In-situ measurement of mechanical properties and dimensional changes of preceramic thermosets during their pyrolysis conversion to ceramics using thermomechanical analysis

Abstract

The study deals with the thermomechanical analysis of the pyrolysis process of four structurally different types of cross-linked preceramic polymers: methylsiloxane, methyl-phenyl-siloxane, hydrosilylation cured methyl-phenyl-siloxane and phenol-formaldehyde resins. During the transformation of polymer to ceramics by pyrolysis, the investigated polymeric materials undergo significant softening and subsequent hardening, which are accompanied by considerable dimensional changes, but crosslinking of the thermoset microstructure greatly limits stress relaxation by plastic deformation. The parallel action of these processes leads to a high risk of cracking at certain temperatures, which is the biggest problem of preceramic polymer technology. To analyse these processes in detail an in-situ thermomechanical experiment using cyclic indentation method was chosen, where a flat-ended cylinder was used as an indenter. To characterize the rheological behaviour development during pyrolysis, a cyclic loading with a trapezoidal pulse with a period of 120 s was applied. Due to the risk of cracking, the experiments were performed at very low applied loads and heating rates. These experiments made it possible to obtain simultaneous temperature characteristics of elastic, viscoelastic and residual deformations as well as overall pyrolysis shrinkage. The acquired results thus provide a comprehensive insight into the gradual changes in mechanical behaviour and volume changes during the thermal transformation of thermosets. A model of the thermomechanical and rheological response on the indentation during pyrolysis was developed to analyse deeply obtained TMA data.