In this paper, we establish connections between the thresholds and mechanisms of the damage and white-light generation upon femtosecond laser irradiation of wide-bandgap transparent materials. On the example of Corning Willow glass, evolution of ablation craters, their quality, and white-light emission were studied experimentally for 130-fs, 800-nm laser pulses. The experimental results indicate co-existence of several ablation mechanisms which can be separated in time. Suppression of the phase explosion mechanism of ablation was revealed at the middle of the irradiation spots. At high laser fluences, air ionization was found to strongly influence ablation rate and quality and the main mechanisms of the influence are analysed. To gain insight into the processes triggered by laser radiation in glass, numerical simulations have been performed with accounting for the balance of laser energy absorption and its distribution/redistribution in the sample, including bremsstrahlung emission from excited free-electron plasma. The simulations have shown an insignificant role of avalanche ionization at such short durations of laser pulses while pointing to high average energy of electrons up to several dozens of eV. At multi-pulse ablation regimes, improvement of crater quality was found as compared to single/few pulses.

EPJ Web of Conferences, open-access proceedings in physics and related sciences

Amorphous silicon carbide (a-SiC) films are promising solution for functional coatings intended for harsh environment due to their superior combination of physical and chemical properties and high temperature stability. However, the structural applications are limited by its brittleness. The possible solution may be an introduction of nitrogen atoms into the SiC structure. The effect of structure and composition on tribo-mechanical properties of magnetron-sputtered a-SiC_xN_y thin films with various nitrogen content (0–40 at.%) and C/Si close to one deposited on silicon substrates were evaluated before and after exposure to high temperatures up to 1100 °C in air and vacuum. IR transmission spectroscopy revealed formation of multiple C-N bonds for the films with N content higher than 30 at.%. Improvement of the organization in the carbon phase with the increase of nitrogen content in the a-SiCN films was detected by Raman spectroscopy. Nanoindentation and scratch test point out on the beneficial effect of the nitrogen doping on the tribo-mechanical performance of a-SiC_xN_y coatings, especially for the annealed coatings. The improved fracture resistance of the SiCN films stems from the formation of triple C≡N bonds for the as deposited films and also by suppression of SiC clusters crystallization by incorporation of nitrogen atoms for annealed films. This together with higher susceptibility to oxidation of a-SiCN films impart them higher scratch and wear resistance in comparison to SiC films before as well as after the thermal exposure. The best tribo-mechanical performance in term of high hardness and sufficient level of ductility were observed for the a-Si_0.32C_0.32N_0.36 film. The enhanced performance is preserved after the thermal exposure in air (up to 1100 °C) and vacuum (up to 900 °C) atmosphere. Annealing in oxidizing atmosphere has a beneficial effect in terms of tribological properties. Harder films with lower nitrogen content suffer from higher brittleness. FIB-SEM identified film-confined cracking as the initial failure event in SiC, while it was through-interface cracking for SiCN at higher loads. This points out on the higher fracture resistance of the SiCN films where higher strains are necessary for crack formation

The scratch test, as probably the most widespread technique for assessment of the adhesive/cohesive properties of a film–substrate system, fully depends on reliable evaluation based on assessment of critical loads for systems’ failures. Traditionally used evaluation methods (depth change record and visual observation) may sometimes give misleading conclusions about the failure dynamics, especially in the case of opaque films. Therefore, there is a need for another independent evaluation technique with the potential to complete the existing approaches. The nondestructive method of acoustic emission, which detects the elastic waves emitted during film cracking and delamination, can be regarded as a convenient candidate for such a role even at nano/micro scale. The strength of the combination of microscopic observation of the residual groove and depth change record with the acoustic emission detection system proved to be a robust and reliable approach in analyzing adhesion/cohesion properties of thin films. The dynamics of the gradual damage taking place during the nano/micro scratch test revealed by the combined approach is presented for SiC and SiCN thin films. Comparison of critical load values clearly reflects the higher ability of the AE approach in detecting the initial material failure compared to the visual observation.

The tribological behaviors of NiTi alloy produced by the spark plasma sintering (SPS) method before and after annealing were investigated at various loading conditions via indentation, scratch, and wear tests. Indentation tests were performed by a pyramidal Berkovich indenter, while scratch and wear tests were carried out by a diamond Rockwell spherical indenter at room temperature. The annealing of the as-prepared samples was performed at 350, 450, and 550 °C in the ambient atmosphere. The influence of the annealing temperature on the sample behavior towards tribological study was investigated here. The results indicated that the alloy annealed at 350 °C showed a higher hardness compared to the other annealed alloys. The scratch and wear resistance of the annealed sample at 350 °C showed a better performance in comparison to the other samples under a constant load. The high wear resistance of the annealed NiTi alloy may be attributed to its metastable R-phase. The detection and evaluation of acoustic emissions during the nano/micro-tribomechanical testing proved to be a very effective way for the exploration of the durability of SPS NiTi alloys.