The effects of weathering on the mechanical performance of automotive paint systems

The durability of automotive paint systems continues to be a great concern to both auto companies and their coating suppliers. Recent advances in assessing the durability of coatings by measuring weathering-induced chemical composition changes have greatly increased our ability to discern superior from inferior coatings. However, different coatings will likely tolerate different amounts of weathering-induced chemical composition changes while still maintaining their mechanical integrity. Thus, a means of linking chemical composition changes to changes in relevant mechanical properties would be highly desirable. The fracture energy, the amount of mechanical energy required to propagate a crack in a material, is a sensitive measure of the brittleness of a material and is relevant to a number of potential failure mechanisms in automotive paint systems. The fracture energy of clearcoats can vary widely depending on the formulation of the clearcoat (initial chemical composition and additive package) and on the amount of weathering. Weathering embrittles most coatings. Weathering-induced changes in the fracture energy are related to chemical composition changes occurring in the clearcoat. Because the brittlest materials will not crack without an applied stress, the stress distribution in complete paint systems as a function of weathering must also be known to accurately anticipate mechanical failures. Measuring thermoelastic constants of individual layers allows for computation of the stresses in complete paint systems. Stresses tend to increase with weathering. The presence of flaws in the clearcoat changes the stress distribution dramatically. Coupled with fracture energy measurements, the stress measurements provide additional insight into paint system failure mechanisms.     

Residual stresses in alumina–zirconia laminates

Significant residual stresses can arise in hybrid ceramic laminates during the densification and cooling processing cycles. The densification stresses in alumina–zirconia laminates were calculated assuming the layers to be linear viscous with data obtained by cyclic loading dilatometry. These stresses placed the zirconia layers in biaxial tension and even at 1 MPa or less, they were sufficient to cause a type of linear cavitation damage. The methodology was also applied to asymmetric laminates, successfully predicting their observed curling behaviour. Thermal expansion mismatch stresses arise during cooling, again placing the zirconia layers in residual biaxial tension and leading to the formation of transverse (channelling) cracks. The stresses were calculated using both elastic and viscoelastic formulations and were confirmed with indentation measurements. Additions of alumina to the zirconia layers were effective in reducing both sources of residual stress and allowed crack formation during processing to be avoided. Residual stresses were also shown to improve mechanical performance.     

On the combined use of UVA, HALS, photooxidation, and fracture energy measurements to anticipate the long-term weathering performance of clearcoat/basecoat automotive paint systems

Transmission UV spectroscopy measurements of clearcoat ul traviol et light absorber (UVA) disposition, electronspin resonance (ESR) spectroscopy measurements of clearcoat and basecoat Active hindered amine light stabilizer (HALS) disposition, and transmission Fourier transform infrared (FTIR) measurements of photooxidation have been carried out on 5 μm thickslioes of clearcoat/basecoat/primer/e-coatpaint systems on steel panels as a function of outdoor exposure. These analysis results are combined with clearcoat fracture energy measurements to assess the possibility that a clearcoat/basecoat paint system will resistcatastrophic cracking/peeling failure at long times. Taken together, all results indicate that these nontraditional paint weathering performance metrics should be added to the existing repertoire of paint weathering performancemetrics to ensure that inferior clearcoat/basecoat automotive paint systems are not introduced into service. Ford Research Laboratory, MD 3182SRL, P.O. Box 2053. Dearborn, MI 48124     

Hot Pressing of Annular Alumina–Zirconia Composites

An analysis is performed to predict the densification during and the state of residual stress after hot pressing of annular alumina/zirconia (3Y-TZP) composites. The objective of the analysis was to study the residual stresses resulting from stress gradients during pressing and those from thermal expansion mismatch during the cooling of the compact from the pressing temperature to room temperature. It is predicted that the residual stresses are affected by the respective densification rates of the core and the annulus, their elastic modulus, and thermal expansion coefficient. For the system analyzed in this study, it is predicted that hot pressing reduces the residual stresses that result from the mismatch in thermal expansion coefficients. This is due in part to the high densification rate and in part to the high elastic modulus of the alumina annulus compared to the zirconia core. For surface compression strengthening, a system where the annulus would have similar elastic modulus but lower densification rate and lower thermal expansion coefficient than the core would be more beneficial.     

Evaluation of the weathering performance of basecoat/clearcoat automotive paint systems by electrochemical properties measurements

The aim of the present study was to investigate the possibility of evaluating the weathering performance of a basecoat/clearcoat automotive paint system through the determination of its electrochemical properties. To this end, the electrochemical properties of a basecoat/clearcoat automotive paint in a 3.5% solution of NaCl in deionized water were measured at different weathering exposure times. A constant phase element (CPE) was used for describing the electrochemical behavior of the coatings under test. The values of the CPE parameters, i.e. Y₀ (the CPE constant) and n (the CPE power) were subsequently correlated to the extent of photo-oxidation (as measured by appearance, surface roughness, FTIR, surface tension and adhesion measurements) of clearcoat at the surface, in the bulk and at the interface between the basecoat and the clearcoat. The result showed that the electrochemical parameters Y₀ and n provide ready means for comparing the weathering performances of basecoat/clearcoat automotive paint systems. Increases in the value of Y₀ together with decreases in the value of n with increasing weathering exposure times suggest increased possibilities for the onset of cracking in the clearcoat itself in addition to its propagation towards the basecoat. Additionally, sudden variations in the values of Y₀ and n are indicative of increases for the clearcoat to peel off.     

An improved accelerated weathering protocol to anticipate Florida exposure behavior of coatings

A new accelerated weathering protocol has been developed which closely replicates the performance of automotive and aerospace coating systems exposed in South Florida. IR spectroscopy was used to verify that the chemical composition changes that occurred during accelerated weathering in devices with a glass filter that produced a high fidelity reproduction of sunlight’s UV spectrum matched those that occurred during natural weathering. Gravimetric water absorption measurements were used to tune the volume of water absorption during accelerated weathering to match that which occurred during natural weathering in South Florida. The frequency of water exposure was then scaled to the appropriate UV dose. A variety of coating systems were used to verify the correlation between the physical failures observed in the accelerated weathering protocol and natural weathering in South Florida. The new accelerated weathering protocol correctly reproduced gloss loss, delamination, cracking, blistering, and good performance in a variety of diverse coating systems. For automotive basecoat/clearcoat paint systems, the new weathering protocol shows significant acceleration over both Florida and previous accelerated weathering tests. For monocoat aerospace systems, the new weathering protocol showed less acceleration than for automotive coatings, but was still an improvement over previous accelerated tests and was faster than Florida exposure.     

Constrained Densification of Alumina/Zirconia Hybrid Laminates, I: Experimental Observations of Processing Defects

Various forms of damage were observed in pressure-less-sintered Al₂O₃/ZrO₂symmetric laminates and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel cracks in the ZrO₂ layers, Al₂O₃ edge-effect cracks parallel to the layers, delamination in the Al₂O₃layers, and debonding between the Al₂O₃and ZrO₂layers. Based on detailed microscopic observations, the defects were attributed to sintering rate and thermal expansion mismatch between the layers. Cracks or cracklike defects were formed in the early stages of densification, and these cracks either opened during sintering or acted as preexisting flaws for thermal expansion mismatch cracks. Consequently, the extent of cracking could be reduced or even eliminated by decreasing mismatch stresses during the sintering and cooling stages. This can be accomplished by reducing the heating and/or cooling rates or by adding Al₂O₃in the ZrO₂layers. The sintering mismatch stresses were estimated from the degree of curling in asymmetric laminates and from layer viscosities that were obtained by cyclic loading dilatometry. The measured curvature was an indication of the mismatch in sintering strain between Al₂O₃and ZrO₂and were consistent with the dilatometric data that were obtained for the component layers.     

Residual stress measurements in melt infiltrated SiC/SiC ceramic matrix composites using Raman spectroscopy

Raman spectroscopy was utilized to characterize the chemical composition and residual stresses formed in melt infiltrated SiC/SiC CMCs during processing. Stresses in SiC fibers, in SiC chemical vapor (CVI) infiltrated matrix, in SiC melt infiltrated matrix, and in free silicon were measured for two different plates of CMCs. Stresses in the free silicon averaged around 2?GPa in compression, while stresses in the matrix SiC were 1.45?GPa in tension. The SiC CVI phase had stresses ranging between 0.9?GPa and 1.2?GPa in tension and the SiC fibers experienced stresses of .05–0.7?GPa in tension. These results were validated with the proposed model of the system. While the mismatch in the coefficients of thermal expansion between the constituents contributes to the overall residual stress state, the silicon expansion upon solidification was found to be the major contributor to residual stresses within the composite.     

Constrained Densification of Alumina/Zirconia Hybrid Laminates, II: Viscoelastic Stress Computation

To analyze the inhibited densification during sintering and differential shrinkage during cooling of Al₂O₃/ZrO₂symmetric and asymmetric laminates, viscoelastic formulations, in which the viscosity and elastic modulus vary with time, have been developed. The viscoelastic mismatch stresses have been numerically computed over the entire processing cycle, including the heating period, the isothermal period, and the cooling period. The viscosity and free sintering rates that are needed for stress computation have been obtained by modifying the parameters that are measured for a normal isotropic densifying compact using cyclic loading dilatometry. The modification is based on the available sintering models to account for the effect of strain history on compact viscosity and sintering rates. The stress calculation shows that, with the exception of the initial heating period, the viscoelastic stress is identical to the viscous stress that is calculated solely from the strain rate mismatch. Sintering damage in the laminates is shown to occur during densification under conditions where the differential sintering stress is smaller than the intrinsic sintering pressure. The magnitude of residual stress in hybrid laminates on cooling is dependent on the cooling rate, and slower cooling rates are capable of almost completely relaxing the expansion mismatch stress at temperatures of >1200°C.     

Finite Element Simulation of Thermal Residual Stresses in Joining Ceramics with Thin Metal Interlayers

Finite element analysis was used to estimate thermal residual stresses developed in silicon nitride bodies bonded by metallic interlayers. Stresses were calculated for various characteristic metals, namely, Ni, Al, and Si, assuming elastic and elastic-plastic behavior. The relative importance of the metal properties, such as thermal expansion coefficient, stiffness and ductility, has been evaluated. Two different joint geometries, butt and lap, have been used in stress calculations, and special care was taken in the mesh generation, to obtain comparable results. The yield stress of the interlayer material rather than thermal expansion mismatch is the critical factor in thermal residual tensile stress buildup inside ceramic adherents.     

Interfacial Peeling Moments and Shear Forces at Free Edges of Multilayers Subjected to Thermal Stresses

Stresses normal to interfaces, i.e., interfacial peeling stresses and interfacial shear stresses, exist locally at edges of multilayers because of both the thermal mismatch between layers and the free-edge effect. These peeling and shear stresses can result in modes I and II edge delamination, respectively. However, because of the complexity of the problem, exact closed-form solutions for these stresses are very difficult if not impossible to derive even for bilayered systems. Hence, instead of the detailed stress field at edges, both the interfacial peeling moment resulting from the localized peeling stresses and the interfacial shear force resulting from the localized shear stresses are analyzed here. Exact closed-form solutions for the peeling moment and the shear force at each interface in elastic multilayered systems are derived. To illustrate the application of present closed-form solutions, specific results are calculated for five-layered thermal barrier coating systems, and a finite-element analysis is also performed to confirm the analytical results.     

The weathering performance of acrylic melamine automotive clearcoats containing hydrophobic nanosilica

This work is an attempt to study the effect of weathering on various properties of a typical acrylic melamine clearcoat containing various loads of nanosilica. It was found that nanosilica caused an incomplete curing process of the acrylic melamine clearcoat, leading to a lower crosslink density and also less Young’s modulus of clearcoat compared to the pure clearcoat. Thermo-mechanical analyses showed that incomplete curing of nanocomposites induced post-curing reactions during weathering, leading to an increase of crosslinking density and Young’s modulus of nanocomposites. Viscoelastic studies showed that incorporation of nanosilica into acrylic melamine resulted in more plastic behavior and higher elongation at break of clearcoats during weathering. In addition, gloss retention, surface topology, and chemical structure investigations revealed that nanosilica particles enhanced weathering performance of clearcoats. This was explained by UV–Visible spectroscopy which confirmed the ability of silica to absorb the harmful incident rays, protecting the clearcoat against weathering degradation.     

Effect of bimodal pores on sintering and residual stresses of YSZ membranes prepared by non-solvent induced phase separation method

Densification mismatch and residual stresses of tri-layered yttria-stabilized zirconia (YSZ) membranes prepared by a non-solvent induced phase separation (NIPS) method were investigated. The tri-layered membrane consisted of sponge-like structures and finger-like voids in macroscale. The densification of the two structures were characterized to elaborate their contribution to the densification mismatch, which led to residual stresses of hundreds of megapascal retained in the sintered membranes. The profile of residual stress suggested that it was related to the strain rate mismatch within the NIPS membranes, which was further quantified with an in-situ monitored camber evolution.     

Contact-Induced Transverse Fractures in Brittle Layers on Soft Substrates: A Study on Silicon Nitride Bilayers

An analysis of transverse cracks induced in brittle coatings on soft substrates by spherical indenters is developed. The transverse cracks are essentially axisymmetric and geometrically conelike, with variant forms dependent on the location of initiation: outer cracks that initiate at the top surface outside the contact and propagate downward; inner cracks that initiate at the coating/substrate interface beneath the contact and propagate upward; intermediate cracks that initiate within the coating and propagate in both directions. Bilayers consisting of hard silicon nitride (coating) on a composite underlayer of silicon nitride with boron nitride platelets (substrate), with strong interfacial bonding to minimize delamination, are used as a model test system for Hertzian testing. Test variables investigated are contact load, coating/substrate elastic-plastic mismatch (controlled by substrate boron nitride content), and coating thickness. Initiation of the transverse coating cracks occurs at lower critical loads, and shifts from the surface to the interface, with increasing elastic-plastic mismatch and decreasing coating thickness. This shift is accompanied by increasing quasi-plasticity in the substrate. Once initiated, the cracks pop in and arrest within the coating, becoming highly stabilized and insensitive to further increases in contact load, or even to coating toughness. A finite element analysis of the stress fields in the loaded layer systems enables a direct correlation between the damage patterns and the stress distributions: between the transverse cracks and the tensile (and compressive) stresses; and between the subsurface yield zones and the shear stresses. Implications of these conclusions concerning the design of coating systems for damage tolerance are discussed.     

底面合一阴极电泳漆的研究

采用丙烯酸树脂与环氧树脂冷拼工艺制备的阴极电泳漆解决了环氧类阴极电泳漆作为底面合一涂层时老化性能差的问题，针对乳液在施工应用中的稳定性，探讨了亲水性溶剂、酸值对乳液及电泳漆漆膜的影响，通过液相色谱仪分析槽液中树脂比例变化，并用人工加速老化实验验证了涂膜的老化水平，结果表明，底面合一型电泳漆在老化、盐雾等方面达到了和谐统一，可满足有关汽车底盘及零部件的综合要求。     

Humidity Drying of Plastic Clay

The humidity drying cycle gives rise to surface and body cracks in plastic clay. The cracks appear in the initial stages when the clay is being heated in a saturated atmosphere. Various factors affecting cracking in this initial heating stage were studied using a purified kaolin in the plastic state, and appropriate experimental models were used to isolate certain aspects for study. It is shown that the condensation of moisture on the clay surface is the major factor influencing the cracking of the plastic clay. The effect of thermal diffusion is insignificant and the loss of mechanical strength is of secondary importance. The balance af evidence from the literature and from this study indicates that capillarity plays a major part in the movement of moisture and the production of stresses during drying. When air spaces are present in the body, thermal capillarity may become important. Stresses caused by thermal expansion oppose those caused by drying shrinkage, while the thermal expansion of capillaries will have a negligible effect.     

Interfacial behavior on Al2O3/HAYNES® 214™ joints fabricated by solid state bonding technique with Ni or Cu–Ni–Cu interlayers

The residual stresses due to the difference in thermal expansion between ceramic and metal is a significant parameter to control during the fabrication of ceramics/metal joint. In this work, residual stress distribution, after solid state bonding of different joints, was measured using X-ray diffraction (XRD) and Vickers Indentation Fracture (VIF) methods. Tensile stress concentration in alumina caused by the thermal expansion mismatch in the Al₂O₃/Ni/Ni alloy (HAYNES^® 214?) joint severely deteriorated the assembly and caused cracks in alumina. To solve this problem, this paper shows that the use of a Cu/Ni/Cu multi-layer, associated with the direct copper bonding method (DCB), by pre-oxidation of copper, reduces significantly the tensile residual stresses in alumina material. Consequently, this process offers the possibility of producing an interlayer with a high melting temperature and hence joints which can withstand high-temperatures.     

Anticipating paint cracking: The application of fracture mechanics to the study of paint weathering

The relationships between the fracture energy (G_c), film thickness, stress level, and weathering time were investigated for a number of automotive clearcoats. As weathering progressed, most clearcoats embrittled due to the combined effects of photooxidation, hydrolysis, and other degradation mechanisms. This embrittlement was measured and related to the chemical composition changes that take place in the clearcoat during weathering. When the fracture energy dropped below the driving energy for cracking, G, brittle cracking occurred. The driving force for cracking was shown to depend on stress level and film thickness. The importance of fatigue loading was also qualitatively investigated. These techniques were successfully used to anticipate the long-term weathering behavior of automotive clearcoats. Materials Science Department, Dearborn, MI 48121.     

Effect of Incorporation of Cylindrical Inhomogeneities on the Strength Distributions of Brittle Materials

Cylindrical inhomogeneities are often deliberately incorporated into engineering ceramics (e.g., fibers, vias, electrical feedthroughs). The thermal expansion mismatch between the matrix and inhomogeneity creates a state of localized stress. We show that for radial cracks around such inhomogeneities, there may be conditions of crack stability even in the presence of an external, destabilizing field. This stability, and the nature of the stress intensity factor due to local stresses, modifies the strength distribution of the matrix. A fracture-mechanics approach allows the prediction of the new strength distributions. As an illustration of this approach, calculations for commonly used ceramic–metal inhomogeneity material pairs are discussed. Depending on the inhomogeneity/flaw size ratio, the new strength distributions can have lower or higher strength variability than the matrix. If the inhomogeneity radius ( R ) is chosen such that a majority of the cracks in the matrix are >0.25 R , the material will have the highest possible strength and reduced variability.     

Investigation of Strut Crack Formation in Open Cell Alumina Ceramics

An investigation was made into the source of strut cracking during the fabrication process of open cell ceramics that are produced by coating a polymeric foam. Several sources for the stress that produces these cracks were considered, viz., differential drying, thermal expansion mismatch between the polymer and the green ceramic coating, and the gas pressure produced by pyrolysis of the organic skeleton. Thermogravimetric analysis of the polymeric foam was used to estimate the gas evolution rate associated with the pyrolysis process, but this was found to be very low compared to the pressures required to cause strut damage. SEM observations on samples taken by interrupting the fabrication procedure showed the cracks were not produced during drying but rather at a temperature near the melting/decomposition point of the polymer and prior to pyrolysis. It was then deduced that the differential thermal expansion between the polymer and the ceramic coating was the source of the stress. The strut cracking is observed to occur primarily in the region of the highly curved strut edges of the polymer foam, at which the ceramic coating is often rather thin. Techniques to change the processing procedure to overcome the strut cracking are discussed.