Preparation and mechanical properties of machinable alumina/mica composites

doi:10.1016/j.jeurceramsoc.2005.03.258

Journal of the European Ceramic Society

Volume 26, Issue 9, 2006, Pages 1687-1693

https://doi.org/10.1016/j.jeurceramsoc.2005.03.258 Get rights and content

Abstract

Using a mica-crystallizing glass powder in which a large amount of mica crystal was precipitated and a larger amount of MgF₂ component was contained as the raw materials of mica, machinable alumina/mica composites were obtained at 1400 °C. In the firing process, magnesia component in the mica crystals reacted with alumina to form spinel at 1150–1200 °C. The reaction made the mica crystals melt. However, the mica crystals were precipitated again during the cooling. Because a larger amount of MgF₂ component was contained in the mica-crystallizing glass powder, the nucleation of the mica crystals was caused during the cooling by the residual magnesium and fluorine in the liquid phase and succeedingly the mica crystals were precipitated. The precipitated mica crystals grew to anisotropicaly larger size than alumina grains, which lowered the bending strength and Vickers hardness and little heightened the fracture toughness.

Introduction

Alumina ceramics are well-known typical ceramics and show a favorable combination of physical and chemical properties, e.g. high strength and hardness, high heat resistance, excellent chemical durability and low electrical resistance, so such alumina ceramics are applied in the various fields as engineering materials, electrical insulators, biomaterials and so on. However, the alumina ceramics are difficult to be machined by conventional metal tools due to their hardness and brittleness as well as many other ceramics. Therefore, the machining costs are very high, which limits the application fields. The improvement of the machinability is very effective in not only the reduction of the machining costs but also the fabrication of complex shape ceramics, precision machining and machining efficiency.

Machinable Al₂O₃/h-BN,¹ Al₂O₃/Ti₃SiC₂² and Al₂O₃/LaPO₄ composites,3, 4, 5 which utilize the machinability of h-BN, the plate or layered shaped structure of Ti₃SiC₂ and the weak interface between oxide and phosphate, respectively, have been reported. They could be cut and drilled using conventional machining tools. On the other hand, mica glass-ceramics have been well-known machinable ceramics since Beal developed a glass ceramic containing mica by controlling crystallization of the base glass in 1971.6, 7 The excellent machinability originates in the cleavage and the interlocking microstructure of mica platelet crystals.6, 7 Utilizing the machinability of mica, we succeeded in fabricating machinable cordierite/mica⁸ and spinel/mica⁹ composites. These composites could be prepared at low temperatures of 1100–1200 °C by the sintering of cordierite or spinel raw materials and mica-composition (KMg₃AlSi₃O₁₀F₂) glass powder mixtures and the crystallization of mica from the powder mixtures. While the machinable composites containing h-BN1, 5, 10, 11, 12 and Ti₃SiC₂² are fabricated by hot pressing and spark plasma sintering, our machinable composites containing mica8, 9 can be fabricated without the special sintering apparatuses. Moreover, comparing with machinable composite containing phosphates,3, 4, 5, 13 our machinable composites containing mica8, 9 have a possibility of low-temperature sintering due to sintering through the liquid phase formed by melting of the additive glass. Thus, the fabrication at low temperatures using conventional apparatuses is the merit of our machinable composites. However, alumina/mica composites had been never prepared by the sintering of alumina and mica-composition glass powder mixtures. Because the mica-composition glass powder reacted with alumina to form spinel, mica was not crystallized.¹⁴

In this study, our aim was to fabricate machinable alumina/mica composites at low temperatures. So not only the mica-composition glass powder but also the mica-crystallizing glass powder which contained a large amount of mica crystals and an excess of MgF₂ component over the mica-composition glass powder was used as the raw materials of mica. Both or either of these powders were mixed with alumina powder and sintered. In consequence, machinable alumina/mica composites were obtained at 1400 °C. In this paper, the sintering behavior of the powder mixtures and the mechanical properties of the obtained composites are reported.

Section snippets

Experimental procedure

The reagents of K₂CO₃, MgO, Al₂O₃, SiO₂ and MgF₂ were mixed in the chemical composition corresponding stoichiometric fluorophlogopite (KMg₃AlSi₃O₁₀F₂) composition, calcined at 900 °C, melted in a sealed platinum container at 1450 °C and then quenched. The obtained glass was pulverized by a ball milling for 48 h. In this way, the mica-composition glass powder was prepared. The above-mentioned reagents were mixed in the chemical composition which was an excess of 10 mass% MgF₂ over the

Phase change and sintering

XRD patterns of the G20 and G10C10 specimens are shown in Fig. 4, Fig. 5, respectively. In the G20 specimen, very small peaks of mica were observed at 1300 and 1350 °C while relatively large peaks of spinel appeared at ≥1250 °C. On the other hand, in the G10C10 specimen, distinct peaks of mica appeared at ≥1300 °C, and simultaneously the peaks of spinel were observed. Also, in the C20 and G20C20 specimens, mica and spinel were observed, of which XRD patterns are not shown in this paper. These XRD

Conclusions

As raw materials of mica, not only the mica-composition glass powder but also the mica-crystallizing glass powder in which a large amount of mica crystal was precipitated and a larger amount of MgF₂ component was contained were used. By adding the 10 mass% mica-composition glass powder and the 10 mass% mica-crystallizing glass powder to alumina powder or by adding the 20 mass% mica-crystallizing glass powder, dense machinable alumina/mica compositions were obtained at 1400 °C. In the firing

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Preparation and mechanical properties of machinable alumina/mica composites

Abstract

Introduction

Section snippets

Experimental procedure

Phase change and sintering

Conclusions

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