Gas Turbine Coatings – an Overview

Engineering Failure Analysis 26 (2012) 355–369

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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal

Review

Gas turbine coatings – An overview R. Rajendran ⇑ Gas Turbine Research Establishment, CV Raman Nagar, Bangalore 560 093, India

a r t i c l e

i n f o

Article history: Received 10 January 2012 Received in revised form 20 July 2012 Accepted 23 July 2012 Available online 31 August 2012 Keywords: Coatings Properties Characterization

a b s t r a c t The components of a gas turbine operate in an aggressive environment where the temperature of service varies from ambient to near melting point of materials which introduce a variety of degradation on the components. Some components that lose their dimensional tolerance during use require repair and refurbishment when high cost replacement is avoidable. Erosion of fly ash and sand particles damages compressor blades which cause engine failure at an early stage. Dovetail roots of the compressor blades are subjected to fretting fatigue due to the oscillatory motion caused by vibration. Casing of the compressor comes in contact with rotating blades due to shaft misalignment, ovality of the casing and or inadequate clearance which cause blade and casing damage. Close clearance control that has bearing on the efficiency of the engine is therefore required in addition to preventing fire where titanium to titanium rubbing might occur. Wear out of the several contact surfaces which undergo rotating and reciprocating motion occur during the running of the engine need protection. Hot gases that are produced by burning the contaminated fuel in the combustion chamber will cause oxidation and corrosion on their passage. In the hot section rotating and stationary components need thermal insulation from higher operating temperature leading to enhanced thermodynamic efficiency of the engine. This wide range of functional requirements of the engine is met by applying an array of coatings that protect the components from failures. Current overview, while not aiming at deeper insight into the field of gas turbine coatings, brings out a summary of details of these coatings at one place, methods of application and characterization, degradation mechanisms and indicative future directions which are of use to a practicing industrial engineer. Ó 2012 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4. 5. 6. 7.

8. 9.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Build up and repair coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erosion resistant coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anti-fretting coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abradable coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wear resistant coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidation and corrosion resistant coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Diffusion coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Overlay coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal barrier coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Tel.: +91 80 25040169; fax: +91 89 25241507. E-mail addresses: [email protected], [email protected] 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.07.007

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R. Rajendran / Engineering Failure Analysis 26 (2012) 355–369

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

1. Introduction When a substrate material has to be chosen for its bulk design properties that are in contradiction with the requirements for its surface design properties, coating is applied to the substrate to meet its engineering requirement. The bulk material provides necessary mechanical strength for the component. Coating protects the component effectively from a variety of environmental degradation factors such as abrasion, erosion, wear, fretting, oxidation and corrosion. Surface modification in terms of a coating material and a process for the application of it are chosen taking into account the environment in which the component has to perform its intended function. There is no single coating that meets all the environmental degradation issues. The chemistry and process of the coating have to meet the functional requirements in terms of physical, mechanical, chemical and environmental compatibilities. Coating can be applied on the component by either diffusion process or overlay process. While diffusion process forms a good metallurgical bond with the substrate in terms of inter diffusion zone, overlay thermal spray coating derives its strength from mechanical bonding. Electron Beam Physical Vapour Deposition (EB PVD), which is kind of physical vapour deposition process, derives its strength from chemical bonding, whereas conventional physical vapour deposition gets its strength from mechanical locking. Diffusion process has advantages like forming thin coating of a few microns, bulk processing possibility and hence is economically cheap. The greatest drawback of this process is that there is no fine control over the chemistry of the coating. Overlay coatings can be done up to even 3 mm thickness. The chemistry of the coating can be finely controlled by engineering the chemistry of the powder. The drawback is, it is line of sight process and fine thickness control is not possible. Overlay coatings are also carried out on worn out component after which machining is done on the component to meet the dimensional requirements as part of refurbishment process. Wear out of the shaft makes the fit lose at its bearing supports and seals. This is made up by chromium electroplating as it requires a build up of a few microns of the material. As the gas turbine is working in the most demanding environment [1], it uses a variety of coatings for its different components. Fig. 1 shows the sketch of application of various types of coatings in a gas turbine. Erosion resistant coatings are applied to the compressor blades and vanes. Anti-fretting coatings are used on the dovetail root of the compressor blade that comes in contact with the disc during running. Abradable coatings are used in casings, tip coatings and seals. Diameter adjustment coatings are applied to different parts of the engine. Gas turbine blades and vanes are protected by corrosion and oxidation resistant coatings. Blades, vanes, flow path cowl, jet pipe liner and burner of the gas turbine are protected by thermal barrier coatings. This overview presents an integrated approach on the selection of the coatings, their materials, processes employed, characterization, performance and failure of various coatings. 2. Build up and repair coatings Sometimes it is necessary to build up a worn out, damaged or mismatched component [2] for reuse instead of replacing them. The coating thickness may vary anywhere from a few microns to 2.54 mm. The build up coating needs correct dimen-

Fig. 1. Metallurgical and high temperature coatings in a gas turbine.

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sional tolerance and it has to comply with surface smoothness requirements. The rule of thumb is to use a build up material similar to that of a base material. If dissimilar materials are to be used, the properties of the coating should be similar to that of the substrate in terms of both co-efficient of thermal expansion and resistance to galvanic coupling. Build up coatings have a variety of compositions such as iron based and nickel based materials. The preferred deposition processes is air plasma spraying (APS) as well as high velocity oxy fuel (HVOF) coating due to their higher bond strength. Fig. 2a shows the microstructure of Amdry 625 (Ni21.5Cr9Mo3.6 (Cb + Ta) 2.5Fe) Sulzer Metco power. The powder particle is produced through gas atomization process, has a spheroidal shape with a size of is 90 + 45 lm [3]. Plasma sprayed Amdry 625 to a thickness of 948 lm on a titanium-64 compressor third stage disc inner diameter for build up to make up the wear is shown in Fig. 2b. Porosity of the Amdry 625 coating was less than 1.2%, unmelted particle was less than 0.5%. The HV0.3 average hardness was 625 and the bond strength was 36 MPa. Fig. 3a shows the feedstock of Metco 450 (Ni5Al) Sulzer Metco powder. The particle size is 90 + 45 lm with clad morphology. The Ni5Al powder that was produced through gas atomized route is shown in Fig. 3b. After plasma spraying on titanium-64, the Ni5%Al coating is shown in Fig. 3c. The hardness of the coating was Rb 82. Thermal spray process provides typically a thickness of near about 10lm for every pass of the torch. Therefore, it is not possible to apply it where the build up of materials is of the order of a few microns. In such cases electroplating is done on the component wherein a fine control of the thickness of the coating is possible with good adhesion strength. Fig. 4 shows the microstructure of chromium electroplated bearing steel for a thickness of 12 lm.

Fig. 2a. Microstructure of feedstock of Amdry 625 powder.

Amdry 625coating

Substrate

Fig. 2b. Microstructure of Amdry 625 plasma sprayed coating.

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Fig. 3a. Microstructure of Metco 450 mechanically clad.

Fig. 3b. Microstructure of Metco 450 gas atomized powder.

Ni-5Al coating

Ti-64 substrate

Fig. 3c. Microstructure of Metco 450 (Ni–5Al) plasma sprayed coating.

3. Erosion resistant coatings Erosion is an incremental material loss from a solid surface due to mechanical interaction between that surface and a fluid or solid particles [4]. It is caused by impinging solid particles or water droplets. The compressor of gas turbine aero engine are prone to performance losses due to erosion of the compressor blades when being operated in regions with dusty and sandy atmosphere and when sand, fly ash, salt and ice crystals or volcanic ashes are ingested [5]. Erosion resistant coatings prevent compressor blades from premature loss of material. Erosion resistant coatings are built over a bond coat. The multilayered top TiN–Ti coating has a typical layer thickness of 3 lm. The overall thickness of alternate layer of ceramic and materials is 25 lm [6]. During impact of a solid particle the brittle ceramic layer cracks but its propagation is arrested by the soft metallic layer. Typical Vicker’s hardness of the coating is 2800–3200 and the operating temperature range is 60 °C to

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Chromium Plating

Steel substrate 100 μm

Fig. 4. Build up of bearing housing and seal by chromium plating.

600 °C. The influence of ductile interlayer material on the particle erosion resistance of multilayer magnetron sputtered, physical vapour deposited (MS PVD) coating for AM355 steel were investigated [7] to bring out that the hardest coating TiN/Zr multilayer exhibited worst ductility and TiN/Nb coating provided the best ductility in most conditions. TiN/Ti coatings showed best ductility against the alumina particles [8]. 4. Anti-fretting coatings Fretting is an accelerated surface damage that occurs at the interface of contacting materials subjected to small radial oscillatory motions combined with centrifugal load [9,10]. The amplitude of the oscillatory motion is of the order of tens of microns [11]. Compressor blades of gas turbine undergo centrifugal forces during running. These centrifugal forces in combination with vibratory load cause fretting motion between the dovetail joint of the compressor blade and the disc. As the material is removed from the contacting areas pits and grooves (galling) are left on them [12]. The debris that is trapped between the contact areas gets oxidized to become harder than the contact material. This in turn introduces microcracks on the fretting surfaces which form the locations for fatigue crack initiation. The factors that affect the severity of fretting are atmosphere, temperature, load, amplitude, fretting cycles, frequency of oscillations and hardness. In inert atmosphere, metals that are less prone to oxidation the fretting rate is slow. As humidity can act as lubricant between the parts that come in contact, it retards the fretting damage. At higher temperature, the rate of oxidation with oxygen becomes higher that accelerates fretting damage. Increasing load, fretting cycle and amplitude increase the fretting rate. Fretting wear comes down as the hardness of the material is increased. At higher fretting frequencies, fretting life is less [9]. Shot peening, which is the process of the material surface with special steel shot, glass or ceramic beads, has been used for the purpose of producing plastic yielding and residual stress at the surface which improves fretting fatigue resistance. Several authors have worked on the effect of shot peening on the fretting fatigue life [13,14]. A typical compressor blade root has copper nickel indium (Cu 38Ni or Cu 36Ni 5 In) anti-fretting coating on its contact surfaces of the dovetail root [15] that had undergone shot peening. Over and above that, a polymer bonded molybdenum disulphide (commercially known as Molydag) thin film of near about 15 lm thickness having a co-efficient of friction 0.07 is provided for additional lubrication [16]. The thickness of anti-fretting copper nickel indium coating varies from 13 to 100 lm [17,18]. Typical bond strength of the coating is 20 MPa with a hardness of 48Rb. The manufacturing routes for the anti-fretting coating are atmospheric plasma spray, combustion powder spray and arc spray. Attainable porosity level is 0.5% and surface roughness for as sprayed coating is 5–10 lm Ra and after grinding/lapping is 2–3 lm. 5. Abradable coatings Abradable coatings are applied to the rubbing surface of the casing of the rotating components of the gas turbine to minimize the clearance between casing and the rotating part to have enhanced gas turbine efficiency [19]. At rotating speeds of the order of 10,000 rpm, rotating blade tip may rub against the stationary casing, due to either thermal expansion, misalignment or rotation induces strains [20,21]. Abradable seals act as sacrificial layers between the blades and the casing and are soft enough to avoid significant wear to blade tips, thus allowing much smaller clearances. In case of compressor bearings, the inner race is coated with abradables for reducing friction [22]. High temperature abradable seals are used in high pressure gas turbine of jet engines [23]. In the absence of abradable seals, the cold clearance between the blade tips and shrouds become large enough to prevent significant contact during operation [20]. When the blade impacts over the surface of an abradable coating, its tip strikes the coating particles which are protruding over the surface of the coating. The force of impact drives the individual particle into the coating. As the stored elastic energy of the abradable coating is more than its adhesion strength, it rebounds and debonds. The ejected debris, that is higher in volume because of the newly created free

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surface is typically of the size less than 100 lm and has to escape through the rough blade tips. In changing thermal environment of the compressor with titanium material systems, rubbing of blade must release particulate matter without endangering a fire or debris impacting down stream components [24]. Surface speed of a blade tip is given as the angular velocity x (2p times the number of rotations of the blade per second) multiplied by the radial distance from the axis of rotation to the tip of the blade. For surface speeds of the blades less than 100 m/s, the blades expel the chips forward, for surface speeds above 100 m/s expelled particles are released backward. As thicker blade tips trap materials and destroy interface, the blade tip needs to be within 1–3 mm. For enhanced abradability, the energy for particle detachment must be reduced. Abradable coating will therefore have small matrix particles, a polymer or a fugitive phase to generate porosity and solid lubricants or release agents to act as dislocators within the coating. During thermal spray deposition, the polymer phase lowers the coating stresses, thereby allowing the deposition of thick coatings. When the polymer phase is burnt-out in a subsequent heat treatment of the coating, porosity is introduced in places where polymer phase was present which further improves the abradability of the coatings. As abradable coatings must inherently porosity for their desired abradability and low bond strength, the base material of the gas turbine component has to be protected from corrosion and high temperature oxidation [25,26]. In addition to that, the abradable coatings are not self bonding. Dimensional restoration of the casings is at times required before going for abradable coating. In order to overcome these shortcomings, a dense bond coat, typically Ni–5Al intermetallic is provided on the substrate over which the required functional abradable coating is sprayed. Ni–5Al bond coat is sprayed with a porosity that is less than 2%, bond strength of typically 20 MPa, service temperature limit of 800 °C and a surface roughness of 3.6 lm after grinding. The recommended coating processes are APS, combustion plasma spray and HVOF [27]. The selection of a particular type of abradable coating primarily depends on the application temperature of the coating [23]. Aluminium silicon polyester abradables are used at the titanium compressor casings of the engine for temperatures up to 325 °C (due to the temperature limitation of polymer in the abradable) [28]. The porosity level desired is 2% with a corresponding superficial Rockwell Hardness HR15Y of 40–50. Al–Si–BN is used for titanium alloys up to a temperature of 480 °C (due to the temperature limitation of aluminium in the abradable) [28]. The porosity level desired is 15–20% with a corresponding superficial Rockwell Hardness HR15Y of 40–50. Combustion spray of abradable coating can produce very high porosity that gets compacted during machining. APS and HVOF give coating of desired porosity through process control parameters [29]. For temperature applications up to 760 °C, Ni or Co based alloy powders are commonly used as the basis of the abradable coating matrix [30]. Polymeric materials which generate porosity in the coating act as release agents are added to the base material to make it abradable. Ceramic abradables are used for temperatures above 760 °C. The most widely used material is yttria stablized zirconia (YSZ) which is usually mixed with a fugitive polymer phase. The blade is reinforced with abrasive grits such as cubic boron nitride (cBN). Abradable that can operate at temperatures above 900 °C and be used without abrasive blade tip treatments was invented by GE as Alstom GT54 coating [31]. Typically, the thickness of the bond coat goes up to 125 lm and the abradable coat each go up to 1000 lm. The bond strength of the coating is obtained as per ASTM C633 (01)-2008 [32]. The erosion resistance of the coating is obtained in accordance with GE E50TF121CL-A specification [23]. In this test, a weight quantity of 600 g of alumina particles of 50 lm impinges on a coated surface at a 20° angle at a stand off of 100 mm. The deepest point of erosion is measured with a ball point micrometer and the GE erosion number is calculated as

Erosion number ¼

Test time ðsÞ Depth of Erosion ðin:Þ 1000

ð1Þ

The GE erosion number is expressed in s/mil and represents the time in seconds necessary to erode 25.4 lm of the coating thickness. A higher GE erosion number means better erosion resistance. A study on the correlation between coating hardness and GE erosion brought out that erosion limit of 2 s/mil is the lower acceptable limit for abradables [33]. An abradable coating that has a hardness of 30–40 at HR 15Y scale gives a GE erosion number of 4 which is a compromise between abradability and strength of the coating for titanium based alloys. An inverse correlation between hardness and porosity was observed [34] for abradable coatings from which it can be said that hardness, porosity, and GE erosion number are interrelated. The rate at which the rotating blade penetrates into the abradable coating is called incursion speed. While the blade tip speed can be determined with certainty, the incursion speeds are difficult to determine. Wear maps that validate the correct abradability under a range of blade tip speed, incursion rate and temperature are generated for the abradable coating with Sulzer Metco high temperature abradability test rig [35]. These wear maps give great insight into the blade wear mechanism at the specified blade tip speed and percentage porosity of the abradable at a given temperature [35]. Wear map results when examined along with the coating microstructures help in determining the ideal abradability to meet the specific design requirements. However, for reasons like time and economy, abradability tests are not routinely carried out at laboratory level. Elastic modulus and Poisson’s ratio are the basic properties that are required for numerical modeling of the elastic response of the abradable coating. The elastic modulus of the coating is obtained by two layer beam method [20,36]. The elastic modulus of the commonly used bond coat Ni–5Al for the abradable coating is much higher (typically 256 GPa [20]) than that of the abradable itself. The elastic modulus for Metco 601 (5% porosity) is 2.1 GPa, Metco 308 is 3.95 GPa (15% porosity), Metco 313 is 3.1 GPa (8% porosity) [14]. The corresponding HR 15 Y values are 60–70, 65–70 and 65–75 respectively. The calculated Poisson’s ratio using finite element analysis varies from 0.23 to 0.3 [37].

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6. Wear resistant coatings Wear and corrosion resistant coatings are most frequently based on transition metal carbides (WC, TiC, Mo2C, TaC, NbC, Cr3C2) and their alloys (NiCoCrAlY) and some hard oxides (Al2O3, TiO2, Cr2O3) [32–34,38–40]. They are usually applied by APS, detonation process and HVOF. Pure carbide powders cannot be melted at high temperatures and deposited even at high enthalpy plasma jets. This is due to the reason that at such high temperatures, oxidation, decarburization and thermal decomposition generally occur. Therefore carbide particles are embedded into easily melted binder materials with high ductility such as Ni, Co, Cr and their mixtures and alloys respectively. Such composite coatings are usually known as cermets. Tungsten carbides are used at temperatures below 540 °C and chromium carbides are applied up to 815 °C [41]. Thermally sprayed WC–Co coatings having high hardness and good adhesion with substrate exhibit satisfactory wear resistance [34]. Al2O3–TiO2 coatings are composed of TiO2 as reinforcement in the Al2O3 matrix [42]. The Al2O3 matrix distributes the stresses in the composite material homogeneously whereas TiO2 mechanically reinforces the material. This type of coating is prepared by blending the matrix with the reinforcement during powder production and by plasma spraying [36] and can take temperatures up to 1100 °C [16]. The microstructure of the WC–17Co powder (commercially known as Sulzer Metco Diamalloy 2005 NS) is shown in Fig. 5a. The powder is spray dried and sintered to a particle size of 53 + 11 lm. A WC–17Co coating on the Amdry 625 build up coating on the Ti-64 substrate is shown in Fig. 5b. The Amdry 625 coating was first applied on the component to build up the worn out dimension to 948 lm. The WC–17Co coating is in general dense

Fig. 5a. Microstructure of feedstock of diamalloy 2005 (WC–17Co).

Ti-64 substrate

Amdry 625 948 μm

Diamalloy 2005 462 μm

Fig. 5b. The two layer coat of Amdry 625 and diamalloy 2005 (WC–17Co) on Ti-64.

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with porosity less than 1.3% and unmelted particles less that 0.2%. The thickness of the coating was 462 lm and its bond strength was 36 MPa with a microhardness of 1187 Hv0.3. A magnified view of the WC–17 Co coating is shown in Fig. 5c where oxide inclusion is seen. The XRD patterns of diamalloy 2005 powder and coating are shown in Fig. 5d. As it is evidently observed from [17], the WC and Co phases present in the powder were preserved in the coating. Nanostructured wear resistant coatings provide superior hardness, toughness and wear resistance [43]. This is because the fracture toughness and hardness have inverse relation with the square root of the grain size (Hall–Petch effect) [44]. A grain size below which (typically of the order of 10 nm) the material becomes amorphous by loosing its crystalline nature is the limiting point beyond material softening takes place due to inverse Hall–Petch effect. The Hall–Petch effect breaks down at a critical grain size below which the grains are not able to support dislocation pile ups [45]. The Hall–Petch slope becomes negative below this critical grain size. Nanoparticles cannot be successfully thermal sprayed because of their low mass and their inability to be carried (momentum which is mass multiplied by the velocity of the powder particle is less) in a moving gas stream and deposited on a substrate. In order to make use of conventional thermal spray units that are commercially available, nanosized particles are agglomerated with a binder followed by certain degree of sintering for making feed-

Oxide inclusion

Fig. 5c. Microstructure of diamalloy 2005 coating substrate.

Fig. 5d. XRD of diamalloy 2005 powder and coating.


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stock. Typically, conventionally HVOF sprayed feed WC–17Co powder has a particle size of 53 + 11 lm whereas agglomerated nanopowder 45 + 5 lm. The grain size of the nanopowder is 100–500 nm [46]. While using conventional plasma spray systems, critical plasma spray parameters such as torch temperature, particle temperature and particle loading effect (which are essentially voltage and current of the torch and flow rate of the argon gas) are obtained through robust process control, optimization of properties and quality control [47].

7. Oxidation and corrosion resistant coatings High temperature components of gas turbine are exposed to a wide range of thermal and mechanical loads in addition to an oxidizing and corrosive environment that may contain contaminant such as chlorides, sulphates and erosive particles. Aluminium and chromium contents are kept at a reduced level (5–12% and 3.4–5% respectively [48–50]) in superalloys in order to obtain their desired high temperature strength and microstructural stability. This situation leads to a fall in oxidation and corrosion resistance of the material. Therefore appropriate coatings are applied to protect the surface of the component from oxidation and corrosion. Before going into the details of the various coatings, a brief introduction to the operating mechanism of oxidation and corrosion is appropriate. When the gas turbine materials are exposed to oxygen and oxygen containing gases at elevated temperatures, they convert some or all of the metallic elements into oxides [51]. The oxides create a protective phase that remains adherent and form Thermally Grown Oxides (TGOs) on the metallic coatings by reacting with hot gases. TGO serves as a diffusion barrier to the next oxide. Some of the coatings such as Cr2O3 protects up to 871 °C while Al2O3 protects at all temperatures up to the melting point of the blade alloy [52]. In order to form adequate thickness of Al2O3 in a reasonable time, sufficiently high temperatures are required. Without the protective oxide, the coating is exposed to rapid environmental degradation of its microstructure. A protective scale can be maintained when aluminium, silicon or chromium does not fall below a critical level of 4–5% weight [53]. The TGO can be removed by the erosion of the particles from the hot gases. A rapid spallation of the TGO can also be caused by diffusion of oxygen into the coating. Presence of impurities in the hot gas like sulphur can cause the disruption and spallation of the TGO. Yttrium or hafnium is added to the coating to improve the adherence of aluminium or chromium oxides to the coating. Hot corrosion takes place in a gas turbine due to the presence of contaminants such as Na2SO4, NaCl and V2O5 in the gases that combine to form molten deposits which damage the protection surface oxides. Hot corrosion is divided into two types: Type I which is High Temperature Hot Corrosion (HTHC) and Type II which is Low temperature Hot Corrosion (LTHC). LTHC is observed mainly within the temperature range of 650–800 °C [54]. It forms typical pitting, resulting from the formation of mixture of Na2SO4–CoSO4 with low melting temperatures (the melting temperature of Na2SO4–CoSO4 eutectic is 540 °C). Similarly, the formation of Na2SO4–NiSO4 eutectics from the nickel takes place at a temperature of 670 °C [55]. A high partial pressure of SO3 in the gas phase is required for LTHC reaction to occur in contrast to HTHC. The localized nature of attack is related to the localized failure of scale as a result of local chloride attack, thermal cycle or erosion. As opposed to Type I corrosion, Type II corrosion has neither incubation period nor microscopic sulphidation, only chromium depletions are in general observed [56,57]. HTHC or Type I corrosion is observed within the temperature range of 850–950 °C [56,57]. HTHC starts with the condensation of fused alkali metal salts on the surface of the component which attacks the protective oxide film and progresses to deplete the chromium element from the coating. With chromium depletion, oxidation of the coating accelerates and porous scale begins to form. The presence of sodium chloride removes the incubation period which is otherwise typical of HTHC. K2SO4 behaves in the same way as Na2SO4 with regard to HTHC. When Vanadium, which is an unavoidable contaminant in certain liquid fuels, comes in contact with components exposed to high temperature, accelerated hot corrosion occurs [56]. Extremely aggressive liquid phase of vanadium forms at temperatures as low as 535 °C, depending on the ratio between Na and V [56]. In addition to their own relative low melting, the vanadium compounds markedly increase the solubility of oxide when mixed with Na2SO4 [58]. The macroscopic appearance of HTHC is characterized by severe peeling of the coating and by significant colour changes (green tone resulting from the formation of NiO) in the area of accelerated attack [51,58]. Oxidation and corrosion resistant coatings are applied to the gas turbine engine components through two broad methods [59,60]: They are: (1) diffusion coating and (2) overlay coatings [60]. For diffusion coating, an inter diffusion zone is formed between the substrate and the coating whereas for overlay coating no such zone is formed. Diffusion coating is formed by diffusion of one or more elements into the surface of the metal to be protected. Typical diffusion coatings are simple aluminide and platinum or chromium modified aluminide. Overlay coatings are coatings of specific composition applied as ‘add on’ to the surface to be protected by plasma spray or physical vapour deposition. Typical overlay coatings are MCrAlX type where M is usually nickel or cobalt; X is Y, Si, Ta, Hf, etc. which is mostly less than 1% by weight. The selection of appropriate coating composition depends on the environment of the coating and the substrate they are applied on. Because of the complex interaction between the environment, coating and substrate, compromises between the mechanical strength, oxidation/corrosion resistance and adhesion are inevitable. It is relevant here to take a look at the role of each element of the high temperature oxidation and corrosion resistant coatings [61]. Nickel is the base element for overlay coatings on Ni base substrates to minimize the interdiffusion. Nickel minimizes the chemical activity of aluminium. Cobalt is the base element for overlay coatings on Co base substrates to minimize the interdiffusion. Cobalt raises the chemical activity of aluminium. Aluminium forms Al2O3 in coatings and Ni and Co based superalloys which protects against oxidation

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up to 1200 °C and also is a c0 former in c/c0 Ni based superalloys. Chromium forms Cr2O3 that protects against hot corrosion and oxidation up to 900 °C. It also reduces the critical level of Al needed to form protective Al2O3. Silicon forms SiO2 and is effective against Low Temperature Hot Corrosion. Silicon promotes the formation of Al2O3 in Ni–Al alloys. Silicon significantly improves cyclic oxidation resistance, however it also decreases the melting point of the coating [62]. A wt% of 5 of Si is adequate to lower the melting temperature to about 1140 °C. There is also evidence that it affects phase stability. For cyclic oxidation at 1000 °C, 2.5 wt.% Si was found to be the optimum content. Further additions were observed to be detrimental. Yttrium improves adherence of Al2O3 and Cr2O3 scales on Ni and Co base alloys. It changes the oxide growth from cation to anion diffusion and reduces the oxidation rate of Cr2O3. Hafnium improves adherence of in situ grown Al2O3 and Cr2O3 scales. Platinum delays transformation of b-NiAl into c0 -Ni3Al in aluminides, which improves the high temperature oxidation resistance. Tantalum strengthens the solid solution of c and c0 phases, reduces interdiffusion of Ni, improves oxidation and hot corrosion resistance and reduces thermal expansion coefficient. Titanium accelerates formation of Cr2O3 at metal/oxide interface and reduces thermal expansion coefficient in Ni base alloys [63]. The degradation mechanism for both diffusion and overlay coatings is the transformation of b-NiAl at the coating-substrate interface to c0 -Ni3Al and its outward growth to the top surface. The coating is said to have lived its life once the transformation of the b-NiAl to c0 -Ni3Al is complete across the thickness of the coating. 7.1. Diffusion coatings Diffusion coatings produce a corrosion/oxidation resistant thermally grown oxide by enriching the surface either with Al, Cr or Si through diffusion. They have homogeneous microstructure with good thermomechanical properties and they are applied using chemical vapour deposition (CVD) process which includes pack cementation, slurry cementation and various forms of gas phase coating. Diffusion coatings are based on intermetallic compound b-NiAl. In diffusion coating, the superalloy components to be coated is cleaned, masked and placed in a retort reaction chamber. They are then – immersed in a pack containing aluminium or pre-alloyed powder known as donor alloy, a halide energizer (NH4Cl) that transport the aluminium from the pack to the components to be coated and an inert oxide dilutant (Al2O3) to prevent pack sintering. The retort is heated to the required temperature under an inert gas or hydrogen atmosphere to prevent subsequent oxidation [50,64]. Diffusion aluminide coatings are classified as ‘inward’ or high activity and ‘outward’ or low activity. In low activity/outward process where the aluminium content is low, formation of coating occurs mainly by Ni diffusion and results in the direct formation of NiAl. The temperature of the process is 1000–1100 °C. These coatings are also subjected to a subsequent diffusion treatment for 4 h [65]. In service, the interdiffusion with the substrate is very limited and the gradient of aluminium in b is low [60,62]. In high activity/inward process, where the aluminium content is high, the formation of the coating occurs mainly by the inward diffusion of aluminium and results in the formation of d-Ni2Al3 and possibly b-NiAl at temperature range of 700–950 °C. There is high aluminium concentration in the coating and also significant interdiffusion in the substrate during service. The d-Ni2Al3 coating that is formed during inward process is then further transformed into a b-NiAl coating by a subsequent diffusion treatment at 1080 °C for 4 h [65]. Aluminide coatings lack ductility below 750 °C. In high activity coatings, aluminium ingress into the alloy is more rapid and the coating thus formed incorporates other phases and elements from the alloy. This inhomogeneity tends to adversely affect oxidation resistance properties when compared with low activity coatings, where substantial outward diffusion of nickel is also involved in the reaction [66]. Nickel aluminide coatings suffer from strong interdiffusion with the substrate that forms c0 at the expense of b-NiAl. Introduction of diffusion barrier is done through platinum aluminide coatings. The blades and vanes of the turbine are electroplated with platinum for a thickness of 5–10 mm [67] and given diffusion heat treatment at about 1000 °C for 5 h in an argon atmosphere [68] after which they are pack aluminized. Because platinum modified aluminide coatings can increase the life of blades up to three times [69], the cost of platinum electroplating is easily compensated. A platinum aluminide coating without prior platinum diffusion process contains PtAl2 which is rich in aluminium reservoir with trace of Ni2Al3 [70]. The phases present in the platinum aluminide coating on CM247LC DS blade without prior Pt diffusion is shown in Fig. 6a [71]. The diffusion treatment of platinum electroplating allows enriching the superalloy surface with platinum, which form a b-NiAl phase with platinum in substitution, that is, b-(Ni, Pt) Al phase during aluminizing. Electroplated platinum after heat treatment goes down in its weight percentage to as low as 47% with an increase in thickness of nearly up to five times and the diffusion of Ni, Co, Cr and Al [72]. However, during aluminizing treatment, the intermetallic compound PtAl2 is also observed which modifies the mechanical properties of the coating [73]. Aluminizing temperature in platinum aluminide coating should be controlled so as to avoid precipitation of secondary harmful phases such as l and R. Furthermore, the initial platinum electroplating thickness influences the percentage of platinum and nickel in the aluminized coating. Platinum promotes selective oxidation [74–77]. It acts as a catalyst promoting the reaction between O and Al to form aAl2O3 possibly by increasing the dissociation rate of O2, thus decreasing the rate for continuous a-Al2O3 to form [76]. It improves the adhesion between the coating and the substrate [73,75] by suppressing the formation of coating-Al2O3 interface [78–80] and aids the retention of Al within the interdiffusion zone, suppressing the deleterious spinal formation, preventing both nucleation of oxides and the reaction transformed spinals and the nucleation of spinals itself [66], delays b–c transformation [81], c–d, h–a Al2O3 phase transformation [82], lowers the scale growth [81,83], remains most concentrated at the coating gas interface thereby retarding the diffusion of certain refractory elements to the coating-Al2O3 interface which leads to improved isothermal (and cyclic) oxidation resistance, improves oxidation resistance when compared with conventional

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PtAl2 PtAl2 PtAl2

PtAl2

PtAl2

PtAl2

PtAl2 PtAl Ni2Al3

10000

AAU19-P1-3

δ-Al2O3 γ-Al2O3

unts 20000

0 40

50

2θ

60

70

80

Fig. 6a. The phases present in the platinum aluminide coating on DS CM247 LC blade without prior platinum diffusion [71].

Intact coating

Cracked coating

IDZ

DS CM247LC substrate

Fig. 6b. Degradation of platinum aluminide coating in service [71].

oxidation resistant coatings due to increase scale adherence and cracking resistance [84,85]. The microstructure of platinum aluminide coating that has degraded and cracked to a major extent after aluminium depletion is shown in Fig. 6b [71]. Chromium aluminide coatings are developed by first pack chromizing at 1060 °C for 40 min followed by a high activity pack aluminizing at 750 °C for 3 h followed by usual diffusion heat treatment for high activity process [65]. 7.2. Overlay coatings For protection against Type II hot corrosion about 25–40% of chromium is required whereas for Type I hot corrosion protection usually 12–20% of chromium is recommended [86]. Overlay coatings with 18–22% Cr and 8–12% Al are designed to withstand corrosion above 900 °C [87]. CoCrAlY alloy coatings with 17–22% Cr and 10–12% Al have the resistance to attack at high temperature, but not with most severe salt environment. For low temperature corrosion protection, CoCrAlY alloy with 25–35% chromium level is required for maximum protection [88]. The application of MCrAlY coatings is limited to 1100 °C because of relatively thick oxide scales which are formed, followed by enhanced local spallation particularly when thermal cycling is encountered [89,90]. Greater amount of cobalt controls Type I hot corrosion while greater amount of nickel gives higher ductility [88,91]. Overlay coatings are applied through various processes like APS, HVOF, Low Pressure Plasma Spray (LPPS) and Electron Beam Physical Vapour Deposition (EB PVD). HVOF coating gives denser microstructure than APS. LPPS gives superior microstructure and mechanical properties but is expensive. 8. Thermal barrier coatings Thermal barrier coatings (TBCs) are two layer (duplex) coating systems which comprise of an oxidation and corrosion resistant inner layer caller ‘bond coat’ and an insulating ceramic outer layer called ‘top coat’ [92–95]. The bond coat serves two purposes: (1) it protects the metallic substrate from the ingression of hot gases and their attack on the substrate; (2) it serves as an intermittent layer that gives better adhesion between the substrate and the top coat. As a detailed discussion was made on the oxidation and corrosion resistant coating in the preceding section, focus in this section is made only on the insulating ceramic coat with barest minimum reference to the bond coat. The TBC system will have a Thermally Grown Oxide

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(TGO) that forms between the bond coat and the top coat. This TGO is the a-Al2O3 that forms a protective layer on the bond coat to prevent the inner surface of it from further oxidation. The TGO is grown to 2–3 lm on the bond coat before the application of the ceramic top coat by a suitable heat treatment process to enhance the adhesion of the latter on the former [96]. The TGO in some cases is grown during the ceramic coat deposition [97]. The growth of the TGO to a thickness of 8–10 lm during the service condition leads to spallation of the TBC [98]. The growth of TGO is controlled mainly by the inward ingression of oxygen anions, rather than the outward diffusion of cations [94]. Ingression of oxygen towards the top coat/bond coat interface can occur by two mechanisms, namely oxygen gaseous permeation through the network or interconnected cracks and voids of the top coat or oxygen diffusion thorough the lattice. Over the range of operating temperatures of TBCs, oxygen transport through the top coat by permeation is expected to dominate. However, the rate of TGO thickening is cubic with respect to time and is largely controlled by the oxygen diffusion through the TGO itself. The TGO with its very high elastic modulus (360 GPa) undergoes thermal stress cycle during every run of the gas turbine due to the large mismatch of its coefficient of thermal expansion (8 106 K1) with the bond coat (typically 17 106 K1) which contributes to its cracking as its thickness grows [99]. With TBC, the operation temperature of the turbine blade can be increased to beyond its substrate melting temperature [94]. By the application of TBC the operating temperature goes up by 70–150 °C without any increase in metal temperature. Conversely, TBCs have the ability to reduce the mass flow of the coolant while maintaining the operating temperature of the turbine [99]. TBC systems typically consist of yttria stablized zirconia (YSZ) top layer, which has low thermal conductivity that is almost constant with increasing temperature, is chemically inert in combustion atmospheres and has a co-efficient of thermal expansion which is reasonably compatible with nickel based superalloys [100]. Seven percent partially YSZ is used as the TBC material because it retains its metastable tetragonal phase up to 1200 °C without any phase transformation. TBCs are sprayed either by APS or by Electron Beam Physical Vapour Deposition (EB PVD) [101]. At 25 °C, the thermal conductivity of APS coatings varies from 0.8 to 1.0 W/m K whereas EB PVD coatings vary from 1.5 to 1.9 W/m K. Therefore APS coatings provide superior thermal insulation. But the spallation resistance of EB PVD coatings is 8–10 times more than that of APS coatings due to the superior in plane compliance of EB PVD coating [102,103]. La2Zr2O7 has a co-efficient of thermal conductivity that is close to APS 7%YSZ. But La2Zr2O7 has a co-efficient of thermal expansion (9 106 K1) that is less than that of 7%YSZ (10–11 106 K1) which leads to higher thermal stresses from thermal expansion mismatch [104–109]. The powders that are used for 7%YSZ consist of mostly spherical particles, but with some slightly distorted spheres [94]. A spherical geometry is beneficial for good flow in the powder feed line, which in turn can lead to high feed and deposition rate. The mean particles sizes of the powder are 30–70 lm. Narrow particle size distribution and low population of microparticles are preferred to avoid nozzle build up [102]. The presence of many fine microcracks and pores in the APS microstructure results in low stiffness that prevents large stress from being generated on the top coat. As APS is cheaper than EB-PVD, and has reasonable strain tolerance due to the microcracks and pores, it is applied for many gas turbine components such as combustion chambers, nozzle guides and abradable seals. Columnar growth microstructure of EB PVD TBC is applied on rotating components such as blades due to its higher strain tolerance [110,111]. Columnar coating morphology of EB PVD has higher thermal conductivity than the splat morphology of APS which leads to greater radiation of heat transport of the former than the latter. The characteristic columnar structure of EB-PVD coatings does not hinder thermal transport in the through-thickness direction, since most of the porosity in the form of voids and interspaces is aligned parallel to the direction of heat flux. Modification of this pore architecture, however, might lead to thermal conductivities comparable to that of APS YSZ TBCs [102]. Young’s modulus of as-sprayed APS TBC between 8 and 12 GPa when measured by cantilever bending whereas by nanoindentation it was 146 GPa [94]. This is because, nanoindentation gives the hardness of the individual splat whereas cantilever beam bending gives the aggregate of the splates, microcracks and pores. Young’s modulus of EB PVD TBC that was measured through nanoindentation was reported to be was between 230 and 250 GPa [112]. The temperature of the substrate that is desired for good adhesion of EB PVD coating is 0.47 times the melting temperature of the coating material [113]. Substrate rotation is required during EB-PVD to obtain sufficient inter- and intracolumnar porosity. The porosity range of APS 7%YSZ is 10–20% [114]. This pore volume fraction is desirable from the point of view that strain tolerance is achieved with minimum thermal conductivity as lower pore percentage leads to higher thermal conductivity [102] and less strain tolerance. The Poisson’s ratio for the TGO is 0.22, bond coat is 0.3 and the TBC is 0.22 [115]. Nano-TBC is obtained by synthesizing nanostructured 7%YSZ powders to microagglomerates so as to make feedstock that is suitable for being fed and thermally sprayed using conventional powder feeders or through solution precursor technique [116]. Nano-TBC through agglomerated powder route showed low elastic modulus, high thermal diffusivity and low sintering effects. Solution precursor plasma spray TBCs were deposited up to 3 mm thickness and showed lower thermal diffusivity, improved thermomechanical durability, higher in plane fracture toughness and low cooling requirements.

9. Conclusions This overview presented the details of various gas turbine coatings, their methods of application and characterization, degradation mechanisms and possible future directions. Build up of several components that have lost their dimensions during service through coating route is essential for economic use of the engine. Erosion resistant coatings protect the compressor blade from sand particles and fly ash thereby improving their performance and life. Alternate layers of soft and hard


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coatings are applied to the compressor blades and vanes for enhanced erosion resistance. Anti-fretting coating protects the contact area of the dovetail part of the compressor blade root from fretting fatigue failure. Abradable coatings offer close clearance control thereby increasing the engine efficiency. A fugitive phase with the solid lubricant and a brittle cermet form the desirable composition of the abradable coating. As abradable coatings are not self bonding a bond coating is applied as a bottom layer before the application of it. Wear resistant coatings give extended life of the parts that undergo rotary and reciprocating rubbing motion. Agglomerated nano-WC–17Co coating is gaining currency with superior hardness and toughness over conventional microsized powder coating. Oxidation and corrosion resistant coatings are applied through either diffusion or overlay process. While diffusion process is economical for bulk processing, there is less scope in it for altering the chemistry of the coating. High temperature low activity coating is preferred to low temperature high activity coating as the latter forms several undesirable compounds during the process which are detrimental to the coating life. Overlay coatings has the advantage of provision for engineering the chemistry of the coating but it is a line of sight process. 7% yttria stabilized zirconia thermal barrier coatings offer increased component life with a decrease in operating temperature of the metal. Solution precursor plasma spray is gaining momentum for higher thickness application and agglomerated nanopowder of yttria stabilized zirconia provides lower thermal diffusivity, improved thermomechanical durability, higher in plane fracture toughness in comparison with conventional plasma spray powders. 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