Exploration of an atomic-scale boron additive in SiAlON ceramics

SiAlON ceramics are of interest for high-risk applications such as biomedical implants and combustion engine turbines. This work is part of a larger study aimed at leveraging atomic- or molecular-scale additives to control the densification, microstructures, and ultimate structural properties of SiAlONs. Here, we investigate the effects of boron in silicon nitride-based ceramics. The present work demonstrates a possible chemical method for controlling the microstructural development of SiAlONs by incorporating boric acid (H₃BO₃) into the starting powder blend. Raman spectroscopy and ¹¹B solid-state magic angle spinning nuclear magnetic resonance cooperatively indicate that after sintering, boron exists in threefold coordination with nitrogen in the turbostratic boron nitride (t-BN) structure. The results of this work indicate that the incorporation of boron and generation of t-BN bonding in the SiAlON system result in a narrower grain size distribution, a suppression of second phases such as yttrium aluminosilicates, and ultimately, increased flexure strength. A separate fractographic study showed that SiAlONs fabricated with 3 wt% boric acid exhibited fracture origins such as subtle surface flaws or cracks, while lower dopant levels and undoped SiAlONs typically failed from flaws such as inclusions or large grains. It is argued that the modification of the intergranular glass chemistry and resulting generation of t-BN reduces atomic diffusion through the grain boundary phase and inhibits the crystallization of second phases as well as exaggerated grain growth that often characterizes the development of β′-SiAlON.