**1. Introduction**

In recently years, zirconia and alumina have been recognized as the most relevant ceramic materials due to their outstanding properties, such as hardness, fracture toughness, Young's modulus, chemical stability, wear and mechanical strength. Due to these excellent properties, ZrO2 and Al2O3 are appropriate materials for a wide range of applications, such as the manufacture of sensors, fuel cells, thermal barriers, implants and structural engineering applications [1–3]. Recent studies claim that the

incorporation of alumina into the zirconia matrix, alumina-reinforced zirconia (ATZ) materials, improves the mechanical properties (i.e., hardness, toughness and wear resistance) [4, 5], as these composites combine the unique properties of alumina and zirconia. These characteristics make ZrO2 and Al2O3 promising composites for prosthetic and dental implant applications. There are a number of reports on the sintering of ATZ composites in the literature. Li et al. [4] have sintered a ZrO2 (3YTZP) + 20% wt% Al2O3 composite by spark plasma sintering (SPS) a non-conventional sintering technique. The hardness and fracture toughness values achieved for these samples sintered by SPS at 1400°C were 12.5 GPa and 5.3 MPa-m1/2, respectively. The ATZ (ZrO2 (3YTZP) + 10% vol. Al2O3) composite was also studied by Meena and Karunakar [5]; in this case, the maximum hardness values for samples sintered by SPS at 1300°C were 19.8 GPa. In summary, the final properties and microstructure of the materials depend on the densification process of the material, sintering mechanisms and methods [6–9].

Substantial improvements in dental prostheses and implants have been achieved through the employment of ceramic-based materials, mainly thanks to the advent of yttria-stabilized zirconia polycrystalline (Y-TZP) as biomaterials [10–13]. The replacement of metal parts in orthopedic and dental applications with ceramics is currently on the rise. Ceramic materials provide several advantages over metals, such as biocompatibility, outstanding mechanical properties and esthetics.

However, an important characteristic of these materials is completely stabilized zirconia tetragonal (t) phase with the addition of an oxide (Y2O3, CeO2,…), for example, with ~3.0 mol% of yttria for dental applications. This is important because when Y-TZP materials are subjected to humidity at 25–280°C [14–16], a sharp decline in their mechanical properties occurs over time. This phenomenon is referred to as low-temperature degradation (LTD) [17, 18]. The t-m transformation produces volume changes and defects in the material, and the mechanical and esthetic properties are affected. Thus, it is highly relevant to research the susceptibility of Y-TZP-based materials to LTD. LTD process is influenced by different factors, such as the tetragonal stabilizing dopant, the grain size or the porosity. Several of these elements are related to the process of sintering and its heating mechanisms.

In the late 1990s, the Y-TZP femoral heads used for the hip replacement failed catastrophically within the human body and these failures were attributed to the hydrothermal aging process [19, 20]. The conditions that promote LTD are found in the dental cavity; therefore, it is critically necessary to investigate the effects of LTD on Y-TZP-based materials for these uses.

Often, ceramics are full consolidated using a thermal treatment at high temperatures (>1200°C), where the temperature and dwell time are the most significant parameters since they establish the mechanical properties and microstructure of the densified material. Conventional sintering requires long-processing times and high temperatures and consequently high-energy expenditure. For this reason, non-conventional and fast sintering methods, such as microwave heating technology, are being implemented in the last decade. Microwave sintering is founded on the absorption of electromagnetic radiation, resulting in an increase in the temperature of the material [21–23]. The mechanism of microwave heating differs from the one used in conventional sintering, since the temperature gradient is, conversely, from the inside to the outside. It is referred to as volumetric heating [24]. Other investigations have also explored the influences of the processing requirements, the addition of second phases such as Al2O3 or Nb2O5, and the incorporation of Y2O3 stabilizers on the behavior of LTD for dental materials sintered by the traditional method [25, 26].

*Design and Development of Zirconia-Alumina Bioceramics Obtained at Low Temperature… DOI: http://dx.doi.org/10.5772/intechopen.102903*

Nevertheless, no comprehensive research has been conducted on the effect of microwave sintering on 3Y-TZP/Al2O3 composites when exposed to LTD conditions.

The aim of this research was to assess the impact of microwave sintering on the LTD resistance of dental materials based on zirconia-alumina nanocomposites by comparing them with materials sintered by the traditional technique. This includes the evaluation of surface roughening, monoclinic phase transformation progression, as well as the mechanical properties as a function of the degradation time in simulated laboratory settings.
