Measurement of coatings' elastic properties by mechanical methods: Part 2. Application to thermal barrier coatings

Elastic properties of a thermal barrier ceramic coating composed of an NiCoCrAIY bond coat and a ZrO₂(Y₂O₃) top coat were measured by a four-point bending rig in the temperature range 20°C–900°C. Different types of specimens (i.e., with bond coat only or with bond coat and top coat, on one side or on both sides) were employed. Test procedures were based on the theory discussed in Part 1 to enhance accuracy and to estimate confidence intervals. In particular, the method employed at high temperature was calibrated at room temperature by comparing the results with those obtained by methods with low sensitivity to layer thicknesses. For the bond coat, Young's modulus was found to be temperature independent up to about 500°C; a decreasing trend was observed above this temperature. For the top coat, a slightly temperature range examined. A possible explanation is given on the basis of phase transformation and the microstructure of the two layers. At room temperature, Poisson's ratio for the bond coat was found to be near 0.3, whereas a near zero value was measured for the top coat.     

Phase Composition of Oxidised Layers Grown on Steel Exposed to Liquid Lead at 749 K

The corrosion resistance of AISI 316 LN stainless steel was studied in contact with stagnant, oxygen-saturated liquid lead at 749 K for times up to 1200 h. The reaction products were analysed by means of optical and scanning electron microscopy, electron probe microanalysis, X-ray diffraction analysis and Mössbauer spectroscopy. Thin layers of largely variable thickness formed on the surface, mainly consisting of Fe₃O₄, with small amounts of FeO in the inner regions, Fe₂O₃ and an Fe-Pb-O ternary product in the outermost regions. The alloying elements Cr and Ni diffused to a different extent into Fe₃O₄. A mechanism is proposed to explain formation and growth of the reaction products.