Thermochemical treatment of titanium alloys in powder media

Processes of solid-phase diffusion saturation of titanium alloys with carbon, nitrogen, aluminum, boron, and silicon are considered. Dependences of the thickness of the diffusion layers on the temperature and time parameters of the treatment and on the composition of the saturation medium are studied. The phase composition of the obtained thermal diffusion coatings is determined. The coatings are tested for wear and heat resistances and for corrosion strength. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 53–59, May, 2007.     

Mathematical modeling of the formation of diffusion coatings with periodic structure

To obtain coatings of thickness to 0.7 mm or more with controllable porosity, having in contrast with the traditional coatings a more uniform chemical composition and properties that are uniform through the thickness, we propose to create on the surface of the part that is being coated a periodic relief of a definite geometry, consisting of alternating protuberances (ribs) and depressions, after which diffusional saturation is performed. The inter-rib channels provide for accelerated diffusion of the saturating element through the entire thickness of the coating, equal to the rib height. Thus, the structure of the diffusion coating will be determined not only by the technological factors of diffusional saturation (temperature, duration, chemical composition of the medium and of the material being coated) but also by the geometry of the relief, which makes it possible to broaden the capabilities for varying the structural state of the diffusion coatings. The multifactor nature of the proposed method leads to certain difficulties in developing the regimes for obtaining coatings with the desired periodic structure. In this connection it is advisable to formulate a mathematical model that will make it possible to predict the structure of the obtained coatings. The mathematical model consists of a system of differential equations that can be solved numerically and describe the changes in time of the concentration of the coating elements and the location of the interphase boundaries. The adequacy of the proposed mathematical model is confirmed herein.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 6–9, June, 1994.     

Carbo-oxidising of titanium by diffusion treatment in carbon–oxygen containing media

Abstract

The surface engineering of titanium with TiC_xO_1?x coatings through diffusion carbo-oxidising from graphite in oxygen containing media is investigated. The effect of oxygen partial pressure on phase composition of coatings during carbo-oxidising is evaluated. The interval of oxygen partial pressure allowing the carbo-oxide TiC_xO_1?x to form is defined as 10^?2–10³ Pa. The effects of process temperature and time on evolution of TiC_xO_1?x composition and morphology of surface layer have been investigated. It is determined that increases in temperature and process time favour an increase in carbon content in TiC_xO_1?x. It is revealed that at temperatures above the transformation temperature T_α?β, TiC_xO_1?x is formed not only in coating but also throughout grain boundaries of diffusion layer. The stages of carbo-oxide formation during diffusion carbo-oxidising of commercially pure titanium are explained as follows: surface saturation with oxygen; formation of TiO₂ film; oxide dissolution and formation of diffusion layer; formation of non-stoichiometric TiC_x and TiO_x and their interaction resulting in TiC_xO_1?x formation. Corrosion properties of carbo-oxide coatings tested in 80%H₂SO₄ are compared with nitride and oxide coatings. It is revealed that carbo-oxide coating demonstrates better corrosion resistance. The tribological properties of carbo-oxide, nitride and oxide coatings tested with bronze counterbody are compared, and carbo-oxide coating demonstrates better wear resistance.     

The Effect of Surface-Layer Modification of Nickel Alloys on the Heat Resistance of Aluminide Coatings Applied on Gas-Turbine Blades

A method for the preparation of a complex diffusion coating by preliminary saturation of the surface layer of heat-resistant nickel alloys with nickel and by subsequent chrome aluminizing in vacuum using powder mixtures at processing temperatures of 1000 and 1200°C is analyzed. The characteristics of the chemical composition and structure of coatings and comparative data on the heat resistance at temperatures of 1000, 1050, and 1100°C under the conditions of isothermal oxidation are given. It is shown that the complex coating exhibits increased heat resistance owing to an increase in the uniformity of the surface alloy layers obtained by the diffusion nickel plating.     

Chromaluminizing of nickel and its alloys

Chromaluminide diffusion coatings on nickel and nickel-base superalloys, EI 867 and IN 100, have been formed by a two-step pack cementation technique. The effect of pack composition on coating formation and structure has been discussed. It has been stated that the appropriate choice of coating parameters enables the enrichment of aluminide coating in chromium either near its surface or near the coating/substrate interface.     

An evaluation study of aluminide and chromoaluminide coatings on IN-100

Aluminide diffusion coatings are commonly used to protect aircraft gas turbine blades and vanes from oxidation and hot-corrosion attack. These coatings are based on NiAl intermetallic compound with other alloying elements like Cr and Ti either diffused from the superalloy substrate or incorporated in a separate coating step. The present investigation is mainly concerned with the development of both aluminide and chromoaluminide coatings on IN-100, a cast Ni-base superalloy. The coating structure and composition have been characterized and the cyclic oxidation and hot corrosion properties have been evaluated for the different types of coatings. The difference in the hot-corrosion properties between the aluminide and the chromoaluminide coatings has been rationalized in terms of the coating chemistry. The mode of coating degradation under hot-corrosion conditions has also been analyzed.     

Corrosion resistance of titanium with diffusion carboxide coatings

Corrosion and electrochemical properties of diffusion coatings on titanium formed by saturating from the carbon-oxygen-containing medium in aqueous 80% sulfuric acid solutions are studied. It is found that morphology of diffusion coatings affects the protective properties of titanium. It is shown that in extra-corrosive sulfuric-acid media carboxide coatings provide titanium with high protective properties, which exceed the corrosion resistance of carbide and oxide coatings. Original Russian Text ? S.V. Guryn, I.N. Pogrelyuk, V.N. Fedirko, I.V. Dyug, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 3, pp. 306–311.     

Thermodynamic modeling of calorizing by the circulation method and computation of the phase composition of a coating

In recent years methods of surface impregnation have benefited from the use of modeling of the process of preparing coatings to determine optimum technological regimes of saturation. The present work concerns the main stages in the formation of coatings by the circulation method of diffusion saturation. A thermodynamic analysis of the state of the system is conducted and an isothermal section of the Ni - Al - Cl phase diagram is computed for a temperature of 1273 K. A computation technique is suggested for determining the phase composition of the coating as a function of the technological factors. The results can be used for developing regimes for surface impregnation that is realized under conditions close to equilibrium.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 34 – 37, May, 1996.     

AZ91D镁合金外加电场下自封孔微弧氧化膜层微观形貌及耐蚀性的研究

微弧氧化技术可以实现对金属表面的高耐蚀、耐磨等改性,传统微弧氧化所得陶瓷膜具有多孔结构,影响了其耐蚀性能及高温氧化性能。本文针对氧化膜多孔结构与腐蚀性能之间的关系开展基础研究。采用外加电场微弧氧化技术实现自封闭孔结构,并研究了不同孔结构膜层的耐蚀性能;讨论了封孔过程中胶体运动-电位-孔结构表征之间的规律性关系,评价了自封孔后膜层腐蚀性能。主要研究结果表明：膜层中的多孔结构是腐蚀介质的通道,自封孔后耐蚀性能提高,此外,耐蚀性与孔隙率及封孔填充物的成分和形态具有极大的相关性,通过调整外加电场强度和时间可以实现对自封孔的调控,从而改善耐蚀性能。     

Diffusion permeability of yttrium-based heat-resistant ion-plasma coatings

The diffusion permeability of yttrium-containing ion-plasma coatings on substrates made of the low-alloy chromium-based Cr–0.7V–0.17La alloy has been studied. It has been found that protective coatings of this type have a strong barrier effect on diffusion fluxes in the substrate–coating–environment system and that YCr + YCrO₃ coatings ensure the best protection of the substrates against gas saturation.     

Formation of Pore Structure and Its Influence on the Mass Transport Property of Vacuum Cold Sprayed TiO2 Coatings Using Strengthened Nanostructured Powder

The pore structure in nano-porous TiO₂ coatings influences the ion diffusion property and the photovoltaic performance of dye-sensitized solar cells. In this paper, TiO₂ coatings were deposited by vacuum cold spray (VCS) using a strengthened nanostructured powder. The pore structure, ion diffusion, and dye infiltration properties were examined to understand the coating deposition mechanism. Results showed that the pores in the VCS TiO₂ coatings presented a bimodal size distribution with two peaks at ~15 and ~50?nm. Based on the impact behavior of spray powder particles, a deposition model was proposed to explain the formation mechanism of the pores in the VCS coating using strengthened nanostructured powder. It was found that, compared to the conventional unimodal-sized nano-pores in TiO₂ coatings, the bimodal-sized nano-pores contributed to a higher ion diffusion coefficient of the coatings and thereby a higher photovoltage of the solar cells.     

Microstructure and tribology of TiC(Ag)/a-C:H nanocomposite coatings deposited by unbalanced magnetron sputtering

TiC(Ag)/a-C:H nanocomposite coatings with various Ag concentrations were fabricated on Si p(100) substrates. The composition and structure of as-deposited nanocomposite coatings were systemically investigated, and the friction and wear behaviors were also evaluated under the ambient, high temperature and high vacuum, respectively. Results show that the TiC nanocrystallites were formed in the amorphous hydrogenated carbon matrix near the substrate. The co-dopant Ag possessed nanocrystalline structure in the as-fabricated coatings whilst it formed Ag clusters (10–50 nm) on the surface. Furthermore, the introduction of Ag caused a significant reduction in the residual compressive stress without considerable decrease of the hardness and improved the adhesive strength of nanocomposite coatings. Tested as-deposited and after annealed at 500 °C coatings, the TiC(Ag)/a-C:H coatings showed a reduction of friction coefficients and wear rates with increment of Ag concentration. Under high vacuum condition, the TiC(Ag)/a-C:H coatings presented superlow friction behavior where the friction coefficient was reduced from 0.01 to 0.005 and lifetime increased from 0 to 1500 cycles. The significant improvement in tribological properties was mainly attributed to the low shear strength of Ag clusters on the surface as well as Ag diffusion to surface and wear track of coatings. The superior friction and wear behaviors of TiC(Ag)/a-C:H coatings make them good candidates as solid lubrication materials in space and aircraft applications.     

沉积温度对高功率脉冲磁控溅射AlCrSiN涂层结构和性能的影响*

采用高功率脉冲磁控溅射(HiPIMS)技术在不同沉积温度下制备了Al-Cr-Si-N涂层。系统研究了沉积温度对涂层结构、成分、显微形貌、力学和摩擦学性能的影响。结果表明:随着沉积温度由100℃升至350℃,涂层内部开始由非晶向纳米晶转化,300℃时出现fcc-AlN相;涂层平整性和致密性逐步改善,膜/基结合强度逐渐提高,在300℃达到最大值77 N,但温度继续升高至350℃时,严重的轰击刻蚀作用使临界载荷骤降至25 N;涂层硬度逐渐增加,在350℃达到最大值19.4GPa;涂层内应力整体呈下降趋势,由–0.8 GPa逐渐降低至–0.4 GPa左右。     

高熵合金涂层的研究进展

高熵合金涂层由于具有优于块体高熵合金和传统金属涂层的综合性能,在航空航天、核反应堆等极端服役环境下表现出了巨大的应用潜力。涂层低维形态产生的尺寸效应与高熵合金独特的多主元特征效应相耦合,使高熵合金涂层具有成分均匀、组织致密、结构稳定、性能优异等特点。概述了近年来高熵合金涂层的主要制备技术,简述了不同制备方法的原理、优势及工艺参数对涂层组织性能的影响。探讨了高熵合金中主要组元元素的作用、相结构的调控准则、多相转变行为等微观组织结构的特征与影响机制。论述了高熵合金涂层的服役性能特点,包括力学性能、抗氧化、耐腐蚀、抗辐照及耐磨损性能,并分析了成分/工艺-组织-性能的关联及相关作用机理。最后,总结了目前研究工作中存在的关键科学难题与挑战,对高熵合金涂层的研究方向与应用前景进行了展望。     

Effect of nitriding on the electrochemical behaviour of Ti‐Al‐Mn alloy

The effect of technological conditions of nitriding such as process time duration and chemical composition of saturating medium, on the corrosion behaviour of nitrided coatings in 14 M solution of sulphuric acid was analyzed. The investigations were done on the alloy Ti‐5,0 Al‐2,0 Mn. The nitriding was carried out in nitrogen both at atmospheric pressure and rarefied nitrogen pressure (1 Pa) at the temperature 850°C and time processing in the range from 5 to 20 h in nitrogen‐containing gas only, and in powder electrode graphite and nitrogen‐containing gas. It was shown that technological conditions of nitriding determine the protective properties of nitrided coatings. It was indicated that the optimal structure of the nitride layer for best corrosion protection is the thin nitride TiN_x with high surface quality and a gas‐saturated layer. Nitriding in graphite powder effects positively the protective properties of nitride coatings due to reducing the nitride‐forming process.     

Electrodeposition of amorphous Ni–P layers,thermal treatment and corrosion behaviour

The investigation reported comprises an experimental study of the thermal and corrosion properties of electrodeposited nickel-phosphorus amorphous alloy layers. The structure of the species was determined by means of X-ray diffraction spectroscopy. Samples with hypo- and hyper-eutectic compositions were synthesised out of modified classical and customised electrolytic solutions on copper substrates. The composition of the samples was determined via energy dispersive spectroscopy. A short review of the development of the composition of existing plating baths for Ni–P electrodeposition has also been presented. Thermal treatment of the samples has been carried out to crystallise the amorphous structure and to compare the thermal and the corrosion properties of the species with both types of structure. Differential thermal measurements were used to examine the thermal effects in the Ni–P samples obtained. The corrosion parameters of initial as-plated and thermally treated binary Ni–P alloy coatings were determined from Tafel plot analysis.     

Investigation of the pore-formation process during the chromizing of steel 40Kh by the circulation method

During the chromium plating of carbon steels, a carbide possessing elevated hardness and wear resistance is formed on the surface. It is known that the development of diffusion porosity, which is associated with the different diffusion mobilities of the atoms, is possible in the coatings. Use of carbides as an anti-corrosion protective layer requires its continuity; this is difficult to achieve by chromium plating in accordance with applicable regimes. In the literature, there are virtually no studies in which pore formation processes are addressed as applies to carbide coatings. This paper examines diffusion processes that take place in the surface layers of steel 40Kh during long-term annealing after chromium plating by the circulation method. The composition and properties of the chromium plated layer are studied. Possible mechanisms of pore formation and recommendations for the use of diffusion carbide coatings are proposed.Translated from Metaliovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 11–14, May, 1994.     

热障涂层失效机理、改进方法及未来发展方向

热障涂层(Thermal Barrier Coatings,TBCs)是用于航空发动机及燃气轮机的一种高效功能性隔热涂层,常用材料为氧化钇(质量分数6％～8％)部分稳定氧化锆(YSZ).首先,从TGO生长、高温烧结、CMAS腐蚀、盐雾腐蚀和热膨胀失配等方面介绍了YSZ的失效机理,以上因素会从不同程度上造成涂层分层、开裂...     

Erfahrungen mit Hochtemperatur-Korrosionsschutzschichten in stationären Gasturbinen

Experience with hot corrosion protective coatings in stationary gas turbines The article reviews the hot corrosion phenomena occurring in stationary gas turbines, shows up the requirements for coatings, and discusses the properties of diffusion and overlay coatings. Based on laboratory and field experience guidelines are given which coating system and what coating composition would be the optimum choice under given corrosion conditions. The problems to be solved for coating developments at highest temperatures, i.e. above about 1000 °C, are outlined.