Initial Oxidation Behavior of AlCoCrFeNi High-Entropy Coating Produced by Atmospheric Plasma Spraying in the Range of 650 °C to 1000 °C

As protective coatings for the thermal parts of aero-engines, AlCoCrFeNi coatings have good application prospects. In this study, atmospheric plasma spraying (APS) was used to prepare AlCoCrFeNi high-entropy coatings (HECs), which were oxidized from 650 °C to 1000 °C. The mechanism of the oxide layer formation and the internal phase transition were systematically investigated. The results show that a mixed oxide scale with a laminated structure was formed at the initial stage of oxidation. The redistribution of elements and phase transition occurred in the HECs' matrix; the BCC/B2 structure transformed to Al-Ni ordered B2 phase and Fe-Cr disordered A2 phase.

Effect of APS Spraying Parameters on the Microstructure Formation of Fe3Al Intermetallics Coatings Using Mechanochemically Synthesized Nanocrystalline Fe-Al Powders

The present paper presents a study of the behaviour of Fe₃Al intermetallic powders particles based on 86Fe-14Al, 86Fe-14(Fe5Mg), and 60.8Fe-39.2(Ti37.5Al) compositions obtained by mechanochemical synthesis at successive stages of the plasma spraying process: during transfer in the volume of the gas stream and deformation at the moment of impact on the substrate. The effect of the change in current on the size of powder particles during their transfer through the high-temperature stream and the degree of particle deformation upon impact with the substrate was determined. It was found that during transfer through the plasma jet, there was an increase in the average size of sputtering products by two-three times compared to the initial effects of mechanochemical synthesis due to the coagulation of some particles. In this case, an increase in current from 400 to 500 A led to a growth in average particle size by 14-47% due to the partial evaporation of fine particles with an increase in their heating degree. An increase in current also led to a 5-10% growth in particle deformation degree upon impact on the substrate due to the rising temperature and velocity of the plasma jet. Based on the research, the parameters of plasma spraying of mechanically synthesized Fe₃Al intermetallic-based powders were determined, at which dense coatings with a thin-lamellar structure were formed.

Two-Steps Method to Prepare Multilayer Sandwich Structure Carbon Fiber Composite with Thermal and Electrical Anisotropy and Electromagnetic Interference Shielding

Carbon fiber (CF) composites performance enhancement is a research hotspot at present. In this work, first, a sandwich structure composite, CF@(carbon nanotube/Fe₃O₄)/epoxy (CF@(CNT/Fe₃O₄)/EP), is prepared by the free arc dispersion-CFs surface spraying-rolling process method, herein, CFs in the middle layer and (CNT/Fe₃O₄)/EP as top and substrate layer. Then, CF@(CNT/Fe₃O₄)/EP (on both sides) and CFs (in the middle) are overlapped by structure design, forming a multilayer CF@(CNT/Fe₃O₄)/EP-CFs composite with a CFs core sheath. A small amount of CNT/Fe₃O₄ is consumed, (CNT/Fe₃O₄)/EP and CFs core sheath realize thermal and electrical anisotropy and directional enhancement, and multilayer sandwich structure makes the electromagnetic interference (EMI) shielding performance better strengthened by multiple absorption-reflection/penetration-reabsorption. From CF-0 to CF-8, CNT/Fe₃O₄ content only increases by 0.045 wt%, axial thermal conductivity (λ_‖) increases from 0.59 W/(m·K) to 1.1 W/(m·K), growth rate is 86%, radial thermal conductivity (λ_⟂) only increases by 0.05 W/(m·K), the maximum λ_‖/λ_⟂ is 2.9, axial electrical conductivity (σ_‖) increases from 6.2 S/cm to 7.7 S/cm, growth rate is 24%, radial electrical conductivity (σ_⟂) only increases by 0.7 × 10^-4 S/cm, the total EMI shielding effectiveness (EMI SE_T) increases by 196%, from 10.3 dB to 30.5 dB. This provides a new idea for enhancing CFs composite properties.

Microstructural, Corrosion Resistance, and Tribological Properties of Al2O3 Coatings Prepared by Atmospheric Plasma Spraying

An usual material, EN-GJL-250 cast iron, used for automotive braking systems, was covered with a ceramic material (105NS-1 aluminium oxide) using an industrial deposition system (Sulzer Metco). The main reason was to improve the corrosion and wear (friction) resistance properties of the cast-iron. Samples were prepared by mechanical grinding and sandblasting before the deposition. We applied two and four passes (around 12-15 µm by layer) each at 90° obtaining ceramic coatings of 30 respectively 60 µm. The surface of the samples (with ceramic coatings) was investigated using scanning electron microscopy (SEM), dispersive energy spectroscopy (EDS) and X-ray diffraction (XRD). Scratch and micro-hardness tests were performed using CETR-UMT-2 micro-tribometer equipment. The better corrosion resistance of the base material was obtained by applying the ceramic coating. The results present a better corrosion resistance and a higher coefficient of friction of the coated samples.

Condition Monitoring of a Three-Cathode Cascaded Plasma Spray Torch Regarding Process Reliability

The TriplexPro™-210 plasma spray torch (Oerlikon Metco) is a three-cathode plasma generator. It became a kind of workhorse for the wide range of tasks handled at the Jülich Thermal Spray Center (JTSC). Compared to conventional single-cathode torches, the cascaded design of the nozzle suggests low fluctuations of the arc and thus high stability. However, after a certain time, degradation sets in even with such a torch, impairing the reliability of the process. It is therefore important to detect indications of performance loss in time and not only during the inspection of the deposited layer. In this study, standard samples of YSZ thermal barrier coatings were sprayed regularly over a period of two years. Operational data and feedstock characteristics were collected and correlated with the area-specific mass deposition. It turned out that the measured substrate surface temperature showed a distinct correlation. Searching for the reasons for the temperature variations, several process parameters could be ruled out as they are monitored by calibrated sensors, controlled, and their time course is recorded by the control unit. Moreover, there are other parameters, which can have a considerable impact such as the robot alignment or the substrate cooling conditions. However, the purposeful experimental variation of such variables resulted in a variability of the mass deposition being considerably smaller than observed over the two years. Thus, it can be concluded that torch degradation had a pronounced effect, too. The substrate surface temperature can be used as indicator for the torch status and the reliability of the spray process.

Ni-Cr Powders Modified with Rhenium as a Novel Coating Material-Physical Properties, Microstructure, and Behavior in Plasma Plume

The aim of this work was to develop a new coating material based on Ni20Cr alloy modified with up to 50%wt. rhenium. The modification was carried out by the mechanical mixing of the base powder and ammonium perrhenate with the subsequent thermoreduction in an H₂ atmosphere. The obtained powder consists of a nickel-chromium core surrounded by a rhenium shell. The characterization of the powders-including their microstructure, phase and chemical composition, density, flowability, particle size distribution, and specific surface area-was performed. The influence of plasma current intensity and hydrogen gas flow on in-flight particle temperature and velocity were investigated. The results indicate that there is interdiffusion between the base Ni20Cr and the rhenium shell, resulting in intermediary solid solution(s). The modified powders have a higher specific surface area and a lower flowability, but this does not prevent them from being used as feedstock in plasma spraying. In-flight measurements reveal that increasing the content of rhenium allows for the higher temperature of particles, though it also reduces their speed.

Microstructure and Corrosion Behavior of Atmospheric Plasma Sprayed NiCoCrAlFe High Entropy Alloy Coating

High entropy alloys (HEAs) are multi-elemental alloy systems that exhibit a combination of exceptional mechanical and physical properties, and nowadays are validating their potential in the form of thermal sprayed coatings. In the present study, a novel synthesis method is presented to form high entropy alloy coatings. For this purpose, thermal sprayed coatings were deposited on Stainless Steel 316L substrates using atmospheric plasma spraying technique with subsequent annealing, at 1000 °C for 4 h, to assist alloy formation by thermal diffusion. The coatings in as-coated samples as well as in annealed forms were extensively studied by SEM for microstructure and cross-sectional analysis. Phase identification was performed by X-ray diffraction studies. The annealed coatings revealed a mixed BCC and FCC based HEA structure. Potentiodynamic corrosion behavior of SS316L sprayed as well as annealed coatings were also carried out in 3.5% NaCl solution and it was found that the HEA-based annealed coatings displayed the best corrosion resistance 0.83 (mpy), as compared to coated/non-annealed and SS 316 L that showed corrosion resistance of 7.60 (mpy) and 3.04 (mpy), respectively.

Recent Research Advances in Plasma Spraying of Bulk-Like Dense Metal Coatings with Metallurgically Bonded Lamellae

Although thermal spray metallic coatings have been widely used for materials protection from wear, corrosion and oxidation, its porous feature limits the full utilization of materials potential. Moreover, the oxidation inherent to thermal spraying in the ambient atmosphere is detrimental to interlamellar bonding formation, which further degrades the performance of thermal spray metal coatings. How to tape out the full potential of spray materials in the form of the coating is a still great challenge to thermal spray coating technology. Facing such challenge, recent efforts have been made to deposit dense metallic coatings with sufficiently bonded lamellae by oxide-free molten droplets through atmospheric plasma spraying. In this paper, the strategies for depositing bulk-like metal coatings will be reviewed. The formation of the bulk-like coating through post-spray treatments is briefly reviewed including post-spray heat treatment and laser remelting following the brief introduction to the features of thermal spray metallic coatings. The effect of the substrate preheating temperature on the splat formation and subsequently the adhesion formation was examined to reveal the dominant limitation of resultant oxide scale. Then, the role of the deposition temperature on the formation of bulk-like metal deposits with neglecting the effect of oxidation during spraying by vacuum plasma spraying practices is shortly presented. The recent progress on the new strategies to develop spread-fusing bonding mechanism and in-situ in-flight deoxidizing mechanism through developing ultra-hot metallic droplets will be introduced. The thermodynamics and kinetics requirements for the in-situ in-flight deoxidizing through deoxidizer elements adding to alloy spray powders for achieving oxide-free molten droplets in the ambient atmosphere are examined. The conditions to develop the spread-fusing mechanism during the spreading of impacting molten metal droplet for metallurgical bonding are presented. It is obvious from this review paper that the realization of two mechanisms depends on both the spray materials design and heating control of in-flight particles. Through the generation of ultra-hot droplets by plasma spraying to achieve oxide-free molten droplets, strategically it will be possible to deposit bulk-like dense metallic coating through spread-fusing of splat surfaces with limited post-spray oxidation. Such strategies will tape out the full potential of coating materials and open up the new application fields for plasma spraying.

Effect of Vacuum Heat Treatment on the Element Diffusion Behavior and Corrosion Resistance of Al2O3-3wt.%TiO2 Coating of Q235 Steel

In this study, we address the effect of vacuum heat treatment on the morphology of Al₂O₃-3wt.%TiO₂ coating, element diffusion behavior, coating hardness, and corrosion resistance. The pores, cracks, and non-liquefied particles on the as-heat treated coating surface of the vacuum-heat-treated coating were observed and compared with the as-sprayed coating using a scanning electron microscope. The diffusion behavior of the elements in the coating was demonstrated by using a line scanning of a cross-section of the coating. Hardness and corrosion-resistance test results were used to judge the effect of a vacuum heat treatment on the coating. The research results show that compared with atmospheric heat treatment, the vacuum heat treatment had less effect on the pores, cracks, and non-liquefied particles on the surface of the coating. However, in the absence of new oxide formation, the pores and cracks in the cross-section of the coating were significantly improved by the vacuum heat treatment. The surface hardness and corrosion resistance of the coating were significantly improved. The crack defects were eliminated, and the uniformity of TiO₂ distribution was improved, which are the main factors that improved the coating performance after vacuum heat treatment. The combination of the coating and the substrate is strengthened, and an Al₂O₃ and TiO₂ interdiffusion zone is formed when the coating undergoes vacuum heat treatment, which is the main mechanism improving the performance of the AT3 coating.

Capturing the Influence of Jet Fluctuations on Particles in Plasma Spraying

Instabilities and fluctuations of the plasma jet in a thermal spray process can have a significant influence on the particle in-flight temperatures and velocities, affecting the properties of resulting plasma-sprayed coatings. Presented in this paper is a novel method for capturing the effects particles are exposed to in the plasma spraying process. High-speed camera images of a plasma jet generated by a cascaded three-cathode plasma generator (TriplexPro-210) were recorded for varying operating conditions. The images are processed using the inverse Abel transform. This transformation accounts for the fact that the images represent a 2D projection of the 3D jet and generates more accurate intensity values that the sprayed particles would experience. These images are then combined with particle tracks resulting from CFD simulations of the plasma jet to match the particles path with the recorded plasma jet. This new method allows a precise description of the plasma intensity experienced by individual particles with a high temporal resolution. The results show a high sensitivity of the method, even detecting the influence on the particles of the plasma jet originating from the cascaded triple arc plasma generator, which is considered as rather stable.

Plasma-Sprayed (Bi2O3)0.705 (Er2O3)0.245 (WO3)0.05 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs)

Stabilized bismuth oxide material with fluorite structure (δ-Bi2O3) has been studied as a promising electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its high oxygen ion conductivity in mediate temperature. Especially, the ternary system Bi2O3-Er2O3-WO3 is widely concerned for its high ionic conductivity and thermal stability. In this study, regarding its low melting point, the possibility to deposit dense Bi2O3-Er2O3-WO3 ((Bi2O3)0.705 (Er2O3)0.245 (WO3)0.05, EWSB) electrolyte by plasma spraying was examined. It was confirmed that the sintered EWSB bulk presents a high ion conductivity of 0.34 S cm-1 at 750 °C and excellent stability that indicates no structure transformation and conductivity degradation after annealing at 600 °C for 1000 h. The phase structure and cross-sectional microstructure of plasma-sprayed EWSB were characterized by XRD and SEM. Results showed that the as-plasma-sprayed EWSB presents a dense microstructure with well bonded lamellae. The XRD showed the formation of EWSB with δ-phase and a trace of β-phase, while the β-phase disappeared after annealing at 750 °C for 10 h. The deposited EWSB electrolyte presented the excellent ionic conductivity of 0.26 S cm-1 at 750 °C which can be directly applied to SOFC at intermediate temperature.

The Corrosion Resistance and Mechanism of AT13/Fe-Based Amorphous Composite Coatings

Due to high strength, high wear resistance and high corrosion resistance, the amorphous metallic glasses were investigated widely. In the present study, the corrosion resistance of amorphous coating and composite coatings with various proportions of AT13 (Al₂O₃–13 wt.% TiO₂) ceramic as additions in 3.5 wt.% NaCl solution were studied. The corrosion resistance was improved obviously as the addition of AT13, and when the content of AT13 was 15 wt.%, the composite coating had the lowest corrosion current density (1.75 × 10⁻⁶ A cm⁻²) and the highest corrosion potential (−411 mV), which was 5.14 × 10⁻⁵ A cm⁻² and −580 mV for Fe-based metallic glassy coating, respectively. The corrosion mechanism was proposed according to the long-time immersion corrosion test.

Microstructure Evolution of FeNiCoCrAl1.3Mo0.5 High Entropy Alloy during Powder Preparation, Laser Powder Bed Fusion, and Microplasma Spraying

In the present study, powder of FeCoCrNiMo_0.5Al_1.3 HEA was manufactured by gas atomization process, and then used for laser powder bed fusion (L-PBF) and microplasma spraying (MPS) technologies. The processes of phase composition and microstructure transformation during above mentioned processes and subsequent heat treatment were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and differential thermal analysis (DTA) methods. It was found that gas atomization leads to a formation of dendrites of body centered cubic (BCC) supersaturated solid solution with insignificant Mo-rich segregations on the peripheries of the dendrites. Annealing leads to an increase of element segregations till to decomposition of the BCC solid solution and formation of σ-phase and B2 phase. Microstructure and phase composition of L-PBF sample are very similar to those of the powder. The MPS coating has a little fraction of face centered cubic (FCC) phase because of Al oxidation during spraying and formation of regions depleted in Al, in which FCC structure becomes more stable. Maximum hardness (950 HV) is achieved in the powder and L-PBF samples after annealing at 600 °C. Elastic modulus of the L-PBF sample, determined by nanoindentation, is 165 GPa, that is 12% lower than that of the cast alloy (186 GPa).

Sealing Treatment of Plasma-Sprayed Cr3C2-NiCr/Al2O3-TiO2 Coating by Aluminum Phosphate Sealant Containing Al2O3 Nanoparticles

A typical structure of thermal spray coatings consisted of molten particles, semi-molten particles, oxides, pores, and cracks. These factors caused the porosity of sprayed coatings, leading to a significant influence on the coating properties, especially their wear-corrosion resistance. In this study, a post-spray sealing treatment of Cr3C2-NiCr/Al2O3-TiO2 plasma-sprayed coatings was carried out, and then, their corrosion properties were evaluated, before and after the treatment. For the sealing process, aluminum phosphate (APP) containing Al2O3 nanoparticles (~10 nm) was used. The permeability of APP into the sprayed coating was analyzed by SEM-EDS. The treatment efficiency for porosity and corrosion resistance of sprayed coatings was evaluated by electrochemical measurements, such as the potentiodynamic polarization and electrochemical impedance spectroscopy. The wear-corrosion resistance of the coating was examined in 3.5 wt.% NaCl circulation solution containing 0.25% SiO2 particles. The sealing efficiency was evaluated by the percentage of the treated open pores in the coating. The obtained results showed that APP penetrated deeply through the coating and the incorporation of Al2O3 nanoparticles into APP sealant improved the sealing efficiency by 20% of open pores in comparison with the sealant without nano-Al2O3. The effect of the post-treatment on corrosion protection of the sprayed coating has been discussed.

Effects of Multi-Components on the Microwave Absorption and Dielectric Properties of Plasma-Sprayed Carbon Nanotube/Y2O3/ZrB2 Ceramics

Carbon nanotube (CNT)-reinforced Y₂O₃/ZrB₂ ceramics were fabricated via planetary ball milling and atmospheric-pressure plasma spraying for the first time. The phase composition, micromorphology, and electromagnetic (EM) wave absorption performance of the Y₂O₃/ZrB₂/CNT hybrid was investigated from 8.2 to 12.4 GHz. Both the real and imaginary parts of the complex permittivity were enhanced as the ZrB₂ and CNT content increased. The Y₂O₃/ZrB₂/CNT hybrids corresponded to a ZrB₂ content of 15 wt.%, and the CNT content was 2 wt.% and showed an exceptional EM wave absorption capability, with a minimum reflection loss of −25.7 dB at 1.9 mm thickness, and the effective absorption band was in a full X-band. These results indicate that an appropriate CNT or ZrB₂ content can tune the complex permittivity and absorption performance of the Y₂O₃/ZrB₂/CNT ceramics.

Plasma-Sprayed High-Performance (Bi2O3)0.75(Y2O3)0.25 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs)

Rare earth element-doped bismuth oxides with the fluorite structure (δ-Bi2O3) exhibit high oxygen ion conductivity at low temperature, which is promising electrolyte materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, traditional co-sintering process is not applicable to the manufacturing of IT-SOFCs using low melting point Bi2O3-based electrolyte, while further high-temperature processing is not required for deposition Bi2O3-based electrolytes. In this study, plasma spraying was used to examine the possibility to deposit high-performance Bi2O3-based electrolytes without the following high-temperature process. (Bi2O3)0.75 (Y2O3)0.25 (YSB) spray powders were prepared by the sinter-crushing method. The YSB electrolytes were fabricated by plasma spraying at different deposition temperatures. The effects of deposition temperature on the coating microstructure, crystalline stability, and ion conductivity were investigated. Results showed that the as-sprayed YSB electrolytes present a dense microstructure with well-bonded lamellar interfaces. The pure δ-phase YSB electrolyte was deposited with 37.5-75 μm powders at a deposition temperature of 350 °C. The deposited YSB electrolyte presented the excellent ionic conductivity of 0.19 S cm-1 at 700 °C in comparison with 0.21 S cm-1 for sintered bulk.

Effect of Powder Particle Size and Spray Parameters on the Ni/Al Reaction During Plasma Spraying of Ni-Al Composite Powders

It was known for long that Ni-Al composite powders can be used to deposit self-bonding coating as a bond coat for common ceramic coatings due to the exothermic reaction between Ni and Al. However, it was found that with commercial Ni-Al composite powders with a large particle size, it is difficult to ignite the self-propagating reaction between Ni and Al to form Ni-Al intermetallics by plasma spraying. In this study, Ni-Al composite powder particles of different sizes were used to prepare Ni-Al intermetallics-based coatings by plasma spraying. The dependencies of the exothermic reaction between Ni and Al and the coating microstructure on powder particle size and spray parameters were investigated. The phase composition, microstructure, porosity and oxide content of the coatings were characterized by x-ray diffraction, scanning electron microscope and image analyzing. The results show that particle size of Ni-Al composite powders is the dominant factor controlling the exothermic reaction for the formation of Ni-Al intermetallics during plasma spraying. When the powders larger than about 50 μm are used, the reaction forming aluminide cannot complete even by heating of plasma flame generated at high plasma arc power. However, when smaller powders less than 50 μm are used, the exothermic reaction can completely occur rapidly in plasma spraying, contributing to heating of Ni-Al droplets to the highest temperature for development of the self-bonding effect. The positive relationship between molten droplet temperature and tensile adhesive strength of the resultant coatings is recognized to confirm the contribution of high droplet temperature to the adhesive or cohesive strength.

Changes in the Coating Composition Due to APS Process Conditions for Al2O3-Cr2O3-TiO2 Ternary Powder Blends

Thermally sprayed coatings from the single oxides and binary compositions of the Al2O3-Cr2O3-TiO2 system show multifunctional properties. Ternary compositions are promising for further improvement in their performance. The stability of the composition during coating formation is an important issue for blended feedstock powders in order to obtain the desired properties. This work focuses on the compositional changes of a ternary blend of Al2O3, Cr2O3 and TiOx powders of equal content by mass in a conventional atmospheric plasma spraying (APS) process using an Ar/H2 plasma gas mixture. By increasing the argon flow rate at constant hydrogen flow rate, the total plasma gas flow rate and the Ar/H2 ratio were varied. For the highest argon flow rate, this resulted in an average particle velocity of 140% and an average particle temperature of 90% of the initial values, respectively. Coating composition and microstructure were studied by optical microscopy, SEM, including EDS analyses, and XRD. In addition, the coating hardness and electrical impedance were also measured. Differences in the "difficulty of melting factor" (DMF) and the thermal diffusivity of the three oxides appear to be responsible for the dramatic change of the coating composition with an increasing argon flow rate. For the highest argon flow rate applied, besides TiO2, the coating contains only 8 wt.% Al2O3, while the Cr2O3 content remained almost constant. At the same time, the change of the Ar/H2 ratio resulted in the formation of stoichiometric TiO2 in the coating by oxidation of TiOx in the feedstock powder. Moreover, a small content of titanium was found in the Cr2O3 splats, showing that there are only limited interactions between the large oxide powder particles. Thus, the study has shown that stability of the chemical composition during spraying of ternary powder blends is strongly influenced by the process conditions.

Application of Plasma Sprayed Cu Intermediate Layers in the Soldering Process of Graphite Composite to 6060 Aluminum Alloy

The work focuses on issues related to the soldering of graphite composite to 6060 aluminum alloy. The graphite composite is of great interest of the transportation industry as it is widely used in slides responsible for current collection from overhead catenary. The slides should meet various criteria resulting from relatively complex working conditions, e.g., in terms of electrical conductivity, self-lubricating, resistance to changing weather conditions, etc. Such an application has extensive requirements, mainly for a joint of graphite slide with aluminum body. The direct soldering of slide plates made of graphite composite to aluminum alloy collector head causes many technological problems and is not possible. In this study, the application of thin plasma sprayed (APS) copper intermediate layers is investigated for that purpose. After soldering process, the microstructural analysis confirmed the proper joint formulation, i.e., the soldering gap of 0.2 mm was well-filled with the solder over the entire width of joint. The soldered joints were then subjected for static shear testing. The obtained shear strength was in a relatively wide range of 13.04 to 20.50 MPa, which was influenced by various fracture mechanisms. Finally, the fact that reaction zones were not formed in investigated joints during soldering was confirmed by EDS analysis and micro hardness values, which were very similar to the ones of raw materials.

Microstructure and Mechanical Properties of Vacuum Plasma Sprayed HfC, TiC, and HfC/TiC Ultra-High-Temperature Ceramic Coatings

To improve the oxidation resistance of carbon composites at high temperatures, hafnium carbide (HfC) and titanium carbide (TiC) ultra-high-temperature ceramic coatings were deposited using vacuum plasma spraying. Single-layer HfC and TiC coatings and multilayer HfC/TiC coatings were fabricated and compared. The microstructure and composition of the fabricated coatings were analyzed using field-emission scanning electron microscopy and energy dispersive X-ray spectroscopy. The coating thicknesses of the HfC and TiC single-layer coatings were 165 µm and 140 µm, respectively, while the thicknesses of the HfC and TiC layers in the HfC/TiC multi-layer coating were 40 µm and 50 µm, respectively. No oxides were observed in any of the coating layers. The porosity was analyzed from cross-sectional images of the coating layers obtained using optical microscopy. Five random areas for each coating layer specimen were analyzed, and average porosity values of approximately 16.8% for the HfC coating and 22.5% for the TiC coating were determined. Furthermore, the mechanical properties of the coating layers were investigated by measuring the hardness of the cross section and surface roughness. The hardness values of the HfC and TiC coatings were 1650.7 HV and 753.6 HV, respectively. The hardness values of the HfC and TiC layers in the multilayer sample were 1563.5 HV and 1059.2 HV, respectively. The roughness values were 5.71 µm for the HfC coating, 4.30 µm for the TiC coating, and 3.32 µm for the HfC/TiC coating.

Improvement of Mechanical Properties of Plasma Sprayed Al2O3-ZrO2-SiO2 Amorphous Coatings by Surface Crystallization

Ceramic Al₂O₃-ZrO₂-SiO₂ coatings with near eutectic composition were plasma sprayed using hybrid water stabilized plasma torch (WSP-H). The as-sprayed coatings possessed fully amorphous microstructure which can be transformed to nanocrystalline by further heat treatment. The amorphous/crystalline content ratio and the crystallite sizes can be controlled by a specific choice of heat treatment conditions, subsequently leading to significant changes in the microstructure and mechanical properties of the coatings, such as hardness or wear resistance. In this study, two advanced methods of surface heat treatment were realized by plasma jet or by high energy laser heating. As opposed to the traditional furnace treatments, inducing homogeneous changes throughout the material, both approaches lead to a formation of gradient microstructure within the coatings; from dominantly amorphous at the substrate-coating interface vicinity to fully nanocrystalline near its surface. The processes can also be applied for large-scale applications and do not induce detrimental changes to the underlying substrate materials. The respective mechanical response was evaluated by measuring coating hardness profile and wear resistance. For some of the heat treatment conditions, an increase in the coating microhardness by factor up to 1.8 was observed, as well as improvement of wear resistance behaviour up to 6.5 times. The phase composition changes were analysed by X-ray diffraction and the microstructure was investigated by scanning electron microscopy.

Enhanced Tribocorrosion Resistance of Hard Ceramic Coated Ti-6Al-4V Alloy for Hip Implant Application: In-Vitro Simulation Study

Developing coatings for various applications is an area of research of uttermost importance, to protect surfaces from severe damage by improving the wear and corrosion resistance of the materials. Recently, there has been increasing interest in ceramic coatings for biomedical applications, as the surface may become more inert in nature for the biological reactions and potentially increase the lifespan of the implants and minimize the side effects on the patients. Hence this study is focused on the tribocorrosion behavior of the ceramic coatings for the hip implant application on commonly used implant titanium alloy. The three types of the ceramic coatings are conventional monolithic micron alumina (IDA), micron alumina-40 wt % yttria-stabilized zirconia (YSZ) composite coating (IDAZ), and by-layer nanostructured alumina-13 wt % titania/YSZ (IDZAT) on Ti-6Al-4V alloy. A series of tests, under free potential and potentiostatic mode, were conducted using a hip simulator tribocorrosion setup under simulated joint fluid (bovine calf serum with protein concentration 30g/L). The tribological conditions are pin-on-ball contact with a load of 16N (approximately contact pressure of 50 MPa), the frequency of 1 Hz (walking frequency), and with an amplitude of 30°. The tribocorrosion studies clearly revealed that the coatings have better wear and corrosion resistance and the predominant damage mechanism was mechanical wear rather than corrosion. Among the coatings, the IDZAT shows enhanced tribocorrosion performance by exhibiting more positive OCP, no induced current, and a lower coefficient of friction.

Deposition of Multiscale Thickness Graphene Coating by Harnessing Extreme Heat and Rapid Quenching: Toward Commercialization

Deposition of graphene as a coating material over large-scale areas is an intense topic of research because of complexities involved in the existing deposition techniques. Higher defects and compromised properties restricted in realizing the full potential of graphene coating. This work aims to deposit graphene coatings by adopting a traditional technique, that is, plasma spraying, which has inherent merits of extremely high cooling rate (∼10⁶ K/s) and low plasma exposure time (∼0.1-10 μs). Graphene nanoplatelets (GNPs) were spray-dried into spherical agglomerates (∼60 μm dia.) and coatings were deposited over a wide range of surfaces. Continuous monitoring of temperature and velocity of in-flight GNPs was done using a diagnostic sensor. Deposition of GNP coatings was the result of striking of quasi-2D melted GNPs with higher velocity (∼197 m/s) toward the substrate. Postcharacterizations confirmed that GNPs did not collapse even after being exposed to harsh environments in plasma. Instead, high temperatures proved to be beneficial in purifying the commercial GNPs. The coatings were transparent even in the short-wavelength infrared region and remained electrically conductive. A proof-of-concept was established by carrying out preliminary corrosion and antifriction tests. Outstanding reduction of ∼3.5 times in corrosion rate and 3 times in coefficient of friction was observed in GNP-deposited coating. It is envisaged that graphene coating by plasma spraying can bring a revolution in commercial sectors.

Plasma-Sprayed Bioactive Ceramic Coatings with High Resorption Resistance Based on Transition Metal-Substituted Calcium Hexaorthophosphates

Calcium (titanium, zirconium) hexaorthophosphates with a [NZP] (sodium zirconium phosphate) structure belonging to the NaSiCon (Na Superionic Conductor) family were deposited by atmospheric plasma spraying onto the surfaces of Ti6Al4V substrates. (NaSiCon sensu strictu refers to solids with the chemical formula Na_1+xZr₂Si_xP_3-xO₁₂, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements). The microstructure of the coatings revealed the incongruent melting of the precursor material as ascertained by electron probe microanalysis (EPMA). The adhesion of the coatings to the substrate surface was within the limits specified for biomedical coatings. The solubility of the coatings was tested by immersion in 0.2 molar tris-hydroxymethyl-amino-methane-HCl (TRIS-HCl) buffer and found to be at least one order of magnitude lower than that of conventional hydroxylapatite coatings deposited under comparable conditions. In vitro biocompatibility tests with primary rat bone marrow cells (BMCs) showed a substantial cell proliferation in the presence of fetal bovine serum. Animal tests confirmed that coatings based on calcium (titanium, zirconium) hexaorthophosphates applied to Ti6Al4V rods implanted in the femoral medulla of sheep led to the strong neoformation of dense bone at a stable interface implant-bioceramic coating without coating delamination. Hence, based on their multifarious advantageous properties in the biomedical context, CaTi_4-xZr_x(PO₄)₆ ceramics may be considered the 'Sleeping Beauty' of osseoconductive coatings for the stem of hip endoprostheses and dental root implants, osteosynthetic fixation devices, and bioelectric devices including bone growth stimulators.

Modeling of High Nanoparticle Exposure in an Indoor Industrial Scenario with a One-Box Model

Mass balance models have proved to be effective tools for exposure prediction in occupational settings. However, they are still not extensively tested in real-world scenarios, or for particle number concentrations. An industrial scenario characterized by high emissions of unintentionally-generated nanoparticles (NP) was selected to assess the performance of a one-box model. Worker exposure to NPs due to thermal spraying was monitored, and two methods were used to calculate emission rates: the convolution theorem, and the cyclic steady state equation. Monitored concentrations ranged between 4.2 × 10⁴–2.5 × 10⁵ cm⁻³. Estimated emission rates were comparable with both methods: 1.4 × 10¹¹–1.2 × 10¹³ min⁻¹ (convolution) and 1.3 × 10¹²–1.4 × 10¹³ min⁻¹ (cyclic steady state). Modeled concentrations were 1.4-6 × 10⁴ cm⁻³ (convolution) and 1.7–7.1 × 10⁴ cm⁻³ (cyclic steady state). Results indicated a clear underestimation of measured particle concentrations, with ratios modeled/measured between 0.2–0.7. While both model parametrizations provided similar results on average, using convolution emission rates improved performance on a case-by-case basis. Thus, using cyclic steady state emission rates would be advisable for preliminary risk assessment, while for more precise results, the convolution theorem would be a better option. Results show that one-box models may be useful tools for preliminary risk assessment in occupational settings when room air is well mixed.

Instant Tuning of Superhydrophilic to Robust Superhydrophobic and Self-Cleaning Metallic Coating: Simple, Direct, One-Step, and Scalable Technique

We present a simple, direct, one-step, scalable technique for instant tuning of all the different states of wetting characteristics using atmospheric plasma spray (APS) technique. We observed that, just by changing the process parameters in the APS technique, the wetting characteristics of an intrinsically hydrophilic aluminum metallic surface can be tuned to superhydrophilic (contact angle (CA): 0°), hydrophilic (CA: 19.6°), hydrophobic (CA: 97.6°), and superhydrophobic (CA: 156.5°) surfaces. Also, tuned superhydrophobic surface showed an excellent self-cleaning property. Further, we demonstrated that these surfaces retain their superhydrophobic nature even after exposure at elevated temperatures (up to 773 K) and on application of mechanical abrasion. Manipulation in different wetting behavior was possible mainly due to the presence of varying degrees of smooth surface as well as micropillars, which incorporated the multiscale roughness to the surface. "Re-entrant"-like microstructures such as mushroom, cauliflower, and cornet microstructures were observed in the case of tuned superhydrophobic surface, which is well-known for achieving the excellent water repellency over the hydrophilic surface.

[Physical and mechanical properties of hydroxyapatite plasma sprayed materials applied in implantology]

BACKGROUND

The research involved the evaluation of physical and mechanical properties of hydroxyapatite (HAp) bio-ceramics sprayed on titanium substrate of the type Ti-grade 2 (CP-Ti) by means of the plasma method. An innovative method of coating is applied when using implantology for healing bone defects in the body.

MATERIAL AND METHODS

Hydroxyapatite coating was prepared in order to conduct research. The powder was made using wet-chemical technology consisting in separating the solids from the solution. Next, a titanium substrate was prepared, onto which hydroxyapatite was applied with a plasma technique.

RESULTS

As a result of the research it has to be noted that the properties of the obtained coating may be used for covering large surfaces of implants of any shape.

CONCLUSIONS

During the research into the physical and mechanical properties of hydroxyapatite composites it has been observed that they show sufficient mechanical properties to be used in implantology. The further aim of the research will involve selecting technological parameters of spray coating in order to increase adhesion and cohesion of HAp coatings. Med Pr 2018;69(6):651-661.

Titanium Plasma-Sprayed Coatings on Polymers for Hard Tissue Applications

The paper presents the results of titanium plasma spraying (TPS) on polymer substrates. Polyethylene (PE300), polyamide PA6, and fiber glass-reinforced polyamide (PA6.6-GF30) were used as substrates. The PE300 and PA6.6-GF30 substrates exhibited appropriate behavior during the TPS process, whereas the PA6 substrate did not “accept” Ti during plasma spraying, and the coating did not form. The TPS coatings exhibited low porosity and high homogeneity, and they had a typical multilayer structure composed of Ti and its oxides. The nanoindentation test showed good mechanical properties of the coatings and demonstrated a hardness and a Young’s modulus of approximately 400 HV and 200 GPa, respectively. The bending test confirmed the good adhesion of the titanium coatings to the polymer substrates. The Ti coatings did not fall off the substrate after its significant bending deformation.

Comparison of Microstructure and Properties of In-Situ TiN- and WC-Reinforced NiCrBSi Composite Coatings Prepared by Plasma Spraying

In this study, NiCrBSi-30 wt.% TiN composite (NTC) coating was produced on carbon steel via plasma spraying, with NiCrBSi-30 wt.% WC composite (NWC) coating as the comparison object. The microstructure and phase constituents of the composite coatings were characterized using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) techniques, transmission electron microscopy (TEM) and x-ray diffraction (XRD). Atomic force microscopy (AFM) was used to measure electronic work functions. The microhardness and wear performance of coatings were also investigated. The average microhardness of the NTC and NWC coatings was 1000 HV and 850 HV, respectively. In addition, the NTC coating had a wear volume loss of 0.8118 mm³, less than 1.4772 mm³, the volume loss of the NWC coating. This was due to the presence of TiN in the form of nanograins in the composite coating and tighter binding to the matrix.

Micro/Nano Structural Tantalum Coating for Enhanced Osteogenic Differentiation of Human Bone Marrow Stem Cells

Recently, tantalum has been attracting much attention for its anticorrosion resistance and biocompatibility, and it has been widely used in surface modification for implant applications. To improve its osteogenic differentiation of human bone marrow stem cells (hBMSCs), a micro/nano structure has been fabricated on the tantalum coating surface through the combination of anodic oxidation and plasma spraying method. The morphology, composition, and microstructure of the modified coating were comprehensively studied by employing scanning electron microscopy (SEM), X-ray diffraction (XRD) as well as transmission electron microscopy (TEM). The effects of hierarchical structures as well as micro-porous structure of tantalum coating on the behavior for human bone marrow stem cells (hBMSCs) were evaluated and compared at both cellular and molecular levels in vitro. The experimental results show that a hierarchical micro/nano structure with Ta₂O₅ nanotubes spread onto a micro-scale tantalum coating has been fabricated successfully, which is confirmed to promote cell adhesion and spreading. Besides, the hierarchical micro/nano tantalum coating can provide 1.5~2.1 times improvement in gene expression, compared with the micro-porous tantalum coating. It demonstrates that it can effectively enhance the proliferation and differentiation of hBMSCs in vitro.

Application of Plasma Sprayed Zirconia Coating in Dental Implant: Study in Implant

The aim was to investigate the osseointegration of a novel coating-plasma-sprayed nanostructured zirconia (NSZ) in dental implant. Nanostructured zirconia coating on non-thread titanium implant was prepared by plasma spraying, the implant surface morphology, surface roughness and wettability were measured. In vivo, nanostructured zirconia-coated implants were inserted in rabbit tibia and animals were respectively sacrificed at 2, 4, 8 and 12 weeks after implantation. The bond strength between implant and bone was measured by removal torque (RTQ) test. The osseointegration was observed by scanning electron microscopy (SEM), micro computed tomography (Micro CT) and histological analyses. Quantified parameters were calculated, including removal torque, Bone Volume to Tissue Volume (BV/TV), Trabecular Thickness (Tb. Th), Trabecular Number (Tb. N), Trabecular Separation/Spacing (Tb. Sp), and Bone-Implant contact (BIC) percentage. The statistical differences were detected by two-tail Mann-Whitney U test (SPSS 20.0). The surface roughness (1.58µm) and wettability (54.61°) of nanostructured zirconia coated implant was more suitable than those of titanium implant (0.598µm and 74.38°) for osseointegration and hierarchical surface morphology could be seen on zirconia coating. The histological analyses showed that zirconia coated implant induced earlier and more condensed bone formation than titanium implant at 2 and 4 weeks. Quantified parameters showed the significant differences between these two groups at early healing period, but the differences between these two groups decreased with the increase of healing period. All these results demonstrated that plasma sprayed zirconia coated implant induced better bone formation than titanium implant at early stage.

Ablation Behavior of Plasma-Sprayed La1-xSrxTiO3+δ Coating Irradiated by High-Intensity Continuous Laser

Laser protection for optical components, particularly those in high-power laser systems, has been a major concern. La_1-xSr_xTiO_3+δ with its good optical and thermal properties can be potentially applied as a high-temperature optical protective coating or high-reflectivity material for optical components. However, the high-power laser ablation behavior of plasma-sprayed La_1-xSr_xTiO_3+δ (x = 0.1) coatings has rarely been investigated. Thus, in this study, laser irradiation experiments were performed to study the effect of high-intensity continuous laser on the ablation behavior of the La_1-xSr_xTiO_3+δ coating. The results show that the La_1-xSr_xTiO_3+δ coating undergoes three ablation stages during laser irradiation: coating oxidation, formation and growth of new structures (columnar and dendritic crystals), and mechanical failure. A finite-element simulation was also conducted to explore the mechanism of the ablation damage to the La_1-xSr_xTiO_3+δ coating and provided a good understanding of the ablation behavior. The apparent ablation characteristics are attributed to the different temperature gradients determined by the reflectivity and thermal diffusivity of the La_1-xSr_xTiO_3+δ coating material, which are critical factors for improving the antilaser ablation property. Now, the stainless steel substrate deposited by it can effectively work as a protective shield layer against ablation by laser irradiation.

Combining 3D human in vitro methods for a 3Rs evaluation of novel titanium surfaces in orthopaedic applications

In this study, we report on a group of complementary human osteoblast in vitro test methods for the preclinical evaluation of 3D porous titanium surfaces. The surfaces were prepared by additive manufacturing (electron beam melting [EBM]) and plasma spraying, allowing the creation of complex lattice surface geometries. Physical properties of the surfaces were characterized by SEM and profilometry and 3D in vitro cell culture using human osteoblasts. Primary human osteoblast cells were found to elicit greater differences between titanium sample surfaces than an MG63 osteoblast-like cell line, particularly in terms of cell survival. Surface morphology was associated with higher osteoblast metabolic activity and mineralization on rougher titanium plasma spray coated surfaces than smoother surfaces. Differences in osteoblast survival and metabolic activity on titanium lattice structures were also found, despite analogous surface morphology at the cellular level. 3D confocal microscopy identified osteoblast organization within complex titanium surface geometries, adhesion, spreading, and alignment to the biomaterial strut geometries. Mineralized nodule formation throughout the lattice structures was also observed, and indicative of early markers of bone in-growth on such materials. Testing methods such as those presented are not traditionally considered by medical device manufacturers, but we suggest have value as an increasingly vital tool in efficiently translating pre-clinical studies, especially in balance with current regulatory practice, commercial demands, the 3Rs, and the relative merits of in vitro and in vivo studies. Biotechnol. Bioeng. 2016;113: 1586-1599. © 2015 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.

Characterization of a micro-roughened TiO2/ZrO2 coating: mechanical properties and HBMSC responses in vitro

Previous studies have shown that using ZrO2 as a second phase to bioceramics can significantly increase the bonding strength of plasma-sprayed composite material. In the present study, micro-roughened titanium dioxide/zirconia (TiO2/ZrO2) (30 wt% ZrO2) coating and TiO2 coating were plasma-sprayed onto Ti plates. The micro-structural characteristics and mechanical properties of both coatings were investigated. Furthermore, the biological behavior and osteogenic differentiation of human bone marrow mesenchymal stem cells (HBMSCs) on both TiO2/ZrO2 and TiO2 coatings were compared. The results indicated that the shear bond strength and microhardness of TiO2/ZrO2 coating were statistically higher than those of TiO2 coating. Scanning electron microscope observation revealed that more irregularly shaped protuberances and denser pores were formed on the surface of TiO2/ZrO2 coating compared with those of TiO2 coating. Further comparative analysis of HBMSC proliferation and osteogenic differentiation on both coatings showed that significantly higher cellular alkaline phosphatase activity and expression levels of Runx2 and Osterix at day 10 after osteogenic culture were found on TiO2/ZrO2 coating compared with TiO2 coating, while no statistically significant difference in cell proliferation and extracellular calcium deposition was observed. The present study suggests that TiO2/ZrO2 coating may be favorable for dental implant applications.

Preparation of MgO/B₂O₃ coatings by plasma spraying on SUS304 surface and effects of heat-resistant

This study mainly deals with the preparation of MgO/B2O3 coatings by plasma spraying on the SUS304 surface and the effects of heat-resistant. The power materials of low thermal conductivity were selected to control the heat divergent performance of high temperature parts. The reticular micro-structure between the cover thermal layer and the substrate was prepared by using the plasma spraying method. The powder mixture of MgO and B2O3 were selected as spraying materials and the SUS304 was used as the substrate material. The MgO/B2O3 coating was prepared on the surface of the SUS304 to provide better cover thermal performance. The properties of the microstructures and the morphologies were studied by Optical Microscope, Scanning Electron Microscope, Electron Probe Microanalyzer, and X-ray Diffraction. The results showed that the cover thermal performance has been improved.

In vitro biological response of plasma electrolytically oxidized and plasma-sprayed hydroxyapatite coatings on Ti-6Al-4V alloy

Plasma electrolytic oxidation (PEO) is a relatively new surface modification process that may be used as an alternative to plasma spraying methods to confer bioactivity to Ti alloy implants. The aim of this study was to compare physical, chemical and biological surface characteristics of two coatings applied by PEO processes, containing different calcium phosphate (CaP) and titanium dioxide phases, with a plasma-sprayed hydroxyapatite (HA) coating. Coating characteristics were examined by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, surface profilometry, and wettability tests. The biological properties were determined using the human osteoblastic cell line MG-63 to assess cell viability, calcium and collagen synthesis. The tests showed that PEO coatings are significantly more hydrophilic (6%) and have 78% lower surface roughness (Ra) than the plasma-sprayed coatings. Cell behavior was demonstrated to be strongly dependent on the phase composition and surface distribution of elements in the PEO coating. MG-63 viability for the TiO2 -based PEO coating containing amorphous CaPs was significantly lower than that for the PEO coating containing crystalline HA and the plasma-sprayed coating. However, collagen synthesis on both the CaP and the TiO2 PEO coatings was significantly higher (92% and 71%, respectively) than on the plasma-sprayed coating after 14 days. PEO has been demonstrated to be a promising method for coating of orthopedic implant surfaces.