Spallation oxide-metal interface

H. E. Evans, Predicting oxide spallation from sulplnir-contaminated oxide/metal interfaces. Oxidation Metab, 79 (2013), 3 14. 

Spallation of oxide scale may occur when the elastic strain energy contained within that part of the scale is equal to the energy required to form new surfaces from the underlying oxide-metal interface (Evans, 1995) ... 

Extensive SEM studies performed on the prevailing materials after the discontinuous oxidation revealed that in respect to extent as well as the mode of scale spallation, the studied alloys could be divided into two groups, i.e. those containing Zr and/or Hf compared to MRef , -i-Si , +C (Table 7.3). In the former case a cohesive mode of spallation, which occurred within the scale prevailed, whereas in the latter case the scales mainly spalled at the oxide/metal interface, i.e. in an adhesive mode (Fig. 7.2). 

Coke deposits from a commercial ESC plant were also examined. Bulk metals concentrations were determined by ashing at 900°C prior to emission spectroscopy analysis. High levels of metals were found at the radiant coil-coke interface due, presumably, to oxide spallation, but concentrations of metals (primarily chromium, iron and titanium,totalling ca200 ppm) were also found into the bulk of the coke and at the coke-process stream interface (Table I). SXAPS examination of coke heated under vacuum for several hours at 900°C also revealed the appearance of up to 1% concentration of metals at the surface of some coke samples, and significantly chromium, iron and titanium were again the metals observed. 

Figure 8.4 presents schematic diagrams of how an oxide film may respond to a compressive stress. One possibility is for the oxide and metal to separate at the interface hy buckling of the oxide (Figure 8.4(a)), i.e. the oxide spalls. The spallation of a compressively stressed film will occur when the elastic strain energy stored in the intact film exceeds... 

The spallation of oxide scales generally involves the separation of cracked scale either by decohesion at the alloy-scale interface or by fracture on planes within the scale itself (Evans, 1995 Chan, 1997). The failure mode which predominates depends on the stress state in the scale and on the extent of adhesion at the alloy-scale interface. Evans and Lobb (1984) showed that, for scale with good adhesion and a thickness exceeding ca. 1 pm, scale-metal decohesion should occur by a wedging fracture process when the temperature drop is greater than a critical value, AT, given by ... 

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