Thermo-ablative properties of polymer-derived ceramic matrix composites for aerospace applications

Zancan, Elia (2025). Thermo-ablative properties of polymer-derived ceramic matrix composites for aerospace applications. University of Birmingham. Ph.D.

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Abstract

The interest around polymer-derived ceramics (PDCs) and their extensive flexibility has been steadily growing for the past 30 years. A potential utilisation only explored to a limited extent thus far is their potential as a matrix material in ceramic-based composites for ultra-high temperature
(UHT) applications such as hypersonic flight. Si-based preceramic precursors can achieve simultaneously matrix densification and the introduction of a silica-forming phase for oxidation protection, which in traditionally studied materials is typically achieved through powder SiC. The employment of PDCs offer advantages such as low processing temperatures, large shape flexibility, and fine control of the microstructure through both the precursor’s chemistry and the ceramisation procedure. The latter includes the option of introducing transition metals in the polymeric structure for tuning various properties, especially increasing the high temperature stability of the ceramic itself and improving the stability of the oxide scale developed in high temperature oxidative environments. The main issue associated with the infiltration of polymeric precursors is the volume loss that occurs during the conversion to ceramic, which translates to an iterative process, called polymer-infiltration and pyrolysis (PIP), required to sufficiently reduce the open porosity.
In this project, carbon fibre-reinforced compositeswere manufactured starting from ZrB2 powders-containing green bodies densified via repeated infiltration and pyrolysis cycles of a SiCN precursor. The first objective was obtaining a production process capable of allowing direct control of the output parameters, particularly the composite’s phases content, while maintaining the number of PIP cycles at a minimum. This is necessary both to reduce the overall manufacturing process length and also to limit the polymer-derived content as a too large fraction of glass-forming phase is detrimental for the behaviour in dynamic oxidative conditions. The second target was determining the effect of the addition of a limited amount of hafnium to the preceramic polymer on the oxide layer stability. The performance of ZrB2-based materials was evaluated both under static conditions at 1400–1600°C and under a subsonic ablative flame above 2000°C. Monolithic samples were used for isothermal static oxidation to study the microstructure evolution at different temperatures and exposure times (1 min, 1 h, 20 h) and to select the optimal Hf content to be used in composite manufacture. All compositions showed limited long term oxidation resistance with a continuous mass increase experienced across the entire test time. The best behaviour was displayed by the lowest dopant content corresponding to a Hf:Si molar content of 0.05 with a 7.8% mass increase over 20 hours. The composite materials were then oxidised with oxypropane and oxyacetylene torches at Mach 0.6 showing promising ablation resistance capability with negligible crater formation after 60 s despite the low diboride fraction and high carbon content. However, the addition of a limited Hf content did not produce any relevant change in the performances in torch tests.
An easy fabrication composite process capable of delivering repeatable results was obtained, showing excellent phase distribution and requiring only 4 total re-infiltration cycles to reduce the porosity below 10 vol.%, which is the normal target for iterative PIP densification processes. Although
spallation of the oxide layer occurred owing to the low ZrB2 content and the relatively large gas evolution from C and SiCN oxidation, the last iteration of composites showed solid ablation performances with no visible crater and excellent thermal stability at 2500°C, showing potential
for sustaining even harsher conditions. The lowest Hf content proved to be optimal for the static oxidation resistance but had no relevant impact during the ablation testing.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):