Influences of curing depth and layer thickness on forming accuracy and mechanical properties of biphasic calcium phosphate bioceramics fabricated by vat photopolymerization

Vat photopolymerization (VPP) is a high potential and efficient method for fabricating advanced ceramics. However, the selection of printing strategy importantly determines the geometrical accuracy and mechanical strength of ceramic parts prepared by VPP. In this study, the effects of the curing depth (C_d) to layer thickness (L_t) ratio and L_t on forming accuracy, microstructures, mechanical properties, surface roughness of biphasic calcium phosphate (BCP) bioceramic green bodies and sintered bodies were investigated. When the C_d/L_t ratio was smaller than 4.0, and the L_t was not over 50 μm, the green bodies presented high forming accuracy (the dimension errors ˂ 3.50 %). Inappropriate C_d/L_t and L_t caused lots of defects including different types of cracks and delamination on green bodies and sintered bodies, and impaired their mechanical strength. The optimal C_d/L_t ratio and L_t were obtained to be 4.0 and 50 μm, in which parameters there were few cracks and delamination on the sintered BCP bioceramics. The flexural strength and the fracture toughness of sintered BCP increased when the C_d/L_t ratio increased from 3.0 to 4.0. However, the flexural strength and the fracture toughness decreased when the C_d/L_t ratio further increased to 5.0. The flexural strength of sintered BCP increased at first and then decreased when the L_t increased from 25 μm to 50 μm, and the fracture toughness and hardness of BCP decreased continually. The flexural strength of sintered BCP showed a maximum value of 113.21 MPa, and the maximum fracture toughness was 0.71 MPa∙m^1/2. The elastic modulus and hardness of sintered BCP also reached 24.16 GPa and 3.04 GPa, respectively. The printing surface roughness of BCP bioceramics decreased when the C_d/L_t ratio or L_t increased. The lateral surface roughness decreased with the increase of C_d/L_t ratio, and increased with the increase of L_t. This new result provides a clear printing strategy for fabricating ceramics with high geometrical accuracy and mechanical properties by VPP technique.