Mechanism Of Coating Formation Research Articles

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.

Development of Cr and Al Pack Cementation Coatings on Co-Based γ/γ′ Superalloys

Co-based superalloys have been developed as candidate materials to replace Ni-based superalloys in hot sections of turbine engines, however, their oxidation resistance is limited. Therefore, in this work chromium and aluminum diffusion coatings were developed via the pack cementation process for novel γ/γ′-strengthened superalloys of the Co–Ni–Al–W system. This alloy system also offers the opportunity to vary the cobalt-to-nickel ratio and the tungsten content systematically to investigate their influence on the coating process. The coating process was shown to successfully enrich the surfaces of such alloys either in aluminum or chromium. For Al, the result was the formation of (Co, Ni)Al on top of an interdiffusion zone, while for chromium a Cr-rich layer in solid solution with the substrate along with the formation of σ-phase were observed. The coating formation mechanisms are discussed for both coatings in the light of activities and phase formation.

Preparation and anticorrosion properties of electrodeposited calcium phosphate (CaP) coatings on Mg–Zn–Ca metallic glass

In order to improve the corrosion resistance and bioactivity of Mg–Zn–Ca metallic glass, a series of calcium phosphate (CaP) coatings were synthesized on Mg66Zn30Ca4 metallic glass via the electrodeposition method in this paper. As compared with the Mg-based metallic glass, the electrodeposited CaP coatings show considerable improvement in corrosion resistance and bioactivity. Based on the experimental results, the formation and anti-corrosion mechanisms of CaP coatings were proposed, suggesting that electrodeposited CaP coating can promote the precipitation of calcium phosphate and thereby induce the mineralization process. By constructing the predominance area diagram, the relationship between the composition of the CaP solution and the precipitation process of calcium phosphate (CaHPO4·2H2O, Ca5(PO4)3OH) at 25 °C was accurately described. These results provide insights into the electrodeposition process of CaP type coatings.

Material priority engineered metal-polyphenol networks: mechanism and platform for multifunctionalities

Engineering the surface of materials with desired multifunctionalities is an effective way to fight against multiple adverse factors during tissue repair process. Recently, metal-polyphenol networks (MPNs) have gained increasing attention because of their rapid and simple deposition process onto various substrates (silicon, quartz, gold and polypropylene sheets, etc.). However, the coating mechanism has not been clarified, and multifunctionalized MPNs remain unexplored. Herein, the flavonoid polyphenol procyanidin (PC) was selected to form PC-MPN coatings with Fe3+, and the effects of different assembly parameters, including pH, molar ratio between PC and Fe3+, and material priority during coating formation, were thoroughly evaluated. We found that the material priority (addition sequence of PC and Fe3+) had a great influence on the thickness of the formed PC-MPNs. Various surface techniques (e.g., ultraviolet–visible spectrophotometry, quartz crystal microbalance, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy) were used to investigate the formation mechanism of PC-MPNs. Then PC-MPNs were further engineered with multifunctionalities (fastening cellular attachment in the early stage, promoting long-term cellular proliferation, antioxidation and antibacterial activity). We believe that these findings could further reveal the coating formation mechanism of MPNs and guide the future design of MPN coatings with multifunctionalities, thereby greatly broadening their application prospects, such as in sensors, environments, drug delivery, and tissue engineering.Graphical

A dense Fe-Al/Al2O3 coating as tritium permeation barrier on CLAM steel by hot-dipping aluminizing

Exploring tritium permeation barriers with high permeation reduction factor (PRF) by an industrial scale process is crucial for tritium systems in fusion reactors. Herein, we fabricated a dense Fe-Al/Al2O3 coating by a simple hot-dipping aluminizing (HDA) method and optimized oxidation strategy. The microstructure, chemical composition, and crystallographic characteristic of obtained samples were investigated. And the permeation of deuterium through the sample was determined by gas-driven permeation (GDP) experiments. The results show that the obtained oxide coating mainly consisted of γ-Al2O3 layer and FeAl gradient transition layer. Notably, the Al2O3 layer formed by the oxidation of FeAl alloy coating possessed a high PRF value of 2394 at 550 °C. Compared with the direct oxidation of HDA sample, the oxide coating prepared by the alloy topcoat oxidation strategy was denser and had less defects. Meanwhile, the effect of Al topcoat and different FeAl alloy topcoats on the microstructure and performance of the oxide coating were investigated, respectively. And the formation mechanisms of oxide coatings prepared by different oxidation strategies were also analyzed.

Characterizations the deposition behavior and mechanical properties of detonation sprayed Fe-based amorphous coatings

In this study, the spreading morphologies and deposition behaviors of molten droplet in a single detonation sprayed shot under different oxygen-fuel ratio conditions are characterized in detail to reveal the formation mechanism of Fe-based amorphous coatings. Additionally, the porosities, micro-hardness, amorphous phase content and wear behavior of the coatings were also carefully evaluated to correlate the relationship between preparation process parameters, particle deposition behavior, spreading morphology, and mechanical properties. The results indicate that the spraying oxygen-fuel ratio (OF) parameter can determine the microstructure change of the coating, which is considered to be a reflection of the molten state of the sprayed particles. The low OF reduces the combustion efficiency, making the coating particles deposited in a form similar to cold spraying, resulting in uneven coating structure. In the case of medium OF (2.00), amorphous particles can form completely melted droplets and spread well on the substrate, and produce a good metallurgical bond with the substrate. Thanks to the good spread of the amorphous droplets, the OF-2.00 coating displays a very uniform and dense structure and uniformly distributed microhardness, which also makes its wear resistance (0.63 × 10−6 mm3 N−1 m−1) increased by about 8 times compared with OF-1.43 coating.

Preparation and formation mechanism of anodic coatings co-doped with Ga, Si, and P ions on Ti–6Al–4V alloys

In this study, gallium (Ga) ions, known to be effective against both bacteria and cancer, was incorporated into micro-arc oxidation (MAO) coatings on Ti–6Al–4V alloys. In a fixed solution of 5 g/L Na3PO4, the influences of Ga(NO3)3 concentration, Na2SiO3 concentration, Na2SO4 concentration and treatment time on coating properties were studied by using an orthogonal experiment to prepare Ga-Si-P coatings. In addition, the roles of each electrolyte on coating formation were clarified by using signal factor experiments. The results show that the raised Ga(NO3)3 concentration and current density continually increase the Ga content, showing that the Ga ions enter into MAO coatings by both diffusion and electromigration. Ga(NO3)3 is harmful for coating formation, while Na2SO4, Na2SiO3 and Na3PO4 act as film-forming agents. The Si content is negatively related to the P content resulted from the competition between Na2SiO3 and Na3PO4 to take part in the coating formation, while the P content is positively related to the Ga content attributed to the main existence form of Ga ions in MAO coatings as amorphous Ga2O3 and GaPO4.

Influence alloying elements of Al and Y in Mg Li alloy on the corrosion behavior and wear resistance of microarc oxidation coatings

Microarc oxidation (MAO) coatings were performed on different Mg Li alloys (Mg 14Li, Mg-14Li-3Al, Mg-14Li-3Al-1Y) in silicate electrolyte. The morphology, thickness, corrosion behavior and wear resistance of the coatings were characterized and analyzed. Results indicate that, compared to Mg 14Li alloy, adding Al and Y in the substrate refines the grains and forms the AlLi and Al 2 Y phases, contributing to the differences in corrosion behaviors of the Mg Li alloys. The alloying elements and uniformly fine microstructure modify the MAO growth process and promote the formation of thick coatings with fewer defects. The MAO coating on the Mg-14Li-3Al-1Y alloy possesses the best corrosion resistance with the i corr of 7.485 × 10 −6 A/cm 2 , due to the densest coating and the thickest inner layer. The reduced porosity and smoother surface enhance the hardness and wear resistance of the MAO coatings on the Mg-14Li-3Al and Mg-14Li-3Al-1Y alloys. It is worth considering the refinement of microstructure and incorporation of passive elements for preparing compact MAO coatings with excellent performances on Mg Li alloys. • The formation mechanism of MAO coatings was influenced by Al and Y in Mg 14Li alloy. • A compact and thick MAO coating was formed on Mg-14Li-3Al-1Y alloy. • High corrosion and wear resistant MAO coating can be obtained on Mg-14Li-3Al-1Y.

Formation and oxidation behavior of refractory high-entropy silicide (NbMoTaW)Si2 coating

A single high-entropy silicide (NbMoTaW)Si2 coating was prepared on NbMoTaW refractory alloy by a halide-activated pack cementation process. The formation mechanism and growth kinetics of the (NbMoTaW)Si2 coatings were analysed. The oxidation behavior of the (NbMoTaW)Si2 coatings was investigated at 1300 ℃ for 24 h in air. The results showed that the high entropy effect and sluggish effect of the high-entropy NbMoTaW alloy makes it form a single-phase silicide (NbMoTaW)Si2 during the Si diffusion growth process. The lattice distortion effect of substrate provides a channel for the diffusion of Si to accelerate its diffusion rate. The (NbMoTaW)Si2 coating effectively protects the NbMoTaW alloy from oxidation at 1300 ℃. The oxidation resistance of the coating is attributed to the preferential oxidation of Si to produce a dense continuous SiO2 scale and the precipitation of WSi2 to suppress the interdiffusion of the silicide layer and the alloy substrate.

Effect of reinforcement particles on cold-sprayed Al-based coatings

ABSTRACT In this study, malleable and heavy Ta particles were introduced into soft Al powders to prepare Al-Ta composite coatings through a low-pressure cold-spray process; for comparison, Al-A2O3 composite coatings were also deposited. The surface and cross-sectional morphologies, deposition efficiency, microstructure, hardness and composition of the coatings were investigated. The results demonstrated that the Al-Ta coatings had roughness values that were almost twice as large as those of the Al-Al2O3 coatings for the same volumetric fraction of reinforcement particles used in the feedstock. Ta particles were more effective in improving the deposition efficiency and densification than were Al2O3 particles. The formation mechanism of the Al-Ta coatings was mainly attributed to the malleable and heavy nature of Ta particles, which allowed them to bond with the formed coating and impact the coating strongly through the peening effect.

Design of Magnesia–Spinel Bricks for Improved Coating Adherence in Cement Rotary Kilns

It is well known that doloma bricks present better coating adherence than magnesia–spinel bricks when applied in cement rotary kilns, which is related to the different coating formation mechanism. The coating has an essential role in prolonged operation by protecting the refractory lining; thus, it is important to improve its adherence on magnesia–spinel refractories. The objective of this investigation is to study different compositions of magnesia–spinel bricks, achieved by varying additives used (calcined alumina, limestone, hematite and zirconia) and firing temperature (1500 °C and 1700 °C), to enhance the coating adherence measured by the sandwich test. The results have pointed out that the use of higher firing temperature contributes positively to physical adherence due to well-sintered refractory structure and elevated permeability, attaining coating strength superior to 2 MPa. For the chemical adherence, the addition of 2 wt.% of limestone increased the coating strength to 3 MPa, but resulted in a drop in hot properties. In this context, the most suitable approach to improve adherence of clinker coating and maintain hot properties in suitable levels is to increase the firing temperature.

Bioinert TiC ceramic coating prepared by laser cladding: Microstructures, wear resistance, and cytocompatibility of the coating

Recent advance in biological applications such as artificial joint replacement are focused towards fabricating reliable bearing surface to extend their service life. Given this perspective, incremental attempts have been made to enhance the wear resistance of titanium alloys used widely in artificial joint and mechanical reliability of brittle TiC ceramic, but plan often don't go along with reality. It is proposed that both high mechanical reliability and wear resistance can be hold if a thin TiC ceramic coating is formed onto high toughness Ti6Al4V alloy used widely in biological applications. In this paper, bioinert TiC ceramic coating was fabricated on Ti6Al4V substrate surfaces through laser cladding using preplaced TiC nanoceramic powders on the substrate surfaces. The microstructural characteristics of the TiC ceramic coating were investigated in detail. Further, the rapid melting mechanism of the TiC nanoparticles and the microstructural formation mechanism of the coating were revealed. Moreover, the wear mechanism and cytocompatibility of the coating were identified. The results showed that the original TiC nanoparticles completely melted and solidified to form dendritic TiC ceramic coating owing to the small size effect of the nanoparticles and the thermodynamic effect of the high-energy laser beam . The wear resistance of the TiC ceramic coating fabricated by laser cladding was three times higher than that of the Ti6Al4V substrate, and the coating had the same good cytocompatibility as that shown by Ti6Al4V. Microstructure-phase-property relationships in TiC ceramic coating were revealed and discussed according to our research findings. The designed TiC coating and Ti alloy hybrid might be potential application in artificial joint surfaces and other implants with strong wear-resistance requirements. • Bioinert TiC ceramic coatings on Ti6Al4V substrate was fabricated by laser cladding using preplaced TiC nanoparticles. • TiC dendrites were generated owing to the high-temperature diffusion of carbon atoms and Ostwald ripening mechanism. • The formation mechanism of ceramic coatings fabricated by laser cladding using TiC ceramic nano particles was proposed. • The coating showed comparable cytocompatibility to Ti alloy and did not affect cell adhesion, spreading, and proliferation.

Microstructure characterization of in-situ ZrC composite coating with graceful toughness and improved tribological properties prepared by plasma spraying

Substantial brittleness is still a problem to be solved for ceramic coatings. ZrC composite coating was prepared by plasma spraying ZrO2-SiC-Al composite powder utilizing in-situ reactions. As a comparison, ZrC-ZrSi2-Al2O3 composite powder was also used to prepare ZrC composite coating by plasma spraying via ex-situ approach. The reactions occurred in ZrO2-SiC-Al system during the plasma spraying process could be described as the thermite reaction between ZrO2 and Al and the self-propagating reaction between Zr and SiC. The corresponding formation mechanism of the composite coatings were also revealed. The resultant materials could melt fully under the effect of the plasma jet and the exothermic reactions. In-situ fabricated ZrC composite coating was featured with higher densification and better interlayer compactness compared to the coating prepared by ex-situ method. In-situ generated fine-grained microstructure, which was further confirmed by TEM, was homogenously distributed in the coating prepared by ZrO2-SiC-Al composite powder. Elevated toughness and excellent wear resistance observed in the coating prepared by plasma spraying ZrO2-SiC-Al composite powder could be attributed to the denser microstructure tailored by in-situ reactions.

Plasma electrolytic oxidation of copper in an aluminate based electrolyte with the respective additives of Na3PO4, NaH2PO4 and NaH2PO2

Plasma electrolytic oxidation of copper is first investigated in an aluminate-based electrolyte using trisodium phosphate (TP), sodium dihydrogen phosphate (DP) and sodium hypophosphite (HP), respectively, as additives. Black coatings can be successfully formed in the electrolytes of DP and HP. Detailed characterizations show that an initial thin layer of aluminum phosphate and alumina (AAP) was formed on cooper in DP electrolyte, which helps the fast establishment of plasma discharges, leading to successful coating formation. The AAP layer also formed in the electrolyte with TP as additive, but the AAP layer may be too thin to ensure stable plasma discharges. The coating formation mechanism is drastically different in HP: pitting corrosion along with loose Cu(OH)2 deposition occurs at initial stage, which may be a dual effect of local acidification and the destruction of the passive layer by the reducing hypophosphite ions. However, the deposited Cu(OH)2 and the subsequent formation of AAP layer may re-passivate the copper surface and provide an insulating layer for plasma discharging and coating formation. Optical emission spectroscopy (OES) and Mott-Schottky tests have also been used to investigate the features of plasma discharges and the semiconductor properties of the coatings, respectively.

Microstructure evolution of in-situ SiC-HfB2-Si ternary coating and its corrosion behaviors at ultra-high temperatures

An in-situ SiC-HfB2-Si ternary coating was deposited on C/C composites (C/Cs) via slurry panting plus gaseous Si infiltration composite method, to improve the oxidation and ablation resistance of C/Cs above 1773 K. The coating formation mechanism was investigated by microstructural analyses and thermo-dynamic calculations. The oxidation behavior of the coated specimens subjected either to high-temperature testing at 1773 K and 1973 K in static air furnace or to ablation testing with oxyacetylene torch upon ultra-high temperature service were studied, base on thermo-dynamic computations, numerical simulations and microstructure evolution. The SiC-HfB2-Si coating protected C/Cs against oxidation at 1773 K for more than 1507 h which is longer than that of the reported SiC-HfB2-based coatings, due to the as-prepared compact mosaic coating filled with HfB2-rich Si-based multiphase and the consequently formed dense Hf-Si-O oxide layer. Moreover, a good ablation resistance with relatively low linear and mass ablation rates of −0.72 μm/s and 0.07 mg/s, respectively, was achieved due to the stable oxide scale with high viscosity.

Thermal insulation performance and char formation and degradation mechanisms of boron-containing hydrocarbon intumescent coatings

Boron compounds are effective ingredients, widely used in passive fire protection. In this work, epoxy-based hydrocarbon intumescent coatings, containing one of three boron compounds, zinc borate (ZB), ammonium pentaborate (APB), and boric acid (BA), were compared in thermal insulation performance at UL1709 heating conditions, char morphology, viscoelastic behavior, thermal degradation, and composition of the char surface layer. Results show that the char structure provides the most reliable indication of the thermal insulation performance. For APB- and BA-containing coatings, results were similar, while the ZB-containing ones showed a distinctive performance, especially the ones with the high borate concentration. The latter was attributed to the role of ZB in the intumescence and oxidation reactions, where the zinc, in the presence of boron, led to a compact char with less oxidation and high residual mass. In the char surface layer of one sample with a high concentration of ZB, a crystalline phase of boron trioxide, anticipated to be one important origin of the compact char, was detected. The present work contributes with an understanding of the essential roles of boron compounds, as well as the binder and pigment contents, and provides guidelines for seeking more sustainable and environmentally friendly alternatives.

Comparison of plasma sprayed NbB2-NbC coatings obtained by ex-situ and in-situ approaches

NbB2-NbC composite coatings were fabricated on the surface of TC4 by plasma spraying NbB2-NbC and Nb-B4C composite powders. The microstructure and properties of the as-prepared coatings were investigated, and the reaction mechanism of the Nb-B4C composite powder in the plasma jet and the formation mechanism of the NbB2-NbC coatings were revealed. During the plasma spraying process, NbB2-NbC composite powder underwent melting-depositing and no phase transformation occurred. The formation mechanism of the coating by plasma spraying Nb-B4C composite powder was solid phase diffusion-reaction-melting-deposition, and NbB2 and NbC were formed in situ by the solid phase diffusion reaction between Nb and B4C in the plasma jet. The coating obtained by reactive plasma spraying Nb-B4C composite powder has obvious lamellar structure, low porosity, high density, higher microhardness, good toughness, and better wear resistance compared with the coating prepared by NbB2-NbC composite powder, which is attributed to the exothermic reaction between Nb and B4C.

Nb silicide coatings processed by double pack cementation: Formation mechanisms and stability

Niobium silicide coatings were processed on Nb metal using double pack cementation procedures at temperatures in the range 1000 °C–1200 °C for up to 6 h. Coating thickness and hardness were affected by processing parameters. Lower processing temperatures resulted in single-layer NbSi2 coatings while the highest processing temperature (1200 °C) produced coatings with two layers, NbSi2 and Nb5Si3, the latter at the NbSi2/Nb interface. Analysis of the results shows that Nb5Si3 forms by decomposition of NbSi2 and interdiffusion of Si and Nb. The influence of processing parameters on vacancy density and of vacancy density on the coating formation mechanisms and properties is discussed. Silicide-coated Nb had superior oxidation resistance to pure Nb. However, under the conditions tested a two-layer scale of Nb2O5 and SiO2 was formed. Analysis of the data indicates that Nb2O5 forms rapidly on the surface of the coatings at lower temperatures whereas SiO2 forms at the top of the oxide scale. It is concluded that pre-oxidation treatment is essential to ensure formation of an SiO2 scale, which is associated with enhanced oxidation performance.

Preparation of high performance Fe-based amorphous coating by resistance seam welding

High performance Fe-based amorphous coatings were first fabricated on 304 stainless steel by resistance seam welding. The microstructure, microhardness, wear and corrosion resistance of the coatings under different welding current were systematically studied. The coatings consisted of amorphous phase and crystalline phases of M23C6, Fe3C and α-Fe. Moreover, coating formation mechanism was characterized by welding temperature and microstructure variation of powders at different welding current. The dense amorphous coating was fabricated at about 750 K in welding current of 24 A and was seriously crystallized at 990 K in 30 A. The optimal hardness and wear rate of the coating reached 1280 HV0.1 and 2.71 × 10−6 mm3/N·m, respectively, which exhibited better performance compared with the amorphous coating fabricated by the traditional methods. In 3.5 wt% NaCl solution, the corrosion resistance of all coatings was superior to that of the substrate, and corrosion occurred in the crystallization regions. As the welding current increased, the amorphous rate in the coating decreased, resulting in a decrease in the hardness, wear resistance and corrosion resistance of the coating.