Effect of the pressureless post-sintering on the hot isostatic pressed Al2O3 prepared from the oxidized AlN powder

The effect of the pressureless post-sintering in hydrogen on the structural and mechanical properties of the hot isostatic pressed Al2O3 prepared by oxidized AlN powder has been studied. The micrometer size AlN powder has been oxidized in air at 900° C and sintered by hot isostatic pressing (HIP) at 1700 °C, 20 MPa nitrogen atmosphere for 5 h. Pressureless sintering (PS) has been applied for all HIP sintered samples in H2 gas at 1800° C for 10 h. It has been shown that the oxidation caused a core–shell AlN/Al2O3 structure and the amount of Al2O3 increased with increasing of the oxidation time of the AlN powder. For the first time, the green samples obtained from oxidized AlN powder have been successfully sintered first by HIP followed by post-sintering by PS under hydrogen without adding any sintering additives. All post-sintered samples exhibited the main α-Al2O3 phase. Sintering in H2 caused the full transformation of AlN to α-Al2O3 phase and their better densification. Therefore, the hardness values of post-sintered samples have been increased to 17–18 GPa having apparent densities between 3.11 and 3.39 g/cm3.

Aluminum nitride (AlN) is an alternative refractory ceramic material being used in various range of applications such as optics, electronics and computer circuits for its unique thermal and electrical properties. It has a really high degree of thermal stability and wear resistance while exhibiting a low density 1 . AlN can be obtained either by carbo-thermal reduction of alumina (Al 2 O 3 ) or by nitridization of aluminum (Al) 1,2 . AlN exhibits covalent bonding and generally has been sintered around temperatures higher than 1600 °C under the presence of sintering additives acting as oxygen absorbers 2 . On the other hand, Al 2 O 3 is a simple covalent oxide of aluminum which is generally formed at the surface of pure aluminum. The growing trend of the key issue of the microstructure of the oxide layer and its effect on the oxidation behavior of AlN ceramics is still unclear 3,4 . Al 2 O 3 has some known phase allotropes. The most commonly identified phase although other intermediary phases evolve during the oxidation process is the γ-Al 2 O 3 5 . However, these phases are mostly unstable and disintegrate at higher temperatures 5 . These thin aluminum oxide films have been increasingly used in various types of electronic devices as dielectric and tunneling barriers 6 . Zheng et al. fabricated the AlN-Al 2 O 3 composite ceramic by heat treating Al 4 O 4 C porous ceramic under N 2 atmosphere above 1500 °C. They showed, that the granular AlN and Al 2 O 3 particles integrated with each other and closely connected at their grain boundary 7 . Oxidation of AlN ceramics is complicated because of the process is influenced by various factors 8 . Moreover, the oxidation of AlN has been shown to lead to improvements in the adhesion of deposited metal layers in several electronic package applications 8 . Yeh et al. studied the oxidation mechanism of AlN particles through microstructure observation 9 . They confirmed the formation of porous oxide layer on the surface of AlN. The oxidation kinetics was therefore fast and this reaction induced an increased in thickness of oxide layer. The reaction stopped when the pores were no longer interconnected. Korbutowicz et al. studied the oxidation rates of aluminum nitride thin films 10 . They observed the quick diffusion and the oxygen gradient in AlN layers: aluminum nitride inside has been infected with oxygen, due to the surface of aluminum oxide layer revealed a high porosity. The mentioned results are in good agreement with investigations made by Zheng et al. 9  . On the other hand, the processing method is influencing the obtained microstructure, reduces the grain size and increases the densification of final sintered ceramic. Hot isostatic pressing (HIP) has unique advantages in promoting the compactness of parts, eliminating void defects, reducing segregation and improving the mechanical properties of the ceramics. The presence of more vacancies and pores in oxide core layer can enhance the sintering by offering a higher chance for lattice diffusion 13 . The HIP sintering of Al 2 O 3 ceramics has a long history of development, therefore is the most familiar for use in the processing of the many existing ceramics materials 14 16 . Hydrogen can facilitate the detachment of protective oxide layer from the metals and alloys. The degradation is usually accelerated at elevated temperatures in many industrial applications 17 19 . The sintering had certain effects on mechanical properties of the composites. The toughness of the composites is enhanced by a crack bridging mechanism or by microcrack toughening. However, the strength of the composites is decreased significantly as the microcracks are formed 19 . Our previous study of the structural and mechanical characterizations of HIP sintered AlN-Al 2 O 3 was discussed in 16 . A combination of HIP and PS post-sintering is proposed in this paper to obtain high-density bodies with higher hardness. In this work, the effect of pressureless sintering in hydrogen on hot isostatic pressed AlN-Al 2 O 3 prepared from oxidized AlN powder was studied.

Materials and experimental
Base AlN ceramic powders with purity of 98 wt% and the average size of 1.3 ± 0.5 μm (H.C. Starck GMBH, Berlin) have been oxidized in ambient atmosphere at 900 °C for 3, 6, 10 and 20 h respectively. The oxidized AlN powders have been pressed by dry press at 7t. After it, the green bodies have been embedded to BN powder in a graphite crucible and sintered by hot isostatic pressing (HIP, ABRA type) at pressure of 20 MPa, at 1700 °C in an inert gas (N 2 ) environment for 5 h. As a post-sintering step, the HIP sintered ceramics have been pressureless sintered (PS) at 1800 °C for 10 h under H 2 environment simultaneously applying 0.1 MPa pressure. The schematic view of experimental procedure is shown in Fig. 1.
The morphology and the microstructure of the powders and sintered samples have been characterized by scanning electron microscopy (SEM). Leo 1540XH Gemini with lens under SEM-SE mode has been used for the powders and Thermo-scientific Scios 2 for the sintered samples. The surface of the sintered samples have been covered by thin carbon coating to have better resolution and conduction. X-ray diffractometry (XRD) has been carried out using Bruker AXS D8 Discover diffractometer for phase analysis of both the powders and sintered samples. The numbering of the samples after each preparation processes has been indicated in Table 1.
The apparent density of the sintered samples has been measured using Archimedes method where the samples with surface porosity have been immersed in soap water for three days ensuring the complete filling of the pores. The equation used for calculation has been provided in Eq. (1). where H v is the Vickers hardness, F is the applied force (N) and d is the diagonal length (mm). . The studies confirmed that the oxidation mechanism may be described as a reaction process together with a diffusion process. The oxidation process for AlN has been founded in temperatures ranging from 550 to 1100 °C [21][22][23][24] . The nearly globular micrometer sized AlN powder has been oxidized at 900 °C from 3 to 20 h in ambient atmosphere (Fig. 2). AlN powder before oxidation showed mainly globular character with average grain size of ~ 1 µm (Fig. 2a). The presence of only the AlN phase has been confirmed by the elemental composition (Fig. 3) analysis. No morphological changes after 3 h oxidization (Fig. 2b) have been observed. The EDS confirmed the presence of oxygen (Fig. 3a) and the quantitative analysis proved the AlN : Al 2 O 3 ratio to be 19 : 81 wt% (Fig. 3b). Increasing of oxidation time to 6 h slightly increased the grain size of oxidized AlN (Fig. 2c) and the AlN : Al 2 O 3 ratio is 4 : 96 wt% (Fig. 3).

Results and discussion
(1) ρ apparent = wt of dry sample wt of dry sample − wt of immersed sample · ρ water   (Fig. 2d,e). The results are in good agreement with the works of Maghsoudipour et al. 11 and Cao et al. 12 . The particle clustering can also be observed in cases of samples with oxidization time above 6 h.
In our previous study, it has been confirmed simultaneous growth of two different phases of aluminum oxide, α-Al 2 O 3 and the intermediary θ-Al 2 O 3 (Fig. 3b). Although the second phase of aluminum oxide can be observed only in the powders after 10 and 20 h of oxidation time 16 . These measurements are in agreement with Tabary et al. 20 .
Sintering of Al 2 O 3 ceramics by hot isostatic pressing (HIP) has a long history 25 . The advantage of HIP over conventional sintering processes is in obtaining of the very high dense samples. In the case of HIP_10 and HIP_20, the presence of α-Al 2 O 3 phase has been only observed. It can be explicable by the sintering process in nitrogen and high temperature disintegrated the non-stable θ-Al 2 O 3 phase 16 . As a post-sintering step pressureless sintering (PS) in hydrogen was applied to as processed HIP samples. Pressureless sintering of AlN-Al 2 O 3 has been applied to further densify the samples with complex shapes after HIP sintering. The sintered samples (PS_3-PS_20) have been heated at 1800 °C, 0.1 MPa under H 2 environment to complete the conversion cycle of AlN to Al 2 O 3 .
Comparison of the phase composition of the oxidized powders, HIP sintered and PS sintered samples have been performed by X-ray diffractometry (XRD) (Figs. 4a, 5a, 6a, 7a, 8a). In all samples, hexagonal AlN (JCP2:03-065-1902) and uniform rhombohedral α-Al 2 O 3 (JCP2:00-010-0173) have been observed as major phases. The oxidation of AlN powders created two new distinct phases of aluminum oxide; major α-Al 2 O 3 and minor θ-Al 2 O 3 , as it has been shown in our previous work 16 . During the oxidation of AlN, besides α-Al 2 O 3 , formation of various intermediary phases of aluminum oxide, like θ-Al 2 O 3 and γ-Al 2 O 3 were observed. However, these oxide phases are unstable and disintegrate at temperatures above 1100 °C 26,27 .
The phase and structural transformation of pure AlN (Fig. 2a) without oxidation has been studied as reference (Fig. 4). The transformation of the part of the AlN to α-Al 2 O 3 during HIP sintering (Fig. 4b) has been observed. The subsequent PS sintering effected the grain growth from 1 μm to ~ 5 μm (Fig. 4b) at 1800 °C for 10 h. Besson  www.nature.com/scientificreports/ and Abouaf reported that this effect has not been observed only if the pressureless sintering prolonged 100 h at the temperature of 1400 °C 28 .
The comparative phase analysis of the 3 h oxidized AlN and HIP-post PS processes have been presented in Fig. 5. The higher volume of α-Al 2 O 3 (Fig. 3b) helped the strong phase transformation of remnant AlN to α-Al 2 O 3 and the sintering in H 2 finished this process (Fig. 5a). This fact has been confirmed by the XRD results (Fig. 5a).   www.nature.com/scientificreports/ The major reflections corresponding to stable α-Al 2 O 3 phase occured at 25°, 35°, 43°, 52° and 57° 2θ positions. The PS sintered Al 2 O 3 has been consisted from the non-uniform morphology (Fig. 5b). The grain size has been still around 5 μm but compared to non-oxidized reference (Fig. 4b), the surface was smoother. Oxidation above 6 h induced the presence of intermediary θ-Al 2 O 3 phase. This phase could be topotactically transformed from γ-Al 2 O 3 , which is stable under higher heat-treatment temperatures 800 °C 29 . θ-Al 2 O 3 is more stable at higher temperatures of ~ 950-1000 °C where kinetic factors play a lesser role 29 . The HIP and PS as well are using comparable higher sintering temperatures, which occured the transformation of the metastable θ-Al 2 O 3 (Fig. 6a). XRD measurements confirmed mainly α-Al 2 O 3 with minor AlN phase (Fig. 6a). The morphology of PS sintered sample is shown in Fig. 6b, the sample is characterized by the average grain size ~ 5 μm.
The oxidization above 10 h had the effect on content of AlN. This fact has been supported by the phase and morphological study illustrated in Figs. 7 and 8. In both cases, the presence of metastable θ-Al 2 O 3 have been proved after oxidation (Figs. 7a, 8a). In all the sintered samples, the composition of Al 2 O 3 increased as function of oxidation time. The second heating cycle (PS) eliminated all intermediary oxide phases and transformed the substrate into a uniform α-Al 2 O 3 phase. Therefore, combined sintering (HIP + PS) associated with complete conversion of base AlN to Al 2 O 3 (Corundum) in the case of longer oxidation time. "The grain size of post-sintered ceramics slightly increased to 10-20 μm in the case of 10 h or 20 h oxidation times. " The apparent density measurement can help in the valuable information to control the quality of a ceramic with respect to the porosity. The apparent densities of sintered samples (HIP, PS) are shown in Fig. 9. The comparative study of densities of HIP sintered and PS sintered samples showed the similar tendency. The HIP and PS sintered base (reference) AlN exhibited the lowest apparent density (2.57 g/cm 3 , Fig. 9). Increasing of the oxidation time of base AlN powder caused the increasing of density values from 2.87 to 3.38 g/cm 3 for HIP_3-20 and from 3.11 to 3.27 g/cm 3 for PS_3-20, respectively. Kim 30 . The presence only the major α-Al 2 O 3 predicted the higher densification during sintering process (independently on sintering type) (Fig. 9). AlN phase blocked the fully densification and caused the formation of the bigger grains, porosities and impurities in sintered ceramics (Figs. 4,5,6,7,8). The highest apparent density 3.39 g/ cm 3 (85% relative density) has been observed in a case of sample oxidized for 10 h. Hardness values have been characterized as function of oxidation time (Fig. 10). The similar tendency of hardness behavior has been observed for both sintering techniques. The increasing of hardness has been influenced by increasing of oxidization time of base AlN powder, minimal presence of AlN and grain size of α-Al 2 O 3 . In addition, reduction of porosity resulted in closer packing, denser structure and improvement the hardness of sintered samples. The highest hardness values between 17 and 18 GPa have been observed for PS sintered α-Al 2 O 3 oxidized between 3 and 10 h. These values are comparable with results of other research groups [31][32][33] .

Conclusions
Bulk sintered Al 2 O 3 has been prepared by oxidization of AlN powder and combined sintering process, hot isostatic pressing (HIP) in N 2 and pressureless sintering (PS) in H 2 atmosphere. The HIP followed by PS postsintering of oxidized AlN powder without sintering additives has been successfully developed for the first time. The micrometer sized AlN has been oxidized for 3, 6, 10 and 20 h in ambient atmosphere. The volume of Al 2 O 3 increased with the increasing of oxidation time of AlN powder. Oxide layer caused porosities and the grains slightly growth. Above 10 h oxidation, "heat-treatment" metastable θ-Al 2 O 3 phase has been observed. High temperature HIP sintering transformed θ-Al 2 O 3 and only two major phases α-Al 2 O 3 and minor AlN have been stabilized. PS post-sintering in 1800 °C for 10 h caused the phase transformation to α-Al 2 O 3 which had effect on the apparent density and hardness of PS sintered ceramics. The highest apparent densities 3.11-3.39 g/cm 3 (78-85% relative densities) and highest hardness values (17-18 GPa) have been measured for PS sintered α-Al 2 O 3 prepared from base powder oxidized between 3 and 10 h.