Depth-profiling study of a thermal barrier coated superalloy using femtosecond laser-induced breakdown spectroscopy D.K. Das a, ⁎ , J.P. McDonald b , S.M. Yalisove b , T.M. Pollock a a Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA b Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, MI 48109, USA Received 29 May 2007; accepted 26 October 2007 Available online 12 November 2007 Abstract Depth-profiling of a thermal barrier coating (TBC) system was carried out using femtosecond laser-induced breakdown spectroscopy (fs- LIBS). The TBC system consisted of an outer 7% yttria stabilized zirconia (7YSZ) ceramic coating and an inner Pt-modified bond coat on a Ni- based superalloy single crystal substrate. In the absence of the ceramic layer, it was possible to qualitatively differentiate between the bond coat and the substrate, and also between the two layers of the bond coat. The spatial location of the interface between the two bond coat layers could also be obtained accurately from the spectral profile of W. In presence of the ceramic coating, it was also possible to easily and accurately determine the ceramic/bond coat interface from the spectral profile of Al. Potential use of fs-LIBS in combination with fs laser machining to produce superior quality cooling holes on thermal barrier coated superalloy components is discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Bond coat; Thermal barrier coating; Superalloy; LIBS (laser-induced breakdown spectroscopy); Depth-profiling; Femtosecond laser 1. Introduction Thermal barrier coatings (TBCs) are widely employed on Ni- based superalloy components of gas turbine engines for protection against extreme thermal, chemical and mechanical service conditions. A thermal barrier coating system consists of a ceramic outer layer, an underlying metallic bond coat and the Ni-based superalloy substrate. Zirconia partially stabilized by 7% yttria, often referred to as 7YSZ, has emerged as the most preferred ceramic coating material for TBC systems [1]. The YSZ coating is typically deposited by electron beam physical vapor deposition (EB-PVD) technique. A platinum-modified aluminide, also referred to as β-(Ni,Pt)Al, bond coat [2,3] is usually used in combination with the above 7YSZ coating [1]. While the outer ceramic coating protects against the heat load, the bond coat enhances the oxidation resistance of the substrate alloy. For internal cooling of the hollow thermal barrier coated components, cooling holes that extend from the external surface to the internal surface, need to be machined on these components. Machining of cooling holes using Nd-YAG pulsed lasers has been an accepted process for aero-engine components over past few decades [4]. Despite their widespread use, the holes produced by Nd-YAG lasers, which employ nanosecond (ns) and microsecond (μs) pulse widths, are often associated with deficiencies such as the formation of recast layer, high heat affected zone and micro-cracks [4,5]. Drilling of holes on thermal barrier coated components additionally causes delami- nation of the ceramic layer [5,6]. During the past decade, the development of chirped-pulse technology has led to the commercial availability of powerful and reliable femtosecond (fs) pulsed lasers. Due to the ultra-short pulse width of fs laser pulses, which is of the order of 100 fs or 100×10 - 15 s, negligible heat diffusion to the surrounding material occurs during the laser–material interaction and the material removal occurs mostly by direct ablation from solid to vapor [7,8]. Using appropriate laser conditions, melting and the consequent formation of the recast layer in the material during machining can Spectrochimica Acta Part B 63 (2008) 27 – 36 www.elsevier.com/locate/sab ⁎ Corresponding author. E-mail address: dkdas@umich.edu (D.K. Das). 0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.10.048