The functional TiO2-biodegradable plastic composite material produced by HVOF spraying process

Photocatalytic TiO2 coatings on bio-degradable plastic(polybutylene succinate: PBS) were prepared by HVOF spraying using three kinds of agglomerated powders (P200: 200 nm, P30: 30 nm, P7: 7 nm). The microstructures of the coatings were characterized with SEM and XRD analysis, and the photocatalytic efficiency of the coatings was evaluated by photo degradation of gaseous acetaldehyde. For both the HVOF sprayed P200 and P30 coatings, high anatase ratio of 100% was achieved, regardless of the fuel gas pressure. On the other hand, for the HVOF sprayed P7 coating, the anatase ratio decreased from 100% to 49.1% with increasing fuel gas pressure. This decrease may be attributed to the much higher susceptibility to heat of the 7 nm agglomerated powders than the 30 nm and 200 nm agglomerated powders. In terms of the photocatalytic efficiency, HVOF sprayed P200 and P30 coatings seemed to outperform the P7 coatings because of their higher anatase ratios. However, the HVOF sprayed P7 coatings did not show photocatalytic activity possibly because of the extremely small reaction surface area to the photo-catalytic activity and low anatase ratio. Therefore, the present study found that functional PBS plastic with photocatalytic performance could be produced by spraying of ceramics such as TiO2.     

Effects of sol infiltration on the screen-printed lead zirconate titanate thick films

Ferroelectric PZT(60/40) thick films were fabricated by the screen-printing method and the PZT precursor solution was spin-coated on the thick films to obtain a densification. Structural and electrical properties of the thick films with the treatment of sol coating were investigated. The lattice constant of PZT thick films was 0.4073 nm and there is no dependence on the number of sol coatings. The thickness of PZT films obtained by one screen-printing was approximately 16 μm. The relative dielectric constant increased and dielectric loss decreased with the increase in the number of sol coatings, and the values of the 15-coated PZT-15 film were 369 and 2.16% at 1 kHz, respectively. The remanent polarization, coercive field, and breakdown strength of the PZT-15 thick film were 23.2 μC/cm², 18.03 kV/cm and 78 kV/cm, respectively.     

Electro-fragmentation analysis of dielectric thin films on flexible polymer substrates

An electro-fragmentation method was developed as a fast alternative to the time consuming fragmentation test carried out in situ in a microscope, to investigate the failure of dielectric inorganic coatings on polymer substrates. An ultrathin conductive layer was used to probe the onset of tensile failure in the dielectric coating through changes of its electrical resistance. A careful selection of the conductive layer has been carried out to avoid artifacts resulting for instance from a change of the cohesive properties (e.g. internal stress state) of the investigated structures. Au layers were found to be too ductile, contrary to Al-Ti layers that were too brittle, which invalidated the use of both materials to probe the failure of the dielectric coatings. In contrast, for structures on high-temperature polymer substrates, a 10 nm thick amorphous graphite (a-G) layer was found to accurately reproduce the cracking of the coating. The Young's modulus and coefficient of thermal expansion of the a-G layer are low enough not to impact the internal strain, hence the crack onset strain of the dielectric coating. The a-G layer is also sufficiently brittle, and its cohesive failure and resulting increase of electrical resistance is triggered by the failure of the dielectric coating. The a-G electro-fragmentation method is presently limited to polymers substrates with a glass-transition temperature higher than 100 °C.     

Nanoindentation-induced deformation behaviour of diamond-like carbon coatings on silicon substrates

The deformation behaviour of diamond-like carbon (DLC) coatings on silicon substrates induced by indentation has been investigated. DLC coatings, deposited by a plasma-assisted chemical vapour deposition technique, were subjected to nanoindentation over a range of maximum loads from 100 mN to 300 mN. The resulting load-displacement plots displayed pop-ins for maximum loads of 200 mN and above, with no distinct pop-out for any of the loads studied. Compressive deformation of the coating, up to a strain of ∼ 9%, was observed. The coating-substrate composite was devoid of cracks at lower loads, but at the maximum load of 300 mN, ring cracks in the coating and a median crack in the substrate were observed. Furthermore, cracking, {111} slip and localized phase transformations were observed in the silicon substrate. The onset of these structural changes was correlated to the mechanical behaviour during indentation.     

Low-temperature synthesis of magnetically recoverable,superparamagnetic, photocatalytic,nanocomposite particles

A new, simple, low-temperature method for the synthesis of superparamagnetic, photocatalytic, nanocomposite particles for applications in the decomposition of pollutants in water is presented. The method is based on the coating of clusters of superparamagnetic maghemite (γ-Fe₂O₃) nanoparticles with a photocatalytic anatase layer using the hydrolysis of aqueous TiOSO₄. The clusters of an appropriate size between 100 and 200 nm form by the simultaneous agglomeration of the aminopropyl-triethoxy-silane-grafted maghemite nanoparticles with a size of approximately 15 nm in a suspension of diluted TiOSO₄. During a sudden increase of pH with the addition of NaOH the titania is heterogeneously nucleated at the cluster surfaces. If the hydrolysis was conducted at an elevated temperature of 90 °C, the titania layer was nanocrystalline anatase. The composition of the nanocomposite particles, i.e., the thickness of the anatase layer, can be controlled simply by changing the starting TiOSO₄/Fe₂O₃ ratio for low titania contents, and by multiple coatings to get high titania contents. The photocatalytic activity of the nanocomposites was evaluated in the photocatalytic decomposition of formic acid. The activity seems to increase with an increase in the thickness and the crystallinity of the anatase coating, whereas it decreased after the calcination of the as-synthesized nanocomposite. The coating of the maghemite nanoparticles with a thin layer of insulating silica also slightly improves the photocatalytic activity.     

In vitro electrochemical corrosion behavior of functionally graded diamond-like carbon coatings on biomedical Nitinol alloy

The anti-corrosion property plays an important role in determining the biocompatibility of metal implants. In this study, functionally graded diamond-like carbon coatings were deposited on the Nitinol substrate by hybrid magnetron sputtering and plasma enhanced chemical vapor deposition. A scratch test was adapted to study the adhesion strength of the coatings. The Si/SiC graded layer (up to 150 nm thick) provided good adhesion between the coating and the substrate, up to approximately 47.17 ± 2.1 N. The effectiveness of corrosion protection for the coated specimen was investigated in Tyrode's simulated body fluid. It was found that adhesion strength had a great influence on the effectiveness of corrosion protection, and the better adhesion strength, the better corrosion resistance. Compared to the others, the coating with a 150 nm thick Si/SiC graded layer provided better corrosion protection, and there existed no large amount of debonding and cracking of the coating around corrosion pits after the potentiodynamic polarization tests.     

Influence of processing factors on properties of anodic coatings obtained on Mg–1.0Ca alloy

The influences of four factors including NaOH concentration, sodium phytate concentration, treatment time and duty cycle on properties of anodic coatings formed on Mg–1.0Ca alloy were systematically investigated using the Taguchi experiment. The effect order of factors on the corrosion resistance is ranked as sodium phytate concentration > treatment time > duty cycle > NaOH concentration, while the sequence on the coating thickness is sodium phytate concentration > treatment time > NaOH concentration > duty cycle. The increasing sodium phytate concentration is benefit for the corrosion resistance of the anodized magnesium alloys by developing thick coating with stable composition.     

Structural and oxidation behavior of atmospheric heat treated plasma sprayed WC–Co coatings

In this study, structural and oxidation behavior of WC–Co coatings was analyzed during atmospheric heat treatment process between 150 °C and 1100 °C. Two types of WC–12%Co coatings with different particle size and morphology were deposited on steel substrates using Air Plasma Spraying. The coated samples were heat treated in atmosphere in different temperatures between 500 and 1100 °C. Microstructural evaluation, X-ray diffraction analysis and microhardness testing were performed before and after heat treatment. In this case, the results showed that, regarding increase hardness of coating samples based on increasing applied temperature, coatings kept their properties up to 500 °C. In addition, by increasing heat treatment temperature up to 1100 °C, oxidation process in coated layer accelerated and caused coating detachment from the coating-substrate interface.     

Comparison of microstructure and phase transformation for nanolayered CrN/AlN and TiN/AlN coatings at elevated temperatures in air environment

CrN/AlN and TiN/AlN multilayer coatings with modulation period of 4 nm and thickness ratio equal to 1.0 were manufactured by RF magnetron sputtering. Both films were annealed at temperatures of 800 °C in air for 1 h and then for an additional 9 h. Both coatings in as-deposited and after heat treatment were evaluated with a transmission electron microscope (TEM) equipped with EDS. After heat treatment at 800 °C for 1 h, a thick oxide layer around 260 nm was formed on the surface of the TiN/AlN coating. The oxide layer which formed on the coating was composed of three different regimes, including Al-enriched oxide with excess oxygen on the top surface, a crystalline Al-depleted TiO₂ layer 30-80 nm thick above the nitride coating and in between, mixed nano-crystalline Al₂O₃ and TiO₂ films. In comparison, only one oxide layer smaller than 50 nm in thickness was found in the annealed CrN/AlN coating. This amorphous or nanocrystalline oxide layer identified by EDS was a metal-deficient oxide, in which Al₂O₃ and Cr₂O₃ were mixed together forming a solid solution. As a result, the CrN/AlN coating exhibited superior stability compared to the TiN/AlN coating at elevated temperatures.     

Intrinsic stresses and stress relaxation in TiN/Ag multilayer coatings during thermal cycling

Morphology, structure and thermal behavior of magnetron sputtered TiN/Ag multilayer thin films deposited at 150 °C with a bilayer thickness Λ in the range of 75-600 nm are characterized. The films are thermally cycled and the relationship between bilayer thickness Λ, film structure and stress development is analyzed. The results indicate that the residual stresses in the as-deposited films and the behavior during heating are determined by the morphology and the mechanical properties of the Ag interlayers. The increasing crystallite size of Ag with increasing Λ and the initial porosity in the Ag layer are the reason for significant changes in the stress-temperature behavior. While coatings with Λ = 75 nm behave like a single-phase coating up to 380 °C, coatings with higher Λ show a different behavior when exceeding the deposition temperature, which is related to the densification of the Ag layers. During cooling, all coatings exhibit linear thermo-elastic behavior, where the slope of the stress-temperature curves also depends on Λ.     

Modelling of the scratch resistance of particle filled sol-gel coatings on aluminium

Sol-gel derived coatings with 12 nm and 0.5-10 μm silica fillers were prepared on aluminium to evaluate the effect of particle size and filler content on coating properties The measured maximum crack-free thickness and hardness strongly depended on the type of particles used and the filler volume fraction. The scratch resistance primarily depended on coating thickness and much less on the mechanical properties of the coating. This finding was interpreted via modelling of the stresses under the scratching stylus. It was established that the initial yield occurred in the substrate for most of the coatings and the load needed for this initial yield had a relatively small dependence on the coating properties but it was strongly influenced by the coating thickness.     

Biaxial fragmentation of thin silicon oxide coatings on poly(ethylene terephthalate)

Crack patterns of 53 nm and 103 nm thick silicon oxide coatings on poly(ethylene terephthalate) films are analyzed under equibiaxial stress loading, by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The biaxial stress and strain are modeled from classical elastic membrane equations, and an excellent agreement is obtained with a finite element method. In the large pressure range, the derivation of the biaxial strain from force equilibrium considerations are found to reproduce accurately the measured data up to 25% strain. The examination of the fragmentation process of the coating under increasing pressure levels reveals that the crack onset strain of the oxide coating is similar to that measured under uniaxial tension. The fragmentation of the coating under biaxial tension is also characterized by complex dynamic phenomena which image the peculiarities of the stress field, resulting in considerable broadening of the fragment size distribution. The evolution of the average fragment area as a function of biaxial stress in the early stages of the fragmentation process is analyzed using Weibull statistics to describe the coating strength.     

Adhesive and bending failure of thermal sprayed hydroxyapatite coatings: Effect of nanostructures at interface and crack propagation phenomenon during bending

Hydroxyapatite (HA) coatings have shown promising effects on rapid bone remodeling and suitable functional life in orthopedic and dental applications. However, the major problem encountered by the HA-coated implants is the failure of the coating due to its insufficient mechanical properties. The present study investigated the influence of the microstructure near to the coating/substrate interface on the adhesion of the coatings. In addition, the crack propagation behavior within the coatings was studied through 4-point bend test. Results showed that nanostructures (30-110 nm) within the HA coatings were achieved by high velocity oxy-fuel (HVOF) spraying. Comparison among HVOF HA coatings, which were deposited using different starting feedstock, suggests detrimental effect of the perpendicular-to-substrate nano-cuboids on adhesion of the coatings. The presence of the grains with hexagonal shape (<250 nm in length and <50 nm in diameter) triggered a deteriorated adhesion. Granular nanosized grains at the interface give rise to enhanced adhesion through improved mechanical interlocking. Formation mechanism of the nanosized grains was discussed in this paper. Furthermore, the 4-point bend test revealed consistent crack propagation path that the cracks actually grow within the coating with a direction parallel to the interface, and approximately several to 20 microns thick coatings were remained on the substrate. The critical strain energy release rate exhibited a value of ∼1.15 kJm⁻². During the crack propagation, kinking and trapping of the bending cracks were decided by the flaws within the coating, which were mainly located at splats’ interface. The interface between the first layer (with one splat thickness) and the second is believed to be the weakest zone in the nanostructured coating.     

Increase of hardness and oxidation resistance of VN coating by nanoscale multilayered structurization with AlN

AlN/VN nanoscale multilayered coatings with various bilayer periods from 2.1 nm to 10.0 nm have been prepared by alternating deposition of AlN and VN. The maximum hardness, 45.4 GPa, was obtained for the AlN/VN nanoscale multilayered coating with bilayer period of 2.1 nm, which is an increase of 54% over the rule of mixture hardness value (29.5 GPa) of AlN and VN coatings. The mass gain of heat-treated AlN/VN nanoscale multilayered coating at temperatures from 600 °C to 700 °C in air was measured to be less than that of the VN single layered coating, which indicates that the oxidation resistance as well as hardness of the VN coating can be increased by nanoscale multilayered structurization with AlN.     

Optical and morphological characterization of photocatalytic TiO2 thin films doped with silver

Silver-doped titanium dioxide thin films were deposited on glass substrates by the sol-gel process. Undoped films and films doped with 1 and 3 mol% Ag were annealed at 100 or 500 °C. The optical and morphological properties of the films were analyzed by optical absorption spectroscopy and scanning electron microscopy. Additionally, the films were evaluated for their ability to degrade methylene blue. It was found that the photocatalytic activity is barely sensitive to silver doping for films annealed at 100 °C, whereas the effect of silver resulted in enhanced photocatalytic activity for films annealed at 500 °C. For the latter annealing temperature, the photocatalytic activity increased with increasing Ag doping concentration.     

Structural and photoluminescence study of thin GaN film grown on silicon substrate by metalorganic chemical vapor deposition

GaN epitaxial layer was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). The structure consists of 50 nm thick high-temperature grown AlN buffer layer, 150 nm thick AlGaN layer, 30 nm low-temperature grown AlN layer, 300 nm GaN layer, 50 nm AlGaN superlattice layer, followed by 100 nm GaN epitaxial layer. The low-temperature AlN interlayer and AlGaN superlattice layer were inserted as the defect-blocking layers in the MOCVD grown sample to eliminate the dislocations and improve the structural and optical properties of the GaN layer. The dislocation density at the top surface was decreased to ∼ 2.8 × 10⁹/cm². The optical quality was considerably improved. The photoluminescence emission at 3.42-3.45 eV is attributed to the recombination of free hole-to-donor electron. The observed 3.30 eV emission peak is assigned to be donor-acceptor transition with two longitudinal optical phonon side bands. The relationship of the peak energy and the temperature is discussed.     

Microstructure and corrosion behavior of Ti nanoparticles reinforced Ni–Ti composite coatings by electrodeposition

A survey on electrochemical codeposition of Ti nanoparticles in Ni matrix coating is given. The influences of Ti nanoparticle loadings in electrolyte on the microstructure, microhardness and corrosion behavior of Ni–Ti coatings were investigated. The results showed that a pyramidal surface structure evolved into a spherical surface structure of the coatings with increasing Ti nanoparticle loading. The content of Ti in the Ni–Ti coatings first increased and reached the maximum value of 7.1 vol.% at the loading of 16 g/L, then decreased due to agglomeration of nanoparticles. The [2 0 0] preferred orientation gradually evolved to [1 1 1] orientation with increasing Ti nanoparticle loading. At Ti nanoparticle loading of 16 g/L, the minimum crystallite size (44 nm) and maximum microstrain (0.25%) were obtained. The microhardness of the Ni–Ti coatings was improved and obtained the maximum value at the loading of 16 g/L. The anti-corrosion behavior of the Ni–Ti coatings had increased trend with increasing Ti nanoparticle loading. The pitting corrosion and the selective dissolution of Ti nanoparticles happened in corrosion of Ni–Ti coating electrodeposited at the loading of 16 g/L in a 3.5 wt.% NaCl solution.     

Effect of nano-TiO2 particles size on the corrosion resistance of alkyd coating

The coating system containing various sizes (∼10, 50, 100, 150 nm) of nano-TiO₂ were prepared and investigated for corrosion protection of carbon steel in 1.0 M H₂SO₄ using polarization, EIS and transmission electron microscopy (TEM) techniques. It was found that nano-TiO₂ particles improved the corrosion resistance of alkyd coatings. The corrosion resistance occurs via physical adhesion on the metal surface. O₂ and H₂O permeability of coating decreased with decrease in the nano-TiO₂ size. The inhibition efficiency was found to increase with decreasing the size of nano-TiO₂ and with decreasing the temperature.     

Study on tribological property of CrCN coating based on magnetron sputtering plating technique

The influence of carbon doping on tribological properties of CrCN coating was studied through preparation of coatings deposited on single crystal silicon and M2 high-speed steel (HSS) substrate using closed-field unbalanced magnetron sputtering ion plating technique. The friction coefficients were measured by pin-on-disc set-up and the wear traces of the coatings were observed by optical microscope. The microstructure and bond states of the coatings were characterized by atomic force microscope (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the friction coefficient of coating decreases from 0.75 to 0.38, the micro-hardness increases from 1930 HV to 2300 HV, and the specific wear ratio of the coatings decreases from 8.351 × 10⁻¹⁵ m³/Nm to 3.859 × 10⁻¹⁵ m³/Nm with the increasing of carbon target current (I_C) from 0 A to 1.5 A. The grain size and the roughness of the coatings both decrease with the increasing of carbon target current, at the same time the coatings transform from crystalline state to amorphous state.     

Electrical and mechanical properties of nano-structured TiN coatings deposited by vacuum cold spray

Titanium nitride (TiN) coatings were fabricated by vacuum cold spray (VCS) process at room temperature with nano-sized starting powder (about 20 nm in size). The microstructure of the powder and coating was examined by scanning electron microscope and X-ray diffraction. The porosity and pore distribution of the VCS TiN coatings were measured by the N₂ adsorption-desorption method. The microhardness and fracture toughness of the coatings were evaluated by using the micro-indentation technique. The sheet resistance and electrical resistivity of the coatings were characterized by the four-point probe method. The results show that the sheet resistance of coatings is significantly reduced from 13565 to 127 Ω with increasing the coating thickness. A minimum electrical resistivity of 1.8 × 10⁻³ Ω m is achieved. The VCS TiN coatings with high porosity ranging from 58.3 to 67.6% exhibit low hardness of 279-490 HV and relatively good fracture toughness of about 3.12 MPa m^1/2.