1. Introduction

Commercial applications of aluminum oxide (Al₂O₃) or alumina have increased in biomedical area such as orthopedics implants, bionics prosthetics, oxygen sensors, dentistry tool, scaffolds [¹]. The physical properties of alumina components depend on the particle size, porosity, and purity of the ceramic [²], which previously depend on the manufacture of raw material. The principal method used to produce alumina powders is the Bayer process which starts from bauxite ore [²-³]. Basically, bauxite is a hydrated alumina with impurities elements such as clay, silica, magnesium oxide and iron oxide, therefore the principal application of alumina for this route is to aluminum smelting industry [³-⁴]. Other route to obtain high purity alumina is the sol-gel technique [²,⁴-⁵]. Under moderate reaction conditions produced through a colloidal suspension, a pseudo-crystallized aluminum hydroxide gel is obtained [⁴,⁶]. Hence pseubohemite (AlOOH.xH₂O) is a synthetic aluminum hydroxide gel that acts he as a precursor to transition of alumina: AlOOH, γ, δ and α-Al₂O₃, depending on the heat-treatment conditions [²,⁷].

Nowadays, advanced alumina-based composite materials are developed with different manufacturing technologies [⁸,⁹]. Composite materials are fabricated using reinforcements particles which are added to ceramic powder in order to create heterogeneous materials called ceramic matrix composites [⁸,⁹]. Principal application for ceramic matrix composites (CMCs) is for lightweight applications. Aluminum oxide matrix could be reinforced with different metal or ceramics materials. Those reinforcing materials are presented in addition to different forms such as particles, whiskers or fibers. The use of each form depends on the desired characteristics to be search like high strength or temperature stability. Silicon carbide (SiC) reinforcement is a frequently material used together aluminum oxide to increase the fracture toughness resistance at high temperature [⁸,⁶].

Mechanical, structural and electrical properties of composite materials are a function of the sintering method. One of the most common methods to produce advanced ceramic materials and CMCs, such as Al₂O₃ and SiC systems, is called conventional route or powder metallurgy route [⁸]. Conventional route of powder processing involves the mixing of matrix and reinforcement in a suitable aqueous or non-aqueous media, drying of the mixture, powder compaction, and densification at high-temperature [⁸-¹⁰]. That process must be carried out carefully. In some cases, the reinforcement can agglomerate during mixing of powders obtaining a non-homogeneous mixture or poor compaction. Consequently, these problems can produce defects within the green compact and cause important defects in the composite after sintering.

This paper is focused on the synthesis of α-Al₂O₃ powders in order to manufacture composite compacts and the study of the effect of previous heat treatment (pre-sintering) applied to Al₂O₃/SiC green compacts before sintering at high temperature.

2. Methodology

Pseudoboehmite powders was synthesized by sol-gel technique starting from preparation of a solution of commercial aluminum sulfate salt (Al(OH)₃.18H₂O) in distilled water at 0.5 M. The solution was stirred for 24 h. Subsequently, peristaltic pump was used to drop aluminum sulfate solution into an ammonium hydroxide solution at 60°C and pH>10 [⁶]. The product obtained was filtered and washed with distilled water and then was dried at 100°C for 24 hours in air. Thereafter, the pseudoboehmite was sieved at 400 mesh and calcined at 1000°C and 1200°C for 5 hours in air. Pseudoboehmite powders were characterized by particle size, X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques.

Commercial SiC powder (≤97.5% purity) of ALDRICH Chemistry was used as reinforcement. Morphology of SiC particles was angular form, the analysis of particle size was 20.6 μm average size. For the fabrication of Al₂O₃/SiC composite, a powder mixture of Al₂O₃ with 5wt.% of SiC was made by rotary evaporation technique using isopropyl alcohol as solvent. The solvent was eliminated at 80°C for 30 minutes during the mixture process. Then, two series of green compacts were produced at 440 MPa by uniaxial pressing; the first compacts were obtained with the mixture powders (Al₂O₃+SiC) and the second compacts with Al₂O₃ powder without reinforcement. The green density in the compacts was about 47%. A set of green samples with 5 wt.% of SiC as reinforcement were a pre-sintered (heat treatment) at 1200°C for 6 h in air before sintering process. Sintering was carried out in a vertical dilatometer Linseis L75 V at 5°Cmin up to the temperature of 1500°C for 2 h under argon atmosphere. The composites produced were transversely cut and polished in order to analyze the interaction of the reinforcing particles in the ceramic matrix by SEM JEOL JSM 7600F. Through the sintering data obtained by the dilatometer, a densification graph was plotted and analyzed to understand the behavior of the material during the heating process.

3. Results and discussion

X-ray diffraction pattern of the pseudoboehmite before and after heat treatment at 1000°C and 1200°C are shown in Fig. 1. Because pseudoboehmite without heating is an aluminum hydroxide AlO(OH) phase, water alters interplanar distances in a crystalline structure showing widening peaks [¹⁰-¹¹]. The transitional metastable phases of Al₂O₃, α, θ, δ and γ were identified after heat treatment at 1000°C. When heating was at 1200 ^- C, the α-Al₂O₃ phase is exclusively present, corresponding to the most stable phase of aluminum oxide [⁵-⁷, ¹⁰-¹¹]. Therefore, 1200°C is the appropriate temperature to produce α-Al₂O₃. Results of particle size distribution after calcination reveals principally a bimodal behavior that is shown in the graph in Fig. 2a). Sieving at 400 mesh controls the size of particles smaller than 38.5 μm. Consequently, graph show a particle size distribution from 30 μm to 0.3 μm, with a mode of 21.69 μm and size average of 6.21 μm respectively. That result is verified in the SEM image at Fig. 2 (b). Particle morphology was spherical, with an observed size of less than 10 μm and longer than 20 μm. Larger particles are agglomerates of finer particles which are composed of an extended interconnected porous structure, this phenomenon is called vermicular structure. The sintering of vermicular structures should be carried out at high temperature and pressure to achieve highly densified bodies [¹¹].

Figure 1 XRD patterns of pseudoboehmite powders before and after heat treatment at 1000°C and 1200°C. 

Figure 2 α-Al2O3 Powders analysis by particle size, a) particle size graph and b) SE-SEM image of powder morphology. 

After characterization of α-Al₂O₃ micrometer powders, they were used as a matrix to mix with reinforcement silicon carbide powder to produce green compacts. Therefore, three different samples were analyzed: 1) monolithic samples called Al₂O₃, 2) composites with 5 wt.% of SiC called Al₂O₃-SiC and 3) same compacts but heat treated (HT) at 1200°C for 6 h in air before sintering called Al₂O₃-SiC HT. Figure 3 a) shows the shrinkage as a function of time during the sintering cycle for the different samples. In this graphic it is observed a low expansion of samples at the initial heating, as the temperature rises from approximately at 1260°C, the contraction of compacts begins, whereas sintering is activated, and it continues until the heating process ends (1500°C2h). Al₂O₃ sample exhibit the greater shrinkage, as well as the greatest densification of the material, which occurs during the holding time at 1500°C, but the contraction continues during all process. The Al₂O₃ ceramic is a material that can be easily sintered. It’s not necessary the use of pressure and heating simultaneously (non-conventional route) to obtain a consolidate compact, and it can even be sintered without an inert atmosphere [¹]. Al₂O₃-SiC and Al₂O₃-SiC HT composites presents a constant contraction at 1500°CHowever, in both cases the axial contraction is slower compared with the monolithic sample (Al₂O₃). Al₂O₃-SiC HT sample reaches a shrinkage of around 2 more than Al₂O₃-SiC. The relative density was calculated with data obtained as a function of the temperature shown in Fig. 3b).

Figure 3 a) Shrinkage and b) densification obtained during the sintering process in the dilatometer (1500°C/2h) for the three different compacts (Al₂O₃, Al₂O₃-SiC-5% and Al₂O₃-SiC-5% HT). 

Initially Al₂O₃ compact had 46.5% of green density. Subsequently, the heating increase the relative density since 53.9% to 1500°C. Composite materials had a green density of approximately 47.5% before sintering. When sintering is achieved, Al₂O₃-SiC sample reaches 49.9% of relative density, and Al₂O₃-SiC HT reaches 51% of relative density. However, in both cases a high density is not reached, which can be attributed to the conditions of the starting powders, low density obtained in the green compact through uniaxial pressure, and temperature used in the conventional sintering route.

Figure 4 shows SEM images of microstructure of the three different samples. The area of analysis is a cross-section near the outer surface of samples that were in contact with argon atmosphere. The surface of Al₂O₃ compact in Fig. 4a) presents a vermicular microstructure similar to described by E. Yalamac et al. [¹²], he observed low intrinsic nucleation accordingly singly crystal grains of α-Al₂O₃ at micrometer scale. Those grains were surrounded by continuous pores, which generated a compact of low density. The compact obtained had many pores due the development of low grain boundary. It was as low as reported by Y. Hirata et al. [¹³]. Despite ceramic particles are in contact during the formation of neck, the micropores are still open, this indicates an early stage of the sintering process due the development of low grain boundary [¹⁴]. Sintered composites in Fig. 4b) and 4c) exhibits a decomposition of SiC reinforcement on the analysis area. The sample Al₂O₃-SiC, which was sintered at 1500°C without pre-sintered, show macro pores. Silicon carbide oxidation can occur in the temperature range of 1200-1400°C as reported by J. Costello and R. Tressler [¹⁵]. This means SiC powder is still susceptible to oxidation under the atmosphere of 99.98% pure inert gas. Otherwise, the cross-section of Al₂O₃-SiC HT in Fig. 4c) show a smaller amount of pores. Because a slightly film of SiO₂ was created on the surface of SiC particles during the sintering step, it acted as barrier in the sintering process. The oxidation of SiC in Al₂O₃-SiC was associated with the low contraction of the composites observed in the dilatometry graph. Unlike to Al₂O₃-SiC HT, the first undergoes the decomposition of the reinforcement and the densification of the material simultaneously.

Figure 4 SE-SEM images of morphology of sintered compacts at 1500°C/2h, a) Al₂O₃, b) Al₂O₃-SiC and c) Al₂O₃-SiC HT. 

To analyze the behavior of the reinforcement particles (SiC) away from the influence of inert atmosphere, an elemental mapping by SEM was carried out inside of samples. Figure 5 show a cross-section of samples, the area of analysis of each one was just on the middle of the piece.

Figure 5 Elemental mapping obtained by SEM of the sintered samples, a) Al₂O₃-SiC composite and b) Al₂O₃-SiC HT composite. 

Figure 5a) show these have vermicular microstructure and non-volumetric damage is observed. The reinforcement particles surrounded by the Al₂O₃ matrix in both composites is clearly and degradation isn’t observed. Therefore, SiC particles did not undergo oxidation when reinforcement particles were directly exposed to gas flow and temperature conditions. As a result, particles retaining its original from and shape. The Al₂O₃-SiC HT characteristic reported in Fig. 5b) is similar to 5a).

It was observed that SiC particles remain without any damage or problem within the matrix, also Al₂O₃ matrix presents the vermicular structure that indicates low density. The SiC starting powders have an average size of 20 μm with irregular morphology. In the central part of the compact, these characteristics are maintained. On the other hand, in the upper and lower part of compact, a reduction in size is clearly exhibited (as seen in Fig. 4). The rounding and size decrease of reinforcement particles probably occurred during the sintering of green compact, where silicon carbide being an oxide-free material, is susceptible to oxidation at high temperatures [¹⁶]. Analysis of XRD in Fig. 6 show three phases formed in the composite sintered at 1500°C in presence of protective argon atmosphere. The reinforcement of SiC reacted with Al₂O₃ matrix to form SiO₂ and aluminum silicate (mullite). As the time is longer, more aluminum silicate is formed. Continuous oxidation of SiC diminishes the particle size until all SiO₂ behaves mullite. If SiC particle size is too big, the final result is a pore with SiC-particle shape surrounded of mullite. Likewise, it has been studied that silicon commonly forms a thin layer of silicon dioxide (passive oxidation) on its surface to suppress further oxidation [¹³,¹⁷]. That is, at very low diffusion rates, a protective layer of SiO₂ is formed. The formation of superficial SiO₂ protects SiC particles from total decomposition of particle and they prevent the appearance of macropores in the Al₂O₃ matrix.

Figure 6 XRD patterns of composite Al₂O₃-SiC sintering at 1500°C. 

4. Conclusions

The results showed a uniform Al₂O₃ powder morphology of submicron size by synthesizing sol-gel technique. Crushing and sifting are necessary prior the heating of PB. To obtain α-Al₂O₃ phase, calcination at temperatures above 1000°C is necessary. At lower heating, the transition phases γ, δ or θ of Al₂O₃ are obtained, which can affect subsequent processes.

Through the sintering curves obtained by the dilatometer, the densification of the material during heating was analyzed. The results reveal that the relative density obtained in sintered composite only reaches 51%, because SiC is susceptible to decomposition under the atmosphere of inert gas. Monolithic Al₂O₃ densification increases at higher temperature and longer holding time. By means conventional route, it is possible produce porous compacts with 53.9% relative density. Elemental mapping showed a homogeneous distribution of the reinforcing particles in the ceramic matrix. Microstructural analysis reveals that the two different sintered composites present a decomposition of SiC reinforcement particles on their surface, leaving pores in the material. However, when pre-sintered is performed at 1200°C and then sintering, the reinforcement particles do not decompose. Instead, they present a decrease in size and rounding of particles attributed to the formation of SiO₂ on the surface of the SiC. Thus, experimental results demonstrate that pre-sintered treatment reduce the decomposition of SiC particles on the surface.