Mechanism Of Coating Formation Research Articles

Microstructure and high-temperature oxidation resistance of Ti-6Al-4V alloy with in-situ SiC-SiO2 nano-composite coating by LPDS technique

A SiC-SiO2 nano-composite coating was prepared via a novel liquid-phase plasma-assisted particle deposition and sintering (LPDS) method to enhance the oxidation resistance of the Ti-6Al-4V alloy. For comparison, a conventional plasma electrolytic oxidation (PEO) ceramic coating is fabricated on the Ti-6Al-4V alloy. The microstructure and formation mechanisms of both ceramic coatings were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results indicated that the thickness of the SiC nano-composite coating (∼40 μm) increased significantly by 285% compared to the PEO coating (∼14 μm). The microstructural evolution and isothermal oxidation performance of the PEO and SiC-SiO2 nano-composite coating were comparatively investigated at 800°C. After 100 h, the thickness gain of the SiC-SiO2 nano-composite coating (∼14 μm) was lower than that of the PEO coating (∼26 μm). The improved oxidation performance is primarily attributed to the outermost layer containing abundant SiC nanoparticles, which transform into SiO2 during the oxidation process and effectively inhibiting the inward penetration of oxygen.

The growth behavior and performance of microarc oxidation coating on AZ91/Ti composite: Influence of Ti-reinforcement phase and electrolyte

Magnesium matrix composites with both high strength and ductility have been achieved by introducing pure Ti particles. However, the properties of the surfaces of the composites need to be improved by surface technology, such as micro-arc oxidation (MAO). In this study, we investigated the influence of the Ti-reinforcement phase on coating growth and evolution by subjecting both AZ91 alloy and AZ91/Ti composite to MAO treatment using silicate-based and phosphate-based electrolytes. Results revealed that the Ti-reinforcement phase influenced the MAO process, altering discharge behavior, and leading to a decreased cell voltage. The vigorous discharge of the Ti-reinforcement phase induced the formation of coating discharge channels, concurrently dissolving and oxidizing Ti-reinforcement to produce a composite ceramic coating with TiO2. The MAO coating on the AZ91/Ti composite exhibited a dark blue macromorphology and distinctive local micromorphological anomalies. In silicate electrolyte, a “volcano-like” localized morphology centered on the discharge channel emerged. In contrast, treatment in phosphate-based electrolyte resulted in a coating morphology similar to typical porous ceramic coatings, with visible radial discharge micropores at the reinforcement phase location. Compared to the AZ91 alloy, the coating on the AZ91/Ti composite exhibited lower thickness and higher porosity. MAO treatment reduced the self-corrosion current density of the AZ91/Ti surface by two orders of magnitude. The silicate coating demonstrated better corrosion resistance than the phosphate coating, attributed to its lower porosity. The formation mechanism of MAO coatings on AZ91/Ti composites in phosphate-based and silicate-based electrolytes was proposed.

Effect of Al3+ on the surface microstructure and film formation mechanism of Zr-based conversion coatings with improved corrosion resistance performance on cold rolled steel

Most Zr conversion coatings are prepared on Al alloys, with Al3+ from the substrate contributing significantly to the film growth process. Thus, the effect of Al3+ on the electrochemical characteristics, surface microstructure and film-formation mechanism of Zr-based conversion coatings on old- rolled steel (CRS) substrates was investigated by growing the coatings with and without Al3+ as an additive. Electrochemical tests indicated that the anticorrosion performance of the Zr-Al coatings prepared in the presence of Al3+ was greatly superior to that of Zr conversion coatings obtained in the absence of Al3+. Field-emission scanning electron microscopy and X-ray photoelectron spectroscopy analysis indicated that adding of Al3+ in the conversion bath changed the surface microstructure of the Zr coating, greatly promoted the deposition of Zr and F on the CRS substrate and increased the film thickness and density. Both coatings were embedded into the etched CRS substrates. The presence of Al3+ additive induced the formation of different coating components. The Zr coating mainly consisted of ZrO2 and some FeFx, whereas the Zr-Al coatings comprised of ZrO2, ZrOxFy, Al2O3, AlFx and some FeFx. The present study results confirm the key role of Al3+ as an additive in the film formation and growth process of Zr coatings and its effect on enhancing the anticorrosion performance of CRS substrates in NaCl solutions.

Formation Mechanism of Al<sub>2</sub>O<sub>3</sub>/HfO<sub>2</sub> Eutectic Coating on SiC Substrate Using Light Focusing Method

In this paper, we discuss our previous experimental results and recent considerations on the preparation of solidified oxide eutectic coating on non-oxide ceramics by unidirectional solidification method using light focusing apparatus. As an example, an explanation is given here with regard to the formation mechanism of Al2O3-HfO2 eutectic coatings on SiC substrate. The Al2O3 component in oxide melt easily reacts with SiC and primary HfO2 phase solidified on SiC substrate. However, the solidified HfO2 phase also react with SiC substrate and HfC-HfO2 graded functional layer is formed on SiC substrate and Al2O3-HfO2 eutectic microstructure is formed on the graded functional layer

Growth and defect formation mechanism of CVD-prepared SiC coatings based on cross-scale simulation

The high quality and efficient production of third-generation semiconductor materials has recently been a hot research topic. At the same time, the substrate material properties could significantly influence the MOCVD quality. In this study, the CVD SiC process on graphite substrate was efficiently controlled by molecular dynamics (MD)-finite volume (FVM) cross-scale simulations. The key deposition parameters such as MTS-H2 flow rate, deposition pressure, and reaction source nMTS:nH2 were first simulated by MD. Subsequently, the results obtained from the MD simulations were input into the FVM software, and CFD simulations were performed in a home-made reactor. The cross-scale simulation results from microscopic chemical reactions and crystal structures to macroscopic airflow fields and deposition rate distributions were obtained. The defects of CVD SiC were optimized by visualizing the simulation process. The tests showed that the graphite substrate was effectively protected by the SiC coating on the surface. The optimal deposition parameters for CVD SiC coatings were finally obtained. This study laid the foundation for the mass production of high-quality semiconductor materials.

Robust transparent superhydrophilic antifogging coatings on glass substrates by a facile rapid thermal process

In this study, a rapid thermal process, which can be achieved via the conventional glass tempering process, was utilized to fabricate silica-based superhydrophilic antifogging coatings with micro-nano hierarchical roughness on glass substrates. The effects of SiO2 nanoparticle and triethylborate loadings on the coatings’ properties have been systematically investigated. Moreover, the formation mechanism of such coatings has been explained. The optimal coatings demonstrate a high level of hydrophilicity with a water contact angle (WCA) of ∼ 0°. Meanwhile, such coatings exhibit good antifogging effect and robustness as well.

The relation of formation mechanism, microstructural evolution and mechanical properties of Ni-based coatings produced via high pressure cold spray deposition

The two kinds of NiCr and NiCo superalloy coatings are deposited by high pressure cold sprayed (CS) technique, in this work. The emphasis is placed at the relation of formation mechanism, microstructural evolution and mechanical properties of the coatings. Compared with the NiCr powder, the NiCo powder needs higher spraying parameters to form the dense coating. The NiCr coating has a higher density and a lower hardness than the NiCo coating, but both of the coatings display better mechanical properties, and thus could play a protective role on the 316L stainless steel. However, there is a difference of formation mechanism between the NiCr and NiCo because of the particle sizes and chemical properties. The deformation of NiCr coating occurs mostly in the interior of particles, and that of NiCo coating is more prominent at the boundaries between the particles. The defects formed during deposition, including twins and dislocations, are benefit for the improvement of mechanical properties of the coatings.

Numerical simulation study on the formation mechanism of hydroxyapatite-silver gradient bioactive ceramic coatings under wide-band laser cladding

Numerical simulation study on the formation mechanism of hydroxyapatite-silver gradient bioactive ceramic coatings under wide-band laser cladding

Forming characteristics and mechanisms of micro-arc oxidation coatings on magnesium alloys based on different types of single electrolyte solutions

To systematically investigates the forming ability and properties of micro-arc oxidation (MAO) coatings on magnesium alloys using different types of single electrolytes and further enhance the understanding of the coating formation mechanism from multiple perspectives. MAO treatments were performed in three types of single electrolyte solutions containing either sodium silicate, potassium fluoride, or sodium hydroxide, which were severally designed in terms of main salts, additives, and alkaline substances. The results show that MAO coatings could still be formed in the three single electrolyte solutions; however, the sparks and observable phenomena on the sample surface differed obviously from those in multicomponent electrolytes. In potassium fluoride and sodium hydroxide electrolytes, sparks occurred continuously at fixed positions on the sample surface, forming concave defects with uneven distributions of Mg, F, and O on the coating surface. In addition, a larger response current was observed in the sodium hydroxide electrolyte during MAO; this is disadvantageous in reducing energy consumption. The coating obtained in the potassium fluoride electrolyte had the highest thickness and number of micropores because of the synergy between the electrochemical process and the spark state on the sample surface. In contrast, the coating prepared in the sodium silicate electrolyte exhibited better corrosion resistance because of a certain thickness and good compactness.

Influence of Feedstock in the Formation Mechanism of Cold-Sprayed Copper Coatings

The aim of this article is to investigate the characterizations and formation mechanisms of cold-sprayed coatings using gas-atomized and electrolytic powders. The study highlights the importance of reaching the particles’ critical velocity for optimal deposition. The main findings reveal that the morphology and stacking conditions of the coatings have a significant impact on their mechanical properties. Coatings made with gas-atomized powders exhibited noticeable pores and higher plastic deformation, while electrolytic powder coatings had greater density and smoother interfaces with the substrate. Adhesion strength relied on the physical bonding resulting from the plastic deformation energy between the spraying powders and the substrate. Gas-atomized powders showed higher adhesion compared to electrolytic powders, with dendritic powders resulting in lower adhesion due to dispersed impact force. The interaction between thermal and kinetic energy during the cold spraying process facilitated plastic deformation and particle deposition by softening and eroding the substrate surface. Insufficient plastic deformation with dendritic powders led to incomplete overlap, pore formation at the interface, and the persistence of the oxide layer along powder boundaries. Overall, these findings provide valuable insights into the influence of powder properties on coating morphology, adhesion strength, and overall performance, contributing to the understanding and optimization of cold spray processes.

Studies on the growth mechanism of aluminide coating on K444 alloy surface by chemical vapor deposition

The studies of the growth mechanism of aluminide coatings by chemical vapor deposition (CVD) is an important prerequisite for optimizing the thermal protection properties of aluminide coatings and improving the CVD process parameters; however, the current research on the growth mechanism of CVD aluminide coatings on nickel-based alloys is relatively limited, and there is a lack of systematic studies in this area. In this work, aluminide coatings were prepared on the surface of nickel-based superalloys by the CVD method, and elemental diffusion and microstructure effects between coating and substrate at different deposition temperatures and times, as well as the growth kinetics and formation mechanism of CVD aluminide coatings were investigated. The results showed that the coating prepared on the surface of K444 nickel-based high-temperature alloy by CVD was an externally diffused aluminide coating with a bilayer structure: the outer layer of the coating was the β-NiAl phase. The interdiffusion layer was mainly composed of nickel-aluminum compounds, carbides, and topologically close-packed phase. The growth of the CVD aluminide coating was mainly controlled by the grain boundary diffusion mechanism.

Direct Fabrication and Characterization of Zirconia Thick Coatings on Zirconium Hydride as a Hydrogen Permeation Barrier

The present work attempted to produce thick zirconia coatings formed by micro-arc oxidation as a hydrogen permeation barrier on zirconium hydride alloy. A novel multiphase zirconia coating was achieved, exhibiting superior hydrogen permeation barrier performance. The growth dynamics, formation mechanism, and phase evolution behavior of thick zirconia coatings were explored, and the hydrogen permeation barrier performance was evaluated by means of vacuum dehydrogenation experiment. The hydrogen desorption quantity was monitored by analyzing pressure changes with a quadruple mass spectrometer (QMS). Experimental results show that the multiphase coatings were composed of monoclinic ZrO2 (m-ZrO2), tetragonal ZrO2 (t-ZrO2), and a trace of cubic ZrO2 (c-ZrO2). The coatings were generally divided into a dense and uniform inner, intermediate layer, and a porous top layer. The quantitative analysis indicates an increased amount of m-ZrO2 toward the coating surface and an increased amount of t-ZrO2 toward the oxide/metal interface. This novel multiphase thick zirconia coating can noticeably improve hydrogen permeation resistance, and the permeation reduction factor (PRF) value is improved by nearly 13 times compared with bare zirconium hydride. It is demonstrated that hydrogen desorption is retarded to some extent in the presence of thick zirconia coating. Hydrogen desorption of the sample with ceramic coating started at 660 °C, which was apparently higher than that of the sample without coating.

Microstructure evolution, growth kinetics and formation mechanisms of silicon-rich NbSi2 coatings on Nb substrate

A Si-rich NbSi2 coating was prepared on Nb substrate by hot dip silicon plating process, which mainly consists of Nb5Si3 interfacial layer, NbSi2 intermediate layer and silicon-rich NbSi2 outer layer. The Si-rich NbSi2 coating has flat surface and dense structure, and the surface roughness is only 0.218–0.548 μm. Raising the deposition temperature can also significantly increase the growth rate of the Si-rich NbSi2 coating. The hot-dip silicon plating process has an extremely low reaction activation energy (249.99 kJ mol−1), which indicates that there are a large number of activated molecules during hot dip silicon plating. The silicon atoms in the molten bath deposited on the Nb surface and maintained a high chemical potential gradient continuously. Therefore, the solid diffusion reaction of Nb–Si was continuous, and the Nb5Si3 phase was generated and consumed continuously, and transformed into NbSi2 phase, which led to a higher growth rate of coating (1.7 × 10−13 to 4.3 × 10−13 m2 s).

Technical characteristics and coating formation mechanism of gilded silver products unearthed from the Consort Tomb of Emperor Shengzong of the Liao dynasty

Compared with other gold plating processes, mercury gilding was widely used in ancient China due to the less consumption of gold and excellent adhesion between the gold layer and substrate. Herein, the comprehensive analyses of the silver crown and boots unearthed from the Consort Tombs of Emperor Shengzong (圣宗萧贵妃) of the Liao (辽) Dynasty reveal the surface composition and structural characteristics and also the unique multilayered structure and cross-sectional compositional distribution of the gold layer. First, the existence of mercury on the golden surface of cultural relics, as well as the unique porous structure and granular surface morphology, indicate that the artifacts were probably processed using mercury gilding. In addition, the intermetallic compound, i.e., Au5Hg, in the gold layer indicates that the processing temperature was 388–419 ℃; thus, the formation mechanism was the transformation of solid solution in the amalgam to intermetallic compound during the heating of Au-Hg system. Moreover, the cross-sectional transition layer is mainly composed of Ag and Hg due to the interstitial diffusion of mercury atoms at room temperature and vacancy diffusion of silver atoms during the heating process, bonding the gold layer with the substrate. Obviously, the bonding mechanism of the gold layer and substrate was interstitial diffusion and vacancy diffusion within the Ag-Hg system. The current work reveals the special multilayered cross-section of silver gilding relics and shows that mercury played a crucial role in both the formation of the gold layer and bonding between the gold layer and substrate during the silver gilding process.

Antibacterial Activity and Cell Viability of Biomimetic Magnesian Calcite Coatings on Biodegradable Mg.

Mg is a material of choice for biodegradable implants. The main challenge for using Mg in temporary implants is to provide protective surfaces that mitigate its rapid degradation in biological fluids and also confer sufficient cytocompatibility and bacterial resistance to Mg-coated surfaces. Even though carbonate mineralization is the most important source of biominerals, such as the skeletons and shells of many marine organisms, there has been little success in the controlled growth of carbonate layers by synthetic processes. We present here the formation mechanism, antibacterial activity, and cell viability of magnesian calcite biomimetic coatings grown on biodegradable Mg via a green, one-step route. Cell compatibility assessment showed cell viability higher than 80% after 72 h using fibroblast cells (NCTC, clone L929) and higher than 60% after 72 h using human osteoblast-like cells (SaOS-2); the cells displayed a normal appearance and a density similar to the control sample. Antimicrobial potential evaluation against both Gram-positive (Staphylococcus aureus (ATCC 25923)) and Gram-negative (Pseudomonas aeruginosa (ATCC 27853)) strains demonstrated that the coated samples significantly inhibited bacterial adhesion and biofilm formation compared to the untreated control. Calcite coatings grown on biodegradable Mg by a single coating process showed the necessary properties of cell compatibility and bacterial resistance for application in surface-modified Mg biomaterials for temporary implants.

The Aqueous Chemistry of Zirconium as a Basis for Better Understanding the Formation of Zirconium Conversion Coatings: Updated Thermodynamic Data

This work tackles the aqueous chemistry of Zr, aiming to contribute to a better understanding of Zr conversion coatings as one of the important contemporary means of corrosion protection. Equilibrium predominance diagrams based on experimentally confirmed Zr–OH and Zr–F aqueous species concerning Zr amorphous solid phase, along with an updated Zr E−pH (Pourbaix) diagram, are constructed. Since ZrO2+ existence had been conclusively disproven in both the aqueous and solid states, we chose to depict mononuclear species with ZrOH3+ and polynuclear with Zr4(OH)8 8+. The formation of the Zr solid phase is assumed to include Zr tetrameric species, Zr4(OH)8 8+, as a fundamental building block thereof. The role of F– and ZrF6 2– ions in Zr conversion baths and subsequently formed coatings is described. The introduction of ZrF6 2– anions keeps Zr solvated in the form of a complex, thus preventing the early onset of hydrolysis. The conversion of Zr species and the coating formation mechanism are further discussed from electrochemical and sol-gel perspectives, aiming to give a foundation for future predictions and rationalisation of Zr conversion coating formation, with the possibility of extensions to various bath additives.

Phenolic-Compound-Based Functional Coatings: Versatile Surface Chemistry and Biomedical Applications.

Phenolic-compound-based functional coatings that allow for flexible modulation of chemical and surface properties have found widespread uses in a diverse range of biomedical applications from antibiofouling and antioxidation to bioimaging, therapeutics, and controlled drug delivery. It is imperative to understand the formation mechanism of phenolic coatings to fully meet the needs of their emerging applications in controlling the surface properties of biomaterials and medical devices. In this Perspective, we highlight the versatile chemical and self-assembly approaches to generate phenolic coatings with tailored surface properties and reactivities and also discuss how the surface properties and chemical reactivities impart functional materials for translational research.

Reactive air aluminizing of a nickel-based superalloy (IN738LC): Coating formation mechanism

Reactive air aluminizing (RAA) based on a slurry route is a low-cost method with high efficiency to form aluminide coatings on surfaces where final coating microstructures are temperature and time-dependent. This study aims to incorporate the RAA method into standard heat-treatment cycles of IN738LC. Painted samples are solutionized, solutionized then aged, and only aged. Different samples are prepared at different stages of aluminide formation. FE-SEM surface morphology, BSE-SEM cross-sectional observations, surface XRD characterization, and comparing EDS elemental composition with Al-Ni-Cr phase diagrams are utilized to discuss the formation mechanism thoroughly. A classical high activity type of aluminide coatings with a triple-layered structure of β-NiAl and Cr-rich precipitates at the top coat, β-NiAl in the middle coat, and an IDZ is formed by the RAA method. A three-stage formation mechanism, including the reactive stage (interaction between molten Al and solid substrate), Al-inward diffusion at low and Ni-outward, and Al-inward diffusions at high temperatures, is proposed in detail. After heat-treatment cycles, a ~3 μm aluminum oxide protective scale is detected on the sample surfaces. γ-Al2O3 and α-Al2O3 are formed during only aging at 845 °C and solutionizing at 1120 °C on sample surfaces, respectively.

Interfacial oxidation and boundary amorphization deposition mechanisms of GaN powder on metallic substrate by low-pressure cold spraying

Understanding powder adhesion mechanism in cold spray is primordial to control the coating formation. Previous studies have suggested that metallic coatings are governed by metallurgical bonding, but no consensus has been established regarding the bonding mechanisms of ceramic particles. In this study, the deposition mechanism of agglomerated gallium nitride (GaN) particles cold sprayed on stainless steel substrates was investigated. The evolution of the oxide layer thickness and coating/particle microstructures were analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy. The results revealed that although mechanical interlocking and coating thickness were enhanced by the substrate surface roughness, grain refinement was not the key factor for the brittle particle deposition, considering the diameter of the nano-sized particles. Instead, a new interfacial oxide layer was formed because of the destruction and removal of native oxide films upon impact. Local heteroepitaxy, which occurred near the interface of the newly formed gallium oxide, enhanced the coating formation. Compared with the GaN feedstock, the coating exhibited boundary amorphization and a high dislocation density induced by the high strain rate. Thus, this study proposed considerable insight into the formation mechanisms of cold-sprayed GaN coatings, which can support a more expanded range of ceramic/metal combinations in the future.

Effect of spray angle and substrate material on formation mechanisms and properties of HVAF sprayed coatings

Thermally sprayed coatings are often used to enhance the surface properties (wear resistance, corrosion resistance, etc.) of engineering components in order to extend their performance and service lifetime. Typically, the industrial components to be coated possess complex geometries and are fabricated using different materials, which can influence the deposited coating's microstructure and performance. High-velocity air fuel (HVAF) process is a relatively new thermal spray processing technique that has shown tremendous potential to deposit high performance coatings for durable industrial components. However, no detailed studies have been reported on HVAF sprayed coating formation mechanisms so far in relation to the spray angle and substrate properties, and the influence of coating material on the above. The objective of this work was to study the influence of spray angles and substrate materials on splat characteristics, coating microstructure evolution, properties and performance for two distinct coating materials.In this study, one cermet (WC-CoCr) and one metallic (Inconel 625) feedstock were deposited onto three different substrates (aluminium alloy, carbon steel and Hastelloy-X) utilising different spray angles (40°, 60° and 90°). The coating evolution was analysed utilising SEM/EDS, image analysis, and micro-indentation. To determine the tribological performance, coatings were subjected to dry sliding wear test utilising alumina ball as counter surface and specific wear rates were obtained.The results showed that initial splat characteristics were substantially altered on changing the substrate and the spray angle. However, the final coating properties were not affected significantly even though the deposition rate was reduced significantly at lower spray angle, suggesting the versatility of the HVAF process.