Field of residual stresses in a coating with crack

We consider an object with coating deposited on its plane surface. The coating is subjected to the action of known uniform uniaxial tensile residual stresses. We analyze the field of residual stresses formed in the coating as a result of the formation of a long flat crack perpendicular both to the direction of action of tensile stresses and to the surface of the coating. The problem is studied in the two-dimensional statement. The stress field is investigated in the sections perpendicular both to the crack plane and to the surface of the coating. Two cases are analyzed, namely, the case of a through crack (through the entire thickness of the coating) and the case of a nonthrough crack with plastic zone on its continuation. The performed numerical analysis enables us to estimate the possibility of subsequent fracture of the coating after crack initiation. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 2, pp. 80–88, March–April, 2006.     

Dimensional effects observed for the electrical, dielectrical and optical properties of TiO2 DC magnetron thin films

In this paper, we present the dimensional effects observed for TiO₂ films deposited by DC circular magnetron sputtering. TiO₂ thin films are deposited in an Al–TiO₂–Al structure to investigate their non-linear characteristics, from which the carrier effective mass and the barrier potential at the Al–TiO₂ interface is calculated, and an important relationship between the effective carrier mass and film thickness is observed. The dielectric constant of TiO₂ thin films is also investigated, and is observed to vary with TiO₂ film thickness. Further, TiO₂ band gap is observed to vary with film thickness.     

Distribution of temperature in a plate with a single-layer coating subjected to intense heating

We establish the distribution of temperature formed under conditions of abrupt changes in the surface temperature near the adhesional contact between the base and the coating in a plate of infinite length made of molybdenum or niobium both sides of which are covered with silicide coatings. The problem of heat conduction for a multilayer plate subjected to thermal cycling is solved by the method of finite integral transformations. It is shown that a silicide coating with a thickness of 60–100 μm leads to the formation of a significant temperature gradient in the base material under conditions of cyclic variation of temperature. This work was partially supported by the Soros International Program in the Field of Exact Sciences (ISSEP), Grant No. PSU 062034. Translated from Problemy Prochnosti, No. 3, pp. 128–133, May–June, 1998.     

Optimization of slot-coating processes: minimizing the amplitude of film-thickness oscillation

Slot coating is one of the most common coating methods for high-precision coatings. The flow limits of steady-state operation have been extensively analyzed in the literature. However, even the best-designed slot-coating operations are subjected to small oscillations on process conditions, such as flow rate, vacuum pressure and gap fluctuations. These time-dependent events lead to thickness variations on the deposited liquid layer that may be unacceptable for product performance. Therefore, the design of slot-coating processes, e.g. the specification of process conditions and die geometry, has to take into account not only the behavior of steady-state flow but also how the flow responds to small periodic disturbances. The process parameters should be such that the film-thickness oscillation is minimized. In this study, coating-thickness variations related to an ongoing oscillation of the coating gap is studied for different process parameters and frequency of the perturbation by solving the transient Navier–Stokes equations with appropriate boundary conditions for free-surface flow. The amplitude of the deposited film-thickness oscillation is used as the objective function of a bound-constrained optimization algorithm. The results show that at a fixed web speed and wet thickness, the film-thickness amplitude may be reduced by a factor of 4 by adjusting other process parameters.     

Electrophoretic deposition of zirconia layers for thermal barrier coatings

This paper presents an exploratory investigation of a production route for thermal barrier coatings (TBCs) from a stable zirconia nanopowder suspension by means of electrophoretic deposition (EPD) on a low carbon steel substrate, followed by a consolidation by hybrid microwave sintering.Since the thickness of the green deposit is restricted to a critical thickness, which depends on the curvature of the substrate, a method of repeated deposition and sintering was applied in order to produce thick coatings. The low electric conductivity of the sintered ceramic coating does not inhibit the deposition of additional layers. This method enabled the production of porous coatings with a sintered thickness of 150–300 μm.A finite element model was applied to evaluate the thermal conductivity of the coatings. Values of the order of 0.8 W/mK were obtained which is comparable to that of coatings deposited by other techniques. Hence, the multiple deposited coatings with their thick and porous microstructure have a high potential for thermal barrier coating (TBC) applications.     

On the dynamics of localization and fragmentation: V. Response of polymer coated Al 6061-O tubes

In this series of papers, we investigate the mechanics and physics of localization and fragmentation in ductile materials. The behavior of ductile metals at strain rates of about 4,000–15,000 per second is considered. The expanding ring experiment is used as the vehicle for examining the material behavior in this range of strain rates. In Parts I–III, we examined the response of rings with cross-sectional aspect ratios in the range of 1–10, exhibiting a transition from diffuse necking to sheet-mode localization. In Part IV, we reported on experimental observations of high strain-rate expansion of Al 6061-O tubes. In the present paper, we investigate the effect of polymer coatings on Al 6061-O tubes; specifically, polyurea and polycarbonate coatings are investigated as coating materials. It is demonstrated that there are two effects of the polymer coating on the overall deformation of the Al 6061-O tube specimens. First, the additional mass of the coating material results in an inertial effect; therefore, thicker coatings result in a slower overall expansion and hence for a limited impulse loading, less overall straining of the metallic specimen. Second, the flow resistance of the polymer dissipates additional energy and can prevent failure in the metallic specimen. This effect is much more pronounced in polycarbonate, the material with a larger strength. Finally, it is demonstrated that the polymer coating does not significantly influence the dynamic forming limit of the metallic material.     

An effective approach to activate 316L stainless steel for biomimetic coating of calcium phosphate: electrochemical pretreatment

In this paper, an electrochemical (EC) method to activate 316L stainless steel (denoted as 316L) surface for biomimetic calcium phosphate (Ca–P) coatings was reported. After EC treatment, a gel-like Ca–P film with a thickness of 150 nm was generated on the stainless steel surface after treatment, which was composed of amorphous phase of calcium phosphate with a large number of crystal nuclei of octacalcium phosphate (OCP) inside. This Ca–P thin film is the main factor that causes Ca–P formation under biomimetic condition. The effectiveness of EC treatment was also compared with alkali heat (AH) pretreatment in producing biomimetic coating on 316L. A uniform Ca–P coating formed on EC treated samples after samples were immersed in saturated calcium solution (SCS) for several hours, while only some island-like deposits were found on the sample surface with AH treatment followed by immersion in SCS for several days. This work has explored a novel and effective pretreatment method to activate 316L implant surface, which can be expected to be applied to activate other metal implants.     

Reflectionless absorption of electromagnetic radiation in a magnetic layer

The conditions for occurrence of total reflectionless absorption of electromagnetic waves in a layer of magnetic material applied to a metal substrate have been found. Their dependence on the layer thickness and the magnetic properties of the coating has been investigated. The frequency band within which this phenomenon can exist has been determined. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 5, pp. 19–22, September–October, 2007.     

Microfluidics and rheology of carbon black suspensions for In-Mold Coating applications: some insights into the slip flow phenomena

Microfluidics and rheology are critical to modeling material processes such as In-Mold Coating (IMC). In the IMC process, a carbon black suspension is injected onto the surface of the molded part while the part is still in the mold. Due to the microscopic length scale of the IMC flow (10–25 μm), a study of the slip flow and rheological properties at high shear rates of the coating liquid becomes significant. A customized microslit rheometer was developed and used to measure the viscosity of a coating material being considered for IMC in the channel gaps between 25 and 100 μm (C. Aramphongphun Ph.D. Dissertation, The Ohio State University, 2006). The results were interpreted assuming true slip at the wall. In this paper, we present further insights into the slip flow phenomena. It is assumed that there is a fluid layer of lower viscosity near the wall, which accounts for the appearance of slip. The previously obtained data is interpreted using this assumption.     

Sintering characteristics of nano-sized Ag–Pd–glass composite powders with high Pd content

Nano-sized Ag–Pd (50–50) alloy powders coated with Pb-based glass material with low and high glass transition temperature are directly prepared by high-temperature flame spray pyrolysis. Nano-sized Ag–Pd–glass composite powder is formed from the evaporated vapors by nucleation and growth process, and then glass material moves out to the outside of the powder by crystallization process of alloy. The thickness of the glass coating layer measured from the TEM image is 2.8 nm. The mass changes of the Ag–Pd alloy and Ag–Pd–glass composite powders in the TG analysis under 900 °C are 10.9 and 6.8%, respectively. Glass materials improve the uniformity and density of the Ag–Pd electrode layers by act as sintering agent and adhesion improvement. The Ag–Pd electrode formed from the composite powders with high glass transition temperature glass material has thin and uniform thickness. The specific resistances of the electrodes formed from the nano-sized Ag–Pd–glass composite powders are 0.27, 0.09, and 0.03 mΩ cm at firing temperatures of 700, 800, and 900 °C, respectively.     

Kinetics of microwave drying of a free-flowing organic material

The kinetics of drying of a dense buckwheat layer in a microwave electromagnetic field of frequency 2.45 GHz has been investigated for different amounts of the material charged into a working chamber. Analysis of the kinetics curves has shown that the drying of the material studied is divided into the periods of heating, drying with a constant rate, and drying with a decreasing rate. The influence of the power supplied as well as the mass and dimensions of a sample on the rate of its drying has been investigated and a formula for calculating this rate has been obtained. It has been established that, in the process of drying of a disperse material, the amount of microwave energy converted into heat energy depends not only on the mass of a sample, but also on the thickness and area of its surface layer. Generalized equations for calculating the moisture content in a layer of a free-flowing material and its temperature have been obtained. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 3, pp. 123–127, May–June, 2006.     

Weight function for a circular crack in the temperature problem of the theory of elasticity

A method for the construction of the weight function for a circular crack in an elastic body is proposed for problems of thermoelasticity. The case of an infinite body is studied in more detail. We deduce equations for the stress intensity factors in the cases where either temperature or the heat flow is specified on the crack surface and present some examples of their application. Kiev International University of Civil Aviation, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 6, pp. 91–98, November–December, 1998.     

A plane dynamic axisymmetric problem for a hollow cylinder

A dynamic problem for an elastic hollow cylinder whose surfaces are subjected to the action of arbitrary forces is solved by a new method based on the use of finite differences only with respect to time [12]. The numerical analyses of the time dependences of stress concentration are carried out for hollow cylinders of different thickness under impact loading. The proposed method is tested by solving a problem of circular hole in an infinite body subjected to the action of impact loads over the entire surface. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 1, pp. 7–13, January–February, 2008.     

Resonant nature of the sensitivity of gas-analytical chemical sensors using a composite acoustic resonator

It is shown that the sensitivity of gas-analytical chemical sensors can be drastically enhanced when the thickness of the film deposited on the resonator surface corresponds to a quarter of the acoustic wavelength in the resonator. Pis’ma Zh. Tekh. Fiz. 25, 76–80 (July 26, 1999)     

Effect of coatings on the distribution of heat in bar-disk tribosystems

We develop an analytic method for the evaluation of the surface temperature and the coefficient of distribution of heat in a tribosystem formed by an immobile bar and a rapidly rotating disk with coating. The analysis of numerical results demonstrates that surface temperature increases with the thickness of the coatings if the thermal conductivity of the coating is lower than the thermal conductivity of the disk. For coatings of fixed thickness, surface temperature decreases as the angular velocity increases. We also study the dependences of the coefficient of distribution of heat between the bodies on their geometric parameters and thermal properties. Franko L'viv University, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 43–49, May–June, 1997.     

Mass transport in a high-gradient thermal field in the course of friction stir welding and modification of the upper layer

We analyze the mass transport in high-gradient thermal fields formed in microvolumes of the material in the course of friction stir welding (FSW) and modification of the upper layer (friction stir processing). A simplified Fe–Cr diffusion system with unidirectional thermodynamic excitations is analyzed. The character of mass transport is determined by thermal diffusion, which affects the final distribution of the concentration of diffusing elements in the friction region. The chemical potentials of the components of the process lead to phase transformations typical of the (Fe–C)–Cr ternary system. We present the results of analysis of these phenomena. The verification of solutions is realized by analyzing an example of adhesive chromium layer deposited on a matrix of S235JR and S355 low-alloyed steels. A modeled heat source in the form of laser radiation is used in the microvolumes of the analyzed friction unit.     

Microstructure and solid particle erosion of carbon-based materials used for the protection of highly porous carbon–carbon composite thermal insulation

Multiparticle erosion tests were performed on candidate coating (colloidal graphite paints) and cladding (dense carbon–carbon composites and graphite foil) materials employed to protect porous carbon–carbon composite thermal insulation in vacuum and inert-gas furnaces that utilize inert gas quenching. The dependence of the erosion rate on the angle of incidence of the erodent was examined and related to the microstructure and the mechanisms of material removal as observed by SEM. In addition, the effect of a thin chemical vapour deposited (CVD) carbon layer on top of a colloidal graphite paint coating and a graphite foil clad was investigated. The coating and cladding materials displayed a greater erosion resistance at all angles of incidence compared to the porous carbon–carbon composite. In general, the greatest erosion rate was found at an angle of incidence of 90°, where the erodent stream is perpendicular to the erosion surface, and brittle fracture was the predominant mechanism of material removal. The exception was the graphite foil material which displayed maximum erosion at an angle of incidence of 60°. For this material, two mechanisms were effective: disruption of the graphite flakes, which are mainly held together by mechanical locking, and a ploughing-like mechanism. The addition of a thin CVD carbon layer to colloidal graphite paint improved performance, whereas the erosion resistance of the graphite foil was slightly degraded as the CVD layer was too thin to prevent the ploughing-like mechanism. This revised version was published online in November 2006 with corrections to the Cover Date.     

On an approach to numerical solution of stress-strain state problems for nonthin plates and shells

Numerical solution of elastic equilibrium problems for nonthin plates and shells of constant and variable thickness is based on using a curvilinear-mesh method in combination with the Vekua method — reduction of three-dimensional elastic equations to a recurrent sequence of boundary problems in a two-dimensional region. Coefficients of the first and second quadratic forms for a conditional mediane surface are calculated by means of a metric of boundary faces without any derivatives with respect to their local bases. Specific examples of numerical solution of the test problems for a thick plate bending case, for which the exact and approximate solutions can be also found by other methods, have proven the proposed numerical approach to be efficient (in terms of a rapid convergence and accuracy). A numerical solution has been obtained for a problem of bending for a nonthin plate of constant and variable thickness made of an orthotropic material and for a shell with a circular recess under axial compression. __________ Translated from Problemy Prochnosti, No. 2, pp. 41–54, March–April, 2008.     

Diffusion of solvent across the surface of a resist film during centrifuging

The influence of surface stresses induced by an external force on the diffusion of solvent across the surface of a resist film during centrifuging is investigated. The proposed model can be used to estimate the formation time of the resist film and the thickness of the protective coating. Pis’ma Zh. Tekh. Fiz. 25, 91–94 (December 12, 1999)     

A study of the dissipative properties of homogeneous materials deposited as coatings. Part 1. Method for the determination of the amplitude dependence of the true vibration decrement of the coating material

A method is proposed for the determination of the amplitude dependence of the true vibration decrement of homogeneous materials deposited as coatings on a substrate of rectangular cross section. The method is based on the self-consistent calculation of energy loss factors for the substrate and coating materials using experimental amplitude dependences of vibration decrement for coated substrate, obtained under the conditions of cantilever specimens of rectangular cross section. The applicability of the method is illustrated by the determination of values of the true vibration decrement of Co-20 wt.% Fe alloy deposited as a coating on VT1-0 titanium alloy. __________ Translated from Problemy Prochnosti, No. 6, pp. 134–143, November–December, 2007.