Infiltration of alumina sol into SLS processed porous Al2O3-Al4B2O9 ceramic composites

Abstract

Selective laser sintering (SLS) is a form of the rapid prototyping technology which produces geometrical objects directly from a three-dimensional computer image without part-specific tooling or human intervention [1]. These rapid prototyping techniques have recently been developed to overcome some of the barriers of conventional manufacturing techniques, such as difficulties in tooling complex-shaped ceramic parts and the long production time in fabricating prototypes. The computer image is generated using CAD/CAE software or a computer imaging process. The three-dimensional computer image is numerically sliced into a series of two-dimensional cross-sections. The SLS machine regenerates these two-dimensional patterns on the powder bed by selectively scanning the powder bed with a focused laser beam and binding the loose powder. The thickness of the sintered powder layer is generally 125–250 μm. The sintered layer is lowered from the sintering plane and a new layer of the powder is spread again. The laser scans again, resulting in sintering of the powder particles, and bonding of the present layer to the underlying previous layer. The desired object is generated by laying down a number of such layers and successively sintering them [2]. The primary advantage of the SLS process is the flexibility of selection of material systems compared to other SFF techniques [3]. The two-phase powder approach to SLS, which involves binding high temperature ceramics such as alumina and silicon carbide with a low melting inorganic binder, is a promising technology to fabricate ceramic composite parts [4–6]. The selection of an optimum materials system for this approach depends on materials properties such as the melting point of the binder material and interparticle wetting between the components in the composite powder blend [7]. In a suitably chosen system, the low melting phase melts completely or partially under the laser beam and binds the high melting phase particles. Furthermore, it can react in some cases with the high melting phase or with the atmosphere. Further reaction can also takes place in a subsequent heating step. As a result, either another compound may incorporate into the matrix or a single phase compound may result. The samples made by SLS and simple post-thermal processing of an alumina-boron oxide composite system had only about 35–40% of the theoretical density due to a lower powder bed density arising from a lower apparent density (34% of theoretical density) of 15 μm alumina powder. This resulted in poor bend strengths (around 6 MPa) of the test bars. Therefore, it was necessary to increase the density for functional applications of SLS parts. Infiltration with colloids and solutions into porous SLS parts is an attractive option to increase the density [8]. The infiltration technique densifies a porous material by filling or partially filling the interconnected porosity of a particular compact with a liquid, melt, or vapor [9]. Infiltration can incorporate additional phases into a body that was formed by well-established techniques such as pressing, extrusion, tape casting, slip casting, and injection molding. Furthermore, infiltration may provide a uniform mixing of various phases. During subsequent heat treatment of a porous powder compact infiltrated with a suitable liquid infiltrant, processes such as decomposition of infiltrant, solid-state reaction, liquid phase formation, and densification can take place depending on the system [10–12]. For the infiltration study, test bars with dimension of 0.076 m× 0.025 m× 0.00625 m (3′′ × 1′′ × 0.25′′) made from alumina-25 wt.% boron oxide (melting point; 450 ◦C) powder blend with a laser energy density between 15.1 and 27.2 joules/cm2 fired at 900 ◦C for 6 h were selected. During that post-thermal processing step, boron oxide powder reacted with alumina powder, resulting in aluminum borate (2Al2O3 · B2O3) whiskers at the surface of the alumina particles [4]. The energy density is defined as laser power/(scan spacing× scan speed) [13]. The SLS operational parameters employed to fabricate these test bars are given in Table I. An infiltrant such as alumina sol was selected for densification of alumina-aluminum borate (2Al2O3 · B2O3) composites. Dispal alumina sol provided by the Vista chemical company was employed. The solid content was 25 wt.%, which was confirmed by a direct drying experiment and the particle size was 150 nm. X-ray diffraction (Fig. 1) of the dried gel reveals it to be boehmite (γ -AlOOH). At about 500 ◦C, boehmite transforms to γ -Al2O3 which has a defect spinel structure with 1/3 of the cation sites