Microstructural and Thermal Evolution of Turbostratic and Crystalline hBN Using SPS With In‐Situ YAG

ABSTRACT This study presents a comparative investigation of the microstructure and thermal properties of hexagonal boron nitride (hBN) ceramics produced by spark plasma sintering (SPS) with an in‐situ formed yttrium aluminium garnet (YAG) phase. This study used both turbostratic hBN (hBN‐S), which was synthesized from boric acid (H 3 BO 3 ) and urea (CO(NH 2 ) 2 ), and well‐crystallized, commercially available hBN. The samples were sintered at different temperatures (1700°C–2000°C) with 30 wt% sintering additives consisting of 3 mol of alumina (Al 2 O 3 ) and 5 mol of yttrium oxide (Y 2 O 3 ), resulting in the in‐situ formation of the Y 3 Al 5 O 12 (YAG) phase. Comprehensive characterization via XRD, FTIR, SEM, Raman spectroscopy, and thermal analysis revealed that both the sintering temperature and the morphology of the initial powder had a significant effect on the crystallinity, grain orientation, and thermal conductivity of the hBN ceramics. The crystalline hBN samples exhibited higher densification, microhardness, and thermal conductivity compared to hBN‐S. The crystalline hBN samples exhibited YAG formation; however, residual boron oxide (B 2 O 3 ) in hBN‐S inhibited YAG crystallization, resulting in liquid‐phase flow during sintering and lower density. Nevertheless, hBN‐S exhibited a high degree of grain orientation at lower temperatures. These findings highlight the significant influence that sintering conditions, powder morphology, and sintering additives have on the texture and anisotropic thermal behavior of hBN ceramics. This is important for developing high‐performance hBN‐based ceramics for thermal management applications.