presents a concise conclusion and an overall summary of the findings.

Tab.1 Literature review

S/N	Reference	System	Lubrication	NF concentration	Machining settings					Machining responses analyzed
					vs	vw/(mm·min−1)	ae/µm	fu/kHz		ft/N	fn/N	Ra/µm	U/(J·mm−3)	µ	Grinding temperature/°C	Vb/µm	Σ/MPa
1	Singh H. et al. [53]	CG	MQL/canola oil	GNPs (1.5 wt.%)	22 m/min	3000	10	‒		3.567	14.094	0.250	‒	‒	‒	‒	‒
2	Mukhopadhyay and Kundu [54]	CG	RQL/blasocut water	‒	14 m/min	‒	10	‒		26.92	58.53	0.64	̶̵	0.6512	‒	‒	‒
3	Guo and Yen [55]	CG	Flood	‒	26.4 m/s	9	40	‒		54	82	̶̵	60	0.66	‒	‒	316.22
4	Rajeshkumar et al. [56]	CG	UMQL/sunflower oil	5 vol.%	20 m/s	6	10	‒		26	70	0.5	‒	‒	‒	‒	‒
5	Sadeghi et al. [57]	CG	MQL	‒	15 m/s	20	7	‒		34	38	025	‒	‒	‒	‒	‒
6	Kacalak et al. [58]	CG	Flood	‒	30 m/s	25	20	‒		70	94	0.46	16	0.665	‒	‒	‒
7	Abbas et al. [59]	Turning	MQL/ester oil	Al2O3 (4 vol.%)	32.1 m/s	111.6	25	‒		‒	‒	0.515	‒	‒	‒	1.65	‒
8	Zhang et al. [60]	CG	Cryo-NMQL/ester oil	0.35%	4.02 m/min	1.12	15	‒		52.37	̶̵	̶̵	62.84	̶̵	183.9	‒	‒
9	Rajeshkumar et al. [56]	UAG	MQL/sunflower oil	1‒10 vol.%	20 m/s	6	10	20		27	70	0.48	‒	‒	‒	‒	‒
10	Kashyap et al. [27]	CG	MQL	‒	22 m/s	3000	10	‒		5.5	̶̵	0.4	‒	‒	‒	‒	‒
11	Ibrahim et al. [61]	CG	MQL/palm oil	GNP (0.1%–0.4%)	1800 r/min	4	10	‒		10.24	50.3	0.278	24.12	0.21	‒	‒	‒
12	Singh et al. [6]	CG	MQL/canola, olive, sunflower oils	GNO (1.5%)	22 m/s	3000	10	‒		3.567	14.094	0.250	13	0.253	‒	‒	‒
13	Wang et al. [62]	CG	MQL/palm oil	Al2O3 (2 wt.%)	30 m/s	3000	20	‒		‒	‒	0.11	35.17	0.27	‒	‒	‒
14	Dambatta et al. [63]	CG	MQL/canola	SiO2 (0.2–6.5 wt.%)	32 m/s	1000	5	‒		23	68	0.16	‒	‒	‒	‒	‒
15	Seid Ahmed and González [64]	CG	MQL/palm oil	Graphene (0–0.1 wt.%)	30 m/s	1000	20	‒		89	115	0.58	18.23	0.228	‒	‒	‒
16	Zhang et al. [65]	CG	Cryo MQL	Al2O3 (0.35%)	15.2 m/s	4000	15	‒		52.37	‒	‒	62.84	‒	190	‒	‒
17	Lopes et al. [66]	CPG	MQL + CA	ITW Acculube 79053D	32 m/s	0.75	5	‒		‒	‒	0.45	‒	‒	‒	9	‒
18	Cui et al. [8]	CG	Cryo MQL	Synthesized lipid	24 m/s	4000	10	‒		46.8	61.4	0.468	‒	‒	151.2	̶̵	‒
19	de Moraes et al. [67]	CPG	MQL + CA	ITW Acculube 79053D	32 m/s	0.5	5	‒		11	‒	0.60	‒	‒	‒	7	‒
20	Zhang et al. [60]	CG	Cryo-NMQL	Synthesized lipid	24 m/s	4000	10	‒		‒	‒	0.375	51.96	0.6	155.9	̶̵	‒
21	Mukhopadhyay and Kundu [54]	CG	SQL/MQL	Propylene glycol, sodium nitrite, soap water, blasocut	30.17 m/s	1400	15	‒		23	55	1.5	38	0.6	‒	‒	‒
22	Liu et al. [68]	CG	Cryo MQL	Plant-based F 30A	30 m/s	4000	30	‒		48.6	51	0.42	42.66	0.32	232.1	‒	‒
23	Dambatta et al. [69]	UAG	MQL/canola, corn, sunflower	SiO2	31.42 m/s	1000	10	20		29.4	65	0.456	55.43	0.4519	‒	‒	‒

Fig.2 Article structure.

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2 Materials and methods

In this section, the characterization of the NPs utilized for developing the NF is explained thoroughly. Similarly, the steps followed to develop the NFs/lubricants and the tribological/physical tests conducted on each of the lubricant are discussed succinctly. The two-step mixing technique was used to produce the NFs, and tribo-physical investigations were conducted through frictional and tribological tests.

2.1 Preparation of nanolubricants

The monotype- and tripartite-based NF samples were produced using different percentage weight concentrations of Al₂O₃, GO, and ZnO in a vegetable oil lubricant (Tab.2 shows the physical properties). The NF concentrations considered included 0.1%, 0.5%, and 1%. Tab.2 and Tab.3 present the process variables and the combination of variables considered to produce the NFs. Moreover, the average size of each NP was between 20 and 40 nm. Similarly, the dispersion behavior of the lubricants on the workpiece surface was analyzed by measuring the contact angle of their droplets. This dispersibility behavior affects the thermal evacuation and lubricity of the lubricants. The contact angle was measured based on Young’s contact angle formula, which provides the relation between the fluid, and surrounding air molecules [70]. The steps taken to produce the NF involved the two-step NF production technique, and an in-depth analysis of dynamic viscosity, friction behavior, and wettability of each lubricant followed. A schematic illustration of the preparation method and tribological tests conducted on the lubricants is shown in Fig.3.

Tab.2 Characteristics of NPs [71,72]

S/N	Properties	Unit	Al2O3	GO	ZnO
1	Specific heat	J/(kg∙K)	880	710	501
2	Thermal expansion coefficient	10−6/°C	8.1	−67	6.5
3	Melting point	°C	2055	3600	2248
4	Density	kg/m3	3600	1800	5606
5	Thermal conductivity at 330 K	W/(m∙K)	36	18	50
6	Hardness	Mohs	9	‒	5
7	Compressive strength	MPa	62.4	76	67
8	Young’s modulus	GPa	215	230	140
9	Fracture toughness	MPa∙m1/2	3.5	6.7	2.44
10	Color	‒	White	Black	White

Tab.3 Process parameters

S/N	Mode	NF concentration/wt.%	Composition
1	Monotype NP	0	100% Al2O3
2	Al2O3 based	0.1	100% ZnO
3	GO based	0.5	100% GO
4	ZnO based	1.0	50%:10%:40% (Al2O3/ZnO/GO)
5	‒	‒	50%:40%:10% (Al2O3/ZnO/GO)
6	‒	‒	50%:10%:40% (GO/Al2O3/ZnO)
7	‒	‒	50%:40%:10% (GO/Al2O3/ZnO)
8	‒	‒	50%:10%:40% (ZnOAl2O3/GO)
9	‒	‒	50%:40%:10% (ZnO/Al2O3/GO)

Fig.3 Illustration of the NF preparation and tribological investigations (coefficient of friction, dynamic viscosity, and contact angle ψ). 1. Computer; 2. Connectors; 3. Microscopic lens; 4. Fluid droplet; 5. Moveable lever.

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Palm-oil-based NFs were prepared by suspending the NPs in palm oil based on the total weight percentage of base oil, surfactant, and NP. The process variables and experimental run settings for manufacturing the NF are provided in Tab.3 and Tab.4, respectively. The formula for weight percentage used in this work is shown in Eq. (1):

Tab.4 Settings for experimental runs

Sample No.		NF concentration/wt.%	Composition
1	–	0.1	Al2O3
2	–	0.1	GO
3	–	0.1	ZnO
4	–	0.5	Al2O3
5	–	0.5	GO
6	–	0.5	ZnO
7	–	1.0	Al2O3
8	–	1.0	GO
9	–	1.0	ZnO
10	Al2O3 based	0.1	50% Al2O3 + 40% ZnO + 10% GO
11		0.1	50% Al2O3 + 40% GO + 10% ZnO
12		0.5	50% Al2O3 + 40% ZnO + 10% GO
13		0.5	50% Al2O3 + 40% GO + 10% ZnO
14		1.0	50% Al2O3 + 40% ZnO + 10% GO
15		1.0	50% Al2O3 + 40% GO + 10% ZnO
16	GO based	0.1	50% GO + 40% ZnO + 10% Al2O3
17		0.1	50% GO + 40% Al2O3 + 10% ZnO
18		0.5	50% GO + 40% ZnO + 10% Al2O3
19		0.5	50% GO + 40% Al2O3 + 10% ZnO
20		1.0	50% GO + 40% ZnO + 10% Al2O3
21		1.0	50% GO + 40% Al2O3 + 10% ZnO
22	ZnO based	0.1	50% ZnO + 40% GO + 10% Al2O3
23		0.1	50% ZnO + 40% Al2O3 + 10% GO
24		0.5	50% ZnO + 40% GO + 10% Al2O3
25		0.5	50% ZnO + 40% Al2O3 + 10% GO
26		1.0	50% ZnO + 40% GO + 10% Al2O3
27		1.0	50% ZnO + 40% Al2O3 + 10% GO
28	–	Pure palm MQL	–

(1)$w_{\text{nf}}=\frac{w_{\text{np}}+w_{\text{surf}}}{w_{\text{np}}+w_{\text{surf}}+w_{\text{oil}}},$

where w_nf is the total weight percent of NF, w_surf is the total weight percent of surfactant and anti-oxidant, w_np is the total weight percent of individual NP, w_oil is the total weight percent of base oil (i.e., palm oil), and w_surf = 0.3w_np.

The resultant NF was then mixed using a magnetic stirrer for one hour to obtain a homogeneous mixture. Then, the produced NF was further mixed in an ultrasonic bath at a frequency of 40 kHz for another hour at room temperature to enhance dispersibility and prevent agglomeration. Previous studies have highlighted the importance of ultrasonication times on the stability and dispersibility of the NFs. Selecting an appropriate mixing period based on the properties of the individual NPs involved is vital because excessive sonication could lead to substantial physical and chemical deformations of the NPs [73–75]. Finally, a fully mixed homogenous NF was obtained. The NF samples were then kept at room temperature to observe the overall sedimentation of the fluid visibly.

2.2 Tribology testing of nanolubricants

The tribological experiments were conducted at atmospheric pressure at an approximate altitude of 67 m above sea level (Huangdao, Qingdao, China). The coefficient of friction (CoF) of each lubricant sample was obtained using a Brukers’ universal mechanical tester (UMT) at the speed and load of 2 kHz and 20 N, respectively. The frictional tests were conducted on the UMT tribometer according to the standards of the American Society for Testing and Materials (ASTM) [52,76]. The tests were performed using a TC4 workpiece material of size 30 mm × 30 mm × 10 mm. The TC4 workpiece was polished to a mirror surface-finish (Ra = 10 nm) using a Technipol PRO VS rotary polisher. The TC4 flat sample moved in reciprocating form with a sliding speed of 0.1 m/s and stroke of 6 mm for 20 min. A G5 precision-grade zirconia ball with a diameter of 9.5 mm was kept fixed and subjected to a normal load of 20 N. Before conducting each frictional test, the test samples were thoroughly eviscerated using acetone to remove any dirt. To ensure improved accuracy of the results, the experiment for frictional coefficients was repeated thrice, and the average values were recorded.

2.3 Viscosity

The dynamic viscosity of the pure palm oil and palm-oil-based NFs was measured using a Brookfield viscometer at shear rates of 50 s⁻¹. The dynamic viscosity of each fluid was determined using a rotating UL adapter. The experiment for the dynamic viscosity in each fluid was performed thrice, and the average readings were recorded. The dynamic viscosity was measured at temperatures of 30, 50, 75, and 90 °C. Pure palm oil exhibited the lowest dynamic viscosity among all the measured lubricants across different temperatures. Similarly, the hybrid NFs had higher dynamic viscosities than the monotype ones. The dynamic viscosity of the NF improved substantially as the NF concentration increased. In the tripartite-based NF samples, the ZnO and Al₂O₃ NPs had a higher effect on the dynamic viscosity than the GO NPs. However, at higher measurement temperatures, the dynamic viscosities across all the NFs were similar. Consequently, the overall values of the dynamic in all the NFs reduced drastically as the temperature increased.

2.3.1 Theoretical model for viscosity of hybrid NFs

Many theoretical models have been developed for calculating the viscosity of fluids. The earliest were presented based on the volume concentration of particles and the fluid. Hence, the volume concentration of the NF (Ψ_nf) in Eq. (2) can be obtained by modifying Eq. (1).

(2)${\mathrm{\Psi }}_{\text{nf}}=\frac{{\displaystyle \frac{w_{\text{np}}}{{\rho }_{\text{NPs}}}}+{\displaystyle \frac{w_{\text{surf}}}{{\rho }_{\text{surf}}}}}{{\displaystyle \frac{w_{\text{NPs}}}{{\rho }_{\text{NPs}}}}+{\displaystyle \frac{w_{\text{surf}}}{{\rho }_{\text{surf}}}}+{\displaystyle \frac{w_{\text{oil}}}{{\rho }_{\text{oil}}}}}\times 100\mathrm{\%}.$

Einstein [77] presented the viscosity formula of a fluid according to the particles present in the fluid. The model deliberated on “small rigid spheres suspended in a liquid”. Several studies have been done to compute the dynamic viscosity of NFs through physical, thermodynamic, and nonrheological variables. The models developed often neglected the effect of different rheological variables on the viscosity of the fluid. Further, the Einstein theoretical model for viscosity calculation (Eq. (3) [77]) was only accurate for fluids with concentration below 0.0245 vol.%. Likewise, Einstein’s equation did not account for the behavior of the hybrid or multiple particles present inside the base fluid. Over the years, many modifications have been made to the viscosity equation due to the discovery of NPs of different shapes.

(3)$\frac{{\mu }_{\text{nf}}}{{\mu }_{\text{bf}}}=1+2.5\mathrm{\Psi },$

where ${\mu }_{\text{bf}}$ is base fluid’s viscosity, and $\mathrm{\Psi }$ < 0.0245 is the volumetric fraction.

Batchelor [78] modified Einstein’s equation to consider the Brownian motion and entropy within the fluid and arrived at Eq. (4):

(4)$\frac{{\mu }_{\text{nf}}}{{\mu }_{\text{bf}}}=1+2.5\mathrm{\Psi }+6.5{\mathrm{\Psi }}^2.$

Furthermore, upon the discovery of NPs, Masoumi et al. [79] modified the theoretical models. The Masoumi model is shown in the following equation [30,79]:

(5)$\frac{{\mu }_{\text{nf}}}{{\mu }_{\text{bf}}}=1+2.5\mathrm{\Psi }+\frac{{\rho }_{\text{p}}+V_{\text{B}}+d_{\text{p}}^2}{72\cdot \delta \cdot C_{\text{f}}},$

where V_B is the Brownian velocity of particles, ρ_p is the density of NP, d_p is the diameter of NPs, C_f is the correlation factor, and δ is the average distance between the NPs.

Moreover, the factors V_B, C_f, and δ may be simplified into Eqs. (6)–(8), respectively.

(6)$V_{\text{B}}={\left(\frac{18RT}{\text{π}{N}_{\text{A}}{\rho }_{\text{p}}{d}_{\text{p}}^3}\right)}^{1/2},$

(7)$C_{\text{f}}=\frac{{10}^{-10}\cdot T\cdot {\mathrm{\Psi }}^{-0.002T-0.284}}{{\mu }_{\text{bf}}},$

(8)$\delta ={\left(\frac{\text{π}{d}_{\text{p}}^3}{6\mathrm{\Psi }}\right)}^{1/3},$

where R is the gas constant, t is the temperature, and N_A is the Avogadro’s number [79].

Udawattha et al. [80] presented the recent improvements to the Masoumi equation considering the weight of multiple NPs present in the fluid. Equation (9) indicates the theoretical model of the viscosity of hybrid NFs presented by Udawattha et al. [80].

(9)$\frac{{\mu }_{\text{nf}}}{{\mu }_{\text{bf}}}=1+2.5\sum _{i=1}^{n_{\text{NPs}}}{\mathrm{\Psi }}_{\text{E}i}+\sum _{i=1}^{n_{\text{NPs}}}{\mu }_{\text{dy}i},$

where n_NPs is the number of NPs and Ψ_Ei is the volume concentration of NPs.

Therefore, the viscosity of the NF depends on the density of the NPs and the distance between the particles in the fluid [30]:

(10)$\sum _{i=1}^n{\mu }_{\text{dy}i}=\frac{{\rho }_{\text{p}i}+V_{\text{B}i}+d_{\text{p}i}^2}{72\cdot {\delta }_i\cdot C_{\text{f}}}.$

The improved Udawattha model encompasses the parameters neglected by the earlier models; hence, it is regarded to be more precise. The improved Udawattha model considers the density of individual NPs and their distance apart. Likewise, recent studies have shown that the utilization of statistical and intelligent-based techniques built on existing theoretical models can be used to predict the viscosity of NFs with greater accuracy [30,81]. However, in recent peculiar cases with multiple NPs (hybrid) present in a mixture of two or more base fluids [54], the density component of Eq. (2) then includes the individual density of each fluid present.

Theoretical models of calculating the dynamic viscosity of an NF often relate to the mixing enthalpy and activation energy of the fluid mixture. Moreover, few research works have focused on the effect of the shapes of multiple NPs on the dynamic viscosity of the NF [82]. Hence, using the improved Einstein’s equation by Kondratiev and Jak [83], Eq. (3) can be modified into Eq. (12). Likewise, the viscosity of the NF can be obtained by considering the concentration of the NF, by further modifying Eq. (12) [83] into Eq. (13).

(11)$\frac{{\mu }_{\text{nf}}}{{\mu }_{\text{bf}}}=(1-\overline{R}\mathrm{\Psi }{)}^{-n},\mathrm{\Psi }<0.0245,$

(12)$\mu ={\mu }_{\text{bf}}(1-\alpha \mathrm{\Psi }{)}^{-n},$

where $\overline{R}$ and n are predetermined variables. $\overline{R}$ is 1.35 for similar-shaped NPs and 2.5 for dissimilar-shaped NPs. n is 1.0 for particles with the same shape/size and 2.5 for dissimilar-shaped particles. α = transpose of the coefficient of solid phases [84].

2.4 Wetness ability

The wetness ability of a lubricant is an important property that affects the overall grinding performance of MQL [22]. Hence, the wetness ability of each lubricant must be determined. The wetness ability, often referred to as wettability, can be determined by analyzing the contact angle ψ of the lubricant droplet on the surface of the titanium alloy [52,85].

Previous researchers have indicated a direct relation between the surface tension and the contact angles of a lubricant. Li et al. [86] explained the mathematical relationship between the contact angle and the surface tensions on a fluid droplet where they related the contact angle with the surface tension as cosψ = (τ_ac − τ_ab)/τ_bc. A higher surface tension led to an increase in the contact angle and the area covered by the droplet [87]. Fig.4 illustrates the method used to measure the contact angles of each lubricant sample using a dropping of the fluid on the surface of the TC4 alloy. The contact angle of each lubricant sample was measured, and the image of the oil droplet was obtained according to ASTM D7334-08 standards [88]. The contact angles of each lubricant sample was measured using a 2009JC120 Powereach goniometer manufactured by Shanghai Zhongchen Digital Technology Apparatus Company. The average contact angle was obtained for each lubricant sample after three measurements, and the work material was cleaned thoroughly using acetone before each experiment.

Fig.4 Illustration of measurements of surface tension and contact angle of a fluid droplet.

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2.5 Characterization of materials

The composition of each NP was obtained using a Hitachi S-3400N scanning electron microscope (SEM). Before the SEM analysis, the surface of each sample was prepared with Au particles in a DC magnetic sputtering machine at 10 Pa and 10 mA DC for 60 s. Fig.5(a)–Fig.5(c) show the SEM images with the elemental compositions of the Al₂O₃, GO, and ZnO NPs, respectively. The spectrum analysis shows the existence of the identified elements through their atomic weights. The ZnO and Al₂O₃ NPs were round and spherical, whereas the GO were flat and flake-like.

Fig.5 SEM and EDX compositions of NPs: (a) Al2O3, (b) GO, and (c) ZnO.

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3 Grinding experimentation

This section fully explains the grinding experiments conducted on the TC4 alloy using the optimal NF concentration. The experimental design was developed to provide a basis for comparison between the tripartite-hybrid-based NF and the other lubricants.

The machining was performed on a model K-P36 grinding machine using a CBN grinding wheel. The dimension of the CBN grinding wheel was 300 mm × 20 mm × 76.2 mm. Additionally, the ejection pressure and fluid flow rate of the MQL system were set as 0.35 MPa and 100 mL/h, respectively. To enhance fluid delivery, the settings for nozzle distance from the contact zone and its angle of inclination were selected based on optimized values of authors [6,42,54,89]. The grinding experiments were then performed to assess the overall performance of the palm-oil-based NFs. The experimental runs were repeated thrice, and the average values of each response was recorded. Furthermore, the grinding wheel was fully dressed before each experimental run. The grinding operations were done on a 40 mm × 30 mm × 30 mm TC4 alloy material (i.e., TC4 with composition of Ti-89.5%, Al-6.12%, V-3.74%, Fe-0.3%, and others-0.34%). The physical and mechanical properties of the titanium alloy are presented in Tab.5.

Tab.5 Physical properties of the TC4 alloy material

S/N	Physical property	Value	Unit
1	Thermal conductivity	7.955	W/(m·K)
2	Specific heat	526.3	J/(Kg·K)
3	Density	4.42	kg/m3
4	Modulus of Elasticity	114	GPa
5	Poisson’s ratio	0.342	‒
6	Shearing strength	700	MPa
7	Hardness	35	GPa (Rockwell)
8	Yield strength	880	MPa

The grinding operations were conducted using the MQL system with pure and hybrid palm-oil-based NFs with the machine settings presented in Tab.6. Furthermore, the grinding forces (i.e., tangential and normal) were obtained with the help of a YDM-III99 dynamometer. The setup of the grinding machine and MQL system are shown in Fig.6. The magnitude of multiaxial grinding forces generated during each grinding pass was recorded from the dynamometer at a sampling frequency of 1 kHz over 3401 cycles. A Zishu USB3214 multifunctional voltage data acquisition card, which was connected to a digital/analog analyzer (DAQ-Explorer), was then used to analyze the output from the YDM-III99 dynamometer.

Tab.6 Parameter settings of grinding operation

S/N	Parameter	Symbol	Value	Unit
1	Grinding depth	ae	10	µm
2	Wheel speed	vs	31	m/s
3	Table feed	vw	1000	mm/min
4	MQL nozzle angle of inclination	α	22	°
5	MQL ejection distance	d	20	mm
6	Droplets flow rate	Qm	80	mL/h
7	Ejection pressure	P	0.35	MPa

Fig.6 Grinding machine and MQL set-up. 1. MQL system; 2. MQL fluid tank; 3. CNC control panel; 4. Magnetic table; 5. Dynamometer; 6. TC4 workpiece; 7. CBN grinding wheel; 8. Hydraulic control; 9. MQL nozzle.

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Moreover, the surface quality of the workpiece material was measured quantitatively using surface roughness, whereas qualitative analysis was conducted using SEM. The ground sample material was properly cleaned in an alcohol bath to remove grinding debris and dirt. Then, the surface quality, roughness (surface roughness Ra and another roughness component Rz), and texture of the ground samples were acquired using a 20 × magnification in an Olympus laser microscope (OLS5000). Furthermore, to optimize the energy consumption during machining, the performance of each lubricant sample was assessed using the specific grinding energy and CoF obtained from Eqs. (14) and (15), respectively.

(13)$U=\frac{v_{\text{s}}{f}_{\text{t}}}{a_{\text{e}}{v}_{\text{w}}b},$

(14)$\mu =\frac{f_{\text{t}}}{f_{\text{n}}},$

where f_t is the tangential grinding force, f_n is the normal grinding force, v_w is the feed rate, v_s is the wheel speed, b is the wheel width, and a_e is the grinding depth.

4 Results and discussion

This section presents the results and discussion of the experiments in the preceding sections. First, the stability of the NFs was studied using the visual analysis technique. Similarly, the tribological performance of each lubricant was evaluated using the frictional tests and measurement of dynamic viscosity. Moreover, the wettability of the NFs was analyzed, and the best lubricant sample was acquired. Lastly, the grinding performance of the NFs was examined based on the grinding forces and final surface topography of the ground TC4 alloy.

4.1 Analysis of the nanolubricant performance

4.1.1 Visual observation samples

The NF samples were studied over regular time intervals, and their settling processes were carefully observed. The settling was visually assessed at regular intervals, and the image after each study period was captured. Previous researchers used the visual assessment process, which confirmed it to be a viable technique for assessing the stability of NFs [90,91]. In addition, the dispersive stability of the hybrid NFs was evaluated over some time. The fluids were placed unagitated at room temperature until the total time needed for complete agglomeration at the bottom was attained. The images in Fig.7 show the state of the manufactured NF studied for 36 days. The agglomeration of the NFs was noticeable within 14 days of ultrasonic mixing. Moreover, the NFs exhibited settling times. In the monotype NFs, the ZnO NF settled faster than the Al₂O₃ and GO-based NFs. Furthermore, the concentrations of 0.1 and 0.5 wt.% attained full sedimentation in 12 days, whereas the 1 wt.% attained full sedimentation after 21 days. In addition, Fig.7 shows that the GO and Al₂O₃ monotype NF at 1% concentration had the highest settling times. Moreover, the monotype Al₂O₃ and GO NF at 1% NF concentration exhibited enhanced tribological characteristics compared with the ZnO-based NF. However the tribological properties of the monotype NF were lower than those of the tripartite-hybrid-based NFs. Further, the 1% concentration tripartite hybrid NFs had a superior stability compared with the monotype ones. The results ascertained that the monotype ZnO NF settled faster than the Al₂O₃- and GO-based NFs. Additionally, the concentrations of 0.1 and 0.5 wt.% attained full sedimentation within 300 h, whereas the 1 wt.% concentration achieved sedimentation in about 21 d.

Fig.7 Images of monotype NF visual stability analysis.

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The tripartite hybrid NFs at 1% concentration had the longest settling times compared with pure palm oil, 0.1%, and 0.5% concentration NFs (Fig.8). The tripartite hybrid NF concentrations of 1 wt.% demonstrated superior dispersibility and stability. Moreover, as shown in Fig.8 for the tripartite hybrid based NFs, the samples with smaller percentages of GO NPs exhibited superior suspension capacity. This outcome could be attributed to the higher surface adsorption energy and the resultant effect of gravity on the GO NPs. For instance, sample No. 14 had only 10% of GO, which was lower than the 40% sample number 15, and the former exhibited faster sedimentation. Similar observations were made across all the samples produced using 40% or higher weight percentage of GO NPs.

Fig.8 Images of tripartite hybrid based NF visual stability analysis.

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In addition, the tripartite hybrid Al₂O₃-based NF had the longest settling time. In the case of the GO tripartite hybrid NPs, the NF contained a high amount of black deposits spread at the bottom of the mixture in the first five days of production. Additionally, the effect of the surfactant was more apparent in the 1% concentration tripartite hybrid based NFs due to the higher amount of the surfactant used. This increase in the amount of the surfactant and antioxidant supplement in the 1% concentration samples could explain its improved miscibility and stability.

Lastly, a notable observation was seen in the GO-based NFs of sample No. 21 after two months of producing the lubricants. The NF condensed due to thorough mixing and interaction of the GO and Al₂O₃ NPs, and formed a sludge-like and highly viscous liquid.

The performance of the nanolubricants manufactured through suspension of different weights of the NP in the based oil was further analyzed. The experimental results indicated a resultant increase in tribological performance by increasing the concentration of NP from 0% to 1%. Generally, the contact angles of NF were higher than those of pure oil. The Al₂O₃- and ZnO-based NFs presented similar tribological results, which were better than the GO NF. Similar results were observed for the other responses, namely, dynamic viscosity, wear, and CoF. Moreover, the tripartite hybrid NF (i.e., Al₂O₃/GO/ZnO) showed improved results compared with the monotype NFs. The highest values for each response parameter were recorded when the NF concentration was 1 wt.%. This outcome agreed with the findings of previous researchers who stated that the 1% NF concentration for hybrid/dual NPs produced optimal tribological behaviors [70,92,93]. The findings indicated that the addition of NPs to pure palm oil helped increase the spritz area of the manufactured fluid. Regarding the dynamic viscosity, the monotype GO NF illustrated enhanced thermophysical behavior compared with the Al₂O₃ and ZnO NFs. However, the tripartite hybrid NF exhibited the best thermal behavior, which also increased with higher NF concentrations. This outcome was evident from the results of dynamic viscosity obtained at elevated temperatures. However, the dynamic viscosities of the mono and tripartite hybrid NFs at the extreme temperature of 90 °C were similar for all NFs. Regarding the contact angle, the addition of NPs into the palm oil produced higher spreading and contact angle of the resultant NF samples. The NF with a higher proportion of alumina and ZnO NP presented the highest contact angle compared with the other tripartite hybrid and mono NF samples.

4.1.2 Coefficient of friction

The total amount of energy consumed during the grinding operations largely depends on the CoF existing between the various chaffing components, that is, the grinding wheel and the work material. The amount of energy expended in grinding can be reduced considerably by appropriate lubrication and grinding conditions. Nonetheless, the complex interaction in the contact region involves multiple force types, and the impetuous hammering actions of the wheel grits on the work material produce a tremendous amount of grinding-induced damage. Hence, obtaining an effective lubrication that can substantially decrease the frictional coefficient is essential. Tribological investigation on the UMT machine is one of the ways to determine the frictional coefficient of a lubricant by the continuous chafing actions [61,94]. The operation of the Bruckers UMT is equivalent to scaling down the grinding to the operation of a single grit. The frictional behavior of a lubricant can be studied on the UMT machine because its operation imitates continuous scratching, similar to what exists in the grinding process’s contact zone.

After the tribological analysis of each lubricant on the UMT machine, the final topography of the ZrO₂ ball and TC4 workpiece was analyzed using the Olympus OLS5000 laser microscope. Fig.9 shows that the surface quality of the ball and the work material differed and were highly dependent on the lubricity of the lubricant utilized. The surface texture of the rubbed surfaces from oil samples 5 and 20 were used to illustrate the effect of frictional coefficient on the surface quality of the components. The results showed a remarkable difference between the topography of ground parts using monotype and hybrid tripartite NF.

Fig.9 Topography of samples from frictional/tribological tests: (a) ZrO2 ball from sample 5, (b) ZrO2 ball from sample 20, (c) scratched TC4 from sample 5, and (d) scratched TC4 from sample 20.

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Fig.10 reveals the results of CoF for the different lubricants obtained by rubbing the ZrO₂ ball and the titanium alloy material. The highest CoF was recorded when pure palm oil was used as the lubricant. The average CoF values for the pure palm oil at 15 and 600 s were 0.425 and 0.326, respectively. However, the CoF was reduced considerably when NF concentration was increased (Fig.10). CoF drastically reduced between times 20 to 120 s. Beyond the 300 s mark, the magnitude of CoF obtained in each sample was not considerably different.

Fig.10 Influence of NF concentration on CoF: (a) 0.1%, (b) 0.5%, and (c) 1%.

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Fig.11 shows that the performance of each lubricant sample corresponded to the properties of the dominant NP in the fluid. Regarding the tripartite-hybrid-based NF, multiple NPs helped considerably improve the CoF, and the lowest and highest CoFs were obtained in the 1% and 0.1% NF concentration samples, respectively. The CoF can be increased by increasing the NF concentration. Although previous works have shown that the CoF decreases when the NF concentration was increased from 0% to 0.1% [61], our findings indicate a steady increase of the CoF as the NF concentration was increased from low to high concentrations (i.e., 0%, 0.1%, 0.5%, and 1%). Likewise, the 1% concentration of Al₂O₃-based tripartite hybrid NF (sample 14) exhibited the lowest CoF (0.226 µm) after 600 s of the analysis. A closely related result was obtained in the 1% concentration of ZnO-based tripartite hybrid NF (sample 27).

Fig.11 Influence of nanoparticle types on CoF: (a) Al2O3 based, (b) GO based, and (c) ZnO based.

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In addition, the results obtained from the tribological testing confirmed that the individual NPs exhibited different mechanical behavior when subjected to intense shearing stress. Nevertheless, the GO-based NFs delaminated after about 120 s of shearing action in the UMT machine. This outcome could be attributed to the frail structural bonding existing in the GO NPs. The delamination of the GO NP may seem like a setback in its tribological behavior, but it facilitated the lubricity of the GO-based NF during the rubbing operations. This finding corroborated the explanations of previous authors [70,95]. Moreover, the synergistic interaction between the three oxide-based NPs remarkably improved the dynamic viscosity of the tripartite hybrid NFs, especially at higher NF concentrations. Finally, the CoF of the tripartite hybrid based NFs was higher than those of the monotype NFs and pure palm oil.

4.1.3 Dynamic viscosity

The dynamic viscosity of a fluid measures its resistance to steady mechanical motion. Because the viscosity of NFs depends on several variables, the NFs can exhibit Newtonian and non-Newtonian characteristics. Hence, the results obtained in this work show that the tripartite hybrid based NF exhibited the characteristics of a Newtonian fluid as the temperature was increased from 30 to 90 °C. Fig.12 visually compares the different results for the dynamic viscosity of each lubricant sample at several temperatures based on the dominant NPs in the fluid. In each lubricant sample, an increase in the temperature caused a lower value of the dynamic viscosity (i.e., inverse relation). This phenomenon can be explained by the behavior of the fluid from the basic principles of physics. The rising temperature caused higher entropy and energy state of the individual atoms and thereby increased the spacing between the NP and the palm oil.

Fig.12 Influence of different NF types on dynamic viscosity: (a) Al2O3 based, (b) GO based, and (c) ZnO based.

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The results of dynamic viscosity showed that compared with the smaller NF concentrations, the 1% NF concentration presented a higher dynamic viscosity at elevated temperature of 90 °C. Additionally, critically analyzing the performances of the monotype NFs indicated that the Al₂O₃ based NF presented a superior thermophysical behavior compared with the GO and ZnO NPs. This outcome is evident from the higher values of dynamic viscosity measured in the monotype-based Al₂O₃ NFs. Furthermore, the best-performing samples in terms of dynamic viscosity from the experimental study were the tripartite hybrid NF from samples 14 and 27. Samples 14 and 27 exhibited similar thermodynamic responses. Fig.12(a) illustrates the result of the dynamic viscosity of the Al₂O₃-based lubricants at different temperatures. Dynamic viscosity was directly proportional to NF concentration but inversely related to temperature. Likewise, Fig.12(b) presents the result of the dynamic viscosity of the GO-based NFs. The tripartite hybrid NF from sample No. 23 with composition (50% GO:40% Al₂O₃:10% ZnO) produced the best results for the dynamic viscosity measured at different temperatures. Moreover, at high temperatures, the difference in magnitude of the dynamic viscosity between the 0.5% and 1% concentration NFs was not evident. Equally, in Fig.12(c), the tripartite hybrid NF of sample No. 27 with a mix-proportion of 50% ZnO + 40% Al₂O₃ + 10% GO presented the best results of dynamic viscosity among all the ZnO-based NF samples.

Fig.13 presents a graphical comparison of the dynamic viscosity based on the NF concentrations. The 1% NF concentration exhibited the highest magnitude of dynamic viscosity at high and low temperatures (i.e., 30 and 90 °C). This outcome could be attributed to an increase in the NP clusters, higher heat absorption by the multiple NPs, and increased mixing capacity found in the high-concentration NFs. Moreover, at 90 °C, no substantial difference in the dynamic viscosity measurements was observed between the low- and high-concentration NF samples. Lastly, the tripartite hybrid based NF from sample 25, which was composed of 50% ZnO, 40% Al₂O₃, and 10% GO at 0.5 wt.% NF concentration, had an unusually high magnitude of dynamic viscosity at 75 °C.

Fig.13 Influence of NF concentration on dynamic viscosity: (a) 0.1%, (b) 0.5%, and (c) 1%.

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4.1.4 Contact angle, ψ

The contact angle measurement of a lubrication fluid is necessary to determine the effect of the surface tension and wetness ability of the fluid for usage as a lubricant. The performance of the NF in an MQL system depends majorly on these properties for effective lubrication activity. In this work, the contact angle was measured using the edge inclination created by a droplet of each lubricant on a mirror-finished surface of the TC4 alloy.

Fig.14 shows the values of contact angles obtained in each NF sample based on the concentration of NPs in the fluid. The 1 wt.% concentration tripartite hybrid based NF sample exhibited the highest values of contact angle. The contact angle of the pure palm oil (α = 23.5°) increased by adding NPs into the oil. Furthermore, the results indicated that the tripartite hybrid Al₂O₃-based NF had the highest contact angle value compared with the other samples.

Fig.14 Influence of NF concentration on contact angle: (a) 0.1%, (b) 0.5%, and (c) 1%.

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Considering the contact angle values in the monotype and tripartite hybrid NFs, the Al₂O₃-based NFs had the highest contact angles compared with the other NF compositions. As shown in Fig.14(a)–Fig.14(c), the NF samples with compositions 50% Al₂O₃:40% ZnO:10% GO and 50% ZnO:40% Al₂O₃:10% GO produced similar results for the contact angle measurements at different NF concentrations. Palm oil with Al₂O₃ NF had better wettability than palm oil with GO and palm oil with ZnO NFs.

Previous researchers have explained that the Al₂O₃-based NPs can create an unbalanced variation in the interacting forces between the NP and fluid, thereby increasing the wettability of the fluid [96,97]. This finding is evident from the results of the Al₂O₃-based NFs, where a positive effect on the contact angles was induced. Similarly, the 1% NF concentration of the tripartite hybrid NF had the highest magnitude of contact angles (Fig.14(c)). The lowest contact angle was obtained in the pure oil and the GO NF samples. The effect of the NPs on the palm oil was not apparent in the low-concentration lubricants (0 to 0.5 wt.%). However, the measured contact angles drastically increased when the NF concentration was increased from 0.5% to 1%.

The 1% concentration tripartite hybrid NF had higher contact angles compared with the pure oil and monotype NFs. This enhancement can be explained by the higher surface tension caused by the interactions of the individual NPs with the mixed fluid. Moreover, the uneven rate of agglomeration of the individual NPs in the tripartite hybrid NF contributed to the improved spread/wetting capacity. Furthermore, explanations from previous works indicated that different types of NPs when mixed at higher NF concentrations led to an increase in the magnitude of the attractive Van der Waals forces within the fluid [49,98]. Equally, the increased inspissation realized in the tripartite hybrid NF sample No. 14 was due to the improved bonding rate of the individual NPs as a result of the suitable combination ratio.

Fig.15 compares the results for the measured contact angles in the pure palm oil and the NF. The result presentation was analyzed according to the dominant NPs present in the fluid. Finally, the best-performing NF samples in terms of contact angle measurements were the tripartite hybrid NF samples (14, 21, and 23).

Fig.15 Influence of different NF types on contact angle: (a) Al2O3 based, (b) GO based, and (c) ZnO based.

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4.2 Grinding performance of the lubricants

The grinding operations were conducted on a TC4 alloy workpiece material using the same machine setting for each lubricant type to provide a basis for comparison in terms of performance between all the lubricant types. Because the 1 wt.% NF was the optimal NF concentration, the 1 wt.% concentration samples were used to conduct the experiments (sample types in Tab.7). Likewise, the MQL variables and machine settings indicated in Tab.6 were used to perform the grinding operations on the TC4 alloy workpiece material. The experimental results from the grinding experiments are explained in this section.

Tab.7 Experimental settings for grinding tests

Experimental run	Parameter
	NF concentration/wt.%	NF
1	0	Pure palm oil
2	1	Al2O3
3	1	GO
4	1	ZnO
5	1	50% Al2O3 + 40% ZnO + 10% GO
6	1	50% Al2O3 + 40% GO + 10% ZnO
7	1	50% GO + 40% Al2O3 + 10% ZnO
8	1	50% GO + 40% ZnO + 10% Al2O3
9	1	50% ZnO + 40% Al2O3 + 10% GO
10	1	50% ZnO + 40% GO + 10% Al2O3

4.2.1 Tangential and normal grinding forces and force ratio

Grinding forces are often used to demonstrate the performance of lubricants in grinding. A smaller magnitude of the grinding forces is more desirable because it indicates a lower amount of energy expended to remove a unit volume of the workpiece material. Hence, in MQL-based experiments, a lower grinding force indicates a better lubrication capacity of the lubricant [3,46,99–102]. The two types of forces predominant during grinding operations are the forces along the x and y axes, referred to as the tangential and normal grinding forces, respectively. The axial force, which is along the z-axis, is negligible.

An overview of the results obtained from the grinding experiments of this work indicated that the different MQL fluids expended different magnitudes of the tangential (f_t) and normal (f_n) grinding forces. The Al₂O₃-based tripartite hybrid NF sample from experimental run 5 in Tab.7 exhibited the lowest amount of normal and tangential grinding forces. This result indicated a substantial decrease in the frictional forces and drag during the grinding operations. This result agreed with the results obtained in the preliminary tribological and thermophysical tests. Further, the tripartite hybrid based NFs generally had a lower grinding force compared with the pure palm oils in the MQL system. Additionally, the hierarchy of lubrication performance of the monotype NFs in descending order was Al₂O₃ > ZnO > GO.

Comparisons among the tripartite hybrid NFs indicated that the Al₂O₃-based lubricants produced lower values of grinding forces. The optimum NF sample, which was composed of 1 wt.% (50% Al₂O₃ + 40% ZnO + 10% GO), reduced the f_t and f_n by 41.5% and 30%, respectively, compared with the pure palm oil-based lubricant. The rates of reduction effected by each tripartite hybrid lubricant sample on f_t and f_n compared with the pure oil are illustrated in Fig.16 and Fig.17, respectively. The poor performance of the pure palm oil-based MQL system was due to the susceptibility of the molecules within the pure oil to be dislodged easily when under intense pressure. The tripartite hybrid based NFs can withstand the high pressure exerted by the grinding wheels. Moreover, the tripartite hybrid NFs that consisted of a higher amount of Al₂O₃ NPs exhibited lower grinding forces compared with the ZnO- and GO-based ones, which indicated a superior tribological performance of the Al₂O₃-based NF.

Fig.16 Tangential grinding force ft.

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Fig.17 Normal grinding force fn.

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Regarding the energy consumed during the grinding operations, a similar trend of the result obtained in the tangential grinding forces was observed. The specific grinding energy is a function of the grinding power, meaning it is the amount of energy required to remove a unit material from the workpiece by a grinding wheel during the grinding operations. Most of the energy generated during the grinding operations is often lost in the form of heat, so achieving a sustainable machining involves reducing the specific grinding energy to the barest minimum. Hence, any lubricant that can obtain the lowest values of U exhibits the optimum lubrication capacity [12,103,104]. The NF samples from experimental runs 5 and 9 exhibited the lowest specific grinding energy. This result also indicates the superior lubricity of the tripartite hybrid NF produced using a higher amount of Al₂O₃ and ZnO NPs. Moreover, the monotype-based NF of these NPs presented a slightly higher amount of energy expended during the grinding operations. This result indicated the effectiveness of the hybrid NPs in improving the tribological and thermophysical performance of the lubricant.

Fig.18 shows the values of the specific grinding energy obtained from the grinding operations. The tripartite hybrid NFs generally decreased the magnitude of the specific grinding energy compared with pure palm oil. The hybrid NF, which consisted of 10% GO NP (experiment runs 5 and 9), expended the lowest amount of specific grinding energy. The results obtained showed that the tripartite hybrid NF if properly mixed can achieve about 40% reduction in the specific grinding energy compared with that when pure palm oil is used.

Fig.18 Specific grinding energy U.

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The grinding force ratio (frictional coefficient) is obtained by taking the ratio of f_t and f_n obtained from the grinding operation. It is one of the machining responses often used to quantify the lubricity of a lubricant in a material removal process [99,105,106]. The force ratio is also a measure of the frictional force between the wheel and the workpiece material. Any lubricant that exhibits a higher value of the force ratio is deemed to have poor lubricity. Therefore, a smaller value of the force ratio is more desired in any grinding operation. The results in Fig.19 illustrate that the lowest grinding force ratio was achieved by sample 5, whereas the highest grinding force ratio was obtained by sample 1. Pure palm oil when used alone did not offer much reduction to the friction between the workpiece and the wheel. Moreover, the tripartite hybrid NF considerably reduced the grinding force ratio by 16.5%.

Fig.19 Grinding force ratio µ.

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4.2.2 Surface quality

The final surface quality of a machined component is a key component used to analyze the effectiveness of a machining system. The surface quality can be assessed qualitatively using SEM/visual analysis and quantitatively through surface roughness measurements.

To understand the source of the spikes on a workpiece material, during grinding operations, the grits on the grinding wheel were embedded into the workpiece material. Similarly, the grits on the grinding wheel were not uniformly distributed, and a larger part of the wheel was occupied by the softer bond material. In addition to factors such as chatter, friction, and excessive heat, this caused waviness and a nonuniform surface texture on the ground surface. The friction and continuous chopping of the work material by the grits encompassed the whole material removal and thereby created furrows and surface defects. Furthermore, grinding created a plastic deformation layer along the furrows of the grinding path and formed a rough top layer.

The performance of the grinding operation can then be evaluated quantitatively by measuring the surface roughness or qualitatively by visual microscopic analysis. Ra refers to the average magnitude of the miniature-sized ridges and spikes on the machined surface. Ra is measured by obtaining the arithmetic mean deviation of the surface of the ground component. Further, another roughness Rz can be obtained by comparing the average distance between the maximum peak and lowest valley point on the surface of the workpiece material. Hence, in this section, the surface quality performance of lubricants using the Ra and Rz values of the machined sections is analyzed.

In this work, the average roughness (Ra and Rz) of the machined workpiece were obtained from roughness measurements taken along a defined plane in the grinding direction. The magnitude of the surface roughness was used to analyze roughness profiles of the ground TC4 material. The results of the surface roughness measurements for Ra and Rz in each experiment are given in Fig.20(a) and Fig.20(b), respectively. Furthermore, the surface morphology of the ground workpiece sections was analyzed qualitatively using the OLS-5000 laser microscope.

Fig.20 Surface roughness measurements: (a) Ra and (b) Rz.

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Fig.21 presents the microscopic results of the surface morphology in each of the ground samples. The tripartite hybrid NFs when used in MQL systems produced much superior surface quality compared with pure palm oil. The inferior lubrication capacity of the pure oil inflicted intense grinding damages such as grooves, wide ridges, pores, and burns on the workpiece material, as shown in the image of experimental run 1 (Fig.21). Moreover, the different NF compositions exerted dissimilar capacities of lubrication on the grinding because the final texture of the ground components differed substantially from one another. In the monotype-based NFs, the Al₂O₃-based NFs presented a better surface finish with lower surface roughness values compared with the GO- and ZnO-based NFs. The hierarchy of surface quality performance in the monotype-based NFs in ascending order was GO < ZnO < Al₂O₃.

Fig.21 Surface quality analysis with the 3D textural view of the ten machined parts. Bottom right number represents the experimental run number.

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Additionally, a proper combination of the NF mixture in the tripartite hybrid NFs can substantially influence the surface quality of the machined parts. Likewise, an excess amount of the GO NPs in the NF caused lower lubrication performance of the MQL system. However, the surface quality of the samples machined using the Al₂O₃-based tripartite hybrid NF in MQL system had the least occurrence of surface deformations, and the lowest values of Ra and Rz.

The samples from experiments 5 and 9 produced the smoothest surfaces with the lowest surface roughness values of 0.169 and 0.212 µm, respectively. These results can be attributed to the layer of enhanced load-bearing capability of the tribofilm formed by these NFs. The formation of tribofilms by the vegetable-oil-based NFs has been explained by many researchers to be the reason for excessive reduction of chatter, improved heat removal capacity, and low friction in MQL machining. Finally, the lubrication performance of the tripartite hybrid NFs in terms of surface quality responses in ascending order was GO-based NFs < ZnO-based NFs < Al₂O₃-based NFs. Hence, based on the findings, the composition of the tripartite hybrid NF should have the Al₂O₃ as the dominant NP.

Fig.22 presents the surface textures of the ground TC4-alloy workpiece material using different lubricants in the MQL system. The images were obtained using the SEM machine. As a result of the hardness of the CBN grits, many grinding-induced damages were noticeable on the machined surfaces. Surface damages such as wedges, micro/macro groves, fracturing/delamination, debris redeposition, and microwelds were apparent on the analyzed workpieces. For instance, the sample from experimental run 1, which was ground using pure palm oil in the MQL system, suffered severe delamination due to discontinuous motion existing in the grinding region. This result confirms the improper, ineffective lubrication and intense thermally-induced damage by the lubrication system.

Fig.22 SEM analysis of ground TC4-alloy workpiece using different lubricants in the MQL system.

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Likewise, macrowelds were observed on the samples ground with pure palm oil. Additionally, the use of tripartite hybrid NF as the MQL lubricant considerably improved the surface quality of the ground workpiece. Due to their different compositions, each lubricant sample offered a distinctive surface outcome on the ground workpiece material. The tripartite hybrid NF remarkably improved the surface quality of the ground compared with those machined with monotype NF and pure palm oils. The samples ground with monotype-based NFs exhibited microplow marks, intense delamination, and exfoliations. However, a smoother surface was obtained in the workpieces ground with the ZnO- and Al₂O₃-based tripartite hybrid NFs. The superior surface quality of the components can be attributed to the tribological enhancement presented by the individual NPs in the tripartite hybrid NF. Equally, the GO-based tripartite hybrid NF produced the worst surface quality among all the developed tripartite hybrid NFs. The sample from experiment number 5 produced a smoother surface with minimum surface impairments compared with the remaining ground surfaces.

Fig.22 reveals the poor surface quality found in the samples from experiments 1, 3, 4, 7, 8, and 10 caused the high surface roughness in these experiments. This figure also indicates poor lubricity in the grinding operations. The Al₂O₃- and ZnO-based hybrid NFs produced a better surface finish. The improvement in the surface quality can be attributed to the higher wettability and lubricity of the tripartite hybrid NFs.

5 Conclusions

Recently, the utilization of infinitesimal amounts of lubricant through the MQL systems in the machining of advanced engineering materials has gained popularity due to the effectiveness and low cost offered by the MQL process. The system has been proven to be eco-friendly and efficient in grinding superhard engineering materials. Recent studies have shown that the use of NFs as the lubricant of the MQL system can further improve the overall machining performance of the lubrication system. The NF plays an important role in reducing friction and thermal loads around the machining/contact zone, and thereby improves machining efficiency and surface quality.

In this work, the novel uses of a tripartite hybrid NF manufactured using Al₂O₃, ZnO, and GO NPs are introduced. The hybrid NF was mixed in pure palm oil using a 5:4:1 mix ratio for the three NPs at different weight-percent concentrations of 0.1%, 0.5%, and 1%. Tribological and physical tests were conducted on each of the manufactured NF, and the results were compared with those of pure palm oil. The pure, monotype, and tripartite hybrid NFs were compared based on their dynamic viscosity at different temperatures, CoF, and wettability. Further, the optimum NF combination with superior tribological features was ascertained. Grinding tests were performed on a TC4 alloy using the lubricants with an MQL system. According to the results obtained, the following conclusions can be made:

1) Tripartite hybrid NF exhibited superior lubrication behavior compared with pure palm oil and monotype NFs.

2) Increasing the NP concentration from 0% to 1% caused a steady reduction in the friction between the ZrO₂ ball and TC4 alloy. Likewise, in the frictional tests, the wear on the ZrO₂ balls was much lower when the tripartite hybrid NFs were utilized. In the tribological test, the 1% NF concentration of the hybrid sample with a mix ratio of 50% Al₂O₃:40% ZnO:10% GO was the optimum composition because it exhibited the lowest frictional coefficient of 0.224.

3) The stability analysis showed that the Al₂O₃-based tripartite hybrid NF had the longest settling times among all the studied NFs.

4) In addition, the contact angles made by a droplet of each lubricant on the TC4 surface were used to investigate the wettability of the lubricants. The tripartite hybrid Al₂O₃ NF exhibited the highest contact angle compared with the pure oil, monotype NF, and the other tripartite NFs. The smallest and highest contact angles were measured in the pure oil sample and the hybrid sample with a mix ratio of 50% Al₂O₃:40% ZnO:10% GO. Similarly, the results obtained indicated that all the NFs had a higher contact angle than the pure oil. Hence, the hybrid NFs remarkably enhanced the wettability of palm.

5) In the Al₂O₃-based NFs, the 1% concentration tripartite hybrid NF produced using the NF composition of 50% Al₂O₃:40% ZnO:10% GO had the highest dynamic viscosity at different temperatures. Moreover, among the GO-based tripartite hybrid NF, the 1% concentration sample with a mix ratio of 50% GO:40% Al₂O₃:10% ZnO achieved the best results of dynamic viscosity. Similarly, regarding the ZnO-based NFs, the 1% concentration tripartite hybrid sample with NF mixture of 50% ZnO:40% Al₂O₃:10% GO obtained the highest results of dynamic viscosity. The Al₂O₃ NPs played an important role in improving the dynamic viscosity of the NFs because all the samples where it was dominant exhibited superior tribological behaviors. Finally, the dynamic viscosity of the tripartite-hybrid-based NFs increased by 12%, 5%, and 11.5% for the Al₂O₃-, GO-, and ZnO-based hybrid NFs compared with the pure oil samples, respectively.

6) The use of tripartite hybrid NF in the MQL system during grinding operations reduced the tangential and normal grinding forces by 41.5% and 30%, respectively, compared with the use of pure palm oil. This outcome indicated the enhanced lubrication capacity of the tripartite hybrid NFs. Similarly, compared with the pure palm oils, the tripartite hybrid based NF decreased the grinding force ratio and specific grinding energy by 16.5% and 40%, respectively.

7) In terms of surface quality, the Al₂O₃ monotype-based NFs produced a superior surface quality compared with the other monotype NFs and pure palm oil. The TC4 alloy workpiece machined using the tripartite hybrid based samples with mix ratios of 50% Al₂O₃:40% ZnO:10% GO and 50% ZnO:40% Al₂O₃:10% GO were the smoothest and exhibited the lowest surface roughness of 0.169 and 0.212 µm, respectively. The surface quality of the ground TC4 workpiece using the MQL system improved by 42%‒71% compared with the samples machined with pure palm oil as the lubricant.

Researchers have mostly focused on the monotype and dual-based hybrid NFs as lubricants for MQL systems. However, in this work, a novel tripartite-based hybrid NF manufactured using Al₂O₃, GO, and ZnO NPs in a 50%:40%:10% mix-ratio is presented. Increasing the NF concentration enhanced the tribological behaviors of the NFs. An optimum NF combination was obtained, and grinding experiments were conducted on the TC4 alloy with the MQL system. Our work extends from preparation, tribo-physical investigations, and novel application of the NFs in the grinding of TC4 alloys. The findings from our work can be useful for developing eco-friendly NFs with enhanced tribological performances, which are useful for machining advanced engineering materials with MQL systems. Lastly, further investigating the performance of the tripartite-based NFs in different compositions such as 70%:20%:10%, 60%:30%:10%, and 40%:30%:30% is recommended.

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Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Grant Nos. 52305477, 52375447, 52305474, and 52205481), the Major Special Projects of Aero-engine and Gas Turbine, China (Grant No. 2017-VII-0002-0095), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20210407), the Special Fund of Taishan Scholars Project, China (Grant No. tsqn202211179), the Youth Talent Promotion Project in Shandong, China (Grant No. SDAST2021qt12), Qingdao Postdoctoral Researchers Applied Research Project, China (Grant No. QDBSH20230102050), the Support plan for Outstanding Youth Innovation Team in Universities of Shandong Province, China (Grant No. 2023KJ114), and Qingdao Science and Technology Planning Park Cultivation Plan (Grant No. 23-1-5-yqpy-17-qy).

Conflict of Interest

Changhe LI is a member of the Editorial Board of Frontiers of Mechanical Engineering, who was excluded from the peer review and all editorial decisions related to the acceptance and publication of this article. Peer review was handled independently by the other editors to minimize bias.

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