Grain growth in (Ca1−x,Mg x )Zr4(PO4)6 ceramics

Grain growth in (Ca_1–x ,Mg _x )Zr₄(PO₄)₆ (CMZP) ceramics in the final stage of sintering has been investigated. The grain growth in CMZP ceramics obeys the isothermal grain-growth kinetics with time exponent,n, lying between 1.8 and 2.4 which depends on magnesium content, indicative of a change in grain-growth rate. The time exponent for the grain growth of CMZP can be taken as 2.0 which implies that a normal grain growth develops in the CMZP ceramics. The apparent activation energy for grain growth demonstrates a maximum atx = 0.0 and a minimum atx = 0.1, with 103.2 and 39.4 kcal mol^–1, respectively, indicating that a small amount of magnesium promotes grain-boundary migration. The critical grain size for initiating microcracks in the CMZP increases with increasing magnesium and reaches 9–12 m whenx = 0.4.     

Microstructure and properties of ceramic coatings produced on 2024 aluminum alloy by microarc oxidation

The microstructures of the microarc oxidation coatings and 2024 aluminum alloy substrate were observed using the scanning electron microscope (SEM) and the phase composition of the coatings was analyzed by X-ray diffraction (XRD). Furthermore, the profiles of the nanohardness, H, and elastic modulus, E, along the coating depth were first determined using the mechanical properties microprobe. The microarc oxidation coatings consist of two layers—a loose layer and a compact layer. The H and E in the compact layer are about 18–32 GPa, 280–390 GPa, respectively. The H and E profiles are similar, and both of them exhibit a maximum value at a same depth of the coatings. The distribution of -Al₂O₃ phase content determines the H and E profiles in the coatings. The changes of -Al₂O₃ and -Al₂O₃ contents result from the different cooling rates of the molten alumina in the microarc discharge channel at the different depths of the coatings. After the microarc oxidation treatment, the microstructure of the alloy substrate, even near the Al/Al₂O₃ interface, has not been changed.     

Alkali corrosion resistant coatings for Si3N4 ceramics

The use of ceramic oxide coatings on silicon nitride is one method to improve its alkali corrosion resistance. Four oxide coatings, including (Ca_0.6, Mg_0.4) Zr₄(PO₄)₆ (CMZP), zirconia, mullite and alumina, were examined. These coatings were applied on Si₃N₄ using both sol–gel and dip coating techniques. The coated and uncoated samples were exposed to sodium molten-salt and sodium-containing atmospheres at 1000 °C for 50 h. The weight loss of all the coated samples was less than that of the uncoated Si₃N₄ with CMZP-coated samples exhibiting the smallest weight loss. There was no decrease in the flexural strength of Si₃N₄ after coating with zirconia and CMZP, and a decrease in strength after coating with either mullite or alumina. After alkali exposure, the strength of the CMZP and zirconia coated samples were significantly higher than those of the mullite-coated, alumina-coated, and uncoated Si₃N₄. The observed behaviour is explained in terms of the microstructure and protection mechanisms. This revised version was published online in November 2006 with corrections to the Cover Date.     

Oxidation resistance of diffusion coatings formed by pack-codeposition of Al and Si on γ-TiAl

Oxidation resistance of the aluminide and silicide diffusion coatings pack-deposited on -TiAl were studied in air over the temperature range of 800 and 850°C for up to 4596 h. The oxidation kinetics of the coatings was monitored by intermittent weight gain measurement at room temperature. The XRD and SEM/EDS techniques were used to identify the oxide scales formed during the oxidation process and to assess the thermal stability of the coatings at the oxidising temperatures. It was revealed that the TiAl₃ coating underwent preferential Al oxidation to form the Al₂O₃ scale in the early oxidation stage, which resulted in Al depletion and formation of TiAl₂ in the subsurface of the coating. The Al depletion could not be sufficiently compensated by Al diffusion from the inner layer of the coating and eventually, in the late oxidation stage, led to the Ti oxidation and formation of the TiO₂ phase in the scale. The preferential Si oxidation was the main oxidation mechanism for the coatings with an outer silicide layer and an inner TiAl₃ layer with the formation of SiO₂ as the stable oxide scale. The thermal stability of the coatings over the temperature range up to 850°C was discussed in relation to the high-temperature stability of diffusion couples of different coating layers.     

Electrical resistivity of plasma-sprayed titanium diboride coatings

Plasma-sprayed TiB₂ coatings (50–600 m thick) on alumina substrates have been developed and characterized. X-ray diffraction studies, thermal analysis and oxygen analysis of the coatings show that there is appreciable oxidation of TiB₂ during the spray process. Partial oxidation of the boride during spraying strongly influences the electrical conductivity of the coatings, which is found to be 100–1000 times less than that of pure TiB₂. Although use of argon as shield gas during the spray process brings down the resisitivity substantially, partial oxidation of TiB₂ could not be totally avoided.     

High-performance polymer coatings for carbon steel heat exchanger tubes in geothermal environments

The most critical issue in developing thermal conductive coatings for the interior surfaces of heat exchanger tubes made from mild carbon steel (MCS), which are used in geothermal power plants at temperatures ranging from 110° to 89°C, is the deposition of scales. These scales, induced by the brine, chemically adhere to the coating surfaces. One of the major factors governing the formation of a strong interfacial bond at interfaces between the coatings and scales was the brine-promoted hydrothermal oxidation of the coatings. In seeking coating unsusceptible to hydrothermal oxidation, two semi-crystalline thermoplastic polymers, polyphenylenesulfide (PPS) and polytetrafluoroethylene (PTFE)-blended PPS, were applied as interior surface coatings to the zinc phosphated MCS tubes. The PPS coating surfaces suffered some oxidation caused by their chemical affinity for FeCl₂ in geothermal brine. FeCl₂-promoted oxidation of PPS surfaces not only incorporated more oxygen into them, generating a sulfide sulfone sulfonic acid conformational transformation within the PPS, but also caused the disintegration of PPS, yielding fragmental polychloroaryl compound and ferrous sulfate (FeSO₄) derivatives. The FeSO₄ reaction product formed at the interfaces between the scale and PPS coating was soluble in water, so that the coatings could be easily removed by highly pressurized water. The oxidation of PPS was considerably inhibited by blending PTFE into it, forming coating surface unsusceptible to hydrothermal oxidation reactions with hot brine. The major reason for such inhibition of oxidation was the formation of a chemically inert PTFE layer segregated from the PPS layer at the outermost surface site of the coating. Hence, the scale easily flaked off from the PTFE-blended PPS coating surfaces. This characteristic of surface was similar to that of the stainless steel surfaces. Nevertheless, both PPS and PTFE-blended PPS coatings can be classified as scale-free coatings.     

Polyphenylenesulphide-sealed Ni–Al coatings for protecting steel from corrosion and oxidation in geothermal environments

Polyphenylenesulphide (PPS) polymer was applied as a sealing material to flame-sprayed nickel–aluminium (Ni–Al) coatings to protect the interior surfaces at the ends where the mild carbon steel (MCS) heat-exchanger tubes are jointed to the tubesheet. The aim of applying PPS is to prevent their corrosion, oxidation and abrasive wear, in a low pH, hypersaline brine geothermal environment at 200°C under a hydrothermal pressure of 1.6 MPa. Although the Ni–Al coatings had an excellent thermal conductivity and a good wear-resistance, the inherent open structure of these coatings allowed the hot brine to permeate them easily under such pressure, causing the development of corrosion-induced stress cracks in the MCS. Furthermore, under these conditions, the coatings underwent oxidation with the formulation of Al₂O₃ as the major scale compound and NiO as the minor one. PPS sealant was used to solve these problems. However, one major drawback of PPS was its susceptibility to oxidation reaction with hot brine. This reaction not only incorporated more oxygen into the PPS, generating a sulphide sulphone transformation within PPS, but also it caused the decomposition of PPS, yielding polychloroaryl compound, and sodium sulphate, and also evolving SO₂ gases. The SO₂ gases had a chemical affinity for oxide scales in Ni–Al, forming water-soluble Al₂(SO₄)₃ and NiSO₄ salt reaction products at the PPS/Ni–Al interfaces. Despite the occurrence of such oxidation damage in PPS, an exposure for 14 days showed that there was no development of corrosion-caused cracks at the interfaces between the underlying steel and Ni–Al, nor was a striking oxidation of the sealed coating panels.     

Investigation of microstructure of molybdenum—copper black electrodeposited coatings with reference to solar selectivity

Optical and microstructural properties of electrodeposited molybdenum-copper (Mo-Cu) black coatings have been studied with reference to their selectivity in absorption of solar radiation. Such coatings were found to have a solar absorptance, , about 0.87 and low thermal emittance, , such that the selectivity, /, was 3.6. Electrodeposited molybdenum-black coatings generally have selectivity /3. The oxidation state of molybdenum in (Mo-Cu) black coatings as determined by X-ray photoelectron spectroscopy is about + 5 (which is fairly close to that of Mo₄O₁₁). Large numbers of irregular particles were found on the surface of molybdenum-copper black coatings. There is evidence that the particles contain copper oxide.     

Degradation of thermal barrier coatings due to thermal cycling up to 1150°C

The degradation of thermal barrier coatings (TBCs) due to thermal cycling up to 1150°C has been studied. During thermal cycling, the bond coat in the TBCs was oxidised to form an alumina and a mixed oxide layer between the top coat of yttria stabilised zirconia (YSZ) and the bond coat of MCrAlY alloy. The mixed oxide layer mainly consists of -Cr₂O₃ and (Ni,Co)(Cr,Al)₂O₄ spinel phases, which are formed above the -alumina layer. Interestingly, the alumina layer gradually disappeared during the oxidation while the content of chromium in the mixed oxide increased with increasing oxidation time. As the oxidation accelerated after the disappearance of the alumina layer, cracks initiated and propagated in the mixed oxide layer near the YSZ. Eventually, the crack propagation induced the spallation of some YSZ top coatings after the 2000 h oxidation.     

Properties of magnetron-sputtered electrically insulating Al2O3 coatings on copper

Amorphous Al₂O₃ coatings were deposited on Cu substrates with metallic bond layers by different magnetron-sputtering processes. Such sputtering conditions as the type of discharge, target material, total pressure, sputtering gas composition and substrate temperature were varied to study the process effects on the structure and properties of the coatings deposited. The structure and general properties were found to be strongly dependent on the type of process and parameters. The breakdown voltages did not show any significant lowering as the temperature was increased from 20 to 400–500 C. The maximum temperature without formation of cracks in the Al₂O₃ coating on Cu was about 700 C. The d.c. electrical conductivities of the Al₂O₃ were similar to that of bulk Al₂O₃ at different temperatures. The results reveal the potential use of magnetron-sputtered Al₂O₃ coatings on Cu for electrical insulation and oxidation protection in different high-temperature applications, e.g. on metallic magnetohydrodynamic electrode and insulator modules.     

Mechanical, microstructural and oxidation properties of reactively sputtered thin CrN coatings on steel

Thin (40 nm and 160 nm) CrN coatings were deposited on steel by reactive magnetron sputtering deposition, varying the N₂ flow. The coatings were characterized in the as-deposited condition and after annealing in air at 500 °C for 1 h, by X-Ray Diffraction, Transmission Electron Microscopy, Raman and Fourier Transform Infrared spectroscopies. Hardness was measured by nanoindentation. Coatings have a nanocrystalline microstructure with the phase shifting from Cr₂N to CrN, increasing grain size, thermal stability and resistance to oxidation with increasing N₂. Also intrinsic coating hardness is influenced by both N₂ flow during deposition and film thickness, as a result of changes in phase composition and microstructural properties.     

Oxidation behavior of Mo≤5Si3C≤1 and its composites

The oxidation behavior of Mo₅Si₃C₁ and its composites was studied in air over the temperature range of 500°C–1600°C. Experiments revealed poor oxidation resistance of monolithic Mo₅Si₃C₁ at high temperature. The oxidation was quite rapid at 1200°C and above, resulting in complete oxidation of specimens in a short time. The addition of 2.0 wt% boron was found to produce a Mo₅Si₃C₁ composite with three other phases of MoB, MoSi₂, and SiC, and showed remarkable improvement in oxidation resistance. The mechanism for the improvement was attributed to the viscous sintering of the scale to close the pores formed during the initial oxidation period. Oxidation tests were also conducted on SiC-Mo₅Si₃C₁ composite at 800°C, 1300°C and 1600°C for more than 100 hours. The oxidation resistance of the composite was found to be very good. The results demonstrate that, though oxidation resistance of monolithic Mo₅Si₃C₁ is far insufficient for high-temperature applications, boron-modification and/or composites with SiC are viable methods to improve oxidation resistance to a practically acceptable level.     

MoSi2 diffusion barrier for the passivation of copper at elevated temperatures

Sputter deposited MoSi₂ coatings (200 and 400 nm thick) on copper have been studied in an attempt to prevent or at least reduce the oxidation of copper. Samples were exposed to an air ambient at temperatures ranging from 600–850 °C for up to 15 min. Sputter-deposited MoSi₂ was amorphous upon deposition and crystallized on annealing. Silicon from the MoSi₂ was found to diffuse into the copper causing the MoSi₂ to transform to lower suicides. The primary oxidation product for MoSi₂-coated samples was CuO (with small amounts of Cu₂O), which is in contrast to uncoated copper where Cu₂O is the main oxidation product. The amount of copper consumed by oxidation, for a 200 nm MoSi₂ barrier relative to uncoated copper, was reduced by 140 times at 600 °C and 30 times at 800 °C. A 400 nm MoSi₂ coating yielded an improvement of 420 times at 600°C, and 200 times at 850 °C. For the 400 nm barrier exposed to air for 15 min, this corresponds to a 35 nm CuO layer at 600°C and a 300 nm thick oxide layer at 850 °C.     

Microstructure, electrochemical surface and electrocatalytic properties of IrO2+Ta2O5 oxide electrodes

X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used to characterize physical structure of IrO₂+Ta₂O₅ films over the whole composition range by thermodecomposition of chloride solutions heated at 450°C. Solid solubilization between Ta component and IrO₂ rutile in the mixed films was measured, and three typical surface morphologies of the oxide coatings were observed. The surface electrochemical properties of Ti/IrO₂-Ta₂O₅ electrodes were studied by cyclic voltammetry at varying potential scan rate, and a double-layer electrochemical structure containing the inner and outer layers has been distinguished. The voltammetric charge appears to decline with the decrease of grain size of oxide coatings as a result of the effect of surface tension. However, the coatings of 70% IrO₂+30% Ta₂O₅ with the finest grains still exhibit the highest apparent activity for oxygen evolution evaluated by the anodic current at a constant potential. This result is interpreted by the measurements of open-circuit potential (E _oc) and double-layer capacitance (C _dl) using electrochemical impedance spectroscopy (EIS). Thereby, the reliability of voltammetric charge obtained in double-layer potential region in determining the real electrocatalytic activity for O₂ evolution has been discussed.     

Sol-gel processing of lithium disilicate

Lithium disilicate (Li₂Si₂O₅) coatings were prepared by spin-coating alkoxide solutions on to substrates [Si, SiO₂, polycrystalline (poly) Si, sapphire] and heating isothermally at 500–600 °C. The effects of solution chemistry, coating thickness and substrate type on crystallization behaviour and microstructure development were investigated using atomic force microscopy, X-ray diffraction and transmission electron microscopy. Amorphous dried coatings began to crystallize into Li₂Si₂O₅ at 500–550 °C. Coatings prepared on Si substrates (with a thin native oxide) using Li-Si methoxyethoxide solution crystallized into microstructures with large grain sizes (ca. 2–5 m diameter) as compared with the coating thickness (<0.3 m). Nucleation rate in these coatings could be increased (and hence transformation rate increased and grain size decreased) by: (1) adding H₂PtCl₆to the solution to act as nucleation agent; (2) increasing the thickness of the coating; or (3) using a crystalline substrate (sapphire or poly Si). Coatings prepared using Li-Si ethoxide solution had fine-grained microstructures (0.5 m diameter) for all substrates. Chemical heterogeneity in the ethoxide system may have increased nucleation rate. Nucleation rate in this system could be decreased by using partially hydrolysed tetraethylorthosilicate as the Si precursor. The relationship between solution chemistry and microstructure was used to tailor microstructures in multilayer coatings.     

Growth and structure of TiC coatings chemically vapour deposited on graphite substrates

TiC coatings were grown on graphite substrates by the chemical vapour deposition technique, using gas mixtures of CH₄-TiCl₄-H₂ at a total pressure of 10.7 kPa and at temperatures of 1400 and 1425 K. The growth rate and structure of the TiC coatings were investigated as a function of CH₄ and H₂ concentrations. The deposition rate of TiC increased with increasing CH₄ flow rate, but did not change with H₂ flow rate. This behaviour was explained by a mass transport theory. Thermodynamic analyses based on minimization of Gibbs' free energy predicted carbon codeposition with TiC. X-ray diffraction and Auger electron spectroscopy (AES) studies and microstructural observations, however, suggested that free carbon did not form. Textural analyses indicated that the growth of TiC coatings was initiated as randomly oriented crystallites, and as the thickness of the coatings increased, preferentially oriented columnar grains developed. The textures of TiC coatings with the same thickness changed from the 110 orientation to the 100 orientation with decreasing H₂ flow rate for a constant CH₄ flow rate. The CH₄ concentration also greatly influenced the preferred orientation of the coatings.     

Plasma- and vacuum-plasma-sprayed Cr3C2 composite coatings

Plasma- and detonation-sprayed chromium carbide-Nichrome coatings have long been used for applications requiring superior wear resistance at temperatures up to 820°C. The coatings are typically sprayed from mechanical blends of powder containing from 17 to 35 wt.% Nichrome. These coatings are susceptible to non-uniformity of microstructure because of segregation of the blended powders. Oxide formation occurs in both ambient atmosphere plasma and detonation-applied coatings.A new Cr₃C₂Nichrome composite powder was developed for application by the plasma and vacuum plasma processes. The developed material consists of 50 wt.% Cr₃C₂ clad with 50 wt.% 80-20 Nichrome. Unlike powder blends, each Cr₃C₂ powder particle is clad with an essentially continuous layer of Nichrome. The developed material is sized ?270 mesh + 5 μm.Coatings of the composite Cr₃C₂ material were sprayed using the conventional non-transferred arc plasma and the low pressure low oxygen vacuum plasma processes. These coatings were compared with coatings sprayed from a commercially available blend of 75 wt.% Cr₃C₂25wt.% Nichrome. Unlike the blend, the microstructure of both composite coatings showed Cr₃C₂ to be present as discrete second-phase particles embedded in the Nichrome matrix. The vacuum-plasma-sprayed composite coating showed no visible oxide. Macrohardness (Rockwell C hardness) and microhardness (diamond pyramid hardness for a load of 300 gf) readings of the conventionally sprayed coatings were 50 HRC and 600 HDP 300 respectively. The hardness values for the vacuum-plasma-sprayed composite were 60 HRC and 860 HDP 300. The abrasive slurry wear resistance of the conventionally sprayed composite was three times that of the blend, while the wear resistance of the vacuum-plasma-sprayed composite was four times that of the standard blend coatings.The air-plasma-sprayed composite Cr₃C₂Nichrome coatings are expected to exhibit performance characteristics comparable with similar detonation-applied coatings. The vacuum-plasma-sprayed composite coatings combine superior wear resistance with low oxide and are recommended for severe high temperature wear environments.     

Fabrication of Mg- and Ti-Doped Submicron-Grained Alpha-Alumina-Based Ceramics

Nanocrystalline alumina ceramics were prepared by magnetic-pulse compaction followed by pressureless sintering. The relative density of green compacts exceeded 0.7. The medium in which the starting powders were classified (gas, water, or ethanol) was shown to have a significant effect on their sinterability. The effects of Mg and Ti additions on the sintering behavior of Al₂O₃ were studied under optimized heat-treatment conditions. Mg-doped -Al₂O₃ ceramics containing uniformly distributed MgAl₂O₄ (4 wt %) and offering hardness values of up to 24 GPa were prepared. In the course of sintering, the grain size of these ceramics increases from 110 to 300 nm, and their relative density rises from 0.94 to 0.98.     

Air-plasma spraying colloidal solutions of nanosized ceramic powders

Coatings prepared from nanosized powders were obtained by spraying ethanol-based colloidal solutions into a plasma plume. The powders investigated included 40 nm -Al₂O₃, 75 nm 8 wt% Y₂O₃-ZrO₂, and 750 nm 25 wt% CeO₂-ZrO₂. Spray distances from approximately 50 to 63 mm were required to achieve significant coating deposition. As observed in the TEM, the typical lamella morphology of air plasma sprayed oxide coatings was not observed in coatings fabricated from 40 nm -Al₂O₃, which was comprised of spherical powders that had partially sintered. However, lamellae were observed in the coatings prepared with both nanosized zirconia powders. The characteristic size of the lamella and the grains that comprised the zirconia coatings were nominally a few nanometers.     

Low cost coatings for flat plate solar collectors

Low cost, optically efficient, selective coatings are required for the flat plate collectors envisioned for solar heating and cooling applications. Plated coatings and paint coatings are two attractive types of coatings for this application. Black nickel (NiSZnS), black chrome (CrO_x) and black iron (FeO_x) were found to be attractive plated coatings from the standpoint of optical efficiency, durability and cost. Black nickel with α = 0.95 and ε = 0.07 had the best optical properties. Black chrome, which showed only minor degradation after 192 h exposure to the MIL-STD-810B humidity test, had the best durability. Black iron, with an estimated materials cost of 5

ft^?2, had the lowest cost. Paint coatings are potentially the lowest cost selective coating. The materials cost for a selective paint coating would be about 0.5

ft^?2. However, paints lack the optical efficiency of the plated coatings. The best optical performance achieved with a paint coating was α = 0.90 and ε = 0.30.