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
]; 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 (Figure 1
) 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
]. 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 (Figure 3
). 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 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.