Intermediate PVD layers as diffusion barriers in turbine coating systems I. E. Ali, D. Wett, T. Grund, D. Nestler, B. Wielage, T. Lampke Institute of Materials Science and Engineering, Technische Universitaet Chemnitz, GER Abstract Standard thermal barrier coating systems consist of yttria-stabilized zirconia (YSZ) top coats on so called M-CrAlY bond coats, where M most often stands for a Co, Ni or CoNi base alloy. During their service under combined heat and oxygen load, a reaction zone is formed at the interface between the YSZ heat insulation and the metallic bond layer. The reaction zone (TGO) consists of different thermally grown metal oxides like (Cr, Al)2O3, (Ni, Co)(Cr, Al)2O4, NiO α-Al2O3. A high content of a dense, slow growing and continuous α-Al2O3 phase is beneficial for the service lifetime of the TBC system due to its barrier effect on oxygen diffusion. However, the other simultaneously grown oxides lead to increased interfacial stresses between top and bond coat followed by micro and macro cracks and consequently by spalling of the insulation layer. In the present study, thin Al and AlOx films were deposited by DC-magnetron sputtering on the raw surface of ther-mally sprayed CoNiCrAlY metallic bond coats. They act as thin interlayers between the thermally sprayed bond and top coats and heighten the Al concentration in this zone. The newly designed interlayers were compared concern-ing their surface morphology, microstructure and influence on the systems’ thermal cycling behaviour. Also, the effect on the constitution and the thickness of the TGO was investigated. As a result, the applied thin metallic Al interlayers fully transform into thin aluminium oxide films in the top coat/bond coat interface. This in-situ formed dense oxide layers obviously act as diffusion barriers for oxygen. The coating systems then showed less cracks and an enhanced mechanical stability. A further reduction of cracks was found in case of the AlOx interlayers. Thus, both the in-situ formed and the deposited oxide layer have the potential to reduce the formation of detrimental tran-sition metal oxides and hence to extend the TBC system’s durability. 1. Introduction Thermal barrier coatings (TBC) are applied for the protection of metallic components that suffer degrada-tion due to corrosion, oxidation or excessive heat load during service in high-temperature environments [1-4]. For example, an up-to-date insulation layer of 0.25 mm TBC can reduce the temperature on the surface of turbine components by up to 170 K in com-parison to the operating gas temperature [2]. The most widely used TBC systems are produced by atmospheric plasma spraying (APS) or electron beam physical vapour deposition (EB-PVD) processes [5]. Recently high velocity oxygen-fuel spraying (HVOF), cold gas spraying (CGS) and others have been used for the bond coat (BC) layer deposition [6-8]. TBC systems can be considered as three-component coat-ing systems on a suitable substrate material. The resulting four-component system hence generally comprises [9]: (1) a metallic substrate which commonly is a Ni-based super alloy in turbine applications, but also titanium alloy in aircraft engines, niobium alloy in exhaust noz-zles, steel in diesel engines or a Co-based alloy in stationary applications [10, 11], (2) An oxidation resistant metallic bond coat, usually high alloyed M-CrAlY (where M is generally Ni, Co, Fe or alloys/combinations of them) [1, 12-15], (3) a thermally grown oxide layer (TGO), which is a reaction product that forms at the interface between metallic BC and ceramic top coat (TC), resulting from reactions with oxygen that reaches the surface of the metallic bond coat via interconnected pores and cracks within the ceramic TC [16-20]; the TGO shows a parabolic growth rate during thermal cycling [15, 21],

(4) a ceramic top coat, most often consisting of yttria-stabilised zirconia (YSZ, Y2O3 mass content usually 7-8 %) which is usually built up as a 0.25 to 0.35 mm thick layer and which is widely used on combustion chamber components of gas turbines [11, 22-26]. TGO layers strongly influence the work life of TBC. They are the main factor causing TBC failure due to fast lateral growth and subsequent wrinkling of oxides as well as thermal expansion mismatches between TGO, ceramic TC and metallic BC, which all result in lateral cracks, delamination and spalling [27, 28]. The wrinkling of the TGO occurs due to preferential lateral oxide formation, which is implicitly followed by in-creased compressive stresses, TGO distortions and consequent contact loss at the interfaces to TC and BC. Lateral growth and wrinkling can be reduced by controlling the oxide grain grow in the TGO, which for example can be achieved by Y addition into the BC material to gain columnar growing oxides instead of equiaxed grain structures [26]. TGO layers generally include different oxide types according to the BC alloy components, like chromium oxides, spinel phases and nickel oxides. It is reported that TGO of the described standard TBC system are constituted of Cr2O3, NiO, and (Ni, Co)(Cr, Al)2O4 [29]. Each of these oxides has its own growth rate [1, 15, 22]. α-Al2O3 is the most beneficial oxide [17]. It shows a hexagonally closed packed crystal structure of oxygen anions wherein two thirds of the octahedral interstices are filled by triva-lent aluminium cations. Due to this, it lowers the oxy-gen diffusion into the BC material and hence the TGO growth rate as well as the oxygen diffusion through the BC to the substrate material [1, 8]. Therefore, a TGO layer is required to be materially engineered in order to achieve low growth rate, low stress formation,

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highly stable oxide phases, continuity and coherence, low crack and pore content and high adhesion to both BC and TC layers. All can be gained by a high α-Al2O3 content [1, 15, 17, 19, 26]. 2. Experimental work The chosen standard TBC system was APS sprayed and comprised an Inconel 600 (2.4816) substrate, a 150 µm thick CoNiCrAlY BC layer and a 150 µm thick YSZ TC layer. The processed CoNiCrAlY powder (CoNi32Cr21Al8Y0.5, GTV 60.95.1, GTV GmbH, Luckenbach) had a fraction of 20 µm < d < 53 μm. The YSZ powder (YSZ 92/8, Y2O3 mass content of 8 %, GTV 40.23.1, GTV GmbH) had a fraction of 20 µm < d < 45 μm. The used APS system was an F6 plasma torch (GTV GmbH). The applied APS parame-ters were standard parameters optimised to gain low-porosity coatings from the applied feedstock materials and were provided by the supplier. The interlayer were produced using DC magnetron sputtering PVD (in the following referred to as PVD) in a vacuum chamber of p = 8 · 10-3 mbar and using a pure-Al tar-get (Al 99.999, EVOCHEM Advanced Materials GmbH, Offenbach). The interlayer were deposited onto the BC with an aim thickness of 1-2 µm. The sputtering gas consisted of pure Ar (50 sccm) in case of applying the Al film and of an Ar/O2 (30 sccm/20 sccm) mixture in case of the AlOx film. Thermal cycling tests were carried out in 40 cycles. In each cycle the samples were exposed 25 min to air at 1150 °C, then cooled down to 100-150 °C using com-pressed air and heated up again to 1150 °C. Heating and cooling rate were 400 K/s and -400 K/s, respec-tively. The temperature effect on the interface trans-formation was studied on samples without YSZ TC. These samples were exposed to air at 1150 °C for 3 h followed by slow cooling. For comparative reasons, the standard TBC system without interlayer was in-cluded into the thermal tests. After the thermal tests, all samples were materialographicly investigated, including optical light and scanning electron micros-copy (OLM, SEM), energy dispersive X-ray analysis (EDXS) and X-ray diffractography (XRD). Investigated characteristics and values were the coatings’ micro-structures before and after the thermal tests, the TGO phases, formations and thicknesses. 3. Results 3.1. Microstructure and surface morphology of as

sprayed samples The cross sectional microstructure characteristics of two BC layers covered by Al and AlOx, respectively, are shown in Fig. 1. The Al layer has a thickness of around 2 µm (Fig. 1a), while small areas (marked by “A”) have not been fully covered by the PVD-Al. This is either due to shadowing effects caused by the rough as-sprayed surface or due to differences in the grain growth kinetics caused by geometrical and chemical irregularities on the BC surface. Cutting arti-

facts originating from the metallographic sample prep-aration cannot be excluded, too. The AlOx interlayer is not visible in the light microscopic images (Fig. 1b). Its thickness, however, is estimated to be about 210 nm according a cross section SEM image of a smooth reference sample coated under similar conditions. The microstructures of TBC systems with and without intermediate layers are shown in Fig. 2. A good con-tact was observed at the pure YSZ/CoNiCrAlY inter-face (Fig. 2a). Some direct contact zones between the BC and the TC layer (marked by “B”) appear at the TBC system containing a PVD-Al interlayer (Fig. 2b). They result from the above mentioned discontinuities of the initial Al-layer or are caused by the impacting YSZ particles that may partly remove the Al material. Fig. 2b shows the TBC system containing an AlOx interlayer. The surface morphologies of the raw and the PVD coated CoNiCrAlY BC layers before the top coating step are shown in Fig. 3. The secondary electron micrograph of the CoNiCrAlY BC shows flattened splats and partially molten particles (Fig. 3a). The corresponding EDX spectrum (not presented) reveals the composition of the BC surface to be identical to that of the feedstock powder. XRD analysis on the surface (not presented) proof existence of a cubic Co and a cubic Cr2Ni3 phase. The Al coated BC consists of globular grown Al grains that more or less cover the BC surface. Few large grains embedded in smaller grains result in a rough, cauliflower shaped surface

10 µm a Co‐Ni‐Cr‐Al‐Y

A

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Figure 1: Optical microscope microstructure for sam-ple (a) with Al layer (b) with AlOx layer. Positions marked by “A” correspond to areas where have not been covered fully by PVD-Al.

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(Fig. 3b). The corresponding EDX spectrum (not pre-sented) shows a higher intensity for Al. Elements from beneath the Al-PVD layer were also detected, as the interlayer thickness ranges within the EDX information depth. This effect also reveals in XRD measurements. However, the strongest reflexes were those of Al. The surface of the PVD-AlOx interlayer shows small undefined shaped grains dispersed on the BC surface (Fig. 3c). The corresponding EDX spectrum (not pre-sented) shows a higher intensity for Al and O in com-parison to the samples without and with Al interlayer. Signals for the BC elements were also detected due to the low thickness of the AlOx coating. The obtained XRD signals equaled those of as-sprayed CoNiCrAlY.

3.2. Microstructure and surface morphology of ther-mally treated samples

The XRD patterns of samples without YSZ TC after annealing treatment for 3 h in air can be seen in Fig. 4. The analysis on the pure BC surface (Fig. 4 bottom curve) detected different oxide phases, like cubic spinel phases and rhombohedral Cr2O3, in addi-tion to signals originating from the unaffected bond coat material (summarised as “BC”). According to the state of knowledge, the formed spinel phases could be (Ni, Co)(Cr, Al)2O4 (e.g. as reported in [29]). α-Al2O3 could not be found. As M. Shibata et al. report-ed [30], this may be due to the high particle surface temperatures during the APS process leading to

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Figure 2: Optical microscope microstructure for aTBC sample (a) without intermediate interlayer(conventional), (b) with Al interlayer and (c) withAlOx interlayer.

a

b

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Figure 3: SEM surface morphology of a bondcoat (a) bare (b) covered by 2 µm Al and (c) cov-ered by 0.25 µm AlOx.

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Figure 2: X-ray diffraction patterns of the YSZ free samples after 3 h oxidation in air at 1150 °C.

Al evaporation. Therefore, the sprayed CoNiCrAlY lacks the capability to rapidly form a protective α-Al2O3 layer. A weak peak of -Al2O3 was detected after 100 h of oxidation process. Another reason is the consumption of Al for the formation of spinel phases like CoAl2O4 during thermal cycling. XRD analysis on the sample surface of oxidised PVD-Al coated BC (Fig. 4 middle curve) show intense peaks of a (pre-sumable thick) spinel phase. Peaks of α-Al2O3, Cr2O3 and originating from the BC material were also found. The XRD analysis on the sample surface of PVD-AlOx covered BC (Fig. 4 top curve) show in additon to the BC signals, Cr2O3 and a spinel phase. Compared to the Al covered sample, the intensity of peaks related to spinel are lower and the Cr2O3 intensity is higher, most probably due to the low AlOx thickness. The deposited thin AlOx layer was not detected. The cross sectional microstructure for the TBC sam-ples after 40 thermal treatment cycles are shown in Fig. 5. The sample without an interlayer (Fig. 5a) shows wider horizontal and vertical cracks inside the ceramic TC compared to the samples with Al or AlOx interlayers (Fig. 4b and c, respectively). By an image analysis software the percentage of cracks and pores in the YSZ TC was estimated for each sample type. It was found to be 10 % for samples without interlayer, 7 % for samples with PVD-Al interlayer and 4 % for samples with PVD-AlOx interlayer. The average TC porosity excluding cracks was measured in uncracked areas and was found to be about 4 %. Hence, qualita-tive crack content in the TC strongly decreases from TBC systems without to TBC systems containing PVD-Al and -AlOx interlayers. Furthermore, the TBC system without an interlayer shows a comparative strong TGO formation (dark grey regions marked by ‘‘C’’ in Fig. 5a) at the TC/BC

interface, from where large horizontal cracks initiate into the ceramic TC layer. This is a typical phenomena and already reported elsewhere (e.g. in [1, 8, 29]). The sample with a former Al interlayer features a more homogeneous and less porous TGO formation (Fig. 5b) compared to the sample without interlayer (Fig. 5a). Generally, the TGO thickness in these sam-ples is lower, beside a few areas, where the thickness is similar to that of the standard in Fig 5a. These are-as may be those that have not completely been cov-ered by PVD-Al, as previously illustrated in Fig. 2b. The otherwise homogeneous TGO formation is be-lieved to result from the otherwise rapidly and full oxi-dised thin PVD-Al interlayer during the thermal treat-ment. The oxygen diffused through the YSZ TC and reached the TC/interlayer or TC/BC interface, respec-tively. The typical TGO reaction zone was built up then, but due to the existence of a high amount of Al in this area, the oxidation results in a high share of Al-oxides and lowered the formation of other oxide phas-es, since the formation of aluminium oxide rich barrier layer quickly lowered the diffusion of oxygen into the pure BC material. As a result, residual stresses creat-ed by growth of the typical TGO phases is reduced and, consequently, the crack formation in the ceramic TC layer is reduced in comparison to the standard TBC system without PVD-Al interlayer. The SEM photograph of the interface microstructure of the TBC system containing an AlOx interlayer shows a homogeneous, fully closed layer at at the TC/BC interface (Fig. 5c). Its optical contrast to the BC material is another compared to the previously discussed two sample types. This may correlate with the above described results of the XRD measure-ments on the oxidised sample surface without YSZ TC, that revealed the existence of Cr2O3 on the sur-

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face and a comparatively low intensity of spinel phas-es. As reported, the growth of spinel phases, especial-ly the spinel structure (Ni, Co)(Cr, Al)2O4, is accompa-nied with rapid local volume growth [29]. As a result,

the nearly non-existent spinel phases at the TC/BC interface limit or slow down the increase of the residu-al stresses and consequently effectively limit the me-chanical load on the ceramic TC layer. 4. Conclusion The interface between the CoNiCrAlY bond coat and the YSZ top coat of an APS sprayed standard TBC system was modified by thin DC-magnetron sputter deposited interlayers of Al and AlOx, respectively. After thermal exposure in and thermo-cycling tests in air, a reduced TGO growth and a reduced number of oxide phases at the TC/BC interface were detected in comparison to equally investigated TBC systems without additional interlayers. The metallic PVD-Al interlayer fully transformed into an comparative thin TGO mainly consisting of a spinel phase, though α-Al2O3 was also detected. Thermla cycling tests reveal that the existence of the transformed PVD-Al interlay-er reduced the oxygen diffusion into the BC material and consequently limited rapid growth of typical high-volumenous oxide phases at the TC/BC interface. This effect could be enstrongened by applying a PVD-Al interlayer directly. Therefore, TBC systems comris-ing an PVD-Al or PVD-AlOx interlayer showed a lim-ited tendency to crack formation within the YSZ top coat of the investigated TBC systems after the same number of cycles during thermal cycling tests. Accord-ing to the investigations, a 210 nm thick PVD-AlOx interlayer was found to be more effective than a 2 µm thick interlayer of PVD-Al. 5. References [1] A.C. Karaoglanli, E. Altuncu, I. Ozdemir, A.

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c

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a

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Co‐Ni‐Cr‐Al‐Y

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Co‐Ni‐Cr‐Al‐Y

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Co‐Ni‐Cr‐Al‐Y

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Co‐Ni‐Cr‐Al‐Y

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Figure 5: Microstructure for the samples after 40 cy-cles (a) without interlayer, (b) with Al interlayer and (c)with AlOx interlayer.

C

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