Scale spallation

In the present analysis, large-scale spallations of alumina scales on NiAl were always connected with the presence of interfacial voids. In the absence of large voids and for small strain rate, it was found that the scale was well adherent under compressive substrate deformation even for large specimen deformations (cf. Fig. 7a).This suggests the presence of an effective stress relief mechanism by a slight scale wrinkling or by oxide Coble creep. 

Fig. 2. Total weight change (sample + spalled oxide) during lOOh cycles at 1200°C for p-NiAl and Fe3Al, with and without a Zr alloy addition. p-NiAl + Zr shows almost no scale spallation during 10 cycles, while the other alloys show significant spallation.

Fig. 4. SEM secondary electron plan views of the a-Al203 scale formed after oxidation in 1 atm 02 for lOOh at 1200°C on (a) Fe-28Al-5Cr-0.1Zr (FAL) and (b) Ni-50Al-0.04at%Zr.Some scale spallation is observed on FAL but not on Zr-doped [3-NiAl.

Fig. 14. Total weight change (sample + spalled oxide) during 100h cycles at 1300°C for several materials. As at 1200°C, the ODS Fe3Al outperformed cast Fe3Al + Zr but showed more scale spallation than ODS FeCrAl or P-NiAl+Zr.

The corrosion attack increases with increasing temperature, too. In Fe-A110-Cr2 the depth of internal corrosion after 82 cycles was increased from 2 p.m at 650°C to about 15 p.m at 850°C. Significant scale spallation was not detected under any of the test con-... 

The oxidation behaviour of TiAl at 900°C and 1000°C (Figs. 2 and 3) was characterised by increasing attack with increasing temperature, as expected by protective type kinetics (rate decreasing with increasing time) initially but then after 50h at 900 C and 30 h at 1000°C, coincident with the onset of substantial scale spallation, by an increasing oxidation rate. The kinetics of cumulative spallation were similar to those of oxidation. As at 800°C the extent of oxidation of FeCrAlY coated TiAl at both... 

Deposition effects of rare earth oxides on the surfaee of iron and stainless steels have also been reported [58]. The eorrosion rate eonstants decreased significantly by the coating in corrosion tests under isothermal eonditions. In thermal cyclic conditions, protective scale spallation completely disappeared for eoated samples. The rare earth effect is more remarkable with elements located on the left part of the lanthanide series (lighter rare earths). For example, eeria coatings strongly modify the microstructure and texture of the wustite (FeO) scale formed during low pressure oxidation of pure iron [59]. Cerium is located in the wustite matrix as a CeFeOs phase which dissolves in FeO in time. 

The buckling stress. Equation (5.40), increases as the square of the scale thickness such that, for thick scales, buckling may not be feasible. In this case, shear cracks can form in the oxide and, if Equation (5.39) is satisfied, lead to scale spallation by a wedging mechaiusm, which is shown schematically in Figure 5.35(b). 

The vertical segmentation cracks improve the in-plane strain tolerance of the TBC and are at present deliberately introduced in some of the advanced industrial coatings. Contrary, delamination is highly unfavourable since large scale spallation of the TBC would terminate the service life of the component. Accordingly, any further improvement of the plasma sprayed TBCs with respect to easier segmentation and more difficult delamination requires in-depth understanding of the fracture... 

Analytical treatments for predicting the changes in the subsurface alloy composition during cyclic oxidation have been developed by Whittle (1972) and Wahl (1983) for complete scale spallation after each cycle, and by Nesbitt (1989) for partial spallation (i.e., a thin layer of scale remains on the alloy). The main purpose of these treatments was to arrive at a criterion to enable prediction of the minimum bulk solute concentration required for repeated reformation of a protective solute-oxide scale during cyclic oxidation. The most advanced treatment is that by Nesbitt (1989), who eombined a dif-... 

In agreement with Eq. (5-29), Barrett and Lowell (1975) concluded from their study of the cyclic oxidation behavior of various commercial alloys at 1150 °C that the most critical factor in minimizing scale spallation is to keep the scale as thin as possible. They reported that Cr203-forming alloys with the most complex compositions tend to spall the most owing to the formation of thicker scale products. 

The dominant scale-spallation mechanism on NiAl is the formation of interfacial voids (Giggins et al., 1974 Smialek, 1978 Hindam and Smeltzer, 1980b Pint, 1997b). Both faceted and smooth voids are shown in Fig. 6-lOf (the smaller grain imprints indicate where the substrate and oxide were still in contact). Because of the large amount of void formation, AI2O3 scales on NiAl are not adherent when the scale thickness exceeds ca. 1 pm. 

Figure 6-11. Specimen weight change during 1-h cycles at 1150 °C for a variety of Ni-Al alloys (at.%). Weight losses indicate scale spallation. Near stoichiometry there is very little effect on spallation for undoped alloys. Alloys of 45 at.% Al or below formed more Ni-rich oxides and a larger amount of spallation was observed. Addition of Hf and Pt significantly improved scale adhesion during this test (data from Pint and Wright (1998) and Pint etal. (1998a)).

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