An investigation of carbon nanofluid minimum quantity lubrication for grinding unidirectional carbon fibre-reinforced ceramic matrix composites

Journal Pre-proof An investigation of carbon nanofluid minimum quantity lubrication for grinding unidirectional carbon fibre-reinforced ceramic matrix composites

Shuoshuo Qu, Yadong Gong, Yuying Yang, Wenwen Wang, Chunyou Liang, Bing Han PII:

S0959-6526(19)34223-4

DOI:

https://doi.org/10.1016/j.jclepro.2019.119353

Reference:

JCLP 119353

To appear in:

Journal of Cleaner Production

Received Date:

24 August 2019

Accepted Date:

16 November 2019

Please cite this article as: Shuoshuo Qu, Yadong Gong, Yuying Yang, Wenwen Wang, Chunyou Liang, Bing Han, An investigation of carbon nanofluid minimum quantity lubrication for grinding unidirectional carbon fibre-reinforced ceramic matrix composites, Journal of Cleaner Production (2019), https://doi.org/10.1016/j.jclepro.2019.119353

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Journal Pre-proof An investigation of carbon nanofluid minimum quantity lubrication for grinding unidirectional carbon fibre-reinforced ceramic matrix composites

Shuoshuo Qu1, Yadong Gong1*, Yuying Yang1, Wenwen Wang2, Chunyou Liang1, Bing Han1 1School of Mechanical Engineering and Automation, Northeastern University, Shenyang, 110819, China 2CHENGWU ZHIZHUAN, Chenwu 274200, China Corresponding author: Gong Yadong E-mail address: [email protected] Tel: +86 13940518488 Shuoshuo Qu E-mail address: [email protected]

Tel: +86 18842525889

Yuying Yang E-mail address: [email protected]

Tel: +86 18842487828

Wenwen Wang E-mail address: [email protected]

Tel: +86 18804012248

Chunyou Liang E-mail address: [email protected]

Tel: +86 18842383046

Bing Han E-mail address: [email protected]

Tel: +86 18804012246

Journal Pre-proof

NMQL system

Fibre pullout and outcrop

Surface roughness and grinding force

Journal Pre-proof An investigation of carbon nanofluid minimum quantity lubrication for grinding unidirectional carbon fibre-reinforced ceramic matrix composites Shuoshuo Qu1, Yadong Gong1*, Yuying Yang1, Wenwen Wang2, Chunyou Liang1, Bing Han1 1School

of Mechanical Engineering and Automation, Northeastern University, Shenyang,

110819, China 2CHENGWU

ZHIZHUAN, Chenwu, 274200, China

ABSTRACT Grinding is the most effective processing method for carbon fibre-reinforced ceramic matrix (Cf/SiC) composites (Liu et al., 2018). The effects of dry, flood, minimum quantity lubrication (MQL) and carbon nanofluid MQL conditions on grinding performance were studied in this research. Moreover, a method for effectively dispersing carbon nanoparticles was proposed. The experimental results showed that carbon nanofluid MQL conditions can provide significantly higher surface quality and lower grinding forces than the dry, flood and MQL conditions. In addition, the nanofluid poses a negligible threat to the environment and to workers, which means that carbon nanofluid has good application prospects in grinding processes. The influences of the nanoparticle concentration, air pressure, flow rate, nozzle distance and nozzle position on the grinding forces and surface quality were also investigated in this paper. According to the experimental results, the optimum parameters can be concluded as follows: the nanoparticle concentration was 5 g/L, the air pressure was 7 bar, the fluid flow rate was 80 mL/h, and the nozzle distance was 60 mm. According to the ground surface microtopography, matrix cracking, fibre pullout and fibre outcrop were the primary defect forms. In the grinding process, the debonding depth between the matrix and the carbon fibre depended on the sharpness and lubrication state of the abrasive grains. Long fibre debris, short fibre debris, fibre microparticles and SiC particles were the basic components of the grinding debris according to scanning electron microscopy (SEM) micrographs of the typical grinding chips from the Cf/SiC composites. This research is expected to investigate the application potential of carbon nanofluid MQL and propose a greener and more efficient lubrication method. 1

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Keywords Unidirectional carbon fibre-reinforced ceramic matrix composites; Carbon nanofluid minimum quantity lubrication; Surface quality; Grinding forces.  Yadong Gong [email protected] 1. Introduction Carbon fibre-reinforced ceramic matrix (Cf/SiC) composites are a new type of material that are being incrementally used in aerospace, defence and transportation fields because of their excellent wear resistance, hardness and low mass (Qu et al., 2018; Zhang et al., 2016). According to the composition of Cf/SiC composites, the physical and mechanical properties of the carbon fibre reinforcement and the SiC matrix are significantly different, which indicates that the machinability of the overall composite is poor (Tawakoli et al., 2011). Difficult machinability and high processing costs limit further application of Cf/SiC composites, although the preparation technology is becoming increasingly mature. According to the related literature (Ding et al., 2017; Liu et al., 2018), high-quality surface finish and high processing efficiency can be achieved by grinding Cf/SiC composites with a diamond wheel. It is remarkable that the high hardness and heterogeneity of Cf/SiC composites easily lead to high wheel wear and surface defects during the grinding process. Aiming at this new material with great application potential, the mechanisms of grinding and the influences of grinding parameters on grinding performance are the research emphases for most researchers. Cao et al. (2013) proposed 3D surface-characterization parameters to quantitatively investigate the ground surface quality of 2.5D SiO2/SiO2 composites. Compared to traditional surface roughness parameters, such as Ra and Rz, the sampling area of 3D parameters is far greater than that of 2D parameters, which indicates that 3D parameters can more accurately reflect the surface quality. Qu et al. (2018) investigated the influences of grinding depth and fibre orientation on grinding performance in unidirectional Cf/SiC composites. Their experimental results showed that different fibre orientations produced completely different grinding mechanisms. In addition, many small grinding chips 2

Journal Pre-proof were suspended in the air during the grinding process, which may be inhaled or absorbed by the respiratory system. To investigate the removal mechanisms of Cf/SiC composites in greater detail, a series of single-grain sliding experiments was devised and carried out by Li et al. (2019). Their results showed that the grinding forces increased with increasing grinding depth and reducing grinding speed. Moreover, they determined that low grinding forces would occur in the transverse surface. Azarhoushang et al. (2011) proposed a novel sonotrode that achieves ultrasonic processing on a conventional grinder. Two different Cf/SiC composites were investigated in their grinding experiment. Moreover, they studied the effects of feed speed, grinding speed and vibration amplitude on the grinding force and wheel wear. Their results indicated that the sonotrode can significantly reduce grinding forces and surface roughness. In the traditional grinding process, dry grinding and flood grinding are the most common cooling methods. Negative grain rake working is the main grinding characteristic, which implies that substantial heat and high grinding forces are generated during grinding. According to the results of Mao et al. (2013), most of the heat would be absorbed by the wheel and sample. As the grinding wheel temperature increases, abrasive particles on the surface of the wheel fall off more easily, which accelerates wear and reduces the life of the wheel. Therefore, good surface quality and long service life of grinding wheels cannot be achieved through dry grinding (Guo et al., 2017). Flood grinding can take away a great deal of heat by using a large amount of flowing grinding fluid. In addition, many grinding chips are washed away by the grinding fluid, which inhibits chip adhesion and improves the working environment. However, the grinding fluid contains many harmful, synthetic and refractory organic compounds that can cause serious environmental pollution. Thus, additional costs are needed to address failed grinding fluids. Moreover, although conventional flood grinding provides effective lubrication, this approach cannot transfer heat out of the grinding zone in time (Lawal et al., 2015). In addition, grinding fluid vapour after evaporation has great toxicity, leading to severe respiratory diseases (Pashmforoush et al., 2018). Based on the work defects of dry grinding and flood grinding, a new technology of minimum quantity lubrication (MQL) has been proposed. Barczak et al. (2010) investigated the influences of MQL on surface grinding from evaluation standpoints of surface roughness, 3

Journal Pre-proof grinding force and temperature. Their results indicated that MQL can provide lubrication performance that is not inferior to flood grinding. Muaz et al. (2019) researched the important role of the viscosity of the cutting fluid in an MQL system. Their results indicated that low-viscosity fluids can improve the lubrication performance of an MQL system. The effects of MQL parameters on grinding performance have been studied in detail by Tawakoli et al. (2010). Their results showed that excellent lubrication performance can be obtained when the angle of the spray nozzle and the machined surface is 10-20°. According to an MQL grinding experiment of hardened steel, response surface regression equations of tangential grinding force and surface roughness were established by Hadad (2015). However, industrial synthetic oil is usually selected as a lubricating medium in the MQL system, which means that some harmful vapour may be produced during grinding (Shokoohi et al., 2016). Certainly, compared to flood grinding, MQL grinding produces a very low concentration of harmful vapor. In addition, although MQL conditions can provide good lubrication performance, the cooling performance is not ideal (Zhang et al., 2015). This result indicates that there is still much room for improvement in the application of MQL grinding. According to the classical heat conduction theory (Hadad et al., 2015), adding nanoparticles can significantly improve the cooling performance of lubricating oil in the machining zone. Alberts et al. (2015) carried out a study to investigate the lubrication performance of graphite nanoplatelets in the grinding process of tool steel. According to their experimental results, an effective application method was proposed. The influences of nanodiamond particles on grinding force and surface quality in the grinding process of titanium alloy were studied by Lee et al. (2018). Their results indicated that the grinding force and grinding tool life can be significantly decreased and increased by the addition of nanodiamond particles in the MQL grinding process, respectively. Wang et al. (2017) applied Al2O3 nanoparticles in the MQL grinding process of Ni-based alloys. Their conclusions showed the excellent lubrication performance of Al2O3 nanoparticles in decreasing grinding forces and alleviating grinding burning. Sharma et al. (2016) reviewed the relevant literature about the rheological behaviour of nanofluids. They concluded that the nanoparticle size, proportion, shear rate range and dispersion considerably affect the rheological behaviour. Singh et al. (2017) proposed a hybrid nanofluid consisting of graphene nanoplatelets and Al2O3 nanoparticles. The results 4

Journal Pre-proof indicated that the thermal conductivity and viscosity can be significantly improved. It was also confirmed that the wear can be inhibited by improving the nanoparticle concentration in the tribological test. Sharma et al. (2019) applied this hybrid nanofluid in an MQL system to turn AISI 304 stainless steel. Compared to alumina-based lubricants, the hybrid nanolubricants observably decreased the tool wear and temperature. Sharma et al. (2018) investigated the tool tip temperature in turning. A hybrid nanofluid was used to determine the temperature. Those authors researched the temperature distribution via conjugate heat transfer analysis. The preparation technologies of Cf/SiC composites and traditional grinding processes have been reported in related literature (Liu et al., 2009; Qu et al., 2019a, b). However, many environmental pollution and health problems have received insufficient attention from related researchers. According to the available literature, the influences of MQL or nanofluid MQL (NMQL) on the machining performance of traditional materials have been extensively investigated. At present, only a few studies have investigated the effects of MQL in grinding Cf/SiC composites. Esmaeili et al. (2019) investigated the grinding performance of Cf/SiC composites under dry, fluid and MQL conditions. Their results showed that MQL can significantly improve grinding efficiency and surface quality. However, there are few studies concerning the influences of NMQL on the surface quality and grinding forces in the grinding process of Cf/SiC composites. Based on this consideration, the primary objective of this paper is to research the lubrication and cooling performance of NMQL in the grinding process of Cf/SiC composites. In this paper, carbon nanoparticles were selected as the main components of the nanofluid, and a detailed introduction can be found in Section 2.2. Carbon nanoparticles not only provide good thermal conductivity but also do not harm the environment. It is remarkable that this is the first time that carbon nanofluids have been studied as grinding fluids, let alone in the process of grinding Cf/SiC composites. Therefore, for the first time, the influences of carbon nanofluids on the surface quality and grinding forces of Cf/SiC composites have been thoroughly studied in this paper. Moreover, according to the surface topography, the grinding mechanisms of the Cf/SiC composites are investigated. The results of this paper are helpful in achieving a reasonable combination of grinding parameters and increasing workpiece quality. 5

Journal Pre-proof 2. Experimental details 2.1. Experimental workpiece There are many classifications of Cf/SiC composites based on the weaving structure of carbon fibres, such as unidirectional, 2D laminated and 2.5D needled Cf/SiC composites. First, the influence of NMQL on grinding performance is the main research goal of this study. Second, unidirectional Cf/SiC composites have strong anisotropy and heterogeneity, which causes the evaluation parameters of grinding quality to change significantly under different grinding conditions. Therefore, unidirectional Cf/SiC composites were selected as the experimental workpiece in this paper. First, in the production process of composites, carbon fibres should be orderly laid in high-temperature, high-pressure environments. Then, the SiC matrix would be formed in the gaps and surfaces of carbon fibres via chemical vapour infiltration (CVI) technology. The detailed structural diagram and topography images of the unidirectional Cf/SiC composites are shown in Fig. 1. These images show that the fibres are fixed firmly by the SiC matrix. However, the strength of the bonding interface is the weakest in the whole Cf/SiC composites based on the preparation method. Additional details regarding the performance of Cf/SiC composites can be found in related literature (Zhang et al., 2011; Yu et al., 2013; Mei et al., 2013). According to previous research (Qu et al., 2019a, b), grinding processes in the radial direction of carbon fibres has the greatest research value. Therefore, the radial direction was selected as the grinding surface.

Fig. 1. Structural diagram and topography images of unidirectional Cf/SiC composites: (a) structural diagram, (b) axial direction of carbon fibres and (c) radial direction of carbon fibres. 2.2. Preparation process of carbon nanofluid 6

Journal Pre-proof According to the good stability and small particle size of carbon nanoparticles (CABOT VXC72), this research selected carbon nanoparticles as the dispersed phase to create nanofluids. The specific surface area, density and particle size are 254 m2/g, 443 kg/m3 and 30 nm, respectively. However, carbon nanoparticles easily aggregate into larger granules during production and application. Moreover, carbon nanoparticles are strongly hydrophobic, which indicates that a special dispersion processing method is necessary to ensure that carbon nanoparticles can be effectively dispersed in deionized water. In this paper, the carbon nanoparticles are fully dispersed in deionized water after oxidation modification, surface active agent addition and ultrasonic vibration. Detailed macroscopic and microscopic images of the dispersed carbon nanoparticles are shown in Fig. 2. After the carbon nanofluids are prepared and stored for 24 hours, the carbon nanoparticles aggregate, as shown in Fig. 2(a). This image shows that stratification cannot be clearly observed, which indicates that carbon nanoparticles remain well dispersed in the nanofluid. Therefore, the pipelines of the MQL system are not easily clogged by carbon nanofluids. This result also indicates that these carbon nanofluids prepared by the above method can be used in the MQL system. To acquire scanning electron microscopy (SEM) micrographs of the dispersed carbon nanoparticles, droplets of the carbon nanofluids were placed on a copper sheet. Fig. 2(b) is a typical SEM image, which shows that carbon nanoparticles are indeed dispersed in the fluid. However, to achieve clearer micrographs of the carbon nanoparticles, transmission electron microscopy (TEM) was utilized in this research. First, the diluted carbon nanofluids were dripped onto the front of the copper mesh with a diameter of 30 μm. After natural drying, the micrographs of the carbon nanoparticles can be observed via TEM. According to the TEM image, as shown in Fig. 2(c), the particle sizes of the dispersed carbon nanoparticles were counted. The TEM image also shows that the surfaces of the carbon nanoparticles are smooth and defectless. Moreover, each particle has a different shape and presents an obvious layer structure. These characteristics illustrate that carbon nanoparticles can provide excellent lubrication performance in the grinding process. The statistical histogram, as shown in Fig. 2(d), shows that the average particle size of the dispersed carbon nanoparticles is 40 nm. This value is very close to the value provided by the manufacturer, which also means that the carbon nanoparticles have been fully dispersed. 7

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Fig. 2. Macroscopic and microscopic morphology images of dispersed carbon nanoparticles: (a) the overall morphology of dispersed nanofluids after 24 hours of storage, (b) an SEM micrograph of the dispersed carbon nanoparticles, (c) a TEM micrograph of the dispersed carbon nanoparticles and (d) a statistical histogram of the particle size of the dispersed carbon nanoparticles. 2.3. Experimental conditions The research was conducted using a diamond grinding wheel on unidirectional Cf/SiC composites with a precision surface grinding machine (M7120A, Tianjin Machine Tool Co., Ltd, China.), as shown in Fig. 3(a). A Jinshuang micro-lubrication system (KS-2107, Shanghai Jinzhao Energy Saving Technology Co., Ltd, China.), as shown in Fig. 3(b), and matching micro-lubrication oil were used as the MQL basic cutting fluid in this research. Table 1 reports the relevant experimental parameters. A schematic diagram of the grinding system is shown in Fig. 3(c). According to the preparation method of carbon nanofluids, the base fluid was deionized water. Meanwhile, the base fluid for wet grinding was Castrol Syntilo XPS. The diamond grinding wheel was dressed with a diamond dresser before every experiment to obtain similar conditions for every experiment. The dressing process was 8

Journal Pre-proof carried out in three passes, causing an actual depth value of 30 μm. The blunt abrasive grains and other impurities would be removed by the dressing process, which keeps the grinding wheel sharp. To avoid errors caused by the tilt of the workpiece placement, effective experiments were conducted after the 15th pass. According to the related surface roughness evaluation literature (Cao et al., 2013; Zhang et al., 2016; Qu et al., 2018), 3D surface roughness evaluation parameters, such as surface arithmetic mean deviation Sa and surface maximum height difference Sz, provide a more reliable evaluation system than 2D parameters. Therefore, to quantitatively investigate the ground surface quality, a MICROMEASURE 3D surface profiler was utilized in this paper. A field emission scanning electron microscope was used to investigate the detailed micromorphologies of the ground surface. A Kistler 9257B dynamometer, a Kistler charge amplifier and a collection card were utilized to record the grinding forces. The obtained grinding force data were analysed with DynoWare software. To avoid errors, the surface roughness and grinding force data were recorded four times, and the average data were used as effective results. Table 1 Relevant experimental parameters.

M7120A

Diamond grinding wheel

Grinding conditions

Parameters of MQL and NMQL

Grinding scope Principal axis power Clamping system Grain size Binder Concentration Abrasive thickness Linear velocity of the wheel (vs) Work speed (vw) Depth of grinding (ap) Lubrication conditions

-

Air pressure (P) Lubrication liquid flow rate (Q) Nozzle distance to 9

mm kW # % mm

200×630×320 1.1 Electromagnetic chuck 120 Resin 100 5

m/s

26

m/min μm

bar

3 30 Dry, wet, MQL, carbon NMQL 3, 5, 7, 9

mL/h

40, 60, 80, 100

mm

40, 60, 80, 100

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°

15

g/L

1, 3, 5, 7

Fig. 3. (a) Grinding machine setup, (b) MQL system and (c) schematic diagram of the grinding system under MQL or NMQL conditions. 3. Results and discussion 3.1. Effects of lubrication conditions

To investigate the effects of dry, flood, MQL and NMQL conditions on the grinding performance of unidirectional Cf/SiC composites, evaluation indicators, such as grinding forces, surface roughness, surface topography, subsurface damage and grinding chips, under different lubrication conditions have been studied in detail in this section. 3.1.1 Grinding forces and surface roughness

The influences of lubrication conditions on the surface roughness and grinding forces are displayed in Fig. 4. The results showed that dry grinding caused poor grinding quality and high grinding forces, wherein the surface maximum height difference Sz and the normal grinding force Fn were as high as 7.12 μm and 28.2 N, respectively. Moreover, the grinding force ratio (μ = Ft Fn) was 0.65, which indicates that substantial friction exists between the grinding wheel and the workpiece under dry grinding conditions. Therefore, there is a very 10

Journal Pre-proof large improvement space for this grinding method. Fig. 4 shows that compared to dry grinding, flood grinding can significantly reduce the surface roughness and grinding forces. Moreover, the grinding force ratio decreased from 0.65 to 0.6 under flood grinding conditions. A substantial amount of grinding heat and debris can be removed by the continuous flow of grinding fluid under flood grinding conditions. Therefore, the surface quality and grinding wheel life can be significantly improved by switching from dry grinding to flood grinding. However, only a small amount of grinding fluid can enter the grinding zone (Barczak et al., 2010; Guo et al., 2017), which means that the lubrication of flood grinding is insufficient. According to the related literature about MQL, droplets of MQL can enter the grinding zone effectively. To ensure the reliability of the equipment, a lubricating oil (mineral synthetic oil) matching MQL system was used in this paper. The results show that the grinding quality can be improved significantly by implementing MQL conditions. Compared to flood grinding, MQL grinding greatly improves the utilization rate of grinding fluid and reduces environmental hazards. The main components of the carbon nanofluids used in this paper are carbon nanoparticles, deionized water and a small amount of surfactants. These carbon nanofluids are hardly harmful to the environment and workers. According to the experimental results, the values of surface roughness and grinding forces were the smallest under NMQL. In addition, the grinding force ratio was 0.48. The above analysis shows that NMQL not only achieves optimal grinding performance but also ensures the safety of workers and the environment, which indicates that this grinding approach has great prospects.

Fig. 4. Influences of lubrication conditions on the surface roughness (a) and grinding forces 11

Journal Pre-proof (b) of Cf/SiC composites. Other NMQL parameters are as follows: C = 5 g/L, P = 7 bar, Q = 80 mL/h, and L = 60 mm. 3.1.2 Surface topography

Unidirectional Cf/SiC composites, as typical ceramic matrix composites, exhibit remarkable anisotropy and heterogeneity. Therefore, the removal forms of the matrix and fibres are significantly different in the grinding process. To investigate the topography in detail, the ground surface of the unidirectional Cf/SiC composites was analysed via SEM, as shown in Fig. 5. According to the typical surface topography, fibre pullout, fibre outcrop, matrix cracking and interfacial debonding are the primary defect types. Fig. 5(b) shows a schematic diagram of the grinding process. According to previous research (Qu et al., 2018 and 2019a, b), the fracture modes of fibres are mainly related to the parameters of the grinding process, the sharpness of the abrasive grains and the lubrication of the wheel workpiece. In this research, the grinding wheel would be dressed under the same parameters before each experiment in this research, which indicates that the initial states of the abrasive grains are consistent in each experiment. Therefore, the micromorphology of the ground surface should be investigated based on the lubrication conditions between the grinding wheel and the workpiece. According to the indentation fracture mechanics theory, a large number of microcracks initiate and propagate in ceramic matrices. Unlike traditional brittle materials, because of the existence of a fibre reinforcement phase, matrix cracks may exhibit bridging, deflection and even suspension during the grinding process. However, the strength of the interface is much lower than that of the matrix and the fibres based on the preparation method of Cf/SiC composites. Therefore, interfacial debonding would occur between the fibres and matrix under the action of abrasive particles, as shown in Fig. 5(b). Interfacial debonding may lead to inconsistencies between the fibre fracture surface and the matrix removal surface, resulting in fibre pullout and outcrop. Fig. 6 shows the detailed fracture SEM micrographs of fibre pullout and outcrop, which indicate that different removal forms of fibres and matrix lead to significantly different machined surfaces. Moreover, interfacial debonding can be observed from the border areas of the fibre and matrix. According to the grinding forces under different lubrication conditions, as shown in Fig. 4(b), dry grinding has the maximum grinding force and grinding force ratio, which indicates that the corresponding Cf/SiC 12

Journal Pre-proof composites have suffered serious shearing and extrusion. The unidirectional Cf/SiC composite removal mechanism, achieved by previous research (Qu et al., 2019a), led to long cracks and deep interfacial debonding. Therefore, most fibres would be removed by crushing caused by extrusion or shearing, which signifies that the proportions of fibre pullout and outcrop are the largest in the composite subjected to dry grinding. The surface quality is the worst in the composite subjected to dry grinding, which is consistent with the results shown in Fig. 4(a). With the improvement in the lubrication and cooling conditions, the friction between the grits and the grinding wheel decreases gradually, which means that the length and depth of matrix cracking and interfacial debonding would be constrained, thereby improving the surface quality. According to the above analysis, good cooling and lubrication conditions are conducive to restraining processing defects and improving surface quality.

(b)

Workpiece Grinding chips

Fibre pullout

Grinding wheel

ap

Grain

Fibre outcrop

Fiber fracture Fiber outcrop Debonding

Matrix

Carbon fiber

Fiber pullout

Fig. 5. Typical ground surface microtopography (a) and schematic diagram of the grinding process (b) of the unidirectional Cf/SiC composites. SiC SiC

Fibre

Fibre

Fig. 6. Typical fracture SEM micrographs of the carbon fibre: (a) fibre pullout and (b) fibre outcrop. 3.1.3 Subsurface damage 13

Journal Pre-proof The definition of surface quality includes surface roughness, surface morphology and subsurface damage (Sun et al., 2017). To more accurately investigate the grinding performance under different lubrication conditions, the machined workpiece was cut along the axial direction of the fibre. Then, the cross section of the workpiece was polished with diamond polishing paste. SEM micrographs of subsurface damage are shown in Fig. 7. However, because of the lack of support, the fibre outcrops are easily destroyed in the sample preparation process. There is a large error in the actual results, and the research value is not great. According to the structural characteristics of the fibre pullout regions, the original morphology can be accurately preserved, which objectively reflects the actual damage. Therefore, the subsurface damage characteristics of the fibre pullout regions are the main investigation objective in this section. Fig. 7 shows that there are significant differences in the depth of fibre pullout under different lubrication conditions. When dry grinding is selected, large grinding forces lead to a significant increase in the debonding depth, which weakens the support of the matrix. Moreover, crack propagation may not be completely inhibited by the fibre reinforcement. This finding indicates that fibres are easily destroyed when the tension caused by cracks is much greater than the strength of the fibres, as shown by the deep fibre pullout in Fig. 7(a). With the improvement in the lubrication conditions, the defect depth decreases gradually. The results show that the defect depth is the shallowest under carbon NMQL conditions. The depths of damage results also show that MQL, especially carbon NMQL, can improve the grinding quality of unidirectional Cf/SiC composites.

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Fig. 7. Typical SEM micrographs of subsurface damage in the Cf/SiC composites: (a) dry grinding, (b) flood grinding, (c) MQL grinding, (d) and carbon NMQL grinding. 3.1.4 Grinding debris

The grinding debris from Cf/SiC composites is similar to that from ceramic materials. The typical features of this grinding debris are small size and irregular shape, as shown in Fig. 8. Under the action of air agitation caused by the grinding wheel rotation, the grinding debris from the Cf/SiC composites can drift into the air. Therefore, grinding debris may cause the following problems: grinding debris inhaled by workers may cause respiratory and pulmonary diseases; grinding debris may enter the interior of the machine, thereby damaging machinery, especially precision machinery; and grinding debris easily adheres to the surface of the grinding wheel, which reduces the service life of the grinding wheel and the quality of workpiece processing. Flood, MQL and NMQL conditions can effectively avoid the abovementioned problems. However, grinding debris can hardly be collected under flood, MQL or NMQL conditions. Moreover, the debris collected through filter paper may also lose the actual structure. Therefore, we investigate the basic morphology of wear debris only under dry grinding conditions. According to the micromorphology and origin of the grinding debris, grinding debris from the Cf/SiC composites include long fibre debris, short fibre debris, fibre microparticles and SiC particles, as shown in Fig. 8. This figure shows that the size of the grinding debris is on the micron or even nanometre scale, which means that dust pollution is easy to occur in the grinding process of Cf/SiC composites. Hence, from this point of view, wet, MQL and NMQL grinding have great advantages to dry grinding.

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Fig. 8. Typical grinding chips SEM micrographs of the Cf/SiC composites under dry grinding conditions. Zhang et al. (2013) reported that the amount of MQL used was only approximately 1/1000 of flood grinding. Meanwhile, a layer of stable film would be formed on the interface of diamond grit and workpiece, which provides excellent lubricating effect (Guo et al., 2017). According to the previous grinding investigations of Cf/SiC composites (Qu et al., 2019a), surface and subsurface damage are easily detected from the machined surface. In addition, the use of grinding fluid not only improves the production cost but also consumes resources and poses a threat to the environment. Compared with dry grinding and wet grinding, Qu et al. (2019c) showed that MQL show excellent lubricating effects. MQL technology has exhibited a significant improvement in surface quality and environmental safety, especially NMQL. Compared to dry grinding, Sz and Fn can be reduced 54.9% and 61.7%, respectively, under NMQL conditions. 3.2. Effects of carbon NMQL parameters

The main parameters of the NMQL system include the particle concentration, air pressure, flow rate, nozzle distance and nozzle position. However, according to the research results by Tawakoli et al. (2010), an air barrier would be formed around the wheel periphery 16

Journal Pre-proof due to the high-speed rotation. The presence of this phenomenon indicates that the droplets need to overcome greater resistance to penetrate into the grinding areas. The lubrication performances of spraying to the workpiece, the grinding wheel and the grinding areas are inferior to that of spraying along the airflow direction in the air barrier. Therefore, the nozzle direction has an angle (approximately 15°) with the grinding surface in this research. The effects of other carbon NMQL system parameters were investigated by single factor experiments as described hereafter. 3.2.1 Concentration of carbon nanoparticles

The improvement in the grinding environment is mainly realized by carbon nanoparticles in the carbon NMQL grinding process. This realization indicates that the concentration of carbon nanoparticles has a significant influence on the grinding performance of Cf/SiC composites. The influences of the concentration of carbon nanoparticles on the surface roughness and grinding forces of the Cf/SiC composites are shown in Fig. 9. When C increased from 1 to 5 g/L, Sa and Sz decreased from 0.6 to 0.35 μm and 6.3 to 3.21 μm, respectively. This finding implies that the grinding surface quality improved. However, when C increased from 5 to 7 g/L, the surface roughness decreased, indicating that the grinding quality deteriorated, which may be caused by the agglomeration of carbon nanoparticles. The agglomerated nanoparticles lead to the following defects: blocking the pipeline, increasing the mass of the droplet, increasing the air resistance, and decreasing the motive power. The above defects imply that the droplets are more difficult to penetrate into the grinding zones. Similarly, when C increased from 1 to 5 g/L, the tangential (Ft) and normal (Fn) grinding forces decreased from 10.5 to 5.2 N and 18.3 to 10.8 N, respectively. When C was 5 g/L, the minimum grinding force was obtained. In fact, carbon nanoparticles with a layered structure can provide excellent lubrication. Under the action of shear loads, the carbon nanoparticles slide each other. Therefore, the frictional force would decrease in the grinding areas. In addition, when a continuous film is formed in the grinding areas, the lubrication performance is excellent, which means lower surface roughness and grinding forces, as shown in Fig. 9. Moreover, with increasing concentration, the number of effective nanoparticles entering the grinding zones increased, indicating that the continuous film area and absorbed grinding heat increased. These characteristics can significantly improve grinding performance and inhibit 17

Journal Pre-proof grinding wheel wear. However, carbon is a typical hydrophobic material, which implies that agglomeration occurs easily in water, particularly at high nanoparticle concentrations. Limited improvement in carbon nanoparticle dispersion can be achieved in water through the addition of a surface active agent. When C increased from 5 to 7 g/L, the optimum dispersion concentration achieved by the current process was exceeded and the carbon nanoparticles began to agglomerate. Therefore, weakened lubrication leads to increased surface roughness and grinding forces. In addition, the grinding force ratio (μ = Ft Fn) is an index that reflects the lubrication performance in the grinding areas (Zhu et al., 2019; Li et al., 2016). A larger grinding force ratio corresponds to worse lubrication performance. According to Fig. 9(b), μ can be determined under different concentration conditions. The calculation results show that the lubrication performance is the best when the concentration is 5 g/L, which is consistent with the above analysis.

Fig. 9. Influences of concentration of carbon nanoparticles C on the surface roughness (a) and grinding forces (b) of Cf/SiC composites. the other NMQL parameters are as follows: P = 7 bar, Q = 80 mL/h, and L = 60 mm. 3.2.2 Air pressure

To provide good lubrication, the droplets must have sufficient kinetic energy to break through the air barrier around the grinding wheel during the grinding process. The kinetic energy of the droplets primarily comes from the air pressure. The influences of the air pressure on the surface roughness and grinding forces of the Cf/SiC composites are shown in Fig. 10. When P increased from 3 to 7 bar, Sa and Sz decreased from 0.58 to 0.49 μm and 6.12 18

Journal Pre-proof to 3.21 μm, respectively. With increasing air pressure, more nanofluid droplets can break through the air barrier and enter the grinding zones, eventually forming an effective continuous lubrication film that improves the grinding environment and grinding quality. Therefore, as shown in Fig. 10(b), the grinding forces decrease with increasing air pressure. In addition, grinding chips can be effectively removed under high-pressure conditions. However, excessive pressure can lead to a significant reduction in the droplet diameter. According to the characteristics of the surface wettability of droplets, smaller droplet diameters lead to smaller continuous wetting areas, which indicates that the effective continuous film formed by carbon nanoparticles would be decreased. Therefore, as shown in Fig. 10, the surface roughness and grinding forces were obviously decreased when P increased from 7 to 9 bar. This finding implies that excessive air pressure is not conducive to improving the grinding performance of unidirectional Cf/SiC composites. Moreover, according to the grinding force ratio results, the lubrication state can also be verified. When P increased from 3 to 7 bar, the lubrication performance improved. However, when P increased from 7 to 9 bar, the lubrication performance decreased instead. Therefore, the changes in the grinding force ratio are consistent with the surface roughness and grinding forces. In addition, it is expensive to generate high-pressure air. Moreover, excessive air pressure creates potential safety hazards in industrial production processes (Tawakoli et al., 2010).

Fig. 10. Influences of the air pressure P on the surface roughness (a) and grinding forces (b) of Cf/SiC composites. The other NMQL parameters are as follows: C = 5 g/L, Q = 80 mL/h, and L = 60 mm. 3.2.3 NMQL flow rate 19

Journal Pre-proof Material removal is a very complex process in grinding. Hou et al. (2003) noted that most grains on the surface of the grinding wheel may not be involved in the grinding process. This realization indicates that the larger the wetting area is, the greater the possibility of carbon nanofluids participating in the grinding process. With increasing lubrication liquid flow rate, the number of carbon nanoparticles sprayed out increased, indicating that the wetting area is growing and that the grinding performance is improving. The influences of the lubrication liquid flow rate Q on the surface roughness and grinding forces of the Cf/SiC composites are shown in Fig. 11. When Q increased from 40 to 100 mL/h, Sa and Sz decreased from 0.65 to 0.31 μm and 6.81 to 2.92 μm, respectively, which indicates that the surface quality was significantly improved. Moreover, when Q increased from 40 to 100 mL/h, Ft and Fn decreased from 12.1 to 4.5 N and 18.9 to 10.1 N, respectively, and the grinding force ratio decreased from 0.64 to 0.45. The above results on grinding forces show that increasing the flow rate can significantly improve lubrication performance in the grinding process of unidirectional Cf/SiC composites. Although a large flow rate has a great effect on improving grinding performance, a larger flow rate also means a high processing cost. In addition, when the fluid flow rate gradually increased, the rate of increase gradually decreased, as shown in Fig. 11. Therefore, by considering both cost and lubrication efficiency, 80 ml/h is selected as the most reasonable flow rate.

Fig. 11. Influences of the fluid flow rate Q on the surface roughness (a) and grinding forces (b) of Cf/SiC composites. The other NMQL parameters are as follows: C = 5 g/L, P = 7 bar, and L = 60 mm. 20

Journal Pre-proof 3.2.4 Nozzle distance In the working process of MQL, Hadad (2009) found that droplets would form aerosols in the air under the action of air pressure. Then, the aerosols would be taken to the grinding zone by the air, thereby providing sufficient lubrication and cooling. According to the working characteristics of MQL, all droplets would form a conical shape. However, droplets suffer from gravity and air resistance in the forward process of droplets, which indicates that the routes of droplets are similar to a parabola. The longer the nozzle distance is, the greater the influences of air resistance and gravity become. The influences of the nozzle distance L on the surface roughness and grinding forces of the Cf/SiC composites are shown in Fig. 12. Another interesting note is that when L increased from 40 to 60 mm, Sa and Ft decreased from 0.45 to 0.35 μm and 6.3 to 5.2 N, respectively. An excessively small nozzle distance may cause droplets in the aerosol to fail to accumulate in time, which signifies that droplets must break through the air barrier in the form of a fog. However, the trajectories of the small droplets are easily disturbed by the air flow caused by the rotation of the grinding wheel. Therefore, it is difficult to form effective films in the grinding zone. When L was 60 mm, the increased droplet volume helped the droplets break through the gas barrier and enter the grinding zone, which indicates that the grinding performance would be improved, as shown in Fig. 12. However, when L increased from 60 to 100 mm, Sa and Ft increased from 0.35 to 0.62 μm and 5.2 to 11.6 N, respectively. This finding indicates that the excessive distance would reduce the kinetic energy and permeability of the droplets, thereby leading to a reduction in lubrication performance. Therefore, an excessively long or short nozzle distance is not conducive to good lubrication performance in the MQL grinding process.

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Fig. 12. Influences of the nozzle distance l on the surface roughness (a) and grinding forces (b) of Cf/SiC composites. The other NMQL parameters are as follows: C = 5 g/L, P = 7 bar, and Q = 80 mL/h. In general, the grinding performance of Cf/SiC composites is mainly determined by the lubrication and cooling conditions under the same grinding parameters (the linear velocity of the wheel, work speed, and depth of grinding). According to the above experimental results, NMQL technology has an importantly influence on the lubrication state in the grinding area. The air pressure can determine the size and kinetic energy of oil droplets, thus affecting the permeability of the droplets. Similarly, the flow rate determines the density of oil droplets, the nozzle distance affects the kinetic energy of oil droplets, and the concentration of nanoparticles affects the agglomeration of nanofluids. Therefore, it is of great significance to investigate the influences of the air pressure, flow rate, nozzle distance and concentration of nanoparticles on the grinding performance of Cf/SiC composites. 4. Conclusions In the present research, the effects of dry, flood, MQL and carbon NMQL conditions on the grinding forces, surface roughness, surface topography, subsurface damage and grinding chips were studied in detail via grinding tests with unidirectional Cf/SiC composites. To assess the carbon NMQL conditions for grinding Cf/SiC composites, the influences of the NMQL parameters were also investigated in this paper. The following outstanding results can be concluded from the present research: 22

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Carbon nanoparticles can be dispersed into deionized water via oxidation modification and by adding a small amount of surfactant. The structure of this mixture is stable, which means that the pipelines of the MQL system would not be easily clogged by these nanofluids. Therefore, the produced nanofluid satisfies the preconditions for use in an MQL system.

2.

Compared to the dry, flood and MQL grinding conditions, carbon NMQL conditions can provide excellent lubrication and cooling, which lead to high surface quality, small grinding forces and minor subsurface damage. Carbon NMQL can significantly decrease the consumption of grinding fluid, which helps protect the ecological environment. Consequently, the cost of purchasing and subsequent processing of grinding fluid can be dramatically reduced.

3.

According to the evaluation parameters, such as the surface roughness and grinding force, the optimum parameters of the NMQL system can be concluded to be the following: the nanoparticle concentration, air pressure, fluid flow rate, and nozzle distance are 5 g/L, 7 bar, 80 mL/h and 60 mm, respectively.

4.

In the grinding process of unidirectional Cf/SiC composites, matrix cracking, fibre pullout and fibre outcrop are the primary defect types. Long fibre debris, short fibre debris, fibre microparticles and SiC particles are the basic components of the grinding debris.

In the future, more theoretical and mechanism studies should be carried out to better understand the physics behind the effect of carbon nanofluid on the grinding process. Carbon nanoparticles easily agglomerate in deionized water, which can block the pipe of the MQL system. Therefore, a more rational dispersion method should be researched and proposed to improve the application scope of carbon nanofluid. Acknowledgements The author wishes to thank National Natural Science Foundation of China (No. 51775100) and Fundamental Research Funds for the Central Universities of China (No. N180306001) for financial assistance.

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Journal Pre-proof Dear Editors: We would like to submit the enclosed manuscript entitled "An investigation of carbon nanofluid minimum quantity lubrication for grinding unidirectional carbon fibre-reinforced ceramic matrix composites", which we wish to be considered for publication in "Journal of Cleaner Production". No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. Thank you and best regards. Yours sincerely, Yadong Gong

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Carbon nanoparticles were added to a minimum-quantity lubrication grinding system.

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Effects of the lubrication conditions on the grinding characteristics were studied.

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Effects of the lubrication parameters on the grinding characteristics were studied.

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This study meets the demands of cleaner manufacturing.