Rheological properties of aqueous alumina–alumina-doped Y-PSZ suspensions

The effect of the substitution of alumina (Al₂O₃) by 0.3 wt% Al₂O₃-doped 3 mol% yttria-partially stabilized zirconia (Y-PSZ) on the rheological properties of concentrated aqueous slips was studied. Al₂O₃–Al₂O₃-doped Y-PSZ aqueous suspensions with different Al₂O₃-doped Y-PSZ contents: 0, 22 and 50 vol% were prepared using ammonium polyacrylate (NH₄PA) as dispersant. The particle size distributions of Al₂O₃ and Al₂O₃-doped Y-PSZ powders were similar; however, the particle shape and the surface coating of alumina conferred a markedly higher specific surface area to the Al₂O₃-doped Y-PSZ powder. The substitution of Al₂O₃ by Al₂O₃-doped Y-PSZ in the mixtures decreased the negative surface charge of the powders at pH 9, thereby increasing the amount of NH₄PA adsorbed and consequently the electrosteric repulsion between particles. However, the viscosity and yield stress values increased with increasing Al₂O₃-doped Y-PSZ content for all the solid loading studied. This could be explained by a larger interaction size of the Al₂O₃-doped Y-PSZ particles which resulted in a higher effective volume solid fraction and a lower amount of free-liquid available for flow.     

Rheological characteristics of alumina platelet–Hydroxyapatite composite suspensions

The rheological behaviour of aqueous suspensions of alumina platelet–hydroxyapatite mixtures for slip casting was investigated. The stabilisation of the suspensions requires the use of a dispersing agent and the breakdown of powder agglomerates. The addition of alumina platelets to the HAP powder does not modify significantly the behaviour of the suspensions which remains always quasi-Newtonian. Nevertheless, this behaviour becomes shear-thinning at low shear rates for high alumina contents when platelets of small size are used. The viscosity increases at low shear rates with the increase of small platelets content. These modifications are assumed to result from orientation phenomena of alumina disks under shear stress in the direction of flowing. The disk-shaped morphology of alumina is detrimental to the preparation of high density green composites. Suspensions containing between 50 and 70 wt% of powder are castable but the best rearrangement of solid particles during the casting process is reached for suspensions containing 65 wt% of powder.     

Thermal shock behaviour of mullite–cordierite refractory materials

Abstract

The characterisation of thermal shock damage in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware has been investigated. Two different refractory compositions (termed REFO and CONC), characterised by different silica to alumina ratios, were studied. Thermal shock damage was induced in as received samples by water quenching tests from 1250°C. Thermal and mechanical properties were measured at room temperature by means of standard techniques and then the thermal shock resistance parameter R was calculated. The fracture toughness of selected samples was measured before and after thermal shock by the chevron notched specimen technique. The reliability of this technique for evaluation of small differences in fracture toughness after a given number of thermal shock cycles was investigated. The suitability of K _Ic measurements by the chevron notched specimen technique to characterise the development of thermal shock damage in refractory materials was proved in this investigation.     

Corrosion resistant behaviour of PANI–metal bilayer coatings

The present work discusses on the corrosion resistant behaviour of polymer metal bilayer coatings, viz. polyaniline (PANI), polyaniline–nickel (PANI–Ni), nickel–polyaniline (Ni–PANI), polyaniline–zinc (PANI–Zn) and zinc–polyaniline (Zn–PANI). The coatings were synthesized by means of cyclic voltametric method. The coatings thus obtained were uniform in nature and highly adherent to the mild steel substrate. The effectiveness of the coatings in preventing corrosion was tested by electrochemical impedance studies (EIS) using Nyquist and Bode plots and potentiodynamic polarization studies as well. Among the various coatings synthesized, the PANI–Zn coating was found to offer the maximum protection, followed by PANI–Ni coatings. Metal–PANI coatings were found to offer the least resistance to corrosion. The coatings thus obtained were characterized by scanning electron microscopic (SEM) analysis and the results are discussed.     

The effect of water on the behaviour of alumina–paraffin suspensions for low-pressure injection moulding (LPIM)

The properties of ceramic powder–paraffin suspensions for low-pressure injection moulding (LPIM) is of critical importance to the successful production of high quality ceramic parts. Due to the high hygroscopicity of fine alumina powder, water introduced into the suspension at any stage of the production prior to sintering, may significantly influence its rheological behaviour and hence determine the process parameters as well as the properties of the sintered ceramics. In the industrial environment humidity cannot easily be avoided. Its effect is usually detrimental — it causes flocculation, which is usually the source of defects in sintered ceramics. However, according to the results of the present work, the effect can also be put to good use. Penetration of water into the as-moulded green parts facilitates binder removal and prevents defect formation during this processing step.     

Effect of the experimental setup in the behaviour of sol–gel coatings

Electrochemical behaviour of the sol–gel-coated AA2024-T3 samples was evaluated using electrochemical impedance spectroscopy (EIS) in a three-electrode arrangement cell using Na₂SO₄ 0.1 M solution as electrolyte. The effect of the reference electrode leakage was examined using as reference a low-leakage Ag/AgCl electrode and a platinum wire. The results show that very low-chloride concentrations in solution are able to induce heavy dissolution of intermetallic precipitates. That corrosion process masks information on the barrier properties of sol–gel coatings otherwise available through EIS measurements.     

Plasma enhanced aerosol–gel deposition of Al2O3 coatings

The new plasma enhanced aerosol–gel technique has been used for alumina films preparation, in this work. This process integrates aerosol–gel deposition of films and their plasma treatment in one reactor. The alumina films deposited by aerosol–gel method on Si substrate were plasma or thermally treated. The influence of deposition and condensation conditions on properties of the films was studied. Produced coatings were characterized in terms of surface morphology (SEM, AFM) as well as crystalline and chemical structure (FTIR, XRD). Plasma discharge used for modification of the substrates prior to the deposition process improved homogeneity of produced coatings. Coatings obtained at room temperature exhibit boehmite structure which was transformed into γ-Al₂O₃ after annealing. A similar transformation was induced by low temperature oxide plasma discharge treatment for sufficiently thin coatings.     

Deposition,structure and tribological behavior of silver–carbon nanocomposite coatings

Silver–carbon nanocomposite coatings were deposited by plasma-enhanced chemical vapor deposition and d.c. magnetron sputtering of a silver target. Coatings with various metal concentrations were prepared by changing of acetylene and argon gas mixture ratio (C₂H₂/Ar), and concentrations of more than 40 at.%Ag was achieved in this study. Transmission electron microscope revealed that silver metallic grains with typically 15 nm were dispersed in amorphous carbon host matrix. Size of the grains increased with decrease of the gas mixture ratio due to secondary or triangularly formed metal grains. Tribological behavior of the coatings was investigated using reciprocating tribometer with in-situ electrical contact resistance measurement. Low and stable friction coefficient was achieved in the specimen with relatively low Ag concentration. Elemental mapping results on the ball after the friction tests reveal that tribofilm was formed on the ball when low and stable friction was achieved, and the tribofilm was mainly composed of C and Ag. It can be concluded that formation of the tribofilm is necessary for achieving low and stable friction.     

Electro-deposition and characterizations of nickel coatings on the carbon–polythene composite

Nickel coating on the carbon–polythene composite plate was prepared by electrodeposition in a nickel sulfate solution in this work. The morphology and cross-sectional microstructure of the nickel coating were examined by scanning electron microscope (SEM) and optical microscope (OM), respectively. The influence of bath temperature on the nickel deposition rate was investigated experimentally. The adhesion between the coating and the substrate was evaluated by the pull-off test. The corrosion behavior of the coating in an aqueous solution of NaCl was studied by electrochemical methods. The results showed that the nickel electrodeposition rate could reach up to 0.68 μm min⁻¹ on average under conditions of cathodic current density of 20 mA cm⁻² and bath temperature of 60 °C. It was confirmed that increasing the bath temperature up to 50 °C had a positive effect on the nickel deposit rate, while an adverse effect was observed beyond 60 °C. The adhesion strength between the nickel coating and the substrate can be more than 2.3 MPa. The corrosion potential of the bright coating in the NaCl solution was more positive than that of the dull coating, and the anodic dissolution rate of the bright coating was also far lower at the same polarization potential compared with the dull coating.     

Corrosion resistance of Pb–Sn composite coatings reinforced by carbon nanotubes

Carbon nanotubes/Pb–Sn composite coatings were prepared by electrodeposition technology. The polarization curves and electrochemical impedance of the Pb–Sn coatings and carbon nanotube/Pb–Sn composite coatings were studied in 3.0 wt% HCl, 10 wt% NaOH, and 3.5 wt% NaCl electrolyte solutions, respectively. The results show that the corrosion potential of carbon nanotubes/Pb–Sn composite coatings were improved in the three kinds of corrosive medium, especially in 3.5 wt% NaCl electrolyte solution, where it increased significantly from −0.592 V (vs SCE) to −0.535 V (vs SCE). In addition, composite coatings have higher electrochemical impedance. Carbon nanotubes can improve the corrosion resistance of lead–tin electroplated coatings.     

Electrophoretic deposition of titania–carbon nanotubes nanocomposite coatings in different alcohols

The suspensions of titania nanoparticles in different alcohols (methanol, ethanol and butanol) were prepared using triethanolamine (TEA) as a dispersant. The optimum concentration of TEA was 16.67, 8 and 0.33 mL/L in methanol, ethanol and butanol, respectively. Two component suspensions of titania (20 g/L) and carbon nanotubes (CNTs) (0.1, 0.2, 0.5 and 1 g/L) were prepared in different alcohols without and with optimum concentration of TEA. The finer and positively charged titania nanoparticles were heterocoagulated on the surface of coarser and negatively charged CNTs and generated the titania–CNT composite particles with the net positive charge. In the presence of TEA, titania nanoparticles completely covered CNTs surface due to their higher positive surface charge. At same CNT concentration, the deposition rate was faster for suspensions with TEA additive due to the faster mobility of the composite particles. The photocatalysis efficiency of coatings for methylene blue degradation increased as CNTs were incorporated in their microstructure.     

Effect of pH and carbon nanotube content on the corrosion behavior of electrophoretically deposited chitosan–hydroxyapatite–carbon nanotube composite coatings

In the first stage, chitosan (CH)–hydroxyapatite (HA)-multiwalled carbon nanotube (MWCNT) composite coatings were synthesized by electrophoretic deposition technique (EPD) on 316L stainless steel substrates at different levels of pH and characterized by X-ray diffraction (XRD), Raman spectroscopy, FTIR and field emission scanning electron microscopy (FESEM). A smooth distribution of HA and MWCNT particles in a chitosan matrix with strong interfacial bonding was obtained. In the next stage, effects of pH and MWCNT content of the suspension on the corrosion behavior and deposition mechanism were studied. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) curves revealed that increasing pH level of the suspension increases the corrosion protection properties of the deposited composite coating in simulated body fluid (SBF). Furthermore, Nyquist plots showed that increasing MWCNT content of the suspension resulted in higher amounts of R_p, but because of the capillary properties of MWCNTs and degradability of the chitosan matrix, corrosion protection level of the coatings containing HA–CH–MWCNT was lower than those of coatings containing solely HA–CH. Amperometric curves in different pH levels of the suspension revealed that the system is diffusion controlled at elevated pH values.     

Nanomechanical behaviour of Ni – YSZ nanocomposite coatings on superalloy 690 as diffusion barrier coatings for nuclear applications

Ni-YSZ nanocomposite coatings with different Ni content in the range of 0–50 wt.% were developed on Inconel 690 substrates using an electron beam physical vapour deposition method to optimise the Ni concentration in order to enhance the durability of coatings for nuclear applications. X ray diffraction confirmed the formation of cubic phases of Ni and YSZ in the Ni-YSZ nanocomposite coatings. Increased addition of Ni was found to increase the crystallite size of Ni which resulted in lower strain. FESEM analysis of cross-sectional view of the compositionally graded Ni-YSZ coating with low concentration of Ni showed dense columnar structure and exhibited increased porosity along the columnar boundaries with higher concentration of Ni. FESEM and XRD analyses suggest that the grains of the columnar growth could be along (111) plane of the YSZ phase. The elemental composition of individual layers constituting the compositionally graded Ni-YSZ was confirmed by Energy Dispersive Spectroscopy analysis. The nanoindentation analysis of the as deposited coatings showed an increase in the hardness from 1.7 to 9.1 GPa, reduced Young’s modulus from 48 to 168 GPa, elastic recovery from 15.03% to 32.78% and resistance to plastic deformation from 0.0021 to 0.027 with the increased Ni content. Scratch test confirmed superior adhesion of the Ni-YSZ (50 wt.%: 50 wt.%) nanocomposite coating with the substrate. Also, investigation on the scratch track of Ni-YSZ (50 wt.%:50 wt.%) coating did not reveal chipping or spallation of the coating throughout the scratch track indicating a good adherence of coating with the substrate. The structural and the nanomechanical properties of Ni-YSZ (50 wt.%:50 wt.%) nanocomposite coating suggest that it could be used as diffusion barrier coating in the components of nuclear vitrification furnaces which are operated at higher temperatures.     

Microstructure and oxidation behavior of a novel bilayer (c-AlPO4–SiCw–mullite)/SiC coating for carbon fiber reinforced CMCs

A novel cristobalite aluminum phosphate particle (c-AlPO₄) modified SiC whisker toughened mullite coating (c-AlPO₄-SiC_w-mullite) was prepared on SiC coated carbon fiber reinforced SiC composites (C/SiC) by a new sol-gel method combined with air spraying to improve the oxidation resistance of SiC_w-mullite coating. Results show that c-AlPO₄-SiC_w-mullite coatings with 10 and 20 wt.% of c-AlPO₄ exhibited obviously improved oxidation resistance at 1773 K in ambient air for 100 h than SiC_w-mullite coating. Moreover, the oxidation resistance of c-AlPO₄-SiC_w-mullite coatings were rapidly declined when the c-AlPO₄ in c-AlPO₄-SiC_w-mullite coating were set to 30 and 40 wt.%. The c-AlPO₄-SiC_w-mullite coating with 20 wt.% of c-AlPO₄ showed most pronounced oxidation resistance, the weight loss rate after the oxidation in ambient air for 210 h was merely 3.00 × 10^?5 g·cm^?2 h^?1. The failure of c-AlPO₄-SiC_w-mullite coating with 20 wt.% of c-AlPO₄ was due to the generation of penetrative micro-cracks and micro-holes in the coating, which cannot be self-healed by the silicate glass layer after long time oxidation at 1773 K.     

The effect of antioxidants on the oxidation behaviour of magnesia–carbon refractory bricks

Oxidation of carbon is the main problem in magnesia–carbon refractories. The effects of various antioxidants, Al, Si, SiC and B₄C on the oxidation resistance of magnesia–carbon bricks were investigated at temperatures of 1300 °C and 1500 °C. Carbon losses as wt.% of the bricks were calculated and oxidized areas of the bricks were examined by XRD, SEM and EDS. B₄C was found to be the most effective antioxidant at both temperatures. Magnesium–borate (Mg₃B₂O₆) compound was determined to be present by characterization studies on B₄C added specimens. Magnesium–borate, which is in liquid state above 1360 °C, had an excellent effect on the oxidation resistance of the bricks by filling up the open pores and forming a protective layer on the surface. Forsterite (Mg₂SiO₄) and spinel (MgAl₂O₄) provided similar effects on the Si and Al added specimens respectively at both temperatures. The SiC added specimens had similar phases with Si added specimens, but SiC was the least effective antioxidant at both temperatures.     

Mullite coatings on ceramic substrates: Stabilisation of Al2O3–SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spraying

The present work deals with the preparation of stable alumina + silica suspensions with high solid loading for the production of spray-dried composite powders. These composite powders are to be used for reactive plasma spraying whereby the formation of mullite and the coating on a ceramic substrate are achieved in a single step process. Electrostatic stabilisation of alumina and silica suspensions has been studied as a function of pH. Silica suspensions are most stable at basic pH whereas alumina suspensions are stable at acidic pH. The addition of ammonium polymethacrylate (APMA) makes it possible to stabilise alumina and prepare a stable 50 wt% alumina + silica suspension at pH 10. The optimum amounts of dispersant and binder have been determined by zeta potential, viscosity and sedimentation measurements. Spray drying of the suspension yields composite powders whose morphology, size distribution and flowability have been characterized before realizing reactive plasma spraying tests.     

Nanocrystalline diamond coatings on the interior of WC–Co dies for drawing carbon steel tubes: Enhancement of tube properties

Tungsten carbide (WC–Co) dies are commercially used for the tube drawing process. However they wear out progressively and are unable to meet the high demands required by the industry. In this study, the effect of nanocrystalline diamond (NCD) coatings on the interior of WC–Co drawing dies using a hot filament chemical vapour deposition technique is reported. A field trial was conducted on the production line for drawing AISI 1541 steel tubes to investigate the quality of the drawn tubes. The surface roughness of the tubes drawn through the NCD coated die was lower (R_a = 381 nm) when compared to the tubes drawn through a regular carbide die (R_a = 527 nm). The average residual stress of tubes drawn through the NCD coated drawing die was lowered by 25%. A pin-on-disc sliding wear test, carried out to estimate the coefficient of friction, showed that the coefficient of friction in the case of the NCD coated die was almost half that of the regular WC–Co dies. The excellent thermal conductivity and lower friction coefficient of NCD coatings also helped to decrease the working temperature of the tube drawing process, thereby resulting in a superior product.     

Corrosion degradation of mullite subject to carbon monoxide atmosphere at 1000oC–1600°C

Thus far, studies on the damage to refractory materials under carbon monoxide atmospheres have mostly concentrated on the effects of carbon deposition, and the testing temperature was always set at approximately 500°C to promote the deposition of carbon. However, this testing temperature is far below the operating temperature of most refractories. In this study, mullite, a widely used high-temperature structural material, was subjected to a carbon monoxide atmosphere at 1000°C-1600°C to investigate its phase and microstructural evolutions. Changes to the grain boundaries were initially observed in mullite specimens treated at 1000°C and 1200°C. After treatment at 1400°C, the specimen surface comprised α-Al₂O₃, a glass phase, and a small amount of mullite. However, treatment at 1600°C resulted in only α-Al₂O₃ and a small amount of glass phase on the surface. Additionally, pores and voids were found in the glass phase on the surface and in the bulk of the specimens treated at 1400°C and 1600°C. This study demonstrated the stability of pure mullite in a carbon monoxide atmosphere and revealed that impurities accelerating generation of the liquid phase in Al₂O₃–SiO₂ system significantly affect the stability of mullite in a carbon monoxide atmosphere.     

Electrochemical synthesis of polythiophene–polyacrylamide composite coatings used for pseudocapacitors

Polythiophene films have been electrochemically deposited onto uncoated and coated with polyacrylamide steel mesh. It has been found that the specific capacitance of the polythiophene film deposited on a sublayer of polyacrylamide was 30% higher than of the film of polythiophene deposited on the steel mesh without the underlayer.     

Zinc–magnesium-pigment rich coatings for corrosion protection of aluminum alloys

Chromate (Cr(VI))-based pigments have been widely used for corrosion protective coatings because of their outstanding protection efficiency especially for aluminum alloy products. However, due to environmental issues associated with Cr VI, more and more requests are being made for alternative solutions. In the presented work zinc was modified by alloying with magnesium to achieve a combination of properties – cathodic protection and less reactivity during production, storage and application of the pigments. The magnesium content leads to a lowering of the electrochemical potential which allows the cathodic protection of aluminum alloys. zinc–magnesium pigments were prepared in different compositions with special attention to the intermetallic phases MgZn, Mg₂Zn₃, and MgZn₂. Pigments were produced and a zinc–magnesium rich coating was formulated and compounded. Pickled samples of AA 2024 unclad were coated and the corrosion behavior investigated. A durability of more than 10,000 h in salt spray test could be achieved.