TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb–SiC composite powder

Niobium carbide composite coatings with Nb₂C, NbC, Nb₃Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb₃Si region, nano-Nb/Nb₃Si cellular eutectic region, and equiaxed Nb₂C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb₃Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb₃Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb₃Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb₂C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system.     

Comparing ablation properties of NbC and NbC-25 mol.% ZrC coating on SiC-coated C/C composites

In this work, ablation properties of NbC and NbC-25 mol.% ZrC coating, deposited on SiC-coated C/C composites by supersonic atmospheric plasma spraying, were tested by an oxyacetylene torch. Results showed that, for NbC coating, an unexpected smooth liquid film mostly composed of niobium suboxides (such as NbO₂ and NbO), rather than pure Nb₂O₅, generated during ablation for 45 s. Mechanical erosion resulted from the molten SiO₂, and the relatively low viscosity of the outer oxide layer owing to insufficiently high melting point of niobium suboxides were the key factors for the failure mechanism of NbC coating. While NbC–ZrC coating abated for 90 s has a 97.49 and 66.53% decrease of linear and mass ablation rate relative to NbC coating ablated for 45 s, since ZrO₂ hindering the evaporation of SiO₂ droplets, and more thermal-stable Nb–O–Zr liquid film endow (NbC–ZrC)/SiC/C/C composites with an outstanding anti-ablation property.     

Low pressure plasma-sprayed Al2O3 and Al2O3/SiC nanocomposite coatings from different feedstock powders

This paper describes a preliminary investigation of a nanocomposite ceramic coating system, based on Al₂O₃/SiC. Feedstock Al₂O₃/SiC nanocomposite powder has been manufactured using sol-gel and conventional freeze-drying processing techniques and then low pressure plasma sprayed onto stainless steel substrates using a CoNiCrAlY bond coat. Coatings of a commercial Al₂O₃ powder have also been manufactured as a reference for phase transformations and microstructure. The different powder morphology and size distribution resulting from the different processing techniques and their effect on coating microstructure has been investigated. Phase analysis of the feedstock powders and of the as-sprayed coatings by X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) showed that the nano-scale SiC particles were retained in the composite coatings and that equilibrium α-Al₂O₃ transformed to metastable γ- and δ-Al₂O₃ phases during plasma spraying. Other minority phases in the sol-gel Al₂O₃/SiC nanocomposite powder such as silica and aluminosilicate were removed by the plasma-spraying process. Microstructure characterisation by scanning electron microscopy (SEM) of the as-sprayed surface, polished cross-section, and fracture surface of the coatings showed evidence of partially molten and unmolten particles incorporated into the predominantly lamella microstructure of the coating. The extent of feedstock particle melting and consequently the character of the coating microstructure were different in each coating because of the effects of particle morphology and particle size distribution on particle melting in the plasma.     

Corrosion resistance of Al2O3-Y2O3-SiC coating on depleted uranium prepared by cathode plasma electrolytic deposition

Al₂O₃-Y₂O₃-SiC composite coatings were prepared on depleted uranium by cathode plasma electrolytic deposition in Al(NO₃)₃, Y(NO₃)₃, SiC nanoparticles and anhydrous ethyl alcohol mixture. The resulting coating consisted of an inner barrier layer and an outer porous layer. The SiC nanoparticles were incorporated into the composite coating and decreased the coating porosity by filling the pores. The potentiodynamic polarization test and neutral salt spray test revealed that the corrosion resistance of depleted uranium was enhanced by the composite coating. Moreover, with increasing the content of SiC nanoparticles in the coating, the coating corrosion resistance was improved gradually.     

Cathode plasma electrolytic deposition of Al2O3 coatings doped with SiC particles

Semiconductor particles doped Al₂O₃ coatings were prepared by cathode plasma electrolytic deposition in Al(NO₃)₃ electrolyte dispersed with SiC micro- and nano-particles (average particle sizes of 0.5–1.7?µm and 40?nm respectively). The effects of the concentrations and particle sizes of the SiC on the microstructures and tribological performances of the composite coatings were studied. In comparison with the case of dispersing with SiC microparticles, the dispersion of SiC nanoparticles in the coatings was more uniform. When the concentration of SiC nanoparticles was 5?g/L, the surface roughness of the composite coating was reduced by 63%, compared with that of the unmodified coating. Friction results demonstrated that the addition of 5?g/L SiC nanoparticles reduced the friction coefficient from 0.60 to 0.38 and decreased the wear volume under dry friction. The current density and bath voltage were measured to analyze the effects of SiC particles on the deposition process. The results showed that the SiC particles could alter the electrical behavior of the coatings during the deposition process, weaken the bombardment of the plasma, and improve the structures of the coatings.     

Laser damage resistance of plasma-sprayed alumina and honeycomb skeleton/alumina composite ceramic coatings

Al₂O₃ and honeycomb skeleton-Al₂O₃ composite coatings on Titanium alloy (Ti–6Al–4V) were prepared by atmospheric plasma spraying. A laser ablation experiment on as-sprayed coatings was performed. In this paper, the laser damage resistance, microstructure, phase composition of Al₂O₃ coatings were examined. 3D Dimensional Confocal Microscopy, Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Energy Dispersive Spectrometry (EDS) characterized the laser damage morphology, microstructure, phase composition, and element analysis, respectively. The influence of the honeycomb skeleton on the laser ablation damage on as-sprayed coatings was investigated by a comparative analysis of the laser damage morphology with different laser ablation times and gas flow. The results show that the honeycomb skeleton raises thermal conductivity and thermal diffusivity. Moreover, a “tower”-like dendrite was generated during the laser irradiation of the composite coating. The honeycomb skeleton refined the structure, suppressed crack propagation, and reduced the influence of gas flow on cracks. Under the same experimental laser ablation parameters, the laser damage area of the honeycomb skeleton-Al₂O₃ composite coating was smaller than that of the Al₂O₃ coating. It was demonstrated that the laser damage resistance of the honeycomb skeleton-Al₂O₃ composite coating was superior to that of the Al₂O₃ coating.     

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.     

Formation of Al2O3–Nb2O5 composite oxide films on low-voltage etched aluminum foil by complexation–precipitation and anodizing

Niobium pentaoxide (Nb₂O₅) thin films were deposited on etched aluminum foils by complexation–precipitation followed by heat treatment. Then the Al₂O₃–Nb₂O₅ (Al–Nb) composite oxide films were formed by anodizing to increase the capacitance of anodized aluminum foils which are used in aluminum electrolytic capacitors. The composition and structure of niobium deposition layer were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), and the microstructures and dielectric properties of anodic oxide films were investigated by scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS) respectively. The results show that the niobium deposition layer after heat treatment existed in the form of crystalline Nb₂O₅. The aluminum foil with Nb₂O₅ coating can be anodized with higher efficiency and energy saving. Compared with that of normal anodized aluminum foils, the effective area of the anodized aluminum foils with Al–Nb composite oxide films had no apparent change. The specimens with Al–Nb composite oxide films anodized at 30 V exhibited about 20% higher specific capacitance than that of those with pure aluminum oxide films. It suggests that the method of complexation–precipitation is an effective way to increase the specific capacitance of anodized aluminum foils used in aluminum electrolytic capacitors.     

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.     

Oxidation behaviors of compositionally complex MAX phases in air

The oxidation behaviours of a newly synthesized MAX phase composite mainly containing a multi-component 413 MAX phase with Ti, Nb and Ta equally and evenly distributed at M site were investigated at 1000–1400 °C in air. Results indicated that the multi-component MAX phase exhibited superior oxidation resistance compared with traditional monolithic 413 MAX phases such as Nb₄AlC₃ and Ta₄AlC₃. Dense and passivating Al₂O₃ layers that formed at the interfaces between the substrate and the oxidation scale is the origin of the high oxidation resistance. The presence of Cr–Al alloy phases is essential for the formation of protective Al₂O₃ scale.     

Comparative study of the corrosion resistance of thermally sprayed ceramic coatings and their bulk ceramic counterparts

A comparative study of the corrosion properties of thermally sprayed ceramic coatings (Al₂O₃, Al₂O₃–TiO₂ with different ratios, mullite, and ZrSiO₄) and their sintered bulk ceramic counterparts was performed. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) spraying processes. The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄ at 85 °C, respectively. The coating microstructures and phase compositions, as well as the corrosive environment were shown to have a strong effect on the corrosion resistance of the coatings. Al₂O₃–coatings were more sensitive to these factors than Al₂O₃–TiO₂ coatings were.The corrosion resistance of the bulk ceramics was superior to that of the thermally sprayed coatings. This is mainly because the coatings exhibited specific microstructure and contained amorphous and/or metastable phases not appearing in the bulk ceramics.     

Preparation of Al2O3–SiC composite powder from kyanite tailings via carbothermal reduction process

ABSTRACT

Al₂O₃–SiC composite powders were prepared from kyanite tailings mixed with 20% excess carbon coke via carbothermal reduction (CR) reaction. The optimised synthesis condition for synthesising Al₂O₃–SiC composite powders was at 1600°C for 6?h. The equilibrium relationship curves of the condensed phases were presented and the temperature dependence of the phase composition was also studied. The results show that irregular Al₂O₃ and SiC grains first formed at 1500°C, and the elements C, O, Al, and Si randomly distributed in the each crystal particles. The amount of Al₂O₃–SiC composites increased with the increasing synthesis temperature and reaction time. Finally, Al₂O₃–SiC composite bulk materials were further prepared by pressureless sintering using the synthesised Al₂O₃–SiC composite powders as raw materials, and their mechanical properties were investigated in detail. All these results indicate that the CR method can offer a niche application for the development of kyanite tailings.     

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

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

Mechanical and tribological properties of nano/micro composite alumina coatings fabricated by atmospheric plasma spraying

Adding nano particles can significantly improve the mechanical properties and wear resistance of thermal sprayed Al₂O₃ coating. However, it still remains a challenge to uniformly incorporate nano particles into traditional coatings due to their bad dispersibility. In the present work, nanometer Al₂O₃ (n-Al₂O₃) powders modified by KH-560 silane coupling agent were introduced into micrometer Al₂O₃ (m-Al₂O₃) powders by ultrasonic dispersion to afford nano/micro composite feedstock, and then four resultant coatings (weight fraction of n-Al₂O₃: 0%, 3%, 5% and 10%) were fabricated by atmospheric plasma spraying. The features and constitutes of feedstock and as-sprayed coatings, as well as their porosity, bonding strength, microhardness and frictional behaviors were investigated in detail. Results show that the nano/micro composite feedstock with uniform microstructure can be better melted in the spraying process, thereby obtaining coatings with denser microstructure, higher hardness and bonding strength. Added n-Al₂O₃ has no obvious effect on the friction coefficient of composite coatings, whereas can improve their wear-resistant and reduce the worn degree of counterpart. The wear mechanism of traditional coating is brittle fracture and lamellar peeling, while that of composite coating with weight fraction of n-Al₂O₃ of 10% is adhesive wear.     

An evaluation of plasma-sprayed coatings based on Al2O3 and Al2O3–13 wt.% TiO2 with bond coat on pure titanium substrate

In this study, the effects of bond coat on the properties of Al₂O₃ and Al₂O₃–13 wt.% TiO₂ coatings, which is plasma sprayed onto a commercial pure titanium substrate with and without Ni–5 wt.% Al (METCO 450 NS) as bond coating layer were investigated in terms of microhardness, bonding strength and surface roughness. Optical and scanning electron microscopy (SEM) examinations revealed that there is a uniform coating layer with no spalling and delamination. However, there is a little amount of porosity. The results indicated that the application of bond coat layer in the plasma spraying of Al₂O₃ and Al₂O₃–13 wt.% TiO₂ on pure titanium substrate has increased the hardness and bonding strength of coatings. While the adhesive bonding is dominant without bond coat, the cohesive bonding is dominant with the application of the bond coating layer. It has been observed that percentage of cohesion strength was about three times higher than that of adhesion 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.     

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.     

Synthesis and strengthening mechanism for (Ti,Nb)3SiC2/Al2O3 ceramics: A combined experimental and first-principles investigation

(Ti,Nb)₃SiC₂/60 vol%-Al₂O₃ composite ceramic samples with different Ti/Nb atomic ratios were prepared by hot-pressing sintering using Ti/Si/Al/TiC/NbC/Al₂O₃ powders as starting materials. X-ray diffraction analysis showed that the solid solubility of Nb is approximately 7.5 at%. Excessive NbC powder addition can lead to the generation of an impurity phase. Scanning transmission electron microscopy and X-ray photoelectron spectroscopy images reveal that most of the Nb-doped atoms aggregate at 2a-sites, as this configuration exhibits the lowest system energy. The optimized hardness, flexural strength and fracture toughness of (Ti,Nb)₃SiC₂/Al₂O₃ were determined to be 16.0 GPa, 672 MPa and 6.9 MPa m^1/2, which corresponds to an increase of 15.9%, 18.7% and 25.4% respectively, compared with Ti₃SiC₂/Al₂O₃ composite ceramics. By means of first-principle calculations, the strengthening mechanism is derived from both intragranular improvement in (Ti,Nb)₃SiC₂ solid solutions and intergranular enhancement of (Ti,Nb)₃SiC₂/Al₂O₃ grains interfaces.     

Dried particle plasma spray in-flight synthesis of spinel coatings

Powder particle diameters currently used for spraying are generally between 5 and 100 μm with a preferred size range around 40–60 μm. Future trends in plasma spraying involve the use of fine or ultrafine powders and the reduction of the number of steps between raw material preparation and coating. The use of non-sintered spray dried ceramic aggregates as feedstock material for plasma spraying has accordingly been investigated. Al₂O₃ based coatings were prepared by this route of dried particle plasma spray (DPPS). The microstructure and crystallographic phases of these deposits were characterised using scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). Given the intimate mixing of the starting oxides, reactions occur during spraying leading to the formation of spinel (MgAl₂O₄ and/or ZnAl₂O₄) and zinc aluminum oxide (Zn₄Al₂₂O₃₇). The layered structure of the deposit is characteristic of conventional plasma-sprayed coatings but the features are smaller in size. Depending on the operating conditions (plasma characteristics and powder injection), two different melting modes of the particles were identified; the first leads to individual well-melted droplets that splash regularly even if generating some fingers and the second leads to aggregates that are well-melted on their outer parts and strengthened in their cores.     

Ultrafine-grained β-Sialon fabricated by two-stage sintering and study on diffusion toughening of Ti–22Al–25Nb

The ultrafine-grained β-Sialon ceramics were fabricated by spark plasma sintering at different temperatures with inorganic Al₂O₃–Y₂O₃ and Ti–22Al–25Nb intermetallic powder as composite additives. The research showed that β-Sialon ceramics achieve two-stage sintering densification. Al₂O₃–Y₂O₃ inorganic additives promoted the synthesis and densification of β-Sialon ceramics at 1125–1215°C. Ti–22Al–25Nb intermetallic powder diffused Ti and Nb elements at 1240–1425°C, thereby improving the fracture toughness of β-Sialon ceramics. The maximum fracture toughness (∼9.69 MPa m^1/2) under 19.6 N was obtained for β-Sialon ceramics sintered at 1600°C.