Α-sialon Phase Research Articles

Formation behavior, microstructure and mechanical properties of multi-cation α-sialons containing calcium and neodymium

Abstract Dual modifying cation (Ca+Nd) α-sialons were hot pressed for the x=1.0 composition ((Ca,Nd)xSi12-3nAl3nOnN16-n) with mixtures of CaO:Nd2O3 in the molar ratios of 0:1, 0.3:0.7, 0.5:0.5, 0.7:0.3 and 1:0. Experimental results showed that except for pure Nd- and Ca-α-silaon compositions, the main crystalline phase in samples hot-pressed at 1750°C for 1 h was α-sialon together with a small amount of melilite solid solution and AlN polytypoid. The higher CaO:Nd2O3 ratio in the starting composition, the higher α-sialon content and the lower melilite solid solution content in the final phase assembly. The α-sialon lattice also expanded with increasing substitution of Ca for Nd. Elongated grain morphology was observed in (Ca+Nd) α-sialon. EDS analysis revealed higher solubility of Ca than Nd in the α-sialon lattice and lower solubility of Ca than Nd in the intergranular phase. The hardness increased, ranging from 15.87 to 18.79 MPa, when the amount of α-sialon phase in the material became higher. The fracture toughness varied from 4.2 to 5.1 MP am1/2 for (Ca+Nd) α-sialon compositions, in which the composition with CaO:Nd2O3=0.5:0.5 possessed the highest value.

Thermal shock resistance of α/β-sialon ceramic composites

Abstract The thermal shock properties of α/β-sialon composites have been evaluated with an indentation-quench method based on propagation of Vickers cracks. Nominally the z value of the β-phase (Si6-zAlzOzN8-z) was held constant at 0.6, and the composition of the α-sialon phase YxSi12-(m+n)Alm+nOnN16-n was x=0.33, m=1.0, n=1.2. Different α/(α+β) ratios were tested, and the amount of sintering aid (a yttrium-containing glass phase) was varied between 0 and 20 vol.%. The thermal shock resistance increases with increasing fraction of β-phase, and the presence of a glass phase also has a positive influence on the thermal shock resistance.

Implications of kinetically promoted formation of metastable α-sialon phases

α-Sialon ceramics are interesting materials, because alone or together with β-sialon they can form in-situ reinforced microstructures which offer the best combinations of strength, hardness and toughness. At temperature above 1200°C, the thermal stability of α-sialon phases has been debated since 1992, however, and it has been discussed if any α-sialon phase can be formed in Ce-, La-, Eu- and Sr-doped sialon systems. Using a novel rapid densification process named spark plasma sintering (SPS), which allows preparation of fully dense compacts of sialon ceramics within a few minutes, we here show that α-sialon phases are initially formed in these systems and that subsequent in situ and ex situ post heat-treatment results in a decomposition of the α-sialon phase. These observations show that cations with a radius larger than 1 A may stabilize the α-sialon phase, which contrasts with previous findings. The thermal stability of these α-sialon phases is strongly dependent on the kinetics of the reactions occurring when one approaches thermodynamic equilibrium. The findings might also have bearing on other sialon systems than those studied here.

Nonepitaxial heterogeneous nucleation of α-sialon in the Ca-doped system

Ca-α-sialon compacts pressureless-sintered to intermediate temperatures, which consisted of both α-sialon and unreacted α–Si3N4 grains, were investigated with transmission electron microscopy for an overall composition Ca1.8Si6.6Al5.4O1.8N14.2. Special attention was paid to identification of the possible crystallographic orientation between a-sialon and the α–Si3N4 particles. In contrast to the frequently occurring heteroepitaxial nucleation of α-sialon in rare-earth-doped samples with low x values, this study showed that most of the newly formed α-sialon grains had no epitaxial orientation relationship with the α–Si3N4 particles, suggesting nonepitaxial heterogeneous nucleation to be a more probable mechanism for the Ca–α-sialon phase with high Ca concentrations.

Transformation and thermal stability of Li-α-sialon ceramics

Two phase α/β and single phase α lithium sialons with different m and n values were produced by hot pressing at 1730–1750°C at 30 MPa for 30–40 min in a graphite resistance furnace. When the two-phase samples were heat-treated at lower (1200–1450°C) temperatures in different packing powders, an increase in the amount of α was observed, due to β-sialon in the as-sintered material reacting with grain boundary liquid to form more α. β→α transformation at low temperatures has not been reported previously in any sialon system and in the present case is believed to occur because the α-sialon phase field in the lithium sialon system shifts slightly towards the β-sialon line at lower temperatures. The thermal stability of lithium α-sialon is good in the centre of the single-phase α region when surrounded by a Li-containing powder bed. However, towards the edges of the single-phase region, compositional changes occur on heat-treatment. Thus, samples with high m, n values decompose into β-sialon plus other Li-containing phases. During heat-treatment of other compositions when surrounded by a BN powder bed, the composition of the α-sialon phase continually readjusts towards the α/β sialon phase region as a function of time and this is followed by decomposition of the α phase. Evaporation of the Li+ stabilising cation is believed to be the main reason for this behaviour. The effects of m and n value, heat treatment parameters and packing powder on the thermal stability of Li α-sialons are discussed.

Synthesis of Mg-α SiAlON powders from talc and halloysite clay minerals

Carbothermal reduction and nitridation (CRN) of SiO2 is an attractive method to manufacture Si3N4 powders with controlled grain morphology. Some inexpensive raw materials were previously used to synthesize β-sialon powder, resulting in cheaper products to be useful for some engineering applications. In this work, Mg-α sialon (MgxSi12−m−nAlm+nOnN16−n) powders were achieved by CRN of mixtures of talc (Mg3(Si2O5)2(OH)2) and NZ halloysite clay (Al2Si2O5(OH)4) minerals. The final products consisted mainly of α-sialon and β-sialon phases. Small amounts of 15R AlN-polytypoid and SiC were also identified in the synthesized powder. By adding a small amount of α-Si3N4 powder as seeds to the starting composition, the conversion rate of α-sialon phase was significantly enhanced and powders with up to 90 wt% of α-sialon were produced.

Formation behavior and microstructure of multi-cation α-sialons containing neodymium and ytterbium or yttrium

Multi-cation α-sialons containing neodymium and ytterbium, as well as neodymium and yttrium, were prepared by hot pressing at the temperatures ranging from 1550 to 1750°C for 2 h with the compositions (Nd 0.18Yb 0.18/Nd 0.18Y 0.18)Si 10.36Al 1.62O 0.54N 15.46. The densification process, reaction sequence, cell dimensions and microstructure were studied in comparison with the corresponding single cation α-sialon. Experimental results have shown that the samples hot-pressed at 1750°C mainly consist of α-sialon phase, whose content (around 95 wt%) is higher than that of counterpart single rare earth doped α-sialon, especially much higher than the one of Nd-α-sialon. On the other hand, the multi-cation α-sialons compositions are beneficial to lower the eutectic temperature in the systems, thus promoting the dissolution of intergranular crystalline melilite phase, and facilitating the precipitation of α-sialon phase. TEM and EDS revealed that the amount of Yb 3+ or Y 3+, which possess relatively smaller ionic radii than Nd 3+, is higher than that of the Nd 3+ in α-sialon grains and more Nd 3+ are remained in the grain boundary phase. Small amount of elongated α-sialon grains were observed by TEM and the preferred orientation also occurred under hot-pressing for such a low x value composition ( x=0.36) used in this study.

Phase relationships and materials design in the Ln-Si-Al-O-N system

Subsolidus phase relationships in the system Ln2O3-Si3N4-AIN-AI2O3, where Ln represents Nd, Sm and Dy, were summarized, with emphasis on the region involving α-sialon, β-sialon and AIN-polytypoid phases. This information is further used in designing the compatible matrix phases of sialon materials with desirable properties. Examples were provided to illustrate the advantage of such a basic approach to materials design.

Hard sialon ceramics reinforced with SiC nanoparticles

Abstract SiC nanoparticles were added to a range of rare-earth doped Ln-sialon (Ln=Nd, Dy, Yb) ceramics to prepare a series of increased hardness sialon/SiC p composites. The composites, of overall composition Ln 0.33 Si 9.3 Al 2.7 O 1.7 N 14.3 and containing 10 and 20 wt.% of SiC particles, were sintered by hot-pressing at 1800°C and then heat-treated at 1450°C to modify the microstructure and properties by α⇔β sialon transformation. Enhanced hardness was found in proportion to the content of SiC particles and with increasing α:β sialon phase ratio in the matrix. The kind of grain boundary phases which occurred also affected the results. In Nd-densified samples with or without SiC particles, the α-sialon phase was unstable under the heat-treatment conditions used and transformed substantially to β-sialon in elongated grain morphology, with Nd-melilite phase as the dominant grain boundary crystalline phase. These materials were tough with a highest indentation fracture toughness K 1C of 7.0 MPa m 1/2 . In contrast, Dy- and Yb-densified samples with or without SiC addition showed good stability for the α-sialon phase under the same conditions, with only a small amount of β-sialon phase produced and Dy- or Yb-garnet as the main grain boundary phase. These composites were hard and brittle with a maximum Vickers hardness HV 10 of 20 GPa.

Effects of Phase Equilibrium on the Oxidation Behavior of Rare-earth-doped a-sialon ceramics

A series of rare-earth-(RE-)doped α-sialons (RExSi12–4.5xAl4.5xO1.5xN16–1.5x, withx= 0.40 for RE = Nd, Sm, Yb, andx= 0.48 for RE = Y) were prepared and heat-treated in air at 1350 °C for 66–727 h (3–30 days), and the variations in composition and structure with time of the formed oxide scales and matrix materials were investigated. In the oxide scales of the Nd-, Sm-, and Y-containing samples a liquid was formed, apparently in (quasi-)equilibrium with the crystalline phases cristobalite and mullite, while only crystalline Yb2Si2O7, cristobalite, and mullite were observed in the Yb sample. Apparently, the liquid plays an important role in the oxidation process. In the depleted zone, located between the scale and the matrix, the liquid attacks the matrix phases, and a process takes place in which the originally formed phases dissolve and reprecipitate as more oxygen-rich phases. In the Nd- and Sm-doped systems, where the α-sialon phase is inherently metastable at 1350 °C, an extensive α → β-sialon transformation takes place, creating still more liquid. As a consequence, the oxidation resistance of α-sialons containing Nd and Sm is much lower than those containing Y and, in particular, Yb.

Effect of different rare earth cations on the densification behaviour of oxygen rich α-SiAlON composition

Five different stabilizing cations (Y, Yb, Dy, Sm, Nd) were investigated on an α-SiAlON composition, deliberately shifted to the Al 2O 3 rich area corresponding to M 0.445Si 9.998Al 2.003O 0.668N1 5.333, with respect to the reaction sequences, densification behaviour and mechanical properties. The analysis of the reaction sequences show that the formation of α-SiAlON involves the intermediate formation of aluminate, garnet, melilite and a nitrogen rich liquid containing the respective cation. The sintering rate is influenced by volume fraction of the garnet and melilite phase which varies for different cations. Dy 2O 3 shows the best densification behaviour which is followed by Sm 2O 3 and Nd 2O 3. The fracture toughness for all are comparable while the hardness value seems to decrease with the increasing ionic size of the cations due to the lower proportions of α-SiAlON phase. The present composition favours the removal of melilite from the as cooled product as well as good densification.

Characteristics of Ca- α-sialon—Phase formation, microstructure and mechanical properties

Ca-α-Sialon (Ca x Si 12-( m+n) Al m+n O n N 16-n ,) ceramics with extensive compositions on the line of Si 3N 4–CaO:3AlN (where m=2 n) ranged from x=0·3 up to x=2·0 were fabricated by hot-pressing. An exploration for Ca-α-Sialon involving reaction sequences, phase compositions, cell dimensions, microstructure and mechanical properties was carried out in the present work.

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.

Thermal Stability of Calcium α-sialon Ceramics

The lack of thermal stability of some rare earth α-SiAlON phases has recently raised serious doubts concerning the high temperature behaviour of such materials. As information on the Ca–SiAlON system is minimal this work investigates the thermal stability behaviour of Ca α-SiAlON (α ′). Compositions near the α′ plane in the Ca–Si–Al–O–N system were prepared by hot-pressing. These materials were subsequently heat treated at temperatures between 1100 and 1500°C for 24 h and for an extended duration, 200 h, at 1450°C. The addition of glass prior to hot-pressing to various compositions enabled the investigation of its effect on the initial phase assemblage and the thermal stability behaviour. It was found that neither the excess glass nor the heat treatment devitrification products, destabilised the Ca α′ phase. The presence of β′ grains appeared to facilitate the transformation behaviour of the Ca α′, however a single crystalline phase Ca α′ with a high value of m was thermally stable under extended heat treatment at 1450°C.

The effect of heat-treatment on the performance of sub-micron SiC p-reinforced α-β sialon composites: II. Heat-treatment studies

The effects of heat-treatment on α → β sialon transformation in dense, agglomerate-free Ln-sialon/ SiC p(LnNd, Yb) composites are reported. The stability of the α-sialon phase is not only dependent on the temperature and time of heat-treatment, but also on the kind of additive and the amount of SiC particles. In Nd-densified samples, the α-sialon phase is very unstable at 1450 °C and transforms almost totally to the β phase with the Nd-M (melilite) phase and NdAlO 3 as grain-boundary crystalline phases. In contrast, Yb 3−-densified samples show good stability for the α-sialon phase at 1450 °C, with only a small amount of β-sialon produced. The crystalline grain-boundary phases produced in this case are Yb-garnet, and Yb-J-phase.

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 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 .

Effect of starting composition, type of rare earth sintering additive and amount of liquid phase on αa ⇆ β sialon transformation

Five different α-β sialon and α-sialon compositions have been prepared using different rare earth oxide additives namely, neodymium, samarium, dysprosium and ytterbium. Post-sintering heat treatments have been carried out at 1450 °C for up to 720 h (one month) to observe α → β sialon transformation. A combination of X-ray and microstructural observations has shown that the high-temperature stability of the α-sialon phase depends on the starting composition, the type of rare earth sintering additives, the amount and viscosity of the liquid phase, and the presence or absence of β-sialon grains in the initial sintered material.

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.

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