Electrodeposition of sol-enhanced nanostructured Ni-TiO2 composite coatings

A novel electroplating method has been developed to produce nanocrystalline metal-matrix nano-structured composite coatings. A small amount of transparent TiO₂ sol was added into the traditional electroplating Ni solution, leading to the formation of nanocrystalline Ni-TiO₂ composite coatings. These coatings have a smooth surface. The Ni nodules changed from traditional pyramid-like shape to spherical shape. The grain size of Ni was also significantly reduced to the level of 50 nm. It was found that the amorphous anatase TiO₂ nano-particles (∼ 10 nm) were highly dispersed in the coating matrix. The microhardness was significantly increased from 320 HV₁₀₀ of the traditional Ni coating to 430 HV₁₀₀ of the novel composite coating with 3.26 wt.% TiO₂. Correspondingly, the wear resistance of the composite coating was improved by ∼ 50%.     

Microstructure and mechanical properties of Al2O3-Cr2O3 composite coatings produced by atmospheric plasma spraying

Al₂O₃, Al₂O₃-Cr₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction. Electron probe microanalyzer was employed to investigate the polished and fractured surface morphologies of the coatings. Mechanical properties including microhardness, fracture toughness and bending strength were evaluated. The results indicate that the addition of Cr₂O₃ is conducive to the stabilization of α-Al₂O₃. Compared with the pure Al₂O₃ and Cr₂O₃ coatings, Al₂O₃-Cr₂O₃ composite coatings show lower porosities and denser structures. Heterogeneous nucleation of α-Al₂O₃ occurs over the isostructural Cr₂O₃ lamellae and partial solid solution of Al₂O₃ and Cr₂O₃ might be occurring as well. Furthermore, grain refining and solid solution strengthening facilitate the mechanical property enhancement of Al₂O₃-Cr₂O₃ composite coatings.     

Mechanical and tribological properties of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

Solid lubricant coatings of WS₂ and Cr-WS₂ (15-50 at.% Cr) prepared using an unbalanced magnetron sputtering system were evaluated for their mechanical and tribological properties. Nanoindentation results indicated that addition of Cr helped in improving the mechanical properties and the elastic recovery ability of Cr-WS₂ coatings. The adhesive strengths of Cr-WS₂ coatings were evaluated using a nanoscratch tester and from the nanoscratch profiles, critical load values and optical images, it was evident that the adhesion of Cr-WS₂ coatings increased with an increase in the Cr content. Further analysis of the nanoscratch data indicated that WS₂ coatings exhibited large amount of plastic deformation compared to Cr-WS₂ coatings which showed a combination of elastic-plastic deformation. However, micro-tribometer measurements at a load of 2 N showed that the tribological properties of Cr-WS₂ coatings deteriorated with an increase in the Cr content. For example, Cr-WS₂ coatings prepared at Cr content ≥ 33 at.% failed after a sliding distance of 1 m. On the other hand, WS₂ and Cr-WS₂ coatings prepared at low Cr contents (15-23 at.% Cr) exhibited a stable friction coefficient (50-60% relative humidity) in the range of 0.10-0.13 for a sliding distance of 14 m. Micro-Raman spectroscopy data of the worn films taken after a sliding distance of 14 m indicated the presence of WS₂ transfer films for WS₂ and Cr-WS₂ coatings prepared at low Cr contents. For Cr-WS₂ coatings with Cr content ≥ 33 at.%, the worn films consisted predominantly of WO₃. After an extended sliding distance of 50 m, Cr-WS₂ coatings (15-23 at.% Cr) outperformed WS₂ coating which failed after 20 m. Further, the coatings prepared at low Cr contents did not show any failure even after a sliding distance of 200 m. At a higher load of 7 N, Cr-WS₂ coating with 15 at.% Cr exhibited the best performance with a friction coefficient of 0.07 up to a sliding distance of 72 m. These results indicate that the amount of Cr in the WS₂ matrix needs to be controlled judiciously to obtain improved mechanical and tribological properties in Cr-WS₂ solid lubricant coatings.     

Effect of applied current on the electrodeposited Ni-Al2O3 composite coatings

Nano-sized Al₂O₃ ceramic particles (50 nm) were co-deposited with nickel using electrodeposition technique to develop composite coatings. The coatings were produced in an aqueous nickel bath at different current densities and the research investigated the effect of applied current on microstructure and thickness of the coatings. The variation in some mechanical properties such as hardness, wear resistance, and the adhesive strength of the composite coatings is influenced by the applied current and this was also studied. The morphology of the coatings was characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The hardness, wear resistance, and bond strength of the coatings were evaluated by Vickers micro-hardness test, pin-on-disc test, and tensile test, respectively. Results showed that the Al₂O₃ particles were uniformly distributed in the coatings, and the coatings deposited at a current density of 0.01 A/cm² was most favorable in achieving a maximum current efficiency which causes the co-deposition of a maximum amount of Al₂O₃ particles (4.3 wt.%) in the coatings. The increase in Al₂O₃ particles in the coatings increased the mechanical properties of the Ni-Al₂O₃ composite coatings by grain refining and dispersion strengthening mechanisms.     

Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Novel thermal barrier coatings based on La₂Ce₂O₇/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La₂Ce₂O₇ (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al₂O₃ in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling.     

Tribological properties of thin hard coatings: demands and evaluation

Thin hard coatings are currently being used in a large number of tribological applications. Some important examples are tools for metal cutting and forming, and machine elements like sliding bearings, seals, valves, etc. The present paper deals with the problems of identifying the limiting factors in given tribological applications of coating composites, and how to experimentally determine their tribological responsse, with special attention given to wear.

As a means for coating development, the intricate relations between coating deposition parameters and the triboological response of the coated composite are indicated by a flow chart of relevant coating and substrate properties. It is demonstrated how the general characteristics (topography, composition, microstructure, etc.) and fundamental properties (chemical, thermal, mechanical, etc.) of coating and substrate combine to generate the tribological properties (resistance against abrasion, erosion, sliding wear, etc.), which given the tribological loading, determines the tribological response in terms of friction and wear.

A number of different test methods for tribological evaluation of coating composites are presented together with illustrative examples. The test range from four-point bending for evaluating coating fracture strength, to designated tribological tests to determine the resistance against abrsion, particle erosion, and sliding wear.     

Computer controlled detonation spraying of WC/Co coatings containing MoS2 solid lubricant

Composite WC/Co + MoS₂ coatings were deposited onto steel substrates by Computer Controlled Detonation Spraying using three spraying modes: very cold, cold and normal. Maximal content of MoS₂ in a sprayed powder was 10 wt.%. Characterization of coatings was made with chemical and phase analyses, microhardness measurement, morphology and microstructure investigation. X-ray diffraction study shows that residual MoS₂ exists only in coatings obtained at very cold and cold spraying modes. At normal spraying mode complete decomposition of the solid lubricant occurs during spraying. From the engineering point of view, the coating applied at the cold mode using a powder containing 10 wt.% MoS₂ is the most promising. Such a coating has microhardness of 650 HV_0.2 and a porosity of 10%.     

First-principles calculations of electronic and optical properties of Ti-doped monoclinic HfO2

Based on the first-principles density-functional theory, the electronic structures and optical properties of monoclinic HfO₂ and Ti-doped m-HfO₂ are comparatively investigated. The calculated lattice parameters of m-HfO₂ are in good agreement with the experimental values and the previous works, and the incorporation of Ti into HfO₂ induces a decrease in the lattice parameters. Electronic structures of m-HfO₂ and Ti-doped HfO₂ are studied through the densities of states (DOS) and band structures. The results indicate that the Ti substitution of Hf sites modifies the conduction band structure of HfO₂, which leads to a reduction of the band gap of HfO₂. The complex dielectric function and refractive index are calculated and the peak position distributions of imaginary parts of the complex dielectric function have been explained. The calculated optical properties are consistent with the experimental measurements for m-HfO₂.     

Effect of nano-sized CeF3 on microstructure, mechanical, high temperature friction and corrosion behavior of Ni-W composite coatings

Ni-W-CeF₃ composite coatings were prepared by electrodeposition in a Ni-W plating bath containing CeF₃ nano-particles. The shape and size of the CeF₃ nano-particles and the surface topography of the composite coatings were observed using an environmental scanning electron microscope, and the component and structure analysis was characterized by means of XRD. A microhardness tester was employed to investigate the microhardness of the coatings. The high temperature friction behavior and corrosion resistance of the coatings against molten glass were investigated by using a high temperature tribometer. It was found that the CeF₃ nano-particles appeared in the coatings as microspheres of a diameter less than 50 nm. The addition of CeF₃ nano-particles led to changes in morphologies of the composite coatings by refining the size of crystalline bulks. Therefore, the Ni-W-nanoCeF₃ composite coatings had more compact and fine granular morphologies. The co-deposited CeF₃ nano-particles were uniformly distributed in the Ni-W matrix and had contribution to greatly increasing the microhardness, high temperature tribological and anti-corrosion properties of the Ni-W alloy, furthermore the mechanism of anti-friction and anti-corrosion is discussed.     

Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.     

Micro-scale sliding contacts on Au and Au-MoS2 coatings

The microtribological properties of Au and Au-MoS₂ coatings were examined using a nanoindentation instrument. MoS₂ was chosen for this study as an additive to Au due to its positive influence on the mechanical and tribological properties. Reciprocating microscratch tests were performed using a diamond indenter with a tip radius of 50 μm and a range of normal loads between 0.2 mN and 5.0 mN. The friction and wear results, with respect to the two coatings, were correlated to different velocity accommodation modes and levels of adhesion. It was found that the addition of 20 mol% MoS₂ to Au reduced the adhesion and limiting friction and also improved the wear resistance significantly. This coating shows potential for applications in microcomponents and microswitches due to its wear resistance, relatively low friction and good electrical conductivity.     

Study of Al2O3 coatings on AISI 316 stainless steel obtained by controlled atmosphere plasma spraying (CAPS)

The tribological behaviour of Al₂O₃ coatings on AISI 316 stainless steel, obtained by the process of controlled atmosphere plasma spraying (CAPS), is studied in this work. Atmospheric plasma spraying (APS) and high pressure plasma spraying (HPPS) were applied in order to produce these coatings. The APS coatings exhibited lower microhardness values compared to the values of HPPS coatings. Regarding the HPPS coatings it was found that plasma composition, through its heat capacity, does influence the heat transfer to particles, and, consequently, their flattening and densification process, which govern coating properties. It was revealed that tribological behaviour of coatings was influenced by the applied spraying method too. Coatings from HPPS under high-enthalpy conditions led to worst wear behaviour. In general, properties, such as microstructure, microhardness, coefficient of friction and wear resistance depended on the processing conditions such as pressure and composition of the spraying chamber atmosphere.     

Effects of CeO2 additive on the microstructure and mechanical properties of in situ TiB2/Al composite

In this paper, CeO₂ was investigated as an additive for in situ preparation of TiB₂/Al composite using an exothermic reaction process via K₂TiF₆ and KBF₄ salts. Experimental results indicated that when 0.5 wt.% CeO₂ additive was added, the dispersion of TiB₂ particles was improved significantly. Meanwhile, α-Al matrix grain was further refined. Compared with the composite without CeO₂, the ultimate tensile strength, yield strength, elastic modulus and tensile elongation increased by 8%, 7%, 26% and 14%, respectively in as-cast condition, and the tensile fracture behavior of the composite with CeO₂ belonged to a typical ductile fracture with microvoid coalescence.     

A novel electroless plating of Ni-P-TiO2 nano-composite coatings

A new processing concept has been developed to produce nano-structured metal-matrix composite coatings. This method combines sol-gel and electroless plating techniques to prepare highly dispersive oxide nano-particle reinforced composite coatings. Transparent TiO₂ sol was added into the standard electroless plated Ni-P solution at a controlled rate to produce Ni-P-TiO₂ nano-composite coatings on Mg alloys. The coating was found to have a crystalline structure. The nano-sized TiO₂ particles (∼ 15 nm) were well dispersed into the Ni-P coating matrix during the co-deposition process. This technique can effectively avoid the agglomeration of nano-particles in the coating matrix. As a result, the microhardness of the composite coatings were significantly increased to ∼ 1025 HV₂₀₀ compared to ∼ 710 HV₂₀₀ of the conventional composite coatings produced with solid particle mixing methods. Correspondingly, the wear resistance of the new composite coatings was also greatly improved.     

Thermal conductivity and thermal cycle life of La2O3 and HfO2 doped ZrO2-Y2O3 coatings produced by EB-PVD

The effects of La₂O₃ and HfO₂ addition on thermal conductivity and thermal cycle life of EB-PVD YSZ coatings were investigated. La₂O₃ and HfO₂ were selected as additives, because they significantly suppress the sintering of YSZ. The developed coating showed low thermal conductivity as well as high resistance to sintering. Burner rig tests confirmed that the developed coating have a superior thermal insulating effect and have a longer life than that of a coating with conventional composition.     

MoSi_2含量对MoSi_2/Al_2O_3复合涂层介电与力学性能的影响(英文)

通过喷雾干燥法制备MoSi2包覆Al2O3的壳核结构混合粉,利用该混合粉以等离子喷涂技术制备MoSi2/Al2O3复合涂层材料。研究MoSi2/Al2O3质量比涂层材料的力学和介电性能的影响。结果表明:随着MoSi2含量从0增加到45%,复合材料的抗弯强度和断裂韧性分别从198MPa和3.05MPa·m1/2增加到324MPa和4.82MPa·m1/2,随后又降到310MPa和4.67MPa·m1/2。在8.2-12.4GHz微波频率波段内,随着MoSi2含量的增加,复合材料的介电损耗增加,而介电常数的实部却呈减小趋势。这主要是由于MoSi2颗粒熔化后的凝聚及导电网络结构的形成导致电导率的增加引起的。     

The effect of plasma spray parameters on the cavitation erosion of Al2O3-TiO2 coatings

This paper reports a study of how the choice of plasma spray parameters, used during deposition of Al₂O₃-13%TiO₂ coatings on carbon steel, influences the cavitation erosion properties of such coatings. The parameters studied are the power feeding rate and hydrogen flow rate. The surface and cross section of coatings before and after cavitation were also observed by scanning electron microscopy (SEM). The phases present in the coatings were characterized by X-ray diffraction method (XRD). The microscopic observations were used to study the inter-lamellar connection, porosity, unmelted particles and so on inside the coating. We also measured the roughness, microhardness, adhesion strength and cavitation erosion of the coatings. The XRD results showed that the coating includes different allotropes of Al₂O₃ such as α and γ. The cavitation erosion studies of the coatings were conducted by ultrasonic cavitation testing on the basis of ASTM G32 standard. It was found that cavitation erosion is accelerated around the unmelted particles and porosities. The results reveal that the cavitation resistance of the coating is determined by its microstructure and that increasing discontinuities (inside the coating) reduce its cavitation resistance. We have found that the coating obtained at hydrogen gas flow rate of 16 L/min and powder feeding rate of 20 g/min has the best cavitation resistance.     

Synthesis and characterization of Y3Al5O12 and ZrO2-Y2O3 thermal barrier coatings by combustion spray pyrolysis

Y₃Al₅O₁₂ and ZrO₂-Y₂O₃ (8 mol% YSZ) coatings for potential application as thermal barrier coatings were prepared by combustion spray pyrolysis. Thermal cycling of as deposited coatings on stainless steel and FeCrAlY bond coat substrates was carried out at 1000 °C and 1200 °C to determine the thermal fatigue response. Structural and morphological studies on Y₃Al₅O₁₂ and 8 mol% YSZ coatings before and after thermal cycling have been carried out. It has been noted that the coatings on FeCrAlY substrates remain intact after 50 cycles between room temperature and 1200 °C, whereas the coatings on stainless steel show some minor damage such as peeling off near the periphery after 50 cycles at 1000 °C. Thermal diffusivity values of Y₃Al₅O₁₂ and 8 mol% YSZ films were measured by using photo thermal deflection spectroscopy and the values are lower than those of coatings produced by conventional techniques such as EBPVD and APS.     

Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings

The influence of nucleation on the microstructure and properties of CVD Al₂O₃ was investigated. The experimental α-Al₂O₃ layers were deposited (a) without nucleation control and (b) with nucleation steps resulting in pronounced , and growth textures. The experimental layers were characterised using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Wear properties of the textured coatings were evaluated in turning. The chemistry of the nucleation surface appeared to be an important factor in pre-determining the phase content and growth textures of the Al₂O₃ layers. Optimised nucleation resulted in substantially improved wear properties and these kinds of α-Al₂O₃ layers were typically composed of relatively small, defect-free grains exhibiting no porosity. The textured α-Al₂O₃ layer showed the best wear resistance.     

Synthesis and photoluminescence of Eu by co-doping Eu and Cl in Sr2P2O7 under air atmosphere

A novel phosphor Sr₂P₂O₇ co-doped with europium ion and chlorine ion was firstly synthesized by solid state reaction under air atmosphere. Its properties were systematically analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and fluorescence spectra. The introduction of chlorine into the system was helpful and necessary to Eu³⁺ substitute Sr²⁺ site and subsequently to reduce Eu³⁺ to Eu²⁺, XPS results confirmed that some amount of Eu³⁺ ions could be reduced to Eu²⁺ ions under air atmosphere at high temperature. The reduction tendency of Eu³⁺ depends not only on the doping Cl⁻ content, but also on the sintering temperature and time. Photoluminescence spectra also revealed that europium ions were present in divalent as well as trivalent oxidation states, the emission peak at 415 nm is ascribed to the typical 5d-4f transition of Eu²⁺, 592 nm and 613 nm assigned to the characteristic transitions of ⁵D₀-⁷F_1,2 of Eu³⁺. Such abnormal reduction was attributed to the electronegative defects formed by nonequivalent substitution of Eu³⁺ on the Sr²⁺ sites in the investigated phosphors.