Fractographic and microstructural study of isothermally and cyclically heat treated thermal barrier coatings

SCT-17366; No of Pages 9 Surface & Coatings Technology xxx (2012) xxx–xxx

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Fractographic and microstructural study of isothermally and cyclically heat treated thermal barrier coatings Robert Eriksson a,⁎, Hå̊kan Brodin a, b, Sten Johansson a, Lars Östergren c, Xin-Hai Li b a b c

Division of Engineering Materials, Department of Management and Engineering, Linköpings universitet, 58183 Linköping, Sweden Siemens Industrial Turbomachinery AB, 61283 Finspå̊ng, Sweden Volvo Aero Corporation, 46181 Trollhättan, Sweden

a r t i c l e

i n f o

Available online xxxx Keywords: Thermal barrier coating TBC Fractography Adhesion Thermal cycling Burner rig

a b s t r a c t The fracture surfaces from adhesion tested thermal barrier coatings (TBC) have been studied by scanning electron microscopy. The adhesion test have been made using the standard method described in ASTM 633, which makes use of a tensile test machine to measure the adhesion. The studied specimens consist of air plasma sprayed (APS) TBC deposited on disc-shaped substrates of Hastelloy X. The bond coat (BC) is of NiCoCrAlY type and the top coat (TC) consists of yttria partially-stabilised zirconia. Before the adhesion test, the specimens were subjected to three different heat treatments: 1) isothermal oxidation 2) thermal cycling fatigue (TCF) and 3) burner rig test (BRT). The fracture surfaces of the adhesion tested specimens were characterised. A difference in fracture mechanism was found for the different heat treatments. Isothermal oxidation gave fracture mainly in the top coat while the two cyclic heat treatments gave an increasing amount of BC/TC interface fracture with increasing number of cycles. Some differences could also be seen between the specimens subjected to burner rig test and furnace cycling. © 2012 Elsevier B.V. All rights reserved.

1. Introduction To increase the efficiency of gas turbines, the combustion temperature is driven to ever increasing levels [1,2]. However, an increase in operating temperature inevitably results in degradation of metallic materials. Thermal barrier coatings (TBCs) offer a means to accomplish such an increase in operating temperature, while avoiding high temperature degradation of metallic parts, by providing insulation and thereby lowering the temperature in the structural parts [2–7]. A typical thermal barrier coating system consists of four parts: substrate, bond coat (BC), thermally grown oxides (TGOs), which develop during high temperature exposure, and a top coat (TC). The BC is often of the type MCrAlX, where M is chosen from Ni and Co, and X is a reactive element, most commonly Y [2,7,8]. The bond coat increases adhesion between substrate and top coat and provides oxidation and corrosion resistance. The TC is most often 6–8% yttria partiallystabilised zirconia (6–8% Y-PSZ) [6,9]; 6–8% has been shown to be the optimum amount of yttria in order to get long fatigue life [10]. The two main techniques for deposition of TBC systems are EBPVD and plasma spraying [2,6,7,11]. Plasma spraying can be conducted in air or vacuum and is consequently referred to as: air plasma spray (APS), and vacuum plasma spray (VPS). For deposition of BC, both types may be used, while the TC is sprayed with APS as it gives

⁎ Corresponding author. Tel.: + 46 13 284410; fax: + 46 13 282505. E-mail address: [email protected] (R. Eriksson).

high porosity and thereby better insulating properties [7]. The signature of the plasma spray process is the typical splat–on–splat structure that arises from the small droplets of molten deposit that impact on the substrate and form splats [12]. As the splat solidifies and cools, it contracts and consequently cracks due to restraints in movement imposed on the splat by the underlying colder material [13–15]; due to inability of the ceramic to plastically deform it cracks and forms a network of microcracks. The contact ratio between individual splats is fairly low, it has been estimated to roughly 20% [16]. The individual splat is built up by a columnar grain structure grown perpendicular to the splat during the rapid cooling [16]. The cooling crack network and the poor contact between splats gives rise to two of the typical defects in APS TC readily visible in cross-sections: insplat microcracks, (also referred to as vertical microcracks), and inter-splat delaminations [13–15], shown in Fig. 1a). Two additional types of defects in APS TC are: globular pores and segmentation cracks [13–15]. The thermally grown oxides form as the TBC system is exposed to high temperature. The oxidation protection of the BC relies on the formation of a slow-growing protective oxide layer between the BC and TC [8,17], such as a scale of predominantly Al2O3 [18,19]. The presence of reactive elements (REs), most commonly Y, increases the adhesion of the protective Al2O3 scale [8,10,18]. The ability of the BC to form Al2O3 depends on the amount of Al in the BC [19]; Davis [20] mentions 4–5% as the lower limit for which a continuous Al2O3layer can be formed on MCrAlY coatings. BCs, consequently, contain large amounts of Al which is predominantly bound in the β-NiAl

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a

5 µm

b

While the study of adhesion properties and interface toughness of the BC/TC interface has been previously examined, comparable little attention has been given to fractographic studies of the fracture surfaces resulting from the ASTM C633 adhesion test. Previous fractographic studies on TBCs include for example Wessel and Steinbech [47] and Harmsworth and Stevens [12] who studied the crack propagation in free-standing plasma sprayed 8% Y-PSZ and found the crack path to depend strongly on the splat morphology. Fractographic studies of fatigue crack growth have been conducted by for example Nesbitt et al. [48] who investigated the spalled-off surfaces from a cyclic oxidation test. Yamazaki et al. [39] studied fracture surfaces from a four-point bending test made on oxidised TBC systems and found the fracture behaviour of TBC systems to depend on the interface TGO thickness. The present study complements previous fractographic studies by comparing and contrasting the fractography of adhesion tested TBC systems subjected to several heat treatments: isothermal oxidation, furnace cycling and burner rig test. In all cases, heat treatments have been stopped before final spallation of the TBC. 2. Experimental 2.1. Material

5 µm Fig. 1. Microcracks and delaminations in the top coat, SEM backscatter electron images. a) An as-sprayed specimen, arrows mark vertical in-splat microcracks and horizontal inter-splat delaminations. b) Evidence of sintering in specimen subjected to 290 h of isothermal oxidation at 1100 °C, arrows mark sintered cracks.

aluminide which acts as an Al reservoir [8,21,22]. As the aluminium is consumed by oxidation and interdiffusion with the substrate, the β-NiAl converts to γ′-Ni3Al and solid solution γ-Ni [2,20,23,24]. As the aluminium is depleted from the bond coat, other elements form oxides: chromia (Cr, Al)2O3, nickel oxide NiO and mixed-elementoxides of spinel type (Ni, Co)(Al, Cr)2O4 [24–26]. TBC systems that are exposed to thermal cycling will, in addition to oxidation, also experience degradation through mechanisms of fatigue [2]. Fatigue crack growth is generally considered to occur in, or close, to the BC/TC interface [2,18,27]. A commonly mentioned mechanism is: crack initiation at, or close to, peaks in the BC/TC interface and crack propagation either in the interface or in the TC close to the BC/TC interface; failure occurs by coalescence of these microcracks into larger cracks that eventually leads to spallation [2,18,28–32]. Another mechanism is edge cracking, in which a single dominant crack, initiated at the coating edge, determines the life of the coating [33]. The fracture surface, after failure, will appear either dark or bright depending on whether the fractured occurred close to the interface TGO or in the TC. The former is referred to as black fracture and the latter white fracture [28]. Fracture surfaces may also contain both types of fracture: mixed-type fracture. Several methods for evaluating the adhesion properties of TBCs are in use [34–39], among them the indentation test to establish interface toughness and the ASTM C633 tensile adhesion test (TAT), according to which one uses a tensile test machine to measure the adhesive strength of a TBC system. The indentation test has been used by, for example, Nusair Khan et al. [15], Chicot et al. [40], Yamazaki et al. [41] to establish the fracture toughness of TBCs, and the adhesion test has been used by for example Hadad et al. [38], Chicot et al. [42], Markocsan et al. [43], Gell et al. [44], Nusair Khan et al. [45], Eriksson et al. [46] to test the adhesion properties of TBCs.

The TBC system studied consists of disc-shaped substrates of Ni-base alloy Hastelloy X (Ni–22Cr–18Fe–9Mo–1.5Co–0.6W, with additions of Mn, C, Si and B.) on which a Ni–23Co–17Cr–12.5Al–0.45Y bond coat and a 7% Y-PSZ top coat have been deposited by air plasma spraying, (all compositions given in weight percent). The material used for the bond coat is a gas atomised powder: AMDRY 365-2 made by Sulzer Metco with a particle size of 45–75 μm. The material used for the top coat is an agglomerated and sintered powder: AMPERIT 827.873 made by H.C. Starck. The coatings are sprayed using a Sulzer Metco F4 gun. The substrate discs are 25.4 mm in diameter and between 5 and 6.35 mm thick, Table 1. The substrate discs were grit blasted with alumina grit and coated by 150 μm of NiCoCrAlY and 300 μm of 7% Y-PSZ. 2.2. Heat treatment The specimens were subjected to three different heat treatments: 1) isothermal oxidation, 2) thermal cycling fatigue (TCF) (performed at Siemens Industrial Turbomachinery) and 3) burner rig test (BRT) (performed at Volvo Aero Corporation). The heat treatments were performed for various lengths of time as summarised in Table 1. During thermal cycling fatigue, the specimens, which are placed on a table, are moved in and out of a furnace. One cycle consists of high temperature exposure at 1100 °C for 60 min followed by cooling for 10 min by compressed air; during cooling the specimens reach a minimum temperature of ~100 °C. Of the 60 min high temperature exposure, approximately 6 min is required to reach maximum temperature. A cycle in the burner rig consists of 75 s heating followed by 75 s of cooling. The heating is done on the coated side of the specimen with oxy-fuel using propane as fuel gas. During this, a temperature gradient is also introduced through the specimens by cooling the uncoated side with air. During the cold part of the cycle the specimen is moved out of the propane flame and cooled on the uncoated side by air. The equipment is further described by Vassen et al. [49]. By the Table 1 Number of specimens used in the present study. Heat treatment

Treatment length

Substrate thickness

Number of specimens used for adhesion test

As-sprayed Isothermal Isothermal TCF BRT

– 1h 23, 47, 111, 290 h 24, 48, 300 cycles 300, 1150 cycles

6.35 mm 5 mm 6.35 mm 6.35 mm 5 mm

4 3 4+4+4+4 4+4+4 3+3

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end of the hot part of the cycle, the temperatures on the coated and uncoated sides reach 1250 °C and 950 °C respectively. A steady-state thermal analysis, using finite element software and thermal conductivity data from Jinnestrand [50], predicts that the BC temperature will reach a maximum of approximately 1140 °C. Further information on the temperature during BRT is available in Liu et al. [51]. 2.3. Adhesion test The heat-treated specimens were subjected to adhesion tests using the ASTM C633 Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings. The method is described in the ASTM standard [52] and in several papers [34,43,53]. The method involves attaching the specimen to two rods using epoxy adhesive, enabling the specimen to be mounted in a tensile test machine equipped with universal joints. The specimen was loaded until fracture and the corresponding fracture stress was recorded. The number of specimens used is shown in Table 1; 3–4 specimens were used for each adhesion test to get a good mean value. The adhesive used was epoxy FM 1000, and the specimens were cured in a gravity bonding fixture (see ASTM C633 [52] for details) at 185 °C. The specimens were loaded with a displacement speed of 1.27 mm/min. 2.4. Specimen preparation and microscopy The fracture surfaces resulting from the adhesion test were analysed in a FEG-SEM Hitachi SU-70 scanning electron microscope

a

3.1. Adhesion test and fractography The results from the adhesion tests are reported in Fig. 2b); the fracture strength of the specimens has been reported as function of the TGO thickness as it enables the comparison between specimens subjected to isothermal and cyclic heat treatment, as well as comparison of TCF with BRT (where an accurate estimation of the high

number of cycles 20

102

10

15

10

5

103

as-sprayed vs. time isothermal vs. time TCF vs. cycles BRT vs. cycles

18

fraction black fracture,%

TGO thickness, µm

3. Results

103

isothermal vs. time TCF vs. time TCF vs. cycles BRT vs. cycles

20

(SEM) equipped with an energy dispersive spectroscopy (EDS) detector from Oxford Instruments. Prior to analysis, the specimens were coated with approximately 25 nm of carbon. The amount of black fracture was measured using EDS mapping; for each specimen 18 1.30 × 0.98 mm maps were established. The fraction of white fracture has been obtained as the area fraction of Zr and the fraction of black fraction has been obtained as the area fraction of areas with a combination of Ni, Co, Cr and Al. The measured area fractions have been normalised so that the area fraction of white and black fracture add up to 100%. EDS measurements on rough surfaces are only approximate. However, this method is considered to give an accurate enough estimate of the amount of black fracture present. In addition to the fracture surfaces, one specimen from each of the heat treatments was cross-sectioned and mounted for microscopy (without being adhesion tested). The specimens were epoxy infiltrated in vacuum, hot mounted and then ground with diamond abrasives down to 1 μm and polished using alumina dispersion.

a

cycles 102

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3

16 14 12 10 8 6 4 2

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100

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300

0 0

time at high temperature,h

50

100

150

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300

time at high temperature,h

b

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35

adhesion, MPa

30 25 20 15 10 as-sprayed isothermal TCF BRT

5 0

0

2

4

6

8

10

12

14

TGO thickness, µm 1 mm

Fig. 2. Interface TGO growth and changes in adhesion with high temperature exposure. a) The TGO thickness as function of time for isothermal oxidation and thermal cycling fatigue (TCF), and as function of cycles for TCF and burner rig test (BRT). b) The adhesion of the coating as function of TGO thickness.

Fig. 3. Fraction of black fracture. a) Fraction of black fracture as function of time for isothermal oxidation and as function of cycles for thermal cycling fatigue (TCF) and burner rig test (BRT). b) Adhesion tested specimen subjected to TCF 300 cycles, backscatter electron image.

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temperature exposure time is difficult). The interface TGO thickness is shown in Fig. 2a). The adhesion was found to vary with high temperature exposure time and number of cycles: the adhesion decreased with time for the two cyclic heat treatments whereas it increased slightly for isothermal oxidation. Specimens either gave white fracture or mixed fracture; none of the heat treatments studied gave entirely black fracture. Fig. 3b) shows a typical mixed fracture surface: a TCF-subjected specimen where black fracture can be seen as dark areas which are exposed Al2O3. The many areas of black fracture can be seen to range in size from ~100 μm to ~1 mm. The isothermal heat treatment gave essentially all white fracture, (≲ 4% black fracture), for all heat treatment times. Both cyclic heat treatments gave increasing amounts of black fracture with number of cycles, as shown in Fig. 3a). TCF-subjected specimens initially gave white fracture, (up to 48 cycles), but at 300 cycles a shift in fracture behaviour had occurred and the specimen showed mixed fracture. The BRT-subjected specimens gave mixed fracture for both 300 and 1050 cycles. The largest amount of black fracture observed was found for the as-sprayed condition.

3.2. Characteristics of white fracture All specimens, regardless of heat treatment, gave about 80–100% white fracture. Despite the different heat treatments the areas of white fracture all show similar characteristics; the characteristics of white fracture for the as-sprayed condition are shown in Fig. 4. The white areas of the fracture surface consists mainly of large areas of interlamellar fracture (fracture between splats), shown in Fig. 4a). In these areas the (pre-heat treatment) cracking of the splats is clearly seen as a network of through-splat microcracks.

a

There are a few examples of translamellar (through-splat) fracture, some examples of which are shown in Fig. 4. Sparse translamellar fracture can give rise to a terrace like fracture, shown in Fig. 4a); Fig. 4a) shows example of an interlamellar crack cutting through a layer of splats continuing in the layer beneath, thus giving a short through-splat crack. Fig. 4b) shows an interlamellar crack taking a short-cut through a valley-like feature in the TC and thereby giving rise to translamellar cracking. Translamellar fracture may also occur at discontinuities such as unmelted or partially melted particles; Fig. 4c) shows extensive translamellar fracture associated with a partially melted TC particle. Fig. 4d) shows translamellar cracking related to a peak in the splat–on–splat structure. In this case the fracture through the peak has occurred just at the BC/TC interface and the fracture surfaces reveals a part of the unoxidised BC.

3.2.1. Characteristics of black fracture Fig. 5 shows areas of black fracture from the four specimens that gave black fracture: as-sprayed specimen, TCF 300 cycles, BRT 300 cycles and BRT 1150 cycles. Fig. 5a) shows the as-sprayed specimen where the black fracture has revealed the unoxidised BC and some pre-heat treatment Al2O3. Fig. 5b) shows black fracture in a specimen subjected to 300 cycles of TCF. The fracture has occurred mainly in the TGO/TC interface leaving the TGO essentially intact except for at large bulky oxide clusters which are usually cut through during fracture; in some cases, the fracture has occurred in the BC/TGO interface thus revealing the metallic BC. Fig. 5c) shows fracture in a specimen subjected to 300 cycles of BRT. The fracture has occurred entirely in the TGO/TC interface; the thin even Al2O3 TGO layer is left completely intact. Fig. 5d) shows black fracture in a specimen subjected to 1150 cycles of BRT. As for BRT 300 cycles, the TGO consists mainly of Al2O3,

b

A C B

10 µm

c

5 µm

d

TC

BC D

E 10 µm

5 µm

Fig. 4. Characteristics of white fracture; backscatter electron images of as-sprayed specimen. a) Terrace like fracture with large areas of inter-splat fracture containing a network of in-splat microcracks, A, and through-splat fracture, B. b) Through-splat cracking associated with a valley, leaving residuals of a splat in the valley, C. c) Through-splat cracking, marked with arrows, associated with an unmelted particle, D. d) Through-splat cracking, E, associated with a peak, in this case revealing the underlying BC.

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Fig. 5. Characteristics of black fracture, backscatter electron images. a) Fracture in an as-sprayed specimen revealing the metallic BC and some Al2O3. b) Specimen subjected to 300 cycles of thermal cycling fatigue showing a cut-through oxide cluster, revealed metallic BC, as well as TGO/TC fracture. c) Specimen subjected to 300 cycles of burner rig test. The fracture has left the TGO intact. d) Specimen subjected to 1150 cycles of burner rig test showing BC/TGO and TGO/TC fracture.

a

b

1 µm

1 µm

c

d

1 µm

1 µm

Fig. 6. Changes in splat grain structure in the top coat, all images are backscatter SEM images. a) Two closely interconnected splats in the as-sprayed specimen; the arrows point out different types of nucleation. b) Grain coarsening in specimen subjected to isothermal oxidation, 290 h. c) Modest grain coarsening in specimen subjected to burner rig test, 1150 cycles. d) Grain coarsening in specimen subjected to thermal cycling fatigue, 300 cycles.

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but here part of the fracture has occurred in the BC/TGO interface in addition to the TGO/TC interface. 3.3. Microstructural development with high temperature exposure 3.3.1. Microstructural changes in the TC The columnar grain structure of the zirconia splats is shown in Fig. 6. In the as-sprayed condition, the columns occasionally join together into larger polygon-shaped groups, as can be seen more clearly in Fig. 7a). Fig. 6a) shows two closely interconnected splats. The interconnection has occurred as the upper of the two has nucleated from the lower; the lower splat, on the other hand, has formed a layer of small equiaxed grains on impact from which the columnar grains have grown. Furthermore, Fig. 6 shows the influence of heat treatment on the in-splat grain structure. Fig. 6b) shows a specimen subjected to isothermal oxidation 290 h and Fig. 6d) shows a specimen subjected to TCF 300 cycles. Both show a coarsening of the columnar grain structure in the splats. Fig. 6c) shows a specimen subjected to 1150 cycles of BRT in which the coarsening of the grain structure is far less advanced compared to the case of TCF 300 cycles and isothermal oxidation 290 h. (It should be noted that the images in Fig. 6 is taken at through-splat microcracks, as those shown in Fig. 4a) and Fig. 7, which means that the displayed columnar grains have been located at a surface throughout the heat treatment.) Fig. 7a) shows in-splat microcracks which arose during spraying; it can be seen that these microcracks are always intergranular; neither do the microcracks split the polygon-shaped groups of columnar grains. Fig. 7b), c) and d) shows healing of in-splat cracks for the three different heat treatments. The crack healing is more prominent for isothermal oxidation and TCF compared to BRT. The isothermal oxidation and TCF both show very similar behaviour, such as: large

a

areas of almost complete crack healing, shown in Fig. 7b), and areas of healing by points, as shown in Fig. 7d). The polygon shaped groups of columnar grains can no longer be observed in specimens subjected to isothermal oxidation or TCF; for BRT-subjected specimens the groups can still be distinguished, mainly as elongated groups as shown in Fig. 7c). Sintering effects between splats are hard to distinguish on the fracture surfaces. Such sintering effects are more readily distinguishable in cross-sections, Fig. 1b). 3.3.2. Development of BC/TC interfacial TGOs The BC/TC interface TGO growth can be seen in Fig. 2a), (further details regarding the TGO thickness measurements can be found in Ericksson et al. [46]). Isothermal oxidation and TCF both give very similar TGO growth. The interface TGO growth for the BRT subjected specimens are reported in Fig. 2a) as a function of cycles. Due to the short high temperature exposure times associated with this heat treatment method, the TGO thickness remains small throughout the testing. Fig. 8 shows some typical oxides that can be found on the areas of black fracture after the adhesion test. Fig. 8a) shows a large area of Al2O3 on top of which there are some small colonies of Cr-rich oxides: (Al, Cr)2O3 and spinels. Fig. 8a) also shows an area where the TGO has undergone extensive cracking; this can be seen in the case of TCF, but has not been observed for BRT, not even after 1150 cycles. Fig. 8b) shows a cross-section of the BC/TC interfacial TGO. The TGO consists of columnar Al2O3 crowned by (Al, Cr)2O3 or possibly spinel. Fig. 8c) displays a bulky oxide cluster which has been cut through during the adhesion test. The centre of the oxide cluster consists of blocky NiO surrounded by granular Cr-rich oxides. Fig. 8d) shows a rather smooth layer of columnar Al2O3 crowned by granular (Al, Cr)2O3. Such granular Cr-rich oxides often make up the Cr oxide colonies shown in Fig. 8a).

b

0.5 µm

c

0.5 µm

d

0.5 µm

0.5 µm

Fig. 7. Changes in splat grain structure and sintering of in-splat cracks, SEM backscatter electron images. a) As-sprayed condition: the dotted line marks a polygon shaped group of grains, arrow marks in-splat cracking. b) Isothermal oxidation 300 h: coarsening of columnar grains, arrows mark closure of an in-splat crack. c) Burner rig test 1150 cycles: slight grain coarsening, arrows mark closure of an in-splat crack. d) Thermal cycling fatigue 300 cycles: image shows grain coarsening, arrows mark closure of an in-splat crack.

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a

TC

b

Al2O3

7

Al2O3

TC

Cr-rich Cr-rich

TC

BC 20 µm

c

2 µm

d

Al2O3 NiO

Al2O3

Cr-rich

Cr-rich /spinel 10 µm

1µm

Fig. 8. Different oxides found on areas of black fracture after adhesion test, images from specimen subjected to 300 cycles of thermal cycling fatigue. a) SEM backscatter electron image. The TGOs consist mainly of Al2O3 and some Cr-rich oxides, arrows mark cracks in the TGO. b) SEM backscatter electron image, cross-section of a columnar interface Al2O3 layer crowned by Cr-rich oxides. c) SEM backscatter electron image of an oxide cluster in the interface TGO layer. The core of the cluster consists of blocky NiO which is surrounded by spinels and chromia. d) SEM secondary electron image of granular Cr-rich oxides on top of interfacial Al2O3.

4. Discussion 4.1. Fractography and adhesion Interlamellar fracture forms from the pre-existing inter-splat delaminations; the tendency of cracks to propagate in the inter-splat delaminations has previously been found for the TC in static fracture [12,47], and fatigue [26,48]. For translamellar cracks, the pre-existing in-splat microcracks offer effective crack paths [47]; this is particularly visible in Fig. 4a) where the saw-tooth shaped terraces, marked by B, clearly follow the pre-existing microcracks. The most extensive areas of translamellar fracture found on the fracture surfaces are often associated with partially melted particles, such as the one shown in Fig. 4c). The transition of fracture mechanism from TC fracture to interface fracture that occurs in TCF-subjected specimens, is due to an increase in interface damage. The increase in TGO thickness alone does not cause a transition from white to black fracture as demonstrated by the isothermal oxidation which gives white fracture for all times. In the case of TCF-subjected specimens, the interface TGO is clearly cracked, as shown in Fig. 8a), such interface TGO cracking is likely to cause a shift from white to mixed fracture. Yamazaki et al. [39] found that even isothermally oxidised TBC systems do change from mainly white fracture to mixed fracture as the interface TGO thickness increases. However, that transition occurred for ageing times well above the ≤ 300 h of high temperature exposure used in this study. The bulky clusters of mixed-element oxides are also affecting the adhesion; especially for TCF, the interface TGO contains a certain amount of bulky oxide clusters. As shown by, for example, Chen et al. [26], oxide clusters may act as effective crack nucleation sites, and the TCF-subjected specimens do display cut-through oxide

clusters on the fracture surface, as also shown by Nesbitt et al. [48]. Such damage in the TGO is likely to decrease adhesion of TBCs and shift the fracture from all white to mixed type fracture, as the BC/TC interface weakens. A shift in fracture mechanism can be seen between BRT 300 cycles and BRT 1150 cycles, as BRT 1150 cycles, in addition to TGO/TC fracture, gives large areas of BC/TGO fracture. Compared to TCF 300 cycles, the amount of BC/TGO fracture is somewhat larger in the case of BRT 1150 cycles. However, the TGO in BRT 1150 cycles still consists of predominantly Al2O3 and cut-through oxide clusters are uncommon in the case of BRT. Comparing the two cyclic heat treatments it can be noted that when the TCF-subjected specimens reach 300 cycles the adhesion has dropped to roughly the same as for 300 cycles of BRT. One might imagine competing mechanisms: 1) beneficial effects increasing the adhesion such as: sintering, relaxation of residual stresses and development of a thin layer of interfacial TGOs (increasing the chemical bonding), and 2) degrading effects decreasing adhesion such as: crack initiation, crack growth and coalescence of cracks. For BRT, the short high temperature exposure times means it is missing the beneficial effects, as they are thermally activated. The TCF on the other hand, initially benefits from heat treatment, but is eventually dominated by degrading mechanisms. However, while high TGO growth may have some beneficial effects, it is also evident that it speeds up the degrading effects of cycling, as evident from Trunova et al. [54] which shows that a longer dwell time reduces the number of cycles the TBC system can withstand before failure. This is also implied by the observation that 300 cycles of BRT gives less than half the fraction of black fracture as TCF gives for 300 cycles; it takes 1150 cycles of BRT to reach the same amount of black fracture as TCF reaches already at 300 cycles. As pointed out by Trunova et al. [54], a shorter dwell time increases

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the number of cycles a TBC system can withstand before failure but decreases the total high temperature exposure time at failure. The as-sprayed condition gave the highest fraction of black fracture. However, the fracture mechanism changes from mixed fracture to all white fracture after only 1 h of heat treatment, as seen in Fig. 3a). This change in fracture mechanism is attributed to the combined effect of relaxation of BC/TC residual stresses [42], and beneficial effects of the development of a thin layer of TGO; the latter a mechanism that has also been suggested by Markocsan et al. [43] after observing an increase in adhesion of isothermally heat treated TBCs. 4.2. Microstructure Since the white parts of the fracture remains interlamellar throughout the heat treatment the sintering between splat is evidently not extensive enough to change the fracture from predominantly interlamellar to predominantly translamellar. It is also possible that the sintering of the microcrack network in splats, to some extent, suppresses translamellar cracking, as it prevents cracks taking short-cuts through splats. Nevertheless, some interlamellar sintering is likely to take place. This is perhaps most clearly demonstrated by Yamazaki [55] who, for TBC systems subjected to various heat treatments, observed that a decrease in interlamellar TC fracture (and consequently an increase in translamellar TC fracture) was accompanied by an increase in Young's modulus and tensile strength, as would be expected in the case of sintering between splats. While the shift from mixed fracture, in the as-sprayed condition, to all white fracture for isothermal heat treatment can be explained by a strengthening of the BC/TC interface, the slight increase in adhesion for isothermal heat treatment indicates that, at least, some sintering between splats occurs. Higher fracture strength associated with isothermal heat treatment is therefore attributed to the combined effect of increased interface strength and sintering between splats. In the case of cyclic heat treatments, (TCF and BRT), it might very well be that the sintering between splats is counteracted by continuous re-cracking of the TC. 5. Conclusions Fractographic studies have been performed on adhesion tested APS TBC systems subjected to isothermal and cyclic heat treatments. The fracture has been found to largely follow the pre-existing defect network in the top coat; the fracture surfaces consisted of predominantly interlamellar fracture for all heat treatments studied here. Furthermore, cyclic heat treatment has been found to promote fracture in the BC/TC interface, as both cyclic heat treatments gave increasing amounts of BC/TC fracture with number of cycles; in contrast, isothermally oxidised specimens gave white fracture for all heat treatment times. By comparing and contrasting the two cyclic heat treatments the following can be said: • Both cyclic heat treatments give larger fractions of black fracture with increasing number of cycles. • BRT-subjected specimens, initially, give somewhat lower adhesion than TCF-subjected specimens, but at 300 cycles of TCF the adhesion has dropped to roughly the same as for BRT-subjected specimens. • The BRT-subjected specimens are exposed to significantly shorter high temperature exposure and have consequently a much thinner TGO and very few bulky oxide clusters. TCF, on the other hand, gives thicker TGO with a larger number of oxide clusters which are usually cut through during adhesion test. • The black parts of the fracture occur predominantly in the TGO/TC interface for both BRT and TCF, but both cyclic heat treatments give

some BC/TGO fracture. In the case of BRT the fraction of BC/TGO fracture increases with number of cycles. • TCF initially gives white fracture but shifts to mixed fracture as the cycling proceeds, while BRT gives mixed fracture even for low number of cycles.

Acknowledgement This research has been funded by the Swedish Energy Agency, Siemens Industrial Turbomachinery AB, Volvo Aero Corporation, and the Royal Institute of Technology through the Swedish research programme TURBO POWER, the support of which is gratefully acknowledged. The authors would also like to accentuate the contribution of Professor Sören Sjöström who participates in the ongoing TBC research at Linköpings universitet. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

[28]

[29] [30] [31] [32]

[33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]

D. Stöver, C. Funke, J. Mater. Process. Technol. 92–93 (1999) 195. X. Zhao, P. Xiao, Mater. Sci. Forum 606 (2009) 1. W. Nelson, R. Orenstein, J. Therm, Spray Technol. 6 (1997) 176. A. Evans, D. Clarke, C. Levi, J. Eur. Ceram. Soc. 28 (2008) 1405. R. Vassen, A. Stuke, D. Stöver, J. Therm, Spray Technol. 18 (2009) 181. L. Chen, Surf. Rev. Lett. 13 (2006) 535. M. Belmonte, Adv. Eng. Mater. 8 (2006) 693. R. Mévrel, Mater. Sci. Eng. A 120 (1989) 13. X. Cao, R. Vaßen, D. Stöver, J. Eur. Ceram. Soc. 24 (2004) 1. S. Stecura, Adv. Ceram. Mater. 1 (1986) 68. C. Levi, Curr. Opin. Solid State Mater. Sci. 8 (2004) 77. P. Harmsworth, R. Stevens, J. Mater. Sci. 27 (1992) 616. P. Bengtsson, Microstructural, residual stress and thermal shock studies of plasma sprayed ZrO2-based thermal barrier coatings, Ph.D. thesis, Linköpings universitet, 1997. G. Antou, G. Montavon, F. Hlawka, A. Cornet, C. Coddet, Mater. Charact. 53 (2004) 361. A. Nusair Khan, J. Lu, H. Liao, Mater. Sci. Eng. A 359 (2003) 129. R. McPherson, Surf. Coat. Technol. 39–40 (1989) 173. P. Niranatlumpong, C. Ponton, H. Evans, Oxid. Met. 53 (2000) 241. H. Evans, M. Taylor, Proc. IMechE 220 (2006) 1. G. Wallwork, A. Hed, Oxid. Met. 3 (1970) 171. J. Davis (Ed.), Heat-Resistant Materials, ASM International, 1999. D. Achar, R. Munoz-Arroyo, L. Singheiser, W. Quadakkers, Surf. Coat. Technol. 187 (2004) 272. K. Ma, J. Schoenung, Surf. Coat. Technol. 205 (2010) 2273. H. Brodin, M. Eskner, Surf. Coat. Technol. 187 (2004) 113. W. Brandl, H. Grabke, D. Toma, J. Kruger, Surf. Coat. Technol. 86–87 (1996) 41. W. Chen, X. Wu, B. Marple, D. Nagy, P. Patnaik, Surf. Coat. Technol. 202 (2008) 2677. W. Chen, X. Wu, B. Marple, P. Patnaik, Surf. Coat. Technol. 201 (2006) 1074. H. Brodin, R. Eriksson, S. Johansson, S. Sjöström, in: D. Zhu, H.-T. Lin, D. Singh, J. Salem (Eds.), Advanced Ceramic Coatings and Interfaces IV, Ceramic Engineering and Science Proceedings, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2010. H. Brodin, Failure of thermal barrier coatings under thermal and mechanical fatigue loading: microstructural observations and modelling aspects, Ph.D. thesis, Linköpings universitet, 2004. E. Busso, J. Lin, S. Sakurai, Acta Mater. 49 (2001) 1529. A. Rabiei, A. Evans, Acta Mater. 48 (2000) 3963. R. Vassen, G. Kerkhoff, D. Stöver, Mater. Sci. Eng. A 303 (2001) 100. S. Sjöström, H. Brodin, in: D. Zhu, H.-T. Lin, S. Mathur, T. Ohji (Eds.), Advanced Ceramic Coatings and Interfaces V, volume 31 of Ceramic Engineering and Science Proceedings, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2010. S. Sjöström, H. Brodin, in: P. Lukáš (Ed.), Procedia Engineering, volume 2, 2010, pp. 1363–1371. M. Arrigoni, S. Barradas, M. Braccini, M. Dupeux, M. Jeandin, M. Boustie, C. Bolis, L. Berthe, J. Adhes. Sci. Technol. 20 (2006) 471. Q. Hongyu, Y. Xiaoguang, W. Yamei, Int. J. Fract. 157 (2009) 71. L.L. Shaw, B. Barber, E.H. Jordan, M. Gell, Scr. Mater. 39 (1998) 1427. P.F. Zhao, C.A. Sun, X.Y. Zhu, F.L. Shang, C.J. Li, Surf. Coat. Technol. 204 (2010) 4066. M. Hadad, G. Margot, P. Démarécaux, D. Chicot, J. Lesage, L. Rohr, S. Siegmann, Surf. Eng. 23 (2007) 279. Y. Yamazaki, A. Schmidt, A. Scholz, Surf. Coat. Technol. 201 (2006) 744. D. Chicot, G. Duarte, A. Tricoteaux, B. Jorgowski, A. Leriche, J. Lesage, Mater. Sci. Eng. A 527 (2009) 65. Y. Yamazaki, S.-I. Kuga, M. Jayaprakash, in: M. Guagliano, L. Vergani (Eds.), Procedia Engineering, volume 10, 2011, pp. 845–850. D. Chicot, G. Marot, P. Araujo, N. Horny, A. Tricoteaux, M.H. Staia, J. Lesage, Surf. Eng. 22 (2006) 390. N. Markocsan, P. Nylén, J. Wigren, X.-H. Li, A. Tricoire, J. Therm, Spray Technol. 18 (2009) 201.

Please cite this article as: R. Eriksson, et al., Surf. Coat. Technol. (2012), doi:10.1016/j.surfcoat.2012.02.040

R. Eriksson et al. / Surface & Coatings Technology xxx (2012) xxx–xxx [44] M. Gell, E. Jordan, K. Vaidyanathan, K. McCarron, B. Barber, Y.-H. Sohn, V.K. Tolpygo, Surf. Coat. Technol. 120–121 (1999) 53. [45] A. Nusair Khan, J. Lu, H. Liao, Surf. Coat. Technol. 168 (2003) 291. [46] R. Eriksson, H. Brodin, S. Johansson, L. Östergren, X.-H. Li, Surf. Coat. Technol. 205 (2011) 5422. [47] E. Wessel, R. Steinbech, Key Eng. Mater. 223 (2002) 55. [48] J. Nesbitt, D. Zhu, R. Miller, C. Barrett, Mater. High Temp. 20 (2003) 507. [49] R. Vassen, F. Cernushi, G. Rizzi, A. Scrivani, N. Markocsan, L. Östergren, A. Kloosterman, R. Mevrel, J. Feist, J. Nicholls, Adv. Eng. Mater. 10 (2008) 907. [50] M. Jinnestrand, Delamination in APS applied thermal barrier coatings: life modelling, Ph.D. thesis, Linköpings universitet, 2004.

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[51] Y. Liu, C. Persson, J. Wigren, J. Therm, Spray Technol. 13 (2004) 415. [52] ASTM Standard C633 - 01, Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings, ASTM International, West Conshohocken, PA, 2008. [53] G. Qian, T. Nakamura, C.C. Berndt, S.H. Leigh, Acta Mater. 45 (1997) 1767. [54] O. Trunova, T. Beck, R. Herzog, R.W. Steinbrech, L. Singheiser, Surf. Coat. Technol. 202 (2008) 5027. [55] Y. Yamazaki, J. Solid Mech. Mater. Eng. 2 (2008) 1275.

Please cite this article as: R. Eriksson, et al., Surf. Coat. Technol. (2012), doi:10.1016/j.surfcoat.2012.02.040