**2. Materials and methods**

A commercial zirconia-alumina powders (ATZ) from the company Tosoh (TZ-3Y20A, Tosoh, Tokyo, Japan) have been studied in this work. Specifically, it is composed of 80 wt% of 3Y-TZP and 20 wt% of Al2O3.

The specimens have been pressed uniaxially (50 MPa of pressure load) with a universal testing machine (Shimadzu AG-X Plus) to obtain circular shapes of 10 mm (diameter) and 3 mm of height. All green samples had a geometric density of ∼55% in relation to the theoretical density.

The sintering of the samples was carried out using two different techniques: microwave technology (a non-conventional fast technique) and conventional oven. Microwave sintering (MW) has been performed in a single-mode circular cavity microwave oven operating in the TE111 mode with a resonant frequency of 2.45 GHz (**Figure 1**, [27]). The microwave sintering conditions have been 1300°C of maximum temperature during 10 min of dwell time and 50°C/min of a heating rate. It is worthy of being mentioned that the zirconia-alumina composites are poorly microwave-absorbent materials at low temperatures, since its dielectric loss factor at room temperature is less than 0.01 [28]. This fact makes hybrid heating necessary, using silicon carbide as a susceptor [29, 30]. The sample temperature is controlled by an infrared radiation pyrometer (Optris CT-Laser GH5, 5 μm), which focuses on the sample surface through the tiny circular opening in the cavity wall. The emissivity and transmissivity of the final temperature material are determined prior to sintering test.

#### **Figure 1.**

*Details of the experimental microwave system of 1 kW at 2.45 GHz connected to a mono-mode circular cavity.*

Conventional sintering (CS) has been performed in an electric oven (Carbolite Gero, HTF 1800) for 120 min in atmospheric conditions at 1500°C and a heating rate of 10°C/min. Both the MW and CS sintering conditions have been extracted from a previous study, where their main mechanical and microstructural properties were analyzed [31]. These chosen settings are based on previous research by our group, in which the sintering conditions of zirconia-based materials were optimized [32, 33].

The study of low-temperature degradation (LTD) of ZrO2-Al2O3 materials has been developed using procedures that simulate and intensify the process of hydrothermal aging. The samples are then autoclaved (MARK) with steam at 125°C and 1.6 bar [19].

The aging samples are characterized after each 20 h of exposition to LTD until they reach 200 h. This characterization consists of the following:

a.quantification of phase content by Raman spectroscopy (Horiba-MTB Xplora). The linear model suggested by Lim et al. is utilized to calculate the content of the monoclinic phases (*m*) in the samples [34]:

$$V\_m = \frac{l\_{su}^{181} + l\_m^{190}}{\mathbf{0}.33 \cdot \left(l\_{\rm t}^{147} + l\_{\rm t}^{265} + l\_m^{181} + l\_m^{190}\right)}\tag{1}$$

where *V*m is the volume fraction of monoclinic phase and *I* is the integrated peak intensity (low peak area). The intensities of the *t*-phase are 147 and 265 cm−1, while those of the *m*-phase are 181 and 190 cm−1 [35].

