On the Early Stage Isothermal Oxidation of APS CoNiCrAlY Coatings

G. Di Girolamo 0 M. Alfano 0 L. Pagnotta 0 A. Taurino 0 J. Zekonyte 0 R.J.K. Wood 0 0 G. Di Girolamo, ENEA, UTTMATB, Brindisi Research Centre , Strada Statale 7 Appia, km 706, 72100 Brindisi, Italy and Department of Mechanical Engineering, University of Calabria , Ponte P. Bucci, Cubo 44C, 87036 Rende, CS, Italy ; M. Alfano and L. Pagnotta, Department of Mechanical Engineering, University of Calabria , Ponte P. Bucci, Cubo 44C, 87036 Rende, CS, Italy ; A. Taurino, Institute for Microelectronics and Microsystems, National Research Council , Via per Monteroni, 73100 Lecce, Italy ; and J. Zekonyte and R.J.K. Wood, National Centre for Advanced Tribology (nCATS), University of Southampton , South- ampton SO171BJ, UK . Contact 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 -Al2O3 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. 1. Introduction Ni-based hot section components of aircraft and land-based gas turbines typically operate in the presence of high temperature (>900-1000 C), high mechanical stresses and oxidizing environments. In addition, the presence of corrosive agents produces severe operating conditions thereby reducing the durability. To this purpose, the application of a protective coating on the surface of these components allows an increase in their lifetime. This reduces through life costs, fuel consumption and cooling air flow requirements as well as the emissions of NOx and CO into the atmosphere. A metal overlay coating, generally composed of a MCrAlY (M = Co, Ni) alloy, is usually applied by thermal spraying in order to improve high-temperature oxidation and hot-corrosion resistance of metal substrate (Ref 1, 2). It also plays a significant role on the durability of thermal barrier coatings (TBCs); indeed, a MCrAlY coating can be also employed as a bond coat in conjunction with an upper ceramic TBC, to increase the operating temperature of turbine components (Ref 3). During in-service high-temperature exposure, a thermally grown oxide (TGO) layer typically grows on the top-surface of MCrAlY coating. A uniform and dense TGO, composed of pure -Al2O3 and with a slow growth rate, may provide further protection against oxygen propagation, thus increasing component lifetime (Ref 4). In this context, the study of the microstructure and the mechanical properties of thermally sprayed MCrAlY coatings during short-time oxidation tests is essential in order to evaluate their oxidation resistance (Ref 5-8). For instance, Belzunce et al. (Ref 5) studied the evolution of the microhardness of CoNiCrAlY coatings after short oxidation cycles; in their work High Frequency Pulse Detonation (HFPD) was employed for fabrication purposes. In turn, the microstructural modifications induced by isothermal oxidation of High Velocity Oxy Fuel (HVOF) CoNiCrAlY coatings were investigated in Ref 6-8. In this case, it was emphasized that higher oxidation rate is observed in the first 10-20 h of thermal exposure and thus it has a strong effect on the oxidation rate of the following steady state stage. This suggests that the analysis of early stage oxidation behavior can provide meaningful information on coating performance. In the 1990s air plasma sprayed (APS) MCrAlY coatings have been employed for environmental protection of Ni-based components such as combustors liners, gas turbines blades and vanes (Ref 9-12). However APS CoNiCrAlY coatings have generally received limited attention in recent years for higher temperature applications because an enhanced oxidation is expected when metal particles are sprayed in standard atmospheric conditions (Ref 8, 13, 14). Indeed, since 2000 the attention of the investigators has been mainly focused on more expensive and less flexible processes, such as vacuum plasma spraying (VPS) or low-pressure plasma spraying (LPPS) and HVOF spraying, respectively (Ref 4, 6-8). Compared to these thermal spraying methods, APS allows to achieve higher productivity and efficiency by a significant reduction of capital investment for equipment and time for manufacturing. So, it is much suitable for large mass production. Moreover, little work has been published about the mechanical properties of thermally sprayed CoNiCrAlY coatings (i.e., Youngs modulus and hardness), and the data available are essentially referred to as-sprayed coatings (Ref 13-16). Therefore, the aim of this study is to investigate the evolution of microstructural and room temperature mechanical properties of APS CoNiCrAlY coatings after early stage isothermal oxidation. To this purpose, the phase composition and the microstructural features of the coatings were investigated by XRD and SEM, respectively. The mechanical properties of as-sprayed and oxidized coatings were in turn measured using the nanoindentation (NI) technique. 2. Material and Methods 2.1 Plasma Spraying The powder feedstock used for coatings fabrication was a commercially available CoNiCrAlY alloy (Amdry 995C, Sulzer Metco, Westbury, NY, USA) with 38Co-32Ni-21Cr-8Al-0.5Y (wt.%) chemical composition and a particle size distribution in the nominal range between 45 and 75 m. The coatings were deposited onto stainless steel substrates (Aisi 310S, 25 25 4 mm3) with a nominal thickness of 150 m. The APS system available at ENEA Research Centre (Brindisi, Italy), equipped with a F4-MB plasma torch (Sulzer Metco, Wolhen, Switzerland) with 6 mm internal diameter nozzle, was employed. Before plasma spraying, the substrates were grit-blasted using alumina abrasive powder (Metcolite F, Sulzer Metco, Westbury, NY, USA), to increase the surface roughness and to improve the mechanical interlocking at substrate/coating interface. They were placed on a rotating sample holder, while the spray gun traversed vertically. Plasma spraying parameters have been set with the purpose to guarantee a powder feed rate of about 50 g/min and a thickness per torch pass of about 18 m. The spraying parameters used in this study are summarized in Table 1. The deposition efficiency was calculated from the ratio between the coating mass and the total feedstock ma (...truncated)