Study on the formation process of Al2O3-TiO2 composite powders

Fine particles of anatase were suspended in solutions of ammonium alum with Al₂O₃/TiO₂ molar ratios from 0.1:1 to 7:1. By spray drying the suspensions and calcining the spray-dried powders, Al₂O₃-TiO₂ composite particles were obtained. The results show that after the spray drying, coatings of ammomium alum are formed on the surface of the anatase particles, leading to composite precursor powders (CCPs) with larger particle sizes. Upon calcining the CCPs, ammomium alum pyrolyzes to amorphous Al₂O₃ and anatase transforms into rutile. Both are mainly responsible for the observed particle size reductions as well as the densification of each composite particle. The in-situ formed α-Al₂O₃ and rutile may have higher reactivities, forming aluminum titanate at 1150 °C, about 130 °C lower than the theoretical temperature for the formation of Al₂TiO₅ by solid reaction. The reaction between α-Al₂O₃ and rutile starts from the interface between the anatase and the alum coating and mainly takes place in the single particles formed by spray drying. The molar ratio of Al₂O₃ to TiO₂ influences the final crystalline phases in the composite powders, but not stoichiometrically.     

Atmospheric plasma spraying coatings from alumina–titania feedstock comprising bimodal particle size distributions

In this work, Al₂O₃–13 wt% TiO₂ submicron-nanostructured powders were deposited using atmospheric plasma spraying. The feedstocks were obtained by spray drying two starting suspensions of different solids content, prepared by adding nanosized TiO₂ and submicron-sized Al₂O₃ powders to water. The spray-dried granules were heat-treated to reduce their porosity and the powders were fully characterised in both untreated and thermally treated state. Comparison with two commercial feedstocks was carried out. Characterisation allowed a temperature for the thermal treatment to be chosen on the basis of the sprayability of the feedstock and the preservation as much as possible of the submicron-sized structure of the unfired agglomerates.Optimisation of the deposition conditions enabled the reconstituted powders to be successfully deposited, yielding coatings that were well bonded to the substrate. The coating microstructure, characterised by SEM, was mostly formed by a matrix of fully molten particles where the presence of semi-molten feedstock agglomerates was also observed.Moreover, microhardness, toughness, adhesion and tribological behaviours were determined, and the impact of the granule characteristics on these properties was studied. It was found that changing the feedstock characteristics allows controlling the coating quality and properties. In general, good mechanical properties were obtained using a feedstock comprising a binary mixture of submicrometric Al₂O₃ and nanometric TiO₂ particles in the spray-dried powder.     

Comparison of dry sliding wear behavior of plasma sprayed FeCr slag coating with Cr2O3 and Al2O3-13TiO2 coatings

The microstructure and dry sliding wear performance of thermally sprayed FeCr slag coating were evaluated in comparison with those of commercially available Al₂O₃-13TiO₂ and Cr₂O₃ ceramic coating powders to assess the applicability of FeCr slag (FS) powder, fabricated from industrial waste, as a ceramic top-coating material against wear. Ceramic top coats and underlying NiCoCrAlY bond coats were deposited on AISI 316L samples via atmospheric plasma spraying (APS), and their tribological properties were assessed using a ball-on-disc test rig at room temperature. As a result, FS coating exhibited the lowest worn volume, although it has the lowest surface hardness. Tribolayer formation was observed on the surface of the samples which were subjected to dry sliding wear tests. Delamination type wear is the dominant wear mechanism for Cr₂O₃ and FS coatings, whereas local spallation areas arising from plastic deformation were observed on the surface of Al₂O₃-13TiO₂ coatings. The results suggested the applicability of FS powder as a candidate ceramic top coating material against wear.     

Characterization of alumina–3 wt.% titania coating prepared by plasma spraying of nanostructured powders

Nanostructured and conventional alumina–3 wt.% titania coatings were deposited by air plasma spraying (APS). The microstructure and phase composition of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties including hardness, adhesion strength, crack extension force (G_C) and sliding wear rate were measured. Equiaxed α-Al₂O₃ grains were observed in the nanostructured Al₂O₃–3 wt.% TiO₂ coating and the diameter of α-Al₂O₃ grains were about 150 to 700 nm in size. The microhardness of both kinds of coating was similar and about 820 HV_0.2. However, the adhesion strength and crack extension force of the nanostructured coating increased by 33 and 80%, respectively, as compared with those of the conventional coating. The wear rate of the nanostructured coating was lower than that of the conventional coating. The results were explained in terms of characteristics of the powders and microstructure of the coatings.     

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.     

Preparation and characterization of Cr2O3-TiO2-Al2O3-V2O5 green pigment

Green pigments with high near infrared reflectance based on a Cr₂O₃-TiO₂-Al₂O₃-V₂O₅ composition have been synthesized. Cr₂O₃ was used as the host component and mixtures of TiO₂, Al₂O₃ and V₂O₅ were used as the guest components. TiO₂, Al₂O₃, and V₂O₅ were mixed into 39 different compositions. The spectral reflectance and the distribution of pigment powder were determined using a spectrophotometer and a scanning electron microscope, respectively. It was found that a pigment powder sample S9 with a Cr₂O₃-TiO₂-Al₂O₃-V₂O₅ composition of 80, 4, 14 and 2 wt%, respectively, gives a maximum near infrared solar reflectance of 82.8% compared with 49.0% for pure Cr₂O₃. The dispersion of pigment powders in a ceramic glaze was also studied. The results show that the pigment powder sample S9 is suitable for use as a coating material for ceramic-based roofs.     

Formation mechanism of Al2O3-YAG amorphous ceramic coating deposited via atmospheric plasma spraying

Al₂O₃-YAG (Al₅Y₃O₁₂) amorphous ceramic coatings exhibit excellent crack propagation resistance under harsh wear services due to the amorphous phase contributing to the plastic deformation performance of the coating. However, the formation mechanism of the amorphous phase is ambiguous. This study mainly investigated the formation mechanism of Al₂O₃-YAG amorphous coating prepared by atmospheric plasma spraying from the perspective of crystallization chemistry. Nano and microsized powders with low eutectic point ratio were selected as feedstock for comparison. X-ray diffraction, scanning electron microscope, and electron backscattered diffraction were used to analyze the phase composition, morphologies, phase distribution, and structure of the coating. It is concluded that the significant thermodynamically stable structure of polycompound with high coordination numbers of cations prioritized crystallizing in the Al₂O₃-YAG melt, but it needed more time to crystallize and hardly crystallized in the limited time during plasma spraying. Therefore, the selection of as-sprayable powder should also be considered the critical factor for preparing amorphous coatings. The nanoscale or submicro scale powder distributed uniformly with low eutectic point ratio was chosen as the feedstock to ensure the powder droplets diffuse sufficiently during deposition.     

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.     

Dielectric and microwave absorption properties of TiAlCo ceramic fabricated by atmospheric plasma spraying

Composite TiAlCo powders (TiO₂, Co₃O₄ and Al₂O₃) were synthesized by spray–drying technology. The phase composition and morphology of synthesized powders were characterized by X-ray diffraction and scanning electron microscopy, respectively. Using above synthesized powders as starting materials, TiAlCo ceramic were successfully prepared by atmospheric plasma spraying (APS) with an internally fed powder torch. Electromagnetic parameters and microwave absorption properties of the prepared ceramic coatings were investigated in the frequency range from 8.2 to 12.4 GHz (X-band). It was found that both the real part and imaginary part of dielectric constant decreases with increasing Al₂O₃ content in the whole measured frequency region. Dielectric properties are closely related to the relaxation polarization and interfacial polarization and electric conductivity. Furthermore, by combination of the frequency selective surfaces (FSS) and prepared coatings, a double absorption band of the reflection loss spectra had been observed in X-band. The optimized reflection loss values exceeding −10 dB can be obtained in the frequency range of X-band when the coating thickness is only 1.8 mm, and the reflection loss is insensitive to incident angle from 0° to 45° for both transverse electromagnetic (TE) and transverse magnetic (TM) polarizations.     

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.     

Facile synthesis of TiO2-supported Al2O3 ceramic hollow fiber substrates with extremely high photocatalytic activity and reusability

TiO₂ oxide was deposited on a microstructured α-Al₂O₃ ceramic hollow fiber substrate by a simple one-step solution-immersion process with tetraethyl orthosilicate (TEOS) as a binder. The effects of the starting coating solution's composition on the photocatalytic properties of TiO₂ powders deposited on a substrate was determined by using TiO₂-supported Al₂O₃ ceramic hollow fiber substrates fabricated from coating solutions with different compositions and examining the substrates' effect on the methylene blue (MetB) degradation reaction under ultraviolet light. A strong correlation was observed between the initial coating solution compositions and the final photocatalytic characteristics of the TiO₂-supported Al₂O₃ ceramic hollow fiber substrates. Under optimal conditions, the MetB removal efficiency reached about 91% in a few minutes. To the best of our knowledge, this is the highest and most rapidly attained MetB removal efficiency reported for TiO₂-supported Al₂O₃ ceramic hollow fiber substrates. Furthermore, apart from attaining an extremely high photocatalytic activity within minutes, the fabricated TiO₂-supported Al₂O₃ ceramic hollow fiber substrates exhibited high photocatalytic stability even after several cycles.     

Protective properties of Al2O3 + TiO2 coating produced by the electrostatic spray deposition method

Mechanical resistance of Al₂O₃ + TiO₂ nanocomposite ceramic coating deposited by electrostatic spray deposition method onto X10CrAlSi18 steel to thermal and slurry tests was investigated. The coating was produced from colloidal suspension of TiO₂ nanoparticles dispersed in 3 wt% solution of Al₂(NO₃)₃, as Al₂O₃ precursor, in ethanol. TiO₂ nanoparticles of two sizes, 15 nm and 32 nm, were used in the experiments. After deposition, coatings were annealed at various temperatures, 300, 1000 and 1200 °C, and next exposed to cyclic thermal and slurry tests. Regardless of annealing temperature and the size of TiO₂ nanoparticles, the outer layer of all coatings was porous. The first five thermal cycles caused a rapid increase of aluminum content of the surface layer to 30–37 wt%, but further increase in the number of thermal cycles did not affect the aluminum content. The oxidation rate of coating-substrate system was lower during the thermal tests than during annealing. The oxidation rate was also lower for smaller TiO₂ particles (15 nm) forming the coating than for the larger ones (32 nm). The protective properties of Al₂O₃ + TiO₂ coating against intense oxidation of substrate were lost at 1200 °C. Slurry tests showed that coatings annealed at 1000 °C had the best slurry resistance, but thermal tests had weakened this slurry resistance, mainly due to decreasing adhesion of the coating.     

Effect of Y2O3 addition on tribological properties of plasma sprayed Al2O3-13% TiO2 coating

The effect of Y₂O₃ addition on structure, mechanical properties and tribological properties of Al₂O₃-13 wt% TiO₂ coating was investigated. The addition of 20 wt% Y₂O₃ resulted in better densification, stabilization of alpha (α) alumina phase and improvement in fracture toughness of Al₂O₃-13 wt% TiO₂ coating. Abrasive wear tests were performed over a range of loads and sliding speeds. The stabilization of α alumina phase further increased with an increase in severity of wear test conditions, as noted from X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) analysis of worn coatings. Al₂O₃-13 wt% TiO₂-20 wt% Y₂O₃ coating displayed lower friction coefficient and lower abrasive wear rate than Al₂O₃-13 wt% TiO₂ coating, which was due to synergistic effect of α alumina phase and formation of magneli phase oxide of titanium; Ti₂O₃. Friction energy map was used to rationalize observed wear rates, to identify different regimes of wear and degradation modes of coatings.     

Synergistic effect of CeO2 and Al2O3 nanoparticle dispersion on the oxidation behavior of MCrAlY coatings deposited by HVOF

In this study, the as-received and nano-scaled oxide dispersion strengthened (ODS) MCrAlY coatings were deposited using high-velocity oxy-fuel (HVOF) spraying process. The high-energy planetary ball-milling process was utilized to prepare CeO₂ and Al₂O₃ nanoparticles. ODS-NiCoCrAlY feedstock powders were also developed using the ball-milling process. The various formulations of Al₂O₃ and CeO₂ nanoparticles (0.5 and 1.0 wt%) were chosen to apply different types of ODS-NiCoCrAlY coatings. The microstructure of the as-received and ODS coatings were evaluated by field emission scanning electron microscope (FESEM) as well as the commercial and ODS powders. Furthermore, the microhardness of different compositions of ODS coatings was accordingly investigated and the obtained results were compared with as-received coating. On account of the measurement of oxidation kinetics, the freestanding as-received and ODS coatings were exposed to air at 1000 °C up to 500 h and the thickness growth rate of the α-Al₂O₃ oxide layer was simultaneously examined. The results exemplified that NiCoCrAlY+1.0 wt% nano-CeO₂+0.5 wt% nano-Al₂O₃ coating had a better oxidation resistance and lower oxide scale growth rate under the synergistic effects of both CeO₂ and Al₂O₃ nanoparticles.     

Erosive and corrosive wear performance and characterization studies of plasma-sprayed WC/Cr3C2 coating on SS316

Plasma spray coating with ceramic carbide is a promising approach for improving the surface quality of the materials. In this work, the effectiveness of tungsten carbide (WC), chromium carbide (Cr₃C₂), and the composite coating of the two powders in the weight ratio of 50:50 were investigated. In the erosion test, aluminum oxide (Al₂O₃) particles were combined with a high-speed air-jet and impinged at 90° on the top surface of the material. Electrochemical polarization and electrochemical impedance spectroscopy studies were conducted with a 3.5 wt.% of sodium chloride (NaCl) solution as the electrolyte. Using a scanning electron microscope, the surface morphology of powders and coatings, as well as the mechanisms of erosion and corrosion, were studied. Energy-dispersive X-ray analysis and X-ray diffractometry were used to reveal the composition and elemental distribution of the feedstock powders and coatings. Because of the presence of hard phases, the composite coating shows the highest average microhardness of 1350.2 HV. The composite coating exhibits improved erosive wear resistance with an increase in erodent exposure time. The Cr₃C₂ coating has a reduced corrosion current density of 1.404 × 10⁻⁵ mA/cm² and a higher charge transfer resistance of 2086.75 Ω cm² due to passivation.     

Thermomechanical properties of ceramics in the systems Al2O3-TiO2 and Al2O3-TiO2-mullite

Conclusions We studied certain properties of ceramics in the systems Al₂O₃-TiO₂ and Al₂O₃-TiO₂-mullite, obtained by the use of the double-stage synthesis of aluminum titanate.We established the nature of the change in the high-temperature strength in relation to the ratio of Al₂O₃ and aluminum titanate. The maximum high-temperature strength (bending) at 1200°C is possessed by ceramic with a corundum matrix and a volume proportion of aluminum titanate equal to 40–45%.It is established that the addition of CaO + SiO₂ made in amounts of up to 1.0–1.5% contributes to the partial breakdown of the aluminum titanate in the compositions Al₂O₃-TiO₂ and the production of a ceramic with a bending strength of 160–190 N/mm² at 20–200°C, thermal-shock resistance 650–800°C, and thermal conductivity of 1.9–2.1 W/(m·K).We studied the effect of the mullite concentration on the properties of the ceramic in the system Al₂O₃-TiO₂-mullite. The introduction of mullite in amounts of not more than 50%, containing up to 3% of impurities, contributes to an increase in the ceramic's strength in the range 20–1300°C and in the thermal shock resistance.Translated from Ogneupory No. 2, pp. 22–26, February, 1988.     

Preparation and corrosion resistance of nonstoichiometric lanthanum zirconate coatings

Lanthanum zirconate is a promising thermal barrier coating material owing to its excellent thermophysical properties and La plays the key role in its corrosion resistance. Here, an amorphous precursor is used as raw feedstock material so as to synthesize lanthanum zirconate coatings with tailorable composition by atmospheric plasma spray (APS). Three lanthanum zirconate coatings of La_1.7Zr_2.3O_7.15, La_2.0Zr_2.0O_7.0 and La_2.3Zr_1.7O_6.85 are fabricated. Furthermore, the corrosion resistance of the as-sprayed coatings against CaO-MgO-Al₂O₃-SiO₂ at 1250℃ is investigated. The increased La content promotes the formation of a sealing layer of the crystalline Ca₂La₈(SiO₄)₆O₂ apatite, which slows down the penetration of molten CaO-MgO-Al₂O₃-SiO₂. Therefore, the infiltration rate of the La_2.3Zr_1.7O_6.85 coating decreased up to 42.6 % compared with the other two coatings. This work develops a feasible preparation strategy to control the La composition for the improved corrosion resistance, which is expected to guide the future coating design and synthesis for the materials with big composition changes during the APS process.     

Enhanced dielectric breakdown strength in Ni2O3 modified Al2O3-SiO2-TiO2 based dielectric ceramics

Dielectric ceramics have raised particular interest since they enable pulsed-power systems to achieve high voltage gradient and compact miniaturization. In this work, x wt%Ni₂O₃ doped Al₂O₃-SiO₂-TiO₂ based dielectric ceramics were prepared using conventional solid-state reaction and the effects of Ni₂O₃ on the crystal structure, dielectric properties and dielectric breakdown strength were investigated. It was found that with the doping of Ni₂O₃, the Al₂O₃-SiO₂-TiO₂ based dielectric ceramics became denser and the distribution of each phase was more uniform. For the composition of x?=?2.0, the dielectric breakdown strength was increased into 82.1?kV/mm, more than twice compared with that of the undoped one. In addition, the relationship between the dielectric breakdown strength and the resistance of Al₂O₃-SiO₂-TiO₂ based dielectric ceramics was discussed. The results show that the doping of Ni₂O₃ is a very feasible way to improve the dielectric breakdown strength and optimize the dielectric properties for the Al₂O₃-SiO₂-TiO₂ based dielectric ceramics.     

Tribological properties of selected ceramic coatings

The paper presents the characteristics of some ceramic coatings obtained by a plasma spray method. The ceramic coatings Al₂O₃, Cr₂O₃ and Cr₂O₃?+?5% TiO₂ were evaluated. Also the influence of the NiCr interlayer on the functional properties of sprayed coatings was studied. Other parameters studied included: thickness; microhardness; adhesion of the coatings; resistance to abrasive wear and thermal cyclic loading. The addition of TiO₂ to the Cr₂O₃ material increased the coating density, but did not substantially reduce the hardness. On the other hand, the lowest loss of material thickness was seen for Cr₂O₃; while the Al₂O₃ and the Cr₂O₃?+?5 wt.% TiO₂ material showed a higher loss. The loss in the case of the latter two was about the same. Relatively, higher values of abrasive wear resistance were observed in the Cr₂O₃ coatings, as compared to the reference material (Al₂O₃ coating), and the highest microhardness values were measured in the Cr₂O₃ coating. Finally, the metal interlayers in all coatings increased their resistance to thermal shock. All the coatings, using the interlayer to reduce differences in coefficients of thermal expansion, were suitable for the purpose of the thermal loading up to 1000?°C.     

Thermal aging of ceramics based on Al2O3-TiO2, Al2O3-SiO2 compositions

Conclusions Following an investigation of the thermal aging at 800°C in air of ceramics based on Al₂O₃ TiO₂ (Nos. 1–3) and Al₂O₃-TiO₂-SiO₂ (No. 4), it was established that the ceramics based on Al₂O₃-TiO₂-SiO₂ are the most heat-stable at 800°C over prolonged periods.Translated from Ogneupory, No. 1, pp. 21–23, January, 1990.