Modelling the effect of temperature on crack onset strain of brittle coatings on polymer substrates

The effect of temperature on the crack onset strain (COS) of brittle coatings on polymer substrates was investigated through a series of temperature-controlled tensile tests carried out in situ under an optical microscope. It was observed that the failure of such materials under tensile strain was strongly affected by temperature, but the exact behaviour was heavily dependent upon the type of material used. Below the glass transition temperature, T_g, of the polymer substrate, an increase in temperature led to a decrease in crack onset strain. Above the T_g, the substrate softening effects and corresponding shrinkage behaviour had a presiding role, leading to an increase in COS at elevated temperatures. The experimental COS data were modelled as a linear superposition of an intrinsic COS and the internal strain taking into consideration the respective influences of temperature dependent energy release rate for crack propagation and thermal expansion behaviour. Using adjustable values of the coefficient of thermal expansion and toughness of the coating, the model was found to accurately reproduce the change of COS with temperature of two different coatings on aromatic polyester substrates. The proposed approach enables, for any thin film composite with known material properties, the COS at any given temperature to be predicted.     

Hochreflektierende Beschichtungen auf Kunststoffoptiken

Metallized polymer substrates offering highest reflectance are not yet state of the art. Coating organic substrate materials is still a task that is connected with multiple problems. Insufficient adhesion of coatings on polymer substrates represents one of the main difficulties. We could show by experiment that aluminium and silver layers indicate good coating adhesion on many different polymers if they are deposited by vacuum evaporation considering certain process parameters. High reflectance values and a good climatic stability of the metal coated polymer parts are other important challenges to plastic mirrors. By performing roughness measurements on the different polymer samples and by comparing reflection values obtained after coating these samples the impact of the polymers surface quality on the reflectance after metal coating has been investigated. Particularly high reflectance above 97% was realized with a protected silver mirror as well as with dielectric enhanced aluminium. Applying these layer systems excellent reflection properties has been obtained on several plastic substrates comparable to those on glass mirrors. Furthermore the dielectric layers used for reflection enhancement showed the ability to protect the aluminium coating against climatic influences.     

Stress-corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices

Stress corrosion cracking of transparent conductive layers of indium tin oxide (ITO), sputtered on polyethylene terephthalate (PET) substrates, is an issue of paramount importance in flexible optoelectronic devices. These components, when used in flexible device stacks, can be in contact with acid containing pressure-sensitive adhesives or with conductive polymers doped in acids. Acids can corrode the brittle ITO layer, stress can cause cracking and delamination, and stress-corrosion cracking can cause more rapid failure than corrosion alone.The combined effect of an externally-applied mechanical stress to bend the device and the corrosive environment provided by the acid is investigated in this work. We show that acrylic acid which is contained in many pressure-sensitive adhesives can cause corrosion of ITO coatings on PET. We also investigate and report on the combined effect of external mechanical stress and corrosion on ITO-coated PET composite films. Also, it is shown that the combination of stress and corrosion by acrylic acid can cause ITO cracking to occur at stresses less than a quarter of those needed for failure with no corrosion. In addition, the time to failure, under ~ 1% tensile strain can reduce the total time to failure by as much as a third.     

Investigations of diffusion behaviour in Al-doped zinc oxide and zinc stannate coatings

Multi-layer dielectric/silver/dielectric coating systems have excellent proprieties as heat insulators and for solar energy reflection and electrical conductivity. The largest market is dominated by low-emissivity (low-E) coatings, which are applied to large area architectural glazing to reduce heat losses from buildings. They combine high visible transparency with high reflectance in the far-infrared region, where the thin (~ 10 nm) silver layer reflects long wavelength IR back into the building and the dielectric layers both protect the silver and act as anti reflectance layers.In this study, a range of dielectric coatings has been deposited onto soda-lime glass substrates by reactive sputtering from metallic targets. The magnetrons were driven in DC mode and also in mid-frequency pulsed DC and AC modes. Process variables investigated include operating pressure, oxygen flow rate and magnetron configuration. Selected coatings were annealed at 650 °C and analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscope (AFM).The oxide samples were then over-coated with silver and annealed for a second time. These coatings were analysed by secondary ion mass spectroscopy (SIMS) to determine the diffusion rates of silver and sodium (from the substrate) through the oxide coatings.The results to date, presented here, show the diffusion of silver and sodium atoms through zinc oxide and zinc stannate thin films deposited under a vast range of conditions. Preliminary attempts have been made to estimate diffusion coefficients for these coating systems and to relate these values to processing conditions and the structural variations observed.     

Tensile behaviour of as deposited and heat-treated electroless Ni-P deposits

Electroless Ni-P coatings were deposited on mild steel substrates and the effect of heat-treatment on their structure and tensile behaviour was studied, with the following conclusions. The as-deposited electroless Ni-P coating is amorphous and it remains amorphous up to 300 °C. At 400 °C the coating becomes crystalline and consists of a Ni₃P matrix containing areas of metallic nickel. For the selected coating/substrate thickness ratio, the contribution of the coating in the tensile properties of the coating-substrate system is negligible as expressed by the values of yield strength, ultimate tensile strength and fracture strain in mild steel substrates and coated as-deposited and heat-treated specimens. Extensive cracking of the coating accompanied by spalling was occurred during the tensile tests. The density of cracks was found to increase close to the fracture surface of the tensile specimen and with increasing heat-treatment temperature. The cracks observed on the surface of the coatings are believed to form due to the inability of the brittle coating to accommodate the strain generated in the ductile substrate. Their orientation to the tensile axis is in close relation to the structure of the coating and the failure mechanism that is dictated by this structure. The first cracks on the surface of the coatings were found to form after the yield strength of the tensile specimen has been reached and plastic deformation of the substrate takes place. Their density increases with the accumulation of strain up to fracture.     

Ancillary properties of vapor-deposited carbide coatings

Instrument-size gas-bearing applications demanding hard wear-resistant coatings on lightweight thermally conductive substrates frequently require that properties such as electrical resistivity, magnetic susceptibility and adhesion integrity be evaluated. Titanium carbide and tungsten carbide coatings, approximately 1 to 2 μm thick, were sputter deposited on beryllium oxide substrates. In addition, titanium carbide coatings approximately 50 μm thick were prepared on beryllium oxide substrates by activated reactive evaporation. Electrical resistivity was measured at various locations on the coating surface with a square four-point probe array. Magnetic susceptibility was measured using the Gouy test method in which a specimen is partially suspended in a magnetic field and the resulting force of attraction is measured. Since adhesion integrity is essential to the success of any coating application, a direct tensile pull test was performed on each specimen. The resulting fracture surface showed coating thickness, and the structure and characterization was accomplished by scanning electron microscopy and X-ray analysis.     

Mechanical integrity of thin inorganic coatings on polymer substrates under quasi-static, thermal and fatigue loadings

The interplay between residual stress state, cohesive and adhesive properties of coatings on substrates is reviewed in this article. Attention is paid to thin inorganic coatings on polymers, characterized by a very high hygro-thermo-mechanical contrast between the brittle and stiff coating and the compliant and soft substrate. An approach to determine the intrinsic, thermal and hygroscopic contributions to the coating residual stress is detailed. The critical strain for coating failure, coating toughness and coating/substrate interface shear strength are derived from the analysis of progressive coating cracking under strain. Electro-fragmentation and electro-fatigue tests in situ in a microscope are described. These methods enable reproducing the thermo-mechanical loads present during processing and service life, hence identifying and modeling the critical conditions for failure. Several case studies relevant to food and pharmaceutical packaging, flexible electronics and thin film photovoltaic devices are discussed to illustrate the benefits and limits of the present methods and models.     

Microwave irradiation-assisted method for the deposition of adherent oxide films on semiconducting and dielectric substrates

We report a method for the deposition of thin films and thick coatings of metal oxides through the liquid medium, involving the microwave irradiation of a solution of a metal-organic complex in a suitable dielectric solvent. The process is a combination of sol-gel and dip-coating methods, wherein coatings can be obtained on nonconducting and semiconducting substrates, within a few minutes. Thin films of nanostructured ZnO (würtzite) have been obtained on Si(100), glass and polymer substrates, the nanostructure determined by process parameters. The coatings are strongly adherent and uniform over 15 mm × 15 mm, the growth rate ∼ 0.25 μm/min. Coatings of nanocrystalline Fe₂O₃ and Ga₂O₃ have also been obtained. The method is scalable to larger substrates, and is promising as a low temperature technique for coating dielectric substrates, including flexible polymers.     

New multilayer arrangement of dielectric layers for enhancement of the magnetic shielding absorption at low frequency in the near field

This paper presents a new multilayer arrangement structure based on the dielectric polymer, which allows an effective absorption of magnetic wave radiation, and a selective reflection of desired wavelengths of the shield composite in the low frequency range that is not widely used in general cases. This is caused by the impedance mismatch between conductive films and air, and between conductive films and dielectric polymer layer. The structure is built from alternating conductive films and dielectric polymer layers, in a way that dielectric polymer layers are sandwiched between conductive films symmetrically in both directions. The simulation results of both the absorption and the total shielding effectiveness from the developing analytical model are in good agreement with experimental characterization, and could be further extended to the design of systems operating in other frequency ranges.     

Comparison between poly(methyl methacrylate)-carbon black and polyaniline conductive coatings

This work aims to develop and compare two types of conductive, polymer-based coatings via a dip coating process. The first type of coatings was made by dispersing and incorporating carbon black (CB) nanoparticles in a poly(methyl methacrylate) (PMMA) solution followed by dip coating. The CB content and dipping parameters were varied to explore their effects on the particle dispersion, coating thickness, and conductivity of the coatings. The dispersion of the CB particles in the polymer matrix was examined by scanning electron microscopy (SEM), while the coating thickness and conductivity were measured with a surface profilometer and a four-point probe electrometer, respectively. The good dispersion of carbon nanoparticles in the PMMA matrix was observed in the coatings. The conductivity of the PMMA-CB composite coatings was found to be mainly affected by CB content, particle dispersion as well as coating thickness. On the other hand, polyaniline (PANI) was used to make conductive coatings by dip coating and doping, and the advantages and limitations of the PANI coatings were compared with the PMMA-CB coatings.     

Spherical indentation and scratch durability studies of transparent conducting layers on polymer substrates

The effect of moderate normal and tangential spherical loading on the durability of brittle ceramic films sputtered on polymer substrates is investigated. Indium tin oxide films with various thicknesses are deposited by sputtering on polyethylene terephthalate substrates and their mechanical durability is assessed using a spherical nanoindenter and scratch tester with built-in optical microscopy capability.It is observed that crack initiation occurs at 40 mN normal applied load. As loading increases, formation of brittle ring cracking and of secondary radial cracks is evident. It is also shown that ring crack spacing is thickness dependent. During scratch testing two main coating failure modes are observed. The first is through thickness cracking and the second is buckling spallation of the coating. Both failure mechanisms exhibit thickness dependence.     

Self-assembled multilayers of nanocomponents

We show it is possible to assemble nanoparticle-polymer layers in a controllable manner dictated by the difference in nano-object morphology and dielectric properties. A thin (10-100 nm) layer of the two components is spin coated onto a solid substrate and the system thermally aged to activate a cross-linking process between polymer molecules. The nanoparticles segregate to the solid substrate prior to complete cross-linking if entropic forces are dominant or to the air interface if dielectric (surface energy) forces are properly tuned. Subsequent layers are then spin coated onto the layer below, and the process is repeated to create layered structures with nanometer accuracy useful for tandem solar cells, sensors, optical coatings, etc. Unlike other self-assembly techniques the layer thicknesses are dictated by the spin coating conditions and relative concentration of the two components.     

Evaluation of PVD nitride coatings, using impact, scratch and Rockwell-C adhesion tests

In this work the scratch test, the impact test and the Rockwell-C adhesion test were compared by investigating the adhesion properties of three types of sputtered physical vapour deposition coatings: TiN, CrN and Cr₂N. Each coating type was deposited on polished SAE 52100 steel, with different thicknesses in the nominal range of 2–20 μm, to evaluate the thickness influence on the test results. All the tests showed a ductile behaviour for the TiN coating with small delaminations and a brittle behaviour for the CrN and Cr₂N coatings with relatively large delaminations. An increase of the dimensions of the delaminations with increased coating thickness was detected for all layer types.

With the scratch test a significant increase in upper critical load with layer thickness was observed for all of the coatings. However, there was considerable variability especially for the Cr₂N coatings. The impact tests showed no coating failure for the TiN layer, and a decreasing impact crater volume with increasing layer thickness, whereas the CrN and Cr₂N layers failed after 10³ impacts and showed an increasing impact crater volume with increasing layer thickness.

The studies demonstrate the usefulness of using these test methods for differentiating between the behaviour of different coatings under various contact conditions.     

Composition profiles and adhesion evaluation of conductive diamond coatings on dielectric ceramics

Sintered silicon nitride (Si₃N₄) ceramic substrates were investigated as dielectric substrates for the growth of metal-like boron-doped nanocrystalline diamond (NCD) and microcrystalline diamond coatings via the Hot Filament Chemical Vapor Deposition (HFCVD) technique. The structural, electrical and chemical properties of both the ceramic substrates and the diamond coatings may potentiate their applicability in particular in harsh environments and highly demanding situations. Boron doping was achieved via a boron oxide solution in ethanol dragged into the reaction chamber with argon. The coatings were characterized by scanning electron microscopy, UV μ-Raman scattering, X-ray diffraction, time-of-flight secondary ion mass spectroscopy, Brale indentation for adhesion evaluation and two-point contact probe for resistivity measurements. The HFCVD technique led to a maximal growth rate of about 1 μm/h. Several metal-like boron doped diamond coatings were obtained. It was found that at lower substrate temperature, lower system pressure and higher methane concentration, the resistivity of the conducting NCD coatings is about 3 orders of magnitude higher when compared with samples obtained with higher substrate temperature, higher system pressure and lower methane concentration. Nevertheless, for every metal-like boron-doped coating the use of the Si₃N₄ ceramic substrate guaranteed a superior adhesion level.     

ELECTROPHORETIC COATINGS FOR ADVANCED MATERIALS

We have electrophoretically deposited a variety of coatings for a number of applications. We have also worked extensively with a process for electrophoretically depositing styrene acrylate polymer coatings. These coatings provide useful corrosion protection and dielectric properties for capacitors and electrical insulation. Dielectric breakdown strengths in the order of 1000 V/micrometer have been observed for capacitors with this coating as the dielectric. Various particles have also been dispersed in the electrolyte; these mixtures yield composite coatings of unusual materials such as fissile uranium in a carbon matrix. The process can also be adapted to form very thin, free-standing styrene acrylate films or pellicles. We have also explored the feasibility of depositing a variety of colloidal inorganic particles from liquid suspensions. Our results show that isopropanol works relatively well as a dispersing medium for a large number of powders. Isopropanol slurries can be used to deposit a number of uniform ceramic or glass coatings on metal substrates. Important coating considerations with regard to whether useful coatings can be deposited using this latter type of electrophoretic process include: (1) the average size, size distribution, and shape of the particles, (2) the charge assumed by the powder particles when they are dispersed in a liquid such as isopropanol, and (3) the coefficients of thermal expansion of the substrate and the coating material from the standpoint of the heat treatment or sintering required to obtain sufficient cohesion and adhesion.     

一种三层雷达吸波涂层的制备和性能

在磁性金属微粉的雷达吸波涂层中引入导电纤维层具有减轻质量和展宽吸收频带作用,本文采用磁性金属微粉和短导电纤维制备了一种三层雷达吸波材料.研究表明,三层的相对顺序对吸波涂层的带宽与峰值具有明显影响,在面密度为2.50 kg/m2的条件下,当导电纤维层位于表层时带宽最大,在8～18 GHz频段范围内反射率小于-10 dB的带宽达7 GHz.     

Low-velocity impact damage in brittle coatings

Based on thin plate/elastic foundation theory and strain energy release rate calculations, this paper presents a lateral crack model to analyse the threshold for and development of impact damage in brittle coatings. Numerical solutions indicate that an increase in coating-to-substrate elastic modulus ratio tends to enhance the coating's resistance to impact damage initiation. The size of the lateral crack in a coating layer is a function of the coating thickness. For comparison with the theoretical prediction, experiments were performed on SiC-coated graphite specimens, as well as on glass microscope slides resting on various kinds of substrates.     

The impact behaviour of paints

The split Hopkinson pressure bar has been used to study the impact behaviour of a selection of single and multi-layer paint systems in the form of films of thickness 0.04 mm. Stress-strain curves are presented for systems comprising three coatings, coating A, coating B and coating C, in compression for strain rates of the order 5×10³ s^–1. A comparison is made between the high strain-rate behaviour and that seen at quasi-static strain rates. All tests were carried out at 23 °C. The coatings studied are shown to be strain-rate sensitive, exhibiting almost a two-fold increase in flow/yield stress between the two strain-rate regimes. At low strain rates, all the coatings deformed uniformly with no sign of fracture. At high strain rates, both coating A and coating C underwent catastrophic failure which is indicative of their susceptibility to chipping. However, this was not the case with coating B which shows no signs of fracture at high strain rates for strains up to 45%. However, a combination of coating A and coating B in alternate layers led to catastrophic fracture of the resulting two-coat multi-layer system at high strain rates.     

Facile and rapid fabrication of conductive layers on flexible polymer surfaces and their application to flexible strain sensors

The advantage of building a conductive network on the surface layer of a flexible substrate is that it has less impact on the elastic recovery properties of the substrate, which is particularly important for flexible strain sensors. However, the facile construction of robust conductive layers on the surface of flexible polymers remains a challenge. Herein, a method for constructing robust conductive layers on the surface of thermoplastic polymers was developed by immersing thermoplastic polymers in a solvent/conductive filler dispersion with the assistance of ultrasound. The solubility of the solvent in the flexible polymer and ultrasonic field are key to the preparation of the conductive layer. This method has the advantages of fast preparation and robustness of the conductive layer and can be applied to thermoplastic polymers of different polarities as well as different types of conductive fillers. Based on this method, a flexible strain sensor with a robust carbon black conductive layer on the styrene–butadiene–styrene block copolymer was prepared in as short as 2 s. The advantages of a broad strain detection range (0.1% to 400%) and robust cyclability of the sensors were exhibited. The sensors can be used for human motion monitoring as well as solvent detection.

     

Strain dependence electrical resistance and cohesive strength of ITO thin films deposited on electroactive polymer

Transparent, conducting, indium tin oxide (ITO) films have been deposited, by pulsed dc magnetron sputtering, on glass and electroactive polymer (poly(vinylidene fluoride)—PVDF) substrates. Samples have been prepared at room temperature by varying the oxygen partial pressure. Electrical resistivity around 8.4 × 10^− 4 Ω cm has been obtained for films deposited on glass, while a resistivity of 1.7 × 10^− 3 Ω cm has been attained in similar coatings on PVDF. Fragmentation tests were performed on PVDF substrates with thicknesses of 28 μm and 110 μm coated with 40 nm ITO layer. The coating's fragmentation process was analyzed and the crack onset strain and cohesive strength of ITO layers were evaluated.