Neutron Diffraction Residual Strain Measurements of Molybdenum Carbide-Based Solid Oxide Fuel Cell Anode Layers with Metal Oxides on Hastelloy X

Abstract

Thermal spray deposition processes impart residual stress in layered Solid Oxide Fuel Cells (SOFC) materials and hence influence the durability and efficiency of the cell. The current study which is the first of its kind in published literature, reports results on using a neutron diffraction technique, to non-destructively evaluate the through thickness strain measurement in plasma sprayed (as-sprayed) anode layer coatings on a Hastelloy®X substrate. Through thickness neutron diffraction residual strain measurements were done on three different anode coatings (Mo-Mo 2 C/Al 2 O 3 , Mo-Mo 2 C/ ZrO 2 and Mo-Mo 2 C/TiO 2 ) using the vertical scan mode. The three anode coatings (developed through optimised process parameters) investigated had porosities as high as 20%, with thicknesses between 200 μm to 300 μm deposited on 4.76 mm thick Hastelloy®X substrate discs of 20 mm diameter. The results showed that while the through thickness residual strain in all three anodes was dissimilar for the investigated crystallographic planes, on average it was tensile. Other measurements include X-ray diffraction, nanoindentation and SEM microscopy. As the anode layer microstructures are complex (includes bi-layer alternate phases), nondestructive characterisation of residual strain, e.g. using neutron diffraction, provides a useful measure of through thickness strain profile without altering the stress field in the SOFC electrode assembly.

Diametral compression test method to analyse relative surface stresses in thermally sprayed coated and uncoated circular disc specimens

“…As observed through recent investigation ( [13], Appendix A.1), neutron diffraction residual stress values for the same coating-substrate specimen (Mo-Mo2C/Al2O3 coating on Hastelloy ® X substrate), it was observed that the difference between average residual stress (102 MPa in substrate, 41 MPa in substrate) is about 61 MPa, with about 150 MPa average stress mis-match at the interface. It can be observed that the through thickness residual strain (or stress) profile is complex and ideally it could be superimposed on the compression stress field [13]. However, such experimental data of residual strain (or stress) is not three dimensional and such superimpositions is not trivial for a three-dimensional stress field during the compression test.…”

“…The coated disc specimen was sourced from work completed previous to the investigation [13,32], as shown in Fig. 3 was assumed as 0.30 (Molybdenum Poisson's ratio) and for the substrate ( s ν ) was presumed to be 0.32 [33].…”

“…An assumption was made when calculating stress field at the surface of disc centre (to use Hooke's law for stress values, it is necessary to ensure that strain gauge locations do not undergo plastic deformation), as the strain measured was multiplied by the elastic modulus at the surface of the coating or substrate found by previous work on the same coating-substrate systems [13]. It is acknowledged that the elastic modulus of the substrate varies slightly with the coating applied [13], as shown in Fig. 4 (with average elastic modulus value of 205±82 GPa for coating).…”

“…Considering thermal spray coating formation, residual strain (or stresses) are formed within the coating and substrates due to many processes (quenching stress, peening effect, deposition temperature, lamella structure) and phase differences [13][14][15][16][17][18]. However, traditionally, as presented by Godoy et al [19], residual stresses mainly arise from two different sources: (a) shrinkage of the spray particles after solidification (primary cooling process), and (b) differences between the coating and substrate thermal expansion coefficients (secondary cooling process).…”

“…Non-destructive (laboratory X-ray, synchrotron Xray, neutron, Raman spectroscopy, digital image correlation, photoluminescence piezospectroscopy), semi-destructive (hole-drilling & ring-coring, layer removal, focused ion beam milling, indentation), and miscellaneous other (curvature, modified layer removal, material removal) approaches have been adapted to experimentally evaluate the residual stress fields in thermal spray coatings. The measured values of stress in the coating-substrate system can be sensitive to the stress measurement technique, which in turn can influence the predicted life of coated components [13][14][15][16][17][18]. However, this study will consider a diametral compression destructive testing method on thermally sprayed and uncoated circular disc specimens to compare the surface relative stresses.…”