Implications of the 3D micro scale coal characteristics ... of the 3D micro scale coal...

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Implications of the 3D micro scale coal characteristics along with Raman stress mapping of the scratch tracks G.L. Manjunath, Rajesh R. Nair Department of Ocean Engineering, IIT Madras, Chennai, India abstract article info Article history: Received 21 February 2015 Received in revised form 24 February 2015 Accepted 24 February 2015 Available online 4 March 2015 Keywords: Fracture toughness Micro indentation Micro scratch test Raman stress mapping Macerals Petrography The microscale interfacial characteristics of coal are studied based on the analysis of mechanical behavior of in- dividual elements contributing to coal heterogeneity with respect to high volatile bituminous A of coal, from Singrauli coal eld, Madhya Pradesh, India. Micro-scale delineation of elastic plastic properties of coal by grid micro-indentation test and fracture toughness was accomplished by micro-scratch test on collotenite maceral. Micro-scratch test was performed in different directions on and critical points of failure were diagnosed by mea- suring acoustic emissions and tangential force. The 3D heterogeneity of coal is studied based on the damages at certain point of loads known as critical points with respect to distribution of macerals derived from coal petrog- raphy. Damages were observed at higher critical loads on samples collected from 332.9335.6 m depth compared to 329.7330.9 m depth samples owing to less heterogeneity and increased compaction. Raman stress mapping at critical points of scratch track revealed a spectral shift due to stress inversion. Shift from 15761593 cm 1 for graphite band (G), and from 13441357 cm 1 for disordered Carbon (D) band indicate the nature of stress and the deformation occurred for coal bulk sample. Raman spectra variation for maceral collotelinite is studied and compared with coal bulk samples behaviour a shift from 15741597 cm 1 for G, and from 13461356 cm 1 for D. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The heterogeneous complex structural property of coal affects its be- havior and chemical reactivity. Coal characteristics based on thermo- chemico-mechanical variations must be analyzed thoroughly. Broader perspective of coal heterogeneity is an important requirement for the exploration of deep coal by means of Underground Coal Gasication (UCG) (Shahidzadeh-Bonn et al., 2005). Fundamentally coal heteroge- neity depends on several aspects such as layering and the litho type microlithotype occurrences, and the stress elds that along with miner- al matter cleats orientation, macro-to-nano-porosity and chemical min- eralogy compositions. Exploration of deep coal by UCG requires higher level of cognizance in coal heterogeneity in order to better implement geomechanical de- signs (Kanitpanyacharoen et al., 2011). A fundamental micro scale anal- ysis of geomaterials, their nature and response to loading and stress distribution leading to failure is signicant (Mahabadi et al., 2012). In the case of ceramics, grid micro-indentation test and micro-scratch test play a signicant role in evaluating their mechanical properties (Akono and Ulm, 2011; Akono et al., 2011). These methods are quite ef- fective in providing an overall idea of the global mechanical response of materials based on statistical distribution of the measured parameters. Material characterization techniques such as scanning electron micros- copy, transmission electron microscopy and other non-destructive test- ing methods probe and map the surface and sub-surface structure of a material, thus revealing their chemical composition, composition varia- tion in degree of ordering structure and ultrasonic studies. Rationalizing coal heterogeneity requires measuring the mechanical behavior of indi- vidual elements. Such studies lead to improved measurement of stress mapping and its nature along with the causes of failure. The texture of coal and its heterogeneity can be elucidated by advanced techniques such as Raman analysis (Guedes et al., 2010; Guo and Bustin, 1998; Marques et al., 2009). With recent advances in image analysis, confocal micro-Raman spec- troscopy has been extensively used for wide range of studies. Raman spectroscopy is used in many varied elds and is useful where non- destructive, microscopic, chemical analysis and imaging is required such as characterizing the chemical composition, internal structure, grain ori- entations, texture and phase distribution. It is also useful for mapping stress and strain and quantifying residual stress and fracture strength in 3 dimensions (Becker et al., 2007; Beyssac et al., 2003a,b; Dumpala International Journal of Coal Geology 141142 (2015) 1322 Corresponding author. Tel.: +91 44 2257 4824/5839/5842. E-mail addresses: [email protected] (G.L. Manjunath), [email protected] (R.R. Nair). http://dx.doi.org/10.1016/j.coal.2015.02.009 0166-5162/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo

Transcript of Implications of the 3D micro scale coal characteristics ... of the 3D micro scale coal...

Page 1: Implications of the 3D micro scale coal characteristics ... of the 3D micro scale coal characteristics along with Raman stress mapping of the scratch tracks G.L. Manjunath, Rajesh

International Journal of Coal Geology 141–142 (2015) 13–22

Contents lists available at ScienceDirect

International Journal of Coal Geology

j ourna l homepage: www.e lsev ie r .com/ locate / i j coa lgeo

Implications of the 3D micro scale coal characteristics along with Ramanstress mapping of the scratch tracks

G.L. Manjunath, Rajesh R. Nair ⁎Department of Ocean Engineering, IIT Madras, Chennai, India

⁎ Corresponding author. Tel.: +91 44 2257 4824/5839E-mail addresses: [email protected] (G.L. M

[email protected] (R.R. Nair).

http://dx.doi.org/10.1016/j.coal.2015.02.0090166-5162/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 February 2015Received in revised form 24 February 2015Accepted 24 February 2015Available online 4 March 2015

Keywords:Fracture toughnessMicro indentationMicro scratch testRaman stress mappingMaceralsPetrography

The microscale interfacial characteristics of coal are studied based on the analysis of mechanical behavior of in-dividual elements contributing to coal heterogeneity with respect to high volatile bituminous A of coal, fromSingrauli coal field, Madhya Pradesh, India. Micro-scale delineation of elastic plastic properties of coal by gridmicro-indentation test and fracture toughness was accomplished by micro-scratch test on collotenite maceral.Micro-scratch test was performed in different directions on and critical points of failure were diagnosed bymea-suring acoustic emissions and tangential force. The 3D heterogeneity of coal is studied based on the damages atcertain point of loads known as critical points with respect to distribution of macerals derived from coal petrog-raphy. Damageswere observed at higher critical loads on samples collected from332.9–335.6mdepth comparedto 329.7–330.9 m depth samples owing to less heterogeneity and increased compaction. Raman stress mappingat critical points of scratch track revealed a spectral shift due to stress inversion. Shift from 1576–1593 cm−1 forgraphite band (G), and from 1344–1357 cm−1 for disordered Carbon (D) band indicate the nature of stress andthe deformation occurred for coal bulk sample. Raman spectra variation for maceral collotelinite is studied andcompared with coal bulk samples behaviour a shift from 1574–1597 cm−1 for G, and from 1346–1356 cm−1

for D.© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The heterogeneous complex structural property of coal affects its be-havior and chemical reactivity. Coal characteristics based on thermo-chemico-mechanical variations must be analyzed thoroughly. Broaderperspective of coal heterogeneity is an important requirement for theexploration of deep coal by means of Underground Coal Gasification(UCG) (Shahidzadeh-Bonn et al., 2005). Fundamentally coal heteroge-neity depends on several aspects such as layering and the litho typemicrolithotype occurrences, and the stress fields that alongwithminer-almatter cleats orientation,macro-to-nano-porosity and chemical min-eralogy compositions.

Exploration of deep coal by UCG requires higher level of cognizancein coal heterogeneity in order to better implement geomechanical de-signs (Kanitpanyacharoen et al., 2011). A fundamental micro scale anal-ysis of geomaterials, their nature and response to loading and stressdistribution leading to failure is significant (Mahabadi et al., 2012). Inthe case of ceramics, grid micro-indentation test and micro-scratch

/5842.anjunath),

test play a significant role in evaluating their mechanical properties(Akono and Ulm, 2011; Akono et al., 2011). Thesemethods are quite ef-fective in providing an overall idea of the global mechanical response ofmaterials based on statistical distribution of the measured parameters.Material characterization techniques such as scanning electron micros-copy, transmission electronmicroscopy and other non-destructive test-ing methods probe and map the surface and sub-surface structure of amaterial, thus revealing their chemical composition, composition varia-tion in degree of ordering structure and ultrasonic studies. Rationalizingcoal heterogeneity requires measuring themechanical behavior of indi-vidual elements. Such studies lead to improved measurement of stressmapping and its nature along with the causes of failure. The texture ofcoal and its heterogeneity can be elucidated by advanced techniquessuch as Raman analysis (Guedes et al., 2010; Guo and Bustin, 1998;Marques et al., 2009).

With recent advances in image analysis, confocal micro-Raman spec-troscopy has been extensively used for wide range of studies. Ramanspectroscopy is used in many varied fields and is useful where non-destructive, microscopic, chemical analysis and imaging is required suchas characterizing the chemical composition, internal structure, grain ori-entations, texture and phase distribution. It is also useful for mappingstress and strain and quantifying residual stress and fracture strength in3 dimensions (Becker et al., 2007; Beyssac et al., 2003a,b; Dumpala

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Table 1Stratigraphic formation sequence of Singrauli coalfield (Raja Rao, 1983; Singh et al., 2014).

Formation Lithology Formation thickness in mm

Raniganj Sandstone and shale 150Jhingurdah top seam 131–138Sandstone and shale 39–58Jhingurdah bottom seam 10–15Sandstone, shale with coal string 60

Barren measure Ferruginous clayey sandstone 125Barakar Sandstone and thin coal beds 45–70

Panihari seam (local) 1–2Sandstone and shale 110–125Khadia seam (local) 1–2Sandstone and shale 30–40Purewa top seam 8–12Fine to coarse grained sandstone 0–60Purewa bottom seam 10–40Sandstone and shale 45–75Turra seam 14–23Sandstone and shale 45–90Kota seam 1–3Sandstone and shale 150–250Unconformity

Bijawar Phyllites and quartzites

Fig. 1. Geological map of the Singrauli coal field showing the area of the study (Geological survey of India).

14 G.L. Manjunath, R.R. Nair / International Journal of Coal Geology 141–142 (2015) 13–22

et al., 2014; Ghosh et al., 2008; Kang et al., 2005; López-Honorato et al.,2010; Puech et al., 2000; Ryu et al., 2012; Sadezky et al., 2005).

Raman spectroscopy was coupled with Scanning Microwave Mi-croscopy a chip placed inside the coal this combination to achieve100nm spatial resolutions for coal structure. Along with the surfacemorphology and chemical variations of coal and to overcome theshortcomings of Transmission Electron Microscopy (Potgieter‐Vermaak et al., 2011; Tselev et al., 2014). Similarly, implementingmicro-Raman imaging analysis along with X-ray diffraction (XRD)and Fourier Transfer Infrared spectroscopy (FTIR) to scrutinize thecrystalline structure helps in determining the coal rank (Mastalerzand Bustin, 1993; Sonibare et al., 2010) and also in measuring theproperties of partially ordered materials (Beyssac et al., 2003a,b;Ferrari and Basko, 2013).

This paper investigates 3D heterogeneity of coal at micro scalelevel based on grid micro-indentation test and micro-scratch testcarried out in three different directions on coal sample. The gridindentation techniques for yielding accurate mechanical propertiesof Young's modulus and hardness of the heterogeneity basedon low load applications is performed on coal (Chen and Huang,1963; Das, 1972; Mukherjee et al., 1989; Nandi et al., 1977; Tiryaki,2005). Statistical analysis was performed for analyzing indentationproperties based on histogram in obtaining mean values of the het-erogeneous media.

The specimens were retrieved from two different depth ranges of329.7–330.9 m and 332.9–335.6 m. At the critical points of failures,Raman analysis was performed to acknowledge the nature of stress in-version. The Raman spectrum, along with micro imaging of the criticalscratch track provides the nature of stress, to which it is subjected,based on the mechanical response of coal.The general physical charac-teristics of coal of Singrauli coal field shows basically banded nature.Based on bright and dull bands it is inferred that vitrinite and internite

are the dominant maceral groups with minor concentration of liptinitegroupwith rank of coal vary fromA-F. Chemical properties of formationcontains ash (wt%) : 22.33, voltaile matter (wt%) : 44.64, fixed carbon(wt%): 55.36. Petrographic composition of coals: collotelinite (vol%):~23 to ~33; Macrinite (vol%) : ~1 – ~2.66; Fusinite (vol%) : ~18 to ~23.

The study helps in a thorough understanding of coal 3D heteroge-neity relating to Raman stress mapping and also evaluation ofthe mechanical properties. The outcome of these experiments acts

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Table 2Range of minerals identified by XRD.

Compound Chemical formula Miller indices(lattice planes orthogonal to eachother)hkl

Quartz SiO2 011Graphite C 002Carbon C 005Kaolinite Al2 (Si2O5) (OH)4 001Silicon oxide SiO2 011Lead Arsenate Pb (As2O6) 001Chlorine oxide ClO2 211Boron Carbon nitride B0.47C0.23N0.30 110Boron nitride BN 002Cesium Cs 111

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as input in analysing stress percolation in global finite elementgeomechanical modeling (Burnley, 2013). Mahabadi et al. (2014)used inputs from such micro indentation and scratch tests in

1

2

3

4

5

6

7

Loa

d in

mN

(e)

(a)

FTR1200

100µm

(c) (

50µm

Fig. 2. (a) Surface of the coal sample showing grid indentation by Berkovich indenter. (b) Loaddented area in white light (c) & fluorescence light (d); (e) Schematic diagram of the conical sextracted from depths 329.7–330.9 m and 332.9–335.6 m; (h) Optical image of the sample sefluorescence light (j); (k) Raman micro-imaging of the selected area: purple indicates carbon, rlybdenum, and black indicates the presence of fluorescence.

accurately predicting the tensile strength and failure behaviour ofrock sample.

2. Materials and sample preparation procedures

2.1. Materials

The samples belong to depths ranging from 329.7 to 330.9 mand from 332.9 to 335.6 m. These specimens were collected fromGondhBehra (Ujheni), Singrauli in Madhya Pradesh. The present stud-ied coal field, Singrauli is covered by Raniganj formation. This formationis underline by Barrenmeasures, Barakar and Talchir formations shownin Fig. 1 and samples are collected from Jhingurdah bottom seamTable 1. An East West trending boundary fault (an offshoot of son Nar-mada linement). Covers the northern limit of the study area and fourother faults have been deciphered within the main block. The seven re-gions of coal seams that occur in Barakar formation is comprised withindepth range 244.23–606.75 m (Misra and Singh, 1990) rank of coalranging from A–F (Singh et al., 2014). Structurally this coal block strike:

0

00

00

00

00

00

00

00

0 2000 4000 6000 8000 10000

Displacement in nm

FV

X13

X11

X122

(b)

d)

50µm

vs. displacement for coal taken from two different depths.; Petrography study of coal in-cratch test set up; (f) & (g) Scanning electron microscopy image of scratched area of coallected for Raman imaging; Petrography study of coal indented area in white light (i) anded indicates silicon, blue indicates sulfur, green indicates titanium, sky blue indicates mo-

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(f) (g)

(h) (i)

(j) (k)

Fig. 2 (continued).

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NE-SW with minor swings low dip of 3–5° towards SE. Throw ofmajor fault is ranging from 100 to 120m trending NE-SW withthrow towards west. The other three faults, the throw is rangingfrom 15–30 m.

2.2. Sample preparation

Cutting, sizing and polishing are the major operations involved.The sample was cut to the required shape using a diamond saw cut-ter and the sizing operation was performed using aluminium oxidegrits extending from coarse 240 to fine 3000 to achieve a high polish.The orientation of the core sample is perpendicular to the beddingplane. The faces of the sample were polished enough to parallelizethem to a dimension of 10mm × 10mm × 3mm. Diamond polishing

was done to achieve high degree of surface finish required for theexperiments.

3. Experimental study details

3.1. X-ray diffraction methods

XRD analysis was performed to determine the degree of orderingthe carbon stacking structure in coal, fraction of amorphous carbon,interlayer spacing of crystalline structure and its sizes. The existenceof the peaks confirms the presence of crystalline structures in coaland other background intensity of diffractions associatedwith highlydisordered structures. The mineralogy details based on the XRDstudies along with Miller indices are shown in Table 1. Powderedcoal samples were placed on the glass slide and X-ray was passed

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Table 3Summary of the nano-indentation test results.

Properties Depth in m(329.7–330.9)

Depth in m(332.9–335.6)

Indentation modulus [GPa] 17.3 ± 5.3 18 ± 5.1Hardness [GPa] 0.531 ± 0.261 0.584 ± 0.242

0

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0.04

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10 15 20 25

Pro

babi

lity

Den

sity

Indentation modulus, GPa

Depth 332.90m-335.60mDepth 329.70m-330.90m

0.4

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0 0.5 1 1.5

Pro

babi

lity

Den

sity

Hardness modulus, GPa

Depth 332.90m-335.60mDepth 329.70m-330.90m

(a)(b)

Fig. 3. (a) Indentation modulus histogram; (b) hardness modulus histogram.

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at an angle 4°–70° in 2θ range. More than 400 chemical compoundswere noticed and around 10 major chemical compounds are shownin the Table 2.

3.2. Nano-indentation testing

Coal heterogeneity depends on the presence of nano and micropores and the macerals and individual macerals in that group andnano-indentation useful approach to determine elastoplastic nature ofcoal. Understanding the complex nature of coal geology by grid indenta-tion techniques and measuring the Young’s modulus and hardness. Ex-periments were performed with Agilent instrument using Berkovichindenter. A grid indentation array of 7 × 7 with spacing of 150μm, ap-plied load of 630mN and frame stiffness of 7.08 × 106 N/m, strain rateof 0.05/s for a frequency of 45Hz, 5s pause and depth range of 2750–3250 nm. Fig. 2(a) discloses the nano-indentation images andFig. 2(b) shows the load vs. displacement behavior of the samples. Pe-trography analysis of indented area is shown in Fig. 2(c) and (d), the re-flectance of which is more due to due to the indentation. Petrographystudies based on white and fluorescence’s light is studied.Fig. 2(c) indicates the mineral matter in variegated colour and darkestgray indicates the various grades of vitrinite (collotenite) reflectance.In Fig. 2(d), black colour indicates the organic matter.

3.3. Micro-scratch testing

Micro-scratch testingwas performedwith a CSM instrument using aRockwell diamond conical probe with a half apex angle of θ = 600,hemispherical tip radius of R = 200μm and transition from sphere tocone d/R=0.132. Fig. 2(e) is the schematic representation of the exper-imental set up along with the dimension of the probe. A vertical load ofFV was applied and the resultant scratch depth, acoustic emission,groove size and tangential load FT were recorded by the software. Pro-gressive load of 1–50 N was applied with a loading rate of 49 N/minand with a speed of 3 mm/min up to a length of 3 mm. The fracturetoughness Kc is measured using a conical scratch test based on the fric-tional force in tangential direction FT, diameter of the cone d, and half-apex angle θ as shown in Eq. (1).

FTKcd

3=2

¼ 2

ffiffiffiffiffiffiffiffiffiffitanθcosθ

rð1Þ

Fig. 2(f) & (g) show SEM images of scratch tracks in coal at differentdepths. The sample shown in Fig. 2(g) (332.90–335.60 m higher depthrange) offers more resistance to scratch compared to the sample shown

in Fig. 2(f) (329.70–330.90 m lower depth range), which highlights thedirect dependency of compactness and scratch resistance on depth.

3.4. Raman analysis

Raman measurements were performed with Witec machine usingthe Confocal Raman Microscope alpha-300 system comprising anultra-high throughput system 300 spectrograph coupled with Peltiercooled charge coupled detector. An Nd: YAG 532nm laser was used forexcitation and a 50× objective lenswas used for laser focus. Backscatterlight from the sample was guided to the spectrometer through a 50 μmultimode optical fiber. The Raman image along with the wavelengthdata for the defined areawas obtained as shown in Fig. 2(k). The petrog-raphy studies for the selected areas of coal are studied by Raman analy-sis and the coal macerals are identified to be mainly colloteliniteFig. 2(i) and (j). A defined area was selected over the optical image, asindicated in Fig. 2(h) and a total of 14,400 Raman spectrawere collectedover this area to obtain the micro image, as shown in Fig. 2(k).

4. Results and discussions

4.1. Nano-indentation

The nano-indentation method was preferred due to the presence ofmicro and nano sized pores in the sample same as used by (Tselev et al.,2014), which demands accurate determination of heterogeneity. Thenano elasto-plastic behavior of the coal sample, based on statisticalanalysis, is illustrated in Fig. 3(a) and (b). In order to estimate the hard-ness and Young’smodulus precisely, the indenter should touch the porevalley. The histogram shows the decreased Young’s modulus and in-creased hardness and Fig. 3(a) and (b) shows the effect ofmaceral com-paction from petrography studies Fig. 2(c) and (d) along withheterogeneity. Table 3 summarizes the mechanical properties of thecoal based on the nano-indentation test based on a single run of 7 × 7arrays for each sample. Fig. 3(a) shows indentation modulus values ofthe sample at a lower depth range (332.9–335.6 m) and at a higherdepth range (329.7–330.9 m). Due to high compaction macerals aremore intact, the samples from greater depth are more ductile.Fig. 3(b) represents the hardness modulus of the samples and the

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1.7

1.75

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Ft(

N)

Fn(N)

AE

%

Ft (N)

AE%

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N)

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lity

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sity

Kc=Ft. d-

Kc in X11 directionKc in X12 directionKc in X13 direction

(a) (b)

(c) (d)

Critical loads

Critical loads

(g) (h)

50µm 50µm

500µm

(e) (f)

1µm

Fig. 4. (a) Tangential force and acoustic emission plots obtained against normal loading in X11 direction. (b) Tangential force and acoustic emission plots obtained against normal loadingin X12 direction. (c) Tangential force and acoustic emission plots obtained against normal loading in X13 direction. (d) Histogram indicating fracture behaviormodulus in different layers.(e) SEM images of scratch length at × 1mm. (f) SEM images of scratch length at × 500μm. Petrographic image of scratched area in white (g) and fluorescence light (h).

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1.7

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(a) (b)

(c) Critical loads

Critical loads

(d)

Critical loads

(g) (h)

50 50

500µm

(e) (f)

1µm

µm µm

Fig. 5. (a) Tangential force and acoustic emission plots obtained against normal loading in X11 direction. (b) Tangential force and acoustic emission plots obtained against normal loadingin X12 direction. (c) Tangential force and acoustic emission plots obtained against normal loading in X13 direction. (d) Histogram indicating fracture behaviormodulus in different layers.(e) SEM images of scratch length at × 1mm. (f) SEM. images of scratch length at × 500μm. Petrographic image of scratched area in white (g) and fluorescence light (h).

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Table 4Fracture toughness in different directions.

Directions Fracture toughness MPa√minitial sample depth(329.7–330.9 m)

Fracture toughness MPa√mdeeper sample depth(332.9–335.6 m)

X11 0.0749 ± 0.0157 0.0768 ± 0.0146X12 0.0804 ± 0.0143 0.0777 ± 0.0135X13 0.0696 ± 0.0155 0.07044 ± 0.0145

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samples obtained from greater depth was found to be harder. The pres-ence of lesser cleats is responsible for the increasedhardness of the sam-ple at greater depth along with macerals and microlithotype.

4.2. Micro-scratch test

Figs. 4 to 5 show the results of micro-scratch test on samples collect-ed from two different depths, where acoustic emission and tangentialforce are plotted against progressive normal loads. SEM images of thescratched area are analyzed after the experimentation and a series offailure events were identified by acoustic emission peaks, marked ascritical loads, along with tangential loadings due to change in frictionalcontact as shown in Fig. 4(a–c) obtained from a depth of 329.70–330.90m and 5(a–c) from a depth of 332.90–335.60 m. Observation re-veals that at critical loads of 4, 15 and 38N, the series of failures occur inX11-direction, as shown in Fig. 4(a); at critical loads of 3, 38 and 48 N,the series of failures occur in X12-direction, as in Fig. 4(b); and at criticalloads of 18, 29 and 38 N, the series of failures occur in X13-direction, asin Fig. 4(c) owing to the release of elastic energy. Fig. 4(d) is a histogramthat indicates the fracture behavior of the samples along different direc-tions (X11, X12 and X13) and Table 4 exposes the fracture toughnessvalues based on Eq. (1) corresponding to different directions shown inFig. 2c. The SEM image shows the scratch track; initially, no failure oc-curs and after certain distances a transition in stress inversion fromcompressive to tensile nature leading to shearing as observed inFig. 4(e) and (f). The critically damaged zone is selected for petrographyfromFig. 4(f) shown in studied in Fig. 4(g) and (h). LightGray is vitrinite(collotelinite) and mineral ground mass variegated in colourFig. 4(g) and degraded bitumen showing yellow fluorescence inFig. 4(h).

The experiment was repeated with samples from a different depthand the results are disclosed in Fig. 5. Observation uncovers a series offailure events at critical loads of 8, 22 and, 25 N along X11-direction, asin Fig. 5(a); 46 N along X12-direction, as shown in Fig. 5(b) and 31 and45N along X13-direction, as in Fig. 5(c), on account of the release of elas-tic energy. SEM analysis performed on scratch, shown in Fig. 5(a), indi-cates damages at lengths 0.97mm, 1.58mm and 2.1mm which arerepresented in Fig. 5(e) and (f). Fig. 5(g) shows the fracture behaviorof samples in X11, X12 and X13 directions. Similarly critically damagedzone is selected for petrography from Fig. 5(g) shown in studied inFig. 5(g) and (h). LightGray is vitrinite (collotelinite) andmineral groundmass variegated in colour Fig. 5(g) and degraded bitumen showing yel-low fluorescence in Fig. 5(h). Compared to Fig. 5(g) in Fig. 4(g) themaceral collotelinite is highly damaged.

Micro-scratch test results confirm more fracture toughness alongX13 direction, compared to other directions, for samples from lowerdepth range (329.7–330.9 m). According to Fig. 5(d), for samples be-longing to greater depth range (332.9–335.6 m), fracture toughnesswas found more along the X12 direction, in comparison with other di-rections. The acoustic emission and tangential force curves indicatethe characteristics of the coal samples from different depths. In this re-gard, the sample obtained from a greater depth offered resistance toscratch due to increased hardness and compaction based on maceralsand microlithotype characteristics. Samples exhibited quasi-brittle fail-ures, as a result of shearing at the surface due to high compaction,

around the indented area. The scratch test based on number of acousticemission peaks indicates weakest spots and the level of heterogeneity.

4.3. Raman stress mapping analysis

Since the micro-scratch test is destructive in nature, the appliedprogressive normal load reaches yield limit and causes failures. The crit-ically damaged zone of collotenite from petrography studies shown inFig. 4(g) and (h) is considered for Raman analysis Fig. 6(a) and (b).Raman spectroscopy analysis was performed on the scratch tracks byK-means cluster analysismethod and critically damaged regions aroundthe failure points were analyzed, as in Fig. 6(c) and (d). A criticallyscratched area at a lower depth was selected for Raman analysis as itis more susceptible to damages. The characteristic Raman peak forunstressed and stressed regions of coal bulk sample was studied. Nor-mally, for an unstressed region of coal the Raman peak is noticed at1593 cm−1, known as graphite (G) band and at 1345 cm−1, referredto as disordered (D) band of carbon. For stressed region, a spectralshift from 1576 to 1593 cm−1 is observed for G band and a spectralshift from 1344 to 1357 cm−1 is observed for D band. The behavior ofpeak shifting from a higher to lower side indicates the presence of com-pressive and tensile stresses inside and outside the scratch track, asshown in Fig. 6(e).

A band of spectrum ranging from 1576 to 1593cm−1 is selected toobtain the Raman images and to perform cluster analysis for obtainingdifferent color intensity contrasts Fig. 6(e). The degree of graphitizationwas measured based on G and D peak intensities, which indicates thedisorderliness in the carbon structure. The increase in G band indicateshigher graphite content and the decrease in D band indicates the disor-der in carbon structures. Normally, the ratio of intensity of G to intensityof D indicates the structural properties of coal. A decrease in the value ofthis ratio indicates the destroyed tensile mode, and an increase in itsvalue indicates the compressive mode of coal during the scratch-test(Zhao et al., 2014).

A lower peak value in Fig. 6(e) indicates the tensile state of stressthat makes it less compact, while a higher peak value indicates thecompressive nature of stress, which makes it more compact. Stressmapping at 1593 cm−1 Fig. 6(e) X graph indicates the presence ofcompressive stress and at 1576 cm−1 Fig. 6(e) Z graph indicates thepresence of tensile stress field. Transition from compressive to tensileindicates the severity in deformation and cause of failure events. Testswere repeated focusing on collotelinite and comparedwith coal bulk re-sults shown in Fig. 6(f). The Raman analysis the of an individualmaceralcollotelinite indicates 1354–1592 cm−1 for X, 1346–1597cm−1 Y and1356–1574 cm−1 for Z, no much variation in G band is noticed com-pared to D band shows slight variations.

While comparing the images of scratch at different depths of coal, itwas found that the failure mechanisms were similar and gradual dam-age was observed in a brittle mode at certain lengths. The regionsaway from the scratch tracks experienced the impact of failures due tocompressive shearing by the indenter. Decohesion occurred due tocompressive stresses induced by the indenter and developed a stress in-version from compressive to tensile, leading to shearing and failures.Coal from a greater depth exhibited increased hardness and increasedresistance to deformation due to increased compaction.

5. Conclusions

Scratch characteristics of coal at different depths were selected forexperimental purposes by applying progressive normal load withinthe range of 1–50 N and nano-indentation test was successfully con-ducted to measure the mechanical properties. Scratch tracks were ana-lyzed step-by-step using SEM and Raman spectroscopy. Investigationreveals that events of failure occurred at critical loads of 4, 15, 38 N inX11-direction and critical loads of 3, 38, 48 N in X12-direction; at criticalloads of 18, 29, 38 N, the series of failures occur in X13-direction for

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samples from 329.7–330.9 m depth. The failure events occur at criticalloads of 8, 22, 25 N in X11-direction, 46 N in X12-direction and 31 N,45 N in X13-direction for samples at 332.9–335.6 m depths. Themacerals are more intact at higher depth coal samples based on petrog-raphy analysis. Specific conclusions can be made using Raman stress

(a)

(e)

1597cm-1

1351cm-1

1357cm-1

1593cm-1

1576cm-11344cm-1

X

Y

Z

(c)

50µm

Fig. 6. Petrographic image of scratched area inwhite (a) and fluorescence light (b). (c) Raman in1, 2, 3. (e) Raman spectra for 1, 2, 3 corresponding color images alongwithwave numbers X, Y&wave numbers X, Y & Z graphs of collotelinite.

mapping for measuring strain associated with the stress inversion,from compressive to tensile stresses, leading to failures of brittle nature.Under gradual loading condition, at certain points the gradually dam-aged zones are detected by progressive loadings. The current study isalso beneficial for calculating the stress orientation at different depths

(b)

(d)

1

2

3

X

Y

Z

1592cm-11354cm-1

1597cm-11346cm-1

1356cm-1

1574cm-1

(f)

50µm

tensity imaging on the selected optical area. (d) Cluster analysis for Raman stressmappingZ graphs of coal bulk. (f) Raman spectra of for 1, 2, 3 corresponding color images alongwith

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22 G.L. Manjunath, R.R. Nair / International Journal of Coal Geology 141–142 (2015) 13–22

by providing an understanding of fundamentals of geomechanics, as ap-plied to coal, of selected reservoirs.

Acknowledgement

The authors would like to thank Central Mine Planning & Design In-stitute Limited (CMPDI), Ranchi, Jharkhand, India, for providing the coalsample for carrying out the research.

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