Effect of substrate temperature on microstructure and nanomechanical properties of Gd2Zr2O7 coatings prepared by EB-PVD technique

In the present work, gadolinium zirconate (Gd₂Zr₂O₇) coatings have been developed on Inconel-718 substrates by electron beam physical vapor deposition (EB-PVD) technique. The structural, morphological and mechanical properties as a function of substrate temperature have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), nanoindentation and scratch tests. XRD analysis revealed that the coatings showed cubic defect fluorite phase, and no secondary phase formation was observed in the coatings during deposition. The decrease in the lattice constant of the fluorite phase with increasing deposition temperature was explained on the basis of strain relaxation and vacancy concentration. Increased surface roughness of the coatings has been found with increasing substrate temperature as a result of increased crystallite size. An improved coating adhesion achieved for the coating deposited at higher substrate temperature of 973?K was confirmed by scratch test. Nanoindentation measurements indicated higher hardness (7.7?GPa) and resistance to plastic deformation and better capability to accommodate deformation energy for the coatings prepared at higher deposition temperature.     

Effects of suspension properties on the fabrication of Yb2Si2O7 coatings using electrophoretic deposition

This study examines the electrophoretic deposition of Yb₂Si₂O₇ particles on SiC substrates to produce Environmental Barrier Coatings. To prepare crack-free and homogeneous green coatings, the effect of the solvent, dispersant concentration, and pH were investigated. Ethanol provided a well-dispersed suspension and crack-free coating which was shown by sedimentation tests and microstructure analysis. The effect of the dispersant concentration was investigated with zeta potential measurement and microstructure analysis with a concentration above 0.5 g/L resulting in higher ionic strength and producing cracked and uneven coatings. The ionic strength was also associated with the powder packing density with larger indentation impressions measured for loosely packed coatings. The deposition rate depended on the suspension properties influenced coating integrity with delamination evidenced by analysing the current density drop during deposition. Sintering of the green coatings having different densities and microstructure showed their importance in the preparation of uniform and dense sintered coatings.     

Effect of plasma pretreatment on durability of sol-gel superhydrophobic coatings on laser modified stainless steel substrates

Superhydrophobic surfaces were generated on stainless steel SS 304 substrates, using a combination of physical as well as chemical modification of the surface and tested for use in biomedical applications. Nanosecond pulsed laser was used for physical modification, i.e. creating nanoscaled roughness on the substrates. An additional chemical modification was performed using fluorosilane-based sol-gel nanocomposite coatings to further improve the hydrophobicity. Presently, the key challenge that such surfaces face, is to possess a substantial durability. In this study, a surface activation technique such as plasma pre-treatment was adopted to improve the adhesion of coatings on the laser treated substrates. The coatings deposited using dip coating technique were cured at 150 °C. The surface morphology and the roughness of the processed substrates and the coated samples were characterized using Atomic Force Microscope and Scanning Electron Microscope. The wettability of the surface was monitored and evaluated throughout the study using water contact angle measurements. Weathering tests and scratch resistance measurements using a crockmeter were carried out to evaluate the durability, which revealed that the adhesion could be improved with plasma treatment of the laser textured substrates, prior to coating deposition. Maximum anti-bacterial activity of up to 90% towards the bacterial species Escherichia coli was found on the substrates coated with the fluorosilane-based superhydrophobic coatings for an exposure time of 30 min, without any addition of external anti-bacterial agents. Thus, the preliminary results obtained from the present investigation were found to be promising and were indicative of use of these surfaces for biomedical applications.     

Constrained sintering of YSZ/Al2O3 composite coatings on metal substrates produced from eletrophoretic deposition

Yttria stabilized zirconia/alumina (YSZ/Al₂O₃) composite coatings were prepared from electrophoretic deposition (EPD), followed by sintering. The constrained sintering of the coatings on metal substrates was characterized with microstructure examination using electron microscopy, mechanical properties examination using nanoindentation, and residual stress measurement using Cr³⁺ fluorescence spectroscopy. The microstructure close to the coating/substrate interface is more porous than that near the surface of the EPD coatings due to the deposition process and the constrained sintering of the coatings. The sintering of the YSZ/Al₂O₃ composite coating took up to 200 h at 1250 °C to achieve the highest density due to the constraint of the substrate. When the coating was sintered at 1000 °C after sintering at 1250 °C for less than 100 h, the compressive stress was generated due to thermal mismatch between the coating and metal substrate, leading to further densification at 1000 °C because of the ‘hot pressing’ effect. The relative densities estimated based on the residual stress measurements are close to the densities measured by the Archimedes method, which excludes an open porosity effect. The densities estimated from the hardness and the modulus measurements are lower than those from the residual stress measurement and the Archimedes method, because it takes account of the open porosity.     

Deposition behavior and characteristics of hydroxyapatite coatings on Al2O3, Ti,and Ti6Al4V formed by a chemical bath method

A simple chemical bath method was used to deposit hydroxyapatite (HA) coatings on Al₂O₃, Ti, and Ti6Al4V substrates at ambient pressure by heating to 65–95 °C in an aqueous solution prepared with Ca(NO₃)₂·4H₂O, KH₂PO₄, KOH, and EDTA. The deposition behavior, morphology, thickness, and phase of the coatings were investigated using scanning electron microscopy and X-ray diffractometry. The bonding strength of the coatings was measured using an epoxy resin method. The HA coatings deposited on the three kinds of substrates were fairly dense and uniform and exhibited good crystallinity without any additional heat treatment. A coating thickness of 1–1.8 μm was obtained for the samples coated once. By repeating the coating process three times, the thickness could be increased to 4.5 μm on the Al₂O₃ substrate. The bonding strength of these coatings was 18 MPa.     

Advances in the control of electrophoretic process parameters to tune the ytterbium disilicate coatings microstructure

Suspensions of ytterbium disilicate in isopropanol were prepared using iodine dispersant. Their zeta potential, electrical conductivity, and pH dependence with iodine concentration is detailed. Electrophoretic deposition was performed on silicon substrates at various voltages (100-200 V) and times (until 10 minutes) and the growth dynamic was investigated. It was observed that the deposited mass reaches a maximum value for [I₂] = 0.2 g/L, and the coating microstructure becomes porous at higher iodine concentrations. Current density and voltage measurements allowed to correlate this behavior to the increase of free protons concentration in the suspension. In these conditions, it was proved that porosity increases with the increase in applied voltage, and a compaction occurs as the deposition time increases. This has been related to the coating resistance increase and subsequent decrease in effective voltage in the suspension. The denser coatings (20% of porosity) were obtained in the case of suspension without iodine, at the minimum applied voltage and for the longest deposition times.     

Chemical vapor deposition of ZrN using in situ produced ZrCl4 as a precursor

ZrN is considered a promising material for high performance coatings of various tools, due to its outstanding mechanical properties. To generate ZrN layers, ZrCl₄, a precursor, is effortlessly produced by the reaction of metallic Zirconium with HCl under elevated temperature, utilizing H₂ as a carrier gas. In a subsequent CVD (Chemical Vapor Deposition) reactor the ZrCl₄ reacts with NH₃, forming ZrN coatings. By varying the experimental conditions, such as the H₂ and NH₃ gas flow, as well as addition of N₂ to the reaction gas, the influence on coating thickness, surface morphology and crystal structure of the generated coatings was investigated. Furthermore, the effects of various deposition temperatures in addition to positional differences of the hardmetal samples in the coating reactor were explored. Ultimately, the generated samples were analyzed by evaluation of coating thickness, light optical microscopy as well as SEM, EDX and XRD measurements.     

In vitro assessment of zinc apatite coatings on titanium surfaces

In this paper, coatings of hydroxyapatite partially substituted with zinc (ZnHA) were produced on titanium substrates by a two-step hydrothermal process using a precursor solution rich in calcium, phosphate and zinc. Activation of titanium surfaces was performed by oxidation with an acidic HF/HNO₃ solution. The coated substrates were then converted into HA by immersion in an alkali 0.1 M NaOH solution. The ZnHA samples were characterized by several techniques and their in vitro behavior was studied in comparison to hydroxyapatite (HA) and titanium (Ti-control) samples. A uniform and homogeneous calcium-deficient carbonate apatite coating was obtained for all samples, both doped and undoped with zinc. The percentage of zinc incorporated in the coatings is 7 at%, and the Ca/P ratio is 1.61(±0.01) for both types of samples, suggesting that Zn is incorporated substitutionally, replacing Ca atoms into the HA structure. The incorporation of Zn in the HA structure changed the crystals morphology, reduced crystals sizes and decreased the deposition rate showing that zinc is an inhibitor of the growth of HA crystal. X-ray diffraction showed that HA is the single crystalline phase present after alkali treatment. The coating adhesion strength was evaluated in terms of the critical load (Lc) obtained from scratch tests and no significant difference was found between the two tested groups, indicating the good adhesion of ZnHA to Ti substrates. The in vitro response of human osteoblasts (HOB) exposed to the surfaces of HA and ZnHA coatings was evaluated. The results of Live/Dead tests showed cell viability for all samples surfaces, but the adhesion and proliferation tests showed that ZnHA samples presented better adhered and spread cells compared with HA. ZnHA coatings presented cells with elongated or polygonal shapes and clearly more spread than HA. Quantitative analysis showed that there was a significantly higher number of cells adhered to ZnHA coatings compared to HA, indicating the zinc incorporation stimulates osteoblast proliferation.     

Deposition mechanisms of columnar structured La2Ce2O7 coatings via plasma spray-PVD

Recently, a columnar structured La₂Ce₂O₇ (LC) coating was successfully produced via plasma spray-physical vapor deposition (PS-PVD) but in a relatively narrow processing window. In this paper, spray distances were adjusted in suitable regions of columnar structures based on our previous work attempting to precisely control coating microstructures. The columnar coatings were investigated to be regularly distributed along the axial (spraying) and radial directions of the plasma jet, and can be divided into three types including PVD-like, Closely-packed and Particle-concomitant, respectively. The PVD-like coatings deposited mainly from vapor phase distribute at relatively short spray and radial distances, while the Closely-packed ones distribute at long radial distances (periphery of the samples). In addition, the Particle-concomitant ones distribute at long spray distances. The related deposition mechanisms are discussed and a deposition model is built to provide an additional understanding of PS-PVD.     

Mechanical and erosion properties of CaCO3‐EMAA thermal sprayed coatings

Polymer‐ceramic composite coatings manufactured from calcium carbonate and ethylene‐methacrylic acid copolymer (EMAA) were prepared via a thermal spray process employing different CaCO₃ filler sizes (average size of 2.8, 9 or 36 μm) and loading levels from about 2.5 to 7 wt%. The optimum filler feeding characteristics, deposition efficiency and deposition rate were obtained with a 36 μm sized CaCO₃. Tensile properties, peel strength, and the erosion resistance of a pure EMAA and CaCO₃‐EMAA composite coatings were investigated. It was found that the tensile strain at fracture of the composite coating decreased with the addition of filler to a greater degree than that observed in compression‐molded polymer composites. This is attributed to an inhomogenous distribution of the filler, with more being concentrated at the boundaries of the deposited polymer particles, thereby establishing a rigid framework within the coating. Only a small filler content is necessary to establish large changes in the mechanical properties of the coating. The peel strength of a composite coating decreases with filler content, both on a mild steel substrate and a previously sprayed polymer coating. Bonding to the latter is significantly higher and offers a possibility as a bonding layer between substrates and composite coatings. The coefficient of friction is lowered with the addition of a filler. Erosion testing has shown that the erosion resistance of PF111 is little improved overall with filler addition, although some increase is found for filler contents less than 5 vol%. Polym. Eng. Sci. 44:1448–1459, 2004. © 2004 Society of Plastics Engineers.     

An improved laboratory reattachment method for the rapid assessment of adult barnacle adhesion strength to fouling-release marine coatings

Modifications have been made to the previously described adult barnacle laboratory reattachment method to enhance and improve the overall utility of this technique for rapidly assessing the efficacy of novel fouling-release marine coating technologies. These modifications include the use of an immobilization template to secure barnacles onto the coating surfaces during the underwater reattachment process, the development of a semi-automated push-off device to enable consistent and reproducible force gauge measurements and the implementation of a software tool to measure the diameter of barnacle base plates for adhesion strength calculations. A series of experimental siloxane?Cpolyurethane and control coatings were evaluated with both the original and improved laboratory reattachment methodologies. Significantly higher adhesion strengths were obtained on these coatings using the improved reattachment method. Furthermore, only the improved reattachment method was able to discern significant differences in the performance of the siloxane?Cpolyurethane coatings based on differences in compositional components. In this regard, the siloxane?Cpolyurethane coatings containing the poly-caprolactone end groups attached to the poly(dimethylsiloxane) (PDMS) backbone exhibited significantly higher reattached barnacle adhesion strengths than the aminopropyl-terminated PDMS containing coatings. It was also shown that the utilization of barnacles with 5?C6?mm base plate diameters, rather than 7?C8?mm diameters, significantly enhanced the strength or tenacity of adhesion to the surface of the control coatings. The results of the improved laboratory reattachment evaluation of experimental siloxane?Cpolyurethane and control coatings were in good agreement with barnacle adhesion measurements obtained for the same coatings with static ocean immersion testing in the field.     

Deposition of silicon oxide hard coatings by low‐temperature radio‐frequency plasmas

In this study, the deposition of silicon oxide (SiOx) hard coatings on polycarbonate (PC) substrates was attempted with low‐temperature radio‐frequency (RF) plasmas from tetramethyldisiloxane (TMDSO) with the addition of oxygen. The coating uniformity and deposition rate were investigated in terms of substrate size, glow uniformity, and RF power input. The hardness of the resulting SiOx plasma coatings was examined by the ASTM pencil hardness test method. The hardness of the resulting SiOx plasma coatings was mainly determined by the TMDSO–O₂ ratio in the plasma gas mixture. Ultraviolet–visible transmission spectra showed that these plasma coatings were transparent in the visible light region. Fourier transform infrared–attenuated total reflection analysis results indicated that the resulting SiOx plasma coatings were inorganic in nature. The interfacial adhesion, which is a common problem in the deposition of hard protective coatings on polymeric substrates, was also significantly improved by the deposition of an ultrathin plasma polymer interlayer from TMDSO before the deposition of the SiOx plasma coatings on the PC substrates. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Deposition of electrically conductive zirconium monoxide via plasma spray-physical vapor deposition

A condensed phase of zirconium monoxide (ZrO) was detected in YSZ (Zr_0.92Y_0.08O_2-δ) coatings deposited using plasma spray-physical vapor deposition. The rapid cooling rate of this process can result in the condensation of nonequilibrium states that can be kinetically trapped in the coatings. The columnar microstructure contained a mixture of YSZ, ZrO_2, and ZrO phases. The ZrO phase was expected to be conductive based on density functional theory calculations, and preliminary impedance measurements were performed that supported this prediction. When heated in an oxygen-containing environment, the ZrO phase remains in the coating until ∼450 K, at which point it disappears quickly, as confirmed by X-ray diffraction and thermogravimetric methods. The loss of ZrO in the coating was also linked to a loss in electrical conductivity. However, it was shown that this phase can persist at elevated temperatures of ∼1000 K in vacuum or inert environments for at least 100 h.     

Microstructure and mechanical properties of flame-sprayed PEEK coating remelted by laser process

Poly-ether-ether-ketone (PEEK) is one of the high-performance thermoplastics. It is being increasingly used for many industrial applications due to its excellent properties. In this paper, a flame spraying technique is used to deposit PEEK coating on 304L stainless substrates. CO₂ and Nd:YAG laser treatments are chosen to remelt the as-sprayed polymer coating to get a dense coating. The microstructures of the as-sprayed and remelted coatings are characterized by SEM and XRD. The results show that both CO₂ and Nd:YAG lasers are suitable for densifying the PEEK coating on stainless substrate. However, the remelted coatings present different crystalline structure due to their laser processing parameters. Hardness measurements, tribological and scratch tests are conducted to characterize the mechanical properties of remelted coating. The coatings’ mechanical properties are correlated with their structures.     

Strongly adherent Al2O3 coating on SS 316L: Optimization of plasma spray parameters and investigation of unique wear resistance behaviour under air and nitrogen environment

Plasma spray deposition of Al₂O₃ is a well-established technique for thick ceramic coatings on various substrates to shield them from corrosion and wear. Owing to its high hardness, aluminum oxide is known to protect stainless steel substrates from wear. However, the plasma process requires optimization for desired coating thickness and adhesion strength. It is also necessary to understand the sensitivity of friction and wear resistance of the deposited coating on exposed environment for evaluation of service life. The study offers comprehensive investigation on plasma process parameters for the development of strongly adherent aluminium oxide coatings on SS 316L substrate. Impact of environment like dry air and dry nitrogen on tribological properties of the coatings was also investigated. Dense adherent coatings of alumina could be deposited on SS 316L at a plasma power of 20 kW with an intermediate bond coat of NiCrAlY to enhance the adhesion properties. The effects of stand-off distance and bond coat thickness on adhesion strength were additionally examined. Further, the coatings were characterised for phase composition, microstructure, microhardness and wear resistance potential. Reciprocating wear tests of the coatings were carried out using ball on disc reciprocating tribometer at different loading conditions (5, 10 and 15 N) at constant (5 Hz) sliding frequency. Unlike the coefficient of friction (COF), wear volume was found to increase with an increase in normal load. These adherent coatings revealed promising properties for the applications where the tribological failure of SS 316L in dry air or dry nitrogen environment is to be controlled.     

Protective properties of cataphoretic epoxy coating on aluminium alloy AA6060 modified with electrodeposited Ce-based coatings: Effect of post-treatment

Ce-based conversion coatings (CeCCs) are a promising alternative to toxic chromate coatings on the metal substrates. In this work the CeCCs were electrodeposited on aluminium alloy AA6060 from aqueous solution of Ce(NO₃)₃ at different potentials (−0.95 V, −1.2 V and −1.4 V). Effect of deposition potential and post-treatment in the phosphate solution on morphology and protective properties of CeCCs with top cataphoretic epoxy coating was studied. To assess the differences between the protective systems, originating from the different CeCCs pre-treatments, electrochemical impedance spectroscopy (EIS), polarization measurements, AFM and SEM/EDS analysis were used. The EIS study was undertaken to follow the evolution of corrosion behaviour of epoxy coating/CeCCs protective systems over prolonged time of exposure to the chloride environment (3 wt.% NaCl). Results suggest significantly improved corrosion stability of epoxy coating on AA6060 with as-deposited CeCCs sub-layers with respect to the same epoxy coatings with phosphate post-treated CeCCs. The far best protective properties, i.e., the greatest value of pore resistance and the lowest value of corrosion current density were provided by the epoxy coating/CeCC protective system with CeCC deposited at −1.2 V and without post-treatment.     

Adhesion of extrusion‐coated polymer sealing layers to a fiber‐based packaging material with an atomic layer deposited aluminum oxide surface coating

Adhesion of extrusion‐coated polymer sealing layers on an atomic layer deposited (ALD) aluminum oxide (Al₂O₃) surface coating was investigated with a view to gain information on the applicability of ALD deposited barrier layers in fiber‐based packaging materials. The polymers used for the sealing layer were low density polyethylene (LDPE), polyethylene terephthalate (PET), and polylactide (PLA). They were extrusion‐coated onto the ceramic side of paper/PET/Al₂O₃ substrates, where the Al₂O₃ layer was a few tens of nanometers thick. According to the results, good adhesion was obtained for LDPE coating, whereas the other coatings showed a considerable lack of adhesion. Presumably, the oxidation faced by LDPE in the air gap of the extrusion‐coating process was able to create an extensive number of reactive sites that strongly bonded with the hydroxyl groups on the oxide surface of the substrate. With the PET and PLA coatings, such oxidation did not occur and the adhesion obtained remained at a relatively poor level. With all of the coatings, the adhesion levels were improved using corona discharge equipment as a pretreatment prior to the extrusion‐coating process. POLYM. ENG. SCI., 52:1985–1990, 2012. © 2012 Society of Plastics Engineers     

Comparison of test methods estimating the stiffness of ultrathin coatings

A key engineering parameter of thin coatings is their stiffness. Stiffness characterization of ultrathin coatings with a nanometer scale thickness is experimentally challenging. In this work, three feasible methods have been used to estimate the Young’s modulus of metal coatings on polymer films. The methods are: (1) nanoindentation, (2) strain-induced elastic buckling and (3) peak-force measurements integrated in atomic force microscopy. The samples were prepared by atomic layer deposition of TiO₂ (6 and 20 nm thick) and mixed oxides of TiO₂ and Al₂O₃ (4 and 20 nm thick). The differences in estimated Young’s modulus are interpreted in terms of the underlying assumptions and test conditions. Their specific advantages and drawbacks are also compared and discussed. In particular, the nanoindentation necessitates a sufficiently sharp indenter tip to make localized measurements dominated by the coating. The strain-induced elastic buckling method is simple in practice, but showed a large scatter due to variation in local coating thickness and irregular deformation patterns. The stiffness characterization using atomic force microscopy gave the most consistent results, due to a sharp tip with a radius comparable to the thinnest coating thickness. All methods gave a higher Young’s modulus for the TiO₂ coating than for the mixed oxide coating, with a variation within one order of magnitude between the methods.     

Oxidation protection of graphite materials by single-phase ultra-high temperature boride modified monolayer Si-SiC coating

To improve the oxidation resistance of Si-SiC coating, single-phase ultra-high temperature boride (ZrB₂ or TaB₂) modified Si-SiC coating was designed and established on graphite substrates by combination of dipping and reactive infiltration process. ZrB₂ or TaB₂ phase was introduced in Si-SiC coating by directly mixing raw materials and phenol formaldehyde resin in the slurry, and then the ZrB₂-SiC-Si and TaB₂-SiC-Si coatings were fabricated on the graphite samples by dipping-curing, pyrolysis, and siliconizing. The crystalline phases and microstructure of the as-obtained multiphase coatings were investigated by X-ray diffraction analysis and scanning electron microscopy. The interrupted oxidation tests from room-temperature to 1500?°C were conducted to assess the anti-oxidation property of the prepared coatings. After 1200?h of oxidation at 1500?°C in air (30 times thermal cycles), the mass losses of the graphite substrates coated with ZrB₂-SiC-Si and TaB₂-SiC-Si coatings were 0.086% and 0.537%, respectively, and the high-temperature stability of the modified coatings was greatly improved compared to the Si-SiC coating. The excellent anti-oxidation performances of the compound coatings were attributed to the compact structure of the coatings and the formation of compound oxide layers covering on the surfaces. The compound Zr-Si-O and Ta-Si-O films possessed low oxygen diffusion rate and appropriate viscosity, which can provide appreciable oxidation protection for the internal coatings, thus obtaining the excellent oxidation and spallation resistance property.     

Alumina-silica composite coatings on graphite by CVD at 550°C

Alumina-silica composite coatings were prepared on the surface of graphite paper by chemical vapor deposition using AlCl₃/SiCl₄/H₂/CO₂ as precursor in the temperature range of 300 to 550°C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the phase composition and the microstructure of the coating, respectively. The results indicated that a dense, uniform, and adherent alumina-silica composite coating can be prepared on graphite paper substrate by chemical vapor deposition at 550°C. Alumina-silica composite coating is composed of particles or nodules of varying size. Each particle is often composed of a number of finer particles. The phases of the 550°C composite coating include γ-alumina and amorphous silica. The elemental chlorine content in the composite coating decreases with increasing deposition temperature. The surfaces of the alumina-silica composite coatings are affected by deposition temperature. There are some obvious micro-cracks in the 300°C composite coating, which are attributed to a mismatch of the coefficient of thermal expansion between composite coating and graphite paper. The 550°C alumina-silica composite coating can be completely turned into mullite after heat-treatment at 1350°C for 0.5 hr in argon atmosphere.