Thommy Ekström

Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing

Single phaseβ-sialon ceramics, Si6−zAlzOzN8−z, have been prepared from carefully balanced powder mixtures, also taking account of any excess oxygen in the starting materials. Sintering powder compacts in a nitrogen atmosphere (0.1 MPa) at 1800° C or higher transforms the starting mixture into aβ-sialon solid solution atz-values up to about 4.3, but the sintered material has an open porosity. Addition of 1 wt% Y2O3 to the starting mix improved the sintering behaviour somewhat and the density of the sintered compacts reached 95% of the theoretical value. By glass-encapsulated hot isostatic pressing at 1825° C, however, sintered materials of virtually theoretical density could be obtained, with or without the 1 wt% Y2O3 addition. These latter samples have been studied by X-ray diffraction and electron microscopy, and their hardness and indentation fracture toughness have been measured. It was found that the maximum extension of theβ-sialon phase composition at 1825° C and 200 MPa pressure is slightly below 4,z∼ 3.85 and about 4.1 at atmospheric pressure, and that the hexagonal unit cell parameters are linear functions of thez-value. The single-phaseβ-sialon ceramics had no residual glassy grain-boundary phase. The grain shape was equi-axed and the grain size increased from about 1μm at lowz-values to 5μm at highz-values. At lowz-values the hardness at a 98 N load was 1700 and the fracture toughness 3, whereas an increase inz above 1 caused both the hardness and fracture toughness to decrease significantly. Addition of 1 wt % Y2O3 to the starting mix prior to the HIP-sintering gave rise to a small amount of amorphous intergranular phase, changes in grain size and shape, a clear increase in fracture toughness and a moderate decrease in hardness.

Reversible α⇌ sialon transformation in heat-treated sialon ceramics

Abstract Typical β-sialon and mixed α-β-sialon starting compositions have been densified by pressureless sintering using either Ln2O3 or equimolar mixtures of Ln2O3 + Y2O3 where Ln = Sm, Dy, Yb. The resulting materials were heat treated at 1000–1500°C to devitrify the grain boundary glass into crystalline oxynitride phases. The effects of rare earth ionic size and heat-treatment temperature on the stability of devitrified phases, microstructure and mechanical properties have been studied. Microstructural characterisation of the α-sialon phase formed in these systems revealed that α is only stable at high temperatures and transforms to β-sialon plus grain boundary phases at lower temperatures; this provides an excellent mechanism for controlling the mechanical properties of the final material.

Notes on phases occurring in the binary tungsten-oxygen system

Abstract The phases occurring in the binary tungsten-oxygen system in the composition region WO 3 WO 2 have been clarified by electron microscopy and powder X-ray diffraction in the temperature range from 723 to 1373 K. There are five structure types in the binary system, besides WO 3 , viz., the {102} CS structures, the {103} CS structures, W 24 O 68 , W 18 O 49 , and WO 2 . The {102} and {103} CS structures, and W 24 O 68 structures, were always disordered and true equilibrium was not achieved even after 5 months of heating at 1373 K. The lowest temperature for the formation of the CS phases was of the order of 873 K, and the disordered W 24 O 68 structure formed at somewhat higher temperatures. The formation of the latter phase was also slower than the formation of the CS phases. The results suggest that elastic strain energy is of importance in controlling the microstructures found in the nonstoichiometric regions.

Ytterbium-stabilized -sialon ceramics

Dense, Yb-doped -sialon ceramics of the composition , with , together with some Yb-based - and -sialon ceramics containing an excess of glassy phase, have been prepared by hot-pressing at . These materials were subsequently heat-treated in the temperature range for different periods. It was found that Yb-doped -sialon forms within a wide composition region, with x ranging from 0.3 to more than 1.0, and that -sialon is stable over a large temperature interval and during heat treatment times of up to 30 days. The -sialon phase coexists with a liquid phase at and with crystalline phases such as the Yb garnet and/or J phase at . Further heat treatment at revealed that initially (during the first 24 h) the -sialon phase reacts with the residual liquid grain-boundary phase, yielding a material which contains an Yb-stabilized garnet phase, besides the -sialon phase, but no glassy phase. This phase assembly is stable upon further heat treatment (30 days). Samples containing Yb-doped -sialon in conjunction with an amorphous intergranular phase show very high hardness and fracture toughness in the range of at room temperature, and mechanical data in the same range were obtained for samples exposed to prolonged heat treatment at . Elongated -sialon grains were found in samples with high x values, but their presence did not affect the mechanical properties significantly.

On the extension of the α-SiAlON solid solution range and anisotropic grain growth in Sm-doped α-SiAlON ceramics

Abstract α-SiAlON samples of the overall composition SmxSi12−(m+n)Alm+nOnN16 − m, with m and n in the ranges 0.6 ≤ m ≤ 1.44 and 1.3 ≤ n ≤ 1.7 (m = 3x), were prepared by a hot-pressing technique at 1800 and 1700 °C. Based on X-ray powder diffraction studies and elemental analysis of individual α-SiAlON grains in the obtained ceramic compacts, the extension of the Sm-doped α-SiAlON solid solution range was mapped out. The m-value was found to vary between 0.89 and 1.52 while n was always less than 1.23. Elongated α-SiAlON grains were found preferentially in compacts having overall compositions located slightly outside the oxygen-rich border-line of the homogeneity region of the Sm-doped α-SiAlON phase, i.e. for m ≈ 1.2 and n ≈ 1.3. We also noticed that elongated α-SiAlON grains were formed preferentially perpendicular to the pressure applied in the sintering procedure. All samples exhibited HV10-values in the range 21–22 GPa and KIC-values in the range 4.0–4.7 M Pa m 1 2 .

Formation of an Y/Ce-doped α-sialon phase

Abstract α-β-Sialon materials were prepared with a constant molar addition of Y2O3 + CeO2 sintering aid with varied composition: 0, 25, 50, 75 and 100 mol% Y2O3. The α-sialon grains formed in the microstructure were carefully investigated by analytical and high-resolution electron microscopy. It was found that the α-sialon structure can accept a small amount of cerium to enter the large interstitial cages, if it is stabilized by simultaneous additions of yttrium. Extended crystal defects were found to occur in the α-phase framework; this has not been observed with only yttrium added.

α-SiAlON Grains with High Aspect Ratio—Utopia Or Reality?

It is generally accepted that β-sialon grains can occur in elongated shapes, but that α sialon can only form equi-axed grains. Unexpected evidence of the formation of elongated α-sialon grains was noticed in our recent work, however. The morphology of α-sialon grains was found to vary significantly with composition, and elongation occurred more frequently with certain rare earth dopants. Elongated α-sialon grains were obtained for the most oxygen-rich compositions, i. e. in samples where extended transient liquid formation occurs. In this presentation, we will discuss how the morphology of α-sialon grains varies with composition, with the type of rare earth dopant used, and with the processing parameters applied.

Effect of composition, phase content and microstructure on the performace of yttrium SiAlON ceramics☆

Abstract α-, β- or mixed α-β-SiAlON ceramics have been prepared by changing the overall composition in the YSiAlON system. The relative amounts of the formed constituent SiAlON phases, their grain size and morphology and the intergranular phase are shown to determine physical properties such as hardness and indentation fracture toughness. The single-phase SiAlON ceramics without an integranular phase have a chunky grain habit and in general a brittle behaviour with a fracture toughness below 3.5 MPa m 1 2 , but they are hard. The single-phase β-SiAlON ceramics have a Vickers hardness of up to 1700 kg mm −2 at a 98 N load and the single-phase α-SiAlON ceramics reach values up to 1950 kg mm −2 . The glass-containing β-SiAlON or mixed α-β-SiAlON ceramics can be manufactured simply by pressureless sintering and have good combinations of hardness and toughness. The β-SiAlON ceramics have a toughness of 5 MPa m 1 2 and a hardness of 1500 kg mm −2 ; the hardness increases by increasing the α-SiAlON phase content in the α-β-SiAlON materials whereas the toughness decreases. The value of these latter ceramics as metal cutting tools for turning of cast iron or aerospace alloys is demonstrated. It is shown that SiAlON ceramics are good cutting tool materials for machining cast iron with the best behaviour obtained for a β-Si 3 N 4 ceramic, and that the α-β-SiAlON is the outstanding material for aerospace alloys.

HIP-sinteredβ- and mixedα-β sialons densified with Y2O3 and La2O3 additions

Various fully dense sialon materials sintered by the glass-encapsulated hot isostatic pressing technique were synthesized using Y2O3 and/or La2O3 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/La2O3:100/0, 75/25, 50/50, 25/75, 0/100. The samples were sintered at two different temperatures, 1550 and 1825° C. At the lower temperature, unreactedα-Si3N4 was present in the samples in addition toβ-sialon and secondary phases. The samples sintered at 1825° C showed that yttrium but not lanthanum favouredα-sialon formation. The amount of intergranular phase increased by about 50% when Y2O3 was replaced by La2O3. The La-sialon ceramics have as good an indentation fracture toughness as the Y-sialon ceramics, about 5 MPam−1/2, but the Vickers hardness is slightly lower, being 1400 kg mm−2 at a 98 N load.

Pressureless sintering of Y2O3-CeO2-doped sialons

Various sialon materials have been prepared by pressureless sintering at 1775 and 1825 °C using Y2O3 and/or Ce02 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/CeO2: 100/0, 75/25, 50/50, 25/75, 0/100 corresponding to 6.0 and 9.25 wt% for the pure Y2O3 and pure CeO2, respectively. Only one of the compositional series reached full density at 1775 °C with cerium replacing yttrium, whereas at 1825 °C all compositional series except one became dense. The samples sintered showed that yttrium but not cerium stabilizes the α sialon phase in these ceramics. The dense cerium-sialon ceramics sintered at 1825 °C have as good hardness and indentation fracture toughness as the corresponding yttrium-sialon ceramics, or even higher for the β sialon type of materials. For the mixed α-β sialon materials the hardness decreased as the amount of a sialon phase decreased by increasing cerium-doping.