Effects of heat treatments on oxidation resistance and mechanical properties of ultra high temperature ceramic coatings

A critical point in the development of future generation space vehicles is the availability of materials able to withstand the extreme temperatures generated during orbital re-entry. UHTC (Ultra High Temperature Ceramics) materials, such as Zirconium diboride based ceramic composites, exhibit outstanding oxidation and thermal shock resistance, high emissivity and very high melting temperature: all these properties make ZrB2 based materials possible candidates in designing thermal protection systems for the hottest structures of re-entry vehicles such as wing leading edges and nose-tip. An innovative, proprietary way to deposit ZrB2 based coatings by plasma spraying has been developed. In the present work, the influence of high temperature exposure on oxidation resistance and mechanical properties has been investigated. Several coatings constituted by different amounts of SiC particles dispersed in a ZrB2 matrix were produced; some samples were tested in as sprayed conditions, other ones were submitted to a high temperature heat treatment before testing. Oxidation resistance was determined by exposing samples in air at 2073 K for 180, 1800 and 2520 s. Oxidation kinetics were analyzed and modelled. Evaluation of flexural strength and elastic modulus by means of four-points bending tests were carried out on samples showing better oxidation behaviour; mechanical tests were performed also on samples exposed at a typical operative temperature (1800 s at 2073 K). Results showed that heat treatment does not affect oxidation resistance, whilst it significantly improves mechanical properties.