Formation of aluminide coatings on Fe-based alloys by chemical vapor deposition

Aluminide and Al-containing coatings were synthesized on commercial ferritic (P91) and austenitic (304L) alloys via a laboratory chemical vapor deposition (CVD) procedure for rigorous control over coating composition, purity and microstructure. The effect of the CVD aluminizing parameters such as temperature, Al activity, and post-aluminizing anneal on coating growth was investigated. Two procedures involving different Al activities were employed with and without including Cr–Al pellets in the CVD reactor to produce coatings with suitable thickness and composition for coating performance evaluation. The phase constitution of the as-synthesized coatings was assessed with the aid of a combination of X-ray diffraction, electron probe microanalysis, and existing phase diagrams. The mechanisms of formation of these CVD coatings on the Fe-based alloys are discussed, and compared with nickel aluminide coatings on Ni-base superalloys. In addition, Cr–Al pellets were replaced with Fe–Al metals in some aluminizing process runs and similar coatings were achieved.     

Growth kinetics and oxidation behavior of WSi2 coating formed by chemical vapor deposition of Si on W substrate

The growth kinetics of WSi₂ coating formed by chemical vapor deposition (CVD) of Si on a W substrate at temperatures between 1000 and 1200 °C using SiCl₄–H₂ gas mixtures was investigated and its isothermal oxidation resistance in 80% Ar–20% O₂ atmosphere was evaluated at temperatures between 800 and 1300 °C. WSi₂ coating grew with a parabolic rate law after an initial incubation period, indicating the diffusion-controlled growth. The activation energy for growth of WSi₂ coating was about 42.5 kcal/mol. The isothermal oxidation rate of WSi₂ coating increased with increasing oxidation temperature but rapidly decreased at 1300 °C. The oxidation product of WSi₂ coating was composed of the WO₃ particles embedded in the amorphous SiO₂ matrix at below 1200 °C but consisted of only SiO₂ phase at 1300 °C. The fast oxidation behavior of WSi₂ coating at below 1200 °C was attributed to the formation of many cracks and pores, i.e. short-circuit diffusion path of oxygen, within the oxide scale, which resulted from the internal stress generated both by the large volume expansion caused by the oxidation reactions of WSi₂ and by the evaporation of WO₃ phase. The slow oxidation behavior of WSi₂ coating at 1300 °C was due to the exclusive formation of a slow-growing continuous SiO₂ scale by the rapid evaporation of WO₃ phase.     

Effect of NaCl vapor on the oxidation of Ni-Cr alloys

Ni-Cr alloys are known for their resistance to high temperature oxidation. The kinetics of scale formation and the nature of the scale in these alloys are affected by NaCl liquid or vapor. There have been a few investigations dealing with the influence of NaCl on long-time exposure. But the nature of reaction at short times can provide information on the initiation of such attack. In this investigation, Ni-Cr alloys with Cr varying from 0 to 25 wt% were exposed to NaCl vapor at 850°C for a few minutes. The surface chemistry of these alloys along with the unattached ones was analyzed by Auger electron spectroscopy. The nature of scale and the distribution of chlorine was found to vary with the Cr content in the alloys, which has a direct bearing on the rate of oxidation of these alloys in NaCl vapor.     

Formation of mullite coatings on silicon-based ceramics by chemical vapor deposition

Dense, uniform, mullite coatings have been deposited by chemical vapor deposition on SiC substrates, using a AlCl₃-SiCl₄-CO₂-H₂ system. The typical coating microstructure consisted of a thin layer of nanocrystallites of γ-Al₂O₃ in vitreous silica at the coating-substrate interface, with columnar mullite grains over this interfacial layer. The composition of the coating was graded such that the outer surface of the coating was highly alumina rich. The changes in the coating microstructure with processing parameters are discussed. The ability of mullite to incorporate such large composition variations is discussed in the light of vacancy formation as theAl/Si ratio is increased and the ordering of these vacancies leads to changes in lattice parameters. The formation of domains was studied by measuring the spacing of superlattice spots in electron diffraction patterns and the relationship between domain size andAl/Si ratio is discussed.     

Influence of water vapor on the oxidation behavior of aluminized coatings under low oxygen partial pressure

FeCrNi alloy after aluminizing was oxidized at 1000 °C in dry and humid (2.23 vol.% water) H₂. Experimental results showed that H₂ promotes the formation of θ alumina and its transformation to α alumina. The morphology of surface alumina coating does not change significantly, but the oxidation rate of the aluminized layer accelerates by the addition of water vapor. As a result, more cracks are found beneath the alumina layer when water vapor is present. The addition of water vapor seems having a favorable effect on the selective oxidation of Al and concentration of oxygen vacancy in the aluminized alloys.     

Temperature and water vapour effects on the cyclic oxidation behaviour of Fe-Cr alloys

Binary Fe-Cr alloys were subjected to cyclic oxidation at 600, 700 and 950 °C in flowing gases of Ar-20O₂ and Ar-20O₂-5H₂O (vol.%). The minimum chromium concentration required to achieve protective scale growth decreased as temperature increased from 600 to 700 °C. This change is attributed to faster chromium diffusion at higher temperature. Conversely, this minimum chromium level increased when the temperature was raised from 700 to 950 °C. This is attributed to faster scale growth, leading to its rapid mechanical failure, along with formation of slow-diffusing austenite. Water vapour accelerated scaling, leading to a need for higher chromium concentrations to resist breakaway oxidation.     

Oxidation behavior of ceramic coatings on Ti–6Al–4V by micro-plasma oxidation

Compound ceramic coatings prepared on Ti–6Al–4V alloy by pulsed bi-polar micro-plasma oxidation (MPO) in NaAlO₂ solution were oxidized under different temperature in air. The phase composition and surface morphology of the coatings before and after oxidation were investigated by X-ray diffractometry and scanning electron microscopy, respectively. Meantime, the weight gains and the high temperature oxidation characteristics of the coated samples were investigated. The results show that the coatings prepared by MPO were composed of a large amount of Al₂TiO₅ and a little -Al₂O₃ and rutile TiO₂. And the oxidation process of the coated samples included the decomposition of the Al₂TiO₅ in the coating, the oxidation of the substrate and the changes of the coating structure. After high temperature oxidation, the increase of -Al₂O₃ in the coating was due to the decomposition of Al₂TiO₅, whereas the increase of rutile TiO₂ in the coating was attributable to both the decomposition of Al₂TiO₅ and the oxidation of the Ti substrate. The main crystalline of the coatings became rutile TiO₂ after the oxidation of 1000 °C for 1 h. The decomposition of Al₂TiO₅ in the coating occurred at 900 and 1000 °C, and its half decomposition time was less than 1 h at 1000 °C. Increasing oxidation temperature or extending oxidation time, the weight gains of coated samples was increased to different extent. However, the weigh gains of the coated samples was much lower than that of the substrate, so the ceramic coatings improved the oxidation resistance of Ti alloy greatly under the experimental conditions.     

Effect of enamel top coating on oxidation behavior of multi-arc ion plating NiCrAlY coating in oxygen containing water vapor

The oxidation behavior of Ni-Cr-Al-Y coating produced by multi-arc ion plating with and without an enamel coating was investigated in flow oxygen and oxygen containing water vapor at 900 ℃. The results show that Ni-Cr-Al-Y coating exhibits low oxidation rate at 900 ℃ in pure oxygen and the oxidation kinetics follow the parabolic rate law. The presence of water vapor accelerates the oxidation rate of Ni-Cr-Al-Y coating and K38G alloy.The enamel top coating is very effective in protecting the Ni-Cr-Al-Y coating from water vapor corrosion attack, but the corrosion attack beneath the enamel coating is not observed.     

The low-temperature oxidation of calcium by water vapor

The oxidation kinetics of calcium in water vapor have been studied over the temperature range 25–300°C. There is a change in the form of the oxidation kinetics with temperature, from essentially linear at temperatures below 150°C to logarithmic at 300°C. This is coupled with a change in the manner of growth of the oxide layer as well as the chemical composition of the reaction product. In addition, the oxidation rate decreases with temperature, reaching a minimum at about 150°C. At temperatures below 150°C oxidation appears to be a result of the formation of cracks or fissures in the oxide film. Above 150°C no single oxidation mechanism can be deduced.     

The effect of cerium on the oxidation of Ni-50Cr alloys

The oxidation behavior of Ni-50Cr alloys with minor cerium additions was studied between 800 and 1100° Cin oxygen, air, and oxygen at reduced partial pressures. Optical and scanning electron metallography, X-ray diffraction, and electron-probe microanalysis techniques were used to characterize the changes in scale and substrate morphology and to identify the oxidation products. Platinum markers were used to determine the direction of ionic transport. The effects of cold work, initial alloy phase distribution, and cyclic oxidation were also studied. The Cr ₂ O ₃ scales on the cerium-containing alloys grew while being largely separated from the metal substrate. Oxidation rate, oxide grain growth, and the tendency of scales to spall on cooling were reduced substantially with increasing alloy cerium content. The first two effects are suggested to result from the interaction of cerium ions and cerium oxide particles with oxide grain boundaries in reducing grain-boundary diffusion and oxide-boundary mobility. The third is suggested to result from the thinner, finer-grained scales formed on the Ce-containing alloys.This work is based on a portion of a thesis by G. M. Ecer submitted to the University of Pittsburgh in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Metallurgical and Materials Engineering.Formerly graduate student. Department of Metallurgical and Materials Engineering, University of Pittsburgh.     

铝及铝合金的阳极氧化研究综述

较为系统地介绍了铝及其铝合金的阳极气体工艺技术,对传统的阳极氧化方法进行了总结,探讨了铝阳极氧化多孔膜的性能及微观结构,以及作为功能材料铝阳极氧化膜广泛的应用,对铝阳极氧化技术最近所取得的应用上的进展进行了总结。     

钛及钛合金高温氧化行为研究

对工业纯钛TA2、TC4及Ti60合金分别在600、700、800℃进行氧化,得到氧化增重曲线.利用SEM、XRD等手段对氧化表面形貌、氧化产物构成及分布进行研究并分析了3种钛材在不同温度下的氧化机制.结果 表明:随着温度升高,TA2、TC4以及Ti60三种钛材的抗氧化能力均有所下降,且抗氧化能力由强到弱依次为Ti60...     

NiCrAlY涂层的表面状态对高温氧化行为的影响

将原始喷涂态和表面抛光的NiCrAlY涂层在1050℃恒温氧化300h，利用XRD、SEM，EDS方法，测定涂层的氧化物及其相转变，分析表面氧化膜的生长破坏行为，结果表明：两种涂层在1050℃保温，在150h以内均能生成α-Al2O3氧化膜，150h后，抛光态涂层保护性氧化膜被破坏，抗氧化能力下降，喷涂态涂层表面粗糙，连续Al2O3保护膜的形成较晚，氧化早期氧化膜中存在微裂纹，可释放应力，有利于氧化膜与涂层的结合。氧化动力学曲线符合抛物线规律，氧化至300h，表面氧化膜只有少量微裂纹，无剥落，说明喷涂涂层的长期恒温抗氧化能力比抛光态涂层强。     

The influence of superficially applied oxide powders on the high-temperature oxidation behavior of Cr2O3-forming alloys

The effects of superficially applied CeO₂, mixed rare earth oxides, Co₃O₄, and Cr₂O₃ powders on the isothermal and cyclic oxidation of Ni-Cr alloys and the effects of CeO₂ and MgO powders on the isothermal oxidation of Fe-25 wt.% Cr have been studied over the temperature range 940–1150°C in pure oxygen and dry air. The rates of oxidation of both the Ni- and Fe-base alloys were markedly reduced by the application of CeO₂ powder. The presence of CeO₂ also improved the scale adherence and resulted in marked changes in the oxidation morphology. The presence of Co₃O₄ or Cr₂O₃ powders on Ni-Cr alloys or MgO on Fe-Cr also produced changes in the oxidation morphology but did not decrease the rate of oxidation. These results are interpreted in terms of the influence of the oxide powders on the development of scale microstructure and their effectiveness in decreasing grain boundary transport in Cr₂O₃.This paper is based in part on the Ph.D. thesis of G. M. Ecer (1975) and in part on the M.S. thesis of R. B. Singh (1977).     

The influence of TiAlN and enamel coatings on the corrosion behavior of Ti6Al4V alloy in the presence of solid NaCl deposit and water vapor at 450 °C

The corrosion behavior of Ti6Al4V alloy with solid NaCl deposit and water vapor at 450 °C has been investigated. Serious corrosion occurred. In order to improve the corrosion resistance, TiAlN and enamel coatings were prepared, and their effectiveness and corrosion resistance were evaluated. The results indicated that both coatings were effective in protecting the alloy against corrosion induced by NaCl deposit and water vapor at 450 °C. The protection mechanisms of these two coatings were also studied.     

Microstructure and high temperature oxidation behavior of hot-dip aluminized coating on high silicon ductile iron

High silicon ductile iron was coated by hot-dipping into an Al molten bath. The oxidation behavior of the aluminized alloy and the bare substrate was studied in air at 750 °C. The results showed that the coating layers consisted of three layers, in the sequence of Al, Fe-Al intermetallic and Si pile-up layers from the external topcoat to the substrate. The intermetallic layer was composed of outer FeAl₃ and inner Fe₂Al₅. The outer rod-shaped FeAl₃ dispersed in the aluminum topcoat, while the inner tongue-like Fe₂Al₅ formed in the metallic layer becoming the major phase in the aluminide coating layer. Those three layers of aluminum, Fe₂Al₅ and silicon pile-up layer exhibited hardness of HV 50, HV 1100 and HV 450, respectively. In this study, when the as-coated specimens were examined, Fe-Al-Si compounds could not be found. But the silicon pile-up at the interface between the substrate and the Fe-Al intermetallic layer could be seen in all the as-coated specimens. The graphite nodules were noticed in the substrate. The presence of graphite nodules in the substrate might be markers of hot-dipping. After hot-dipping in Al all the specimens contained graphite nodules in the aluminide layer.The oxidized graphite nodules resulted in cracks propagating in aluminide coating. Even though graphite nodules meant the existence of crack in the aluminide coating, the high temperature oxidation experiments indicated that the aluminide coating could prevent the oxidation of substrate effectively even at 750 °C.     

Influence of the mode of introduction of a reactive element on the high temperature oxidation behavior of an alumina‐forming alloy. Part I: Isothermal oxidation tests

Different modes of introduction of yttrium have been tested with regard to the influence on the high temperature oxidation behavior of a FeCral alloy. Y₂O₃ sol‐gel coatings, Y₂O₃ metal‐organic chemical vapor deposition (MOCVD) coatings, implanted yttrium ions and yttrium as alloying element (0.1 wt.%) in the same Fe‐20Cr‐5Al alloy were oxidized at 1100°C in air under atmospheric pressure. Whatever the mode of introduction of the reactive element, the oxidation rates were not decreased compared to the oxidation rate of the blank specimen. The observation of the oxidized surface indicated that the alumina scale largely spalled from the blank alloy. Spallation was reduced for the Y₂O₃ sol‐gel coated, the Y₂O₃ MOCVD coated alloys and the yttrium ion implanted steels. The Y‐containing alloy did not exhibit any detachment of the oxide scale, indicating the best high temperature oxidation behavior, at least from the viewpoint of scale adherence.     

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe-30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al₂O₃ and Cr₂O₃ films doped with Sm₂O₃ or Nd₂O₃ were prepared via the metal-organic chemical vapor deposition technique. Single Cr₂O₃, Al₂O₃, Nd₂O₃ and codeposited Cr₂O₃-Nd₂O₃, Al₂O₃-Nd₂O₃, Al₂O₃-Sm₂O₃ coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11-18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO₃ or NdTi₂₁O₃₈ and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.     

Effect of water vapor on annealing scale formation on 316 SS

The oxidation behavior of 316 stainless steel (SS) annealed in air containing 0.1 atm water vapor at temperatures ranging from 800 to 1030 °C was investigated. A kinetic study of the oxidation was made by employing thermal-gravimetric analysis (TGA). The morphology, composition and structure of the scale were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The experimental results showed that significant breakaway oxidation occurred, resulting in substantial weight increase, as the steel was annealed in moist air at temperatures above 950 °C. The scaling behavior of 316 SS in wet air at 1030 °C could be divided into two stages based on the alteration of the oxidation rate. In each stage, the scale on 316 SS exhibited a different structure and morphology. The complex process of the formation of scale in wet air was discussed and proposed.     

Effects of the restructuring of Fe catalyst films on chemical vapor deposition of carbon nanotubes

We investigated the restructuring of Fe catalyst films during high temperature processing and its effects on the morphologies of carbon nanotubes (CNTs) arrays synthesized using the thermal chemical vapor deposition (CVD) techniques. Results show that annealing time and gas environment deeply influence the particle size distribution, which determines the morphologies of the corresponding CNT arrays. Fe particles with narrow size distribution were obtained after proper high temperature processing in NH₃, from which higher purity CNT arrays with better-alignment and uniform diameter distribution have been grown.