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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CuKα irradiation in the range 2Θ=9-54 o with a step of 0.02 o . Specialized software DIFFRAC.Eva (Bruker, Germany) was used to identify the individual phases and their relative ratio. Porosity was measured using a solid density analyzer Ultrapyc 5000 Foam (Anton Paar, Austria). Compressive strength test was performed according to the ASTM C 1424:2004 Standard using ZWICK Roell Vibrophore 100 testing machine (ZWICK Roell, Germany). Three samples of each group were subjected to compression test and the average value of the maximum fracture load was used in the analysis. 3. Results obtained The initial powder, used for the samples preparation, consisted of HA (Fig. 2). After sintering at high temperature (1450 o C), the monophasic phase composition of the cylindrical specimens changed to biphasic: 66 % HA and 34 % tetra-calcium phosphate (TTCP).

Fig. 2. Phase composition of HA bioceramic samples: (a) initial powder and (b) specimen sintered at 1450 o C.

Macro- and microstructure of the samples of the two groups are shown in Fig. 3. The all samples are characterized with inhomogeneous porous microstructure and presence of several large cracks in the volume (Fig. 3a,c). On the top of the sample, a layer of well-shaped elliptical pores of similar sizes is observed (Fig. 3b,d). In the specimens of Group 1, this layer is characterized with higher thickness compared to Group 2, representing half of their height. At the boundary between the top layer and the main volume, a large crack (indicated by arrows in Fig. 1 and Fig. 3c) is observed nearly in all samples of the two groups leading to easy destruction. In the samples of Group 2, the layer under the top layer (region 2 in Fig. 3c) is characterized with interconnected pores with rough surfaces (Fig. 3d). Three large cracks are observed bellow the geometrical center of the specimen shown in Fig. 3c. The bottom layer at the same picture consists of comparatively dense structure, while the side regions (regions 4 and 5 in Fig. 3c) are characterized with long and narrow pores of a wide variety of sizes. The average pores sizes vary in the range 146 180 μm in the top region, 96-144 μm in the center and 109-228 μm in the side areas (Fig. 4). The average porosity of the samples is 70.1 % in Group 1 and 65.5 % in Group 2 (Fig. 5a). As the samples of the two groups are characterized with irregular cylindrical shape (Fig. 1), the maximum breaking force under compression is evaluated instead of the compression strength. The average value of the fracture load of the investigated samples is shown in Fig. 5b. It is evident, that the samples of Group 1 break at a lower force in the range of 135-225 N (average 173 N). While the specimens of Group 2 show greater resistance to compressive loading with average breaking force of 350 N varying in lower range of 335-365 N. It is observed, that the destruction of the