Synthesis of SiAlON Ceramics with Molecular Precursors as Additives
New York State College of Ceramics at Alfred University. Inamori School of Engineering.
Silicon nitride (Si3N4) and its related materials have been the subjects of research for 70 years and have garnered interest for applications ranging from cutting tools and bearings to turbine blades and spinal implants. The highly covalent nature of Si3N4 lends it exceptional structural properties such as high strength and hardness, but simultaneously renders it difficult to sinter. A few techniques are employed to ameliorate this challenge. The first is the generation of a solid solution of Al and O in the Si3N4 lattice, commonly through the use of Al2O3 powder, thereby reducing the covalency of the system and resulting in what is known as a SiAlON. The second is the incorporation of liquid phase sintering additives which enable a dissolution-reprecipitation sintering mechanism but which reside at the grain boundary after cooling as a relatively low-temperature glass. The present work investigates the incorporation of additives, including Al and O, via molecular-level precursors in order to tailor the sintering, microstructural evolution, and resultant structural properties of SiAlON ceramics. The first portion of this work demonstrates the incorporation of Al and O atoms with a very fine-scale homogeneous distribution via organometallic precursor aluminum tri sec-butoxide (ASB). A combination of chemical mapping, X-ray diffraction, thermogravimetric analysis, and differential thermal analysis was employed to investigate the pyrolytic decomposition of the organometallic precursor. Rietveld refinements were performed to assess the effectiveness of solid solution formation via the molecular precursor route to SiAlONs, in direct comparison to conventional Al2O3 powder-derived SiAlONs. Homogeneous distribution of Al which persists to at least 1000°C was achieved by the deposition of the organometallic precursor on starting Si3N4 powder surfaces, with no evidence of Al2O3 particle formation. Lattice refinements revealed that for various liquid phase concentrations and dwell times, the Al-organometallic more effectively facilitated the SiAlON solid solution than did Al2O3 powder. The second portion of this work investigates the incorporation of boron into the SiAlON system via precursor boric acid (H3BO3). Inspired by ultrahigh temperature polymer-derived ceramic SiBCN, this body of work aims to assess the roles of boron in a powder-route silicon-based ceramic system in the context of bonding, structural development, and ultimate structural properties. It was found through Raman spectroscopy and 11B SS MAS-NMR that boron exists in threefold coordination with nitrogen in the turbostratic boron nitride (t-BN) structure, similarly to in SiBCN. Increasing boron concentration in resultant SiAlONs results in a decrease in the population of both residual α-Si3N4 and second phases in the grain boundary, until a single phase β'-SiAlON was obtained at 3 wt% H3BO3. The grain size distributions of resultant SiAlONs were significantly narrowed by incorporating boron. Ultimately, fracture strength was increased from ~850 MPa to >1000 MPa by incorporating 3 wt% H3BO3. Subsequent in-depth fractographic analysis indicated that fracture origins of low-boron SiAlONs were predominantly inclusions consisting of either native material or foreign material from processing. However, boron-rich SiAlONs tended to fail from more elusive, less severe surface flaws such as machining cracks. It is proposed that the incorporation of boron reduces grain boundary diffusivity, mitigating abnormal grain growth or crystallization of second phases, effectively eliminating the worst flaw population in the present SiAlONs.
Thesis completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University