**3. Results and discussion**

### **3.1 Microstructure and phase transformation**

The relative densities of sintered composite obtained by microwave technology at 1300°C with 10 min of dwell time (MW1300\_10) and conventional oven at 1500°C

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

**Figure 2.** *FE-SEM micrograph of ATZ composites under different sintering conditions: (a) MW1300\_10 and (b) CS1500\_120.*

with 120 min of dwell time (CS1500\_120) were around 99.8 and 99.2%, respectively, and its grain sizes below 500 nm (**Figure 2**).

The relative density is calculated from the theoretical density of the ATZ sample, 4.89 g/cm3 . The results indicated that all specimens reached a high degree of densification. It should be noted that the samples sintered by MW at 2.45 GHz exhibited a relative higher density than samples sintered by CS, achieving 99.8% for MW1300\_10. It is important to note that the sintering temperatures and holding times employed in microwave sintering are considerably lower than in a conventional process. In conclusion, while the sintering time needed to achieve relative densities above 99% with conventional sintering is 350 min, microwave technology produces denser samples in only 35 min. It should be noted that the final economic cost is considerably reduced due to the decrease in processing time and energy consumption and, consequently, the environmental impact also decreases. Therefore, microwave technology is considered to be a more environmentally friendly technique than conventional sintering.

The FE-SEM micrographs of the MW and CS densified ATZ composite are shown in **Figure 2**. The samples are very dense and have a high homogeneity, since the alumina grains (the darker ones) are uniformly dispersed in the zirconia matrix. These results are consistent with the relative density values. The darker grains correspond to alumina, while the lighter grains are zirconia. The average grain sizes of zirconia and alumina have been measured from their micrographs. The grain size increases with residence time and sintering temperature.

As for the sample densified by (MW1300\_10), the average grain size of ZrO2 and Al2O3 reached approximately 280 and 400 nm, respectively. In the case of the sample sintered by conventional furnace, the evolution of the average grain sizes was similar to that of MW; the higher the sintering temperature, the larger the grain size. In sample HC1500\_120, the grain sizes of ZrO2 and Al2O3 were 330 and 450 nm, respectively [31]. Wu et al. [37] researched the effects of Al2O3 addition in 3Y-TZP on the mechanical properties and microstructure of the composite. The increase in alumina content favored slightly the grain growth during the densification. Thus, the average grain size of ZrO2 is slightly higher than that reported in the literature [38].

Raman spectra for conventional and microwave-sintered ATZ composites are shown in **Figure 3**. It is possible to check the phase transformation by Raman spectroscopy, thanks to the characteristic doublet of the *m*-phase at 181–190 cm−1.

#### **Figure 3.**

*Raman spectrums of different LTD times of exposure for sintered ATZ material by (a) MW1300\_10 and (b) CS1500\_120.*

As can be seen, no monoclinic phase peaks are present in any of the samples after sintering (0 h of exposure to LTD) and also after 20 h of LTD exposure. As ATZ samples are exposed to LTD for longer, the intensity of the monoclinic phase peaks increases. After 40 h, such peaks can be recognized for sample CS1500\_120, while for MW1300\_10, they appear after 80 h. As the degradation time increases, the transformation becomes rather important with a distinct presence at 181–190 cm−1.

If both sintering methods are compared, differences in the intensities of the doublet characteristic of the monoclinic phase can be observed between the spectra. The monoclinic peaks are higher in the sample CS1500\_120. This fact suggests a greater vulnerability to transformation induced by LTD in conventionally sintered samples than by microwave technique [39].

The phase transformation can be quantified from the volume fraction of the m-phase, Vm, measured by Raman spectra (Eq. 1). The results of the quantification of Vm are shown in **Figure 4**.

#### **Figure 4.**

*Volume fraction of the m-phase, Vm, a result of the LTD exposure time for ATZ material in either sintering conditions (MW1300\_10 and CS1500\_120).*

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

The lines show differences in the kinetics of the phase change. The MW1300\_10 sample sintered by MW degrades more slowly compared with the conventionally densified CS1500\_120 sample. After 200 h of degradation, Vm is 24% for MW1300\_10 and 37% for CS1500\_120. Therefore, the sample obtained by microwave is less susceptible to LTD.

After these results, it is verified that adding alumina to zirconia, forming an ATZ composite causes the zirconia to degrade more slowly compared with monolithic zirconia. Presenda et al. concluded that Y-TZP transforms approximately 90% after 100 h of testing under the same conditions as this work [17]. Ultimately, the resistance to aging is increased with the addition of alumina in the material composition.

## **3.2 Topography and surface roughness**

A volume increases of about 3–4% accompany the t- to m-phase transformation of ZrO2-based composites. **Figure 5** shows the AFM images of the ATZ composite sintered by CS and MW, where it is possible to analyze the surface changes induced by LTD exposure. The average surface roughness, Ra, has also been identified as a way to measure this variation.

As the exposure time increases, the topography becomes more irregular, increasing the surface roughness. After 200 hours of exposure to LTD, the surfaces of the samples tested are found to have increased in roughness, with the appearance of bulging.

In purpose of comparing the roughness of the samples sintered by different sintering methods, the Ra values at different exposure times have been determined. These values are presented in **Figure 6.**

Sample MW1300\_10 has given lower Ra values than CS1500\_120, showing greater variability in rugosity, particularly beyond the first 20 h. A strong increase

#### **Figure 5.**

*AFM topographic images of ATZ composite at various exposure times of LTD for MW1300\_10: (a) 0 h, (b) 200 h; and CS1500\_120: (c) 0 h and (d) 200 h.*

**Figure 6.** *Average surface roughening at various LTD times for each specimen.*

is observed after the first 20 h for CS1500\_120, reaching about 5.1 nm after 200 h of LTD. However, it was not until after 80 h that a sharp change in Ra was observed in sample MW1300\_10. This performance suggests that the more meaningful topographical changes vary depending on the sintering method.

These results are consistent with the transformation of the *t*-phase to *m*-phase according to the Raman spectra obtained (**Figure 3**); because after 80 h, the monoclinic peak begins to be observed in the Raman spectra for the sample MW1300\_10, which is when the sudden jump in the Ra values appears. The same happens for the sample CS1500\_120.

Although in the Raman spectra of both samples there are no monoclinic peaks until after 20 h of LTD exposure, by observing the surface changes at the submicrometric scale with AFM, it can be seen these microstructure changes are occurring before the peaks appear. As the exposure time of the samples to LTD increases, the irregularities in the surface of the samples also increase because of the push of the grains to the surface to explain the expansion in volume.
