Infrared radiative properties of plasma-sprayed BaZrO3 coatings

The room temperature directional-hemispherical reflectance and transmittance spectra of free-standing atmospheric plasma-sprayed BaZrO₃ coatings with different thicknesses were measured in the wavelength range of 0.8–15.0 μm, and the absorption coefficient and scattering coefficient as a function of wavelength were extracted using the modified four-flux model. Results showed that BaZrO₃ is a high scattering, low absorption material at the wavelength <6 μm, where turbine engine thermal radiation is most concentrated. The absorption coefficient of BaZrO₃ starts to increase rapidly at wavelength of 6 μm, indicating that BaZrO₃ is high absorbing and opaque in the long wavelength range. A pronounced absorption peak occurs at a wavelength of 7 μm, and is associated with a BaCO₃ coating impurity. The scattering coefficient of BaZrO₃ decreases with the increase of wavelength in the whole measured wavelength range, caused by the decrease of the relative size of the scattering center compared with the wavelength.     

Rare-earth modified zirconium diboride high emissivity coatings for hypersonic applications

Sharp features of hypersonic vehicles increases heat transfer to the surface during flight. This thermal energy can be reduced via increasing the radiation and conduction heat transfer away from the surface. In this study, an emissivity modifier was incorporated into an ultra-high-temperature-ceramic coating system (ZrB₂/SiC) to increase its surface radiation heat transfer rate by increasing the emissivity of the surface. The rare-earth were incorporated into the coatings via mechanical mixing Sm₂O₃ or Tm₂O₃ with ZrB₂/SiC or chemically infiltrating Sm(NO₃)₃/ethanol solution into ZrB₂/SiC. Coatings were fabricated using shrouded air plasma spray. Total hemispherical emissivity results show that the Sm(NO₃)₃ infiltrated ZrB₂/SiC coating had a higher emissivity compared to the baseline ZrB₂/SiC coatings up to 1200 °C. The thermal conductivity of all coatings presently studied was below 12 W/m/K. The presence of rare-earth in the boria-rich surface glasses formed during oxidation increases the glass evaporation rate of the coatings compared to the ZrB₂/SiC coating.     

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.     

Laser modified microstructures in ZrB2, ZrB2/SiC and ZrC

Microstructural evolution of spark plasma sintered ZrB₂, ZrB₂/20 vol.% SiC (ZS20) and ZrC ultra high temperature ceramics (UHTCs) during laser heating has been investigated. Laser heating at temperatures between 2000 and 3750 °C for up to 300 s, in air or vacuum, resulted in extensive bubble and crater formation on the surfaces of 10 mm diameter samples. However, even after exposure to ultra high temperatures, samples did not disintegrate. X-ray diffraction of exposed faces of ZrB₂ and ZS20 samples laser heated in air up to 2700 °C detected only crystalline zirconia. A wide range of morphologies, including nodules, needles, nanofibres and lamella, were observed. The surface of ZrC samples, laser heated in vacuum up to 3750 °C, were characterised by dendritic and eutectic morphologies. Other features associated with melting, such as solidification cracks and trapped porosity, were also observed. A complex array of mechanisms involving solid, liquid and vapour phases led to formation of these various morphologies including melting, oxidation, volatilisation and liquid flow     

The electromagnetic absorbing properties of plasma-sprayed TiC/Al2O3 coatings under oblique incident microwave irradiation

High performance absorbing coatings attract great interest because of the urgent demand for radar stealth technology. However, most reported absorbing coatings focused on the incident microwave were perpendicular to the surface of the coatings. Here, a kind of absorber of TiC/Al₂O₃ was prepared by plasma spraying method and the influence of TiC content and oblique incident microwave irradiation on the absorbing properties of TiC/Al₂O₃ were reported. The absorbing performance firstly increased and then decreased with increasing the TiC content due to the impedance matching and attenuation constant cannot match well at the same time. Meanwhile, compared with TE polarization, TM polarization of the plasma-sprayed TiC/Al₂O₃ coatings was more sensitive to the incident angle. This study demonstrates the feasibility of plasma-sprayed TiC/Al₂O₃ absorbing coatings for electromagnetic applications.     

Reaction behavior and wear properties of in-situ air plasma-sprayed Al2O3-TiB2 composite coatings

In this study, Al₂O₃-TiB₂ coating was successfully deposited on steel substrates by in situ plasma spraying (IPS) using H₃BO₃, Al, and TiO₂ reactants. Beside TiB₂ and Al₂O₃, Al₁₈B₄O₃₃ was formed as a by-product with ratio of about 13 wt%. The effect of milling time of reactant and the reaction behavior was also explored. Milling process for at least 10 h can promote efficiency of reaction and milling for efficient production of Al₂O₃-TiB₂ composite. Wear behavior was examined in terms of hardness, wear track width, and wear rate of the coatings with respective measured values of 797.6 HV, 1061.3 µm, and 4.2 × 10⁻³ mm³/N.m. Based on the FESEM observations, the thickness of abrasive coating was 417 µm, delamination and adhesion were the main wear mechanisms in Al₂O₃-TiB₂-coated specimens.     

Influence of Y2O3 addition on the mechanical and oxidation behaviour of carbon fibre reinforced ZrB2/SiC composites

The influence of Y₂O₃ addition on the microstructure, thermo-mechanical properties and oxidation resistance of carbon fibre reinforced ZrB₂/SiC composites was investigated. Y₂O₃ reacted with oxide impurities present on the surface of ZrB₂ and SiC grains and formed a liquid phase, effectively lowering the sintering temperature and allowing to reach full density at 1900 °C. The presence of a carbon source (fibres) led to additional reactions which resulted in the formation of new secondary phases such as yttrium boro-carbides. Mechanical properties were significantly enhanced compared to the un-doped composite. Further tests at high temperatures resulted in strength increase up to 700 MPa at 1500 °C which was attributed to stress relaxation. Oxidation tests carried out at 1500 °C and 1650 °C in air showed that the presence of the Y-based secondary phases enhanced the growth of ZrO₂ grains, but offered limited protection to oxygen due to the lower availability of surficial SiO₂ formed from SiC.     

Thermophysical and radiative properties of pressureless sintered SiC and ZrB2-SiC laminates

The evaluation of thermal and radiative properties of materials to be used as a hot part of thermal protection systems is a key issue for the design process of the HTC and UHTC components. Ceramic laminates with composition 100?vol%SiC and 80?vol%ZrB₂-20?vol%SiC were prepared by the tape casting technique and pressureless sintered. Thermal properties such as the thermal expansion coefficient, specific heat, thermal diffusivity and conductivity were measured; in addition the total emissivity was evaluated. A comparison of the thermal behavior of these two kinds of laminates is made. Moreover their possible integration in a unique structure is discussed.     

Fabrication and characterization of Yb2Si2O7-based composites as novel abradable sealing coatings

Ceramic matrix composites (CMCs) have gradually replaced superalloys for use in hot sections of aero-engines to meet the requirements for increasingly high engine temperatures. However, the abradable sealing coatings (ASCs) commonly applied to superalloys are not suitable for CMCs owing to factors such as the difference in the coefficient of thermal expansion, therefore it is necessary to develop a new system suitable for CMCs. Yb₂Si₂O₇ exhibits good high-temperature phase stability and relatively low hardness and may be a suitable material for ASCs. In the present study, Yb₂Si₂O₇-based composite coatings with different hexagonal boron nitride (hBN) contents (0, 5, 10, and 15 wt%) were deposited by air plasma spray (APS), and the microstructure, mechanical properties, and abradability of the coatings were characterized. The results showed that the system of Yb₂Si₂O₇ combined with hBN exhibits lower hardness, lower friction coefficient, higher wear rate, and smaller IDR value than the pure Yb₂Si₂O₇ coating and is appropriate for use in ASCs. However, excessive hBN caused severe wear of the coatings. The relevant wear mechanisms of the coatings were analysed, and this study may provide guidelines for the development of ASCs suitable for CMCs.     

Phase evolution in plasma sprayed Nb2O5 coatings

Nb₂O₅ polymorphism and defect chemistry depend on the temperature, pressure, atmosphere composition and the initial crystallography. Plasma spray of Nb₂O₅ is a pathway to form coatings with in-situ metastable and nonstoichiometric phases, however so far unexplored. This study aimed to understand the phase evolution of plasma sprayed Nb₂O₅ coatings, and its effect on their morphology and properties. Phase evolution from H-Nb₂O₅ in the feedstock, to T-Nb₂O₅, TT-Nb₂O₅, N-Nb₂O₅, H-Nb₂O₅, Nb₁₂O₂₉ and NbO₂ in the coatings depends on the plasma Ar/H₂ ratio and its related enthalpy. The microstructure shows a layered distribution of nonstoichiometric phases at the splat boundaries and splat cores composed of T-Nb₂O₅ or TT-Nb₂O₅. The presence and distribution of these phases are related to the thermomechanical and electrical properties. The mechanisms driving the formation of these coatings are based on the Nb₂O₅ incongruent vaporization which promote retention of nonstoichiometric phases and the rapid solidification of metastable phases.     

Phase and microstructure evolution in plasma sprayed Yb2Si2O7 coatings

Yb₂Si₂O₇ coatings were deposited on Si/SiC substrates by atmospheric plasma spray (APS). The different power and plasma chemistries used in this work produced mainly amorphous crack-free coatings with compositions shifted to lower SiO₂ content with higher power and H₂ flow. Differences in microstructure and thermomechanical properties (crystallization behavior, thermal expansion coefficient and thermal conductivity) of as-deposited and thermally treated coatings were directly related to the evolution from amorphous to crystalline phases. A Yb₂SiO₅ metastable phase was identified after thermal treatments at temperatures ～ 1000 °C that transformed to its stable isomorph at 1220 °C. This transformation, followed by the growth of the crystal cell volume, promoted the coating expansion and the “healing” of microcracks present in the amorphous as-sprayed coating.     

Ablation behavior of C/SiC with YSZ and ZrB2 coatings under high-energy laser irradiation

Carbon fiber-reinforced silicon carbide (C/SiC) composites are important candidates for laser protection materials. In this study, ablation mechanism of C/SiC coated with ZrO₂/Mo and ZrB₂–SiC/ZrO₂/Mo under laser irradiation was studied. ZrB₂–SiC multiphase ceramic and ZrO₂ ceramic were successfully coated on C/SiC composite by atmospheric plasma spraying technology with Mo as transition layer. Phase evolution and morphology of composite were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Moreover, ablation behavior of the composite was investigated by laser confocal microscopy. Results showed that ablation mechanism of C/SiC composite was controlled by phase transformation, thermal reaction, and thermal diffusion, with solid–liquid transition of ZrB₂ and ZrO₂ being dominant factor. Endothermic reaction and good thermal diffusivity of coatings were also important factors affecting ablation performance. Reflectivity effect of ZrO₂ coating was limited under high-energy laser irradiation. Compared with ZrO₂/Mo single-phase-monolayer coating, designed ZrB₂–SiC/ZrO₂/Mo coating showed better ablation performance, and breakdown time of C/SiC increased from 10 to 40 s. The depletion of liquid phase in molten pool was identified as an important factor responsible for rapid failure of C/SiC. The coating failed when the entire liquid phase was consumed within molten pool, followed by rapid damage of C/SiC substrate. Results of this study can provide theoretical guidance and research ideas for design and application of laser protective materials.     

多孔Al2O3/SiC、MoSi2/SiC复合材料的制备及吸波性能

以Si、Al₂O₃、MoSi₂微粉和生物竹材为原料,采用包埋烧结法分别制备出SiC多孔材料、Al₂O₃/SiC、MoSi₂/SiC复合材料。采用XRD、SEM及波导法测试其物相组成、显微结构及吸波性能。结果表明：MoSi₂/SiC复合材料的厚度为2 mm时有明显的吸波特性,有效吸收带宽在X波段的9.65~12.4 GHz频率范围内达2.75 GHz,且最低反射损耗为-38.27 dB。Al₂O₃/SiC复合材料孔道内的Al₂O₃与SiC晶须交缠,形成大量电偶极矩,产生介电损耗;MoSi₂/SiC复合材料除介电损耗外还存在电阻损耗,使得复合材料电磁损耗增加,是较有前途的结构功能吸波材料。     

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.     

Phase transition and interface evolution of Al2O3/ZrO2 particles in plasma-sprayed coatings

Alumina and zirconia ceramic particles exhibit high hardness and excellent wear resistance at high temperature, and hence are used as ceramic reinforcement phases in some plasma sprayed coatings. In this study, the interface evolution of a zirconia/alumina eutectic ceramic and the phase transition of zirconia in a plasma-sprayed coating were investigated. Scanning electron microscopy and transmission electron microscopy combined with focused-ion beam and energy dispersive X-ray were used to analyze the microstructure and composition of the ceramic interface. The results showed that the eutectic ceramic particles consisted of alumina (outer) and columnar zirconia (inner) before and after the plasma spraying process. The inner zirconia part showed the martensitic transformation of t-type zirconia to stripe-like m-type zirconia. After the plasma spraying, the interface between alumina and zirconia changed significantly, which formed a new oxide layer. The phase transition mechanism in the ceramic particle and oxide layer formation mechanism at the alumina/zirconia interface were investigated.     

Microstructure and mechanical properties of plasma sprayed TiC/Ti5Si3/Ti3SiC2 composite coatings with Al additions

The mass production of MAX phase coatings such as Ti₃SiC₂ and Ti₃AlC₂ using the plasma spraying method is highly challenging due to its ultra-high temperature and short reaction time. In this study, agglomerate powders of 3Ti/SiC/C/xAl with various Al contents (x = 0–1.5) were prepared to form TiC/Ti₅Si₃/Ti₃SiC₂ composite coatings using the plasma spraying technique. The effect of the Al addition on the microstructures and mechanical performances of the as-sprayed coatings was investigated. The addition of Al decreased the TiC content of the coatings while increasing their Ti₃SiC₂ content significantly. The addition of even small amounts of Al improved the MAX phase fraction of the coatings from 8.95 wt% (x = 0) to 34.05 wt% (x = 0.2) and 41.60 wt% (x = 0.5). Excess Al did not affect the Ti₃SiC₂ content of the coatings. The composite coatings showed a lamellar structure with pores and microcracks. With the addition of Al, the microhardness of the coatings increased slightly, while the fracture toughness improved significantly. The composite coatings with Al showed better wear resistance than those without Al. The wear mechanism of the coatings was a combination of adhesive wear, abrasive wear, and oxidative wear.     

Deposition of graded SiO2/SiC coatings using high-velocity solution plasma spray

Carbon/carbon (C/C) composites are widely used in structural components, particularly in the aerospace and aeronautics sectors. However, the application of C/C composites is limited by low oxidation resistance at high temperatures. In order to overcome this problem, graded SiO₂/SiC coatings were deposited on C/C composites by a high-velocity solution plasma spray (HVSPS) process. Graded coatings were formed by reactions between the Si(OH)₄ sprayed liquid precursor and the C/C substrate; these reactions were promoted by the high temperature of the plasma torch. The morphologies, microstructures, and chemical compositions of the coatings were investigated by X-ray diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. By altering the deposition time, the coating thickness was controlled, therefore demonstrating SiC formation and realizing graded SiO₂/SiC coatings.     

Influence of the SiC grain size on the wear behaviour of Al2O3/SiC composites

Dry ceramic block-on-steel ring wear tests were performed at high loads in several Al₂O₃/20 vol.% SiC composites as a function of the SiC grain size, which ranged from 0.2 to 4.5 μm in d₅₀. The wear resistance of the monolithic alumina was radically improved by the addition of the SiC particles, reducing down to one order of magnitude wear rate. Two different behaviours were identified according to the microstructural observations on the worm surfaces: intergranular fracture and grain pull-out in the monolithic Al₂O₃, and plastic deformation and surface polishing in the composites. The wear resistance of the Al₂O₃/SiC composites increased with the SiC grain size due to their fracture toughness enhancement.     

Electrophoretic deposition of ZrB2 coatings in NaCl-KCl-AlF3 melt containing synthesized ZrB2 nanoparticles

A novel method for preparation of ZrB₂ coatings has been proposed by combination of molten salt synthesis of ZrB₂ nanoparticles and subsequent electrophoretic deposition of the as-synthesized ZrB₂ nanoparticles in the same molten system in this paper. Nanoscale ZrB₂ particles have been produced by borothermal reduction of ZrO₂ in NaCl-KCl-AlF₃ melt at 980°C. Then electrophoretic deposition of the as-synthesized ZrB₂ nanoparticles has been achieved in the resulting molten suspension at a reduced temperature of 900°C, yielding relatively dense ZrB₂ coatings on graphite substrates with a thickness of around 25 μm. Moreover, the effect of different cell voltages ranging from 0.8 V to 1.4 V (i.e., electric field 0.4–0.7 V/cm) on the prepared ZrB₂ coatings has been investigated. Finally, during the tests at 600°C–800°C in air, ZrB₂ coatings deposited at a cell voltage of 1.2 V have exhibited good high-temperature oxidation resistance.     

Effect of microstructure and mechanical properties on wear behavior of plasma-sprayed Cr2O3-YSZ-SiC coatings

In this study, the microstructure and mechanical properties of the atmospheric plasma-sprayed Cr₂O₃ (C), Cr₂O₃-20YSZ (CZ), and Cr₂O₃-20YSZ-10SiC (CZS) coatings were evaluated and also compared with each other, so as to explain the coatings wear behavior. Microstructural evaluations included X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and porosity measurements. Mechanical tests including bonding strength, fracture toughness, and micro-hardness tests were used to advance our understanding of the correlation between the coatings properties and their wear behavior. The sliding wear test was conducted using a ball-on-disk configuration against an alumina counterpart at room temperature. Addition of multimodal YSZ and subsequent SiC reinforcements to the Cr₂O₃ matrix resulted in an increase in the fracture toughness and Vickers micro-hardness, respectively. It was found that the composite coatings had comparable coefficients of friction with pure Cr₂O₃ coatings. When compared with the C coating, the CZ and CZS composite coatings with higher fracture toughness exhibited superior wear resistance. Observation of the wear tracks of the coatings indicated that the lower wear rates of the CZ and CZS coatings were due to the higher plastic deformation of the detached materials. In fact, improvement in the wear resistance of the composite coatings was attributed to a phase transformation toughening mechanism associated with tetragonal zirconia which created more ductile tribofilms during the wear test participated in filling the pores of coatings.