Damage Evaluation of Yttria Stabilised Zirconia (YSZ) Thermal Barrier Coatings

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1 More info about this article: Abstract: Damage Evaluation of Yttria Stabilised Zirconia (YSZ) Thermal Barrier Coatings B.B. Jha 1, R.K. Satpathy 2, T.K. Sahoo 1 & R. Sahoo 1 1. CSIR - Institute of Minerals & Materials Technology (IMMT), Bhubaneswar 2. Centre for Military Airworthiness & Certification (CEMILAC), Koraput bbjha@immt.res.in The present work involved studies that contributed to the understanding of service induced deterioration and life cycle evaluation of thermal barrier coatings (TBCs). Conventional 7 wt% YSZ (top coat) on substrate Nickel base superalloy (BZL12Y) with bond coat of NiCoCrAlY having service exposure of 500 and 1000 hours have been used for this study. The microstructural examinations were carried out using optical and scanning electron microscopy techniques. Isothermal oxidation, micro-scratch test coupled with Acoustic Emission (AE) sensors, and nano-indentation test have been carried out to identify the various degrading mechanism operating during service exposure of the TBC system. Growth of thermally grown oxide (TGO) layer and inter-diffusion of species within the layers including the substrate have been identified as the parameters for estimating the exhausted life fraction of the degraded coatings. Influence of severity of deterioration on hardness degradation obtained by nanoindentation test has also been examined. Finally, it has been concluded that parameters like TGO layer growth, deficiency of Al in bond coat, root mean square (RMS) voltage level of AE signals and hardness degradations have the potential to assess the life cycle evaluation of TBCs. Keywords: Thermal barrier coatings, RMS voltage level, Hardness degradation, Exhausted life fraction, Nano-indentation. 1. Introduction: Thermal Barrier Coatings (TBCs) are refractory oxide ceramic coatings applied to the surface of hottest metallic parts of gas turbine engines enabling these modern engines to operate at significantly higher temperatures than their predecessors [1,2]. Typically made of 7wt% Yttria ( Y 2 O 3 ) Stabilised Zirconia (7YSZ) Ceramics TBCs, provide thermal insulation to the metallic/super alloy parts thus introduces a temperature gradient enabling high combustion temperatures while avoiding high temperature degradation such as creep, oxidation, hot corrosion, erosion etc. As a result, TBCs have been the subject of scientific investigation for the past two decades and are being exploited for a range of commercial applications, particularly in the aviation and automobile sector. Plasma sprayed YSZ coatings with tetragonal prime (t') structure is one of the most commonly used TBC because of its low thermal conductivity and phase stability at operating temperatures <1200 C [3], However to increase the efficiencies of these ceramic materials, attempts are on to further lower down their thermal conductivity & 295 Non-Destructive Evaluation 2016

2 metallurgical structure enabling them to operate at higher working temperatures. Development of nanostructured coatings having bimodal structure is an effect in these directions [4, 5]. Thermal Barrier Coating systems are made up of three parts (1) Substrate (Component) (2) Bond Coat (BC) & (3) Top Coat (TC). In addition with increase in time at higher operating temperatures, a layer of Thermally Grown Oxides (TGO) develops between the bond coat and top coat [6]. Top Coat having low thermal conductivity provides necessary thermal insulation where as the metallic bond coat ensures good adhesion of Ceramic Coatings (TC) and provide oxidation resistance [7]. The top coat, thus introduces a temperature gradient enabling high inlet temperature of combustor while avoiding high temperature degradation of substrates made up of Superalloys. The use of TBCs in gas turbine components is, however, are not entirely without its problems. During service several kinetic processes occur in parallel. They are mainly interdiffusion between the bond coat and underlying super alloys, Al-diffusion from bond coat to form TGO layer and stress development at interface due to growth of TGO layer. Since all these are thermally activated processes, the rates at which they occur are expected to increase exponentially with temperature leading to failure of TBC by spallation of the top coat which deprives the TBC system of its insulating capability. The present work has involved studies that contributed to the understanding of service induced deterioration & life cycle evaluation of TBCs. Conventional 7 wt% YSZ (top coat) on substrate Nickel base superalloy (BZL12Y) with bond coat of NiCoCrAlY having service exposure of 500 & 1000 hours have been used for this study. Isothermal oxidation, microscratch test coupled with Acoustic Emission (AE) sensors, Nano-indentation as well as microscopic studies have been carried out to identify the various degrading mechanism operating during service exposure of these TBCs system. Life predictions both from the view point of growth of TGO layer and inter diffusion of species within the layers including the substrate have been attempted & discussed in the paper. 2. Experimental: 2.1 Deposition of Coatings: Nickel base superalloy (BZL12Y) material has been used as substrate for this study. The coating process was carried out using a plasma spray system supplied by M/s Metallization, UK. This is a typical atmospheric plasma spray system working in non-transferred arc mode. The set up assembles a no. of sub units like a plasma torch mounted on a six-axes robot, power supply, powder feeders, mass flow controller, plasmagen gas supply, water chiller and rotating turn table for sample rotation. The entire assembly is housed inside an acoustic chamber and is operated by a control console. In this study high purity Ar & He were used as primary and secondary plasmagen gas respectively. A roughned Nickel base superalloy (BZL12Y) substrate (commercially procured) was fixed on a turn table and 7YSZ coatings powders was sprayed & 296 Non-Destructive Evaluation 2016

3 deposited on the substrate. Prior to the top coat, NiCrCoAlY bond coats of thickness about 35µm were applied on Inconel Substrates through HVOF (M/s MEC, INDIA) by using set of parameters given in Table-1 below. Table 1: Operating Parameters of HVOF process (Bond Coat) Gas Operating Range Pressure in bar Flow rate in lit/min Oxygen LPG 7 70 Air Carrier Gas (N 2 ) These coated samples were then given exposure to the extent 500 and 1000 hours in real life situations and taken out for further study. One of the coated samples also oxidized at C in air for 10 hours and then cooled in air for comparative study. 2.2 Optical metallography: Specimens of size 20x10x2 mm were cut from differently samples which were service exposed and oxidized to various degrees of treatments. These samples were suitably mounted in a cold setting resin and were ground using successive grades of SiC papers and polished mechanically using 0.25 µm diamond paste for final scratch free polishing. Chemical etching was then done using a solution of FeCl 3, HCl, CuCl 2, and SnCl 4 in water for 10 seconds maximum. Optical microscopy of chemically etched and dried specimens were carried out using Leica make metallurgical microscope (Model: DMI 3000M). Complete scanning was done at lower magnifications to know the representative microstructural features. These representative microstructural features were then captured at higher magnifications and presented as observations. 2.3 Scanning electron microscopy (SEM): All the treated samples were first examined using SEM (Hitachi Model: S N) for topographical study. Energy dispersive X- Ray analysis (EDX) attached with SEM was used for determination of phase composition. X-Ray images showing the elemental distribution of various elements of different phases was also obtained. These analyses helped in identifying the concentration of different elements in substrate, bond coat, and top coat separately. 2.4 Micro scratch test: 297 Non-Destructive Evaluation 2016

4 Micro scratch test has been used to evaluate the adhesion characteristics of TBC coating when exposed to various degrading conditions. CETR (Model: CETR- 476 Micro-scratch test system) make micro scratch test has been used for this purpose. Top surface of the exposed samples were subjected to such examinations under progressive load varying conditions (2 N to 50 N). RMS voltage of the AE signals and depth of penetration of the indenter has been taken as the parameters for evaluation of micro scratch test results. 2.5 Nano-indentation test. Nano-indentation test has been used to determine the mechanical properties of degraded coatings. Fischer- Crips make (Model: UIMS nano- indentation system) has been used for this purpose. In Nano indentation test, a Berkovich indentor (which has three sided pyramid geometry) has been used for indentation. This indentor is allowed to penetrate on the surface till it reaches a user defined value of load (30 N in the present study) and then released. During the course of the indentation process, a record of depth of penetration is made, and then the area of the indent is determined using a known geometry of the indentation tip. While indenting various parameters such as load and depth of penetration were measured. A record of these values is then plotted on a graph to create a load-displacement curve. These curves are then used to extract mechanical properties of the material. 3. Results and Discussion: (a) (b) 298 Non-Destructive Evaluation 2016

5 (c) (d) Fig. 1. SE Micrographs of top surface of YSZ coatings: (a) unused, (b) exposed for 500 hours, (c) exposed for 1000 hours, (d) Oxidized at C for 10 hours. Figure 1 shows the scanning electron microscope of the top surface of the YSZ coatings for unused samples along with the samples which have been exposed in service for 500 and 1000 hours as well as the same which has been exposed at C for 10 hours in air. Typical flowery structures of the unused samples are clearly identified. As the samples are exposed in service the flowery structures get more and more flatten with increase in their porosity content. Pore sizes also get increased as a result of service exposure. Sample oxidized at C in air for 10 hours also indicated the flattened flowery structure with increased population of pores. This clearly indicated that population and the size of the pores get increased with an increase in the severity of the degradation of the coatings. (a) (b) 299 Non-Destructive Evaluation 2016

6 (c) (d) Fig. 2. Optical Micrographs of cross sections showing substrate, bond coat, TGO layer and top coat: (a) unused, (b) exposed for 500 hours, (c) exposed for 1000 hours, (d) Oxidized at C for 10 hours. Figure 2 shows the optical micrograph of cross section of the samples which has been exposed to various kinds of degradation viz. 500 hours, 1000 hours and oxidized for 10 hours at C. Formation of thermally grown oxide (TGO) layer is prevalent in samples which have been service exposed for 500 and 1000 hours (Fig. 2 (b) and (c)). TGO layer is almost nonexistent in unused samples (Fig. 2 (a)). However, for samples which have been oxidized for 10 hours at C showed severely grown TGO layer at the expense of bond coat. We could not observe the formation of oxide stringers in the bond coat as a result of its internal oxidation. Presence of TGO layer and its growth with severity of degradation of YSZ coatings indicates that this could be considered as one of the life prediction criterion for deterioration of these coatings. (a) Wt % Al = Non-Destructive Evaluation 2016

7 (b) Wt % Al = 2.53 (c) Wt % Al =0.59 (d) Wt % Al = 0.7 Fig.3. EDX spectrum of TGO Bond coat region of YSZ coating: (a) unused, (b) exposed for 500 hours, (c) exposed for 1000 hours, (d) Oxidized at C for 10 hours. (a) 301 Non-Destructive Evaluation 2016

8 (b) (c) (d) Fig.4 Micro scratch test results of YSZ coating: (a) unused, (b) exposed for 500 hours, (c)exposed for 1000 hours, (d) Oxidized at C for 10 hours. Figure 3 indicates the EDX spectrum of typical Bond coat TGO layer of unused and degraded samples. It is observed that Al content is almost constant in samples which were unused and service exposed to 500 hours (Fig. 3 (a), (b)). However, for samples which has been exposed to 1000 hours and oxidised at C for 10 hours showed negligible presence of Al in the region of Bond coat TGO region. In fact for the oxidised sample only the growth of TGO layer has found to be very large (Fig. 2 (d)). It has been reported that TGO layers remains to be protective as long as it consists predominantly Al 2 O 3 -rich scale [7, 8]. Higher growth rate of 302 Non-Destructive Evaluation 2016

9 TGO layer for samples oxidized at C is clearly indicative of the absence of Al 2 O 3 rich scale and as a result Al content in the region of Bond coat TGO layer has drastically reduced. This clearly indicates that concentration of Al in bond coat could be identified as one of the parameter for life cycle evaluation of the TBCs. Figure 4 shows the micro scratch test results of unused and degraded coatings under progressive loading conditions (from 2N to 50N). Improved adhesive strength of unused coatings is clearly revealed in the plot of RMS voltage levels of AE signals. During the progressive application of load on the top surface of unused coatings; continuous RMS voltage level plot showed enhancement only at a level of 40 N of load (Fig. 4 (a)). However, for all other coatings which have been degraded to various degrees, continuous RMS voltage plots showed the enhanced signals at much lower load values ranging from 15 to 22 N. These plots not only indicate the decrease in adhesion strength of degraded TBCs but also confirm the possibility of acoustic emission technique (AET) to be used as an on-line tool for detection of in-situ deterioration of TBCs while in service. Fig.5. Load displacement plot of top surface of YSZ coatings: (a) unused, (b) exposed for 500 hours, (c) Oxidized at C for 10 hours. Table 2: Mechanical properties of unused and degraded YSZ coatings Conditions Elastic modulus Hardness Depth of 303 Non-Destructive Evaluation 2016

10 (GPa) (GPa) penetration ( m) Unused * Exposed to 500 hours * Oxidised at C for 10 hour * Figure 5 shows the load displacement curve of unused and degraded coatings for various degrees. Table 2 below also shows the different mechanical properties obtained as a result of Nano-indentation test done at constant load of 30 mn. From the values tabulated in Table 2, it is clearly revealed that as a result of degradation the hardness values of the coatings show a decreasing trend. This has also been reflected in the depth of penetration which increases from γ.7 m for unused samples to 4.7 m for samples which has been oxidised at C for 10 hours. However, degradation of coatings didn t get reflected in the values of their elastic modulus. This could be possibly due to very low thickness of the coatings taking part in loading and unloading process of nano-indentation test. 4. Conclusions: Present investigation indicated that damage evaluation of YSZ coatings could be carried out based on micro-structural characterisation of the deteriorated coatings. An unused sample of YSZ TBCs has a flowery appearance, which gets progressively flattened as a result of service degradation, with the appearance of distinctly visible increase in the size and population of the pores. The growth of thermally grown oxide (TGO) layer present between top coat and bond coat is an indicative of the degradation of TBCs and hence is identified as one of the important parameter for the life cycle evaluation of the coatings. TGO layer remains to be protective only as long as it consists predominantly Aluminarich scales. Growth of TGO layer as a result of service degradation of coatings could be attributed to the deficiency of Alumina- rich scales in it. Micro scratch test along with RMS voltage level of AE signals could be used to identify the adhesive strength of the degraded coatings. Increase in the values of RMS voltage of AE signals during scratch test is indicative of lower adhesive strength of degraded coatings. Hardness values obtained from nano-indentation test and its degradation trend with exhausted service life could be used as one of the criteria for life cycle evaluation the TBCs. 304 Non-Destructive Evaluation 2016

11 References: (1) A.G.Evans, D.R. Mumm, J.W.Hutchinson, G.H.Heir, F.S. Fettit, Progress in Mat.Sci, 46, 05 (2001). (2) A.G.Evans, D.R. Clarke,C.G.Levi J. of Evr. Ceram. Soc. 28, 1405 (2008). (3) R.S.Lima, B.R. Marple, Jr. Thermal Spray Technology, 16 (1) pp ,( 2007). (4) R. Miller, Jr. of Thermal Spray Technology, Vol-6, no.1, pp.35-42,( 1997). (5) S.Mantry, B.B.Jha, A.Mandal, D.K.Mishra, B.K.Mishra, and M.Chakraborty, International journal of smart and nano materials, Volume 5, pp ( 2014). (6) M.Belmonte, Adv. Engg. Mater, Vol-8, No-8, pp , (2006). (7) H.Evans&M.Tayler, Proc. I Mech E, Vol-220, pp -1-10, (2006). (8) J. Davis, Ed. Head Resistant materials, ASM internationals, (1999). 305 Non-Destructive Evaluation 2016