Effect of cycle length on the oxidation performance of iron aluminide coatings

One of the lifetime issues of particular concern for application of iron aluminide coatings is the possible compatibility problems between Fe-Al coatings and substrates which can have substantially different coefficients of thermal expansion (CTEs). This difference could cause deformation or cracking and reduce coating lifetime. The present study has focused on the effect of cycle length (1 h vs. 100 h) on the cyclic oxidation behavior of aluminide coatings on representative commercial ferritic (Fe-9Cr-1Mo) and austenitic (type 304L stainless steel) alloys at 700°C in air with 10 vol.% H₂O. The ferritic and austenitic steel specimens were aluminized in a laboratory-scale chemical vapor deposition (CVD) reactor. Testing of the coating specimens indicated that high frequency thermal cycling (1 h cycle time) could significantly degrade the coating performance. Comparison of these results with those from similar specimens with a longer cycle time (100 h) or after isothermal exposure showed that this degradation was not due to Al loss from the coating into the substrate by interdiffusion, but most likely was caused by the thermal expansion mismatch between the coating and substrate.