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Short Communication Proc IMechE Part B: J Engineering Manufacture 1–6 Ó IMechE 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954405417733019 journals.sagepub.com/home/pib Silver and molybdenum disulfide nanoparticles synthesized in situ in dimethylformamide as dielectric for micro-electro discharge machining Soumen Mandal 1,2 , Rajulapati Vinod Kumar 1 and Nagahanumaiah 1,2 Abstract The research focuses on the applicability of silver (Ag) and molybdenum disulfide (MOS 2 ) nanoparticle synthesized in situ in dimethylformamide solution as dielectric material for micro-electro discharge machining. Ag nanoparticles (~120nm size) and MOS 2 nanoparticles (~20nm size) were synthesized in dimethylformamide solution using a combination of nanoparticle solution synthesis routes. A setup for micro-electro discharge machining was developed in-house with an arrangement to generate spark at varying voltages. The setup was integrated with a precise linear height gauge to mea- sure the spark gap during the experiments where Ag and MOS 2 nanoparticles in dimethylformamide solution served as dielectric. The debris was collected and was characterized for each of the experiments. The feature size of the crater generated during the micro-electro discharge machining was also studied. The experiments were repeated with silver and MOS 2 nanoparticle powder mixed with dimethylformamide as dielectric. It was observed that in situ prepared nano- particles in dimethylformamide offered much better machining performance in terms of process stability, crater size and material removal rates. On use of in situ synthesized nanoparticle dielectric, the material removal rate increased by nearly two to three times whereas the spark gap increased by about two times. Keywords Micro-electro discharge machining, spark gap, silver nanoparticle, MOS 2 nanoparticle, process stability, dielectric constant Date received: 27 July 2017; accepted: 27 August 2017 Introduction Powder-mixed dielectric for micro-electro discharge machining (EDM) is gaining significant popularity in research of late due to its large number of advantages such as high material removal rate (MRR), enhanced spark gap, higher process stability and better shape of machined features. 1,2 In powder-mixed micro-EDM, electrically conductive powder is mixed and well dis- persed in the dielectric of micro-EDM, which reduces the insulating strength of the dielectric fluid and increases the spark gap between the tool and the work- piece. As a result, the process becomes more stable thereby improving the MRR and surface roughness. 3,4 The performance of such powder-mixed dielectric depends on factors such as powder size, conductivity of the powder and uniformity with which the powder is mixed with the dielectric. Although size and conductiv- ity of the power-mixed fluid can be controlled easily, the uniformity while mixing is difficult to be assured as the physics behind mixing of a solid powder with a fluid can be of varying nature such as solid suspension and solid deagglomeration. 5 To ensure the uniformity of the mixed powder in the dielectric during EDM process, several researchers have proposed methods to impose external energy to the powder–dielectric mixture. Few of them include use of ultrasonic bath during EDM process, 6,7 use of external stirrer for maintaining uniformity of the dielectric 8 and so on. Although such methods were found to be effec- tive to disperse the powder uniformly in the solution, 1 Microsystem Technology Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India 2 Academy of Scientific and Innovative Research, Chennai, India Corresponding author: Soumen Mandal, Microsystem Technology Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur 713209, West Bengal, India. Email: [email protected]; [email protected]

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Page 1: Proc IMechE Part B: Silver and molybdenum disulfide ...soumenmandal.in/wp-content/uploads/2017/10/Silver-and-Molybednu… · Soumen Mandal1,2, Rajulapati Vinod Kumar1 and Nagahanumaiah1,2

Short Communication

Proc IMechE Part B:J Engineering Manufacture1–6� IMechE 2017Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954405417733019journals.sagepub.com/home/pib

Silver and molybdenum disulfidenanoparticles synthesized in situ indimethylformamide as dielectric formicro-electro discharge machining

Soumen Mandal1,2, Rajulapati Vinod Kumar1 and Nagahanumaiah1,2

AbstractThe research focuses on the applicability of silver (Ag) and molybdenum disulfide (MOS2) nanoparticle synthesized in situin dimethylformamide solution as dielectric material for micro-electro discharge machining. Ag nanoparticles (~120 nmsize) and MOS2 nanoparticles (~20 nm size) were synthesized in dimethylformamide solution using a combination ofnanoparticle solution synthesis routes. A setup for micro-electro discharge machining was developed in-house with anarrangement to generate spark at varying voltages. The setup was integrated with a precise linear height gauge to mea-sure the spark gap during the experiments where Ag and MOS2 nanoparticles in dimethylformamide solution served asdielectric. The debris was collected and was characterized for each of the experiments. The feature size of the cratergenerated during the micro-electro discharge machining was also studied. The experiments were repeated with silverand MOS2 nanoparticle powder mixed with dimethylformamide as dielectric. It was observed that in situ prepared nano-particles in dimethylformamide offered much better machining performance in terms of process stability, crater size andmaterial removal rates. On use of in situ synthesized nanoparticle dielectric, the material removal rate increased bynearly two to three times whereas the spark gap increased by about two times.

KeywordsMicro-electro discharge machining, spark gap, silver nanoparticle, MOS2 nanoparticle, process stability, dielectricconstant

Date received: 27 July 2017; accepted: 27 August 2017

Introduction

Powder-mixed dielectric for micro-electro dischargemachining (EDM) is gaining significant popularity inresearch of late due to its large number of advantagessuch as high material removal rate (MRR), enhancedspark gap, higher process stability and better shape ofmachined features.1,2 In powder-mixed micro-EDM,electrically conductive powder is mixed and well dis-persed in the dielectric of micro-EDM, which reducesthe insulating strength of the dielectric fluid andincreases the spark gap between the tool and the work-piece. As a result, the process becomes more stablethereby improving the MRR and surface roughness.3,4

The performance of such powder-mixed dielectricdepends on factors such as powder size, conductivity ofthe powder and uniformity with which the powder ismixed with the dielectric. Although size and conductiv-ity of the power-mixed fluid can be controlled easily,the uniformity while mixing is difficult to be assured as

the physics behind mixing of a solid powder with afluid can be of varying nature such as solid suspensionand solid deagglomeration.5

To ensure the uniformity of the mixed powder in thedielectric during EDM process, several researchers haveproposed methods to impose external energy to thepowder–dielectric mixture. Few of them include use ofultrasonic bath during EDM process,6,7 use of externalstirrer for maintaining uniformity of the dielectric8 andso on. Although such methods were found to be effec-tive to disperse the powder uniformly in the solution,

1Microsystem Technology Laboratory, CSIR-Central Mechanical

Engineering Research Institute, Durgapur, India2Academy of Scientific and Innovative Research, Chennai, India

Corresponding author:

Soumen Mandal, Microsystem Technology Laboratory, CSIR-Central

Mechanical Engineering Research Institute, Durgapur 713209, West

Bengal, India.

Email: [email protected]; [email protected]

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they require continuous source of external energy whichis region-specific to the powder-mixed dielectric anddepends on the location of energy source. Furthermore,the mixed powder particles settle down rapidly as aresult of agglomeration of the dispersed particles due tolack of capping (stabilizing) agent in the powderparticles.9

To alleviate these issues, we propose a new tech-nique of in situ nanoparticle (NP)-synthesized dielectricfor micro-EDM in which NPs of less than 150nm sizeare prepared in dimethylformamide (DMF) as solventusing standard NP synthesis routes. To universalize theprocess, we selected metallic (silver) NPs and semicon-ducting (MOS2) NPs synthesized in DMF as dielectricmedium for micro-EDM. The performance measuresof synthesized NP in situ in DMF are compared withNP powder mixed in DMF as dielectric for micro-EDM in terms of spark gap, debris formed, amount ofmaterial removed and size of crater.

Materials and methods

Dielectric synthesis and characterization

We used DMF as the solvent for preparation of Ag andMOS2-NP-based dielectric as it is simple to synthesizeNPs in situ in DMF using regular synthesis routes.10

Briefly, for synthesis of Ag-NPs, 20mg polyvinylpyrro-lidone (PVP) as capping agent was mixed in 40mLDMF. The mixture was heated at 150��C for 5min withcontinuous stirring. About 10mg AgNO3 and 5mgNaBH4 were mixed with the solution. The entire mix-ture was stirred magnetically at 1240 r/min for 24 h.The resultant brownish colored solution was subjectedto ultrasonication for 1 h at 70��C and the solution wasfiltered using grade-4 filter paper. The solution was sub-jected to centrifugation at 9000 r/min for 15min andthe supernatant was separated. The supernatant servedas the solution containing silver NPs.11

Synthesis of MOS2-NPs was conducted by mixingand ultrasonication of 20mg MOS2 powder with 40mLDMF containing 10mg PVP using a probe sonicatorfor 1 h followed by centrifugation of yellowish blackmixture at 7000 r/min for 10min. The supernatanthosted MOS2-NPs synthesized in situ in DMF solution.

Synthesis of Ag nanopowder- and MOS2nanopowder-mixed DMF (dielectric) was conducted bymixing these commercially available nanopowders (sil-ver nanopowder \ 150nm particle size and MOS2nanopowder ;90nm particle size from Sigma–Aldrichcompany) in DMF and continuous ultrasonication dur-ing experiments. The amount of nanopowders mixedwas controlled in a manner such that the density ofnanopowder-mixed dielectric and the in situ NP-synthesized dielectric is equal. Accordingly, 0.12mg Agnanopowder and 0.6mg MOS2 were mixed in DMFfor preparation of powder-mixed dielectric.

The in situ synthesized NP dielectric was character-ized using field emission scanning electron microscope

(FESEM). The NP solutions were also characterizedusing NP analyzer (NanoSight NS500). The factor-based increase/decrease in dielectric constant of in situsynthesized Ag and MOS2-NP dielectric and thepowder-mixed dielectric over DMF solution was evalu-ated using cyclic voltammetry (CV) using PARSTAT4000A potentiostat. A standard test cell with platinumworking electrode, Ag/AgCl reference electrode andglassy carbon counter electrode were used for CVsetup. The capacitance of each of the dielectric was cal-culated using equation (1)12

C=iavgs

ð1Þ

where iavg is the average value of maximum currentsweep magnitude in the CV curve, s is the scan rate andC is the capacitance measured.

The relative change in dielectric constant for the NP/nanopowder dielectric over DMF solution was assessedusing equation (2)

et =Cm

Crð2Þ

where Cr is the measured capacitance of the referencesample (DMF), Cm is the measured capacitance of thenanopowder/NP containing dielectric and et is the fac-tor by which the dielectric constant of the nanopowder/NP containing dielectric is higher/lower than DMF.

Experiments for micro-EDM

A setup for micro-EDM was developed in-house whichcomprised a micro-EDM tool, a container for housingthe liquid dielectric, the workpiece, a spark generationcircuit and a linear height gauge for measurement ofspark gap. The linear height gauge was fixed with a lin-ear translation stage which can move along the Z-axisthus moving the tool up or down. The micro-EDM toolwas fixed on the linear translation stage. The tool andthe workpiece were connected to an electrical continuitytesting instrument (multimeter). The experimental setupand the spark generation circuit are shown in Figure 1.

Micro-EDM was conducted on Aluminum-6061 atthree different spark gap voltages, namely, 20, 25 and30V for DMF, in situ synthesized MOS2-NP in DMF,in situ synthesized Ag-NP in DMF, powder-mixedMOS2-NP in DMF and powder-mixed Ag-NP inDMF as dielectric. For generation of spark and mea-surement of spark gap, the Z-axis stage was lowered sothat the tool attached to it touched the work. The tool–workpiece touch was confirmed using continuity test,and the reading on the height gauge was recorded. Thetool was again retraced back, and the spark generatorcircuit was activated. The tool was lowered slowly, andthe spark occurring at a certain height was identifiedby a pop sound. The height gauge reading was againrecorded at the moment the pop sound occurred.The difference between height gauge reading at

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tool–workpiece touch point and spark point renderedthe spark gap length.

After each experiment, the dielectric fluid was col-lected separately. The fluid was centrifuged at 7000 r/min which led to settlement of the debris out from thedielectric solution. The debris was dried and was char-acterized using FESEM. Furthermore, the experimentswere repeated for 50 times (50 sparks) to collect mea-surable amount of debris. The weight of debrisobtained from 50 sparks at each gap voltage for eachdielectric was measured using a precise balance. Themicro-machined samples with single spark were ana-lyzed for their crater size using FESEM.

Experimental results

The CV plots for DMF, in situ synthesized MOS2-NPin DMF, in situ synthesized Ag-NP in DMF, powder-mixed MOS2-NP in DMF and powder-mixed Ag-NP inDMF are shown in Figure 2.

The relative changes in dielectric constant of thematerials with reference to DMF were calculated usingequations (1) and (2). The dielectric constant ofpowder-mixed Ag-NP dielectric in DMF, in situ

synthesized Ag-NP dielectric in DMF, powder-mixedMOS2 dielectric in DMF, in situ synthesized MOS2dielectric in DMF were 6.57, 14.47, 4.07 and 9.47 timeshigher than DMF, respectively.

The FESEM images of in situ synthesized MOS2-NP in DMF and in situ synthesized Ag-NP in DMFare shown in Figure 3.

The NP solutions were also characterized using NPanalyzer (NanoSight NS500) where the size of Ag-NPwas found to be 120nm and the size of MOS2-NP was20nm. The spark gap obtained during micro-EDMprocess at 20, 25 and 30V discharge voltage with DMF,in situ synthesized MOS2-NP in DMF, in situ synthe-sized Ag-NP in DMF, powder-mixed MOS2-NP inDMF and powder-mixed Ag-NP in DMF as dielectricare shown in Table 1.

The FESEM images of debris collected after eachexperiment for spark gap voltage of 25V are shown inFigure 4.

The weight of the debris generated for 50 sparks atthree different gap voltages with different dielectricmaterials is shown in Table 2.

The crater morphology as found using FESEM atspark gap voltage 25V is shown in Figure 5.

Figure 1. (a) Spark generator circuit used for micro-EDM experiments and (b) experimental setup for micro-EDM andmeasurement of spark gap.

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The following could be inferred from the experimen-tal results:

1. The use of in situ NP-synthesized dielectric rendersbetter micro-EDM results as compared tonanopowder-mixed dielectric in terms of processstability. The spark gap while using in situ NP-synthesized dielectric increases twofold as com-pared to nanopowder-mixed dielectric (Table 1).

This might be due to better stability and lack ofagglomeration of NPs in solution due to use ofstabilizing agent for in situ NP-synthesizeddielectric.

2. The increase in spark gap can be justified fromFigure 1 and the dielectric constants of in situsynthesized NP dielectric and -mixed dielectric. Asstated earlier, the dielectric constant of powder-mixed Ag-NP dielectric in DMF, in situ

Figure 2. Cyclic voltammetry plots for (a) DMF, (b) in situ synthesized MOS2-NP in DMF and powder-mixed MOS2-NP in DMF and(c) in situ synthesized Ag-NP in DMF and powder-mixed Ag-NP in DMF.

Figure 3. FESEM image of the in situ synthesized: (a) Ag-NP in DMF and (b) MOS2-NP in DMF.

Table 1. Spark gaps at discharge voltage 20, 25 and 30 V with DMF, in situ synthesized MOS2-NP in DMF, in situ synthesized Ag-NPin DMF, powder-mixed MOS2-NP in DMF and powder-mixed Ag-NP in DMF as dielectric.

Dielectric material Spark gap (mm) at different gap voltages

20 V 25 V 30 V

DMF 8.6 9.2 10.8Powder-mixed MOS2-NP in DMF 35.6 39.1 42.7Powder-mixed Ag-NP in DMF 50.3 58.7 64.7In situ synthesized MOS2-NP in DMF 75.5 86.1 93.2In situ synthesized Ag-NP in DMF 120.8 130.3 143.4

DMF: dimethylformamide.

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synthesized Ag-NP dielectric in DMF, powder-mixed MOS2 dielectric in DMF and in situ synthe-sized MOS2 dielectric in DMF were 6.57, 14.47,4.07 and 9.47 times higher than DMF, respectively.The spark gap for powder-mixed Ag-NP dielectricin DMF, in situ synthesized Ag-NP dielectric inDMF, powder-mixed MOS2 dielectric in DMFand in situ synthesized MOS2 dielectric in DMFalso increased by approximately similar factors(Table 1). This shows that there is a direct correla-tion between the dielectric constant of the dielectricand spark gap length.

3. The FESEM images of the debris as shown inFigure 4 show that the debris generated while usingpowder-mixed dielectric were smaller and withoutany agglomeration, whereas the debris generatedwhile using in situ nanopowder-based dielectric werelarger and shows agglomeration. This indicates thefact that in micro-EDM process, in situ synthesizeddielectric leads to higher spark gap energy as com-pared to nanopowder-mixed dielectric. The higherenergy may be due to higher dielectric constant (bet-ter polarizability) of in situ synthesized dielectric ascompared to nanopowder-mixed dielectric.

4. The weight of debris on use of in situ NP-synthe-sized dielectric increased two- to threefold thanpowder-mixed dielectric (Figure 4). This confirmsthe fact that use of in situ NP-synthesized dielectricrenders higher MRR in micro-EDM process thanpowder-mixed dielectric.

5. The higher MRR can also be confirmed fromFigure 5 wherein the size of machined crater washigher on use of in situ NP-synthesized dielectric.Furthermore, on use of in situ NP-synthesizeddielectric, uniform circular crater structure could befound which indicates uniform distribution of energyin the spark gap. This is due to better and uniformdistribution of the NPs in situ NP-synthesized dielec-tric as compared to nanopowder-mixed dielectric.

Conclusion

The use of powder-mixed dielectric for improvement ofmachining parameters for micro-EDM is an emergingarea of research. In this work, a modification of thepowder-mixed EDM process was conducted whereinNPs were synthesized in situ in a dielectric media(DMF) instead of continuous mixing and stirring of

Table 2. Weight of debris collected after 50 sparks using different dielectric materials.

Dielectric material Weight of debris at different spark gap voltages collected after 50 sparks (mg)

20 V 25 V 30 V

DMF 1.12 1.63 1.81Powder-mixed MOS2-NP in DMF 2.80 3.11 4.55Powder-mixed Ag-NP in DMF 3.77 3.91 4.07In situ synthesized MOS2-NP in DMF 5.11 5.83 6.23In situ synthesized Ag-NP in DMF 8.66 9.02 9.54

DMF: dimethylformamide.

Figure 4. FESEM images of debris generated during the experiments with dielectric: (a) DMF, (b) powder-mixed Ag-NP in DMF, (c)powder-mixed MOS2 in DMF, (d) in situ synthesized MOS2 in DMF and (e) in situ synthesized Ag-NP in DMF.

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the powder with the dielectric. The results and infer-ences indicated that use of in situ synthesized NPdielectric in micro-EDM process enhances the MRRby two to three times and spark gap by two times whilemaintaining uniformity in distribution of spark gapenergy. Moreover, increase in spark gap enhances thestability of micro-EDM process by eliminating unne-cessary arcing phenomenon. Additionally, higher anduniform spark gap energy leads to higher MRR andbetter shape of the machined features. The process isparticularly beneficial for machining industries whereprocess stability and enhancement of MRR in micro-EDM is a major concern. The process also bears bene-fits in terms of energy conservation as external stirringor energy needs not to be applied continuously to thedielectric.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interestwith respect to the research, authorship and/or publica-tion of this article.

Funding

The author(s) received no financial support for theresearch, authorship and/or publication of this article.

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Figure 5. FESEM images of machined craters with dielectric: (a) DMF, (b) powder-mixed Ag-NP in DMF, (c) powder-mixed MOS2 inDMF, (d) in situ synthesized MOS2 in DMF and (e) in situ synthesized Ag-NP in DMF.

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