Sintering behavior of alumina whisker reinforced zirconia ceramics in hot oscillatory pressing

Abstract: Alumina whisker reinforced zirconia ceramic composite was prepared by both hot oscillatory pressing (HOP) and conventional hot pressing (HP). The results show that compared with HP, HOP can significantly increase the final density and densification rate of the material. Analysis of densification kinetics reveals that the predominant densification mechanism transits from grain boundary sliding in the beginning to the diffusion in the later stage. The main effect of the oscillating pressure is to increase the densification rate in the process of grain boundary sliding. The current study suggests that HOP is a promising technique for densifying whisker reinforced ceramics.

Keywords: hot oscillatory pressing (HOP); densification behavior; grain boundary sliding; whisker reinforced ceramic composite

1 Introduction

Incorporating whiskers into the ceramic matrix to form the whisker-reinforced ceramic composites is one of the most effective methods to strengthen ceramic materials ^[1–5]. The composites exhibit significantly improved mechanical properties compared with their monolithic counterparts, thus having many important engineering applications in turbine engines ^[6], cutting tools ^[7], ballistic protection ^[8], and biological applications ^[9]. However, sintering the composites to high density is a difficult job. Because the rigid ceramic whiskers hardly accomplish the shrinkage in the process of sintering ^[10–12], the shrinkage of ceramic matrix would generate high stresses around the whiskers, which hinders the matrix shrinkage and causes the densification of composites difficult ^[13–15]. Therefore, the whisker-reinforced ceramic composites are usually prepared by pressure-assisted processes at very high temperatures and pressures, such as hot pressing (HP) ^[16] and hot isostatic pressing ^[17]. In these conventional pressure-assisted processes, static pressure was applied to provide additional driving force for promoting material transfer to overcome the restriction caused by whiskers ^[18]. 

Recently, a new pressure-assisted sintering technique, hot oscillatory pressing (HOP), was developed ^[19]. Unlike the conventional HP which uses static pressure, HOP uses oscillatory pressure. The technique has been applied to many material systems ^[20–27]. It shows that compared with conventional HP, HOP can enhance densification, suppress grain growth, and improve mechanical properties for almost all material systems tested so far. The proposed mechanisms for the improved sinterability and mechanical properties including the oscillatory pressure can break powder agglomeration, promote grain-boundary sliding ^[28], cause plastic deformation ^[24], and facilitate liquid phase distribution ^[21]. HOP has also been applied to prepare the whisker-reinforced ceramic composites ^[29–34]. These works revealed that HOP can also improve the densification and mechanical properties of the composites. However, these earlier works reported the final density only, but no information on the entire densification process, and therefore lacked an in-depth understanding of the densification mechanism. 

In this study, we prepared the Al₂O₃-whisker reinforced ZrO₂ composite by both HOP and HP. As an oxide–oxide system, the composite has natural oxidation resistance, and is one of the excellent candidates for structural applications at high temperatures and oxidizing environments. This study shows that HOP not only increases the final density, but also increases the densification rate. This study also shows that although the oscillatory pressure does not change the sintering mechanism of the materials, but accelerates the grain boundary sliding in the sintering process of initial stage and intermediate stage.

2 Experimental 

The starting materials used in this study were 3 mol% yttria-doped tetragonal zirconia powders with a mean particle size of 20–50 nm (3YSZ, 99.9% purity, Beijing HWRK Chem Co., Beijing, China) and alumina whiskers with a mean diameter of ~3 μm and a mean length of ~90 μm (Al₂O_3w, 95% purity, Shandong Dongheng Chem. Co., Dongying, China). 10 wt% Al₂O₃ whiskers (corresponding to 16.5 vol%) were first mixed with the ZrO₂ powders by low-speed ball milling for 48 h using water as the milling medium. The resulting slurry was dried by rotary evaporation at 65℃ . The powder mixture was then ground in an agate mortar, followed by sieving with an 80-mesh screen. 

The resulting powder mixture was finally sintered in vacuum using an HOP apparatus (HOP2020, Chengdu Efield Materials Technology Co., Ltd., Chengdu, China) at 1200, 1300, and 1400℃ . First, the mixture was placed in a cylindrical graphite die with an inner diameter of 30 mm and placed in the apparatus. The sample was then heated to 1200, 1300, and 1400℃ at a heating rate of 8 ℃/min under a pressure of 5 MPa. When the temperature reached 1200, 1300, and 1400℃ , the pressure was quickly increased to the desired level in 3–4 min. After soaking for 120 min, the pressure was removed, and the sample was cooled down to room temperature with the furnace. In order to reveal the difference between HOP and HP, the sintering was conducted at three different conditions (Table 1): HP at 70 MPa (HP-70), HOP at 70±10 MPa (HOP-70±10), and HOP at 60±10 MPa (HOP-60±10). HP-70 and HOP-70±10 have the same mean pressure. HP-70 and HOP-60±10 have the same maximum pressure, and thus the pressure of HOP-60±10 is never higher than that of HP-70 all the time. 

The densities of the resulting samples were measured using the Archimedes method. Five measurements were made for each sample. The phase compositions were analyzed by the X-ray diffractometer (XRD, PANalytical EMPYRAN, the Netherlands) using Cu Kα (λ = 0.15406 nm) as radiation. The microstructure was characterized on the scanning electron microscope (SEM; S-4700, Hitachi, Tokyo, Japan). The average grain sizes were determined from more than 300 individual grains by the method of linear intercept through the statistical software of Nano Measurer. The Vickers hardness (HV) was determined by the hardness tester (HXD-1000TMC, Shanghai Taiming Optical Instruments Co., Ltd., Shanghai, China) under a load of 4.9 N for 15 s. 

3 Results and discussion 

The relative densities of the three samples sintered under 1200℃ are measured and listed in Table 1. It can be seen that the two samples prepared by HOP 

Table 1 Relative densities and average grain sizes of the resulting samples

have higher densities than that prepared by HP. The sample prepared by HOP at 70±10 MPa has the highest relative density, which is ~1% higher than that prepared by HP at 70 MPa, although the two samples were prepared at the same mean pressure. It is more interesting that the sample prepared by HOP at 60±10 MPa also has higher relative density than that prepared by HP at 70 MPa, although the former was prepared at a lower pressure. This suggests that the final density should not be determined by diffusion, because diffusion-controlled densification should depend on the magnitude of the applied pressure rather than the pressure mode. 

In order to understand the densification mechanism, the densification curves of the three samples were derived from the final relative densities, and the corresponding displacement curves by using the procedure described in Ref. ^[35]: 

where ρ_i is the instantaneous relative density; ρ_f is the final relative density; h_f is the final height of the sintered sample; and h_i is the instantaneous height of the sample during sintering, which equals to h_f +∆h , where ∆h is the displacement relative to the final height. In order to avoid the effect of the thermal expansion of graphite die/indenter on the onsite calculation of densification curve, an extra dwell time of 30, 20, and 10 min before pressure was applied in 1200, 1300, and 1400℃ , respectively, to achieve a steady state of thermal expansion. Moreover, a control experiment of blank specimen comparison was used to eliminate the effect of thermal expansion of graphite die/indenter. The obtained densification curves under the three process conditions are presented in Fig. 1. Figure 1 reveals that all the three curves exhibit three stages: fast densification stage, retardation stage, and zero-densification stage, as observed in HP ^[36,37]. It also shows that compared with HP, HOP not only leads to higher final relative density, but also requires much shorter time to reach the same relative density, e.g., it took about 60 min for HP at 70 MPa to achieve the relative density of 92%, while it took less than 20 min for HOP at 70±10 MPa to reach the same relative density. Even at 60±10 MPa, it took a little more than 35 min for HOP to reach the relative density. Therefore, the improvement of the densification by HOP mainly occurs at the sintering process of initial stage and intermediate stage. When the density reaches a certain value, the density difference between the HOP samples and the HP sample gradually decreases. 

Fig. 1 Relative densities as a function of sintering time for the three samples prepared at different conditions as labeled. 

In order to better compare the densification process of the three samples, diffusion rates ( dρ/( ρdt ) ) were calculated from the curves in Fig. 1. Figure 2 plots the densification rate as a function of relative density for the three samples. It can be seen that the three curves take the similar trend, that is, the densification rate first slowly decreases as the increase of relative density, and when the relative density reaches a certain value, the densification rate decreases rapidly and approaches to zero. This suggests that the densification mechanism in the low-density region is different from that in the high-density region ^[37,38]. Figure 2 also reveals that the densification rates of HOP are always higher than that of HP at the same relative density even if the mean pressure of HOP is lower than that of HP, especially in the low-density region. It indicates that the oscillatory pressure can indeed increase the densification rate, especially in the low-density region. This is consistent with the results shown in Fig. 1, which shows that the improved densification of HOP mainly occurs at the sintering process of initial stage and intermediate stage. 

Fig. 2 Densification rate as a function of relative density for the three samples prepared at different conditions as labeled.

According to Bernard-Granger and Guizard ^[39], the densification rate of HP can be expressed by Eq. (2):

where ρ_i is the instantaneous relative density, t is the soak time, A is a material-related and dimensionless constant, D is the diffusion coefficient, b is the Burgers vector, μ_eff is the effective shear modulus, K is the Boltzmann’s constant, T is the absolute temperature, G is the grain size, σ_eff is the effective applied pressure, m is the grain size exponent, and n is the stress exponent. μ_eff and σ_eff can be calculated by using Eq. (3): 

where σ_a is the macroscopic applied pressure; ρ_o is the relative density of green body, which can be detected by the difference between the height of the empty graphite die and pre-pressed graphite die; E is the Young’s modulus of the material, determined to be 241.85 GPa; and υ_eff is the effective Poisson’s ratio, being 0.29 ^[39,40]. At a constant temperature and no grain growth, Eq. (2) can be rewritten as Eq. (4): 

where C is a constant for a fixed sintering temperature. Equation (4) suggests that the log–log plot of the modified densification rate e (1/μ_eff  dρ/ρdt) and the normalized effective pressure ( σ_eff/μ_eff) should be a straight line; and the slope of the line is the stress exponent. Note that this model can only be applied for less than 90% relative density ^[41]. 

Figure 3 is the plot of Eq. (4) for the three samples. In order to avoid the influence of grain size, the relative density range in which grain size remained almost unchanged should be selected. References ^[42,43] have shown that open pores can hinder grain growth at the intermediate stage of sintering. It has also been proved that the grain size of ZrO₂ almost unchanged in the relative density less than 90% ^[44]. Thus, the data up to 85% relative density were used in this study. For the two HOP samples, instantaneous effect pressures were calculated by using the mean pressure as the applied macroscopic pressure. The straight lines suggest that the model is applicable for the intermediate stage in the sintering of the three samples. The three straight lines are almost parallel, suggesting that the densification mechanism of the three samples in the sintering process of initial stage and intermediate stage is the same, independent of the pressure mode. The slope of the lines is slightly higher than 1. There are two densification mechanisms with the stress exponent of 1 ^[45]: diffusion and grain boundary sliding accommodated by diffusion flow. As discussed above, diffusion-controlled densification rate should only depend on the magnitude of the applied pressure and be independent of pressure mode, which contradicts the experimental results. Therefore, the densification process of the three samples should be grain boundary sliding, followed by diffusion. Figure 3 also shows that at the same effective pressure, the densification rates of the two HOP-processed samples are higher than that of the HP-processed sample, indicating that the oscillatory pressure significantly enhanced grain boundary sliding process. It is also seen that the densification rate obtained at 60±10 MPa is lower than that obtained at 70±10 MPa. This indicates that the mean pressure has intrinsic effect on grain boundary sliding. 

Fig. 3 Modified densification rate as a function of effective pressure for the three samples prepared at different conditions as labeled. 

It has been shown that diffusion and grain boundary sliding accommodated by diffusion flow are in fact the same mechanism, and it is the diffusion that is rate controlling in both cases ^[46]. Thus, the difference in the densification rate must come from the repeated sliding caused by the oscillatory pressure. There are two possible reasons for the difference in the densification rate: (1) The oscillatory pressure will increase the grain boundary temperature due to the internal friction existing in grain boundary and interfaces of grains and whiskers ^[47]; (2) the oscillatory pressure of the reciprocal cycle will cause the grain boundaries to become flatter, which will benefit the densification rate and inhibit grain growth. 

Figure 2 also shows that the transition from grain boundary sliding to diffusion occurred at higher densities for the HOP samples compared with for the HP samples. This indicates that oscillatory pressure can make grain boundary sliding to occur at lower pressure levels. 

In order to investigate the effect of the oscillatory pressure on the microstructure, the resulting samples were characterized by the SEM (Fig. 4). The low-magnification images (Figs. 4(a)–4(c)) reveal that the alumina whiskers are uniformly distributed in the zirconia matrix, and there are almost no pores. The high-magnification SEM images (Figs. 4(d)–4(f)) show that the grain sizes and distribution of the three samples are similar to each other. These results indicate that the pressure mode has no significant effect on the development of the microstructure of the material. 

Fig. 4 SEM images of the samples sintered in 1200 at (a, d) 7 ℃ 0 MPa, (b, e) 60±10 MPa, and (c, f) 70±10 MPa. The arrows in (a–c) indicate the presence of the pores, and the insets in (d–f) are grain size distributions. 

The obtained samples, together with the powder mixture, were also analyzed using the X-ray diffraction (Fig. 5). The diffraction patterns reveal that all the four samples consist of t-ZrO₂ and α-Al₂O₃ phases. It is noted that the ratio of the diffraction peak intensity of (002) plane to (110) plane of the HOP-70±10 sample is much higher than that of the HP-70 sample, and also higher than that of the powder mixture. This indicates that the sintered samples have a certain degree of (002) oriented texture; and the oscillatory pressure promoted such texture. The mechanism governing such change is not clear at this moment, and desires further study. 

Fig. 5 XRD patterns of the ceramic composites sintered at different conditions as labeled.

In order to prepare the higher-performance Al₂O_3w–ZrO₂ composites, the HOP experiments were conducted at higher temperatures. The relative densities and Vickers hardness of the composites prepared at different conditions are shown in Fig. 6. It can be found that the relative densities and Vickers hardness of the samples increase with pressure and temperature increasing. Obviously, the specimen sintered at 1400℃ of the hot oscillatory pressure (70±10 MPa) presents the highest relative density and Vickers hardness of 99.3% and 17 GPa, respectively. This value of Vickers hardness is higher than that of the conventional sintering (1500℃) sample of 15.11 GPa ^[48] and the spark plasma sintering condition (1500 ) of 16.27 GPa ℃ ^[49]. Thus, the HOP sintering technique is likely applicable to prepare high-performance whisker-reinforced ceramic systems. 

Fig. 6 Relative densities and Vickers hardness of the ceramic composites sintered at different conditions: (a) different pressures at 1200 and (b) ℃ the same pressure at different temperatures

4 Conclusions 

Alumina whisker reinforced zirconia ceramic composites are prepared by both HOP and conventional HP at different pressures. The density measurement reveals that at the same mean pressure, the relative density of the samples prepared by HOP is ~1% higher than that prepared by HP. Even the sample prepared by HOP at lower mean pressure has higher density than that prepared by HP at higher pressures. The time required for HOP to sinter the material to the same density is much shorter than that for HP. The kinetics analysis reveals that the densification mechanism for HOP and HP is the same, grain boundary sliding at lower densities and diffusion at higher densities. The oscillating pressure mainly increases the density and densification rate by increasing grain boundary sliding rate. Microstructural analysis reveals that the oscillating pressure does not change the grain structure of the resulting materials, but causes a higher degree of texture. The sample prepared by HOP-70±10 at 1400℃ presents the highest relative density of 99.3% and Vickers hardness of 17 GPa. 

References: Omitted