Improved Thermally Grown Oxide Scale in Air Plasma Sprayed NiCrAlY/Nano-YSZ Coatings

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 520104, 9 pages http://dx.doi.org/10.1155/2013/520104 Research Article Improved Thermally Grown Oxide Scale in Air Plasma Sprayed NiCrAlY/Nano-YSZ Coatings Mohammadreza Daroonparvar, Muhamad Azizi Mat Yajid, Noordin Mohd Yusof, and Mohammad Sakhawat Hussain Department of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia Correspondence should be addressed to Mohammadreza Daroonparvar; Received 26 December 2012; Accepted 13 February 2013 Academic Editor: Fathallah Karimzadeh Copyright © 2013 Mohammadreza Daroonparvar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Oxidation has been considered as one of the principal disruptive factors in thermal barrier coating systems during service. So, oxidation behavior of thermal barrier coating (TBC) systems with nanostructured and microstructured YSZ coatings was investigated at 1000∘ C for 24 h, 48 h, and 120 h. Air plasma sprayed nano-YSZ coating exhibited a trimodal structure. Microstructural characterization also demonstrated an improved thermally grown oxide scale containing lower spinels in nanoTBC system after 120 h of oxidation. This phenomenon is mainly related to the unique structure of the nano-YSZ coating, which acted as a strong barrier for oxygen diffusion into the TBC system at elevated temperatures. Nearly continues but thinner Al2 O3 layer formation at the NiCrAlY/nano-YSZ interface was seen, due to lower oxygen infiltration into the system. Under this condition, spinels formation and growth on the Al2 O3 oxide scale were diminished in nano-TBC system compared to normal TBC system. 1. Introduction Conventional TBC system usually consists of yttria stabilized zirconia (YSZ) as top coat (TC), an MCrAlY (M=Ni and/or Co) oxidation-resistant metallic bond coat (BC) and Nibased superalloy as a substrate [1, 2]. Recently, air plasma sprayed nano-YSZ coatings have shown better performance than the conventional YSZ coatings [3, 4]. In this regard, nanostructured YSZ layer showed better thermal shock resistance and lower thermal diffusivity compared to that of conventional YSZ coating [4]. Oxygen transfer through the TC towards the BC would occur at elevated temperatures by microcracks and interconnected pinholes inside the TC. Therefore, an oxidized scale would be formed on the BC which is called thermally grown oxide (TGO) layer which is mainly related to the oxidation of the BC. TGO layer also plays an important role in the failure of TBC, due to the growth of the TGO layer during oxidation [1, 2, 5]. On the other hand, the TGO thickness can be increased during oxidation process which is accompanied by evolution of stresses at the BC/YSZ interface. This stress would cause the delamination of the coating at the BC/YSZ interface [6]. It has been reported that the stresses in TBC increase with a growing TGO layer [7]. Hence, the thicker TGO layer has larger stresses than the thinner one [6, 8]. Nowadays, atmospheric plasma sprayed nanozirconia coatings have been investigated by many investigators, because they provide superior properties in comparison to normal TBC (YSZ) coatings [9–11]. It can be speculated that nanostructured YSZ layer would have less pinholes and voids because of the compactness and homogeneity of the nanostructure. So, it is anticipated that nanostructured YSZ layer could considerably reduce the penetration of oxygen into the TBC system at higher temperatures. Continues but thinner Al2 O3 (TGO) layer formation at the BC/TC interface of TBC system was seen by Chen et al. [5] under low oxygen pressure condition. So, in this paper, lower oxygen activity at the BC/nano-YSZ interface can be expected during oxidation which would help nearly continues but thinner Al2 O3 (TGO) layer formation with slow growth in TBC system [5, 12]. It was also found that, spinels formation on the thicker and discontinues TGO (Al2 O3 ) layer is much more in comparison to thinner and nearly continuums TGO 2 Journal of Nanomaterials Table 1: Used materials in this paper for production of normal and nano-TBC systems. Used materials Nano-YSZ powders (granulated) Normal YSZ powders Normal NiCrAlY powders Inconel Function As top coat As top coat As bond coat As substrate Brand Nanox Powder S4007 (Inframat, USA) Metco 204 NS-G Amdry 962 738 Size range of powders 15–150 𝜇m −106 + 11 𝜇m −106 + 52 𝜇m — Nano-YSZ particles (a) (b) Figure 1: Granulated nano-YSZ powders at different magnifications: (a) 250x and (b) 50000x. layer [12]. It means that TBC system with lower TGO thickness has better oxidation behavior compared to TBC system with thicker TGO layer. A few researchers have studied TGO formation and growth at the BC/TC interface of nano-TBC systems during high temperature oxidation. Therefore, isothermal oxidation behavior of TBC systems with micro- and nano-YSZ coatings was explored at 1000∘ C for 24 h, 48 h, and 120 h. TGO growth in TBC systems was elaborated after oxidation at 1000∘ C for different times of oxidation. 2. Experimental Procedures Table 2: Parameters of air plasma spraying method. Parameters Current (A) Voltage (V) Primary gas, Ar (L/min) Secondary gas, H2 (L/min) Powder feed rate (g/min) Spray distance (cm) Normal NiCrAlY Normal YSZ Nano-YSZ 450 50 550 70 620 65 85 38 35 15 17 6 15 35 30 15 7.5 8 2.1. Used Materials and As-Sprayed TBCs. Table 1 shows used materials in this paper for production of normal and nanoTBC systems. It should be mention that the coatings were deposited through air plasma spray machine (Sulzer Metco with 3 MB gun). The surface of substrates (Inconel 738) was grit blasted with 24–50 mesh alumina grit and under a pressure of 0.28–0.32 MPa before spraying the coatings. They were then preheated at 70–100∘ C and followed by the coatings were sprayed on them. The primary and the secondary plasma gases were argon and hydrogen, respectively. In this regard, optimized parameters of air plasma spray method are listed in Table 2. 2.3. Microstructural Characterization. Surface and crosssection of the coatings before and after oxidation test were characterized by using field emission scanning electron microscopy (FESEM) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). In the meantime, in order to detect the type of formed oxide phases on the bond coat after 120 h of oxidation, XRD was conducted (Siemens-D500) by using Cu K𝛼 line generated at 40 kV and 35 mA. 2.2. Oxidation (Isothermal Oxidation) Test at 1000∘ C. Samples were put in an electrical furnace with air atmosphere at 1000∘ C for 24 h, 48 h, and 120 h. The specimens were then furnace cooled. The sample removal before c (...truncated)