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An investigation of graphite nanoplatelets as lubricant in grinding
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 49 (2009) 966–970
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
An investigation of graphite nanoplatelets as lubricant in grinding Matthew Alberts a, Kyriaki Kalaitzidou a,b,, Shreyes Melkote a a b
G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, United States School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, United States
a r t i c l e in f o
a b s t r a c t
Article history: Received 20 April 2009 Received in revised form 8 June 2009 Accepted 10 June 2009 Available online 21 June 2009
Cooling and lubrication are very critical to ensure workpiece quality in grinding due to the high friction and intense heat generation involved in the process. Liquid lubricants have traditionally been used in flood form or minimum quantity lubrication (MQL), raising however, major environmental and economic concerns. The focus of this study is to evaluate the performance of graphite nanoplatelets as a lubricant in surface grinding. The role of graphite’s characteristics such as form, size and concentration; and the effect of the carrying medium and the graphite’s application method are determined based on an experimental study. The results indicate that graphite nanoplatelets significantly reduce the grinding forces, specific energy, and improve surface finish during surface grinding of hardened D-2 tool steel. A comparison with results obtained in conventional MQL grinding is also provided. The proper selection of graphite, carrying medium and application method can lead to a low cost, nontoxic and simple alternative to solid lubrication or MQL grinding. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Surface grinding MQL Graphite nanoplatelets Solid lubrication
1. Introduction Surface grinding, an abrasive material-removal technique, generates significant heat and high cutting forces, especially at the workpiece–wheel interface. Cooling and lubrication are necessary to protect the workpiece and wheel from damage such as thermal burn, residual stresses, phase transformations and microcracks [1]. Conventionally, liquid coolants in flood form are employed in grinding. However, issues such as restricted accessibility of coolants in the grinding zone [2] and ineffective heat transfer [3] limit their value in grinding. Additionally, many of these fluids are health hazards raising major environmental concerns [4], and cumbersome to recycle and manage, which significantly increases the total manufacturing cost [5]. A few vegetable-based cutting/grinding oils have been developed, but do not perform as conventional cutting fluids [6]. An alternative to flood cooling is minimum quantity lubrication (MQL) or use of solid lubricants. Dry grinding is not an option because it results in workpiece damage, poor surface finish and accelerated wheel wear due to insufficient chip removal. MQL grinding can produce results very similar to flood cooling if the coolant in MQL does not evaporate due to the grinding heat before it reaches the workpiece–wheel interface [7]. Solid lubricants for grinding should satisfy the following requirements: be able to
Corresponding author at: MaRC Rm 438, 817 Ferst Drive, Atlanta, GA 303320405, United States. Tel.: +404 385 3446; fax: +404 894 9342. E-mail address: [email protected] (K. Kalaitzidou).
0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.06.005
sustain the high temperatures present in the grinding process, be nontoxic, easy to apply and be cost effective [8]. Materials that have been employed as solid lubricants in surface grinding are graphite [8–11], molybdenum trioxide [8] and molybdenum disulfide [11–13], and calcium and barium fluoride [8]. Graphite and molybdenum disulfide are the most common ones, owing their lubrication properties to their layered morphology and crystal structure [14]. Their morphology consists of a hexagonal arrangement of carbon atoms which form stable planar lattices due to strong covalent bonds. Parallel planes stay together due to weak inter-layer bonding, mainly van der Waals forces. The tangential frictional force present during the grinding process breaks these weak electron bonds causing inter-layer slip with a low friction coefficient [14]. In addition to the morphology and crystal structure of the solid lubricant particles, the way the particles are introduced at the workpiece–wheel interface, and their size and quantity also play a dominant role in the lubrication achieved during the grinding process. One approach is to add the solid lubricant in powder form directly to the grinding zone using an automated feeder [12]. Although the resulting lubrication is sufficient, there is still a need for flushing action and tool cleaning making solid lubrication less attractive than conventional liquid-lubrication methods. Instead of powder form, a thick paste of the solid lubricant made from water-soluble oil and general purpose grease has also been used [8]. However, it yielded significantly lower forces: wheel loading due to ineffective removal of the lubricant paste and the formed chips is the major limitation of this application method. Finally, a third application method is to disperse the particles in waterbased oils and perform grinding under MQL conditions [13]. The
ARTICLE IN PRESS M. Alberts et al. / International Journal of Machine Tools & Manufacture 49 (2009) 966–970
solid lubricant used was molybdenum disulfide with an average particle size of 250 nm at concentrations of 5 and 20 wt% with respect to the oil. The results are very encouraging but the required amount of nanomaterial significantly increases the overall cost. The above-mentioned studies indicate the great potential of using solid lubricants for low cost and environment-friendly grinding under the condition that the proper material and application method are identified. This study focuses on investigating the effect of graphite nanoplatelets in solid lubrication grinding and aims at developing an application method that eliminates the use of any oils or toxic organic lubricants. Specifically, the role of graphite size, concentration and nature on the forces, specific energy and surface finish in surface grinding are evaluated.
2. Materials and methods 2.1. Graphite nanoplatelets The graphite nanoplatelets with the trade name xGnPTM (XG Sciences, East Lansing, Michigan) were used in this study. They are made from synthetic, acid-intercalated graphite based on a microwave exfoliation method [15]. Two different types of exfoliated graphite nanoplatelets, xGnP, are used in this study with the only difference being their diameters. Both types have an average platelet thickness of 5–10 nm and their diameter is either 1 mm (for the xGnP-1 type), or 15 mm (for the xGnP-15 graphite type) shown in Fig. 1a. Each platelet consists of 10–15 graphene sheets, as indicated in Fig. 1b showing two neighboring platelets, which are kept close together by van der Waals forces and they can easily slip relative to each other under the application of shear loads [16]. The material is highly crystalline, has a large surface area of 60 m2/g, is nontoxic and costs 5–10 $/ lb depending on the platelet diameter. Two other types of graphite platelets supplied by Timcal (Houston, TX) were also used for comparison purposes. These include TimrexSFG-15 and TimrexKS4, both synthetic, with average diameters of 15 and 4 mm, respectively, and corresponding surface areas are 9.5 and 26 m2/g, respectively. 2.2. Grinding experiments The experimental study conducted investigated the effects of graphite type, concentration and size on the forces, specific energy and surface finish in surface grinding of hardened D-2 tool steel
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(62 HRC). The graphite was dispersed in isopropyl alcohol (IPA), a common low-cost and relatively nontoxic organic solvent with boiling point 80 1C. A sonicator (Misonic Sonicator 4000, probe diameter 12.7 mm) was used at 35% amplitude for 30 min to disperse the graphite in the solvent. The resulting solution was used immediately for grinding in order to avoid agglomeration and/or precipitation of the graphite platelets. The graphite–IPA dispersion was introduced into the grinding process by (i) spraying the workpiece–wheel interface using two common hand pump spray bottles with a combined flow rate of 200 ml/min, or (ii) spray coating the workpiece surface prior to grinding. For comparison purposes, graphite was also dispersed in a common cutting fluid called Trim SC200, which is a semisynthetic, water-based emulsion with boiling point 102 1C, and applied to the grinding zone using the same method as the graphite–IPA dispersion. Furthermore, xGnP was compared to the Timcal graphite. Note that the latter graphite type is typically recommended for use as a lubricant in concentrations of 5–40 wt% with grease as the carrying medium. The solid lubrication results were compared to results obtained for dry and MQL (just Trim SC200 fluid without graphite) grinding. The experimental parameters that were investigated and their corresponding range are summarized in Table 1. A Kistler 9257B 3-Component Dynamometer was used to measure the normal and horizontal grinding forces. The specific grinding energy was calculated from the measured force data as the ratio of the grinding power ( ¼ horizontal force peripheral velocity of the grinding wheel) to the volumetric removal rate. The surface finish was characterized using microscope white light scanning interferometer-based surface texture measuring instrument (ZYGO New View 200). The surface grinder utilized was equipped with a 304.8-mm-diameter, 25.4-mm-wide aluminum oxide grinding wheel (32A46-HVBE). The dynamometer was placed in the center of the magnetic chuck on the grinder and a 152 mm 101 mm 12.7 mm block of hardened D-2 tool steel (62 HRC) was bolted to it. The wheel rotational speed was set to
Table 1 Summary of the experimental parameters studied and their investigation range. Parameter
Investigation range
1. xGnP concentration 2. xGnP diameter 3. Graphite source 4. Carrying medium 5. Application method
0–2 wt% 1 and 15 mm xGnP and Timcal IPA and Trim SC200 Spraying, coating
Fig. 1. (a) ESEM micrograph of xGnP-15 (scale bar:100 mm) and (b) TEM side view of xGnP-1 (scale bar:5 nm) [15].
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Table 2 Grinding conditions. Grinding conditions Workpiece material Hardness Table speed Wheel peripheral speed Wheel rotational speed Wheel diameter Depth of cut Width of cut
D-2 tool steel 62 HRC 0.75 m/s 25 m/s 1600 rpm 304.8 mm 50 mm 25.4 mm
1600 rpm yielding a peripheral wheel speed of 25 m/s. The table (workpiece) speed was set to .75 m/min, which is below standard table speeds to ensure that adequate data could be gathered in a single pass of the grinding wheel. A total of five adjacent grinding wheel passes were used for each test condition. Note that the wheel was dressed similarly prior to each set of five passes in order to maintain the same initial wheel condition for each test condition. The grinding conditions are given in Table 2.
Fig. 2. Normal grinding force as a function of graphite diameter and concentration, and carrying medium.
3. Results and discussion 3.1. Effects of graphite characteristics and dispersing medium The effects of graphite characteristics such as diameter and concentration (wt%) in the grinding fluid (Trim SC200) on the normal and horizontal grinding forces and the specific energy are shown in Figs. 2–4, respectively. The error bars in the plots represent the standard deviation of the average grinding forces obtained in the tests carried out at each condition. A comparison between Trim SC200 and isopropyl alcohol, a low-cost and nontoxic solvent, is also presented. The results show that pure IPA is not as good a lubricant as pure Trim SC200. This is because IPA by itself does not possess good lubrication properties. However, as soon as graphite nanoplatelets are added to IPA, the grinding forces and specific energy are significantly reduced even for concentrations of graphite as low as 0.5 wt%. Addition of graphite in Trim SC200 shows a similar trend but the effects on forces and specific energy are noticeably weaker compared to those with pure Trim SC200. This observation points toward synergy between graphite and IPA which can be explained based on the lubrication mechanism and the interactions between the materials involved. Graphite is a good lubricant due to its layered structure. In the presence of shear loads, the platelets can slide pass each other, thus reducing the frictional forces at the wheel–workpiece interface. Lubrication can be achieved at very low concentrations of graphite if the platelets form a continuous thin film at the interface. Graphite is hydrophobic, so the platelets tend to agglomerate when dispersed in the water-based Trim SC200 and form a thicker but not necessarily a continuous film at the interface. On the other hand, the presence of IPA facilitates the formation of a continuous graphite film. Graphite stays well dispersed in IPA, which starts evaporating (80 1C) as soon as the graphite–IPA solution is delivered to the interface. As IPA evaporates, it moves the platelets which start connecting to each other forming a continuous film. Another advantage of IPA compared to Trim SC200 is its low viscosity which allows for more efficient entry into the workpiece–wheel interface. In addition to the effect of the carrying medium, the concentration of graphite in the solution and the platelet diameter also affect the lubrication properties of graphite. As shown in Figs. 2–4, both the grinding forces and the specific energy reach a
Fig. 3. Horizontal grinding force as a function of graphite diameter and concentration, and carrying medium.
Fig. 4. Specific energy as a function of graphite diameter and concentration, and carrying medium.
plateau at 1 wt% of graphite for any of the two media used, i.e., IPA and Trim SC200. Higher graphite concentration increases the cost without offering any additional lubrication. The larger diameter platelets are more effective in lubrication because of the larger contact area with the wheel surface. The effect of platelet diameter is more obvious in the case of graphite
ARTICLE IN PRESS M. Alberts et al. / International Journal of Machine Tools & Manufacture 49 (2009) 966–970
dispersed in Trim SC200. Another possible mechanism for energy absorption in case of larger graphite platelets is their flexibility, which permits them to bend, buckle or even roll-up forming hollow tubes as has been experimentally observed for large graphite platelets dispersed in a polymer matrix [15]. However, an optimum platelet diameter size is also expected because larger platelets have a higher tendency to agglomerate but most importantly they will not be able to easily access the grinding zone. Although in some cases presented in Figs. 2–4 the error bars overlap, the observed trends are statistically significant according to the t-test statistics at a confidence level of 95%. The specific trends are as follows: (i) the larger platelets are more effective than the smaller ones for all three graphite concentrations tested and for both carrying media; (ii) IPA is more effective than Trim SC200 for both graphite sizes and all three concentrations used.
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surface area which is 60 m2/g for xGnP and 9.5 and 26 m2/g for the Timcal-4 and Timcal-15, respectively. The lack of information about the crystallite size of the two graphite types does not allow for a complete understanding of the lubrication mechanism. Although on average, xGnP-15 yields slightly better results than Timcal-15, it can be concluded that the graphite source does not have a significant effect on the grinding forces and specific energy indicating that the inherent lubrication ability of graphite overcomes the above differences.
3.3. Effect of application method
In addition to the diameter and concentration of graphite, its intrinsic properties such as surface area, crystallinity, impurities and platelet thickness also affect its lubrication ability. A comparison of the two graphite sources (xGnP from XG Sciences and graphite from Timcal) in terms of grinding forces and specific energy is provided in Figs. 5–7. IPA was used as the carrying medium. It is known that the two graphite platelet types are made from synthetic graphite, have similar diameters and differ in their
The application method, that is how the lubricant is introduced into the grinding zone, can have a strong effect on the lubrication process. The basic application method employed in this study is the graphite–IPA delivered by spraying during grinding. An alternative approach is to coat the workpiece surface with the graphite–IPA solution prior to grinding. The two are compared in terms of grinding forces and specific energy in Fig. 8. It is concluded that the coating method leads to better lubrication. The basic difference is that the graphite layer formed during coating is uniform and homogenous completely covering the workpiece area, whereas in the case of spraying it is possible that some platelets are pushed away from the grinding zone so they do not really contribute to lubrication. Again, according to the t-test statistics, the differences are significant for confidence level 95% for both graphite concentrations tested.
Fig. 5. Normal grinding force as a function of graphite diameter and concentration, and source.
Fig. 7. Specific energy as a function of graphite diameter and concentration, and source.
Fig. 6. Horizontal grinding force as a function of graphite diameter and concentration, and source.
Fig. 8. Grinding forces and specific energy for 1 wt% xGnP-15/IPA as a function of the application method.
3.2. Effect of graphite source
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quently minimum cost. The carrying medium used is IPA, a low cost and nontoxic solvent that, due to its low evaporation point, eliminates issues related to management and recycling of liquid lubricants. IPA itself has no lubrication action but is a good dispersing medium for graphite nanoplatelets, and its low viscosity allows for easy entry of the graphite–IPA solution into the grinding zone. Finally, both application methods employed in this study, spraying and coating, are simple to implement, lead to significant reduction of the grinding forces and the specific energy during surface grinding, and improve the surface finish.
Acknowledgements The authors acknowledge the support from Prof. W.J. Wepfer, Chair of the Woodruff School of Mechanical Engineering and Prof. Melkote’s Woodruff Faculty Fellowship. In addition, the authors thank XG Sciences and Timcal for providing the graphite; and Braddock Metallurgical for heat treating the D-2 samples. References
Fig. 9. Surface roughness as a function of graphite diameter and concentration, dispersing medium and application method.
3.4. Roughness characterization Finally, the lubrication ability of graphite was also evaluated in terms of the roughness of the final surface. The root mean square (RMS) roughness was measured both perpendicular and parallel to the grinding direction and the results for all the parameters (graphite size and concentration, carrying medium, application method) are presented in Fig. 9. As shown, IPA reduces the surface roughness by 30% and 45% compared to Trim SC200 for the cases xGnP-1 and xGnP-15, respectively. A smoother surface is obtained in case of xGnP-15 dispersed in IPA and applied by the (no-spray) coating method. This is the combination that also resulted in lower grinding forces and specific energy. 4. Conclusions The use of graphite nanoplatelets as a solid lubricant in surface grinding was systematically studied through experiments. It is concluded that the most important parameters are the diameter of the graphite platelets, the carrying medium and the application method. The larger diameter (15 mm) platelets are more effective than the smaller ones (1 mm) from the point of view of reducing grinding forces, specific energy and roughness. The graphite concentration was optimized to 1 wt%, so that lubrication is achieved at the minimum possible graphite content and conse-
[1] E. Brinksmeier, C. Heinzel, M. Wittmann, Friction, cooling and lubrication in grinding, Ann. ClRP 48 (2) (1999) 581–598. [2] S. Malkin, C. Guo, Thermal analysis of grinding, Ann. CIRP 56 (2) (2007). [3] T.D. Howes, Assessment of the cooling and lubricative property of grinding fluids, Ann. CIRP 39 (1) (1990) 313–315. [4] T.D. Howes, H.K. Toenshoff, W. Heuer, Environmental aspects of grinding fluids, Ann. CIRP 40 (2) (1991) 623–629. [5] E. Brinksmeier, A. Walter, R. Janssen, P. Dierson, Aspects of cooling lubrication reduction in machining advanced materials, Proc. Inst. Mech. Eng. P. B J. Eng. Manuf. 213 (B8) (1999) 769–778. [6] S. Malkin, Grinding Technology, Wiley, New York, 1989. [7] L.R. da Silva, E.C. Bianchi, R.Y. Fusse, R.E. Catai, T.V. Franca, P.R. Aguiar, Analysis of surface integrity in minimum quantity lubricant—MQL in grinding, Int. J. Mach. Tools Manuf. 47 (2007) 412–418. [8] S. Shaji, V. Radhakrishnan, Investigations on the application of solid lubricants in grinding, Proc. Inst. Mech. Eng. P. B J. Eng. Manuf. 216 (2002) 1325–1343. [9] S. Shaji, V. Radhakrishnan, An investigation on surface grinding using graphite as lubricant, Int. J. Mach. Tools Manuf. 42 (2002) 733–740. [10] S. Shaji, V. Radhakrishnan, Application of solid lubricants in grinding: Investigations of graphite sandwiched grinding wheels, Mach. Sci. Technol. 7 (1) (2003) 137–155. [11] N.S. Reddy, P.V. Rao, Experimental investigation to study the effect of solid lubricants on cutting forces and surface quality in end milling, Int. J. Mach. Tools Des. 46 (2006) 189–198. [12] D. Mukhopadhyay, S. Banerjee, N.S. Reddy, Investigation to study the application of solid lubricant in turning AISI 1040 steel, Trans. ASME 129 (2007) 520–526. [13] B. Shen, P. Kalita, A. Malshe, A. Shih, Performance of novel MoS2 nanoparticles based grinding fluids in minimum quantity lubrications grinding, Trans. NAMRI/SME 36 (2008) 357–364. [14] P.J. Bryant, P.L. Gutshall, L.H. Taylor, A study on mechanisms of graphite friction and wear, Wear 7 (3) (1964) 118–119. [15] H. Fukushima, L.T. Drzal, Graphite nanoplatelets as reinforcements for polymers: structural and electrical properties, in: Proceedings of the 17th Annual Conference of the American Society for Composites, Purdue University, 2003. [16] K. Kalaitzidou, H. Fukushima, L.T. Drzal, Mechanical properties and morphological characterization of exfoliated graphite–polypropylene nanocomposites, Compos P. A-Appl. Sci. 38 (2007) 1675–1682.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
An investigation of graphite nanoplatelets as lubricant in grinding Matthew Alberts a, Kyriaki Kalaitzidou a,b,, Shreyes Melkote a a b
G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, United States School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, United States
a r t i c l e in f o
a b s t r a c t
Article history: Received 20 April 2009 Received in revised form 8 June 2009 Accepted 10 June 2009 Available online 21 June 2009
Cooling and lubrication are very critical to ensure workpiece quality in grinding due to the high friction and intense heat generation involved in the process. Liquid lubricants have traditionally been used in flood form or minimum quantity lubrication (MQL), raising however, major environmental and economic concerns. The focus of this study is to evaluate the performance of graphite nanoplatelets as a lubricant in surface grinding. The role of graphite’s characteristics such as form, size and concentration; and the effect of the carrying medium and the graphite’s application method are determined based on an experimental study. The results indicate that graphite nanoplatelets significantly reduce the grinding forces, specific energy, and improve surface finish during surface grinding of hardened D-2 tool steel. A comparison with results obtained in conventional MQL grinding is also provided. The proper selection of graphite, carrying medium and application method can lead to a low cost, nontoxic and simple alternative to solid lubrication or MQL grinding. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Surface grinding MQL Graphite nanoplatelets Solid lubrication
1. Introduction Surface grinding, an abrasive material-removal technique, generates significant heat and high cutting forces, especially at the workpiece–wheel interface. Cooling and lubrication are necessary to protect the workpiece and wheel from damage such as thermal burn, residual stresses, phase transformations and microcracks [1]. Conventionally, liquid coolants in flood form are employed in grinding. However, issues such as restricted accessibility of coolants in the grinding zone [2] and ineffective heat transfer [3] limit their value in grinding. Additionally, many of these fluids are health hazards raising major environmental concerns [4], and cumbersome to recycle and manage, which significantly increases the total manufacturing cost [5]. A few vegetable-based cutting/grinding oils have been developed, but do not perform as conventional cutting fluids [6]. An alternative to flood cooling is minimum quantity lubrication (MQL) or use of solid lubricants. Dry grinding is not an option because it results in workpiece damage, poor surface finish and accelerated wheel wear due to insufficient chip removal. MQL grinding can produce results very similar to flood cooling if the coolant in MQL does not evaporate due to the grinding heat before it reaches the workpiece–wheel interface [7]. Solid lubricants for grinding should satisfy the following requirements: be able to
Corresponding author at: MaRC Rm 438, 817 Ferst Drive, Atlanta, GA 303320405, United States. Tel.: +404 385 3446; fax: +404 894 9342. E-mail address: [email protected] (K. Kalaitzidou).
0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.06.005
sustain the high temperatures present in the grinding process, be nontoxic, easy to apply and be cost effective [8]. Materials that have been employed as solid lubricants in surface grinding are graphite [8–11], molybdenum trioxide [8] and molybdenum disulfide [11–13], and calcium and barium fluoride [8]. Graphite and molybdenum disulfide are the most common ones, owing their lubrication properties to their layered morphology and crystal structure [14]. Their morphology consists of a hexagonal arrangement of carbon atoms which form stable planar lattices due to strong covalent bonds. Parallel planes stay together due to weak inter-layer bonding, mainly van der Waals forces. The tangential frictional force present during the grinding process breaks these weak electron bonds causing inter-layer slip with a low friction coefficient [14]. In addition to the morphology and crystal structure of the solid lubricant particles, the way the particles are introduced at the workpiece–wheel interface, and their size and quantity also play a dominant role in the lubrication achieved during the grinding process. One approach is to add the solid lubricant in powder form directly to the grinding zone using an automated feeder [12]. Although the resulting lubrication is sufficient, there is still a need for flushing action and tool cleaning making solid lubrication less attractive than conventional liquid-lubrication methods. Instead of powder form, a thick paste of the solid lubricant made from water-soluble oil and general purpose grease has also been used [8]. However, it yielded significantly lower forces: wheel loading due to ineffective removal of the lubricant paste and the formed chips is the major limitation of this application method. Finally, a third application method is to disperse the particles in waterbased oils and perform grinding under MQL conditions [13]. The
ARTICLE IN PRESS M. Alberts et al. / International Journal of Machine Tools & Manufacture 49 (2009) 966–970
solid lubricant used was molybdenum disulfide with an average particle size of 250 nm at concentrations of 5 and 20 wt% with respect to the oil. The results are very encouraging but the required amount of nanomaterial significantly increases the overall cost. The above-mentioned studies indicate the great potential of using solid lubricants for low cost and environment-friendly grinding under the condition that the proper material and application method are identified. This study focuses on investigating the effect of graphite nanoplatelets in solid lubrication grinding and aims at developing an application method that eliminates the use of any oils or toxic organic lubricants. Specifically, the role of graphite size, concentration and nature on the forces, specific energy and surface finish in surface grinding are evaluated.
2. Materials and methods 2.1. Graphite nanoplatelets The graphite nanoplatelets with the trade name xGnPTM (XG Sciences, East Lansing, Michigan) were used in this study. They are made from synthetic, acid-intercalated graphite based on a microwave exfoliation method [15]. Two different types of exfoliated graphite nanoplatelets, xGnP, are used in this study with the only difference being their diameters. Both types have an average platelet thickness of 5–10 nm and their diameter is either 1 mm (for the xGnP-1 type), or 15 mm (for the xGnP-15 graphite type) shown in Fig. 1a. Each platelet consists of 10–15 graphene sheets, as indicated in Fig. 1b showing two neighboring platelets, which are kept close together by van der Waals forces and they can easily slip relative to each other under the application of shear loads [16]. The material is highly crystalline, has a large surface area of 60 m2/g, is nontoxic and costs 5–10 $/ lb depending on the platelet diameter. Two other types of graphite platelets supplied by Timcal (Houston, TX) were also used for comparison purposes. These include TimrexSFG-15 and TimrexKS4, both synthetic, with average diameters of 15 and 4 mm, respectively, and corresponding surface areas are 9.5 and 26 m2/g, respectively. 2.2. Grinding experiments The experimental study conducted investigated the effects of graphite type, concentration and size on the forces, specific energy and surface finish in surface grinding of hardened D-2 tool steel
967
(62 HRC). The graphite was dispersed in isopropyl alcohol (IPA), a common low-cost and relatively nontoxic organic solvent with boiling point 80 1C. A sonicator (Misonic Sonicator 4000, probe diameter 12.7 mm) was used at 35% amplitude for 30 min to disperse the graphite in the solvent. The resulting solution was used immediately for grinding in order to avoid agglomeration and/or precipitation of the graphite platelets. The graphite–IPA dispersion was introduced into the grinding process by (i) spraying the workpiece–wheel interface using two common hand pump spray bottles with a combined flow rate of 200 ml/min, or (ii) spray coating the workpiece surface prior to grinding. For comparison purposes, graphite was also dispersed in a common cutting fluid called Trim SC200, which is a semisynthetic, water-based emulsion with boiling point 102 1C, and applied to the grinding zone using the same method as the graphite–IPA dispersion. Furthermore, xGnP was compared to the Timcal graphite. Note that the latter graphite type is typically recommended for use as a lubricant in concentrations of 5–40 wt% with grease as the carrying medium. The solid lubrication results were compared to results obtained for dry and MQL (just Trim SC200 fluid without graphite) grinding. The experimental parameters that were investigated and their corresponding range are summarized in Table 1. A Kistler 9257B 3-Component Dynamometer was used to measure the normal and horizontal grinding forces. The specific grinding energy was calculated from the measured force data as the ratio of the grinding power ( ¼ horizontal force peripheral velocity of the grinding wheel) to the volumetric removal rate. The surface finish was characterized using microscope white light scanning interferometer-based surface texture measuring instrument (ZYGO New View 200). The surface grinder utilized was equipped with a 304.8-mm-diameter, 25.4-mm-wide aluminum oxide grinding wheel (32A46-HVBE). The dynamometer was placed in the center of the magnetic chuck on the grinder and a 152 mm 101 mm 12.7 mm block of hardened D-2 tool steel (62 HRC) was bolted to it. The wheel rotational speed was set to
Table 1 Summary of the experimental parameters studied and their investigation range. Parameter
Investigation range
1. xGnP concentration 2. xGnP diameter 3. Graphite source 4. Carrying medium 5. Application method
0–2 wt% 1 and 15 mm xGnP and Timcal IPA and Trim SC200 Spraying, coating
Fig. 1. (a) ESEM micrograph of xGnP-15 (scale bar:100 mm) and (b) TEM side view of xGnP-1 (scale bar:5 nm) [15].
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Table 2 Grinding conditions. Grinding conditions Workpiece material Hardness Table speed Wheel peripheral speed Wheel rotational speed Wheel diameter Depth of cut Width of cut
D-2 tool steel 62 HRC 0.75 m/s 25 m/s 1600 rpm 304.8 mm 50 mm 25.4 mm
1600 rpm yielding a peripheral wheel speed of 25 m/s. The table (workpiece) speed was set to .75 m/min, which is below standard table speeds to ensure that adequate data could be gathered in a single pass of the grinding wheel. A total of five adjacent grinding wheel passes were used for each test condition. Note that the wheel was dressed similarly prior to each set of five passes in order to maintain the same initial wheel condition for each test condition. The grinding conditions are given in Table 2.
Fig. 2. Normal grinding force as a function of graphite diameter and concentration, and carrying medium.
3. Results and discussion 3.1. Effects of graphite characteristics and dispersing medium The effects of graphite characteristics such as diameter and concentration (wt%) in the grinding fluid (Trim SC200) on the normal and horizontal grinding forces and the specific energy are shown in Figs. 2–4, respectively. The error bars in the plots represent the standard deviation of the average grinding forces obtained in the tests carried out at each condition. A comparison between Trim SC200 and isopropyl alcohol, a low-cost and nontoxic solvent, is also presented. The results show that pure IPA is not as good a lubricant as pure Trim SC200. This is because IPA by itself does not possess good lubrication properties. However, as soon as graphite nanoplatelets are added to IPA, the grinding forces and specific energy are significantly reduced even for concentrations of graphite as low as 0.5 wt%. Addition of graphite in Trim SC200 shows a similar trend but the effects on forces and specific energy are noticeably weaker compared to those with pure Trim SC200. This observation points toward synergy between graphite and IPA which can be explained based on the lubrication mechanism and the interactions between the materials involved. Graphite is a good lubricant due to its layered structure. In the presence of shear loads, the platelets can slide pass each other, thus reducing the frictional forces at the wheel–workpiece interface. Lubrication can be achieved at very low concentrations of graphite if the platelets form a continuous thin film at the interface. Graphite is hydrophobic, so the platelets tend to agglomerate when dispersed in the water-based Trim SC200 and form a thicker but not necessarily a continuous film at the interface. On the other hand, the presence of IPA facilitates the formation of a continuous graphite film. Graphite stays well dispersed in IPA, which starts evaporating (80 1C) as soon as the graphite–IPA solution is delivered to the interface. As IPA evaporates, it moves the platelets which start connecting to each other forming a continuous film. Another advantage of IPA compared to Trim SC200 is its low viscosity which allows for more efficient entry into the workpiece–wheel interface. In addition to the effect of the carrying medium, the concentration of graphite in the solution and the platelet diameter also affect the lubrication properties of graphite. As shown in Figs. 2–4, both the grinding forces and the specific energy reach a
Fig. 3. Horizontal grinding force as a function of graphite diameter and concentration, and carrying medium.
Fig. 4. Specific energy as a function of graphite diameter and concentration, and carrying medium.
plateau at 1 wt% of graphite for any of the two media used, i.e., IPA and Trim SC200. Higher graphite concentration increases the cost without offering any additional lubrication. The larger diameter platelets are more effective in lubrication because of the larger contact area with the wheel surface. The effect of platelet diameter is more obvious in the case of graphite
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dispersed in Trim SC200. Another possible mechanism for energy absorption in case of larger graphite platelets is their flexibility, which permits them to bend, buckle or even roll-up forming hollow tubes as has been experimentally observed for large graphite platelets dispersed in a polymer matrix [15]. However, an optimum platelet diameter size is also expected because larger platelets have a higher tendency to agglomerate but most importantly they will not be able to easily access the grinding zone. Although in some cases presented in Figs. 2–4 the error bars overlap, the observed trends are statistically significant according to the t-test statistics at a confidence level of 95%. The specific trends are as follows: (i) the larger platelets are more effective than the smaller ones for all three graphite concentrations tested and for both carrying media; (ii) IPA is more effective than Trim SC200 for both graphite sizes and all three concentrations used.
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surface area which is 60 m2/g for xGnP and 9.5 and 26 m2/g for the Timcal-4 and Timcal-15, respectively. The lack of information about the crystallite size of the two graphite types does not allow for a complete understanding of the lubrication mechanism. Although on average, xGnP-15 yields slightly better results than Timcal-15, it can be concluded that the graphite source does not have a significant effect on the grinding forces and specific energy indicating that the inherent lubrication ability of graphite overcomes the above differences.
3.3. Effect of application method
In addition to the diameter and concentration of graphite, its intrinsic properties such as surface area, crystallinity, impurities and platelet thickness also affect its lubrication ability. A comparison of the two graphite sources (xGnP from XG Sciences and graphite from Timcal) in terms of grinding forces and specific energy is provided in Figs. 5–7. IPA was used as the carrying medium. It is known that the two graphite platelet types are made from synthetic graphite, have similar diameters and differ in their
The application method, that is how the lubricant is introduced into the grinding zone, can have a strong effect on the lubrication process. The basic application method employed in this study is the graphite–IPA delivered by spraying during grinding. An alternative approach is to coat the workpiece surface with the graphite–IPA solution prior to grinding. The two are compared in terms of grinding forces and specific energy in Fig. 8. It is concluded that the coating method leads to better lubrication. The basic difference is that the graphite layer formed during coating is uniform and homogenous completely covering the workpiece area, whereas in the case of spraying it is possible that some platelets are pushed away from the grinding zone so they do not really contribute to lubrication. Again, according to the t-test statistics, the differences are significant for confidence level 95% for both graphite concentrations tested.
Fig. 5. Normal grinding force as a function of graphite diameter and concentration, and source.
Fig. 7. Specific energy as a function of graphite diameter and concentration, and source.
Fig. 6. Horizontal grinding force as a function of graphite diameter and concentration, and source.
Fig. 8. Grinding forces and specific energy for 1 wt% xGnP-15/IPA as a function of the application method.
3.2. Effect of graphite source
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quently minimum cost. The carrying medium used is IPA, a low cost and nontoxic solvent that, due to its low evaporation point, eliminates issues related to management and recycling of liquid lubricants. IPA itself has no lubrication action but is a good dispersing medium for graphite nanoplatelets, and its low viscosity allows for easy entry of the graphite–IPA solution into the grinding zone. Finally, both application methods employed in this study, spraying and coating, are simple to implement, lead to significant reduction of the grinding forces and the specific energy during surface grinding, and improve the surface finish.
Acknowledgements The authors acknowledge the support from Prof. W.J. Wepfer, Chair of the Woodruff School of Mechanical Engineering and Prof. Melkote’s Woodruff Faculty Fellowship. In addition, the authors thank XG Sciences and Timcal for providing the graphite; and Braddock Metallurgical for heat treating the D-2 samples. References
Fig. 9. Surface roughness as a function of graphite diameter and concentration, dispersing medium and application method.
3.4. Roughness characterization Finally, the lubrication ability of graphite was also evaluated in terms of the roughness of the final surface. The root mean square (RMS) roughness was measured both perpendicular and parallel to the grinding direction and the results for all the parameters (graphite size and concentration, carrying medium, application method) are presented in Fig. 9. As shown, IPA reduces the surface roughness by 30% and 45% compared to Trim SC200 for the cases xGnP-1 and xGnP-15, respectively. A smoother surface is obtained in case of xGnP-15 dispersed in IPA and applied by the (no-spray) coating method. This is the combination that also resulted in lower grinding forces and specific energy. 4. Conclusions The use of graphite nanoplatelets as a solid lubricant in surface grinding was systematically studied through experiments. It is concluded that the most important parameters are the diameter of the graphite platelets, the carrying medium and the application method. The larger diameter (15 mm) platelets are more effective than the smaller ones (1 mm) from the point of view of reducing grinding forces, specific energy and roughness. The graphite concentration was optimized to 1 wt%, so that lubrication is achieved at the minimum possible graphite content and conse-
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