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Laser ablation of ultra-high temperature ceramics

Connect no. 169

Extreme environment materials are becoming increasingly important in a number of strategically vital areas.

Ultra-high temperature ceramics (UHTCs), for example, have been identified as having an important role to play in nuclear power generation and radioactive waste storage. Hypersonic aviation, where scramjet intake and re-entry vehicle leading edge temperatures exceed 2000°C, is another important application area.

However, determining the performance of UHTCs at such temperatures is challenging and costly owing to the limited availability and complex nature of appropriate test facilities. Consequently, ready access to more affordable techniques for evaluating the properties of UHTCs, and other extreme environment materials, is desirable.

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(a) HfB2 specimen, sat on carbon block, prior to and (b) During collimated laser beam (CLB) testing at 44MW/m2, (c) Pyrometer output during CLB testing of HfB2, and examples of the surface appearance of (d) HfB2 and (e) HfB2 + 20%Sic UHTC 5mm diameter specimens after CLB testing

TWI is currently working with Imperial College London, Loughborough University and DSTL to develop the collimated laser beam (CLB) test; a rapid, cost effective means of assessing the oxidation resistance of UHTCs, such as HfB2, at temperatures in excess of 2500°C.

Initially developed by Steve Westgate (TWI), the CLB test uses a collimated (parallel sided) laser beam. Using a high brightness ytterbium (Yb) fibre laser beam, only 5mm in diameter, enables UHTC samples to be completely bathed in high heat flux laser light, ie up to 44MW/m2, enabling testing to temperatures approaching at least 4000°C.

Another key feature of the CLB test 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 beam can be released for one second or, potentially, an hour and the heat flux within that beam is controllable to within a few watts.

Microstructural changes in UHTC specimens (determined by post-test microstructural characterisation) along with in-test pyrometry (under development at TWI) can then be used to infer the maximum temperatures reached during testing. (Figure c).

The depth to which UHTC specimens are oxidised during CLB testing is measured during subsequent microstructural characterisation. However, an immediate qualitative assessment of their oxidation resistance can usually be obtained by post-test visual inspection (Figures d-e).

In ongoing work, the CLB test facilities at TWI are being modified to include the option of introducing active flows of different gases across the surface of UHTC specimens during testing to better simulate anticipated service conditions.

If you are interested in the use of lasers for the high temperature testing of materials, either for this or other applications, please contact lasers@twi.co.uk

Another key feature of the CLB test 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 beam can be released for one second or, potentially, an hour and the heat flux within that beam is controllable to within a few watts.

For more information please email:


contactus@twi.co.uk