Self-Reinforced α-SiAlON Ceramics with Improved Damage Tolerance Developed by a New Processing Strategy

Alpha-SiAlON ceramics with a refined self-reinforced microstructure, i.e., containing acicular grains with dimensions much smaller than those obtained in previous studies, embedded in a matrix consisting of submicrometer-sized isotropic grains, were prepared by applying a rapid one-step sintering procedure. To suppress the overabundant formation of α-SiAlON nuclei, a combination of stabilizing cations, Y + Yb, was used; to encourage formation of acicular α-SiAlON grains, a small amount of an extra liquid (∼3 vol%) was introduced; to avoid abnormal grain coarsening resulting from dynamic ripening, the final sintering temperature was set to just slightly above the minimum temperature threshold for activating grain growth (1700°C). The fully dense compacts obtained exhibited excellent thermal-shock resistance, and hardness and fracture toughness values of 20 GPa and 5.1 MPa·m^1/2, respectively.     

Control of Microstructures in α-SiAlON Ceramics

The effects of sintering cycles and doping elements on the microstructures of Ln-α-sialon were studied. The results showed that microstructures with an elongated α-sialon morphology could be obtained through high-temperature post-heat treatment (1800–1900°C) or by prolonging soaking times during sintering. Different rare-earth elements had a profound effect on the microstructure of the resulting α-sialon. The Ln-α-sialon doped with low- Z -value elements could easily develop elongated grains with higher aspect ratio.     

Microstructure Control of In-Situ-Toughened α-SiAlON Ceramics

Single-phase α-SiAlON with elongated grains is obtained from α-Si₃N₄ powder for a broad range of compositions of practical interest. Following the concept of nucleation and growth, two-step firing is used for microstructure control. This method takes advantage of the slow transformation reaction from α-Si₃N₄ to α-SiAlON at low temperature when the composition is near the α-SiAlON phase boundary and, hence, is marginally stable. For more-stable compositions, the seeding of α-SiAlON crystals is more effective, because it allows elongated grains to grow onto the seed crystals. The fracture toughness is strongly correlated with the microstructure and is enhanced greatly in the optimized materials.     

R-Curve Behavior of In Situ Toughened α-SiAlON Ceramics

R -curves of single-phase Y- and Ca-containing α-SiAlON ceramics have been measured. They range from flat ones for fine-grain ceramics to pronounced rising ones when large elongated grains are present. The highest toughness measured reached 11.5 MPa·m^1/2 over a crack extension of about 1000 μm.     

Liquid-Phase Growth of Small Crystals for Seeding α-SiAlON Ceramics

Single-phase small crystals of Li-, Mg-, Ca-, Y-, Nd-, and Yb-α-SiAlONs have been obtained by liquid-phase sintering for various compositions and processing conditions. These crystals are suitable for seeding grain growth in α-SiAlON ceramics. The influence of chemical and processing parameters (starting composition and powders, green density, liquid content, heating schedule, nitrogen pressure, and temperature) on the size and morphology of seed crystals has been investigated. The results are compared with those for β-Si₃N₄ crystal formation, and the differences are discussed in terms of nucleation and growth kinetics during liquid-phase sintering.     

α-SiAlON Ceramics Obtained by Slip Casting and Pressureless Sintering

Dense α-SiAlON ceramics were obtained by pressureless sintering of green compacts prepared using slip casting. The rheological properties of the reaction SiAlON suspension were optimized to achieve a high degree of dispersion with a high solids volume fraction, which resulted in homogeneous and relatively dense green bodies with high sintering ability, which could be densified by pressureless sintering at 1750°C for 2 h. The sintered samples revealed a high degree of uniformity and almost fully dense microstructures that consisted of many small, elongated grains homogeneously dispersed in the fracture surfaces, which had properties comparable with those of other SiAlONs obtained using hot pressing.     

Influence of Oxygen Content on Formation of Yttrium α-SiAlON Ceramics

Low porosity α- and β-SiAION composite material was prepared when the powder mixture intended for preparation of yttrium α-SiAlON, with the formula Y_0.4Si_12–m+n Al_m+nO_nN_16–n, was attritor milled in isopropyl alcohol or contained excess oxygen (n > 0.6). The region of stability of single-phase yttrium α-SiAlON was smaller at lower temperatures. Wet milling (in isopropyl alcohol) of AIN powder was found to introduce excess oxygen into the milled powder.     

Photoluminescence of Cerium-Doped α-SiAlON Materials

Cerium-doped α-SiAlON (M _x Si_{12−( m + n )}Al _{m + n} O _n N_{16– n} ) materials have been prepared by gas-pressure sintering and post-hot-isostatic-press (HIP) annealing, using four powder mixtures of α-Si₃N₄, AlN, and either (i) CeO₂, (ii) CeO₂+ Y-α-SiAlON seed, (iii) CeO₂+ Y₂O₃, or (iv) CeO₂+ CaO. Cerium-containing CeAl(Si_{6– z} Al _z )(N_{10– z} O _z ) (JEM) phase, rather than Ce-α-SiAlON phase, forms in the sample with only CeO₂, whereas a single-phase α-SiAlON generates in samples with dual doping (CeO₂+ Y₂O₃ and CeO₂+ CaO). On ultraviolet-light excitation, JEM gives one broad emission band with maximum at 465 nm and a shoulder at 498 nm; α-SiAlON shows an intense and broad emission band that peaks at 500 nm. The unusual long-wavelength emissions in JEM and α-SiAlON are due to increases in the nephelauxetic effect and the ligand-field splitting of the 5 d band, because the coordination of Ce³⁺ in JEM and α-SiAlON is nitrogen enriched.     

Machinable α-SiAlON/BN Composites

Dense machinable α-SiAlON/BN composites were fabricated by hot-pressing using turbostratic boron nitride (tBN) obtained from nitridation of melamine diborate. The tBN was added to the starting powders, or introduced as a coating that formed in situ on α-Si₃N₄ carrier powders during nitridation, and was subsequently converted to hexagonal boron nitride (hBN) during hot pressing by solution reprecipitation. These composites maintain high strength at 1000°C and their strength/hardness are much higher than similar composites prepared using commercial hBN powder, which yielded a coarser microstructure. Good machinability was achieved despite a flat R curve.     

Synthesis of α-SiAlON Seed Crystals

Single-phase seed crystals of Ca- and Y-α-SiAlONs have been synthesized for tailoring microstructure of α-SiAlON ceramics. The influence of composition, sintering temperature, and nitrogen pressure on the size and morphology of seeds has been explored. Guidelines for α-SiAlON seed preparation and morphology control are provided.     

Experimental and Finite Element Study of the Thermal Conductivity of α-SiAlON Ceramics

The thermal conductivity of monolithic Y-Sm/α-SiAlON was evaluated using experimental data and finite element analysis. The thermal diffusivities of Y, Y-Dy, and Y-Ce/α-SiAlON ceramics were also investigated experimentally for comparison. The maximum achievable thermal conductivity of Y-Sm/α-SiAlON has been calculated by the linear extrapolation of the temperature-based experimental inverse diffusivity data and was used for the numerical calculations. Two-dimensional model microstructures were built on the base of real microstructure images and applied for calculations. Experimental data and numerical calculations were compared for Y-Sm/α-SiAlON, and it was revealed that both results are in good agreement.     

Preparation of Multication α-SiAlON Containing Strontium

The possibility of having Sr as an interstitial metal cation in α-SiAION has been investigated in two systems: a single-cation system (Si₃N₄-SrO-AlN) and a multication system (Si₃N₄-(Y₂O₃/SrO/CaO)-AlN). It was found that Sr alone does not form α-SiAlON and that Sr could only be accommodated in α -SiAION in conjunction with Y and Ca. The Sr content of α-SiAION increased as the total content of (Y + Ca) increased and appeared to reach a limit at 0.5 at.%, or 0.15 atom per α-SiAlON. Unexpectedly, some of the α-SiAlON that contained (Sr + Y + Ca) was present as laths or fibers with the c -axis perpendicular to the hot-press direction.     

Synthesis of Cerium α-SiAlON with Nuclei Addition

Cerium α-SiAlON ceramics were made from a powder mixture of Si₃N₄-AlN-CeO₂ that contained 1 wt% yttrium α-SiAlON powder. Plasma-activated sintering was used to examine the effect of the cooling rate on the formation of α-SiAlON. The formation of cerium α-SiAlON was suggested to be controlled by the nucleation at the surface of α-SiAlON nuclei, because α-phase formation could not occur without the addition of SiAlON powder. The solubility of cerium in the α-SiAlON was shown to be less than a previously predicted critical value.     

Formation of AlN-Polytypoid Phases during α-SiAlON Decomposition

Phase transformations from α– to ß–SiAlONs (i.e., from α' to ß') have been recently reported in a number of rare-earth SiAlON systems during postsintering heat treatment. In the present work, this transformation process in a Sm (α+ß)-SiAlON material is studied by using XRD, TEM, and EDS X-ray mapping techniques. It is observed that in addition to the formation of ß' and M' phases, the α'-to-ß' transformation is accompanied by a significant increase in the amount of an AlN-polytypoid phase. The results suggest that some α' phases are thermodynamically unstable at temperatures lower than the material sintering temperature and will decompose when conditions allow. For the composition studied in this work, the α-SiAlON decomposition can be described in general as α'→ß'+ M'+ AlN polytypoid.     

Effect of Seeding on the Microstructure and Mechanical Properties of α-SiAlON: II, Ca-α-SiAlON

Seeding effects on the microstructure and mechanical properties of single-phase Ca-α-SiAlON ceramics have been investigated. Whereas a small amount of seeds can transform the microstructure from one of fine equiaxed grains to one consisting of many needle-like grains, the highest fracture toughness of 8 MPa·m^1/2 is not reached until 8% seeding. This contrasts with the much higher seed efficiency in Y-SiAlON, where the peak toughness is reached at 1% seeding. The difference and the general trend of mechanical properties of seeded α-SiAlONs are discussed in terms of α-SiAlON formation and toughening mechanisms.     

Fabrication of Elongated α-SiAlON via a Reaction-Bonding Process

A reaction-bonding process, which offers low sintering shrinkage and is a low-cost process, was applied to fabricate Y–α-SiAlON ceramics. The green compacts composed of Si, Y₂O₃, Al₂O₃, and AlN were nitrided and subsequently postsintered. Dense single-phase Y–α-SiAlON with elongated grain morphology could be achieved in the specimen postsintered at 1900°C. The material exhibited high hardness (1850 HV10) and high fracture toughness (5.1 MPa·m^1/2).     

Novel Faceted α-SiAlON Micro-Crystals Prepared by Combustion Synthesis

Novel faceted α-SiAlON micro-crystals were prepared by combustion synthesis with proper additives. These crystals exhibited various end shapes, including smooth flat hexagonal faces, pyramids bounded by six small isosceles triangles, and partial pyramids with the sharp roofs removed. The formation mechanisms of different crystal shapes were discussed, and a terraced epitaxial nucleation mode for α-SiAlON was proposed. By this nucleation mode, two interesting crystal morphologies were formed: a terraced tower structure and a T-like shape. Transmission electron microscopy results showed that the fast growth direction of rod-like α-SiAlON crystals was parallel to the c -axis.     

Hollow Beads Composed of Nanosize Ca α-SiAlON Grains

Carbothermal reduction—nitridation (CRN) of SiO₂ is an attractive method to manufacture Si₃N₄ powders with controlled grain morphology. Moreover, β-SiAlON powders could also be synthesized from either pure powder mixture or some inexpensive raw minerals by CRN and the resulting powders favored the sintering of SiAlON product. However, there have been few works on preparing α-SiAlON powders so far. In this work, Ca α-SiAlON powder was synthesized by CRN of a SiO₂—Al₂O₃—CaCO₃ mixture. An unusual morphology of hollow beads 200 to 500 nm in diameter with a great deal of nanosize α-SiAlON particles around 10 to 30 nm in diameter was observed from the resultant Ca α-SiAlON powders, which has not been reported for SiAlON ceramics before.     

The Effect of Powder Mixing Procedures on α-SiAlON

Various procedures of mixing starting powders for hot-pressing α-SiAlON ceramics were studied. They included different milling methods (attrition milling, ball milling, and sonication), liquid medium (water, isopropyl alcohol, and pyridine), and atmospheres (ambient air and nitrogen). These mixing procedures resulted in markedly different densification behavior and fired ceramics. As the powders experienced increasing oxidation because of mixing, the densification temperature decreased, the amount of residual glass increased, and α-SiAlON was destabilized and replaced by β-SiAlON and AlN polytypes during hot pressing. These effects were mitigated when pyridine, nitrogen, and sonication were used. Several protocols that yielded nearly phase-pure, glass-free dense α-SiAlON were thus identified.     

Grain Growth of α-SiAlON in the Calcium-Doped System

Two calcium-doped α-SiAlON compositions (Ca_0.6Si_10.2Al_1.8−O_0.6N_15.4 and Ca_1.8Si_6.6Al_5.4O_1.8N_14.2) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from α-Si₃N₄ to α-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of α-SiAlON. Elongated α-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the α-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development.