High temperature materials like Ni-based superalloys can be oxidized if exposed to prolonged operations in high temperatures. Oxidation in such instances can lead to failures and serious consequences. Various metallic components complete their lifetime early. To protect metallic parts in high temperature applications, thermal barrier coatings (TBCs) are the preferred choice for use as protective coatitgs, as they are good thermal insulators. Bond coats are deposited on a substrate to obtain better adhesion and to decrease thermal expansion mismatch with a top coat. TBCs also provide oxidation and corrosion resistance. Generally, NiCr, NiAl, NiPt or MCrAlY are used as bond coat materials. Top layer consists of a ceramic material that has low thermal conductivity and high temperature resistance [1,2,3,4,5,6]. In the production of a bond coat, thermal spray techniques such as atmospheric plasma spray (APS) or high velocity oxygen fuel (HVOF) are often utilized, as they offer low cost and direct applicability. In an APS technique, powders can be sprayed on a substrate as molten or semi-molten materials. All materials can be deposited using this plasma technique. APS coatings have laminar structure, porosity and oxide containing microstructures [7,8,9]. NiCr and NiCr-based coatings are used to provide resistance against corrosion, oxidation or erosion [10,11]. They were widely adopted in exhaust nozzle of rockets as bond coat material. YSZ is the preferred top coat material in TBC systems [12,13,14]. YSZ has high coefficient of thermal expansion, high fracture toughness and low thermal conductivity compared to other ceramic materials, and it can be used in temperatures up to 1200°C [15,16].
In this study, Inconel 718 was used as substrate, NiCr was sprayed as a bond coat and YSZ was deposited as a top coat. Ni-based superalloys are typically subjected to 800-1100°C operating conditions thus produced TBCs were exposed to 1000°C in furnace at varying times. Oxidation behavior of TBCs with respect to time was evaluated.
2 Experimental Procedure
2.1 TBC Sample Production and Preparation
Inconel 718 with 25.4 mm diameter and 5 mm thickness was used as a substrate. Its chemical composition mainly consisted of Ni, Fe, Cr, Nb and Mo. Substrates were sandblasted before the bond coat production. NiCr (80/20) powders (firm: GTV, 80.20.1 code, particle: -53 +20 μm,) were sprayed on Inconel 718 using APS technique. In Figure 1, images of a) NiCr and b) YSZ powders are given with their XRD analyses, respectively. NiCr powders have a spherical shape and consist of γ-Ni and NiCr phases. Similarly, YSZ powders have spherical shape, yet they consist of only t-ZrO2.
After depositing the bond coat, YSZ (ZrO2+8%Y2O3) powders (firm: Sulzer Metco 204F, particle: −45 + 15 μm) were sprayed on NiCr bond-coated substrates. Spray parameters of bond and top coat are given in Table 1. According to Table 1, there are small differences between deposition parameters due to plasma atmosphere such as used gas flow rates (standard lit per minute (slpm)), spray distance (mm) or arc current (A).
2.2 Oxidation Tests and Characterization
Image J software program was used to calculate average porosity and oxide content as percentage, information obtained from 5 different cross-sectional SEM images at 1kx magnification. In lab atmosphere, oxidation tests were performed at 1000°C for 8 h, 24 h and 50 h, using PLF 130/12 Protherm high-temperature furnace. Crosssections of oxidized TBCs were analyzed using SEM (Tescan, Maia3). XRD (Rigaku, CuKα) analysis of TBCs was performed to determine material phases. Thermally grown oxide (TGO) values were calculated using Image Pro Plus 6 software program by taking average of 10 measurements from interfaces of 10 images at 3kx magnification for each oxidation periods.
Ethical approval: The conducted research is not related to either human or animals use.
3 Results and Discussion
SEM and elemental mapping microstructures of as-sprayed TBC are shown in Figure 2. Bond coats have porosity and show presence of oxides. Top coat has porosity the same as the bond coat, which is caused by the characteristic properties of APS technique. Average oxide and porosity content of as-sprayed 100 μm-NiCr coating were 5.91% and 1,59%, respectively while as-sprayed 300 μm-YSZ coating had 5.58% porosity according to Image analysis. Elemental distribution shows that there are locally oxidized regions consisting of mainly Cr2O3 phases.
YSZ has an ionic conductivity due to the its crystal structure. At high temperature, oxygen diffuses from top coat to bond coat with affecting both, YSZ structure and existing porosities [16,17]. Cr is more reactive compared to Ni according to Ellingham diagrams . For this reason, Cr first reacts with oxygen forming protective Cr2O3 layer at the interface during this oxidation reaction. Cr2O3 layer acts as an oxygen barrier and that is why TGO layer mainly consists of Cr2O3 phase considering all oxidation periods [19,20].
In Figure 3, SEM microstructures of oxidized TBCs at 1000°C for 8 h, 24 h and 50 h are shown. TGO layer thickness increased with the increase of oxidation time. CrO3 phase can be volatilized at the temperature higher than 1000°C [l9,20,21]. However, volatilization of CrO3 was not detected with use of YSZ, as it provided thermal barrier and caused temperature drop.
At the end of oxidation periods, spallation was not observed, as evident from Figure 4. Top coat shows little densification, whereas bond coat layer shows localized oxidation. Based on elemental distributions, there was no evidence of presence of other oxides, such as NiO or NiCr2O4 which are undesired phases due to higher growth rate . This shows that NiCr is a suitable bond coat material for TBCs at 1000°C applications.
Figure 5 shows XRD and TGO growth behavior of TBCs. According to XRD results, phase transformation on the top coat did not occur at the end of the oxidation and the top coat only consisted of tetragonal ZrO2 (t-ZrO2). Semi-tetragonal ZrO2 phase is durable at temperatures up to 1173°C, therefore, phase transformation has not occurred . With increased time, TGO layer thickness increased and, with the highest increase observed in initial oxidation stage. Interestingly, TGO thickness exhibits similar growth but lower growth rate compared to CoNiCrAlY TBCs in literature [22,24] for isothermal oxidation tests at 1000°C.
However, internal oxidation rate can be higher in NiCr TBCs. Thus, results can change at higher dwell times.
Bond and top coat were successfully deposited on Inconel 718 substrate. TBCs exposed to oxidation experiments and other tested specimens withstood to oxidation periods. TGO layer consisting mainly of Cr2O3 layer preserved its uniformity. During the oxidation, YSZ did not experience phase transformation and did not result in crack formations. The thickness of TGO layer increased with an increased time. Normally, 1000°C temperature is not preferred with NiCr coating due to the volatilization of CrO3 phase. However, with the use of YSZ, a temperature drop has been achieved. As a result, the present study was able to demonstrate that a low cost NiCr bond coating combined with low cost APS technique can be used in 1000°C applications. In future studies, the effects of different production techniques on durability of high temperature NiCr TBCs will be investigated.
This investigation was financially supported by Scientific Research Projects (BAP) Coordinatorship of Karabuk University with project code of KBÜBAP-17-DR-202.
Doleker K.M., Ahlatci H., Karaoglanli A.C., Investigation of Isothermal Oxidation Behavior of Thermal Barrier Coatings (TBCs) Consisting of YSZ and Multilayered YSZ/Gd2Zr2O7 Ceramic Layers, Oxid. Met., 2017, 88(1-2), 109-119. Web of ScienceCrossrefGoogle Scholar
Clarke D.R., Materials selection guidelines for low thermal conductivity thermal barrier coatings. Surf. Coat. Tech., 2003, 163, 67-74. Google Scholar
Zhu D., Miller R.A., Development of advanced low conductivity thermal barrier coatings. Int. J. Appl. Ceram. Tec., 2004, 1(1), 86-94. Google Scholar
Liu S.H., Li C.X., Zhang H.Y., Zhang S.L., Li L., Xu P., Yang G.J., Li C.J., A novel structure of YSZ coatings by atmospheric laminar plasma spraying technology. Scripta Mater.,2018, 153, 73-76. CrossrefWeb of ScienceGoogle Scholar
Richer P., Yandouzi M., Beauvais L., Jodoin B., Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying. Surf. Coat. Tech., 2010, 204(24), 3962-3974. Google Scholar
Espallargas N., Berget J., Guilemany J.M., Benedetti A.V., Suegama P.H., Cr3C2–NiCr and WC–Ni thermal spray coatings as alternatives to hard chromium for erosion–corrosion resistance. Surf. Coat. Tech., 2008, 202(8), 1405-1417. Web of ScienceCrossrefGoogle Scholar
Ozgurluk Y., Doleker K.M., Karaoglanli A.C., Hot corrosion behavior of YSZ, Gd2Zr2O7 and YSZ/ Gd2Zr2O7 thermal barrier coatings exposed to molten sulfate and vanadate salt, Appl. Surf. Sci., (in press), CrossrefWeb of ScienceGoogle Scholar
Ma K., Schoenung J.M., Isothermal oxidation behavior of cryomilled NiCrAlY bond coat: homogeneity and growth rate of TGO. Surf. Coat. Tech., 2011, 205(21-22), 5178-5185. CrossrefWeb of ScienceGoogle Scholar
Gaskell D.R., Laughlin D.E., Introduction to the Thermodynamics of Materials, 5rd ed., Taylor & Francis Group, New York, 2017. Google Scholar
Tsai S.C., Huntz A.M., Dolin C., Growth mechanism of Cr2O3 scales: oxygen and chromium diffusion, oxidation kinetics and effect of yttrium, Materials Science and Engineering: A, 1996, 212(1), 6-13. CrossrefGoogle Scholar
Sidhu T.S., Prakash S., Agrawal R.D. Studies on the properties of high-velocity oxy-fuel thermal spray coatings for higher temperature applications. Materials Science, 2005, 41(6), 805-823. CrossrefGoogle Scholar
Nath S., Manna I., Majumdar J.D., Kinetics and mechanism of isothermal oxidation of compositionally graded yttria stabilized zirconia (YSZ) based thermal barrier coating, Corr. Sci., 2014, 88, 10-22. CrossrefGoogle Scholar
Krogstad J.A., Krämer S., Lipkin D.M., Johnson C.A., Mitchell D.R.G., Cairney J.M., Levi C.G., Phase stability of t′-zirconia-based thermal barrier coatings: Mechanistic insights. J. American Ceram. Soc. 2011, 94(S1), 168–177. CrossrefWeb of ScienceGoogle Scholar
Karaoglanli A.C., Doleker K.M., Demirel B., Turk A., Varol R., Effect of shot peening on the oxidation behavior of thermal barrier coatings, Appl. Surf. Sci. 2015, 354, 314-322. CrossrefWeb of ScienceGoogle Scholar
About the article
Published Online: 2018-09-21
Conflict of interest: Authors state no conflict of interest.
Citation Information: Open Chemistry, Volume 16, Issue 1, Pages 876–881, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2018-0096.
© 2018 Kadir Mert Doleker et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0