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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increased from 1300 °C to 1350 °C. The lower temperatures resulted in incomplete sintering, with pore walls exhibiting microporosity. This is a prerequisite for deterioration of the mechanical properties. Zhang et al. (2009) found that the pore morphology and microstructure of porous HA ceramics could be adjusted by adding gelatin to the ceramic suspension. After sintering at 1300°C of samples containing 50% HA and 6% gelatin, scaffolds with a porosity of 46.65% were obtained. The authors concluded that the presence of gelatin improves the interconnectivity of the pores. Later, Zuo et al. (2010) demonstrated that polyvinyl alcohol (PVA) has a similar effect. They used a suspension of 50% HA and 6% PVA and obtained scaffolds with interconnected pores and a total porosity of 62.3% after sintering at 1300 °C. Although an increased sintering temperature results in higher compression strength, only a few studies on the microstructure and properties of HA bioceramic samples sintered at high temperatures were found. This determined the aim of the present work: to investigate the microstructure, phase composition, porosity and mechanical properties of HA bioceramics obtained by high temperature sintering.

2. Materials and Methods 2.1. Samples preparation 2.1.1. Preparation of bioceramic suspension

In the beginning, 5 wt.% gelatin (Merck, Germany) was added to distilled water and mixed using a magnetic stirrer (MS10-H500-Pro, DLABScientific Co. Ltd., Beijing, China) until the temperature reached 60°C. Afterwards, 30 wt.% hydroxyapatite (BLD Pharm, Germany) and 0.25 wt.% polyvinyl alcohol (Merck, Germany), which acts as a binding agent, were added to the suspension. The as-obtained suspension was stirred for 15 minutes at a speed of 300 rpm and deaerated using a vacuum desiccator for 10 minutes at a pressure of 0.1 Bar to remove trapped air bubbles. 2.1.2. Freezing of the suspension

The suspension was cast into cylindrical polyethylene molds with a diameter d=15 mm and a height h=30 mm (Kulzer, Germany). The samples were divided into two groups (Fig. 1): 9 molds in Group 1 were filled up to h=15 mm, while 7 molds in Group 2 were filled to the top. The as-prepared molds were placed in a freezer for 24 hours at a temperature of -20°C. 2.1.3. Freeze drying (lyophilization)

The frozen samples were removed from the molds and dried 24 hours in a freeze dryer (2-4 LD plus, Christ, Germany) at a temperature of -50°C and a pressure of 0.05 mBar. 2.1.4. Sintering The freeze dried samples were subjected to preliminary heat treatment at 500°C for 2 hours to remove the organic phase. After that the samples were sintered in a high temperature furnace (Everest Therm 4180, KaVo Dental GmbH, Germany) at 1450°C for 6 hours. 2.2. Samples characterization The microstructure, phase composition, porosity and compressive strength of the obtained samples were investigated. The microstructure of the scaffolds was examined using an optical microscope Leica M80 Leica Mikrosysteme Vertrieb GmbH, Germany). The phase composition of the initial powder and sintered samples was investigated by XRD analysis using a Bruker D8 Advance diffractometer (Billerica, MA, USA). The analysis was performed under Fig. 1. As-sintered HA bioceramic samples of Group 1