*2.2.4 Glass infiltration*

The glass infiltration processing is a powerful technique for the fabrication of ceramic/glass composite with exceptional mechanical properties and low shrinkage.

**Figure 5.** *SEM micrographs of the porous ceramic after sintering [71].*

*Recent Advances in Ceramic Materials for Dentistry DOI: http://dx.doi.org/10.5772/intechopen.96890*

Porous Y-TZP nano-ceramics, with hierarchical heterogeneities, were prepared by partial sintering method from meso-porous powder [72]. The results showed that the products have a crystallite sizes between 34 and 71 nm for relative densities between 54 and 81.7%. They also revealed a surface area of 18 m2 /g, a thermal conductivity of 0.63–1.88 W.m−1.K−1, an elastic modulus of 32–156 GPa, and a strength in the range of 70 and 540 MPa.

Yang et al. [73] investigated the effects of process parameters and material characteristics in glass infiltration of gel cast zirconia-toughened alumina (ZTA) ceramic for dental applications. They showed that the strength of the obtained ceramic was 291 MPa and the shrinkage was 1.8548%.

In another work [74], biocomposites were obtained by infiltrating porous alumina-titania (Al2O3-TiO2) substrates with a lanthania-rich (La2O3) glass. Al2O3-TiO2 substrates were fabricated using high energy milled powder mixtures of two different compositions. The sintered substrates presented *α*-Al2O3 and *β*-Al2TiO5 as crystal phases and relative densities ranging between 65.5 ± 2 and 69.4 ± 1.2%. These products were then infiltrated by lanthania containing glass at a higher temperature (1140 °C) for 2 hours. These ceramics showed a fracture toughness up to 2.6 MPa.m1/2, a fracture strength in the order of 218–254 MPa, a high density of 94–99% (**Figure 6**), and a Vickers hardness in the order of 895–1036 HV. However, phase identification of the samples by XRD indicated the decomposition of aluminum titanate into alumina and titania besides the formation of lanthanum borosilicate (LaBSiO5). In addition, all studied compositions presented non-cytotoxic behavior and low chemical solubility (inferior to 75 μg/cm<sup>2</sup> ).

### *2.2.5 Slip casting and sintering*

In-Ceram zirconia bulk composites were synthesized via slip casting of alumina or zirconia. Slip was a dispersion of particles of ceramic powders in a liquid (such as water). Thus, the pH of water was then regulated to the desired value to charged particles.

### **Figure 6.**

*Variation of relative density with sintering temperature of the Al2O3-TiO2 and 3Al2O3-TiO2 composites after glass infiltration [74].*

### *Advanced Ceramic Materials*

Kim et al. [75] fabricated dense zirconia compacts by slip casting and sintering from zirconia nanopowders. Thus, the green compacts obtained from slip casting were cold isostatic pressed to enhance the close packing and densified by sintering at 1450 °C for 2 h. Highly dense zirconia compacts with a relative density of 99.5% and grain size of 350 nm were obtained based on the powder type and solid loading in the slurry. The microstructure and mechanical hardness of the sintered specimen after slip casting were dependent on the yttria content in the 3 mol% yttria-stabilized tetragonal zirconia polycrystal powder and the solid loading within the slurry.

Additionally, Kim et al. [76] prepared dental zirconia implants by sintering. They showed that the zirconia blocks have many surface cracks that lead to the deterioration of mechanical strength and the failure of the implant in the body. Thus, highly dense 3Y-TZP samples with a relative density of 99% and grain size of 200–400 nm were obtained at a solid loading of 50–65 wt%. Recently, removable partial dentures (RPD) cobalt-chromium (Co-Cr) alloys are fabricated using a casting technique [77]. New additive manufacturing processes based on laser-sintering has been developed for quick fabrication of RPD metal frameworks at low cost. **Figure 7** illustrates the SEM micrograph of the fractured surface of Co-Cr alloy after casting. As can be seen, the Co-Cr alloy exhibited smaller grain size, higher microstructural homogeneity, and low porosity (2.1–3.3%). It has been shown that laser sintered alloys are more precise and present better mechanical and fatigue properties than cast alloys for RPD.

## *2.2.6 Hot isostatic pressing*

For a decade, hot isostatic pressing (HIP) has been used successfully by manufacturers around the world to increase productivity. HIP was used to eliminate pores and remove casting defects (such as oxides and carbides) to dramatically increase the material properties.

Gionea et al. [78] synthesized zirconia powders by HIP at 500 °C for 2 h. The results showed that a pure cubic phase, with average particle dimension

**Figure 7.** *SEM image of the fractured surface of Co-Cr alloy after casting [77].*

*Recent Advances in Ceramic Materials for Dentistry DOI: http://dx.doi.org/10.5772/intechopen.96890*

about 70 nm, was obtained. Thus, the obtained samples presented a mixture of monoclinic-tetragonal or monoclinic-cubic phases. Final dense ceramic materials (relative density of 94%) were achieved. However, ZrO2-CaO ceramics have high biocompatibility and excellent mechanical properties characterized by strength of 500–708 MPa and Young's modulus of 1739–4372 MPa. Hu et al. [79] synthesized tetragonal zirconia polycrystalline (3Y-TZP) ceramics by HIP. The grain size of the final products reached about 138 nm. This fine grain size leads to an increase in Vickers hardness to achieve 13.79 MPa. These materials also revealed an elevated transmittance (in the range of 76–78%). The result showed that HIP was an effective process to prepare infrared-transparent 3Y-TZP ceramics with small grain size and with good optical and mechanical properties. Similarly, Klimke et al. [80] fabricated ZrO2 ceramics by HIP. They demonstrated that the particle size, determined by TEM, was less than 50 nm (**Figure 8**) and the maximum in-line transmission was about 77%, which observed at IR wavelengths in the range of 3–5 μm.
