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.

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.

R-curves of single-phase Y-and Ca-containing ␣-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 MPa⅐m 1/2 over a crack extension of about 1000 m.

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.