Sintering Behavior and Mechanical Properties of Biphasic Calcium Phosphate Ceramics
The sintering behavior and the mechanical properties of a mechanical mixture of hydroxyapatite and tricalcium phosphate (BCP) ceramics with the composition of 30% HA and 70% TCP are experimentally investigated in the temperature range between 1000°C and 1300°C. The results show that consolidation, grain growth, and Vickers hardness generally increase with increasing sintering temperature up to 1200°C. However, microstructure observation indicates that cracks are formed along the grain boundaries as well as in the bulk of the grains after sintering at 1200°C. Moreover, the best values of compressive strength, modulus of elasticity, and toughness are achieved in the samples sintered at 1100°C. These properties at 1100°C decay with sintering at 1200°C and increase again after sintering at 1300°C.
In composite bioceramics world, many researchers have focused their interest on the development of biphasic calcium phosphate (BCP) ceramics, especially with hydroxyapatite (HA: Ca10(PO4)6(OH)2) and tricalcium phosphate (TCP: Ca3(PO4)2). Different HA : TCP ratios for BCP ceramics display different bioactive and bioresorbing capacities. Generally, this difference is directly related to porosity of the BCP. In other words, sintering behavior and mechanical properties of BCP ceramics can play a crucial role for the difference with respect to porosity, as one of the major physical properties of bioceramics [1–3].
As a major part of BCP composite ceramics, TCP is a mobile phase, which means that it has an intrinsic tendency to form agglomerations with increasing sintering temperature. There are experimental findings which show that TCP-containing samples suffer from abnormal grain growth, as observed in their microstructure. It is known that β-ΤCP → α-TCP phase transformation occurs at elevated temperatures. In samples of TCP fired at high temperatures, α-TCP has been largely recorded in X-ray diffraction analysis. Best values for compressive strength, modulus of elasticity, and toughness in cylindrical specimens of TCP sintered are determined at 1100°C. Density of TCP is increased with increasing sintering temperature but never reached the theoretical value of β-TCP (3.07 g/cm3) [4–8]. Some studies for BCP ceramics show that the mean thermal expansion coefficient of dense sintered HA/β-TCP biphasic ceramics increases almost linearly with the increase in β-TCP content. Grain growth which occurs at 1200°C has a more important effect than the increase of TCP loading in the properties of a HA/TCP biphasic material [9–11].
Due to the fact that BCP is a mixture of HA and TCP, it may be assumed that, on one hand, the BCP is an initially homogeneous material when HA : TCP ratio approaches zero; that is, the formation nearly becomes pure TCP. On the other hand, it turns relatively to a porous material when 60% ≥ HA. According to that interesting feature of BCP ceramics, an investigation of BCP with composition range of 0–60% HA seems to be critical. Composition of 60% HA and 40% TCP is generally preferred due to its appropriate bone substitution/regeneration capabilities and good mechanical properties . At that point, a question arises: what is the critical HA : TCP ratio where behavior of good bone regeneration starts? In order to answer this question, partly, half of 60% HA (i.e., BCP with 30% HA and 70% TCP) is worth examining where acceptable use of HA : TCP ratio might offer advantages biomechanically.
The purpose of this study is to investigate the sintering behavior of the BCP ceramics with a complex system (30% HA and 70% TCP) and to determine the influence of the composition on the mechanical properties in the temperature range between 1000 and 1300°C.
2. Material and Methods
Cylindrical biphasic calcium phosphate ceramic samples (BCP) are put together by mixing dry HA and β-TCP (MERCK, Darmstadt, Germany). The catalog numbers for HA and β-TCP are Merck #2196 and Merck #2143, respectively. According to the British Standard BS 7253, pellets (approximately 11 mm in diameter and 8 mm in height) are prepared by using uniaxial cold pressing in hardened steel dies. Under a heating rate of 4 K/min, the pressed samples are sintered in an open atmospheric furnace at different temperatures, specifically 1000, 1100, 1200, and 1300°C, for 4 h. For each temperature, 13 cylindrical samples of BCP with composition of 30% HA and 70% TCP are produced. After sintering at 1000, 1100, 1200, and 1300°C, their diameters are 12.39 ± 0.31, 11.07 ± 0.08, 10.52 ± 0.1, and 10.52 ± 0.14 mm and heights are 8.46 ± 0.41, 7.96 ± 0.38, 7.55 ± 0.8, and 7.9 ± 0.45 mm, respectively. The density of the sintered samples is determined by the Archimedes method.
The identification of the crystalline phases is done in an X-ray diffractometer (Rigaku ZSX Primus II, Japan). The microstructural analysis is carried out by a scanning electron microscope (SEM, JEOL JSM 7000F). The Vickers hardness measurements are realized with a microhardness testing machine (Shimadzu HMV-2, Japan), using 200 g load applied for 20 s (dwell time). A universal tensile testing machine (Devotrans FU 50kN, Turkey) is used for compression strength tests under a loading rate of 1 mm/min.
The experimental results are analysed with a general purpose statistical data analysis program WINKS SDA (Version 6.0.91 Professional Ed., TexaSoft, Houston, TX). In these statistical analyses, independent group t-test/ANOVA with comparison type Newman-Keuls and any difference at the 5% level are considered.
3. Results and Discussion
The X-ray diffractograms of the sintered BCP samples, shown in Figure 1, suggest that amount of HA decreases with increasing sintering temperature. It is also noted that a weak peak of monetite is recorded at the highest sintering temperature (1300°C).
In this study, it is assumed that phase transformation between β and α phase takes place between 1100°C and 1200°C; that is, β-TCP forms below 1180°C and α-TCP between 1180°C and 1430°C . This assumption is confirmed with the results of the mechanical properties reported in the following paragraphs.
The influence of sintering temperature on the microstructure of the samples is observed in the SEM images of Figure 2. The samples are poorly sintered at 1000°C, since only necks between the ceramic grains are observed and a large porosity throughout the whole bulk of the sample still exists (Figure 2(a)). Significant consolidation occurs after heat treatment at 1100°C (Figure 2(b)). However, small porosity still exists, whereas there is no evidence for extensive grain growth.
In Figure 2(c) extensive grain growth is observed in the samples sintered at 1200°C. Regions with local intergranular cracks are marked with the upper black arrow. The other two (lower) black arrows mark areas with intragranular cracks with a zigzag profile. These cracks are probably activated cleavage planes located in a single grain. The fracture initiation can only be attributed to the effect of phase transformation. Apparently, these cracks, and mainly the intergranular ones, should cause a reduction of the mechanical properties of the sintered ceramics. In general, intergranular cracks grow faster than intragranular ones.
The grains pronouncedly grow at 1300°C (Figure 2(d)). At this sintering temperature, material with large grains is more fragile than material with fine-grained microstructure. In Figures 2(c) and 2(d), the variation in the contrast of the particles may be due to the rounded aspect of the particles. That is, the white dots are corresponding to the upper flat part of the particles .
The mean values of density and Vickers microhardness, along with the corresponding values of the standard deviations, are listed in Table 1. It is clear that the density of the BCP samples increases with increasing sintering temperature up to 1200°C. There is actually no significant change of the densification of the samples after sintering at 1300°C since values of the densities at 1200 and 1300°C are not significantly different (). However, it is worthy to note that the measured density does not reach the theoretical value of TCP that is 3.07 g/cm3 .
The good sintering regime achieved at the high sintering temperatures is also concluded by looking at the values of the Vickers microhardness. However, a small decay of the hardness is observed in the samples sintered at 1300°C. Statistically, the values of all groups (with respect to the sintering temperature) are significantly different among each other ().
Figures 3 and 4 present the major mechanical properties of the BCP samples, that is, modulus of elasticity, compression strength, and toughness. These plots suggest that the optimum sintering temperature is 1100°C. On one hand, although results of modulus of elasticity and compression strength at 1000 and 1200°C are not significantly different (), those at 1100 and 1300°C are significantly different (). On the other hand, results of toughness at 1200 and 1300°C are not significantly different ().
For the BCP samples sintered at 1100°C, the modulus of elasticity is evaluated as 37.11 GPa (Figure 3), which is significantly higher that the values of TCP reported by Metsger et al.  and Wang et al. , which are 21 and 24.6 GPa, respectively. Additionally, in Figure 4, value of toughness in this work at 1100°C is 4.34 J/cm3 (Figure 4), which is much higher than the value 2.34 J/cm3 reported by Metsger et al. .
Also, one can find interesting similarities between the results of compression strength in the present study (Figure 3) and the results of flexural strength of HA + 11% TCP ; that is, the strength (whether compressive or flexural) increases up to 1100°, decreases at 1200°C, and finally it again increases at 1300°C.
A similar comparison can be easily done with the values of Vickers hardness from this work and the studies of [11, 13, 15, 16]. The behavior of these mechanical properties with increasing sintering temperature does not follow the evolution of density values with increasing sintering temperature (Table 1) and, therefore, it can be attributed to the phase transformation of TCP between β and α phase between 1100°C and 1200°C.
As a specific comparison in the sintering temperature range of 1040–1100°C, a graphical representation in  indicates that the modulus of elasticity and Vickers hardness of BCP (30% HA and 70% TCP) samples sintered at around 1045°C are 109 and 5.1 GPa, respectively. However, the present study shows that modulus of elasticity and Vickers hardness of BCP (30% HA and 70% TCP) samples sintered at 1100°C are 34 and 1.94 GPa, respectively.
The experimental results of this study, which agree fairly well with the results of earlier studies reported in the literature, suggest that the sintering behavior and the mechanical properties of the BCP-containing ceramics strongly depend on the sintering temperature. With regard to the consolidation, good sintering regime can be achieved at 1200°C, which is reflected in the high values of density and the Vickers hardness as well as in the highly dense microstructure. However, the formation of inter- and intragranular cracks, observed in the samples sintered at 1200°C, attributed to the transformation of β-TCP into α-TCP, jeopardizes the good mechanical properties. This transformation phase of TCP should occur between 1100°C and 1200°C. This transformation explains the influence of sintering temperature on the values of compressive strength, modulus of elasticity, and toughness, that is, the achievement of the best values at 1100°C, the decay of these properties at 1200°C, and their increase again at 1300°C.
It can be also concluded that some results are different from those in the literature. In general, this may be caused by different sample preparation techniques. This study shows that a function of HA : TCP ratio sintering temperature is still being investigated for estimating the mechanical properties of the BCP ceramics with respect to the preparation techniques.
Conflict of Interests
The author declares that there is no conflict of interests regarding the publication of this paper.
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