Bioactive nanocrystalline sol-gel hydroxyapatite coatings

Sol-gel technology offers an alternative technique for producing bioactive surfaces for improved bone attachment. Previous work indicated that monophasic hydroxyapatite coatings were difficult to produce. In the present work hydroxyapatite was synthesized using the sol-gel technique with alkoxide precursors and the solution was allowed to age up to seven days prior to coating. It was found that, similar to the wet-chemical method of hydroxyapatite powder synthesis, an aging time is required to produce a pure hydroxyapatite phase. A methodology that has been successfully used to produce nanocrystalline hydroxyapatite thin film coatings via the sol-gel route on various substrates including alumina, Vycor glass, partially stabilized zirconia, Ti–6Al–4V alloy and single crystal MgO is described. Coatings produced on MgO substrates were characterized by X-ray diffraction and atomic force microscopy, while the analogous gels were examined with thermogravimetric and differential thermal analyses. The coatings were crack free and the surface was covered with small grains, of approximately 200 nm in size for samples fired to 1000 °C. Coating thickness varied between 70 and 1000 nm depending on the number of applied layers.     

Properties of nanocrystalline arc PVD TiN-Cu coatings

TiN-Cu metal-ceramic coatings grown by vacuum arc PVD and containing 0–20 at % copper consist of titanium nitride and copper, which is X-ray amorphous up to 10 at % Cu. The presence of copper in the coatings reduces the crystallite size of the nitride phase from 100 to 20 nm. The hardness of the coatings increases from 20 to 49 GPa as the copper content of the coatings increases to 3.5 at %. Further increase in copper content, up to 20 at %, accompanied by a decrease in the crystallite size of the nitride phase, leads to a drop in hardness to 14–15 GPa, which is caused by the effect of the soft, plastic metal and porosity. We have studied the tribological and adhesive-cohesive properties of the coatings grown on hard-alloy substrates.     

Vacuum-deposited carbon coatings

In recent years, highly favorable results have been obtained using low temperature isotropic pyrolytic carbons in prosthetic devices requiring a high degree of thromboresistance. The development of vacuum-deposited carbon coatings was undertaken to extend the application of carbon to geometries and configurations that cannot be fabricated from low temperature isotropic carbon. Vacuum-deposited coatings have been produced on a variety of metallic and polymeric substrates.The different vacuum deposition processes which have been investigated include electron beam gun evaporation using high vacuum, gas scattering and ion- plating conditions. In addition, sputtering processes using ion beams and magnetically confined plasmas were studied.The surface morphology, structure and preferred orientation of the coatings produced by the different processes were characterized by scanning and transmission electron microscopy. Film purity and interfacial characteristics were examined by Auger electron spectroscopy.The scanning electron microscopy study shows that thin carbon films generally have a smooth and featureless surface morphology. However, other surface morphology features are obtained in thicker films, depending on the processing conditions. The transmission electron micrographs show the absence of structure and growth features. Electron diffraction indicates that the films consist of a turbostratic phase and a non-crystalline phase. The apparent crystallite sizes are small, and there is no three-dimensional ordering. Generally, the films are isotropic and consist of relatively pure carbon, with the degree of disorder dependent on the process conditions.     

Structural nanocrystalline Ni coatings on periodic cellular steel

This study develops a new type of hybrid material that is a composite of a plain carbon steel micro-truss and a structural nanocrystalline Ni coating. The plain carbon steel micro-truss was made by a simple stretch–bend sheet forming method. It created a low density cellular material (～5% relative density), combining the low embodied energy and cost of the starting precursor material with the structural efficiency of pyramidal micro-truss architecture. The nanocrystalline Ni structural coating was designed to provide both corrosion protection and inelastic buckling resistance. Because the ultra-high strength material was optimally located at the furthest distance from the neutral bending axis, only a thin coating of nanocrystalline Ni (～50 μm) is needed to double the inelastic buckling resistance of the 1.13 mm × 0.63 mm plain carbon steel struts.     

Biological evaluation of ultrananocrystalline and nanocrystalline diamond coatings

Nanostructured biomaterials have been investigated for achieving desirable tissue-material interactions in medical implants. Ultrananocrystalline diamond (UNCD) and nanocrystalline diamond (NCD) coatings are the two most studied classes of synthetic diamond coatings; these materials are grown using chemical vapor deposition and are classified based on their nanostructure, grain size, and sp³ content. UNCD and NCD are mechanically robust, chemically inert, biocompatible, and wear resistant, making them ideal implant coatings. UNCD and NCD have been recently investigated for ophthalmic, cardiovascular, dental, and orthopaedic device applications. The aim of this study was (a) to evaluate the in vitro biocompatibility of UNCD and NCD coatings and (b) to determine if variations in surface topography and sp³ content affect cellular response. Diamond coatings with various nanoscale topographies (grain sizes 5–400?nm) were deposited on silicon substrates using microwave plasma chemical vapor deposition. Scanning electron microscopy and atomic force microscopy revealed uniform coatings with different scales of surface topography; Raman spectroscopy confirmed the presence of carbon bonding typical of diamond coatings. Cell viability, proliferation, and morphology responses of human bone marrow-derived mesenchymal stem cells (hBMSCs) to UNCD and NCD surfaces were evaluated. The hBMSCs on UNCD and NCD coatings exhibited similar cell viability, proliferation, and morphology as those on the control material, tissue culture polystyrene. No significant differences in cellular response were observed on UNCD and NCD coatings with different nanoscale topographies. Our data shows that both UNCD and NCD coatings demonstrate in vitro biocompatibility irrespective of surface topography.     

Horizontally oriented carbon nanotubes coated with nanocrystalline carbon

Single-walled carbon nanotubes (CNTs) and multi-walled CNTs of length 2-5 mm were grown from Fe/Mo nanoparticles and Fe thin film catalyst, respectively, by thermal chemical vapor deposition. Following CNT growth, the CNTs were in-situ coated with nanocrystalline carbon shells of thickness 100-1500 nm. Horizontally oriented CNTs with carbon shells in the direction of the feeding gas were visible under a regular optical microscope. They were easily manipulated by optical manipulators, and CNT probes can thus be fabricated.     

Long-term strength of SiC fibers with nanocrystalline SiC coatings

The long-term strength σ_t of SiC fibers coated with SiC nanoparticles is approximately equal to30·10 ⁷ pa for t=200h at 1500K. The long-term strength of coated fibers is lower than for fibers without coatings by 25–50%. Owing to their enhanced reaction characteristics, the nanocrystalline SiC coatings are sintered at T<1500K, which is lower than the temperature of sintering of self-bonded SiC by 500 K. For this reason, we can recommend coated SiC fibers for manufacturing SiC/SiC composites by sintering at a temperature of 1500K because, at this temperature, SiC fibers do not degrade. Shevchenko National University, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 95 – 99, January – February, 1998.     

Deposition of nanocrystalline nonstoichiometric chromium oxide coatings on the surface of multiwalled carbon nanotubes by chromium acetylacetonate vapor pyrolysis

Nanocrystalline coatings of nonstoichiometric chromium oxide have been obtained for the first time on the surface of multiwalled carbon nanotubes (MWCNTs) by the method of metalorganic chemical-vapor deposition using chromium acetylacetonate as a precursor. The new hybrid nanomaterial (Cr₂O_2.4/MWCNT) has been characterized by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. It is established that oxidation of the hybrid nanomaterial in air under soft conditions (at 380°C) leads to the formation of nanocrystalline chromium oxide (Cr₂O₃) on the surface of MWCNTs.

     

Annealing of sputter-deposited nanocrystalline Cr-Ta coatings in a low-oxygen-containing atmosphere

As a protective hard coating on glass molding dies, Cr-Ta coatings were fabricated on binderless tungsten carbide substrates with a Ti interlayer by RF magnetron sputtering. The nanocrystalline Cr-Ta coatings were deposited at 550 °C, which revealed one nanocrystalline phase for the Ta-rich coating and two nanocrystalline phases for the Cr-rich coating. Annealing treatment was conducted at 600 °C in a 12 ppm O₂-N₂ atmosphere to evaluate the coating performance in a realistic glass molding environment. Both Auger electron spectroscopy and X-ray photoelectron spectroscopy depth profiles verified the outward diffusion of Cr, which formed a protective coating for the Cr-rich coatings. A scale of Cr₂O₃ and a Cr-depleted transition zone near the surface were identified by conducting a transmission electron microscopy investigation on the annealed Cr_0.71Ta_0.29 coating. The Cr-rich coating absorbed a smaller amount of oxygen, exhibited greater hardness, and maintained nanoscale surface roughness after annealing in the glass molding atmosphere, thus making it an appropriate protective coating for the die material.     

Mechanical and tribological properties of nanocrystalline and nanolaminated surface coatings

The mechanical and tribological (friction/wear)properties were determined for nanocrystalline aluminum, as-sputtered Ti, Zr and Cu films, and nanolaminated Al-Al₂O₃, TiTiN, TiCu and TiZr composite films. The aluminum grain size varied between 15 and 10⁶ nm. The Al and Ti layer thicknesses in Al-Al₂O₃ and TiTiN composites ranged from 70 to 500 nm and from 150 to 450 nm, respectively. Within the grain size range of 15–100 nm, the hardness of the aluminum follows a Hall-Petch type relationship. The hardness of the TiTiN and Al-Al₂O₃ fllms also follows a Hall-Petch relationship with the Ti orAl layer thickness. The TiCu composites show a softening effect with decreasing Ti layer thickness. The coefficient of friction and wear rate for nanolaminated TiTiN and Al-Al₂O₃ composites are consistently reduced as the metal layer thickness is reduced. The micromechanisms responsible for the differences in mechanical and tribological properties are examined and the results compared with published literature.     

SPM characterization of pulsed laser deposited nanocrystalline CrN hard coatings

Nanocrystalline CrN coatings, widely required for surface engineering application covering wear and corrosion resistance, need to be investigated for atomic scale morphology, surface roughness, local stiffness, phase uniformity, and homogeneity. Evolution of these properties as a function of thickness need to be studied. In this paper, we have attempted to address these issues through use of a multimode scanning probe microscope (SPM) equipped to carry out Atomic Force Microscopy (AFM) and Atomic Force Acoustic microscopy (AFAM) of Chromium nitride films (100-500 nm thick) on Si prepared under high vacuum by pulsed Laser Ablation using Nd-YAG Q-switched laser. Prior to SPM analysis, the coatings were annealed in N2 atmosphere at 700 degrees C for 30 minutes for improving crystallanity and coating substrate adhesion. The GIXRD patterns of these annealed specimens showed formation of nanocrystalline CrN. Also signature of amorphous phases was seen. The grain size was estimated to be less than 30 nm. Contact mode AFM imaging revealed a roughness value less than 50 nm. Local stiffness values were calculated from AFM force-distance curves. Imaging of frictional force and surface flaws are being investigated by Frictional Force Microscopy (FFM), resonance spectroscopy, and AFAM, respectively. The contrast in AFAM images is seen due to variation in surface elasticity in reference and CrN samples. Stiffness constant and elastic modulus were calculated for both the samples and compared.     

Protective coatings for carbon fibers

Protective refractory nanocoatings on carbon bundles and tapes have been investigated. Processes have been developed for growing thin layers of refractory oxides—alumina, zirconia, and silica—on continuous carbon fibers and tapes using sol-gel processing. ZrC/ZrO₂ bilayer coatings have been produced by chemical vapor transport. The surface morphology, phase composition, and elemental composition of the coatings have been studied by x-ray diffraction, high-resolution scanning electron microscopy, and qualitative energy-dispersive x-ray microanalysis. The results demonstrate that the refractory oxide coatings are homogeneous along the length and perimeter of individual fibers and adhere well to the fiber surface, with no peeling. Their thickness is within 200–300 nm. The effect of the nature of the coating on the oxidation resistance of carbon materials is analyzed.     

Superior hardness and Young's modulus of low temperature nanocrystalline diamond coatings

Nanocrystalline diamond (NCD) coatings with thickness of about 3 μm were grown on silicon substrates at four deposition temperatures ranging from 653 to 884 °C in CH₄/H₂/Ar microwave plasmas. The morphology, structure, chemical composition and mechanical and surface properties were studied by means of Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Raman spectroscopy, nanoindentation and Water Contact Angle (WCA) techniques. The different deposition temperatures used enabled to modulate the chemical, structural and mechanical NCD properties, in particular the grain size and the shape. The characterization measurements revealed a relatively smooth surface morphology with a variable grain size, which affected the incorporated hydrogen amount and the sp² carbon content, and, as a consequence, the mechanical properties. Specifically, the hydrogen content decreased by increasing the grain size, whereas the sp² carbon content increased. The highest values of hardness (121 ± 25 GPa) and elastic modulus (1036 ± 163 GPa) were achieved in NCD film grown at the lowest value of deposition temperature, which favored the formation of elongated nanocrystallites characterized by improved hydrophobic surface properties.     

High-temperature protective coatings for carbon fibers

Literature data are used to examine the current status of the problem of effective oxidation protection for carbon materials; materials capable of protecting fibers from oxidation, including inorganic polymers, such as oxides, carbides, nitrides, borides, silicides, and their combinations; coating procedures and conditions; thickness and fracture toughness of coatings; materials design limitations related to the reactivity of the components of fiber-reinforced materials; and new approaches to the selection of protective coating materials.     

Ferromagnetic composite coatings on carbon fibers

Alumina- and titania-based composite coatings containing ferromagnetic nanoparticles have been produced on UKN-5000P carbon fibers by an in situ organo-inorganic hybrid sol-gel process using appropriate aqueous metal chloride solutions containing Fe(III) and Co(II). The morphology, phase composition, and elemental composition of the coatings have been studied by high-resolution electron microscopy, X-ray diffraction, and energy-dispersive X-ray microanalysis. The results demonstrate that the oxide coatings containing ferromagnetic nanoparticles are uniform in thickness along and across the fibers and adhere well to the fiber surface. The coatings range in thickness from 100 to 500 nm. In all of the systems studied, the coatings produced on carbon fibers differ in phase composition from powders prepared from identical hydrosols.     

Grain growth behavior of nanocrystalline Inconel 718 and Ni powders and coatings

Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled powders, as well as the HVOF-derived coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth by maintaining a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good agreement with the experimental results.     

CVD nanocrystalline diamond coatings on Ti alloy: A synchrotron-assisted interfacial investigation

Diamond coating on Ti-6Al-4V alloy was carried out using microwave plasma enhanced CVD with a super high CH₄ concentration, and at a moderate deposition temperature close to 500 °C. The nucleation, growth, adhesion behaviors of the diamond coating and the interfacial structures were investigated using Raman, XRD, SEM/TEM, synchrotron radiation and indentation test. Nanocrystalline diamond coatings have been produced and the nucleation density, nucleation rate and adhesion strength of diamond coatings on Ti alloy substrate are significantly enhanced. An intermediate layer of TiC is formed between the diamond coating and the alloy substrate, while diamond coating debonding occurs both at the diamond-TiC interface and TiC-substrate interface. The simultaneous hydrogenation and carburization also cause complex micro-structural and microhardness changes on the alloy substrates. The low deposition temperature and extremely high methane concentration demonstrate beneficial to enhance coating adhesion strength and reduce substrate damage.     

Solid or liquid phase sintering of nanocrystalline WC/Co hardmetals

Nanocrystalline WC/Co hardmetal powder has been sintered at various temperatures in vacuum and HIP sintered under different pressures. In addition, hot stage optical microscopy (HSM) and differential thermal analysis (DTA) was performed on a similar powder. All the results indicate that the matrix of the nanopowder starts to melt at about 1150 °C; however, the melting is confined to locations that are still nanocrystalline. With increasing temperature, more material melts and at about 1330 °C the melting of the matrix is complete. High density can be achieved below this temperature, but not below 1200 °C. Compared with previous sintering results on micron sized powder, the conclusion is that the nanopowder densifies mostly in the solid state, while the latter mostly in the liquid state.