Transparent aluminium oxynitride (AlON) ceramics were prepared by hot isostatic pressing (HIP) with 0.08~0.14 wt% Y2O3 or 0.01~0.04 wt% La2O3 as sintering additives. Both Y2O3 and La2O3 additives prompted sintering process and accelerated grain growth. When higher amount of Y2O3 (0.12 wt%) or La2O3 (0.03 wt%) additives were doped, extensive twins occurred accompanied with the coarsing of grain size (~100μm). Electron backscatter diffraction (EBSD) analysis showed that twin boundaries increased with the increasing of Y2O3/La2O3 concentrations. By co-doping of Y2O3 -La2O3 additives with reduced doping level, the formation of twins was successfully suppressed, which was necessary to limit the additional light scattering of twin boundaries.
© 2015 Optical Society of America
Transparent aluminium oxynitride (AlON) polycrystalline ceramic has been considered as an alternative to traditional glass and single crystal for a diverse range of applications including IR/visible windows, electron microscope domes and transparent armours due to its higher melting point, higher strength and wider transparent region [1–3]. To use AlON as a transparent material, its microstructure should be optimized since transmittance is sensitive to microstructures, such as pores, grain boundaries, second phases, and impurities etc . AlON has a cubic crystal structure and is free of birefringence, hence scattering at pores is considered as the most significant factor which degrades transparency. There has been considerable work on the elimination of pores to achieve highly transparent AlON with limited light scattering [5,6]. Despite the influence of residual pores, grain boundaries may also play an important role in light scattering if they are not just clean interfaces with zero thickness, but have a variety of microstructural features and varying roughness. As described by Hartnett et. al, grain boundaries are potential optical scattering sites when structural defects, such as twins, are located at grain boundaries . Recently, we have successfully fabricated highly transparent AlON ceramics using a sinter plus HIP approach with Y2O3-La2O3 as sintering additives, where we observed the presence of twin boundaries (TBs) in HIPed AlON for the first time . Although TBs have been extensively investigated in face-centred cubic metals (Cu, Al and Ni) because of their strengthened effect [9–11], only a few studies have reported the existing of TBs in ceramic materials. For example, Wei et al fabricated c-axis textured α-alumina through gel-casting process and observed the presence of Σ3 twin boundaries by electron backscatter diffraction (EBSD) technique . Vonlanthen et al investigated the distribution of coincidence site lattice grain boundaries in texture-free alumina and zirconia ceramics and found that the number fractions of twin- related Σ3 grain boundaries were ~16% and ~11% for alumina and zirconia, respectively . However, the mechanism by which twins are formed and the effects of TBs on the properties of ceramics are still incompletely understood. With regards to twins in transparent AlON ceramics, no data was given in previous literatures. So experimental studies on the development of TBs should be carried out and the effects of TBs on optical properties of AlON ceramics have yet to be revealed.
In this paper, AlON transparent ceramics were fabricated by HIP sintering with different amounts of Y2O3 or La2O3 as sintering additives. The evolution of TBs in HIPed AlON ceramics as a function of Y2O3/ La2O3 concentrations was investigated and the relationship between TBs and optical performance of AlON ceramics was discussed.
2. Experimental procedure
Single phase AlON powders were lab-synthesized by carbothermal reduction and nitridation method. Commercial rare earth oxides of Y2O3 (99.99%) and La2O3 (99.99%) were used as additives. The ball milled AlON powders were weighed and dry pressed into φ40mm × 6mm discs under 20MPa, and then cold-isostatic-pressed under 200MPa. The pressed discs were sintered at 1930°C for 2 h and further treated by HIP at 1900°C for 2h under 190MPa in Ar atmosphere. The HIPed ceramics were ground and mirror polished for transmittance measurement by a Cary 5000 UV/VIS/NIR (Varian, US) spectrophotometer. Sintered density was measured by the Archimedes’ principle. Relative density was calculated using a theoretical density of 3.71g/cm3. For microstructure observation by scanning electron microscopy (SEM, JSM-6360LV, JEOL, Tokyo, Japan), the ceramics were chemically etched in acid solution for 5 minutes. Grain size was measured (based on 200 grains) from the average linear intercept length multiplied by 1.56. The EBSD maps were collected using JSM-6700F field emission gun (FEG) SEM, equipped with a fully automatic HKL Technology EBSD attachment. Grain misorientations were determined by EBSD in SEM and grain boundaries with misorientation angle of 60° between two neighbouring orientation points were considered as twin boundaries . The parameters used for indexing were 1 μm step size, using 1-4 frames of averaging and 8 × 8 binning for the large area scans. To ensure statistical significance, the average values of twin boundaries were calculated based on three maps obtained from different 1250 × 1250μm2 areas in each specimen.
3. Results and discussion
Figure 1 shows the SEM images of HIPed AlON ceramics single doped with various Y2O3 contents from 0.08 wt% to 0.14 wt%. Obviously, the micro-morphologies of the samples changed notably with the increase of Y2O3 content. The 0.08 wt% -Y2O3 doped ceramic had fine and well shaped grains, but a large amount of residual pores situated at triangle boundaries (Fig. 1(a) inset). When the Y2O3 content was added to 0.10 wt%, the elimination of pores was significant. However, the grain size grew slightly and the grain size distribution became broader. Further increasing of Y2O3 contents to 0.12~0.14 wt% not only resulted in relatively larger micron-scale pores at triangle boundaries and heterogeneous microstructures with abnormal grain growth, but also introduced the TBs (marked in red). During sintering, additives reacted with oxide (AlxO) on the surface of AlON to form eutectic liquid phase. Under the action of capillary force, the liquid phase wetted and penetrated among solid particles. This would facilitate their rearrangements and greatly promote the densification and elimination of pores. At the final stage of sintering, the transient liquid phase slowly decomposed and incorporated into the AlON host lattice . Therefore, the densification effect mainly depends on the amount of liquid phase. Lower Y2O3 concentration (i.e. 0.08 wt%) may form no or very low amount of liquid phase and the sintering was greatly controlled by solid-state diffusion with lower densification rate, leading to a large number of micropores in sintered sample . In contrast, when a higher amount of sintering additives were added (i.e. 0.12~0.14 wt%), the volume of formed liquid phase would increase significantly. Large pores were left at triangle boundaries after the liquid phase incorporated into AlON lattice at the final stage of sintering . Seen from the SEM of the 0.01~0.04 wt% single-La2O3 doped AlON (Figs. 2(a)-2(d)), twins, large pores as well as accelerated grain growth were also observed with increasing La2O3 concentration from 0.01 wt% to 0.04 wt%, similar to the microstructural changes that happened in single-Y2O3 doped AlON. This result is different from that of Wang’s report, in which La2O3 acts as a grain growth inhibitor in the pressureless sintering of AlON ceramics . Presently, the opposite effect of La2O3 on the mass transfer process may be attributed to the different sintering mechanisms and kinetics between HIP and pressureless sintering. Microstructural analysis of the HIPed AlON revealed that a lower doping level of both Y2O3 and La2O3 can assist in the elimination of micropores, but higher amount of Y2O3 /La2O3 resulted in rapid grain growth as well as twinning behavior.
For the understanding of twins, there remain somewhat conflicting theories : some researchers thought that rapidly growing grains caused stacking fault accidents to occur and twin structures might take place from those growth accidents; others believed that different grains with twin orientation might form a coherent twin boundary because of grain encounter; in addition, twins could also be produced by emission of partial dislocations from the migrating grain boundaries. The orientation map of HIPed AlON ceramic in Fig. 3(a) revealed that no preferred orientation was observed, suggesting that grain growth was not controlled by selective growth of particular orientation or driven purely by a texture mechanism. Details of the reason for the twinning behavior in AlON ceramics are unclear. However, it was probable that the face-centered cubic structure of AlON was an important reason for the occurrence of twins, because twins were occasionally reported as structural defects in face-centred cubic metals and metal alloy nanoparticles where two subcrystallites share a facet to form a mirror image of each other.
For detailed analysis of the amounts of TBs as a function of Y2O3/La2O3 additives, the number fractions of TBs in each AlON were measured by EBSD technique and were shown in Fig. 3. The number fraction of TBs increased from 0% to 9.5 ± 0.5% as the Y2O3 content increased from 0.08 wt% to 0.14 wt%, while that of TBs increased from 0% to 4.5 ± 0.5% as the La2O3 content increased from 0.01 wt% to 0.04 wt%. The result combined with the micrographs shown in Fig. 1 and Fig. 2 demonstrated that twins were rarely observed in lower Y2O3/La2O3 doped ceramics, but happened extensively at higher doping levels. It is worth noting that the occurence and increasing of twins were accompanied by the rapid grain growth of AlON grains and twins happened more frequently in larger grains than in small ones, especially in exaggerated grains. This result agrees with Pande’s work: there is a grain size effect on twinning, the larger the grain size, the higher the twin density, and twinning behavior does not take place below a threshold grain size . Presently, regardless of the different doping strategies, TBs were much more frequently observed in AlON ceramics with average grain size of 100~115μm, but were rarely detected when the average grain size was below ~60μm. So the influence of Y2O3/La2O3 additives on twin density may be realized mostly through grain size, once a particular grain size has been obtained at certain additive concentration, twin density is determined by that grain size. Considering the fact that at given conditions, larger grain size is associated with higher grain boundary velocity. Previous literatures on alloys have shown that the annealing twins can be easily developed by higher strain amount that favors the higher velocity of grain boundary migration during annealing . In the present case, higher additive concentration can accelerate grain boundary velocity during sintering, which may contribute to the higher probability of twin nucleation. On the other hand, the high solubility of larger Y3+ (0.9Å) / La3+(1.06Å) ions into matrix may cause lattice distortion, as well as a modification of the interatomic distances and thus resulting in different stress levels, dislocations and vacancies in AlON. The subsequent relaxtion of internal stress and readjustment of dislocations may lead to twin nucleations against the rapid migrating grain boundary .
Figure 4 shows the transmittance curves of HIPed AlON ceramics doped with single-La2O3, and single-Y2O3 additives, respectively. From these figures, it can be seen that, when the sintering additives were increased, the transmittance decreased continually. The transparency of polycrystalline ceramic is affected by many factors, such as second phases, pores, grain boundaries, et al. Based on Miller’s report that the solubility limits of La3+ and Y3+ in AlON were 498 ± 82ppm and 1775 ± 128ppm, respectively . Presently, the amounts of Y2O3/La2O3 additives were below the solubility limits of La3+ and Y3+ in AlON. Besides, wang et al have investigated the 0.12 wt% Y2O3- 0.12 wt% La2O3 codoped AlON ceramics by using TEM and no second phases could be found . So the decrease of transmittance may not be attributed to the light scattering of additives-rich regions segregated into the grain boundaries. Combined with the relative densities listed in Table 1 and the corresponding SEM micrographs in Fig. 1 and Fig. 2, lower relative densities and larger micropores obtained at higher La2O3 doping levels can explain the rapid decrease of transmittance (at 1100nm) from 75.6% to 58.5% as the La2O3 content increased from 0.01 wt% to 0.04 wt%. For the single -Y2O3 doped samples, a slight increase of relative density from 99.17 ± 0.02% to 99.61 ± 0.02% as the Y2O3 content increased from 0.08 wt% to 0.10 wt% did not result in higher transparency. A possible cause of the lower transmittance of the 0.10 wt% Y2O3 doped sample is the pore diameter inhomogeneity and the relatively broader grain size distribution, as revealed by Apetz et al that the transmittance not only depends on the amount of pores but also on the size distribution of the grains and residual pores . Furthermore, compared with the 0.12~0.14 wt% Y2O3 doped ceramics that contained TBs, the 0.03~0.04 wt% La2O3 doped ones had a relatively higher transmittance, although they possessed similar densities (Table 1) and grain morphologies. Despite the residual pores, the lower transmittance of Y2O3 doped ceramics may be also related to their relatively higher amount of TBs within grains. Because twin lamellar usually contained a large amount of structural defects, such as triangles, Shockley partial dislocations associated stacking faults , which can serve as additional sources for scattering when the light travels through one grain. It is well known that scattering losses are most intense if structural defects scale with the wavelength, i.e. for the visible range the most critical pores are about 0.1~1μm range . From Fig. 3(c) and 3(d), the twin thickness (i.e., the mean distance between two adjacent twin boundaries) presented in AlON varies from nano-scale to micro-scale, because the twin areas are not infinitesimally thin boundaries, but extend over a finite thickness with concave or convex band. The thickness of some twin lamellars may be equal to the wavelength of the incident light, resulting in significant scattering.
To achieve full density and simultaneously restrain grain growth as well as twinning behavior, the lowest possible Y2O3 and La2O3 concentration was chosen to investigate the microstructure of HIPed AlON ceramic with Y2O3 -La2O3 composite additives. Figure 5 depicts the SEM and transmittance of the 0.08 wt% Y2O3 −0.01 wt% La2O3 codoped ceramic. As expected, the 0.08 wt% Y2O3 −0.01 wt% La2O3 ceramic demonstrated a pore-free and homogeneous microstructure without the presence of twins as a result of the low additive concentration. Small amount of liquid phase formed from the La2O3-Y2O3-Al2O3 ternary system promoted the densification successfully. The transmittance of 85.2% at 1100nm was achieved by HIP sintering with Y2O3 -La2O3 concentration as low as 0.09 wt%. The doping level is much lower compared to that required in pressureless sintering, where 0.12 wt% Y2O3 −0.09 wt% La2O3 were doped . This result was similar to Lee’s observation that a sinter plus HIP approach could reduce the amount of SiO2 additives necessary to achieve transparent Nd:YAG ceramic . The pressure applied during HIP creates a significantly higher driving force for densification and it should reduce the additive concentration, and thus limit the possibility of forming additive-related secondary phases, abnormal grain growth as well as grain size-related twin boundaries.
Dense transparent AlON ceramics without presence of TBs were obtained by HIP sintering with very low content of Y2O3 -La2O3 composite additives. It was noted that either Y2O3 or La2O3 additives could eliminate micropores and prompt sintering process at a low doping level. But excessive Y2O3 /La2O3 additives not only resulted in larger pores and coarsing microstructure (average grain size above 100μm) but also contributed to the formation of extensive twins within AlON grains, which would introduce additional light scattering and deteriorate optical properties of AlON ceramics.
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