High temperature mechanical and oxidation behavior of amorphous silicon carbonitride processed via chemical nanoprecursor route.

摘要

Polymer precursor derived ceramics have been studied since last four decades. They promise higher purity and excellent properties with lower processing temperatures in comparison to the traditional route of processing ceramics from powders. The main focus in the literature for polymer derived materials has been on the study of cross linking and pyrolysis of precursors. Relatively fewer reports are available on processing bulk components and property characterization. The polymer precursor determines the nanostructure of the resulting amorphous material and is therefore termed nanoprecursor.; In the present dissertation the processing of nanoprecursor to obtain bulk ceramics is studied, with development of an innovative process to fabricate dense defect free materials. The properties of these defect free materials are characterized. Commercially available oligo-ureamethylvinylsilazane (Ceraset™-SN) was used in the present dissertation. The pyrolysis at 1000°C in nitrogen or argon resulted in an amorphous ceramic with chemical composition SiC _0.86N_0.82. The specimens obtained by the new process are called cast specimens. Mechanical properties such as flexural strength, hardness, Young s modulus and fracture toughness were determined. The cast specimens showed much better properties as compared to nanoprecursor derived ceramics processed by methods published in the literature.; Nanoprecursor derived SiCN show excellent creep resistance at 1350°C at uniaxial stresses ranging from 25–100 MPa. Though a small deformation was measured (~2% in 7 hours), careful analysis showed that this deformation was not creep deformation. The deformation was sintering like, but resulted mainly from the reduction of the activation volume of the amorphous material.; The oxidation behavior of the silicon carbonitrides was studied. The material showed passive oxidation, in air at temperature of 1350°C and exhibited parabolic kinetics. Growth rate of the oxidation scale was about a factor of 4 higher than the lowest reported value of CVD Si₃N ₄ and SiC.; Overall the nanoprecursor derived SiCN exhibits excellent thermo-mechanical properties and is a potential candidate for high temperature structural applications.