X-ray photoelectron spectroscopy investigation of the carburization of 310 stainless steel

The surface of 310 stainless steel (310SS) samples was investigated by X-ray photoelectron spectroscopy (XPS) after 500 h cyclic exposure to two carburizing atmospheres: CH₄ (2%)–H₂ (98%) at 800 °C, and CH₄ (10%)–H₂ (90%) at 1100 °C. The depth distribution of various elements in the surface region was obtained by XPS after successive cycles of argon etching. The microstructure of the alloy was observed by scanning electron microscopy (SEM) and the phases formed during the exposure were analyzed by X-ray diffraction (XRD). The results showed that the major phases that were formed within few micrometer depth during exposure at 800 °C include both iron and chromium carbides. (Mn, Cr) oxide was also formed as a result of the reaction with the residual oxygen of the atmosphere. A region of few microns width that was relatively depleted of chromium was formed under the surface as a result of the outwards diffusion of chromium. The exposure to the reducing atmosphere at 1100 °C led to the formation of various iron and chromium carbides. No oxide was formed during exposure. In all exposed samples, the surface was Cr enriched while nickel remained buried under the surface region that reacted with the atmosphere.     

In situ surface analytical investigation of the thermal oxidation of Ti–Al intermetallics up to 1000 °C

The low pressure high temperature oxidation behavior of Ti–Al intermetallics are of interest to power technology aiming to synthesize this material by sintering of powders. This paper presents in situ surface analytical studies of the composition of a two-phase TiAl/Ti₃Al bulk microcrystalline system after oxidizing the same (sputtered) reference surface for 30 min at various oxygen partial pressures and temperatures varying between room temperature (RT) and 1000 °C. The results show that oxidation already begins at very low (<5×10⁻¹⁰ mbar) oxygen pressure, producing Al₂O₃ and the lower oxidation states of Ti. As the oxygen pressure and oxidation temperature increases, TiO₂ becomes dominant up to 900 °C. No phase transition of Al₂O₃ has been observed in this range. No sign of a blocking behavior of the oxide layer is seen. At 1000 °C a new oxide phase, Al₂TiO₅ appears, changing the composition and behavior of the surface drastically. The observed results can be explained by qualitative thermodynamic arguments. The thickness and composition of the oxide overlayer is, however, primarily determined by the oxygen supply.     

Thermal stability of the structural features in the superhydrophobic boehmite films on austenitic stainless steels

Boehmite thin film with 50–100 nm surface flake structure has been synthesized on AISI 316 type austenitic stainless steel by immersing boehmite gel film into boiling water. When further coated with hydrolyzed (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane (FAS), the boehmite film becomes superhydrophobic with a contact angle for water of 152°. The superhydrophobic property results from both the nanoscale surface flake structure and the low surface energy of the FAS top layer. The topography of such film was revealed by atomic force microscope (AFM) and a set of roughness parameters of such film was discussed. The degradation of superhydrophobicity of the surface was studied as a function of the heat-treatment temperatures. Below 600 °C, the surface remained to be superhydrophobic with the FAS top layer. Above 700 °C, the surface was not superhydrophobic anymore due to a gradual loss in surface roughness which was revealed by field emission scanning electron microscope (FESEM). A phase change from boehmite to γ-Al₂O₃ occurred during the heat-treatments from 700 to 900 °C which was studied by the selected area electron diffraction (SAED) patterns from the transmission electron microscope (TEM) measurement.     

Influence of CrN surface compound on the initial stages of high temperature oxidation of ferritic stainless steel

Segregation phenomena and formation of surface compounds on Fe-17Cr (ferritic stainless steel) were studied at temperatures up to 800 °C upon annealing and kinetically controlled oxidation by photoelectron spectroscopy and inelastic electron background analysis. The results revealed the formation of a chromium nitride surface compound during annealing in ultrahigh vacuum at temperatures exceeding 527 °C. Surface enrichment of P, As, and other trace elements became more prominent at higher temperatures approaching 800 °C. It was found that nitrogen was buried below the surface oxide layer during oxygen exposure, yet it had little effect on the rate of oxidation. However, the formation of CrN surface compound promoted the selective oxidation of Cr initially, which is of great importance in processing and application environments involving high temperature and controlled atmosphere conditions.     

Comparison of the CO and D2 oxidation reactions on Pd supported on MgO(100), MgO(110) and MgO(111)

Oxidation of D₂ and CO on oxygen pre-exposed 200 nm thick Pd films, epitaxially grown on MgO(100), MgO(110) and MgO(111), has been investigated in the temperature range 100–300°C. Oxygen initial sticking coefficients have been determined to be close to 1 for the 100 and 110 films, and around 0.8 for the 111 film. The sticking coefficient and reactive sticking coefficient for CO oxidation on Pd/MgO(100) is also close to 1, and the maximum reactive sticking coefficient for hydrogen oxidation is determined to be around 0.9 at temperatures above 200°C. It is shown that the reactivities for the different surfaces vary strongly with surface and oxygen coverage, and the consequence of this for supported particle catalysts is pointed out.     

Growth and thermal properties of ultrathin cerium oxide layers on Rh(1 1 1)

Ultrathin layers of cerium oxide have been deposited on a Rh(1 1 1) surface and their growth morphology, structure, and thermal stability have been investigated by LEED, STM, XPS, and valence band resonant photoemission. STM and LEED indicate that the ceria grows epitaxially in form of ordered CeO₂ islands at elevated substrate temperature (250–300 °C), with (1 1 1) faces parallel and orientationally aligned to the main azimuthal directions of the substrate. The ultrathin ceria films contain significant amounts of reduced Ce³⁺ species, which appear to be located predominantly at the ceria–Rh interface. For thicker films (>6 equivalent monolayers) stoichiometric CeO₂ is detected in XPS. Vacuum annealing produces morphologically well-defined hexagonal islands, accompanied by partial reduction and the formation of oxygen vacancies at the ceria surface. The thermal stability and the degree of reduction is a function of the oxide layer thickness, with thinner layers being thermally less stable. At temperatures >800 °C, the ceria decomposes and Ce–Rh alloy phases are identified.     

An evaluation of surface properties and frictional forces generated from Al–Mo–Ni coating on piston ring

Surface properties of the Al–Mo–Ni coating plasma sprayed on the piston ring material and the frictional forces obtained by testing carried out under different loads, temperatures and frictional conditions were evaluated.

Al–Mo–Ni composite material was deposited on the AISI 440C test steel using plasma spraying method. The coated and uncoated samples were tested by being exposed to frictional testing under dry and lubricated conditions. Test temperatures of 25, 100, 200, and 300 °C and loads of 83, 100, 200, and 300 N were applied during the tests in order to obtain the frictional response of the coating under conditions similar to real piston ring/cylinder friction conditions. Gray cast iron was used as a counterface material. All the tests were carried out with a constant sliding speed of 1 m/s.

The properties of the coating were determined by using EDX and SEM analyses. Hardness distribution on the cross-section of the coating was also determined. In addition, the variations of the surface roughness after testing with test temperatures and loads under dry and lubricated conditions were recorded versus sliding distance.

It was determined that the surface roughness increased with increasing loads. It increased with temperature up to 200 °C and then decreased at 300 °C under dry test conditions.

Under lubricated conditions, the roughness decreased under the loads of 100 N and then increased. The roughness decreased at 200 °C but below and above this point it increased with the test temperature.

Frictional forces observed under dry and lubricated test conditions increased with load at running-in period of the sliding. The steady-state period was then established with the sliding distance as a normal situation. However, the frictional forces were generally lower at a higher test temperature than those at a lower test temperature. Surprisingly, the test temperature of 200 °C was a critical point for frictional forces and surface roughness.     

Evolution of steel surface composition with heating in vacuum and in air

X-ray photoelectron spectroscopy (XPS) has been used to investigate the changes in surface composition of three steels as they have undergone heating. The steels were mild steel, and two austenitic stainless steels, commonly designated 304 and 316 stainless steels. XPS measurements were made on the untreated samples, and then following heating for 30 min in vacuo and in a 1 × 10⁻⁶ Torr partial pressure of air, at temperatures between 100 °C and 600 °C.Mild steel behaves differently to the two stainless steels under the heating conditions. In mild steel the iron content of the surface increased, with oxygen and carbon decreasing, as a function of increasing temperature. The chemical state of the iron also changed from oxide at low temperatures, to metallic at temperatures above 450 °C.In both stainless steels the amount of iron present in the surface decreased with increasing temperature. The decrease in iron at the surface was accompanied by an increase in the amount of chromium at the steel surface. At temperatures above 450 °C the iron in both 304 and 316 stainless steels showed significant contributions from metallic iron, whilst the chromium present was in an oxide state. In 316 stainless steel heated to 600 °C there was some metallic chromium present in the surface layer.The surfaces heated in air showed the least variation in composition, with the major change being the loss of carbon from the surfaces following heating above 300 °C. There was also a minor increase in the concentration of chromium present on both the stainless steels heated under these conditions. There was also little change in the oxidation state of the iron and chromium present on the surface of these steels. There was some evidence of the thickening of the surface oxides as seen by the loss of the lower binding energy signal in the iron or chromium core level scans.The surfaces heated in vacuum showed a similar trend in the concentration of carbon on the surfaces, however the overall concentration of oxygen decreased throughout the heating of these steels. There were also significant changes in the oxidation state of the iron and chromium on these surfaces with significant amounts or iron and chromium present in the metallic form following heating up to 600 °C.It appears that the carbon contamination on the surfaces plays an important role in the fate of the surface oxide layer for all of the steels heated in a vacuum environment.     

Synthesis and physical behaviour of In2S3 films

In₂S₃ layers have been grown by close-spaced evaporation of pre-synthesized In₂S₃ powder from its constituent elements. The layers were deposited on glass substrates at temperatures in the range, 200–350 °C. The effect of substrate temperature on composition, structure, morphology, electrical and optical properties of the as-grown indium sulfide films has been studied. The synthesized powder exhibited cubic structure with a grain size of 63.92 nm and S/In ratio of 1.01. The films grown at 200 °C were amorphous in nature while its crystallinity increased with the increase of substrate temperature to 300 °C. The films exhibited pure tetragonal β-In₂S₃ phase at the substrate temperature of 350 °C. The surface morphological analysis revealed that the films grown at 300 °C had an average roughness of 1.43 nm. These films showed a S/In ratio of 0.98 and a lower electrical resistivity of 1.28 × 10³ Ω cm. The optical band gap was found to be direct and the layers grown at 300 °C showed a higher optical transmittance of 78% and an energy band gap of 2.49 eV.     

Oxidation and annealing of thin FeTi layers covered with Pd

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O₂ and H₂O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a (4 or 20 nm) thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 300°C in vacuum, air and 10⁻⁵ mbar O₂. The layered structure remains intact up to 150°C. At 200°C in air and O₂, Fe and (some) Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 150°C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 200°C, 20 nm Pd may still provide sufficient protection against oxidation.     

XPS and RBS investigations of Si–Er–O interactions on a Si(1 0 0)-2x1 surface

The interactions among erbium, oxygen and silicon atoms on a Si(1 0 0)-2x1 reconstructed surface have been studied by means of X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Erbium and oxygen were deposited at 600 °C on the Si surface and their behavior has been observed after different thermal processes. It was found that at 600 °C, the formation of a stable surface complex Er–O–Si is obtained together with Si oxidation; after an 800 °C annealing, the amount of oxygen bound to Si decreases and the remaining O atoms are mainly bonded to Er. An abrupt change was observed after 900 and 1000 °C annealings, which bury the Er atoms about 60 Å below the substrate surface. Our results give some hints to hypotise the O diffusion towards the Si bulk.     

Low temperature fabrication of vanadium oxide films for uncooled bolometric detectors

Vanadium oxide films with temperature coefficient of resistant of −2.6% K⁻¹ have been fabricated on Si₃N₄-film-coated Si substrates by ion beam sputtering in a controlled Ar/O₂ atmosphere, at a relatively low growth temperature of 200 °C. The as-deposited films show no semiconductor-to-metal phase transitions even heated up to 150 °C. X-ray diffractometry shows that the main compound of the VO_x film is a metastable phase of vanadium dioxide (VO₂(B)) and the VO₂(B) film can be transformed into VO₂ film by post-growth annealing at 450 °C in flowing Ar atmosphere.     

Surface modification and oxidation kinetics of hot-pressed AlN–SiC–MoSi2 electroconductive ceramic composite

The effects of oxidation on the microstructural modification and on the electrical resistivity and mechanical strength of a hot-pressed AlN–SiC–MoSi₂ electroconductive ceramic composite were studied. The kinetic of the oxidation was also evaluated. After the oxidation at temperatures below 1000 °C samples do not gain weight, due to simultaneous formation of SiO₂ and evaporation of MoO₃ formed by the oxidation of MoSi₂. However, the AlN/SiC matrix disables the “pesting” phenomena and strength degradation, despite the fact that at these temperatures MoSi₂ oxidizes rapidly. At temperatures above 1000 °C, the composite gains weight due to protective mullite layer formation on the surface, that provides a good oxidation resistance for use at higher temperatures. The kinetics of the oxidation follows the parabolic law. The possible rate-controlling mechanism is the diffusion of oxygen through the mullite-rich surface oxide scale.     

Growth of β-MnO2 films on TiO2(110) by oxygen plasma assisted molecular beam epitaxy

We have used oxygen plasma assisted MBE to grow epitaxial films of pyrolusite (β-MnO₂) on TiO₂(110) for thicknesses of one to six bilayers (BL). We define a bilayer to be a layer of Mn and lattice O and an adjacent layer of bridging O within the rutile structure. The resulting surfaces have been characterized in situ by reflection high-energy electron diffraction, low-energy electron diffraction, X-ray photoelectron spectroscopy and diffraction, and atomic force microscopy. Well-ordered, pseudomorphic overlayers form for substrate temperatures between 400 and 500°C. Mn–Ti intermixing occurs over the time scale of film growth (1 BL/min) for substrate temperatures in excess of 500°C. Films grown at 400–500°C exhibit island growth, whereas intermixed films grown at temperatures of 500–600°C are more laminar. 1 BL films grown at 450°C are more laminar than multilayer films grown at the same temperature, and form a well-ordered surface cation layer of Mn on the rutile structure with at most 10% indiffusion to the second cation layer.     

InAs wetting layer evolution on GaAs(0 0 1)

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 °C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 × 4) structure. For a disorientation of 0.7°, InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 × 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 °C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying.     

Microstructure and oxidation of hot-dip aluminized titanium at high temperature

High-temperature diffusion of a hot-dip aluminized titanium is conducted to study microstructure changes and oxidation behavior of the aluminized titanium. After aluminizing, the titanium substrate is covered by a black layer in which tiny block-shaped TiAl₃ particles are scattered in aluminum matrix. Based on the diffusion experiment results, the thickness of the aluminum diffusion layer at 800 °C increases with diffusion time. However, the aluminum diffusion layer at 900 °C grows and reaches its maximum thickness in 6 h, and then the thickness of the aluminum diffusion layer is reduced with prolonged diffusion time. An inversion of the diffusion layer thickness versus time appears for the aluminized titanium treated at 1000 °C, and the thickness of the diffusion layer keeps declining with diffusion time. The phases present in the outer and middle sublayers are titanium-rich TiAl₃ and equilibrium TiAl₃, respectively. However, the phase in inner sublayer changes from titanium-rich TiAl₃ to TiAl₂ and TiAl as diffusion temperature and time increase. Through energy-dispersive X-ray and X-ray diffraction analysis, the oxides formed in the oxidation process are Al₂O₃ and Al₂TiO₅. Although the oxide scale formed on the surface of the aluminized titanium has an insufficient stability and integrity, the thermal oxidation resistance of the aluminized titanium is still improved by over 5 times compared with that of the pure titanium.     

Dynamic surface behaviour of VPO catalysts under reactive and non-reactive gas compositions: an in situ XAS study

The surface of an activated vanadium phosphorus oxide (VPO) catalyst was investigated by means of in situ X-ray absorption spectroscopy in the total electron yield mode. We observed significant changes of the V L₃-near edge X-ray absorption fine structure (NEXAFS) when the material was transferred from room temperature to working conditions at 400 °C in the reaction atmosphere. We studied the same VPO material under different gas compositions comprising the reaction mixture of n-butane and oxygen, pure oxygen and vacuum to elucidate the influence of the gas–surface interaction and the effect of the temperature. The results of this extensive study indicate a dynamic response of the catalyst surface to the applied conditions.     

Investigation of phosphorus surface segregation by X-ray scattering measurements

The surface segregation of phosphorus in silicon at low temperatures is studied by using δ doping structures grown by molecular beam epitaxy. The samples are characterized by X-ray crystal truncation rod (CTR) scattering using synchrotron radiation as the light source. The 1/e decay length of P segregation and segregation barrier energy are obtained by fitting the CTR curves within kinematical approximation of X-ray diffraction theory. The surface segregation of P is strong at a growth temperature of 450 °C, with a 1/e decay length of 14 nm, while for growth temperatures below 350 °C, P segregation is negligible with a 1/e decay length not larger than 4 nm. The segregation barrier energy is determined to be 0.43 eV.     

RF-sputtered CrB2 diffusion barrier for Ni/Au Ohmic contacts on p-CuCrO2

Ohmic contacts to p-type CuCrO₂ using Ni/Au/CrB₂/Ti/Au contact metallurgy are reported. The samples were annealed in the 200–700 °C range for 60 s in flowing oxygen ambient. A minimum specific contact resistance of 2 × 10⁻⁵ Ω cm² was obtained after annealing at 400 °C. Further increase in the annealing temperature (>400 °C) resulted in the degradation of contact resistance. Auger Electron Spectroscopy (AES) depth profiling showed that out-diffusion of Ti to the surface of the contact stacks was evident by 400 °C, followed by Cr at higher temperature. The CrB₂ diffusion barrier decreases the specific contact resistance by almost two orders of magnitude relative to Ni/Au alone.     

Oxygen exchange kinetics of La0.58Sr0.4Co0.2Fe0.8O3 at 600 °C in dry and humid atmospheres

The time-dependent degradation of the oxygen exchange kinetics of the solid oxide fuel cell cathode material La_0.58Sr_0.4Co_0.2Fe_0.8O_3 − δ (LSCF) is investigated at 600 °C. Special emphasis is placed on systematic long-term dc-conductivity relaxation measurements (t > 1000 h) in dry as well as in humidified atmospheres in order to obtain representative trends for the application of LSCF in intermediate-temperature SOFCs. The determination of the chemical surface exchange coefficient k_chem of oxygen is combined with investigations of the elemental surface compositions and depth profiles of fresh and degraded samples by X-ray photoelectron spectroscopy (XPS), providing further insight into the mechanisms of degradation. The slow decrease of k_chem by a factor of 2 during exposure of the sample to a dry O₂-Ar reference atmosphere for 1000 h at 600 °C can be ascribed to an enrichment of La and Sr in correlation with an elevated oxygen concentration within about 30-35 nm depth. The interpretation of the XPS core level spectra indicates the formation of SrO and La₂O₃ secondary phases in this zone. The subsequent treatment in a humidified atmosphere for 1000 h results in a pronounced initial decrease of k_chem by an additional factor of 10, followed by a time dependent decay of about 15% kh^− 1. A Sr-rich silicate layer of about 10 nm thickness is identified by XPS as the major cause of the degradation in humidified atmosphere. The evidence of Si-poisoning over the whole sample surface could also be confirmed by post-test SEM analysis. In addition, indications of a re-structuring of the sample surface during the degradation are shown. These results indicate, that with LSCF as a cathode in ambient (humid) air in SOFC stacks containing various Si-sources, such as glass or glass-ceramic seals, and thermal insulation materials a significant decrease of the surface oxygen exchange coefficient can occur, even at temperatures as low as 600 °C. In order to prevent a severe Si-induced degradation, dry air should be used as an oxidant. However, even in dry atmosphere a minor decrease of k_chem can occur during long-term operation due to changes in the relative cation and oxygen content at the surface.