Advanced Chemical Vapor Aluminizing Technology

Proceedings of ASME Turbo Expo 2012 GT2012 June 11-15, 2012, Copenhagen, Denmark

GT2012-70135

ADVANCED CHEMICAL VAPOR ALUMINIZING TECHNOLOGY: CO-DEPOSITION PROCESS AND DOPED ALUMINIZED COATINGS Hristo Strakov Ionbond AG Olten, Switzerland

Renato Bonetti Ionbond AG Olten, Switzerland

Vasileios Papageorgiou Ionbond AG Olten, Switzerland

Val Lieberman Ionbond AG Olten, Switzerland

ABSTRACT Chemical Vapor Aluminizing (CVA) is used for more than 20 years to protect blades and vanes in the hot section of aeroand land based turbines against oxidation and hot corrosion [1]. Modern CVA is an advanced process capable of controlled alloying the coating with additional elements using metal chlorides and tight control of the coating composition. CVA processes offer a number of advantages over conventional pack and above-the-pack cementation. This paper deals with the industrial CVA technology to produce multi-component coatings using different metal chloride generating devices. Specific examples illustrate the influence of the coating parameters and base materials on the NiAl-based coatings microstructure and composition. Advanced co-deposition CVA processes with addition of different metal elements to the aluminide coatings are presented. Modified coating properties and structures of multiple metal coatings with elements like Al, Cr, Si, Co, Y and others will be discussed.

INTRODUCTION Gas turbine engines of various size and complexity are widely used in aerospace, marine industries as well as for power generation and various aero-dedicated applications. Over the last few decades, thanks to advances in engineering, aerodynamics and material science gas turbine engines have delivered substantial improvements in fuel consumption, environmental emissions, engine noise and operating costs. One of the key ingredients of improving the fuel efficiency of the engine is an increase in the combustion (inlet) temperature. Obviously, this path requires special materials and

Audie Scott Ionbond AG Olten, Switzerland

protective coatings, capable to provide long-term mechanical stability at the operating temperatures of up to 1500° C. With this in mind, new high-temperature materials with increased creep and yield strengths were successfully developed. Direct solidification and single crystal production methods further improved mechanical performance of the materials at high temperature. However, the mechanical properties improvement is often accompanied with degradation of the environmental resistance, namely high temperature corrosion and oxidation resistance. Respect to this, special coatings, which are capable of protecting the bulk material against adverse effects of the environment, become essential for providing longevity to the hot-section engine’s components. Considering that the upside for further increase in temperature capability of traditional super alloys is limited, development of more efficient coatings is one of the attractive avenues to further improve the engine efficiency. Modern protective coatings on the components of the turbine’s hot section typically comprise two layers – underlying bond layer and top thermal barrier coating (TBC). Each layer of the coating serves its own purpose – thermally insulating TBC film decreases the surface temperature of the component, while the bond layer provides protection against high-temperature oxidation/corrosion and, in addition, acts as a bonding layer for the TBC coat (which cannot securely adhere to the uncoated bulk alloy). These two layers, performing in unison, enable stable high temperature operation of such components as blades and vanes of the engine’s hot section. There are essentially two types of bond coatings: diffusion and overlay. In the diffusion coating the deposited material is diffused into the substrate to form a continuous gradient in

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concentration. In the overlay coating the deposited material is at the surface of the substrate [2]; no diffusion interaction occurs. Most frequently used bond coats consist of inward grown nickel aluminide (β-NiAl) coatings, produced by diffusion of aluminum in the nickel-containing base material. Nickel aluminide provides protection against high temperature oxidation due to formation of thermally grown (TGO) layer of alumina, naturally occurring during exposure of the coating to high temperature. NiAl coat is often grown in presence of platinum, which slows down growth rates of alumina, thus extending the life of the coating. Generally, any doped or codeposited material, which decreases the oxidation rate of the bond coat, will positively impact the coating durability. Chromium and silicon are among the elements that are often cited as providing substantial increase in oxidation resistance [3]. However, co-deposition of these elements along with aluminum, particularly in a single deposition cycle presents a challenge. Conventional methods of aluminizing such as pack and above-the-pack cementation [4-5], cannot easily accommodate simultaneous deposition of two or more elements due to in-situ synthesis of all participating metal halides. Alternative deposition technique, pure CVD (also known as CVA – Chemical Vapor Aluminizing) implies generation of halide vapors outside the deposition zone and their transport to the reactive zone. CVA technology offers substantially more flexibility in producing alloyed coatings by offering substantially better process control and process repeatability. CVA also offers a number of advantages for producing the internal coatings, in particular in cooling channels of airfoils. Pack cementation processes have only limited capabilities to feed and remove the reactants into and from the channels. In these cases the chemical vapor deposition offers a better solution for aluminizing of such channels. A comparison between the different methods for aluminide coatings is shown in Fig.1.

CVA PROCESS: EXPERIMENTAL APPROACH AND MACHINE TECHNOLOGY Chemical Vapor Aluminizing (CVA) is process of producing Al-based coatings out of reactive Al chlorides, which are reduced by hydrogen and form Al deposit on metal surfaces. Al chlorides are routed to the surfaces to be coated through a special gas distribution system. In case of nickel-containing super alloys, the end-product is an inter-metallic coating with the composition of AlNiy (3 ≥ y ≥ 1/3), which is obtained through the following reaction (for y = 1): 3AlCl (g) + 2Ni (s) → 2AlNi (s) + AlCl3 (g)

This reaction occurs at temperature of 850-1150°C and reduced pressure of about 100mbar. H2 and Ar gases are used respectively as reduction agent and carrier gas. After arriving at the substrate surface, Al diffuses inwards, forming inter-metallic compounds with nickel. Special fixtures for loading components like turbine blades or vanes in the reactor chamber are typically used. These fixtures are designed to uniformly distribute reactive mixture among the parts and also promote it inside the cooling channels when required. If the coating is to be applied only to external surfaces, the reactive gas mixture is flown directly in the reactor and distributed among the different reactor loading levels. The components and/or samples are usually heated up to the required temperature by using a furnace with 3 to 5 heating zones. The process pressure is controlled by vacuum pump and pressure control valve. The vacuum pump is connected to neutralization system where the exhaust gas is cleaned (Fig. 2). Volatile aluminum sub-chlorides AlCln (n < 3) are used for the aluminizing reactions, which react with the surface of a nickel-base super alloy to deposit aluminum following the reaction (1) with n = 1. The formation of the aluminum subchloride AlCl occurs at temperatures above 800°C in the reactor chamber through following reaction [6-7]: AlCl3 (g) + 2Al (l) → 3AlCl (g)

Figure 1. COMPARISON OF ALUMINIZING PROCESSES

(1)

(2)

The above reaction of extraction type requires sources of Al and AlCl3. In practice this can be done with pure liquid Al at temperatures above 800°C, which is brought in contact with gaseous AlCl3. Al alloys (e.g. Al-Cr), which remain solid at this temperature, can also be used. For the industrial scale process pure Al metal is preferred. In that case, molten Al is contained in a special generator that is located directly in the reactor. The reactive gas flows through this internal generator at process temperature and the sub-chloride generation occurs according the reaction (2). The efficiency of the reaction depends on the temperature and the geometric setup of the generator. In order to increase the flexibility to load Al, running advanced processes with additional metal elements and increase the efficiency of the generator a special design is used.

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External Al generator is used as a source of AlCl3, which is required to produce sub-chloride. The AlCl3 gas is generated in the external Al generator at 300°C where solid Al reacts with HCl according to the reaction: Al (s) + 3HCl (g) → AlCl3 (g)

(3)

The formation of the AlCl3 and the amount can be easily controlled by the flow of gaseous HCl and the temperature of the generator. This feature makes CVA process very precise and better controlled as compared to other aluminizing techniques. The exhaust gases and unused chlorides are pumped out of the system, directed in the neutralization system and neutralized with NaOH. After the neutralization the exhaust gas contains only Ar and H2. The schematic of the CVA equipment is shown in Figure 2.

Figure 2. CVA – GAS & MATERIAL FLOW Ionbond AG manufactures ALUVAP line of CVA coaters, which are widely used in the industry for producing aluminide and modified aluminide coatings. A typical coating unit has two working positions, two hot wall reactor chambers and one vacuum furnace. Typically, the reactor has usable dimensions of 470 mm diameter and 1100 mm length; other dimensions are offered as well. The bell jar type furnace is designed to be moved over two working positions. When the furnace is not in process use, it is located in the furnace stand-by position. The cooling time can be reduced by using fan assisted high speed cooling shroud (Fig. 3).

The development work described in this article was carried out in the industrial unit described above. The unit was modified to accommodate co-deposition processes to enable introduction of such materials as Cr, Si, Co, Y, Zr and Hf. In order to make such co-deposition possible a special equipment tool was developed – high temperature generator. This generator operates similarly to the external Al generator but designed to enable high temperature (T > 800°C) chlorination reactions in reducing atmosphere. The required metal is loaded as granulate in the generator and heated up to the chlorination temperature. Formation of different volatile chlorides of metal elements like Cr (CrCln) achieved through reaction of the metal with HCl (H2 and Ar as carrier gas). Such generated metal chlorides are used in the reactor chamber as precursors for addition of such elements in the coating [8]. This equipment tool is allowing simultaneous deposition or single step deposition of coating structures with Al-Cr, Al-Co and Al-Y. Experimental results presented in this paper are related mainly to multi-step processes (first one metal element, followed by the next metal element) with the aim to show inter-diffused coating structures. Processes of simultaneous deposition (co-deposition) of a multi-elemental coating are a subject of the ongoing study and are intended for a separate publication. In this case the advantage of the CVA technology is that each elementary deposition process can be controlled independently, e.g. modified, started or stopped when required, thus producing various structures, impossible to obtain otherwise. For the deposition of Si-based coatings Si-halides in liquid form were used as a precursor. In our experimental program SiCl4 was evaporated and introduced in the reactor chamber before, after or simultaneously with the Al deposition. While the focus of the current work was on deposition of aluminides with addition of Cr and Si, the experimental setup also allowed introduction of such elements as Zr and Hf. Formation of the volatile Zr- or Hf-chlorides is controlled by the generator temperature and the HCl amount. The coating structure is then determined by deposition of Al and codeposition of Zr or Hf by simultaneous introduction of Al and Zr- or Hf-precursors. Addition of these elements is planned for the future development program. Substrates for the experiments were high temperature materials – nickel-base super alloys Inconel IN-600, IN-625 and IN-718 and pure Ni. The IN-718 sample dimensions were coupons with diameter 25 mm by 10 mm thick with polished surface. The IN-600, IN-625 and pure Ni base materials were used in form of tubes with internal diameter of 25 mm. The samples were cut, mounted and polished for optical and scanning electron microscope examination. The coating structure was analyzed by SEM. The device used for this analysis was Philips SEM 535M. The elements profile was detected by EDX technique using a module of the same device. The coating thickness was evaluated with optical microscope Leica DMLM (DFC425).

Figure 3. IONBOND CVA ALUVAP 190 BL COATING UNIT

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RESULTS AND DISCUSSION At the first stage formation of NiAl on different base alloys in CVA process has been investigated in order to produce reference data. As a second step, these coatings have been modified by addition of metal elements such as Si and Cr. Different concentrations of Si and Cr in the coating have been studied with variation of process temperature and gas composition. The goal of the first step was to understand the coating growth and the coating structures deposited. Coatings were deposited at different temperatures (1020, 1050 and 1060°C) and constant Al concentration for Al containing vapor. The table below is showing the coating thicknesses measured on IN-718 super alloy. It can be clearly seen that the inter-diffusion zone thickness is rapidly increasing with the temperature.

Nr. 1 2 3

Temperature (°C) 1020 1050 1060

Total thickness (µm) 26.6 34.2 38.4

Inter-diffusion zone (µm) 9.6 13.4 17.5

The CVA coating structure depends on the concentration of aluminum sub-chloride and the deposition temperature. Based on these two main factors different coatings diffusion structures can be obtained: LTHA (Low Temperature High Activity) – Inward Al diffusion with 3 zones structure (heat treatment required) HTLA (High Temperature Low Activity) – Outward Ni diffusion with 2 zones structure HTHA (High Temperature High Activity) – Inward Al diffusion with 2 or 3 zones structure The first two coating mechanisms are very well known [910] with commercial available coating systems (PWA73 and MDC 150L). The following examples demonstrate the formation of these coating while using the CVA technology. The formation of HTHA coating structures in the chemical vapor deposition technique combines two mechanisms above and the result is inward grown δ-phase Ni2Al3 [11]. In order to increase the process efficiency for the high activity CVA NiAl coatings a liquid aluminum generator for AlCl (conversion of AlCl3 to AlCl) formation (higher Al activity compared to other techniques) has been used. Next examples (Fig. 4 and Fig. 5) are demonstrating typical results of Al-diffusion experiments and comparing the coating structures deposited at the same process conditions on different base materials. Figure 4 depicts NiAl diffusion coating formed on pure nickel.

Figure 4. ALUMINIDE COATING ON NICKEL BY CVA NiAl layer is separated from the substrate material by a thin layer that was shown to be Ni3Al (Fig. 4). On pure nickel materials the aluminide coatings usually grow outwards. Since the Ni outwards diffusion is faster than the inward diffusion of Al, Kirkendall porosity could be formed below the original surface.

Figure 5. ALUMINIDE COATING ON IN-625 BY CVA

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As shown in Figure 5, the type of substrate material influences the coating structure and composition, even when it deposited at the same conditions. Here a coating structure and elemental profile of aluminide coating with clearly defined Al-rich δNi2Al3 phase on IN-625 is presented. The deposition parameters were the same as above for pure nickel. Different coating structures have been formed on the nickel-based IN-718 super alloy (Fig. 6). These examples prove that the aluminide coating structure strongly depends on the composition of the substrate.

Figure 7. INFLUENCE OF HEAT TREATMENT: a) BEFORE, b) AFTER Figure 8 presents HTLA coating on IN-600 substrate. In this case two coating zones are clearly identifiable with β-NiAl being present.

Figure 6. STRUCTURE AND ELEMENT PROFILE OF ALUMINIDE COATING ON IN-718 BY CVA The coating presented on Figure 6 has a typical for CVA HTHA coating structure. The structure and elemental profile in this case are also strongly dependent on the base material and different from the coatings deposited on pure Ni, IN-600 or IN625. Three distinct layers can be identified in the coating, with Al-rich δ-Ni2Al3 phase located on the surface (Al content up to 60%). Since this phase is hard and brittle, additional heat treatment process in order to form β-NiAl phase is usually applied. [12]. The heat treatment process with a purpose of further diffusion of Al (to flatten the Al profile) and converting δ-Ni2Al3 in to β-NiAl, could be done in the same equipment after the CVA process. Figure 7 shows an example of the impact of the heat treatment after CVA process on the coating structure and composition.

Figure 8. STRUCTURE AND ELEMENT PROFILE OF ALUMINIDE COATING ON IN-600 BY CVA The next examples present some more advanced coating structures. Based on the Al diffusion technology and the process flexibility of CVA, the addition of different metal elements has been studied. Two examples show the possibility to add elements like Si and Cr (Fig. 9, 10 and Fig. 11) to the NiAl diffusion coatings in order to increase their performance. Two general types of environmental attack are oxidation and hot corrosion. At moderate temperatures of about 860°C and lower, general uniform oxidation is not a major problem. At higher temperatures super alloys are attacked by oxygen and oxidation is taking place. However, another type of surface degradation is accelerated oxidation, induced by the deposition of molten alkali metal sulfates on the surface. This effect is known as hot corrosion with two type classifications: high

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temperature hot corrosion (Type I) at temperature range 800925°C and low temperature hot corrosion (Type II) at temperature range 680-780°C. The addition of elements such as Cr or Si to diffusion aluminide coatings has been reported to provide increased protection under hot corrosion and oxidation environments. There are several commercial available coatings on turbine blades including such elements (e.g. PWA 70-73 with Cr-Al and SermaLoy J with Si-Al) and their improved corrosion resistance is reported in [13-14].

Figure 10. STRUCTURE AND ELEMENT PROFILE OF ALUMINIDE COATING ON IN-600 WITH Si ADDITION (12%) BY CVA

Figure 9. STRUCTURE AND ELEMENT PROFILE OF ALUMINIDE COATING ON IN-600 WITH Si ADDITION (5%) BY CVA Figure 9 shows Si doped coating with 5% Si added in the 6 µm coating (gradient content increase in the surface area). This coating was produced in 1.3 hours process time using AlCl concentration of 1.3 mol% and SiCl4 concentration 0.2 mol%. In addition to this by increasing the process time to 2 hours and the SiCl4 concentration to 0.3 mol%, coatings with 12% Si content have been deposited on IN-600 base material (Fig. 10).

As it can be seen on the examples above the structure of the coating does not differ too much when the Si content changes in this range. In case of Si addition the required improvement of the hot corrosion resistance is achieved by adding Si content in the coating in the amount of 10-20% [14] (done by slurry technique). In our research program NiAl coatings have been formed at different temperatures (900-1000°C) with Si content on the surface between 2-15%. Increasing Si content beyond 15 % by means of CVD processing is impossible due to thermal properties of the NixSiy compound, which forms during coating. Since the melting point of NixSix lies at 990°C and vapor pressure of SiClx is relatively low at temperatures below 900°C, it is not possible to cross over the above concentration limitations. At temperatures over 1000°C, both the vapor pressure of SiClx and its diffusion force will increase, but sublimation of NixSiy will start taking place as well, reversing the deposition process. On the other hand, by decreasing the temperature below 900°C, there will not be enough vapor pressure of SiClx, which will dramatically decrease gas phase concentration and its diffusion potential. Besides Si-modified coatings, Cr-modified aluminide coatings promise substantial improvements and protection against hot corrosion phenomena caused by deposits of fused alkali sulfates [3]. The HTHA chromium aluminide coating was produced in the temperature range 1000-1060°C in two steps by first chromizing followed by aluminizing step. The 3-zone structure is demonstrating inward aluminum diffusion during the coating cycle by leaving the chromium content almost intact

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and as an under-layer between the “hypo-stoichiometric” δNi2Al3 and the inter-diffusion zone, as shown in Figure 11. In addition, the chromium content near the surface is low (10-16% Cr, 23% Ni, 10% Fe and 55% Al) as compared with its content in the LTHA chromium aluminide coating [13]. Additionally, the inter-diffusion zone is similar to that found in other aluminide coatings by containing significant amounts of alpha chromium precipitates (α-Cr), which probably have been transported from the base material (IN-718).

Figure 11. STRUCTURE AND ELEMENT PROFILE OF ALUMINIDE COATING ON IN-718 WITH Cr ADDITION BY CVA Such capabilities of CVA technology as multi-step processing and precise control of process parameters, make possible sequential processing: e.g. aluminizing as a first step, followed by the next coating, like chromizing. Naturally, this sequence can be reversed, when the aluminizing process becomes the last step of the coating process. This flexibility is inherent for the CVA process and offers numerous advantages in comparison to conventional cementation, where combination of the various deposition steps is impossible in the single batch and requires replacement of the reactive powders. For special processing it was found that some process adjustments are needed e.g. temperature or pressure before introducing the next reactive species. Some advanced structures also with ternary codoping (Cr-Al-Si) have been studied, but not fully validated at the present time.

SUMMARY Chemical vapor aluminizing (CVA) process was applied for depositing nickel aluminide coatings of three different structure types (LTHA, HTLA and HTHA). Feasibility of applying CVA for producing nickel aluminide coatings with various aluminum contents was proven. Deposition processes for modification of the coating with Cr or Si were developed and investigated. It was shown that the aluminizing process with additional elements requires precise control of the process parameters and often must be carried out in a few steps. In this case, each step is characterized by unique process parameters. Further knowledge of the impact of alloying elements to NiAl on both oxidation and hot-corrosion resistance is needed and potential synergistic effects of certain additions have still to be determined. Results and discussions, selected for presentation in this paper, were intentionally limited to demonstration of the influence of the process conditions and base material on the coating structure and composition as well as the CVA process control for doped or un-doped NiAl coatings. The balance of the data and new developments will be presented in the future. REFERENCES [1] G. W. Goward, 1998, Surface and coating Technology, 108 – 109, pp. 73 – 79 [2] S. Bose, 2007, “High Temperature Coatings”, Elsevier Inc. [3] R. L. McCarron, N. R. Lindblad, D. Chatterji, 1976, Corrosion, 32 (12), pp. 476 – 481 [4] E. Fitzer, H.-J. Mäurer, 1978, Arch. Eisenhüttenwesen, 49 (2) [5] W. Betz, H. Huff, W. Track, 1976, Z. Werkst.-Techn. , 7, pp. 161 – 166 [6] D. B. Rao, V. V. Dapade, 1966, J. Phys. Chem., 70, pp. 1349 – 1353 [7] T. Kikutschi, T. Kurosawa, T. Yagihashi, 1964, Trans. J. Inst. Met., 5(2), pp. 122 – 126 [8] H. Strakov, R. Bonetti, M. Auger, 2005, Fifteenth European Conference on Chemical Vapor Deposition, SPM-19 [9] P. C. Patnaik, J. P. Immarigeon, 1989, Materials & Manufacturing Process, 4 (3), pp. 347 - 384 [10] G. W. Goward, D. H. Boone, 1971, Oxidation of Metals, 3 (5), pp. 475 - 495 [11] M. Thoma, A. Scrivani, C. Giolli, A. Giorgetti, 2011, Proceedings of the ASME TurboExpo 2011 [12] J. R. Nicholls, D. J. Stephenson, 1991, Met. Mater., 17 (3), pp. 156 – 163 [13] R. E. Malush, P. Deband, D. H. Boone, 1988, Surf. Coat. Technol., 36 (1-2), pp. 13 – 26 [14] G. W. Goward, 1986, J. Eng. Gas Turbines Power, 108 (2), pp. 421 – 425

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