Preparation of Al2O3 coating on TiN coating by polymer-assisted deposition to improve oxidation resistance in coking inhibition applications

In order to inhibit the metal catalytic coking and improve oxidation resistance of single TiN coating, the TiN/Al₂O₃ double layer coatings were designed as a chemically inert coating for methylcyclohexane supercritical pyrolysis. Internal TiN coatings were prepared by atmospheric pressure chemical vapor deposition using TiCl₄–H₂–N₂ system. The external Al₂O₃ coatings with different thicknesses were prepared on the TiN surface by polymer-assisted deposition, and the coating with the most suitable thickness was further annealed at different temperatures of 600, 700, 800 and 900 °C. The morphology, elemental and phase composition of TiN/Al₂O₃ coatings were characterized by SEM, EDX and XRD respectively. The chemical state information of the coating elements was based on Ti 2p, Al 2p core level X-ray photoelectron spectroscopy (XPS) spectra. The results indicated that the external Al₂O₃ coating will partially peel off at 900 °C annealing temperature. The thermogravimetric analysis results indicated that all TiN/Al₂O₃ coatings show better oxidation resistance than single-layer TiN coating. The anti-coking test with methylcyclohexane supercritical pyrolysis showed that the TiN/Al₂O₃ coatings can effectively cover the metal catalytic sites and eliminate metal catalytic coking. However, the acid sites of external Al₂O₃ coating slightly promoted coking, so the anti-coking ratios of TiN/Al₂O₃ coatings were smaller than that of TiN. Thus, the addition of external Al₂O₃ coating can greatly improve the oxidation resistance of TiN coatings with little loss of coking resistance.     

Nickel–alumina graded coatings obtained by dipping and EPD on nickel substrates

Nickel substrates have been coated by Ni/Al₂O₃ composite films by a dipping process using aqueous suspensions that contain a temporary binder. Two-layer and three-layer graded coatings have been produced, consisting of pure Ni powder and Ni/Al₂O₃ composites with Al₂O₃ contents of 15 and 30 vol.% as intermediate layers to release sintering and thermal stresses. The laminates were further coated with a ceramic layer of Al₂O₃/ZrO₂ that was deposited by electrophoretic deposition using a non-aqueous suspension. A continuous, thin Al₂O₃ layer surrounding Ni grains developed at the intermediate composite layer of Ni/Al₂O₃ allows the ceramic coating to maintain strongly adhered to the nickel substrate by means of a porous substrate/coating interface.     

Electrochemical behavior of diamond-like-carbon coatings deposited on AlTiC (Al2O3 + TiC) ceramic composite substrate in HCl solution

The electrochemical characterization/corrosion behavior of diamond-like carbon thin films is worthwhile to study and needed in the field (as there has been limited comprehensive evaluation of this across all types of DLC in the literature). In this paper, newly developed tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H) films, prepared by filtered cathodic arc deposition (FCVA) and plasma enhanced chemical vapor deposition (PECVD) respectively were deposited over AlTiC (Al₂O₃ + TiC) ceramic composite substrate. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance in 2 M HCl solution. This ceramic substrate is used widely for the hard disk drives and read and write heads in computer. The memory of the hard disks can be increased by improving the surface quality and decreasing the pinholes. The DLC coatings were modified under different preparation conditions by changing the nitrogen-doping ratios as an attempt for improving the surface distribution and minimizing the surface coating defects.     

Improved adhesion of TiN coatings on Al2O3–carbide composites by DFT calculations and experimental arc-PVD synthesis

Electronic and atomic structures of interfaces between TiN coatings and Al₂O₃–WC composites were investigated by Density functional theory (DFT) calculations. As typical examples, interfacial orientation relationships of TiN(001)/Al₂O₃(0001), TiN(001)/WC(0001), TiN(111)/Al₂O₃(0001), and TiN(111)/WC(0001) were chosen. It was found that the TiN(111)/Al₂O₃(0001) and TiN(111)/WC(0001) interfaces have interfacial structural units of six-fold coordinated triangular prisms centered on Ti atoms. In contrast, those of the interfaces with TiN(001) tend to have more distorted structure units due to their larger lattice misfits. Theoretical works of separation showed that interfacial strength is much more increased at the TiN(111) interfaces, as compared to those at the TiN(001) interfaces. Accordingly, experimental controls of TiN-coating orientations on Al₂O₃–WC composites were attempted by using the cathodic arc ion plating method. It was found that orientations of (111) in the TiN coatings can be more enhanced and then interfacial mechanical strengths and hardnesses of the TiN coatings can increase more with rising bias voltages.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.     

Microstructure and interface-adhesion of thermally sprayed continuous gradient elastic modulus FeCrAl-ceramic coatings

The bond strength between thermally sprayed metal bond-coats and ceramic top-coats is a key factor in determining their service life. However, most studies focus on interface modifications. In this research, based on FeCrAl bond-coats prepared by arc spraying, top-coats (Al₂O₃-40 wt% TiO₂) were prepared by plasma spraying, and heat treatment was carried out in a hypoxic atmosphere. Continuous gradient elastic modulus FeCrAl-ceramic coatings were successfully prepared, and the microstructural and mechanical properties from the substrate to the top-coats were systematically investigated. The Al₂O₃ content gradually decreased from the top-coats to the substrate, forming continuous gradient elastic modulus FeCrAl-ceramic coatings. The oxide formed during the heat treatment filled the defects in the bond-coats and greatly improved the mechanical properties of the coating. The bonding strength of the continuous gradient elastic modulus coating was 21.7% greater than that of the as-received coating.     

Improved corrosion resistance of Al2O3 ceramic coatings on AZ31 magnesium alloy fabricated through cathode plasma electrolytic deposition combined with surface pore-sealing treatment

In this study, we explored the phase compositions and morphologies of the ceramic coatings from different aluminum sources (aluminum isopropoxide, aluminum nitrate, or a mixture of the two) prepared using cathode plasma electrolytic deposition (CPED) onto AZ31 magnesium alloys. Scanning electron microscopy and X-ray diffraction analyses of these coatings indicate that the deposited ceramic made from aluminum isopropoxide was composed of γ-Al₂O₃ whereas the one made from aluminum nitrate was composed of MgA1₂O₄, and that the former was more compact and uniform than the latter. A composite coating was prepared using epoxy resin as a protective layer that sealed the micropores on the CPED coating, thereby further improving its anticorrosion property. The elemental distribution of the cross-section of the composite coating was examined via energy dispersive spectroscopy. Corrosion resistance was investigated using potentiodynamic polarization curves and electrochemical impedance spectroscopy in a 3.5?wt% NaCl medium, and a salt spray test. The results indicate that the corrosion protection property of the Al₂O₃/epoxy resin coating of the magnesium alloy was better than that of the single Al₂O₃ coating. A cross-cut test revealed that the adhesion of the Al₂O₃/epoxy resin composite coating to the magnesium alloy surface was better than that of the single epoxy resin coating. The approach presented herein provides an attractive way to modify the surface of magnesium alloys to improve anticorrosion.     

Corrosion resistance and anti-reaction mechanism of Al2O3-based refractory ceramic under weak static magnetic field

A slag resistance experiment of the Al₂O₃-based refractory ceramic with CaO–Al₂O₃–SiO₂ slag at 1600°C under a milli-Tesla static magnetic field was conducted. The magnetic flux density effect on the corrosion at the two- and three-phase interfaces of the Al₂O₃-based refractory ceramic, excluding the ‘electromagnetic damping’, was studied. The slag resistance of the Al₂O₃-based refractory was enhanced and quasi-volcanic corrosion at the three-phase interface was eliminated gradually with an increase in the magnetic flux density. A hypothesis and mechanism for the inhibition effect of the static magnetic field based on the free radical pair reaction model was proposed.     

Alumina ceramic tool material with enhanced properties through the addition of bionic prepared nano SiC@graphene

Graphene-coated SiC nanoparticles containing graphene floating bands (SiC@G) were prepared by a liquid-phase laser irradiation technique, and SiC@G nanoparticles with high dispersivity were incorporated into an Al₂O₃ matrix. An Al₂O₃-based composite ceramic tool was prepared by spark plasma sintering (SPS), and the effects of SiC@G nanoparticles on the mechanical and cutting properties and microstructure of the materials were further investigated. Analysis of the cross-sectional morphology shows that SiC@G nanoparticles containing graphene floating bands were homogeneously dispersed in the composite, which resulted in tighter bonds between the Al₂O₃ particles. This particular core-shell structure increased the contact area between the graphene and the matrix due to the formation of a graphene 3D mesh by extrusion, which enhanced the difficulty of relative sliding of graphene. Second, this special core-shell structure also made the crack propagation path more tortuous, further increasing the energy consumed in the fracture process, which is conducive to improving the mechanical properties of ceramic tools. The addition of SiC@G nanoparticles improves the mechanical properties of Al₂O₃-based composite ceramic tools. The fracture toughness (7.2 Mpa·m^1/2) and flexural strength (709 MPa) increased by 75.6% and 28.7%, respectively. Cutting experiments with Al₂O₃/SiC/G composite ceramic tool and Al₂O₃/SiC@G composite ceramic tools on 40Cr hardened steel were performed. The results prove that the addition of SiC@G nanoparticles improves the cutting life by 18.1% and reduces the cutting force and friction coefficient by 6.3% and 14.8%, respectively.     

Nanostructured ceramic composite coating prepared by reactive plasma spraying micro-sized Al–Fe2O3 composite powders

Nanostructured ceramic matrix composite coating was prepared in-situ by reactive plasma spraying micro-sized Al-Fe₂O₃ composite powders. The microstructure, toughness and Vickers hardness of these coatings were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that the coating exhibited nanostructures which consisted of FeAl₂O₄, Al₂O₃, Fe (or Fe solid solution) and a little FeAl. The composite coating showed significantly higher toughness and wear resistance than the conventional Al₂O₃ coating.     

Cathodic plasma electrolytic deposition of ZrO2/YSZ doped Al2O3 ceramic coating on TiAl alloy

ZrO₂/yttria-stabilized zirconia (YSZ) doping Al₂O₃ ceramic coating was fabricated via cathodic plasma electrolytic deposition (CPED) technique. The microstructures and the chemical and phase compositions of the doped coating were characterized, the mechanical properties and the high temperature oxidation resistance were evaluated, and the doping mechanism was also discussed in detail. The results showed that, doped Zr⁴⁺ and Y³⁺ ions could effectively reduce the working voltage during CPED process and increase the content of metastable γ-Al₂O₃ in the coating. Accordingly, the doped ZrO₂/YSZ significantly refined the grain size of Al₂O₃, as well as remarkably improved the high temperature oxidation resistance, the micro-structural compactness and hardness of the Al₂O₃ CPED coating. This study displayed here constructed an efficiently method for the fabrication of multifunctional coating on the surface of TiAl alloy.     

Influence of different introduction modes of metal Ag into plasma sprayed Al2O3 coating on tribological properties

Al₂O₃ coating and Al₂O₃/Ag (10%) composite coating were prepared on the surface of GH4169 superalloy by the atmospheric plasma spraying technology. And an in-situ synthesis method was applied to introduce the Ag particles into a part of Al₂O₃ coatings to obtain Al₂O₃/Ag(synthesis) composite coating. Then, the microstructure and mechanical properties of these three Al₂O₃-based coatings were systematically studied in this work. In order to reveal the lubrication characteristics of Ag, their friction tests were carried out at room temperature (RT), 400 °C, 600 °C and 800 °C, respectively. The results showed that both microstructure and mechanical properties of Al₂O₃/Ag(synthesis) composite coating were better than that of Al₂O₃/Ag (10%) composite coating because many pores and cracks produced during the direct spraying. Although the friction coefficients of two kinds of composite coatings were close to that of Al₂O₃ coatings at RT, their wear rates were both greatly decreased due to the introduction of Ag. In addition, the lubricating performance of Ag was not enough to reduce their friction coefficients when friction temperature is lower than 600 °C. However, the friction coefficients of these composite coatings were both reduced to about 0.3 at 800 °C . At this time, the Al₂O₃/Ag(synthesis) composite coating also exhibited a lower wear rate because of its dense microstructure and excellent mechanical properties.     

Compressive strength and impact resistance of Al2O3/Al composite structures fabricated by digital light processing

The combination of multi-materials is an alternative way to meet the diverse requirements for various applications. However, the processing difficulty especially in ceramic forming limited the structural innovations. In this paper, a combined methods of ceramic digital light processing (DLP) and metal infiltration is proposed to fabricate the Al₂O₃/Al composite structures with controllable ceramic skeletons. The interfacial, compressive and impact resistance properties were studied. The results showed that the Al₂O₃ grains were closely bonded, and no destructive defects occurred at the interface between Al₂O₃ and Al. The energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) results showed that the formation of protective phases contributed to the improvement of bonding strength. The compressive tests showed that the composite structures had better capabilities to absorb and resist the applied loads compared with Al structures. Finally, the impact resistance of the structures was discussed, the finite element analysis and the experimental results showed that the composite structures had advantages in dissipating the energy of incident objects and reducing the penetration depth. Based on these results, the damage model of Al₂O₃/Al structures was established, and the roles of different materials were revealed.     

Fabrication of high-quality alumina coating through novel,dual-particle aerosol deposition

High-quality alumina (Al₂O₃) coating has an extensive demand in the fields of optoelectronics, solar cells, and corrosion/impurity resistant coatings and cutting tools. The quality of alumina coating depends on its hardness and transparency. To obtain hard and highly transparent alumina coating, which is also a widely used ceramic material, a novel, aerosol deposition approach is presented in which the starting powder, used to fabricate Al₂O₃ ceramic coatings, is composed of angular and spherical Al₂O₃ particles. Films fabricated using angular:spherical Al₂O₃ particle mixtures with ratios of 10:0, 7:3, 5:5, 3:7, and 0:10, showed significant variation in surface roughness and microstructure. The dramatic morphology modulation of the 3:7 angular:spherical Al₂O₃ mixture film, resulting from the superposition hammering effect caused by the aerosol mixture, improved transmittance (84.7%) and hardness (13.6 GPa). Previous studies used high-energy approaches to optimize Al₂O₃ film properties. This dual-particle approach, however, produces Al₂O₃ film with excellent transmittance and hardness while achieving a fast coating speed (32 mm² × μm/min) without additional thermal treatment. Our proposed approach provides a novel and energy efficient method to produce transparent Al₂O₃ films with superior durability.     

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Growth rate effect on colony formation in directional solidification of Al2O3/YAG/ZrO2

Mechanical properties of Al₂O₃/Y₃Al₅O₁₂/ZrO₂ ternary eutectic ceramics are strongly affected by structural defects as pores or colonies. Experimental investigation of the microstructure of this ternary composite indicates that the colonies are generally observed when the solidification occurs at high rates. In this work, the influence of the growth rate on the solid-liquid interface shape and formation of colonies in directional solidification of Al₂O₃/Y₃Al₅O₁₂/ZrO₂ by Bridgman, Edge-defined Film-fed Growth (EFG), and Czochralski (Cz) methods is numerically and experimentally investigated. Numerical modeling of the Bridgman growth process shows large curvatures of the solid-liquid interface when the pulling rate is increased up to 80 mm/h. The ingots solidified at rates between 5 and 80 mm/h exhibit colony type microstructure. The analysis of EFG growth of ceramic ribbons reveals less curved solid-liquid interfaces in this system. Numerical modeling shows significant increase in the interface curvature with increasing pulling rate. The microstructure of ribbons grown at pulling rates between 6 and 12 mm/h exhibits colonies only for the ingots solidified at higher rate. Simulations carried out for Czochralski growth process show that the solidification front is almost plane in this system. These results are in agreement with experimental observations showing good structural quality of Cz grown crystals with a flat solid-liquid interface. Finally it is concluded that formation of colonies in directional solidification of this ternary eutectic composite is linked to large curvatures of the growth interface.     

Wettability and interfacial reactions of a low Hf-containing nickel-based superalloy on Al2O3-based,SiO2-based,ZrSiO4, and CoAl2O4 substrates

To understand the wetting behavior and interfacial phenomena between molten superalloys and ceramic materials, the wettability and interfacial reactions of a low Hf-containing Nickel-based superalloy on the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ substrates were studied using the sessile drop method at 1773 K. The wetting angles of the alloy on the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ substrates were 141.4°, 143.5°, 135.7°, and 128.4°, respectively. This indicated that the wettability of the alloy on the Al₂O₃-based substrate was comparable to that on the SiO₂-based substrate, and the wettability of the CoAl₂O₄ system was the best among the four systems. The microstructure characteristics of the interface implied that Hf has a strong tendency to react with ceramic substrates, even at low contents. Additionally, the interfacial reactions transformed the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ ceramic substrates into (Al₂O₃ + HfO₂), (Al₂O₃ + HfO₂), (Al₂O₃ + HfO₂ + ZrO₂), and (Al₂O₃ + HfO₂ + Co), respectively, which were in contact with the alloys. The experimental results demonstrated that the wettability of the system was governed by the properties of the reaction products.     

Plasma electrolytic deposition of α-Al2O3 on TiNb fibres and their mechanical properties

A novel TiNb fibre with an α-Al₂O₃ coating was fabricated by cathodic plasma electrolytic deposition (CPED), which has enormous potential for use in intermetallic matrix composites (IMMCs). This study aims to clarify the microstructural evolution of α-Al₂O₃ coatings on TiNb fibres and to systematically evaluate the mechanical properties of such modified fibres. The results revealed that the CPED process can be divided into three stages as voltage and deposition time increased: gas film formation, spark discharge, and spark fading, where the coating successively underwent local nucleation, uniform deposition, micropore self-sealing, and loose structure formation. The optimum deposition parameters of the deposition voltage of 300–400 V and deposition time of 3–4 min were determined, under which the α-Al₂O₃ coating combined tightly with the TiNb fibre matrix, micropores were completely self-sealed, and the loose structure and detrimental phase transitions in TiNb were effectively avoided. The fracture strength calculated by the Weibull method suggested that the fracture strength of the modified Al₂O₃/TiNb fibre was enhanced by more than 30%; this improved strength maintained high stability, benefiting from the intact α-Al₂O₃ ceramic coating. In particular, the fibre coated at 300 V for 4 min had the highest strength reaching 1620 MPa. The fracture morphology presented marked necking and shear lip characteristics, indicating excellent plasticity.     

Effects of tool coating materials and coating thickness on cutting temperature distribution with coated tools

Coated tools are currently widely used tool technology in machining. The influence of tool coating on heat transfer has become an active field of research enjoying constantly increasing attention in the field of machining. This paper is devoted to the cutting temperature in machining H13 hardened steel with monolayer coated tools (TiN, TiAlN, and Al₂O₃) and multilayer coated tools (TiN/TiC/TiN and TiAlN/TiN). Equivalent composite thermal conductivity and thermal diffusivity of multilayer coated tools were calculated using the equivalent approach. The established heat transfer analytical models estimated coating temperature in turning. The effect of tool coating in steady and transient heat transfer was studied, as well as the cutting temperature distribution. It reveals that the tool coating material and coating thickness can influence the cutting temperature distribution of coated tool. Thermal conductivity of coating material affects the steady cutting temperature distribution, and thermal diffusivity of coating material affects the transient cutting temperature distribution of coating tools.     

Effect of TiN content on properties of NiFe2O4-based ceramic inert anode for aluminum electrolysis

NiFe₂O₄-based ceramic inert anodes for aluminum electrolysis doped with various TiN nanoparticles were prepared by a two-step cold-pressing sintering process to investigate how TiN affected the sintering behavior and properties of the composites. The differential scanning calorimetry-thermogravimetry (DSC-TG), X-ray diffraction (XRD), and microstructure analysis results indicated that the Ti and N were evenly distributed in the NiFe₂O₄ matrix to form a solid solution. The maximum linear shrinkage and linear shrinkage rate were enhanced with the increase of TiN nanoparticles contents, and the sintering activation energy of initial stage was lowered from 382.63 to 279.58 kJ mol⁻¹ with the TiN nanoparticles additive range from 0 to 9 wt%. When the content of TiN nanoparticles was 7 wt%, the relative density, bending strength, and elastic modulus reached their maximum values of 97.24%, 73.88 MPa, and 3.77 GPa, respectively, whereas the minimum static corrosion rate of NiFe₂O₄-based ceramic of 0.00114 g cm⁻² h⁻¹ was obtained, mainly attributed to the relatively dense and stable microstructure. The electrical conductivity of NiFe₂O₄-based ceramics presented a clear ascending trend with increasing TiN nanoparticles content and elevated temperature, attributed to the increased concentration and migration rate of carrier.