Effect of arc current on microstructure,texturing and wear behavior of plasma sprayed CaZrO3 coatings

In order to enhance the biocompatibility of metallic implants, various ceramic coatings are currently in vogue. CaZrO₃, a promising candidate material, was deposited through plasma spraying on stainless steel (316L) substrates at arc currents of 400, 500 and 600 A. The coatings were characterized using a SEM, XRD, surface profilometers and a tribometer. It was found that the arc current had profound effects on the thickness, microstructure, phase evolution, crystallinity and wear behavior of the coatings. The cross-sectional images and fractographic analysis showed that a denser coating with better inter-splat fusion was produced at arc current of 600 A. The average roughness (Ra) of the coatings increased from 3.62 to 6.68 μm as the arc current was increased from 400 to 600 A. The feedstock (powder) and the coatings were predominantly composed of CaZrO₃ along with a minor amount of CaZr₄O₉ phase. The rise in the arc current resulted in a slight increase in the relative proportion of the CaZrO₃ phase. Also, the coating produced at arc current of 600 A exhibited highest crystallinity. The detailed XRD analysis of (002) and (200) reflections of the ferroelectric CaZrO₃ revealed the preferred orientation of crystals in the coatings. The presence of this texture is explained on the basis of shifting the unstable Zr⁴⁺ ion in oxygen octahedral cage preferably in one direction. The increase in the arc current decreased the coefficient of friction and, as a result, relatively better wear resistance was observed for the coating produced using higher arc current. Moreover, the coating fabricated using arc current of 600 A reduced the volumetric weight loss by 13 times during the wear test as compared to the substrate. Plasma sprayed CaZrO₃ coating not only enhanced the wear resistance of the stainless steel but also showed the potential to furnish a bioactive surface.     

Photocatalytic decomposition of benzene with plasma sprayed TiO2-based coatings on foamed aluminum

Plasma spraying technique was used to deposit thin TiO₂-based photocatalytic coatings on foamed aluminum. Before plasma spraying, the composites of nano-TiO₂ powder (P25) and nano-ZnO/CeO₂/SnO₂ powders were agglomerated into microsized powders by spray-drying process. X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photocatalytic activity evaluation by the decomposition of gas-phase benzene (C₆H₆) were applied to characterize the starting powders and the coatings, respectively. The results showed that all the three plasma sprayed TiO₂-based coatings were the mixture phases of anatase and rutile. On the splats’ surfaces of the as-sprayed coatings, fine nano-crystalline particles were observed. However, grain growth occurred on the surface of plasma sprayed 90%TiO₂–10%ZnO coating. The XPS spectra revealed that the Ti, Zn, Ce and Sn elements existed on the surfaces of plasma sprayed TiO₂-based coatings as the chemical states of Ti⁴⁺, Zn²⁺, Ce⁴⁺ and Sn⁴⁺, respectively, whilst, the oxygen element was composed of three kinds of chemical states, i.e. crystal lattice oxygen, hydroxyl oxygen and physical-adsorbed oxygen. It was found that plasma sprayed 90%TiO₂–10%CeO₂ coating and 90%TiO₂–10%SnO₂ coating exhibited similar photocatalytic activity, which was higher than that of plasma sprayed 90%TiO₂–10%ZnO coating. The photocatalytic activity is not only dependent on the anatase content but also on the surface morphology and the hydroxyl content formed on the surface of plasma sprayed TiO₂-based coatings as well as the additive character.     

Structure and properties of plasma sprayed BaTiO3 coatings: Spray parameters versus structure and photocatalytic activity

Plasma spraying enables the creation of layers with thickness in a millimeter range adhering on various substrates. This paper provides a study of phase composition, optical properties and photocatalytic activity of BaTiO₃ coatings prepared by atmospheric plasma spraying. The spraying was carried out by a direct current gas-stabilized plasma gun. BaTiO₃ was fed into the plasma jet as a feedstock powder prepared by a reactive sintering of micrometer-sized powders of BaCO₃ and TiO₂. Microstructure and phase composition are reported and discussed in connection with optical properties and photocatalytic activity. The spraying was carried out by a direct current gas-stabilized plasma gun which normally utilizes spray distance (SD) in frames from 100 to 150 mm. Besides conventional SD 100 mm also extremely high SD 190 mm was used. The color of the sprayed coating is different for each SD and also differs from sintered BaTiO₃. X-ray diffraction and also SAD mode of HR-TEM show certain content of amorphous fraction in the coating. The hydrogen content in the coating was found to be higher in the coatings than in the sintered bulk. The diffuse reflectance was measured by UV–VIS spectrophotometry and corresponding band-gap energy was estimated. X-ray photoelectron spectroscopy confirms specific stoichiometric and structural disorder observed also at bang-gap evaluation and by Raman spectroscopy. HR-TEM images for crystalline and amorphous zones are given. Photocatalytic decomposition of acetone was tested and BaTiO₃ coatings compared with a sintered bulk.     

Infrared radiative performance and anti-ablation behaviour of Sm2O3 modified ZrB2/SiC coatings

To improve the emissivity of ZrB₂/SiC coatings for serving in more serious environment, ZrB₂/SiC coatings with varying contents of high emissivity Sm₂O₃ were fabricated using atmospheric plasma spraying. The microstructure, infrared radiative performance and anti-ablation behaviour of the modified coatings were investigated. The results showed that as the content of Sm₂O₃ increased, the density of the coatings increased because of the low melting point of Sm₂O₃. When the content of Sm₂O₃ was 10 vol%, the coating had the highest emissivity in the 2.5–5 μm band at 1000 °C, up to 0.85, because of the oxygen vacancies promoting additional electronic transitions. Due to the high emissivity, the surface temperature of the coating modified with 10 vol% Sm₂O₃ decreased by 300 °C, which led to little volatilisation of the sealing phase. Further, the mass ablation ratio of the above coating was 3.19 × 10^?4 g/s, decreasing 31% compared to that of a ZrB₂/SiC coating. The formed dense surface structure of the coatings showed considerable oxygen obstructive effects. These findings indicate that the modified coatings show considerable anti-ablation performance, which provides effective anti-ablation protection for the C/C composite substrate.     

Plasma-sprayed TiO2 coatings: Hydrophobicity enhanced by ZnO additions

Titanium dioxide (TiO₂) powder mixed individually with 10 and 30 weight percentage (wt%) ZnO was thermally sprayed onto a grade B API 5 L carbon steel substrate by atmospheric plasma spraying. The effect of the addition of ZnO (10 wt% and 30 wt%) on the microstructures and wettability properties of the TiO₂/ZnO coatings was investigated. The characterization of the coatings was carried out using scanning electron microscopy, X-ray diffraction (XRD), laser confocal microscope, and sessile droplet system. The XRD analysis of the coating revealed that the anatase phase of TiO₂ in the powder state transformed into rutile phases for the produced TiO₂/ZnO coatings. Surface microstructure analysis revealed that the coatings had typical micro-roughened surfaces of plasma spraying products. The coating with 30 wt% ZnO produced a coating with remarkable pores and microcracks compared with the TiO₂ coating and coating with 10 wt% ZnO. Additionally, the increase in the wt% of ZnO increased the surface roughness value of the produced coatings and substantially changed the wettability properties of the TiO₂ coating from hydrophilic to hydrophobic.     

Production of Re doped La2Zr2O7 based TBCs and numerical analysis of their use on IC engine piston surface

In this study, first of all, a metallic bond layer was coated on the metal substrate using the HVOF method. Then, Gd and Yb doped La₂Zr₂O7 powders, which were specially produced to obtain a low thermal conductivity value, were coated on the metallic bond layer by atmospheric plasma spraying method. The coatings were produced in single-layer and double-layer designs using YSZ as the buffer layer. In the microstructure analysis, it was observed that the coatings exhibited the characteristic microstructure properties of the materials produced by atmospheric plasma spraying method. In the phase analysis, it was found that the Gd and Yb doped La₂Zr₂O₇ was in the form of defect fluorite type structure after plasma spraying. The thermal conductivity of the YSZ coating ranged from 0.88 to 1.00 W/mK, while the thermal conductivity of the doped La₂Zr₂O₇ coatings was measured between 0.38 and 0.68 W/mK. Especially, the lowest thermal conductivity values were obtained in the double-layer Gd doped coating. As a result of modeling these coatings on the piston surface of a diesel engine using the finite element method, it was found that the maximum and minimum surface temperatures of the pistons increased by 69% and 60%, respectively. There was also a reduction of up to 6.5% in the temperature of the piston substrate surface.     

Influence of powder size on characteristics of air plasma sprayed silicon coatings

Silicon powders with different medium sizes (114 μm, 79 μm and 31 μm, respectively) were used to fabricate coatings by air plasma spraying. The velocity and temperature of in-flight silicon particles during plasma spraying were determined. The composition and microstructure of the coatings were characterized and some physical properties of the coatings were measured. The obtained results showed that the size of silicon particles had great influence on their velocity and temperature in plasma flame. The oxidation of silicon particles in the spraying process was observed and is higher for particles of smaller sizes. Areas of silicon oxide in micrometer size are embedded and randomly distributed in the coating. The surface roughness and void content of silicon coatings increase with an increase in the particle size of the powders. The microhardness and oxygen content of coatings decrease with an increase in the particle size. However, the size of silicon particles has little impact on the deposition efficiency of silicon under the same deposition conditions.     

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

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

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.     

Microstructure and properties of niobium carbide composite coatings prepared by plasma spraying

Niobium carbide composite coatings were prepared on titanium alloy surface by plasma spraying NbC–Al₂O₃, Nb–SiC and Nb–SiC–Al composite powders, respectively. The phase composition, microstructure and formation mechanism of the three composite coatings were analyzed and their microhardness, toughness and scratch resistance were compared. The phases of the NbC–Al₂O₃ system did not change during the plasma spraying process, and new phases (Nb₂C, NbC and Nb₃Si) were formed in the Nb–SiC and Nb–SiC–Al systems. TEM results of the Nb–SiC composite coating indicate that the new phases nanocrystalline Nb₂C, submicron NbC and nanocrystalline Nb₃Si were formed during the plasma spraying process. Compared with the NbC–Al₂O₃ composite coating, the microstructure of the Nb–SiC and the Nb–SiC–Al composite coatings were uniform, and the porosity were relatively low, and the hardness was higher. The Nb–SiC–Al composite coating was denser than the Nb–SiC composite coating, the lamellar structure was obvious and the number of pores in the coating was the least, which is attributed to the better molten state of the composite powder by the addition of the Al to the Nb–SiC system. The Nb–SiC–Al composite coating had better toughness and scratch resistance.     

Electrochemical behaviour of thermally sprayed Cr3C2–NiCr coatings in 0.5 M H2SO4 media

The electrochemical behaviour of coated Cr₃C₂–NiCr steel in aerated 0.5 M H₂SO₄ solution was studied by means of electrochemical a.c. and d.c. measurements. A complete structural characterization of the coated steel before and after electrochemical tests was also carried out to access the corrosion mechanism of coated steel, electrolyte penetration through the coating, and to confirm the results obtained using electrochemical techniques. Two types of Cr₃C₂–NiCr coatings produced by a high velocity oxy-fuel spraying system (HVOF) were studied. Differences between coated steels are related to the spraying parameters reflecting their behaviour against corrosion phenomena. The electrochemical behaviour of the coated steel was strongly influenced by porosity and the presence of microcracks in the coating. Once the electrolyte reaches the steel substrate, it corrodes in a galvanic manner resulting in coating detachment from the steel.     

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO₂ powders were prepared by adding HfO₂ powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO₂ bond layer, Yb₂Si₂O₇ intermediate layer and Yb₂SiO₅ surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO₂ improved the thermal cycling performances of the coating. The HfO₂ in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO₂ powders.     

The influence of gun transverse speed on electrochemical behaviour of thermally sprayed Cr3C2-NiCr coatings in 0.5 M H2SO4 solution

In the present study, different types of 75% Cr₃C₂-25% NiCr coatings were applied on a steel substrate by means of high velocity oxygen fuel spraying (HVOF), and studied using ac and dc electrochemical measurements in an aerated and unstirred 0.5 M H₂SO₄ solution. Structural characterization was determined before and after electrochemical tests. Differences between all sprayed systems are related to the gun transverse speed and number of deposited layers, which strongly affected the electrochemical characteristics of the coated steels. The coating obtained with a higher torch speed showed better resistance against corrosion. The electrochemical impedance results were analyzed using an equivalent circuit where porosity of the coatings and substrate oxidation were considered.     

Fabrication of graphene oxide reinforced plasma sprayed Al2O3 coatings

Graphene oxide (GO) reinforced Al₂O₃ ceramic coatings were prepared on the surface of medium carbon steel by plasma spraying. The microstructure of the raw materials and coatings were characterized and analyzed by XPS, XRD, Raman and SEM. The bonding strength of the coatings was studied using a scratch method. The wear resistance of the coatings was assessed by the sliding test. The results showed that, after adding GO, the porosity of the coating reduced by about 31%, the hardness increased by approximately 10%, the bonding strength improved by 250%, and the wear rate reduced by 81% (Load: 30 N) and 84% (Load: 60 N), respectively.     

Effect of Ti particle size on mechanical and tribological properties of TiCN coatings prepared by reactive plasma spraying

Titanium carbonitride (TiCN) coatings were successfully fabricated by reactive plasma spraying (RPS) from agglomerated Ti-graphite feedstock. The effect of Ti particle size on the microstructure and phase composition of plasma sprayed TiCN coatings was investigated. The Vickers microhardness of coatings was measured by a Microhardness Test and the corresponding Weibull distribution were also analyzed. In addition, a pin-on-disk tribometer was employed to determine the trobological properties of coatings. Results show that all the coatings consist of TiC_xN_1−x (0 ≤ x ≤1) and minor Ti₂O phases, and the amount of Ti₂O increases with the increase of Ti particle size. The Weibull distribution of Vickers microhardness of all the coatings shows apparent scattering, while the coating sprayed with Ti particle size of 28 µm exhibits a relatively even distribution. Compared with the coating sprayed with Ti particle size of 14 µm or 48 µm, the coating sprayed with Ti particle size of 28 µm exhibits improved mechanical and tribological properties, which are attributed to the high microhardness and strong bonding strength.     

Phase composition,structural, and plasma erosion properties of ceramic coating prepared by suspension plasma spraying

Y₃Al₅O₁₂ (YAG), Y₂O₃, and Al₂O₃ ceramic coatings were manufactured to investigate the plasma erosion properties. The X‐Ray Diffraction (XRD) analysis confirmed that YAG coating was synthesized successfully by Y₂O₃ and Al₂O₃ mixture suspension using the plasma spraying method. Meanwhile, metastable phases were found in Y₂O₃ and Al₂O₃ coatings due to the quenching in cooling process of melted droplets. The coating surface morphology and microstructure of cross sections were characterized by SEM. The results reveal that coatings are composed by ultrafine splats and exhibit dense lamellar structure. The plasma erosion properties were evaluated at different etching test power under Ar/CF₄/O₂ plasma gas. The experimental results clarify that both of YAG and Y₂O₃ coatings show the better plasma erosion resistance than Al₂O₃ coatings. The formation of fluorination layer surface prevents the coatings from further erosion with plasma gas. Moreover, the etching rate of coatings depended on the fluorination and removing rate of fluoride layer.     

Microstructural,mechanical and tribological properties of carbon nanotubes reinforced plasma sprayed molybdenum disulphide composite coatings

The development of 1-Dimensional (1D) and 2-Dimensional (2D) materials have gained considerable attention towards achieving solid-state lubricity. Herein, we present the effect of carbon nanotubes (1D) reinforcement into the molybdenum disulphide (2D) coatings. Plasma sprayed MoS₂ coatings reinforced with 2-4 wt% CNTs were fabricated using shroud plasma spraying over steel substrates. The shroud attachment envelops the plasma plume and cut down its exposure to surroundings, which minimizes the oxidation of MoS₂ powder during spraying. The microstructural analysis revealed the presence of MoS₂ and CNTs in the composite coating. The mechanical hardness and elastic modulus of MoS₂ coating improved by 2–3 folds in the composite coating. In tribological performance, the coefficient of friction (COF) decreased from 0.13 to 0.07 in M2C coating. The wear weight loss was estimated as 0.89 ± 0.07 mg, 0.18 ± 0.02 mg and 0.39 ± 0.03 mg for M, M2C and M4C coatings respectively. It can be attributed that tubular CNTs acted as bearing on MoS₂ layers. This work opens an impressive stepping for the synergistic mixture of 1D (CNTs) and 2D (MoS₂) material to obtain high-quality wear-resistant coatings.     

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

La₂Ce₂O₇ (LCO) is a promising candidate material for thermal barrier coatings (TBCs) application because of its higher temperature capability and better thermal insulation property relative to yttria stabilized zirconia (YSZ). In this work, La₂Ce₂O₇ TBC with segmentation crack structure was produced by atmospheric plasma spray (APS). The mechanical properties of the sprayed coatings at room temperature including microhardness, Young's modulus, fracture toughness and tensile strength were evaluated. The Young's modulus and microhardness of the segmented coating were measured to be about 25 and 5 GPa, relatively higher than those of the non-segmented coating, respectively. The fracture toughness of the LCO coating is in a range of 1.3–1.5 MPa m^1/2, about 40% lower than that of the YSZ coating. The segmented TBC had a lifetime of more than 700 cycles, improving the lifetime by nearly two times as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

Sol-enhanced electroplating of nanostructured Ni-TiO2 composite coatings—The effects of sol concentration on the mechanical and corrosion properties

Novel sol-enhanced Ni-TiO₂ nano-composite coatings were electroplated by adding a transparent TiO₂ sol into the traditional electroplating Ni solution. It was found that the structure, mechanical properties and corrosion resistance of the nano-composite coatings were largely determined by the sol concentration. The higher sol concentration in the plating electrolyte led to a higher content of TiO₂ nano-particles in the coating matrix. The coating prepared at the sol concentration of 12.5 mL/L had the best microhardness, wear resistance and corrosion resistance. Adding excessive sol to the electrolyte changed the surface microstructure, caused cracking on the coating surface and deteriorated the properties. It was demonstrated that the corrosion resistance of the composite coatings is determined by two factors: surface microstructure and incorporation of TiO₂ nano-particles.     

High temperature oxidation resistance of Y2O3 modified ZrB2-SiC coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at high temperature, Y₂O₃ modified ZrB₂-SiC coating was fabricated on C/C composites by atmospheric plasma spraying. The microstructure and chemical composition of the coatings were characterized by SEM, EDS, and XRD. Experiment results showed that the coating with 10 wt% Y₂O₃ presented a relatively compact surface without evident holes and cracks. No peeling off occurred on the interface between the coating and substrate. The ZSY10 coating underwent oxidation at 1450 °C for 10 h with a mass loss of 5.77%, while that of ZS coating was as high as 16.79%. The existence of Y₂O₃ played an important role in inhibiting the phase transition of ZrO₂, thus avoiding the cracks caused by the volume expansion of the coating. Meanwhile, Y₂SiO₅ and ZrSiO₄ had a similar coefficient of thermal expansion (CTE), which could relieve the thermal stress inside the coating. The ceramic phases Y₂SiO₅, Y₂Si₂O₇ and ZrSiO₄ with high thermal stability and low oxygen permeability reduced the volatilization of SiO₂.