Stress monitoring in thin polymer coatings using time resolved fluorescence

Stress monitoring in thin polymer coatings was monitored using time resolved fluorescence from organic molecules. The decay time of fluorescence from an organic molecule in a uniaxially stretched polymer coating decreases with increasing tensile stress. The substance 9-methylanthracene (9-MeAn) is an effective dye for detecting internal stresses up to 10 MPa. Compared with the traditional bimetallic method, time resolved fluorescence of 9-MeAn gave reliable values for internal stresses in a thin polymer coating. The internal stress in a polymer coating cured on a glass plate was measured during exposure to an outdoor weathering test. The internal stress diminished significantly in three days. The decrease in the internal stress was caused mainly by light irradiation. Because 9-MeAn degraded in sunlight, it was a useful probe for stress monitoring only for periods less than two weeks.     

Residual Stress Analysis of Multilayer Environmental Barrier Coatings

Silicon-based ceramics (SiC, Si₃N₄) are promising materials systems for high-temperature structural applications in gas turbine engines. However, the silica layer that forms on these materials is susceptible to attack from water vapor present in combustion environments. To protect against this degradation, environmental barrier coatings (EBCs) have been developed to shield the underlying substrate and prevent degradation. Here we report on elastic and thermal properties, as well as internal stresses of candidate multilayer coatings, as measured in situ using microfocused high-energy X-rays in a transmission diffraction geometry. Doped aluminosilicate coatings were investigated for their stability on a SiC/SiC melt-infiltrated substrate. The coatings consisted of a Ba_{1− x} Sr _x Al₂Si₂O₈ topcoat with a mullite or mullite+SrAl₂Si₂O₈ interlayer, and a silicon bond coat. A numerical model was used to compare the stress results with an ideal coating system. Experiments were carried out on as-sprayed and heat-treated samples in order to analyze the strain and phase evolution as a function of multilayer depth and temperature. The phase transformation of the topcoat promoted healing of cracks in the EBC and reduced stresses in the underlying layers and the addition of SAS to the interlayer reduced stresses in thermally cycled coatings, but did not stop cracks from forming.     

The corrosion protection of mild steel by single layered polypyrrole and multilayered polypyrrole/poly(5-amino-1-naphthol) coatings

Electrochemical synthesis of polypyrrole (PPy) and top coat of poly(5-amino-1-naphthol) (PANAP) on PPy films from oxalic acid solution was achieved on mild steel (MS) by cyclic voltammetry technique. The morphology and the structure of the films were investigated by scanning electron microscopy (SEM). The corrosion performance of this multilayer coating and single PPy coating were investigated in 3.5% NaCl solution by using open circuit potential (E_ocp)–time curves, polarization curves and electrochemical impedance spectroscopy (EIS). It was found that the multilayer PPy/PANAP coating could provide much better protection than single PPy coating for corrosion of MS. It was observed that corrosion performance of coatings was increasing with immersion period. This was explained by auto-undoping properties of PPy coatings during immersion in corrosive solution. The improved corrosion performance in the presence of PANAP top coat on PPy was explained by increase in barrier effect of bilayer films.     

Internal stresses and adhesion of thin silicon oxide coatings on poly(ethylene terephthalate)

The effect of internal stresses on the cohesion and adhesion of a thin silicon oxide (SiO_x) oxygen-barrier coating, evaporated on a poly(ethylene terephthalate) (PET) film substrate was studied. Internal stresses were generated during annealing in the temperature range for recrystallization of the PET,during calendering in a multilayer structure where two SiO_x /PET films were laminated together with a polypropylene film, and during long-term thermal aging below the glass transition temperature of the polymer. The cohesion of the coating and its adhesion to the polymer substrate were derived from fragmentation tests, in which the failure of the oxide coating was analyzed as a function of the applied stress during uniaxial tensile loading of the substrate. The intrinsic coating strength at critical length and the interfacial shear strength were found to be equal to 1350 MPa and 73 MPa, respectively. It was found that none of the thermal treatments investigated altered the interfacial interactions. Rather, these treatments induced shrinkage of the PET substrate, which increased the coating internal compressive stress and the SiO_x /PET interfacial shear strength. A linear relationship between the SiO_x /PET interfacial shear strength and the coating internal stress was determined from a stress transfer analysis. The coefficient of this linear relationship, equal to-1.34 · h _c/l _c, where h _c is coating thickness and l _c is the critical stress transfer length, reproduces the experimental data with good accuracy.     

The effect of annealing on mechanical and tribological properties of diamond-like carbon multilayer films

The diamond-like carbon (DLC) multilayer films have been deposited by plasma CVD deposition onSi wafer substrate. The deposited films have then been post-annealed in vacuum at 250 °C for 2 h. Changes in internal stress, hardness, critical load, friction coefficient and wear have been investigated toassess the influence of annealing on mechanical and tribological properties of DLC multilayer films. At the same time, DLC single layerfilms are also deposited and annealed in the same method for a comparison.The results show that there is 28–33% decrease in internal stress and 10–13% decrease in hardness of theDLC single layer films after the anneal treatment. However, for the DLC multilayer films, there is 41–43% decreasein internal stress and less than 2% decrease in hardness. In addition, the annealed DLC multilayer filmhas the same friction and wear properties as that un-annealed film. This result indicates that the anneal treatment isan effective method for the DLC multilayer films to reduce the internal stress and to increase the critical load.The by-effect of the annealing, decrease of hardness and wear resistance of the multilayer film, can be restrictedby the multilayer structure.     

Residual stress development in a bi-layer coating

Measurements have been made of the residual stress development in a solventdeposited thermoplastic coating and a two-component thermoset by monitoring the bending of the steel substrate using strain gauges. The stresses in single layers were found to be significant when compared to the strength of the coatings. When the thermoset was applied on top of the thermoplastic (undercoat), the stress in the top coat was much less than that obtained when the thermoset was applied directly to the steel substrate. This was deduced using a new elastic analysis for tri-layers (substrate plus bi-layered coating).     

Failure modes in organic coatings studied by scanning acoustic microscopy

Scanning acoustic microscopy (SAM) has been found to be well suited to study physically and chemically induced changes and defects in polymer coatings exposed to corrosive environments. In this work, SAM was used to investigate sub-surface migration and blister formation in polymer coatings of different layer structure after exposure to a corrosive solution. Two model systems consisting of base coat and clear coat on steel substrates where studied. The time evolution of sub-surface migration fronts and blister initiation and their growth were investigated by analysing SAM images after different exposure times. Depending on the layer structure, it was possible to differentiate between transport of the electrolyte solution (i) through the coating and (ii) along the coating/substrate interface. Samples without clear coat typically showed randomly distributed blisters at the coating/substrate interface, irrespective of the location of initial defects. The random distribution of blisters is related to diffusion of the electrolyte solution through the coating layer followed by “nucleation” at weak spots of the substrate, at the interface between polymer and substrate or within the polymer. In contrast, samples with a clear coat acting as a diffusion barrier showed a sub-surface migration front of 2–4 μm height, propagating along the coating/substrate interface, starting at initial defects. The linear propagation of this front cannot be explained by Fickian diffusion and is discussed in terms of an accelerated diffusion or crack growth kinetics. Since blistering started only at regions, where the migration front has already passed, the presence of electrolyte solution or water at the coating/substrate interface was found to be a prerequisite for the nucleation of blisters.     

OEM汽车涂装车间技术的环境生命周期评价（一）

生命周期评价（LCA）是一种评价产品系统的环境影响和环境权衡的标准化方法。伊士曼化工公司完成了一项＂从涂料到成品车涂装＂的LCA研究,在该研究中,比较了5种有代表性的OEM汽车涂装车间工艺配置的温室气体（GHG）影响和挥发性有机化合物（VOC）排放性能。涂装车间数据由IHS提供。这些工艺配置包括：1）三涂两烘（3C2B）水性（WB）底色漆和1K罩光清漆;2）3C2B WB底色漆和2K罩光清漆;3）3C2B低固含溶剂型底色漆和1K罩光清漆;4）三涂一烘（3C1B）高固含溶剂型底色漆和1K罩光清漆;5）3C1B WB底色漆和1K罩光清漆。本研究的目标是通过比较通用工艺技术类别来帮助理解差异和制定决策。本研究的结论是,3C1B高固含溶剂型工艺可以实现最低的GHG释放量,同时达到欧洲现行的VOC排放限值35 g/m2。但是,如果世界上某些地区的OEM选择不采用VOC减排技术,那么为了达到排放限值35g/m2,需要采用WB技术。这项LCA研究表明,高固含溶剂型技术是对环境影响最小,有利于可持续性发展的技术,建议政策制定者和利益相关者予以考虑。     

In situ measurement of stresses in diamond-like carbon films

In situ determination of stresses in thin films can be used as an important tool to assist process development as well as to understand the thermodynamics of film formation. A simple technique for the measurement of stresses in growing films is described here. The technique consists of measuring the displacement of a laser beam reflected from the film surface. Displacement is induced by changes in the radius of the curvature of the substrate resulting from stresses in the film. The detector sensitivity at the used wavelength (635 nm) is approximately 12 mV μm⁻¹, for which our experimental set-up is equivalent to 4 mV μrad⁻¹. The actual data collected consist of the reflected beam displacement vs. time, and provides at any instant the value of the average stress. By knowing the deposition rate, time is directly correlated with film thickness, and the local stress can be determined. Examples of measurement of stresses in tetragonally bonded amorphous carbon films prepared by filtered cathodic arc are presented, as well as how this technique can be used to design the deposition process to virtually eliminate intrinsic stresses.     

Interfacial processing and adhesion of diamond,diamond-like,and TiN films on metallic and polymeric substrates

Diamond, diamond-like, and titanium nitride (TiN) films have extremely desirable chemical, electrical, and mechanical properties for a variety of applications ranging from corrosion- and erosion-resistant coatings to electronics packaging of microelectronic devices. However, many of these applications are limited by the poor adhesion of these films to metal and polymer substrates. The adhesion of a film is determined primarily by internal stresses in the film, thermal and lattice mismatch, and most importantly by interfacial bonding. We have developed methods based on mechanical interlocking, chemical bonding, grading of interatomic potentials, and the multilayer discontinuous thin films approach to control stresses and strains in thin films. A substantial improvement in adhesion and wear properties is obtained by using these methods selectively. We review issues related to the adhesion of diamond, diamond-like carbon, and TiN films on metal and polymeric substrates.     

Residual Stresses and Their Effects on the Durability of Environmental Barrier Coatings for SiC Ceramics

Qualitative residual stresses in current environmental barrier coatings (EBCs) were inferred from the curvature of EBC-coated SiC wafers, and the effects of EBC stresses on the durability of EBC-coated SiC were evaluated. The magnitude of substrate curvature correlated fairly well with the EBC–SiC coefficient of thermal expansion (CTE) mismatch, EBC modulus, and thermally induced physical changes in EBC. BSAS (1− x BaO· x SrO·Al₂O₃·2SiO₂, 0≤ x ≤1) components in the current EBCs, i.e., Si/mullite or mullite+BSAS/BSAS or yttria-stabilized zirconia (YSZ: ZrO₂–8 wt% Y₂O₃), were the most beneficial for reducing the EBC stress in as-sprayed as well as in post-exposure EBCs. The reduced stress was attributed to the low modulus of BSAS. The addition of a YSZ top coat significantly increased the substrate curvature because of its high CTE and sintering in thermal exposures. There were clear correlations between the wafer curvature and the EBC durability. The Si/mullite+20 wt% BSAS/BSAS EBC maintained excellent adherence, protecting the SiC substrate from oxidation, while the Si/mullite+20 wt% BSAS/YSZ EBC suffered delamination, leading to severe oxidation of the SiC substrate, after a 100 h −1300°C exposure in a high-pressure burner rig.     

Damage Evolution and Stress Analysis in Zirconia Thermal Barrier Coatings during Cyclic and Isothermal Oxidation

The failure mechanisms of air-plasma-sprayed ZrO₂ thermal barrier coatings with various microstructures were studied by microscopic techniques after thermal cycling. The elastic modulus ( E ) and hardness ( H ) of the coatings were measured as functions of the number of thermal cycles. Initially, both E and H increased by ∼60% with thermal cycling because of sintering effects. However, after ∼80 cycles (0.5 h at 980°C), the accumulated damage in the coatings led to a significant decrease of ∼20% of the maximum value in both E and H . These results were correlated with stresses measured by a spectroscopic technique to understand specific damage mechanisms. Stress measurement and analysis revealed that the stress distribution in the scale was a complex function of local interface geometry and damage in the top coat. Localized variations in geometry could lead to variations in measured hydrostatic stresses from −0.25 to −2.0 GPa in the oxide scale. Protrusions of the top ZrO₂ coat into the bond coat were localized areas of high stress concentration and acted as damage-nucleation sites during thermal and mechanical cycling. The net compressive hydrostatic stress in the oxide scale increased significantly as the scale spalled during thermal cycling.     

POLYELECTROLYTE MULTILAYER STAMPING IN AQUEOUS PHASE AND NON-CONTACT MODE

Polyelectrolyte multilayer (PEM) transfer printing has been previously achieved by stamping under dry conditions. Here, we show for the first time, that PEM can be transferred from a stamp to the base substrate under aqueous conditions whereby the two surfaces are in a non-contact mode. Degradable multilayers of (PAA/PEG)(10.5) followed by non-degradable multilayers of (PDAC/SPS)(80.5) were fabricated under acidic pH conditions on either PDMS or glass (stamp), and subsequently transferred over top of another multilayer prepared on a different substrate (base substrate), with a spacing of ~ 200 μm between the stamping surface and the base substrate. This multilayer transfer was performed under physiological pH conditions. This process is referred to herein as non-contact, aqueous-phase multilayer (NAM) transfer. NAM transfer can be useful for applications such as fabricating three-dimensional (3-D) cellular scaffolds. We attempted to create a 3-D cellular scaffold using NAM transfer, and characterized the scaffolds with conventional and fluorescence microscopy.     

Measurement of residual stresses in injection‐molded polymer parts by time‐resolved fluorescence

Time‐resolved fluorescence properties of 9‐methylanthracene (9MAn) dispersed in film of polyvinylchloride (PVC) containing carbon black were studied under tensile loadings. The fluorescence lifetime of 9MAn decreased from 5.70 to 5.55 ns, whereas the stresses acting on the films increased from 0 to 3 MPa. The change in fluorescence lifetimes of 9MAn during the stress relaxation process showed that the fluorescence lifetimes were correlated with the stresses, not with the strains. The results suggest that 9MAn is a useful probe for monitoring stresses acting on the matrix. With the use of the fluorescence properties of 9MAn, the residual tensile stresses on the skin‐layer of PVC injection‐molded test pieces were estimated. The estimated residual stresses were about ～ 1 MPa. The residual stresses were relaxed to 0 MPa with annealing at 100°C. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2600–2603, 2002     

Evaluation of mar/scratch resistance of a two component automotive clear coat via nano-indenter

Nano-indenter as a technique for characterization and evaluation of mar/scratch resistance of a two component automotive clear coat is used. Different responses of the coatings to the marring stress and critical forces are discussed. The plastic deformation compared to the elastic part and the damage width is decreased by adding nano-silica in the coatings formulation.     

基体条件对Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力的影响

采用有限元分析软件ANSYS对等离子喷涂Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力分布进行了数值仿真。结果表明:基体厚度不同时,涂层界面Sm2Zr2O7/YSZ及界面YSZ/NiCoCrAlY对应应力及应力梯度基本不变,表明应力及应力梯度与基体厚度无关;但基体材质热膨胀系数对涂层系统界面的径向、轴向及剪切应力梯度有决定性的影响,且各应力梯度随金属基体的热膨胀系数差异增加而增大,表明基体材质是影响涂层界面径向残余热应力及应力梯度的根本原因。采用多层陶瓷结构并合理选择各层材质的热膨胀系数将更加有利于降低涂层应力梯度,进而改善涂层性能,延长涂层寿命。     

Decay Lifetimes of Para-Substituted Polystyrene in Solid Films and in Dichloromethane

The fluorescence decay times of polystyrene, poly(4-bromostyrene), poly(4-chlorostyrene), poly(4-methylstyrene), poly(α-methylstyrene), poly(4-methoxystyrene), and poly(4-tert butylstyrene) were measured in solid films and in dichloromethane solution. A detailed analysis of the emission profile performed by nanosecond time resolved fluorescence spectroscopy confirmed the presence of monomer fluorescence as well as excimer fluorescence in both media. Monomer fluorescence decay times gave shorter times compared with that of excimer decay times in dichloromethane solution. Fluorescence from substituted polystyrene was mainly excimer fluorescence with shift of maximum emission to longer wavelength. The ratio of monomer to excimer contributions was found to be dependent on the emission wavelength, but was not affected by polymer concentration. Both monomer and excimer fluorescence lifetimes as well as excimer intensity increase with increasing emission wavelength. An accompanying decrease in monomer contribution is also observed is solution in comparison with that in solid films.     

Mass diffusion coefficients of polymer films using real-time holographic interferometry

The mass diffusion coefficient is obtained using real-time holographic interferometry. The stresses induced during desorption and absorption of moisture are monitored as a function of time at various relative humidities. A mathematical relationship is applied to relate these stresses to the moisture diffusion through the thickness of the films. The advantage of the holographic interferometry technique is that both the swelling stress as a function of time and the mass diffusion coefficient can be quantified simultaneously during the same experiment. Several films and coatings were investigated: polyimides (PMDA–ODA and BPDA–PAA), photographic gelatin, and cellulose acetate. Each exhibited characteristic Fickian diffusion behavior. A gravimetric technique was employed to validate the holographic interferometry results. The moisture diffusion coefficients determined by holographic interferometry agreed well with reported literature values. © 1996 John Wiley & Sons, Inc.     

In situ measurement of water at the organic coating/substrate interface

In situ and quantitative information on the water layer at the organic coating/substrate interface is crucial for understanding and preventing the failure of organic coating systems. A technique, based on a two-layer model derived rigorously from internal reflection theory, has been developed for measuring in situ the thickness and amount of the water layer at the organic coating/substrate interface. The technique gives new insight into the processes by which water degrades the coating/substrate bonds. In this technique, a transparent or an opaque organic coating of sufficient thickness is applied to an internal reflection element (IRE) with or without a thin metallic film, which is used as the substrate. A water chamber is attached to the organic-coated specimen. After adding water to the chamber, Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectra are taken automatically at specified time intervals without disturbing the specimens or the instrument. Water uptake in the coating and FTIR-MIR spectra of water on the coating-free substrate are also used for the analysis. Examples of clear and pigmented coatings on untreated and treated substrate surfaces are given to demonstrate the technique. Results of water accumulation at the coating/iron interface with and without applied electrical potentials are given. In addition to measuring water at the coating/substrate interface, the technique provides a means for studying the transport of water through a coating adhered to a substrate. Information on water at the interface and its transport properties through coatings applied to a substrate is valuable for interpreting corrosion, blistering and delamination of organic coating systems, and for developing models for use in predicting the serivce lives of protective coatings.     

New soluble teflon for coating onto plastics

Teflon PTFE, FEP, and low melting HFP/TFE copolymers can be made to have excellent adhesion to substrates if residual stresses from large changes in temperature can be eliminated. PTFE and FEP can be adhered using a high boiling saturated perfluorinated oligomer to plasticize the fluoropolymer at the interface. Low melting HFP/TFE copolymers can be deposited as very thin films from solution in common perfluorinated solvents. Highly durable coatings can be prepared owing to the intimate degree of contact obtained between the substrate and the fluoropolymer chains. The use of low boiling solvents allow coatings on temperature sensitive substrates, such as plastics, rubber, and paper. Possible commercial applications range from mold release coatings, clear protective coatings on photographs, paintings, and documents and optical coatings on plastic and glass.