The effects of zinc bath temperature on the coating growth behavior of reactive steel

The purpose of this work is to identify the influence of zinc bath temperature on the morphology and the thickness of reactive steel (Fe–0.1 wt.%Si alloy) coatings. The Fe–0.1 wt.%Si samples were galvanized for 3 min at temperatures in the range of 450–530 °C in steps of 10 °C. The coatings were characterized by using scanning electron microscopy/energy dispersive X-rays analysis. It was found that the coating thickness reaches the maximum at 470 °C and the minimum at 500 °C, respectively. When the reactive steel is galvanized at temperatures in the range of 450–490 °C, the coatings have a loose ζ layer on the top of a compact δ layer. With the increase of the galvanizing temperature, the ζ layer becomes looser. When the temperature is at 500 °C, the ζ phase disappears. With the increase of temperature, the coatings change to be a diffuse-Δ layer (δ+ liquid zinc).     

Effect of saturation parameters on the interaction of titanium alloys with carbon-nitrogen-containing media

We study some specific features of phase formation and gas saturation in the course of chemical and thermal treatment of VT1-0, OT4, and VT14 titanium alloys in carbon-nitrogen-containing (graphite and nitrogen under atmospheric pressure) and nitrogen-containing (molecular nitrogen under atmospheric pressure) media. Saturation was carried out at 750–1100°C, and the time of isothermal holding was 5 or 20 h. We have shown that, irrespective of the composition of the saturating medium, within the range 750–1000°C, the phase composition of coatings is identical and consists of TiN and Ti₂N nitrides. The quantitative proportions between them depend on the temperature and time of treatment. Owing to the specific features of the composition of a carbon-nitrogen-containing medium, the depth of the diffusion sublayer increases, and the stoichiometry of cubic nitride decreases, which leads to an increase in the surface microhardness of nitride coatings. Carbonitride coatings are formed only at high saturation temperature (1100°C). Their maximum hardness is 2.5 GPa higher than that of nitrides formed under similar conditions.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 3, pp. 81–87, May–June, 2004.     

On the CVD of MoSi2: an experimental study from the MoCl4–SiCl4–H2–Ar precursor with a view to the preparation of C/MoSi2/SiC and SiC/MoSi2/SiC microcomposites

The chemical vapour deposition of MoSi2 on plane substrates (graphite or sintered-SiC) and ceramic fibres has been studied from MoCl4–SiCl4–H2–Ar gas mixtures at 900相似文献   

Pressure-pulsed chemical vapour infiltration of SiC to porous carbon from a gas system SiCl4-CH4-H2

SiC matrix was deposited into porous carbon from a gas system SiCl₄-CH₄-H₂ in the temperature range 900–1200 °C using pressure-pulsed chemical vapour infiltration (PCVI) process. At 1000 °C, silicon single phase, a mixed phase of (Si + SiC), and SiC single phase, were detected by X-ray diffractions for specimens obtained with the reaction time per pulse of 1, 2–3, and 5 s, respectively. At 1100 °C, SiC single phase was obtained with a reaction time of only 0.3s. Between 1050 and 1075 °C, deposition rate accelerated suddenly. The increase of SiCl₄ concentration increased the deposition rate linearly up to 4%–6%. The residual porosity decreased from 29% to 6% after 2×10⁴ pulses of CVI at 1100 °C, and the flexural strength was 110 MPa.     

Chemical vapour growth of HfC whiskers and their morphology

HfC whiskers were prepared from a gas mixture of HfCl₄ + CH₄ + H₂ + Ar in the presence of metal impurities, and the growth conditions and morphology were examined. The HfC whiskers preferentially grew at an H/Cl ratio of above 8, an HfCl₄ gas flow rate of 10–20 standard cm³ min^–1, a CH₄ flow rate of 10–20 standard cm³ min^–1, and at temperatures above 1050 °C. HfC whiskers, 60–170 m long, with a ball-like tip and periodically varying diameters, were obtained at 1250 °C using a cobalt impurity.     

Structure and oxidation resistance of plasma sprayed Ni–Si coatings on carbon steel

Ni–Si coatings consisting of mainly NiSi₂ and NiSi were deposited on a carbon steel by air plasma spraying. Isothermal oxidation tests of the carbon steel substrates with the Ni–Si coatings at 500–800 °C have been carried out. The result indicated that a protective SiO₂-based oxide scale was formed on the surface of the coatings after oxidation. On the other hand, during oxidation, phase transformation occurred among the NiSi₂, NiSi and Ni₂Si phases constructing the Ni–Si coatings. This was caused by the extraction of silicon from the silicides and the reformation of silicides at the silcide/Si-blocks interface. Above 700 °C, the outward diffusion of iron and carbon became very fast and consequently decarburization happened at the coating/substrates interface, which induced the formation of pores in the substrates near the interface. In addition, grain boundary oxidation of Cr in the steel substrate was observed above 700 °C.     

Evaluation of hot workability of particle reinforced aluminum matrix composites by using deformation efficiency

The high temperature deformation behavior of Al 6061 composites reinforced with SiC and Al₂O₃ particles has been studied in the temperature range of 300–550°C and the strain rate range of 0.1–3.0/sec by hot torsion test. The deformation efficiency , given by (2m/m + 1), where m is the strain rate sensitivity, is calculated as a function of temperature and strain rate to obtain iso-efficiency contour map. The composite reinforced with SiC particle exhibited a domain of dynamic recrystallization (DRX) with a peak efficiency of 40% at the temperature range of 450–500°C and strain rate range of 0.2–0.5/sec. On the other hand, the composite reinforced with Al₂O₃ particle showed the DRX domain at the temperature range of 450–480°C and strain rate range of 0.1–0.2/sec. The characteristics of these domain have been investigated with the help of microstructural observation and hot ductility measurements.     

Effect of pressure on mass transfer in heterogeneous chemical processes in a fluidized bed

A heterogeneous process in a fluidized-bed reactor with continuous feed and removal of material was investigated at pressures ranging from 0 to 196.1·10⁴ N/m². A general relationship connecting the rate of mass transfer between the gas flow and the particles in the bed in the Re range 15–274 with the pressure from 0 to 196.1·10⁴ N/m² is found.     

Kinetics of direct nitridation of pelletized silicon grains in a fluidized bed: experiment, mechanism and modelling

A fluidized-bed nitridation of pelletized silicon grains having a wide size distribution was carried out in the temperature range 1200–1300°C under conditions free of external heat and mass transfer effects. N2(30%–90%)–H2(5%–50%)–Ar (balance) mixtures were used as the nitriding gas at atmospheric pressure. Both the yield of -Si3N4 and the final overall conversion of silicon are affected by temperature and nitrogen gas concentration in a nitriding atmosphere, but hydrogen gas has a minor effect on either of these. After accounting for some of the structural changes that occur during nitridation, a simple model was derived. The model has shown that the pseudo-asymptotic exponential conversion trend in the second nitridation stage could be explained by various reaction mechanisms, adjusted for properties of the size distribution of silicon grains and the experimentally observed spalling of the product scale from the silicon surface. In the investigated range of experimental conditions, nitridation could be considered as having an apparent activation energy of Eapp340 kJ mol-1. © 1998 Chapman & Hall     

The nanostructured phase transition of superhard Ti-Si-C-N coatings

Ti-Si-C-N coatings with different Si contents were synthesized by means of pulsed direct current plasma-enhanced chemical vapor deposition using a TiCl(4), SiCl(4), CH(4), N(2), H(2) and Ar gas mixture. A complex phase transition has been identified which is strongly controlled by the silicon content in the coatings. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) results indicate that increasing Si content leads to a phase transition from a solid solution (Ti, Si)(C, N) to a dual-phase Ti?(C,N)+TiSi(2) structure, and then to a two-phase Ti?(C,N)+SiC structure, and that the changes in grain size and lattice parameter coincide with the phase transition. The Ti-Si-C-N coatings with high Si contents (≥4.3?at.%) possess a superhigh hardness (43-52?GPa) due to grain refinement/grain boundary hardening and compressive stress/dispersion hardening of the hard, nanosized crystalline TiSi(2) or SiC dispersed in the matrix.     

Mechanism of Ferrite Spinel Formation Revisited

Data on the volume changes of the starting reagents and reaction products were used to analyze the reactions taking place in the ferrite-forming systems MgO–Fe₂O₃, Mn_0.75Mg_0.25O–Fe₂O₃, and NiO–Fe₂O₃ in the temperature range 1255–1315°C. It was shown that, under these conditions, there is no oxygen transport through the gas phase. The possible formation of Fe²⁺ ions is attributed to partial electron compensation for the charge on the M²⁺ cations as a result of counterdiffusion. The presence of excess Fe in the spinel phase at the intermediate stages of ferrite formation is due to the transformation of -Fe₂O₃ into -Fe₂O₃ at a certain M²⁺ concentration.     

Crystallization and phase transformation behaviour of electroless nickel-phosphorus deposits with low and medium phosphorus contents under continuous heating

Electroless nickel-phosphorus deposits with 5–8 wt% P and 3–5 wt% P were analysed for the effects of continuous heating on the crystallization kinetics and phase transformation behaviour of the deposits. The as-deposited coatings consist of a mixture of amorphous and microcrystalline nickel phases, featuring in their X-ray diffraction patterns. Continuous heating processes to 300°C–800°C at 20°C/min were carried out on the deposits in a differential scanning calorimetric apparatus. The subsequent X-ray diffraction analyses show that the sequence of phase transformation process was: amorphous phase + microcrystalline nickel f.c.c. nickel + Ni₃P stable phases. Preferred orientation of nickel (200) plane developed in the deposits after the heating processes. Differential scanning calorimetry of the deposits indicates that the crystallization temperatures increased with decreasing phosphorus content, and increasing heating rate. Crystallization activation energies of the deposits (230 and 322 kJ/mol, respectively) were calculated using the peak temperatures of crystallization process, from the differential scanning calorimetric curves at the heating rates ranging from 5 to 50°C/min. It was found that the deposit with lower phosphorus content has higher activation energy.     

Nanostructure transition in Cr-C-N coatings deposited by pulsed closed field unbalanced magnetron sputtering

Cr-C-N coatings with different compositions, i.e. (C + N)/Cr atomic ratios (x) of 0.81-2.77, were deposited using pulsed closed field unbalanced magnetron sputtering by varying the chromium and graphite target powers, the pulse configuration and the ratio of the nitrogen flow rate to the total gas flow rate. Three kinds of nanostructures were identified in the Cr-C-N coatings dependent on the x values: a nano-columnar structure of hexagonal closed-packed (hcp) Cr₂(C,N) and face-centered cubic (fcc) Cr(C,N) at x = 0.81 and 1.03 respectively, a nanocomposite structure consisting of nanocrystalline Cr(C,N) embedded in an amorphous C(N) matrix at x = 1.26 and 1.78, and a Cr-containing amorphous C(N) structure at x = 2.77. A maximum hardness of 31.0 GPa and a high H/E ratio of 1.0 have been achieved in the nc-Cr(C,N)/a-C(N) nanocomposite structure at x = 1.26, whereas the coating with a Cr-containing amorphous C(N) structure had a minimum hardness of 10.9 GPa and a low H/E ratio of 0.08 at x = 2.77. The incorporation of carbon into the Cr-N coatings led to a phase transition from hcp-Cr₂(C,N) to fcc-Cr(C,N) by the dissolution into the nanocrystallites, and promoted the amorphization of Cr-C-N coatings with the precipitation of amorphous C(N). It was found that a high x value over 1.0 in the Cr-C-N coatings is the composition threshold to the nanostructure transition.     

Sol-gel processed barium titanate ceramics and thin films

Ferroelectric barium titanate (BaTiO3) ceramics and thin films have been prepared from barium acetate (Ba(CH3COO)2) and titanium (IV) isopropoxied (Ti((CH3)2CHO)4) precursors by a sol–gel technique. The as-grown powder and thin films were found to be amorphous, which crystallized to the tetragonal phase after annealing at 700°C in air for 1 h. Both the ceramics and thin films showed well-saturated polarization–field (P–E) hysteresis loops at room temperature. The value of the spontaneous polarization, PS, remnant polarization, Pr, and coercive field, Ec, of the ceramics and thin films determined from the P–E hysteresis loop were found to be 19.0 and 12.6; 14.0 and 3.2 G cm–2, and 30 and 53 kV cm–1, respectively. The coercive field of the film determined from the capacitance–voltage, C–V, characteristics is slightly lower than that determined from the P–E hysteresis loop (43 kV cm–1). The room-temperature dielectric constant, , of the ceramics and films was found to be 1135 and 370, respectively. Both the films and ceramics showed dielectric anomaly peaks at 125 °C, showing ferroelectric to paraelectric phase transition. © 1998 Kluwer Academic Publishers     

Optimization of PVD Parameters for the Deposition of Ultrahard Ti–Si–B–N Coatings

Multicomponent Ti–Si–B–N coatings were deposited on high-speed steel (HSS) substrates by reactive magnetron sputtering using a SHS TiB + 20 wt% Si target. The influences of the substrate temperature, bias voltage, and nitrogen partial pressure on the structure and the elemental compositions of the films were studied. The films were characterized by high-resolution transmission electron microscopy (HRTEM), Auger spectroscopy (AES), and X-ray diffraction (XRD). The results of HRTEM analysis indicated the formation of an ordered–disordered structure with fine crystalline grains of hexagonal Ti(B,N) _x phase and amorphous integrain layers. The stoichiometry of the Ti(B,N) _x phase was strongly affected by PVD process parameters. The films were characterized in terms of their microhardness and wear resistance. The reasons for the high value of microhardness appear to be the result of stoichiometric phase composition, compressive residual stress, and dense and fine microstructure of the Ti–Si–B–N coatings. The tribological wear test results indicated the superior wear-resistant properties of Ti–Si–B–N coatings compared to TiN and Ti(C,N) coatings.     

Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapor deposition at low temperature

Vertically-aligned carbon nanotubes(CNTs) with multi-walled structure were successfully grown on a Fe-deposited Si substrate at low temperature below 330°C by using the microwave plasma chemical vapor deposition of methane and carbon dioxide gas mixture. This is apparently different from the conventional reaction in gas mixtures of hydrogen and methane, hydrogen and acetylene, and hydrogen and benzene ... etc. High quality carbon nanotubes were grown at lower temperature with CO₂ and CH₄ gas mixture than those used by the previous. After deposition, the microstructure morphology of carbon nanotubes was observed with scanning electron microscope and high-resolution transmission electron microscope. The characteristics of carbon nanotubes were analyzed by laser Raman spectroscopy. The results showed the variation of the flow rate ratio of CH₄/CO₂ from 28.5 sccm/30 sccm to 30/30 sccm and the DC bias voltage from –150 V to –200 V, at 300 W microwave power, 1.3–2.0 kPa range of total gas pressure, and substrate temperatures between 300°C and 350°C. Vertically aligned carbon nanotubes with the diameter of about 15 nm and multi-walled structure were illustrated by SEM and HRTEM. However, the highest yield of carbon nanotubes of about 50% was obtained at low temperature below 330°C by MPCVD for the CH₄/CO₂ gas mixture with properly controlled parameters.     

Chemical vapour growth of HfP whiskers and their properties

Whiskers and ribbon-like single crystals of -HfP (hexagonal) have been prepared from HfCl₄+PCl₃+H₂+Ar gas mixtures at 1100–1200 °C using a metal impurity-activated chemical vapour deposition process. The growth conditions, morphology and chemical properties were examined. The 3.5–6.5 mm (average 4 mm) long HfP whiskers were obtained at 1200 °C using Si+Pt or Si+Pd mixed impurities. The HfP whiskers were very stable against oxidation up to 3 h exposure at 1000 °C and for 80 min immersion in concentrated HCl solution at 50 °C.     

Deposition rates of titanium nitride plates prepared by chemical vapour deposition of TiCl4+NH3 system

Titanium nitride plates (TiN_x,x = 0.74–1.0, about 2 mm thick maximum) were prepared by chemical vapour deposition (CVD) using TiCI₄, NH₃ and H₂ as source gases. The effects of CVD conditions, i.e. gas molar ratio (m _N/Ti = NH₃/TiCI₄) and deposition temperature (T_dep), on deposition rates and surface morphology were examined, and the deposition mechanism of the CVD-TiN_x plates was discussed. The relationship between m_N/Ti and deposition rates showed a maximum peak at certainm _N/Ti, and this maximum peak shifted to lowerm _N/Ti with increasing_{T dep}. The activation energy for the formation of CVD-TiN_x plates was about 80 kJ mol^–1 in the lower temperature range. The decomposition reaction of NH₃ gas could be associated with the rate-controlling step. At higher temperatures, the diffusion process may be the rate-controlling step, and a large amount of powder (mainly NH₄Cl) was formed in the gas phase. The highest deposition rate obtained in the present work was 1.06×10^–7 ms^–1 (0.38 mmh^–1) atT _dep = 1773 K andm _N/Ti = 0.87.     

In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM

Multilayer alumino-silicide and silicide coatings were formed by in situ chemical vapour co-deposition of Al and Si on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy for improving its high-temperature oxidation resistance. MoSi₂ and Mo (Si, Al)₂ layers were formed in the inner and the outer layers, respectively in the case of alumino-silicide coating. Whereas silicide coating consisted of Mo₅Si₃ and MoSi₂ phases in the inner and the outer layers, respectively. 24–100-μm thick coatings were formed by optimizing the pack mixture of Al and or Si, NH₄F and Al₂O₃ powders and conducting the experiments at 1000 °C for 8–36 h. MoSi₂ layer showed a faster growth rate and presence of columnar grains. A small weight gain at the initial stages was observed during the oxidation tests of the coated samples under continuous or cyclic heating at 1300 °C in air. Neither cracks nor peeling of the coating layers were noticed after oxidation tests.     

Effect of negatively charged species on the growth behavior of silicon films in hot wire chemical vapor deposition

The deposition behavior of silicon in hot wire chemical vapor deposition was investigated, focusing on the generation of negatively charged species in the gas phase using a gas mixture of 20% SiH₄ and 80% H₂ at a 450 °C substrate temperature under a working pressure of 66.7 Pa. A negative current of 6–21 µA/cm² was measured on the substrate at all processing conditions, and its absolute value increased with increasing wire temperature in the range of 1400 °C–1900 °C. The surface roughness of the films deposited on the silicon wafers increased with increasing wire temperature in the range of 1510 °C–1800 °C. The film growth rate on the positively biased substrates (+ 100 V, + 200 V) was higher than that on the neutral (0 V) and negatively biased substrates (− 100 V, − 200 V, − 300 V). These results indicate that the negatively charged species are generated in the gas phase and contribute to deposition. The surface roughness evolved during deposition was attributed to the electrostatic interaction between these negatively charged species and the negatively charged growing surface.