Α-sialon Grains Research Articles

Inhomogeneous grain growth and elongation of Dy-α sialon ceramics at temperatures above 1800°C

A series of samples with dysprosium α-sialon compositions and different amounts of sintering additives has been fabricated from α-Si 3N 4, AlN, Al 2O3 and Dy 2O 3 starting powders, using pressureless sintering (PLS) at 1800°C plus gas pressure sintering (GPS) at the same and higher temperatures under a relatively low gas pressure of 0.9 MPa N 2. The resultant α-sialon grains show significantly different features, such as regularly fine and equiaxed, elongated, and even a few extraordinarily large with high aspect ratio grains in the fine matrix, which has rarely been observed in α′ ceramics. It is suggested that the temperature strongly influences the grain morphologies of α-sialon, playing an important role particularly in the latter stage of the “nucleation-growth” process. Such a kind of microstructural morphology as mixed with equiaxed and elongated grains toughens the α-sialon ceramics and leaves them still hard as usual. Overly high temperature treatment leads to the preferential growth of a few grains which become excessively large and long, and do not significantly improve the materials toughness.

Effect of AlN-polytypoid on formation of elongated α-Sialon

The microstructures of (Y+Sm)α-Sialon with different amounts of AlN-polytypoid were studied by SEM and EDS. The results showed that with increasing addition of AlN-polytypoid, more α-Sialon grains exhibited the elongated morphology. The mechanism of forming elongated α-Sialon grains in α′-AlN-polytypoid dual-phase area was discussed.

Synthesis of .ALPHA.-SiAlON from Different Particle Size of .BETA.-Si3N4.

β-Si3N4 powders with different particle size, AlN and Y2O3 were used as starting materials to fabricate α-SiAlON by hot-pressing. Five mass% of commercial α-SiAlON powder was also added as seeds to these mixtures to increase the formation rate of α-SiAlON. No significant increase in the α-SiAlON content was found by seed addition when coarse β-Si3N4 powder was used. However, in samples sintered from fine β-Si3N4 powder, the addition of seeds promoted α-SiAlON formation and a content of 100mass% (compared to the 60mass% without seed) could be achieved. When the starting powder β-Si3N4 was coarse, large α-SiAlON and β-Si3N4 grains were obtained, the residual β-Si3N4 phase being located within the α-SiAlON grains also could be found. In principle fine-grained β-Si3N4 and α-SiAlON could both form from fine β-Si3N4 powder, residual β-Si3N4 was hardly deserved within α-SiAlON grains.

New developments in [formula omitted]-SiAlON ceramics

Two different α-sialon compositions have been densified by either pressureless sintering or capsul-free sinter HIPing using either single cation namely strontium, lanthanum, cerium, neodymium or their equimolar mixtures with calcium/ytterbium as a principal cation. The resulting materials have been heat-treated at 1450 °C for a maximum of 720 h to observe the α → β sialon transformation. Effect of starting compositions on the phase stabilities, microstructure, room temperature mechanical properties and optical transparent nature of sintered materials have been investigated. SEM observations for oxygen rich starting compositions revealed that α-sialon grains exist in needle like morphology in all the systems studied after both pressureless sintering and HIPing. Suprisingly, the nitrogen rich starting compositions give rise to coloured, optically transparent α-sialon ceramics after capsul-free HIPing. Clear evidence of α-sialon formation with incorporation of large cations namely, La 3+, Ce 3+, Nd 3+ and Sr 2+, has been established by XRD, SEM and EDX analysis. The observed phenomenon could possibly extend the usefulness of α-sialons ceramics to both structural and functional applications.

Stability of α-Sialons in Low Temperature Annealing

Phase and microstructural change due to post-sintering annealing of Nd, Dy, Y and Yb-α-sialon ceramics have been investigated for the composition of α-sialon-rare earth aluminum garnet (RAG) systems. For R= Dy, Y and Yb, only α-sialon was observed as the crystalline phase after sintering at 1750°C. After subsequent post-sintering annealing at 1450°C for 72 h, no phase transformation from α-to β-sialon was observed, although the crystallization of garnet phase at grain boundaries was detected. However, for R=Nd, α-,β-sialon and melilite phase (M’) were observed after sintering at 1750°C. The β-sialon and melilite phases increased with low-temperature heat treatment, implying that the transformation from α- to β-sialon occurred. It was concluded that the α-sialon phase stabilized by small ions like Y, Dy or Yb has high thermal stability and does not transform to β-sialon when there is no chemical reaction between α-sialon grains and the grain boundary phase.

Elongated α-sialon grains in pressureless sintered sialon ceramics

Calcium α-sialons, containing elongated grains, were developed by pressureless sintering. Preliminary electron microscopy analyses revealed the high aspect ratio of the α′ grains resulted from preferred crystal growth in the [001] direction. Such grain morphologies offer hope for in-situ toughening of α-sialon materials via the promotion of crack deflection and bridging during failure.

Sialon ceramic with gradient microstructures

In order to combine the high hardness of α-Sialon and high toughness of β-Sialon, a graded Sialon ceramic was developed in this work. The microstructures and the properties of the graded Sialon ceramic change gradually from spherical α-Sialon grains on the surface (HV10 = 19 GPa, HV0.1 = 34 GPa, and K 1c = 4.3 MPam 1/2) to elongated β-Sialon grains in the inner core (HV10 = 15.7 GPa, HV0.1 = 24 GPa and K 1c = 7.3 MPam 1/2). Mixtures of the α-Sialon grains and the β-Sialon grains were observed in the transitional zone with hardness HV0.1 = 24 ~ 30 GPa. The graded Sialon ceramic was produced in situ by using a green compact starting from a homogeneously mixed powder mixture.

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

α-SiAlON samples of the overall composition Sm xSi 12−(m+n)Al m+nO nN 16 − m, with m and n in the ranges 0.6 ≤ m ≤ 1.44 and 1.3 ≤ n ≤ 1.7 ( m = 3 x), 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 K IC -values in the range 4.0–4.7 M Pa m 1 2 .

The effect of heat-treatment on the performance of submicron SiC p-reinforced α-β sialon composites: III. Mechanical properties

Agglomerate-free SiC p-reinforced Ln-sialon (LnNd and Yb) composites were fabricated by hot-pressing and by pressureless sintering to evaluate the improvement in microstructure and mechanical properties that could be achieved by incorporating sub-micron SiC particles into a sialon matrix and then inducing α → β sialon transformation by heattreatment. Both Nd 2O 3 and Yb 2O 3 were successful in producing dense samples by hot-pressing, but Yb 2O 3 produced a more stable α-sialon phase than Nd 2O 3. As a result, Yb-sialon/SiC p composites showed higher hardness ( H v ), attributable to the higher percentage of α-sialon grains (formed in acicular morphology) in these materials. Under comparable heat-treatment conditions, the α-sialon phase present in Nd-sialon/SiC p composites was very unstable and almost completely transformed to β-sialon, with the aluminium-containing melilite phase (M′) forming at pockets in grain junctions as the dominant grain-boundary phase. Even though the Nd-densified samples contained a large proportion of β-sialon, the fracture toughness( K IC ) was not significantly improved by SiC p addition because of the decrease in aspect ratio and the coalescence of grains caused by the longer heat-treatment which resulted in degradation of mechanical properties, especially K IC .

On the extension of the α-sialon phase area in yttrium and rare-earth doped systems

The extension of the α-sialon phase area in Y, Yb, Dy and Nd doped sialon systems has been mapped out on the basis of element analysis of individual α-sialon grains. The α-sialon-forming area expands with decreasing size of the M ion in M xSi 12−(m+n)Al m+nO nN 16−n. The maximum n value, n max is 1̃.0 for M = Nd and 1̃.2 for the other dopants. The m value varies from 1̃.0 in all systems studied to a value of 2.75 in the Yb-α-sialon system, indicating that the substitution of AlO units for SiN in α-sialon is more restricted than substitution of AlN for SiN. It is shown that the lattice expansion of the α-sialon phase with increasing content of M ions is due to the substitution of AlN units for SiN, while the lattice parameters are not at all or only very slightly dependent on the ionic radius of the M ion. Based on the elemental analysis of individual α-sialon grains and the lattice parameters obtained, the following empirical relation between the sizes of the a and c axes and the m and n values of the α-sialon phase was obtained: a( A ̊ ) = 7.752 + 0.036 m + 0.02 n c( A ̊ ) = 5.620 + 0.031 m + 0.04 n

Preparation and properties of stable dysprosium-doped α-sialon ceramics

Dense ceramics with overall compositions DyxSi12-4.5xAl4.5xO1.5xN16-1.5x, where 0.2≤x≤1.0, along the Si3N4–Dy2O3·9AlN tie line were prepared by hot-pressing at 1800°C. The dysprosium-doped α-sialon phase formed in the composition range 0.3≤x≤0.7. Sintered materials of different compositions were post-heat-treated at temperatures in the range 1300–1750° C for different times and it was shown that the Dy-α-sialon phase is stable over a large temperature interval and during heat treatment times up to 30 days. Unlike corresponding neodymium- and samarium-doped α-sialons, dysprosium-doped α-sialon does not decompose into β-sialon and rare-earth-rich grain-boundary phase(s) at temperatures below 1550°C. The α-phase can coexist with a liquid phase at temperatures ≥1550°C and with the Dy-M′-phase (Dy2Si3-xAlxO3+xN4-x) at lower temperatures. When heat treated at 1450°C, any residual liquid grain-boundary phase reacted with minor amounts of the α-sialon phase and devitrified to Dy-M′-phase, yielding a glassy phase-free material. The Dy-M′-phase formed had the maximum aluminium substitution, i.e. x≈0.7. Dysprosium-doped α-sialon exhibited very high hardness (Hv10=22 GPa) and a fracture toughness of 4.5 MPa m1/2, and the hardness and toughness decreased only slightly after devitrification of the glassy phase. Some elongated α-sialon grains were formed at high x values in glassy phase-containing materials, but their presence did not affect the toughness significantly.

Plasma Etching of α‐Sialon Ceramics

Plasma etching of β‐Si3N4, α‐sialon/β‐Si3N4 and α‐sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β‐Si3N4 grains was observed. The etching rate of α‐sialon grains and of the grain‐boundary glassy phase was distinctly lower than that of β‐Si3N4 grains. The size, shape, and distribution of β‐Si3N4 grains in the α‐sialon/β‐Si3N4 composite ceramics were revealed by the present method.

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.