Oxidation Resistance of Niobium Coated with Titanium Disilicide

Coatings based on TiSi₂ have been used to protect niobium alloys from corrosion at temperatures up to 1300°C. Kinetic oxidation curves are given for these coatings on niobium. The phase compositions of the coatings have been determined and also of the layers formed during oxidation. EPMA has been applied to the element distributions in the coating, from which it is concluded that the silicon is mobile. The high resistance to oxygen of coatings based on TiSi₂ is due to the formation of films of TiO₂ and SiO₂ on them.     

Effect of Properties of Titanium Aluminide Powders and Detonation Spraying Conditions on Phase and Structure Formation in Coatings

We have studied phase formation in detonation coatings sprayed from Ti - 50 at.% Al powders. The powders of the alloy were obtained by various methods: crushing an ingot and mechanical alloying of Ti and Al. Using polyphase nanostructural materials activated by mechanical alloying makes the process of phase formation in the gas-thermal sprayed coatings based on them more general-purpose and controlled due to the more active and more subtle reaction of the material with the gaseous atmosphere. We have shown that from mechanically alloyed Ti - 50 at.% Al powder, using the detonation-gas spraying method we can consolidate a coating based on Al₂TiO₅ by oxidizing action of the working gas on the powder and also a coating based on titanium aluminides with TiN inclusions by nitriding action. The phase composition of the cast microstructural γ-TiAl powder is inherited by the coating.     

Oxidation Resistance of Niobium Coated with Titanium Disilicide

Coatings based on TiSi₂ have been used to protect niobium alloys from corrosion at temperatures up to 1300°C. Kinetic oxidation curves are given for these coatings on niobium. The phase compositions of the coatings have been determined and also of the layers formed during oxidation. EPMA has been applied to the element distributions in the coating, from which it is concluded that the silicon is mobile. The high resistance to oxygen of coatings based on TiSi₂ is due to the formation of films of TiO₂ and SiO₂ on them.

     

Effect of heat treatment on the strength and tribotechnical properties of material of LNKhB grade

Conclusions The Nichrome alloy was alloyed with niobium to produce the materials of the LNKhB7 and LNKhB8 grades with a heterogeneous structure which consist of an alloyed solid solution and nickel and a high-hardness phase, i.e., molybdenum carbide. Heat treatment (normalizing, aging) greatly hardens the material as a result of the formation of the high-hardness phase (Ni₃Al).The materials hardened with the high-hardness molybdenum carbide and the phase have high and stable strength properties in the temperature range 20–700°C (800°C); at temperatures higher than the temperature of hardening heat treatment the strength properties of the materials rapidly decrease.Alloying the grade LNKhB materials with niobium greatly improves the tribotechnical characteristics as a result of the formation of a surface film in the form of complex compounds on the friction surfaces.Translated from Poroshkovaya Metallurgiya, No. 7(283), pp. 74–77, July, 1986.     

The stability behaviour of hard material coatings on alloyed steel substrates

The stability behaviour of hard material coatings made by CVD on different alloyed and carbon steel substrates depends on the properties of both substrate and coating. SEM in-situ investigations on the tension loaded surface of bended samples may be used to observe crack nucleation and growth in brittle hard material coatings such as TiC, TiCN on low alloyed and carbon steels. From the crack distance distributions some important parameters of mechanical stability such as the threshold value of fracture σ_th, the Weibull-parameter β and the ratio of the strength distribution function F and a characteristic length D may be estimated. The threshold value for fracture in the coatings σ_th decreases with increasing coating thickness for all steel-coating combinations. The threshold value also decreases if a heat treatment is made on the steel-coating system. This is due to the relaxation of the internal stress in the coating. The mean strength of the TiC-coatings decreases and the strength variance (measured by the Weibull-parameter β) increases with increasing coating thickness and therefore the mechanical stability of coatings decreases with growing thickness. The ratio of the strength distribution function F and the characteristic distance between defects D as a measure of crack density depends on the deformation behaviour of the steel as well as on the strength and the defect structure of the coating.     

OXIDATION PROTECTION MECHANISMS DURING THE SINTERING OF Mo/FeSi2 MIXTURES

Abstract

A powder mixture of molybdenum and iron disilicide can be caused to react exothermically without excessive oxidation of the molybdenum owing to the formation of an unreactive and self-healing coating. The method depends upon a chemical means, the formation of a surface barrier of a molybdate that provides an environment in which molybdenum losses are suppressed by the exclusion of oxygen. A sodium compound added to the mixture of metal powders to be alloyed reacts with the molybdenum oxide to form a sodium molybdate. During the sintering process, the sodium molybdate migrates to the surface where it forms a protective glaze which prevents oxygen in the environment from reacting with the metallic molybdenum during alloying. Once the alloy is formed, molybdenum losses through oxidation and sublimation cannot occur since the molybdenum is in the form of MoSi₂. A discussion of the chain of chemical reactions believed to occur in the sintering process is based upon identification of evolved reaction products, as well as X-ray identification of solid reaction products.     

Diffusion-coating stability and the effects of copper and sulfur on refractory metal boriding

Studies have been made on the effects of copper and sulfur on structure and phase formation as well as growth in boride coatings on refractory metals (niobium, tantalum, molybdenum, and tungsten), in addition to measurements on the thermal stability of the diffusion layers.     

Formation of eutectic silicide coatings on molybdenum

General features of structural changes in silicide coatings on molybdenum during vacuum annealing in the temperature range between 1850 and 1920°C have been studied by electron microscopy, metallography, and x-ray phase analysis. The kinetics of eutectic MoSi₂-Mo₅Si₃ formation and structure are studied. It is found that fusion of the eutectic compound coating makes it possible to reduce defects substantially and thus improve surface layer quality.     

Effect of Electron-Beam Treatment on Wear-Resistant Coatings Applied by Electroexplosive Sputtering

TiC–Mo, TiC–Ni, TiB₂–Mo, and TiB₂–Ni coatings applied to the surface of Hardox 450 steel by electroexplosive sputtering are subjected to electron-beam treatment, After electroexplosive application, the surface relief of the coatings includes features such as deformed solidifying microglobules, buildup, microcraters, microcracks, and peeling. After electron-beam treatment, the microglobules, buildup, microcraters, and microcracks disappear from the coating surface. A polycrystalline structure containing cellular elements is formed. After electron-beam treatment, the surface roughness is 1.1–1.2 μm. The thickness of the layers modified by the electron beam in the electroexplosive coatings depends linearly on the surface energy density. The greatest coating thickness is observed when using the TiB₂–Mo system; the coating thickness is least for the TiC–Ni system. That may be attributed to the thermophysical properties of the coatings. The following substructures are observed in the coatings: cellular, striated, fragmented, and subgranular. Grains with chaotically distributed dislocations and reticular dislocations are also observed. Electron-beam treatment leads to the formation of composite filled structure over the whole cross section of the remelted layer. The structure formed in this layer is more disperse and uniform than in coatings formed without electron-beam treatment. The inclusions of titanium carbide or titanium diboride in the molybdenum or nickel matrix are 2–4 times smaller than immediately after electroexplosive sputtering. Within the molybdenum or nickel grains and at their boundaries, rounded particles of secondary phase (titanium carbide or titanium diboride) are observed. They may be divided into two classes by size: particles of the initial powder (80–150 nm) that have not dissolved on irradiation; and particles formed on solidification of the melt (10–15 nm). In the electroexplosive powder coatings, the structure is mainly formed by dynamic rotation of the sprayed particles, which form a vertical structure both in the coating and in the upper layers of the substrate. The coatings have excellent operational properties: nano- and microhardness, elastic modulus of the first kind, and wear resistance in dry slipping friction.     

Surface Modification of AL9 Alloy by Electric-Spark Alloying with Materials of the AlN – Ti(Zr)B2 – Ti(Zr)Si2System

Using x-ray phase, electron-probe microanalysis, and scanning electron microscopy, we have established that heterophase coatings based on an aluminum matrix with a quasi-layered finely dispersed structure reinforced with titanium (zirconium) diboride and borosilicide, and aluminum nitride (alloyed with oxygen) are formed by electric-spark alloying of AL9 alloy with A1N – Ti(Zr)B₂ – Ti(Zr)Si₂ composite ceramics. The wear resistance of the A1N – ZrB2-based coating approached that of the ceramic itself with the sliding speed increased to V ≥ l m/sec. The formation of secondary structures of oxide solid solutions on the coating surface during tribooxidation was investigated.     

Conceptual Protection Model for Especially Heat-Proof Materials in Hypersonic Oxidizing Gas Flows

This article is a continuation of the publication cycle of the authors on the subject matter “Multifunctional Protective Coatings for Especially Heat-Loaded Constructional Elements of Hypersonic Systems.” A conceptual physicochemical operation model of the protective coating in a high-speed high-enthalpy oxidizing gas flow taking into account and leveling main surface fracture sources by the gas flow is proposed. The model is successfully implemented when developing a whole series of alloys of the Si–TiSi₂–MoSi₂–B–Y system intended to form thin-layer coatings from them by any method of the stratified deposition providing the reproduction of the structure, phase composition, and morphological features of the deposited material in the coating. During the deposition, the formation of a microcomposition layer is provided. This layer is a refractory silicide framework with the cells filled by a low-melting (relative to the melting point of framework-forming phases) eutectic structural component. This layer transforms into a multilayer system with a series of functional layers (anticatalytic, reradiative, antierosion, heat-proof, and barrier-compensation layers) of micron and submicron thicknesses during high-temperature interaction with oxygen-containing media (the synergetic effect). The protection ability is provided by the formation of self-restoring oxide vitreous film based on alloyed silica. The self-restoring effect consists of rapid filling of random defects with a viscoplastic eutectic component and protective film formation accelerated when compared with known coatings. The high resistance to the erosion carryover is provided by the presence of a branched dendritic-cellular refractory framework. Coatings MAI D5 and MAI D5U, designed in the scope of the proposed concept, are successfully approved in high-speed high-enthalpy oxygen-containing gas flows affecting the samples and constructional elements made of especially heat-proof material of various classes (niobium alloys, carbon–carbon and carbon–ceramic composite materials, and graphitized carbon materials). The protective ability of coatings of 80–100 μm in thickness in flows with the Mach number of 5–7 and enthalpy of 30–40 MJ/kg is no shorter than 600 s at T_w = 1800°C, 200 s at 1900°C, and 60 s at 2000°C, including the constructive elements with sharp edges.     

Transformation of mechanically alloyed Nb-Sn powder to Nb<Subscript>3</Subscript>Sn

Nb₃Sn was processed via mechanical alloying (MA). The powder mixture comprised of stoichiometric proportions of elemental niobium and tin powder was mechanically alloyed for 3 hours and the mechanically alloyed powder mixture was heat treated. While MA resulted in Nb-Sn solid solution, the reaction leading to the formation of Nb₃Sn occurs during the subsequent heat treatment of the powder mixture.     

Structure and properties of nickel-chrome diboride composite electrolytic coatings

Conclusions The addition, to the CEC based on nickel, of up to 6% fine-dispersion powders of chrome diboride (in their position in the vertical cathode) and of more than 16.4% micropowders (on the horizontal cathode) with subsequent vacuum annealing at temperatures ranging from 400 to 1100°C makes it possible to produce wear and corrosion-resisting coatings of three types: the coatings consisting of a nickel matrix alloyed with chrome, with uniformly distributed inclusions of Ni₃B, coatings consisting of an eutectic boride frame filled with a solid solution of chrome in nickel, and coatings consisting of an eutectic boride matrix which contain specially oriented columnar crystals of the boride of complicated composition (Ni₂CrB₂).Isothermal annealing of the nickel-chrome boride CECs results in high adhesion to the steel substrate and gas-abrasive resistance and increases by an order of magnitude the wear resistance in sliding friction in the corrosion-active acid medium. This is explained by the formation, in the coating, of eutectic structures with spatially oriented crystals of the boride which contains nickel and chrome.Translated from Poroshkovaya Metallurgiya, No. 3(315), pp. 54–59, March, 1989.     

Formation of electrospark coatings of the VK8 hard alloy with the Al2O3 additive

The results of studying the formation of coatings by electrospark alloying with the use of an electrode material based on the VK8 tungsten-containing alloy with the addition of an aluminum oxide nanopowder are presented. It is shown that introducing Al₂O₃ into the hard alloy promotes an increase in the mass transfer coefficient, microhardness, and wear resistance of the formed coatings. The duration of electric discharges exerts a significant effect on the phase composition of the alloyed layer.     

High-temperature oxidation-resistant coatings on niobium and its alloys

The formation of high-temperature oxidation-resistant coatings on niobium by multicomponent activated diffusion saturation in vacuum is investigated. Thermodynamic calculations of potential chemical reactions that result in anticorrosive protective coatings are performed. Comparative tests of high-temperature oxidation resistance of niobium with protective coatings are conducted at 1700°C in open air.     

Influence of Metal Properties on the Formation and Evolution of Metal Coatings During Mechanical Coating

Powders of Cu, Ti, Ni, Fe, and Zn metals were used to prepare coatings on the surfaces of Al₂O₃ balls by the mechanical coating technique. The coated Al₂O₃ balls were characterized with XRD and SEM. The results showed that all the metal powders except Ni formed continuous metal coatings. The evolution of metal coatings during mechanical coating was also investigated. The analysis indicates that as long as continuous metal coatings can be formed, the evolution can fall into the following stages: nucleation, formation and coalescence of discrete islands, formation and thickening of continuous coatings, and exfoliation of continuous coatings. Electronegativity of the metal was shown to have a major effect on the adhesion of the tiny metal particles on the surfaces of the Al₂O₃ balls during the initial stage of mechanical coating. The lower the electronegativity of the metal, the greater the coverage of the metal on the Al₂O₃ ball and the easier the adhesion of the tiny metal particles. Further, the better the plastic deformability of metal, the easier the cold welding among metal powder particles and the greater the thickness of the continuous metal coatings.     

Thermodynamic analysis of gaseous phase equilibrium compositions in producing iron-and cobalt-based alloys by diffusion saturation from point sources. I. Calculation of thermodynamic characteristics of chemical processes in mixtures of iron or cobalt with chromium, nickel, or molybdenum

The paper presents a thermodynamic analysis of chemical equilibrium characteristics of Fe-Cr-Ni-Mo and Co-Cr-Ni-Mo alloys used to produce retention coatings on dentures by diffusion saturation from point sources under 0.1 MPa at 500–1500 K. It is shown that metals are transported through the gaseous phase with FeCl and FeCl₂, CoCl and CoCl₂, CrCl and CrCl₃, NiCl and NiCl₂, and MoCl₃ and MoCl₄ molecules. Chromium, nickel, and molybdenum in the iron-based system should be expected to be alloyed with iron and chromium transport should be expected in the cobalt-based system.     

Characterization and In Vitro Corrosion Investigations of Thermal Sprayed Hydroxyapatite and Hydroxyapatite-Titania Coatings on Ti Alloy

In the current investigation, hydroxyapatite (HA) powder was mixed with titania (TiO₂) in 50:50?wt?pct for depositing composite coatings on a Ti-alloy substrate using a thermal-spray coating technique. The coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS) analyses. The corrosion behavior of the coatings was studied by electrochemical corrosion testing in simulated human body fluid. After the corrosion testing, the samples were analyzed by XRD and SEM/EDS analyses. HA and TiO₂ (rutile) were the main phases observed in the developed coatings. Bulk HA coating was amorphous; however, the addition of TiO₂ effectively improved the crystallinity of HA in HA-TiO₂ coating. The SEM analysis confirmed the formation of a well-formed HA-TiO₂ composite coating. HA coating exhibited higher bond strength (67.8?MPa) compared with HA-TiO₂ composite coating (37.6?MPa). The electrochemical study showed a significant improvement in the corrosion resistance of the Ti alloy after the deposition of the coatings.     

Structural transformations occurring in flame-sprayed Ni60Nb40 alloy coatings during heating in the presence of oxygen

Conclusions During the heating of flame-sprayed Ni₆₀Nb₄₀ alloy coatings in the presence of oxygen the process of crystallization of their amorphous matrix occurs at T > 750°K through the formation of the niobium oxide Nb₂U₅ and precipitation of nickel. Decomposition of the amorphous matrix is accompanied by a decrease in length of the coatings. The oxidation of niobium at 750–890°K prevents the formation of the equilibrium phases of the Ni-Nb system during the crystallization of the amorphous structure. During heating in the 890–1110°K range the metastable Ni₈Nb phase is present in the coatings.Translated from Poroshkovaya Metallurgiya, No. 12(300), pp. 21–26, December, 1987.     

氯磺酚S光度法测铌时钼的干扰研究

GB/T223.39-94氯磺酚S光度法测定钢铁中Nb含量时因受到Mo的干扰,仅适用于Mo含量不大于25 μg/50 mL的显色体系。本文研究了氯磺酚S光度法显色体系中Mo和Nb的反应,讨论了测定Nb时Mo的干扰。结果表明,当Mo含量在25～500 μg/50 mL范围内时,Mo含量与其对应的吸光度有良好的线性关系,且Mo的表观摩尔吸光系数（4.75×102 L·mol-1·cm-1）远小于Nb的表观摩尔吸光系数（3.21×104 L·mol-1·cm-1）。采用基体匹配法消除了测定Nb时Mo的干扰,从而使氯磺酚S光度法测Nb时Mo的允许量由25 μg/50 mL扩大到500 μg/50 mL。该方法适用于测定Mo含量高而Nb含量低的钢铁类试样。