Analysis of Multi-scale Mechanical Properties of Ceramic Trusses Prepared from Preceramic Polymers

To better understand the impact of complex structure on mechanical properties in additively manufactured ceramics, truss structures were 3D printed in preceramic polymer and mechanically evaluated in the pyrolyzed SiOC state. Specimens were printed using digital light processing with a siloxane polymer resin blend. Four different designs were printed: two bending-dominant Kelvin cell structures, a stretching-dominant octet structure, and a mixture of the two with geometries chosen for equivalent stiffness. Mechanical characterization was done at multiple length scales: uniaxial compression to evaluate the entire truss structure, and three-point flexure to assess individual beam elements. After pyrolysis, it was found that truss designs exhibited different shrinkages at the beam element scale despite being composed of the same preceramic polymer and exhibiting isotropic shrinkage at the macro-truss scale. This manner of nonuniform shrinkage has rarely, if ever been reported, as it is standard practice in additive manufacturing to report only bulk linear shrinkage. In uniaxial compression, Kelvin structures with thicker beams exhibited the highest strength of 10 MPa, and octet structures exhibited the lowest strength of 3.8 MPa. In beam element flexure however, the octet beams had the highest strength, 1.9 GPa, four times stronger than the Kelvin beam elements and 500 times stronger than the octet bulk structure. Lastly, the implications for interchangeable truss structures are discussed.