Oxidation behavior of niobium aluminide intermetallics protected by aluminide and silicide diffusion coatings

The isothermal and cyclic oxidation behavior of a new class of damage-tolerant niobium aluminide (Nb₃Al-xTi-yCr) intermetallics is studied between 650 °C and 850 °C. Protective diffusion coatings were deposited by pack cementation to achieve the siliciding or aluminizing of substrates with or without intervening Mo or Ni layers, respectively. The compositions and microstructures of the resulting coatings and oxidized surfaces were characterized. The isothermal and cyclic oxidation kinetics indicate that uncoated Nb-40Ti-15Al-based intermetallics may be used up to ∼750 °C. Alloying with Cr improves the isothermal oxidation resistance between 650 °C and 850 °C. The most significant improvement in oxidation resistance is achieved by the aluminization of electroplated Ni interlayers. The results suggest that the high-temperature limit of niobium aluminide-based alloys may be increased to 800 °C to 850 °C by aluminide-based diffusion coatings on ductile Ni interlayers. Indentation fracture experiments also indicate that the ductile nickel interlayers are resistant to crack propagation in multilayered aluminide-based coatings.     

铂改性铝化物涂层的应用与发展

作为重要的高温防护涂层或热障涂层的粘结层, 扩散型铂改性铝化物涂层（ PtAl 涂层） 在国外已获得批量 化应用, 自 PtAl 涂层商业化应用至今约 50 年时间内, PtAl 涂层在制备工艺技术、 微观结构和成分控制等方面得 到了系统优化和发展, 已形成了系列化涂层。 在 PtAl 涂层技术方面, 国内与国外尚存较大的差距, 在单晶高温 合金发展和燃气涡轮发动机对高性能高温防护涂层需求的驱动下, 国内大力开展了 PtAl 及改性 PtAl 涂层的研制 工作。 在此背景下, 本文对 PtAl 涂层的发展、 应用、 性能特征、 Pt 的作用机理和最新研究进展进行了综述, 并系统介绍了 PtAl 涂层组织结构特征及其制备技术, 最后展望了国内在 PtAl 涂层发展方向。     

Electrochemical synthesis of protective coatings of refractory compounds of group IVa–VIa metals from ionic melts

Data are given concerning the production of protective coatings of refractory compounds of Group IV–VI metals using high-temperature electrochemical synthesis from ionic melts.     

Structure and oxidation resistance of a slurry-diffusion coating on niobium

Conclusions A study was made of the phase composition of slurry-diffusion Fe-Ti-Si coatings on niobium during their formation and atmospheric oxidation. It has been established that such coatings possess excellent resistance to oxidation by air at temperatures of up to 1300°C. The coatings lose their protective qualities after the complete oxidation of their outer zones, containing titanium and iron suicides.Translated from Poroshkovaya Metallurgiya, No. 11(227), pp. 69–73, November, 1981.     

Diffusion aluminide coatings for protecting the surface of the internal space of single-crystal turbine blades made of rhenium- and rhenium-ruthenium-containing high-temperature alloys: Part II

The production of heat-resistant aluminide and chromium aluminide protective coatings with a barrier layer, which is based on the carbides of the refractory elements of a nickel superalloy, is considered for the powder methods of diffusion chromium and aluminum saturation of the surface of the internal space of single-crystal high-pressure turbine blades made of high-temperature rhenium- and rhenium-ruthenium-containing alloys for a gas turbine engine. The barrier layer is shown to prevent the formation of a secondary reaction zone, which softens an alloy, under a heat-resistant coating during long-term high-temperature holding. The kinetics of powder-assisted aluminizing and chromizing-aluminizing of high-temperature rhenium- and rhenium-ruthenium-containing alloys is studied, aluminizing and chromizing-aluminizing conditions are determined, and the effect of heat-resistant diffusion coatings with a barrier layer on the mechanical properties of the high-temperature alloys is investigated.     

Protecting niobium and tantalum from high-temperature gas corrosion (review)

The survey deals with methods of raising the heat resistance of niobium, tantalum, and alloys, based on them, and the main lines of development are distinguished for protective coatings that increase the working life of refractory-metal specimens. Ternopol Pedagogic Institute. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 85–93, January–February, 1997.     

Self-propagating high-temperature synthesis for the deposition of thermal-sprayed coatings

The paper reviews the relevant literature and results of the author’s research into self-propagating high-temperature synthesis (SHS) used to deposit protective thermal-sprayed coatings. The compositions of different SHS-produced powders, which are used to deposit thermal-sprayed coatings, are indicated. The combination of spraying and synthesis of the material to form coatings by the interaction of composite powder particles is considered. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 105–125, 2008.     

Composition and structure of complex Nb - Cr carbide coatings on carbon steels

Coatings of chromium and niobium carbides deposited on steels 20, 45, 60, U8, and U10 have been studied. The structure, chemical and phase compositions of diffusion coatings were determined by x-ray layer-by-layer analysis and electron-probe microanalysis for simultaneous saturation with niobium and chromium. The coatings were found to have a layered structure with layers of markedly different compositions. The lattice constants of the phases were determined over various depth of the coatings. The microhardness of the layers ranged from 16.5 to 21.0 GPa. The cavitation-corrosion resistance of complex niobium-chromium coatings was higher than that of single-phase niobium carbide coatings.     

The Effects of Impurity Content on Plastic Deformation and Microstructure Evolution in Niobium at Temperatures from 1473 K to 1773 K

Metallurgical and Materials Transactions A - A mechanism is established by which interstitial impurity content increases the high-temperature strength of niobium. A Type 2 niobium sheet material...     

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.     

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.     

Carboborosilicide and oxide composite coatings on carbon materials

Complex carboborosilicide and oxide coatings on carbon materials prepared by vacuum activated diffusion impregnation predominantly through a liquid phase, vacuum fusion, and enamelling are studied. It is established that coating formation from the vapor phase hardly reduces the original porosity of carbon materials. The complex protective coatings developed that are formed by vacuum activated diffusion impregnation through a liquid phase lead to a reduction in the original porosity and gas permeability of carbon materials thereby increasing their high-temperature strength and heat resistance at elevated temperatures. It is shown that higher heat resistance in the temperature range 1800–2000 °C is provided by multilayer coatings based on carbides and borides of silicon, titanium and zirconium, borosilicides of tungsten, hafnium, molybdenum, and also oxides of silicon, aluminum, zirconium, and hafnium. It is established that the protective properties of coatings depend markedly both on their composition, structure, preparation method and on the form of the original carbon materials. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 21–27, March–April, 2006.     

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 palladium on the formation and protective properties of molybdenum-silicon coatings on niobium

Conclusions The addition of 0.5% Pd to a sinter-bonded molybdenum coating on niobium intensifies the process of formation of a Nb-Mo solid solution zone. The rates of siliconizing of an alloyed molybdenum coating and of the diffusional reaction of the resultant alloyed siticide coating with the substrate are higher than those observed with unalloyed coatings as a result of the formation of liquid eutectic interlayers containing Pd and Si. Alloying with palladium improves the oxidation and thermal fatigue resistance of MoSi₂ coatings on niobium.Translated from Poroshkovaya Mettallurgiya, No. 1(229), pp. 26–32, January, 1982.     

Producing Granules of High-Purity Niobium Compounds

The feasibility of obtaining granules of high-purity materials from a melt has been demonstrated, using niobium compounds as an example. A distinguishing feature of the method developed is that a concentrated flux of light energy is used to melt high-purity materials in a crucible with a thick protective layer of sintered powder of the material being melted. Moreover, the melt pelletizer is made of ceramics with a protective layer of the material being granulated or a material with a similar chemical composition. Spectral analysis of the composition of the granules obtained from a charge consisting of Nb₂O₅ and LiNbO₃ showed that the content of all the monitored impurities did not increase relative to the initial material. The proposed method can be recommended for prepreparation of mixtures of high-purity materials for growing single crystals or spraying protective coatings.     

Review of domestic designs in the field of protecting carbonaceous materials against gas corrosion and erosion in high-speed plasma fluxes

The article is a continuation of the publication cycle of the authors the on the theme “Multifunctional Protective Coatings for Especially Thermally Loaded Elements of Constructions of Hypersonic Systems.” Modern domestic approaches to the formation of single-layer and multilayered high-temperature protective coatings of various classes such as glass–ceramics ones obtained by various methods, coatings based on oxide ceramics, oxygen-free reaction-coupled coatings, and microcomposition coatings of the synergetic type are analyzed. Attention is paid to the fact that the scientific-and-applied investigation directed at the development of the structural and physicochemical models of the coating architecture and substantiation of their functioning with high-speed gas streams are almost absent. The selection of the chemical composition of coatings and production procedures of their formation is performed mainly based on the empirical basis. A series of coatings claimed as workable in conditions in a medium of tranquil or weakly perturbed air up to temperatures of 1600–2000°C is presented; however, their protective ability in conditions of the effect of high-enthalpy supersonic and hypersonic gas streams cannot be judged because of the insufficiency or absence of required data. Microcompositional coatings of the synergetic type based on the Si–TiSi₂–MoSi₂–B–Y system, which were developed in terms of an original conceptual approach to the formation of a multilevel system of heat protection of thermally stressed constructional elements made of heat-resistant materials of hypersonic aircrafts and their propulsion systems, are especially emphasized. A detailed analysis of their functioning in the composition of a unique construction wall with protected carbonaceous materials will make it possible to determine the tendencies and directions of subsequent investigations by the authors with the purpose of improving the developments.     

Protective Coatings La–Mn–Cu–O for Stainless-Steel Interconnector 08Х17Т for SOFC,Obtained by the Electrocrystallization Method from Non-Aqueous Solutions

A novel method for the formation of the protective layer on stainless steel interconnectors for solid oxide fuel cells was developed. The method was based on the electrocrystallization of metals from non-aqueous solutions on the stainless-steel interconnector with consecutive thermal treatments. Suggested method was applied for the stainless-steel 08X17T. Chemical composition of the electrolyte for the electrocrystallization was made in order to obtain the oxide protective layer of the stainless-steel interconnector of the following composition: LaMn_0.9Cu_0.1O₃. As a result, a uniform oxide layer was formed on the stainless-steel interconnector surface, protected the stainless-steel from the high-temperature oxidation leading to degradation of the functional properties of the interconnector. Forming coatings were characterized by means of grazing incidence X-rays diffraction, X-rays photoelectron spectroscopy and scanning electron microscopy. Elemental analysis and phase composition have shown that the main components of the protective coatings are found to be compounds with perovskite and spinel structures. The protective coating in the contact with cathode material based on lanthanum-strontium manganite shows significant decrease of chromium propagation from the stainless steel as a result of the diffusive firing in comparison with the sample of the stainless steel without the protective coating. Electrical resistance of the interconnector with the protective coating does not show noticeable degradation during at least 500 h at the temperature 850°C in ambient air.     

Structure and properties of Nb-Al alloys prepared by powder metallurgy

The structure and short-time strength of Nb-Al alloys of two compositions prepared by powder metallurgy are studied. The mechanical alloying of niobium with aluminum in a planetary ball mill in air is shown to result in simultaneous alloying of niobium with oxygen. During subsequent vacuum high-temperature sintering, disperse particles of a complex oxide, whose tentative composition is (AlNb)₂O₃, form in the alloy structure. The short-time strength at 1250°C of the prepared alloys exceeds that of nickel-aluminum superalloys.     

Influence of electrospark alloying on the heat resistance of steels

Conclusions The high-temperature oxidation of electrospark coatings on R9K5, 5KhNV, and 50 steels obtained by electrospark alloying with standard (T15K6, VK8, VK6M) and carbide-, nitride-, and boride-base tungsten-free alloys was investigated by the differential thermal analysis method. It was shown that in increase in heat resistance the refractory compound-base electrode materials used for production of coatings are in the order (by base component) boride-carbidenitride. A significant factor increasing the high-temperature oxidation resistance of the coatings is the increase in their coverage. With a high coating coverage on carbon steel its high-temperature oxidation resistance approaches that of die steel. Therefore, in a number of cases, replacement of expensive alloy steels with carbon steels is possible.Translated from Poroshkovaya Metallurgiya, No. 3(303), pp. 69–74, March, 1988.     

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.