Vincent Guipont

Residual Stress Analysis of Laser-Drilled Thermal Barrier Coatings Involving Various Bond Coats

The gas turbine combustion chamber of aero-engines requires a thermal barrier coating (TBC) by thermal spraying. Further heat protection is achieved by laser drilling of cooling holes. The residual stresses play an important role in the mechanical behaviour of TBC. It could also affect the TBC response to delamination during laser drilling. In this work, studies of the cracking behaviour after laser drilling and residual stress distribution have been achieved for different bond coats by plasma spray or cold spray. From interface crack length measured pulse-by-pulse after laser percussion drilling at 20° angle, the role of the various bond coats on crack initiation and propagation are investigated. It is shown that the bond coat drastically influences the cracking behaviour. The residual stresses profiles were also determined by the incremental hole-drilling method involving speckle interferometry. An original method was also developed to measure the residual stress profiles around a pre-drilled zone with a laser beam at 90°. The results are discussed to highlight the influence of TBCs interfaces on the resulting residual stresses distribution before laser drilling, and also to investigate the modification around the hole after laser drilling. It is shown that laser drilling could affect the residual stress state.

Cold Spray Coating of Submicronic Ceramic Particles on Poly(vinyl alcohol) in Dry and Hydrogel States

We report an approach using cold spray technology to coat poly(vinyl alcohol) (PVA) in polymer and hydrogel states with hydroxyapatite (HA). Using porous aggregated HA powder, we hypothesized that fragmentation of the powder upon cold spray could lead to formation of a ceramic coating on the surface of the PVA substrate. However, direct spraying of this powder led to complete destruction of the swollen PVA hydrogel substrate. As an alternative, HA coatings were successfully produced by spraying onto dry PVA substrates prior to swelling in water. Dense homogeneous HA coatings composed of submicron particles were obtained using rather low-energy spraying parameters (temperature 200-250xa0°C, pressure 1-3xa0MPa). Coated PVA substrates could swell in water without removal of the ceramic layer to form HA-coated hydrogels. Microscopic observations and inxa0situ measurements were used to explain how local heating and impact of sprayed aggregates induced surface roughening and strong binding of HA particles to the molten PVA substrate. Such an approach could lead to design of ceramic coatings whose roughness and crystallinity can be finely adjusted to improve interfacing with biological tissues.

New advances for laser adhesion test (lasat): From thick to thin coatings

This paper present recent developments in LAser Adhesion Test. This technique sollicitates material interfaces with laser driven shock wave. Results show the ability of the technique to do a quantitative adhesion test for wide range of material and configuration. Edge effect principle and ultra-short shock wave give perspectives for new applications for thick and thin coatings. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.This paper present recent developments in LAser Adhesion Test. This technique sollicitates material interfaces with laser driven shock wave. Results show the ability of the technique to do a quantitative adhesion test for wide range of material and configuration. Edge effect principle and ultra-short shock wave give perspectives for new applications for thick and thin coatings. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.