Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: III, Comparison of Modifying Cations

Single-phase in situ toughened SiAlON ceramics containing various modifying cations and single-crystal seeds were studied. The modifying cations include rare-earth cations from the smallest to the largest allowed in the α-SiAlON structure (Yb to Y, to Nd), and from monovalent to trivalent (Li to Ca, to rare earths). At low seeding levels, the aspect ratio of grains increases with the size of modifying cations, giving rise to rather different appearances of the microstructure in different SiAlONs. A one-to-one correspondence between seed crystals and large grains at low seeding levels is also observed. An optimal amount of seeds is required to maximize the fracture toughness, which is controlled by grain pullout with the fracture energy that scales with the fraction of elongated grains, their width, and their aspect ratio. The optimal amount of seeds required to reach maximal toughening increases with the aspect ratio of grains and is the lowest (1%) in Y- and Yb-SiAlONs

Liquid-Phase Growth of Small Crystals for Seeding alpha-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

Effect of Heating Schedule on the Microstructure and Fracture Toughness of alpha-SiAlON--Cause and Solution

The effect of heating schedule on microstructure and fracture resistance has been investigated in single-phase Nd-, Y-, and Yb-α-SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phase stability on α-SiAON nucleation and the amount of transient/residual liquid during processing. The addition of 1% of α-SiAlON seeds to the starting powders nearly completely obliterates such effect, while it simultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa∙m1/2 and peak R-curve toughness of ~11 MPa∙m1/2 have been obtained for seeded Y-α-SiAlON ceramics using heating rates from 1oC/min to 25oC/min

Development of Tough Alpha-SiAlON

The development of tough α-SiAlON with elongated grains in the last five years is summarized. This progress has been guided by the improved understanding of phase relations and nucleation/growth kinetics in SiAlON ceramics. Although most α-SiAlON compositions can be processed to contain some elongated grains, their microstructure, fracture toughness and R-curve behavior vary greatly. Such variability is due to the different phase stability of α-SiAlONs and the varying physical chemistry of the competing phases, including the transient/residual liquid. For this reason, microstructure control of α-SiAlON must pay close attention to the composition, starting powder and heating schedule. Seeding with single crystals of an appropriate α-SiAlON composition provides an attractive alternative that simplifies the task of microstructure control, since such seeds are thermodynamically stable and they completely dominate the nucleation statistics. Tough and hard α-SiAlON ceramics containing Ca, Y, Nd, and Yb stabilizers have been obtained using this method, some with toughness exceeding 10 MPam1/2. The ability of maintaining a uniform microstructure of highly elongated grains is the key to high toughness material

Synthesis of alpha-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

\u3ci\u3eR\u3c/i\u3e-Curve Behavior of \u3ci\u3eIn Situ\u3c/i\u3e Toughened alpha-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∙m1/2 over a crack extension of about 1000 μm

Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: I, Y-SiAlON

The effect of seeding on the microstructure and mechanical properties of single-phase Y-α-SiAlON ceramics with elongated grains has been studied. Seeds of the intended α-SiAlON compositions but with different size, shape, and number of grains have been compared for their effects. The microstructure, resistance (R-curve) behavior, and Weibull modulus are strongly correlated to the number density of the seeds. The highest fracture toughness reached is ~12 MPa∙m1/2 and can be obtained with as little as 1% seeding. The thermodynamic stability of seeds has been examined and is attributed to their chemical composition

Formation of β-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: I, Phase, Composition, and Shape Evolutions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65704/1/j.1151-2916.2003.tb03517.x.pd