Ni-Co-TiO2 nanocomposite coating prepared by pulse and pulse reversal methods using acetate bath

Ni-Co/nano TiO₂ (Ni-Co-TiO₂) composite coatings were prepared under pulse current and pulse reverse current methods using acetate bath. The microstructure and corrosion resistance of the coatings were characterized by means of XRD, SEM and EIS. Both the Ni-Co alloy and composite coatings exhibited single phase of Ni matrix with face centered cubic (fcc) crystal structure. The crystal orientation of the Ni-Co-TiO₂ composite coating was transformed from crystal face (2 0 0) to (1 1 1) compared with Ni-Co alloy coatings. The results showed that the microstructure and performances of the coatings were greatly affected by TiO₂ content on the deposits prepared by PC and PRC methods. The microhardness and corrosion resistance were enhanced in the optimum percentage of TiO₂ composite coatings. The PRC composite coatings were exhibited from compact surface, higher microhardness and good corrosion resistance compared with that of the PC composite coating.     

Preparation and wear resistance of pulse electrodeposited Ni-W/Al2O3 composite coatings

The aim of this work is to obtain the electroplating parameters for preparation of Ni-W/Al₂O₃ composite coating with high tungsten content, high micro-hardness and excellent wear resistance by pulse plating procedure. Our results showed that the duty cycle is a dominant parameter for the tungsten content in the coating and the tungsten content increases significantly with increasing duty cycle. The further analysis showed the great influence of tungsten content on micro-hardness of the coating. A maximum micro-hardness of about 859 Hv was obtained in pulse electrodeposited Ni-W/Al₂O₃ composite with tungsten content of 40 wt.% at a peak current density of 20 A/dm², a duty cycle of 80%, a pulse frequency of 1000 Hz and a particle loading of 10 g/L alumina in the plating bath. Although the hardness of Ni-W/Al₂O₃ composite coating was only slightly affected by the alumina content of the deposits prepared in present investigation, the alumina content effect on the tribological characteristic of Ni-W/Al₂O₃ composite coatings is significant. The friction coefficient was lowered to 0.25 and the wear loss was reduced to 1.05 mg by setting the control factors according to the values mentioned above for obtaining the coating with the highest micro-hardness.     

Microstructure evolution and electromagnetic properties improvement of Al18B4O33w/Co composite powders through heat-treatment

Al₁₈B₄O₃₃w/Co composite powders were prepared through electroless depositing Co on Al₁₈B₄O₃₃ whiskers and the microstructure of the prepared composite powders was adjusted through heat-treatment. The included Co oxide is reduced and the density as well as the crystal perfection of the coatings is improved when annealed at 400 °C in H₂ atmosphere. An increase of 10⁵ S m⁻¹ in conductivity together with an increase of 28% in M_s is obtained, resulting in a prominent increase of the permittivity and the permeability. The increase of permittivity, specifically the dielectric relaxation is attributed to the increase of conductivity. The increase of permeability is attributed to the increase of M_s and the microstructure evolution. The increase of electromagnetic parameters in 2-18 GHz band is believed to enhance the electromagnetic wave absorbing performance of the Al₁₈B₄O₃₃w/Co composite powders.     

Microstructure of laser melted Al18B4O33w/2024Al composite

The microstructure of a laser treated Al₁₈B₄O₃₃w/2024Al composite has been investigated using transmission electron microscope (TEM), low-angle (glancing angle) X-ray diffraction (XRD) techniques. Various surface microstructures were observed in the laser treated composite. The Al₁₈B₄O₃₃ whisker on the surface of the composite was decomposed during laser surface melting, various decomposition products were studied in the laser treated composite. Eutectic phases and the precipitation in the matrix of the composite with laser-treated were observed. The main phases detected in the molten zone were aluminum and decomposition products Al₂O₃. The effect of laser treatment on the hardness of the composite was also examined. A surface hardness of 400 Hv was noted.     

Electrodeposition and characterization of Ni-Y2O3 composite

Nano-sized Y₂O₃ particles were codeposited with nickel by electrolytic plating from a nickel sulfate bath. The effects of the incorporated Y₂O₃ on the structure, morphology and mechanical properties (including microhardness, friction coefficient and wear resistant) of Ni-Y₂O₃ composite coatings were studied. It is observed that the addition of nano-sized Y₂O₃ particles shows apparent influence on the reduction potential and pH of the electrolyte. The incorporated Y₂O₃ increases from 1.56 wt.% to 4.4 wt.% by increasing the Y₂O₃ concentration in the plating bath from 20 to 80 g/l. XRD results reveal that the incorporated Y₂O₃ particles favour the crystal faces (2 0 0) and (2 2 0). SEM and AFM images demonstrate that the addition of Y₂O₃ particles causes a smooth and compact surface. The present study also shows that the codeposited Y₂O₃ particles in deposits decrease the friction coefficient and simultaneously reduce the wear weight loss. Ni-Y₂O₃ composite coatings reach their best microhardness and tribological properties at Y₂O₃ content 4.4 wt.% under the experiment conditions.     

Plasma sprayed Al2O3/FeCrAl composite coatings for electromagnetic wave absorption application

Al₂O₃/FeCrAl composite coatings were fabricated by atmosphere plasma spraying technique. Microstructure and dielectric properties in the frequency range from 8.2 to 12.4 GHz were investigated. The microstructure of composite coatings shows a uniform dispersion of metal particles with litter pores and microcracks in the composite coatings. The relaxation polarization and interfacial polarization in the coatings would contribute to enhance ?′ with rising FeCrAl content, and the associated loss could be considered as a dominating factor enhancing ?″. By calculating the microwave-absorption as a single-layer absorber, for the composite coatings with 41 wt.% FeCrAl content, the reflection loss values exceeding −10 dB are achieved in the frequency range of 9.1-10.6 GHz when the coating thickness is 1.3 mm.     

Correlation between Al2O3 particles and interface of Al-Al2O3 coatings by cold spray

Al-Al₂O₃ composite coatings with different Al₂O₃ particle shapes were prepared on Si and Al substrate by cold spray. The powder compositions of metal (Al) and ceramic (Al₂O₃) having different sizes and agglomerations were varied into ratios of 10:1 wt% and 1:1 wt%. Al₂O₃ particles were successfully incorporated into the soft metal matrix of Al. It was found that crater formation between the coatings and substrate, which is typical characteristic signature of cold spray could be affected by initial starting Al₂O₃ particles. In addition, when the large hard particles of fused Al₂O₃ were employed, the deep and big craters were generated at the interface between coatings and hard substrates. In the case of pure soft metal coating such as Al on hard substrate, it is very hard to get proper adhesion due to lack of crater formation. Therefore, the composite coating would have certain advantages.     

Microstructure and mechanical properties of Al2O3-Al composite coatings deposited by plasma spraying

Al₂O₃ and Al₂O₃-Al composite coatings were prepared by plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction (XRD), while optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphology of impacted droplets, polished and fractured surface, and the element distribution in terms of wavelength-dispersive spectrometer (WDS). Mechanical properties including microhardness, adhesion and bending strength, fracture toughness and sliding wear rate were evaluated. The results indicated that the addition of Al into Al₂O₃ was beneficial to decrease the splashing of impinging droplets and to increase the deposition efficiency. The Al₂O₃-Al composite coating exhibited homogeneously dispersed pores and the co-sprayed Al particles were considered to be distributed in the splat boundary. Compared with Al₂O₃ coating, the composite coating showed slightly lower hardness, whereas the coexistence of metal Al phase and Al₂O₃ ceramic phase effectively improved the toughness, strength and wear resistance of coatings.     

Preparation and characterization of nickel nano-Al2O3 composite coatings by sediment co-deposition

Ni-Al₂O₃ composite coatings were prepared by using sediment co-deposition (SCD) technique and conventional electroplating (CEP) technique from Watt's type electrolyte without any additives. The microstructure, hardness, and wear resistance of resulting composites were investigated. The results show that the incorporation of nano-Al₂O₃ particles changes the surface morphology of nickel matrix. The preferential orientation is modified from (2 0 0) plane to (1 1 1) plane. The microhardness of Ni-Al₂O₃ composite coatings in the SCD technique are higher than that of the CEP technique and pure Ni coating and increase with the increasing of the nano-Al₂O₃ particles concentration in plating solution. The wear rate of the Ni-Al₂O₃ composite coating fabricated via SCD technique with 10 g/l nano-Al₂O₃ particles in plating bath is approximately one order of magnitude lower than that of pure Ni coating. Wear resistance for SCD obtained composite coatings is superior to that obtained by the CEP technique. The wear mechanism of pure Ni and nickel nano-Al₂O₃ composite coatings are adhesive wear and abrasive wear, respectively.     

Microstructure and properties of TiB2-containing coatings prepared by arc spraying

Low cost arc spraying and cored wires were used to deposit composite coatings consisting of TiB₂ and TiB₂/Al₂O₃ hard particles in a Ni(Cr) and stainless steel 304L matrix. Four coatings were prepared namely Ni(Cr)-TiB₂, Ni(Cr)-TiB₂/Al₂O₃, 304L-TiB₂ and 304L-TiB₂/Al₂O₃. The microstructural characteristics of powders and coatings were observed by scanning electron microscopy (SEM). Phase compositions of powders were analyzed by X-ray diffraction (XRD). Although all the analyzed coatings exhibited similar lamella structure, remarkable differences not only in the morphology of hard phase and matrix but also in the size and distribution of hard phases were observed from one coating to another. Tribological behavior of the coatings was analyzed in room temperature dry sliding wear tests (block-on-ring configuration), under 75 N at low velocity (0.5 m/s). The coatings showed far high wear resistance than low carbon steel substrate under same conditions examined. Wear loss of 304L-TiB₂ and Ni(Cr)-TiB₂ coatings were lower nearly 15 times than that of steel substrate. TiB₂ hard phases in coatings bonded well with metal matrix contributed to high wear resistance.     

Investigation of the characteristics and corrosion resistance of Al2O3/TiN coatings

Al₂O₃ /TiN double and Al₂O₃/Cr/TiN triple coatings were produced on stainless steel substrates using plasma-detonation techniques. Investigation of the microstructure and characteristics of the coatings after the preparation was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). The corrosion resistance of the coatings was studied in several electrolytic solutions (0.5 M H₂SO₄, 1 M HCl, 0.75 M NaCl) using electrochemical techniques (open circuit potential, cyclovoltammetry and potentiodynamic polarization). The obtained results showed, in most of the cases, an improvement of the corrosion resistance, except in NaCl solutions. The effect of the controlled thickness of TiN and Cr layers as well as the additional treatment with a high-current electron beam was also investigated. Nuclear reaction analysis (NRA), Rutherford backscattering spectroscopy (RBS) and scanning electron microscopy (SEM) were applied for the characterization of the samples before and after the corrosion experiments.     

Preparation and properties of electroless Ni-P-SiO2 composite coatings

Dispersible SiO₂ nanoparticles were co-deposited with electroless Ni-P coating onto AISI-1045 steel substrates in the absence of any surfactants in plating bath. The resulting Ni-P/nano-SiO₂ composite coatings were heat-treated for 1 h at 200 °C, 400 °C, and 600 °C, respectively. The hardness and wear resistance of the heat-treated composite coatings were measured. Moreover, the structural changes of the composite coatings before and after heat treatment were investigated by means of X-ray diffraction (XRD), while their elemental composition and morphology were analyzed using an energy dispersive spectrometer (EDS) and a scanning electron microscope (SEM). Results show that co-deposited SiO₂ particles contributed to increase the hardness and wear resistance of electroless Ni-P coating, and the composite coating heat-treated at about 400 °C had the maximum hardness and wear resistance.     

Interfacial reactions and oxidation behavior of Al2O3 and Al2O3/Al coatings on an orthorhombic Ti2AlNb alloy

The uniform and dense Al₂O₃ and Al₂O₃/Al coatings were deposited on an orthorhombic Ti₂AlNb alloy by filtered arc ion plating. The interfacial reactions of the Al₂O₃/Ti₂AlNb and Al₂O₃/Al/Ti₂AlNb specimens after vacuum annealing at 750 °C were studied. In the Al₂O₃/Ti₂AlNb specimens, the Al₂O₃ coating decomposed significantly due to reaction between the Al₂O₃ coating and the O-Ti₂AlNb substrate. In the Al₂O₃/Al/Ti₂AlNb specimens, a γ-TiAl layer and an Nb-rich zone came into being by interdiffusion between the Al layer and the O-Ti₂AlNb substrate. The γ-TiAl layer is chemically compatible with Al₂O₃, with no decomposition of Al₂O₃ being detected. No internal oxidation or oxygen and nitrogen dissolution zone was observed in the O-Ti₂AlNb alloy. The Al₂O₃/Al/Ti₂AlNb specimens exhibited excellent oxidation resistance at 750 °C.     

Microstructure and mechanical properties of alumina coatings prepared by double glow plasma technique

Low-temperature growth (600 °C) of α-Al₂O₃ coatings on the stainless steel substrate by double glow plasma technique was achieved. The compositions and microstructures of the coatings prepared at different oxygen flow rates were characterized, respectively, by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectrometry. A phenomenological mechanism for the formation of the Al₂O₃ ceramic coatings during the oxidation process was proposed on the basis of the experimental results. It was obvious that the oxygen flow rates had a great effect on the surface structure of the prepared Al₂O₃ coatings. The dense and smooth Al₂O₃ coatings were prepared at the oxygen flow rate of 15 sccm. In addition, the correlations between the mechanical properties of Al₂O₃ coating and oxygen flow rates were also discussed. The coating prepared at 15 sccm oxygen flow rate exhibited the best mechanical properties with a maximum hardness of 31 GPa and elastic modulus of 321 GPa. The corresponding critical load of scratch adherence for this sample was 47 N.     

Effect of TiO2/Al2O3 film coated diamond abrasive particles by sol-gel technique

The diamond abrasive particles were coated with the TiO₂/Al₂O₃ film by the sol-gel technique. Compared with the uncoated diamonds, the TiO₂/Al₂O₃ film was excellent material for the protection of the diamonds. The results showed that the incipient oxidation temperature of the TiO₂/Al₂O₃ film coated diamonds in air atmosphere was 775 °C, which was higher 175 °C than that of the uncoated diamonds. And the coated diamonds also had better the diamond's single particle compressive strength and the impact toughness than that of uncoated diamonds after sintering at 750 °C. For the vitrified bond grinding wheels, replacing the uncoated diamonds with the TiO₂/Al₂O₃ film coated diamonds, the volume expansion of the grinding wheels decreased from 6.2% to 3.4%, the porosity decreased from 35.7% to 25.7%, the hardness increased from 61.2H_RC to 66.5H_RC and the grinding ratio of the vitrified bond grinding wheels to carbide alloy (YG8) increased from 11.5 to 19.1.     

Development of electroless Ni-Zn-P/nano-TiO2 composite coatings and their properties

Ni-Zn-P-TiO₂ composite coatings were successfully obtained on low carbon steel by electroless plating technique. Deposits were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive analysis (EDS) studies. The hardness and microstructure of as plated and heat treated Ni-Zn-P and Ni-Zn-P-TiO₂ composite coatings were analyzed. The change in microstructure and higher hardness was noticed for heat treated composite. The corrosion resistance behavior of as plated and heat treated Ni-Zn-P and Ni-Zn-P-TiO₂ coatings was investigated by anodic polarization, Tafel plots and electrochemical impedance spectroscopic (EIS) studies in 3.5 wt% NaCl solution. The composite coating exhibited enhanced corrosion resistance property over Ni-Zn-P coating.     

The electrical properties of dielectric stacks of SiO2 and Al2O3 prepared by atomic layer deposition method

We produced dielectric stacks composed of ALD SiO₂ and ALD Al₂O₃, such as SiO₂/Al₂O₃, Al₂O₃/SiO₂, and SiO₂/Al₂O₃/SiO₂, and measured the leakage currents through the stacks in comparison with those of the single oxide layers. SiO₂/Al₂O₃ shows lowest leakage current for negative bias region below 6.4 V, and Al₂O₃/SiO₂ showed highest current under negative biases below 4.5 V. Two distinct electron conduction regimes are observed for Al₂O₃ and SiO₂/Al₂O₃. Poole-Frenkel emission is dominant at the high-voltage regime for both dielectrics, whereas the direct tunneling through the dielectric is dominant at the low-voltage regime. The calculated transition voltage between two regimes for SiO₂ (6.5 nm)/Al₂O₃ (12.6 nm) is −6.4 V, which agrees well with the experimental observation (−6.1 V). For the same EOT of entire dielectric stack, the transition voltage between two regimes decreases with thinner SiO₂ layer.     

Microstructures and tribological properties of plasma sprayed WC-Co-Cu-BaF2/CaF2 self-lubricating wear resistant coatings

A promising WC-Co-Cu-BaF₂/CaF₂ self-lubricating wear resistant coating was deposited via atmospheric plasma spraying (APS) process by using homemade feedstock powders composed of WC-Co, Cu and BaF₂/CaF₂ eutectic. The as-prepared cermet coatings had better frictional behavior comparing with the WC-Co coating. Moreover, the often-occurred decarburization of WC in APS process was noticeably improved due to the binding of copper and BaF₂/CaF₂ phase, which not only offered effective solid lubrication, but also acted as bind phases to mend the microstructure and protected WC from decomposition. The optimized specimen contained 10 wt.% Cu and 10 wt.% BaF₂/CaF₂ in a WC-Co matrix, which had excellent frictional and wear performance. The wear mechanism of the self-lubricating wear resistant coating was discussed with the microstructures, compositions and mechanical properties of the composite materials in detail.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

Electrodeposited nickel-cobalt composite coating containing MoS2

Ni-Co/MoS₂ composite coatings were prepared by electrodeposition in a Ni-Co plating bath containing nano-sized MoS₂ particles to be co-deposited. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The friction and wear behaviors of the Ni-Co/MoS₂ composite coatings were evaluated with UMT-2MT test rig in a ball-on-disk contact mode. The morphologies of the original and worn surfaces of the composite coatings were observed on scanning electron microscope (SEM). It was found that the introduction of MoS₂ nano-particulates in the electrolyte caused the shift towards larger negatives of the reduction potential of the Ni-Co alloy coating, and the co-deposited MoS₂ showed no significant effect on the electrodeposition process of the Ni-Co alloy coating. However, the co-deposited MoS₂ led to changes in the surface morphology and structure of the composite coating as well. Namely, the peak width of the Ni-Co solid solution for the composite coating is broader as compared to that of the Ni-Co alloy coating. The co-deposited MoS₂ particulates were uniformly distributed in the Ni-Co matrix and contributed to increase tribological properties of the Ni-Co alloy coating.