Surface Review and Letters

Thermal barrier coatings (TBCs), which protect metallic components from high-temperature environments, have been widely applied to the fields of high-temperature and corrosion-resistant structural parts such as gas turbine engines, diesel engines, and power generation systems. Yttria-stabilized zirconia (YSZ) is one of the most widely used materials for TBCs owing to its excellent shock resistance, low-thermal conductivity, and relatively high coefficient of thermal expansion. In this paper the properties of YSZ and the recent developments of YSZ-TBCs are reviewed. The failure mechanism of YSZ-TBCs and corresponding methods for lengthening the lifetime of YSZ-TBCs are discussed. The advantages of graded thermal barrier coatings and the problems in processing are elucidated.

Thermal barrier coatings of Al₂O₃–ZrO₂ were prepared by air plasma spraying on the surface of 20G steel. Phase constitution, microstructures and elemental distributions of the coatings were studied by X-ray diffraction, scanning electron microscope and electron probe X-ray microanalysis. The results show that the plasma spray coating mainly consists of α-Al₂O₃, c-ZrO₂, and t-ZrO₂. The bond state of the interface between the top layer and bond layer is fine, and the bond layer has a good combination with the substrate. ZrO₂ and Al₂O₃ structures can closely integrate together and form compact top layer system.

The oxidation behavior of the coatings at 1100°C in air was investigated using electron probe microanalyzer, scanning electron microscopy, and X-ray diffractometer. Compacted oxide scales, mainly consisting of oxides of Al and Y, were formed on the coating surface, and thus the oxidation resistance of the coatings was enhanced. Cone oxides are formed on the scale surface and normal to the coating surface due to the characterization of preferential growth.

The microelectronic devices are formed by a substrate that supports the functional thin film material. The thermal, electrical, and mechanical properties of the system depend strongly on the interfacial properties between a film and a substrate. The interfacial nature in a film/substrate system originates the thermal contact resistance (R_tc). We discuss the thermal and the electrical behavior in a film/substrate system (bimaterial system) making emphasis on the R_tc of the interface. Au/glass samples with different thicknesses were prepared by thermal evaporation for experimentation. The bimaterial system was heated by a DC electrical current to obtain thermal profiles. Film and substrate thermal profiles acquired with high resolution combined with a developed bimaterial model are used as an alternative method to estimate the R_tc value at atmospheric pressure, the electrical resistivity ρ, and the thermal resistive coefficient α_r in the bimaterial system. The calculated R_tc values ranged from 7.7 × 10^-4 to 1.2 × 10^-3 m² K/W for the Au/glass system, in good agreement with previously reported values. The ρ values obtained from the thermal profile data present a more reliable value due to the global character than the local values measured by the four-probe technique. Dependence on film thickness was also found in the α_r coefficient determination.

Zirconium silicate (Zr_xSi_1-xO₂) thin films were deposited by pulsed metal-organic chemical vapor deposition (MOCVD) using zirconium tert-butoxide (ZTB) and tetrakis-diethylamido silane (TDEAS). The growth temperature of 200–300°C was used to deposit films with uniform thickness. The grown films showed the Zr-rich composition, which is thought to induce the Zr-silicide formation at the interface of the silicate and Si substrate. The film composition and chemical binding states were investigated by XPS depth profiling measurements.

The electroluminescence (EL) studies on the aluminum-doped nanoporous silicon (NPSi:Al) have been conducted. Nanoporous silicon (NPSi) layers have been prepared by anodically etching the unpolished p-type Si[100] wafer with surface resistivity of 4–8 Ω cm^-1 in hydrofluoric solution at 1:1 ratio of ethanol. Aluminum (Al) was doped on NPSi using cathodic electrodeposition composed of aluminum chloride (AlCl₃) and ethanol electrolyte. A diode structure has been fabricated comprising a semi-transparent Au/NPSi:Al/p-Si/Al ohmic contact electrode showing rectification on a forward bias I–V curve. EL from NPSi and NPSi:Al/p-Si has also been observed using the diode structure. The NPSi:Al/p-Si device shows increasing EL quantum efficiency at about 30%, and blue-shift EL spectra are observed. Possible reasons for the enhancement will be discussed.

Amorphous carbon nitride (a-CN_x) films were deposited on quartz substrates by newly developed surface wave microwave plasma chemical vapor deposition (SWMP-CVD) of alcohol camphoric carbon plasma source at room temperature. Then the a-CN_x films were heat-treated at various annealing temperatures (AT) in the 100–500°C range. The effects of heat treatment on the structural modifications were studied by Visible-Raman spectroscopy through the evolution of D and G peaks. The spectral evolution observed on heat-treated a-CN_x shows progressive formation of crystallites. Raman spectra have revealed the amorphous structure of as-grown a-CN_x films and the growth of nanocrystallinity upon increase of AT. These structural changes were further correlated with optical band gap and fraction of sp³ bonded carbons present, derived respectively from the UV-visible and photoelectron spectroscopy. The wide range of optical absorption coefficient characteristics is observed depending on the AT. The optical band gap of as-grown a-CN_x films is found to be approximately 2.8 eV; it gradually decreases to 2.5 eV for the films heat-treated at 300°C and then it decreases rapidly to 0.9 eV at 500°C. The results obtained are discussed and compared with the literatures.

We have studied the influence of the methane gas (CH₄) flow rate on the composition and structural and electrical properties of nitrogenated amorphous carbon (a-C:N) films grown by surface wave microwave plasma chemical vapor deposition (SWMP-CVD) using Auger electron spectroscopy, X-ray photoelectron spectroscopy, UV-visible spectroscopy, four-point probe and two-probe method resistance measurement. The photoelectrical properties of a-C:N films were also studied. We have succeeded to grow a-C:N films using a novel method of SWMP-CVD at room temperature and found that the deposition rate, bonding and optical and electrical properties of a-C:N films are strongly dependent on the CH₄ gas sources, and the a-C:N films grown at higher CH₄ gas flow rate have relatively high electrical conductivity for both cases of in dark and under illumination condition.

Nitrogen doped amorphous carbon (a-C:N) thin films were deposited on silicon and quartz substrates by microwave surface-wave plasma chemical vapor deposition (SWMP-CVD) technique at low temperatures (<100°C). We used argon (Ar), camphor dissolved in alcohol, and nitrogen (N) as carrier, source, and dopant gases, respectively. Optical band gap (E_g) decreased from 4.1 to 2.4 eV when the N gas concentration increased from 0% to 4.5%. The films were annealed at different temperatures ranging from 150°C to 450°C in Ar gas environment to investigate the optical and electrical properties of the films before and after annealing. Both E_g and electrical resistivity (ρ) decreased dramatically up to 0.95 eV and 57×10³ Ω cm at 450°C annealing. The structural modifications of the films leading them to become more graphitized as a function of the annealing temperature were confirmed by the characterization of Raman spectra.

The spin-dependent electron transport in a two-dimensional electron gas (2DEG) modulated by a stripe of magnetized ferromagnetic metal under an applied voltage was investigated theoretically. It is revealed that highly spin-polarized current can be achieved in this kind of nanosystems. It is also shown that the spin polarity of the electron transport can be switched by adjusting the applied voltage to the stripe in the device. These interesting properties may provide an alternative scheme to spin polarize electrons into semiconductors, and this device may be used as a voltage-tunable spin filter.

Surface morphology of the {001} faces of MMTWD crystals grown from by the temperature-lowering method has been studied. Monolayer and multilayer steps elongate along the a direction, which is determined by the crystal structure. Apart from that, the elementary steps have narrower terraces than the bunched ones, which may be resulting from the faster growth rates of the former than the latter. The formation of the protuberances at the step fronts is primarily associated with the uneven growth rates. The hollow cavities also elongate along the a direction, which demonstrates that the formation of them is also restricted by the crystal structure. Cracks are supposed to occur during harvesting, handling, or temperature stress afterwards. Growth of the 3D hillocks in high density can probably cause large stress and induce structure mismatch and serious cracks at the later stage.

The cold spraying process is analyzed by numerical modeling of the impact between a single spherical particle and a substrate; the effects of impact velocity and spray angle of a particle on the particle and substrate deformation behavior were investigated. It was found that much localized stress, localized strain, and localized heating are present near the particle/substrate interface. When impact velocity exceeds a critical velocity, a jet-type flow of material at the interface was formed. High particle/substrate contact pressures and better developed interfacial jets appear to be the major factors controlling the strength of interfacial bonding. As the particle impact velocity or spray angle increases, the length of interfacial jets increases and it may strengthen the bonding of the particle and the substrate.

Titanium nitride (TiN) films were deposited on high-speed steel (HSS) using cathodic arc physical vapor deposition (CAPVD) technique. The effect of substrate bias on the crystallography, microstructure, deposition rate, coating thickness and composition, hardness, and adhesion strength of TiN films was investigated. The crystallography of the films was investigated using X-ray diffraction with glazing incidence angle technique. The coating microstructure and elemental composition analysis were carried out using field emission scanning electron microscopy (FE-SEM) together with energy-dispersive X-ray. Crystallography of the films revealed that the effect of substrate bias shows complex symmetry in crystal structure. The resputtering effect due to the high-energy ion bombardment on the film surface influenced the thickness as well as the color of deposited coatings. By increasing the substrate bias from 0 to - 150 V, the size and amount of macrodroplets decreased, whereas the micro-Vickers hardness decreased from 2530 HV_0.05 to 1500 HV_0.05. Scratch tester used to compare the critical loads for coatings and the adhesion achievable at substrate bias of - 50 V was demonstrated, with relevance to the various modes.

The fretting wear behavior of tin plated copper alloy contacts and its influence on the contact resistance are addressed in this paper. Based on the change in the area of contact zone as well as the wear depth as a function of fretting cycles, a model was proposed to explain the observed low and stable contact resistance. The extent of wear of tin coating and the formation of wear debris as a function of fretting cycles were assessed by scanning electron microscopy (SEM). Energy dispersive X-ray line scanning (EDX), X-ray mapping, and EDX spot analysis were employed to characterize the nature of changes that occur at the contact zone. The study reveals that the fretted area increases linearly up to 8000 cycles due to the continuous removal of the tin coating and attains saturation when the fretting path length reaches a maximum. The observed low and stable contact resistance observed up to 8000 cycles is due to the common area of contact which provides an electrically conducting area. Surface analysis by SEM, EDX, and X-ray elemental mapping elucidate the nature of changes that occurred at the contact zone. Based on the change in contact resistance as a function of fretting cycles, the fretting wear and fretting corrosion dominant regimes are proposed. The interdependence of extent of wear and oxidation increases the complexity of the fretting corrosion behavior of tin plated contacts.

Laser cladding is an advanced surface modification technology with broad prospect in making wear-resistant coating on titanium alloys. In this paper, the influences of laser cladding processing parameters on the quality of coating are generalized as well as the selection of cladding materials on titanium alloys. The microstructure characteristics and strengthening mechanism of coating are also analyzed. In addition, the problems and precaution measures in the laser cladding are pointed out.

The low bonding strength between plasma sprayed hydroxyapatite (HA) coatings and substrates is one of the problems, which should be solved. In this paper, the as-sprayed HA coatings were retreated by laser remelting. The microstructure and element analysis in coatings were studied by electron probe microanalyzer (EPMA) and the accessory of energy spectrum analyzer. The results show that the bonding state can be improved greatly after laser remelting and it is a metallurgical combination between transition layers and substrates. Generally, with the increase of laser power and the decrease of scanning speed, the bonding state between the surface and transition layer will be improved much more, but the value of Ca/P ratio will deviate much more from 1.67. In this experiment, the optimum technological parameter is that the laser power is 600 W and the scanning speed is 11.2 mm/s.

The surface temperature field model of hot filament chemical vapor deposition (HF CVD) diamond films on WC–Co alloys was constructed and calculated by taking into account the influences of both thermal source properties and physical properties of substrates. Under the certain conditions of some parameters, the effects of the influence factors such as the maximum specific heat flux (q_m), the heat conductivity coefficient (λ) of the substrate and the substrate height (h) on the surface temperature field were exhibited quickly and clearly by the computer modeling. The theoretical calculating data are close to the experimental results. It is found that the influences of the thermal source parameter, the substrate height, and the heat conductivity coefficient of the substrate materials on the surface temperature field of diamond films on WC–Co alloys are almost equally important.

The 5-parameter Morse potential (5-MP) of an O–Pd system and the improved extended LEPS potential of an O₂–Pd system were constructed. The adsorption and dissociation of oxygen atoms and oxygen molecules on flat and stepped palladium single crystal surfaces were studied detailedly, and the data of adsorption sites, adsorption geometry, binding energy, eigenvibration, dissociative mechanism, etc. were obtained. Theoretical results show that the high-symmetry sites are the preferred adsorption sites for tilted adsorption of oxygen molecules, and the tilted angle depends on the number of atoms per unit surface area. Meanwhile, a variety of dissociation pathways are analyzed in detail related to the O₂–Pd(111) system. These results show that the adsorptive dissociation of oxygen molecules on the Pd(111) hollow site is precursor-mediated, on the Pd(111) bridge site is direct dissociation and on the Pd(111) top site is not only depended on the translational energy, but also depended on the vibrational energy. Moreover, the role of the step defect for oxygen atoms and oxygen molecules is also discussed related to flat surface. Theoretical results are in good agreement with the experimental results, and explain well the divergence [N. Salanov and V. N. Bibin, Surf. Sci.441 (1999) 399; P. D. Nolan, B. R. Lutz, P. L. Tanaka and C. B. Mullins, Surf. Sci.419 (1998) L107] related to the O₂ dissociation mechanism.

In this paper, we study the kinetics of potassium adsorption on SrTiO₃(100) surface at room temperature. The study took place in UHV using AES, EELS, LEED, TPD, and WF measurements. Potassium grows in two-dimensional islands with maximum coverage of 3.9 × 10¹⁴ at/cm². During adsorption charge transfer from the K-4s to Ti-3d energy level has been observed. No K–O compound has been measured. After heating at 1050 K most of the adsorbed potassium desorbs and the SrTiO₃(100) (1 × 1) LEED pattern reappears. There is not any indication of potassium intercalation into the substrate, either at room or at elevated temperature.

Pulsed laser deposition (PLD), which is a novel technique in producing thin films in the recent years, shows unique advantages for the deposition of ferroelectric films. Effects of technical parameters on the pulsed laser deposited ferroelectric films, including substrate temperature, oxygen pressure, post-annealing, buffer layer, target composition, energy density, wavelength, target-to-substrate distance, and laser pulse rate, are systematically reviewed in order to optimize these parameters. Processing-microstructure-property relationships of ferroelectric films by PLD are discussed. The application prospect is pointed as well.

The surface segregation, structure, and valence band density of states of Pt₃Ni(100), (110), and (111) single crystals are characterized with low energy ion scattering (LEIS), low energy electron diffraction (LEED), and ultraviolet photoemission spectroscopy (UPS). The results of LEIS clearly reveal the complete surface segregation of Pt to the top layer on all crystal alloys. LEED indicates the (5 × 1) surface reconstruction on the Pt₃Ni(100), while (110) and (111) surfaces show (2 × 1) and (1 × 1) patterns, respectively, identical to Pt single crystals. The valence bands density of states on Pt₃Ni alloys are compared to those of Pt single crystals via UPS measurements.

Fractal analysis of electrode surfaces by means of electrochemical techniques is an efficient tool to reveal surface structures via a geometrical model. Since this is based on the concept of "diffusion toward electrode surfaces", detectable scale depends on the diffusion coefficient, and it is in the range of 1–100 μm for conventional electrochemistry. By taking this issue into account, a simple approach is proposed to perform fractal analysis at nanoscale, which is an important requirement in surface studies. Increasing the electrolyte viscosity and decreasing the environment temperature lead to slower diffusion. Thus, the detectable fractality scale, which is proportional to the square root of diffusion coefficient, can be reduced. This provides an opportunity for fractal analysis at nanoscale. In this case, the possible errors due to natural convection (i.e., an important obstacle in this method) will also be significantly reduced. This simple approach in company with the previously developed gold-masking approach provides an acceptable generality for fractal analysis of electrode surfaces.

Microscale to nanoscale carbon and carbon nitride particles and films were synthesized by using plasma discharge sputtering deposition techniques. Experimental results indicated that sizes and distributions of the particles were directly determined by both discharge voltage and bias voltage. Sputtering deposition at high discharge voltages yielded large sizes (micrometer order) of particles with a high disorder of distribution. Whereas sputtering deposition at low discharge voltages yielded nanoscale of particles that uniformly distributed on the surface of the substrate. Ar laser beam in combination with an optical microscope has been used, enabling one to remove these particles and to achieve preferred distributions of the particles. Low growth rate of the films was found at the low voltage discharge sputtering deposition. Bias voltage was employed during the experiments in order to speed up the growth rate and increase nitrogen content inside the carbon nitride film. With an increase of the bias voltage up to 5 kV, nanoparticles appeared in two-dimensional, sunflower type of cluster distributions. Typical G, D, and C=N bands in the Raman spectra of the samples were identified.