Structure and Crystallization of Borate-Based Glasses for Machinable Material Applications

Abstract

Abstract-This study investigated bulk crystallization at pre-set temperatures of B 2 O 3 -SiO 2 -Al 2 O 3 -BaO-MgO glass system. Heat treatment at pre-set temperature was carried out between 900 and 1200°C on clear glasses to form non-transparent glass-ceramics. Structure of glasses were studied by Fourier transform infrared spectroscopy, phase evolution in the bulk crystallized samples was studied by x-ray diffraction, microstructures were observed by scanning electron microscope and micro-hardness was measured by Vickers method. Aluminum borate whiskers/rods were in the nano to micro sized regime and dispersed randomly within the remaining glass matrix. Whiskers had different sizes and morphologies depending on the heat treatment temperature. Increasing heat treatment temperature led to an increase in crystal size, length and the formation of hollow tubes. The surface hardness decreased when the morphology changed from fine rods into hollow tubes. The synthesized glass-ceramics in this study can be applied as machinable materials in various applications because their microstructures can resist microcracking.Index Terms-Glass, crystallization, microstructure, whiskers, hardness. I. INTRODUCTIONFibers and whiskers reinforced glasses and ceramics can lead to significant improvements in mechanical properties, such as increased fracture toughness, bending strength, hardness and thermal shock resistance [1]. The possibility of using whiskers is limited due to the health hazard posed by loose particles with a high aspect ratio shape [2]. Aluminum borate whisker (9Al 2 O 3 ⋅2B 2 O 3 ) is a refractory compound (melting point 1440°C) possessing excellent physical properties, such as low density (2.94 g/cm 3 ), high hardness, high Young's modulus (400 GPa), high electrical resistivity and low coefficient of thermal expansion [3]. Such characteristics suggest that aluminum borate whisker could be used to reinforce ceramics to give ceramic-ceramic composites with increased fracture toughness and also reinforce light metals such as aluminum alloys. The metal matrix composites (MMCs) can be useful for the automobile and aerospace industries. Manuscript received February 12, 2012; revised March 24, 2012 Aluminum borate whiskers have been synthesized by various methods such as vapor phase method, solid-state reactive firing and flux method using borates [4][5]. However, the product yields were found to be very low, 10% or less. The in-situ direct growth of whiskers in a glass to form glass-ceramic is an attractive alternative method. This is analogous to the creation of fluorcansite platelets in toughened glass-ceramics. Unfortunately, oxide systems that can be exploited are limited. Recently, the formation and crystallization behavior of glasses in the Al 2 O 3 -B 2 O 3 -SiO 2 system has been investigated [6], showing that whisker-shaped aluminum borate crystals can be obtained after thermal treatment.This study investigated a specially formulated glass compositions based on a composition similar to that proposed by Lee et al...

In-situ Quasi-Instantaneous e-beam Driven Catalyst-Free Formation Of Crystalline Aluminum Borate Nanowires

“…Despite the numerous variations in experimental approaches, the underlying rationale of the synthesis process of Al 5 BO 9 NWs is similar in all cases, namely, one enhances the atomic mobility within bulk quantities of an either amorphous241011 or crystalline7813 precursor (powders, gels, pellets) by injecting energy into it. A considerable amount of free-volume is generated within the precursor during the process (through evaporation, for example).…”

“…Aluminium borate NWs have been produced via glass crystallization4, calcination of powders567, porous compacts89 and sol-gels1011 or through chemical vapor deposition over sapphire substrates1213 to name a few. Interestingly, the use of catalysts is avoided in almost all reports, thus, a growth mechanism other than the vapor-liquid-solid (VLS) mechanism must be considered.…”

Commercial alkaline earth boroaluminosilicate glasses for sealing solid oxide cell stacks. Part I: Development of glass‐ceramic microstructure and thermomechanical properties

“…The impact from surface‐nucleation on devitrification of borate‐free phases complies with a more general behavior of alkaline earth silicate glasses discussed in literature, where the dominant nucleation mechanism changes from a surface to a bulk nucleation model with increasing particle size . The reports in literature, that suggest that crystallization in boroaluminosilicate glasses or borosilicate glasses contrastingly are controlled mainly by homogeneous nucleation, cannot unambiguously be eluded from the microstructures of c30b and cf30, cf Figures and .…”

Commercial alkaline earth boroaluminosilicate glasses for sealing solid oxide cell stacks. Part II: Characterization of devitrification and glass‐ceramic phase assemblages

“…This is quite in concord with reports in literature on alkaline earth silicate glasses, where the effect of this dominant nucleation mechanism can be reduced with increasing particle size . The crystallization of borates, as seen in c30b and cf30, is more difficult to assess in light of reports in literature that shows nucleation and growth mechanisms in boroaluminosilicate glasses or borosilicate glasses are controlled mainly by homogeneous nucleation.…”