The direct observation and investigation of the oxidation of aluminum in the transmission electron microscope

The authors studied the oxidation of thin aluminum films free of oxide layers in situ prepared by evaporation directly in the electron microscope under ultra-high-vacuum conditions. The oxidation was realized at various temperatures (350–500°C) and at various oxygen pressures (1–10^–3 Pa). The formation and growth of the amorphous and crystalline products have been studied.     

In situ oxidation studies on (001) copper-nickel alloy thin films

The (001)-oriented, single crystalline thin films of Cu-3% Ni, Cu-4.6% Ni, and Cu-50% Ni alloy were prepared by vapor deposition onto (001) NaCl substrates. The films were subsequently annealed at around 1100°K and oxidized at 725°K at an oxygen partial pressure of 5×10^–1 N · m^–2 (5× 10^–6 atm). High-resolution transmission electron microscopy was employed to observe the changes in situ. For all the alloy concentrations Cu₂O and NiO were observed to nucleate and grow independently; no mixed oxides were noted. The shape and growth rates of Cu₂O nuclei were similar to those found in previous work on pure copper films. For low-nickel alloy concentrations the NiO nuclei were larger and the number density of NiO was less when compared to the oxidation of pure nickel films. For the Cu-50% Ni films the shape and growth rates of NiO were identical to those for the oxidation of pure nickel films. Low nickel concentrations exhibited a reduced induction period for Cu₂O when compared to pure copper films. Cu-50%Ni films showed surface precipitation and growth of NiO first, followed by Cu₂O in a typical through -thickness growth after a prolonged induction period. The results are consistent with the previously established oxidation mechanisms of pure copper and pure nickel films.This work was performed at the Ames Research Center and funded by NASA Grants NCA2-OP390-403 and NSG-2025.     

The oxidation and resistance of tantalum-nitride thin films

The oxidation of reactively-sputtered, tantalum—nitride thin films has been studied between 473 and 773 K in air. Films with thicknesses that correspond to sheet resistances of 43, 75, and 150 / were evaluated in this study. X-ray diffraction revealed that the films and the oxidation products were amorphous. The oxidation products were characterized with AES and XPS. The oxidation process was monitored by measuring the change in sheet resistance with time. Sheet resistance measurements were performed with a four-point probe. The oxidation kinetics can be represented by the equation R/R ⁿ =kt. R/R is sheet resistance change in %, t is time, and k is the rate constant. The oxidation kinetics, for the thickest film, were parabolic above about 598 K, and approximately quartic below this temperature. The 75 / films deviated from parabolic kinetics below 603 K, and approached approximate cubic kinetics at 498 K. However, the data were forced to fit parabolic kinetics with good correlation. For the thinnest film, no deviation from parabolic kinetics was observed. The rate constants obeyed the Arrhenius relation. An activation energy of 147 kJ/mole (1.52eV) was calculated, for the thickest film, during parabolic kinetics. This value is approximately equal to the activation energy obtained during quartic kinetics. A model that explains the deviation from parabolic to quartic oxidation kinetics at lower temperatures is presented. In this model, the change in resistance as a function of time was due to simultaneous change in film resistivity, as a result of oxygen dissolution into the film, and oxide-scale growth. The temperature at which this deviation occurs should be a function of film thickness.     

Observations on the growth of Al–Ni–Co decagonal thin films by atomic force microscopy and transmission electron microscopy

Thin films of Al–Ni–Co alloy were produced by vacuum deposition technique using a substrate material of amorphous carbon thin-foil. Attempts were made to obtain a homogeneous decagonal quasicrystalline film, where the preparation technique was based on the direct evaporation of pre-alloyed ingot of Al₇₂Ni₁₅Co₁₃ onto the heated substrates. In order to explore early stages of the decagonal film growth, the Al–Ni–Co films with different thicknesses ranging from 2 nm to 30 nm were deposited on either substrates heated at 500 °C or non-heated substrates. The film samples so obtained were examined mainly by atomic force microscopy in combination with transmission electron diffraction and imaging techniques. On the basis of these observations, deposition conditions necessary for the growth of decagonal phase in the resulting films as well as the growth mechanism of the decagonal film will be discussed.     

Oxide removal and desorption of oxygen from partly oxidized thin films of copper at low pressures

Partly oxidized copper films were annealed in a controlled vacuum of 10^–7 Pa at a temperature of 450° C. The changes discussed below were observed in situ with a specially designed high-resolution transmission electron microscope. The thin, (100)-oriented, single-crystal films of copper had been oxidized immediately prior to the annealing studies at the same temperature and at an oxygen partial pressure of 7×10 ^–1 Pa, until the desired fraction of the copper film was converted to oxide. It was observed that the oxide disappeared during annealing as long as some copper was left unoxidized. The disappearance of the oxide is explained as being due to dissociation of the oxide at the oxide-metal interface followed by diffusion of oxygen into the metal and desorption of oxygen from the surface of the unoxidized copper. The rate of disappearance of the oxide was found to be proportional to the surface area of unoxidized copper, i.e., the desorption was found to be the rate — limiting step. In the case of heavily oxidized films (>50%), holes were observed to develop in the oxide near the oxide-metal interface after an annealing period of 2–3 hr. Upon resumption of the oxidation, these holes first disappeared, and the normal oxidation behavior was then resumed. The formation of holes may be explained by vacancy clustering. When completely oxidized films were annealed, recrystallization of the oxide was observed.This work was performed at the Ames Research Center and funded by NASA Grants Nos. NCA2-OP390-403 and NSG-2025.     

组织和温度对Cu表面氧化速率的影响

利用扫描电镜、X射线衍射仪和氧化称重实验对铜表面氧化速率进行研究,分析了不同组织、晶粒和温度对铜氧化性能的影响。结果表明：铜的组织结构是影响其氧化性能的主要因素,非密排的（100）晶面上界面能高、晶面原子堆垛相对疏松和原子尺度上粗糙,且氧化膜生长连续,氧化速率高于密排的（111）晶面;温度升高,铜表面的内能增大,铜原子较易成为活化原子与氧气发生反应,同时增大了铜原子在氧化膜中的扩散速率,加速了铜的氧化;铜试样经过等径角挤压（ECAP）后,（111）晶面铜表面的面积增大,氧化速率降低,晶界不是影响铜氧化速率的主要因素,能量较小的晶面原子所占的面积增大,铜的氧化速率减小。     

A comparison of grain boundary evolution during grain growth in fcc metals

Grain growth of Cu and Ni thin films, subjected to in situ annealing within a transmission electron microscope, has been quantified using a precession-enhanced electron diffraction technique. The orientation of each grain and its misorientation with respect to its neighboring grains were calculated. The Cu underwent grain growth that maintained a monomodal grain size distribution, with its low-angle grain boundaries being consumed, and the Ni exhibited grain size distributions in stages, from monomodal to bimodal to monomodal. The onset of Ni’s abnormal grain growth was accompanied by a sharp increase in the Σ3 and Σ9 boundary fractions, which is attributed to simulation predictions of their increased mobility. These Σ3 and Σ9 fractions then dropped to their room temperature values during the third stage of grain growth. In addition to the Σ3 and Σ9 boundaries, the Σ5 and Σ7 boundaries also underwent an increase in total boundary fraction with increasing temperature in both metals.     

The initiation of breakaway oxidation of Fe-9Cr-1Mo in a high pressure CO2 atmosphere

The scale existent at the initiation of breakaway oxidation kinetics for an Fe-9Cr-1Mo alloy, as oxidized in 1% CO-CO₂ at 600°C, is chemically and microstructurally characterized by the transmission electron microscopy of cross-sectional specimens. The relative importance of chemical and mechanical instabilities in the history of the scale development is thus deduced and the importance of changes in the CO/CO₂ ratio at the inward growing interfaces of the scale emphasised.     

Hot-stage scanning electron microscope for high-temperaturein-situ oxidation studies

The design of a hot stage for a Cambridge S 4 scanning electron microscope (SEM) is described. Provision is made for the entry of noncondensible reactive gases to the specimen chamber through a leak tube connected to a microvalve. Preliminary oxidation studies on copper at 850°C and = 3 × 10^–4 have shown exceptional promise for the continuous observation of the scalegas interface at magnifications of up to 20,000x. Growth strains resulted in fractures at the grain boundaries of the oxide scale. Growth and random nucleation of oxides are all important features which have been observed here, and clearly show the usefulness of the modified stage of the SEM for future studies of oxidation of metals and alloys.     

TEM investigations of the early stages of TiAl oxidation

The early stages of TiAl oxidation at 900°C and 1000°C in air have been investigated by transmission electron microscopy (TEM). The investigations revealed that at the beginning of oxidation, i.e., after 4 min, TiO₂ and Al₂O₃ grow in a preferential orientation on the -TiAl substrate. After 4 h of oxidation an oxide scale structure can already be found similar to that known from long-term oxidation. In addition, besides -Al₂O₃, the formation of a second aluminum oxide phase and of titanium nitrides is observed. The processes at the metal-oxide interface of oxidation in the early stages, consisting of a repeated cycle of Al₂O₃ formation, Al₂O₃ dissolution, outward migration of Al through the scale, and reprecipitation of Al₂O₃ in the outer scale, are described by a model. The four stages observed in the kinetics of TiAl oxidation are explained on the basis of the results obtained and the structure of the oxide scale.     

In situ synthesis of nanostructured titania film on NiTi shape memory alloy by Fenton＇s oxidation method

Fenton＇s oxidation method was successfully used to synthesize an ideal titania film in situ on NiTi shape memory alloy（SMA） for medical applications. Characterized with scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffractometry, inductively coupled plasma mass spectrometry and electrochemical tests, it is found that the titania film produced by Fenton＇s oxidation method on NiTi SMA is nanostructured and has a Ni-free zone near its top surface, which results in a notable improvement in corrosion resistance and a remarkable decrease in leaching of harmful Ni ions from NiTi SMA in simulated body fluids. The improvement of effectiveness to corrosion resistance and the reduction in Ni release of NiTi SMA by Fenton＇s oxidation method are comparable to those by oxygen plasma immersion ion implantation reported earlier.     

Behavior of porosity and copper oxidation in W/Cu composite produced by plasma spray

A spherical tungsten (W)/copper (Cu) composite powder was plasma-sprayed onto a boron-nitride-coated graphite substrate to produce heat sink material for the electronic packaging by atmospheric plasma spray using different input powers. At the surface of the deposited layer, Cu became oxidized to cuprite (Cu₂O). The degree of oxidation at the surface layer did not vary significantly with input power. Very little Cu₂O was detected at the inner layers after grinding off the 70 μm from the surface. The input powers had a strong effect on the porosity in the deposit layers. The porosity in the deposit layers at 25 kW was very small, about 3 vol%. The microstructures of W/Cu composite were discussed.     

A critical appraisal of the logarithmic rate law in thin-film formation during oxidation of copper and its alloys

Investigations have been carried out on the kinetics of thin film formation of copper and its alloys in the temperature range of 75–100°C. The experimental results have been found to follow the logarithmic rate law except for the copper-chromium system at 100°C. The kinetic data have been analyzed in light of William and Hayfield's theory, and the various parameters of the logarithmic rate equation match very well to those calculated by others. The estimated activation energy value did not show significant variation. The major contribution to the rate of film growth has been ascribed to the available number of Fermi electrons.     

The oxidation of 63Sn/37Pb at typical soldering temperatures

63Sn/37Pb is a common solder used in the electronics industry. The oxidation of this alloy on a soldering iron in air follows the parabolic rate law from 204–454°C and 0–600 sec. Transmission electron microscopy was used to determine the scale thickness, composition, and morphology.     

夹杂物对铜在钢氧化过程中富集行为的影响

对于含有0.3%残余元素铜的中碳钢,在1100℃下氧化1800s和3600s,利用扫描电镜和能谱仪观察氧化层与钢基体界面处铜的富集析出情况。并分析S、Ti等元素含量对于残余元素铜在氧化层附近富集情况的影响。结果表明,钢中硫化锰夹杂物数量的增多有利于更多的铜在硫化锰上析出,从而有效减轻铜在钢氧化层的富集偏析以及浸润晶界现象。     

Analytical electron microscopy of a selective oxide scale formed on 20% Cr-25% Ni-Nb stainless steel

A selective oxidation process has been developed to protect a 20%Cr/25%Ni/Nb stainless steel during exposure to carbon dioxide based environments at 700°C. The composition and microstructure of transverse sections of the 0.6 m thick selective oxide scale produced has been examined by analytical electron microscopy. This has highlighted the role of easy diffusion paths in the steel in enabling chromium movement to form the outer Cr₂O₃ layer and the mechanism by which the inner amorphous silica layer grows. The implications of the physical and chemical characteristics of the selective oxide scale upon the degree of protection it affords are discussed.     

Isothermal oxidation of interstitial-free steel in air at different temperatures

The oxidation behaviour of interstitial-free (IF) steel at high temperatures was evaluated at 868, 977, 1087, and 1200°C in air for 120?s. Scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction analysis were performed to investigate the cross-sectional morphologies, microstructures, and phase constituents of IF steel after isothermal oxidation. Kinetic results showed that the weight gains of IF steel increased with increasing temperature and exhibited an exponential relationship with temperature. Linear and parabolic laws were found to exist in the weight gain process. The activation energy was 89.12 and 274.12?kJ?mol^?1 during the linear and parabolic stages, respectively. Non-adherent scales were formed on the substrates of IF steel. FeO was determined to be the major component based on the iron–oxygen atom ratios of the scales after oxidation at different temperatures.     

电子束选区熔化增材制造TiAl合金的高温硬度及氧化行为研究

钛铝合金是航空航天领域具有广阔应用前景的轻质高温合金,电子束选区熔化（EBM）增材制造技术是制备复杂结构TiAl合金有效途径,但目前对其高温性能研究较少。本文重点研究了电子束选区熔化增材制造Ti-48Al-2Cr-2Nb合金的显微组织、高温硬度及其高温氧化行为。结果表明,EBM成形Ti-48Al-2Cr-2Nb合金呈现出由等轴γ晶粒和双相区组成的独特层状组织;在800 ℃下恒温氧化100 h,表现出较低的氧化速率常数,形成的氧化膜主要由TiO2、Al2O3及TiO2 / Al2O3混合交替层组成,抗氧化性能优于传统方法制备的Ti-48Al-2Cr-2Nb合金和其他TiAl合金。此外,900℃以下,该合金具有良好的高温硬度,显微硬度随温度的升高未发生明显的下降趋势。     

Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al₂S₃ or Cr₂S₃ layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al³⁺ ions into NbS₂ and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al₂S₃ layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr₂O₃ layer in the oxide scales is formed on Cr-rich Cr-Nb and Cr-Ta alloys due to outward diffusion of Cr³⁺ ions. In contrast, continuous alumina layer cannot be formed on the Al-Nb and Al-Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.