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.     

Effect of increased water vapor levels on TBC lifetime with Pt-containing bond coatings

To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycle tests were performed at 1150 °C in air with 10 vol.% water vapor (similar to natural gas combustion) and 90 vol.%. Either Pt diffusion or Pt-modified aluminide bond coatings were applied to specimens from the same batch of a commercial second-generation single-crystal superalloy and commercial vapor-deposited yttria-stabilized zirconia (YSZ) top coats were applied. Three coatings of each type were furnace cycled to failure to compare the average lifetimes obtained in dry O₂, using the same superalloy batch and coating types. Average lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor. In contrast, the average lifetime of Pt-modified aluminide coatings was reduced by more than 50% with 10% water vapor but only slightly reduced by 90% water vapor. Based on roughness measurements from similar specimens without a YSZ coating, the addition of 10% water vapor increased the rate of coating roughening more than 90% water vapor. Qualitatively, the amount of β-phase depletion in the coatings exposed in 10% water vapor did not appear to be accelerated.     

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.     

Interdiffusion behavior of Al‐rich oxidation resistant coatings on ferritic–martensitic alloys

Interdiffusion of thin Al‐rich coatings synthesized by chemical vapor deposition (CVD) and pack cementation on 9Cr ferritic–martensitic alloys was investigated in the temperature range of 650–700 °C. The compositional changes after long‐term exposures in laboratory air and air + 10 vol% H₂O were examined experimentally. Interdiffusion was modeled by a modified coating oxidation and substrate interdiffusion model (COSIM) program. The modification enabled the program to directly input the concentration profiles of the as‐deposited coating determined by electron probe microanalysis (EPMA). Reasonable agreement was achieved between the simulated and experimental Al profiles after exposures. The model was also applied to predict coating lifetime at 650–700 °C based on a minimum Al content (C_b) required at the coating surface to re‐form protective oxide scale. In addition to a C_b value established from the failure of a thin CVD coating at 700 °C, values reported for slurry aluminide coatings were also included in lifetime predictions.     

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.     

化学气相沉积硬质合金TiN/TiCN/Al_2O_3/TiN多层涂层的抗氧化性能

对化学气相沉积(CVD)法制备的硬质合金TiN/TiCN/Al2O3/TiN多层涂层试样在600~950℃温度范围内进行了氧化质量增加试验,采用X射线衍射(XRD)和扫描电子显微镜(SEM)分析试样氧化前后相组成及微观组织。结果表明:复合涂层最外层为α-Al2O3时,涂层具有最佳的抗氧化能力;增加TiN/TiCN/κ-Al2O3/TiN复合涂层中TiCN和κ-Al2O3的厚度能大大提高涂层高温抗氧化性。TiN和TiCN涂层经600℃以上氧化后,产物均为金红石结构的TiO2,氧化后TiN/TiCN间的界面消失;经900℃以上氧化时,κ-Al2O3转变为α-Al2O3。     

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。     

原位生长碳纳米管对化学气相沉积SiC涂层的影响

以碳/碳复合材料为基体,MTS为先驱体原料,采用化学气相沉积法在复合材料表面制备CNT-SiC/SiC复合涂层;研究原位生长的碳纳米管(CNTs)对SiC沉积速度和微观形貌的影响。结果表明:CNTs加快SiC的沉积,涂层的平均质量增加速率提高5%,提高沉积的均匀性,且晶粒更细小;经1 100℃恒温氧化10 h后,单一SiC涂层、CNT-SiC/SiC涂层的质量损失率分别为41.11%和34.32%;经(1 100℃,3 min)(室温,3 min)热循环15次后,单一SiC涂层和CNT-SiC/SiC涂层的质量损失率分别为33.17%和30.25%,部分区域涂层脱落及涂层表面形成的气孔是涂层试样质量损失的主要原因。     

氧化时间及铬含量对Fe-Cr钢高温氧化行为的影响

为了测定不同氧化时间以及铬含量对高温条件下钢材表面氧化铁皮组织和厚度的影响,将Fe-5Cr钢与Fe-10Cr钢在1000 ℃空气条件下氧化60~180 min,采用增重法绘制其氧化动力学曲线,并利用光学显微镜、能谱仪和X射线衍射仪对氧化铁皮的断面形貌和物相进行研究。结果表明,两试验钢氧化初期为“气-固”反应,中后期为“气相-氧化铁皮-固相”反应。两试验钢氧化铁皮结构均由外氧化层、内氧化层和内氧化区域组成。当氧化时间为180 min时,Fe-10Cr钢检测到了内氧化物Cr₂O₃。空气中氧元素向内扩散与基体中铬元素发生反应生成内氧化物Cr₂O₃,再与氧化铁皮层中的FeO发生固相反应生成尖晶石结构产物FeCr₂O₄。随着氧化时间的增加,由于内氧化物Cr₂O₃不断受到内氧化层的包裹而转为外氧化,内外氧化的转变使得基体不断被腐蚀,氧化铁皮厚度不断增加。     

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).     

Room temperature process for chemical vapor deposition of amorphous silicon carbide thin film using monomethylsilane gas

The silicon carbide thin film formation process, which was completely performed at room temperature, was developed by employing a reactive silicon surface preparation using argon plasma and a chemical vapor deposition using monomethylsilane gas. Time-of-flight secondary ion mass spectrometry showed that silicon-carbon bonds existed in the obtained film, the surface of which could remain specular after exposure to hydrogen chloride gas at 800 °C. The silicon dangling bonds formed at the silicon surface by the argon plasma are considered to easily accept the monomethylsilane molecules at room temperature to produce the amorphous silicon carbide film thicker than monolayer. Thus, the entire silicon carbide thin film formation process at room temperature is possible.