Search

• The oxidation behavior of (γ'+β) nialdy coatings treated by LSP is investigated. • The isothermal oxidation resistance of the coatings is optimized by LSP. • LSP stabilizes β phase, improves TGO homogeneity and refines Al2O3 grain size. • The LSP-treated coatings undergo undesirable TGO spallation during cyclic oxidation. • LSP limits the synchronous deformation of the TGO/coating in early oxidation stage. Laser shock processing (LSP), as a novel surface modification technology, exhibits tremendous potential to improve the oxidation resistance of alloys. In this paper, (γ'+β) two-phase Ni-34Al-0.1Dy coatings were prepared by electron beam physical vapor deposition (EB-PVD) and then treated by LSP with different pulse energy (4 and 5 J). Their oxidation behavior (including isothermal oxidation and cyclic oxidation) at 1150 °C was compared. In the isothermal oxidation case, the LSP-treated samples exhibited good oxidation resistance due to high dislocation density and refined grains caused by the LSP. For the cyclic oxidation, however, the LSP-treated samples were subjected to undesirable thermally grown oxide (TGO) spallation. In the early oxidation stage (within 30 min), the residual compressive stress introduced by the LSP rapidly released, resulting in a severe plastic deformation at the coating/TGO interface. As a result, the mismatch between coating and TGO gave rise to the TGO spallation. With the increase in the oxidation time, work hardening, which continuously limited the synchronous deformation of the TGO/coating interface, became the dominant mechanism controlling the TGO spallation of 5 J-treated coatings. The continuous TGO spallation affected the Al supply and thus deteriorated the cyclic oxidation behavior. [Display omitted] [ABSTRACT FROM AUTHOR]

Laser-generated stress wave profiles with rarefaction shocks (almost zero post-peak decay times) have been uncovered in different types of glasses and presented in this communication. The rise time of the pulses was found to increase with their amplitude, with values reaching as high as 50 ns. This is in contrast to measurements in other brittle crystalline solids where pulses with rise times of 1–2 ns and post-peak decay times of 16–20 ns were recorded. The formation of rarefaction shock is attributed to the increased compressibility of glasses with increasing pressures. This was demonstrated using a one-dimensional nonlinear elastic wave propagation model in which the wave speed was taken as a function of particle velocity. The technological importance of these pulses in measuring the tensile strength of very thin film interfaces is demonstrated by using a previously developed laser spallation experiment in which a laser-generated compressive stress pulse in the substrate reflects into a tensile wave from the free surface of the film and pries off its interface at a threshold amplitude. Because of the rarefaction shock, glass-modified waves allow generation of substantially higher interfacial tensile stress amplitudes compared with those with finite post-peak decay profiles. Thus, for the first time, tensile strengths of very strong and ultra thin film interfaces can be measured. Results presented here indicate that interfaces of 185-nm-thick films, and with strengths as high as 2.7 GPa, can be measured. Thus, an important advance has been made that should allow material optimization of ultra thin layer systems that may form the basis of future MEMS-based microelectronic, mechanical and clinical devices. [Copyright &y& Elsevier]