PSI - Issue 82

Tsanka Dikova et al. / Procedia Structural Integrity 82 (2026) 9–15 Dikova et al. / Structural Integrity Procedia 00 (2026) 000–000

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Fig. 5. (a) Porosity; (b) Maximum breaking force under compression of HA bioceramic samples.

4. Discussion Increasing the sintering temperature leads to raising the density and mechanical properties as well as to changes of the phase composition and microstructure of the HA scaffolds. The high temperature sintering of HA samples at 1450 o C ensures a compressive strength (13.5 MPa) and elastic modulus (about 380 MPa) that are similar to that of the human trabecular bone (Yin et al. (2021). Additionally, it influences positively on the biocompatibility by increasing the cells proliferation. However, at this temperature, the fluoridated HA decomposes and small amount of β-TCP is found. It is revealed by Bulina et al. (2021) that the HA remain stable up to a temperature of 1300 o C. In the temperature range 1000-1360 o C, the HA gradually releases its OH‾ ions and converts into oxyhydroxyapatite (OHA) (Liao et al. (1999). Above this temperature, partial decomposition of OHA into β-TCP and TTCP takes place, as at about 1450 o C, the β-TCP transforms into α-TCP (Wang et al. (2009). The researchers report different composition after sintering of HA at temperature of 1450 o C: HA, α-TCP and TTCP (Liao et al. (1999), Bulina et al. (2021) or HA and β-TCP (Yin et al. (2021). It should be mentioned that presence of α-TCP and TTCP in the microstructure of HA scaffolds can lead to decreasing of their strength and increasing of resorption. Our results partially confirmed the findings of the above mentioned researchers. After sintering at 1450 o C temperature, the phase composition of the samples changed to biphasic, consisting of HA and TTCP. The microstructure along the specimen height is similar to that observed by Yin et al. (2021): dense at the bottom, elliptical pores in the top layer and mostly lamellar pores in the middle. Our findings about increasing the compression fracture force with decreasing the samples porosity is in good agreement with the results of Fu et al. (2008-1) and Farhangdoust et al. (2013). 5. Conclusion Based on the results in the present study, following conclusions can be drown: • After sintering at 1450 o C temperature, the phase composition of HA samples is biphasic: 66 % HA and 34 % tetra-calcium phosphate. • The sintered HA samples are characterized with inhomogeneous porous microstructure: dense at the bottom, well-shaped elliptical pores in the top layer and mostly lamellar pores in the middle. The top layer in the samples of Group 1 is thicker compared to Group 2 and is about 50 % of the whole sample’s height. A large crack at the boundary between the top porous layer and the main volume as well as several large cracks in the volume are observed. • Inverse proportional relationship is confirmed between the porosity and breaking force in compression of the investigated bioceramic specimens: the average porosity in Group 1 (70.1%) is higher than that in Group 2 (65.5%) leading to fracture of the samples in the first group at lower force (173 N) compared to the second (350 N).