Papers by Keyword: Aluminide Coatings

Abstract: The microstructure of the palladium modified and non-modified aluminide coatings was examined by the EDS and the positron annihilation spectroscopy methods. Both coatings have a double layer structure: β-NiAl phase or β-(Ni,Pd)Al phase on the top and the interdiffusion zones with the chromium and molybdenum rich phases in the β-NiAl or or β-(Ni,Pd)Al phase below. Palladium, that forms the β-(Ni,Pd)Al phase and substitutes for nickel atoms causes the increase of the positron lifetime value due to the increase in the number of open volume defects in the lattice which are jogs or vacancies on dislocation lines.

Abstract: Thermal Barrier Coatings (TBC) is the most advanced system for protection of turbine blades and vanes against high temperature, and oxidation. They are used in most advanced jet engines. In present article the new Plasma Spray Physical Vapour Deposition Technology was used to obtain yttria stabilized zirconia oxide coating with columnar structure. In research the different process parameters were changed. It was observed that powder feed rate had big influence on coating thickness. The large amount of Ar in plasma gasses combined with high powder feed rate resulted in partial evaporation of ceramic powder and splat-type structure. The same effect was observed when the power current was decreased form 2400 to 1600 A as well as pressure was increased to 200 Pa when the powder feed rate was 30 g/min. The obtained results showed that full evaporation of ceramic powder requires very low feed rate of ceramic material (2 g/min), high power current and high He content into plasma.

Abstract: A slurry aluminising process was utilised to produce duplex Si-modified aluminide MCrAlY coatings for superalloy GTD-111. MCrAlY coating was applied by means of high velocity oxy-fuel (HVOF) metal spray technique. Cyclic oxidation behaviour of the aluminide/MCrAlY coating were compared with plain MCrAlY coating. Oxidation performance of the coated samples was investigated by exposing samples to 1 h cyclic oxidation at 1100 ^°C. Oxidation test results demonstrate the Si-aluminide/MCrAlY coating exhibited much better oxidation resistance than the the uncoated superalloy due to the superior oxidation resistance of the alumina-silica scale at 1100 ^°C.

Abstract: The paper presents results of microstructural analysis of Hf-modified aluminide coatings. The coating was obtained using chemical vapour deposition (CVD) method at 1040°C using BPX-Pro 325 S equipment (Iond Bond). The deposition process time was 960 mintutes. The IN-718, IN-100 as well as CMSX-4 single-crystal nickel superalloys were the substrate material. The observation of coating was carried out using scanning electron microscopy. Chemical composition was analyzed using EDS method. The results showed that hafnium accumulates mainly on diffusion/additive layer interface and forms a „chain” of small precipitations. Hafnium was found in the additive NiAl layer of aluminide coating deposited on IN-100 superalloy. Its amount did not exceed 0.3 at %.

Abstract: In the article the hafnium modified aluminide coatings deposited using chemical vapour deposition (CVD) method were analyzed. The influence of surface treatment (grinding, sandblasting with different pressures) on microstructure of coatings were described. The Re 80 and M-247 nickel superalloys were used as substrate. Thickness of the obtained aluminide coating was in the range 32-45 mm on Re 80 and 40-45 mm on M-247 respectively. The average amount of Al in the additive layer was 22-24 wt% on Re 80 and about 21 wt % on M-247 base alloy. The total amount of hafnium in coatings did not exceed 2.5 wt % - usuallly below 0.5 wt %. The conducted research has shown that there is no strong influence of surface preparation methodology on microstructure of aluminide coatings obtained by CVD method.

Abstract: The paper presents the comparison of the structures of the zirconium modified aluminide coatings deposited on pure nickel by the CVD and method for different conditions, that is the gas flow and the time of deposition. The time of the aluminizing processes varied from 1.5 to 10 hours and the gas (HCl) flow varied from 0.4 to 1.4 l/min. Aluminum was deposited from the AlCl₃ and zirconium from the ZrCl₃ gas phases at 1040 oC. The obtained coatings were examined using an optical microscope (microstructure and coating thickness) a scanning electron microscope (chemical composition on the cross-section of the modified aluminide coating) and an XRD phase analyzer. Microstructures and phase compositions of coatings obtained at different process parameters do not differ significantly. In all cases, it is a triple zone structure. Chemical compositions of zones correspond to β-NiAl, γ’-Ni₃Al and γ-Ni (Al) phases. The elongation of the time of zirconium-aluminizing process from 1.5 to 10 hours leads to the increase of the coating thickness from 30 to about 60 μm. The EDS analysis and concentration profiles of the cross-section of the coating showed the nickel outward diffusion from the substrate and the aluminum inward diffusion from the surface to the nickel substrate. In coatings deposited at a slow gas flow porosity was observed on the border between β-NiAl and γ’-Ni₃Al layers. In coatings deposited at fast gas flow, zirconium does not form any inclusions but dissolves in the matrix. The Kirkendall porosity was not observed.

Abstract: In this study the oxidation behavior of diffusion aluminide coating containing layers of TiAl₃ and TiAl₂, develop on a substrate of Zr-Y doped α₂-Ti₃Al/γ-TiAlCrNb intermetallic alloy using pack aluminizing method, was investigated isothermally at 800°C, 900°C, and 1000°C under atmospheric air pressure. The pack cementation was carried out at 850°C for 25 hours in a pack containing 20%-wt Al, 2%-wt NH₄Cl, and 78%-wt Al₂O₃.The phases in the coatings and oxide layers were examined by optical and scanning electron microscopy as well as X-ray diffraction method, while chemical composition of the oxides and phases were examined with EDS attached on the SEM. The experimental results showed that the addition of Zr and Y increases the oxidation resistance of the coating by formation of complex oxides mainly of Al₂O₃ at the coating surfaces and sub-surface. Combination of oxidation and interdiffusion process cause transformation of TiAl₃ layer to TiAl₂ that decrease the oxidation resistance through the formation of TiO₂ rod crystals on the junction between TiAl₂ and Al₂O₃ in the outer layer.

Abstract: The paper presents results of research into the aluminizing process of TiAl intermetallics. The substrate was Ti48Al2Cr2Nb intermetallic alloy. The BPX Pro 325S CVD system was used for aluminizing process. Used in the experimental were four types of activators: AlCl₃, AlF₃, ZrCl₄ and HfCl₄. During the aluminizing process 2 kg of Al-Cr granules were put in a container. The deposition process was carried out in argon atmosphere for a duration of 4 hours at the temperature of 1000°C. The XRD and chemical analysis were conducted. The results showed than aluminide coatings contained TiAl₂ and TiAl₂ phases were formed using an AlF₃ activator. In other processes the amount of Al in the coatings was smaller than in the substrate. The obtained results showed that for the aluminizing process use of aluminum fluorides is necessary.

Abstract: An investigation was conducted to synthesize βNiAl coating on the nickel based superalloy Inconel 625 in the low activity chemical vapor deposition process (CVD). The deposition was carried out for 8 hours at 1050°C using the BPXpro3252 IonBond company equipment. Surface morphology and cross-section microstructure of the diffusion coating were studied and compared using an optical microscope, an X-ray diffractometer and a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope. It was found that 29 μm thick aluminide coating consisted of two layers: an outer one and the inner interdiffusion one. The outer layer consisted of the βNiAl phase. The inner one consisted of the βNiAl phase with chromium, molybdenum and niobium carbides (M₂₃C₆ and MC type) inclusions. Outer layer hardness was about 564 HV0.002 while interdiffusion layer hardness was about 725 HV0.002. Thermal diffusivity of Inconel 625 superalloy with and without coating was measured using a NETZSCH model 427 laser flash diffusivity apparatus. The thermal diffusivity measurements were conducted in the argon atmosphere at the temperature interval 20 - 1200 oC. Thermal diffusivity of the uncoated Inconel 625 Ni-base superalloy at the room temperature is about 2 mm²/s, while for the coated superalloy thermal diffusivity is about 2.8 mm²/s. The increase of the temperature from 20 to 1200 oC leads to the increase of the thermal diffusivity of the coated sample from 2.8 to 5.6 mm²/s. Cyclic oxidation tests for both coated and uncoated superalloys were performed at 1100°C for 1000 h in the air atmosphere. The aluminized samples exhibited a small mass increase and the α-Al₂O₃ scale was formed during the oxidation test.

Abstract: Zirconium, hafnium or platinum modification of NiAl phase increases the oxidation resistance of diffusion aluminide coatings. Small hafnium addition to aluminide coatings decreases the oxidation rate of nickel superalloys at 1100 °C.The paper presents comparison of structures of hafnium modified aluminide coatings deposited in two different ways on pure nickel. In the first way double layers of hafnium 3 μm thick and aluminum 3 μm thick were deposited by the EB-PVD on the nickel substrate. The double layers were subjected to diffusion treatment at 1050 °C for 6 h and 20 h. In the second method, a hafnium layer was deposited by the EB-PVD method, whereas aluminum was deposited by the CVD method. The obtained coatings were examined by the use of an optical microscope (microstructure and coating thickness) and a scanning electron microscope (chemical composition on the cross-section of the modified aluminide coating). Microstructures and phase compositions of coatings obtained by different methods differ significantly. Diffusion treatment for 6 h leads into formation of the Ni₅Hf phase. The elongation of the diffusion time from 6 to 20 h decrease the volume fraction of the Ni₅Hf phase. An aluminide coating deposited by the CVD method at 1050 °C at the nickel substrate with prior hafnium layer (3 μm thick) has a triple zone structure. An outer zone consists of the NiAl phase, a middle zone consists of the Ni₃Al phase, and the Ni(Al) phase forms an inner zone, close to the substrate. An NiHf intermetallic phase is between the outer and the middle zone, whereas Ni₃Hf is between the inner zone and the substrate.