Showing papers in "Surface & Coatings Technology in 2010"

Overview on advanced thermal barrier coatings

Abstract: During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200 °C which necessitates its substitution in advanced gas turbines. In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed.

High power pulsed magnetron sputtering: A review on scientific and engineering state of the art

Abstract: High power pulsed magnetron sputtering (HPPMS) is an emerging technology that has gained substantial interest among academics and industrials alike. HPPMS, also known as HIPIMS (high power impulse ...

On the film density using high power impulse magnetron sputtering

Abstract: The influence on thin film density using high power impulse magnetron sputtering (HIPIMS) has been investigated for eight different target materials (Al, Ti, Cr. Cu, Zr, Ag, Ta, and Pt). The densit ...

Ni–TiO2 nanocomposite coating with high resistance to corrosion and wear

Abstract: Ni–TiO 2 nanocomposite coatings with various contents of TiO 2 nanoparticles were prepared by electrodeposition in a Ni plating bath containing TiO 2 nanoparticles to be codeposited. The influences of the TiO 2 nanoparticle concentration in the plating bath, the current density and the stirring rate on the composition of nanocomposite coatings were investigated. The composition of coatings was studied by using energy dispersive X-ray system (EDX). The wear behavior of the pure Ni and Ni–TiO 2 nanocomposite coatings were evaluated by a pin-on-disc tribometer. The corrosion performance of coatings in 0.5 M NaCl, 1 M NaOH and 1 M HNO 3 as corrosive solutions was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy methods (EIS). The microhardness and wear resistance of the nanocomposite coatings increase with increasing of TiO 2 nanoparticle content in the coating. With increasing of TiO 2 nanoparticle content in the coating, the polarization resistance increases, the corrosion current decreases and the corrosion potential shifts to more positive values.

Cu and CuO nanoparticles immobilized by silica thin films as antibacterial materials and photocatalysts

Abstract: CuO nanoparticles with average diameter of about 20 nm were accumulated on surface of sol–gel silica thin films heat treated at 300 °C in air. Heat treatment of the CuO nanoparticles at 600 °C in a reducing environment resulted in effective reduction of the nanoparticles and penetration of them into the film. While the thin films heat treated at 300 °C exhibited a strong antibacterial activity against Escherichia coli bacteria, the reducing process decreased their antibacterial activity. However, by definition of normalized antibacterial activity (antibacterial activity/surface concentration of coppers) it was found that Cu nanoparticles were more toxic to the bacteria than the CuO nanoparticles (by a factor of ∼ 2.1). Thus, the lower antibacterial activity of the reduced thin films was assigned to diffusion of the initially accumulated copper-based nanoparticles into the film. The CuO nanoparticles also exhibited a slight photocatalytic activity for inactivation of the bacteria (∼ 22% improvement in their antibacterial activity). Instead, the normalized antibacterial activity of the Cu nanoparticles covered by a thin oxide layer highly increased (∼ 63% improvement) in the photocatalytic process. A mechanism was also proposed to describe the better antibacterial activity of the Cu than CuO nanoparticles in dark and under light irradiation.

Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying

Abstract: This paper examines and compares the microstructure and oxidation behaviour of CoNiCrAlY coatings manufactured by the APS, HVOF and CGDS deposition techniques. The coatings microstructural features were characterized by means of SEM and XRD analyses. Coating samples were then subjected to isothermal heat treatments at 1000 °C. Oxide growth rates were obtained from a series of mass gain measurements while oxide scale compositions were determined from SEM, XRD and EDS analyses. Results obtained in this study show that the as-sprayed CGDS and HVOF coatings exhibit similar microstructures, whereas the APS coating features high levels of visible defects and oxide content. Oxidation experiments revealed low oxide growth rates for both the CGDS and HVOF coatings as a result of low porosity and oxide content. The oxide scale on the CGDS and HVOF coatings after 100 h of oxidation were composed mainly of alumina without the presence of detrimental fast-growing mixed oxides. The presence of Cr 2 O 3 and dispersed NiO was however also observed for the HVOF coating. As expected, the APS coating featured the onset of mixed oxides in the early stages of oxidation. From these results, it appears that potential improvements to the bond coat oxidation behaviour can be achieved using low-temperature processing methods such as CGDS.

Quantitative measurements of sp3 content in DLC films with Raman spectroscopy

Abstract: National Natural Science Foundation [50672079]; National Natural Science Foundation of China [60837001]

Deposition of metallic coatings on polymer surfaces using cold spray

Abstract: Current coating technologies such as plasma spray, High Velocity Oxygen Fuel (HVOF) or laser cladding involve the delivery of molten materials during the deposition process However, such techniques are not well suited to the deposition of metallic coatings on polymers and composites Cold spray (CS) has attracted much industrial interest over the past two decades In this method, a material in powder form is accelerated on passage through a converging–diverging nozzle to high speeds via a high pressure coaxial carrier gas jet The high impact kinetic energy deforms the particles, which creates effective bonding to the substrate This paper presents the results of an initial study on the potential of the CS process to produce metallic coatings on non-metallic surfaces such as polymers and composites for engineering applications Experimental and Computational Fluid Dynamics (CFD) results when spraying copper, aluminium and tin powder on a range of substrates such as PC/ABS, polyamide-6, polypropylene, polystyrene and a glass-fibre composite material are presented and analyzed

The structure and properties of chromium nitride coatings deposited using dc, pulsed dc and modulated pulse power magnetron sputtering

Abstract: The time averaged ion energy distributions and ion fluxes of continuous dc magnetron sputtering (dcMS), middle frequency pulsed dc magnetron sputtering (PMS), and modulated pulse power (MPP) magnetron sputtering plasmas were compared during sputtering of a Cr target in an Ar/N 2 atmosphere in a closed field unbalanced magnetron sputtering system. The results showed that the dcMS plasma exhibited a low ion energy and ion flux; the PMS plasma generated a moderate ion flux of multiple high ion energy regions; while the MPP plasma exhibited a significantly increased number of target Cr + and gas ions with a low ion energy as compared to the dcMS and PMS plasmas. Cubic CrN coatings were deposited using these three techniques with a floating substrate bias. The structure and properties of the coatings were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nanoindentation, microscratch and ball-on-disk wear tests. It was found that the deposition rate of the MPP CrN depositions was slightly lower than those of the dcMS depositions, but higher than in the PMS depositions at similar average target powers. The coatings deposited in the dcMS and PMS conditions without the aid of the substrate bias exhibited large columnar grains with clear grain boundaries. On the other hand, the interruption of the large columnar grain growth accompanied with the renucleation and growth of the grains was revealed in the MPP CrN coatings. The MPP CrN coatings exhibited a dense microstructure, fine grain size and smooth surface with high hardness (24.5 and 26 GPa), improved wear resistance (COF = 0.33 and 0.36) and adhesion, which are the results of the low ion energy and high ion flux bombardment from the MPP plasma.

A novel electroless plating of Ni–P–TiO2 nano-composite coatings

Abstract: A new processing concept has been developed to produce nano-structured metal-matrix composite coatings. This method combines sol-gel and electroless plating techniques to prepare highly dispersive oxide nano-particle reinforced composite coatings. Transparent TiO2 sol was added into the standard electroless plated Ni–P solution at a controlled rate to produce Ni–P–TiO2 nano-composite coatings on Mg alloys. The coating was found to have a crystalline structure. The nano-sized TiO2 particles (∼ 15 nm) were well dispersed into the Ni–P coating matrix during the co-deposition process. This technique can effectively avoid the agglomeration of nano-particles in the coating matrix. As a result, the microhardness of the composite coatings were significantly increased to ∼ 1025 HV200 compared to ∼ 710 HV200 of the conventional composite coatings produced with solid particle mixing methods. Correspondingly, the wear resistance of the new composite coatings was also greatly improved.

Investigation of wear behavior of titanium oxide films, produced by anodic oxidation, on commercially pure titanium in vacuum conditions

Abstract: The wear performances of titanium oxide films, produced by anodic oxidation, in vacuum conditions were investigated. Anodic oxidation treatments were carried out at − 3 and 40 °C temperatures using H 2 SO 4 (1.5 M)–H 3 PO 4 (0.3 M) solution and voltage of 200 V. Wear tests were performed at ambient air, pressures of 10 − 3 mbar and 10 − 6 mbar. Anodizing process produced a porous oxide layer on the surface. Although the pore size decreased with increasing process temperature, surface roughness decreased. Hardness results showed that anodic oxidation at lower temperatures produced oxide film with higher hardness. Wear rate of CP-Ti significantly decreased with anodic oxidation treatment in all wear conditions since oxide film acted like a solid lubricant. The best wear resistance was obtained from the hard-anodized samples both in ambient air and vacuum conditions.

Influence of process parameters on electrolytic plasma discharging behaviour and aluminum oxide coating microstructure

Abstract: In this study, a plasma electrolytic oxidation process (PEO) was used to produce oxide coatings on commercially pure aluminum (Al 1100) at two different current modes, pulsed unipolar and bipolar modes. Optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285 nm–800 nm) was employed to investigate the PEO plasma. The emission spectra were recorded and plasma temperature profile versus processing time was constructed using line intensity ratios method. Scanning Electron Microscopy (SEM) with energy dispersive x-ray analysis (EDS) was used to study the coating microstructure and coating cross section. It was found that the plasma discharge behavior significantly influenced the microstructure and the morphology of the oxide coatings. The main effect came from the strongest discharges which were initiated at the interface between the substrate and the coating. Through manipulation of process parameters to control or reduce the strongest discharge, the density and quality of the coating layers could be modified. This work demonstrated that by adjusting the ratio of the positive to negative pulse currents as well as their timing in order to eliminate the strongest discharges, the quality of the coatings was considerably improved.

Self-accumulated Ag nanoparticles on mesoporous TiO2 thin film with high bactericidal activities

Abstract: Antibacterial activity of sol–gel synthesized Ag–TiO 2 nanocomposite layer (30 nm) deposited on rough anatase ( a ) TiO 2 thin film (∼ 200 nm in thickness) was investigated against Escherichia coli bacteria, in dark and also in exposure to UV light. The nanocomposite thin films were transparent with a surface plasmon resonance absorption band at a wavelength of 410 nm. The metallic silver nanoparticles with an average diameter of 30 nm and fcc crystalline structure were self-accumulated on surface of a mesoporous and aqueous TiO 2 layer with a capillary pore structure having a pore radius of 3.0 nm. By adding the silver nanoparticles in the TiO 2 layer, recombination of the photoexcited electron–hole pairs in the ( a )TiO 2 thin film was delayed, while the pore structure was unchanged. Decrease in the recombination rate and accumulation of the silver nanoparticles on the film surface let the mesoporous Ag–TiO 2 /( a )TiO 2 nanocomposite thin films have excellent antibacterial activity against E. coli bacteria. It was found that the relative rate of reduction of the viable bacteria in dark (in exposure to the UV light) for the Ag–TiO 2 /( a )TiO 2 nanocomposite thin film was 3.2 × 10 –2 min –1 (26 × 10 –2 min –1 ) which was 4.6 (2.0) times greater than the corresponding value for the ( a )TiO 2 thin film. The behavior of silver ion release showed that the dominant mechanism of the release process in long time was based on water diffusion through the capillary mesoporous of the TiO 2 layer, unlike the usual diffusion of water on the surface of silver-based bulk materials. Therefore, the synthesized Ag–TiO 2 /( a )TiO 2 nanocomposite thin film can be utilized as a promising and effective bactericidal material in the future.

Residual stress evaluation at the micrometer scale: Analysis of thin coatings by FIB milling and digital image correlation

Abstract: In this report, an optimised method for residual stress determination at the microscopic scale is presented. The newly proposed approach involves incremental Focused Ion Beam (FIB) milling of annular trenches at material surface, combined with high resolution SEM imaging of a previously deposited marker pattern. Digital image correlation (DIC) analysis of the relative displacements between markers with respect to the undisturbed state provides a measure of strain relief. Results of finite element modeling show that the proposed configuration gives complete strain relief when the annular trench depth becomes comparable with the diameter of the remaining stub, thus allowing analytical calculation of the average residual stress from measured strain components. Basing on results of modeling, the experimental methodology has been developed and optimised for residual stress analysis in thin coatings. In order to cover a wide range of material properties and residual stress states, two different materials have been selected: TiN CAE-PVD coating (hard and stiff, with compressive residual stress) on WC–Co substrate, and also an Au MS-PVD coating (soft and compliant, with tensile residual stress). The procedure for the optimization of FIB milling parameters is reported. Results are validated by comparison with residual stress evaluation by X-ray diffraction and curvature measurement on the two different specifically selected PVD coatings.

A numerical model of severe shot peening (SSP) to predict the generation of a nanostructured surface layer of material

Abstract: Generation of a surface layer of material characterized by grains with dimensions up to 100 nm by means of severe plastic deformation is one of the most interesting methods to improve the mechanical behaviour of materials and structural elements. Among the ways to obtain a surface layer with this characteristic, shot peening is one of the most promising processes, since it is applicable to very general geometries and to all metals and metal alloys without high-tech equipments. Notwithstanding the fact that the ability of shot peening to obtain nanostructured surfaces by using particular process parameters (mainly high impact energy and long exposure time) is proved, deep knowledge of the correct choice of quantitative values of process parameters and their relation to the grain size and the thickness and uniformity of the nanostructured layer is still lacking. In this paper a finite element model of severe shot peening (SSP) is developed with the aim of predicting the treatment conditions that lead to surface nanocrystallization. After having assessed the accuracy of the model as regards mesh parameters and constitutive law of the material, the results are discussed and interpreted in terms of induced residual stresses and surface work hardening. A method to assess the formation of nanostructured layer of materials based on the value of the equivalent plastic strain is developed. The comparison with experimental results allow to affirm that the model is a useful tool to predict the generation of a nanostructured surface layer by shot peening and to relate the peening parameters with the treated surface layer in terms of residual stresses, work hardening, and depth of the nanostructured layer.

Inhibitor-doped sol–gel coatings for corrosion protection of magnesium alloy AZ31

Abstract: This work presents new anticorrosive coatings for the AZ31 magnesium alloy, based on hybrid sol–gel films doped with a corrosion inhibitor. The sol–gel coatings were prepared by copolymerization of 3-glycidoxypropyltrimethoxysilane and zirconium (IV) tetrapropoxide. 8-Hydroxyquinoline (8-HQ) was chosen as a corrosion inhibitor to be incorporated into the sol–gel films at two different stages of synthesis, either before or after hydrolysis of the sol–gel precursors. The effectiveness of 8-HQ for corrosion suppression on AZ31 was verified by Scanning Vibrating Electrode Technique. Electrochemical Impedance Spectroscopy was used to monitor the evolution of the substrate/film systems in the course of immersion in 0.005 M NaCl. The morphology and the structure of the sol–gel films were characterized with SEM/EDS and TEM techniques. The sol–gel films exhibit good adhesion to the metal substrate and prevent the corrosive attack during 2 weeks under immersion test. Results showed that addition of inhibitor into the sol–gel films enhances the corrosion protection of the magnesium alloy and does not lead to deterioration of the barrier properties of the sol–gel matrix.

Self cleaning TiO2 coating on polycarbonate: Surface treatment, photocatalytic and nanomechanical properties

Abstract: A developed route to form TiO 2 self cleaning coatings on polycarbonate substrates is reported. TiO 2 coatings on plastics may find widespread application in auto and construction industries if possess desired photocatalytic and mechanical properties. A chemical surface treatment method was used to create hydrophilic groups on the surface. X-ray photoelectron spectroscopy showed the treatment led to the oxidation of surface groups. TiO 2 deposition was based on wet coating using an anatase sol of TiO 2 nanoparticles of 30 nm size. The sol was synthesized using a sol–gel route. A pre-coat of peroxotitanium complex was employed to improve adhesion and inhibit the substrate degradation. The coating reduced the transparency for 10–15%. The photocatalytic activity was found linearly dependent on the thickness. The reaction rate constant for methylene blue degradation was estimated 0.024 s − 1 for films of 150 nm thickness. The mechanical properties were also improved after coating, as demonstrated by nano-indentation and nano-scratch tests. The hardness and scratch resistance were improved by 2.5 and ∼6.4 folds.

Surface characterisation of DC plasma electrolytic oxidation treated 6082 aluminium alloy: Effect of current density and electrolyte concentration

Abstract: Plasma electrolytic oxidation (PEO) is a specialised but well-developed process which has found applications in aerospace, oil/gas, textile, chemical, electrical and biomedical sectors. A novel range of coatings having technologically attractive physical and chemical properties (e.g. wear- and corrosion-resistance) can be produced by suitable control of the electrolyte as well as electrical parameters of the PEO process. Oxide ceramic films, 3 to 40 μm thick, were produced on 6082 aluminium alloy by DC PEO using 5 to 20 A/dm 2 current density in KOH electrolyte with varied concentration (0.5 to 2.0 g/l). Phase analysis (composition and crystallite size) was carried out using X-ray diffraction and TEM techniques. Residual stresses associated with the crystalline coating phase (α-Al 2 O 3 ) were evaluated using the X-ray diffraction Sin 2 ψ method. Nanoindentation studies were conducted to evaluate the hardness and elastic modulus. SEM, SPM and TEM techniques were utilised to study surface as well as cross-sectional morphology and nano features of the PEO coatings. Correlations between internal stress and coating thickness, surface morphology and phase composition are discussed. It was found that, depending on the current density and electrolyte concentration used, internal direct and shear stresses in DC PEO alumina coatings ranged from − 302 ± 19 MPa to − 714 ± 22 MPa and − 25 ± 12 MPa to − 345 ± 27 MPa, respectively. Regimes of PEO treatment favourable for the production of thicker coatings with minimal stress level, dense morphology and relatively high content of α-Al 2 O 3 phase are identified.

Tribological properties of Cr- and Ti-doped MoS2 composite coatings under different humidity atmosphere

Abstract: Solid-lubricant MoS 2 coatings have been successfully applied in high vacuum and aerospace environments. However, these coatings are very sensitive to water vapor and not suitable for applications in moist environments. In this work, Cr- and T-doped MoS 2 composite coatings were developed. The results demonstrated that these composite coatings are promising for applications in high humidity environments. MoS 2 –Cr and MoS 2 –Ti composite coatings with different Cr or Ti content were deposited on high speed steel substrate by unbalanced magnetron sputtering. The composition, microstructure, and mechanical properties of the as-deposited MoS 2 -metal composite coatings were analyzed by energy dispersive analysis of X-ray (EDX), X-ray diffraction (XRD), and nanoindentation experiments. The tribological properties of the coatings were evaluated against an alumina ball under different relative humidity atmosphere using a ball-on-disc tribometer. The MoS 2 –Cr and MoS 2 –Ti coatings showed a maximum hardness of 7.5 GPa and 8.4 GPa at a dopant content of 16.6 at.% Cr or 20.2 at.% Ti, respectively. The tribological test results showed that, with a small amount of Cr and/or Ti doping, the tribological properties of MoS 2 coatings under humid atmosphere could be significantly improved. The optimum doping level was found to be around 10 at.% for both MoS 2 –Cr coatings and MoS 2 –Ti coatings to show the best tribological properties, with both the lowest friction coefficient and wear rate. The excellent tribological properties of the MoS 2 –Cr and MoS 2 –Ti coatings with an appropriate metal doping level in moist atmosphere are found due to their ability to form stable transfer layer on the surface of the counterbody, which supplies lubrication for the contact surface.

Comparison of the tribological and antimicrobial properties of CrN/Ag, ZrN/Ag, TiN/Ag, and TiN/Cu nanocomposite coatings

Abstract: Nanocomposite coatings including CrN/Ag, ZrN/Ag, TiN/Ag and TiN/Cu with varying silver or copper contents were produced by co-deposition in a dual pulsed magnetron sputtering system. The compositions and structures of the coatings were characterised using energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM), and the physical and tribological properties were assessed by means of nanoindentation and thrust washer wear testing. Although increasing silver or copper content provided a reduction in the coefficient of friction, this was accompanied by reductions in hardness for all the coatings and wear resistance for some of the coatings. Zones of inhibition were used to determine the extent of silver ion release from the coating surfaces, and a NBT (nitro-blue tetrazolium) redox dye was used to determine the antimicrobial effectiveness of the coatings following incubation. The microorganisms tested were Pseudomonas aeruginosa and Staphylococcus aureus. For the NBT assays, significant reductions in the number of viable cells were observed with increasing Ag or Cu content, compared to the ‘pure’ nitride surfaces. Whilst no zones of inhibition were observed for S. aureus, on any of the surfaces, the diameter of the ‘kill’ zones generally increased with increasing silver content for P. aeruginosa.

Effect of laser cladding process parameters on clad quality and in-situ formed microstructure of Fe-TiC composite coatings

Abstract: Over the past decade, researchers have demonstrated interest in tribology and prototyping by the laser cladding process. In-situ laser cladding enables the formation of a uniform clad by melting the powder to form desired composition from pure powder component. In this research pure Ti, graphite, and Fe with max particle sizes of 40 μm (0.04 mm) are used for in-situ laser cladding on a steel substrate. The effects of laser parameters on the quality of an in-situ formed TiC–Fe based composite clad are investigated. Laser parameters have an important role in clad quality and crack formation. They affect the bonding between clad/substrate and cooling rate. Diverse microstructures have been detected in the clad. Finally a model is developed in order to explain the formation and morphology of TiC. The melting and solidification stages of TiC formation and matrix confirm the suggested model. TiC particles increase the clad hardness to an average of four times greater than substrate's hardness. Experimental methods such as, XRD and SEM are used for phases characterization.

The influence of ceramic particles on bond strength of cold spray composite coatings on AZ91 alloy substrate

Abstract: Al–Al 2 O 3 composite coatings were produced on AZ91D magnesium alloy substrates using kinetic metallization (KM), which is a special type of cold spray using a convergent barrel nozzle to attain sonic velocity. The effect of the volume fraction of Al 2 O 3 particles and KM spray temperatures on the microstructure, hardness of the composite coatings, the deposition efficiency, and the bond strength between the coating and substrate was studied. Results show that addition of Al 2 O 3 particles not only significantly improves the density of the coating, but also enhances the deposition efficiency to an optimum value. The bond strength of the composite coatings with the substrate was found to be much stronger than the coating itself, measured using a specially designed lug shear method. Furthermore, based on bond strength data and SEM analysis, higher Al 2 O 3 content resulted in a failure mode transition from adhesive failure to cohesive failure. This is considered a result of a competition between the strengthening of the ceramic reinforcing particles at the coating/substrate interface, and the weakening of coating cohesive strength due to an increase in the proportion of weaker Al–Al 2 O 3 bonds compared with stronger Al–Al bonds. Characterisation of the composite coating in terms of hardness, porosity and microstructure was also conducted.

The role of anions in the formation and corrosion resistance of the plasma electrolytic oxidation coatings

Abstract: Combination of KOH with each of Na 2 SiO 3 , Na 3 PO 4 and NaAlO 2 , formed three different coating solutions to produce plasma electrolytic oxidation (PEO) coatings on the surface of AM50 magnesium alloy. The surface morphology, cross section, chemical composition, corrosion resistance and structure of each of the coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS). The results showed that different anions, i.e., SiO 3 2− , PO 4 3− and AlO 2 − , influence the coating characteristics such as thickness, chemical composition and coating structure. The results showed that thicknesses of the Si-, P- and Al-coatings are 8, 4 and 1 µm, respectively. Moreover beside MgO existing in structure of all three coatings, specific phases namely Mg 2 SiO 4 , Mg 3 (PO 4 ) 2 and MgAl 2 O 4 were formed in the structure of the Si-, P-, and Al-coatings, respectively. It was revealed that usage of SiO 3 2− instead of PO 4 3− or AlO 2 − led to formation of a coating layer with better corrosion protection properties. The better performance of the Si-coating compared to P- or Al-coatings is considered to be due to the fact that the thickness, the number of open pores and the resistance of the barrier layer are formed under such optimum conditions which result in a higher corrosion resistance.

Effects of nitrogen pressure and pulse bias voltage on the properties of Cr-N coatings deposited by arc ion plating

Abstract: Cr-N coatings were deposited on 1Cr18Ni9Ti stainless steel in the pure N(2) atmosphere by arc ion plating (AIP). The relationships between deposition parameters and coating properties were investigated. X-ray diffraction showed a phase transformation from CrN+Cr(2)N+Cr-->CrN+Cr-->CrN and the CrN preferred orientation changed from (200) to (220) as N(2) pressure increased. Increasing bias voltage led to CrN preferred orientation changed from (200) to (220) and the formation of Cr(2)N. XPS results indicated that chemical composition of the coatings changed as N(2) pressure increased but it changed little with bias voltage. The lower melting point of chromium nitride formed on target surface induced the increase of macroparticles and deposition rate with increasing N(2) pressure; and bias voltage had an obvious effect on reducing macroparticles of the Cr-N coatings. Residual stresses were measured by substrate curvature technique, and the changing tendency coincided with the microhardness of the coatings. (C) 2009 Elsevier B.V. All rights reserved.

PEO coatings obtained on an Mg–Mn type alloy under unipolar and bipolar modes in silicate-containing electrolytes

Abstract: Protective surface layers with high corrosion resistance (Rp = 3.3·105 ohm cm2) and significant microhardness (H = 4.8 GPa), as compared to the substrate material, were obtained on MA8 magnesium alloy by bipolar Plasma Electrolytic Oxidation (PEO) in a silicate–fluoride electrolyte. The phase and elemental composition and morphology of the coatings were investigated. It was found that the application of the bipolar PEO mode enables one to synthesise on the alloy's surface a high-temperature phase of magnesium silicate, forsterite (Mg2SiO4) having good anticorrosion and mechanical properties.

Thermal stability of AlCoCrCuFeNi high entropy alloy thin films studied by in-situ XRD analysis

Abstract: High entropy alloys (HEAs), containing five to thirteen metallic elements, with a concentration in the range of 5 to 35 % for each element, exhibit very interesting properties (mechanical, tribological, formability, magnetism...). Their high mixing entropy promotes the formation of simple solid solutions with amorphous or nanocrystallized structure. Bulk pieces of these alloys are known to be stable at relatively high temperature (until 800°C). We study the stability of AlCoCrCuFeNi thin film at temperatures in the range 110 – 810 °C. HEA thin films are deposited by magnetron sputtering of mosaic targets. In-situ X-ray diffraction performed during annealing evidences damages of the film above 510°C depending on the initial structure (or chemical composition) of the as-deposited HEA. Energy Dispersive Spectroscopy (EDS) and Scanning Electron Microscopy (SEM) analysis carried out before and after annealing on both studied samples, show that partial evaporation of the thin film, crystalline phase transformation and chemical reaction with the substrate may take place during annealing.

Hydroxyapatite coatings for biomedical applications deposited by different thermal spray techniques

Abstract: Thermally sprayed hydroxyapatite (HAp) coatings are widely used for various biomedical applications due to the fact that HAp is a bioactive, osteoconductive material capable of forming a direct and firm biological fixation with surrounding bone tissue. Bioceramic coatings based on nanoscale HAp suspension and microscale HAp powder were thermally sprayed on Ti plates by high-velocity suspension flame spraying (HVSFS) technique and atmospheric plasma spraying (APS) as well as high velocity oxy fuel spraying (HVOF) technique. HVSFS is a novel thermal spray process developed at IMTCCC, for direct processing of submicron and nano-sized particles dispersed in a liquid feedstock. The deposited coatings were mechanically characterized including surface roughness, micro hardness and coating porosity. The bond strength of the layer composites were analyzed by the pull-off method and compared for the different spray techniques. Phase content and crystallinity of the coatings were evaluated using X-ray diffraction (XRD). The coating composite specimen and initial feedstock were further analysed by scanning electron microscope (SEM) and rheology analysis.

Comparison of calcium phosphate coatings on Mg-Al and Mg-Ca alloys and their corrosion behavior in Hank's solution

Abstract: article i nfo Calcium phosphate coatings were prepared on AZ31 and Mg-1.0Ca magnesium alloys by electrochemical deposition. Hydrogen evolution testing and potentiodynamic electrochemical technique were employed to investigate the corrosion behavior of the coated alloys in Hank's solution. The results show that the deposited coatings mainly consist of flaky brushite (DCPD, CaHPO4·2H2O) crystallites. The obvious increase in free corrosion potential (open circuit potential) and decrease in corrosion current density of the coated samples indicate that the coating significantly enhanced the corrosion resistance of the substrates. The hydrogen evolution rates of the coated samples were much lower than that of the substrates during initial immersion in Hank's solution. However, the corrosion rate of the coated samples increased rapidly once the samples suffered pit corrosion. Furthermore, the corrosion properties of the coated samples have much to do with the corrosion properties of their substrates.

Superhard nanocomposites: Origin of hardness enhancement, properties and applications

Abstract: The original finding of Veprek et al. that in nc-TiN/a-Si 3 N 4 and in nc-TiN/a-Si 3 N 4 /TiSi 2 nanocomposites, deposited under conditions which allow complete phase segregation by spinodal mechanism, the maximum hardness of ≥ 45 and > 100 GPa, respectively, is achieved when the thickness of the interfacial Si 3 N 4 is about 1 monolayer, has been recently confirmed by both experiments and theory. First principle calculations explain why the decohesion and shear strength of a TiN-SiN x -TiN sandwich is higher than that of bulk SiN x . Combined ab initio DFT calculations of shear resistance of the interfaces, their averaging according to Sachs for randomly oriented polycrystalline material to obtain tensile yield strength, Tabor's criterion, Hertzian analysis and pressure-enhanced flow stress explain in a simple way the experimentally achieved high values of hardness of > 100 GPa, in excess of diamond. Friedel oscillations of the valence charge density, originating from negative charge transfer to the strengthened SiN x interface, cause decohesion and ideal shear to occur between Ti-N bonds near that interface. The extraordinary mechanical properties of these and related quasi-binary superhard nanocomposites can be understood in terms of nearly flaw-free strong materials with no need to invoke any new mechanism of strengthening. We shall present selected examples of industrial applications of the superhard nanocomposite coatings.

The effect of shot peening on fatigue and corrosion behavior of 316L stainless steel in Ringer's solution

Abstract: Stainless steel 316L is one of the most common biomaterials utilized for producing orthopedic implants. But it has low resistance to fatigue and wear. Therefore surface treatments such as shot peening are used to modify the surface properties. In the present research, the influence of shot peening treatment on hardness, fatigue and corrosion behavior of 316L stainless steel in Ringer's solution was investigated. For this purpose, the steel specimens were shot peened for 5, 10, 15, 20 and 25 min. Hardness, fatigue and electrochemical tests were performed on each specimen before and after shot peening treatment. The open circuit potential (OCP) of the specimens, after 2 h of equilibrium time, was measured in Ringer's solution for 300 s. The cyclic potentiodynamic polarization tests were performed with 5 mV/s scan rate. According to the results, the shot peening treatment increases the surface hardness and fatigue resistance. In addition, this treatment decreases the break-down potential of the passive layer and increases the corrosion current density in shot peened specimens up to 10 min, which shows a reduction in resistance to pitting corrosion. However, the break-down potential of the passive layer begins to increase and the corrosion current density decreases at upper times. This trend continues such that even the conditions of resistance to pitting corrosion improve in comparison with un-shot peened specimens at longer times of shot peening. The morphology of the fractured surfaces of samples was investigated by scanning electron microscopy (SEM).