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RESEARCH PRODUCT

New methods for the determination of local residual stresses

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Splat solidification in thermal sprayed coatings relies on phenomena of various origins, time constants, and dimensional scales which altogether lead to complex residual stress states. As a result, in-service performance may be under or overestimated. Hence, this work aims at characterizing residual stress fields using high resolution instruments: a Raman spectrometer and a tailor-made Near-Field Scanning Microwave Microscope (NFSMM). Investigation areas are of 1 µm and 0.020 µm diameter respectively, both techniques are non-destructive and rely on electromagnetic interaction with matter.A preliminary study was conducted on sintered trigonal Cr2O3 with NFSMM as the technique is used for the first time on a ceramic material. Electromagnetic response was analyzed regarding both signal and phase outcomes, as well as discussed in terms of depth of investigation and dispersion.Results obtained from plasma sprayed trigonal Cr2O3 on steel substrate are then presented. NFSMM mapping reveal some uniaxial gradients or/and patterns between frequencies, none of which are observed on Raman spectroscopy mapping. Through thickness results are discussed in comparison with Incremental Hole Drilling (IHD) investigation and results from the preliminary study: response from 4.5 to 10.5 GHz in terms of stress measurement is questioned. Mapping from Raman spectrometry exhibits randomly arranged stress levels which are relatively well balanced around a compressive stress state (- 180 MPa) and which dispersion is similar between coatings. This is consistent with literature and the fact that coatings exhibit high hardness values. IHD results mostly indicate compressive stress levels within the coatings whereas XRD reveals tensile stress (superior to 100 MPa). All results are discussed in terms of uncertainty, depth of investigation, and physical principle of measurement.Regarding unexpected results from NFSMM further methodological and theoretical work was engaged questioning our analysis technique and revealing magnetic organization carried by Cr3+ ions is very likely to influence electromagnetic response at microwave frequencies.