Crystalline Coatings Research Articles

Effects of substrates on the crystal structure, texturing and optical properties of AlN coatings deposited by inverted cylindrical magnetron sputtering

AlN films of thicknesses 670–780 nm were deposited on (1 1 1) silicon wafer, (0 0 · 1) sapphire, float glass and fused silica substrates by reactive mid-frequency (41 kHz) ac sputtering of dual hollow Al targets in a mixture of Ar and N2 gases. Nanostructured films of hexagonal wurtizite phase of AlN were prepared and characterized by UV-visible spectroscopy and X-ray diffraction. The novel geometry of dual target inverted cylindrical magnetrons produces very dense plasma and ion bombardment of the growing film, and facilitates the formation of highly crystalline AlN coatings without any deliberate substrate heating. Substrates showed significant influence on the crystal structure of AlN films; sapphire forms hexagonal AlN phase with very strong texturing toward (0 0 · 2) crystallographic orientation, silicon wafer grows small amount of metastable high-pressure cubic phase of AlN along with the predominant, thermodynamically stable hexagonal phase while the amorphous glass substrates form hexagonal AlN phase with a random orientation of the crystallites.

Properties of Thixocast Spring Steel Ratchets

Forging is state of the art for the production of hand tools in industrial scales. Due to high demands on stiffness and fracture toughness high-strength forged steels are used to provide components with high mechanical load capacity. In spite of all advantages forging is restricted by some means like the freedom of scope. However, because of the extreme thermal load and the hardly avoidable casting defects, hand tools are nowadays not cast. By thixocasting steel technical difficulties can be reduced and new options are provided, which allow manufacturing of components with much higher complexity than forging. During this work ratchets were thixocast successfully using spring steel type 61SiCr7. Based on the STC-concept, a new method to prevent the shot sleeve from damage was tested. Dies made of hot forming tool steel X38CrMoV5 were optimized by simulation and coated by plasma chemical deposition of crystalline Al2O3 protective coatings. The cast parts were heat treated successfully to enhance the toughness, which was measured by tensile and Charpy impact tests.

Lanthanum zirconate thermal barrier coatings deposited by spray pyrolysis

The fabrication of thick nanostructured lanthanum zirconate thermal barrier coatings deposited by spray pyrolysis from aqueous nitrate based precursor solutions is presented. After deposition, the green coatings are decomposed to introduce vertical cracks which are beneficial to enhance the thermo-mechanical compliance of the coatings. The cracks were stable after crystallization of the coatings to the cubic pyrochlore structure by further heat-treatment. Nanocrystalline mono- and multi-layered coatings with contrasting crack patterns were produced based on the knowledge of the influence of the different parameters. Multilayered coatings with small crack spacing and crack opening exhibited a higher density, a lower thermal diffusivity, and a higher thermal conductivity of ~0.34W/(m·K) at elevated temperatures compared to monolayered coatings of similar thickness with larger crack spacing and crack opening. The low thermal properties of the crystalline coatings were attributed to the nanostructure of the coatings.

Structure characterisation and mechanical properties of crystalline alumina coatings on stainless steel fabricated via sol–gel technology and fibre laser processing

Abstract The surface modification of stainless steel by coating with alumina (Al 2 O 3 ) was carried out using sol–gel coating technology in combination with laser processing. Alumina coatings have been synthesised via a sol–gel route and deposited on stainless steel substrates by dip coating. The coated substrates were then treated with pulsed ytterbium fibre laser radiation ( λ = 1064 nm) in continuous wave mode with different specific energies. The composition and structure of the coated surfaces after laser processing were characterised by ATR-FTIR, XRD, SEM and contact angle measurements, whilst the mechanical properties of modified surfaces were determined using nano-indentation. The results showed that the alumina xerogel films coated on the substrates are successfully converted into crystalline alumina ceramic coatings by the laser irradiation, the structure of resulting coatings being dependent on the irradiation conditions, with increase of laser specific energy leading to the formation of initially γ-Al 2 O 3 with increasing amounts of α-Al 2 O 3 at higher energy. Nano-indentation results reveal that the laser processing results in significant improvement in hardness and Young's modulus of the alumina-coated surface and, at optimum, can achieve the mechanical properties at the same level as pure α-alumina ceramic, much higher than those of the as-dried xerogel coating.

Influence of different annealing temperatures and cooling rates on amorphous and crystalline composite coating

Influence of different annealing temperatures and cooling rates on FeNiBSiNb amorphous and crystalline composite coatings was investigated. Coatings were annealed at different temperatures as 500°C, 600°C, 700°C, 800°C and 900°C for 30min and the crystallization behavior of the amorphous composite coating was investigated. The intensity of crystalline phase diffraction peaks gradually increases with the increasing annealing temperature. Apparently, the relative volume fractions of amorphous phases decrease with an increase of annealing temperature. The amount of crystalline grains in the coating decreases with the increasing cooling rate. In brief, an equiaxed structure in the clad layer is beneficial to increase the glass forming ability (GFA) of the coating.

Carbonated Hydroxyapatite Coatings with Aligned Crystal Domains

Carbonated hydroxyapatite coatings were formed on seeded titanium substrates by hydrothermal crystallization in the presence of urea. Decomposition of urea to ammonia and carbon dioxide under hydrothermal conditions was found to have a dramatic effect on crystal morphology and composition of the coatings. Some of the carbon dioxide released by urea decomposition was converted to carbonate ions that substitute for a small fraction of the phosphate groups in the hydroxyapatite crystals. The added urea also suppressed growth of the crystallographic c-axis relative to a-axis, resulting in a decrease in thickness and an increase in density of the coatings. Repeated hydrothermal growth steps in the presence of urea resulted in dense, highly crystalline coatings with near perfect alignment of the crystallographic c-axis normal to the coating surface. The dense coatings can be doped with a variety of other ions, including magnesium, potassium, fluoride, and lanthanum, by addition of appropriate salts in the hydrothermal synthesis. Recent reports have shown that ions move preferentially along the c-axis of carbonated apatites at elevated temperature. The morphology of the coatings is therefore ideal for promoting ion transport through the coating to produce electrically polarized bioelectret coatings or ion conducting membranes for electrochemical devices.

Crystallization Mechanisms in Convective Particle Assembly

Colloidal particles are continuously assembled into crystalline particle coatings using convective fluid flows. Assembly takes place inside a meniscus on a wetting reservoir. The shape of the meniscus defines the profile of the convective flow and the motion of the particles. We use optical interference microscopy, particle image velocimetry, and particle tracking to analyze the particles' trajectory from the liquid reservoir to the film growth front and inside the deposited film as a function of temperature. Our results indicate a transition from assembly at a static film growth front at high deposition temperatures to assembly in a precursor film with high particle mobility at low deposition temperatures. A simple model that compares the convective drag on the particles to the thermal agitation explains this behavior. Convective assembly mechanisms exhibit a pronounced temperature dependency and require a temperature that provides sufficient evaporation. Capillary mechanisms are nearly temperature independent and govern assembly at lower temperatures. The model fits the experimental data with temperature and particle size as variable parameters and allows prediction of the transition temperatures. While the two mechanisms are markedly different, dried particle films from both assembly regimes exhibit hexagonal particle packings. We show that films assembled by convective mechanisms exhibit greater regularity than those assembled by capillary mechanisms.

Structure and Dissolution of l-Leucine-Coated Salbutamol Sulphate Aerosol Particles

L-Leucine formed different crystalline coatings on salbutamol sulphate aerosol particles depending on the saturation conditions of L-leucine. The work emphasizes a careful characterization of powders where structural compartments such as crystal size and particle coating may affect the performance of drug when administered. The sublimation of L-leucine from the aerosol particles took place 90°C lower temperature than the bulk L-leucine which was attributed to result from the sublimation of L-leucine from nano-sized crystalline domains. The dissolution slowed down and initial dissolution rate decreased with increasing L-leucine content. Decreasing crystalline domains to nano-scale improve heat and mass transfer which was observed as the lowered decomposition temperature of the drug salbutamol sulphate and the sublimation temperature of surface material L-leucine as well as the altered dissolution characteristics of the drug. The structure of the coated drug particles was studied by means of thermal analysis techniques (DSC and TG), and the dissolution of salbutamol sulphate was studied as an on-line measurement in a diffusion cell.

Surface Crystallization of Rapamycin on Stents Using a Temperature Induced Process

Metallic drug eluting stents (DES) are usually prepared by coating with a drug-polymer matrix as a rate controlling diffusion barrier. However, coating materials may display numerous problems, thus carrier-free DES are desired, yet releasing drug over long period of time. For this, we are reporting a novel temperature induced (TI) crystallization process for coating rapamycin on stents. Rapamycin crystals with a defined morphology and target drug load were applied from supersaturated solution. This method enables fabrication of controllable and homogeneous crystalline coatings on stent scaffolds and allowing the drug to release for several weeks.

Microstructure and tribological behavior of amorphous and crystalline composite coatings using laser melting

Four composite coatings were fabricated by laser melting. Amorphous phase appeared in the Fe43.2Ni28.8B19.2Si4.8Nb4 and Fe43.2Co14.4Ni14.4B19.2Si4.8Nb4 coatings but not in the Fe30Co30Ni15B17Si8 and Fe39Ni36Mo2B18Si5 coatings. The growth of crystalline grain in the coatings was suppressed greatly due to the large cooling rates caused by fast laser scanning. But the crystallization in the coatings cannot be avoided completely and an amorphous and crystalline composite coating was formed in melted zone. Amorphous phase can increase the hardness of coatings greatly and the highest hardness is related to the amount of amorphous phase in the coatings. The amorphous and crystalline composite coatings exhibit excellent performance of abrasion.

Relation between microstructure and hardness of nano-composite CrN/Si3N4 coatings obtained using CrSi single target magnetron system

Sintered targets with mixed Cr and Si (10, 20, 30, 35 and 40 at. %) elemental powders were sputtered in mixture of argon and nitrogen on silicon substrates kept either at ambient temperature or at 600 °C. The microstructure was characterized using transmission electron microscopy, while mechanical properties were estimated based on hardness measurements. Observations showed that magnetron sputtering of targets with high silicon content on substrates at ambient temperature resulted in the formation of gradient nano-composites that are partly amorphous and partly crystalline. The amorphous phase dominated in the zone close to the substrate, while the nano-crystalline character was more pronounced nearer to the coating surface. The volume of crystalline phase in the coatings was decreasing with increasing silicon content to such an extent that those obtained from CrSi40 target were fully amorphous. The coatings deposited on substrates resistively heated up to 600 °C showed fine columnar microstructure with diameter decreasing with increasing silicon content, which changed to nano-composite and eventually fully amorphous for those obtained using CrSi35 and CrSi40 targets. The Elemental mapping by electron energy filtered imaging helped prove that in the nano-composite coatings the CrN–type crystalline phase and the amorphous phase are enriched respectively in chromium and silicon. The hardness of the coatings deposited at ambient temperature and characterized by mixed crystalline and amorphous microstructure was close to 20 GPa, except those containing layers enriched in chromium and silicon, which fall down to 15 GPa or even 10 GPa. The hardness of crystalline coatings deposited at 600 °C was increasing from 20 to 30–35 GPa with decreasing average column diameter from ∼40 to ∼35 and ∼25 nm (obtained from CrSi10, CrSi20 and CrSi30 targets). The higher silicon content in the coatings caused both further diminishing of the average crystallite size below 10 nm and simultaneous formation of an amorphous phase. The hardness of the coatings with such nano-composite microstructure was however decreased to ∼25 GPa. Finally, the hardness of fully amorphous coatings obtained from CrSi40 target under the same deposition conditions was at the level of 10 GPa. Our experiments proved that it is possible to obtain extremely fine columnar (Cr,Si)N coatings twice as hard as pure CrN, although a basic one target magnetron system and oxygen-contaminated CrSi targets were only used.

Low temperature preparation of crystalline ZrO2 coatings for improved elevated-temperature performances of Li-ion battery cathodes

Ultrathin crystalline ZrO(2) nanofilms have been facilely deposited on LiMn(2)O(4) particles at 120 °C using atomic layer deposition. The ZrO(2) coating shows high crystallinity, conformality and homogeneity, which contribute to considerably improved electrochemical performance of LiMn(2)O(4) at elevated temperature in lithium-ion batteries.

Structure and thermal conductivity of yttria-stabilized hafnia ceramic coatings grown on nickel-based alloy

Yttria-stabilized hafnia (YSH) coatings were produced by sputter-deposition onto nickel (Ni) based super alloy substrates. The deposition was made by varying the growth temperature from room-temperature (RT) to 500°C. The microstructure and thermal properties of the YSH coatings were evaluated employing grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and phot-acoustic measurements. GIXRD studies indicate that the coatings grown at RT are amorphous while those grown at 300°C and higher are crystalline. The crystalline YSH coatings crystallize in cubic hafnia phase and exhibit slightly enhanced lattice parameter compared to pure hafnia. The average grain size increases with increasing growth temperature. Thermal measurements indicate an effective reduction in thermal conductivity of YSH coatings compared to pure hafnia.

Crystalline coats or hollow crystals as tools for product design in pharmaceutical industry

The coating of pharmaceutical compounds is a field of high interest. As most of the coating materials form an amorphous layer around the material, the studies on crystalline coatings are rare. In this work the progress in this domain should be summarized and innovative results concerning crystalline hollow needles as coating material are presented. Since the first reports on needles formed via a solvent-mediated phase transition from solvates to hydrates, the field could be widened to hydrate-to-anhydrate and anhydrate-to-hydrate transformations. Novel investigations on hollow theophylline monohydrate and carbamazepine dihydrate needles are presented. It is shown that the inclusion of substances into the hollow needle crystals is feasible by simple means, which enable an application in industry as coating for sensitive materials.

Pulsed electrodeposition of Co–W amorphous and crystalline coatings

Replacement of hard chromium plating has been of particular interest to many industrial applications, including automotive, aircraft, and machinery parts that require high hardness and wear/corrosion resistance. Co–W alloy coatings, owing to their eco-friendly processing and high hardness/wear resistance, are promising for electrolytic chromium replacement. In the present study, pulsed electrodeposition of amorphous and crystalline Co–W coating is reported. Systematic investigations on the effect of pulse duty cycle and pulse frequency on development of surface microstructure, phases, composition, surface roughness, and micro-hardness are presented. Furthermore, detailed analysis of micro-/nano-mechanical (using ball-on-disc wear tester and nanoindentor) and corrosion (using potentiodynamic polarization method) behavior of optimized Co–W coatings is presented.

ZnO nanoparticles and porous coatings for dye-sensitized solar cell application: Photoelectrochemical characterization

Nanoscale zinc oxide (ZnO) powder with Brunauer–Emmelt–Teller surface area of 43 m 2 g − 1 has been synthesized by soft chemistry at low temperature via reaction of zinc acetate dehydrate (Zn(CH 3COO) 2.2H 2O) and sodium hydroxide (NaOH). The influence of the pH value of the sol on the structure and morphology of ZnO powder have been investigated by X-Ray Diffraction, Scanning Electron Microscopy and High Resolution Transmission Electron Microcopy. Their thermal properties have been determined by simultaneous Differential Thermal Analysis and Thermogravimetry coupled to Mass Spectroscopy analysis. The nanoparticles are single crystals with (101) preferred orientation but agglomerated. Their crystallite size can be adjusted from 15 nm to 35 nm by controlling the pH value between 7 and 13. Thick porous crystalline coatings have been obtained by doctor blade coating on conducting SnO 2:F glass substrates using pastes prepared by wetting the crystalline powders with polyethylene glycol and water. After sintering at 400 °C and Ruthenium 535 dye sensitization, the coatings have been tested in a three electrode electrochemical cell containing an appropriate electrolyte in the dark and under 450 W Xenon lamp illumination. The influence of the electrolyte iodine concentration, the film thickness and the light intensity on the current density are presented and discussed. Such coatings appeared promising for the realization of dye sensitized solar cells.

Synthesis of Fe–Ni–B–Si–Nb amorphous and crystalline composite coatings by laser cladding and remelting

Amorphous and nanocrystalline composite coatings are synthesized by laser cladding and remelting. The result of X-ray diffraction shows that an amorphous structure and some crystalline phases occur in the coatings. The microstructure of the coatings is different with different amounts of Si in the coatings. The fracture appearance suggests that brittle fracture occurs in coatings. There are some oxides, borides and metal compounds appearing on the surface of Fe 32Ni 32Si 16B 18Nb 2 and Fe 30Ni 30Si 20B 18Nb 2 coatings. The abrasion resistance of the coatings will be improved with the increase of amorphous phases. The coatings along the depth direction can be divided into three regions which are an amorphous and crystalline composite layer, a clad layer and a diffusion layer, respectively. The experiments show that Si plays a critical role in fabricating a Fe-based amorphous composite coating.

Nanostructured core–shell Ni deposition on SiC particles by alkaline electroless coating

In this study, core–shell nanostructured nickel formation on silicon carbide (SiC) ceramic powders was achieved through the electroless deposition method using alkaline solutions. To produce a nano core–shell Ni deposition on the SiC surfaces, process parameters such as pH values, the type of reducer material, deposition temperature, stirring rate and activation procedure among others were determined. Full coverage of core–shell nickel structures on SiC surfaces was achieved with a grain size of between 100 and 300 nm, which was approximately the same deposition thickness on the SiC surfaces. The surface morphology of the coated SiC particles showed a homogenous distribution of nanostructured nickel grains characterized by scanning electron microscopy and X-ray diffraction techniques. The nanostructures of the crystalline Ni coatings were observed to be attractive for achieving both good bonding and dense structure. The thin core shell-structure of Ni on the SiC surfaces was assessed as a beneficial reinforcement for possible metal matrix composite manufacturing.

Effects of the Nano-Tubular Anodic TiO2 Buffer Layer on Bioactive Hydroxyapatite Coating

We studied the effect of nano-tubular anodic TiO2 buffer layers on hydroxyapatite (HA) coating. The pulsed laser deposition (PLD) method was used to deposit HA on a well arranged nano-tubular anodic TiO2 (NT-ATO) buffer layer prepared by an electrochemical anodization technique. The surface morphology and chemical composition of HA coatings were characterized by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and contact angle measurement. We found that crystalline HA coatings show well arranged porous morphologies with a favorable surface wettability. We also found that an anodic nano-tubular TiO2 buffer layer with a relatively short tube length shows a better coating morphology. The deposition process of HA on the nanotubular TiO2 buffer layer was also proposed.

Diamond deposition on thin cylindrical substrates

Diamond coatings were deposited onto different cylindrical substrates (Cu, SiC, W and Mo) by the hot filament chemical vapor deposition, (CVD) method. Continuous, adhered and well-faceted crystalline coatings of diamond were obtained on Cu-wire using a special pretreatment with a mixture of diamond and metal powders as well as carefully controlled deposition at lower power. Diamond deposition on SiC-fiber gave continuous and uniform coatings when only the filament power was properly selected. Uniform, homogeneous, euchedral diamond coatings on W- and Mo-wires, attained at a higher filament power, confirmed once more the convenience of refractory metals as substrates for diamond deposition by the CVD technique. Characterization of the obtained coatings was realized using scanning electron microscopy (SEM). The obtained results are compared with the literature data. Differences are discussed with regard to the chemical nature of the substrates as well as their thermophysical characteristics.