Ultra high temperature testing of ceramics using a collimated laser beam

Researcher: Dr Doni Daniel

Supervisor: Professor Bill Lee

Funding: DSTL

Laser heating/melting has been used extensively to study UHTCs and other ceramics used at extreme temperatures including refractories and nuclear fuels. Laser heating/melting offers several benefits over traditional methods for studying refractory materials, e.g., ultra high temperatures are readily and rapidly achieved and large test samples are not required. The figure shows a typical set up of the laser beam test at The Welding Institute (TWI), Cambridge, UK. Using a high brightness ytterbium (Yb) fibre laser beam enables UHTC samples to be completely bathed in high heat flux laser light, i.e., up to 44 MW m^-2, enabling testing temperatures approaching at least 4000^°C. Another key feature of CLB testing is its flexibility. Over twenty specimens can be evaluated in a day and the heat flux/duration of each test can be adjusted easily to simulate a range of environments. The laser beam can be applied for one second or, potentially, an hour and the heat flux within that beam can be controllable to within a few watts. The depth to which UHTC specimens are oxidized during CLB testing can be measured during subsequent microstructural characterization. However, an immediate qualitative assessment of their high temperature oxidation resistance can usually be obtained by post-test visual inspection.

The figure shows photos of UHTC samples namely HfB₂-2 wt% La₂O₃, HfB₂-20 vol.% SiC, HfB₂-20 vol.%SiC-2 wt% La₂O₃ and ZrB₂-20 vol.%SiC-2 wt% La₂O₃ (top exposed and bottom surfaces) laser tested at 44 MW m^-2 for 1 s. It is apparent that HfB₂ / 2 wt% La₂O₃ performed much better than the other samples. Even after exposure to 44 MW m^-2 heat flux for 1 s, the sample remained in excellent condition retaining even the edges whereas the other samples were severely damaged. It is interesting to note that the surface of all samples was observed to be black or dark grey in colour. This might be attributed either to the solidification of HfB₂ on the top surface or to the formation of black oxide. Black oxide coatings can form as a result of a chemical reaction of the metal atoms, which can be formed by the reduction of oxides in low oxygen partial pressure, with an oxidizing agent such as air, aqueous solution and molten salts. The advantage of adding TaSi₂ to UHTC to form black oxide to increase the emissivity is well known.

The figure shows SEM images of cross-sections of samples laser tested at 44 MW m^-2 for 1 s. These indicate that HfB₂-2 wt% La₂O₃ performed better than the other UHTC samples, with a thin top layer thickness (100 µm) and an absence of other defects such as pores and cracks. Microstructural observation indicates that the SiC reinforcement and La₂O₃ additive which help to improve the oxidation behaviour at intermediate temperatures (1600°C) by altering the chemical composition of the top oxidised layer and intermediate layers are not helping in an extreme heat flux environment as they produce gases which further deteriorate the performance of the sample.