L'abstract è  presente nell'allegato / the abstract is in the attachmen

DI FRANCIA, Elisabetta

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L'abstract è  presente nell'allegato / the abstract is in the attachmen

DI FRANCIA, ELISABETTA

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

04 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEA systematic study of laser ablation efficacy as a low-invasive cleaning technique for Cu-based alloys / DI FRANCIA,Elisabetta. - (2019 Jul 08), pp. 1-133.OriginalA systematic study of laser ablation efficacy as a low-invasive cleaning technique for Cu-based alloysPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2740996 since: 2019-07-10T09:30:49ZPolitecnico di TorinoPolitecnico di TorinoDoctoral dissertationDoctoral program in Materials Science and Technology (31th cycle)A systematic study of laser ablation efficacy as a low-invasivecleaning technique for Cu-based alloysElisabetta Di FranciaSupervisorsProf. Emma Angelini, SupervisorProf. Sabrina Grassini, Co-supervisorDr. Eng. Ruth Lahoz, Co-supervisorDoctoral Examination CommitteeDr. Esther Rebollar, Referee, Instituto de Química Física Rocasolano-CSIC, SpainDr. Nicola Emmerson, Referee, Cardiff University, UKProf. Claudio Badini, Politecnico di Torino, ItalyProf. Jean Marc Christian Tulliani, Politecnico di Torino, ItalyDr. Alessandro Re, Università degli Studi di Torino, ItalySummaryThis dissertation is focused on a systematic study of the efficacy of laser ablationas a low-invasive cleaning procedure for Cu-based alloys. On one hand, the researchwas motivated by the increasing use of lasers in our society in recent decades andin a specific and important field like Cultural Heritage conservation. On the otherhand, little information is available within the literature on laser-surface materialinteractions during the ablation process on metals and, in particular, on Cu-basedarchaeological and historical artefacts. Furthermore, no systematic studies havebeen done on both laser parameters and material composition.There are two innovation in this research. The first is the use of a near-infrared(1064 nm) pulsed laser coupled with a galvanometric scanner. The laser scanner iscoupled with a vector graphic editor, with CAD-like capability, which enables theuser to perform rapid, precise and complex surface treatments in a repeatable way.The other innovation is the preparation of artificially-corroded coupons that mimicsome of the possible corrosion scenarios found in archaeological Cu-alloys to favourthe study of laser interaction mechanisms.The main issue in laser-cleaning procedures for Cu-based materials in the Cul-tural Heritage field is the proper setting of laser parameters, in order to ablate(remove) only the unwanted materials (e.g. reactive corrosion products), leaving aprotective layer (e.g. copper oxides corrosion products) and without affecting themetallic substrate. A problem arises when both the unwanted materials and theprotective layer have similar ablation thresholds, due to the similarities in their ab-sorption values. Proper setting of both laser (i.e. power, fluence, irradiance, pulseduration) and geometrical parameters due to the scan system (i.e. interlining, scanspeed), makes it possible to ablate only the reactive corrosion products, avoidingthermal diffusivity in the metallic bulk, that could result in structural transforma-tion and even melting phenomena.Screening tests to investigate and optimise the effect of laser power, pulse dura-tion and scan speed were carried out on artificially-aged Cu-based samples. Then,the optimised parameters (in terms of best ablation without affecting the metallicsubstrate) were applied on archaeological bronze coins in order to validate the treat-ments as a low-invasive procedure.A second issue was also studied: the possibility of re-oxidation phenomena dur-ing the ablation process. To this aim, more intense laser conditions were applied inorder to force the possible laser-material interactions between the outer layers andthe oxygen present in the air. Tests were performed in a vacuum chamber filledwith synthetic air marked by the presence of 18O (a less common oxygen isotope) atatmospheric pressure. This way, if interactions with the oxygen present in the airoccurred, the re-oxidised products would be traced with the 18O isotope. Resultsshow that no abnormal amount of 18O % was detected by means of an advancedspectroscopic technique (ToF-SIMS). This phenomenon was also verified for the op-timised condition used on the coins.Results show that the scanning laser system has proven to be a powerful tool inperforming rapid, precise and complex surface treatments in a repeatable way andthat a low-invasive laser-cleaning procedure on complex stratified corrosion layers,present in archaeological Cu-based alloys, is possible. Furthermore, no substantialphenomenon of re-oxidation occurred; only slight incremental trend was found whenlaser intensity was increased, in extreme conditions far from optimal. This showsthat lasers, applied in optimised conditions on Cu-based archaeological artefacts,do not induce re-oxidation interactions at a sufficient level to be detectable by ad-vanced spectroscopic techniques. A model of the laser-surface material interactionmechanisms during the ablation process is therefore proposed.2

A systematic study of laser ablation efficacy as a low-invasive cleaning technique for Cu-based alloys

https://iris.polito.it/retrieve/handle/11583/2740996/256838/PhD-thesis-Summary-DiFrancia.pdf

A systematic study of laser ablation efficacy as a low-invasive cleaning technique for Cu-based alloys

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