Caroline Richard

Laser peening processing effect on mechanical and tribological properties of rolling steel 100Cr6

This study is divided into two parts. In the first part, the influence of laser peening on the surface properties of a steel was studied. Investigative observations and comparative tests were effected on 100Cr6 rolling steel, untreated and treated by one and two passes of laser peening. Three principal parameters were analysed: surface morphology, mechanical state (residual stresses and hardness). These analyses showed that laser peening processing (LP) created a hardened layer. In the second part, the influence of the LP on the tribological behaviour of 100Cr6 rolling steel was studied. A linear rolling-sliding contact between 100Cr6 steel and a roller of 40CrMoV13 steel was used for different types of friction tests: firstly, a comparative study of the tribological behaviour of the steel untreated and treated by one and two passes of laser peening. Secondly, investigative tests were conducted under various contact pressures on steel which had been either untreated or treated by one laser peening pass. The experiments showed that there is a critical pressure which limits the protective properties of the treatment.

Animal urine as painting materials in African rock art revealed by cluster ToF-SIMS mass spectrometry imaging.

The rock art site at the village of Songo in Mali is a very important Dogon ritual place where, since the end of the nineteenth century until today, takes place the ceremony of circumcision. During these ceremonies, paintings are performed on the walls of the shelter with mainly three colors: red, black and white. Ethnological literature mentions the use of animal urine of different species such as birds, lizards or snakes as a white pigment. Urine of these animals is mainly composed of uric acid or urate salts. In this article, time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to compare uric acid, snake urine and a sample of a white pigment of a Dogon painting coming from the rock art site of Songo. ToF-SIMS measurements in both positive and negative ion modes on reference compounds and snake urine proved useful for the study of uric acid and urate salts. This method enables to identify unambiguously these compounds owing to the detection in negative ion mode of the ion corresponding to the deprotonated molecule ([M-H](-) at m/z 167.01) and its fragment ions. Moreover, the mass spectra obtained in positive ion mode permit to differentiate uric acid and urate salts on the basis of specific ions. Applying this method to the Dogon white pigments sample, we show that the sample is entirely composed of uric acid. This proves for the first time, that animal urine was used as a pigment by the Dogon. The presence of uric acid instead of urate salts as normally expected in animal urine could be explained by the preparation of the pigment for its application on the stone.

Caractérisation par émission acoustique de l'adhérence et de l'endommagement d'un revêtement : cas d'un revêtement WC-Co sur acier

Abstract This note presents a characterization method of the ‘adhesion’ of a coating by acoustic emission technique. In situ acoustic emission measurements which were investigated in terms of amplitude, absolute energy, position of the event, etc. were performed on WC–Co coated specimens prepared by HVOF (high velocity oxy fuel) during four-point bending tests. The microscopic observations of the specimens show two types of cracks: transversal cracks (regularly spaced on the coating surface) and interfacial cracks. Acoustic emission results show two different types of acoustic events in terms of absolute energy and amplitude, which are representative of two cracking mechanisms.

New trends in laser shock wave physics and applications

Recent applications for laser-induced shock waves have been demonstrated in the aeronautical and nuclear industries, due to the development of new generations of lasers that enable high cadency rates with rather small designs. In this paper, we first aim at making an overview on basic physical processes involved in Laser Shock Processing, and a presentation of pressure loadings generated by different laser conditions. In a second part, a specific focus is given to new ranges of applications like wear resistance, uniform and localized corrosion or modeling of fatigue behaviour after LSP. For instance it is demonstrated that the pitting corrosion behaviour of 316L steel in saline medium can be improved by laser-induced pure mechanical effects surrounding inclusions. It is also shown that wear rates of a 100Cr6 tool steel can be reduced after LSP provided applied pressures are kept below a material deposit threshold. Last but not least, the fatigue cracking behaviour of 2024-T351 aluminum alloy after LSP was improved and calculated through a computed program taking into account work hardening together with residual stress effects.