The effect of tape casting on the orientation degree and properties of hot-pressed hexagonal boron nitride–based ceramic materials

This study reports the fabrication of hexagonal boron nitride (h-BN)-based ceramic materials with different tape-cast layer thicknesses (50, 100 and 200 μm) via tape casting followed by hot pressing. We systematically investigated the thickness of the individual tape-cast layers influencing the microstructure, grain orientation, mechanical properties and thermal conductivity of the prepared ceramic samples. Tape casting before hot pressing significantly enhanced the crystallographic texture of the h-BN-based ceramic materials. The orientation degree, quantified by the index of preferred orientation (IOP), was highly dependent on the layer thickness. The highest orientation degree (IOP = −2931.54) was achieved with a layer thickness of 100 μm, resulting in exceptional directional thermal conductivity. The thermal conductivity in the Layer∥ direction (parallel to lamellar h-BN grains) reached 158.79 W m−1 K−1, which is 16.6-fold higher than that in the Layer⊥ direction (perpendicular to lamellar h-BN grains; 9.58 W m−1 K−1). The mechanical properties were strongly influenced by the layer thickness, primarily affecting densification and porosity. The 50-μm thick layered sample exhibited the highest mechanical performance, with a flexural strength of 127.59 ± 7.11 MPa and a fracture toughness of 3.66 ± 0.53 MPa m1/2, which is attributed to its substantially low open porosity (0.40%), ensuring effective load transfer and a high degree of grain alignment. Conversely, increasing the layer thickness to 200 μm results in debinding-related defects and high porosity, severely degrading the mechanical strength (17.27 MPa). Thus, optimising the tape-cast layer thickness is crucial for balancing the trade-off between achieving a high degree of texture for thermal conductivity and maintaining a high density to achieve high mechanical strength.