Microstructural effects on hardness and optical transparency of birefringent aluminosilicate nanoceramics

Abstract

Transparent nanoceramics, synthesized at extreme conditions of high pressure and temperature, are new classes of materials highly attractive for photonic applications, such as optical windows, which require additional increased hardness and toughness. In this study, mechanical properties of transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite (~91.4 vol%) and trigonal Al2O3 corundum (~8.6 vol%) fabricated at high pressure (10 GPa) and temperature (1200‐1400°C) were investigated. It is already known that the optical transparency of kyanite‐based nanoceramics increases with decreasing average grain size. The present study shows that the hardness of these ceramics increases with decreasing grain sizes down to ~70 nm according to the Hall‐Petch strengthening. This grain size seems to mark a transition range where an inverse Hall‐Petch effect is indicated due to signs of a moderate hardness decrease at a smaller grain size of ~35 nm. The observed hardness‐grain size relation can fairly be described by an existing composite model, which considers the crystals to be harder than the noncrystalline grain boundaries. Within the range of average grain sizes examined, the kyanite habit changes from more equant to more columnar. This behavior is associated with the observed strong crack deflection by the columnar kyanite grains with aspect (length to diameter) ratios ranging from ~2 to 10 and may positively affect the fracture toughness.

Elasticity of nanocrystalline kyanite at high pressure and temperature from ultrasonic and synchrotron X‐ray techniques

“…Additional details can be found elsewhere. 17,22 The high-pressure synchrotron in situ X-ray and ultrasonic measurements were implemented with the 1500-ton multianvil large volume press SPEED1500 located at SPring-8 (beamline BL04B1). 19 For this, the carefully diamond-mediated polished NPK (~0.8 µm mirror-surface; sample size of ~2.2 mm in diameter and ~0.7 mm in height) was fit into a MgO sleeve and both covered by (Mg, Co)O pressure medium (1)…”

“…It has been previously shown that the hardness of the NPK is positively affected by the Hall-Petch behavior. 17 It is tempting to apply Equation (13) to the NPK, but it may be speculative because it is explicitly for non-nanocrystalline materials. However, the calculated hardness value (H V,calc = 18.2(4) GPa) of the NPK by Equation (13) using our obtained bulk and shear modulus is consistent to the corresponding experimentally measured one (H V,exp = 18.3(11) GPa).…”

“…Polycrystalline ceramics consisting of the variety of low‐symmetry birefringent crystals can be made of the same quality if the crystal sizes are grown in the low‐micro‐to‐nanometer scale. Recently, highly transparent and reinforced aluminosilicate nanoceramics composed of triclinic kyanite with minor amounts of trigonal α‐alumina crystals were manufactured by high P ‐ T ‐induced crystallization from a glass 17,22 . This ceramic is referred to as nano‐polycrystalline kyanite (NPK) in the following text.…”

“…15,16 Recent studies have demonstrated that the synthesis of materials under elevated pressures effectively tunes the physical characteristics, such as mechanical or optical ones. 17 A well-known technique for exploring the bulk modulus of materials and its pressure evolution is in situ synchrotron X-ray diffraction under high pressure. 5 This process is followed by applying equation of states (EoS) to the acquired pressure-volume (P-V) data, for example, the Birch-Murnaghan EoS 18 written in its third order as Equation (1) relates the unit-cell volume V P,300 at room temperature to the pressure P with the parameters K T0 for the isothermal bulk modulus, K ′ T0 for the first pressure derivation of the isothermal bulk modulus, and V 0,300 as the unit-cell volume at atmospheric pressure and temperature.…”

“…Recently, highly transparent and reinforced aluminosilicate nanoceramics composed of triclinic kyanite with minor amounts of trigonal α-alumina crystals were manufactured by high P-T-induced crystallization from a glass. 17,22 This ceramic is referred to as nano-polycrystalline kyanite (NPK) in the following text. The NPK is substantially harder than kyanite single crystals 24 and only slightly softer than polycrystalline α-alumina.…”