A Microstructural Approach Toward the Quantification of Anomaly Bond Coat Surface Geometry Change in NiCoCrAlY Plasma-Sp
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A Microstructural Approach Toward the Quantification of Anomaly Bond Coat Surface Geometry Change in NiCoCrAlY Plasma-Sprayed Bond Coat Peiman Shahbeigi-Roodposhti1 • Eric Jordan1 • Sina Shahbazmohamadi1
Submitted: 3 April 2017 / in revised form: 16 August 2017 / Published online: 8 September 2017 Ó ASM International 2017
Abstract Three-dimensional behavior of NiCoCrAlY bond coat surface geometry change (known as rumpling) was characterized during 120 h of thermal cycling. The proposed scanning electron microscope (SEM)-based 3D imaging method allows for recording the change in both height and width at the same location during the heat treatment. Statistical analysis using both profile information [two dimensions (2D)] and surface information [three dimensions (3D)] demonstrated a typical nature of rumpling as increase in height and decrease in width. However, it also revealed an anomaly of height reduction between 40 and 80 cycles. Such behavior was further investigated by analyzing the bearing area ratio curve of the surface and attributed to filling of voids and valleys by the growth of thermally grown oxide. Keywords rumpling SEM photogrammetry thermal barrier coatings
Introduction Thermal barrier coatings (TBCs) are used in gas turbines to reduce the metal temperature for a given operating condition. The temperature drop in the TBC can be exploited by running the engine at a higher firing temperature leading to greater fuel efficiency and/or increased durability (Ref 1). A typical structure of a TBC consists of a ceramic topcoat (TC) (usually 7-8% yttria partially stabilized zirconia
& Peiman Shahbeigi-Roodposhti [email protected] 1
Institute of Materials Science (IMS), University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA
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(YSZ)), a thermally grown aluminum oxide (TGO), a metallic bond coat and a superalloy substrate. Thermal expansion mismatch, thermal cycling and geometry change due to oxidation cause slabs of coating to spall off (or rumples) (Ref 1, 2). Rumpling behavior of platinum modified NiAl bond coats is well recognized and deemed an important contributor to TBC failure for such systems (Ref 3-16). Different methods have been used to observe and quantify the rumpling in these systems. The methods include image analysis of series of SEM micrographs of the cross sections of TBCs (Ref 3, 5-8, 11), use of optical profilometers (Ref 4, 9, 13) and three-dimensional (3D) optical interferometers (Ref 11, 14, 16). Also, several parameters have been introduced to quantify the rumpling such as root mean square (RMS) roughness, line tortuosity, true area-to-projected area ratio and average wavelength of surface undulation (Ref 4, 5, 7, 9, 16). In this study, however, it is aimed to characterize the NiCoCrAlY bond coats with radically different surface morphology. The existence of rumpling, as a major contributor to failure in MCrAlY systems, is less well established, but has been reported (Ref 17-25). Jordan et al. (Ref 17) first investigated the bond coat rum
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