On the Early Stage Isothermal Oxidation of APS CoNiCrAlY Coatings

Abstract

The aim of this study is to analyze the evolution of microstructural and room temperature mechanical properties of air plasma sprayed (APS) CoNiCrAlY coatings before and after early stage high-temperature oxidation. To this purpose, selected samples were isothermally heat treated at 1110°C for different durations. Phase analysis and oxide scale characterization were performed using x-ray diffraction. Morphological and microstructural features of as-sprayed and oxidized CoNiCrAlY coatings were analyzed by scanning electron microscopy and energy dispersive x-ray spectroscopy. After heat treatment, a duplex oxide scale, composed of an inner α-Al 2 O 3 layer and an outer spinel-type oxide layer, was observed on coating top-surface. The nanoindentation technique was employed to study the evolution of the mechanical properties. An increase in Young's modulus and hardness with increasing the aging time was observed, this effect was mainly addressed to the partial densification of coating microstructure.

On extracting mechanical properties from nanoindentation at temperatures up to 1000 °C

“…Operation in vacuum additionally prevents deleterious oxidation of the sample or the indenter tip [12,15,16]. Nanoindentation has been applied to superalloy materials [18,19] and their bond coats [20][21][22][23] with high-temperature nanoindentation recorded on CMSX-4 up to 400 • C [7] and René N4 up to 800 • C [24]. However, both works on high-temperature nanoindentation note the experimental difficulties -particular in the measurement of Young's modulus -due to creep during nanoindentation.…”

Nanomechanical behavior of yttria stabilized zirconia (YSZ) based thermal barrier coating

“…It is reported that the observed difference between the Young's modulus of TGO (preferably Al 2 O 3 ) and the bulk Al 2 O 3 is due to the presence of fine microcracks in TGO[19]. It is also reported that the Young's modulus of CoNiCrAlY is increased from 134-154 GPa to 202 -216 GPa due to thermal aging at 1100 °C for 24 h which further increases with increase in time[36]. Hence, the Young's modulus ranging from 174 to 193 GPa is contributed by the presence of CoNiCrAlY phase.…”

Morphology and Microstructure of NiCoCrAlYRe Coatings after Thermal Aging and Growth of an Al2O3-Rich Oxide Scale

“…In other words, the presence of Re in the microstructure is useful to promote the formation of a protective, well adherent, dense and quasi continuous Al2O3 layer at metal surface, so that the formation of mixed oxide scales is reduced at lower extent as well as the internal oxidation occurring at splat boundaries. Therefore, the β phase depletion and the oxidation rate of MCrAlYRe coatings are lower with respect to the conventional MCrAlY ones [6]. The addition of Re influences the morphology and the amount of the phases into the coating.…”

“…In this context, metal coatings can be successfully adopted to cover turbine components in order to improve their environmental resistance, without compromising their structural stability and mechanical strength [3][4][5]. Among them, the well-known MCrAlY (Me = Ni, Co or Ni/Co) coatings can be used as overlay coatings or in conjunction with an upper ceramic TBC [6,7].…”