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.     

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.     

Electrodeposition of Ni-Co composites containing nano-CeO2 and their structure, properties

One of the most powerful rare earth oxides, ceria CeO₂ was incorporated in Ni matrix and the effect of cobalt addition in the matrix, on the structure and properties has been studied. The amount of cobalt incorporated in the Ni matrix was in the range of 25-85 wt.%. The presence of cobalt resulted in a marginal variation in CeO₂ content from 5 to 8 wt.%. The microhardness studies revealed that a maximum value of 540 Hk_50gf was obtained in the presence of 25 wt.% cobalt in Ni-CeO₂ matrix compared to 50 and 85 wt.% addition. The presence of 25 wt.% cobalt in Ni-CeO₂ matrix also improved its wear resistance as seen from Tribology studies. The wear products were identified using Raman Spectroscopy. The X-ray diffraction (XRD) studies showed that an increase in cobalt content from 25 to 85 wt.% resulted in a change in crystal structure from fcc to hcp. A change in surface morphology with variation in cobalt content was seen from scanning electron microscopy (SEM). It was perceived from the thermal stability studies that the presence of 85 wt.% cobalt in Ni-CeO₂ matrix imparted better stability in microhardness at temperatures up to 800 °C. Although, the incorporation of cobalt in Ni-CeO₂ matrix enhanced the microhardness, wear resistance and thermal stability it did not improve the corrosion resistance as noticed from immersion corrosion studies.     

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 properties of Ni-Co/nano-Al2O3 composite coatings by pulse reversal current electrodeposition

Ni-Co/nano-Al₂O₃ (Ni-Co/Al₂O₃) composite coatings were prepared under pulse reversal current (PRC) and direct current (dc) methods respectively. The microstructure of coatings was characterized by means of XRD, SEM and TEM. Both the Ni-Co alloy and composite coatings exhibit single phase of Ni matrix with face-centered cubic (fcc) crystal structure, and the crystal orientation of the Ni-Co/Al₂O₃ composite coating was transformed from crystal face (2 0 0) to (1 1 1) compared with alloy coatings. The hardness, anti-wear property and macro-residual stress were also investigated. The results showed that the microstructure and performance of the coatings were greatly affected by Al₂O₃ content and the electrodeposition methods. With the increasing of Al₂O₃ content, the hardness and wear resistance of the composite coatings enhanced. The PRC composite coatings exhibited compact surface, high hardness, better wear resistance and lower macro-residual stress compared with that of the dc composite coatings.     

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.     

Low-voltage cathodoluminescence property of Li-doped Gd2−xYxO3:Eu

Cathodoluminescent (CL) spectra of Li-doped Gd_2−xY_xO₃:Eu³⁺ solid-solution (0.0?x?0.8) were investigated at low voltages (300 V-1 kV). The CL intensity is maximum for the composition of x=0.2 and gradually reduces with increasing the amount of substituted Y content. In particular, small (∼100 nm) particles of Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ are obtained by firing the citrate precursors at only 650°C for 18 h. Relative red-emission intensity at 300 V of this phosphor is close to 180% in comparison with that of commercial red phosphor Y₂O₃:Eu³⁺. An increase of firing temperature to 900°C results in 400-600 nm sized spherical particles. At low voltages (300-800 V), the CL emission of 100 nm sized particles is much stronger than that of 400-600 nm sized ones. In contrast, the larger particles exhibit the higher CL emission intensity at high voltages (1-10 kV). Taking into consideration small spherical morphology and effective CL emission, Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ appears to be an efficient phosphor material for low voltage field emission display.     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

Effect of nano-Y2O3 on microstructure of laser cladding cobalt-based alloy coatings

Cobalt-based alloy coatings with and without nano-Y₂O₃ particles produced by a 5 kW CO₂ laser on Ni-based superalloy were introduced. Solidified microstructure, phase compositions and distribution of nano-particles were investigated by using optical microscope, SEM, TEM and XRD. The results showed that metastable phases, such as γ-Co and Cr₂₃C₆ existed in cobalt-based alloy coatings. Another two phases (Y₂O₃ and ?-Co) were found by adding nano-Y₂O₃. Without nano-Y₂O₃, rapid directional solidified microstructure of columnar dendrite appeared. Fine and short dendritic microstructure and columnar to equiaxed transition (CET) occurred by adding nano-Y₂O₃ particles. With the increase in amount of nano-Y₂O₃, fully equiaxed crystallization appeared and the formation mechanism was analyzed. The results also showed that the sub-microstructure of the coatings changed from dislocation to stacking fault by adding nano-Y₂O₃.     

Characterization of Y2O3 gate dielectric on n-GaAs substrates

Physical and electrical properties of sputtered deposited Y₂O₃ films on NH₄OH treated n-GaAs substrate are investigated. The as-deposited films and interfacial layer formation have been analyzed by using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). It is found that directly deposited Y₂O₃ on n-GaAs exhibits excellent electrical properties with low frequency dispersion (<5%), hysteresis voltage (0.24 V), and interface trap density (3 × 10¹² eV⁻¹ cm⁻²). The results show that the deposition of Y₂O₃ on n-GaAs can be an effective way to improve the interface quality by the suppression on native oxides formation, especially arsenic oxide which causes Fermi level pinning at high-k/GaAs interface. The Al/Y₂O₃/n-GaAs stack with an equivalent oxide thickness (EOT) of 2.1 nm shows a leakage current density of 3.6 × 10⁻⁶ A cm⁻² at a V_FB of 1 V. While the low-field leakage current conduction mechanism has been found to be dominated by the Schottky emission, Poole-Frenkel emission takes over at high electric fields. The energy band alignment of Y₂O₃ films on n-GaAs substrate is extracted from detailed XPS measurements. The valence and conduction band offsets at Y₂O₃/n-GaAs interfaces are found to be 2.14 and 2.21 eV, respectively.     

Effect of the dispersibility of ZrO2 nanoparticles in Ni-ZrO2 electroplated nanocomposite coatings on the mechanical properties of nanocomposite coatings

ZrO₂ nanoparticles was uniformly co-deposited into a nickel matrix by electroplating of nickel from a Watts bath containing particles in suspension which were monodispersed with dispersant under DC electrodeposition condition. It was found that morphology, orientation and hardness of the nanocomposite coatings with monodispersed ZrO₂ nanoparticles had lots of difference from the nanocomposite coatings with agglomerated ZrO₂ nanoparticles and pure nickel coatings. Especially, the result of hardness showed that only a very low volume percent (less than 1 wt.%) of monodispered ZrO₂ nanoparticles in Ni-ZrO₂ nanocomposite coatings would result in higher hardness of the coatings. The hardness of Ni-ZrO₂ nanocomposite coatings with monodispersed and agglomerated ZrO₂ nanoparticles were 529 and 393 HV, respectively. The hardness value of the former composite coatings was over 1.3 times higher than that of the later. All these composite coatings were two-three times higher than that of pure nickel plating (207 HV) prepared under the same condition. The strengthening mechanisms of the Ni-ZrO₂ nanocomposite coatings based on a combination of grain refinement strengthening from nickel matrix grain refining and dispersion strengthening from dispersion state of ZrO₂ nanoparticles in the coatings.     

Intense ultraviolet and blue upconversion emissions in Yb-Tm codoped stoichiometric Y7O6F9 powder

Stoichiometric Y₇O₆F₉ powder codoped with Yb³⁺-Tm³⁺ was synthesized via co-precipitation and subsequent calcining route. The results of X-ray diffraction and transmission electron microscopy reveal that when the calcining temperature is beyond 800 °C, orthorhombic YF₃ nanoparticles can be completely oxidized into orthorhombic Y₇O₆F₉ powder. Under the excitation of a 980 nm laser, Y₇O₆F₉ powder exhibits multicolor UC emission in regions spanning the UV to the NIR. In addition, the upconversion emission intensities of YF₃, Y₇O₆F₉ and Y₂O₃ powders were compared under the same dopant condition (Yb/Tm=5/0.5 mol%). The low phonon energy revealed by Raman spectra helped to understand the high efficient upconversion emission of Y₇O₆F₉ and the main phonon vibration of Y₇O₆F₉ lies at 472 cm⁻¹, which is far lower that of Y₂O₃ (at 708 cm⁻¹). Our results indicate that orthorhombic rare earth ions doped Y₇O₆F₉ is an efficient matrix for UV and blue UC emission, and has potential applications in color displays, anti-counterfeiting and multicolor fluorescent labels.     

Cathodoluminescence characteristics of particles and film of (Y, Zn)2O3:Eu phosphor prepared by spray pyrolysis

Eu-doped Y₂O₃ particles with spherical shape and fine size were prepared by spray pyrolysis. The cathodoluminescence of Y₂O₃:Eu³⁺ powder was optimized by substituting small amount of zinc atoms in place of yttrium sites. As a result, the optimized (Y, Zn)₂O₃:Eu³⁺ phosphor showed 60% improved cathodoluminescence compared with Y₂O₃:Eu³⁺ particles. The prepared (Y, Zn)₂O₃:Eu³⁺ phosphor had spherical shape and 0.726 μm in mean size. Using these particles, the thickness of the phosphor film was controlled by varying the phosphor loading. The brightness and luminous efficiency of phosphor films prepared were monitored with varying the accelerating voltage ranges from 4 to 14 kV. The dependency of the luminous efficiency on the accelerating voltage was very sensitive to the phosphor loading. As increasing the accelerating voltage from 4 to 14 kV, the brightness of phosphor films prepared was monotonically increased from 200 to 1085 cd/cm²_, but the saturation in the luminous efficiency appeared at 10 kV. The highest efficiency was achieved when the number of phosphor-particles layer was about 3. More details about the luminous efficiency and brightness were discussed with changing the phosphor loading.     

Synthesis and characterization of wire-like and near-spherical Eu2O3-doped Y2O3 phosphors by solvothermal reaction

Eu₂O₃-doped yttrium oxide (3 mol%) [Y₂O₃:Eu(3 mol%)] with wire-like and near-spherical morphologies were prepared by a solvothermal treatment using water, ethanol, ethylene glycol and glycerol as reaction media followed by calcination. The powders prepared in water and ethanol possessed wire structure, where the powder treated in water showed high aspect ratio and that in ethanol showed low aspect ratio. The powders prepared in ethylene glycol and glycerol possessed well-dispersed near-spherical powders, which showed almost the same level of photoluminescence emission intensity as that of submicron particles prepared without solvothermal treatment.     

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 synthesis of spinel nickel ferrite (NiFe2O4) nano-sheets

The present investigation is related to the deposition of single-phase nano-sheets spinel nickel ferrite (NiFe₂O₄) thin films onto glass substrates using a chemical method. Nano-sheets nickel ferrite films were deposited from an alkaline bath containing Ni²⁺ and Fe²⁺ ions. The films were characterized for their structural, surface morphological and electrical properties by means of X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and two-point probe electrical resistivity techniques. The X-ray diffraction pattern showed that NiFe₂O₄ nano-sheets are oriented along (3 1 1) plane. The FT-IR spectra of NiFe₂O₄ films showed strong absorption peaks around 600 and 400 cm⁻¹ which are typical for cubic spinel crystal structure. Microstructural study of NiFe₂O₄ film revealed nano-sheet like morphology with average sheet thickness of 30 nm. The room temperature electrical resistivity of the NiFe₂O₄ nano-sheets was 10⁷ Ω cm.     

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.     

Structural and optical properties of Er, Yb co-doped Y2O3 thin films

Thin Er³⁺, Yb³⁺ co-doped Y₂O₃ films were grown on (1 0 0) YAG substrates by pulsed laser deposition. Ceramic targets having different active ion concentration were used for ablation. The influence of the rare-earth content and oxygen pressure applied during the deposition on the structural, morphological and optical properties of the films were investigated. The films deposited at the lower pressure, 1 Pa, and at 1/10 Er to Yb doping ratio are highly textured along the (1 1 1) direction of the Y₂O₃ cubic phase. In addition to the crystalline structure, these films possess smoother surface compared to those prepared at the higher pressure, 10 Pa. All other films are polycrystalline, consisting of cubic and monoclinic phases of Y₂O₃. The rougher surface of the films produced at the higher-pressure leads to higher scattering losses and different behavior of the reflectivity spectra. Optical anisotropy in the films of less than 0.004 was measured regardless of the monoclinic structure obtained. Waveguide losses of about 1 dB/cm at 633 nm were obtained for the films produced at the lower oxygen pressure.     

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.     

Enhanced optical activation of Er in Er-doped Al2O3 powders by Y codoping in a sol-gel method

Enhanced photoluminescence (PL) mechanism of Er³⁺-doped Al₂O₃ powders by Y³⁺ codoping at wavelength 1.53 μm has been investigated through PL measurements of 0.1 mol% Er³⁺- and 0-20 mol% Y³⁺-codoped Al₂O₃ powders prepared at a sintering temperature of 900 °C in a non-aqueous sol-gel method. PL intensity and lifetime of Er³⁺-Y³⁺-codoped Al₂O₃ powders composed of γ-(Al,Er,Y)₂O₃ and θ-(Al,Er,Y)₂O₃ phases increased with increasing Y³⁺-codoping concentration. The 10-20 mol% Y³⁺ codoping in 0.1 mol% Er³⁺-doped Al₂O₃ powders intensified the PL intensity by about 20 times, with a PL lifetime prolonged from 3.5 to 5.8 ms. A maximal increase of the optical activity of Er³⁺ in 0.1 mol% Er³⁺-Y³⁺-codoped Al₂O₃ powders about one order was achieved by 10-20 mol% Y³⁺ codoping. It is found that the improved PL properties for Er³⁺-Y³⁺-codoped Al₂O₃ powders are mainly attributed to enhanced optical activation of Er³⁺ in the Al₂O₃ by Y³⁺ codoping, and to the slightly increased radiative quantum efficiency of Er³⁺ in the Al₂O₃.