Consolidation of alumina toughened zirconia by two-step sintering

Sol-gel-synthesized ZrO 2 -based nanoparticles, containing 1.5 mol% Y 2 O 3 used as dopant and 25 mol% Al 2 O 3 (abbreviated as ZrO 2 (1.5Y)-25Al 2 O 3 ), were densiﬁed by pulsed electric current sintering (PECS) accompanied with a two-step heating proﬁle. Particularly, sintering was carried out in the temperature range from 1050 to 1200 °C for 10 min, followed by dwell at 1325 °C for 10 min. The ZrO 2 -based ceramics achieved by the two-step sintering was fully dense, containing 98 vol.% of tetragonal ZrO 2 phase. Morphology of the sintered samples showed the submicron grains with uniform size distribution. Although the second-step sintering temperature was ﬁxed at 1325 °C, the mean grain size was increased along with the increase in the ﬁrst-step sintering temperature. In addition, the material characterization of the samples after the ﬁrst-step heating was tested to investigate the inﬂuence of the ﬁrst-step sintering on the consolidation of ceramics achieved via the two-step sintering.


I. Introduction
Zirconia (ZrO 2 ) and alumina (Al 2 O 3 ) have been employed for a diverse variety of industrial and commercial applications, such as dentistry, automotive equipment and refractories, owing to their superior mechanical properties [1,2]. ZrO 2 ceramics has high flexural strength and fracture toughness. However, this material exhibits a poor wear resistance as compared to that of Al 2 O 3 [3]. Meanwhile, Al 2 O 3 possesses a superb hardness which is only inferior to diamond, while the inherent brittleness is a significant hurdle to the development of this material. Thus, the Al 2 O 3 -toughened ZrO 2 ceramics is a promising solution to overcome the drawbacks of zirconia and alumina. Further, such combination exhibits the outstanding integrated mechanical properties [4][5][6][7].
The critical point with ZrO 2 ceramics is the existence of three different polymorphs comprising of monoclinic, tetragonal and cubic phases. In production, the transition from metastable tetragonal to stable monoclinic phase during cooling induces the residual stress due to the variation in volume between tetragonal and monoclinic phases, facilitating crack propagation in the bulk material. Therefore, ZrO 2 is usually stabilized in the tetragonal phase by Y 2 O 3 doping [8][9][10]. Meanwhile, mechanical properties of Al 2 O 3 -reinforced ZrO 2 ceramics are commonly enhanced by a fine-grain structure through Hall-Petch strengthening, which can be obtained by pressure assisted sintering [11][12][13]. Pulsed electric current sintering (PECS) has been extensively studied as an advanced sintering method owing to the low sintering temperature and short duration [14,15]. It is reported that Al 2 O 3 -toughened ZrO 2 ceramics was fully dense after PECS at 1300°C under 50 MPa [16]. The bulk samples exhibited grain size of ∼200 nm and Vickers hardness of 12.5 GPa. In addition, Li et al. reported that Al 2 O 3 -ZrO 2 ceramics underwent PECS at 1400°C under 44.6 MPa using the starting powder with particle size smaller than 100 nm [17]. The product yielded a good hardness of more than 12 GPa. Hong et al. investigated the advantageous characteristics of PECS processed ZrO 2 (3.0Y 2 O 3 )-20 mol% Al 2 O 3 ceramics as compared to the hot pressed samples [18]. The samples sintered by PECS at 1300 and 1400°C possessed a fully dense structure with well-distributed fine grain. Moreover, the maximum values of flexural strength, fracture toughness and Vickers hardness were more than 1000 MPa, 6.6 MPa·m 1/2 and 14 GPa, respectively. These results achieved by PECS surpassed ones attained by hot pressing at 1400°C.
We reported that ZrO 2 -based ceramics containing 75 mol% ZrO 2 , 1.5 mol% Y 2 O 3 , and 25 mol% Al 2 O 3 , abbreviated as ZrO 2 (1.5Y)-25Al 2 O 3 , was successfully synthesized by sol-gel method, and subsequently densified by PECS at 1325°C under an applied uniaxial pressure of 60 MPa [19,20]. Recently, the effort has been carried out to optimise the sintering stage and the twostep heating method has been proposed as a promising approach for ceramic advanced sintering. Chu et al. [21] formulated the two-step sintering routine which starts with a relatively low temperature in the first step, followed by a higher temperature stage. The products obtained by the two-step heating process can exhibit high relative density and ultrafine or fine grain size while the sintering duration is appreciably shortened due to a rapid heating rate [21][22][23]. In this study, parameters of the two-step heating profile were selected according to the single-step sintering mode that provided the fully dense ZrO 2 (1.5Y)-25Al 2 O 3 ceramics [19,20]. The influence of the two-step sintering process on the consolidation of ceramics, including tetragonal volume proportion, grain size, relative density and hardness was validated.

II. Experimental
The starting material was the ZrO 2 -based ceramic nanoparticles synthesized by sol-gel method [19]. The powder included 75 mol% ZrO 2 doped with 1.5 mol% Y 2 O 3 and 25 mol% Al 2 O 3 with particle size of 10 nm. First, calcination of the powder was conducted at 700°C for 9 h, followed by dwell at 900°C for 1 h. Subsequently, the calcined powder was compacted by cold isostatic pressing under the pressure of 200 MPa. The green pellets were densified by PECS (Dr Sinter 515, Sumitomo, Japan) using a graphite die under a uniaxial pressure of 60 MPa with the heating rate of 50°C/min in Ar gas. The sintering duration was performed according to the two-step routine. The first-stage sintering temperatures were set at 1050, 1100, 1150 and 1200°C for 10 min, and the second stage was subsequently carried out at 1325°C for 10 min. After sintering, polished samples were prepared for materials characterization.
X-ray diffractometer (XRD, Rint 2000, Rigaku) using CuK α radiation was adopted for the phase identification. Besides, Garvie et al. [24] claimed that the volume proportion of monoclinic ZrO 2 (m-ZrO 2 ) and tetragonal (t-ZrO 2 ) phases could be calculated using peak intensities of (111) and (111) reflections of m-ZrO 2 and (111) reflection of t-ZrO 2 . Relative densities of the bulk samples were determined based on the Archimedes method. Meanwhile, theoretical density (D x ) of ZrO 2 (1.5Y)-25Al 2 O 3 ceramics was obtained using the volume fractions of t-ZrO 2 and m-ZrO 2 phases. Estimated D x of t-ZrO 2 and m-ZrO 2 doped with 1.5 mol% Y 2 O 3 were 6.0510 and 5.7725 g/cm 3 , respec-tively, according to Rietveld analysis [19], and D x of α-Al 2 O 3 equals 3.987 g/cm 3 . Therefore, D x of ZrO 2based ceramics could be evaluated using D x and the volume proportions of t-ZrO 2 , m-ZrO 2 and α-Al 2 O 3 . To obtain microstructure with apparent grain boundary, the bulk samples were fractured and the fractured surface was subsequently observed by a field emission scanning electron microscope (FE-SEM, JSM-7001FD, JEOL Ltd.). The grain sizes were measured by the linear intercept method using SEM images [25]. In addition, Vickers hardness was determined using a Vickers hardness tester (VMT-7, Matsuzawa) with an applied load of 19.6 N for 15 s.

Characterization after the first sintering stage
XRD patterns of the powders before and after calcination are shown in Fig. 1. Before calcination, the powder is characterised by broad peaks, revealing a poor crystallinity. Meanwhile, the powder after calcination exhibited a good crystallinity, which is shown by sharpened and well-defined peaks. The XRD reflections of the calcined powder correspond to both t-ZrO 2 and m-ZrO 2 phases. However, there are no peaks belonging to Al 2 O 3 phase, suggesting a poor crystallinity of Al 2 O 3 phase in the calcined powder.  Figure 2 represents XRD patterns of the ceramic samples sintered at various first-step temperatures, namely 1050, 1100, 1150 and 1200°C. Overall, the identified phases included tetragonal zirconia (t-ZrO 2 ), monoclinic zirconia (m-ZrO 2 ) and alpha alumina (α-Al 2 O 3 ). Although the Al 2 O 3 -toughened ZrO 2 ceramics contained 1.5 mol% of Y 2 O 3 , no peak reflecting Y 2 O 3 phase was detected due to the minimal fraction of Y 2 O 3 , which agreed with previous studies [26][27][28]. In the case of the sample sintered at 1050°C, the broad peaks show existence of t-ZrO 2 and m-ZrO 2 phases with poor crystallinity. When the first-step temperature increased from 1050 to 1200°C, intensities of the t-ZrO 2 peaks gradually increased, whereas intensities involving m-ZrO 2 K.Q. Dang et al. / Processing and Application of Ceramics 17 [2] (2023) 164-171 diminished. It has been well documented that the phase transformation between t-ZrO 2 and m-ZrO 2 arises in the pure zirconia at temperature above 1170°C [29] and is even much lower for ZrO 2 containing Y 2 O 3 . Therefore, the XRD pattern of the sample sintered at 1200°C indicates that t-ZrO 2 phase was intense and well-defined, revealing good crystallinity, while the intensity of peaks of m-ZrO 2 phase declined. Meanwhile, no reflection of Al 2 O 3 phase was present in the case of the samples sintered at 1050°C. Conversely, the α-Al 2 O 3 phase was detected at temperatures higher than 1100°C, which was explained by the phase stability of α-Al 2 O 3 at relatively high temperature above 1100°C [30,31].
The volume proportions of t-ZrO2 to overall amount of ZrO 2 were calculated according to the peak intensities of the XRD patterns as depicted in Fig. 3. Already at 1050°C volume amount of t-ZrO 2 was 70%. ZrO 2 was completely transformed into t-ZrO 2 when the temperature reached 1200°C. Volume proportion of t-ZrO 2 linearly increased along with the increase in tempera- ceramics sintered at various temperatures ture from 1050 to 1200°C (approximately 10% for every 50°C). The high portion of t-ZrO 2 confirms that Y 3+ ions are incorporated in the structure and contribute to the stabilization of tetragonal phase. Figure 4 shows bulk and relative densities of the ZrO 2 (1.5Y)-25Al 2 O 3 ceramics as a function of the firststep sintering temperature. Along with the increase in temperature, both relative and bulk densities rose linearly, demonstrating that samples are intensely densified at the temperatures from 1050 to 1200°C. Additionally, due to the higher calculated theoretical density of t-ZrO 2 of 6.0510 g/cm 3 than 5.7725 g/cm 3 of m-ZrO 2 , the bulk density of ceramics with increasing the temperature from 1050 to 1200°C came from on the increase in t-ZrO 2 volume proportion (Fig. 3). Microstructure of the samples sintered at various first-step temperatures is depicted in Fig. 5. Apparently, the grain growth could be observed along with the increase in temperature. In addition, the grain sizes were well distributed and no noticeable local grain growth was detected. Figure 6 displays average grain sizes of the samples as a function of the first-step sintering temperature. When the temperature extended from 1050 to 1100°C, the average grain size rapidly increased, while a marginal increase in grain size was observed at temperature higher than 1100°C.

Characterization of two-step sintered ceramics
XRD patterns of the samples sintered via two-step heating profiles with various first-step temperatures are presented in Fig. 7. Overall, only t-ZrO 2 , m-ZrO 2 and α-Al 2 O 3 phases could be identified. However, m-ZrO 2 and α-Al 2 O 3 reflections were ambiguously defined, while t-ZrO 2 peaks appeared sharp and wellintense. The domination of t-ZrO 2 phase could be explained by high enough sintering temperature and efficient stabilization caused by uniform distribution of Y 3+ ions in ZrO 2 grains. Figure 8  portion was approximately 90% at 1050°C and subsequently increased to 98% at 1100°C before levelling off at temperatures more than 1100°C. The t-ZrO 2 volume fraction of the sample sintered at 1200°C followed by 1325°C was identical to that of the sample sintered only at 1200°C (Fig. 3), verifying that the phase transformation to t-ZrO 2 completed at 1200°C. In addition, although the second-step sintering temperature was 1325°C for all the samples, t-ZrO 2 volume proportion changed slightly according to different first-step sintering temperatures, especially in the case of 1050 and 1100°C. Consequently, it is worthy to note that the first sintering step had a remarkable impact on the t-ZrO 2 volume content. Figure 9 shows relation between bulk and relative densities of the ZrO 2 (1.5Y)-25Al 2 O 3 ceramics obtained by the two-step sintering as a function of the first-stage temperature. As t-ZrO 2 phase accounted for over 90%, no significant difference in bulk densities of final products was recorded. Nearly 100% relative densities indicated that all the samples sintered via the two-step heating profile were fully dense. The fractured morphology of the sintered samples shows that grain sizes were submicron, and well-defined without any remarkable local grain development (Fig. 10). However, microstruc- ture involving the first sintering temperature of 1200°C presented a more intense grain growth than other samples. Average grain sizes were in the range from 150 to 250 nm (Fig. 11). When the first-step temperature increased up to 1150°C, the grain size was mildly increased. In the case of 1200°C, the mean grain size severely grew to 224 nm. Apparently, the higher sintering temperature yields the higher activation energy for atoms, leading to grain growth [32,33]. In this work, the average grain size gradually increased at the firststep temperature up to 1150°C before rapidly rising at 1200°C. Hence, the first sintering step at 1200°C ar- guably promoted a significant diffusion at grain boundaries, inducing the rapid grain growth during the second stage.
Polished microstructure of the sample sintered at 1150°C is shown in Fig. 12. The dark and white regions reflect Al 2 O 3 and ZrO 2 phases, respectively [34]. Elemental composition obtained by EDS mapping (not shown here) shows that the percentage of Zr was dominant over that of Al, which was equal to the compositional portion of initial powder.
To validate preliminarily the impact of the first-step sintering on mechanical properties of the prepared ce- ramics, Vickers hardness of the sintered samples was measured and shown in Fig. 13. Apparently, the samples obtained by PECS yielded outstanding hardness of more than 15 GPa owing to the fine grain and 100% relative density. When the first-step temperature increased, hardness linearly increased to a peak at 1150°C before decreasing markedly at 1200°C, which could be elucidated by the grain growth at 1200°C, as mentioned in Figs. 10 and 11. The achieved results proved that the two-step sintering had a direct effect on the phase transformation and consolidation of the ZrO 2 (1.5Y)-25Al 2 O 3 ceramics. Specifically, t-ZrO 2 volume amount after the first sintering stage was proportional to temperature. This demonstrates that duration of the first sintering stage facilitates phase transformation of t-ZrO 2 . Regarding consolidation, although all the samples sintered via two-step profiles had nearly full density, the sample sintered only at 1200°C was fully dense. In other words, ZrO 2 -based ceramics encountered a severe densification at 1200°C, so diffusion at grain boundary was significantly enhanced when sintering was further conducted at the secondstep temperature of 1325°C. As a result, microstructure of the final samples obtained by the first-step sintering at 1200°C showed coarse grains as compared to those attained by the lower first sintering temperatures (Fig. 11). These results were further supported by comparison with ones achieved by the single-step sintering, which was previously reported by our research group (Table 1). Although the bulk samples are fully dense in both cases of single-step and two-step sintering, grains relating to the single-step sintering developed more significantly than ones involving two-step sintering. Furthermore, the t-ZrO 2 volume proportion and hardness obtained by two-step sintering surpassed ones attained by single-step sintering.

IV. Conclusions
In this research, the influence of the two-step sintering profile on the consolidation of ZrO 2 (1.5Y)-25Al 2 O 3 ceramics fabricated by PECS was investigated. After heating at the different first-step temperatures, t-ZrO 2 content linearly boosted with rising temperature. The analogous trend was observed with bulk and relative densities. Meanwhile, the grain size was markedly increased when temperature extended from 1050 to  1100°C before the mild increase at more than 1100°C. In the case of two-step sintering, t-ZrO 2 quantity in the final products slightly varied with rising the first sintering temperature and reached 98%. The mean grain sizes were from 150 to 250 nm and grains became coarser when the first-step temperature increased. However, the grain size was well-defined with all first-step temperatures. Additionally, the relative densities presented no remarkable change among various first-step temperatures and all the samples exhibited nearly 100% relative density regardless of the first-step temperature. Ultimately, Vickers hardness of the as-sintered samples reached a peak when the first-step temperature was 1150°C, and subsequently declined at 1200°C.