Solid particle erosion of thermal spray and physical vapour deposition thermal barrier coatings

Wear 271 (2011) 2909–2918

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Solid particle erosion of thermal spray and physical vapour deposition thermal barrier coatings F. Cernuschi a,∗ , L. Lorenzoni a , S. Capelli a , C. Guardamagna a , M. Karger b , R. Vaßen b , K. von Niessen c , N. Markocsan d , J. Menuey e , Carlo Giolli f a

RSE – Ricerca per il Sistema Energetico, Via Rubattino, 54, 20134 Milano, Italy Forschungszentrum Jülich GmbH, Institut für Energieforschung IEF-1, 52425 Jülich, Germany c Sulzer Metco AG, Rigackerstr. 16, CH-5610, Wohlen, Switzerland d University West, Dept. of Engineering Science, Gustava Melins gata 2, SE-461 86 Trollhattan, Sweden e Snecma, 1 Rue Maryse Bastié, 86100 Châtellerault, France f Turbocoating SpA, Via Mistrali 7, Rubbiano di Solignano, 43030, (PR) Italy b

a r t i c l e

i n f o

Article history: Received 21 December 2010 Received in revised form 27 May 2011 Accepted 9 June 2011 Available online 16 June 2011 Keywords: Solid particle erosion Thermal spray coatings High temperature Electron microscopy

a b s t r a c t Thermal barrier coatings (TBC) are used to protect hot path components of gas turbines from hot combustion gases. For a number of decades, in the case of aero engines TBCs are usually deposited by electron beam physical vapour deposition (EB-PVD). EB-PVD coatings have a columnar microstructure that guarantees high strain compliance and better solid particle erosion than PS TBCs. The main drawback of EB-PVD coating is the deposition cost that is higher than that of air plasma sprayed (APS) TBC. The major scientific and technical objective of the UE project TOPPCOAT was the development of improved TBC systems using advanced bonding concepts in combination with additional protective functional coatings. The first specific objective was to use these developments to provide a significant improvement to state-of-the-art APS coatings and hence provide a cost-effective alternative to EB-PVD. In this perspective one standard porous APS, two segmented APS, one EB-PVD and one PS-PVDTM were tested at 700 ◦ C in a solid particle erosion jet tester, with EB-PVD and standard porous APS being the two reference systems. Tests were performed at impingement angles of 30◦ and 90◦ , representative for particle impingement on trailing and leading edges of gas turbine blades and vanes, respectively. Microquartz was chosen as the erodent being one of the main constituents of sand and fly volcanic ashes. After the end of the tests, the TBC microstructure was investigated using electron microscopy to characterise the failure mechanisms taking place in the TBC. It was found that PS-PVDTM and highly segmented TBCs showed erosion rates comparable or better than EB-PVD samples. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Ceramic thermal barrier coatings (TBCs) are widely applied for protecting hot path components of gas turbines from combustion gases. By using this refractory ceramic porous layer deposited on surfaces of base materials of vanes, blades, transition pieces and combustion chambers, the temperature on metallic substrates can be reduced by 30–150 ◦ C depending on the thickness and on the specific properties of the coating [1]. The state-of the-art of these TBCs is represented by yttria (partially) stabilized zirconia (YPSZ) (7–8 wt.% Y2 O3 + ZrO2 ) deposited onto the components either by air

∗ Corresponding author. Tel.: +39 02 3992 4577; fax: +39 02 3992 5626. E-mail address: [email protected] (F. Cernuschi). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.06.013

plasma spray (APS) or by electron beam physical vapour deposition (EB-PVD) [1]. Owing to the deposition process, APS TBC show a porous microstructure consisting of a superposition of ceramic lamellar shaped splats, interlamellar fine penny shaped pores and either trans granular or intergranular microcracking with the major axis oriented perpendicular and parallel to the TBC surface, respectively. On the other hand, EB-PVD coatings have mostly columnar microstructure (even the porosity among columns shows a columnar structure) that guarantees higher strain compliance but lower thermal insulation compared to APS TBC [2–5]. To improve strain compliance and erosion resistance of APS TBC, keeping the deposition costs lower than EB-PVD coatings, dense vertically cracked APS TBC have been developed within the last decades. These coatings consist of a quite dense microstructure segmented by vertical cracks penetrating most of the TBC thick-

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ness. Depending on the crack density, the strain compliance of these coatings can be significantly improved. At the same time the dense microstructure guarantees a better resistance to solid particle erosion. A thermal conductivity comparable with EB-PVD coating is the main drawback of these coatings compared to standard APS TBC [6]. In aero gas turbines, blades & vanes and combustors & transitions are usually coated by EB-PVD and APS TBC, respectively. On the contrary, for land based applications hot path components of most of the engines are coated by APS TBC. Main failure mechanism of TBCs during service (or testing at high temperatures) is governed by the thermally grown oxide (TGO) at the interface between the metallic bondcoat and the ceramic coating and by the mismatch of thermal expansion coefficients between the ceramic layer and the metallic substrate. Depending on the operating (or testing) conditions one of these two phenomena can be predominant compared to the other one. In any case, both these effects are the driving forces for nucleation, propagation and coalescence of cracks parallel to the interface between ceramic top coat and the metallic bondcoat up to the TBC spallation [7–13]. Solid particle erosion is another failure mode of TBC. This is especially true for aero gas turbines operating in sandy (or ashy) environments, but even for land based gas turbines, where air is filtered before entering the compressor stage, solid particle erosion can take place owing to particles escaped from filters, or produced either within compressor stages or in the combustion chamber, depending on the materials and on the operating conditions of the specific engine. Owing to their inertia, solid particles do not move along the flow streamlines and thus they impact on components eroding the protective coatings from the base materials. Pressure loss, change in blade geometry and overheating of base metals are the main effects of erosion in gas turbines [14–16]. Erosion mechanisms in APS and EB-PVD coatings differ significantly because of their different microstructure, as described in the next section. In this work, the main results of high temperature solid particle erosion tests on three innovative and on two reference TBCs systems are reported. The effect of impingement angle and speed on erosion rates has been investigated. The different failure modes for the five tested systems have been studied by scanning electron microscopy. 2. Review of erosion mechanisms in TBCs 2.1. APS coatings Following the work of Eaton and Novak, three different types of solid particle erosion can be distinguished in APS TBCs [14]: • primary scars as principal observable feature on the erosion surface (low erosion rate), • occurrence of fractures around the impact area on the coating surface (moderate erosion rate), • tunnel formation on the surface (high erosion rate). In the first case impacting particles produce mainly indentations on the surface and erosion takes place as material loss caused by successive impacts on deformed material. In the second type of mechanism impact produces crack propagation along splat boundaries. In the third case the kinetic energy transferred from the particles to the target is high enough to connect pre-existing pores inside the TBC eroding clusters of several splats each time. They also found a correlation between the strength of TBC as measured by four point bending test and the erosion rate (the

higher the first the lower the second). When the overall porosity is fixed, the erosion rate increases as a function of the specific surface area of the porosity. Starting from these results they proposed a linear relationship between the erosion rate we and the ratio of the normalised specific area C (i.e. the area per unit of weight and of volumetric porosity content) to the strength : we = a

C +b 

(1)

where a and b are constants. As also predicted by models for bulk ceramics, erosion rate is strongly dependent on the ratio of the coating to particle micro-hardness, independently from the porosity fraction, as shown by Janos et al. [17]. When the particle microhardness is fixed, the dependence is just on the coating microhardness. Nichols et al. and Li et al. describe the erosion of APS TBC as occurring through spalling off surface lamella resulting from impact of erodent particles [18,19]. Accordingly, the erosion of the coating is controlled by crack propagation along the interface between neighbour lamellae. In other words, APS fails by propagation of cracks around splat boundaries and through the microcrack network. This means that the higher the percentage and/or the strength of the bonded interface among lamellae the lower the erosion rate. Starting from the McPherson modelling of a plasma sprayed coatings [20], Li et al. describe the erosion rate as proportional to the lamellar interface bonding ratio ˛, the lamellar thickness ı and the effective surface energy of lamellar material  c [19]: we ∝

c Eeff 2c ˛

x

(2)

where c and Eeff are the density of the target and the fraction of the kinetic energy per unit mass of impacting particles promoting cracking, respectively. Here 2 c ˛ is the fracture toughness of the TBC. They also report that if a weaker bonding between lamellae of two different passes is observed a higher erosion rate occurs [19]. Since sintering process promotes the bonding between lamellae, an increase of the erosion resistance of APS TBC is reported in the literature for aged samples [19,21]. 2.2. EB-PVD coatings In the case of EB-PVD coatings the columnar structure is responsible for damage modes not comparable with those typical of bulk ceramic materials and APS coatings. In particular, Wellman and Nicholls and Chen et al. describe three possible modes [21–25]. 2.2.1. Mode I (near surface cracking/lateral cracking) When small particles impact on EB-PVD TBC surface with a sufficiently low speed, the top 20 ␮m of the individual columns are cracked due to impact. Following Chen, in this experimental condition the response of the TBC is only elastic and cracking parallel to the surface are caused by tensile stresses promoted by the elastic waves propagating forward and backward along each single columns all around the impingement site. Reduced erosion rates correspond to this damage mode. This damage mode has been observed both at RT and at high temperature even if at high temperature the erosion rates differ because of the temperature dependence of elastic modulus hardness and fracture toughness of TBC. A limited amount of cracks occur even deeper than 20 ␮m. This type of cracks usually is initiated at the base of the dendritic column edge structure [21–24]. 2.2.2. Mode II (compaction damage) Due to impingement of particles with slightly higher momentum (speed and or mass) compared to the previous case, a

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densification of a shallow top layer (1–20 ␮m) of the TBC caused by a plastic deformation is observed. This is a typical multi particles phenomenum. As a matter of fact, each impact is not able to promote cracking, but a wide densified top layer is obtained as the results of many impacts. At the interface between this densified layer and each underlying column, cracks can nucleate and propagate due to stress concentration induced by subsequent impacts. Usually some overlapping areas can be observed where both Mode I and Mode II take place on the same sample [21–24]. 2.2.3. Mode III (foreign object damage FOD) When particles with high momentum (high speed large particles) impact on the EB-PVD TBC, most of the kinetic energy is absorbed by plastic deformation and densification bending. Deformation is accompanied by kink bands around the perimeter of the plastic zone. These crack bands propagate conically toward the coating and bend close to the interface between TBC and bondcoat promoting delaminations: the higher the temperature the higher the plastic deformation. A different type of FOD type has been recently observed at high temperature. In particular, column buckling without relevant cracking and densification is observed. In this case only subsequent impacts increase the buckling degree up to a threshold level that promotes cracking and thus material loss. These two types of FOD were found to occur in the same sample and thus they are not mutually exclusive [21–24]. As concerns ageing at high temperature, an increase of the erosion rate for EB-PVD coatings contrarily to the case of APS coatings, is observed. In fact, sintering allows cracks to propagate easily along more neighbouring sintered columns in all the three damage modes [21,26,27]. Furthermore, Nicholls et al. report a linear dependence from impingement speed for both APS and EB-PVD coatings but a seven to 10 times higher erosion rate for the former coating type has been observed [18]. On the contrary, EB-PVD and dense vertically cracked APS TBC show similar erosion rates [21]. 3. Experimental 3.1. Testing facility The solid particle erosion resistance of the TBC systems has been tested using a Jet tester (see Fig. 1) consisting in:

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i. The test chamber containing the nozzle, the sample holder and the system for the erosive particle speed measurement. ii. Two furnaces for heating the carrier gas of the erosive particles. iii. The erosive particle powder feeder. iv. The cyclone for powder filtering and removal. v. The control board. Two coils inserted within the two furnaces allow the carrier gas (typically compressed air) to be heated up to the desired temperature (from RT up to 700 ◦ C). The solid erosive particles (proportioned by a powder feeder) are injected within the main gas stream and, through the nozzle, they are allowed to impinge at the desired angle on the surface of the tested specimen. The erosive particle speed is directly related to the pressure of the carrier gas. Two small furnaces inside the testing chamber keep the sample and nozzle at the desired temperature, respectively. The exhausted powder is then removed and filtered by the cyclone. Erosive particle speed measurement is performed using the rotating double-disk method [28]. The powder feeder is positioned on a precision balance to monitor the erosive impacted mass. Typical erosive flow rates and erosion loads range from about 2 g min−1 up to 5 g min−1 and 1–4 kg m−2 s−1 , respectively. The test procedure consists in the following steps: • fix all the experimental parameters; • weigh the specimen; • position the specimen on the holder and wait until the testing temperature has been reached; • make a fixed amount of powder impinge on the specimen surface; • weigh the specimen. This procedure is repeated until either an established total mass of erosive has impacted on the specimen surface or the top layer is completely eroded. After the end of the test, for each specimen, the evaluation of the erosion rate, as the slope of the straight line best fitting the experimental data, is performed by plotting the eroded weight as a function of the impacted mass. The incidence angle between the erosive particles and the specimen surface is set up by mounting samples within sample holders having the desired inclination in respect to the impinging particle stream 3.2. Samples Solid particle erosion resistance of one standard porous APS, two segmented APS, one EB-PVD and one Plasma Spray-PVDTM TBCs has been experimentally investigated. Table 1 resumes the main microstructural features of the five TBC systems. In particular, TBC thickness, porosity and crack density (or column width) have been estimated by image analysis of secondary and back scattered electron images of the as deposited samples. Fig. 2 shows the microstructure of all the five TBC systems. Moreover, as reported within Table 1, for the APS coatings Vickers microhardness (3 N) has been measured along the section. CMSX4 single crystal Ni-base superalloy was used as substrate, APS TBC have been sprayed onto a 100–300 ␮m CoNiCrAlY bondcoat (Amdry 995) deposited by LPPS (Low Pressure Plasma Spray) technique. For both PVD TBCs, the bondcoat was a diffusion Pt-Al coating. For impingement angles equal to 90◦ and to 30◦ , 4 mm thickness 25 mm disks and rectangular 40 × 15 × 4 mm samples have been considered, respectively. In the following a short description of each TBC system is given.

Fig. 1. Overall view of the solid particle erosion (SPE) facility: (1) test chamber, (2) heating furnaces, (3) powder feeder, (4) cyclone, and (5) control board.

3.2.1. APS standard porous TBC The ceramic top coating has been deposited by air plasma spray, using ARTEC SpA F4A type torch and YPSZ commercial powder

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Table 1 Main microstructural features of the five TBC systems. TBC system

TBC thickness [␮m]

Porous APS Segmented APS Highly segmented APS PS-PVDTM EB PVD

416 580 408 146 156

± ± ± ± ±

28 36 30 5 4

TBC porosity [%]

TBC microhardness HV300g

Crack density [mm−1 ]

Column width [␮m]

Secondary arms thickness [␮m]

± ± ± ± ±

571 ± 105 1143 ± 197 1130 ± 122 – –

– 2.7 7.2 – –

– – – 8–17 2–12

– – – 1 <1

19 8.7 5.0 12.5 10.0

1 1 0.5 1 0.5

(Amperit 827.7 produced by H.C. Starck) with a particle size distribution in a range from 45 to 90 ␮m. The cubic and tetragonal phase content is 98 and 1 wt%, respectively; whereas the monoclinic phase content is 1 wt% in the as-received feedstock.

3.2.2. APS segmented TBC The topcoats were atmospheric plasma sprayed using a single cathode F4 gun (Sulzer Metco, Wohlen, Switzerland) and homogenized oven spheroidised (HOSP) 8YSZ powder (Metco 204 B-NS).

Fig. 2. Scanning electron images (SEI) of a section of (a) PS-PVDTM , (b) segmented APS, (c) highly segmented APS, (d) porous APS, and (e) EB-PVD TBC systems.

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The development work behind the spraying technique used to apply the topcoats was carried out in the TOPPCOAT project1 and it was mainly targeted towards a quick and cost effective method for spraying strain tolerant zirconia coatings. In this technique the coating is basically deposited by a single passage in vertical direction of the spray gun while the substrate sample is executing a horizontal movement with a relative velocity of 500–800 mm s−1 . Apart the high relative velocity between the spray gun and substrate, of highest significance are also the high energy of the plasma (45 ± 3 kW) and the high powder flow rate of 200 ± 50 g min−1 . A preheating of 500 ± 50 ◦ C is usually applied to the substrate prior to spraying the topcoat. The coatings sprayed by this technique are usually in the interval of 400–800 ␮m in thickness whereas the segmentation cracks density varies between 2 and 8 cracks mm−1 as well as the coatings porosity between 7 and 15%. 3.2.3. Highly segmented TBC The powder feedstock for the highly segmented coatings was fused and crushed Yttria stabilized zirconia with an Y2 O3 content of 8 wt% (8YSZ, d10 = 9.2 ␮m, d50 = 20.5 ␮m, d90 = 42 ␮m), supplied by Treibacher Industrie AG (Treibach, Switzerland). Segmented coatings were atmospheric plasma sprayed using a three-cathode Triplex II plasma torch in an atmospheric plasma spray unit (both supplied by Sulzer Metco, Wohlen, Switzerland). The spray parameters for highly segmented coatings are based on experimental details given elsewhere [29–31]. The key parameters for inducing vertical cracks are (i) a high substrate temperature and (ii) a high thickness per pass. Both properties were adjusted with optimised process parameters like spraying distance, powder feed rate and plasma torch movement speed. Prior to the topcoat deposition, the substrates were pre-heated up to 500 ◦ C with the plasma flame without any powder feed. More information about the highly segmented coatings is described in [32]. 3.2.4. PS-PVDTM TBC The PS-PVD process is based on the ChamProTM technology of Sulzer Metco which comprises all those thermal spray processes performing under a defined and controlled atmosphere like LPPS, VPS and LVPS [33]. The PS-PVDTM process operates at lower pressures down to 0.5–2 mbar. Under these conditions, the properties of the plasma jet change substantially [34]. Even though the PS-PVDTM work pressure (∼1 mbar) is still much higher than the one used in conventional PVD processes (∼10−3 mbar), the combination of a high energy plasma gun operated at a low pressure environment enables a defined evaporation of the injected powder material. This allows one to produce a controlled deposition out of the vapour phase. In EB-PVD, the transport of the vaporized coating material towards the substrate is done through a diffusion process having a limited throughput and producing coatings at low growth rates. In contrast to that, in the PS-PVDTM process the vaporized coating material is transported in a hot and supersonic gas stream (2000–4000 m s−1 , 6000–10,000 K) which is expanding in a 1 mbar atmosphere. This leads to high growth rates and the possibility to coat undercuts and areas which are not in the line-of-sight. To ensure the evaporation of the injected coating material, a fine grain sized powder feedstock (<25 ␮m) is used. The 8 wt% YSZ powder Metco 6700 has been developed specifically for this purpose. With this particular powder and new PS-PVDTM technology, it is possible to produce coatings having a columnar micro-structure similar to EB-PVD. Typical substrate temperatures during the spray process can vary between 900 and 1100 ◦ C depending on the

1 TOwards design and Processing of advanced, comPetitive thermal barrier COATing systems – TOPPCOAT, STREP 6 project no. AST4-CT-2005-516 149, project report, 2010.

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Table 2 Testing parameters. Parameter

Value

Testing temperature Impingement angles Erosive particles feeding rate Maximum impaction speed Maximum impaction speed @ 30◦ 2nd

700 ◦ C 30◦ and 90◦ 2 g min−1 40 ± 5 104 ± 3

spray conditions and substrate material. More details about the PSPVDTM deposition technique and are can be found in the literature [35]. 3.2.5. EB-PVD TBC The topcoat has been deposited via EB-PVD process. The substrates were mounted on a rotating holder positioned perpendicular to the ingot axis and heated in the range 900–1000 ◦ C. The evaporation source material was an ingot with standard ZrO2 7–8 wt% Y2 O3 chemical composition. Evaporation of the ingot was achieved by applying an electron power gun. During deposition the chamber pressure was kept in the 10−3 mbar range. 3.3. Testing conditions As summarised in Table 2, testing temperature, impingement speed and angles have been set up equal to 700 ◦ C, 30◦ and 90◦ and 40 ± 5 m s−1 , respectively. Microquartz (SiO2 in the crystallographic hexagonal phase) powder (d10 = 68 ␮m, d50 = 122 ␮m, d90 = 204 ␮m) was used as erosive particles, quartz being one of the main erosive constituents of sand. Furthermore, the feed rate of erosive particles was set up to 2 g min−1 . Limited to 30◦ impingement angle, some additional tests have been performed at higher speed (104 ± 3 m s−1 ) to study the dependence of erosion rate from the speed of erosive particles. For each TBC system and each experimental testing condition, three samples have been tested for statistical purposes. 4. Results and discussion 4.1. Erosion rates Figs. 3 and 4 summarise the results of the testing activity. TBC systems with columnar structure show a significantly higher erosion resistance compared to porous TBC. In particular, PS-PVDTM

Fig. 3. Comparison of the erosion rate of the five TBC systems at 700 ◦ C. Data refer to impaction speed v = 40 ± 5 m s−1 and impaction angle 30◦ and 90◦ .

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ties of ceramics, but also to the geometrical feature related to the testing set up. In fact, it might be expected that the erosion rate we for brittle materials is proportional to the impingement speed component perpendicular to the target surface: we ∝ (v sin ϕ)n

(3)

where ϕ, v and n are the impingement angle, the particle speed and an exponent (for bulk ceramic n ∼ = 3 [38]), respectively. In the present case, this means that geometry should contribute to the reduction of the erosion rate by a factor equal to 2 or higher, depending on the value of the index n. Starting from the experimental data for each TBC system, the estimation of the index n can be performed considering the ratio of erosion rates of tests carried out either:

Fig. 4. Comparison of the erosion rate of the five TBC systems at 700 ◦ C. Data refer to impaction angle ϕ = 30◦ and to the two speeds v = 40 m s−1 and v = 104 m s−1 .

seems to have the best solid particle erosion resistance. Highly segmented APS TBC shows erosion rates comparable or better than EB-PVD system, as reported in the literature [22]. As expected, tests performed at lower impingement speed show lower erosion rates, as summarised within Table 3. Similarly, a decrease of erosion rate is observed by lowering impingement angle from 90◦ to 30◦ . By increasing the speed from 40 to 104 m s−1 , erosion rates increase by a factor which ranges from 4 to 8.7, depending on the specific TBC system. The erosion rate decrease observed between 90◦ and 30◦ impingement angle is related not only to material fracture properTable 3 Ratio of erosion rates obtained in different experimental conditions and corresponding estimated values for the index n. TBC system

30◦ /90◦

30◦ low/30◦ high

n (30◦ /90◦ )

n (low/high)

Porous APS Segmented Coating APS High segmented coating APS PS-PVDTM EB-PVD

0.15 0.33

0.11 0.16

0.4 0.6

2.3 1.9

0.60

0.20

1.3

1.7

0.81 0.54

0.13 0.25

3.3 1.1

2.1 1.4

- at the same speed at two different angles; - or at the same angle at two speeds. Table 3 summarises the results of both the estimations approaches. A part from the porous APS TBC, the index n ranges from 1.1 up to 3, showing a behaviour intermediate between bulk ceramics (n = 3) and the linear dependence (n = 1) reported in the literature for both APS and EB-PVD TBCs [22]. The erosion rates for all the samples resulted two orders of magnitude lower compared to those reported in the literature [19,21,36,37]. Micro-hardness and impingement speed of erosive particles significantly lower in this study compared to those considered in the literature are considered the main reasons for this relevant difference. In particular, results in the literature refer to tests carried out using alumina erosive particles with typical impingement speeds and grain size in the range 140–360 m s−1 and 27 ␮m up to 200 ␮m, respectively [14–22]. Since Mohs’ hardness for ZrO2 , SiO2 and Al2 O3 are 8, 6.5 and 9, respectively, in this testing activity the target has an hardness higher than that of erosive particles; on the contrary in literature studies erosive particles have a hardness higher than the target. The rare presence of deposits consisting in softened erodent particles on the surface just outside the most eroded area supports this explanation. Following Ruff and Wiederhorn model, a correlation between TBC micro-hardness Ht and the erosion rate we is expected [38]: we = aHtb

(4)

Fig. 5. Erosion rates as a function of the Vickers microhardness, as measured on the APS samples tested within this work. The trend observed by Janos et al. is also reported [17].

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Fig. 6. Segmented APS sample tested at impaction angle 30◦ and speed 104 m s−1 . (a) Optical image of the eroded surface; a network of cracks promoted by erosion are clearly visible. (b) Overview SEI image of a section of the eroded area. (c) Close up view of the eroded section.

being a a multiplicative factor which depends on the testing conditions, target and erosive features. Fitting the experimental data by Eq. (4), trends similar to that reported in the literature for a set of APS TBC coatings are obtained. Although there is extremely limited data available, it is worth noting that the exponential coefficient b found for erosion rates referring to low and high impingement speeds (40 and 104 m s−1 ) is quite similar to that reported by Janos et al., as shown within Fig. 5, even if the multiplicative factor a significantly differs from test to test because of the different testing conditions (T = 1093 ◦ C, 27 ␮m alumina, v = 244 m s−1 , ϕ = 30◦ [19]).

4.2. Failure mechanisms of the TBCs Optical macrographs and SEM images confirm that for all the APS TBC the damage mechanism is related to single or multiple splat removal through crack propagation along the boundaries of single splat or cluster of splats (see Figs. 6–8). Depending on the microstructure of each system, this crack network is more or less evident. In the case of segmented APS TBC (Figs. 6 and 7) erosion highlights a peculiar bubble structure of the surface (also present in the as sprayed samples) particularly evident for highly segmented TBC (Fig. 7). In the maximum erosion area this structure disappears

Fig. 7. Highly segmented APS sample tested at impaction angle 30◦ and speed 104 m s−1 . (a) Optical image of the eroded surface showing the peculiar bubble structure. (b) Overview SEI image of a section of the eroded area; (c) eroded surface with a cluster of splats almost ready to spall off away from the sample; (d) a deposit of softened microquartz far from the main erosion area.

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Fig. 8. Porous APS sample tested at impaction angle 30◦ and speed 104 m s−1 . (a) Optical image of the eroded surface. (b) Overview SEI image of a section of the eroded area; (c) close up view of the erosion area; (d) a deposit of softened microquartz far from the main erosion area.

Fig. 9. PS-PVDTM sample tested at impaction angle 30◦ and speed 104 m s−1 . (a) Optical image of the eroded surface. (b) Overview SEI image of a section of the eroded area. (c) Densified top layer of the coating; (d) a deposit of softened microquartz far from the main erosion area.

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Fig. 10. EB PVD sample tested at impaction angle 30◦ and speed 104 m s−1 . (a) Optical image of the eroded surface. (b) Overview SEI image of a section of the eroded area. (c) Densified top layer of the coating.

and a smooth surface is observed. Rare small deposits within the roughness depression can be observed far from the erosion area on all the samples (see Figs. 6–8). In the case of PS-PVDTM and EB-PVD samples, a densification of a thin (5 ␮m) layer on the top of the TBC occurs owing to the multiple impacts of erodent particles (Figs. 9 and 10). The progressive erosion of columns takes place by crack propagation at the interface between the densified layer and the as deposited columnar structure, as expected for columnar TBC. Moreover, within small depressions present along the TBC surface deposits of softened erosive powder can be occasionally observed (see Fig. 9(d)). A possible reason for the big difference in the erosion rates of the PVD coatings between this work and the literature is a change in the erosion mechanism. Here both PVD coatings eroded via a compaction/densification mode (mode II) while the literature reports on PVD systems eroding via cracking of the top (near surface cracking) without densification (mode I). Similar damage mechanisms have been observed after erosion at 40 m s−1 both at 30◦ and 90◦ .

5. Conclusions Five different TBC systems were tested at 700 ◦ C in a solid particle erosion jet tester. Tests were performed at impingement angles equal to 30◦ and 90◦ , representative for particle impingement on trailing and leading edges of gas turbine blades and vanes, respectively. Microquartz was chosen as erodent being one of the main constituent of sand and fly volcanic ashes. After the end of the tests, the TBC microstructure was investigated by electron microscope to characterise the failure mechanisms taking place in the TBC. The main results can be summarised as follows: • The PS-PVDTM coatings due to their peculiar columnar structure showed the lowest erosion rate among all the five TBC systems, independently from the testing conditions. • EB-PVD and the highly segmented APS coatings showed comparable erosion rates. • Standard porous APS coating shows a significantly poor erosion resistance. • Erosion rates at low impingement angle are smaller then those at 90◦ , as expected. • The dependence of erosion rate from the impingement speed resulted in the range 1.4–2.3. • In APS coating the damage mechanism is related to multiple splats removal through crack propagation along splat boundaries. • For PS-PVDTM and EB-PVD TBC a densification of a thin layer on the top of the TBC occurs and the erosion takes place by crack

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