Ni-Ti Thin Films by EBE

Ni Al Alloy Coating Deposition by Electron Beam Physical Vapor Deposition

Daniel Soares de Almeida1,a, Cosme Roberto Moreira da Silva1,b, Maria do Carmo Andrade Nono2,c, Carlos Alberto Alves Cairo1,d

1Comando Geral de Tecnologia Aeroespacial - Instituto de Aeronáutica e Espaço, S. J. Campos,

Cep: 12228-904, SP, Brasil 2Instituto Nacional de Pesquisas Espaciais, Lab. Materiais e Sensores, S. J. Campos, Brasil a [email protected], b [email protected], c [email protected], d [email protected]

Keywords: EB-PVD, MCrAlY, TBC, SEM, X-Ray diffraction Abstract. Turbine blades work in adverse conditions, under corrosive atmosphere and high temperature and pressure. One way to improve life or work temperature of blades is by the use of thermal barrier coatings (TBCs) over metallic substrate applied by several techniques. The most usual configuration for the TBCs is a ceramic material, applied over an intermetallic bond coating (BC), both applied over a metallic substrate. The aim of this paper is show the results of the deposition of the bond coating applied by one source EB-PVD. BC has two functions: improve adhesion between ceramic layer and substrate and work as barrier against oxidation of the substrate. The usual techniques for production of BC is by plasma spray or diffusion, these standard methods are limited. Here is used the EB-PVD technique, where a Ni, Cr, Al and Y alloy (MCrAlY) tablet is evaporated using an electron beam. An advantage of EB-PVD is the possibility to deposit multi-component alloy coatings with high production. However, there are drawbacks, due to the difference in the vapor pressure of the individual components, the evaporation of alloys is selective. Several samples obtained in different deposition conditions were analyzed using XRD, SEM and EDS. EDS analysis show the variation in the chemical composition across the cross section of the deposit layer. However, this chemical variation can be controlled, and then it is possible to obtain the phase anticipated by the Al-Ni equilibrium phase diagram. The XRD analysis shows the presence of AlNi and AlNi3 phase. This is in agreement with the phase diagram for the composition of the MCrAlY used as vapor source. Introduction Turbine blades work in adverse conditions, under corrosive atmospheres and high temperature and pressure compromising its integrity. Several techniques are using to producing coatings to working as thermal barrier applied over metallic materials. Theses coatings increase the life or the work temperature of blades. The usual configuration for the TBCs is a ceramic material layer applied over a MCrAlY bond layer, both applied over a metallic substrate. The bond layer function is, essentially, promote a good adhesion between the ceramic layer and the metallic substrate (usually a Ni base superalloy). Such a way, a compatible thermal expansion coefficient must be had. The bond layer also contributes for the inhibition of the substrate oxidation and promotes a good diffusion stability with the substrate and ceramic layer. Usually, yttria stabilized zirconia is used as ceramic layer due to its low thermal conductivity, high thermal expansion coefficient and excellent chemical and mechanical stability. The plasma spray or diffusion techniques are the usual techniques for the production of the BC, however, these conventional methods presents limitations. In this work it was used EB-PVD technique, where sintered tablets of an alloy of Ni, Cr, Al and Y (MCrAlY) had been evaporated using the energy of an electron beam and then condensed in the substrate. As advantage, this technique, allows to get coatings of many components with high productivity. Some drawbacks are: had to the differences in

17º CBECIMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 15 a 19 de Novembro de 2006, Foz do Iguaçu, PR, Brasil.

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the vapor pressures of the individual components and interactions between them, the evaporation of alloys is a selective process that’s produces coatings with composicional gradient. Literature References The EB-PVD process allows attaining coatings with unique properties. The process parameters are adjusted so that the deposit has a columnar grains structure perpendicular to the interface. This morphology maximizes the resistance to strains that arise from differences in thermal expansion coefficients. Others advantages are: aerodynamically favorable smooth surface, better interaction with the substrate, greater thermal cycle tolerance and, hence, greater lifetime comparatively with the plasma spray process [1-9]. There are four primary constituents in a thermal protection system. They comprise: the thermal barrier coating (TBC) itself based usually on ~ 8 wt. % (8.7 mol % YO1.5) yttria stabilized zirconia; the metallic component, treated here as the substrate; a Ni, Cr, Al Y bond coat (BC) located between the substrate and the TBC; and a thermally grown oxide (TGO), predominantly α-alumina, that forms between the TBC and the bond coat. The TBC is the thermal insulator, the bond coat provides oxidation protection, since the zirconia is essentially transparent for the oxygen at high temperatures, and the metallic component, usually a nickel base super-alloy, sustains the structural loads. The TGO is an oxidation reaction product of the bond layer, and plays a role in the metal/oxide adhesion. Each of these elements is dynamic and all interact to control the performance and durability [10-11]. In this configuration, the system presents a soft thermal expansion coefficients transition from the substrate (16.10-6K-1) passing thought the bond layer (11 - 13.10-6/K-1) to the ceramic coating (5 - 10.10-6K-1). This soft transition reduces the thermal stresses generated during the operation of the TBCs [12,13]. The bond coating must have high content of aluminum to work as a reservoir for the formation of the α-alumina during a long period of time without loss of aluminum in the substrate alloy [14]. There are two bond layer categories (fig.1). One is based on the MCrAlY system (where M = Ni, Co and/or Fe), whose coatings are generally two-phase (γ’-NiAl with β-NiAl or γ-Ni-Cr). Figure 2 shows the Ni-Al Equilibrium Diagram and the dark areas represents the possible phases that is possible to find in MCrAlY bond layer. Reactive elements as Si, Ti, Re, Hf and Y are increased to the alloy for the attainment of determined effect as, for example, the Y that is added in low concentrations to promote the adhesion of the TGO, acting as small area of sulphur capture in the solid state. The second category is based on single-phase β-NiAl modified by platinum [4, 15 -17]. Both S and reactive elements can diffuse from the superalloy through the bond coating to influence oxide growth and adherence [18]. NiAl intermetallic alloy systems have comparable strength and superior oxidation resistance to superalloys at elevated temperatures. Researchers have investigated the possibility of alternatives to superalloy for gas turbine blades materials. Such application could, however, be limited by the difficulties in metallurgical processing and machining of this alloy system. To apply such an intermetallic compound as a surface layer on some suitable substrate material could, therefore, be an attractive alternative for the enhancement of oxidation resistance at high temperature while retaining substrate strength and toughness requirements [19]. The phases ratio and distribution, as well as the contents of impurities in the bond layer are influenced by the processing method. The β-(Ni, Pt)Al layer is usually made by electroplating followed of a thermal treatment to promote inter-diffusion. Low pressure plasma thermal spray is used for MCrAlYs. EB-PVD is an alternative route capable to produce both types, however unusual [20, 21].


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A potentially attractive processing route is to deposit both the bond coat and the TBC by a PVD method, giving advantages in terms of both cost and integrity, in permitting sequential deposition during a single coating cycle [19]. After BC application, the components must be submitted to some form of surface preparation before the deposition of the TBC. This treatment can be blasting with ceramic or metallic particles or in abrasive mud bath. Concluded these stages, a thin layer of alumina (TGO), can be grown in these surfaces, being able to present significant variation in the morphology, chemical composition and content of defects depending on the used process [17]. The TGO formation occurs during the use of the coated component, during the deposition of the ceramic layer or, still, it can be produced by different techniques. However, the aluminum oxidation of the BC surface in the vacuum chamber, before the deposition of the ceramic layer, at residual pressure below of 10 torr (1.103 Pa) and temperature of 1000oC [22] (or, as suggested by Hass, 4 hours, 1120oC, 1.10-4 Pa (7.5.10-7 torr) [23]) is the most common method. The physical properties of the coatings gotten by evaporation are strong dependents of its microstructure who, in turn, depend on the used deposition parameters. For the optimization of its properties, the microstructure control is fundamental. The coating granular microstructure classification can be made, qualitatively, using the Diagrams of Structural Zones [24] that is based on the compilation of experimental results and correlate the granular structure with the homologous temperature (Ts/Tm, relation between the melting temperature of material that is being evaporated and the substrate temperature, in K) and the residual pressure of the evaporation chamber [25]. Experimental Procedure The coated substrates were 50 x 10 x 0.2 mm3 plates of SAE 304 stainless steel. The bond coat was EB-PVD deposited using one source 30kV electron beam equipment. It consists of an electron gun with an accelerating voltage of 25 kV and beam current variation from 0 to 1.2 A. The vacuum system has an ultimate pressure of 10-6 Torr (~ 10-4 Pa). A substrate holder assembly is situated above the vapor source at a vertical distance of 150 mm. A tungsten filament is used to heat the substrate by Joule effect to the desired temperature (300 – 750oC), which is measured and maintained by a thermocouple and programmable temperature controller. A water-cooled copper crucible was used for evaporation of sintered targets. The MCrAlY targets (bottoms of 20 mm diameter and mass of 20 gram) were prepared from a Ni-31Cr-11Al-0.65Y (wt.%) powder alloy sintered at 1340oC under vacuum (10-4 Pa). The crystalline phases of bond coating, with an average thickness of 50 µm, are identified by x-ray diffraction using a X’Pert Philips PW 1380/80 diffractometer. The ceramic coating microstructure and grain morphology were observed by SEM and the chemical composition was estimated by EDS using a LEO 435 VPI scanning electron microscope. Results and Discussion Measurements on MCrAlY bond layer fractured cross section, SEM image, using the UTHSCSA – Image Tool software shows columns diameter of (3 ± 1) µm in the sample of Figure 3. Table I presents the parameters used in the production of the bond layers studied in this work and Table II shows the chemical composition of MCrAlY used. One of the most used characterization technique for EB-PVD coatings is X-Ray diffraction. Using this technique is possible to take information about type and amount of crystalline phases in the coating, what is the structure of this phases and what is the lattice parameters. The crystalline phases and lattice parameters are linked with materials properties as, for example, toughness, density, thermal conductivity and durability of the coating. Figure 4 shows the X-ray diffractogram in scanning mode performed on the surface of the sintered MCrAlY bottom.


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Figure 5 shows the typical microstructure of EB-PVD MCrAlY bond layer, as seen by SEM on polished cross-section. The bond layer shows color bands associated with chemical composition changes due to the differences in saturation vapor pressure of the individual components as function of the temperature and complex chemical interactions between them. For these reasons, the evaporation of alloy is a selective process, resulting in depletion and enrichment in the melt pool and, consequently, in the coating. The results of EDS analysis performed on the bond coating cross-section are summarized in Figure 6. The composition of bond coating layer differs point-to-point. Because of the selective process of evaporation, the concentration of Al and Ni in the bond coating have a tendency of increase with the process evolution, i.e., as near of the substrate, lesser is the concentration of this two elements (Figure 6). For Cr, the behavior is opposite. Is possible to observe that from a distance of the substrate (near 20µm), i.e., after a time period of evaporation, there is a tendency of a chemical composition stabilization. These chemical composition variations take place when is used a single electron beam source during evaporation process. Figure 7 shows the X-ray diffractogram in scanning mode performed on the surface of the MCrAlY coating, where the phases AlNi and AlNi3 are detected for all Ts/Tm relation studied. Conclusions This work shows a way to produce MCrAlY bond coatings using a single source EB-PVD equipment. Several samples obtained in different deposition conditions were analyzed using XRD, SEM and EDS. EDS analysis show the variation in the chemical composition across the cross section of the deposit layer. However, this chemical variation can be controlled, and then it is possible to obtain the equilibrium phases anticipated by the Al-Ni Equilibrium Diagram. The XRD analysis shows the presence of AlNi and AlNi3 phase, this is in agreement with the phase diagram for the composition of the MCrAlY used as vapor source. Acknowledgments The authors like to thank FAPESP (process number: 02/06514-1) for the financial support.


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Figure 1. Bond layer categories related with compositions and oxidation and corrosion resistance [16].

Figure 2. Ni-Al Equilibrium Diagram [26]


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Figure 3. SEM of EB-PVD bond coating (Ts/Tm = 0.52) fractured cross-section, column diameter = 3 +/-1 µm.

Table I: Bond Layer Production Parameters

Sample Voltage (kV)

Current (A)

Time (min)

Ts (oC) Ts/Tm

Deposition rate

(µm/kW.h)

Thickness (µm)

M-01 26.0 0.25 70 650 0.57 - - M-02 26.5 0.15 120 940 0.75 1.96 16 M-03 27.0 0.10 200 750 0.63 2.66 24 M-04 26.0 0.10 145 550 0.51 4.77 30 M-05 26.0 0.20 160 300 0.36 3.18 44 M-06 26.0 0.13 180 550 0.51 2.37 24 M-07 27.0 0.11 195 550 0.51 2.38 23 M-08 25.1 0.21 100 563 0.52 - - M-09 25.3 0.20 150 566 0.52 1.58 20 M-10 25.3 0.16 170 571 0.52 1.74 20

Table II: Chemical Analyses (wt. %) of Powder and Sintered MCrAlY

Al Cr Ni YPowder chemical analyses 11.3 31.0 56.3 0.9

Powder EDS 8.9 32.1 58.1 1.0Sintered EDS 10.5 33.3 54.3 1.9


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10 20 30 40 50 60 70 80

200

400

600

800

1000

1200

1400

AlNi3(220)

AlNi3(210)

Ni

AlNi3(111)

AlNi3(200)

Y3Al2(AlO4)3AlNi3(100)

AlNi(100)

AlNi(211)

AlNi(110)

a. u

.

2 θ (degree)

Figure 4. XRD in scanning mode of the sintered MCrAlY.

Figure 5. SEM (BS mode))of an EB-PVD MCrAlY bond layer coating over a stainless steel substrate

(sample M-05).


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0 5 10 15 20 25 30 35 400

10

20

30

40

50

60

70

80

Con

cent

ratio

n (w

t. %

)

Distance from substrate surface (µm)

Ni Cr Al

Figure 6. EDS semi-quantitative analysis performed on the bond layer cross-section (sample M-05).

10 20 30 40 50 60 70 800

1

2

3

4

AlNi(110) AlNi

(211)

AlNi(210)

AlNi(100)

AlNi3(211)

AlNi3(210)

AlNi3(220) AlNi3

(200)

AlNi3(111)

AlNi3(110)

AlNi3(100)

Ts/Tm = 0,75

Ts/Tm = 0,63

Ts/Tm = 0,52

Ts/Tm = 0,36

a. u

.

2 θ (degree)

Figure 7. XRD in scanning mode on the surface of the MCrAlY coating surfaces with different Ts/Tm values.


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Ni-Ti Thin Films by EBE