The role of the surface roughness on the integrity of thermally generated oxide scales. Application to the Al2O3/MA956 system

This work deals with the effect of the metal roughness on the integrity of thermally generated oxide scales. For illustrative purposes, experimental evidence is shown for the alumina forming MA 956 alloy. The experimental results reveal that scale spallation occurs more readily in scales with rough surfaces than in scales with smooth surfaces, preferentially at the crests of the scale profile. In order to explain this feature, the effect of the roughness of both the gas/scale and scale/metal interfaces on the thermal stress distribution was analyzed by the finite element method. This analysis shows that with increasing roughness a gradient of compression stresses develops in the scale, being the maximum value of stresses located near the gas/scale interface. In general, the higher the roughness the higher the difference between the maximum and minimum values of the stresses. However, the average value of the stress distribution through the scale thickness decreases with increasing surface roughness. The effect of a planar gas/scale and a rough scale/metal interfaces was also modelled. In this case, the stress gradient in the scale was found to monotonically increase with increasing roughness although in a lower extension than when a rough gas/scale interface was also considered. On the basis of the experimental results and the stress distribution analyses a sequence of the scale failure during the cooling stage are proposed for both cases. It is concluded that the stress component that is normal to the interface and the shear stress play a key role on the scale integrity.