Investigations on the mechanical properties of hybrid nanocomposite hard coatings on polycarbonate

Hybrid nanocomposite coatings derived from titanium tetraisopropoxide and epoxy or acrylic modified silanes were deposited on polycarbonate (PC) by dip coating employing various withdrawal speeds followed by ultraviolet and thermal curing. The effect of different organic functional groups in the precursors and ageing effect of these sols were systematically studied with respect to thickness, abrasion resistance, pencil scratch test, nanoindentation hardness and transmittance. The gels derived from the freshly prepared and aged sols were structurally characterized by FT-IR and TEM analysis. The viscosities of the sols were monitored with time. The change in viscosity is rapid for sol from epoxy modified silane. The thickness of the coatings increases with increase in viscosity in case of both the silane precursors. The scratch as well as abrasion resistance increases as a function of coating thickness. The pencil scratch hardness improves from 2B for the bare PC to a maximum of 3H for the coating obtained from an aged sol derived from epoxy modified silane. Also, the abrasion resistance of the coatings from same sol was maximum as evidenced by a <6% change in haze after 500 cycles, vis-a-vis 40% for the bare PC. The coatings from a freshly prepared sol of acrylic modified silane and titania showed the maximum nanoindentation hardness of 0.52 GPa, when compared to 0.23 GPa for the bare PC.     

The preparation and characterization of abrasion-resistant coatings on polycarbonate

A series of abrasion-resistant coatings based on methyltrimethoxysilane (MTMS), tetraethoxysilane (TEOS), and methacyloxypropyltrimethoxysilane (MEMO) were synthesized via a sol–gel method and applied to polycarbonate (PC) substrate. Through a comparison with an uncoated PC substrate, hardness, adhesion, and abrasion resistance of coated PC substrates were investigated. Fourier transform infrared (FTIR) spectra of these coatings before and after being cured indicated that the crosslinked structure of Si–O–Si was formed via the hydrolysis–condensation reactions of alkoxy silane. The optical test results showed that coated PC substrates possessed higher transmittance and lower haze than uncoated PC substrates. The hardness and adhesion data demonstrated that PC substrates coated with the hybrid coatings (MTMS/TEOS/MEMO coatings) possessed the greatest hardness and optimal adhesion. After 500 wear cycles, the PC substrate with MTMS/TEOS/MEMO coatings showed some grooves and wear tracks without any spallation or delamination, and there was a decrease of only 1.0–1.3% in transmittance and an increase of only 5.68–7.44% in haze. Whereas uncoated PC substrate and substrates with MTMS and MTMS/TEOS (molar ratio is 1.5:1) coatings exhibited a decrease of 3.3, 3.9, and 2.4% in transmittance and an increase of 30.19, 17.42, and 12.35% in haze, respectively.     

Scratch resistance behavior of model coating systems

Coatings are often subjected to physical deformations caused by car wash brushes, tree limbs, keys, fingernails, and the like, which may result in mechanical abrasion. The resistance of the coating to scratches imposed by such mechanical abrasions has been studied through the utilization of laboratory tests such as nanoindentors, crockmeters, scanning probe microscopes, and taber abraders. Little emphasis, however, has been placed on the influence of coating attributes on measured scratch resistance. In this study we attempt to relate the effect of coating glass transition temperature, crosslink density, and crosslink type in a series of formulated waterborne polyurethane dispersion clearcoats on resultant scratch resistance. Methods utilized to impart the scratches, e.g., scanning probe microscopy, weighted fingernail, and crockmeter, as well as related coating physical properties measured, e.g., viscoelastic behavior, hardness, and tensile strength, are discussed. The scratch resistance of model coating systems analyzed was found to be dependent upon the base resin T_g, which affected the surface hardness of the coating and the toughness of the crosslinked network, as measured by the method of essential work. Coatings that exhibited both hard surfaces and tough-elastic network integrity afforded the optimized scratch resistance behavior. Presented at the 26th International Waterborne, High-Solids, and Powder Coatings Symposium, February 10–12, 1999, New Orleans, LA. 401 Southfield Rd., P.O. Box 6231, Dearborn, MI 48121-6231.     

The dependence of pendulum hardness on the thickness of acrylic coating

Pendulum hardness, which is widely used in the characterization of organic coatings, depends greatly on the thickness of the coating. However, it is still unclear whether a qualitative or quantitative relationship exists between pendulum hardness and coating thickness. In the present article, the pendulum hardness values of acrylic coatings with different thicknesses are measured using a König pendulum hardness tester to clarify the dependence of pendulum hardness on thickness of coating. The results show that the pendulum hardness of acrylic coatings decreases gradually with the increasing thickness of coating within a thickness range, and the sensitivity of pendulum hardness to thickness of coating depends greatly on the glass transition temperature of the coatings. An equation suitable for describing the relationship between logarithmic decrement of the amplitude of pendulum's oscillation and thickness of coating is presented, which can separate the contributions of substrate and coating on the logarithmic decrement. This study demonstrates that the measured value of pendulum hardness is not the bulk hardness value for the coating but the representative value of the system consisting of substrate and coating. An excellent correlation between pendulum hardness and thickness of acrylic coatings is obtained, which is fairly supported by the experimental data.     

Determination of epoxy coating wet-adhesive strength using a standardized ASTM/ISO scratch test

A new approach for evaluating the wet-adhesive strength of epoxy-based coatings was carried out based on a recently standardized ASTM/ISO scratch test. A linearly increasing scratch normal load was applied during scratch to induce progressively increased delamination stress at the coating and steel substrate interface. Thus, the applied critical load to cause coating debonding can be experimentally determined. To find out the corresponding stress magnitude to incur coating debonding, finite element methods (FEM) modeling was conducted to analyze the stress fields around the scratch tip during scratching. The wet-adhesive strength is then quantitatively determined. Based on the above methodology, investigation on a set of model coating systems suggests that the critical load for coating delamination is significantly influenced by water exposure time, coating thickness, and substrate surface roughness. By combining the standardized scratch tests and FEM modeling, the proposed approach is found to be effective for quantitative assessment of epoxy coating wet-adhesive strength and for the development of high performance protective coatings for various industrial applications.     

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.     

A macroscopically nondestructive method for characterizing surface mechanical properties of polymeric coatings under accelerated weathering

The surface of coatings and plastics is the first target in any degradation process initiated by ultraviolet (UV) radiation or mechanical stress (via scratch and abrasion). Surface damage can lead to changes in optical, morphological, and mechanical properties and can result in pathways for ingress of moisture and corrosive agents. Current test methods for monitoring performance of protective coatings focus on chemical properties and optical properties, such as color and gloss measurements, or invasive tests such as abrasion and cross-cut adhesion. In this study, a macroscopically nondestructive performance protocol using nanoindentation metrology via a well-controlled scratch test was applied to evaluate the scratch resistance and monitor the surface mechanical property changes in a protective coating under accelerated weathering. Polyurethane (PU) coatings with different polyol compositions were chosen for this study. Coating specimens were exposed to high-intensity UV radiation at 55°C and 75% RH conditions. Exposed specimens were removed at specified UV exposure times for surface modulus/hardness and scratch resistance characterization via nanoindentation and scratch test. The effect of polyol type and UV radiation dose on the scratch damage (scratch morphology) was investigated and correlated with the surface hardness and modulus of the materials.     

An investigation of scratch testing of silicone elastomer coatings with a thickness gradient

Silicone elastomer coatings are currently being investigated as foul release coatings on ships hulls. Previous tests on silicone duplex elastomer coatings used a progressive load scratch test. It has been shown that the durability of uniform silicone duplex elastomer coatings is a function of thickness, indentation modulus, and stylus and that the failure mechanism depended on coating thickness and stylus. When applying silicone coatings to a ship's hull, there are regions on the ship where the coating is not uniform. This article investigates the effect of a thickness gradient on the durability of a single layer silicone elastomer coating. In these tests, a constant normal load was used as the stylus moved transversely to the surface. It was found that when the scratch test started in the silicone coating and proceeded in the direction of decreasing coating thickness (“Elastomer to Metal”), there was first a scratch tract followed by the initiation of detachment of the coating, then by gross detachment of the coating. When the scratch started on the exposed aluminum surface and proceeded into the silicone in the direction of increasing coating thickness (“Metal to Elastomer”), there was first gross detachment of the coating, followed by recovery (i.e., silicone coating is intact) and a scratch tract into the silicone. It was also found that the coefficient of friction was much higher in the silicone when the scratch test was going in the direction of decreasing coating thickness as opposed to the scratch test going in the opposite direction. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis and characterization of elastomeric polyester coatings for automotive pre-coated metal

An automotive pre-coated metal system (PCM) has been investigated to replace wet coating process, such as pre-treatment, dip coating and spray coating, to address environmental regulations. However, automotive pre-coated metal sheets must have high flexibility and stiffness to overcome harsh conditions such as cutting, press and stamping processes. For these reasons, elastomeric polyester coatings were designed to improve scratch resistance and to impart reflow characteristic for an automotive PCM. The characteristics, curing behavior and viscoelastic property of the resins were measured by ¹H NMR, GPC, RPT and DMA. The flexibility was evaluated using a texture analyzer. A nano scratch tester, equipped with an optical microscope was used to measure the scratch resistance of coatings and the scratched surface morphology. An Amtec laboratory car wash test and a sharp knife were used to evaluate the reflow property of the coatings. Change of the damaged surface morphology was measured using a mini-SEM.     

Improving the mechanical resistance of waterborne wood coatings by adding cellulose nanofibres

In the present study, microfibrillated cellulose (MFC) and nanocrystalline cellulose (NCC) were applied as additives for a waterborne acrylate/polyurethane-based wood coating in order to improve the mechanical resistance of coated wood surfaces. Coating mixtures containing up to 5 wt% nanocellulose were prepared by high-shear mixing and applied to wood substrates. The optical, mechanical and chemical properties of cured coatings were characterized. Surface roughness, gloss, scratch resistance, abrasion resistance and resistance against chemicals were determined according to the relevant European standards. Additionally, nanoindentation (NI) was used to assess the micromechanical properties of modified and unmodified coatings. Owing to a higher surface roughness, cellulose-filled coatings showed significantly lower levels of gloss than the unmodified coating indicating that nanocellulose acts as a matting agent. NI experiments revealed a slightly positive effect of nanocellulose addition on the hardness and modulus of the coatings. While scratch resistance improved consistently with increasing nanocellulose addition, abrasion resistance was found to improve only sporadically. Tensile tests on free-standing coating films revealed a significantly higher tensile strength and modulus for cellulose-filled coatings. Overall, the results suggest that the addition of cellulose nanofibres primarily improves the internal cohesion of the coating layer whereby MFC was more effective than NCC.     

Influence of plasma nitriding treatment on the adhesion of DLC films deposited on AISI 4140 steel by PVD magnetron sputtering

Duplex surface treatments composed of diamond like carbon (DLC) coating followed by plasma nitriding have drawn attention for a while. In this study, AISI 4140 steel substrates were plasma nitrided at different treatment temperatures and times. Then, DLC films were deposited on both untreated and plasma nitrided samples using PVD magnetron sputtering. The effect of different plasma nitriding temperatures and times on the structural, mechanical and adhesion properties of DLC coatings was investigated by XRD, SEM, microhardness tester and scratch tester, respectively. It was found that surface hardness, intrinsic stresses, layer thickness values and phase distribution in modified layers and DLC coating were the main factors on adhesion properties of duplex coating system. The surface hardness and residual stress values of AISI 4140 steel substrates significantly increased with both DLC coating and duplex surface treatment (plasma nitriding + DLC coating). Increasing plasma nitriding temperature and time also increased the diffusion depth and the thickness of modified layers. Hard surface layers led to a significant improvement on load bearing capacity of the substrate material. However, it was also determined that the process parameters, which provided lower intrinsic stresses, improved the adhesion properties of the duplex coating system.     

电镀银/钯/硬金组合耐磨镀层工艺研究

为了提高镀硬金导电环的耐磨性,在黄铜基材上电镀铜/钯/硬金、镍/钯/硬金和银/钯/硬金三种组合镀层,通过工艺实验,组合镀层的外观、厚度、结合力、硬度、接触电阻及表面微观形貌等性能测试,以及装配后耐磨性能考核试验,发现银/钯/硬金组合镀层耐磨性最好。结果表明,该镀层耐磨可达100万次,能够满足产品抗磨性要求。     

Novel thermal and photo curable anti-reflective coatings using fluoroelastomer nanocomposites and self-assembly of nanoparticles

We describe novel optical coatings which require either thermal or photocuring to render them mechanically robust and abrasion resistant. These new coatings are low refractive index fluoroelastomer-nanoparticle composites that form a unique nanostructure during drying of the liquid coating. During drying, the nanoparticles in these liquid coatings migrate towards the substrate. The final, 100-nm-thick anti-reflective coatings are novel and exhibit a unique bilayer structure in which the nanoparticles are ordered and segregated towards the substrate. The coatings are rapidly cured using a new process and exhibit surprising “scratch durability” as measured by aggressively testing with steel wool. Sol gel chemistry is used which involves the reaction of the nanoparticles with an acrylic oxysilane to form nanoparticles which are functionalized with sol gel derived oligomers. The functionalized nanoparticles are combined with a fluoroelastomer containing a free radical initiator and multiolefinic crosslinker, and the composite film is rapidly cured by a thermal or UV process at low temperatures. The final product is a mechanically robust, low refractive index anti-reflective film which is useful for displays and photovoltaic devices. These are unique fluoropolymer nanocomposites which utilize nanoparticle self-assembly to enhance important properties.     

Epoxy coatings with modified montmorillonites

Two types of modified layered aluminosilicates (montmorillonites) with different grain size and gallery spaces were tested as nanofillers in epoxy coating compositions for a steel substrate. Organophilic montmorillonites, in the amount of 2.5 and 5 wt% were introduced to waterborne and to solvent-type epoxy coating materials (based on bisphenol-A epoxy resins and various curing agents). The results of this work indicate that processing properties of coating compositions (leveling and drying time) as well as mechanical properties of coatings (hardness, scratch and abrasion resistance, adhesion to steel) were positively affected by a layered aluminosilicate nanofiller. Enhanced water resistance (lower water absorbance) of coats, especially those formed from waterborne compositions, has been found.     

Investigation of tribological properties of micro-arc oxidation ceramic coating on Mg alloy under dry sliding condition

To enhance wear resistance of Mg alloy, micro-arc oxidation (MAO) ceramic coatings on Mg substrate were prepared in silicate electrolyte under various currents. It was found that the surface roughness and thickness of MAO coating were increased with the increase of current. The dry tribological tests showed that the friction coefficient and wear resistance of thicker coatings (obtained under currents of 3?A and 4?A) were much higher than that of Mg alloy and the thin coating (obtained under current of 2?A), meanwhile the lifetime of the coating obtained under 4?A was longer than the other coatings under higher load. The wear type of thin MAO coating was slight abrasive wear under low load, whereas translated to severe adhesive wear under high load. While the main wear mechanism of thick MAO coating was slight abrasive wear or scratch under the given test condition, which was attributed to the thick intermediate layer improved load support for the soft substrate. The tribological study indicated that the MAO coating obtained under 4?A current had better wear resistance and life time due to its compact microstructure and thickness.     

Some Properties of Composite Panels Made from Wood Flour and Recycled Polyethylene

This study investigated the effect of board type (unmodified vs. MAPE modified) on the surface quality and thickness swelling-water absorption properties of recycled high density polyethylene (HDPE) based wood plastic composites. Additionally, two commercially available coatings (cellulosic coating and polyurethane lacquer coating) were also applied to composite surfaces and their adhesion strength, abrasion and scratch resistance, and gloss values were determined. This study showed that modification of the composites with MAPE coupling agent increased the surface smoothness and reduced the water absorption and thickness swelling of the panels. Abrasion resistance of the composites was also improved through MAPE modification. Regardless of board type, higher scratch resistance and gloss values were observed for polyurethane lacquer coated samples compared to those of cellulosic varnish coated ones. Improvement of adhesion strength was also seen on SEM micrographs.     

Scaling-up of the electrodeposition process of nano-composite coating for corrosion and wear protection

The codeposition of hard nanoparticles into metal matrix electrodeposits usually leads to the increase of the coating hardness and abrasion resistance and causes a change to the microstructure of the deposits leading to more compact, nanostructured coatings with an increased corrosion resistance. Very often the laboratory scale results are not easily transferable to an industrial scale due to the introduction of new process variables such as the geometry and the dimensions of the component to coat.The aim of the present work was the study, in laboratory scale, of nano-composite nickel matrix coatings containing SiC nanoparticles and the transfer of this technology in industrial scale. The deposits have been produced using a Watts type bath containing 20 g/l of nanoparticles, under galvanostatic conditions using a current density of 2 A/dm². The deposits have been studied regarding their microstructure, abrasion and corrosion resistance. Based on the satisfactory results of the laboratory tests, the second part of this work contains the scaling-up and the industrialization of the process and the electrodeposition of the composite coating on ship propeller models and profiles as well as on train axles. The prototype parts were tested under actual working conditions.     

Testing of adhesion of thin films to substrates

The results of the measurement of the adhesion strength of thin films or coatings to substrates are often influenced by system parameters that may be influenced by preparation of the specimen for testing or by deposition of the film itself. In this paper, the scratch, pull, and peel tests are analyzed in terms of criteria for film/coating removal and for the effects of system parameters such as film hardness and ductility, film thickness, substrate hardness, etc.     

紫外固化涂料固化工艺参数的研究

本文研究了UV固化工艺参数与涂层性能的关系,实验显示涂层厚度影响着涂层的硬度、附着力、柔韧性和耐磨擦性能,UV固化光源和制造环境对涂层材料性能同样有较大的影响.设计合理的涂层厚度,选择合适的固化工艺和环境条件,可以获得目标性能的涂层.     

The Effect of Radiation Dosages and UV/EB Radiation on the Properties of Nanocomposite Coatings

The aim of this study was to compare the effects of ultraviolet (UV) and electron beam (EB) radiation on the properties of cured nanocomposite coatings. Surface hardness increased with increasing radiation dosages (number of passes) for all samples. This was due to the increase in crosslinking with increasing radiation dosages. Pendulum hardness, gel content, and thumb twist results were analyzed to choose the appropriate curing dosage for both curing techniques. The selected dosages were then used to cure coatings for scratch and abrasion resistance tests. It was found that the UV curing produced coatings with better abrasion resistance, whereas EB curing was more suitable for producing scratch-resistant coatings.