Cracks In Coatings Research Articles

Dual Monitoring of Cracking and Healing in Self-healing Coatings Using Microcapsules Loaded with Two Fluorescent Dyes.

We report the development of an extrinsic, self-healing coating system that shows no fluorescence from intact coating, yellowish fluorescence in cracked regions, and greenish fluorescence in healed regions, thus allowing separate monitoring of cracking and healing of coatings. This fluorescence-monitoring self-healing system consisted of a top coating and an epoxy matrix resin containing mixed dye loaded in a single microcapsule. The dye-loaded microcapsules consisted of a poly(urea-formaldehyde) shell encapsulating a healing agent containing methacryloxypropyl-terminated polydimethylsiloxane (MAT-PDMS), styrene, a photo-initiator, and a mixture of two dyes: one that fluoresced only in the solid state (DCM) and a second that fluoresced dramatically in the solid than in the solution state (4-TPAE). A mixture of the healing agent, photo-initiator, and the two dyes was yellow due to fluorescence from DCM. On UV curing of this mixture, however, the color changed from yellow to green, and the fluorescence intensity increased due to fluorescence from 4-TPAE in the solid state. When a self-healing coating embedded with microcapsules containing the DCM/4-TPAE dye mixture was scratched, the damaged region exhibited a yellowish color that changed to green after healing. Thus, the self-healing system reported here allows separate monitoring of cracking and healing based on changes in fluorescence color.

A Comprehensive Review of Corrosion Resistance of Thermally-Sprayed and Thermally-Diffused Protective Coatings on Steel Structures

Protective coatings are widely utilized to promote corrosion resistance of the surfaces of steel components that are used in various industrial applications. Different surface engineering methods such as thermal spraying and thermal diffusion techniques can be used to fabricate these protective coatings. Thermal spraying processes have received significant attention due to their ability to deposit a variety of materials. Several metals such as zinc-, aluminum-, nickel, and chromium-based materials and their alloys can be deposited using thermal spraying processes to enhance the corrosion resistance and prolong the service life of steel components. On the other hand, technologies based on thermal diffusion techniques are emerging due to their unique deposition features, which alleviates the issues of cracking and delamination typical of thermal-sprayed coatings, as well as their ability to protect inaccessible and complex components such as the inner surface of long tubing and pipelines. This work is a comprehensive review on short and long-term corrosion resistance of the most effective and commonly-used coatings deposited by a variety of surface engineering techniques to protect steel structures. Moreover, the effect of the spraying process, the addition of alloying elements on the corrosion resistance of these coatings has also been reviewed in this study.

A Damage Mechanics Based Cohesive Zone Model with Damage Gradient Extension for Creep-Fatigue-Interaction

In this paper a novel Cohesive Zone Model (CZM) is derived within the framework of continuum thermodynamics to describe cracking and delamination behaviour of coatings at high-temperatures. The separation variable in the Traction-Separation-Law (TSL) is decomposed into elastic and inelastic part. For evolution of inelastic separation, a power-law in combination with a damage evolution law is used to consider the tertiary stage of inelastic separation of the interface, additionally. Thereby, damage evolution is related to the corresponding thermodynamic driving force and the inelastic opening rate. For reasons of simplicity the resulting thermo-mechanical problem only considers heat conduction through the interface. Due to the fact that standard Newton-Raphson procedure gets unstable (e.g. snap-back) when softening occurs which is the case by using a CZM, this model is enhanced with the damage gradient, similar to approaches in phase field modelling. Further on, this extension is done to investigate if it is possible to overcome the size dependence of CZMs. Finally, the model is reduced to pure Mode I opening and an example for a Double Cantilever Beam (DCB) is analysed by the finite difference method.

Dry Sliding Wear Behavior and Mechanism of a Hot-Dip Aluminized Steel as a Function of Sliding Velocity

Fe-Al intermetallic coatings were prepared on AISI 1045 steel by hot-dip aluminizing and subsequent high-temperature diffusion. The dry sliding wear tests were performed for the aluminized steel with various substrate hardnesses in the velocity range of 0.75-4 m/s on a pin-on-disk configuration. The dry sliding wear behavior and mechanisms of the aluminized steel under various velocities were explored. It was found that the wear rate decreased substantially with an increase in sliding velocity from 0.75 to 1.5 and then slightly varied in the velocity range of 1.5-4 m/s. The wear behavior and mechanisms of the aluminized steel were noticed to be closely related with tribo-layers. As tribo-layers contained more oxides with Fe2O3, the wear resistance of the aluminized steel was markedly improved because of their protective function. The substrate hardness also changed the wear behavior of the aluminized steel by affecting the stability of tribo-layers because the substrate hardness was inversely proportional to the thermal stability of the substrate structure. Under a low velocity of 0.75 m/s, adhesive wear and abrasive wear prevailed owing to the metal-on-metal contact, resulting in a higher wear rate. At 1.5-4 m/s, mild oxidative wear prevailed due to the stable existence of tribo-oxides layers. However, at 1.5 and 4 m/s for the substrate hardness of 45 HRC, the wear rate was slightly increased because the protection of tribo-layers was weakened by a double-layered structure or more cracks in tribo-layers and coatings.

Investigation on laser cladding Ni-base coating assisted by electromagnetic field

Laser cladding Ni-base coatings were prepared on pure iron assisted by an electromagnetic compound field. The microstructure, phase composition, crack and microhardness of the cladding coatings were investigated. Results indicated that laser cladding Ni-base coatings with and without the electromagnetic field was composed of γ-(Fe, Ni), CrB, Cr7C3, Ni3B and Ni2B. The electromagnetic compound field could promote the transformation of γ-(Fe, Ni) to equiaxed crystals. The volume fraction of hard phase CrB can be increased by electromagnetic field. Mechanical scouring effect of electromagnetic stirring makes hard phase Cr7C3 form an independent rod structure. The number of cracks in coatings is decreased under the electromagnetic field. The average microhardness of cladding coatings increased to 923.7 HV0.2. Under the combined action of the electromagnetic stirring and current thermal effect, the temperature gradient in the molten pool had an optimum value. That is, the value of AC current was 1200 A and magnetic flux intensity was 0.2 T.

High-temperature degradation mechanism of aluminide coatings on titanium alloys under cyclic oxidation at 750°C

ABSTRACTAluminide coatings prepared on Ti-6Al-4V substrate were able to improve oxidation resistance of the alloy under cyclic oxidation at 750°C both in dry and moist air conditions due to aluminide’s ability to form a stable alumina oxide scale. However, degradation of the coating due to spallation, cracking, internal oxidation and formation of voids with increased cyclic oxidation reduced the lifespan of the coating and the underneath substrate. The main cause of coating degradation for hot-dip specimens is cracks that initiated and propagated perpendicular to the surface. For the plasma spray specimens, the cracks are parallel to the surface. Initiation of cracks in hot-dip coatings are more accredited to residual stresses due to cooling and presence of brittle intermetallic phases TiAl2 and TiAl. For plasma spray coatings, initiation and propagation of cracks are attributed to presence of entrapped oxides, pores and grain boundaries of the deposited splats whose flattened edges are parallel to the surface of the coating. Presence of water vapor, too, acts as an oxygen carrier and thus promotes oxidation internally, inhibits growth of continuous protective alumina oxide scales and weakens the scale/alloy interfacial toughness. Water vapor therefore accelerates degradation by increasing spallation and cracking rate of the coating.

About the main causes of cracks in-situ cement concrete pavements in joints

In recent years, the use of concrete (cement concrete) bases and coatings of roads and industrial sites has begun to revive in road construction. Employees of the Department of «roads» BGIT together with students for several years under surveillance for cracks in the monolithic cement concrete layers of coatings. The article presents the results of field surveys of cement concrete coatings of highways of the Bryansk region and industrial sites in order to identify the causes of cracks in the expansion joints. We have examined the sections of roads with cement concrete coating on some plates near the seam and found the formed transverse cracks. They were located mainly along the seam, the maximum opening width reached 5 mm. the Maximum distance from the seam 30 cm. Cracks along the seam were located unevenly and had a diverse shape. We have also calculated several variants of pavement with cement concrete coating on different bases. As a result of the survey were tested: the coefficient of compaction of the base material, the compressive strength of concrete in the coating plates, the technology of works on the device of coatings and expansion joints in them, as well as the control of the project materials submitted by the design organization, including the project of works. In conclusion, the following main causes of cracks in monolithic cement concrete coatings of roads and industrial sites can be identified: errors made in the calculation and design of the pavement structure; errors made in the calculation of the length of the plates, the appointment of reinforcing bars in the seams; errors in the construction of cement concrete pavement construction organization (if device transverse control joints and expansion joints in compression, buckling and longitudinal). The study material is generalized, road and construction organizations are given recommendations for the elimination of cracks, some measures to prevent their occurrence are indicated.

FORMATION OF HARD AND WEAR-RESISTANT NIOBIUM CARBIDE COATINGS ON HARD-ALLOY TOOLS BY A VACUUM–ARC METHOD

The temperature fields of a hard-alloy tool surface during niobium ion impact were determined by thermography. The temperature of the surface layers reaches a maximum value of 1100–1250°C after exposure of 40 s to Nb ion, and it does not change with increase in the exposure time to 1 min. Ion exposure for 1 min leads to the formation of a carbide layer (Nb, W)C0.7, in the surface layer with a thickness of about 0.4 μm. Subsequent plasma deposition of Nb in methane atmosphere forms an NbC coating. The hardness of the formed niobium-based and tungsten carbide-based layers reaches 35–65 GPa. Adhesive resistance to cracking and destruction of NbC coatings with a sublayer of (Nb, W)C0.7 increases at least 2 times compared to an NbC coating.

A QTL associated with high seed coat cracking rate of a leading Japanese soybean variety

Seed coat cracking in soybeans [Glycine max (L). Merr.] leads to commercial and agronomic losses. The Japanese elite soybean cultivar ‘Fukuyutaka’ is often used as a parent for breeding, but its high rate of seed coat cracking is an obstacle to its further use in breeding programs. To establish a DNA marker-assisted selection system for seed coat cracking, genetic factors related to high rates of seed coat cracking were surveyed, and a quantitative trait locus (QTL) with a stable effect on seed coat cracking in both years of a two-year replication experiment was detected on chromosome 20. Comparison of a set of near-isogenic lines (NILs) around this locus verified that the presence of the ‘Fukuyutaka’ allele significantly increased seed coat cracking in the kernel. The locus is located in a genomic region spanning 3.2 Mb. Marker-assisted selection for the locus will improve the selection efficiency of ‘Fukuyutaka’-derived breeding populations.

Apparatus for Identifying Microcracks on the Inner Surface of Glazed Pipes

Amethod of identifying cracks on the inner surface of a glazed coating on a pipe with aid of a flaw detector is examined. The technological scheme is presented and the principle of operation of the developed apparatus is presented. The electric resistance of cracks in the glazed coating of a pipe is determined in terms of the parameter readings of the apparatus. A calculation of the basic dimensions of cracks in coatings along the entire length of standard pipes is presented.

A new injection system for spraying liquid nitrates in a low power plasma reactor: Application to local repair of damaged thermal barrier coating

In addition to the search for new Thermal Barrier Coating (TBCs) systems with increased reliability over very long time periods, the repair of current systems is a technological and economic issue for both civilian and military engine end-users. This paper describes the latest version of the deposition process known as the Low-Power Plasma Reactor (LPPR) process, specially developed to repair locally damaged TBCs. The LPPR process enables micro/nanostructured TBCs to be made from nitrate salts in aqueous solutions, which are sprayed in an Ar/O2 plasma discharge at low power (240 W) and transformed into oxide coatings. A new injection device was designed to produce a fairly homogenous and reproducible spray to repair partially spalled APS and EB-PVD TBCs deposited on small flat coupons. The microstructure and the stability of the LPPR TBCs were assessed, in particular using SEM observations, during ageing tests under various time/temperature conditions. The Particle Image Velocimetry (PIV) technique and associated modeling have proved that the nitrates impact the substrate in a liquid state even in the presence of plasma and a vacuum. Due to the liquid state of the precursors, the new LPPR TBC seals the damaged areas and deeply infiltrates all porosities and failure cracks in the original coatings. This research has enabled the new version of the LPPR process to be validated as a simple, efficient, cheap and promising way to repair locally damaged TBCs.

Síntesis y caracterización electroquímica de recubrimientos de multicapas metal cerámico de W/WN, Ti/TiN y WTiN

Corrosion causes great losses and catastrophic damages in industrial plants and infrastructure, among the methods to reduce it are the coatings. The use of metal-ceramic multilayer coatings to reduce corrosion in steels is a viable option, since they offer several advantages over single-layer coatings, among which, they can be deposited with relatively high thicknesses and are able to dissipate the residual stresses that form during the growth of the thin films, which maintains its adhesion to the substrate. The coatings were deposited by magnetron Sputtering, using Ti and W targets. Two multilayer architectures were designed and deposited, including tungsten nitride (WN) and titanium tungsten nitride (WTiN) coatings with alternated bilayers of Ti/TiN and W/WN, those two deposition sequences of layer were identified as A with more Ti and B layers with higher W content; both consist of a nine-layer deposit process. The structure was studied by X-ray diffraction and the morphology and composition by SEM and EDS respectively. The surface was studied with profilometry and atomic force microscopy. The electrochemical behavior was analyzed in a sodium chloride solution by means of potentiodynamic polarizations and electrochemical impedance spectroscopy (EIS). X-ray diffraction results show the presence of two phases of WN. The grain size of coating A is similar to that obtained in coating B. The morphology of the multilayers showed a mostly smooth surface but with the presence of spherical domes, as well as cracks in both coatings, being more abundant in the coating B. In its cross section, the sequence of multilayers composed of layers of Ti, W, TiN, WTiN and WN is observed, however in some cases the change between the individual layers is not observed and two layers remain as one in the images of MEB. The electrochemical results obtained by potentiodynamic polarization curves showed a shift of the corrosion potential towards more noble values. The corrosion current in coating A with more layers of Ti is lower than coating B and both improve the corrosion resistance of the substrate. This behavior was confirmed in the Nyquist diagram obtained by EIS.

Direct comparison of anisotropic damage mechanics to fracture mechanics of explicit cracks

Because damage mechanics modeling involves damage initiation followed by propagation that releases energy, it is often uncritically assumed to be an alternative implementation of fracture mechanics. This paper tests that claim by running side-by-side damage mechanics and explicit crack fracture mechanics simulations within the same code on three separate problems. For a center-notched specimen loaded in tension, damage mechanics can reproduce all features of fracture mechanics, but with three restrictions. First, damage mechanics must implement anisotropic damage mechanics rather then simpler isotropic or scalar damage methods. Second, initiation stress and toughness used in damage mechanics must be calibrated before damage mechanics can reproduce either fracture mechanics or experimental results. Third, damage mechanics properties must scale with absolute size of discretization elements. Two other problems considered were mixed-mode, dynamic fracture and cracking of a brittle coating on a substrate. In each case, anisotropic damage mechanics can be made consistent with fracture mechanics by suitable calibration of properties. An advantage of damage mechanics is its potential to model certain complicated issues better then explicit crack fracture mechanics. Two examples are dealing with a transition to shear failure and modeling of fracture events such as periodic cracking in coatings.

Numerical analyses of the residual stress and top coat cracking behavior in thermal barrier coatings under cyclic thermal loading

Acting as one of the original cracks for interfacial delamination, the horizontal roughness-induced top coat crack plays an important role in the premature failure of the air plasma sprayed thermal barrier coating system. Thus, the residual stress and top coat cracking behavior in thermal barrier coatings under cyclic thermal loading were numerically investigated. The effects of thermally grown oxide swelling, creep, and interfacial roughness were taken into account. The arbitrary top coat cracking was modeled using the extended finite element method. In addition, the effect of the interfacial cracking on top coat cracking and their mutual interaction were investigated using the cohesive zone model. The results showed that the top coat crack initiated before thermal cycling due to large residual stress, the combination of the thermally grown oxide swelling and creep relaxation promoted crack propagation during the early periods, and the crack continued to grow with the thermally grown oxide thickening during the later periods. The initiation and propagation of the interfacial crack between the thermally grown oxide and bond coat accelerated the top coat cracking, and their mutual interaction led to premature interfacial delamination.

Investigation of Geometric Parameters of Cracks in Silicate Enamel Coatings of Steel Pipes

The results of investigation of cracks on the surface of silicate enamel coatings of steel oil pipes of various diameters are presented. It is shown that appearance of cracks and their geometric parameters depend essentially on the quality and thickness of the coatings.

Seed Coat Cracking in a Soybean Cultivar “Satonohohoemi” and Effects of Planting Time on Its Occurrence

Seed Coat Cracking in a Soybean Cultivar “Satonohohoemi” and Effects of Planting Time on Its Occurrence

In Situ Acoustic Monitoring of Thermal Spray Process Using High-Frequency Impulse Measurements

In order to guarantee their protective function, thermal spray coatings must be free from cracks, which expose the substrate surface to, e.g., corrosive media. Cracks in thermal spray coatings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology and coating defects were inspected using light microscopy on metallographic cross sections. Additionally, the resulting crack patterns were imaged in 3D by means of x-ray microtomography.

Damage characterization model of ceramic coating systems based on energy analysis and bending tests

A quantitative damage model of ceramic coating systems was developed based on their load-displacement curves obtained from three-point bending tests. According to the energy mechanism of damage, the normalized damage rate of such systems can be simply expressed using the load and the tangent slope of their load-displacement curves. The experimental results demonstrated the thickness dependence of fracture and damage. In thin coating systems, tensile failure was found to be predominant and multiple transverse cracks appeared in the coatings. In contrast, thick coating systems showed a predominance of interface shear failure and the occurrence of interface delamination. These observations are consistent with previous experimental results. The damage of the systems displayed catastrophic characteristics when the load tended to reach the failure point, i.e., the damage increased rapidly, and the damage rate displayed a power-law singularity at the failure point. These results are consistent with the damage characteristics predicted using the mathematic model. The damage evolution in the case of interface delamination in the thick coating systems was faster than that for transverse cracking in the thin coatings because of the difference in the degree of damage localization. The present model provides an effective method to elucidate the damage behavior of brittle ceramic coating systems, and hence, it is expected to greatly aid the coating design.

THE SUBSTANTIVE FACTORS COMPLEX INFLUENCE ON THE SURFACE LAYERS' INTERNAL TENSIONS IN THE STEEL HARDENED CAR'S PARTS

Досліджено актуальне питання комплексного впливу режимів електролізу на структуру та величину внутрішніх напружень у поверхневих шарах різної товщини насталених автомобільних деталей. Встановлено, що на якість насталених покриттів значною мірою впливають такі вагомі чинники як густина струму, температура та кислотність електроліту. Для визначення комплексного впливу вагомих чинників на внутрішні розтягувальні напруження у поверхневих шарах насталених деталей за різної їх товщини застосовано системний підхід, вихідною передумовою якого є прагнення з максимальною повнотою врахувати початкові та вихідні характеристики об'єкту. На підставі застосування методу планування експерименту отримано регресійні рівняння, які дають змогу оцінити взаємовплив густини струму, кислотності та температури електроліту на внутрішні напруження у поверхневих шарах різної товщини насталених автомобільних деталей. За результатами аналізу регресійних рівнянь й оцінки числових значень та знаків їх коефіцієнтів з'ясовано, що для різної товщини покриття сила й характер впливу чинників є неоднаковими, що дасть змогу вдосконалити технологічні процеси відновлення автомобільних деталей електролітичним нарощуванням.

Preparation and Characterization of Microcapsules Containing Soybean Oil and Their Application in Self-Healing Anticorrosive Coatings

ABSTRACTSynthetic self-healing materials are a new prototype in polymeric coatings. In the present investigation, soybean oil along with drier was encapsulated in urea–formaldehyde (UF) microcapsules, which were successfully prepared by in situ polymerization and their self-healing ability, the corrosion resistance, gloss, and adhesion strength of microcapsules were studied. The synthesized microcapsules were characterized by Fourier transform infrared spectrometer for identification of separated core and shell materials and their surface morphology was studied by scanning electron microscope. The particle size was checked under optical microscope and confirmed with particle size analyzer. Effectiveness of soybean oil-filled microcapsules was investigated for healing of cracks generated in anticorrosive coatings. Cracks in paint coatings were found to be healed when soybean oil was released from microcapsules under simulated mechanical action and the corrosion resistance of healed area was evaluated by immersion study.