Melting Of Surface Layer Research Articles

Wear resistance of WC p/Duplex Stainless Steel metal matrix composite layers prepared by laser melt injection

Laser Melt Injection (LMI) was used to prepare metal matrix composite layers with a thickness of about 0.7 mm and approximately 10% volume fraction of WC particles in three kinds of Cast Duplex Stainless Steels (CDSSs). WC particles were injected into the molten surface layer using Nd:YAG high power laser beam. As a result the microstructure characterized by hard ceramic particles distributed in a metal matrix with the strong bonding to substrate is formed in the surface layer of the treated metal. Dry sliding wear properties of these metal matrix composites layers were measured and compared with the wear properties of the substrate and with surfaces simply remelted by the laser beam. The observed wear mechanisms are summarized and related to detailed microstructural observations. The layers have been found to show excellent interfacial bonding, coupled with substantially improved tribological properties expressed through the wear resistance increase of 8 times. The amount of WC particles was sufficient to reinforce the matrix and the particles have shown a good bonding to the matrix to support the contact stress in the layer.

Development of High-Voltage Vacuum Circuit Breakers in China

This paper introduces research work on the development of high-voltage (HV) vacuum circuit breakers (VCBs) in China from its starting point in 1989 to 2006. In this period, a 126-kV two-breaks VCB prototype and a 126-kV single-break VCB prototype were developed. A latest 252-kV single-break interrupter prototype is introduced. Five HV VCBs technologies are discussed, which include HV vacuum insulation, high current interrupting technology with long contact gaps, increasing nominal current, operating mechanism characteristics, and contact bouncing damping. In vacuum insulation, adding a metal ring at the electrode back can decrease breakdown possibilities in the gap between the electrode back and the main shield. A surface melting layer of contact material could have an influence on voltage withstanding capability of a vacuum gap. For interrupting high current with long contact gaps, a strong axial magnetic field (AMF) is needed for better vacuum arc control. Therefore, a single coil AMF electrode is introduced. Heat radiators are effective to increase the nominal current of VCBs. Thermal analysis can help to give appropriate design parameters. There is an optimum opening characteristic that is helpful to improve the interrupting performance of VCBs. Permanent magnet operating mechanism and spring-type operating mechanism are suitable for HV VCBs. Contact bouncing in HV VCBs can be damped by installing contact spring and bellows on the stationary end of VCBs.

Silicon surface modification by quasistationary compression plasma flow

Modification of silicon single crystal surface by the action of nitrogen quasistationary compression plasma flow (CPF) generated by magnetoplasma compressor is studied. It was found that during single pulse surface treatment regular fracture features are obtained on the Si (111) and Si (100) surface in the target central part. Some of these regular structures can become free from the underlying bulk, formed as blocks ejected from the surface. Also, oriented silicon periodic structures are produced in the target periphery part. These surface phenomena are results of specific conditions during CPF interaction with silicon surface. High plasma flow energy density, large dynamic pressure, thermodynamic parameters gradients and induced magnetic field on treated surface cause rapid heating and melting of surface layer, as well as surface fracturing, long existence of molten layer and fast cooling and recrystalisation.

Selective surface purification via crater eruption under pulsed electron beam irradiation

This letter reports an interesting phenomenon associated with the high-current pulsed electron beam treatment:selective surface purification. The treatment induces crater eruptions that preferentially occur at irregular composition and structure sites. The eruptions of second phase inclusions naturally lead to the purification and homogenization of the melted surface layer. This improves significantly the corrosion resistance of NiTi and 316L alloys.

Comparison of methods for melt detection over Greenland using active and passive microwave measurements

Microwave measurements have been used in various studies to detect melt based on their sensitivity to liquid water present in snow. To contrast different melt detection methods used with different sensors, six different melt detection method/sensor combinations are compared using data for the summer of 2000. The sensors include the Special Spectral Microwave Imager (SSM/I), SeaWinds on QuikSCAT (QSCAT), and the European Remote Sensing (ERS) Advanced Microwave Instrument (AMI) in scatterometer mode. Existing melt detection methods are compared with melt detection based on a simple physical model. The model relates the moisture content and depth of a surface melt layer of wet snow to a single channel melt detection threshold. The model can be applied to both active and passive sensors and improves the consistency between brightness temperature (Tb ) and normalized radar backscatter (σ°) based detection of melt. Model‐based melt estimates from different sensors are highly correlated and do not exhibit the unnatural phenomenon observed with previous methods. Relative merits and limitations of the various methods are discussed.

Igneous rims on micrometeorites

Melting of micrometeorites (MMs) due to atmospheric entry heating causes significant changes in the textures, mineralogies and compositions of particles that obscure their primary properties and greatly complicate interpretation of these extraterrestrial materials. Despite the abundance of melted MMs, the nature of melting processes in these materials is poorly constrained. In this study, mineralogical, textural, and compositional data on 77 MMs with igneous rims are presented, which suggest that fusion of micrometeorites during atmospheric entry occurs initially by surface melting. Textural and mineralogical evidence are presented that demonstrate unequivocally that igneous rims crystallized from a melt surrounding a largely unmelted core and establish melting as a gradational process. The compositions of igneous rims on fine-grained MMs (fgMMs) are broadly similar to those of the unmelted core except for depletions in volatile and moderately volatile elements produced by partial evaporation and suggest the formation of the rims by melting of the fine-grained matrix core. Enrichments in Fe/Si, Ni/Si, and Mn/Si in igneous rims compared with unmelted cores within fgMMs are suggested to occur due to the migration of Fe–S eutectic liquids from the core of the particle into the surface melt layer. The sulphide liquids are probably generated in the core of the particle under reducing conditions resulting by the pyrolysis of carbonaceous materials. The presence of igneous rims on fgMMs is enabled by the thermal decomposition of phyllosilicates and, therefore, indicates that fgMMs were hydrated particles prior to atmospheric entry. In contrast the compositions of igneous rims on coarse-grained MMs (cgMMs) are difficult to reconcile with those of the unmelted core and instead closely resemble those of fgMMs. The igneous rims of cgMMs are, therefore, suggested to form by melting of fine-grained matrix that was present on the exterior of these particles prior to atmospheric entry. Coarse-grained MMs with igneous rims thus were originally composite micrometeoroids, which consist of both fine-grained and coarse-grained materials, and are thought to be samples of chondrule-like objects. The abundance of composite MMs to cgMMs allows a first estimate of the mean chondrule radius within the parent bodies of MMs of ∼625 μm to be made.

High current pulsed electron beam treatment of AZ31 Mg alloy

This paper reports, for the first time, an analysis of the effect of High Current Pulsed Electron Beam (HCPEB) on a Mg alloy. The AZ31 alloy was HCPEB treated in order to see the potential of this fairly recent technique in modifying its wear resistance. For the 2.5J∕cm2 beam energy density used in the present work, the evaporation mode was operative and led to the formation of a “wavy” surface and the absence of eruptive microcraters. The selective evaporation of Mg over Al led to an Al-rich melted surface layer and precipitation hardening from the over saturated solid solution. Due to the increase in hardness of the top surface layer, the friction coefficient values were lowered by more than 20% after the HCPEB treatments, and the wear resistance was drastically (by a factor of 6) improved. The microhardness of the HCPEB samples was also increased significantly down to a depth of about 500μm, far exceeding the heat-affected zone (about 40μm). This is due to the effect of the propagation of the shockwave associated with this HCPEB treatment.

Bulk Texture and Structure Changes in Tubes of Zr Alloys due to the Long-Range Effect of Ion-Plasma Surface Treatment

Irradiation of metal materials with low-energy ions is accompanied by the long-range effect, consisting in distinct texture and structure changes at the depth, exceeding, at least, by 104 times the thickness of the layer of ion retardation. In order to ascertain mechanisms of this effect, a layer-by-layer X-ray study was carried out as applied to ion-plasma treated cladding tubes of Zr- 1%Nb alloy for nuclear reactors. Impacts of pulse treatments with helium plasma by two different regimes and the ion energy of ~ 1 keV were compared. It was found, that only by minimal surface melting the ion-plasma treatment causes unpredictable bulk texture changes, consisting in arising of the axial texture component. The melted surface layer suppresses shock waves, associated with braking of ions and supposedly responsible for stimulation of dislocation processes at long distances from the surface.

Optical switching at ZnSe–Ga interfaces via nanoscale light-induced metallisation

A structural transformation induced by optical impulse excitation of only a few mJ/cm2 fluence affecting the nanoscale surface melt layer of gallium at a ZnSe–gallium interface leads to a fast, substantial and fully reversible change in reflectivity of up to 50%. Recovery occurs within a few μs after excitation is withdrawn and less than 1μs at temperatures more than 4K below gallium’s melting point.

Optical evidence for a self-propagating molten buried layer in germanium films upon nanosecond laser irradiation

This work describes the phase transitions occurring at the film-substrate interface of amorphous germanium films upon nanosecond laser-pulse-induced melting of the surface. Films with thickness ranging from 50 to 130 nm deposited on glass substrates were studied. Real-time reflectivity measurements with subnanosecond time resolution performed both at the air-film and film-substrate interfaces were used to obtain both surface and in-depth information of the process. In the thicker films (⩾80nm), the enthalpy released upon solidification of a shallow molten surface layer induces a thin buried liquid layer that self-propagates in-depth towards the film-substrate interface. This buried liquid layer propagates with a threshold velocity of 16±1m∕s and causes, eventually, melting at the film-substrate interface. In the thinnest film (50 nm) there is no evidence of the formation of the buried layer. The presence of the self-propagating buried layer for films thicker than 80 nm at low and intermediate laser fluences is discussed in terms of the thermal gradient in the primary melt front and the heat released upon solidification.

Determination of the Composition of the Surface Layer of Binary Metal Alloys

We have analyzed the effect of various parameters (surface tensions of the components, molar volumes of the components and the solution, compression and depression in alloy formation, activity coefficients of the components in the melt) on the shape of the surface tension isotherm as a function of the surface layer composition. We compare the analysis results with experimental data. We propose a method for estimating the composition of the surface layer based on a model assuming an additive variation in the surface tension of the solution as a function of the composition of the surface layer. We have calculated the concentration of the surface-active component in the surface layer of melts in the systems Ge - Sn, Ge - Pb, and Ge - Bi.

Burning-Rate Predictor for Multi-Ingredient Propellants: Nitrate-Ester Propellants

The theoretical capability to predict the burning rate of real propellants from their ingredients would be an invaluable aid to formulating new propellants. Despite progress over the last decade on a very few simple ingredients, such as cyclotrimethylenetrinitramine (RDX), and a few simple binary mixtures, no general capability of this sort exists today. This shortcoming is not due to insufficient computational resources, but to a lack of understanding of fundamental combustion mechanisms in the condensed phase and surface/gas interface for typical propellant ingredients and their mixtures. This difficult problem is likely to remain intractable for some time to come. It is demonstrated that our previously published semi-empirical formalism for single ingredients can be successfully extended to treat multi-ingredient propellants. In particular, the study is confined to nitrate‐ester propellants (using M10, M2, M9, and JA2 as examples). However, the method should also be applicable to other classes of homogeneous propellants and even composite propellants where mixing of ingredients in a surface melt layer or sufficiently small particle sizes remove the multidimensional character. The method treats the gas-phase processes on the level of elementary reactions and multicomponent transport. A semi-empirical pyrolysis law coupled with informed estimates of the decomposition products of the condensed phase enables us to finesse the absence of knowledge of the detailed processes occurring in the condensed phase and at the burning surface. Results of a computer code, CYCLOPS, based on this approach, show that both the burning rate and flame structure are well predicted for a series of four U.S. Army gun propellants.

Activity of Components in the Surface Layer of Ternary Mutual Melts

The activity of components in the bulk, in the surface layer, considered as individual phase was calculated from the experimental surface tension values and in terms of the model of regular solutions. The ternary mutual chloride-fluoride, chloride-carbonate, and fluoride-carbonate systems were examined.

Effect of overlaps on phase composition and crystalline orientation of laser-melted surfaces of 321 austenitic stainless steel

Understanding the effect of overlaps on microstructure and performance of laser treated surface layer is of fundamental importance for the development and application of laser surface treatment of engineering materials. In the present work, laser surface melting of 321 austenitic stainless steel with overlaps and single laser track was carried out using a 2 kW CO2 laser and a 3 kW CW Nd:YAG laser respectively. The crystallization of the melted surface layers and their preferred orientation were characterized by X-ray diffraction. It was found that the heat treatment effect at overlap regions caused not only the transformation of some δ-ferrite formed in previous laser tracks reversed into γ-austenite, but also the increase in the extent of the preferred orientation of γ-austenite along [200] direction. Relationships between phase composition and laser process parameters such as laser power lever, beam size, scanning velocity and overlap ratio have been also studied.

Mechanical and elastic properties of modified thick thermal barrier coatings

Mechanical and elastic properties of the phosphate sealed and laser-glazed 8Y 2O 3–ZrO 2 and 22MgO–ZrO 2 thick thermal barrier coatings (TBCs) were studied. Mechanical properties were determined by erosion and abrasion experiments and the elastic properties were evaluated by four-point bending (4PB) tests. Phosphate based sealing treatments improved significantly the erosion and abrasion resistance of the coatings and also increased the coating microhardness, bending strength and stiffness. The improvements of mechanical properties in the phosphate sealed coatings were caused by the lamellae bonding, due to the sealant. The effect of laser-glazing on the mechanical and elastic properties varied between the studied materials. The melted surface layer of the laser-glazed 8Y 2O 3–ZrO 2 coating was very hard and dense and had excellent resistance against mechanical wear. At the same time the vertical segmentation crack network in the coating structure lowered the bending strength and stiffness. In laser-glazed 22MgO–ZrO 2 coating the melted layer was also very hard, but most of the segmentation cracks were curved and branched and under mechanical loads they had tendency to coalesce. For this reason the laser-glazed 22MgO–ZrO 2 coating had poor mechanical properties.

Surface Properties of Manganese – Tin Liquid Alloys

We used the sessile drop method to study the temperature and concentration dependences of the density and surface tension of melts in the manganese – tin system. The density polytherms are linear dependences which are consistent with those calculated by the additivity rule for the specific volumes of the pure components. The temperature dependences of the surface tension are also linear and their temperature coefficient changes from negative values for tin-rich melts to positive values for melts with high manganese content. The surface tension isotherm is satisfactorily described by the Popel-Pavlov equation. We calculated the composition of the surface layer of the studied melts as a function of the composition of the bulk phase. We have shown that the model for the surface layer of melts in the Mn – Sn system at 1300°C is close to monolayer.

Surface morphology of titanium irradiated by high-intensity pulsed ion beam

Surface morphology of pure Ti irradiated by high-intensity pulsed ion beam (HIPIB) with ion current density of 60–250 A/cm 2 and shot number of 1–30 at 220 kV has been investigated by using profilometer, scanning electron microscopy and atomic force microscopy to explore the interaction mechanism between HIPIB and metallic materials. Two kinds of samples were prepared with different initial surface roughness ( R a), i.e. high-roughness and low-roughness Ti, respectively. A similar change trend of R a was found on the irradiated surfaces for both the two kinds of samples that increase of the R a was obtained with few shot number and then continuous decrease of the R a to a surface smoothing was with multi-shot irradiation. However, the increase of R a on the irradiated low-roughness Ti was greatly limited as compared to the high-roughness case. It is demonstrated that surface smoothing and roughening of irradiated Ti can be realized under optimization of the parameters by combining adjustments of ion current density and shot number. The morphology features on the irradiated high-roughness Ti were craters and waviness for roughened surfaces and some vague or apparent texture with disappearance of the craters for smoothed surfaces. For the low-roughness Ti, no distinct craters were formed except for some obvious texture. The micro-non-uniformity (micro-protrusions) on the irradiated surfaces causes the cratering on pure Ti by inducing a selective ablation under HIPIB irradiation. The locally more intense liquid evaporation and droplet ejection from the irradiated surfaces of different morphologies led to disturbance of the molten surface layer in the different scales, resulting in the surface roughening and smoothing.

Activity of components in surface layer of chloride-fluoride mutual melts

The activities of components in the bulk, surface layer of melts of chloride-fluoride ternarymutual systems were calculated in the framework of the regular solution model using surface tension valuesand considering the surface layer is a separate phase.

A new approach to the study of interfacial melting of ice: infrared spectroscopy

Faraday observed in 1850 "that a particle of water which could retain the liquid state whilst touching ice on only one side, could not retain the liquid if it were touched by ice on both" (M. Faraday, Royal Institution Discourse, June 7, 1850; Experimental Researches in Chemistry and Physics (Taylor and Francis, New York, 1991)). Thus began the concept of interfacial melting, and the presence of a liquid water film on the surface of ice at temperatures of 0°C and below. Over the past few decades, there have been a number of measurements of interfacial melting. In some studies, the thickness of the thin film, variously called the quasi-liquid layer (QLL), liquid-like layer, surface melting layer, or premelting layer, has been determined. The results of these measurements demonstrate a striking variation depending on the experimental method and the nature of the ice samples. For example, at –0.1°C, the thickness values range over two orders of magnitude from around 1 to 100 nm. Although the disagreement can be partially explained by the differences in ice samples, the experimental techniques employed in measurements of the QLL thickness are based on different physical principals, and involve a web of assumptions for their deconvolution. We describe here the technique of infrared attenuated total reflection (ATR) spectroscopy that has been directed to the study of interfacial melting of ice for the first time. PACS No.: 83.50Lh

The effect of dense compression plasma flow on silicon surface morphology

The dense compression plasma flow action on silicon surface layers is investigated. Samples of monocrystalline silicon of (111) and (100) orientations were subjected to plasma flow processings. Values of energy absorbed by the silicon surface range from 5 to 25 J per pulse, the latter corresponding to the flow power density range 0.5·10 5–3·10 5 W/cm 2. Microreliefs of sample surfaces and slices were recorded by means of high-resolution scanning microscopy. The action of the compression plasma flow on the sample results in the melting and subsequent modification of silicon material down to the depth of 6 μm. Micrographs of surface layers clearly show regular cylindrical structures that are the first of their kind ever observed. Cylindrical fragments measure 50–100 μm in length and 0.7–1.5 μm in diameter. These fragments are located on the surface of the sample at intervals of 1–2 μm with a surface density of (2–6)·10 6 cm −2. Possible mechanisms of cylindrical periodic structure formation are discussed. The roles of Rayleigh–Taylor instability, Kelvin–Helmholtz instability and Benard instability in transformations of molten surface layers are considered. It may be inferred that structural-phase changes in the state of silicon surface are related to the fast crystallization of molten layer accompanied by the development of various instabilities in the presence of an induced magnetic field.