Surface Structures in Thin Polymer Layers Caused by Coupling of Diffusion‐Controlled Marangoni Instability and Local Horizontal Temperature Gradient

Summary: Surface tension‐driven Marangoni convection causes the formation of regular surface structures in drying polymer layers. The shape of the surface structures formed during solvent evaporation depends on layer and interfacial dynamic parameters as well as external factors. The influence of a horizontal radial temperature gradient produced by a point heat source below the polymer layer on the diffusion‐controlled Marangoni instability has been studied. In the region of the lateral temperature gradient, radial surface flow coupled with the interfacial instability leads to stripe, ladder, chevron and/or labyrinthine surface structures.

Stepped ladder structures in a poly(vinyl butyral) layer produced by interfacial instability and heating with an ultrasonic sonotrode below the layer substrate.     

Compressive Surface Layer to Avoid Edge Cracking in Laminar Ceramic Composite

Previous work has shown that layers within ceramic laminates that contain biaxial compressive stresses will also contain tensile stresses where the layer intercepts the surface. It has also been shown that when the thickness of the layer exceeds a critical value, the tensile stresses at and near the surface can produce a centerline crack that extends along the surface to a depth corresponding to the thickness of the compressive layer. The current work explores the concept of preventing the occurrence of surface cracks with a thin layer of material that places the entire external surface in compression. A recent finite element analysis by Monkowski and Beltz showed that when the external surface layer material was identical to the compressive layers, the tensile stresses at the surface could be reduced to zero when the thickness of the surface layer was 0.6 of the thickness of the compressive layer. In addition, they determined the stress intensity factor function for a surface crack and showed that surface cracking could be avoided when the thickness of the surface layer was ≥0.25 times the thickness of the compressive layer. The experimental results presented here show that a thin compressive layer will prevent surface cracks from forming in the compressive layers; the results also appear to confirm the predictions of Monkowski and Beltz.     

Thermomechanical surface instability at the origin of surface fissure patterns on heated circular MDF samples

When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primary crack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary to common belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that this results from thermomechanical instability. In the present paper, new experimental data are presented for circular samples of the same MDF material. The sample was exposed to radiant heating at 20 or 50 kW/m², and completely different crack patterns with independent eigenmodes were observed at the two heat fluxes. We show that the two patterns can be reproduced with a full 3‐D thermomechanical surface instability model of a hot layer adhered to an elastic colder foundation in an axisymmetric domain. Analytical and numerical solutions of a simplified 2‐D formulation of the same problem provide excellent qualitative agreement between observed and calculated patterns. Previous data for square samples, together with the results reported in the present paper for circular samples, confirm the validity of the model for qualitative predictions and indicate that further refinements can be made to improve its quantitative predictive capability.     

Electrophoretic Deposition of Zirconia on Porous Anodic Substrates

The aim of the present work is the preparation of thin (<20 μm) zirconia layers on porous substrates with the electrophoretic deposition process. The preparation was completed with a cosintering step of substrate and layer. Through adjustment of shrinkage and the shrinkage rate of the deposited zirconia layer on the presintered porous substrate, thin, dense layers without cracks were prepared. A method for direct control of the layer thickness during the electrophoretic deposition process was developed. The solid oxide fuel cell application with porous anode substrates and thin zirconia electrolytes was chosen to demonstrate the potential of the electrophoretic deposition process.     

Failure Modes in Ceramic-Based Layer Structures: A Basis for Materials Design of Dental Crowns

A research program on failure modes induced by spherical indenters in brittle layer structures bonded to polymeric substrates, in simulation of occlusal function in all-ceramic dental crowns, is surveyed. Tests are made on model flat and curved layers bonded onto a dentin-like polymer base, in bilayer (ceramic/polymer) and trilayer (ceramic/ceramic/polymer) configurations. All-transparent systems using glass as a porcelain-like outer or veneer layer and sapphire as a stiff and strong core support layer enable in situ observation of the entire evolution of fracture modes in the brittle layers, from initiation through to failure. With the fracture modes identified, tests are readily extended to systems with opaque polycrystalline dental core ceramics, notably alumina and zirconia. A variety of principal failure modes is identified: outer and inner cone cracks developing in the near-contact region at the top surface; radial cracks developing at the bottom surface along the loading axis; margin cracks from the edges of dome-like structures. All of these modes are exacerbated in cyclic loading by time-cumulative slow crack growth, but inner cones are subject to especially severe mechanical fatigue from hydraulic pumping of water into the crack fissures. Conditions under which each mode may be expected to dominate, particularly in relation to geometrical variables (layer thickness, contact radius) and relative material properties, are outlined. Clinical issues such as crown geometry, overload versus fatigue failure, role of residual stresses in fabrication, etc. are addressed.     

Bimaterial Composites via Colloidal Rolling Techniques: II, Sintering Behavior and Thermal Stresses

Shrinkage behavior and crack formation during firing have been investigated for Al₂O₃/Ce-TZP composites that have been fabricated by colloidal rolling and folding. These composites show improved sinterability and sinter isotropically after repeated rolling. Interface instability in rolling creates corrugated interfaces with large layer waviness; therefore, rolling can substantially alleviate the in-plane sintering constraints, which leads to improved sinterability. A loss of sintering anisotropy also is observed and is directly correlated to the microstructure instability, which is coincident with the laminate-cellular transition. Sintering cracks during heating and thermal cracks during cooling both are limited to the thick Ce-TZP layers in the composites. The critical layer thickness and the normalized crack spacing of the thermal cracks follow the predicted behavior of elasticity theory. Thus, crack-free, high-density Al₂O₃/Ce-TZP composites with either a laminate or cellular microstructure can be obtained, with a layer thickness of 4-60 µm, via pressureless sintering.     

Prevention of Cracks in Functionally Graded SiO2‐Mo Materials Fabricated by Uniaxial Compression Casting and Pressureless Sintering

In industrial high‐intensity discharge lamps, cracks and delaminations occasionally develop at the interface between SiO₂ and the Mo foil in the seal. Here, functionally graded SiO₂‐Mo materials for use in these lamps were fabricated by uniaxial compression casting and pressureless sintering. Consequently, vertical cracks developed across the sintered body layers, and interfacial cracks developed between the 100 wt% SiO₂ and 90 wt% SiO₂‐Mo layers. Therefore, the effects of residual stress, difference in the coefficient of thermal expansion (CTE), and difference in the volume shrinkage on these cracks were investigated. Vertical cracks were suppressed when residual stress was relaxed by annealing near the annealing point of silica glass during the cooling step in the sintering process. Interfacial cracks were suppressed when the difference in the CTE of the interface between the 100 wt% SiO₂ and 90 wt% SiO₂‐Mo layers was relaxed by inserting layers of 95 wt% SiO₂‐Mo between them. Furthermore, the suppression effect became stronger when the difference in the volume shrinkage of the layers was relaxed by sintering to join the separately sintered monolayers. Thus, the development of these cracks was influenced by the residual stress, CTE, and volume shrinkage. Therefore, these cracks can be prevented by optimizing these factors.     

Crack Interactions in Laminar Ceramics That Exhibit a Threshold Strength

Laminar ceramic composites have been fabricated with thin compressive layers, containing a mixture of alumina and mullite, sandwiched between thicker alumina layers. It has previously been shown that a single crack that extends within a thicker alumina layer can be arrested by the compressive layers to produce a threshold strength, i.e., a strength below which the probability of failure is zero. The behavior of multiple cracks within the laminate has been investigated, to observe the mechanisms of crack interaction and measure their influence on the threshold strength. It was found that when the cracks in adjacent thick layers were offset by a distance less than the thickness of two thick layers, the cracks would interact and decrease the threshold strength. The number of interacting cracks, their orientation, and location can also have an effect on the threshold strength.     

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Surface modification of coil coatings with thin plasma polymer films structure and stability

Plasma deposition of a thin top layer with tailored properties is an effective strategy of modification of the organic coating surface. Thin plasma polymer layers are candidates and can provide superior hardness, scratch resistance, modified surface hydrophobicity and easy to clean properties.The present work studies the stability of thin plasma polymer films deposited as top layer on polyurethane coil coating systems. Microwave, hollow cathode and radio frequency plasma polymerization reactors were employed in order to deposit a thin SiO_x based plasma polymer layer.The plasma film stability was studied using surface analysis techniques, ex situ and in situ atomic force microscopy and scanning electron microscopy. Energy dispersive spectroscopy, FTIR spectroscopy and optical measurements confirm the composition and plasma layer properties. The structure of the plasma layers was investigated by means of transmission electron microscopy.The surface morphology together with composition evolution allows the study of the stability of the different coatings. The structure examination of the formed plasma polymer film offers good clarification for coating failure. Decrease of the operating pressure during plasma polymerization and oxygen concentration in precursor mixture lead to formation of compacter layer with higher stability. Introduction of fluorine-containing precursor also increases the anti-weathering performance of the plasma polymer films.     

Detecting sub‐surface cracking in laminated membranes using infrared imaging

This paper presents a new experimental method that utilizes the thermoelastic effect to detect sub‐surface cracks in a laminated polymer membrane. A highly accurate infrared camera is used to measure the thermoelastic and dissipational heat signatures associated with bi‐axial fatigue loading of membranes. Changes in these thermal signatures arise whenever cracks form in any layer of the laminate, including fully embedded layers, thereby providing a novel method for experimentally measuring the initiation and growth of damage in sub‐surface layers. The proposed method is illustrated using a model 3‐layer system of EVOH sandwiched between two polyurethane layers. Bi‐axial fatigue loading was used to initiate cracking in the central EVOH layer without damaging the outer polyurethane layers. Cracking in the central layer resulted in a distinct thermal signature that was plainly visible using the proposed method.     

Influence of Cold Low Pressure Lamination Parameters on the Joining of Low Temperature Co‐Fired Ceramics Green Tapes and Sintered Alumina Substrates

In this study, laminates consisting of sintered alumina substrates and green Low Temperature Co‐fired Ceramics (LTCC) tapes have been produced via Cold Low Pressure Lamination which is based on adhesive tapes for joining of layers at room temperature and pressures <5 MPa. The influences of lamination parameters such as temperature, pressure, and time on the quality of the green and sintered multilayer stack have been determined. If the bottom LTCC layer of an alumina–LTCC–LTCC laminate is metallized by screen printing defects such as crack formation can occur due to stress formation caused by constrained sintering. By adapting the lamination parameters, these stresses can be avoided. Another defect observed is cavities which form along the printed circuit lines. This type of defect is caused by the shrinkage of the circuit line width during firing; by reducing the height of the conductor line during screen printing, the cavity size can be reduced. In addition, different screen‐printed metallization layouts have been tested to determine the influence of line and spaces on the quality of sintered laminates.     

Effect of residual stresses to the crack path in alumina/zirconia laminates

Laminates with alternating layers are well known from nature. The strongly bonded alumina/zirconia (Al₂O₃/ZrO₂) layers can combine high fracture resistance with high strength and stiffness when properly tailored. The presence of compressive residual stresses formed in Al₂O₃ layers can suppress and deflect cracks propagating through the layers. The crack path is governed by both the elastic properties and the internal stress field of individual layers. The laminates with various layer-thickness ratios ranging from 0.1 to 3 were used to investigate the effect of residual stresses and influence of crack formation pattern on the crack path development. The indentation surface cracks observed in various alumina-zirconia laminates exhibit the same crack deflection independently on the level of internal stresses. The crack deflection observed on the fracture surfaces of bending specimens was related to the indentations cracks. The complicated crack path was explained experimentally by 3D reconstruction with the support of numerical simulations.     

Transitions of morphological patterns of crystallizing polycarbonate in thin films

This article reports the transitions of morphological patterns of polycarbonate crystals in thin films by solvent‐induced crystallization (SINC). As a substrate (silica glass) deposited with an amorphous and micron‐thick bisphenol A polycarbonate polymer film is partially dipped into a liquid acetone bath, acetone penetrated rapidly through the polymer film. The rate of acetone penetration is significantly higher than the predicted by Fickian diffusion or anomalous diffusion model, indicating that the capillary force through stress‐induced cracks may have played a major role in the upward transport of acetone through the polymer films. The morphologies of polycarbonate at different vertical positions on a substrate surface were analyzed by scanning electron microscopy and atomic force microscopy. It was observed that depending on the local acetone concentration the polymer morphologies showed quite diverse patterns ranging from stress‐induced cracks to fully developed three‐dimensional spherulites. The diverse morphologies developed during the thin film SINC may serve as a useful platform for further detailed mechanistic analysis of structures and crystallization kinetics. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation of dual‐layer acetylated methyl cellulose hollow fiber membranes via co‐extrusion using thermally induced phase separation and non‐solvent induced phase separation methods

Dual‐layer acetylated methyl cellulose (AMC) hollow fiber membranes were prepared by coupling the thermally induced phase separation (TIPS) and non‐solvent induced phase separation (NIPS) methods through a co‐extrusion process. The TIPS layer was optimized by investigating the effects of coagulant composition on morphology and tensile strength. The solvent in the aqueous coagulation bath caused both delayed liquid–liquid demixing and decreased polymer concentration at the membrane surface, leading to porous structure. The addition of an additive (triethylene glycol, (TEG)) to the NIPS solution resolved the adhesion instability problem of the TIPS and NIPS layers, which occurred due to the different phase separation rates. The dual‐layer AMC membrane showed good mechanical strength and performance. Comparison of the fouling resistance of the AMC membranes with dual‐layer polyvinylidene fluoride (PVDF) hollow fiber membranes fabricated with the same method revealed less fouling of the AMC than the PVDF hollow fiber membrane. This study demonstrated that a dual‐layer AMC membrane with good mechanical strength, performance, and fouling resistance can be successfully fabricated by a one‐step process of TIPS and NIPS. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42715.     

Improved Fracture Behavior of Alumina Microstructural Composites with Highly Textured Compressive Layers

Alumina‐based microstructural composites combining equiaxed and textured layers were fabricated to examine how cracks propagate and the mechanical properties are affected as a function of the residual stress and volume fraction of texture in a multilayer structure. By combining equiaxed and highly textured alumina layers of varying thermal expansion, the embedded textured layers were placed under compressive residual stresses as high as ?670 MPa. Composites with a near constant maximum failure stress of up to 300 MPa were shown to be almost independent of the initial defect size as result of the compressive residual stress in the textured layers. An apparent fracture toughness of up to 10.1 MPa·m^1/2 was obtained for composites with an equiaxed to textured volume ratio of 7.4:1. The high compressive stress in the textured layers arrested cracks, whereas the weak bonding parallel to the basal surfaces of the textured alumina grains caused cracks to deflect within the textured layers. The coupling of these two mechanisms resulted in crack arrest and a maximum work of fracture of ~1200 J/m² or almost 50 times higher than equiaxed alumina. We believe that embedding textured layers having compressive stresses below the surface of multilayer composites represent an important strategy for designing flaw‐tolerant materials with pronounced crack growth resistance and a high work of fracture.     

Cracking of Laminates Subjected to Biaxial Tensile Stresses

During the processing of laminar ceramic, biaxial residual stresses can arise due to differential thermal contraction between unlike layers. A tensile stress can cause preexisting flaws to extend across the layer and into the adjacent layers and then tunnel until they meet either another crack or a free surface. A previous analysis has shown that for a given residual stress there is a critical layer thickness, below which no tunnel cracks will exist, regardless of initial flaw size. Here, the previous analysis was modified to take into account the crack extension into adjacent layers. To determine the validity of the analysis, laminates composed of alternating layers of zirconia and alumina/zirconia were fabricated by a sequential centrifugation technique. The composition of the alumina/zirconia layer was varied to change the biaxial, tensile stresses in the zirconia layer. Observations were then made to determine the critical layer thickness for tunnel cracks and their extension into the adjacent layers. These observations were compared to the theoretical predictions.     

Enhancement of adhesion between inorganic nanoparticles and polymeric matrix in nanocomposite by introducing polymeric thin film onto nanoparticles

Herein, we demonstrate an efficient pathway to improve both affinity and adhesion at interfaces between inorganic nanoparticles and organic polymer matrix by vapor phase incorporation of organic thin layers. The polymeric thin films with an average thickness of a few ～tens of nanometers were introduced onto the surface of inorganic nanoparticle by vapor deposition polymerization method. Diverse polymers such as vinyl polymers and conducting polymers can be coated on the surface of indium tin oxide (ITO) nanoparticles with a relatively simple method. The polymer‐coated ITO nanoparticles showed improved‐compatibility with polymer matrix in nanocomposite owing to the improved adhesion because of the presence of intermediate polymeric layer. POLYM. ENG. SCI., 55:1906–1911, 2015. © 2014 Society of Plastics Engineers     

Crack Propagation Behavior of Y-TZP Ceramics

Y-TZP ceramics present the twofold advantage of a very fine grain size and a phase transformation leading to some reinforcement, making them useful in hip joint head prostheses. The subcritical crack growth of such a material that mainly drives the lifetime of the components has been evaluated for wide crack velocity ranges down to 10^-11 m/s by using both double torsion and static fatigue tests. It appeared that subcritical crack growth was activated by H₂O. It is shown that two types of flaws should be considered: surface flaws caused by machining located in the compressive layer present at the surface, and so-called volume flaws. The subcritical crack growth and the toughness of both types of defects are different. The relevance of subcritical crack growth analysis for such surface cracks is discussed. The compressive layer at the surface leads to a threshold value below which no crack growth occurs.     

Effect of Solute‐ and Solvent‐Derived Marangoni Flows on the Shape of Polymer Films Formed from Drying Droplets

The effects of mixed solvents on the shape of films formed on a hydrophilic surface from polymer solute droplets were investigated experimentally. The film shape depended on the two solvents used, and the mixing ratio. It was also found that the flow direction was dominated by solute‐derived rather than solvent‐derived Marangoni flows. Consequently, the changes in the shape of the film could not be explained only by the Marangoni flow direction.