Effect of physical aging on thermal stress development in powder coatings

Thermal stress and physical aging are inherent to thennosetting systems (such as powder coatings) and may affect the coating durability leading to damage such as detachment and cracking. Both phenomena occur principally below the glass transition temperature (T_g) of the coating and affect each other. It is shown that the measurement of stress, as a function of temperature of coatings aged at different temperatures and during various times, represents a simple and interesting way to study these phenomena. The results obtained which show changes in the stress magnitude with aging are explained in terms of stress relaxation and structural recovery. The latter process is especially evident in the T_g region and can prevent the correct determination of the T_g by means of thermal stress measurements. The thermal expansion coefficient and the elastic modulus, two properties directly affecting the thermal stress magnitude were determined separately, and agree well with the proposed interpretation of experimental data. The linear dependence of thermal (compressive) stress on the logarithm of time indicates the possibility of predicting the effect of physical aging.     

Physical aging of thermosetting powder coatings

The physical aging behaviour of several thermosetting powder coatings has been studied in order to compare the relative stabilities of alternatives to those based on triglycidyl isocyanurate (TGIC). The aging behaviour at ambient temperature is studied by differential scanning calorimetry and is analysed by the peak shift method in order to determine the kinetic parameters of the enthalpy relaxation process. Comparison between the various systems based on carboxyl functional polyester cured with bisphenol-A epoxy resins (hybrid systems), with triglycidylisocyanurate (TGIC) and β-hydroxyalkylamide shows that the aging rate, expressed as the enthalpy loss per decade of aging time, is smallest for those based on TGIC, and hence that the performance of the alternative systems requires improvement as regards their stability.     

Effect of scandia content on the thermal shock behavior of SYSZ thermal sprayed barrier coatings

Nanostructured 4SYSZ (scandia (3.5 mol%) yttria (0.5 mol%) stabilized zirconia) and 5.5 SYSZ (5 mol% scandia and 0.5 mol% yttria) thermal barrier coatings (TBCs) were deposited on nickel-based superalloy using NiCrAlY as the bond coat by plasma spraying process. The thermal shock response of both as-sprayed TBCs was investigated at 1000 °C. Experimental results indicated that the nanostructured 5.5SYSZ TBCs have better thermal shock performance in contrast to 4SYSZ TBCs due to their higher tetragonal phase content and higher fracture toughness of this coating     

Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating

The crystallization behavior of the amorphous phase of the plasma sprayed LaMgAl₁₁O₁₉ (LaMA) coating during thermal aging processes has been investigated. Results indicate that LaMA coating exhibits much similar microstructure and thermal properties such as close coefficient of thermal expansion and specific heat capacity etc. to the sintered LaMA bulk after aging at 1673 K for 20 h. On the other hand, a solid state reaction seems to occur to reform the ideal magnetoplumbite-type LaMA phase coupled with the formations of the La-rich aluminate intermediate phases. When the aging temperature is held between 1273 K and 1473 K, nanosized platelet-like grains as well as sub-grains with high aspect ratios are present. The phase stability has been investigated through the chemical compositions and X-ray diffraction analysis. The recrystallization mechanism of the amorphous LaMA coating has been explored by tracing the microstructure evolutions during thermal aging process.     

Analysis of interfacial crack initiation mechanism of thermal barrier coatings in isothermal oxidation process based on interfacial stress state

In this paper, the interfacial stress state is used to analyze the interfacial crack initiation mechanism of the thermal barrier coatings (TBCs) during isothermal oxidation. The influence of thermal growth stress, initial residual stress, and creep behavior on the stress distribution is considered to have an accurate simulation result. A parameter that integrates the effects of interfacial normal and tangential stress is modified for evaluating interfacial crack initiation. It is found that, in the cooling stage, the interfacial cracks sprout at the top coat (TC)/thermally grown oxide (TGO) interface valley region and the TGO/bond coat (BC) interface peak region, which agrees with the experimental results. Furthermore, the influence of interfacial roughness on crack initiation is investigated. The result shows that different interfacial roughness affects the sprouting region of interfacial cracks and cracks within the TC layer.     

Interplay of the phase and the chemical composition of the powder feedstock on the properties of porous 8YSZ thermal barrier coatings

Some chemical impurities enhance sintering kinetics of ceramic Thermal Barrier Coatings (TBCs) which can cause their premature failure during operation in gas turbine engine by causing reduction in coating’s strain compliance as well as faster bond-coat oxidation due to increased thermal conductivity. Certain chemical impurities are also believed to suppress resistance to tetragonal to monoclinic phase transformation in 8YSZ, which can also be an important factor regarding TBC’s performance. Most of the impurities and some of the monoclinic phase present in the powder feedstocks can survive into the as-sprayed coating. Therefore, there is a general trend towards OEMs requiring the lowest amounts of chemical impurities and the lowest amounts of monoclinic phase in the powder feedstocks. This paper presents a comprehensive investigation aimed at understanding the role and the relative importance of the chemical and phase purities of the powder feedstock for the properties and performance of thick 8YSZ TBCs.     

Effect of oxide growth on the stress development in double-ceramic-layer thermal barrier coatings

A numerical study is conducted to investigate the effect of oxide growth on the stress development within the plasma sprayed double-ceramic-layer thermal barrier coatings. The roles of oxide morphology, growth rate, and oxidation duration are discussed. A two-dimensional periodical unit-cell model is developed, taking into account the different interfacial roughnesses among the coatings layers. Thermal gradient conditions are imposed during the high-temperature period to represent the non-uniform temperature distributions throughout the coatings thickness. It is found that stresses in the regions that close to the interface of the ceramic layers result from the thermal expansion mismatch and the non-uniform temperature field, in which the oxide growth reveals negligible influence on the development of the stresses. The gradually thickening thermally grown oxide (TGO) mainly contributes to the variations of stress and inelastic strain evolutions in its nearby regions. The residual stress fields in the coatings are almost unaffected by the oxide thickness after operating for a sufficiently long time. During long-term operation, the large inelastic deformation is found to be the intrinsic reason responsible for the cracking in the vicinity of TGO.     

Thermophysical behavior of thermal sprayed yttria stabilized zirconia based composite coatings

The effective thermal conductivity of a composite coating depends on intrinsic thermal conductivity of the constituent phases, its characteristics (size, shape) and volume fraction of porosities. The present study concerns studying the effect of CoNiCrAlY and Al₂O₃ content on the coefficient of thermal expansion and thermal conductivity of the YSZ (YSZ-CoNiCrAlY and YSZ-Al₂O₃) based composite coatings developed by thermal spray deposition technique. The coefficient of thermal expansion and thermal conductivity of the composite coatings were measured by push rod dilatometer and laser flash techniques, respectively, from room temperature to 1000 °C. Variation in density, porosity, coefficient of thermal expansion, and thermal conductivity was observed in the composite coatings with the addition of different volume fraction of CoNiCrAlY and Al₂O₃ powders in YSZ-CoNiCrAlY and YSZ-Al₂O₃ composites, respectively. Comparison between the theoretical and experimental thermal conductivities showed a mismatch varying from 4% to 58% for YSZ-CoNiCrAlY composite coatings and from 58% to 80% for YSZ-Al₂O₃ composite coatings. Model based analyses were used to understand the mechanism of thermal conductivity reduction in the composite coatings. It was concluded that the morphology of porosities varied with composition.     

Effect of the substrate and interface on micro-scratch deformation of epoxy-polyester powder coatings

The role of the interface on the deformation response in scratch tests of epoxy-polyester films deposited by electrostatic spraying is investigated. A comparative study of the scratch deformation behaviour of films deposited on micro- and macro-corrugated rigid substrates and on ‘soft’ silicon sub-layers is made. Scratch deformation parameters were evaluated by contact gauge inductive profilometry, whilst morphological examinations of the residual scratch patterns were performed by electron microscopy.     

Effect of nano-sized calcium carbonate on cure kinetics and properties of polyester/epoxy blend powder coatings

Today's strict environmental laws pose significant challenges for coating's formulators to look for eco-friendly products. Powder coatings, particularly polyester/epoxy blends have demonstrated their ability as alternatives to traditional solvent-borne coatings. Recently, the use of nanoparticles such as nano-CaCO₃ (nCaCO₃) has been suggested as a beneficial strategy towards powder coating application with improved properties. Here, we study the effect of nCaCO₃ on morphology, cure behavior, adhesion and hardness of polyester/epoxy systems. The nanoparticles shape, size and dispersion state were investigated through X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) methods. Furthermore, isothermal cure characterization of the neat and filled systems was performed using a torque rheometer. The most important finding based on the rheological studies was the catalytic effect of nCaCO₃ on cure reaction of polyester/epoxy, leading to the shorter curing time. Moreover, the kinetic analyses of rheograms revealed a marked decrease in the activation energy of the cure process upon raising nCaCO₃ content. Interestingly, pull-off adhesion and hardness tests showed that the hardness and adhesion strength were dramatically increased by the addition of nCaCO₃ into the polyester/epoxy system compared to pure blend resin. Therefore, considering the strong competition in powder coating market, the use of nCaCO₃ as a commercial and inexpensive nanofiller is necessary not only to reduce the dwell time which has benefits in terms of the energy consumption and economics, but also to improve the performance of final polyester/epoxy coating.     

High heat flux burner-rig testing of 8YSZ thermal barrier coatings: Influence of the powder feedstock

Burner-rig thermal cyclic testing of Thermal Barrier Coating (TBC) samples fabricated using different 8YSZ powders was conducted to investigate the influence of the chemical and phase compositions of the powder feedstocks. Four different powder feedstocks were selected. The chemical and phase compositions among the 8YSZ powders were systematically varied while the powder particle size and other physical characteristics were kept nominally the same. The coating process was also selected to achieve similar microstructure among the samples. The testing revealed that (1) higher impurity content (esp. silica) is detrimental to the cyclic life of the TBC; (2) coating porosity has a significant influence on the cyclic life of the TBC, the higher the porosity, the higher the cyclic life, for the range of porosity of the tested samples; (3) a low monoclinic content in the feedstock powder has not been shown to have a positive effect on the cyclic life of the TBC.     

Characteristics and thermal cycling behavior of plasma-sprayed Ba(Mg1/3Ta2/3)O3 thermal barrier coatings

Ba(Mg_1/3Ta_2/3)O₃ (BMT) powders were synthesized by the solid state reaction method. BMT thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying (APS). The phase composition and microstructure of the BMT coatings were characterized. The thermal cycling behavior of the BMT coatings was investigated by the water quenching method from 1150 °C to room temperature. The results reveal that BMT powders have an ordered hexagonal perovskite structure, whereas the as-sprayed coating of BMT has a disordered cubic perovskite structure because of the different degree of structural order for different treatment conditions. During thermal cycling testing, the entire spalling of coatings occurred within the BMT coating near the bond coat. This is attributed to the following reasons: (1) the growth of a thermally grown oxides (TGO) layer, which leads to additional stresses in the coatings; (2) the coefficient of thermal expansion mismatch between the BMT coating and bond coat, which develops enormous stress in the coatings; (3) the precipitation of Ba₃Ta₅O₁₅ due to the evaporation of MgO during the spraying process, which changes the continuity of the coatings.     

Substrate-constrained effect on the stiffening behavior of lamellar thermal barrier coatings

Thermal exposure would compromises the compliance and thermal insulating performance of thermal barrier coatings (TBCs). However, most publications were based on free-standing coatings in which the stress resulting from substrate is essentially different from TBCs on superalloy substrate. In this paper, the constrained effect of substrate on the ceramic top-coat of plasma sprayed lamellar TBCs was investigated. Results showed that the structural changes evolve from micro-scale to macro-scale during thermal exposure. In a relatively shorter thermal exposure stage, the inter-splat pores became narrowed, whereas the intra-splat cracks became widened. Consequently, the healing kinetics of inter-splat pores was much faster than that of the intra-splat cracks. In a relatively longer thermal exposure stage, some macroscale cracks appeared in coating surface owing to the gradually stiffening coatings. As a result, the microscale intra-splat cracks near the macroscale cracks were healed rapidly. In brief, the substrate constraint induced structural changes were stage sensitive.     

A model of the infrared cure of powder coatings based on surface absorptivities in-situ measurements

The efficiency of radiative powder coatings curing depends both on the coating radiative properties and the spectral emissivities of the infrared emitters. This investigation is structured to develop a mathematical model using the radiative properties of the coating to describe precisely the coating temperature and deduce the degree of polymerization conversion during the cure. The reflectance measurements results obtained with a FTIR equipped with an integrating sphere are analyzed and implemented in a thermal model. These results show that reflectance values vary with the chemical composition, the pigment nature, and are influenced by the powder coatings cure. The thermal model, solved with the finite volume method, permits to compute the varying temperature within the thickness of the powder-coated metal sample and provides a good prediction of the temperature within the “coating + substrate” system. The experimental cure of powder coatings supporting this study was performed in a small test oven equipped with electrical infrared emitters. Presented at 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 1–3, 2006, in New Orleans, LA.     

Effect of material properties on residual stress distribution in thermal barrier coatings

Residual stress has a significant influence on the crack nucleation and propagation in thermal barrier coatings (TBC) system. In this work, the residual stress in the air plasma spraying (APS) TBC system during cooling process was numerically studied, and the influence of the material properties of each layer on the residual stress was investigated. The morphologies of the interface were described by a piecewise cosine function, and the amplitude for each segment gradually increases. The elasticity, plasticity and creep of top coat (TC), thermally grown oxide (TGO) layer and bond coat (BC) were considered and the elasticity and creep of the substrate layer were taken into account. The material properties of all layers vary with temperature. The results show that the material properties have complex influence on the residual stress during cooling. The effect of the material properties of TC and BC on the residual stress at the interface is relatively large, and that of TGO and substrate is relatively small. These results provide important insight into the failure mechanism of air plasma spraying thermal barrier coatings, and important guidance for the optimization of thermal barrier coating interfaces.     

Concurrent physical aging and degradation of crosslinked coating systems in accelerated weathering

Coating degradation is a combination of both chemical and physical processes; however, physical processes have not received much attention. Physical aging has a non-negligible effect on coatings’ mechanical properties and permeability etc. through the densification that continues as a polymer approaches its thermodynamic equilibrium below the glass transition temperature, T _g. Observations in recent work showed that physical aging affects coatings’ mechanical property response during accelerated weathering and is, itself, affected by the associated chemical degradation. Two crosslinked coating systems were studied in order to compare different chemical compositions, their T _g, and their thermal response in accelerated weathering. During thermal cycling, physical aging measured by enthalpy recovery exhibited different trends in the two coatings. A “rejuvenation” mechanism was observed in the coating with a T _g between the top and bottom limits of the exposure cycle; continued aging was observed for the coating with a high T _g. Stress relaxation tests detected aging and “memory” behavior over periods comparable with accelerated weathering cycles. Both thermal and mechanical responses changed in complicated and different ways as the coatings degraded. Different degrees of coating thickness reduction were observed in both isothermal relaxation and degradation. When various coatings are evaluated, simply judging their performance under the same weathering environment is not reliable since polymer relaxation behavior depends on the relationship between the exposure temperatures and the T _g of each polymer. This paper was awarded First Place in the 2007 Gordon Awards technical paper competition, held as part of the FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in Toronto, ON, Canada, on October 3–5, 2007.     

Effect of UV crosslinking and physical aging on the gas permeability of thin glassy polyarylate films

The effect of crosslinking by UV irradiation on the gas permeation properties of thin films (thickness ≤1 μm) made from two benzophenone-based polyarylates were examined. In addition to the permeation response to UV crosslinking in these two polymers, the effects of crosslinking on the rate of physical aging was also explored. The sequence of physical aging and crosslinking, as well as reversal of the aging process was studied in order to separate the similar effects of aging and crosslinking. The results show that crosslinking very thin films can greatly improve the long-term performance of membranes when compared to noncrosslinked films of similar thickness.     

The interplay of physical aging and degradation during weathering for two crosslinked coatings

Polymer molecular relaxation, or ‘physical aging’, is a very important influence on permeability and mechanical properties of any polymer below its glass transition. ‘Physical aging’ occurs as even an unstressed polymer gradually relaxes towards its equilibrium conformation. This and the shorter term response to stress happen over periods much longer than the typical cycle of an accelerated weathering test, thus important properties of a polymeric coating may be affected by the difference in frequency between natural and artificial exposures, in addition to other factors. Further, ‘physical aging’ is affected by chemical changes to the polymer network caused by the degradation during a weathering exposure. In this investigation, purely physical aging was compared with the effect of concurrent chemical degradation by measuring ‘enthalpy recovery’ and mechanical stress relaxation at a variety of temperatures and at various stages during accelerated weathering exposure. The effect of physical aging was quite apparent in both an epoxy-polyamide coating and a polyester-urethane coating. Changes in physical aging behaviour during degradation were different for the two coatings, which points to further reasons for discrepancy between accelerated weathering and natural exposure.     

A novel approach to the preparation of powder coating—Manufacture of polyacrylate powder coatings via one step minisuspension polymerization

A novel method for manufacturing powder coating through one step minisuspension polymerization is described. The conventional production of powder coating includes six steps—synthesizing resins, mixing the raw material, extrusion, cooling, pre-crushing and pulverization. Comparatively, the present method can simplify the complicated processes, reduce equipment and save energy. Before polymerization, the TiO₂ particles were treated with a reactive silane coupling agent 3-methacryloxypropyltrimethoxysilane (MPTMS) to obtain enough compatibility with the monomers. The powder coating was directly synthesized through employing one step minisuspension polymerization in the presence of titanium dioxide white particles. The powder coating was characterized using Fourier transform infrared spectra (FT-IR) and thermogravimetric analysis (TGA). The results show that TiO₂ particles and polymer are successfully linked up via MPTMS in the powder particles. The morphology of powder coatings produced via different methods was observed by scanning electron microscope (SEM). The powder coatings obtained from minisuspension polymerization consist of regular spherical morphology particles with narrow particle size distribution. The powder flowability and surface film smoothness were enhanced compared to the pulverization powder coating.