Keywords

1 Introduction

The thermal spray technology is a high performance coating process. According to the standard DIN EN ISO 14917 (2017), thermal spraying generally refers to processes, in which spray additives are heated to a plastic or molten state inside or outside the spray gun or torch and then projected onto a prepared surface. Thermally sprayed coatings are used to reduce wear, enhance corrosion protection and apply thermal barrier coatings. Especially the latter are used in aviation and power plant industries. Turbine blades of aero engines are operated in harsh environments such as high temperature and mechanical loads for long-term. As a result, various defects like erosion, distortion, wear, cracks and impact dents occur. In order to increase the service life of such components, maintenance, repair and overhaul is becoming increasingly important in view of the growing competition in the industry. Turbine blades made of nickel-based alloys are mainly used in high-pressure turbines in the aviation industry and in power plant design for stationary gas turbines. Established repair processes are welding and brazing (Henderson et al. 2004; Huang and Miglietti 2011). The repair brazing of turbine blades was the focus of this study. Figure 1 shows the essential steps for repair brazing of turbine blades (Stolle 2004).

Fig. 1
A schematic illustration of a worn turbine blade and its repair process steps are presented as a manual application of filler metal, brazing in high vacuum, machining of excess filler metal, additional coatings, and repositioning of cooling holes.

Steps of turbine blade repair brazing process (Stolle 2004)

Full size image

The process steps are as follows: the coating of the worn turbine blade is stripped down to the base material. The filler metal is applied manually in form of pastes, tapes or melt spun foils, which are also nickel-based alloys. After the brazing process in a high-vacuum furnace, excess filler metal is removed by machining. Subsequently a hot gas corrosion protective coating (e.g. NiCoCrAlY) is applied by thermal spraying, followed by an aluminizing process, to enhance the resistance against hot gas corrosion by the formation of the β-phase (NiAl) (Miracle 1993; Zhan et al. 2009). The aluminizing process is carried out by a chemical vapor deposition process (CVD) by pack cementation, consisting of aluminum, alumina (Al2O3) and an aluminum halide (Pytel et al. 2012). Often, special aluminizing furnaces are also used. The last layer is the thermal barrier coating (TBC, e.g. ZrO2·8Y2O3). After the coating of the turbine blades, the cooling holes are repositioned, for example by laser drilling. This repair process is expensive and includes several process steps. The aim of this study was to develop a hybrid joining and coating technology, consisting of two stages, which allows to shorten the process chain for repair brazing of turbine blades. Reducing the process chain is achieved by applying the materials required for the repair by thermal spraying. The repair coating taken into account consisted of the nickel-based filler metal, the hot gas corrosion protective layer (NiCoCrAlY) and optional additional layers, like aluminum and the TBC. The coated turbine blade was subjected to a heat treatment and a simultaneous brazing and aluminizing process was carried out. The feasibility of the developed hybrid technology could be demonstrated in previous studies (Nicolaus et al. 2017a, b, 2018). Figure 2 shows the principle of shortening the process chain for repair brazing of turbine blades.

Fig. 2
A process flow of repair brazing turbine blades presents steps for filler metal application, brazing, machining, M C r A I Y application, aluminizing, and thermal barrier coating. These lead to repair coating in the first stage and aluminizing in the second stage via a 2-stage hybrid technology.

Process chain for repair brazing turbine blades

Full size image

Inconel 718 flat specimens were used as the substrate. This nickel-based alloy was chosen as its properties are well investigated (Brooks and Bridges 1988; Schirra 1991; Rao et al. 2001; Česnik et al. 2008). The filler metal used was Ni19Cr10Si (also known as Ni650 or B-Ni5). The advantage of this filler metal is that it consists only of three alloying elements, where silicon is the melting point depressant. For the hot gas corrosion protective layer, a MCrAlY (M = Ni and/or Co) alloy was used, which is state-of-the-art. In prior studies, two approaches for the simultaneous brazing and aluminizing process were considered. In the first approach the brazing/aluminizing process was carried out in a pack cementation. The feasibility of this procedure could be shown, but to get a microstructure nearly free of pores, a process duration of about 36 h is necessary (Nicolaus et al. 2017a). The combined brazing/aluminizing process showed that the flow and wetting ability of the thermally sprayed filler metal was maintained, providing good crack infiltration capability. The second approach was to apply the aluminum layer by thermal spraying as well, so that the pack cementation could be avoided. In this case, the process duration could be decreased to less than 30 min (Nicolaus et al. 2017b). While the brazing and aluminizing process is carried out, the NiAl-phase (β-phase) is formed so that the turbine blade is better protected against hot gas corrosion. A more detailed analysis of the coatings’ microstructures also suggest the additional formation of various intermetallic phases, like Ni3Al (γ′-phase), Al9Co2 and Al3Co (Nicolaus et al. 2017b). The formation of these phases is due to a more pronounced aluminum diffusion into the MCrAlY layer with increasing temperature. Figure 3 illustrates the cross sections of the microstructure at different temperatures.

Fig. 3
4 microscopic images proceeding of aluminum diffusion and phase formation with increased temperature present aluminum at the top, N I C o C e A I Y in the middle, and N i 650 at the bottom in as-sprayed, then at 660, 1100, and 1190 degree Celsius.

Micrographs of coated samples at different temperatures

Full size image

The feasibility of carrying out a simultaneous brazing and aluminizing with thermally sprayed aluminum could be shown. The process duration could be reduced compared to the process carried out in a pack cementation. Still, some pores are formed in the micro-structure of the brazed/aluminized coating due to diffusion- and segregation processes as well as the Kirkendall effect (Nakajima 1997).

The microstructure of a thermally sprayed coating is influenced by the spraying parameters. However, in this hybrid technology, the heat treatment parameters have an influence on the microstructure as well. In the present study, the influence or sensitivity of the coating parameters typical for thermal spraying and the impact of the heat treatment parameters of the microstructure of the coating were analyzed. Due to the fact that the combined brazing and aluminizing process could also be carried out successfully by thermally sprayed aluminum, it seems pertinent to integrate the TBC, consisting of yttria stabilized zirconia (YSZ, ZrO2·8Y2O3), in this hybrid technology. The results of a mutual heat treatment of the repair coating system Ni650/MCrAlY/Al/TBC carried out in a vacuum furnace and a shielding gas furnace using an argon/silane mixture as an inert gas are presented below. The interactions of the process parameters on the properties of the repair coating were investigated using electron microscopy analysis as well as adhesive tensile testing.

2 Materials and Methods

2.1 Materials

Inconel 718 flat specimens (30 mm × 30 mm × 2 mm) were used for the substrate. To activate the surface of the material, the samples were corundum blasted using EKF54 with a grain size from 250 to 355 µm. To remove the grid, the blasted specimens were cleaned with isopropanol in an ultrasonic bath, dried with pressurized air and afterwards coated by thermal spraying. The brazing material was the filler metal Ni650 (also known as B-Ni5), which consists of the alloying elements nickel, chromium and silicon, where the latter one is the melting point depressant. The hot gas corrosive protection layer was a MCrAlY layer and the thermal barrier coating (TBC) an yttria stabilized zirconia (ZrO2·8Y2O3). The latter layers are state-of-the-art for turbine blades. The chemical compositions are listed in Table 1.

Table 1 Chemical composition (in wt%) of the materials used
Full size table

2.2 Coating Equipment

The coating of Inconel 718 specimen with the nickel-based filler metal Ni650 (thickness 250 µm), the hot gas corrosion protective material (MCrAlY, thickness 300 µm), aluminum (thickness 80 µm) and TBC (thickness 100 µm) was realized by atmospheric plasma spraying (APS, Delta torch, GTV-Verschleißschutz, Luckenbach, Germany). Table 2 shows the coating parameters employed.

Table 2 Coating parameters
Full size table

2.3 Heat Treatment

The coated specimens underwent a heat treatment in a vacuum furnace (PVA Tepla, Wettenberg, Germany) with a pressure of 10–3 Pa within the furnace chamber. The heating rate was 20 K/min, the brazing/aluminizing temperatures were 1,090 and 1,190 °C and the dwells were 5 and 15 min. This was followed by free cooling under vacuum. For the heat treatment under an inert gas atmosphere, a shielding gas furnace (Kohnle HTE 1200-200/80–1500, Birkenfeld, Germany) was used.

2.4 Design of Experiments

To optimize the coating and heating parameters in the vacuum furnace with a focus on the microstructure of the filler metal, a design of experiments (DoE) approach was used. Parameters which influence the microstructure are: powder particle size (P) of the filler metal, powder feed rate (F), traverse velocity (V) of the torch, brazing temperature (T) and brazing time (t). Using two settings of each parameter with a high (+) and low (–) value plus the specimens in the state as-sprayed, the result is a 25 + 8 = 40 full factorial DoE. The process parameters for the DoE are shown in Table 3.

Table 3 Process parameters DoE
Full size table

Table 4 shows the principle approach of the DoE.

Table 4 DoE approach employed
Full size table

2.5 Characterization of the Coatings

To characterize the coated and brazed/aluminized samples, materialographic cross sections were prepared for microstructural analysis. Both light microscope (Axioplan 2 microscope, Zeiss, Oberkochen, Germany) and electron microscope (FE-REM SUPRA 40 VP, Zeiss, Oberkochen, Germany) images were taken. This provided data about the bonding of the coating to the base material as well as the porosity. The porosity was determined using the software ImageJ (2021). The concentration profile and the distribution of the alloying elements were obtained by EDX analyses. The tensile adhesive strength of the thermally sprayed coatings were determined according to DIN EN ISO14916 (2017). Figure 4 depicts the specimen geometry used for the tensile tests.

Fig. 4
A schematic diagram of the adhesive tensile test specimen presents a loading block of diameter 40 millimeters at the top and bottom, adhesive bond between the loading, and Inconel 718 disk, repair coating between the adhesive bond and Inconel disk 718,

Specimen for adhesive tensile test (DIN EN ISO 14916)

Full size image

Two loading blocks made of mild steel with a diameter of 40 mm were used for the adhesive test sample. A coated Inconel 718 disc was fixed between these loading blocks with an adhesive bond (HTK Ultra Bond 100, HTK Hamburg GmbH, Germany). The prepared test specimens were clamped into a universal testing machine (Walter and Bai AG, Switzerland) and the adhesive strengths of the repair coating were determined.

3 Results and Discussion

3.1 Formation of the Microstructure in the Coating and Heat Treating Processes

Figure 5 exemplarily depicts the cross sections of a coated Inconel 718 flat specimens in the as-sprayed state.

Fig. 5
A microscopic image of 100 micrometers presents 4 layers of aluminum at the top then MCrAIY, then ni-filler metal, and at the bottom Inconel 718 at 1.91 eminus 005 microbar, and signal A equal to N T S B S D.

Cross section of a coated Inconel 718 as-sprayed specimen (SEM picture)

Full size image

Starting from the Inconel 718, the nickel-based filler metal is followed by the MCrAlY hot gas corrosion protective layer and finally the aluminum. In the as-sprayed state, the boundary between the filler metal and the MCrAlY layer is difficult to recognize. As mentioned in the previous section, a DoE was carried out to reduce the pores in the filler metal. Figure 6 shows the measured porosity in the filler metal of the samples.

Fig. 6
A column chart plots porosity in percentage versus sample number for as-sprayed highest value is approximately 1 and the lowest is 2, and for heat-treated highest value is approximately 8 and the lowest is 4.

Porosity of coated specimens, as-sprayed and heat-treated

Full size image

The average porosity of the samples in the as-sprayed state was 5.3% with little scatter, while the porosity of the heat treated samples demonstrated strong variations, especially of the samples 2–20. These samples were coated with the coarse filler metal powder and the porosity varied from 2.2 to 17.6%. The porosity of the samples 21–40, which were coated with the fine filler metal powder, was nearly constant having a value of 4.2%. Figure 7 shows the cross sections of selected specimens.

Fig. 7
5 microscopic images on a scale of 200 micrometers present layers of N i A l, M C r A I Y, N i filler metal, and Inconel 718 in sample numbers 7, 8, 14, 27, and 30. More pore cavities in dark shades are in sample numbers 7 and 8.

Heat treated specimens with different coating and heat treatment parameters

Full size image

After the heat treatment, the boundary between the filler metal and the MCrAlY became clearly visible, because the filler metal had melted and changed the microstructure. Moreover, a grey area with a thickness of approximately 50 µm can be observed on the surface, due to the formation of the NiAl-phase (β-phase). Samples No. 7 and 8 were heat treated at 1,190 °C and pores and cavities were developed. Sample No. 14 underwent a heat treatment at 1,090 °C and only few pores or cavities were observed. The pores or cavities occur due to diffusion processes caused by local differences in concentration of the alloying elements. Furthermore, segregation effects and volume contractions while the filler metal solidifies are responsible for the formation of pores. Sample No. 27 and 30 are representative for the specimens coated with fine filler metal powder. There are no changes in the microstructure. On the one hand, there are less pores in the filler metal layer. On the other hand, the filler metal layer is interspersed with grey seams. These are due to the oxidation of the fine powder during the coating process. Oxides counteract the flow ability of the filler metal, so that the coating and heat treating parameters have no pronounced influence on the microstructure. When using coarse powder, dark and bright areas can be observed in the filler metal layer and the compositions of these areas are different. Exemplarily, Fig. 8 illustrates a brazed seam (sample no. 12) along with the compositions of these areas. The microstructure is similar to sample no. 7 (cf. Fig. 7). Theses samples were heat treated at high temperature (1,190 °C) and long dwell times (15 min).

Fig. 8
A microscopic image of 200 micrometers and 1 data chart presents pore, cavity, composite 1 and 2 in sample number 12. The percentage of nickel, chromium, silicon, iron, cobalt, and aluminum is 62.7, 22.1, 13.1, 0.9, 0.9, and 0.3 and 67.0, 6.2, 25.7, 0.3, 0.8, and 0.2 respectively.

Brazed seam with chemical composition of the areas marked

Full size image

In the ternary phase diagram Ni–Cr–Si different solid solutions and phases can be formed (Gupta 2006; Schuster and Du 2000). Because of the heat treatment, for both compositions within the brazed seam, alloying elements from the Inconel 718 dissolve in the filler metal and are distributed over the filler metal layer. Diffusion processes cause an isothermal solidification that leads to the areas with composition 1. These are formed primarily at the boundaries between Inconel 718/B-Ni5 and B-Ni5/MCrAlY. Due to these diffusion processes, some remaining areas in the liquid state are enriched with silicon. These areas then have the composition 2, which is formed more inside the brazed seam. This composition can be assigned to the eutectic composition in the binary system Ni–Si (Nash and Nash 1987).

When using heat treatment parameters with low values (1,090 °C, 5 min), the isothermal solidification at the boundary of the filler metal to the Inconel 718 and the MCrAlY layer is not pronounced and the areas with compositions 1 and 2 are distributed equally all over the brazed seam. Due to the low temperature and short dwell, the diffusion process is slowed down, which leads to the microstructure shown in Fig. 9.

Fig. 9
A microscopic image of 200 micrometers of sample 10 presents asymmetric structures.

Brazed seam at low heat treatment temperature and short dwell (1,090 °C, 5 min)

Full size image

Due to the Kirkendall effect (Nakajima 1997), some bonding defects of the filler metal to the Inconel 718 can be observed. However, the formation of pores in the brazed seam is substantially reduced, which can also be attributed to a higher viscosity of the filler metal at lower temperature. A detailed analysis of the DoE and the impact of the formed micro-structure is given in (Nicolaus et al. 2021). The DoE offers a set of parameters, which leads to an optimized, low porosity microstructure within the brazed seam. The microstructure and the parameters are shown in Fig. 10. The porosity was determined to be 2.2% (cf. Fig. 6).

Fig. 10
A microscopic image of 200 micrometers of sample number 19 and a data chart presents coating parameters of current in A, argon in L per minute, hydrogen in L per minute, powder feed rate in g per minute, traverse velocity in meter per second, nozzle distance in millimeters, and powder particle size.

Microstructure of a specimen heat treated with optimized parameters

Full size image

To get the lowest porosity, the brazing temperature must be set to a low value and an extended dwell time should be employed. The powder feed rate and the traverse velocity must be set to low values. The reason for the latter is still unclear. It should be noted that the DoE must be adapted to the material system used to account for the individual differences in segregation and diffusion processes.

3.2 Including the Thermal Barrier Coating into the Hybrid Process

It could be demonstrated that the aluminizing can be carried out using a thermally sprayed aluminum layer and that the usage of a pack cementation or a special aluminizing furnace is no longer needed. There are some additional aspects that should be considered if the thermal barrier coating should be integrated into this hybrid process as well: (i) The ternary phase diagram of aluminum-zirconium-oxygen indicates the formation of intermetallic Al-Zr-phases (Harmelin 1993; Zhao and Sun 2001), which leads to the assumption that these phases can improve the bonding of the ceramic thermal barrier coating to the MCrAlY layer. (ii) It is also described in the literature that the aluminizing of a MCrAlY layer by pack cementation increases the lifetime of a thermal barrier coating (Lih et al. 1992).

Based on these data, the developed repair process so far was extended, so that the thermal barrier coating could be integrated into the hybrid technology. Starting from the previous repair coating, the extended layer system filler metal/MCrAlY/Al/TBC, with M = Ni/Co and TBC = ZrO2·8Y2O3 resulted. Figure 11 shows a coated Inconel specimen with a SEM picture of the cross section.

Fig. 11
A photograph of a coated Inconel specimen and its microscopic image present layers of aluminum, T B C, M C r A I Y, filler metal, and Inconel 718.

Coated Inconel 718 specimen with SEM image demonstrating the individual layers

Full size image

3.2.1 Heat Treatment in a Vacuum Furnace

The coated specimen underwent a heat treatment with the parameters obtained from the DoE mentioned in Sect. 3.1. Figure 12 shows the result of the heat treatment carried out in a vacuum furnace.

Fig. 12
A photograph of a treated sample presents effects after brazing at a temperature of 1090 degrees Celsius for 15 minutes coating peeled off. 4 heatmaps present aluminum and Z r distribution from the top and bottom view presents a large amount of Z r in the bottom view.

Element distribution maps of heat treated sample including the TBC

Full size image

After the heat treatment of the coated specimen in vacuum, delamination of the ceramic thermal barrier coating was evident. An analysis of the delaminated area revealed that the delamination progresses between the Al/MCrAlY and the TBC layer. The more detailed analysis of the delaminated coating in Fig. 13 shows the element distribution of aluminum and oxygen at 550 and 1,090 °C.

Fig. 13
2 heatmaps. A presents layers of aluminum, T B C, and MCrAIY at 550 degrees Celsius and delamination of aluminum at 10190 degrees Celsius. B presents layers of T B C, oxygen, and MCrAIY at 550 degrees Celsius and delamination of the oxygen layer at 1090 degree Celsius.

Element distribution of Al and O in heat treated sample at different temperatures

Full size image

At 550 °C, which is below the melting point of aluminum, no delamination of the TBC occurs. At 1,090 °C, the concentration of the aluminum at the top of the MCrAlY layer is higher than at 550 °C. Aluminum diffuses into the MCrAlY layer due to a gradient in the chemical potential between these layers. At 550 °C, oxygen was detected in the MCrAlY layer. At 1,090 °C, oxygen is significantly present at the top of the MCrAlY, but hardly detected within the MCrAlY layer. At higher temperatures, oxygen dissolves and is distributed homogeneously in the MCrAlY layer, thus leading to a dilution of oxygen and the concentration finally is below the detection limit. The detectable oxygen at the top of the MCrAlY layer can be explained as follows: The coated Inconel 718 sample was heat treated in vacuum at 1,090 °C. The pressure within the furnace was 10–8 bar and residual oxygen is present. Yttria stabilized zirconia is an oxygen ion conductor (Yoon et al. 2013) and oxygen diffuses via the TBC towards the aluminum. This leads to the formation of alumina (Al2O3) and causes a delamination of the TBC. The detectable oxygen at the top of the MCrAlY layer at 1,090 °C is not “free” oxygen, but the result of the alumina formation. Regarding the other alloying elements like Ni, Co and Cr, the formation of different chemical compounds and phases is possible. Figure 14 shows the corresponding element distribution at 1,090 °C.

Fig. 14
4 heatmaps present the addition of layers of nickel, cobalt, chromium, and oxygen to the T B C, M r y A I Y, filler metal, and Inconel 718.

Distribution of additional alloying elements of the sample heat treated at 1,090 °C

Full size image

Beside the formation of NiAl (β-phase) and Al2O3, the element distribution suggests that different oxides like NiO, NiAl2O4 and other mixed oxides can be formed, taking into account all alloying elements. This is in agreement with information from literature (Lv et al. 2022; Saltykov et al. 2004). According to the phase diagram Ni-Al (Nash and Nash 1987), the β-phase is a stoichiometric, thermodynamically stable composition, so that the aluminum can be substituted with a NiAl layer. This led to the following repair coating system: filler metal/MCrAlY/NiAl/TBC. The coated Inconel specimen underwent a heat treatment (1,090 °C, 15 min) and no delamination occurred. The result and the corresponding cross sections are shown in Fig. 15.

Fig. 15
2 microscopic images and photographs of the sample in the as-sprayed and heat-treated condition present layers of T b C, Ni A l, M C r A I Y, and filler metal.

Heat-treated sample, using a NiAl layer

Full size image

The reason for the formation of this microstructure is the kinetic of the oxidation of the bond coat used (Al or NiAl). The standard Gibbs free energies of the formation of NiAl, NiO and Al2O3 are −131 kJ/mol, −234 kJ/mol and −1300 kJ/mol, respectively (Róg et al. 2003; Holmes et al. 1986; Yang et al. 2014). From a thermodynamic point of view, the equilibrium is on the side of these components. Clearly, thermodynamic data does not directly provide the kinetics of chemical reactions. Using aluminum as a bond coat, NiAl is formed at the boundary to the MCrAlY layer, while aluminum oxidizes at the boundary to the TBC without an intermediate step. Using NiAl as a bond coat, NiO and Al2O3 are formed at the boundary to the TBC. However, several stages are needed to oxidize the NiAl layer (Unocic et al. 2017), which lower the rate of oxidization, and thus no delamination occurs. Further investigations must be carried out to better understand what happens at the boundary NiAl/TBC in detail (e.g. formation of additional intermetallic phases, like ZrAl, ZrNi etc.).

3.2.2 Heat Treatment in a Shielding Gas Furnace

The results presented so far are based on a heat treatment carried out in a vacuum furnace. The knowledge gained was exploited to transfer the heat treatment process into a shielding gas furnace. Silane (SiH4) doped argon was used as the shielding gas. The addition of SiH4 in the single-digit ppm range is sufficient to quantitatively remove oxygen and water impurities in conventional process gases. In this way, extremely high vacuum adequate process conditions can be established (Bach et al. 2010–2011). The brazing temperature was 1,120 °C and the dwell was 25 min, which represents a standard brazing process under these conditions. Figure 16 shows the cross section of a sample, heat treated in a shielding gas furnace.

Fig. 16
A microscopic image of 200 micrometers presents the T B C layer at the top, then N I A l, M C r A I Y, filler metal, and at the bottom Inconel 718.

Microstructure of a sample heat-treated in a shielding gas furnace

Full size image

The microstructure of the sample is nearly the same as can be seen in Fig. 8 but with less pores and can be explained by the lower brazing temperature used.

3.3 Tensile Adhesive Strength

The previous section described the impact of the heat treatment on the microstructure and the element distribution in the coating. For further assessment, tensile adhesive strength tests of the applied thermal barrier coating in the as-sprayed state and the heat-treated repair coating in a vacuum- and a shielding gas furnace were carried out according to DIN EN ISO 14916 (2017). The results are summarized in Fig. 17.

Fig. 17
A bar chart of tensile adhesive strength for as-sprayed is less than 20, for heat-treated vacuum is more than 20, and for heat-treated shielding gas is less than 60 that is the failure of glue. A photograph presents torn coating at the top then Inconel 718 disc, and in the bottom loading block.

Adhesive tensile strength of differently heat-treated samples

Full size image

The tensile adhesive strength of the thermal barrier coating in the as sprayed state was determined to 16 ± 1 MPa. This is a typical value for ceramic coatings and is in the range of data reported in the literature (Limarga et al. 2005; Lima and Guilemany 2007; Pugacheva et al. 2019). Figure 17 also shows a typical failure pattern of the tested coating due to adhesive and cohesive failure. The sample, which was heat treated in the vacuum furnace, shows an improved strength of 22 MPa at first glance but has a scatter of ±10 MPa. This is caused by cracks formed in the TBC, which led to premature failure of the coating. Obviously, the process parameters must be adjusted, but the trend towards higher tensile adhesive strength is already apparent. In case of heat treated samples in a shielding gas furnace, the tensile adhesive strength is even higher and reached 55 MPa with a scatter of ±4 MPa. The true value of the tensile adhesive strength could not be determined because the failure of glue applied to the coated test specimens. Research is underway to transfer the repair process into actual industrial application.

4 Conclusions

This study demonstrated the development of a hybrid technology for repairing turbine blades of the high pressure turbine. This hybrid technology employs a simultaneous brazing and aluminizing process. Specifically a thermally sprayed repair coating, consisting of the filler metal, the hot gas corrosion protective coating, a nickel aluminum layer and finally the thermal barrier coating is applied. The main results can be summarized as follows:

  1. 1.

    The feasibility of the developed hybrid technology could be shown.

  2. 2.

    With this technology, it is possible to shorten the state-of-the-art repair brazing process of turbine blades.

  3. 3.

    The aluminizing process can be integrated in the hybrid process. This eliminates the need for pack cementation or use of a special aluminizing furnace.

  4. 4.

    The diffusion processes and phase formations in the repair coating of a simultaneous brazing and aluminizing process can be tailored to achieve a well adhering coating with low porosity.