Atomic force microscopy studies of the surface scale formed during oxidation of incoloy MA956

The formation of thin oxide films on {111} and {110} single crystal specimens of Fe20Cr5A1 based oxide dispersion strengthened alloy during the early stages of oxidation up to 1000°C was investigated by atomic force microscopy. The atomic force microscopy results revealed the crystalline character of a corrosion layer. The alumina scale morphology (height and grain size of crystallites) was only slightly dependent on the crystallographic texture of the underlying bulk material. The results show that atomic force microscopy has the potential to study surface structure of oxide layers in the initial stages of an oxidation process

Effect of bondcoat roughness on lifetime of APS-TBC systems in dry and wet gases

Low pressure plasma spraying (LPPS) is a process commonly used for deposition of MCrAlY (M=Ni,Co) bondcoats for air plasma spray thermal barrier coatings (APS-TBCs). LPPS produces bondcoats with a high roughness and good oxidation resistance, which are known to play a key role for long lifetimes of APS-TBC’s. An alternative process for the bondcoat deposition is high velocity oxy-fuel (HVOF), which is substantially cheaper than LPPS but even with well optimized spraying parameters generates intrinsically lower bondcoat roughness. In the present work it is shown that a bi-layer MCrAlY-bondcoat consisting of an HVOF-base layer and an upper, thin APS-flashcoat of the same chemical composition can provide cyclic oxidation TBC-lifetimes, which are similar to those obtained with well optimized LPPS bondcoats. The key points for the extended lifetime are the specific roughness profile and microstructure of the flashcoat, which allow good adhesion of the topcoat combined with an excellent oxidation resistance. Testing of the TBC-system with the APS-flashcoat in the atmosphere with increased amount of water vapour relevant for gas-turbine operation on alternative, hydrogen rich fuels revealed some lifetime shortening with respect to the drier test gas. However, even under these more aggressive conditions, the measured cyclic furnace lifetimes of samples with APS-flashcoat are a factor of 2 to 3 longer than those of the reference TBC-system with the state of the art HVOF bondcoat. Depending on the actually prevailing coating system and test conditions, the life times of the coatings were even longer than for coating systems which were completely manufactured using LPPS. In order to correlate the bondcoat roughness profile with the APS-TBC-lifetime an alternative method based on fractal analysis is proposed. Using this method, a more accurate description of complex bondcoat surface morphologies and a better correlation with the TBC-lifetime are obtained than with the commonly used mean roughness amplitude (Ra) approach

Oxidation of ODS alloys

NiCr, NiCrAl and FeCrAl based Oxide Dispersion Strengthened (ODS) high temperature alloys possess far better oxidation resistance than dispersion free alloys of the same base composition. The presence of yttria dispersions leads to excellent scale adherence and a very selective oxidation of the scale forming element. Most of the observed differences in microstructure and protective properties between oxide scales on ODS and non-ODS alloys can be explained by differences in scale transport mechanisms. Although other dispersions have been found to have a positive effect on oxidation behaviour similar to that of yttria, they seem to possess no major advantage, provided that the yttria content is properly adjusted to the actual alloy system