Chromia

About: Chromia is a research topic. Over the lifetime, 1860 publications have been published within this topic receiving 39167 citations.

TGO growth behaviour in TBCs with APS and HVOF bond coats

Abstract: The growth of thermally grown oxide (TGO) layers and their influence on crack formation were studied for two thermal barrier coating (TBC) systems with CoNiCrAlY bond coats produced by (i) air plasma spray (APS) and (ii) high-velocity oxy-fuel (HVOF) techniques. All samples received a vacuum heat treatment and were subsequently subjected to thermal cycling in air. The TGOs were predominantly comprised of layered alumina, along with some oxide clusters of chromia, spinel and nickel oxide. However, after extended oxidation, the alumina layer formed in the APS-CoNiCrAlY bond coat transformed to chromia/spinel, while that formed in the HVOF-CoNiCrAlY bond coat remained stable. TGO thickening in the APS-CoNiCrAlY bond coat generally exhibited a three-stage growth behavior, which resembles a high temperature creep curve, whereas growth of the alumina layer in the HVOF-CoNiCrAlY bond coat showed an extended steady-state stage. Crack propagation in these two TBCs was found to be related to the growth and coalescence of oxide-induced cracking, connecting with pre-existing discontinuities in the topcoat. Hence, crack propagation during thermal cycling appeared to be controlled by TGO growth.

Evidence for Chromium Evaporation Influencing the Oxidation of 304L: The Effect of Temperature and Flow Rate

Abstract: The influence of temperature and flow rate on the oxidation of 304L steel in O2/H2O mixtures was investigated. Polished samples were isothermally exposed to dry O2 and O2+40% H2O at 500–800°C at 0.02–13 cm/sec flow velocity, for 168 hr. The samples were analyzed by gravimetry, XRD, ESEM/EDX, and AES depth profiling. The oxidation of 304L in water vapor/oxygen mixtures at 500–800°C is strongly influenced by chromium evaporation. The loss of chromium tends to convert the protective chromia-rich oxide initially formed into a poorly protective, iron-rich oxide. The rate of oxidation depends on flow rate; high flow rates result in an early breakdown of the protective oxide. The most rapid breakdown of the protective oxide occurs at the highest temperature (800°C) and the highest gas flow (4000 ml/min=13 cm/sec). The oxide formed close to grain boundaries in the metal is more protective, while other parts, grain surfaces suffer breakaway corrosion. The protective oxide consists of a Cr-rich 50–200-nm thick M2O3 film, while the parts experiencing breakaway corrosion form a 10–30-μm thick Fe-rich M2O3/M3O4 scale. The results show that chromium evaporation is a key process affecting the oxidation resistance of chromia formers and marginal chromia formers in O2/H2O mixtures.