CNT nanofluid for minimal quantity lubrication in Ni-based alloy grinding

International Journal of Machine Tools & Manufacture 99 (2015) 19–33

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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool

Experimental evaluation of the lubrication performance of MoS2/CNT nanofluid for minimal quantity lubrication in Ni-based alloy grinding Yanbin Zhang, Changhe Li n, Dongzhou Jia, Dongkun Zhang, Xiaowei Zhang School of Mechanical Engineering, Qingdao Technological University, 266033 Qingdao, China

art ic l e i nf o

a b s t r a c t

Article history: Received 3 June 2015 Received in revised form 27 August 2015 Accepted 1 September 2015 Available online 3 September 2015

A nanofluid minimum quantity lubrication with addition of one kind of nanoparticle has several limitations, such as grinding of difficult-to-cutting materials. Hybrid nanoparticles integrate the properties of two or more kinds of nanoparticles, thus having better lubrication and heat transfer performances than single nanoparticle additives. However, the use of hybrid nanoparticles in nanofluid minimum quantity lubrication grinding has not been reported. This study aims to determine whether hybrid nanoparticles have better lubrication performance than pure nanoparticle. A hybrid nanofluid consisting of MoS2 nanoparticles with good lubrication effect and CNTs with high heat conductivity coefficient is investigated. The effects of the hybrid nanofluid on grinding force, coefficient of friction, and workpiece surface quality for Ni-based alloy grinding are analyzed. Results show that the MoS2/CNT hybrid nanoparticles achieve better lubrication effect than single nanoparticles. The optimal MoS2/CNT mixing ratio and nanofluid concentration are 2:1 and 6 wt%, respectively. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Grinding Minimum quantity lubrication MoS2/CNT nanofluids Grinding force Coefficient of friction Surface roughness

1. Introduction In general, grinding is applied for cutting negative grain rake, thus consuming more energy per unit material than other machining forms [1]. Currently, minimum quantity lubrication (MQL) and nanoparticle jet MQL have reflected their considerable advantages over dry grinding and flood grinding [2]. As Malkin and Guo [3] reported, MQL grinding technology is environmentally friendly, in which, after mixing and atomizing, only a small quantity of lubricant and gas with certain pressure is jetted to the grinding zone as cooling lubrication. The cooling and chip removal effect is mainly achieved by high-pressure gas highlighted by Li et al. [4]. The flow of lubricants per unit wheel width of MQL and flood grinding is 30–100 mL/h and 60 L/h, respectively. However, according to Tawakoli et al. [5] the lubrication effect of MQL exceeds that of flood grinding, thereby decreasing the quantity of lubricants. Nanofluids of nanoparticle jet MQL grinding are atomized with a high-pressure gas and sent to the grinding zone in the form of jet flow. As Hadad et al. [6] reported, nanoparticles increase the heat exchange ability of a liquid in the grinding zone through a cooling effect. Based on the theory of heat transfer enhancement, the heat exchange ability of solid is better than that of liquid [7]. Barczak et al. [8] showed that the new nanoparticle jet n

Corresponding author. Fax: þ 86 532 85071286. E-mail addresses: [email protected] (Y. Zhang), [email protected] (C. Li), [email protected] (D. Jia), [email protected] (D. Zhang), [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.ijmachtools.2015.09.003 0890-6955/& 2015 Elsevier Ltd. All rights reserved.

MQL technology better resolves heat transfer in the grinding zone while enhancing the lubricating property in the zone. As previously reported [9], nanoparticles can significantly improve lubrication and heat transfer of nanofluids. Nanofluids are suspensions formed by lubricating oil and 1–100 nm nanoparticles. Currently, common nanoparticles mainly include metal nanoparticles (Cu, Ag), oxide nanoparticles (Al2O3, SiO2, CuO), MoS2 nanoparticle, as well as single-walled, double-walled, and multi-walled carbon nanotubes (CNTs). Each kind of nanoparticle has different molecular structural characteristics and chemical characteristics; hence, the corresponding nanofluids have different impacts on lubrication and heat transfer performances. Given their high molecular length–diameter ratio, CNTs have higher heat conductivity coefficient than other nanostructures. As Choi et al. [10] reported, the heat conductivity coefficient CNTs is about thousands of times higher than that of water [0.613 W/(m K)] and about tens of thousands times higher than that of engine oil [0.145 W/(m K)]. Therefore, adding CNTs into base oil would increase the heat conductivity coefficient of a hybrid grinding fluid and improve energy transfer, thus enhancing heat transfer performance of nanofluids. However, shen et al. [11] showed that CNTs could not significantly improve the lubricating performance of nanofluids because of high friction between molecules and high molecular strength. MoS2 nanoparticles are usually spherical, and many molecular MoS2 layers overlap into thin layers. The layers then curl and pile up, forming pomegranate corrugation structure, which endows MoS2 with certain friability, flexibility, and ductility [12]. Therefore, MoS2 nanoparticles can extend into thin physical

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Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

Nomenclature Ft Fn

μ

Ra Rz RSm

tangential grinding force normal grinding force coefficient of friction Arithmetic average height ten-point height mean spacing at mean line

films on friction surface under external shear force. Meanwhile, MoS2 nanoparticles have high surface activity and are easily adsorb onto the friction surface. Fallen physical MoS2 nanoparticle films during friction could be quickly supplemented and renewed during follow-up adsorption, thereby retaining the lubrication effect according to Hu et al. [13]. Nevertheless, these nanoparticles can significantly improve heat transfer performance of nanofluids because of their low heat conductivity coefficient. Kalita et al. [14] performed nanofluid plane grinding of cast iron and EN24 alloy steel by adding MoS2 nanoparticles into paroline and soybean oil. They confirmed the tribological properties of MoS2 nanoparticles through measurement and computation of grinding force, coefficient of friction, specific grinding energy, and G ratio. MoS2 film formation on grains of grinding wheel was observed through SEM. Based on content measurement of chemical elements on grains, they analyzed the lubrication mechanism of MoS2 nanoparticles to a certain extent. The authors [15] also conducted a friction and wear experiment based on grinding head for bonding abrasive particles. They compared the lubrication performance of dry grinding, soybean oil pouring lubrication, soybean oil MQL, and soybean oil-based MoS2/Al2O3 hybrid nanofluid MQL under 20 N pressure, and 200 and 300 mm/ s peripheral velocity. The coefficient of friction was calculated and the workpiece surface quality was observed by SEM to determine the lubrication performance of nanofluid MQL. They concluded that the MoS2/Al2O3 hybrid nanofluid MQL provides more ideal lubrication performance than the three other lubrication methods. Lee et al. [16] added diamond nanoparticles into liquid paraffin, which was used to prepare nanofluids for three lubrication methods, namely, dry grinding, liquid paraffin MQL, and nanofluid MQL. The experimental results demonstrated that nanofluid MQL has significantly lower grinding force and surface roughness value than the two other lubrication ways. They also discovered that smaller nanoparticle size has better surface processing quality. Shen et al. [11] prepared nanofluids from Al2O3 and diamond nanoparticles to grind cast irons. They compared these nanofluids using dry grinding, flood grinding, and MQL grinding to determine their performance in terms of grinding force, surface roughness, and workpiece burning. Their results showed a proportional relationship between G ratio and nanoparticle concentration. Mao et al. [17] implemented surface grinding to AISI52100 steel with an alumina grinding wheel (diameter: 200 mm; width: 12 mm) at 31.4 m/s peripheral velocity of grinding wheel, 10 μm cutting depth, and 0.05 m/s workpiece feed speed. They compared the lubrication performance of dry grinding, flood lubrication, MQL, and Al2O3 nanofluid MQL. Results showed that Al2O3 nanofluid MQL achieved lower grinding force and grinding temperature, as well as better workpiece surface quality, than the three other methods. Zhang et al. [18] applied nanoparticles in the cooling lubrication of grinding and theoretically analyzed the impact of cooling lubrication on the grinding surface in terms of energy ratio coefficient and specific grinding energy. They determined the nanoparticle type and volume concentration that exhibit satisfactory cooling effects and should be added into the grinding fluid.

CV coefficient of variation MQL minimum quantity lubrication CNTs carbon nano tube MoS2 molybdenum disulfide SEM scanning electron microscope SDS lauryl sodium sulfate Nanofluids fluid containing nanometer-sized particles

Ni-based alloys have high strength and oxidation corrosion resistance under 650–1000 °C. Ni-based alloys can be classified into Ni-based heat-resistant and corrosion-resistant alloys according to its main property. According to Dudzinski et al. [19], high-temperature GH4169 Ni-based alloy is widely applied in marine, energy, aerospace, and mechanical and electrical product manufacturing fields. To achieve high surface precision, Ni-based alloy workpieces are often subjected to grinding in the study of Rahim and Sasahara [20]. Given that Ni-based alloys have low heat conductivity coefficient and grinding produces more heat than cutting and milling, grinding of Ni-based alloy workpieces is accompanied by remarkable increase in surface temperature, which deteriorates the surface quality of the workpieces, thereby limiting the application of Ni-based alloys. Although the use of nanoparticles as additive can improve nanofluid lubrication and heat transfer, nanofluids with a single type of nanoparticles have either good lubrication or good heat transfer performance [21]. Consequently, nanofluid MQL using single nanoparticle fails to provide an ideal surface grinding quality of Ni-based alloys. The good lubrication and heat transfer performance of hybrid nanoparticle additives have attracted considerable attention. Integrating properties of two or more types of nanoparticles would result in better lubrication and heat transfer performance than single-nanoparticle additives. To date, hybrid nanoparticles have been applied as additives into lubricating oil to improve antifriction performance. Gu et al. [22] have studied the tribological properties of calcium carbonate, rare earth metals, copper, and iron nanoparticles, as well as their mixture, as lubricant additive successively in recent years. They discovered that adding certain amount of these lubricant additives could significantly improve antifriction performance and endow a self-repair effect. Wang [23] tested the antifriction performance of prepared A12O3–SiO2–MgO inorganic hybrid nanoparticles by using a friction tester and analyzed its characteristics. He found that the modified A12O3–SiO2–MgO hybrid nanoparticles could optimize the synergistic effect of different nanoparticles during friction and wear processes and produce physical and chemical adsorption films on the friction surface. Therefore, lubricating oil with hybrid nanoparticles has antifriction and self-repair effects. Tian et al. [24] added Cu, La2O3, and Ce2O3 nanoparticles into 500SN base oil and investigated the tribological properties of this lubricant additive by using a friction and wear testing machine. Results presented that 64 nm Cu nanoparticle matched the 25 nm La2O3–Ce2O3 nanoparticles well. Lubricating oil with 8% (mass fraction) of Cu/La2O3–Ce2O3 nanofluids (2:1) possesses better antifriction and extreme pressure (EP) properties than those with only Cu/La2O3 or Cu/Ce2O3 as additive. Li et al. [25] studied the tribological properties of CeO2/TiO2 nanoparticles and their composite as lubricant additives and obtained the best tribological properties at about 0.6% total mass fraction of CeO2/TiO2 nanoparticles (wt (CeO2):wt (TiO2)¼ 1:3), which may be attributed to the synergistic reaction mechanism of CeO2 and TiO2 nanoparticles. Previous studies showed that lubricating oil using hybrid nanoparticles as additive has better antifriction performances. However, the use of hybrid nanoparticles in nanofluid MQL

Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

cutting, specifically grinding of difficult-to-cut materials, has not been reported. Different kinds of nanoparticles have different heat transfer effects and lubrication performances in nanofluid MQL owing to their differences in molecular structures and shapes. Some nanoparticles may have good heat transfer performance but poor lubrication effect, or vice versa. Although MoS2 nanoparticles can improve the lubrication performance of grinding fluids of nanofluid MQL, the heat transfer effect of grinding fluids cannot be improved simultaneously because of the low heat conductivity coefficient of the nanoparticles. On the contrary, CNTs have high heat conductivity coefficient and can improve heat transfer effect of grinding fluids. Nevertheless, CNTs cannot improve the lubrication performance of grinding fluids. Therefore, hybrid MoS2 nanoparticles with good lubrication effect and CNTs with high heat conductivity coefficient were prepared in this study. The lubrication performance and workpiece surface quality of MoS2/CNTs nanofluid MQL for Ni-based alloy grinding were analyzed to determine whether the MoS2/CNTs hybrid nanoparticles have better lubrication performance than the MoS2 nanoparticles or CNTs. This paper aims to explore the use of hybrid nanofluids in minimum quantity lubrication grinding, and try to enhance heat transfer and lubricating property maximumly in this way. This paper solves the bottleneck of surface integrity insufficient and providing a new way in MQL grinding.

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surfactant weight. The nanofluid was prepared by dispersing the nanoparticles in synthetic lipids via a two-step method [27]. Sonication was performed for 1 h by using a numerical control ultrasonic oscillator (version KQ3200DB). During the experimental process, the MoS2/CNT hybrid nanoparticles were named as Mix(x:y), where x:y is the MoS2:CNT mass ratio in the hybrid nanoparticles. For example, Mix(1:2) means that the mass ratio of MoS2 and CNTs in the hybrid nanoparticles is 1:2. 2.3. Experimental scheme 2.3.1. Effect of MoS2/CNTs mixing ratio on lubrication performance of nanofluid MQL In the first group of experiments, the effect of MoS2/CNT mixing ratio on the lubrication performance of nanofluids formed by MoS2/CNTs hybrid nanoparticles and MQL base oil was discussed. Grinding forces, coefficient of friction, and workpiece surface quality were used as characterization parameters. Six different mixing ratios were designed: pure MoS2, pure CNTs, Mix(1:1), Mix (1:2), Mix(2:1), and Mix(1:3). The grinding fluids of nanofluids MQL was mixed with 4% mass fraction of above six groups of nanoparticles and synthetic lipids. Grinding force was measured by a 3D dynamometer and subsequently used to calculate the coefficient of friction. Workpiece surface roughness was measured by roughness tester. Table 4 shows the experimental plan.

2. Experimentation 2.1. Work and wheel material The experiments were conducted using a conventional white corundum abrasive grinding wheel (WA80H12V) on GH4169 Nibased alloy material on a K-P36 numerical control precision surface grinder. Tables 1, 2, and 3 present the classification, composition, and mechanical properties of the GH4169 Ni-based alloy, respectively. The size of the workpiece and the wheel was 40 mm  30 mm  30 mm and 300 mm  20 mm  76.2 mm, respectively. The kinematic conditions of the experiment are as follows: grinding speed, 30 m/s; table speed, 3 m/min; depth of cut, 10 μm; and number of passes, 15. Each experiment was repeated for five times, and images of the grinding force measurement were obtained. 2.2. Nanofluids MoS2, CNTs, and MoS2/CNT hybrid were used as additives to obtain nanofluids, with synthetic lipids as the base fluid. The average length of the CNTs was 10–30 nm with a mean diameter of 30 nm; and the mean size of the MoS2 nanoparticles was 30 nm. Lauryl sodium sulfate (SDS) with 1/10 weight of the nanoparticle weight was added as a surfactant to improve the stability of suspensions based on the study of O'connell et al. [26]. The studied nanofluids with different concentrations (mass fraction) are summarized in Tables 4 and 5, which show the type of nanoparticle, weight corresponding to mass concentration, and the Table 1 Symbols of GH4169 Ni-based alloy. Material GH4169 Ni-based alloy Symbols Inconel 718, NC19FeNb (France), UNS NO7718 (USA), W. Nr. 2.4668 (Germany) ASTM B637, ASTM B670, ASME SFA-5.14, ASME Case 2222-1 AMS 5596, AMS 5662, AMS 5663, AMS 5832

2.3.2. Effect of hybrid nanoparticle concentration on lubrication performance of nanofluid MQL The optimal MoS2/CNT mixing ratio was gained from the first group of experiments. The MoS2/CNT hybrid nanoparticles, which served as additive of MQL grinding fluids, changed the viscosity of nanofluids and the contact angle between droplet and workpiece, thus changing the lubrication performance of nanofluids. However, the concentration of MoS2/CNT hybrid nanoparticles in the nanofluids shall be limited within a certain range. Excessive concentration will make nanoparticles unstable and thereby cause agglomeration of nanoparticles, hence destroying nanofluid lubrication. In the second group of experiments, the effect of nanofluid concentration on lubrication performance was discussed by changing the mass fraction of MoS2/CNT hybrid nanoparticles in nanofluids. The optimal concentration was determined. According to the experiment conducted by Kalita [14], we first selected 2% mass fraction, thus the nanofluids were prepared with Mix(2:1) nanoparticles at 2%, 4%, 6%, 8%, and 10% mass fractions. Surface roughness of workpiece was measured by roughness tester. The contact angle between nanofluids and Ni-based alloy was measured by contact angle meter. Table 5 shows the experimental design. 2.4. Estimation of coefficient of friction In the grinding operation, grinding forces can be measured online using the piezoelectric dynamometer (YDM-III99) attached with multichannel charge amplifier and dynoware software. After each experiment, the coefficient of friction was calculated. Coefficient of friction in grinding is the ratio of tangential grinding force (Ft) and normal grinding force (Fn). The typical μ values in grinding lie between 0.2 and 0.7 was found by Rowe et al. [28]. The coefficient of friction was calculated using the following equation:

μ = Ft /Fn

(1)

The lubrication effect on grinding directly influences the surface quality of the ground workpiece, and coefficient of friction is a parameter that evaluates the lubrication effect on the grinding

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Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

Table 2 Composition of GH4169 Ni-based alloy. Element

C

Ni

Fe

Cu

Mn

Mo

Cr

Al

Co

Si

Component/%

0.08

50–55

Remainder

0.3

0.35

2.8–3.3

17–21

0.95

1.0

0.35

3. Results

area [29]. Smaller coefficient of friction corresponds to better lubrication effect.

The experiment was conducted using a K-P36 numerical control precision surface grinder. The main technological parameters are the following: principal axis power, 40 KW; highest rotating speed, 2000 r/min; driving motor power of workbench, 5 KW; grinding scope, 600 mm  300 mm; corundum wheel size, 300 mm  20 mm  76.2 mm; particle size, 80#; highest peripheral velocity, 50 m/s; nanofluid transfer device, Bluebe minimum quantity oil supply system. The experimental setup is shown in Fig. 3. The size of the workpiece and the wheel was 40 mm  30 mm  30 mm and 300 mm  20 mm  76.2 mm, respectively. The kinematic conditions of experiment are as follows: grinding speed, 30 m/s; table speed, 3 m/min; depth of cut, 10 μm; and number of passes, 15. Table 6 shows the uniform grinding parameters of the experiment. The measuring cell YDM-III99 3D dynamometer was used in each experiment to measure and record normal force, tangential force, and axial force. The measured sample frequency of grinding force was 1 kHz. The grinding force signal after sampling was guided by the Dynamic Grinding Force Test System software to filter and obtain the grinding force image document and grinding force data document finally. A total of 100 data points were selected from the stable zone of grinding force in each direction to evaluate the mean and obtain the corresponding average force. The calculated average value of grinding force was used in data processing to calculate the coefficient of friction in each grinding process, which needs five times of experiment for each group of the same grinding condition. According to each grinding force signal diagram, the grinding force and coefficient of friction can be calculated. Subsequently, the mean and variance (used to describe the dispersion degree of data) of grinding force and coefficient of friction in five experiments were calculated to compare the lubrication effect. The surface roughness value of workpiece was reserved under each working condition. Surface roughness of workpiece was measured by TIME3220 roughness tester. A total of 10 points on surface of every workpiece were selected to measure surface roughness. Roughness profile of each point was measured, including the Ra and RSm values. The mean surface roughness of these 10 points was calculated and used in the final evaluation of workpiece surface quality. Two surfaces with the same mean surface roughness have different surface qualities if every group of data deviates from the mean value to different extents. According to mathematical statistics [32], the coefficient of variation (C.V) was used as the standard to evaluate data scattering degree. The calculation formula of C.V is as follows:

2.5. Estimation of surface roughness Grinding is known as finish machining. Hence, workpiece surface quality is an important evaluation standard of grinding performance. Surface roughness is a key parameter of workpiece surface quality and it was used to represent surface quality in this study. Smaller surface roughness value represents higher surface smoothness. Surface roughness significantly affects the usability of machine parts [30]. Yang [31] established a mathematical model to analyze the weights of evaluation parameters (including Ra (arithmetic average height), Rz (ten-point height), RSm (mean spacing at mean line), and Rmr (bearing line length and bearing area curve)) of surface roughness. He discussed the weights of Ra, Rz, RSm, and Rmr and found that Ra contains most information about microcosmic irregularities; hence, Ra may represent surface features and usability of surface roughness. However, Ra could not reflect the spacing characteristics of roughness, but RSm could. Finally, this paper selected Ra as the main evaluation parameter of surface roughness and RSm as an auxiliary parameter. 2.5.1. Arithmetic average height (Ra) The arithmetic average height parameter, also known as the central line average, is the most universally used roughness parameter for general quality control. This parameter is defined as the average absolute deviation of the roughness irregularities from the mean line over one sampling length as shown in Fig. 1. The mathematical definition of the arithmetic average height parameter is defined as follows:

Ra =

1 l

∫0

l

y (x) dx

(2)

2.5.2. Mean spacing at mean line (RSm) This parameter is defined as the mean spacing between profile peaks at the mean line and is denoted as RSm. The profile peak is the highest point of the profile between upward and downward crossing the mean line. Fig. 2 shows how to measure the mean spacing at mean line parameter. This parameter can be calculated from the following equation:

RSm =

1 N

n

∑ Si

(3)

i=1

C . V = (SD/MN ) × 100%

where N is the number of profile peaks at the mean line.

(4)

where SD: standard deviation, MN: average value.

Table 3 Mechanical properties of GH4169 Ni-based alloy. Density (g/cm3)

Modulus of elasticity (GPa)

Thermal conductivity (W/m K)

Specific heat (J/Kg K)

Yield strength (MPa)

Tensile strength (MPa)

Poisson's ratio

8.24

199.9

14.7

435

550

965

0.3

Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

23

Table 4 Experimental design for the effect of MoS2/CNT mixing ratio on lubrication performance of nanofluid MQL. Experiment no. Particle type % Mass concentration

Nanoparticle weight in 100 ml base oil (g)

Weight of MoS2 in hybrid nanoparticles

Weight of CNTs in hybrid nanoparticles

Weight of SDS (g)

1–1 1–2 1–3 1–4 1–5 1–6

3.26 3.26 3.26 3.26 3.26 3.26

– 3.26 1.63 1.09 0.815 2.17

3.26 – 1.63 2.17 2.445 1.09

0.326 0.326 0.326 0.326 0.326 0.326

Pure CNTs Pure MoS2 Mix(1:1) Mix(1:2) Mix(1:3) Mix(2:1)

4% 4% 4% 4% 4% 4%

Table 5 Experimental design for the effect of hybrid nanoparticle concentration on lubrication performance of nanofluid MQL. Experiment no. Particle type % Mass concentration

Nanoparticle weight in 100 ml base oil (g)

Weight of MoS2 in hybrid nanoparticles (g)

Weight of CNTs in hybrid nanoparticles (g)

Weight of SDS (g)

2–1 2–2 2–3 2–4 2–5

1.63 3.26 4.89 6.52 8.15

1.09 2.17 3.26 4.35 5.43

0.54 1.09 1.63 2.17 2.72

0.163 0.326 0.489 0.652 0.815

Mix(2:1) Mix(2:1) Mix(2:1) Mix(2:1) Mix(2:1)

2% 4% 6% 8% 10%

which is 15.32% and 8.79% lower than that of pure MoS2 and pure CNTs, respectively. For Mix(1:1), the coefficient of friction is 0.2925, which is higher than that of Mix(2:1), but 10.17% and 3.23% lower than that of pure MoS2 and pure CNTs, respectively. The coefficient of friction for Mix(1:2) is 0.3125, which is lower than that of pure CNTs but higher than that of pure MoS2. Comparison of the results in Figs. 5 and 6 shows the following observations:

3.1. Effect of MoS2/CNT mixing ratio on lubrication performance of nanofluid MQL 3.1.1. Grinding force and coefficient of friction Fig. 4 shows a typical signal image of a grinding force measurement under six working conditions. Meanwhile, Fig. 5 shows the grinding force of different MoS2/CNT mixing ratios. The grinding forces of the six prepared nanofluids were compared. The Fn and Ft of pure CNT nanofluids are 109.05 N and 35.51 N, respectively. Among the six prepared nanofluids, pure CNT nanofluids show the largest grinding force and standard deviation. Pure MoS2 achieves the lowest grinding force of Fn ¼82.63 N and Ft ¼ 24.98 N, which is 24.23% and 29.65% lower than that of pure CNTs, respectively. Four hybrid nanofluids have lower grinding force than pure CNTs nanofluids, hence presenting certain advantages compared with pure nanofluids. Mix(2:1) achieves the lower grinding force of Fn ¼91.28 N and Ft ¼25.17 N, which is 16.29% and 29.12% lower than pure CNTs, respectively. Only the grinding force of Mix(2:1) is higher than that of the pure MoS2. Moreover, the standard deviation of Mix(2:1) achieves the lowest value. Fig. 6 presents the coefficient of friction of different MoS2/CNT mixing ratios. The values of the coefficient of friction (μ) of the six prepared nanofluids were compared. The μ values of pure MoS2 nanofluid and pure CNT nanofluid are μMoS2 ¼ 0.3023 and μCNTs ¼ 0.3256, respectively. Among the six prepared nanofluids, the pure CNT nanofluid shows highest coefficient of friction and standard deviation. The four hybrid nanofluids have lower coefficient of friction, thus presenting certain advantages over the pure nanofluids. Mix(2:1) achieves the lowest coefficient of friction (μ ¼0.2757), y

 Pure CNT nanofluids are inferior to pure MoS2 nanofluids with





regard to lubrication effect, which is caused by the differences in physical properties and shapes of MoS2 nanoparticles and CNTs. Coefficient of friction of the four hybrid nanofluids is smaller than pure nanofluids, hence indicating better lubrication effect of hybrid nanofluids. Such lubrication improvement is the result of “physical synergistic effect” of hybrid nanoparticles. Four hybrid nanofluids have different lubrication effects. Mix (2:1) achieves the lowest coefficient of friction. By comparing the coefficients of friction of Mix(1:1), Mix(1:2), and Mix(1:3), we determined that the lubrication effect of hybrid nanofluids decreases with the increase in CNT proportion. The coefficients of friction of Mix(1:2) and Mix(1:3) exceed that of pure MoS2 and are only lower than pure CNTs. This finding implies that the “physical synergistic effect” has the optimal mixing ratio, which is determined by the mechanism of the “physical synergistic effect.”

3.1.2. Surface roughness The workpiece for grinding used in milling pretreatment requires that the surface roughness value (Ra) be r2 mm. A typical signal image of the surface roughness measurement under six

yi

Mean Line

x

l

l

l

l

Fig. 1. Definition of the arithmetic average height (Ra).

l

24

Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

S1

S2

S3

Sn-1 Sn

S4 Mean Line

y

x

Fig. 2. Calculating the mean spacing at mean line (RSm).

working conditions and the signal image of the workpiece before grinding are shown in Fig.7. The workpiece surface roughness value (Ra) of MQL grinding using different nanofluids and the corresponding CV are shown in Fig. 8. The workpiece RSm of MQL grinding using different nanofluids and corresponding CV are shown in Fig. 9. In Fig. 8, the mean Ra values of MQL grinding using pure MoS2 nanofluids and pure CNT nanofluids are RaMoS2 ¼0.338 mm and RaCNTs ¼ 0.481 mm, respectively. The workpiece using pure CNT nanofluids in MQL grinding has higher Ra than the other five groups. A significant difference in CV is determined between these two lubrication mechanisms. The CV of Ra using pure MoS2 nanofluids is only 3.82  10  2, but that of pure CNT nanofluids reaches 7.44  10  2. This finding indicates that the workpiece using pure MoS2 nanofluids as grinding fluids has a higher precision workpiece surface. By contrast, the workpiece using pure CNT

Table 6 Grinding parameters. Grinding parameters

Value

Grinding pattern Wheel speed Vs (m/s) Feed speed Vw (mm/min) Cutting depth ap (μm) MQL flow rate (ml/h) MQL nozzle distance (mm) MQL nozzle angle (°) MQL gas pressure (bar)

Surface grinding 30 3000 10 50 12 15 6.0

nanofluids as grinding fluids has a lower precision workpiece surface. The entire workpiece surface precision is poor. The four prepared hybrid nanofluids are superior to the two pure nanofluids in MQL grinding because of their lower Ra

Wheel Nozzle

Workpiece

Dynamometer Workbench

MQL supply device

NC surface grinder

Wheel Nozzle Workpiece

vs vw

Dynamometer

Workbench

Computer

Data collection system

Grinding force measurement Fig. 3. Surface grinding setup.

Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

25

132.7

107.7

Normal force

94.1

74.6

Tangential force

41.5

Axial force

Grinding forces / N

Grinding forces / N

Grinding stage

55.5

16.9

8.4

-24.7

-21.7 0

55

109

164

0

218

54

118.3

106.8

84.3

73.9

50.2

16.2

161

214

146

195

40.1

8.3

-24.8

-17.8 0

51

103

154

205

0

49

(1–3) Mix(1:1)

98

(1–4) Mix(1:2)

124.2

113.2

88.4

81.0

Grinding forces / N

Grinding forces / N

107

(1–2) MoS2

Grinding forces / N

Grinding forces / N

(1–1) CNTs

42.7

16.9

48.7

16.5

-15.7

-18.9 0

59

119

178

237

0

(1–5) Mix(1:3)

58

117

175

233

(1–6) Mix(2:1)

Fig. 4. Grinding force measurement signal image of six working conditions.

(RaMix(1:1) ¼ 0.332 mm, RaMix(1:2) ¼0.337 mm, RaMix(1:3) ¼ 0.367 mm, and RaMix(2:1) ¼ 0.294 mm). RaMix(1:1), RaMix(1:2), RaMix(1:3), and RaMix(2:1) are 33.06%, 29.94%, 23.70%, and 38.88% lower than RaCNTs, respectively. RaMix(2:1) is the lowest of six nanofluids (0.294 mm). RaMix(2:1) is 13.1% and 38.9% lower than RaMoS2 and RaCNTs, respectively, and slightly lower than RaMix(1:1), RaMix(1:2), and RaMix(1:3). Moreover, CVMix(2:1) ¼3.05  10  2, which is slightly higher than CVMix(1:1) ¼3.62  10  2, but lower than the four other nanofluids. In Fig. 9, RSm of the workpiece using pure CNT nanofluids and its CV are 0.0695 mm and 12.27  10  2, respectively, which are higher than those under the working conditions. Mix(2:1) also has the lowest RSm (0.346 mm) and lowest CV (8.71  10  2). This finding indicates that the workpiece using pure CNT nanofluids produces a larger “furrow” on surface, resulting in poor workpiece surface quality. This finding proves that CNTs can easily agglomerate into irregular “tube clusters,” which may enwind abrasive grains to increase the diameter of the “furrow” when abrasive

grains are cutting the workpiece. Thus, using pure CNT nanoparticle as additive of MQL is significantly inferior to other lubrication mechanisms and does not meet the MQL grinding conditions of Ni-based alloy. 3.1.3. SEM micrographs The workpiece SEM micrographs of MQL grinding using different nanofluids are shown in Fig. 10. Based on the comparison results, we observed that:

 The workpiece using pure MoS2 nanofluids as grinding fluids of



MQL achieves better surface quality (lower Ra and higher precision). This finding proves that the physical properties and shapes of MoS2 and CNT nanoparticles could affect the lubrication effect of grinding fluids of MQL (Figs. 8, 10a, and 10b). Compared with pure nanofluids, hybrid nanofluids improve the workpiece surface quality significantly. This finding confirms that hybrid nanoparticles have the “physical synergistic effect,”

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Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

10 1.5 8

91 .28

82 .63

100

25 .17

31 .62

29 .84

40

29 .86

60

24 .98

80

35 .51

Grinding force (N)

120

89 .75

10 2.0 9

10 9.0 5

140

Normal force tangential force

20 0

CNTS

MoS2

Mix(1:1) Mix(1:2) Mix(1:3) Mix(2:1)

Fig. 5. Grinding force of different MoS2/CNTs mixing ratios.

0.45

coefficient of friction (μ)

0.40

0.3256 0.3023

0.35 0.30

0.2925

0.3125

0.3148 0.2757

0.25 0.20 0.15 0.10 0.05 0.00

CNTS

MoS2

Mix(1:1) Mix(1:2) Mix(1:3) Mix(2:1)

Fig. 6. Coefficient of friction of different MoS2/CNTs mixing ratios.



which reduces the coefficient of friction of the grinding zone (Figs. 8 and 10). Investigation of Ra, RSm, and the corresponding CV shows that Mix(2:1) achieves the lowest surface roughness value and the highest precision. In other words, Mix(2:1) exhibits the best workpiece surface quality. This finding proves that grinding fluids of hybrid nanofluid MQL have the best lubrication effect and workpiece surface quality when the mixing ratio of MoS2 and CNTs is 2:1 (Figs. 8, 9, and 10f).

3.1.4. Discussion 3.1.4.1. Lubrication mechanisms of pure MoS2 nanofluids and pure cnt nanofluids. The involvement of nanoparticles improves the lubrication performance of MQL significantly, which is the consequence of the “balling effect” and “filling effect” [33]. Different types of nanoparticles have different lubrication mechanisms because of its different physical properties and shapes, thus resulting in different lubrication effects. This finding could explain the different lubrication effects of pure MoS2 nanofluids and pure CNT nanofluids. The molecular structure and molecular layer of MoS2 nanoparticles are shown in Fig. 11. The lubrication mechanism of MoS2 nanoparticles is shown in Fig. 12. MoS2 nanoparticles are ellipsoidal. The layer structure of MoS2 is a hexagonal crystal system that combines Mo and S through a covalent bond. Each crystal

consists of many MoS2 molecules. Every MoS2 molecular layer is 0.626 nm thick and contains three atom layers, namely, the sulfur atom layer at the top and bottom and the molybdenum atom layer at the middle. Each molybdenum atom is surrounded by six sulfur atoms (at the top of triangular prisms). Only sulfur atoms are exposed on the surface of the MoS2 molecular layer. The bond between molybdenum atom and sulfur atom is short. However, the spacing between sulfur atoms is large. As such, the bond between two adjacent sulfur atom layers is weak. During the grinding process, a plane of low shearing force will be generated because of the strong Mo–S binding, but weak binding of sulfur atoms between molecular layers. This plane will be broken along the molecular layer upon shearing force between molecules, forming a glide plane. Such structure endows the MoS2 nanoparticle with a certain friability, flexibility, and ductility. When a grinding force exists, the MoS2 nanoparticle will extend into thin physical films in the grinding zone, reducing wear and coefficient of friction. By contrast, the edges of the crystal have a higher surface energy because bonding in slices containing the S–Mo–S groups is strong. Under high temperature and pressure at the grinding zone, the MoS2 nanoparticle can be easily adsorbed on the workpiece surface to react with oxygen to produce a surface film containing MoO3 (2MoS2 þ7O2-2MoO3 þ4SO2). The chemical film strongly combines with the workpiece material, protecting the workpiece from friction. However, MoS2 easily lost the lubrication effect under high temperature and high pressure was found by Qi et al. [34]. Fig. 13 shows the structure of CNTs. CNTs have a high modulus and strength because carbon atoms in CNTs adopt the sp2 hybridization, which has a higher proportion of S pathways than sp3 hybridization. When nanofluids have a small CNT content, nanoparticles exist in the grinding zone as “independent microtubule.” CNTs will not be ground into hard film under large loads because of its high strength and hardness. As such, CNTs could be used as “similar bearings” that reduce the coefficient of friction of the grinding zone and improve the lubrication effect of nanofluids. As the CNT content increases, CNTs easily agglomerate. This phenomenon is related to the strong van der Waals force between CNTs and the high length–diameter ratio [35]. CNTs form irregular “tube clusters”. Considering the high strength and high hardness of CNTs, the formed “tube clusters” not only inherit the properties of CNTs but also possess higher strength for the correlation of “micro tubes.” On one hand, “tube clusters” entering the grinding zone lead to a higher coefficient of friction between grinding wheel and workpiece compared with the “micro tubes”. Irregular “tube clusters” may enwind abrasive grains to increase the diameter of the “furrow” when abrasive grains are cutting the workpiece, which will increase the surface roughness value of the workpiece to a certain extent. On the other hand, “tube clusters” will break the lubricating oil film on the surface of the friction pair and increase the coefficient of friction. In particular, CNTs could enhance the heat transfer of nanofluids significantly and improve the lubrication effect of nanofluids to a certain extent. Nevertheless, limited by content, a more significant improvement of heat transfer performance is restricted by the weakening of the lubrication effect. MoS2 and CNT nanoparticles have different physical properties, different shapes, and different lubrication effects in grinding fluids of MQL. Pure MoS2 and pure CNT nanofluids have advantages and disadvantages in lubrication. MoS2 nanoparticles have loose and flexible structures and can be easily carbonized. CNT nanoparticles have high modulus and high strength and could serve as “similar bearings”, but can easily agglomerate to reduce the lubrication effect. In this study, we mixed MoS2 nanoparticles and CNTs to improve the lubrication effect of nanofluids by combining the advantages of the individual nanoparticles.

Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

27

Before ginding (Ra=1.92μm)

(1-1) CNTs (Ra=0.481μm)

( 1-2 ) MoS2 (Ra=0.338μm)

μm

(1-3) Mix(1:1) (Ra=0.322μm)

( 1-4) Mix(1:2) (Ra=0.337μm)

(1-5 ) Mix(1:3) (Ra=0.367μm)

( 1-6) Mix(2:1) (Ra=0.294μm)

Roughness curve (mm) Fig. 7. The typical surface roughness measurement signal image.

0.50

10

0.481

0.40

0.338

Ra (μm)

0.35 0.30

0.322

0.337

0.367 0.294

0.25 0.20 0.15 0.10 0.05 0.00

Coefficient of Variation (×10 )

0.45 8

7.44 6.33

6

5.42 3.82

4

3.05

3.62

2

0

CNTs

MoS

Mix(1:1) Mix(1:2) Mix(1:3) Mix(2:1)

CNTs

MoS

Mix(1:1) Mix(1:2) Mix(1:3) Mix(2:1)

Fig. 8. Workpiece surface roughness (Ra) and corresponding CV of MQL grinding using different nanofluids.

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Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

14

Rsm (μm)

0.05 0.0396

0.0375

0.0358

0.0378

0.0346

0.00

Coefficient of Variation (×10 )

0.0695

12.27

11.67

12 10

8.93

8.91

9.4

8.71

8 6 4 2 0

CNTs

MoS

Mix(1:1) Mix(1:2) Mix(1:3) Mix(2:1)

CNTs

MoS

Mix(1:1) Mix(1:2) Mix(1:3) Mix(2:1)

Fig. 9. Workpiece surface roughness (Ra) and corresponding CV of MQL grinding using different nanofluids.

3.1.4.2. Lubrication mechanism of MoS2/CNT hybrid nanoparticles. Based on the experimental results, the coefficients of friction of four hybrid nanofluids are lower than those of pure MoS2 and pure CNT nanofluids, reflecting their better lubrication effect because of the “physical synergistic effect”. Such “physical synergistic effect”

indicates that MoS2 and CNT nanoparticles develop their advantages in lubrication and prevent the disadvantages. In particular, the low shearing force plane of MoS2 molecular layers could “reduce friction” and the coefficient of friction of the grinding zone in hybrid nanoparticles. With high modulus, high strength, and

Fig. 10. Workpiece SEM micrographs of MQL grinding using different nanofluids.

Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

S Mo S

29

1.54 A 1.54 A

Single-walled carbon Nanotubes

Armchair (n,n)

Double-walled carbon Nanotubes

Zig-Zag (n,0)

3.08 A S Mo S Legend:

1.54 A 1.54 A

Sulfur atoms Molybdenum atoms

Fig. 11. Molecular structure and molecular layer of the MoS2 nanoparticle.

Abrasive

Slip plane

Molybdenum atoms layer Sulfur atoms layer

S-Mo Bonds

Workpiece Fig. 12. Lubrication mechanism of MoS2 nanoparticles.

one-dimensional tube shape, CNT nanoparticles could “resist friction”, which increases rigid lubrication “similar bearings” of hybrid nanoparticles in the grinding zone and prevents lubrication failure under high temperature and pressure. The “physical synergistic effect” may be caused by the following two reasons: MoS2 and CNT nanoparticles were mixed to prepare grinding fluids of MQL at the same mass fraction. On one hand, this mixture of MoS2 nanoparticles and CNTs reduces the mass fraction of CNTs in grinding fluids and prevent CNT agglomeration effectively. On the other hand, considering the “physical encapsulation” of the CNT nanoparticle by the MoS2 nanoparticle, the MoS2 nanoparticle could modify the CNT nanoparticle physically, improving the dispersion stability of CNTs in grinding fluids of MQL. Therefore, the “physical synergistic effect” enables the hybrid nanofluids of MQL to integrate the advantages and eradicate the disadvantages of MoS2 and CNT nanoparticles. This finding explains the better lubrication effect of hybrid nanofluids compared with pure nanofluids.

Multi-walled carbon Nanotubes

Chiral (n,m)m≠n

Fig. 13. Structure of CNTs.

3.1.4.3. Optimal MoS2/CNT mixing ratio under the “physical synergistic effect”. The aforementioned “physical synergistic effect” could provide a macroscopic explanation for the better lubrication performance of hybrid nanofluids than pure nanofluids. However, different lubrication performances of the four hybrid nanofluids were observed in the experiments. Lubrication effect is inversely proportional to CNT proportion, which is related with “physical encapsulation”. “Physical encapsulating” indicates that, during the preparation of the grinding fluids of MQL, the MoS2 nanoparticle encapsulates the CNT nanoparticle, forming tubular materials centered at the CNT nanoparticle and covered by the MoS2 nanoparticle. Nanoparticles become antifriction materials with tenacity and low coefficient of friction. “Physical encapsulation” is caused by local agglomeration of MoS2 and CNT nanoparticles in grinding fluids of MQL. Given that the molecular structure of the CNT nanoparticle is a seamless combination of hexagonal rings without unstable bonds, CNTs have a high chemical stability. Therefore, MoS2 nanoparticles are adsorbed on CNT nanoparticles. MoS2 and CNT nanoparticles are nonpolar molecules. The Van der Waals force between nonpolar molecules is the main dispersion force, which is determined by the molecular weight. Compared with MoS2 nanoparticles, CNT nanoparticles have a stronger Van der Waals force because of their larger molecular weight. Based on the experimental results, CNTs disperse well in grinding fluids of MQL under a low proportion of CNTs in hybrid nanoparticles. Under this circumstance, MoS2 and CNT nanoparticles agglomerate locally. MoS2 nanoparticles are adsorbed on CNT nanoparticles, resulting in the physical modification of CNT nanoparticles, which further increases the dispersion stability of grinding fluids. In the experiments, Mix(2:1) shows the lowest coefficient of friction (0.2757), indicating the best lubrication performance compared with the other three hybrid nanofluids. However, such “physical encapsulation” is vulnerable. As the proportion of CNTs in hybrid nanoparticles increases, the mass fraction of CNTs in grinding fluids of MQL increases accordingly. Dominated by the Van der Waals force between CNT nanoparticles, CNTs agglomerate to different extents, weakening the lubrication effect. This finding is confirmed by the increasing coefficients of friction of Mix(1:1), Mix(1:2), and Mix(1:3). In summary, a lower proportion of CNTs in hybrid nanoparticles than MoS2 nanoparticles could improve the lubrication effect of grinding fluids of MQL. When the proportion of CNTs in hybrid nanoparticles decreases gradually, the lubrication effect of the hybrid nanoparticles will be

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Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

Abrasive

Abrasive

MoS

MoS Sandwich structure

CNTs

CNTs

Physical encapsulating

MoS

Friction plane CNTs--MoS

MoS

Slip plane MoS --MoS

Workpiece

Workpiece

Fig. 14. “Physical encapsulation” of CNTs and MoS2 for Mix(2:1) and Mix(1:1).

0.32

0.30

0.35

coefficient of friction (μ) Ra (μm)

3.2. Effect of the concentration of hybrid nanoparticles on the lubrication performance of nanofluid MQL We conclude from previous experiments that Mix(2:1) is the optimal MoS2/CNT mixing ratio of hybrid nanoparticles. In this group of experiments, the effect of the concentration of nanofluids on the lubrication effect was investigated by changing the mass fraction (2%, 4%, 6%, 8%, and 10%) of Mix(2:1) hybrid nanoparticles in nanofluids. 3.2.1. Coefficient of friction and surface roughness The coefficient of friction and Ra under different concentrations of nanofluids for MQL grinding are shown in Fig. 15.

0.34

0.2923 0.323

0.29

0.332

0.33 0.32

0.28

0.311 0.2757

0.27

0.25

0.2731

0.31 0.30

0.294

0.26

0.29

0.281 0.2445

0.24

2.0%

3.1.4.4. Formation mechanism of “furrow”. As shown in the previous analysis, CNTs easily agglomerate as the CNT content increases. This finding is related to the strong Van der Waals force between CNTs and the high length–diameter ratio. CNTs form irregular “tube clusters”. Considering the high strength and high hardness of CNTs, the formed “tube clusters” not only inherit the properties of CNTs but also possess higher strength for the correlation of “micro tubes”. On one hand, “tube clusters” entering the grinding zone lead to a higher coefficient of friction between grinding wheel and workpiece compared with the “micro tubes.” Irregular “tube clusters” may enwind abrasive grains to increase the diameter of the “furrow” when abrasive grains are cutting the workpiece, which will increase the surface roughness value of the workpiece to a certain extent. On the other hand, “tube clusters” will break the lubricating oil film on the surface of the friction pair and increase the coefficient of friction.

0.3148

Ra (μm)

0.31

coefficient of friction (μ)

approximately similar to that of pure MoS2. Therefore, the optimal MoS2/CNT mixing ratio enables the hybrid nanoparticles to achieve the best lubrication effect. Fig. 14 shows the “physical encapsulation” of CNTs and MoS2. For Mix(2:1) and Mix(1:1), CNTs were covered by two MoS2 molecular layers, forming a “sandwich structure”, is the aforementioned tubular material centered at CNTs and covered by MoS2 nanoparticles. At this moment, shear slip between MoS2 and molecular layer is the main antifriction structure in the friction pair. The low shear force between MoS2 and molecular layer enables the hybrid nanoparticles to achieve the best lubrication effect. For Mix(1:1) and Mix(3:1), the MoS2/CNT molecular interface is the main antifriction structure in the friction pair, which fails to achieve a satisfying lubrication performance.

4.0%

6.0%

Concentration

8.0%

0.28

10.0%

wt%

Fig. 15. Coefficient of friction and Ra under different concentrations of nanofluids for MQL grinding.

In Fig. 15, the coefficients of friction under five concentrations of nanofluids for MQL grinding are μ(2%) ¼ 0.2923, μ(4%) ¼ 0.2757, μ (6%) ¼0.2445, μ(8%) ¼ 0.2731, and μ(10%) ¼0.3148. The coefficient of friction (μ) shows a V-shaped variation trend as the mass fraction of hybrid nanoparticles increases. First, the coefficient of friction decreases from 0.2923 (2%) to 0.2445 (6%) and subsequently reaches the minimum level. Then, the coefficient of friction increases with the continuous increase in mass fraction and reaches 0.3148 at 10%, which is higher than that at 2%. After analyzing the variation law of the coefficient of friction, we conclude that, before 6.0%, the coefficient of friction is inversely proportional to the nanoparticle concentration, which reflects that the lubrication performance of MQL grinding fluids improves gradually. However, after 6.0%, the coefficient of friction is proportional to the nanoparticle concentration, which reflects that the lubrication performance of MQL grinding fluids decreases gradually [36]. Ra under five concentrations of nanofluids for MQL grinding are Ra(2%) ¼ 0.281 mm, Ra(4%) ¼0.294 mm, Ra(6%) ¼0.311 mm, Ra(8%) ¼0.323 mm, and Ra(10%) ¼ 0.332 mm. With the increase in the mass fraction of hybrid nanoparticles, Ra increases from 0.281 mm to 0.332 mm. Based on the variation law of Ra, a higher mass fraction of hybrid nanoparticles will lead to poorer workpiece surface quality. During the grinding process, the coefficient of friction reflects the lubrication performance of the grinding zone, which is mainly determined by the viscosity of MQL grinding fluids. Ra depends on the infiltrate area (effective lubrication area) of MQL

Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

MQL Droplet

θ Workpiece

2.0wt.%

θ=20.51°

4.0wt.%

θ=22.82°

6.0wt.%

θ=25.75°

8.0wt.%

θ=27.01°

10.0wt.%

θ=33.24°

Pure oil

θ=18.80°

Fig. 16. Contact angle between MQL droplet and workpiece.

grinding fluids and is mainly influenced by the contact angle between MQL droplets and workpieces. The contact angles between different concentrations of MQL grinding fluids and the workpiece were measured to determine the relationship between contact angle and mass fraction of nanoparticles. Fig. 16 shows the contact angle between MQL droplet and workpiece. In Fig. 16, the contact angles between five concentrations of MQL droplet and the workpiece are θ(2%) ¼ 20.51°, θ(4%) ¼22.82°, θ(6%) ¼ 25.75°, θ(8%) ¼27.01°, and θ(10%) ¼35.43°. A proportional relationship between mass fraction of nanoparticles and contact angle is observed. 3.2.2. Discussion 3.2.2.1. Effect of the viscosity of nanofluids on lubrication performance. In the grinding zone, the relative motion between grinding wheel and workpiece will produce shear force on the grinding fluids of MQL, such that internal friction will be generated between internal fluid layers of the grinding fluids. The property that influences internal friction is the viscosity of grinding fluids. The main influencing factors of viscosity include the temperature, pressure, and mass fraction of nanoparticles. Suspension liquid is formed by adding the nanoparticle to the base oil. At a low nanoparticle concentration, the distance among nanoparticles is considerable. The attraction effect of Brown random force and colloidal force is weak, and viscosity force is mainly generated by soybean oil viscosity in the continuous phase. Soybean oil viscosity is the main factor that influences the viscosity of the suspension system. The attraction effect of Brown random force and colloidal force is gradually strengthened with the increase in nanoparticle concentration. The distance among particles becomes smaller, and collisions among nanoparticles increase. The experimental results show that viscosity increases correspondingly with the increase in concentration. The strength of the absorption film between the wheel and the workpiece depends on the absorption force in the grinding liquid molecules [37]. Colloidal force, Brown random force, and viscosity among molecules in the grinding liquid increase along with viscosity. As such, the viscosity of MQL liquid will increase gradually with the increase in hybrid nanoparticle concentration. In the grinding process, viscosity is the main influencing factor of the lubrication effect of MQL grinding fluids. On one hand, high viscosity can prevent the lubricating oil from flowing to improve

31

the lubricating property of the contact area between the cutter and the workpiece, reduce friction, and prevent the rapid wear of the cutter. On the other hand, colloidal force, Brown random force, and viscosity among molecules in the grinding liquid increase along with viscosity, enhancing the strength of the absorption film and subsequently improving the lubricating property. Based on previously presented analysis, the viscosity of MQL grinding fluids increases gradually when the concentration of hybrid nanoparticles increases, which improves the lubrication effect of the grinding fluids to a certain extent. This finding is confirmed by the variation law of the coefficient of friction. The variation of the coefficient of friction after 6% is unrelated to viscosity growth because of nanoparticle agglomeration under a high concentration of hybrid nanoparticles. On one hand, agglomerated nanoparticles destroy the continuity of the lubricating film. On the other hand, “tube clusters” formed by CNTs will increase the coefficient of friction of the grinding zone. 3.2.2.2. Effect of the contact angle of nanofluid droplets on surface roughness. MQL grinding fluids are sprayed onto the grinding zone. The droplets from the nozzle enter the space between workpiece and grinding wheel for cooling lubrication. Therefore, the state of the droplets on the workpiece determines the lubrication effect. The state of the droplets refers to the contact angle between the droplet and the workpiece. During the grinding process, a small contact angle represents the infiltrate area of a large droplet (effective lubrication area of MQL grinding fluids). A larger infiltrate area will result in a better lubrication effect and lower Ra. However, a small contact angle will weaken the lubricating film and subsequently decrease the lubrication effect. A large contact angle will result in a small infiltrate area and subsequently achieve the preferred lubrication effect. The surface tension of the droplet is the internal influencing factor of the contact angle between droplet and workpiece. The stronger the surface tension is, the smaller the contact angle will be [38]. A previous research [39] reported that the surface tension of the droplet decreases when the mass fraction of nanoparticles increases. Therefore, the mass fraction of nanoparticles in nanofluids will affect the contact angle. In summary, with the increase in the mass fraction of nanoparticles, the surface tension of the droplet weakens, but the contact angle between droplet and workpiece enlarges. Consequently, the infiltrate area of MQL grinding fluids narrows, thus increasing Ra. In Fig. 12, Ra increases from 0.281 mm to 0.332 mm when the mass fraction of nanoparticles increases continuously. This result reflects the influence of the contact angle on the surface roughness of the workpiece. 3.2.2.3. Optimal concentration of hybrid nanoparticles in MQL grinding fluids. Based on the lubrication performance of MQL grinding fluids, the mass fraction of nanoparticles in MQL grinding fluids influences the coefficient of friction and workpiece surface roughness. Furthermore, the coefficient of friction and Ra vary significantly. The optimal mass fraction is 6% from the perspective of the coefficient of friction, but is only 2% from the perspective of Ra. However, the coefficient of friction and Ra have different effect weights on lubrication performance. On one hand, the coefficient of friction reflects the wear rate of cutters and grinding heat production during the grinding process. A small coefficient of friction could prolong the service life of cutters and produce lower grinding heat. On the other hand, Ra increases slightly with the increase in the mass fraction of nanoparticles, from Ra(2%) ¼0.281 mm to Ra(6%) ¼ 0.311 mm. Such small growth (only 9.6%) is less significant to the considerably large decrease of the coefficient

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Y. Zhang et al. / International Journal of Machine Tools & Manufacture 99 (2015) 19–33

Table 7 The results comparison with references. Author

Lubrication type

Infeed (mm)

Grinding force (N) (Fn)

Grinding force (N) (Ft)

Coefficient of friction

Ra (mm)

This paper Jia et al. Tso Huddedar et al.

hybrid nanofluid MQL MQL Flood Flood

10 10 10 10

91.28 – 146.69 137

25.17 – 91.55 65

0.2445 0.414 0.624 0.474

0.294 – – 0.4

of friction (17.24% from μ(2%) ¼0.2923 to μ(6%) ¼0.2445). In addition, heat transfer under 6% of nanoparticles is improved compared with that under 2%. As a result, 6% is determined as the optimal mass fraction of nanoparticles under the present experimental conditions.

4. Conclusion In the present study, the effects of the use of oil-based MoS2/CNT hybrid nanofluids on grinding force, coefficient of friction, surface roughness, and ground surface in MQL grinding of Nibased alloy (Inconel 718) were investigated. Based on the experimental results, that the following conclusions were drawn: (1) Pure CNTs nanofluid is inferior to pure MoS2 nanofluid with regard to lubrication effect. Compared with the pure CNTs nanofluid, the application of MoS2 nanofluids in MQL grinding achieves lower grinding forces (Fn ¼82.63 N, Ft ¼24.98 N), coefficient of friction (0.3023), and surface roughness (Ra ¼0.338 mm) and better ground surface. Compared with pure nanoparticles, MoS2/CNTs hybrid nanoparticles achieve lower grinding forces, coefficient of friction, and surface roughness, as well as better ground surface, indicating a better lubrication effect of hybrid nanofluids. (2) Based on the analyses, this better lubrication effect is mainly due to the “physical synergistic effect” of MoS2/CNTs hybrid nanoparticles in MQL grinding fluids. Such “physical synergistic effect” integrates the lubrication advantages and prevents the disadvantages of MoS2 and CNTs nanoparticles. (3) MQL grinding fluids with different MoS2/CNTs mixing ratios have different lubrication performances. Among the prepared six mixing ratios, Mix(2:1) achieves the lowest coefficient of friction (i.e., 0.2757) and lowest Ra (i.e., 0.294 mm). Therefore, the optimal MoS2/CNTs mixing ratio is 2:1 under the present experimental conditions. With the increase in CNTs proportion in the hybrid nanoparticles, higher proportion of CNTs in hybrid nanoparticles destroys the MoS2/CNTs “physical encapsulation” structure and weakening the lubrication effect of MQL grinding fluids. (4) Based on the experiments, the mass fraction of the hybrid nanoparticles in the MQL grinding fluids influences the coefficient of friction and workpiece surface roughness during the grinding process. The coefficient of friction (μ) shows a V-shaped variation trend with the increase in the mass fraction of the hybrid nanoparticles, whereas Ra increases from 0.281 mm to 0.332 mm. The main causes of this phenomenon are the viscosity of MQL grinding fluids and the contact angle between droplet and workpiece, which differ under different mass fractions. (5) Considering the effects of the coefficient of friction and workpiece surface roughness on lubrication and heat transfer performance, we determined that 6 wt% is the optimal mass fraction of MoS2/CNT(2:1) nanoparticles in the MQL grinding fluids, which yielded the minimum coefficient of friction of 0.2445 and Ra of 0.311 mm under the present experimental conditions.

At present, the use of hybrid nanoparticles in nanofluid MQL cutting, particularly in grinding of difficult-to-cutting materials, has not been reported. Jia et al. [40] investigated the effect of jet parameters on lubricating properties during surface grinding of Ni-based alloys with uniform grinding parameters, which are similar to the parameters used in the present study. Tso [41] investigated the grinding force of Ni-based alloys with GC grinding wheels. Huddedar et al. [42] investigated the grinding force and surface roughness of Ni-based alloys grinding. The results comparison was shown in Table 7. Compared with these previous results, MoS2/CNTs(2:1) nanofluid showed a lower grinding forces, coefficient of friction and Ra, which indicated a better lubrication effect of the hybrid nanofluid. Finally, this paper is the first approach to use hybrid nanofluids in minimum quantity lubrication grinding to improve the workpiece surface quality of difficult-to-cut materials. Improvement was obtained and lubrication mechanism was analysed, this paper solves the bottleneck of surface integrity insufficient and providing a new way in MQL grinding. Greater understanding of the contribution of hybrid nanofluids jet minimum quantity lubrication grinding was achieved. This can be expanded to several applications in machining, such as turning or drilling, and This can also be expanded to machine of several difficult-to-cutting materials.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ijmachtools.2015. 09.003.

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