Effect of addition of surface modified nanosilica into silica–zirconia hybrid sol–gel matrix

An organic–inorganic hybrid sol (MZ) comprising a methacrylate functionalized silane matrix (M) and zirconium-n-propoxide (Z) was prepared using sol–gel technique. Two methodologies were adopted to modify the hybrid sol for generating nanocomposite coatings viz., (a) addition of acrylic surface modified silica nanoparticles (N) of diameter ～20 nm to the sol to enhance their compatibility with the hybrid sol–gel matrix and (b) in-situ formation of a three dimensional silica network by addition of tetraethoxy silane (T) to the sol MZ. In the first methodology, the sols were prepared with six different weight ratios of the nanoparticles to the sol, i.e. 0, 0.01, 0.05, 0.1, 0.25 and 1 which were labelled as MZ+Nx where x=0, 1, 2, 3, 4 and 5 respectively. The prepared sols were dip coated on 100 mm×100 mm polycarbonate substrates followed by thermal curing at 130 °C. The coatings were characterized for their mechanical properties like pencil scratch hardness, scratch resistance using scratch tester, nanoindentation hardness, and abrasion resistance as well as visible light transmittance. FT-IR studies were also carried out on heat-treated gels derived from the sols. A maximum pencil scratch hardness of 3H was obtained for the MZ+T coatings and these coatings withstood a critical load of 4.3±0.7 N before failure during scratch test. The maximum nanoindentation hardness of 3.8±0.01 GPa was obtained for the MZ+N5 coatings. The abrasion resistance of MZ+T coatings was higher when compared to MZ+N0 and MZ+N5 coatings. The scratch and nanoindentation hardness were seen to be better for an in-situ formed –Si–O–Si– network in the hybrid sol when compared to those obtained from coatings generated by external addition of acrylic surface modified silica nanoparticles. The difference in properties was attributed to the level of interaction between the nanoparticles and hybrid sol–gel matrix.     

Effect of functional groups (methyl,phenyl) on organic–inorganic hybrid sol–gel silica coatings on surface modified SS 316

Organic–inorganic hybrid sol–gel based silica coatings derived from hydrolysis and condensation of organically modified silane precursors like phenyltrimethoxysilane and methyltriethoxysilane along with tetraethoxysilane were deposited on different surface pre-treated (as-cleaned, plasma-treated, shot-blasted) SS 316 grade stainless steel substrate, using dip coating technique. The coatings were heat treated at 150 °C for 2 h in air. The pre-treated surfaces were characterized using X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. The water content of the sols was determined by Karl Fischer titration to evaluate the degree of completion of hydrolysis and condensation reactions. Cured coatings were characterized to evaluate thickness, water contact angle, pencil scratch hardness, gloss, and shrinkage in coating thickness. Impact test was carried out on pigmented coatings derived from sols synthesized using the two silane precursors. The corrosion resistance and water durability tests were carried out to compare the coatings derived from using different precursors and different surface pre-treatments. The corrosion tests were carried out for 1 h and 24 h exposure to a 3.5% NaCl solution by electrochemical polarization measurements. It was found that coatings from methyl substituted organically modified alkoxysilane exhibited better hydrophobicity, scratch hardness, impact resistance and barrier properties with respect to corrosion, when compared to those derived from phenyl substituted trialkoxysilane. The difference in performance of coatings was explained on the basis of difference in hydrolysis and condensation rates between the two organically modified silane precursors used for the sol synthesis.     

Conversion behaviour and resulting mechanical properties of polysilazane-based coatings

Polymer-derived ceramics exhibit a convenient route for the processing of low-dimensional ceramics like coatings or fibres. In previous investigations unfilled and composite coatings have been developed using ammonolysed bis(dichloromethylsilyl)ethane (ABSE) or perhydropolysilazane (PHPS) as precursors and BN, ZrO₂ or glass particles as filler materials. The coating systems provide excellent corrosion and oxidation resistance to underlying metals. This paper reports on the effect of the precursor system and the pyrolysis parameters on the conversion behaviour, shrinkage and mechanical properties, including hardness and Young's modulus, of ABSE- and PHPS-based coatings. Therefore the crosslinking and pyrolysis behaviour as well as the mechanical properties of the coatings were investigated up to pyrolysis temperatures of 1000 °C in nitrogen and in air by ATR-IR, SEM, profilometry and nanoindentation measurements. The coatings pyrolysed at 1000 °C in nitrogen, have hardness values of 13 GPa and Young's moduli up to 155 GPa.     

Investigation of the tribological and biomechanical properties of CrAlTiN and CrN/NbN coatings on SST 304

This study examines the influence of thin layer coatings of CrAlTiN and CrN/NbN, deposited via physical vapor, on the biocompatibility, mechanical, tribological, and corrosion properties of stainless steel 304. The microstructure and morphology of the thin CrAlTiN and CrN/NbN layers were characterized by scanning electron microscopy (SEM), EDX, and X-ray diffraction. The pin on disc wear test was performed on bare and metal-nitride coated SST 304 under a 15 N load at 60 rpm and showed that the wear rates of the thin CrAlTiN and CrN/NbN film coatings were lower than the bare substrate wear ratio. The coefficients of friction (COFs) attained were 0.64, 0.5, and 0.55 for the bare substrate, CrN/NbN coating, and CrAlTiN coating, respectively. Nano indentation tests were also performed on CrAlTiN-coated and CrN/NbN-coated SST 304. The nanohardnesses and Young's moduli of the coated substrates were 28 GPa and 390 GPa (CrN/NbN-coated) and 33 GPa and 450 GPa (CrA1TiN-coated), respectively. For comparison, the nanohardness and Young's modulus of the uncoated substrate were 4.8 GPa and 185 GPa, respectively. Corrosion tests were conducted, and the behaviors of the bare and metal nitride-deposited substrates were studied in CaCl₂ for seven days. The corrosion Tafel test results showed that the metal-nitride coatings offer proper corrosion resistance and can protect the substrate against penetration of CaCl₂ electrolyte. The CrN/NbN-coated substrates showed better corrosion resistance compared to the CrAlTiN-coated ones. In evaluating the biocompatibility of the CrAlTiN and CrN/NbN coatings, the human cell line MDA-MB-231 was found to attach and proliferate well on the surfaces of the two coatings.     

The protective properties of epoxy coating electrodeposited on Zn–Mn alloy substrate

The epoxy coating was cataphoretically deposited on steel and steel modified by electroplated Zn–Mn alloy of different chemical contents. The samples were immersed in 0.5 mol dm⁻³ NaCl solution for 60 days. The electrochemical impedance spectroscopy (EIS) analysis showed that the values of pore resistance for epoxy coating on steel and Zn–Mn alloy with 16 at.% Mn were two orders of magnitude higher, while the capacitance values were two orders of magnitude lower than those for the epoxy coating on Zn–Mn alloy substrates with 5 and 8 at.% Mn. It was assumed that the main reason for such a difference was metallic substrate dissolution during cataphoretic deposition, due to high pH (12.9). This assumption was supported by energy dispersive X-ray spectrometry (EDS) measurements showing that the amount of released Zn in epoxy coatings decreased as Mn percent in the Zn–Mn alloys increased. In addition, Zn–Mn alloy coatings on steel, as well as bare steel, were immersed in 0.1 mol dm⁻³ NaOH solution, pH 12.9, simulating conditions during cataphoretic deposition, and polarization resistance measurement in this solution indicated that Mn inclusions in Zn–Mn alloy substrate prevent Zn dissolution in alkaline medium.     

Formulation and performance evaluation of hydroxyl terminated hyperbranched polyesters based poly (ester–urethane–urea) coatings on mild steel

A low molecular weight, anticorrosive hyperbranched poly (ester–urethane–urea) [HB-P(EUU)] coatings were formulated using 2nd generation hydroxyl terminated hyperbranched polyesters (OH–HBPEs), isophorone di-isocyanate (IPDI) as a cross linking agent and dibutyltin dilaurate (DBTDL) as a catalyst with certain additives. First, NCO terminated prepolymers (HBPEUs) were formulated by reacting OH–HBPEs with IPDI at NCO:OH ratio of 1.1:1 for 4 h at 70–80 °C, then HBPEUs were mixed with DBTDL and various additives and finally coated on pretreated cold rolled mild steel (MS) substrates by dip coating method. Before applying on MS substrates, viscosity and volume solid of coatings were measured. The molecular structure of HBPEUs was characterized by ATR-FTIR and ¹H NMR analysis. Surface morphology of coated panels was characterized by atomic force microscopy (AFM) and found that coating components were homogeneously distributed and surface was smooth and crack free. Performance of coated substrates was evaluated by various tests such as cross hatch and pull off adhesion, abrasion resistance, scratch resistance, impact resistance, flexibility, and pencil hardness. UV stability of coated substrates was evaluated by UV-whether-o-meter and corrosion resistance property was evaluated by salt spray, humidity, polarization and electrochemical impedance (EIS) test. Results were also compared with polyurethane coating based on linear polyester. HB-P(EUU) coatings showed excellent enhancement in mechanical, durability as well as corrosion resistance properties than their linear counterpart.     

Synthesis and characterization of CuO-hybrid silica nanocomposite coatings on SS 304

Pure and CuO-dispersed hybrid silica nanocomposite coatings were generated using sols synthesized from acid catalyzed hydrolysis and condensation of n-propyl trimethoxysilane and tetraethoxysilane in combination with copper nitrate. Coatings were initially deposited on soda lime glass substrates by dip coating followed by heat treatment at 150, 250 and 350 °C for 2 h in air and characterized. Coatings were subsequently deposited by dip coating on stainless steel 304 substrates. An optimized heat treatment temperature of 250 °C was chosen based on the contact angles of coatings on soda lime glass substrates and results of thermogravimetric/differential thermal analysis on the dried gels obtained from the sol synthesized from the combination of n-propyl trimethoxysilane and tetraethoxysilane. Gels heat-treated at 250 °C were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy for crystallinity. Characterization of the coatings was carried out with respect to thickness, water contact angle and adhesion. Corrosion testing of coatings on SS 304 was studied by potentiodynamic polarization measurements and electrochemical impedance spectroscopy after 1 h and 24 h exposure to 3.5% NaCl. The corrosion resistances of CuO-dispersed hybrid silica coatings after 1 h and 24 h exposure to 3.5% NaCl solution were higher than that of pure hybrid silica coatings, both of which had thicknesses ranging from 140 nm–200 nm.     

Analysis of instrumented scratch hardness and fracture toughness properties of laser surface alloyed tribological coatings

The mechanical characteristics of ceramic matrix composite (CMC) coatings are widely different from the same materials in bulk form or the individual constituents and are very important to be assessed to carry out application oriented studies on CMC coatings with novel compositions. In the present work, a composite coating of TiB₂, TiN and SiC is fabricated in-situ through a combination of high temperature chemical reaction and laser surface alloying. The formation of the surface layer is due to the laser-assisted chemical reaction followed by laser melting. A mixture of TiO₂, SiO₂, hBN and graphite in stoichiometric proportions is used as the precursor for the chemical reaction. The presence of all the reaction products in the CMC coatings developed is confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A thorough evaluation of various mechanical properties achieved more insight into the CMC coatings developed. Hardness and fracture toughness of the coatings are measured with a scratch tester. The property evaluations are performed in a similar way for two more coatings fabricated with precursor mixtures containing more than a stoichiometric amount of SiC and hBN respectively. For comparison, a number of composites fabricated through various other routes are characterized afresh with the same set of techniques. Coatings formed with SiC in precursor show higher values of scratch hardness (14.37 GPa), microhardness (24.37 GPa) and fracture toughness (6.63 MPa-m^1/2).     

Nanoindentation,AFM and tribological properties of thin nc-WC/a-C Coatings

Instrumented indentation, AFM (atomic force microscopy) and tribological studies were performed on PE CVD (Plasma Enhanced Chemical Vapor Deposition) nanocomposite WC–C coatings to investigate the effects of surface roughness on the reliability of nanoindentation data and the possibilities of different AFM modes in nanomechanical testing, which can be used as a feedback to optimize deposition technology from the viewpoint of their mechanical properties. It was confirmed that surface roughness below 30 nm is necessary to keep the scatter of indentation modulus, E_IT, and hardness, H_IT, below 15%. PF QNM (Peak Force Quantitative NanoMechanical) mode was successfully applied for qualitative mapping of the elastic modulus of coatings with the stiffness above 300 GPa. LFM (lateral force microscopy) mode showed only weak contrast and quantitative measurements in both AFM modes require precise calibration. Coefficients of friction of the studied WC–C coatings were below 0.2 at RT, but increased to 0.7–0.8 at 450 °C due to the formation of a transfer film. Optimization of the deposition conditions based on nanoindentation resulted in the increase of E_IT from ～220 GPa to 350 GPa and H_IT from ～17 GPa to ～29 GPa.     

Multilayer organic–inorganic coating incorporating TiO2 nanocontainers loaded with inhibitors for corrosion protection of AA2024-T3

Organic–inorganic multilayer coating containing organically modified silicates, epoxy resins and TiO₂ nanocontainers loaded with 8-hydroxyquinoline were produced on AA2024-T3 substrates via dip coating method. The parameters of the curing temperature and time were optimized via variation in a widespread range in order to realize coatings with best anticorrosive properties. Curing temperature at 110 °C for 24 h presented the best anticorrosive behavior. The morphology of the coating was examined via scanning electron microscopy. The composition of the coatings was determined by energy dispersive X-ray analysis and Fourier transform infra-red spectroscopy. Furthermore, the coatings were exposed to corrosive environment and evaluated by electrochemical impedance spectroscopy. We demonstrate that the presence of loaded TiO₂ nanocontainers enhances the anticorrosive properties of the coatings; specifically, the total impedance values were increased about two orders of magnitude compare to the bare substrate after 400 h of exposure to aggressive environment.     

Synthesis of nanodiamond-reinforced aluminum metal matrix composites using cold-spray deposition

A dense nanodiamond–aluminum (ND–Al) composite coating was successfully produced by low pressure cold spray (CS) deposition of ball-milled powders containing 10 wt% ND. High-energy ball milling is a feasible means for the synthesis of composite feedstock powders as it provides excellent control over particle size distribution, crystal size, and the dispersion of ND agglomerates. The resulting CS coatings were characterized with respect to deposition efficiency, particle velocity and mechanical properties. It was found that the CS deposition produced dense, ND–Al composite coatings with increases in both hardness and elastic modulus as compared to the feedstock powders. The coating hardness of the 0.5 h-milled ND–Al composite that has the highest DE (14.2%) in ND–Al composites is 3.02 GPa, an 175% increase over the pristine as-received Al (1.10 GPa). The highest elastic modulus of the composite coatings is 98.3 GPa, a 51.5% increase over the as-received Al powder.     

Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition

Plasma spray physical vapor deposition (PS-PVD) was used to deposit yttria stabilized zirconia (YSZ) coatings with different columnar morphologies by varying the spray distance. Although similar quasi-columnar structures were formed at the spray distances of 600 mm and 1400 mm, the formation mechanisms of particles in the coatings were different. Besides, an electron beam physical vapor deposition (EB-PVD) like columnar coating out of pure vapor was deposited at a spray distance of 1000 mm and the columnar consisted of elongated nano-sized secondary columns. The hardness and Young׳s modulus of the coatings were investigated. Compared to the other two quasi-columnar structures, the EB-PVD like columnar coating exhibited higher hardness (~9.0 GPa ) and Young׳s modulus (~110.9 GPa), mainly due to its low porosity and defect.     

Non-fluorinated,room temperature curable hydrophobic coatings by sol–gel process

Non-fluorinated hydrophobic silica surfaces were generated on soda lime glass (SLG) substrates using hexamethyldisilazane (HMDS) as a surface modifying agent. Silica coatings were fabricated by dip coating of a sol derived from base catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS). Two methodologies were adopted to generate the hydrophobic surface; one where the hydrophilic silica coated surface was treated by immersion into different concentrations of alcoholic solutions of HMDS varying from 2.5 wt% to 15 wt%. In the other method, HMDS was directly added to a mixture of TEOS, water, ethanol, and ammonium hydroxide and coatings were deposited using this sol by dip coating and spray coating. Water contact angles (WCA) were measured to study the effect of HMDS treatment times and concentrations on hydrophobicity in the first case, and in the second case, WCA were measured for dip and spray coated samples. UV–visible transmission, scratch resistance, and thermal stability of the coatings were determined. The WCA increased from 66 ± 2° to 125 ± 4° after the treatment of the silica coatings with HMDS. In case of coatings generated from direct addition of HMDS to silica sol, WCA varied from 145 ± 2° to 166 ± 4° for dip and spray coated surfaces respectively. Surface morphology was studied to explain the difference in hydrophobicity of coatings generated using the two methods.     

Mechanical and electrical properties of micron-thick nitrogen-doped tetrahedral amorphous carbon coatings

The effect of nitrogen doping on the mechanical and electrical performance of single-layer tetrahedral amorphous carbon (ta-C:N) coatings of up to 1 μm in thickness was investigated using a custom-made filtered cathode vacuum arc (FCVA). The results obtained revealed that the hardness and electrical resistance of the coatings decreased from 65 ± 4.8 GPa (3 kΩ/square) to 25 ± 2.4 GPa (10 Ω/square) with increasing nitrogen gas ratio, which indicates that nitrogen doping occurs through substitution in the sp² phase. Subsequent AES analysis showed that the N/C ratio in the ta-C:N thick-film coatings ranged from 0.03 to 0.29 and increased with the nitrogen flow rate. Variation in the G-peak positions and I(D)/I(G) ratio exhibit a similar trend. It is concluded from these results that micron-thick ta-C:N films have the potential to be used in a wide range of functional coating applications in electronics.     

Protective nature of nano-TiN coatings shaped by EPD on Ti substrates

The hardness and corrosion resistance of TiN coatings, processed by Electrophoretic Deposition (EPD) to cover polished and unpolished Ti substrates, have been evaluated. A deposition time of 5 min has been enough to obtain a cohesive layer of 7–8 μm in thickness. The coatings were thermally treated in vacuum atmosphere at 1200 °C for 1 h with heating and cooling rates of 5 °C min^?1. The surfaces have been covered homogeneously optimizing the properties of the Ti substrates. Uniform and dense TiN coatings have been obtained onto polished substrates, while on unpolished Ti the nitrogen diffuses toward the substrate, moderately dissolving TiN coating. The nanohardness values of the polished samples have been increased from 2.8–4.8 GPa up to 6.5–8.5 GPa. Besides, the corrosion current density has been reduced more than one order of magnitude obtaining a protective efficiency of 82%. These values have been compared with other works in literature where authors used complex and costly processing techniques, demonstrating the strong impact of the colloidal processing over the specific properties of the material.     

Synthesis of yttria by aqueous sol-gel route to develop anti-CMAS coatings for the protection of EBPVD thermal barriers

Anti-CMAS yttria coatings have been prepared by sol-gel routes. Yttria powders with controlled morphology are prepared via auto-combustion of yttrium precursors in a polymerized matrix. The influence of key parameters of the water-based sols is assessed. Indeed, the pH of the initial sol and the temperature of thermal treatment play a major role in the morphology and grain size of yttria powders. To prevent infiltration of CMAS, yttria powders are proposed to be synthesized at pH=1 of the aqueous sol, with drying of the sol and heating at 900 °C. After optimization of the synthesis and deposition conditions via sol-gel route, yttria-based coatings with high specific surface area are obtained. They promote the interaction with melt CMAS and consequently limit the degradation of the thermal barrier coatings situated underneath. It was proved that anti-CMAS yttria coating is effective against the infiltration of CMAS at 1250 °C for 15 min and even 1 h.     

Organic–inorganic hybrid coatings prepared from glycidyl carbamate resin, 3-aminopropyl trimethoxy silane and tetraethoxyorthosilicate

Hybrid sol–gel coatings were formulated from glycidyl carbamate (GC) resins, 3-aminopropyltrimethoxy silane (APTMS) and tetraethoxyorthosilicate (TEOS) as the inorganic network former. GC and silane-modified GC resins were synthesized and then characterized using Gel Permeation Chromatography (GPC), Nuclear Magnetic Resonance Spectrometry (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). The resins were crosslinked with amine crosslinkers such as p-aminocyclohexyl methane (PACM), Ancamide 2050, Ancamide 2353 and Epikure 3164 at 1:1 equivalent ratio of the epoxy groups in the synthesized resin and amine crosslinker. The TEOS content in the coatings were varied to understand its effect on the coating properties. The hybrid coatings were cured at room temperature and humidity for more than 20 days as well as oven cured at 80 °C for 1 h. The thermal properties of the post-cured hybrid materials were evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Mechanical property evaluation such as König pendulum hardness measurement, impact resistance and crosshatch adhesion tests of the post-cured samples were carried out. MEK double rub resistance and water contact angle of the coatings were also evaluated. All of the coatings had good adhesion to aluminum 2024-T3 and had good MEK double rub resistance, indicating good crosslinking. Properties such as T_g, hardness and flexibility varied with the amine crosslinker used with Epikure 3164 yielding the lowest T_g, highest flexibility, and lower hardness coatings. Increasing the amount of TEOS modification in the formulations increased the hardness, the T_g, and the thermal stability. The flexibility – determined using reverse impact measurements – also increased with increasing TEOS content.     

Surface defect healing effect of silica coatings on silicon nitride fibers

In this study, silica coatings with different thickness were prepared on silicon nitride fibers by a continuous dip-coating method. The effects of the coatings on the mechanical properties of the silicon nitride fibers were investigated. The SiO₂ coatings with uniform thickness were prepared from a sol solution with a concentration of 0.75 wt% and then heat-treated at 400 °C, and the strength of the fibers was improved by the treated coating. The tensile strength of a coated fiber was approximately 26% higher than that of an uncoated fiber because the thin coating healed the surface defects. Our study also confirmed that the size of sol particles must match that of the flaws on the fiber surface before these flaws could be effectively repaired. Finally, a probable mechanism will be proposed here to explain this effect. The present results demonstrate that the strength of silicon nitride fibers can be enhanced by coating them through the sol–gel process, and the findings are expected to provide guidelines for repairing strength-limiting flaws in other fibers.     

Mechanical and high pressure tribological properties of nanocrystalline Ti(N,C) and amorphous C:H nanocomposite coatings

This paper reports on the mechanical and high pressure tribological properties of nanocrystalline (nc-) Ti(N,C)/amorphous (a-) C:H deposited, using low temperature (～ 200 °C) DC reactive magnetron sputtering. The mechanical properties are affected by the nc-Ti(N,C)/a-C:H phase fraction ratio. For increasing C contents (from 31 to 47 at.%) an increase of the a-C:H phase content and a degradation of the nanocrystalline phase occurs leading to a reduction in nanoindentation hardness (H) values (from 15 to 9 GPa) and reduced modulus (Er) values (from 150 to 80 GPa). A strong correlation between H/E ratio and wear performance was exhibited by the coatings. The synthesized coatings survived up to 100 m sliding distance when tested using pin-on-disc sliding configuration at > 4.5 GPa contact pressures and the measured friction coefficient values were similar for all films (μ ～ 0.21–0.25).     

Nanoindentation-induced deformation behaviour of tetrahedral amorphous carbon coating deposited by filtered cathodic vacuum arc

The nanoindentation-induced deformation behaviour of a ta-C (tetrahedral amorphous carbon) coating deposited on to a silicon substrate by a filtered vacuum cathodic vapour arc technique was investigated. The 0.17-μm-thick ta-C coating was subjected to nanoindentation with a spherical indenter and the residual indents were examined by cross-sectional transmission electron microscopy. The hard (~ 30 GPa) ta-C coatings exhibited very little localized plastic compression, unlike the softer amorphous carbon coatings deposited by plasma-assisted chemical vapour deposition. However, neither through-thickness cracks nor delamination was observed in the coating for the loads studied. Rather, the silicon substrate exhibited plastic deformation for indentation loads as low as 10 mN and at higher loads it showed evidence of both phase transformation and cracking. These microstructural features were correlated to the observed discontinuities in the load-displacement curves. Further, it was observed that even a very thin coating can modify the primary deformation mechanism from phase transformation in uncoated Si to predominantly plastic deformation in the underlying substrate.