Interface oxide scale growth

Application to the role of the oxide/metal interface during oxide scale growth... 

Thus, if sufficiently accurate measurements can be performed on oxidized specimens, comparison between the theoretical and actual positions of these reference planes would provide a quantitative evaluation of the departure from ideal stress-free scale growth and would permit an evaluation of the effective role and the action of scale/substrate interface during scale growth and to interpret it in terms of local strains and stresses within both the oxide scale and its underlying substrate close to the scale/metal interface could become possible. 

All these possible developments could permit a better understanding of the dynamic behaviour of the scale/substrate interface during scale growth. Such an understanding could, in addition, introduce new opportunities to control the oxidation behaviour of high temperature materials and to improve their oxidation resistance. 

Pieraggi B, Rapp R A and Hirth J P (1995), Role of interface structure and interfacial defects in oxide scale growth , Oxid Met, 44(1/2), 63-79. 

As indicated earlier, protective oxide scales typically have a PBR greater than unity and are, therefore, less dense than the metal from which they have formed. As a result, the formation of protective oxides invariably results in a local volume increase, or a stress-free oxidation strain" . If lateral growth occurs, then compressive stresses can build up, and these are intensified at convex and reduced at concave interfaces by the radial displacement of the scale due to outward cation diffusion (Fig. 7.7) . 

The aforementioned requirements on surface stability are typical for all exposed areas of the metallic interconnect, as well as other metallic components in a SOFC stack (e.g., some designs use metallic frames to support the ceramic cell). In addition, the protection layer for the interconnect, or in particular the active areas that interface with electrodes and are in the path of electric current, must be electrically conductive. This conductivity requirement differentiates the interconnect protection layer from many traditional surface modifications as well as nonactive areas of interconnects and other components in SOFC stacks, where only surface stability is emphasized. While the electrical conductivity is usually dominated by their electronic conductivity, conductive oxides for protection layer applications often demonstrate a nonnegligible oxygen ion conductivity as well, which leads to scale growth beneath the protection layer. With this in mind, a high electrical conductivity is always desirable for the protection layers, along with low chromium cation and oxygen anion diffusivity. 

Critical for the coating system lifetime is the formation of a thermally grown oxide scale (TGO) at the interface BC/TBC during service, which mainly consists of alumina [3]. The possible oxygen diffusion in zirconia itself and the open columnar structure of the ceramic coating allows oxidation of bond coat aluminum. The scale s growth and the difference in... 

Alloys of Nb with small additions of Zr exhibit internal oxidation of Zr under an external scale of Nb-rich oxides. This class of alloy is somewhat different from those such as dilute Ni-Cr alloys in that the external Nb-rich scale grows at a linear, rather than parabolic rate. The kinetics of this process have been analyzed by Rapp and Colson. The analysis indicates the process should involve a diffusion-controlled internal oxidation coupled with the linear scale growth, i.e., a paralinear process. At steady state, a limiting value for the penetration of the internal zone below the scale-metal interface is predicted. Rapp and Goldberg have verified these predictions for Nb-Zr alloys. 

Nucleation considerations may dictate that the first oxide to form will have an epitaxial relationship with the substrate. This constraint will result in stress development because of the difference in lattice parameter between the metal and the oxide. This mechanism would only seem to generate significant stresses when the oxide is very thin, i.e., for short oxidation times and low oxidation temperatures. However, there are proposals that the action of intrinsic dislocations, in what amounts to a semi-coherent interface, in amuhilating vacancies during cationic scale growth can lead to sizeable stresses. However, there are also interface dislocation structures proposed, which could annihilate vacancies without generating significant stress. ... 

In common with oxide scales, halide scales can also suffer mechanical damage such as cracks, etc. In addition, the vapour pressure of the halide can be quite high, such that evaporation of the halide from the scale-gas interface can be substanhal. This leads to simultaneous formation and evaporahon of the hahde scale and has also been treated by Daniel and Rapp. The rate of thickening of a scale forming under these conditions then follows paralinear kinehcs" as described for the combined scale growth and evaporahon of chromium oxide in Chapter 4. 

All voids described so far have been formed at, or released from, the metal-oxide interface. Birchenal has discussed the formation and growth of voids within the oxide scale by condensation of vacancies. For this to occur anions must be removed in some way and he has suggested creep and slip in the anion lattice as possible mechanisms. More recently, Cox has used a sensitive porosimeter as well as special metallographic techniques to study strings of voids formed in zirconium oxide as a result of recrystallisation—the work is discussed in more detail below. Although Birchenal s model predicted greater deviations from the parabolic rate law than were in fact found, the phenomenon of pore growth within the scale seems to be real and it is as well to keep the implications for anion movement in mind. 

---