PSI - Issue 82

ScienceDirect Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2026) 000–000 Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2026) 000–000 Available online at www.sciencedirect.com

www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

Procedia Structural Integrity 82 (2026) 9–15

© 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Abstract The aim of the present study is to investigate the microstructure, phase composition, porosity and mechanical properties of hydroxyapatite (HA) bioceramics obtained by high-temperature sintering. Two groups of cylindrical specimens with height h=15 mm (Group 1) and h=30 mm (Group 2) were prepared by freeze drying method. Then the samples were sintered at 1450 o C temperature. The microstructure, phase composition, porosity and maximum breaking force of the samples were investigated by OM, XRD and compression test. It was found that after sintering, the phase composition of HA samples changed to biphasic: 66 % HA and 34 % tetra-calcium phosphate (TTCP). The sintered HA samples were 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 was thicker compared to Group 2. A large crack at the boundary between top porous layer and main volume as well as several large cracks inside the volume were observed. Inverse proportional relationship was confirmed between the porosity and breaking force in compression of the investigated bioceramic specimens: the average porosity in Group 1 (70.1 %) was 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). © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: hydroxyapatite bioceramics; freeze drying; high temperature sintering; structure and porosity; compression test Abstract The aim of the present study is to investigate the microstructure, phase composition, porosity and mechanical properties of hydroxyapatite (HA) bioceramics obtained by high-temperature sintering. Two groups of cylindrical specimens with height h=15 mm (Group 1) and h=30 mm (Group 2) were prepared by freeze drying method. Then the samples were sintered at 1450 o C temperature. The microstructure, phase composition, porosity and maximum breaking force of the samples were investigated by OM, XRD and compression test. It was found that after sintering, the phase composition of HA samples changed to biphasic: 66 % HA and 34 % tetra-calcium phosphate (TTCP). The sintered HA samples were 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 was thicker compared to Group 2. A large crack at the boundary between top porous layer and main volume as well as several large cracks inside the volume were observed. Inverse proportional relationship was confirmed between the porosity and breaking force in compression of the investigated bioceramic specimens: the average porosity in Group 1 (70.1 %) was 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). © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: hydroxyapatite bioceramics; freeze drying; high temperature sintering; structure and porosity; compression test 8th International Conference on Structural Integrity and Durability (ICSID2025) Structure and mechanical properties of hydroxyapatite bioceramic obtained by high temperature sintering Tsanka Dikova a, *, Ivaylo Parushev a , Dzhendo Dzhendov a , Stanislav Mitkov a , Vladimir Dunchev b 8th International Conference on Structural Integrity and Durability (ICSID2025) Structure and mechanical properties of hydroxyapatite bioceramic obtained by high temperature sintering Tsanka Dikova a, *, Ivaylo Parushev a , Dzhendo Dzhendov a , Stanislav Mitkov a , Vladimir Dunchev b a Medical University of Varna, Faculty of Dental Medicine, 84 Tsar Osvoboditel Blvd, Varna 9000, Bulgaria b Technical University of Gabrovo, Faculty of Mechanical Engineering, 4 Hadji Dimitar Str, Gabrovo 5300, Bulgaria a Medical University of Varna, Faculty of Dental Medicine, 84 Tsar Osvoboditel Blvd, Varna 9000, Bulgaria b Technical University of Gabrovo, Faculty of Mechanical Engineering, 4 Hadji Dimitar Str, Gabrovo 5300, Bulgaria

* Corresponding author. Tel.: +359-889-883-125. E-mail address: tsanka.dikova@mu-varna.bg * Corresponding author. Tel.: +359-889-883-125. E-mail address: tsanka.dikova@mu-varna.bg

2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers

2452-3216 © 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 10.1016/j.prostr.2026.04.003