Characterization report of selected RET and SPIFT samples characterized by electron microscopy and X-ray tomography

RET (rain erosion test) samples tested to failure were investigated by electron microscopy at locations with surface erosion, close to areas with surface erosion and very far from the eroded area. In all three cases, cracks were found at the peel-ply interface and at the outermost interface of the laminate. The top coating was found to contain flaky particles consisting of a compound of Si, Al and K. Cracks were seen to develop at the flaky particles close to eroded areas and evidence of degradation by spalling from the flaky particles were found. The top coating was observed to have a high porosity and microscopic cracks was found connecting the holes in the porous structure. A black interface was observed through the whole sample in both the top coating and the filler, indicating that they have been applied as several layers. The black line in the filler was observed to have a porous structure most likely caused by microscopic bubbles. SPIFT samples (single point impact fatigue tester) tested to failure had cracks close to the degraded area that were pointing directly into the top coating and no cracks at the flaky particles in the top coating. In the SPIFT sample tested with 100 impacts at 150 m/s no visual damage was observed on the surface but a crack was observed by X-ray tomography below the surface at the location of the black interface line in the filler. Based on the observations it is concluded that the RET and SPIFTS tests seem to activate different damage mechanisms in the samples. However, a more detailed study needs to be carried out to compare the two methods at all stages of the damage.