Microstructure, microhardness, composition, and corrosive properties of stainless steel 304 I. Laser surface alloying with silicon by beam-oscillating method.

Laser surface alloying (LSA) with silicon was conducted on austenitic stainless steel 304. Silicon slurry composed of silicon particle of 5 μm in average diameter was made and a uniform layer was supplied on the substrate stainless steel. The surface was melted with beam-oscillated carbon dioxide laser and then LSA layers of 0.4–1.2 mm in thickness were obtained. When an impinged energy density was adjusted to be equal to or lower than 100 W mm⁻², LSA layers retained rapidly solidified microstructure with dispersed cracks. In these samples, Fe₃Si was detected and the concentration of Si in LSA layer was estimated to be 10.5 wt.% maximum. When the energy density was equal to or greater than 147 W mm⁻², cellular grained structure with no crack was formed. No iron silicate was observed and alpha iron content in LSA layers increased. Si concentration within LSA layers was estimated to be 5 to 9 wt.% on average. Crack-free as-deposited samples exhibited no distinct corrosion resistance. The segregation of Si was confirmed along the grain boundaries and inside the grains. The microstructure of these samples changed with solution-annealing and the corrosion resistance was fairly improved with the time period of solution-annealing. Received: 2 September 1999 / Accepted: 6 September 1999 / Published online: 1 March 2000     

Effects of pre-placed coating thickness on thermal fatigue resistance of cast iron with biomimetic non-smooth surface treated by laser alloying

In order to enhance the thermal fatigue resistance of gray cast iron with biomimetic non-smooth surface further, studies on laser alloying of Cr powder with different pre-placed coating thickness were performed to change both the composition and the microstructure of non-smooth unit. Additionally, the optimization of coating thickness was done based on the content of Cr in alloyed zone and the thermal fatigue behaviors of non-smooth samples. The results indicated that there was a critical coating thickness which corresponded to the increase of Cr content in alloyed zone under a definitive laser processing conditions, and the critical thickness was 0.3 mm in this paper. Any coating thicker than 0.3 mm would lead to the waste of alloying powder. The thermal fatigue resistance of non-smooth samples was better than that of smooth sample. In addition among all the non-smooth samples, the sample which was treated by the laser alloying of Cr had superior resistance to thermal fatigue compared with laser melting treated samples. And the thermal fatigue resistance increased with increasing of Cr content in alloyed zone which was caused by pre-placed coating thickening.     

Microstructure and corrosion properties of thick WC composite coating formed by plasma cladding

The thick Ni-coated WC coatings, in a matrix of Nickel-based alloys, were prepared on AISI 1045 steel using plasma cladding equipment. A pre-placed layer of uniform mixture, with different weight fractions of Ni-coated WC powder and Nickel-based alloy powder, on the steel substrate was melted at the high temperature of the plasma jet. The coating composition, microstructure and microhardness were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and microhardness testing. The experimental results show that the metallurgical bond was formed between the coating and substrate. The XRD results show that the coatings contain γ-Ni, carbides (such as M₂₃C₆ and M₇C₃) and boride (such as Fe₂B, Fe₃B phases). SEM shows that all the coatings are crack-free with lower porosity (<1%). It is found that the microhardness and the electrochemical behavior of the coatings are depended on the content of Ni-coated WC powder. The corrosion mechanism for the coatings may be due to the microgalvance corrosion between the phases in the cladding coatings.     

Nitriding of steel and titanium surface layers under the action of compression plasma flows

The elemental and phase compositions of St3 steel and VT1-0 titanium surface layers nitrided by the action of compression plasma flows (CPFs) have been investigated. The plasma flow parameters are shown to be correlated with the modified-layer nitrogen content. The basic mechanism by which the steel and titanium surface layers are saturated with nitrogen has been revealed. The performed experiments indicate that an increase in the absorbed energy density leads to a decrease in the nitrogen concentration because a shock-compressed layer is formed in the near-surface region, impeding nitrogen diffusion into the sample. The higher nitrogen concentration of surface layers treated by CPFs is achieved by increasing the pressure of the residual nitrogen atmosphere. It has been established that γ_N-Fe nitrous austenite, α″-Fe(N) and α′-Ti(N) martensitic phases, and γ′-Fe₄N and δ-TiN _x nitrides can be produced by nitriding the surface layers of St3 steel and VT1-0 titanium.     

Process monitoring of powder pre-paste laser surface alloying

Instead of the continuous powder delivery method using a powder feeder for thick layer laser cladding, pre-pasting of the alloying powder on the substrate is a widely used method to supply the coating powders into the melt pool for LSA. A method to monitor the process of laser surface alloying based on the infrared emission from the melt pool using infrared photodiodes was developed. The technique is solely aimed at the process of laser surface alloying using pre-paste metal powder on the substrate surface prior to laser melting. This monitoring technique is able to distinguish the existence or the absence of the pre-paste powder and the consistency of the laser surface alloying process. The technique is of low cost and is simple to implement into the process.     

Adsorption effect in non-reaction wetting: In–Ti on CaF2

The experiments show that the alloying liquid In with only (0.1–0.5) at% Ti dramatically reduces the equilibrium contact angle Θ _∞ formed by In on the surface of CaF₂. The aim of this paper is to clarify whether this practically important and conceptually challenging effect can be explained solely by Ti adsorption at the F-terminated solid–liquid interface without resorting to any other Ti-induced effect. The combination of ab initio calculations and regular solution approximation was proposed for finding the binding energy, ΔE _Ti of Ti adatom with the interface “CaF₂/liquid solutions In–Ti.” With thus obtained ΔE _Ti=1.16 eV, we calculated from the Shishkovsky isotherm the reduction in the solid–liquid interface energy, Δγ _SL induced by Ti adsorption from liquid In with various Ti concentration, C. It was found that Δγ _SL(C) dependence demonstrated close inverse correspondence with Θ _∞(C) and that the theory fitted very well all available experimental data on the concentration and temperature dependence of Δγ _SL. It was concluded that the Ti adsorption effect is large enough to account for the observed wetting improvement. The proposed multiscale modeling approach to the role of adsorption in wetting can be applied also to other nonreactive systems “liquid metal–ceramics” where the substrate determines the surface density of the adsorption sites for the active element.     

High-temperature oxidation behavior of SiO2 protective layer deposited on IN738LC superalloy by combustion CVD (CCVD)

A SiO₂ protective coating was deposited on an IN738LC alloy using CCVD. The physical properties of a SiO₂ protective layer are influenced by the amount of tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si). Therefore, the SiO₂ protective coating was deposited using different TEOS concentrations and deposition times to optimize the conditions. The deposited coating layer was confirmed to be a SiO₂ layer by SEM, EDX, and ESCA analyses. The oxidation resistance of the alloy was evaluated by thermo gravimetric analysis. The oxidation resistance of the SiO₂ protective coating was highest when the coating was processed at a TEOS concentration of 0.05 mol/l, which is the highest concentration of source material used. The surface roughness of the SiO₂ protective layer also increased with increasing TEOS concentration. The surface roughness of the coating had little effect on the oxidation resistance for a film thickness of approximately 1 μm.     

Characterizing the coating and size-resolved oxidative stability of carbon-coated aluminum nanoparticles by single-particle mass-spectrometry

Aluminum nanoparticles are of significant interest in enhancing the rate of energy release from propellants. One of the major impediments to their use is that bare aluminum is highly reactive, while oxide coated aluminum significantly decreases overall performance. We investigate creating aluminum nanoparticles with a thin carbon coating using either a laser induced plasma or a DC plasma-arc. The carbon coating was created by injecting ethylene (C₂H₄) directly downstream of the plasma. The elemental composition of the coated aluminum nanoparticles was measured in real time with a recently developed quantitative single particle mass spectrometer (SPMS). We found that the aluminum nanoparticles were coated with a carbon layer of thickness around 1–3 nm.The thermal and oxidative stability of these particles was determined by passing the aerosols through a heated flow reactor in a carrier flow of either air or argon, and measuring the aluminum, carbon and oxygen content in the particles with the single particle mass spectrometer. We found that below 700°C the coating was stable, but that the coating oxidized above ∼ ∼800°C. In contrast the carbon coating was thermally stable above ∼ ∼900°C. These results indicate that a carbon coating may be a suitable passivating agent.     

Radiation-induced color centers in the near-surface layer of lithium fluoride crystals

For radiation-induced intrinsic color centers, we show that the concentrations of identical centers and the concentration ratios of different centers are quite different in the near-surface layer and within the interior volume of a lithium fluoride crystal. We have established that these differences also depend on the sign of the difference between the temperature at which the crystal was irradiated with γ photons and the vacancy mobility temperature. We provide an interpretation for the results obtained, based on the structural features of the near-surface layer and the concentration ratio of vacancies and electrons in the layer, serving as the starting components for color center formation. We found that the concentrations of centers change over the course of a few days by tens of percent in the layer “emerging” from the interior onto the surface as a result of cleavage of the crystal. We measured the luminescence lifetimes of F₃ (R2) and F₄ centers: 6.6 nsec and 11.7 nsec. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 775–781, November–December, 2006.     

Process and performance of hot dip zinc coatings containing ZnO and Ni-P under layers as barrier protection

A new coating system of under layer for hot dip zinc coating was explored as an effective coating for steel especially for application in relatively high aggressive environments. The influence of different barrier layers formed prior to hot dip galvanization was investigated to optimize high performance protective galvanic coatings. The deposition of ZnO and Ni-P inner layers and characteristics of hotdip zinc coatings were explored in this study. The coating morphology was characterized by scanning electron microscope (SEM) analysis. The hot dip zinc coatings containing under layer showed substantial improvement in their properties such as good adhesion, and high hardness. In addition, a decrease in the thickness of the coating layer and an enhancement of the corrosion resistance were found. Open circuit potential (OCP) of different galvanized layers in different corrosive media viz. 5% NaCl and 0.5 M H₂SO₄ solutions at 25 ± 1 °C was measured as a function of time. A nobler OCP was exhibited for samples treated with ZnO and Ni than sample of pure Zn; this indicates a dissolution process followed by passivation due to the surface oxide formation. The high negative OCP can be attributed to the better alloying reaction between Zn and Fe and to the sacrificial nature of the top pure zinc layer.     

Microstructure evolution of Fe-based WC composite coating prepared by laser induction hybrid rapid cladding

Fe + 50 wt.% WC composite coating was prepared by laser induction hybrid rapid cladding (LIHRC) on steel substrate. The phase and microstructure of the composite coating were investigated by X-ray diffraction (XRD), environmental scanning electron microscope (ESEM) and energy dispersive spectrum (EDS). The results showed that WC particles were dissolved almost completely to precipitate the coarse herringbone M₆C eutectic carbides and the fine dendritic M₆C carbides, and that the partially dissolved WC particles with an alloyed reaction layer were occasionally observed in the whole coating. The phases of the composite coating were composed of supersaturated solid solution α-Fe, retained austenite, Fe₃C, W₂C, M₆C and M₇C₃. The microstructure evolution in the composite coating was represented by the transformation of three parts such as Fe-based metallic matrix, dispersed carbides and incompletely dissolved WC particles. The microhardness of Fe-based WC composite coating was three times much higher than that of the substrate, but was relatively lower than that of Ni-based WC composite coating by LIHRC.     

Microstructure and corrosion behavior of austenitic stainless steel treated with laser

Surface modification of AISI316 stainless steel by laser melting was investigated experimentally using 2 and 4 kW laser power emitted from a continuous wave CO₂ laser at different specimen scanning speeds ranged from 300 to 1500 mm/min. Also, an investigation is reported of the introduction of carbon into the same material by means of laser surface alloying, which involves pre-coating the specimen surfaces with graphite powder followed by laser melting. The aim of these treatments is to enhance corrosion resistance by the rapid solidification associated with laser melting and also to increase surface hardness without affecting the bulk properties by increasing the carbon concentration near the surface. Different metallurgical techniques such as optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure of the treated zone. The microstructures of the laser melted zones exhibited a dendritic morphology with a very fine scale with a slight increase in hardness from 200 to 230 Hv. However, the laser alloyed samples with carbon showed microstructure consisting of γ dendrite surrounded by a network of eutectic structures (γ+carbide). A significant increase in hardness from 200 to 500 Hv is obtained. Corrosion resistance was improved after laser melting, especially in the samples processed at high laser power (4 kW). There was shift in I_corr and E_corr toward more noble values and a lower passive current density than that of the untreated materials. These improvements in corrosion resistance were attributed to the fine and homogeneous dendritic structure, which was found throughout the melted zones. The corrosion resistance of the carburized sample was lower than the laser melted sample.     

Preparation, characterization and electrochemical performance of γ-MnO2 and LiMn2O4 as cathodes for lithium batteries

The spinel LiMn₂O₄ is a very promising cathode material with economical and environmental advantages. LiMn₂O₄ materials have been synthesized by solid state method using γ-MnO₂ as manganese source, and Li₂CO₃ or LiNO₃ as Li sources. γ-MnO₂ is a commercial battery grade electrolytic manganese dioxide (TOSOH-Hellas GH-S) and LiMn₂O₄ samples were synthesized at a calcinations temperature up to 800 °C. γ-MnO₂ and LiMn₂O₄ samples were characterized by X-ray diffraction, thermal and electrochemical measurements. X-ray powder diffraction of as prepared LiMn₂O₄ showed a well-defined highly pure spinel single phase. The electrochemical performance of LiMn₂O₄ and its starting material γ-MnO₂ was evaluated through cyclic voltammetry, galvanostatic (constant current charge-discharge cycling) The electrochemical properties in terms of cycle performance were also discussed. γ-MnO₂ showed fairly high initial capacity of about 200 mAhg⁻¹ but poor cycle performance. LiMn₂O₄ samples showed fairly low initial capacity but good cycle performance.     

Chemical bond strain in molecules of polyamide 6 coatings on the steel surface

The structure and stresses of chemical bonds in a 20 μm thick polyamide 6 (PA 6) coating on the steel surface are studied by IR spectroscopy. It is found that PA 6 molecules in the coating are oriented parallel to the metal surface and the content of metastable crystalline (γ) and mesomorphic (β) phases in it is twice as low as in PA 6 films. In the crystalline α phase, the molecular skeleton is compressed and “lateral” (respective to the skeleton) chemical bonds are extended. In the mesomorphic β phase, the concentration of overstressed (extended to a prerupture state) parts in the molecular skeleton is higher than in the film. These effects are explained by a change in the PA 6 low-frequency vibration spectrum at the PA 6-metal interface.     

Thermal Induced Processes in Laminar System of Stainless Steel – Beryllium

The paper reports on investigation of the laminar system ‘stainless steel 12Cr18Ni10Ti – Be’ at thermal treatment. There have been determined sequences of phase transformations along with relative amount of iron-containing phases in the samples subjected to thermal beryllization. It has been revealed that thermal beryllization of stainless steel thin foils results in γ→α transformation and formation of the beryllides NiBe and FeBe₂. It has also been revealed that direct γ→α- and reverse α→γ-transformations are accompanied by, correspondingly, formation and decomposition of the beryllide NiBe. It is shown that distribution of the formed phases within sample bulk is defined by local concentration of beryllium. Based on obtained experimental data there is proposed a physical model of phase transformations in stainless steel at thermal beryllization.     

Synthesis and characterization of mono-dispersed Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Er<Superscript>3+</Superscript>-coated SiO<Subscript>2</Subscript> nanoparticles by co-precipitation process

In this paper, a facile co-precipitation process for preparing mono-dispersed core–shell structure nanoparticles is reported. The 110 nm SiO₂ cores coated with an yttrium aluminum garnet (Y₃Al₅O₁₂) layer doped with Er³⁺ were synthesized and the influence of the concentration ratio of [urea]/[metal ions] on the final product was investigated. The structure and morphology of samples were characterized by the X-ray powder diffraction, Fourier transform IR spectroscopy and transmission electron microscopy, respectively. The results indicate that a layer of well-crystallized garnet Y₃Al₅O₁₂:Er³⁺ were successfully coated on the silica particles with the thickness of 20 nm. The near infrared and upconversion luminescent spectra of the SiO₂@Y₃Al₅O₁₂:Er³⁺ powders further confirm that a Y₃Al₅O₁₂:Er³⁺ coating layer has formed on the surface of silica spherical particles.     

Electron-beam surface treatment of alloys based on titanium, modified by plasma from an electrical explosion of conducting material

We present the results from investigating the phase composition, structure, and properties of surface layers of titanium alloys subjected to combined treatment that includes plasma alloying the electric explosion of a carbon fiber with a quantity of TiB₂ powder and subsequent irradiation with a high-intensity electron beam. The formation of a multilayer, multiphase structure in the submicro- and nano-sized ranges is observed. It is shown that the properties of a alloyed layer exceed those of a bulk sample many times over.     

Effect of carbon coating process on the structure and electrochemical performance of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>O<Subscript>2</Subscript> used as cathode in Li-ion batteries

LiNi_0.5Mn_0.5O₂ powder was synthesized by a coprecipitation method. LiOH.H₂O and coprecipitated [(Ni_0.5Mn_0.5)C₂O₄] precursors were mixed carefully together and then calcined at 900°C. Surface modified cathode materials were obtained by coating LiNi_0.5Mn_0.5O₂ with a thin layer of amorphous carbon using table sugar and starch as carbon source. Both parent and carbon-coated samples have the characteristic layered structure of LiNi_0.5Mn_0.5O₂ as estimated from X-ray diffractometry measurements. Transmission electron microscope showed the presence of C layer around the prepared particles. TGA analysis emphasized and confirmed the presence of C coating around LiNi_0.5Mn_0.5O₂. It is obvious that the carbon coating appears to be beneficial for the electrochemical performance of the LiNi_0.5Mn_0.5O₂. A capacity of about 150 mAh/g is delivered in the voltage range 2.5–4.5 V at current density C/15 for carbon coated LiNi_0.5Mn_0.5O₂ in comparison with about 165 mAh/g obtained for carbon free LiNi_0.5Mn_0.5O₂ at the same current density and voltage window. About 92% and 82% capacity retention was obtained at 50th cycle for coated LiNi_0.5Mn_0.5O₂ using sucrose and starch, respectively; whereas, 75% was retained after only 30th cycle for carbon free LiNi_0.5Mn_0.5O₂. This improvement is mainly attributed to the presence of thin layer of carbon layer that encapsulate the nanoparticles and improve the conductivity and the electrochemical performance of LiNi_0.5Mn_0.5O₂.     

Carbon precipitation and diffusion in SiGeC alloys under silicon self-interstitial injection

In this work we investigate the diffusion and precipitation of supersaturated substitutional carbon in 200-nm-thick SiGeC layers buried under a silicon cap layer of 40 nm. The samples were annealed in either inert (N₂) or oxidizing (O₂) ambient at 850 °C for times ranging from 2 to 10 h. The silicon self-interstitial (I) flux coming from the surface under oxidation enhances the C diffusion with respect to the N₂-annealed samples. In the early stages of the oxidation process, the loss of C from the SiGeC layer by diffusion across the layer/cap interface dominates. This phenomenon saturates after an initial period (2–4 h), which depends on the C concentration. This saturation is due to the formation and growth of C-containing precipitates that are promoted by the I injection and act as a sink for mobile C atoms. The influence of carbon concentration on the competition between precipitation and diffusion is discussed. Received: 19 October 2001 / Accepted: 19 December 2001 / Published online: 20 March 2002 / Published online: 20 March 2002     

Surface relief and structure of electroexplosive composite surface layers of the molybdenum-copper system

The surface topography and structure of copper layers exposed to multiphase plasma jets of products of electrical explosion of molybdenum and copper foils are studied using profilometry and scanning electron and light microscopy. Such treatment allows deposition of either layered coatings or alloyed composite layers. It is found that the surface layer roughness parameter is R _a = 3.2−4.0 μm. The thickness of some copper and molybdenum layers of coatings is 15–20 μm. Electroexplosive alloying produces layers 25 μm thick. Sizes of copper inclusions in the molybdenum matrix near the surface of such layers vary from 30 nm to 1–2 μm.