Showing papers in "Surface & Coatings Technology in 2003"

Materials selection guidelines for low thermal conductivity thermal barrier coatings

Abstract: Materials selection guidelines are desirable in identifying and developing alternative materials for higher-temperature capability thermal barrier coatings. Some relate to identifying candidate materials that exhibit particularly low values of thermal conductivity at high temperatures and others relate to thermodynamic stability in contact with the thermally grown oxides formed on bond-coat alloys and superalloys. By using existing theories of the minimum thermal conductivity, a materials parameter is developed that can be used to identify candidate alternatives to yttria-stabilized zirconia for high-temperature applications.

Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology

Abstract: Thick alumina coatings were synthesized on 7075 Al-alloy substrates through the microarc oxidation (MAO) route. Different oxidation times namely 1, 3, 5, 10, 20 and 30 min were employed and the coated samples were subjected to coating thickness and surface roughness measurements. Phase composition of the surface layers of the coatings was evaluated through X-ray diffraction. In addition, the surface features of the coatings were studied using scanning electron microscopy. Influence of coating time on the kinetics of coating formation, surface roughness, microhardness, number and size of the microarc discharge channels was investigated. On the basis of the experimental results, a phenomenological mechanism for the formation of the alumina-based ceramic coatings during the MAO process has been proposed.

Recent advances of superhard nanocomposite coatings: a review

Abstract: In this paper, a review of the present status of the research and technological development in the field of superhard nanocomposite coatings is attempted. Various deposition techniques have been used to prepare nanocomposite coatings. Among them, reactive magnetron sputtering is most commonly used. Nanocomposite coating design methodology and synthesis are described with emphasis on the magnetron sputtering deposition technique. Also discussed are the hardness and fracture toughness measurements of the coatings and the size effect. Superhard nanocomposite thin films are obtainable through optimal design of microstructure. So far, much attention is paid to increasing hardness, but not enough to toughness. The development of next generation superhard coatings should base on appropriate material design to achieve high hardness and at the same time high toughness.

Physics and chemistry in a glow dielectric barrier discharge at atmospheric pressure: diagnostics and modelling

Abstract: Glow dielectric barrier discharge (GDBD) appears as an attractive solution to realise an atmospheric pressure cold plasma process suitable for all the surface treatments including thin film coatings. Such a development requires a large understanding of the GDBD physics and chemistry. The objective of this work is to contribute to that understanding. From the analysis of electrical measurements, time resolved emission spectroscopy, short exposure time pictures and numerical model results, it is shown that GDBD which are discharges due to a Townsend initiation and not to a streamer coupling, transit from Townsend to subnormal glow discharge during the current increase. Depending on the maximum current density, they can be considered as a glow or a Townsend discharge. In a glow discharge, the memory effect from one discharge to the following one is based on electrons and ions trapped in the positive column while in a Townsend discharge, it is metastables which stay in the gas and create electrons through cathode secondary emission. In all the cases, the ionisation has to be slowed down by a significant contribution of Penning ionisation.

Deposition characteristics of titanium coating in cold spraying

Abstract: Titanium coating was deposited by cold spraying process using nitrogen and helium gases under different temperatures and pressures. The deposition characteristics of the particle in cold spray were studied by the examination of the microstructure evolution of the deposited spot and coating. The effects of the gas type and temperature on the deposition behavior were examined. The microstructure was examined using optical microscopy and scanning electron microscopy. It was found that the pattern of a sprayed spot in cold spray presents a conical shape. The deposition efficiency of spray particles increases with the increase in gas temperature. Two distinguishable top and inner regions exist in the spot deposit and coating, which are characterized by the porous and dense microstructures. The dense microstructure results from the accumulative effect of tamping on the top porous region by the successive impact of following particles. The tamping effect has great influence on the microstructure of the coating in cold spray.

High power pulsed magnetron sputtered CrNX films

Abstract: Microstructure and macroscopic properties of droplet free CrN films deposited by the recently developed high power pulsed magnetron sputtering (HIPIMS) technique are presented. Magnetron glow discharges with peak power densities reaching 3000 W cm−2 were used to sputter Cr targets in both inert and reactive gas atmospheres. The flux arriving at the substrates consisted of neutrals and ions (approx. 70/30) of the sputtered metal and working gas atoms (Ar) with significantly elevated degree of ionization compared to conventional magnetron sputtering. The high-speed steel and stainless steel substrates were metal ion etched using a bias voltage of −1200 V prior to the deposition of CrN films. The film-to-substrate interfaces, observed by scanning transmission electron microscope cross-sections, were clean and contained no phases besides the film and substrate ones or recrystallized regions. CrN films were grown by reactive HIPIMS at floating potential reaching −160 V. Initial nucleation grains were large compared to conventional magnetron sputtered films, indicating a high adatom mobility in the present case. The films exhibited polycrystalline columnar growth morphology with evidence of renucleation. No intercolumnar voids were observed and the corrosion behavior of the film was superior to arc deposited CrNx. A high density of lattice defects was observed throughout the films due to the high floating potential. A residual compressive stress of 3 GPa and a hardness value of HK0.025=2600 were measured. A low friction coefficient of 0.4 and low wear rates against Al2O3 in these films are explained by the absence of droplets and voids known to contribute to extensive debris generation.

Structure and in vitro bioactivity of titania-based films by micro-arc oxidation

Abstract: Titania-based films on titanium were formed by micro-arc oxidation in electrolytic solutions containing sodium carbonate, sodium phosphate, acetate monohydrate and β-glycerophosphate disodium salt pentahydrate using a pulse power supply. The morphology, elemental composition and phase components of the films were investigated as a function of the electrolytes composition and the applied voltage (in the range of 200–500 V). In vitro bioactivity of the films was evaluated in a most commonly used simulated body fluid as proposed by Kokubo et al. The results showed that the films were porous with 1–8 μm pores and nano-crystallized, without apparent interface to the titanium substrates. The phase components of the films could be anatase, rutile, CaTiO 3 , β-Ca 2 P 2 O 7 and α-Ca 3 (PO 4 ) 2 , strongly depending on the electrolytes composition and the applied voltage. The pore size and the content of Ca and P tended to increase with the applied voltage. Among the prepared titania-based films, only the film containing CaTiO 3 , β-Ca 2 P 2 O 7 and α-Ca 3 (PO 4 ) 2 could induce an apatite layer on its surface, exhibiting bioactivity. The bioactive response of the micro-arc oxidized films to the structural factors and the apatite-induced mechanism were discussed.

Oxidation resistance of Cr1-XAlXN and Ti1-XAlXN films

Abstract: Cr1−XAlXN films were synthesized on mirror-polished stainless steel substrates by the arc ion plating method using Cr1−XAlX alloy targets with diffent Al contents. Oxidation resistance of films was estimated by heating substrates in air at 800, 900 and 1000 °C and subsequent analysis by the X-ray diffraction method (XRD). The XRD peaks from Ti0.7Al0.3N films, annealed at 800 °C for 14 h, disappeared and the peaks from iron oxides consequently appeared. The oxidation resistance of Ti1−XAlXN films improved with increasing Al content X. On the other hand, the peaks from Cr1−XAlXN films which were annealed at 800 °C did not change at all, but Cr1−XAlXN films were slightly oxidized over 900 °C. It is considered that the oxidation resistance of Cr1−XAlXN films was superior to that of Ti1−XAlXN films.

Friction-induced structural transformations of diamondlike carbon coatings under various atmospheres

Abstract: Structural transformations that occur in diamondlike carbon coatings with increasing hydrogen content have been investigated by Raman spectroscopy, transmission electron microscopy, electron diffraction, and electron-energy-loss spectroscopy. Friction tests were performed with uncoated steel balls against coated substrates at contact stresses of 1 GPa in ambient air (RFI = 30-40%), dry air (RH<1%), and dry nitrogen (<1%). The lowest friction coefficient (f<0.02) was obtained for the most hydrogenated sample in dry nitrogen, where the formation of a third-body layer was observed on the steel surface. Raman spectra obtained from the counterfaces after sliding in humid and dry air revealed an increase and narrowing of the 'D' and 'G' peaks with decreasing humidity. Analysis of peak positions and I(D)/I(G) ratios suggest an increasing order and an enlargement of the sp 2 clusters under friction. The shape and position of the carbon K-edge spectra for the transfer layer are affected the same way, although evidence of extended graphite layer formation was not observed. Development of these differing trends was correlated with the hydrogen-to-carbon ratio of the gas precursor used during the synthesis and testing environment.

The tribological performance of ultra-hard ceramic composite coatings obtained through microarc oxidation

Abstract: A dense ceramic oxide coating approximately 100 mm thick was prepared on a 7075 Al alloy by microarc oxidation in an alkali-silicate electrolytic solution. Coating thickness and surface roughness (R ) were measured during coating formation. The a influence of current density, electrolyte temperature and inter-electrode distance on coating kinetics was investigated. Microstructure and phase compositions were analysed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The microhardness of the coating was also measured. The tribological performance of the coatings was evaluated using a dry sand abrasion test, a solid particle erosion test and a pin-on-disc sliding wear test. In addition, the results are compared to detonation-sprayed alumina (Al O ) coating and bulk Al O . The basic mechanism of microarc coating formation is explained. The material removal 23 2 3 mechanism during solid particle erosion was investigated, and structure–property correlations were established. 2002 Elsevier Science B.V. All rights reserved.

Cerium based chemical conversion coating on AZ63 magnesium alloy

Abstract: A CeCl 3 /H 2 O 2 aqueous solution treatment is assessed for the formation of conversion coatings on a AZ63 magnesium alloy. The coating composition and morphology are examined. The conversion coating appears to consist of a thin and cracked coating with ‘dry-mud’ morphology with large agglomerates over cathodic intermetallic particles. The corrosion resistance in NaCl solution has been investigated. The cerium-based conversion process improves the pitting potential of the alloy. Better corrosion resistant surfaces are obtained when the samples are submitted to repeated immersions in the conversion bath for 30 s up to 180 s of total immersion time. The thickness of the cerium conversion coating rapidly grows up in the first 30 s; afterwards it remains nearly constant. An increase of both hydrogen peroxide concentration and immersion time produces a worsening of the alloy to corrosion.

High-temperature oxidation resistance of Cr1−xAlxN thin films deposited by reactive magnetron sputtering

Abstract: The thermal stability against oxidation of Cr1−xAlxN films with 0≤x≤0.63 has been investigated by isochronal (15 min) heating in air at various temperatures up to 1173 K. Cr1−xAlxN thin films were deposited by reactive magnetron sputtering from Cr and Al targets in a mixed Ar/N2 atmosphere at a substrate temperature of 573 K. All the films crystallize in the pseudo binary, rocksalt-type cubic structure, showing a (111) preferential orientation. Oxidation proceeds by de-nitridation and the formation of a pseudo binary, mixed, Cr/Al oxide with the corundum structure. The degree of film oxidation was evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Rutherford backscattering spectroscopy (RBS). The substitution of Cr atoms by Al atoms leads to two oxidation behaviors. Cr1−xAlxN films with low Al content (x 0.2) are more resistant to high temperatures compared to pure CrN. Films with the highest Al content (x=0.63) are stable up to 1173 K due to the formation of an amorphous, aluminum-rich oxide which blocks oxygen diffusion and prevents further film oxidation.

Frictional behavior of diamondlike carbon films in vacuum and under varying water vapor pressure

Abstract: Frictional behavior of diamondlike carbon films in vacuum and under varying water vapor pressure

An in vitro study of Ti and Ti-alloys coated with sol–gel derived hydroxyapatite coatings

Abstract: Electrochemical–corrosion properties of Ti, Ti6Al6Nb and Ti6Al4V implant materials coated with calcium phosphate (CaP) coatings were investigated in vitro at the open circuit potential using non-destructive electrochemical impedance spectroscopy (EIS) technique in simulated physiological-Hank's Balanced Salt Solution (HBSS). Characterization of the structure and composition of CaP coatings obtained by the sol–gel method was performed using X-ray diffraction analysis. The interpretation of the EIS results was based upon a two-layer model of the surface film consisting of an inner barrier TiO2 (CaTiO3) layer and the outer porous hydroxyapatite (HAP)+β-tricalcium phosphate (β-TCP) layer, described by using the high frequency time constant (τ1HF) and the low frequency time constant (τ2LF), respectively. Spontaneously passivated surfaces of naturally grown surface films on all three substrates in HBSS were used for reference. The chemical composition of the substrate as well as the composition and crystallinity of the CaP coating have a direct influence on electrochemical–corrosion properties of the implant materials under in vitro conditions. Well-crystallized HAP and β-TCP exhibited a beneficial corrosion protection effect on the substrate during prolonged exposure to HBSS. In the case of poorly crystallized coatings containing a greater amount of carbonate incorporated in the structure of HAP, the total impedance of the system markedly changed with prolonged exposure to physiological solution due to degradation of the coating.

Depth profiling characterisation of the surface layer obtained by pulsed Nd :YAG laser irradiation of titanium in nitrogen

Abstract: We investigated the composition of the surface layer obtained by pulsed Nd:YAG laser (λ=1.064 μm, τ∼120 ns, ν=1 kHz) irradiation of Ti targets in high pressure nitrogen. The surface morphology, the crystalline state, and the depth distribution of the elements were analysed by scanning electron microscopy, X-ray diffractometry, secondary ion mass spectrometry, X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy. The chemical binding states were studied by deconvolution of the XPS spectra. The layer has a uniform surface, and mainly consists of the tetragonal δ′-TiNx crystalline phase. The nitrogen concentration increases firstly in the depth until approximately 0.15 μm, and then decreases until the greatest measured depth of 2 μm. The TiNx stoichiometry changes from x≈0.8 close to the surface to x≈0.5 at a depth of approximately 0.2 μm, it remains around this value until approximately 0.5 μm, and for greater depths decreases until 0.1 at 1.6 μm. Furthermore, the oxygen concentration decreases quickly and reaches the concentration of bulk Ti at approximately 0.2 μm.

The effects of fuel chemistry and feedstock powder structure on the mechanical and tribological properties of HVOF thermal-sprayed WC–Co coatings with very fine structures

Abstract: We have deposited a series of WC–Co coatings by the high velocity oxy fuel (HVOF) process, using three different powders, and different spray conditions. The powders are a nanocrystalline (Nanocarb), a near-nanostructured powder (Infralloy) containing a proprietary additive aimed at retarding grain growth, and multimodal (mixed micro and nano) powder (Nanomyte). HVOF spray conditions were stoichiometric, fuel rich and oxygen rich. ‘In-flight’ feedstock powder temperature and velocity were measured. The hardness and toughness of the coatings are found to depend on WC-binder adhesion and adhesion between splats. High flame temperatures increase WC-binder adhesion but increase decarburization. The latter is found to decrease adhesion between the splats. Decarburization is most pronounced for nanostructured powders because of their high specific surface. The additive in Infralloy decreases adhesion between WC grains and binder, but it also reduces decarburization. The wear resistance of the coatings increases with hardness and decreases with increasing decarburization. Sliding wear occurs by a attrition of the WC grains and the lifting of entire splats; abrasive wear occurs by ductile cutting, grain loss and lifting of splats; the low wear rate in sliding leads to splat-boundary weakening by fatigue. The effect of decarburization predominates in sliding wear and is less pronounced in abrasion; the high abrasive wear removes material before fatigue becomes important. The coating deposited at high temperature, from the multimodal powder Nanomyte, presents outstanding sliding and abrasive wear resistance but inflicts large wear on the opposing silicon nitride surface in sliding. Coatings deposited with the near nanostructured powder containing an additive present high sliding wear resistance, independent of the deposition parameters, and cause low wear of the opposing silicon nitride. Coatings deposited with spray-dried nanostructured powders offer comparatively low wear resistance, in agreement with previous reports.

Characteristics of martensitic stainless steel nitrided in a low-pressure RF plasma

Abstract: Martensitic grade stainless steel AISI 420 was nitrided in a low-pressure RF plasma using pure nitrogen. With a treatment time of 4.0 h at 400 °C, the nitrogen-rich layer on the sample was 20 μm thick and had a hardness of approximately 4.3 times higher than that of untreated material. The layer thickness was much greater than that obtained on AISI 304 under identical conditions, reflecting the different Cr content of the two alloys. Below the temperature at which CrN forms, the treated layer retains its martensitic structure, but with a larger lattice parameter than the bulk, a phase that can be termed ‘expanded martensite’, by analogy with the situation with austenitic stainless steel. Kinetics of the layer formation was also investigated which yielded an activation energy of 33.4 kJ/mol.

Pulse electrodeposition of nanocrystalline nickel using ultra narrow pulse width and high peak current density

Abstract: The synthesis of nanocrystalline nickel by electrodeposition has been studied for more than 10 years. However, most attention has been on the adjustment of bath composition or development of new chemical additives. In this paper, a new method was developed to synthesize bulk nanocrystalline nickel without using any additives. Pulse plating with ultra narrow pulse width and high peak current density was employed to increase the deposition current density and the nucleation rate. At an on-time of 10 μs and an off-time of 90 μs, it was found that different surface morphologies, grain sizes, textures, and hardness were obtained at different current densities. Grain sizes ranging from 50 to 200 nm were obtained when the current density varied from 300 to 60 A dm −2 . The preferred orientation of the nickel deposit changed from a weak (2 0 0) fiber texture to a strong (2 0 0) fiber texture when the peak current density increased from 40 to 100 A dm −2 . It was obtained however that the intensity of the (2 0 0) fiber orientation decreased when there was a further increase in the current density. The hardness of the nickel deposit was also found to increase with increasing peak current density when it changed from 20 to 150 A dm −2 , but the hardness tended to decrease when the current density was above 150 A dm −2 . These experimental findings are considered to relate to the change in cathodic overpotential which affects both the grain size, the internal stress, the porosity, and the preferred orientation.

Evaluation of pore diameter of anodic porous films formed on aluminum

Abstract: The porosity of anodic films formed in four major electrolytes, which was measured by pore-filling technique, decreases with increasing voltage Comparing the porosity at an identical voltage, it increases in the order of the films formed in sulfuric acid, oxalic acid, chromic acid, and phosphoric acid solutions When the voltage decreases down to lower than 10 V, the porosity increases remarkably, especially in the films formed in phosphoric acid The pore diameter estimated from the porosity and cell diameter measured by pore-filling technique decreases with decreasing voltage down to 5 V However, at the voltages lower than 5 V, the pore diameter increases while the cell diameter decreases Pore and cell diameters of the films formed in the four electrolytes at an identical voltage increase in the order: sulfuric acid, oxalic acid, chromic acid, and phosphoric acid These results are basically consistent with those reported in our previous TEM study, although the pore sizes are slightly larger than the latter's The reason of the discrepancy between the methods is discussed

Effect of molybdenum on the microstructure and wear resistance of cobalt-base Stellite hardfacing alloys

Abstract: The Stellite 6 hardfacing alloys with different Mo contents have been deposited on AISI 1045-carbon steel using a Plasma Transferred Arc (PTA) welding machine. The effect of Mo on the microstructures and wear resistance properties of the Stellite 6 hardfacing alloys were investigated using optical microscopy, scanning electron microscopy, electron probe microanalysis and X-ray diffraction. With an increase in Mo contents, the M 23 C 6 and M 6 C type carbides were formed instead of Cr-rich M 7 C 3 and M 23 C 6 type carbides observed in the interdenritic region of the Mo-free Stellite 6 hardfacing alloy. The size of Cr-rich carbides in interdendritic region decreased, but that of M 6 C type carbide increased as well as the refinement of Co-rich dendrites. The volume fraction of Cr-rich carbides slightly increased, but that of M 6 C type carbide abruptly increased. This microstructural change was responsible for the improvement of the mechanical properties such as hardness and wear resistance of the Mo-modified Stellite 6 hardfacing alloy.

Surface treatment by high current pulsed electron beam

Abstract: Electron beams are becoming an increased subject of interest for materials processing. While continuous electron beams have already found wide applications in drilling, hardening, cutting and welding, the advantage of a pulsed electron beam has just emerged. It generates a high power density up to 108–109 W/cm2 at the target surface. Such a high energy is deposited only in a very thin layer within a short time, and causes superfast processes such as heating, melting and evaporation. A dynamic stress field induced in these processes leads to significant modification effects in the material. The combination of these processes provides the material with improved physicochemical and mechanical properties unattainable with ordinary surface treatment techniques. The present paper reports our recent research work on surface treatment by high-current pulsed electron beam (HCPEB). HCPEB is produced on system ‘Nadezhda-2’ with an energy range of 20–40 kV. A series of pure Al and mold steels were studied. Some of them were pre-coated with C, Cr, Ti or TiN powders. A strong enhanced diffusion effect was revealed: the surface elements diffuse approximately several micrometers in depth into the substrate only after several bombardments. Tribological behaviors of these samples were characterized and significant improvement in wear resistance was found. Finally, TEM analysis reveals the presence of stress waves generated by the coupling of thermal and stress fields, which constitutes the main cause of the enhanced diffusion.

Structure–property relationships in single- and dual-phase nanocrystalline hard coatings

Abstract: The structure of hard coatings deposited by PVD (physical vapor deposition) strongly depends on the growth parameters. In this work, the influence of microstructure and chemical composition on mechanical properties was investigated in detail for several nanocrystalline hard coatings. Single-phase TiN, CrN and TiB2 as well as dual-phase CrN–Cr2N, TiN–TiB2 and TiC–TiB2 coatings were prepared by reactive or non-reactive unbalanced dc magnetron sputtering, respectively. The hardness of the coatings investigated will be discussed with respect to the average grain size and residual biaxial stresses. By optimizing both nanostructure and biaxial stresses of stoichiometric TiN coatings, their microhardness (H) could be improved from 33 to 56 GPa, whereas the reduced Young's modulus (E*) changed from 402 to 480 GPa. Thus, the ratio H3/E*2 which is representative for the resistance to plastic deformation could be increased from 0.222 to 0.806 GPa. For Cr–N coatings, H3/E*2 values up to 0.521 GPa were obtained, depending on their chemical composition and nanostructure. The nanocrystalline dual-phase coatings TiN–TiB2 and TiC–TiB2 even yielded H3/E*2 values up to 1.332 and 1.575 GPa, respectively. Comparing the different single- and dual-phase coatings investigated, the establishment of correlations between structure and mechanical properties enables the development of advanced coatings with high hardness and high resistance against plastic deformation. In addition, this work shows the tremendous importance of a controlled deposition process for optimizing coating properties.

Anodizing of Mg alloys in alkaline solutions

Abstract: The electrochemical behaviors of 99.95 mass% magnesium and its alloys, i.e. AZ31 and AZ91, were investigated in NaOH alkaline solution. Mg and Mg alloy specimens were anodized for 10 min at 3, 10 and 80 V in 1 mol/dm3 NaOH alkaline solution. The films anodized at 3 V on Mg and Mg alloys had the best effective corrosion resistance and these films consisted of comparatively thick magnesium hydroxide. Corrosion resistances of the films anodized at 80 V on Mg and Mg alloys were higher than those at 10 V and lower than those at 3 V, from results of the anodic polarization measurement in 0.1 mass% NaCl solution.

Anti-bacterial silver coatings exhibiting enhanced activity through the addition of platinum

Abstract: Anti-bacterial silver coatings were deposited on thermally sensitive polymeric substrates using a combination of magnetron sputtering and neutral atom beam (Saddle Field) plasma sources. The anti-bacterial activity of silver is dependent on the release of Ag+ ions, which act by displacing other essential metal ions such as Ca2+ or Zn+. This study evaluates the use of platinum to enhance the release of silver ions from the silver coating. In the galvanic series platinum is more active than silver and therefore Pt enhances Ag+ ion formation through galvanic action. In order to evaluate this, potential step (chronoamperometric) experiments were performed on silver/platinum alloys containing 0.5 and 3.0% Pt. The resulting current–time curves demonstrated that Ag+ formation increased with platinum addition by up to 100%. A magnetron sputtering target was fabricated consisting of 1% Pt in a Ag matrix. This was used to sputter Pt/Ag coatings with thicknesses in the range 5–12 nm onto silicone and polyurethane substrates. The bacterial adhesion and bactericidal effects of the coated polymers was assessed using Straphylococcus epidermidis and the cytotoxicity using fibroblast cells. The addition of 1% Pt was found to significantly enhance the anti-bacterial effectiveness of the Ag coatings. Up to a 2 log reduction in bacterial adhesion was achieved for 5 nm thick Ag/1% Pt coatings on silicone, which did not exhibit cytotoxicity.

The influences of sealing methods on corrosion behavior of anodized aluminum alloys in NaCl solutions

Abstract: The influences of sealing methods on the corrosion behavior of three anodized aluminum alloys, 1070, 2024 and 7075, in NaCl solutions were studied. The anodic films were sealed by the methods of boiling water, stearic acid, potassium dichromate and nickel fluoride, respectively. Passivation and pitting behaviors resulting from the sealing treatments were studied using the potentiodynamic polarization, and surface morphology was examined by scanning electron microscopy. The results show that boiling water and potassium dichromate sealed films provide relatively higher corrosion resistance in acidic solution, while nickel fluoride sealed film is better in basic solution. Stearic acid sealed film provides good corrosion resistance both in acidic and in basic solutions. The influencing mechanisms of different sealing methods on corrosion resistance of the anodized alloys were discussed.

Characteristics of PVD-coatings on AZ31hp magnesium alloys

Abstract: Magnesium-based light-metal alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability. However, the limitations according to mechanical strength and the low corrosion resistance restrict their practical application. To improve the surface hardness and the corrosion resistance, PVD-coating techniques offer possibilities to overcome these drawbacks. The paper presented reveals relevant mechanical and chemical properties of various PVD-coatings on high purity (hp) AZ31 magnesium alloy specimens. The industrially very important layer systems TiN, CrN, TiAlN, NbN-(TiAl)N, CrN-TiCN and the multi-layer composite AlNyTiN are discussed in detail. Furthermore, the protective effect of CrN yNbN superlattices has been studied. All of these coatings were deposited by d.c. magnetron sputtering. The sputtering processes have been performed by two different coating devices-PLS 500 (laboratory scale) and HTC 1000y4 ABS (industrial scale). Both the mechanical behaviour and the � corrosion resistance of the specimens have been studied after coating. The chemical composition of the thin films was analysed by GDOES. A special emphasis of the investigation was laid on searching for appropriate stripping procedures due to the fact that the substrate material is very reactive. � 2002 Elsevier Science B.V. All rights reserved.

Structural and optical properties of indium oxide thin films prepared by an ultrasonic spray CVD process

Abstract: Polycrystalline In2O3 thin films have been synthesized by a modified chemical vapor deposition process using the pyrolysis of an aerosol generated by ultrahigh frequency spraying of a volatile precursor solution. The deposition has been carried out on glass substrates (350–550 °C). X-Ray diffraction has shown that deposited films are polycrystalline without second phases. The morphology of the surface as function of the substrate temperature has been studied using scanning electron microscopy (LEO Stereoscan 440). Transmission and reflection spectra of the sprayed films were examined in the 260–2600 nm spectral range and, for investigated films, the transmission coefficient in the visible and infrared regions exceed 85% and is weakly affected by deposition temperature. The optical band gap values, Eg, ranged between 3.57 and 3.68 eV. The Urbach tail parameter E0, was calculated for films deposited at different temperatures.

Properties of (Ti, Cr, Al)N coatings with high Al content deposited by new plasma enhanced arc-cathode

Abstract: A novel super hard coating system of (Ti x Cr y ,Al z )N ( x : y ∼1:2, z =0.63–0.73, x + y + z =1) was synthesized by a newly developed plasma enhanced type of arc-cathode. The effect of arc-cathode type, film composition and process parameters on coating's properties was investigated. It was found that in the deposition of the new (Ti,Cr,Al)N coatings, the plasma enhanced cathode has an effect to reduce the number of macro-particles significantly. Depositions of (Ti,Cr,Al)N with approximately z =0.7 under different substrate biases revealed that there was a bias induced phase change from hexagonal (B4) to cubic (B1) structure approximately 50 V of the substrate bias. The hardness of the coating changed drastically as the crystal structure changed from hexagonal to cubic phase, reached more than 35 GPa under proper bias conditions. Depositions from the targets, which contain different Al ratio revealed that cubic phase was maintained up to Al ratio of 0.73. The onset temperature of the oxidation of the (Ti,Cr,Al)N, which was measured by thermo-balance, was approximately 1000 °C, whereas 850 °C was realized in case of the conventional (Ti,Al)N. Also high speed cutting tests against hardened die-steel were conducted in comparison with conventional (Ti,Al)N.

Surface treatment of metals using an atmospheric pressure plasma jet and their surface characteristics

Abstract: We have treated the surfaces of Al, SUS and Cu metals using an atmospheric-pressure plasma jet generated by nitrogen and oxygen gases under the atmospheric pressure at room temperature The plasma ignition occurred by flowing mixed gases between two coaxial metal electrodes, and the voltage was applied with impulse type and 16–20 kHz frequencies The treated surfaces were basically characterized by means of a contact angle analyzer for the activation property on their surfaces From the results of XPS, FE-SEM, OES and AFM, we could confirm that the main phenomena such as the reactive etching and oxidation were observed on their surfaces as well as even the aggregation of particles by the activated atoms, radicals and metastable species in the plasma space However, all treated surfaces contained only oxygen and carbon without nitrogen, even though the excited nitrogen species were generated in the plasma due to its higher reactivity than oxygen ones observed in the OES data The aging effect on the duration time of the surface energy, moreover, was also studied because of the production cost on the industrial applications in addition

Assessment of wear performance of flame sprayed and fused Ni-based coatings

Abstract: Characterization of the flame sprayed and furnace fused NiCrBSiC alloy coatings with two different carbon contents and 15–45 wt.% WCCo addition is described in terms of microstructure, microhardness and differential thermal analysis. Microstructural development of these coatings before and after fusing treatment is discussed to identify the precipitates in the coatings. Optimum fusing conditions (time and temperature) for wear-testing sample were investigated in terms of microhardness and porosity of the coatings. Wear performance of these coatings was also investigated by two- and three-body abrasive and dry sliding wear experiments. The coating of 35% WC with NiCrBSiC is showing the best quality (the highest hardness and the lowest porosity). However, 25% WC addition is showing the best wear resistance for Sugaru abrasive wear test, while 40% WC addition is showing the best wear resistance for DSRW (dry sand rubber wheel) abrasive wear test. It is also shown that the dry sliding wear resistance of the 20% (and/or 30%) WCNiCrBSiC composite coating is almost 10 times better than that of the quenched and tempered JIS SUJ2 bearing steel.