The application of photoluminescence piezospectroscopy for residual stresses measurement in thermally sprayed TBCs C.R.C. Lima a,b, ⁎, S. Dosta c , J.M. Guilemany c , D.R. Clarke b a Methodist University of Piracicaba, Rod. Luis Ometto, Km 24, Sta. Bárbara d'Oeste, SP, 13451-900, Brazil b John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA c Thermal Spray Center (CPT), University of Barcelona, C/Marti y Franqués, 1, Barcelona 08028, Spain abstract article info Article history: Received 18 March 2016 Revised 16 July 2016 Accepted in revised form 28 July 2016 Available online xxxx Photoluminescence piezospectroscopy (PLPS) was used as a non-destructive technique for the measurement of residual stresses within the thermally grown oxide (TGO) layer beneath plasma-spray thermal barrier coatings (TBC). The technique has proved to be very effective for such measurements in YSZ thermal barrier coatings ap- plied by EB-PVD but its application to thermal sprayed coatings has been hindered by optical scattering in plasma sprayed coatings of usual thicknesses. PLPS experiments were performed on TBCs with cold sprayed bond coat- ings and several different ceramic layer thicknesses after thermal cycling. The results are discussed as a function of the coating characteristics like bond coat spraying process, thickness of both bond and top coat, microstructur- al features, and damage accumulation. © 2016 Elsevier B.V. All rights reserved. Keywords: Photoluminescence piezospectroscopy Residual stresses Thermal barrier coating Thermally grown oxide 1. Introduction Thermal barrier coatings (TBC) are the best way to protect compo- nents of gas turbine engines and the demand for such coatings is be- coming more important as higher temperature engines are being developed [1–3]. A TBC system generally consists of a ceramic top coat as a thermal insulator and a metallic bond coat (BC) on the underlying high-temperature alloy component [4,5]. The ceramic layer is normally 7–8% yttria partially stabilized zirconia (YSZ) [5–8] applied by atmo- spheric plasma spray (APS) or electron bean assisted physical vapor de- position (EB-PVD) [3,8]. The bond coat usually consists of either platinum modified nickel aluminide (Ni, Pt)Al (applied by electroplating and chemical vapor deposition – CVD) or a MCrAlY alloy, where M stands for Ni, Fe, Co or some combination of them. The alloys also usually include Hf, Ta or Re [8,10]. The main functions of the bond-coat alloy are to ensure good bonding between the high-tem- perature alloy component and the top coat as well as providing some oxidation and hot corrosion protection [4,5]. In use, a thin aluminum oxide scale forms on the bond-coat at its interface with the top-coat. The TBC lifetime often depends on the growth and internal stresses of this thermally grown oxide (TGO). Cracks nucleate at the thermally grown oxide and grow over the lifetime of the coating, eventually lead- ing to the coating failure [8–11]. Actually, the failure of a TBC is a com- plex phenomenon that has instigated several research works. It has been accepted that cracks can start both in the TGO and in the YSZ close to the rough TGO mainly due to the complex stress state close to the rough YSZ/TGO/bond coat interface [12–14]. The formation of a dense and uniform α-Al 2 O 3 scale is desirable due to its low oxygen diffusivity and low growth rate compared to other ox- ides. Other oxides, such as Cr and Ni oxides as well as spinels are unde- sirable due to their volume changes as they grow, and in the worst case can create protrusions contributing to the increase in local stresses and consequent failure [5,9]. The morphology, adherence and stresses in the TGO are very important issues in TBC evaluation and for life prediction. It has been observed that in EB-PVD TBC coatings, after a certain period of service life, the failure cracks typically follow the top coat/TGO inter- face. In a less extension, cracks can form at the “ridges” present on the bond coat surface before top-coat deposition [15]. For plasma sprayed TBC coatings, failure cracks propagate partly within the top coat (close to the TGO interface). It should be noted that even after coating general failure some adherent ceramic residues can be found [4,5,16–18]. Air plasma spray (APS), vacuum plasma spraying (VPS) and low- pressure plasma spray (LPPS) are the main techniques to apply bond coat onto superalloys [3,19]. These techniques provide fast manufactur- ing and a strong bonding with the part; but the main drawback is relat- ed to high temperatures that inevitably modify the coating microstructure by forming oxides. The oxide content is the principal problem for the aluminum depletion. It hinders the aluminum diffusion to the top of the BC, triggering the formation of Ni/Cr oxides and spinels in the TGO that are undesirable because of their volume change during formation. This volume change creates protrusions contributing to the increase of stresses and consequent failure. HVOF spraying has been Surface & Coatings Technology xxx (2016) xxx–xxx ⁎ Corresponding author at: Methodist University of Piracicaba, Rod. Luis Ometto, Km 24, Sta. Bárbara d'Oeste, SP, 13451-900, Brazil. E-mail address: crclima@unimep.br (C.R.C. Lima). SCT-21414; No of Pages 10 http://dx.doi.org/10.1016/j.surfcoat.2016.07.084 0257-8972/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Please cite this article as: C.R.C. Lima, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.07.084