Solid State Phenomena Vol. 227

Abstract: The paper presents results of research into thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were formed by overaluminizing of an MeCrAlY–type coating deposited by low pressure plasma spraying. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized. Used as base material was Rene 80 nickel superalloy. The research has shown that as a result of aluminizing by the CVD method there was formed in the bondcoat a deposit zone built of the β-NiAl phase which protects from oxidation. Preserved below increased chromium content ensures resistance to hot corrosion. The outer ceramic layer was characterized by columnar structure similar to that obtained in the EB-PVD process.

Abstract: The turbine blades made of directionally solidified nickel-based superalloys are exposed to combination of high temperature and aircraft environment, in which appear corrosive elements like sulphur, sodium and vanadium (hot corrosion). Corrosion resistance of superalloys is mainly dependent on their structure and chemical composition. Therefore, it is important to be aware of the correlation between the hot corrosion and changes in chemical composition and morphology of a surface of the material. The following paper presents the influence of sulphur on the microstructure of directionally solidified nickel-based superalloy. The research was carried out in Na₂SO₄ environment at two temperatures of 850^oC and 900^oC (below and above the melting point of salt, 884°C). The results show scale morphology on material surface and changes in chemical composition of surface of nickel superalloy.

Abstract: De-ashed coal samples involving from 0 to 0.6 %wt organic sulphur were investigated as solid fuels in a direct carbon solid oxide fuel cell (DC-SOFC). Prior to electrochemical investigations, the corrosion resistance of cell components in the coal bed was investigated at 900°C for 500h. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence of sulphur on the surface of the yttria fully-stabilised zirconia (8 %mol Y₂O₃ in ZrO₂, 8YSZ) as well as the surface of the Ni-8YSZ cermet anode following these tests. The electrochemical oxidation of coal on the surface of 8YSZ as well as that of the Ni-8YSZ cermet anode was investigated at a temperature range of 580–850°C. It was found that the presence of the organic form of sulphur as an impurity of coal caused a more rapid decrease in current during operation of a DC-SOFC with an 8YSZ electrolyte under load. The unfavourable impact of this form of sulphur on the electrochemical oxidation of coal on the Ni-8YSZ surface is more complicated than in the case of the 8YSZ electrolyte. In the DC-SOFC with Ni-8YSZ anode material, the presence of organic form of sulphur in de-ashed coal leads to depreciation of the Ni-8YSZ anode material.

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: We considered the degradation of hollow turbine blades made of ZhS6K nickel-based superalloy after service in an aircraft engine. The blades were coated with a diffusive aluminide coating (Al-Si) to improve resistance to oxidation and hot corrosion. Turbine blades work under extreme conditions and a complex state of stress. During service, creep and fatigue occur. The interaction among hot combustion gases causes oxidation of the surface layer, hot corrosion, and micro-cracking of the coating. Moreover, changes occur in the morphology of the γ’ phase just under the coating, and transformations of the primary carbides take place. The factors limiting the lifetime of a turbine blade are the quality of the aluminide coating and the microstructure of the superalloy, depending on the service parameters (the temperature and the duration service). We found that exposure to high temperatures above the critical value for several seconds substantially decreased the engine power and its durability. We analysed the microstructure, chemical composition, and phase composition of turbine blades after service. An evaluation of the extent of degradation was performed using scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS), and electron backscatter diffraction (EBSD). The EBSD technique was used to analyse the phase composition in micro-areas, especially to identify carbides before and after transformations.

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.

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: The aim of the present work was to determine the influence of chemical composition of the protective coating on the oxidation resistance of the protected alloy. Zirconium modified and nonmodified aluminide coatings were deposited on the MAR M200 nickel superalloy by the CVD method. The oxidation tests were conducted at 1100°C into 23 hour in the air. The chemical composition (EDS) analysis was performed. The kinetic of oxidation of zirconium modified and nonmodified aluminide coatings was similar. Oxides inclusions called pegs were observed on the surface of oxidized aluminide coating. HfO₂ oxide is more stable than Al₂O₃ oxide, hafnium atoms can replace aluminum atoms in Al₂O₃ oxides. This phenomena let to stabilize NiAl phase and increase of oxidation resistance of aluminide coating.

Abstract: The aim of the present work was to determine the influence of chemical composition of the coating protected nickel based superalloys Inconel 713 LC, Inconel 625 and CMSX 4 on the oxidation resistance of aluminide coating. Protective aluminide coatings were deposited in the CVD process. The low activity aluminizing at the presence of AlCl₃ and H₂ was carried out. Cyclic oxidation test for both coated and uncoated superalloys was performed at 1100°C for 1000 h in the air atmosphere. Microstructure of aluminide coatings after oxidation test was investigated by a scanning electron microscopy (SEM) and an energy dispersive spectroscopy (EDS). The best oxidation resistance shows uncoated Inconel 713 LC superalloy. That is due to a relatively high aluminum content in this alloy. The aluminide coating deposited on the surface of Inconel 625 shows the largest oxidation resistance (insignificnt changes of mass for the whole test duration). Excellent oxidation resistance is a result of Al₂O₃ scale formation.

Abstract: The paper presents an advanced technology of Thermal Barrier Coatings (TBCs) deposition called Plasma Spray – Physical Vapor Deposition (PS-PVD). The PS-PVD is a low pressure plasma spray technology to deposit coatings out of vapor phase, which enables obtaining of columnar ceramic coatings. In this paper, the influence of various gas mixtures on properties of deposited coatings has been investigated. The measurement of coating thickness was conducted by a light microscopy method, followed by a statistical analysis. All processes had been conducted at a very low feed rate, which additionally affected ceramic microstructure.