Shuba, Roman
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Publication Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: I, Y-SiAlON(2002-05-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-SunThe 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.Publication Effect of Heating Schedule on the Microstructure and Fracture Toughness of alpha-SiAlON--Cause and Solution(2002-07-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-WeiThe 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.Publication Synthesis of alpha-SiAlON Seed Crystals(2001-07-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-SunSingle-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.Publication R-Curve Behavior of In Situ Toughened alpha-SiAlON Ceramics(2001-04-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-SunR-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.Publication Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: III, Comparison of Modifying Cations(2003-07-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-WeiSingle-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.Publication Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: II, Ca-alpha-SiAlON(2002-05-01) Shuba, Roman; Chen, I-WeiSeeding 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 . m1/2 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.Publication Liquid-Phase Growth of Small Crystals for Seeding alpha-SiAlON Ceramics(2004-06-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-WeiSingle-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.