Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 2. Stability and Heat Resistance of Silicide Coatings on Tungsten and Molybdenum at 1500-2000°C

The kinetics of diffusion redistribution of phases within the system WSi₂ W on heating tungsten silicide in air in the temperature range 1500-2000°C is studied. The stability and heat resistance of silicide coatings on tungsten is mainly governed by the diffusion of silicon towards the interphase boundaries W W₅Si₃, W₅Si₃ WSi₂, and WSi₂ SiO₂, formation at them of diffusion barriers of lower silicide W₅Si₃, and also a protective SiO₂ film at the outer boundary of the silicide coating. It is established that the transition rate for the higher to the lower tungsten silicide WSi₂ W₅Si₃ is on average four times slower than the transition rate for MoSi₂ Mo₅Si₃. It is shown that an increase in silicon concentration in the WSi₂ surface layer stimulates formation of diffusion barrier compounds at interphase boundaries. This leads to an increase in the stability of the phase composition and heat resistance of a silicide coating on metals. In particular at 1700°C the transition rate for molybdenum silicide on tungsten MoSi₂ (Mo, W)₅Si₃ is about twenty times slower than the transition rate for MoSi₂ Mo₅Si₃, and less by a factor of about eleven than the transition rate for WSi₂ W₅Si₃. Here there is also an increase in the heat resistance of silicide coatings on tungsten and molybdenum. It is shown that the SiO₂ film on tungsten silicide does not lose its protective properties up to 2000°C.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 1. Effect of Subsurface Layer on Process of Phase Redistribution in System WSi2 ― W

The effect of a subsurface layer of metal silicide on the phase composition stability of high-temperature MeSi₂-type coatings on refractory metals was investigated. Using the system WSi₂ W as a prototype it was experimentally determined that a subsurface layer of the tungsten disilicide, and the distribution profile of silicon in the diffusion zone upon high-temperature heating have a substantial effect on the formation of a barrier layer of lower silicide which determines the stability of the system as a whole. It is proposed that the search for more stable silicide coatings on refractory metals should be directed toward the creation of diffusion barriers on not only the inner interface MeSi₂ Me, but also the external surface of the coating. The process of coating formation must be accompanied by the formation of a metal silicide on the external surface with the highest possible concentration of silicon.     

Structure and Properties of Detonation Coatings Based on γ-TiAl

Investigations of a coating ― substrate composite before and after oxidation in air at 900°C revealed that the main structural features were: formation of an Al₂O₃ scale on the surface of the TiAlCrSc(γ) coating (as a result of oxidation) and of inner layer at its interface with a 90% titanium substrate (as a result of diffusion in the composite). The observed phenomenon was caused by a Kirkendall effect resulting in the formation of titanium-enriched phases, apparently Ti₃Al and α-Ti, which have a broad homogeneity range. The formation of a diffusion zone in the system with displacement of the Kirkendall plane in the direction of the substrate has a positive effect on the adherence of the coating. Furthermore, the filling of vacancies and pores in the coating with additional material supplied by the diffusion of titanium should have a favorable effect on the strength and durability of the coating, particularly its fatigue resistance.     

Effect of Rare Earth on the Corrosion-Resisting Performance of Zinc-Based Alloy Coatings

The corrosion behavior of coatings of pure zinc and Zn-Al,Zn-Al-RE alloys in NaCl solu-tions was studied by salt-spray experiments,even corrosion experiments and electrochemical measurementsof bi-directional polarization curves and a.c.impedance in weak polarization region consistent regularitieswere obtained by these different methods,viz.,the corrosion resistance of Zn could be enhanced by alloying itwith Al,and particularly with Al-RE.The causes of enhancement of corrosion resistance by RE were alsodiscussed.     

Electrospark Deposition of Fenicrbsic–Meb2 Coatings on Steel

Powder Metallurgy and Metal Ceramics - The effect of TiB2 and CrB2 additions to the commercial self-fluxing FeNiCrBSiC eutectic alloy on the structurization of electrospark coatings was examined....     

Study on Structural Stability of RE/Fe Multilayers

ＳｔｕｄｙｏｎＳｔｒｕｃｔｕｒａｌＳｔａｂｉｌｉｔｙｏｆＲＥ／ＦｅＭｕｌｔｉｌａｙｅｒｓＬｉＳｈｅｎｇｌｉ（李胜利），ＷａｎｇＣｈａｎｇｚｈｅｎｇ（王长征），ＢａｏＷｅｎｌｉａｎｇ（鲍文良）（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳｃｉｅｎｃｅ...     

Effect of Rare Earth on Microstructure of Vacuum Melting Ni-Based Self-Fluxing Alloy Coatings

The Ni-based self-fluxing alloy coating containing RE was acquired by the technique of vacuum melting on the hypoeutectoid steel (Fe-0.45% C) matrix. By X-ray diffraction, SEM and EDX, the microstructure and phase structure of section of coating and the microstructure near the interface between coating and matrix were investigated, and the effect of RE on microstructure of coating was also discussed. The results show that the microstructure of the NiCrBSi alloy coating is composed of Ni-based solid solution and a lot of massive, globular and needle secondary phases CrB, Ni3B, Cr7C3, Cr23C6 among the solid solution. The metallurgical binding between steel matrix and coating is realized. RE makes needle phase of alloy coating vanish. New phases of NiB and Cr6.5Ni2.5Si are precipitated from alloy coating, and secondary phases of alloy coating are sphericized. Consequently, RE also hinders the diffusion of Ni, Cr and Si atoms from coating to matrix and Fe atoms from matrix to coating, holds back the dilution of Fe for NiCrBSi alloy coating, and assures the chemical composition of the alloy coating.     

Effect of Temperature on Microstructure and Formability of Al-10 mass％ Si Coatings

The type-1aluminized(Al-10mass% Si)coating on hot stamped steel is used in commercial applications.The effect of temperature on microstructure and formability of the Al-10mass% Si coating was investigated.Hot-dip aluminized 22MnB5 steel was heated at 870,900,930,and 1 050 ℃ for 5 min.After heat treatment at different temperatures,the microstructure of Al-10mass% Si was characterized by field-emission scanning electron microscopy and energy-dispersive spectroscopy.The results show that when the Al-10mass% Si coating was heated at 870℃for 5min,three types of intermetallic phases were formed:two ternary Fe-Si-Al phases(Fe2SiAl7+Fe2Si2Al5and Fe2SiAl2+Fe2Si2Al5)and one binary Fe-Al phase(FeAl3).The phases in the coating became Fe2SiAl2+Fe2Si2Al5and FeAl3,the binary Fe-Al phase near the interface became Fe2Al5 and the Si-rich layers were decreased with the increase of heating temperature.When the heat treatment was 1 050℃for 5min,the Si-rich layer disappeared,the coating consisted of FeAl and Fe3 Al,and the Kirkendall voids were formed.When the heating temperature was increased through 870,900,and 930℃,the coating hardness was reduced and cracks were formed in the coating after hot stamping.The formability of the coating with temperature change was confirmed by a hot stamping test.When heated at 1 050 ℃,the coating was formable but continuous Kirkendall voids were observed.     

Influence of Structural Design on the Aeroelastic Stability of Brancusi’s Endless Column

Brancusi’s Endless Column (Targu-Jiu, Romania) is an interesting case study in bluff body aeroelasticity. It has been referred to as aeroelastically indifferent owing to its remarkable aeroelastic stability. This stability has been attributed to its unconventional shape. Calculations are presented which show that this strictly aerodynamic view of the column behavior is incomplete, and that the structural dynamics characteristics of the column have a powerful role in ensuring its aeroelastic stability. The calculations show that the column’s design, which provided for significant damping and mass, would assure its aeroelastic stability even if the column had a conventional and aeroelastically less favorable shape, i.e., if it were a circular cylinder (a shape that is unfavorable from the point of view of vortex-induced response) or a square cylinder (a shape that is unfavorable from the point of view of galloping).     

Preparation and Corrosion Resistance of Rare Earth Conversion Coatings on AZ91 Magnesium Alloy

Magnesiumalloysarewidelyusedinweaponin dustry,spacenavigationandhavegoodpotentialof applicationanddevelopmentindailycommunication facilitiesduetotheirpropertiesoflightweightandhighstrength[1].Appliedasstructuralmaterials,mag nesiumalloysmayeasilycrackduet…     

Preparation and Corrosion Resistance of Rare Earth Conversion Coatings on AZ91 Magnesium Alloy

The feasibility of forming Ce-containing rare earth conversion coating (short for RECCs),which is nonpoisonous and green to the environment,onto the AZ91 magnesium alloy surface in order to increase the corrosion resistance was studied. And the optimum technological conditions, such as the appropriate concentrations of the components in the solution,temperature and duration of the coatings formation were also settled. The protection of conversion coatings on magnesium alloy surface was evaluated by moisture/heating test, anodie polarization, etc.     

Effect of External Electric Field on Phase Selection and Stability of Amorphous （Nd0. 1 Fe0.9 ）3 B Alloy

Anexternalelectricfieldiswidelyusedtocontrolthemicrostructureandpropertiesofvariousmaterials ,suchasliquid crystallinepolymers[1,2 ] ,oxideglass es[3～ 5] ,ferroelectricmaterials[6 ,7] .Thesufficientlystrongelectricfieldcaninduceacholesteric to nemat icphasetransformationinliquidcrystallinepolymers .Thephaseseparationkineticsduetotheappliedelec tricfieldwasfirstreportedforoxideglassesbydeVekeyandMajumdar[8] .Subsequently ,Liuetal.systematicallystudiedtheeffectofanelectricfieldonthephasesepara…     

Influence of Lanthanum on Tribological Properties and Microstructure of Laser Clad B ＋ Fe ＋ Si Composite Coatings

The effects of rare earth ferrosilicon on the microstructure and anti-wear properties of laser-clad Fe-based alloy coating were investigated. The composition of Fe, B4C and rare earth ferrosillcon powders with different contents of lanthanum were clad onto a 45 # carbon steel substrate. Microstructural features, chemical compositions, phase structure,hardness, friction and wear properties by scanning electron microscopy (SEM), X-ray photoelectron microscopy (XPS),hardness tester, block-on-ring friction and wear tester of the clad coating were determined. Experimental results show that the friction coefficient of the clad coating doped with rare earth ferrosilicon is reduced while the wear resistance of clad coating doped with rare earth ferrosilicon is enhanced. When the content of lanthanum increases to 1.92%, the clad coating shows the best anti-wear ability, and as the content of lanthanum exceeds 1.92%, the wear weight loss increases quickly. The rare earth ferrosilicon to be doped in the clad coatings helps to disperse the boride phase (Fe2B, FeB, B4C)particles and refine the grain of boride phase. The enhancement of clad coating‘s wear resistance is due to the existence of dispersed boride phases.     

Development of Cr and Al Pack Cementation Coatings on Co-Based γ/γ′ Superalloys

Metallurgical and Materials Transactions A - Co-based superalloys have been developed as candidate materials to replace Ni-based superalloys in hot sections of turbine engines, however, their...     

An investigation on Preparation and Effects of Post Heat Treatment on Electroless Nanocrystalline Ni–Sn–P Coatings

In this study, the electroless Ni–Sn–P coatings with different amounts of Sn were deposited on copper substrates to study the properties of the as-plated and heat treated coatings, especially corrosion behavior in 10 wt% sulfuric acid. Surface composition, surface morphology, and a cross-section of the coatings were studied by Energy Dispersive Spectroscopy (EDS) and field emission electron microscope (FESEM) respectively. Structure and corrosion behavior of coatings were studied by X-ray diffraction (XRD) and electrochemical tests. Results showed that increasing in Sn content and changes in Ni and P content of the coatings led to the uniform and compact surface and changes within the structure of the coating. The coating with higher Sn content had the best corrosion behavior. Post heat treatment of nanocrystalline Ni–Sn–P coating resulted in better corrosion behavior, and the optimum heat treatment temperature was 350 °C.     

Study on Coke Formation and Stability of Nickel-Based Catalysts in CO2 Reforming of CH4

CO2 reforming of CH4 over nickel-based catalysts was investigated by using a fixed-bed reactor. Catalytic activity and amount of carbon deposition effects by nickel loading content, rare earth promoter and promoter contents were evaluated. It is found that nickel loading as well as reaction temperature can influence the activity and carbon deposition amounts, and with the addition of rare earth promoter, it can greatly improve the catalytic activity and exert a strong effect on the anticoking performance of the catalysts. The 5.0% (mass fraction)Ni-0.75% La BaTiO3 catalyst shows great resistance to coke formation and higher thermal stability as well as the catalytic activity.     

A Study on the Thermal Stability of La_2O_3-TiO_2 Supports

In this paper,various techniques including BET,XRD,SEM and XPS were used to study the sintering ofpure and La_2O_3-doped titania.The experimental results show that sintering of titania proceeds via volume diffu-sion.Adding of lanthanum oxide decreases the rate of sintering and hinders the phase transition from anatase torutile crystal by strong surface interaction between the mixed crystals(La_4Ti_9O_(24),La_(0.66)TiO_(2.99))and TiO_2.     

Corrosion Protection of Electro-Galvanized Steel by Green Conversion Coatings

A A cerium-based chemical conversion process was studied. First, zinc coating obtained from a free-cyanide alkaline bath, with derivative of imidazol with new brightener, was investigated, zinc-plated steel specimens were treated with a solution of 50 mmol· L^-1 Ce（NO3 ）3 The corrosion behavior of bare and treated mild galvanized steel was evaluated during exposure to 0.5 mol · L^-1 NaCl for different immersion time, using Tafel polarization curves and electrochemical impedance spectroscopy （EIS） measurements. The surface morphology of the coating was studied using scanning electron microscopy （SEM）. The composition and chemical percent of the coating were examined by X-ray dispersion energy （EDAX）. The results of these measurements showed that the newly developed cerium-based conversion coating process was a promising candidate for replacing the conventional chromate treatments used at present for galvanized steel.