*2.2.2 Sintering*

Sintering is a heat treatment under pressure applied to a powders compact without melting. The final product is a solid or porous mass with excellent properties.

Fan et al. [67] studied the mechanical properties of sintering temperature on the microstructure of dental zirconia-toughened alumina (ZTA). By increasing temperature, they concluded that the mechanical properties of the samples were improved, the crystal structure of ZrO2 was changed (from tetragonal into monoclinic), and the porosity was decreased. However, the ceramics sintered at 1450 °C showed greatest fracture toughness (5.23 MPa.m1/2) and greatest flexural strength (348 MPa). The authors concluded that the properties of ZTA ceramic depend on sintering temperature, and the optimal temperature was about 1200–1250 °C. Ghayebloo et al. [68] revealed that it is possible to fabricate ZLS glass–ceramics by sintering (**Figure 4**). The results showed a highest flexural strength of 255.10 ± 15.44 MPa, a fracture toughness of 3.15 ± 0.62 MPam1/2, a Vickers microhardness of 7.96 ± 0.13 GPa, and a bulk density of 2.63 ± 0.02 g/cm3 . Thus, the lowest water absorption was of 0.11 ± 0.12 and the apparent porosity was of 0.25 ± 0.32.

### *2.2.3 Partial sintering*

Partial sintering is considered as the most straightforward processing route for macro-porous scaffolds and involves the partial sintering of initially porous powder compacts.

**Figure 3.** *Light micrograph of the bulk sample [66].*

A homogenous although closed pore structure can be produced when sintering is terminated before full densification [69]. The pore size and porosity are controlled by the size of the powder particles and the degree of partial sintering, and the size of the raw powder should generally be 2–5 times larger than the desired pore size. Chen et al. [70] studied the properties of YB2C2 ceramics prepared by partial sintering. Thus, a porous YB2C2 ceramics were prepared by partial sintering. The results showed a good mechanical behavior: high porosity (57.17–75.26%) and a high compressive strength (9.32–34.78 MPa). In another work [71], alumina powder agglomerates were prepared by partial sintering. The SEM micrographs of the porous ceramic obtained after sintering are given in **Figure 5**. The final ceramic material is characterized by a hierarchical porous network that can contain three levels of interconnected pores: the voids existing between the agglomerates (≥10 μm in size), the porosity remaining inside the agglomerates after partial sintering (≈100–1000 nm in size), and the pores that may exist within the initial ceramic particles (<100 nm in size). Such porous ceramic structures are developed to be applied in the field of dentistry.
