Protective properties of SiO2 with SiO2 and Al2O3 nanoparticles sol-gel coatings deposited on FeCrAl alloys

Five layer SiO₂ coatings containing SiO₂ or Al₂O₃ nanopowders were deposited on FeCrAl alloy support by sol-gel method. Studies of protective properties of the coatings were carried out during high temperature cyclic oxidation. Changes in surface topography, structure and chemical composition of the surface layer of FeCrAl alloy were investigated. It has been shown that the type of nanofillers present in the SiO₂ coating (about 2.5?wt%) affects morphology of Al₂O₃ growing scale and determines the heat resistance of FeCrAl alloy. The lowest relative mass change (approx. 1.3%) after 10 oxidation cycles in air at 900?°C (one cycle = 12?h) was measured for the samples with coatings containing hydrophilic nanosilica (Aerosil 380) as filler. The protective efficiency of the coatings in the process of high-temperature oxidation is from 66% to 85%. The thickness of the formed scale and the value of the parabolic rate constant depend on the type of nanopowder in the coating.     

Microhardness and wear resistance of Al2O3-TiB2-TiC ceramic coatings on carbon steel fabricated by laser cladding

Al₂O₃-TiB₂-TiC ceramic coatings with high microhardness and wear resistance were fabricated on the surfaces of carbon steel substrates by laser cladding using different coating formulations. The microstructures of these ceramic coatings with the different coating formulations were investigated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometer. The wear resistance and wear mechanism were analyzed using Vickers microhardness and sliding wear tests. The results showed that when the amount of independent Al₂O₃ was increased to 30%, the ceramic coatings had a favorable surface formation quality and strong metallurgical bond with the steel matrix. The cladding layer was uniformly and densely organized. The black massive Al₂O₃, white granular TiB₂, and TiC distributed on the Fe substrate significantly increased the microhardness and wear resistance. The laser cladding ceramic coating had many hard strengthening phases, and thus resisted the extrusion of rigid particles in frictional contact parts. Therefore, the wear process ended with a “cutting-off” loss mechanism.     

Characterization and tribological investigation of sol-gel Al2O3 and doped Al2O3 films

Thin films of Al₂O₃ and doped Al₂O₃ were prepared on a glass substrate by dip coating process from specially formulated ethanol sols. The morphologies of the unworn and worn surfaces of the films were observed with atomic force microscope (AFM) and scanning electron microscope (SEM). The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of obtained thin films sliding against Si₃N₄ ball were evaluated and compared with glass slide on a one-way reciprocating friction tester. XPS results confirm that the target films were obtained successfully. The doped elements distribute in the film evenly and exist in different kinds of forms, such as oxide and silicate. AFM results show that the addition of the doped elements changes the structure of the Al₂O₃ films, i.e., a rougher and smoother surface is obtained. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. As the results, the doped films exhibit better tribological properties due to the improved toughness. Sever brittle fracture is avoided in the doped films. The wear of glass is characteristic of brittle fracture and severe abrasion. The wear of Al₂O₃ is characteristic of brittle fracture and delamination. And the wear of doped Al₂O₃ is characteristic of micro-fracture, deformation and slight abrasive wear. The introduction of ZnO is recommended to improve the tribological property of Al₂O₃ film.     

Protective abilities of nanocomposite coatings containing Al2O3 nano-particles loaded by CeCl3

The protective ability of hybrid nano-composite oxysilane coatings, deposited via sol–gel method on AA2024-T3 – aluminium alloy, were studied by linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) methods in 0.05 M solution of NaCl. Cerium chloride (CeCl₃) was incorporated as an inhibitor into a sol–gel hybrid matrix in two different routes: directly and via filled porous Al₂O₃ nano-particle aggregates with diameters up to 500 nm. The influences of the inhibitor concentration, as well as the influence of nano-particles on the barrier properties and the susceptibility against corrosion, were evaluated and EIS spectra were fitted by appropriated equivalent circuits. The values for C_coat, R_coat, C_oxy and R_oxy were achieved and their evolution over time was investigated. The investigated coatings possess highly expressed barrier properties (10⁶ to 10⁷ Ω cm²). Despite of the chloride ions inside of the matrix, some samples illustrated a significant durability of over 4000 h during exposure to the corrosion medium before first signs of corrosion appeared. The electrochemical results were compared with the neutral salt spray test. Thus, it was proved that the potential of these coatings is to be used as anticorrosive protective materials and are candidate to replace Cr(VI)-based anti-corrosion coatings.     

Relationship between microstructure and abrasive wear resistance of Al2O3-FeAl2O4 nanocomposites produced via solid-state precipitation

The work investigates the correlation between the microstructure and wear behaviour of novel Al₂O₃-FeAl₂O₄ nanocomposites, developed by precipitation of FeAl₂O₄ particles through reduction aging of Al₂O₃-10 wt.% Fe₂O₃ solid solutions in N₂/4%H₂. Reduction aging at 1450 °C for 10 and 20 h resulted in considerable improvements in abrasive wear resistance. The nanocomposites developed from solid solutions doped additionally with ∼250 ppm of Y₂O₃ contained finer intergranular second phase particles (by a factor of ∼2) and showed further improvements in the wear resistance. Doped nanocomposites reduction aged for 20 h at 1450 °C exhibited the minimum wear rate (reduced by a factor of ∼2.5 with respect to monolithic Al₂O₃). The suppression of fracture-induced surface pullout in the presence of intragranular nanosized second phase particles was the major factor responsible for the improved wear resistance of the nanocomposites with respect to monolithic alumina; microstructures without these intragranular nanoparticles showed no improvement. Higher aging temperature led to the presence of coarse (>2 μm) intergranular FeAl₂O₄ particles which had a detrimental effect on the wear resistance.     

Composite Ni/Al2O3 coatings and their corrosion resistance

Composite coatings Ni/Al₂O₃ were electrochemically deposited from a Watts bath. Al₂O₃ powder with particle diameter below 1 μm was codeposited with the metal. The obtained Ni/Al₂O₃ coatings contained 5-6% by weight of corundum. The structure of the coatings was examined by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure, increasing its microcrystallinity and surface roughness. DC and AC electrochemical tests were carried out on such coatings in a 0.5 M solution of Na₂SO₄ in order to evaluate their corrosion resistance. The potentiodynamic tests showed that the corrosion resistance of composite coating Ni/Al₂O₃ is better than that of the standard nickel coating. After 14 days of exposure the nickel coating corrodes three times faster than the Ni/Al₂O₃ coating. The electrochemical behaviour of the coatings in the corrosive solution was investigated by electrochemical impedance spectroscopy (EIS). An equivalent circuit diagram consisting of two RC electric circuits: one for electrode, nickel corrosion processes and the other for processes causing coating surface blockage, were adopted for the analysis of the impedance spectra. The changes in the charge transfer resistance determined from the impedance measurements are comparable with the changes in corrosion resistance determined from potentiodynamic measurements.     

Improved mechanical and wear properties of hybrid Al-Al2O3/GNPs electro-less coated Ni nanocomposite

Al was successfully reinforced with two ceramics Al₂O₃ coated Ni and graphene nanoplatelets (GNPs) coated Ni by electro-less deposition technique to form Al-Al₂O₃/$\mathit{x}$ GNPs hybrid nanocomposite ($\mathit{x}=0,0.2,0.6,1$and $1.4\%$) with improved mechanical and wear properties. Compressive strength, hardness, wear properties and coefficient of friction were investigated. The results indicated that increasing GNPs volume fraction improves compressive strength, hardness and antifriction properties of composites significantly. In comparison with pure aluminum, 1.52- fold increases in the strength, 2.45-fold increase in the hardness and 19.2-fold decreases in the wear rate of Al-10%Al₂O₃/1.4%GNPs nanocomposite are achieved. This improvement is attributed to the remarkable mechanical strength and excellent self-lubrication of grapheme, the reduction of grain size during electro-less deposition process and the increased efficient stress transfer due to the curled structure of GNPs. Additionally, coating GNPs with Ni particles prevent the formation of Al₃C₄ intermetallic phase which lead to this large improvement in the wear rate. In comparison with the available results in the literature, electro-less coating of GNPs with Ni provides 2.1 times larger hardness than composite with uncoated GNPs.     

Effect of Al2O3 on mechanical properties of Ti3SiC2/Al2O3 composite

The effect of Al₂O₃ on mechanical properties of Ti₃SiC₂/Al₂O₃ composite fabricated by SPS was studied systematically. The results show that the hardness of the Ti₃SiC₂/Al₂O₃ composite can reach 10.28 GPa, 50% higher than that of pure Ti₃SiC₂. However, slight decrease in the other mechanical properties was observed with Al₂O₃ addition higher than 5–10 vol.%, which is believed to be due to the agglomeration of Al₂O₃ in the composite.     

Effect of surface-modified Al2O3 on the thermomechanical properties of polybutylene succinate/Al2O3 composites

This study investigates the effect of the incorporation of alumina particles on the thermomechanical properties of polybutylene succinate (PBS)/Al₂O₃ composites. The alumina surface was modified with the carboxylic groups of maleic acid through simple acid-base and in situ polymerization reactions. Scanning electron microscope (SEM) results revealed the introduction of maleic acid treated alumina significantly affect the morphology of the PBS/Al₂O₃ composites as compared to the neat PBS. The thermal conductivity of the composite (0.411?W?m^?1 K^?1) was more than twice that of neat PBS. The composite containing polymerization-modified alumina showed a 50% increase in storage modulus compared with that of neat PBS. In addition, universal testing machine (UTM) and differential scanning calorimetry (DSC) measurements indicated an increase in the tensile strength and degree of crystallinity after the incorporation of modified alumina in the PBS/Al₂O₃ composite.     

Tensile strength and corrosion resistance properties of porous Al2O3/Ni composites prepared with rice husk pore-forming agent

The mechanical performance and chemical stability of porous alumina materials operating under harsh service conditions are of utmost importance in understanding their operational behavior if they are to stand the test of time. In the present study, the joint effect of nickel (Ni) reinforcement and rice husk (RH) pore-forming agent (PFA) on the tensile strength and the corrosion resistance properties of composite porous alumina ceramics was studied. To exploit the potential of this new porous alumina system, plain and Ni-reinforced porous alumina samples (Al₂O₃-xNi-RH; x?=?2, 4, 6 and 8?wt%) were developed through the powder metallurgy technique. Comprehensive investigation on the tensile strength properties of the developed porous alumina ceramics showed that relative to the plain sample (tensile strength and elastic modulus; 6.1?MPa and 1201?MPa), the presence of highly stable Ni₃Al₂SiO₈ spinelloid promoted the tensile strength enhancement (12.6–6.4?MPa) and the elastic modulus decline (897–627?MPa) of the composite samples. Similarly, corrosion resistance test was performed on the composite porous alumina samples in both 10?wt% NaOH and 20?wt% H₂SO₄ hot aqueous solutions. Overall, the composite samples demonstrated superior chemical stability in NaOH solution as compared with the plain sample. On the other hand, the composites were more prone to attack in H₂SO₄ solution, except for the Al₂O₃-2Ni-10RH composite sample which maintained its superiority over the plain counterpart.     

UV laser microprocessing and post chemical etching on ultrathin Al2O3 ceramic substrate

In this study, an Nd:YVO₄ UV laser was used for microprocessing ultrathin (125 μm) ceramic plates for use as a multi-layer microchip substrate. The effects of the UV laser microprocessing parameters, including laser power density, frequency, laser scanning speed, and pass delay on microprocessing accuracy and quality (kerf width and arithmetic average roughness R_a on the kerf sidewall) were investigated by means of a 4 × 4 orthogonal design. The key processing parameters were determined and optimized for small kerf width and minimal R_a of the kerf sidewall while retaining high production efficiency. Subsequent chemical etching of the laser processed areas was performed to reduce the kerf surface and kerf sidewall roughness by removing debris and the thin recast layer for the required size precision and post gilding treatment. The results showed that a clean surface and crack-free kerf sidewall with roughness R_a of 0.16 μm could be achieved by laser microprocessing and chemical etching.     

Mechanical properties of Al2O3 + ZrO2 + nano-SiCp ceramic composites

The mechanical properties of Al₂O₃ matrix composites reinforced by ZrO₂(2 mol% Y₂O₃) and nanometre scale SiC dispersions have been investigated. It is shown that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂(2 mol% Y₂O₃) and nano-SiC particles. The maximum flexural strength and fracture toughness of the composites are 945 MPa and 7.3 MPam^1/2, respectively. The reinforcing effect of both t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) and nano-SiC particles appears to be synergetic.     

Self-healing of Al2O3 containing Ti microparticles

This work explores the possibility of using embedded micron-sized Ti particles to heal surface cracks in alumina and to unravel the evolution of the crack filling process in case of pure solid-state oxidation reactions. The oxidation kinetics of the Ti particles is studied and the results are applied in a simple model for crack-gap filling. An activation energy of 136?kJ/mol is determined for the oxidation of the Ti particles having an average particle size of 10?µm. The almost fully dense alumina composite containing 10?vol% Ti has an indentation fracture resistance of 4.5?±?0.5?MPa?m^1/2. Crack healing in air is studied at 700, 800 and 900?°C for 0.5, 1, and 4?h and the strength recovered is measured by 4-point bending. The optimum healing condition for full strength recovery is 800?°C for 1?h or 900?°C for 15?min. Crack filling is observed to proceed in three steps i.e., local bonding at the site of an intersected Ti particle, lateral spreading of the oxide and global filling of the crack. It is discovered that, although significant strength recovery can be attained by local bonding of the intersected particles, full crack filling is required to prevent crack initiation from the damaged region upon reloading. The experimental results observed are in good agreement with the predictions of a simple discrete crack filling/healing model.     

Microstructure and toughening mechanisms of Al2O3/(W,Ti)C/graphene composite ceramic tool material

To toughen the Al₂O₃ matrix ceramic materials, Al₂O₃/(W, Ti)C/graphene multi-phase composite ceramic materials were fabricated via hot pressing. The effects of the graphene nanoplates (GNPs) content on microstructure and mechanical properties were investigated. Results showed that the fracture toughness and flexural strength of the composite added with just 0.2?wt% GNPs were markedly improved by about 35.3% (~ 7.78?MPa?m^1/2) and 49% (~ 608.54?MPa) respectively compared with the specimens without GNPs while the hardness was kept about 24.22?GPa. However, the mechanical properties degrade with the further increase of GNPs’ content owing to the increased defects caused by agglomeration of GNPs. Synergistic toughening effects of (W, Ti)C and GNPs played an essential role in improving the fracture toughness of composites. By analyzing the microstructures of fractured surface and indentation cracks, besides GNPs pull-out, crack deflection, crack bridging, crack branching and crack arrest, new toughening mechanisms such as break of GNPs and crack guiding were also identified. Furthermore, interface stress can be controlled by means of stagger distributed strong and weak bonding interfaces correlated with the distribution of GNPs.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

Thermodynamic simulation models for predicting Al2O3-MgO castable chemical corrosion

The chemical corrosion of two Al₂O₃-MgO castables (containing distinct binder sources: hydratable alumina or calcium aluminate cement) were evaluated in this work via thermodynamic calculations. Two simulation models were proposed according to the following procedures: (1) firstly the matrix and later the aggregates of the castables were placed, separately, in contact with an industrial basic slag, and (2) the overall chemical composition of the design castables was directly reacted with the molten slag. The theoretical results were further compared with experimental data collected after corrosion cup tests. Although the thermodynamic evaluation of the overall castable compositions was able to identify the phase transformations correctly, a two-step analysis of the matrix components and aggregates particles seems to be the best alternative to evaluate the binder source effect on the corrosion performance of the two Al₂O₃-MgO refractory materials.     

Mechanical properties of Al2O3-Cr2O3/Cr3C2 nanocomposite fabricated by spark plasma sintering

The fine grains of Al₂O₃-Cr₂O₃/Cr-carbide nanocomposites were prepared by employing recently developed spark plasma sintering (SPS) technique. The initial materials were fabricated by a metal organic chemical vapor deposition (MOCVD) process, in which Cr(CO)₆ was used as a precursor and Al₂O₃ powders as matrix in a spouted chamber. The basic mechanical properties like hardness, fracture strength and toughness, and the nanoindentation characterization of nanocomposites such as Elastics modulus (E), elastic work (W_e) and plastic work (W_p) were analyzed. The microstructure of dislocation, transgranular and step-wise fracture surface were observed in the nanocomposites. The nanocomposites show fracture toughness of (4.8 MPa m^1/2) and facture strength (780 MPa), which is higher than monolithic alumina. The strengthening mechanism from the secondary phase and solid solution are also discussed in the present work. Nanoindentation characterization further illustrates the strengthening of nanocomposites.     

Contact damage in a Ce-TZP/Al2O3 nanocomposite

The damage tolerance of a nanocomposite based on Ce-TZP and 30 vol% Al₂O₃ has been studied under monotonic contact with a spherical indenter. The results are compared with those previously known for commercial 3Y-TZP zirconia. It is concluded that the minimum load for ring crack appearance is similar in both ceramics. However, in the nanocomposite the ring cracks penetrate much less into the bulk, because of its higher fracture toughness. Finally, the stress-induced phase transformation of the zirconia component was quantified and mapped by micro-Raman spectroscopy.     

Laser machining of Al2O3-ZrO2 (3%Y2O3) eutectic composite

In this work we present the study of the interaction between NIR pulsed laser and Al₂O₃-ZrO₂ (3%Y₂O₃) eutectic composite. The effect produced by modifying the reference position as well as the working conditions and laser beam features has been studied when the samples are processed by means of pulse bursts.The samples were obtained by the laser floating zone technique using a CO₂ laser system. The laser machining was carried out with a Q-switched Nd:YAG laser at its fundamental wavelength of 1064 nm with pulse-widths in the nanosecond range.Geometric dimensions, i.e. ablated depth, machined width and removed volume as well as ablation yield of the resulting holes have been studied. We have described and discussed the morphology, composition and microstructure of the processed samples.     

Improved corrosion resistance of Al2O3 ceramic coatings on AZ31 magnesium alloy fabricated through cathode plasma electrolytic deposition combined with surface pore-sealing treatment

In this study, we explored the phase compositions and morphologies of the ceramic coatings from different aluminum sources (aluminum isopropoxide, aluminum nitrate, or a mixture of the two) prepared using cathode plasma electrolytic deposition (CPED) onto AZ31 magnesium alloys. Scanning electron microscopy and X-ray diffraction analyses of these coatings indicate that the deposited ceramic made from aluminum isopropoxide was composed of γ-Al₂O₃ whereas the one made from aluminum nitrate was composed of MgA1₂O₄, and that the former was more compact and uniform than the latter. A composite coating was prepared using epoxy resin as a protective layer that sealed the micropores on the CPED coating, thereby further improving its anticorrosion property. The elemental distribution of the cross-section of the composite coating was examined via energy dispersive spectroscopy. Corrosion resistance was investigated using potentiodynamic polarization curves and electrochemical impedance spectroscopy in a 3.5?wt% NaCl medium, and a salt spray test. The results indicate that the corrosion protection property of the Al₂O₃/epoxy resin coating of the magnesium alloy was better than that of the single Al₂O₃ coating. A cross-cut test revealed that the adhesion of the Al₂O₃/epoxy resin composite coating to the magnesium alloy surface was better than that of the single epoxy resin coating. The approach presented herein provides an attractive way to modify the surface of magnesium alloys to improve anticorrosion.