Thermal shock behavior and high-temperature oxidation performance of PDC-based environmental barrier coatings on AISI 441 stainless steel

The present work aimed to investigate the influence of different testing conditions on high-temperature oxidation performance of selected polymer-derived ceramic (PDC) environmental barrier coatings on AISI 441 stainless steel substrates. For this purpose, two types of double-layer PDC-based glass-ceramic coatings containing passive and glass fillers were fabricated using a combination of dip coating and spray coating processes. To evaluate the performance of the coatings in harsh environments simulating real applications, thermal cyclic tests in dry air, long-term static oxidation tests in humidified air, and thermal shock tests were performed. YAG, monoclinic ZrO2, cubic ZrO2, and a newly formed BaAl2Si2O8 phase were identified from XRD patterns of the tested PDC coatings. The SiO2 reflections, detected in the coatings after pyrolysis, were absent after both oxidation and thermal shock resistance tests. The SEM examination indicated that the performance of the studied PDC coatings was mainly affected by the microstructural modifications induced by differential sintering of the glass filler particles, leading to an expansion of existing pores, which represented preferential failure sites in the coating. In contrast, the existence of internal porosity relieved the residual stresses and reduced the driving force for crack extension, resulting in an increased thermal shock resistance. A combined scratch test based on the acoustic emission signal record along with the SEM observation of the resulting scratch tracks were also carried out. The most significant damage event in both tested PDC coatings was partial decohesion and surface chipping, which, however, did not translate into a critical adhesive failure. These findings confirm that the developed PDC coatings can be used for the protection of stainless steel in oxidizing environments up to 900 °C.