Chemical Stability of Tricalcium Phosphate–Iron Composite During Spark Plasma Sintering

Article Chemical Stability of Tricalcium Phosphate–Iron Composite during Spark Plasma Sintering

Mariano Casas-Luna 1, Miroslava Horynová 1, Serhii Tkachenko 1, Lenka Klakurková 1 ID , Ladislav Celko 1, Sebastián Diaz-de-la-Torre 2 and Edgar B. Montufar 1,*

1 Central European Institute of Technology (CEITEC), Brno University of Technology, Brno 61200, Czech Republic; [email protected] (M.C.-L.); [email protected] (M.H.); [email protected] (S.T.); [email protected] (L.K.); [email protected] (L.C.) 2 Centro de Investigación e Innovación Tecnológica (CIITEC), Instituto Politécnico Nacional, México City 02250, Mexico; [email protected] * Correspondence: [email protected]; Tel.: +420-54114-9201

Received: 23 July 2018; Accepted: 23 August 2018; Published: 1 September 2018

Abstract: Tricalcium phosphate (Ca3(PO4)2, TCP) is a ceramic widely used as a bone filler material due to its good osteoconductivity. Nevertheless, its poor mechanical properties do not allow its use for load-bearing applications. Therefore, the option of improving its strength and toughness by adding a biocompatible metallic component is a promising alternative to overcome this drawback, leading to the fabrication of improved bone implants. The present work is focused on defining the thermal stability of alpha-TCP (α-TCP) when it is sintered together with iron (Fe) by spark plasma sintering. The results showed the thermal stability of the composite with no degradation or oxidation in the ceramic or metal phase. A clear advantage from the TCP-Fe composites when compared with others, such as hydroxyapatite-titanium, is the complete retention of the TCP due to the less reactivity with iron respect to titanium. Furthermore, the allotropic phase transformation from alpha to beta-TCP polymorph was reduced by sintering at 900 ◦C. However, the densification of the material was also impaired at this temperature. It is expected that spark plasma sintering will allow the fabrication of TCP–Fe composites free of secondary phases that compromise the mechanical strength of the material.

Keywords: metal matrix composite; iron; tricalcium phosphate; sintering; decomposition

1. Introduction In the ﬁeld of biomaterials, the development of metallic–ceramic composites has attracted the attention of material researchers due to the possibility of combining the mechanical strength and toughness of implantable metals with the osteoconductive properties of calcium phosphate ceramics. Among the different alternatives, titanium and hydroxyapatite are two of the most extensively used materials to treat bone fractures [1,2], so the interactions between them during sintering has been studied in detail [3,4]. The results show that the fabrication of titanium-hydroxyapatite composites is technically feasible. However, the disadvantage of this system is that titanium catalyzes the thermal degradation of hydroxyapatite during sintering, generating mixtures of titanium, hydroxyapatite, calcium and titanium oxides and phosphates [4], which leads to detrimental mechanical properties [5]. In the present work, a different system is considered and studied as a potential candidate to overcome this decomposition drawback. Our hypothesis is that tricalcium phosphate–iron (TCP–Fe) powders will be stable during sintering, therefore allowing the fabrication of a new class of biocompatible composite. This premise is based on the fact that tricalcium phosphate is thermodynamically more stable than hydroxyapatite during heating (Figure1)[ 6,7]. Besides, as hydroxyapatite, tricalcium

J. Compos. Sci. 2018, 2, 51 ; doi:10.3390/jcs2030051 www.mdpi.com/journal/jcs J. Compos. Sci. 2018, 2, 51 2 of 8 J. Compos. Sci. 2018, 2, x FOR PEER REVIEW 2 of 8 phosphatehydroxyapatite, is successfully tricalcium used phosphate in the clinicis successfully to reconstruct used in and the ﬁll clinic bone to reconstruct defects [2]. and Metallic fill bone iron, ondefects its side, [2]. is Metallic less reactive iron, thanon its titanium side, is less [8]; reactive consequently, than titanium it can increase[8]; consequently, the chances it can to increase retain the tricalciumthe chances phosphate to retain phase the tricalcium unreacted. phosphate To validate phase this unreacted. assumption, To validate alpha-tricalcium this assumption, phosphate alpha and‐ ◦ irontricalcium powders phosphate were consolidated and iron powders between were 900 and consolidated 1100 C using between the spark900 and plasma 1100 °C sintering using the technique. spark Theplasma microstructure, sintering crystallinetechnique. phaseThe microstructure, composition and crystalline mechanical phase behavior composition of the resulting and mechanical composite werebehavior studied of by the scanning resulting electron composite microscopy were studied (SEM), by X-ray scanning diffraction electron (XRD) microscopy and micro-indentation. (SEM), X‐ray Sparkdiffraction plasma (XRD) sintering and wasmicro used‐indentation. as the forming Spark plasma technique sintering because was it used allows as fastthe forming heating technique rates (up to hundredsbecause ofit allows◦C·min fast−1) heating regarding rates conventional (up to hundreds sintering of °C technics∙min−1) regarding and applies conventional uniaxial mechanical sintering loadtechnics during and sintering. applies These uniaxial characteristics mechanical allowload reductionsduring sintering. in the processingThese characteristics time and sintering allow temperaturereductions while in the providing processing superior time and mechanical sintering propertiestemperature due while to improved providing powder superior densiﬁcation mechanical [ 9]. A secondproperties advantage due to improved of the TCP–Fe powder system densification is that [9]. it A can second generate advantage a biodegradable of the TCP–Fe composite system is for temporalthat it can biomedical generate applications.a biodegradable On composite the one hand,for temporal iron is biomedical a biodegradable applications. metal On with the reliable one hand, iron is a biodegradable metal with reliable mechanical performance in vivo. In fact, recent mechanical performance in vivo. In fact, recent works show that iron and its corrosion products are works show that iron and its corrosion products are biocompatible [10]. On the other hand, alpha‐ biocompatible [10]. On the other hand, alpha-tricalcium phosphate is slightly more soluble than its beta tricalcium phosphate is slightly more soluble than its beta polymorph, and thus represents a better polymorph, and thus represents a better alternative for the development of a biodegradable composite. alternative for the development of a biodegradable composite.

FigureFigure 1. 1.Schematic Schematic representation representation ofof thethe thermalthermal stability of of stoichiometric stoichiometric hydroxyapatite hydroxyapatite and and crystallinecrystalline tricalcium tricalcium phosphate. phosphate. Data Data obtained obtained fromfrom References [6,7]. [6,7].

2. Materials and Methods 2. Materials and Methods

2.1.2.1. Preparation Preparation of of Tricalcium Tricalcium Phosphate-Iron Phosphate‐Iron Composite Composite Synthesis of alpha‐tricalcium phosphate powder was carried out through solid state reaction at Synthesis of alpha-tricalcium phosphate powder was carried out through solid state reaction at 1400 °C between CaCO3 (Lach‐ner, Neratovice, Czech Republic) and CaHPO4 (Merck, Darmstadt, 1400 ◦C between CaCO (Lach-ner, Neratovice, Czech Republic) and CaHPO (Merck, Darmstadt, Germany). After the chemical3 reaction, the product was quenched in air to retain4 the alpha phase of Germany). After the chemical reaction, the product was quenched in air to retain the alpha phase tricalcium phosphate [11]. Then, tricalcium phosphate was milled using a horizontal high energy of tricalcium phosphate [11]. Then, tricalcium phosphate was milled using a horizontal high energy ball‐mill (Simoloyer CM01, Zoz GmbH, Wenden, Germany) at 1000 rpm for 15 min (until an average ball-millparticle (Simoloyer size below CM01,36 μm) Zoz and GmbH,thoroughly Wenden, mixed Germany) in identical at volumetric 1000 rpm parts for 15 with min commercial (until an average iron particlepowder size containing below 36 3µ m)vol.% and of thoroughly carbon nanotubes mixed in(average identical particle volumetric size of parts 3.7 μ withm. Applied commercial Carbon iron powderNano containingTechnologies, 3 vol.% Pohang, of carbonKorean). nanotubes Carbon nanotubes (average particlewere introduced size of 3.7 intoµm. the Applied composite Carbon to Nanopromote Technologies, the galvanic Pohang, degradation Korean). of iron Carbon [12] at nanotubes the same time were to introducedimprove the intomechanical the composite strength to promoteof the composite the galvanic [13]. degradation However, since of iron safety [12 ]issues at the related same time to carbon to improve nanotubes the mechanicaldepend on several strength

J. Compos. Sci. 2018, 2, 51 3 of 8 of the composite [13]. However, since safety issues related to carbon nanotubes depend on several factors [14], more studies are needed to access their safety or toxicity in this speciﬁc exposure condition.

2.2. Sintering Conditions Consolidation of TCP–Fe powder mixture was conducted using a cylindrical graphite die set and a commercial spark plasma sintering system (Dr. Sinter 1050, Sumitomo Coal & Mining Co. Ltd., now SPS Syntex Inc., Tokyo, Japan) applying a direct current of 600 A at 3 V with an on-off cycle of 12 and 2 ms. The sintering temperature was set at 900, 1000 and 1100 ◦C with a heating rate of 150 ◦C·min−1. For monitoring the process temperature, a K-type thermocouple was placed in the center of the wall of the graphite die. No holding time at the sintering temperature was performed; therefore, samples were allowed to cool down after reaching the sintering temperature under vacuum (8 Pa). A constant uniaxial compaction load of 5 kN (16 MPa) was maintained during all the sintering process. Finally, disc-shaped samples of 5 mm in thickness and 20 mm in diameter were obtained.

2.3. Chemical and Microstructural Characterization After sintering, for the chemical and microstructural characterization, the obtained discs were ground using SiC sandpapers measuring 500-grit up to 1200-grit and polished with a diamond paste of 1 µm of particle size. XRD analyses (Rigaku SmartLab 3 kW CF2, Tokyo, Japan) were conducted to identify the crystalline phases present in the composite. Scans were performed in Bragg–Brentano geometry between 10◦ and 90◦, using Cu Kα radiation and a scan speed of 3◦·min−1. Later, a Rietveld reﬁnement of the XRD patterns was performed using High Score Plus software to determine the percentage of crystalline phases. After the XRD analysis, samples were coated with carbon to observe the microstructure of the polished surfaces by SEM (TESCAN Lyra3, Tescan, Brno, Czech Republic) and prevent charging during observation. Local elemental-composition analyses were performed using an SEM equipped with an energy dispersive X-ray spectrometer (PHILIPS XL30, SEMTech solutions, Billerica, MA, USA). To have an estimation of the degree of consolidation of the tricalcium phosphate phase with respect to the sintering temperature, Vickers hardness measurements were carried out using a microhardness indenter (DuraScan-70, Struers, Puch, Austria) under a load of 100 g and a dwell time of 15 s. A minimum of ten measurements were performed in each sample and the average value is reported.

3. Results Figure2a shows the XRD pattern of the TCP–Fe composite sintered at 1100 ◦C, while Figure2b is a detail of the pattern for a better appreciation of the tricalcium phosphate phase in 2-theta angle between 25◦ and 40◦. According to the peak indexation, the composite is composed of α-iron (ICSD No. 64998), monoclinic tricalcium phosphate (α-phase, ICSD No. 923) and rhombohedral tricalcium phosphate (β-phase, ICSD No. 6191). The presence of other phases, such as hydroxyapatite, calcium oxide, iron oxide or carbonates, was not detected. Rietveld analysis of the diffraction patterns of TCP–Fe composites sintered at 900, 1000 and 1100 ◦C (Figure3) revealed that the average content of β-tricalcium phosphate in the samples was 10.8, 38.5 and 44.2 vol.%, respectively. The lowest level of β-tricalcium phosphate formation was observed at a sintering temperature of 900 ◦C, which means that the reduction in sintering temperature delays the allotropic phase transformation from metastable α-phase into β-phase. At the same time, the average iron content in the surface of the samples was found to be constant with a value of around 14 vol.%. J. Compos. Sci. 2018, 2, 51 4 of 8 J. Compos. Sci. 2018, 2, x FOR PEER REVIEW 4 of 8 J. Compos. Sci. 2018, 2, x FOR PEER REVIEW 4 of 8

FigureFigure 2. 2. XX-ray‐ray diffraction diffraction (XRD) (XRD) pattern pattern of of (a (a) )the the tricalcium tricalcium phosphate–iron phosphate–iron (TCP–Fe) (TCP–Fe) composite composite surfaceFiguresurface after2. after X‐ rayspark spark diffraction plasma sintering (XRD) pattern atat 11001100 of◦ °CC (a and )and the ( b (tricalciumb) detail) detail of of thephosphate–iron the pattern pattern showed showed (TCP–Fe) in (ina) ( ina )composite the in the 2-theta 2‐ thetasurfacerange range between after between spark 25 and plasma 25 40and degrees. 40sintering degrees. at 1100 °C and (b) detail of the pattern showed in (a) in the 2‐ theta range between 25 and 40 degrees.

Figure 3. XRD patterns of the TCP–Fe composite sintered by spark plasma sintering at different temperaturesFigureFigure 3. XRDXRD and patterns patterns the starting of of the the α‐ TCPTCP–Fe TCP–Fe powder. composite composite The patterns sintered sintered are by by presented spark spark plasma plasma between sintering sintering 27 and at36 at different different(2‐theta degrees)temperaturestemperatures to highlight and and the the the starting starting effect α‐ ofα-TCP TCPthe sintering powder. powder. temperatureThe The patterns patterns onare are the presented presented ratio between between between α‐ 27 27and and and β‐ tricalcium36 36 (2 (2-theta‐theta phosphatedegrees)degrees) to to polymorphs. highlight highlight the the effect effect of of the the sintering temperature temperature on the ratio between α‐ α- and β‐β-tricalciumtricalcium phosphatephosphate polymorphs. polymorphs. Figure 4 shows backscattered SEM images of the TCP–Fe composite sintered at different temperatures,Figure 4 showing shows abackscattered biphasic microstructure SEM images with of well the‐defined TCP–Fe boundaries composite between sintered the at phases. different As Figure4 shows backscattered SEM images of the TCP–Fe composite sintered at different cantemperatures, be corroborated showing by the a biphasic elemental microstructure analysis spectra with presented well‐defined in Figure boundaries 5, the dark between phase the corresponds phases. As temperatures, showing a biphasic microstructure with well-deﬁned boundaries between the phases. tocan the be corroboratedceramic phase, by thecomposed elemental of analysis calcium, spectra phosphorus presented and in Figureoxygen, 5, thepresenting dark phase a correspondscalcium to As can be corroborated by the elemental analysis spectra presented in Figure5, the dark phase phosphorusto the ceramic ratio phase,corresponding composed to tricalciumof calcium, phosphate phosphorus i.e., Ca/P and = oxygen,1.5, while presenting the white phasea calcium is iron. to corresponds to the ceramic phase, composed of calcium, phosphorus and oxygen, presenting a calcium Thephosphorus microstructure ratio corresponding of the composite to processedtricalcium atphosphate 900 °C featured i.e., Ca/P a weak = 1.5, connection while the whitebetween phase tricalcium is iron. to phosphorus ratio corresponding to tricalcium phosphate i.e., Ca/P = 1.5, while the white phase is phosphateThe microstructure particles, of exhibiting the composite almost processed free ceramic at 900 °Cparticles featured and a weak several connection pores (Figure between 4a), tricalcium which iron. The microstructure of the composite processed at 900 ◦C featured a weak connection between indicatesphosphate that particles, the material exhibiting was not almost entirely free sintered. ceramic Nonetheless, particles and since several the sintering pores (Figure temperature 4a), which was tricalcium phosphate particles, exhibiting almost free ceramic particles and several pores (Figure4a), increased,indicates thatbetter the densification material was was not observedentirely sintered. with a remainingNonetheless, porosity since themainly sintering presented temperature within wasthe which indicates that the material was not entirely sintered. Nonetheless, since the sintering temperature ceramicincreased, component better densification (Figure 4b,c). was Unexpectedly, observed with less a presence remaining of porosity wasmainly observed presented in the within material the was increased, better densiﬁcation was observed with a remaining porosity mainly presented within sinteredceramic atcomponent 1000 °C in (Figure comparison 4b,c). withUnexpectedly, the higher less temperature presence of porosity1100 °C. was observed in the material the ceramic component (Figure4b,c). Unexpectedly, less presence of porosity was observed in the sintered at 1000 °C in comparison with the higher temperature of 1100 °C. material sintered at 1000 ◦C in comparison with the higher temperature of 1100 ◦C.

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Figure 4. Representative backscattered SEM images of the surface of TCP–Fe composite after spark

Figureplasma 4. Representative sintering at (a) backscattered 900 °C, (b) 1000 SEM °C and images (c) 1100 of the°C. The surface distribution of TCP–Fe of the composite two components after spark plasmaFigurecan sintering be observed,4. Representative at (a i.e.,) 900 iron◦ C,backscattered in ( brightb) 1000 and◦C SEMceramic and images (c )in 1100 dark of◦ theC.grey. Thesurface distribution of TCP–Fe of composite the two components after spark can be observed,plasma sintering i.e., iron at in (a bright) 900 °C, and (b) ceramic 1000 °C inand dark (c) 1100 grey. °C. The distribution of the two components can be observed, i.e., iron in bright and ceramic in dark grey.

Figure 4. Representative backscattered SEM images of the surface of TCP–Fe composite after spark plasma sintering at (a) 900 °C, (b) 1000 °C and (c) 1100 °C. The distribution of the two components can be observed, i.e., iron in bright and ceramic in dark grey.

Figure 5. Representative energy‐dispersive X‐ray spectroscopy corresponding to (a) the dark phase

(TCP) and (b) the bright phase (Fe) observed in Figure 4. Figure 5. Representative energy‐dispersive X‐ray spectroscopy corresponding to (a) the dark phase Figure 5. Representative energy-dispersive X-ray spectroscopy corresponding to (a) the dark phase (TCP)Figure and 6 shows (b) the thebright hardness phase (Fe) values observed measured in Figure on 4.the sintered composite, which were specifically (TCP) and (b) the bright phase (Fe) observed in Figure4. carried out only at the tricalcium phosphate phase of the samples. The highest hardness of the ceramic (1249Figure MPa) was6 shows observed the hardness for the material values measuredsintered at on a temperaturethe sintered ofcomposite, 1000 °C, whichwhich is were followed specifically by the Figure 5. Representative energy‐dispersive X‐ray spectroscopy corresponding to (a) the dark phase carriedFigurecomposite 6out shows sinteredonly at the theat hardness 1100 tricalcium °C with values phosphate a hardness measured phase of 1050 of on MPa.the the samples. sinteredThese values The composite, highest are in accordancehardness which of were to the the ceramic speciﬁcally degree (TCP) and (b) the bright phase (Fe) observed in Figure 4. carried(1249of porosity out MPa) only wasthat at thewasobserved tricalcium observed for the in phosphate materialthe micrographies sintered phase at of(Figure a thetemperature samples. 4), where of Thehigher 1000 highest °C, porosity which hardness wasis followed present of theby in thethe ceramic composite sintered at 1100 °C with a hardness of 1050 MPa. These values are in accordance◦ to the degree (1249samples MPa)Figure wassintered observed6 shows at 1100 the °C. for hardness Hardness the material values was measured not sintered measured on atthe on a sintered temperature the sample composite, sintered of 1000which at 900 wereC, °C which becausespecifically is it followed was of porosity that was observed in the micrographies (Figure 4), where higher porosity was present in the by thenotcarried composite thoroughly out only sintered consolidated, at the tricalcium at 1100 leading ◦phosphateC to with the brittle a phase hardness fracture of the samples. of of 1050the specimen The MPa. highest These during hardness values indentation. of arethe ceramic in accordance samples sintered at 1100 °C. Hardness was not measured on the sample sintered at 900 °C because it was to the degree(1249 MPa) of porositywas observed that for was the material observed sintered in the at a micrographies temperature of 1000 (Figure °C, which4), whereis followed higher by the porosity not thoroughly consolidated, leading to the brittle fracture of the specimen during indentation. was presentcomposite in the sintered samples at 1100 sintered °C with a at hardness 1100 ◦ ofC. 1050 Hardness MPa. These was values not measuredare in accordance on the to the sample degree sintered ◦of porosity that was observed in the micrographies (Figure 4), where higher porosity was present in the at 900 samplesC because sintered it was at 1100 not °C. thoroughly Hardness was consolidated, not measured on leading the sample to thesintered brittle at 900 fracture °C because of theit was specimen duringnot indentation. thoroughly consolidated, leading to the brittle fracture of the specimen during indentation.

Figure 6. Average of the Vickers’ hardness on the ceramic phase of the TCP–Fe composite sintered at

temperatures of 1000 and 1100 °C. No consolidation of the composite at the temperature of 900 °C Figureleads to 6. the Average brittle offracture the Vickers’ of the hardnesssample during on the indentation, ceramic phase giving of the no TCP–Fe results. composite sintered at temperatures of 1000 and 1100 °C. No consolidation of the composite at the temperature of 900 °C leads to the brittle fracture of the sample during indentation, giving no results. Figure 6. Average of the Vickers’ hardness on the ceramic phase of the TCP–Fe composite sintered at Figure 6. Average of the Vickers’ hardness on the ceramic phase of the TCP–Fe composite sintered at temperaturestemperatures of 1000 of 1000 and and 1100 1100◦C. °C. No No consolidation consolidation of of the the composite composite at the at temperature the temperature of 900 °C of 900 ◦C leads to the brittle fracture of the sample during indentation, giving no results. leads to the brittle fracture of the sample during indentation, giving no results.

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4. Discussion The poor mechanical strength and brittle fracture of calcium phosphate ceramics limit their applications, rendering them unsuitable for bone fracture fixation. In contrast, implantable metals are extensively used to fabricate devices for the internal fixation of bone fractures, such as screws, plates, or even hip prostheses. The combination of calcium phosphate ceramics and metallic reinforcements has been considered as a promising alternative to achieve both bioactivity and mechanical strength in one material. The most commonly explored combination is the composite containing hydroxyapatite matrix toughened with titanium particles [3–5]; however, it has been found that the presence of titanium promotes the partial decomposition of hydroxyapatite during sintering [4]. Therefore, in the present work, we investigated a metal–ceramic composite based on iron and tricalcium phosphate, which could be a good alternative to hydroxyapatite–titanium composites. The microstructure of the obtained TCP–Fe composite presented a well-defined boundary between the tricalcium phosphate ceramic phase and the metallic iron phase. Moreover, no signs of reaction between the components was detected. As it was corroborated by XRD and the elemental analysis, the existing phases inside the composite corresponded to pure iron particles and pure tricalcium phosphate. In addition, pores were observed within the tricalcium phosphate phase as a result of poor material densification that can be improved in future by varying the sintering conditions, i.e., increasing the compaction load or the dwelling time. Oxidation of iron was avoided because the spark plasma sintering process operates under vacuum. Furthermore, iron did not present reactivity or catalytic degradation effect on the tricalcium phosphate. This could be explained due to the thermal stability of the tricalcium phosphate (Figure1) and the less reactivity of iron in comparison with other metals like the titanium [15]. In fact, it has been reported that the spark plasma sintering of tricalcium phosphate–titanium powders has led to the partial degradation of the phosphate and the formation of CaTiO3 phases [16]. For the TCP–Fe composite, no formation of CaO has been found as it is typical in the case of the hydroxyapatite–titanium composites. Therefore, the use of iron represents a step forward to retain unreacted the tricalcium phosphate when these compounds are mixed in a composite. However, the allotropic transformation from α- to β-polymorph of tricalcium phosphate occurred (Figure3). There are two main reasons for this transformation. The first is the meta-stability of the alpha phase below 1125 ◦C[7] and the second is the diffusive mechanism of phase transformation that is promoted at higher sintering temperatures. In fact, since the beta phase is thermodynamically more stable at 1000 and 1100 ◦C and the energy for diffusion is relevant at these two temperatures, around the 50% of the α-tricalcium phosphate was transformed into its beta polymorph. Nonetheless, despite the fact that the beta phase is thermodynamically even more stable than the alpha phase at 900 ◦C, the slower kinetics of phase transformation governed the process, leading to a higher α-tricalcium phosphate retention (around 87%). Unfortunately, slow mass diffusion at 900 ◦C caused a poor particle consolidation inside the tricalcium phosphate ceramic phase during sintering. It must be pointed out that both alpha and beta tricalcium phosphate polymorphs are biocompatible and osteoconductive ceramics [2]. Therefore, the biocompatibility of the composite should not be affected by the phase transformation of tricalcium phosphate. The interest on retaining the alpha phase is due to its higher bioactivity and solubility than the beta phase [7], allowing the chance of producing a biodegradable and osteoconductive composite in combination with iron, as a biocompatible reinforcement for higher mechanical stability. The sintering process could be performed above the phase transition temperature in order to retain the alpha phase. However, this option presents technical drawbacks, such as a more expensive process due to the higher energy consumption. In the present work, a K-type thermocouple was used to control the sintering temperature in the center of the wall of the graphite die. The use of higher sintering temperature at our laboratory would require the utilization of a pyrometer to measure the temperature on the surface of the graphite die, farther from the sample, with the consequent loss of accuracy. In fact, the control of the real temperature in the sample during the sintering is one of the biggest drawbacks in the spark plasma sintering process [17]. Numerical models have been applied to determine the real temperature distribution inside the sample, but there is not a general solution since J. Compos. Sci. 2018, 2, 51 7 of 8 it depends on several parameters, e.g., geometry and dimensions of the sintering set-up, electrical and thermal properties of the material and frequency of electrical current pulses [17]. Additionally, the increase in the sintering temperature would prolong the cooling time, so the sample would stay at high temperature for longer time and the conditions would be more favorable for phase transformation of the tricalcium phosphate. Despite of the poor densification with the sintering conditions, the highest hardness of the tricalcium phosphate phase was found to be the half of that reported for bulk tricalcium phosphate ceramics processed by the conventional hot-pressing method [18]. This is an indicator of the superior mechanical properties that can be achieved when using the spark plasma sintering technique in order to decrease the thermal degradation of metal matrix composites that impair the strength and biocompatible properties.

5. Conclusions The spark plasma sintering of iron and tricalcium phosphate powders leads to the consolidation of composite materials free of secondary phases, signs of degradation in its initial components and oxidation. Furthermore, the reduction in the sintering temperature better prevented the phase transformation from alpha to beta phase of tricalcium phosphate in the TCP–Fe composite. Nonetheless, lower temperature reduces the densiﬁcation of the material, starting the consolidation above 1000 ◦C. It is expected that the better retention of tricalcium phosphate during sintering with iron will allow the fabrication of osteoconductive and bioactive orthopedic devices with improved mechanical performance.

Author Contributions: Conceptualization—E.B.M.; Data curation—M.C.-L., M.H., S.T. and L.K.; Formal analysis—M.H. and S.T.; Funding acquisition—L.C. and E.B.M. Investigation—M.C.-L.; Methodology—S.D.-T.; Writing original draft—M.C.-L.; Writing review & editing—L.C., S.D.-T. and E.B.M. Acknowledgments: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie and it is co-financed by the South Moravian Region under grant agreement no. 665860. The authors also acknowledge the project CEITEC 2020 (LQ1601) with financial support from the Ministry of Education, Youth and Sports of the Czech Republic under the National Sustainability Program II. Part of the work was carried out with the support of core facilities of research infrastructure CEITEC Nano of CEITEC-BUT. MCL acknowledges the Brno Ph. D. Talent scholarship founded by the Brno City Municipality. SDT acknowledges Consejo Nacional de Ciencia y Tecnología, Mexico, SNI (P.1777000). Thanks to Dr Zdenek Spotz for his support with Rietveld analysis and Guillermo Diéguez-Trejo for his support in spark plasma sintering. Conflicts of Interest: The authors state that no conflicts of interest exist.

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