Microstructure,mechanical and tribological properties of WC-WCoB coating

A new kind of WC-based coating with superhard WCoB compound as the binder was fabricated by the high velocity oxy-fuel spraying of WC-WB-Co powder. The microstructure, mechanical and tribological properties of the WC-WCoB coating were investigated, together with those of the conventional WC-Co coating for comparison. The results demonstrated that the WC-WCoB coating has simultaneously improved hardness and fracture toughness, and thus remarkably decreased wear rate as compared to the conventional coating. The enhanced tribological properties of the WC-WCoB coating are attributed to the low plastic deformation and the resultant inhibition of the micro-ploughing wear and the increased fracture toughness and interfacial bonding, which can reduce the amount of large cracks. Moreover, the high intrinsic hardness of WC and WCoB, as well as their good interfacial bonding, are more effective in resisting against wear as compared with the conventional coating.     

Preparation of nano-Al2O3 dispersion strengthened coating via coating-substrate co-sintering and underwater shock wave compaction

This paper aims at developing a process technology to fabricate an aluminum dispersion strengthened coating on a copper substrate. First, nanostructured Cu–Al₂O₃ powder was prepared by hydrogen-reduction of mechanical alloyed Cu and Al₂O₃ mixed powders. Low-density coating was then achieved through die pressing and pre-co sintering. Finally, dynamic consolidation by underwater shock wave was used to densify the Cu–Al₂O₃ sintering body, ensuring strong bonding between the coating and substrate. The compressing process was simulated using the commercially available package LS-DYNA. Numerical simulation gave deformation of the coating layer and pressure distribution during the compressing process. Microstructure characteristics indicate that the coating layer has a uniform and fine-grained composite structure and a strong surface bonding between coating layer and copper substrate. The fracture surface, analyzed by SEM, showed intergranular fracturing. Measurement of hardness showed a high hardness value, indicating sound mechanical properties of the coating layer. This method has thus been proven feasible for preparation of aluminum dispersion strengthened coating on copper substrate.     

Role of TiO2 nanoparticles in the dry deposition of NiO micro-sized particles at room temperature

A room-temperature dry-deposition method with TiO₂ powder was used to deposit NiO particles onto a fluorine-doped tin oxide (FTO) substrate. Initially, in the absence of TiO₂ powder, we observed that the NiO particles did not adhere to the substrate; however, the addition of TiO₂ particles facilitated NiO deposition. The volume percentage (vol%) deposition of NiO particles increased with the TiO₂ particle concentration. The inability of the NiO particles to adhere to the FTO substrate was attributed to the absence of deformation and fragmentation in the substrate. This is related to the lower hardness of the FTO substrate, compared with that of the NiO particles. However, the addition of the TiO₂ particles at different vol% during NiO deposition induced deposition, possibly due to the lower hardness of the TiO₂ particles compared with the FTO substrate. The minimum TiO₂ fraction that enabled NiO powder deposition was ~4.8 vol%. Microstructural analysis revealed that TiO₂ powder agglomerates tended to break up as the NiO particles impacted the substrate surface, creating a “deposition complement” from the excess kinetic energy. The deposition mechanism was investigated using microstructural analysis, electron probe microanalysis, and Brunauer–Emmett–Teller (BET) measurements; the results confirmed the influence of the TiO₂ powders on NiO powder deposition, specifically, an improvement in the adhesion and density of the NiO powder and a decrease in the surface roughness of the coating. Therefore, we demonstrated NiO deposition with TiO₂ particles at room temperature, providing potential applications to the supercapacitor and battery industries.     

拉伸法测定涂层界面强度的适用性研究

评述了用于测定涂层界面强度的几种主要测试方法的优点与不足，分析了拉伸法的适用条件与范围。指出拉伸法仅适于测定弹性模量大于金属基体的脆性薄涂,但测试值与界面结合强度的真实值有较大差异。当涂层的变形能力大于金属基体的变形能力时，该法无法测定涂层－金属基体界面结合强度。研究了不同固化工艺的胶粘剂粘涂层在拉伸法中的应变与拉伸载荷的关系，结果表明该法在粘涂层的形变能力较大时无法获得涂层开裂的信号。     

Preparation of nickel coating on ZTA particles by electroless plating

With the aim to effectively improve the interface between ZrO₂ toughened Al₂O₃ (ZTA) particles and metal matrix, nickel was deposited on the surface of ZTA particles by electroless plating method. Formation mechanism of nickel coating and effects of the solution pH, loading capacity of ZTA particles and temperature on the nickel deposition were investigated. Microstructures, thickness and element distributions of nickel coating were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results showed that the nickel was successfully deposited on the surface of ZTA particles by electroless plating without noticeable defects. The process of electroless nickel plating could be explained by combination of atomic hydrogen and electrochemistry theories. The interfacial nucleation of nickel is easier to form than spontaneous nucleation in the solution. Deposited Nickel has priority on the surface of ZTA particles comparing to that in solution. The optimal conditions to coat nickel on the surface of ZTA particles are: solution pH 4.7–4.8, loading capacity 15–20?g/L, and electroless plating temperature 85?°C. The ZTA particle reinforced iron matrix composites prepared by powder metallurgy could have better interfacial bonding between ZTA particle and iron matrix because of the nickel coating on the surface of ZTA particle. Nickel diffuses into the iron matrix during the sintering preparation of composite materials. The interface between ZTA particle and iron matrix presents the evidence of non-chemical bonding.     

Effect of spraying parameter and injector angle on the properties of in-flight particles and alumina coatings on Al alloy with PA-HT

Bonding strength is one of the most important properties of plasma sprayed coatings, especially ceramic coatings on complexly shaped light alloys, which is mainly determined by the properties of in-flight particles and their flattening behaviors on substrate surface. Consequently, the influence of current, voltage, primary gas flow rate and injector angle on temperature and velocity of in-flight alumina particles and morphology of splats on Al alloy with plasma arc-heat treatment (PA-HT) were investigated in detail by DPV Evolution, optical microscope and 3D non-contact surface mapping profile. The bonding strength, hardness and wear behavior of corresponding alumina coatings were measured by universal testing machine, nano-indentation test and wear test. Results showed that particle temperature and velocity increased with increase of input current and voltage, while their trends were reverse with increasing primary gas flow rate. The droplets could melt Al alloy surface and penetrate into its interior that greatly affected the morphology and corresponding shape factor (SF) of splats. Moreover, reducing injector angle caused droplets to glide on substrate surface and then to form “sole-like” splats. The morphology change of these splats obviously influenced the interfacial bonding strength, compactness, hardness and wear resistance of alumina coatings.     

纳米SiO2复合粒子/有机硅低聚物透明超疏水复合涂层的制备及其性能

本文以Stober法制备的胶体SiO2粒子与粉体SiO2粒子结合的SiO2复合粒子在玻璃基底构建粗糙表面,以三乙氧基甲基硅烷（MTES）与正硅酸乙酯（TEOS）为前聚体制备的酸性有机硅低聚物作为粘接剂,使用偶联剂KH540与氟硅烷PFDT进行改性,通过喷涂法在玻璃基底上制备出SiO2复合粒子/酸性有机硅低聚物复合透明超疏水涂层,然后探究SiO2复合粒子、酸性有机硅低聚物、偶联剂KH540以及氟硅烷PFDT对复合涂层的影响。研究表明：当SiO2复合粒子由粒径为110 nm的胶体SiO2粒子与粒径为50 nm的粉体SiO2粒子两种粒子组成,SiO2复合粒子溶液与酸性有机硅稀释液的混合质量比为4:1,添加偶联剂KH540与氟硅烷PFDT的质量比为混合液的1%时,复合涂层在可见光波长范围内透光率可达88%,静态接触角能达155°,在800目砂纸上磨损60 cm后仍能保持超疏水性能,具有良好的自清洁性,为透明超疏水涂层的制备提供一种简便、低成本方案。     

Fabrication of ceramic composite coatings using electrophoretic deposition,reaction bonding and low temperature sintering

We have developed a novel combination of electrophoretic deposition (EPD), reaction bonding and low temperature sintering techniques for the fabrication of yttria stablised zirconia (YSZ)/alumina composite coatings on Fecralloys. A mixture of ethanol and acetylacetone solvent was found to be an effective medium for YSZ and aluminium particle suspension. With the particle size of YSZ and aluminium being significantly reduced during ball milling. By using the EPD process, uniform green form coatings containing YSZ and aluminium particles were produced on Fecralloys. After oxidation of aluminium at 500°C and sintering at 1200°C, a dense and adherent YSZ/Al₂O₃ coating was produced. The presence of aluminium in the green form coatings not only contribute to the bonding between the coating and the metal substrate, but also compensate for the volume shrinkage of the coatings during sintering by the volume expansion arising from oxidation of aluminium to alumina.     

Low pressure plasma-sprayed Al2O3 and Al2O3/SiC nanocomposite coatings from different feedstock powders

This paper describes a preliminary investigation of a nanocomposite ceramic coating system, based on Al₂O₃/SiC. Feedstock Al₂O₃/SiC nanocomposite powder has been manufactured using sol-gel and conventional freeze-drying processing techniques and then low pressure plasma sprayed onto stainless steel substrates using a CoNiCrAlY bond coat. Coatings of a commercial Al₂O₃ powder have also been manufactured as a reference for phase transformations and microstructure. The different powder morphology and size distribution resulting from the different processing techniques and their effect on coating microstructure has been investigated. Phase analysis of the feedstock powders and of the as-sprayed coatings by X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) showed that the nano-scale SiC particles were retained in the composite coatings and that equilibrium α-Al₂O₃ transformed to metastable γ- and δ-Al₂O₃ phases during plasma spraying. Other minority phases in the sol-gel Al₂O₃/SiC nanocomposite powder such as silica and aluminosilicate were removed by the plasma-spraying process. Microstructure characterisation by scanning electron microscopy (SEM) of the as-sprayed surface, polished cross-section, and fracture surface of the coatings showed evidence of partially molten and unmolten particles incorporated into the predominantly lamella microstructure of the coating. The extent of feedstock particle melting and consequently the character of the coating microstructure were different in each coating because of the effects of particle morphology and particle size distribution on particle melting in the plasma.     

A novel approach to the preparation of powder coating—Manufacture of polyacrylate powder coatings via one step minisuspension polymerization

A novel method for manufacturing powder coating through one step minisuspension polymerization is described. The conventional production of powder coating includes six steps—synthesizing resins, mixing the raw material, extrusion, cooling, pre-crushing and pulverization. Comparatively, the present method can simplify the complicated processes, reduce equipment and save energy. Before polymerization, the TiO₂ particles were treated with a reactive silane coupling agent 3-methacryloxypropyltrimethoxysilane (MPTMS) to obtain enough compatibility with the monomers. The powder coating was directly synthesized through employing one step minisuspension polymerization in the presence of titanium dioxide white particles. The powder coating was characterized using Fourier transform infrared spectra (FT-IR) and thermogravimetric analysis (TGA). The results show that TiO₂ particles and polymer are successfully linked up via MPTMS in the powder particles. The morphology of powder coatings produced via different methods was observed by scanning electron microscope (SEM). The powder coatings obtained from minisuspension polymerization consist of regular spherical morphology particles with narrow particle size distribution. The powder flowability and surface film smoothness were enhanced compared to the pulverization powder coating.     

Mechanical and tribological properties of nano/micro composite alumina coatings fabricated by atmospheric plasma spraying

Adding nano particles can significantly improve the mechanical properties and wear resistance of thermal sprayed Al₂O₃ coating. However, it still remains a challenge to uniformly incorporate nano particles into traditional coatings due to their bad dispersibility. In the present work, nanometer Al₂O₃ (n-Al₂O₃) powders modified by KH-560 silane coupling agent were introduced into micrometer Al₂O₃ (m-Al₂O₃) powders by ultrasonic dispersion to afford nano/micro composite feedstock, and then four resultant coatings (weight fraction of n-Al₂O₃: 0%, 3%, 5% and 10%) were fabricated by atmospheric plasma spraying. The features and constitutes of feedstock and as-sprayed coatings, as well as their porosity, bonding strength, microhardness and frictional behaviors were investigated in detail. Results show that the nano/micro composite feedstock with uniform microstructure can be better melted in the spraying process, thereby obtaining coatings with denser microstructure, higher hardness and bonding strength. Added n-Al₂O₃ has no obvious effect on the friction coefficient of composite coatings, whereas can improve their wear-resistant and reduce the worn degree of counterpart. The wear mechanism of traditional coating is brittle fracture and lamellar peeling, while that of composite coating with weight fraction of n-Al₂O₃ of 10% is adhesive wear.     

Fabrication of yttria-stabilized-zirconia coatings using electrophoretic deposition: Effects of agglomerate size distribution on particle packing

Yttria-stabilized-zirconia (YSZ) particles with various size distributions have been electrophoretically deposited (EPD) on Fecralloy substrate to investigate the particle size effect on EPD coatings. The deposition rates, as-deposited particle packing densities, green densities and sintered (for 2 h at 1250 °C in air) coating hardnesses are dependent on particle size. The particle packing arrangement in EPD coatings can be affected by further electric field densification (EFD) of the as-deposited coating in which the wet EPD coating is immersed in pure solvent (acetylacetone) with the application of a constant electric field. The effect of EFD was found to be most effective on small particles (<0.5 μm) when they are co-deposited with large particles (>1 μm). The improvements are reflected in increased mechanical hardness of sintered coatings.     

Influence of heat treatment on alternant-layer structure and mechanical properties of Al2O3-TiO2-MgO coatings

Al₂O₃-TiO₂-MgO ceramic alternant layer coatings were prepared by atmospheric plasma spraying and heat treated at 600, 700, 800, 900, and 1000?°C. The influence of heat treatment on microhardness, fracture toughness, and the structural evolution of the coatings on steel were investigated. Heat treatment promoted alternant layer interdiffusion within ceramic coatings, which could result a transformation from a lamellar morphology to mutual pinning. The interfacial diffusion between the bond coating and substrate was clearly demonstrated after heat treatment at different temperatures. Heat treatment also significantly affected the evolution of the hardness and fracture toughness. Temperature strongly affected the microhardness of the specimens, and the hardness arrived to the highest value at 1000?°C. The formation of a new Mg₂Al₆Ti₇O₂₅ phase and alternant layer mutual pinning were beneficial to hardness improvement, and heat treatment also significantly improved fracture toughness.     

Promotion of the fragmentation and densification for a dense vacuum kinetic-sprayed Y2O3 coating by heat-treatment of feedstock powder

Achieving a proper strength and properties of vacuum kinetic-sprayed (VKS) coatings has been a matter of issue. The heat-treatment of feedstock powder has been suggested as a solution to prevent porous microstructure of coatings. Unfortunately, there is still no study that clearly explains the effect of powder treatment on the two crucial deposition mechanisms in VKS (fragmentation and consolidation). Through observations and characterization of the coating microstructure and single-particle impact testing, the importance of the sufficient impact energy for dense VKS coatings is demonstrated. The results show that heat-treatment of Y₂O₃ powder resulted in improved mechanical properties of the coating and a dense coating structure. It is revealed that the larger heat-treated particles caused further size reduction of crystallites and consolidation via promotion of the intense fragmentation and intimate inter-crystallite bonding.     

Adhesion properties and fracture mechanism of plasma sprayed NiCrAl/Al2O3–13wt.%TiO2 coatings by post annealing

Plasma-sprayed NiCrAl/Al₂O₃–13wt.%TiO₂ coatings (AT13) deposited on mild steel substrate were annealed with varying temperatures in air. The adhesion of the coating was evaluated by tensile adhesive strength test. The microstructure and the fracture mechanism were studied using optical microscopy, X-ray diffraction, and scanning electron spectroscopy/energy dispersive spectroscopy. It was found that the tensile bond strength of the coatings increased with increasing of annealing temperature at first and then decreased with increasing of annealing temperature further. The as-sprayed coating fractured at the interfaces of substrate/bond layer and bond layer/ceramic coating with a brittle–ductile mixed fracture. The measured strength expressed the adhesive strength and internal adhesive strength of the coating. The failure of the coating annealed at 300, 400, and 500 °C took place at the interface of substrate/bond layer and had a mixed fracture surface of transgranular cleavage fracture and localized ductile fracture. The strength obtained is the adhesive strength between the coating and the steel substrate. The coating annealed at 400 °C had a maximum strength of 42.9 MPa. When the temperature is above 600 °C, the bonding strength would be damaged. Therefore, there is a proper annealing temperature which can significantly improve the bond strength of the coating.     

The improvement of anticorrosion properties of zinc-rich organic coating by incorporating surface-modified zinc particle

The corrosion behaviors of zinc-rich coating with various zinc contents, ranging from 0 to 60 volume percent, in thin organic coatings (below 5 μm) were characterized by electrochemical impedance spectroscopy (EIS), free corrosion potential (E_corr) measurement and cycle corrosion test (CCT). It was verified that both coatings with 60 volume percent of zinc powder and without zinc powder showed good corrosion resistance mainly due to the cathodic protection and barrier effect, respectively. On the other hand, coatings with an intermediate concentration (10–40 vol.%) of zinc powder was not successful in protecting a steel substrate efficiently. To improve anticorrosion property of zinc-rich coating, the surface modification of zinc particle was carried out with derivatives of phosphoric and phosphonic acid in the aqueous solution. The effects of the surface modification of zinc particle on corrosion resistance of the coating were investigated with scanning vibrating electrode technique (SVET) and X-ray photoelectron spectroscopy (XPS). The best anti-corrosion performance was achieved when the incorporated zinc particle was treated with phosphoric acid 2-ethylhexyl ester and calcium ion simultaneously, which induced the formation of alkyl-phosphate-calcium complex layer of 190 nm in thickness on zinc particles. Corrosion resistance was improved by the decreased zinc activity and the increased compatibility between the formed complex layer on zinc surface and polymer binder matrix.     

Microstructural evolution and mechanical properties of atmospheric plasma sprayed Y2O3 coating with state of in-flight particle

Y₂O₃ coating on Al₂O₃ substrate was prepared by atmospheric plasma spray (APS). Computational fluid dynamics (CFD) was carried out to predict the state of in-flight Y₂O₃ particles at different powder feeding rates. Microstructure and mechanical properties were found to be affected by the spray distance and powder feeding rate. In this study, the hardness was calculated using a field emission-scanning electron microscope (FE-SEM) because the indentation in the coating is too small to measure using a hardness test machine. The formation of pores causes a decrease in the mechanical property, and the pore length of over 10 μm substantially decreases the hardness. Meanwhile, the solidification behavior is affected by the maximum temperature of the in-flight particles. Based on computational fluid dynamics (CFD) analysis, the maximum temperature of the in-flight particles was found to decrease with increase of the powder feeding rate at the same spray distance. At the powder feeding rate of 60 g/min, a lower adhesion strength was confirmed than that at feeding rate of 30 g/min because splats were insufficiently spread due to the lower maximum temperature of the in-flight particles. The roughness and height of the coating surface were evaluated by confocal microscopy and atomic force microscopy(AFM) analyses. The roughness is the resultant of accumulated splats and the accumulation mechanism of splats is affected by the state of the in-flight particles. Furthermore, there were nano-scale differences of height on the splat surface, on which the nucleation looks like ‘rugged bark’ during solidification of splats when the in-flight particles impact the substrate.     

La0.8Sr0.2Ga0.8Mg0.2O3 electrolytes prepared by vacuum cold spray under heated gas for improved performance of SOFCs

High impact velocity of particles has found its common way into the vacuum cold spray (VCS), but heating gas may further intensify this function, resulting in significantly higher impact velocity. That's the original design idea to realize denser ceramic deposition at low temperature in this paper. In this study, a ~ 10?µm thick La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte layer for SOFCs is prepared by VCS under heated gas. The effects of gas temperature on the deposition behavior, mechanical and electrical properties of the coatings are investigated. Results show improvements in coating density, hardness and ionic conductivity at elevated temperature. Additionally, the output performance of cell with LSGM electrolytes deposited at gas temperature of 400?°C achieved an open circuit voltage of ~ 1.0?V and the maximum power density of 855?mW/cm² at 750?°C. Overall, these findings testify of the promising aspects of VCS method for preparing solid electrolyte films for IT-SOFCs.     

Role of Sn on the adhesion in Cu–Sn alloy-coated steel–rubber interface

Cu–Sn coatings with varying Sn content were deposited on steel substrate by immersion route and the effect of variation of Sn content and the substrate roughness on the interfacial adhesion strength of Cu–Sn-coated steel substrates vulcanized with styrene butadiene rubber were investigated. The surface roughness of the coatings did not vary compared to pristine steel substrate with change in Sn weight% in the coatings. The coated surfaces exhibited bare spots or deep trough as micro-discontinuities in the coatings, where formation of Fe₂O₃ was evident from SEM-EDS, AES, and XPS analysis. Microstructural study of the coating cross-section and coating-substrate interface by transmission electron microscopy of cross-sectioned samples revealed inadequate penetration of coating inside these troughs. Peel test carried out on the Cu–Sn-coated steel–rubber joints showed mixed mode i.e. adhesive and cohesive mode of interfacial fracture irrespective of the coating composition. The peel test further indicated higher interfacial adhesion strength for Cu–Sn-coated samples than pure Cu-coated samples, with an optimum adhesion strength for the coatings containing 3–4?wt.% Sn.     

Si/SiC coated Cf/SiC composites via tape casting and reaction bonding: The effect of carbon content

In this paper, a novel surface modification method for C_f/SiC composites is proposed. Si/SiC coating on C_f/SiC composites is prepared by tape casting and reaction bonding method. The effects of carbon content on the rheological property of the slurries along with the microstructure of the sintered coatings are investigated. The best result has been obtained by infiltrating liquid silicon into a porous green tape with a carbon density of 0.84 g/cm³. In addition, the effect of sintering parameters on the phase composition of the coatings is studied. Dense Si/SiC coating with high density as well as strong bonding onto the substrate is obtained. This Si/SiC coating exhibits an excellent mechanical property with HV hardness of 16.29±0.53 GPa and fracture toughness of 3.01±0.32 MPa m^1/2. Fine surface with roughness (RMS) as low as 2.164 nm is achieved after precision grinding and polishing. This study inspires a novel and effective surface modification method for C_f/SiC composites.