CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

Ceramics International 40 (2014) 16349–16359

Synthesis and sintering of hydroxyapatite derived from eggshellsas a calcium precursor

P. Kamalanathana, S. Ramesha,n, L.T. Banga, A. Niakana, C.Y. Tana, J. Purbolaksonoa,Hari Chandranb, W.D. Tengc

aCenter for Advanced Manufacturing and Materials Processing, Department of Mechanical Engineering, University of Malaya, Kuala Lumpur 50603, MalaysiabDivision of Neurosurgery, Faculty of Medicine, University of Malaya, Kuala Lumpur 50603, Malaysia

cCeramics Technology Group, SIRIM Berhad, Shah Alam 40911, Malaysia

Received 4 June 2014; received in revised form 14 July 2014; accepted 14 July 2014Available online 22 July 2014

Abstract

In the present work, phase pure hydroxyapatite (HA) was successfully prepared using calcined eggshells as a calcium precursor via the wetchemical precipitation method. The sintering of eggshell-derived HA (EHA) compacts was carried out in air over a temperature range of 800–1400 1C. The sintered HA samples were evaluated in terms of phase stability, relative density, grain size, Vickers hardness and fracturetoughness. The results showed that phase pure HA was obtained and remained stable after sintering at 1250 1C. However, secondary phases suchas !-TCP and TTCP were obtained at 1300–1350 1C and melted upon further sintering at 1400 1C. A relatively high density of 97.4% wasachieved in pure HA at 1250 1C whilst a maximum fracture toughness of 1.14 MPa m1/2 was attained at 1000 1C due to the small grain size of0.33 mm obtained at this temperature. The study found that a combination of relative density, the reverse Hall–Petch relationship and grain growthaffected the hardness of the HA samples, where the highest value of 4.96 GPa was achieved at a sintering temperature of 1250 1C.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sintering; C. Mechanical properties; Hydroxyapatite

1. Introduction

Up to now, large bone defects still represent a majorproblem in orthopaedics [1]. Being similar to bone structurein terms of its mineral composition, as well as its highlybiocompatible and bioactive nature, hydroxyapatite (HA) hasmade its mark in the medical and health-related fields as a bonegraft substitute [2–5]. However a distinctive drawback of HAlies in its poor mechanical properties, particularly its lowfracture toughness (KIc) of o1 MPa m1/2. This limits its usageto non-load bearing applications [6,7].

Various methods have been developed to synthesize HAfrom synthetically derived precursors in which the parameterswere varied to produce HA with significant purity and goodmechanical properties [8]. Some of these synthesis methods

include the wet chemical precipitation method [9,10], mechan-ochemical method [11,12], sol–gel method [13,14], andhydrothermal method [15,16]. However, stoichiometric HAprepared from these methods lack the presence of traceamounts of ions in its lattice structure. Bone itself is regardedas a non-stoichiometric HA due to the presence of minoramount of ions in its HA lattice which benefits its ownstructure as well as calcium phosphate based implants[17,18]. On the other hand HA derived from natural resourcesand bio-wastes such as eggshells [19–21], seashells [22,23]and animal bones [24–26] are non-stoichiometric due to thetrace amount of ions incorporated in its crystal structure, suchas Fe2! , Mg2! , Si2! and F" [26,27]. Therefore, the devel-opment of HA from natural resources is of great importance inits usage as bone-like implants.High content of calcium carbonate (CaCO3), as well as the

presence of trace amount of ions such as Na! , Sr2! , andMg2! , in eggshells makes it an attractive waste material to be

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nCorresponding author.E-mail address: ramesh79@um.edu.my (S. Ramesh).

used as a calcium precursor in the synthesis of HA [28,29].The first research on the usage of eggshells as a calciumprecursor in the synthesis of HA was reported by Rivera et al.[30], where a hydrothermal route was employed. Numerousmethods have been adopted in the synthesis of HA fromeggshells, such as the wet chemical precipitation method [31–33], hydrothermal method [19,34,35], and wet mechanochem-ical method [36,37], as well as the microwave irradiationmethod [17,38]. Different calcium precursors and synthesismethods employed can lead to the production of HA withdifferent chemical and physical properties. These variations areevident in terms of the thermal stability of as-synthesized HA.

The wet chemical precipitation method is the most commonsynthesis route due to its simple and low-cost processingtechnique. This method generally produces larger particles inthe micro-scale region [8,39,40]. However, some of thephysical and chemical characteristics of the final productdepend on the specific method used in the synthesis process.For example, the synthesis of fine nano-sized HA particles byIbrahim et al. [33] was attributed to the steady addition of thephosphorous precursor to the calcium precursor derived fromeggshells at a rate of 200 ml/h. This was to ensure amaintained pH in the mixture, translating to the dissolution,formation and maturation of the reactants at an effective rate,thereby contributing to the formation of fine nanoparticles. Onthe other hand, the nano-sized HA particles synthesized byChaudhuri et al. [41] was owing to the length of the reactionperiod between the calcium and phosphorous precursor,ranging from 1 to 7 days.

An effective and efficient usage of synthetic HA inbiomedical applications requires the produced powder to bemorphologically well defined [12,42]. Synthesis of HA basedon eggshells (EHA) has been shown to produce a number ofpowder morphologies. Rod-like and needle-like morphologiesare commonly found in the human hard tissue whereas flower-like morphologies are known to be beneficial in drug loadingand releasing [43–46]. In addition, it is postulated that thepresence of needle-like particles in the bulk material is capableof enhancing fracture toughness. EHA with flower-likemorphologies have been synthesized via a microwave irradia-tion route with the assistance of ethylenediaminetetraaceticacid (EDTA) as a chelating agent to control powder morphology[47]. On the other hand, EHA with rod-like [35] and needle-likemorphologies [34] have also been produced. In these studies, anadditional morphological controlling agent was alwaysemployed. For instance a cationic surfactant such as cetyltri-methyl bromide (CTAB) was added to the phosphorousprecursor prior to synthesis [35], whilst fruit extract solutionssuch as pomelo peels were incorporated into the calciumprecursor solution in order to manipulate the powder morphol-ogy of EHA [34]. Besides these, variations in synthesisdurations have been shown to alter the morphology of EHA.This is highlighted in the study conducted by Wu et al. [34],where EHA particles with a needle-like morphology evolved toresemble a rod-like structure when the hydrothermal reactiontime increased from 24 to 72 h. This is an inherent characteristicof the hydrothermal treatment, which manipulates the growth

and morphology of the crystalline HA nuclei formed during theionic reaction between precursors [8].In the synthesis of EHA, eggshells have been converted to

various forms of calcium precursor such as calcium oxide(CaO) [41], calcium hydroxide (Ca(OH2)) [39], calcium nitrate(Ca(NO3)) [48], and calcium chloride (CaCl2) [34]. The studyconducted by Ahmed and Ahsan [49] highlighted the effect ofdifferent precursors, where EHA was synthesized using Ca(NO3) – derived from CaO (route 1) and CaCO3 (route 2). Thelatter required an additional calcination procedure, resulting inas-synthesized EHA with a higher crystallinity as compared toroute 1. Based on available resources, most of the researchworks conducted on EHA have been limited to the synthesisand characterization efforts on as-synthesized and sinteredEHA samples whilst a full evaluation in terms of mechanicalproperties of sintered EHA samples has hardly been explored.Although the mechanical strength of sintered EHA sampleshas been investigated previously [38], only the relative densityand Vickers hardness were evaluated. Thus, a thoroughmechanical property evaluation on sintered EHA samples isnecessary. Therefore, the aim of the present research is tosynthesize and sinter phase pure HA using waste eggshells asa calcium source. Mechanical characteristic investigations ofsintered EHA samples were evaluated through density, hard-ness and fracture toughness.

2. Experimental procedures

2.1. Powder preparation and sintering EHA compacts

Waste eggshells were thoroughly cleaned and air-dried priorto the removal of the inner membrane layer. Cleaned and driedeggshells were crushed to a fine powder consistency usinga pestle and a mortar. The raw eggshells were then calcined atelevated temperatures to convert CaCO3 inherent in eggshellsto CaO. In order to determine the most suitable calcinationtemperature based on purity and the crystallinity of the CaOproduced, the temperatures of 700, 800, 850, 900, and 1000 1Cwere employed with a 10 1C min"1 ramp rate (heating andcooling) for 1 h. The CaO obtained via calcination of eggshellswere added to a pre-determined amount of distilled water toproduce a Ca(OH)2 solution, hereafter referred to as thecalcium precursor. EHA was then synthesized using thecalcium precursor and H3PO4 (85% purity, Merck) via a wetchemical precipitation (WCP) route [50]. The molar ratio of Ca(OH)2 to H3PO4 was 1.67:1, where the H3PO4 solution wasadded to the calcium precursor at a rate of 10–20 drops/min.The pH of the solution was maintained at a level greater than10.5 by using ammonium hydroxide (NH4OH) (25% purity,Sigma-Aldrich).After the reaction was complete, the produced EHA

suspension was then allowed to mature for 24 h. Subsequently,it was filtered and washed with distilled water before beingdried in an oven at 60 1C for 24 h. The dried EHA powder wascrushed and sieved using a 212 mm sieve to produce uniformEHA particles. Disk samples were then made by uniaxialpressing at 13.5–14.0 MPa followed by cold isostatic pressing

P. Kamalanathan et al. / Ceramics International 40 (2014) 16349–1635916350

at 200 MPa (Riken Seiki, Japan). The compacted samples werethen sintered in air within a temperature range of 800–1400 1Cfor 1 h at a ramp rate of 10 1C min–1 (heating and cooling).Sintered samples were then ground using silicon carbide (SiC)sandpapers and polished to a 1 mm mirror finish using diamondpaste as a polishing medium prior to evaluation.

2.2. Sample characterization and evaluation

The sintered and as-synthesized EHA samples, CaCO3

inherent to eggshells and produced CaO were characterizedby X-Ray Diffraction (XRD) (EMPYREAN, PANalytical,Netherlands) operated at 45 kV and 40 mA using a monochro-matic Cu-K! beam (!#1.5406 Å). The 2" scanning range was20–601 at a step size of 0.021 and a scan speed of 0.51 min-1.Crystallite size of the as-synthesized EHA was calculated byScherrer's formula shown as

t# 0:9!=FWHM $ cos " %1&

where t is the average crystallite size, ! is the wavelength ofthe XRD radiation, FWHM is the full width half maximumvalue of the measured diffraction peak and " is the diffractionangle of the investigated peak [34]. Functional groups ofCaCO3 in eggshells, calcined eggshells, as-synthesized andsintered EHA at 1250 1C were characterized by FourierTransform Infrared Spectroscopy (FTIR) using the KBr pellettechnique in the 4000–400 cm"1 wavenumber range with a4 cm"1 spectral resolution. The microstructures of sinteredsamples were characterized using a scanning electron micro-scope (SEM) (TM-3030, Hitachi). The grain size of thesintered EHA pellet was determined from the SEM imagesbased on a line intercept method [42] whilst powder morphol-ogy and particle size of the as-synthesized EHA powder weredetermined using a transmission electron microscope (TEM)(JEOL, JEM-2100F, Japan), operated at 120 kV. For TEMobservation, the EHA suspension was prepared by dispersingEHA powder in an ethanol solution via ultrasonication. A dropof the suspension was placed onto a copper grid and left to dryprior to the analysis.

Bulk density measurement of sintered samples was carriedout based on the Archimedes principle using distilled water asan immersion medium. Relative densities of the sinteredsamples were calculated by utilizing the theoretical densityof HA (#HA#3.156 g cm"3). The micro-hardness (HV) andfracture toughness (KIc) values of polished sintered sampleswere determined via a Vickers hardness indenter (Shimadzu,Japan) using an applied load of 100–200 g with a dwell time of10 s. Five indentations were made for each sample and anaverage value was taken. The indentation fracture toughnesswas calculated from the following equation derived by Niiharaet al. [51]:

KIc # 0:203ca

! ""1:5HV a% &0:5 %2&

where c is the characteristic crack length and a represents thehalf-diagonal of the indent.

3. Results and discussion

3.1. Calcium precursor analysis

The XRD diffractogram of crushed eggshells is shown inFig. 1. Predominant peaks of CaCO3 are observed in the

0

50000

100000

150000

200000

250000

20 25 30 35 40 45 50 55 60

(012

)

(104

)

(006

) (110

) (113

)

(202

)

(024

)(0

18)

(116

)

(211

)(1

22)

(101

0)

2! (Degree)

Inte

nsity

(Cou

nts)

Fig. 1. The XRD diffractogram of the crushed eggshells. All the peaks belongto CaCO3.

20 25 30 35 40 45 50 55 60

1000°C

2! (Degree)

900°C

850°C

800°C

700°C

CaOCaCO3

(111

)

(200

)

(220

)

(012

)

(104

)

(006

)

(110

)

(113

)

(202

)

(024

) (018

)(1

16)

(211

)(1

22)

Fig. 2. The XRD diffractograms of the CaO produced via calcination atdifferent temperatures.

P. Kamalanathan et al. / Ceramics International 40 (2014) 16349–16359 16351

structure of crushed eggshells where the highest intensity peakis observed at a 2" angle of 29.51 corresponding to a Millerindices of (104). Decomposition of CaCO3 to CaO has beenreported at a low calcination temperature of 700 1C [19] andbetween 830 and 850 1C [29,38,40] in addition to thecommonly reported temperatures 900 1C [39,41] and1000 1C [31,52]. Hence, the calcination temperature waschosen in a temperature range of 700–1000 1C in the presentstudy. Fig. 2 shows the XRD patterns of CaO produced fromcrushed eggshells at different calcination temperatures. Uponcalcination of eggshells at 850, 900, and 1000 1C, the XRDresults indicate only the crystalline phase formation of CaO.However, after calcination at 700 and 800 1C the presence ofboth CaCO3 and CaO is observed. This indicates an incom-plete transformation of the CaCO3 in eggshells to CaO. Thehighest intensity CaO peak is observed at a 2" angle of 37.31corresponding to the (200) lattice plane. At different calcina-tion temperatures the intensity of the strongest CaO peakincreased from 850 1C to 900 1C, followed by a drop at1000 1C. The highest crystallinity was obtained at 900 1C.Therefore, the calcination temperature of eggshells was set at900 1C.

The FTIR spectrum of crushed eggshells shown in Fig. 3(a)further confirmed the carbonate phase present with significantcharacteristic peaks at 1425 ("3), 875 ("2) and 711 cm"1 ("4)which corresponded to the carbonate group in CaCO3. Thesmall frequency bands at 2873, 2515, 2362 and 1799 cm"1 areattributed to the combination modes of different CO3

2" bands[53]. The details of these modes are summarized in Table 1.Bands at 3243 and 2363 cm"1 correspond to H2O vapor andatmospheric CO2 concentration respectively. The presence ofthe H2O band could be mitigated by calibrating the FTIRspectrometer with a known gas mixture [54].

The FTIR spectroscopy of calcined eggshells at 900 1C isshown in Fig. 3(b). Three frequency bands inherent to thestructure of CaO are clearly observed. The broad bands at 1441and 1063 cm"1 are ascribed to the C–O stretching mode withrespect to CO2 adsorbed on the surface of CaO whereas thesharp peak observed at 3640 cm"1 is related to the O–H group[55]. The three characteristic bands of the CO3

2" as well as thebands corresponding to the CO3

2" combination modes thatwere present in CaCO3 are no longer visible in the CaOspectrum, indicating the complete decomposition of thecarbonate phase. This is in agreement with the XRD resultshown in Fig. 2. The calcium precursor used in the presentresearch to synthesize EHA is therefore of high purity.Tables 1 and 2 summarize the frequency bands of both CaCO3

and CaO in comparison with previous studies.

3.2. Phase analysis of EHA

The XRD pattern of the as-synthesized EHA powder isshown in Fig. 4. All peaks were indexed to hexagonal HAwithout secondary phases such as !-TCP and #-TCP. Thisdemonstrated the successful synthesis of single phase EHA viathe WCP route. The highest intensity peak is observed at a 2"angle of 31.831 whilst that of the standard HA (JCPDS:

01-084-1998) is at 31.791. The crystallite size of the as-synthesized EHA powder based on the (002) peak was foundto be nanocrystalline with a value of 35.3 nm. This is slightlylarger than that obtained from previous studies [17,38] withcrystallite sizes in the range of 21–27 nm. This could beattributed to the microwave irradiation route employed tosynthesize EHA in those studies [17,38].

500 1000 1500 2000 2500 3000 3500 4000

Wavenumber (cm-1)

Transmittance (a.u)

Fig. 3. FTIR spectra of (a) crushed eggshell and (b) calcium oxide synthesizedvia calcination of eggshells at 900 1C.

Table 1Infrared (IR) vibration modes of crushed eggshells and references [53,54,70].

Assignments Observed vibrational frequencies (cm"1)

Presentstudy

Reference[54]

Reference[70]

Reference[53]

"4 – Symmetric CO32"

deformation711 – 713 712

"2 – Asymmetric CO32"

deformation875 – 875 874

"3 – Asymmetric CO32"

deformation1425 – 1417 1425

"1!"4 – CO32" deformation 1799 – 1799 1798

"3 – CO2 stretching mode 2362 2349 – –

2"2!"4 – CO32"

deformation2515 – – 2514

2"3 – CO32" deformation 2873 – – 2873

H2O stretching mode 3243 – 3455 –

Table 2Infrared (IR) vibration modes of calcium oxide synthesized via calcination ofeggshells and references [55,70,71].

Assignments Observed vibrational frequencies (cm"1)

Present study Reference [55] Reference [70] Reference [71]

" C–O 1063 866 – 1060" C–O 1441 1417 1417 1415" O–H 3640 3643 3620 –

P. Kamalanathan et al. / Ceramics International 40 (2014) 16349–1635916352

In comparison, EHA powder produced by other researchworks [30,32,36] contained trace amounts of Ca(OH)2, CaO,and H3PO4. This is due to the insufficient usage of either thecalcium or phosphorous precursors. However, in the presentstudy, XRD results indicate the synthesis of single phase andnano-crystalline EHA.

The sintering of EHA was carried out at 800 1C up to1400 1C where pure HA was formed up to a sinteringtemperature of 1250 1C. XRD patterns of EHA at differentsintering temperatures are presented in Figs. 5 and 6. Theintensity of the highest peak, corresponding to the (211) latticeplane, at 2"#31.771, increases from 800 to 1250 1C anddecreases between the temperatures 1300 and 1350 1C. Thisindicated the decomposition of EHA. As shown in the XRDpattern, secondary phases corresponding to !-TCP and TTCPare observed at a sintering temperature of 1300 1C onwards(Fig. 6c and d). The decomposition process became moreprominent at 1350 1C, which is attributed to the increase inintensity of both the !-TCP and TTCP peaks.

At 1400 1C, melting of sintered EHA occurs. This isjustified by the bloating and disfiguration of the sinteredsample. As such, no further analysis was done on this sample.In addition, XRD peak shifting to a lower Bragg's angle wasobserved for the 1350 1C sintered samples which is believed tobe associated with the dehydroxylation of HA structure whenheated in air at elevated temperatures [56–60]. The dehydrox-ylation of HA to OHA is shown in Eq. (3) whereas thedecomposition of HA to TCP and TTCP is shown in Eq. (4)where the value x in Eq. (3) is less than 1 [6,57]. In the presentstudy, the single phase EHA was stable up to 1250 1C whilstEHA was only stable up to 1200 1C in a previous study [38].As such, the results obtained in the present work are encoura-ging since sintering of EHA at 1250 1C in the study conductedby Krishna et al. [38] resulted in the decomposition of EHAwhereas Bardhan et al. [37] only attempted a maximum

0

5000

10000

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20000

25000

30000

35000

20 25 30 35 40 45 50 55 60

Inte

nsity

(Cou

nts)

2 (Degree)

(200

) (1

11)

(002

)

(102

) (1

20)

(121

) (1

12)

(300

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02)

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) (310

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22)

(312

) (1

23)

(231

) (4

10)

(402

) (0

04)

(104

) (2

32)

(133

) (1

42)

(420

)

Fig. 4. The XRD diffractogram of as-synthesized EHA powder via a WCProute. All peaks belong to the HA phase.

20 25 30 35 40 45 50 55 60

2 (Degree)

(a)

(b)

(c)

(d)

(e)

(200

) (1

11)

(201

) (0

02)

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) (2

10)

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) (1

12)

(300

) (2

02)

(301

)

(212

) (130

) (2

21)

(131

)

(113

) (4

00)

(203

) (2

22)

(132

) (2

30)

(213

) (3

21)

(140

) (4

02)

(004

) (1

04)

(322

) (3

13)

(501

) (3

30)

(240

)

Fig. 5. The XRD diffractograms of sintered EHA pellets synthesized via theWCP route at (a) 800 1C, (b) 900 1C, (c) 1000 1C, (d) 1100 1C, and(e) 1150 1C. All peaks belong to the HA phase.

v

20 25 30 35 40 45 50 55 60

2 (Degree)

(a)

(b)

(c)

(d)

(200

) (1

11)

(201

) (002

)

(102

) (210

)

(211

) (1

12) (3

00)

(202

) (3

01)

(212

) (130

) (2

21)

(131

)

(113

) (4

00)

(203

) (2

22)

(132

) (2

30)

(213

) (3

21)

(140

) (4

02)

(004

) (1

04)

(322

) (3

13)

(501

) (3

30)

(240

)

HA -TCP

TTCP

Fig. 6. The XRD diffractograms of sintered EHA pellets synthesized via theWCP route at (a) 1200 1C, (b) 1250 1C, (c) 1300 1C and (d) 1350 1C.

P. Kamalanathan et al. / Ceramics International 40 (2014) 16349–16359 16353

sintering temperature of 1200 1C.

Ca10%PO4&6%OH&2 -Ca10%PO4&6%OH&2"2xOx!xH2O %3&

Ca10%PO4&6%OH&2 -2Ca3%PO4&2!Ca4P2O9!H2O %4&

3.3. FTIR analysis

Fig. 7 and Table 3 present the frequency bands correspondingto vibrational groups inherent to the structure of EHA.Characteristic peaks corresponding to ("3 and "4) PO4

3" modesare observed at frequencies of 1038–1046, 601–604 and568 cm"1 in both as-synthesized and sintered EHA. StructuralOH" vibrational modes are observed only in sintered EHAsamples at 3566 cm"1. However, this band was not visible inas-synthesized EHA samples due to the overlap of the adsorbedH2O band at 3425 cm"1. Broad bands at 1643 cm"1 and3425 cm"1 observed in as-synthesized EHA powder wereattributed to the ("2) bending mode of adsorbed H2O. Thesebands were no longer visible in sintered samples. Frequencybands observed at 2362 cm"1 and 2358 cm"1 in the as-synthesized and sintered EHA are a characteristic of free CO2.The ("3) CO3

2" vibrational mode is clearly observed in as-synthesized EHA at 1421 cm"1. This indicates the presence of aB-type carbonate substitution where the PO4

3" ions present inthe HA lattice are substituted by CO3

2" ions [2]. As shown inFig. 7(b) the aforementioned CO3

2" band disappeared, due to theelimination of CO3

2" ions at elevated temperature. SimilarCO3

2" substitutions in EHA were also observed in previousstudies [34,38]. On the contrary, CO3

2" substitutions were notobserved in the study conducted by Prabakaran and Rajeswari[35] on EHA synthesized via a hydrothermal route. It is believedthat the closed system of the hydrothermal method prevented thereaction of atmospheric CO2 with the precursors, thus inhibitingCO3

2" substitution.

3.4. Relative density

The effect of sintering temperature on the relative density isshown in Fig. 8. A steady increase in density is observed withan increase in sintering temperature up to 1250 1C. Relativedensity increases from 63.8% at 800 1C to a maximum of97.4% at 1250 1C. Beyond this point, consolidation no longercontributed to an increase in density. This is clearly shown inthe slight decrease in relative density at 1300 and 1350 1C to97.1%, even though SEM micrograph in Fig. 9 at thesetemperatures exhibits a rather dense structure. However, adramatic decrease is observed at 1400 1C as density drops to87.2%. The decrease in density beyond 1250 1C is ascribed tothe presence of secondary phases (!-TCP and TTCP) in thesintered EHA as indicated by XRD data in Fig. 6. Essentially,consolidation occurs in three distinct steps which begin withthe formation of necking between particles in the sinteringrange of 700–800 1C. Within this temperature region, densifi-cation did not occur. This is clearly shown in Fig. 8, wheresintering at 800 1C results in a low relative density of 68.3%.Densification occurs in the second stage, where the initial

necking grows to form grain boundaries. Thus a reduction inpore volume and shrinkage of the sample is observed, resultingin a relative density of approximately 90% in sintered samples.

400 900 1400 1900 2400 2900 3400 3900

Wavenumber (cm-1)

Transmittance (a.u)

Fig. 7. FTIR spectra of (a) as-synthesized EHA powder and (b) EHA sinteredat 1250 1C.

Table 3Infrared (IR) vibration modes of as-synthesized EHA, sintered EHA at 1250 1Cand references [17,37,38].

Assignments Observed vibrational frequencies (cm"1)

Present study Reference[38]

Reference[37]

Reference[17]

Synthesized Sintered Synthesized Sintered Sintered

PO43" bend "2 – 442 469 – 480

PO43" bend "4 568 568 560 568 570

PO43" bend "4 604 601 599 602 602

PO43" bend "3 1038 1046 1046 1048 1046

CO32" group "3

(B-typecarbonate)

1421 – 1424 – –

H2O adsorbed "2 1643 – 1637 – 1630"3 – CO2

stretching mode2362 2358 – – –

H2O adsorbed 3425 3444 3435 – 3430OH" structural"S

– 3566 3571 3572 3572

Fig. 8. The variation in relative density and grain size of conventionalpressureless sintered EHA as a function of sintering temperature.

P. Kamalanathan et al. / Ceramics International 40 (2014) 16349–1635916354

This corresponded well to the relative density of 92.4% in thepresent study at a sintering temperature of 1100 1C. The finalstage contributes to the complete isolation of pores resulting inan almost fully dense ceramic [61], which agrees well with thehigh density (97.4%) achieved at 1250 1C in the present work.

3.5. Microstructural evolution and grain size analysis

As-synthesized EHA powder exhibited a needle-like mor-phology with an average length of 48.2 nm where a similarpowder morphology was observed in the study conducted by

Wu et al. [34]. It is believed that the presence of needle-likecrystals in bulk materials can enhance fracture toughness [62].Fig. 9 presents the microstructural evolution of sintered EHAsamples. When the sintering temperature increased from 1100to 1350 1C, the samples exhibited a more densely packed grainmicrostructure that corresponds to the increase in relativedensity of sintered EHA. The increase in density is due tothe fact that pore elimination occurs during densification,where this is achieved via the diffusion of matter through thegrain boundaries of the particles or the particle volume itself[7,61]. Upon an increase in sintering temperature, significant

Fig. 9. Images of (a) TEM micrograph of as-synthesized EHA nanocrystals and SEM micrographs of EHA sintered at (b) 1100 1C, (c) 1200 1C, (d) 1250 1C,(e) 1300 1C, and (f) 1350 1C.

P. Kamalanathan et al. / Ceramics International 40 (2014) 16349–16359 16355

grain growth was observed as shown in Figs. 8 and 9. A nearlinear relationship between grain size and sintering temperaturecan be seen. The smallest grain size (0.330 mm) is attained at1000 1C, whereas the largest grain size (4.45 mm) is obtainedat 1400 1C. Rapid grain growth was observed beyond sinteringat 1250 1C. However, the rate of this exaggerated grain growthwas not as significant compared to that observed in the studyconducted by Ramesh et al. [42], in which grain size increasedfrom approximately 2 to 8 mm within a sintering temperaturerange of 1200–1250 1C. This suggests that the produced EHAin the present study is capable of suppressing rapid graingrowth, resulting in smaller grain sizes which enhancemechanical properties.

3.6. Fracture toughness and Vickers hardness

The relationship between Vickers hardness and sinteringtemperature is shown in Fig. 10. Vickers hardness increasesfrom a low value of 0.68 GPa at 800 1C up to a maximum of4.96 GPa at sintering temperature of 1250 1C. This wasattributed to the increase in density as shown in Fig. 11.Beyond 1250 1C, the hardness decreases from 4.96 GPa to3.48 GPa at 1400 1C. The relatively high hardness obtainedfrom the current research is rather promising as it exceeds thehardness of sintered EHA with an approximate value of4.36 GPa, reported in a previous study [38].

It has been postulated that the hardness of HA increases asrelative density increases [63]. However, in the present study,the aforementioned claim is partially true. As observed inFig. 11, below a critical grain size (Dc#1.72 mm), the Vickershardness was governed by relative density. Therefore, thehardness trend is observed to correlate well with the variationin relative density. However, beyond the Dc, the hardnessappeared to be governed by grain growth rather than relativedensity itself. As shown in Fig. 11, a steady decline in Vickershardness is observed even though the relative densities at1300 1C and 1350 1C had a significantly high value of 97.1%compared to that of 1250 1C. This phenomenon had beenreported previously [56], where a similar trend was observed.The critical grain size reported in their study was approxi-mately 2 mm which agreed well with the Dc in this study.Besides being governed by relative density, hardness is alsoobserved to adhere to the linear nature of the reverse Hall–Petch relationship [64] below Dc. This is highlighted in Fig. 12which unambiguously reveals the linear reduction in Vickershardness with a decrease in grain size. Therefore it can beinferred that the Vickers hardness of EHA is governed byrelative density and the reverse Hall–Petch relationship belowa particular Dc whereas beyond the Dc, hardness is dictated bygrain growth.

The decline in the Vickers hardness of EHA could also bedue to the decomposition of HA to its secondary phase at hightemperatures. The presence of these phases is known to bedetrimental to the mechanical properties of HA owing to itsbrittle nature and weak strength [65,66].

In the present study, the fracture toughness of sintered EHAincreases with sintering temperature up to a maximum value of

1.14 MPa m1/2 at 1000 1C. However, further sintering at highertemperatures resulted in a decrease in KIc as shown in Fig. 13.A fracture toughness of 0.81 MPa m1/2 is achieved in samplessintered at 1250 1C, whereas low values of 0.38 MPa m1/2 and0.56 MPa m1/2 are obtained at 800 1C and 900 1C, respec-tively. This low KIc measured at such low temperatures couldbe attributed to the weak grain boundaries of the HA matrix[61,67]. On the other hand, the high fracture toughnessobtained in the present work (1.14 MPa m1/2) at 1000 1C israther encouraging as it is significantly higher than the reportedmaximum fracture toughness of 0.9 MPa m1/2 achieved at

Fig. 10. The variation in Vickers hardness of conventional pressurelesssintered EHA as a function of sintering temperature.

Fig. 11. The variation of Vickers hardness and relative density of conventionalpressureless sintered EHA as a function of grain size.

Fig. 12. The variation of Vickers hardness of conventional pressurelesssintered EHA as a function of the inverse square root of the grain size basedon the reverse Hall–Petch relationship.

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a sintering temperature of 1050 1C [42]. In addition, the overallfracture toughness of EHA in the present study is greater than0.8 MPa m1/2 up to the point where phase pure HA is attained.This is greater than that observed in previous study [42], wherethe fracture toughness of pure HA is consistently observed to belower than 0.8 MPa m1/2 besides its maximum at 0.9 MPa m1/2 at1050 1C. The high fracture toughness in the present study is alsocomparable to the fracture toughness of 1.18 MPa m1/2 [9]attained via the two-step sintering (TSS) of HA. However, it isgreater than the fracture toughness of 1.02 MPa m1/2 [68] obtainedvia a similar approach. This method generally enhances themechanical characteristics of the sintered sample by attainingdensification while limiting grain growth. This is achieved bysuppressing grain boundary migration while ensuring that grainboundary diffusion remains active [69]. To accomplish this,samples are fired up to a temperature T1, followed by animmediate drop to a lower temperature T2 and held for longdurations. As such, the high fracture toughness attained by Linet al. [9] was only reached after a holding time of 20 h whereasthe 24 h holding time employed by Veljovic et al. [68] onlyresulted in a low fracture toughness. In the present study, a highKIc was achieved with a significantly shorter holding time of 1 h.This clearly emphasizes the superiority of sintered EHA in termsof mechanical property enhancement.

In addition fracture toughness is also affected by grain size,where KIc decreases with an increase in grain size. Thisunambiguous relationship is shown in Fig. 14. A smaller grain

size reduces the inherent flaw size of the sintered microstruc-ture, which in turn translates to an overall enhancement of thesintered sample's mechanical property [63,67].This effect can be explained by the theory proposed by Wang

and Shaw [63]. The theory is based on the formation of inter-granular and trans-granular fracture, where the former enhancesthe fracture toughness. It is postulated that the critical factor for theoccurrence of inter-granular fracture in EHA lies in the probabilityof a crack to arrive in the grain-boundary-affected (GBA) zone.If a crack were to fall in the GBA zone, inter-granular fracture andcrack deflection would occur leading to an increase in KIc.However, if a crack were to occur outside GBA, trans-granularfracture becomes predominant, contributing to a decrease in KIc.The size of the GBA zone varies from one location to another.Nevertheless, the probability of a crack to lie in the GBA zoneincreases as the grain size decreases [63]. This correlates well withthe present study where the high fracture toughness of 1.14 MPam1/2 attained at a sintering temperature of 1000 1C corresponds toa small grain size of 0.33 mm. The small grain size could havesignificantly contributed to the increased probability of a crackfrom the indentation to lie within the GBA zone. Therefore inter-granular fracture along with crack deflection becomes dominant,resulting in shorter crack lengths. It is observed that while Vickershardness is governed by relative density, the reverse Hall–Petchrelationship and grain growth, the enhancement of fracturetoughness is largely due to finer grain microstructure.

4. Conclusions

In the present work, HA was successfully synthesized viathe WCP route using waste eggshells as a calcium precursor toproduce powder with a needle-like morphology and an averagelength of 48.2 nm. Pure eggshell-derived HA (EHA) wasstable up to a high temperature of 1250 1C. However, EHAdecomposed to !-TCP and TTCP upon sintering at 1300 1Cwhereas melting of EHA was observed at 1400 1C as thesample began to deform. The EHA sample achieved a near fulldensity of 97.4% at 1250 1C whilst beyond this temperature,density began to decrease due to the decomposition of EHA.This is concurrent with SEM images, where an increase insintering temperature resulted in the prominence of poreisolation, thus forming a dense ceramic.In terms of mechanical properties, EHA samples exhibited

the highest Vickers hardness of 4.96 GPa at a sinteringtemperature of 1250 1C. The Vickers hardness of EHA wasobserved to be governed by relative density and the reverseHall–Petch relationship below a critical grain size (Dc) of1.72 mm whereas beyond this particular point, hardness wasgoverned by grain growth. On the other hand, the fracturetoughness reached an optimum value of 1.14 MPa m1/2 at1000 1C attributed mainly to the small grain size of 0.33 mmachieved at this sintering temperature.

Acknowledgements

This study was supported under HIR Grant no. H-16001-00-D000027 and PPP Grant no. FP046-2013B.

Fig. 13. The variation of fracture toughness of conventional pressurelesssintered EHA as a function of sintering temperature.

Fig. 14. The variation of fracture toughness of conventional pressurelesssintered EHA as a function of grain size.

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