Comparison of the Microstructural Characteristics and Electrical Properties of Thermally Sprayed Al2O3 Coatings from Aqueous Suspensions and Feedstock Powders

In this work the microstructural characteristics and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) and conventional HVOF spray processes are presented. The electrical resistance at different relative air humidity (RH) levels (from 6 to 97% RH) and values of dielectric strength were investigated by direct current electrical resistance measurements, electrochemical impedance spectroscopy, and dielectric breakdown tests. Relationships between electrical properties and coating characteristics are discussed. At low humidity levels (up to 40% RH) the electrical resistivities of S-HVOF and HVOF coatings were on the same order of magnitude (10¹¹???·m). At a very high humidity level (97% RH) the electrical resistivity values for the S-HVOF coatings were in the range 10⁷-10¹¹???·m, up to five orders of magnitude higher than those recorded for the HVOF coating (orders of magnitude of 10⁶???·m). The better electrical resistance stability of the suspension-sprayed Al₂O₃ coatings can be explained by their specific microstructure and retention of a higher content of ??-Al₂O₃. The dielectric strength E _d of suspension-sprayed coatings was found to be 19.5-26.8?kV·mm^?1 for coating thicknesses ranging from 60 to 200???m. These values were slightly lower than those obtained for conventional HVOF coatings (up to 32?kV·mm^?1). However, it seemed that the dielectric strength of conventionally sprayed coatings was more sensitive to the coating thickness (when compared with the values of E _d determined for S-HVOF coatings) and varied to a greater extent (up to 10?kV·mm^?1) when the coating thickness varied in the range 100-200???m.     

Comparative Study of the Electrical Properties and Characteristics of Thermally Sprayed Alumina and Spinel Coatings

In this study, APS and HVOF processes have been used to prepare alumina (Al₂O₃) and magnesium spinel (MgAl₂O₄) coatings designed for insulating applications. The electrical characteristics, i.e., dielectric strength and electrical resistance (electrical resistivity) were investigated using different methods: dielectric breakdown test, direct current (DC) measurements, and electrochemical impedance spectroscopy (EIS). The electrical resistance was measured at room temperature at different relative humidity (RH) levels (from 6% RH to 95% RH) as well as at 200 °C. The coating microstructure, phase composition, and water vapor sorption were studied. Differences in the electrical insulating properties due to the different coating system characteristics are discussed. Of the coatings and conditions investigated in this study, the HVOF spinel coatings showed superior dielectric breakdown strength and electrical resistance stability at high humidity levels.     

Corrosion Performance of Laser Posttreated Cold Sprayed Titanium Coatings

The recent development of cold spray technology has made possible the deposition of highly reactive, oxygen sensitive materials, such as titanium, without significant chemical reaction of the powder, modification of particle microstructure and with minimal heating of the substrate. However, the presence of interconnected pathways (microscale porosity) within the deposit limits the performance of the metallic coating as an effective barrier to corrosion and substrate attack by corrosive media is usually inevitable. The aim of the present study was to investigate the effects of processing, including a postspray laser treatment, on the deposit microstructure and corrosion behavior. Commercially pure titanium (CP Ti) was deposited onto a carbon steel substrate, using a commercial cold spray system (CGT^TM Kinetiks^? 4000) with preheated nitrogen as both the main process gas and the powder carrier gas. Selected coatings were given a surface melting treatment using a commercial 2 kW CO₂ laser (505 Trumpf DMD). The effect of postdeposition laser treatment on corrosion behavior was analyzed in terms of pore structure evolution and microstructural changes. Optical microscopy, scanning electron microscopy, and x-ray diffraction were employed to examine the microstructural characteristics of the coatings. Their corrosion performance was investigated using electrochemical methods in 3.5 wt.% NaCl (ASTM G5-94 (2004)). As-sprayed titanium coatings could not provide favorable protection to the carbon steel substrate in the aerated NaCl solution, whereas the coatings with laser-treated surfaces provided barrier-like properties.     

Effect of Microstructure on the Electrical Properties of Nano-Structured TiN Coatings Deposited by Vacuum Cold Spray

TiN coatings on Al₂O₃ substrates were fabricated by vacuum cold spray (VCS) process using ultrafine starting ceramic powders of 20 nm in size at room temperature (RT). Microstructure analysis of the samples was carried out by scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. Sheet resistance of the VCS TiN coatings was measured with a four-point probe. The effects of microstructure on the electrical properties of the coatings were investigated. It was found that the sheet resistance and electrical resistivity of TiN coatings were significantly associating with the spray distance, nozzle traversal speed, and deposition chamber pressure. A minimum sheet resistance of 127 Ω was achieved. The microstructural changes can be correlated to the electrical resistivity of TiN coatings.     

High Pressure Cold Sprayed (HPCS) and Low Pressure Cold Sprayed (LPCS) Coatings Prepared from OFHC Cu Feedstock: Overview from Powder Characteristics to Coating Properties

Cold spraying enables high quality Cu coatings to be deposited for applications where high electrical and/or thermal conductivity is needed. Fully dense Cu coatings can provide an effective corrosion barrier in specific environments. The structure of cold-sprayed Cu coatings is characterized by high deformation which imparts excellent properties. Coating properties depend on powder, the cold spray process and post treatments. First of all, powder characteristics have a strong influence on the formation of pure coatings. Secondly, cold spraying provides dense, adherent, and conductive coatings by using HPCS and LPCS. Furthermore, an addition of Al₂O₃ particles to the Cu powder in LPCS process significantly improves coating properties. Also, heat treatments improve electrical conductivity. This study summarizes optimal characteristics of Cu powder optimized for cold spraying, achieving high coating quality and compares properties of HPCS Cu, LPCS Cu and Cu+Al₂O₃ coatings prepared from the same batch of OFHC Cu powder.     

Sinterability and microwave dielectric properties of nano structured 0.95MgTiO3-0.05CaTiO3 synthesised by top down and bottom up approaches

0.95MgTiO₃-0.05CaTiO₃ (MCT) nano powders were synthesised using sol-gel method and high energy ball milling (HEBM). Synthesised powders were characterised using X-ray diffraction analysis to ensure phase purity and HRTEM to determine the fine microstructural features like particle size, interplanar spacing, etc. The powder pellets were heat treated to study the sinterability and microwave dielectric properties and these properties were then compared with the microwave dielectric properties of micron sized sample. Nano powder synthesised using HEBM shows better dielectric properties, sinterability and gets densified to 90% of theoretical density (TD) at 1200 °C/2 h. Dielectric resonators prepared using chemically synthesised nano powder showed poor sinterability and microwave dielectric properties, but, dielectric properties of HEBM samples were very near to that of solid state synthesised samples. Sintered HEBM powders retain the microwave dielectric properties almost to the same level as the solid state synthesised powder with considerable lowering of sintering temperature.     

High temperature wear resistance and thermal fatigue behavior of Stellite-6/WC coatings produced by laser cladding with Co-coated WC powder

The Stellite-6/WC composite coatings were produced on AISI H13 hot work tool steel by laser cladding with mixture of Co-coated WC (WC-12Co) particles and Stellite-6 powder. The phase composition, microstructural characterization, high temperature wear resistance and thermal fatigue behavior of Stellite-6/WC coatings were investigated and compared with the properties of the coatings produced from mixture of WC particles and Stellite-6 powder. The results showed that using the WC-12Co particles alleviated the decomposition of WC and resulted in the weaker intensity of W₂C, CoC_x and Co₆W₆C peaks in the X-Ray Diffraction (XRD) patterns. Compared with using the WC particles directly as the coating material, using the WC-12Co particles could further improve the wear resistance of coatings according to the relative lower width and depth of wear scars at the same WC content. In addition, fewer fatigue cracks were observed on the surface of coatings made by adding WC-12Co particles under the same thermal fatigue conditions, which indicates that using WC-12Co is beneficial to extend the life of Stellite-6/WC coatings.     

Characterization and processing of spray-dried zirconia powders for plasma spray application

The correlation between the performance of plasma spray coatings and feedstock powder properties is not fully understood. To demonstrate this correlation, eight spray-dried zirconia powders containing a mass fraction of 20% Y₂O₃ (yttria) were characterized with respect to their physical, bulk chemical, and surface chemical properties. The same powders were plasma spray deposited as coatings, and their relative performance was evaluated using a thermal rupture test developed by Pratt and Whitney. The specific powder properties studied were chemical composition, binder content, particle size distribution, powder morphology, interface chemistry, thermogravimetry, phase composition, and specific surface area. Among the characterization data, the binder-related properties of the powder correlated most strongly with the thermal rupture test data. Specifically, higher binder contents were associated with poor thermal rupture test performance.     

Investigation and characterization of Cr3C2-based wear-resistant coatings applied by the cold spray process

The purpose of this study was to explore the potential of the cold spray (CS) process in applying Cr₃C₂-25wt.%NiCr and Cr₃C₂-25wt%Ni coatings on 4140 alloy for wear-resistant applications. This article discusses the improvements in Cr₃C₂-based coating properties and microstructure through changes in nozzle design, powder characteristics stand off distance, powder feed rate, and traverse speed that resulted in an improved average Vickers hardness number comparable to some thermal spray processes. Cold spray process optimization of the Cr₃C₂-based coatings resulted in increased hardness and improved wear characteristics with lower friction coefficients. The improvement in hardness is directly associated with higher particle velocities and increased densities of the Cr₃C₂-based coatings deposited on 4140 alloy at ambient temperature. Selective coatings were evaluated using x-ray diffraction for phase analysis, optical microscopy (OM). and scanning electron microscopy (SEM) for microstructural evaluation, and ball-on-disk tribology experiments for friction coefficient and wear determination. The presented results strongly suggest that cold, spray is a versatile coating technique capable of tailoring the hardness of Cr₃C₂-based wear-resistant coatings on temperature sensitive substrates.     

The effects of microstructural features on the performance gap in corrosion resistance between bulk and HVOF sprayed Inconel 625

It is commonly observed that there is a performance gap between the corrosion resistance of thermally sprayed coatings and the equivalent bulk material. This is attributed to the significantly modified microstructure of the sprayed coatings. However, currently there is no detailed understanding of which aspects of microstructural modification are primarily responsible for this performance gap. In this work several deliberately microstructurally modified versions of the Ni-based superalloy Inconel 625 were produced. These were subjected to potentiodynamic electrochemical testing in 0.5 M H₂SO₄ to investigate the links between specific microstructural features and electrochemical behaviour. Samples were prepared by high-velocity oxy-fuel (HVOF) thermal spraying, laser surface remelting using a high power diode laser and conventional powder sintering. Microstructural features were examined by optical and scanning electron microscopy and X-ray diffraction. Potentiodynamic testing was carried out on the following forms of Inconel 625: wrought sheet; HVOF sprayed coatings; sintered powder compacts; laser melted wrought sheet and HVOF sprayed coatings. Using the corrosion behaviour, i.e. passive current density, of the wrought sheet as a baseline, the performance of different forms of Inconel 625 was compared. It is found that a fine dendritic structure (with associated microsegregation) produced by laser remelting wrought sheet has no significant effect on corrosion performance. Up to 12% porosity in sintered powder samples increases the passive current density by a factor of only around 2. As observed previously, the passive current density of HVOF sprayed coatings is 20-40 times greater. However, HVOF coatings subjected to laser surface remelting are found to have a passive current density close to that of wrought material. It is concluded that, whilst porosity in coatings produces some decrease in corrosion resistance, the main contributing factor is the galvanic corrosion of localised Cr-depleted regions which are associated with oxide inclusions within HVOF sprayed samples.     

Wire-array-applied ultrafine tungsten wire coated with Al–Mg alloy layer via electron beam evaporation

Al–Mg alloy coatings were deposited onto superfine tungsten wire substrates via electron beam deposition at electronic beam currents ranging from 80 to 120?mA. The effects of electronic beam currents and baking process on the surface characteristics of the aluminium–magnesium alloy coatings were investigated using scanning electron microscopy. Energy-dispersive X-ray was also used to investigate the composition of the coatings according to the chemical components of the source materials. X-ray diffraction results suggested that the Al–Mg alloy coatings consisted of an Al₁₂Mg₁₇ intermetallic compound and pure aluminium phase. Electrochemical measurements determined the corrosion protection performance of the aluminium–magnesium alloy coatings with different magnesium contents. Specimen tensile properties were related to electron beams and surface roughness. The anti-corrosion performance of the coatings was increased with magnesium content.     

Microstructure and electrical properties of sol–gel derived Ni-doped CaCu3Ti4O12 ceramics

Dielectric properties and varistor performance of sol–gel prepared Ni-doped calcium copper titanate ceramics (CaCu₃Ni_xTi₄O_12+x, x=0, 0.1, 0.2, 0.3) were investigated. SEM and XRD were used in the microstructural studies of the specimens and the electrical properties were investigated for varistors. XRD patterns show that the CCTO ceramics were single phase with no Cu-rich phase. SEM results indicated that the samples had smaller grain sizes than those synthesized by traditional solid-state reaction methods. The experimental results show that the highest dielectric constant and lower dielectric loss occur when x=0.2. When x=0.3, the lowest leakage current is obtained and the maximum value reaches 0.295; meanwhile, the lowest threshold voltage and nonlinear coefficient are found, the minimum values of them are 1326 V/mm and 3.1, respectively.     

Study of the temperature and organic bindings effects in the dielectric and structural properties of the lithium ferrite ceramic matrix (LiFe5O8)

In this study, the effects of the calcination temperature and of the organic bindings in the structural and dielectric properties of lithium ferrite (LiFe₅O₈) were investigated. The organic bindings used were glycerol, PVA (polyvinyl alcohol) and Galactomannan (Adenanthera pavonina). The investigated calcination temperature range was from 773 K to 1073 K. The structural properties were analyzed using differential scanning calorimetry, thermogravimetry, X-ray diffraction and infra-red spectroscopy. The electrical and magnetical properties were investigated using impedance dielectric spectroscopy and Mössbauer spectroscopy. The study of the structural and electrical properties of the lithium ferrites is an important issue in view to their attractive technological properties and low cost of fabrication. This work shows that the binding type, affects significantly the dielectric constant and loss of the LiFe₅O₈ ceramics.     

High dielectric constant of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 multilayer ceramic capacitors with Cu doped Ni electrodes

A nickel paste with Cu dopant was used as the internal electrodes in multi-layer ceramic capacitors (MLCCs) using barium titanate (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ ceramic (BCTZ) with copper endtermination. The sintering behaviors of the MLCC and the interfacial structure between the Ni/Cu electrodes and the BCTZ dielectrics have been investigated. The thermal shrinkage and sheet resistance of the Ni/Cu alloys sintered at 1220 °C for 2 h in a reducing atmosphere were measured using thermal analysis techniques (TMA) and four-point probe equipment. The composition distributions, microstructures and line defects were examined using microstructural analysis techniques (SEM/HRTEM) coupled with energy-dispersive spectroscopy (EDS). The Cu alloyed with Ni significantly improves the continuity of the electrode in the MLCC; this is due to there being no mutual trigger reaction between Ni and BCTZ dielectrics. In terms of the electrical properties, the results showed that Ni paste with Cu dopant improves the dielectric constant of the MLCC, but the dielectric loss (tan δ) is a slightly when higher compared to standard MLCC.     

Formation of Pore Structure and Its Influence on the Mass Transport Property of Vacuum Cold Sprayed TiO2 Coatings Using Strengthened Nanostructured Powder

The pore structure in nano-porous TiO₂ coatings influences the ion diffusion property and the photovoltaic performance of dye-sensitized solar cells. In this paper, TiO₂ coatings were deposited by vacuum cold spray (VCS) using a strengthened nanostructured powder. The pore structure, ion diffusion, and dye infiltration properties were examined to understand the coating deposition mechanism. Results showed that the pores in the VCS TiO₂ coatings presented a bimodal size distribution with two peaks at ~15 and ~50?nm. Based on the impact behavior of spray powder particles, a deposition model was proposed to explain the formation mechanism of the pores in the VCS coating using strengthened nanostructured powder. It was found that, compared to the conventional unimodal-sized nano-pores in TiO₂ coatings, the bimodal-sized nano-pores contributed to a higher ion diffusion coefficient of the coatings and thereby a higher photovoltage of the solar cells.     

Investigation of photoinduced change of dielectric and electrical properties of indium (III) phthalocyanine and fullerene doped nematic liquid crystal

In this work, we have studied photoinduced dielectric and electrical properties of E63 nematic liquid crystal (E63-LC) materials doped with indium (III) metallo phthalocyanine (In-MPc) and fullerene (C₆₀) in dark and under UV illumination. Doping concentration of In-MPc in E63-LC was chosen to be 5 wt%, whereas a trace amount of C₆₀ was used in investigated samples. Real and imaginary parts of dielectric permittivity were investigated in the frequency range of 100 Hz–10 MHz by using dielectric spectroscopy technique (DST). It was observed that doping agents and UV illumination enhanced the real part of dielectric permittivity at low frequencies, which was thought to originate from photoinduced charge transfer between In-MPc and C₆₀ caused by extra dipole strength in LC medium. In addition to that, critical frequency (f_c) and relaxation time (τ) were obtained and analyzed for all investigated samples. Photoelectrical characteristics of hybrid LC structures were carried out by current–voltage measurements. A major increase in electrical conductivity was observed in In-MPc and C₆₀ doped LC structures under UV irradiation.     

激光微纳烧结处理对玻璃陶瓷涂层组织及腐蚀性能的影响

采用高温烧结、球磨破碎的方法制备出了SiO2玻璃与Cr2O3陶瓷包覆型玻璃陶瓷复合粉末.利用常规氧—乙炔火焰喷涂技术在45钢基材表面制备出了玻璃陶瓷保护涂层,并使用激光微纳烧结技术对热喷涂层进行二次处理.研究了激光微纳烧结对玻璃陶瓷涂层组织与性能的影响.结果表明,采用激光微纳烧结对玻璃陶瓷涂层进行二次处理,可提高涂层结构的致密性,使组织均匀化,减少涂层中的微孔和微裂纹;明显提高界面的结合强度和涂层的疏水性能与耐蚀性能.因此激光微纳烧结二次处理技术可以显著提高玻璃陶瓷涂层的综合性能.     

Electromagnetic wave absorbing properties of Fe<Subscript>73</Subscript>Si<Subscript>16</Subscript>B<Subscript>7</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> base P/M sheets mixed with dielectric fine particles

Fe-based nanocrystalline powder sheets with dielectric TiO₂ powder additives were investigated to improve the characteristics of electromagnetic (EM) wave absorption. The amorphous ribbons of Fe₇₃Si₁₆B₇Nb₃Cu₁ (at.%) alloys were prepared by a planar flow casting (PFC) process, and the ribbons were pulverized using an attrition mill. Fe-based flake powder crystallized at 550°C for 1h was mixed with a nano-sized and a micro-sized TiO₂ powder. The powder mixtures were then tape-cast with binders to become EM wave-absorbing sheets. The absorbing properties of the fabricated sheet sample, such as complex permittivity and permeability, were measured by a network analyzer. The properties of EM wave absorption improved with the increase of TiO₂ powder in the mixture. The mixture with micro-sized TiO₂ powder was a little more effective in causing power loss of EM waves than the mixture with nano-sized TiO₂ powder.     

Properties of Reactive Plasma Sprayed TiB2-TiC0.3N0.7 Based Composite Coatings with Cr Addition and Laser Surface Treatment

Thick TiB₂-TiC_0.3N_0.7 based composite coatings were deposited by reactive plasma spraying (RPS) successfully in air. The influence to the coating properties (morphology, Vickers microhardness and corrosion resistant property) with Cr addition in the thermal spray powder and TiB₂-TiC_0.3N_0.7 based coatings treated by laser were investigated. The phase composition, structure and properties of composite coatings were studied using XRD, SEM, EDS, Vickers microhardness and electrochemical testers. The results show that the Vickers microhardness values and the density of laser surface treated coatings are improved significantly. The Cr addition in the thermal spray powder can increase the density, improve the wettability of ceramic phases, uniform the phase distribution and enhance the corrosion-resistant property of coatings. However, due to lower microhardness of metal Cr than ceramic phases in coatings, the Vickers microhardness values of plasma sprayed coatings and plasma sprayed coatings with laser surface treatment are a little lower than that of each coating without Cr addition in the thermal spray powder.     

Corrosion protection of steel by epoxy nanocomposite coatings containing various combinations of clay and nanoparticulate zirconia

Epoxy-based nanocomposite coatings containing various amounts of nano-clay and aminopropyltrimethoxy silane (APS) treated zirconia nanoparticles were prepared via slurry method. Morphology and dispersion of nanoparticles within the nanocomposites were evaluated using XRD and TEM analyses. Corrosion performance of mild steel coated specimens was investigated using EIS and EN techniques. The results showed that the simultaneous addition of the spherical ZrO₂ and layered clay nanoparticles promotes the exfoliation of the clay nanoparticles and in so doing improves the corrosion performance of nanocomposite coatings via enhancing the barrier properties and ohmic resistance.