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Adhesion of coatings to sheet metal under plastic deformation
ARTICLE IN PRESS
International Journal of Adhesion & Adhesives 27 (2007) 480–492 www.elsevier.com/locate/ijadhadh
Adhesion of coatings to sheet metal under plastic deformation Ravi Vayedaa, Jyhwen Wanga,b, a
Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA Department of Engineering Technology and Industrial Distribution, Texas A&M University, College Station, TX, USA
b
Accepted 15 August 2006 Available online 30 October 2006
Abstract This study presents a new technique to evaluate adhesion of coatings to sheet metal under plastic deformation. Based on the concept of the forming limit diagram (FLD), the present work evaluates coating performance with the notch-coating adhesion test, the cross-hatch tape test (ASTM D3359), accelerated conditioning, the uniaxial tensile test, and the rectangular stretch bend test. Experiments were conducted on two types of polyvinylidene fluoride (PVDF)-coated sheet metals: one with polyester primer and other with polyurethane primer. The areas with coating failures were identified, and results of the evaluation were plotted according to major and minor strains depicting coating durability. The constructed durability limit diagram can be used in conjunction with the FLD to determine the feasibility of a complex forming operation. Optical microscopy and energy-dispersive X-ray spectroscopy (EDS) were employed to understand the nature and composition of debonded surfaces after conditioning and application of strain. Comparative results were presented in terms of number of damaged grids in each sample for different applied strains and different durations of conditioning. This procedure combines the key attributes of various experimental techniques and shows that the methodology is successful in characterizing the strength and weakness of two different types of coated materials. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Peel; Delamination; Durability
1. Introduction In the conventional manufacturing process, parts are stamped from sheet metal into the required final shape and then are coated with paint. The curing process in a conventional coating cycle produces harmful substances known as volatile organic compounds (VOCs) or hydrocarbons. The organic solvents used in the paint and the paint thinner contain toxic substances such as xylene, turpenoid, toluene, mineral spirits, acetone and methyl ethyl ketone. The solvent evaporates during the curing process from the coating, releasing these toxic chemicals into the atmosphere. The VOCs react with nitrogen oxides in the presence of sunlight and form ozone in the atmosphere. VOCs can cause health disorders in humans and can destroy natural vegetation. Apart from that, a large amount of waste water, used solvent and paint sludge Corresponding author.
E-mail address: [email protected] (J. Wang). 0143-7496/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2006.08.003
is generated after each cycle of conventional coating process. Various laws and acts have been passed by the governments around the world in order to prevent pollution arising from the paints and coatings industry. Coil coating is an alternative which may reduce VOC emissions and prevent the formation of hazardous wastes during the coating process. It is a continuous and automated process for coating sheet metal prior to stamping parts [1]. Every coil-coating line consists of a number of basic steps, or operations, such as unwinding the metal coil, cleaning and treating the coil with chemicals, applying the primer, curing the primer, applying the top and/or bottom coat, curing the coat, cooling, and finally rewinding the coil for shipment [2]. The process, also known as pre-coating, can lead to control of the VOC emission by incorporation of incinerators and treatment units for waste water and toxic gases in the production lines. Compared to most other painting processes, it is a time- and energy-efficient process which has several other advantages, such as low running cost, low transportation
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and storage costs, and reduced bottlenecks. The coating thickness can be accurately controlled during the process, which ensures consistent chemical and mechanical properties in all the sheets. Since the coated sheet metal coil is free from dirt, residual oil, uneven application and other defects, it provides a better resistance against harsh weather and corrosive environment. The major applications of coil-coated materials are in the areas of construction, transportation, consumer products and the packaging industry. In spite of all the above advantages, the coating on a coated sheet metal has to meet a key technical requirement, i.e. it has to survive the manufacturing process which converts the sheet metal into a final product. The coating should not be damaged during various operations, such as cutting, bending, stamping, embossing, roll forming, or deep drawing. It has to be free from defects, such as cracking, peeling, chipping and scratch. Also the adhesive bond between the coating, primer, and substrate should remain intact during the manufacturing process. Thus, understanding the durability of coating during plastic deformation of coated sheet metal has become a critical concern. Research in adhesive science focuses on materials and mechanics of adhesion. Like adhesives, coatings must also adhere to surfaces. Kinloch [3] and Comyn [4] have described different mechanisms of adhesion such as physical adsorption, mechanical interlocking, chemical bonding, diffusion, electrostatic force, and weak boundary layer theories. While these theories are important to our understanding of the process of adhesion at the atomic and molecular level, experiments and tests are always required for the measurement of coating adhesion at the macroscopic level. Some coatings are brittle while others are ductile, and each behaves in a different manner when subjected to deformation. Today, a number of test methods are used in industries to evaluate the durability of coil coatings. But despite the importance of adhesion in coatings, there are no good, standardized test methods for measuring this property [5]. The most common mechanical tests to evaluate the adhesion of coatings are the mandrel bend test (ASTM D522), the scrape adhesion test (ASTM D2197), the impact failure test (ASTM D2794), the cross-cut tape test (ASTM D3359), the Taber abrasion test (ASTM D4060), the T-bend flexibility test (ASTM D4145), the pull-off test (ASTM D4541), and the mar resistance test (ASTM D5178) [6]. Since all the above tests are mostly static in nature, they do not provide enough data on coating durability relative to a plastically deformed substrate. Thus, there is a need to directly address coating failures in deformation processes where contact mechanisms and plastic deformation play significant roles. To meet this need, this study presents a new testing methodology based on the key characteristics of coating adhesion tests and sheet metal formability tests. Experiments were conducted to assess the performance of sheet metal primed with polyester and polyurethane and
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then coated with polyvinylidene fluoride (PVDF). Based on the concept proposed by Wang et al. [7], the data collected in the experiments was used to construct the durability limit diagram (DLD) for a given coating system. The feasibility of using a DLD to predict coating performance during and after sheet metal forming is discussed. 2. Evaluation of coating durability in forming coated sheet metal As a result of the growing interest in deformable coated materials, work is being conducted on the deformation of polymer-coated sheet metal. Mechanical stress imposed on the coated material is a key factor that leads to coating failure. Various aspects of continuum plasticity (deformation and rupture of coating and substrate), adhesive bonding, and surface contact damage are being investigated. Adhesion between a solid and a stretched elastic material was investigated by Gay [8]. For such materials, it was concluded that the adhesion at a molecular scale is not affected by stretching. However, adhesion at a macroscopic scale is changed due to the elastic response of the material. Chang et al. [9] devised a test method called the notched coating adhesion (NCA) test to estimate the adhesive performance and durability by using special specimens that accelerate humidity conditioning. The specimens were subjected to uniaxial tensile strain and the strain at which the coating de-bonded was used to calculate the critical strain energy release rate. The NCA test method was used to measure the interfacial fracture toughness of certain coatings. Later, Dillard et al. [10] conducted finite element analysis to determine the critical strain energy release rates for the NCA specimen. Even though plastic deformation is developed when the NCA specimens are placed in tension, the NCA test cannot predict the adhesion failure of coatings on deformed substrates in practical situation of complex metal forming. The application of polymer-coated metal to strip ironing was studied by the Jaworski et al. [11]. The experiments demonstrated that the coating integrity was affected by tooling design and processing conditions. Huang [12] and Huang et al. [13] further explored the effects of temperature on the survivability of coating in strip ironing. Coating adhesion after deep drawing was investigated in relation to the pre-treatment of aluminum by Hatanaka et al. [14]. It was concluded that the deterioration of adhesion by drawing was due to the cohesive failure of films resulted from pre-treatment and the change in the underlying surface topography. The performance of polyurethane- and polyester-coated stainless steel after Erichsen cup drawing was evaluated by Deflorian et al. [15]. The adhesion was evaluated through electrochemical impedance spectroscopy. Braunlich et al. [16] evaluated the formability of polymer-coated steels in production conditions. Bending, folding, embossing, deep drawing, and other tests were conducted. Parts were then inspected for gloss loss, coating tear-off, etc. The work provided a means to evaluate coating performance.
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However, use of this approach for coating and application development is costly and time consuming. In sheet metal forming, failure normally occurs by the development of a localized neck in the metal. The strain states at which localized necks are first observed can be experimentally measured for various materials. By conducting the limit dome height (LDH) tests and plotting the measured strain states, the forming limit diagram (FLD) of the material can be established as shown in Fig. 1 [28,29]. Numerous attempts have also been made to predict the FLDs of the materials analytically [17–27]. To evaluate a forming operation, strain states from numerical calculations are compared with the FLD of the material. This widely used strain-based analysis technique deals with plastic deformation of sheet metal only. The coefficient of friction between tooling and sheet metal is an input parameter in finite element analyses. No consideration is given to the durability of the polymer coating. The strain states in sheet forming can be biaxial (e14e240), plane strain (e140, e2 ¼ 0), or deep drawing (e140, e2o0) as characterized in the LDH test. For coated sheet metal, these strain states may result in an increase or decrease of surface area at the coating–primer–substrate interface. The deformation may also create interfacial shear. How the changing and shaping of the interface affects the coating performance requires a thorough investigation. An attempt was first made by Wang et al. [7] to evaluate the coating durability in sheet metal forming using the concepts of FLD and DLD. Experiments were conducted to evaluate the performance of thermoset- and thermo-
Fig. 1. Limit dome height test and forming limit diagram [28,29].
plastic-coated steel under deformation. The methodology combined the NCA test, the sheet metal LDH test, and the ASTM cross-hatch tape test. It was shown that the coating adhesion can be affected by deformation. There was no significant debonding when the material was subjected to biaxial stretching, whereas the adhesive bond deteriorated and coating pickup was observed in the tape test for the deep drawing mode. Since the coating failure was observed only in the deep drawing deformation mode, this mode is considered to be detrimental for coating survival in manufacturing processes. Hence, it is the authors’ concern that there should be in-depth investigation for coated sheet metal in plane strain and deep drawing deformation modes. In order to fill this need, this study focuses on using the tests that would subject the specimens to the required deformation modes. 3. The proposed coating durability evaluation method Based on the concept of the FLD, the present study evaluates coating performance in coated sheet metal under plastic deformation by means of the notch coating adhesion (NCA) test, accelerated conditioning, the crosshatch tape test (ASTM D3359), uniaxial tensile test, and rectangular stretch bend test. Most coatings are susceptible to moisture ingression, but the time required for adhesive bond degradation and moisture equilibrium for a given coating may be few or several years depending on the type of coating, its use, and its exposure to the environment [9]. Thus, the use of notched, conditioned specimens becomes an excellent means for accelerated testing of adhesion. The notch serves as the initial debonding site that could lead to crack propagation and further damage as the material is strained. Instead of the single notch used by Chang et al. and Dillard et al., cross-hatched grids similar to (ASTM D3359) tape test are created in the present test specimens. The grids serve two purposes. First, they accelerate the conditioning of the specimen in different locations by shortening the diffusion path and reducing the time required to saturate the adhesive bond and the interface region. Secondly, similar to the etched or printed circular grids in LDH test, the notched grids can be used for strain measurement. The cross-hatched specimens are placed in a humidity cabinet for the conditioning that will cause coating adhesion to deteriorate and lead to debonding during deformation process. The specimens are subjected to different durations of conditioning. Tape tests are conducted after each conditioning to inspect coating adhesion. To investigate the effects of substrate strain on coating, the specimens are subjected to deformation using the uniaxial tensile test and rectangular stretch bend test. In both tests, the applied displacement determines the strain states obtained from the specimens. As the displacement can be controlled in a universal tensile tester as well as the hydraulic press, various strain states can be created during the tensile tests and stretch bend tests. Post-forming
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Experiments were conducted on two different types of coil-coated sheet metals supplied by a roll coating company. Let us designate the sheet metal materials as A and B. The composition of each is as shown in Table 1. As one can see, both materials contain the same type of coating but the primer and substrate are different.
The first cut was made along the length of the gauge section. The second cut was made at 901 to the first cut along the width of the gauge section creating a cross-hatch grid pattern with 210 squares of 1.5 1.5 mm. Care should be taken to ensure that the cut is made in one steady motion using just sufficient pressure on the blade to have the cutting edge reach the substrate. For the rectangular stretch bend test, the specimen was prepared by cutting the coil-coated sheet metal into a rectangular shape of 180 100 mm. Cross-hatch grids were created on the top surface of the specimen according to ASTM D3359-97. Following the procedure mentioned in the previous paragraph, a grid with 1600 squares of 1.5 1.5 mm was created. The location and number of cuts in the specimen are as shown in Fig. 3. For evaluation of coating under the environmental factors such as moisture and salt, the specimens were subjected to conditioning tests according to modified ASTM D870-97 [32]. Each specimen was submerged in a corrosion resistant closed container filled with a solution of water and salt at ambient temperature. The concentration of salt used was 6 g/l. Batches of specimen were tested for different durations of conditioning. The specimens were conditioned for a period of 1, 4, 7 and 14 days.
4.2. Specimen preparation
4.3. Deformation process
For the uniaxial tensile test, the standard dog bone shape specimen was prepared from coil-coated sheet metal according to ASTM A370-97a [30]. The dimensions of the specimen are as shown in Fig. 2. Cross-hatch grids were created according to ASTM D3359-97 [31] in the gauge section of the specimen. A blade with multiple cutting edges spaced 1.5 mm apart was used to create the grids.
4.3.1. Uniaxial tensile test In this test, the tension test specimens were tested on a universal testing machine according to ASTM A370 and ASTM E8. An electromechanical, testing system with a load frame capacity of 10 kN supplied by United Testing Systems Inc. was used. The system was equipped with a computer and software to monitor the extension, applied
evaluation of the specimens consists of inspection of coating adhesion by means of the tape test and by strain measurement. For a measured strain state, conclusions about coating performance can be made based on various criteria. Once the coating failures are identified, results of the evaluation are plotted according to the major and minor strains to depict the coating durability. The constructed durability limit diagram can then be used in conjunction with the FLD to determine the feasibility of a complex forming operation. Comparative results are presented in terms of percentage of the coating area that is debonded for different applied strain, and different durations of conditioning. 4. Experimental procedure 4.1. Materials
Table 1 Material data Sheet thickness (mm)
Material A Material B
0.584 0.572
Coating
Primer
Substrate material
Material
Thickness (mm)
Material
Thickness (mm)
PVDF PVDF
0.0180 0.0180
Polyester Polyurethane
0.0051 0.0046
HDG, hot dip galvanized; CQ, commercial quality. Galvalumes is an aluminum–zinc alloy metallic coating.
Fig. 2. Uniaxial tensile test specimen (all dimensions in mm).
Galvalume (Grade 50) HDG (CQ)
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Fig. 3. Rectangular stretch bend test specimen (all dimensions in mm).
load, and cross-head displacement. The experimental procedure is summarized as follows: 1. The tension test specimen is clamped from the grip section in the upper and lower jaws of the tensile testing machine. 2. An extensometer is attached to gauge section of the specimen to monitor strain. 3. The cross-head is displaced at the rate of 2.54 mm/min. 4. The movement of the cross-head is stopped when the extensometer indicates yielding in the gauge section. 5. The jaws are unclamped, and the specimen is removed from the machine. In order to evaluate the effect of higher strain states on coating adhesion, several specimens were tested beyond the yield point by increasing the displacement loading up to specimen failure or necking point. 4.3.2. Rectangular stretch bend test In this test, rectangular specimens were tested on a LDH testing machine supplied by MTS Systems Corporation. The system consists of a hydraulic power supply with pressures up to 35 MPa. It was equipped with a computer, display panel and software to monitor the clamping force, punch displacement and reaction force exerted on the punch. The experimental procedure is summarized as follows: 1. In this test, the specimen is clamped with a force of 700 kN by a circular clamp. 2. Displacement loading is applied on the specimen from bottom with a rectangular punch. 3. When the punch is moved upwards, a reaction force is exerted on the punch by the specimen in order to prevent deformation. This force increases until the material yields. It is measured by a sensor and displayed on the display panel.
4. The upward movement of the punch is stopped when a significant drop in the reaction force is observed. The drop in the reaction force signifies a failure in the specimen i.e. the substrate. 5. The punch is brought down at its original position, the clamp pressure is released, and the specimen is removed from the machine. 4.4. Tape pull adhesion measurement In this test, as per ASTM D3359, a pressure-sensitive adhesive tape was applied over the grids on the top of the specimen. Within 2 min of application, the tape was peeled away rapidly from the specimen by seizing the free end and pulling at an angle close to 1801. The grids were inspected for removal of coating. Coating adhesion was qualitatively determined from the severity of the pickup. The number of damaged elements in each sample was reported after the tape test. The tape pull test was conducted at three stages in the entire experimental procedure: 1. After cross-hatching the specimen, tape test was conducted to determine the possibilities of coating pickup due to initial debond. 2. After the accelerated conditioning, tape test was conducted to determine the loss of coating adhesion due to salt and moisture penetration. 3. After the application of strain i.e. after the uniaxial tensile test and rectangular stretch bend test, tape test was conducted to evaluate coating durability under plastic deformation of substrate. 4.5. Strain measurement The strain in the deformed specimen was measured using the Automated Strain Analysis and Measurement
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Environment (ASAME) Target Model (TRM), a digital camera-based system that measures surface geometry and strain distribution over an area of a deformed part that has been grided with a pattern of squares. The measurement procedure is as follows: 1. Two photographs of the measurement area (region with grids) with the target cube (a cube for reference) in the view are taken from different positions. 2. The photos are then processed to correct any defects in the grids. Defects can occur due to grid application error, excessive peeling during forming or uneven lighting conditions. 3. The digital photographs are then used to determine the three-dimensional coordinates of the intersections on a square grid by the ASAME software. 4. The measured three-dimensional coordinates, along with the size of the original undeformed square, are used to determine the surface strains. A measurement accuracy of 2.5% strain is obtained with this system. 5. The results are plotted in the form of a durability limit diagram showing the values of effective, major and minor strains at which debonding is observed in each specimen.
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to detect the type and amount of elements present in the debonded spots. A JEOL JSM-6400 scanning electron microscope and a Princeton Gamma-Tech unit equipped with a prism digital spectrometer and Spirit program (version 1.07) were used. The EDS was performed at 15 KV accelerating voltage for 60 s with working distance of 15 mm and a take off angle of 57.21. The detector material was Si (Li). The analyzed thickness was approximately 1 mm. The debonded spots were examined at different magnifications and the images were recorded photographically. 5. Results and discussions Based on the proposed methodology, tensile and stretch bend specimens were prepared from both materials A and B. Qualitative observation of adhesion test was made for specimens subjected to different durations of conditioning and different levels of straining. Samples (three specimens each) were taken from each conditioning variable (0, 1, 7, and 14 days) to count the number of debonded grids and to construct the durability limit diagram. The results and discussions are presented as the followings.
4.6. Optical microscopy and image processing Olympus 3M microscope was used to analyze small and large debonded spots on the specimen using an optical magnification ranging from 50 to 500 . The main purpose of conducting optical microscopy was to understand the physical nature of debonded spots for the two different types of materials used in this study. 2D and 3D images were produced using ImageJ [33], a java-based image analysis program, to obtain a clear picture of debonded locations. Multiple images of each debonded spot were taken along the optical axis. Using the ‘‘Extended Depth of Field’’ plugin available with ImageJ, infocus images were created by merging the stack of photographs taken at different focal lengths into a single totally focused composite image. 4.7. Energy-dispersive X-ray spectroscopy (EDS) The debonded surfaces on the specimen were characterized using EDS. The purpose of conducting the EDS was
5.1. PVDF coating, polyester primer on Galvalume steel substrate 5.1.1. Effect of conditioning and rectangular stretch bend test No damage was observed on the coating for a conditioning duration of up to 4 days. But when the samples were subjected to conditioning for 7 days some of the square grid elements showed scattered, swelled spots adjacent to the grids. Fig. 4 shows a swelled spot of about 0.5 mm in diameter. This was an indication that the adhesive bond within the primer and the coating had deteriorated due to penetration of salt and water. But the spots did not peel away when subjected to tape tests. Although the adhesive bond had deteriorated, it was strong enough to overcome the peel force and keep the coating attached to the substrate. When tape tests were conducted on the specimens subjected to straining, the swelled spots peeled off. Fig. 5 shows a peeled spot of approximately 0.5 mm in diameter. That is, strain
Fig. 4. Swelled spot from sample with polyester primer: (a) 2D image and (b) 3D image.
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caused complete failure of the swelled and partially debonded spots. It was observed that peeled spots contained polymer residue adhered to the metal substrate. In order to confirm the presence of polymer on the peeled surface, EDS was performed on some spots from few specimens. As shown in
Fig. 6, it was found that the major composition (more than 90% by weight) of the spots was carbon, which suggested the presence of polymer. Other significant elements found were aluminum and zinc, which showed the presence of material from the galvanized coating layer. Since the spots were affected by conditioning, chlorine (an element of salt)
Fig. 5. Debonded spot from sample with polyester primer: (a) 2D image and (b) 3D image.
Fig. 6. EDS of peeled spot from sample with polyester primer.
Fig. 7. Durability diagram for samples with polyester primer subjected to rectangular stretch bend test.
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was also detected in the spots. Other elements such as boron, chromium and molybdenum were also found in trace amounts in the spots. The major and minor strain values of the grids collected from the samples are shown on the durability diagram in Fig. 7. The color contour shows the effective strain at different locations in the sample. The damaged grids are selected by white dots at its corners in each specimen. The triangular marks on the graph represent the major and minor strain values of the damaged grids. The maximum effective strain in the specimens for rectangular stretch bend test was around 35% at substrate failure. One can see that most of the peeling occurred in grids with low values of major (between 5% and 20%) and minor (between 2% and 6%) strains. This was because moisture penetrated at these spots and weakened the adhesive bond. When the spots were subjected to straining, the adhesive bond failed leading to debonding of the coating. Many grids subjected to high strain states were found to be undamaged. This suggested that the coating and primer
Table 2 Number of damaged grids for samples with polyester primer subjected to rectangular stretch bend test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 17 14 24
10–20
20–30
30 and 4
0 0 15 21
0 0 3 5
0 0 0 0
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in those grids were not affected by strain. Thus it can be concluded that this coating has the quality of sustaining high strain states without being damaged if it is not subjected to severe conditioning. When the samples were subjected to conditioning for 14 days, the size of the scattered, swelled spots increased to 1 mm in diameter. The number of spots remained approximately the same as for the conditioning of 7 days. Some of the spots debonded and peeled off when subjected to peel tests. This indicated that a period of 14 days of conditioning caused the adhesive bond to deteriorate completely, leading to coating failure in certain grids prior to application of strain. When the specimens were subjected to strain, all the swelled spots peeled from the substrate. This showed that the application of strain caused complete failure of all the swelled and partially debonded spots. The results for samples subjected to rectangular stretch bend test in terms of applied strain and duration of conditioning were summarized in the form of number of damaged grids. From Table 2 it can be concluded that the coating performed well at all values of effective strain when subjected to 0 and 1 days of conditioning. Damaged grids were found in samples subjected to 0–10% and 10–20% effective strain and a conditioning duration of 7 days. Most of the grids at 20–30% effective strain and above were able to survive the substrate deformation. This was because these grids showed good resistance to conditioning. 5.1.2. Effect of conditioning and uniaxial tensile test Similar results were obtained when the samples were subjected to uniaxial tensile test. The debonding was more severe in tensile samples. The numbers of failed grids were higher per specimen as well as for the overall results when
Fig. 8. Durability diagram for samples with polyester primer subjected to uniaxial tensile test.
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compared with samples subjected to rectangular stretch bend tests. Also higher values of major and minor strains were achieved before substrate failure occurred in the tensile tests. The durability diagram for the tensile experiment is shown in Fig. 8. The maximum effective strain in the samples for tensile test was around 40% at substrate failure. Table 3 shows the number of damaged grids. At 0–10% and 10–20% applied strain, almost double the numbers of grids were damaged in the tensile samples compared to samples in rectangular stretch bend test. For 20–30% strain and above, the number are same for both the tests. Also it is clear from the results that the longer the duration of conditioning the higher the number of damaged grids. Thus, for the PVDF-coated samples with polyester primer, it can be concluded that although the applied strain caused complete failure of the adhesive bond, the major damage was caused by moisture and salt penetration. This coating showed good performance at high values of strain but was susceptible to severe damage when subjected to conditioning. So one should take precautions before using this coated sample in humid and corrosive environment.
5.2. PVDF coating, polyurethane primer on hot dip galvanized steel substrate 5.2.1. Effect of conditioning and rectangular stretch bend test The samples were subjected to a conditioning duration from 1 to 14 days. No damage was observed on the coating Table 3 Number of damaged grids for samples with polyester primer subjected to uniaxial tensile test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 32 14 48
10–20
20–30
30 and 4
0 0 21 27
0 0 1 1
0 0 0 0
after any of these conditioning durations. The samples did show some moisture penetration through the grids but there was no swelling or debonding of the coating on the grids. Fig. 9(a) shows grids with moisture penetration after 7 days of conditioning duration. This was an indication that the grids were subjected to penetration of salt and water, even though the coating did not peel away when subjected to tape tests. Although the grids were affected, the adhesive bond within the coating and primer was strong enough to overcome the peel force and keep the coating attached to the substrate. When tape tests were conducted on the samples subjected to straining, the affected spots on the grids debonded. Fig. 9(b) shows debonded grids after the application of strain. On comparing Fig. 9(a) and (b), one can see that only the locations that show moisture penetration within the grids debonded after the application of strain. Strain-induced damage to the adhesive bond in the grids that were affected by salt and moisture penetration and caused debonding. Other grid locations on the sample did not show any signs of damage. The type damage observed in coating with polyurethane primer was different compared to the coating with polyester primer. There was debonding of the coating but no peeling was observed during the tape test. Similar to the samples with polyester primer, the debonded spots in this sample also contained polymer residue adhered to the metal substrate. Fig. 10 shows a debonded spot with polymer residue and exposed substrate. In order to confirm the presence of polymer on the debonded surface, EDS was performed on few spots. As shown in Fig. 11, it was found that the major composition (more than 89% by weight) of the spots was carbon which suggested the presence of polymer. Other significant elements found were boron, oxygen and iron. Aluminum and zinc were also found in trace amounts which showed the presence of galvanized coating layer. Chlorine (an element of salt) was also detected in the spots. The values of major and minor strains of the grids are shown on the durability diagram in Fig. 12. The triangular spots on the graph represent the major and minor strain values of the damaged grids. The maximum effective strain
Fig. 9. Sample with polyurethane primer: (a) moisture penetration in grids due to conditioning and (b) debonding in grids due to strain.
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Fig. 10. Debonded spot from sample with polyurethane primer: (a) 2D image and (b) 3D image.
Fig. 11. EDS of debonded spot from sample with polyurethane primer.
Fig. 12. Durability diagram for samples with polyurethane primer subjected to rectangular stretch bend test.
in the specimens with polyurethane primer subjected to rectangular stretch bend test was around 35% at substrate failure. One can see that most of the peeling occurred in grids with high values of major (between 15% and 30%) and minor (between 6% and 10%) strains. During conditioning, moisture penetrated at several locations in
the grids but debonding occurred only in grids that were subjected to high strain states. Many grids subjected to low strain states were found to be undamaged. This suggested that the adhesive bond within the coating and primer in those grids was not affected by strain or conditioning. Thus, it was noted that
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this coating performed well against conditioning and low strain states but was not able to sustain high strain states. When the samples were subjected to strain after a conditioning duration of 14 days, the number of damaged grids was slightly higher compared with the number of damaged grids after conditioning of 7 days. This indicated that a period of 14 days of conditioning resulted in deterioration of the adhesive bond in a higher number of grids, leading to failure after the application of strain. The results for samples subjected to rectangular stretch bend test in terms of applied strain and duration of conditioning were summarized in the form of number of damaged grids. From Table 4, it can be concluded that the coating performed well at all values of strain when subjected to 0 and 1 days of conditioning. But for a conditioning duration of 7 and 14 days, some damaged grids were noted in the samples. Most of the grids at 0–10% effective strain were able to survive the deformation of the substrate. This was because these grids showed good resistance to conditioning as well as straining. But many
Table 4 Number of damaged grids for samples with polyurethane primer subjected to rectangular stretch bend test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 0 14 4
10–20
20–30
30 and 4
0 0 20 29
0 0 46 41
0 0 1 6
girds subjected to 10–20% and 20–30% effective strain were damaged. Thus this coating was susceptible to damage at high strain states. 5.2.2. Effect of conditioning and uniaxial tensile test Different results were obtained when the samples were subjected to uniaxial tensile test. Girds were found to be damaged at low as well as high strain states. The debonding was more severe in tensile samples. The numbers of failed grids were higher per specimen as well as for the overall results when compared with samples subjected to rectangular stretch bend tests. The durability diagram for the tensile samples is shown in Fig. 13. The maximum effective strain in the samples for tensile test was around 50% at substrate failure. As one can see, the damaged grids are present all the way from 5% to 50% effective strain. Table 5 shows that at 0–10% and 10–20% effective strain almost double the numbers of grids were damaged in the tensile samples compared to samples in rectangular stretch bend test. For 20–30% strain and above, the number is slightly lower for the tensile tests. Many grids were found to be damaged at 30% and higher values of effective strain. Thus low as well as high strain states were detrimental for the coating survival in this (tensile) deformation mode. Thus for the PVDF-coated samples with polyurethane primer, it can be concluded that the applied strain caused complete failure of the adhesive bond. The coating showed resistance to damage due to conditioning but it was unable to withstand high substrate deformation. So one should take precautions before using this coated material in conditions where high strain states are involved.
Fig. 13. Durability diagram for samples with polyurethane primer subjected to uniaxial tensile test.
ARTICLE IN PRESS R. Vayeda, J. Wang / International Journal of Adhesion & Adhesives 27 (2007) 480–492 Table 5 Number of damaged grids for samples with polyurethane primer subjected to uniaxial tensile test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 13 14 41
10–20
20–30
30 and 4
0 0 45 51
0 0 20 28
0 0 36 16
491
coated sheet metals for suitable applications. It is also believed that additional work is needed to develop analytical or numerical models to investigate the stress states at the polymer–metal interface under different deformation conditions. Acknowledgments
6. Conclusions The commonly used tests for adhesion of coatings are not effective in evaluating the coating performance in the presence of plastic deformation of the substrate. On the other hand, the formability test used in the sheet metal industry does not evaluate coating durability. In this paper, a testing methodology combines the key attributes of various experimental techniques is presented to effectively address the concern. Based on the proposed methodology, experimental investigations showed that coating adhesion can be affected by plastic deformation. Two coated sheet metal with different primer and metallic coating were tested for durability. In samples with polyester primer, a number of scattered, swelled spots were observed on the coating adjacent to the grids after conditioning. Some of the spots peeled off prior to application of strain. The durability diagram showed that debonding occurred in grids that subjected to low values of major and minor strain whereas many grids with high strain states were still found to be unaffected. It was concluded that moisture penetrated at certain spots through the grids and weakened the adhesive bond. When these spots were subjected to straining, the adhesive bond failed, leading to coating failure. Thus, the coating with polyester primer showed better performance under plastic deformation of substrate but was susceptible to damage by humid and corrosive environment. In samples with polyurethane primer, no significant damage patterns were observed in the coating after conditioning. But when subjected to straining, the coating showed debonded spots adjacent to the grids. The durability diagram showed that debonding occurred in grids subjected to high values of major and minor strain, whereas the areas with low strain states were found intact. The higher substrate strain caused the adhesive bond to deteriorate, leading to coating failure. Thus, the coating with polyurethane primer showed better resistance to humid and corrosive environment but was susceptible to damage under plastic deformation of the substrate. The assessment methodology presented in this paper can be used to investigate coating adhesion under conditions of substrate deformation. The experimental results suggested that the proposed testing method is effective in evaluating
The authors gratefully acknowledge the support of National Science Foundation (NSF Grant award DMI0448789). References [1] National Coil Coating Association, Prepaint FAQs /http:// www.coilcoatinginstitute.org/faqs/#whatlookS [accessed November 2005]. [2] SpecialChem Coatings and Inks, Coil coating part II—the process, coil coating part III—the coatings and their markets /http:// www.specialchem4coatings.com/resources/articles/article.aspx?id=2869S [accessed November 2005]. [3] Kinloch AJ. Adhesion and adhesives—science and technology. New York: Chapman & Hall; 1987. [4] Comyn J. Adhesion science. Cambridge: Royal Society of Chemistry Paperbacks; 1997. [5] Weldon DG. Failure analysis of paints and coatings. Chichester: Wiley; 2001. [6] D3794-00 Standard guide for testing coil coatings. ASTM Int, 2005. [7] Wang J, Alexander R, Pauly S. An experimental investigation of coating durability in forming pre-coated sheet metal. SME Trans XXXI, N Am Manuf Res Inst 2003:57–63. [8] Gay C. Does stretching affect adhesion? Int J Adhes Adhes 2000;20:387–93. [9] Chang T, Sproat EA, Lai YH, Shephard NE, Dillard DA. A test method for accelerated humidity conditioning and estimation of adhesive bond durability. J Adhes 1997;60:153–62. [10] Dillard DA, Chen B, Chang T, Lai YH. Analysis of the notched coating adhesion test. J Adhes 1999;69:99–120. [11] Jaworski J, Schmid SR, Wang J. An experimental investigation of the formability and friction characteristics of tin coated and polymer laminated steels. J Manuf Sci Eng 1999;121:232–7. [12] Huang CH, Schmid S, Wang J. Thermal effects on polymer laminated steel formability in ironing. J Manuf Sci Eng 2001;123:225–30. [13] Huang CH. Tribology of polymer films in manufacturing. PhD thesis, University of Notre Dame, 2001. [14] Hatanaka K, Fukui M, Mukai Y, Toyose K. Influence of drawing on organic-coating adhesion property in aluminum. J Jpn Inst Light Met 1989;39(9):621–6. [15] Deflorian F, Fedrizzi L, Rossi S, Benedetti L, Bonora P. Effect of mechanical deformation on the degradation of coil coated stainless steel. Metall Ital 1999;91(5):60. [16] Braunlich H, Demmler M. Behavior of organic coatings during forming. Met Form 1998:36–40. [17] Keeler SP, Backofen WA. Trans ASM 1965;56:25–48. [18] Meuleman DJ, Siles JL, Zoldak JJ. The limiting Dome height test for assessing the formability of sheet steel. SAE technical paper #850005, Warrendale, Pennsylvania, 1985. [19] Marciniak Z, Kuczynski K. Limit strains in the process of stretch forming sheet metal. Int J Mech Sci 1967;9:609–20. [20] Sowerby R, Duncan JL. Failure in sheet metal in biaxial tension. Int J Mech Sci 1971;13:217–29. [21] Yamaguchi K, Mellor PB. Thickness and grain size dependence of limit strains in sheet metal stretching. Int J Mech Sci 1976;18:85–90. [22] Parmer A, Meller PB. A new model for the prediction of instability and limit strains in thin sheet metal. Int J Mech Sci 1977;19:389–98.
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[23] Graf A, Hosford WF. Calculations of forming limit diagrams. Metall Trans 1990;21A:87–94. [24] Shi MF, Gerdeen. Effect of strain gradient and curvature on forming limit diagrams for anisotropic sheets. J Mater Shap Technol 1991;9: 253–68. [25] Xu S, Weinmann KJ, Chandra A. Analysis of forming limits using the hill 1993 yield criterion. J Eng Mater Technol 1998;120:236–41. [26] Klocke F, Breuer D, Raedt HW. An advanced method to describe the forming limit of metals. SME/NAMRI Trans 2003:33–40. [27] Chow CL, Jie M, Hu SJ. Forming limit analysis of sheet metal based on generalized deformation theory. J Eng Mater Technol 2003;125: 260–5.
[28] Kalpakjian S, Schmid SR. Manufacturing engineering and technology. Englewood Cliffs, NJ: Prentice Hall. [29] Hosford WF, Caddell RM. Metal forming—mechanics and metallurgy. Englewood Cliffs, NJ: Prentice-Hall; 1983. [30] ASTM A 370-97a, Standard test methods and definitions for mechanical testing of steel products, ASTM International, 2006. [31] ASTM D 3359-97, Standard test methods for measuring adhesion by tape test, ASTM International, 2006. [32] ASTM D 870-97, Standard Practice for testing water resistance of coatings using water immersion, ASTM International, 2006. [33] Rasband WS. ImageJ, US National Institutes of Health, Bethesda, MD, USA, /http://rsb.info.nih.gov/ij/S, 1997–2005.
International Journal of Adhesion & Adhesives 27 (2007) 480–492 www.elsevier.com/locate/ijadhadh
Adhesion of coatings to sheet metal under plastic deformation Ravi Vayedaa, Jyhwen Wanga,b, a
Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA Department of Engineering Technology and Industrial Distribution, Texas A&M University, College Station, TX, USA
b
Accepted 15 August 2006 Available online 30 October 2006
Abstract This study presents a new technique to evaluate adhesion of coatings to sheet metal under plastic deformation. Based on the concept of the forming limit diagram (FLD), the present work evaluates coating performance with the notch-coating adhesion test, the cross-hatch tape test (ASTM D3359), accelerated conditioning, the uniaxial tensile test, and the rectangular stretch bend test. Experiments were conducted on two types of polyvinylidene fluoride (PVDF)-coated sheet metals: one with polyester primer and other with polyurethane primer. The areas with coating failures were identified, and results of the evaluation were plotted according to major and minor strains depicting coating durability. The constructed durability limit diagram can be used in conjunction with the FLD to determine the feasibility of a complex forming operation. Optical microscopy and energy-dispersive X-ray spectroscopy (EDS) were employed to understand the nature and composition of debonded surfaces after conditioning and application of strain. Comparative results were presented in terms of number of damaged grids in each sample for different applied strains and different durations of conditioning. This procedure combines the key attributes of various experimental techniques and shows that the methodology is successful in characterizing the strength and weakness of two different types of coated materials. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Peel; Delamination; Durability
1. Introduction In the conventional manufacturing process, parts are stamped from sheet metal into the required final shape and then are coated with paint. The curing process in a conventional coating cycle produces harmful substances known as volatile organic compounds (VOCs) or hydrocarbons. The organic solvents used in the paint and the paint thinner contain toxic substances such as xylene, turpenoid, toluene, mineral spirits, acetone and methyl ethyl ketone. The solvent evaporates during the curing process from the coating, releasing these toxic chemicals into the atmosphere. The VOCs react with nitrogen oxides in the presence of sunlight and form ozone in the atmosphere. VOCs can cause health disorders in humans and can destroy natural vegetation. Apart from that, a large amount of waste water, used solvent and paint sludge Corresponding author.
E-mail address: [email protected] (J. Wang). 0143-7496/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2006.08.003
is generated after each cycle of conventional coating process. Various laws and acts have been passed by the governments around the world in order to prevent pollution arising from the paints and coatings industry. Coil coating is an alternative which may reduce VOC emissions and prevent the formation of hazardous wastes during the coating process. It is a continuous and automated process for coating sheet metal prior to stamping parts [1]. Every coil-coating line consists of a number of basic steps, or operations, such as unwinding the metal coil, cleaning and treating the coil with chemicals, applying the primer, curing the primer, applying the top and/or bottom coat, curing the coat, cooling, and finally rewinding the coil for shipment [2]. The process, also known as pre-coating, can lead to control of the VOC emission by incorporation of incinerators and treatment units for waste water and toxic gases in the production lines. Compared to most other painting processes, it is a time- and energy-efficient process which has several other advantages, such as low running cost, low transportation
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and storage costs, and reduced bottlenecks. The coating thickness can be accurately controlled during the process, which ensures consistent chemical and mechanical properties in all the sheets. Since the coated sheet metal coil is free from dirt, residual oil, uneven application and other defects, it provides a better resistance against harsh weather and corrosive environment. The major applications of coil-coated materials are in the areas of construction, transportation, consumer products and the packaging industry. In spite of all the above advantages, the coating on a coated sheet metal has to meet a key technical requirement, i.e. it has to survive the manufacturing process which converts the sheet metal into a final product. The coating should not be damaged during various operations, such as cutting, bending, stamping, embossing, roll forming, or deep drawing. It has to be free from defects, such as cracking, peeling, chipping and scratch. Also the adhesive bond between the coating, primer, and substrate should remain intact during the manufacturing process. Thus, understanding the durability of coating during plastic deformation of coated sheet metal has become a critical concern. Research in adhesive science focuses on materials and mechanics of adhesion. Like adhesives, coatings must also adhere to surfaces. Kinloch [3] and Comyn [4] have described different mechanisms of adhesion such as physical adsorption, mechanical interlocking, chemical bonding, diffusion, electrostatic force, and weak boundary layer theories. While these theories are important to our understanding of the process of adhesion at the atomic and molecular level, experiments and tests are always required for the measurement of coating adhesion at the macroscopic level. Some coatings are brittle while others are ductile, and each behaves in a different manner when subjected to deformation. Today, a number of test methods are used in industries to evaluate the durability of coil coatings. But despite the importance of adhesion in coatings, there are no good, standardized test methods for measuring this property [5]. The most common mechanical tests to evaluate the adhesion of coatings are the mandrel bend test (ASTM D522), the scrape adhesion test (ASTM D2197), the impact failure test (ASTM D2794), the cross-cut tape test (ASTM D3359), the Taber abrasion test (ASTM D4060), the T-bend flexibility test (ASTM D4145), the pull-off test (ASTM D4541), and the mar resistance test (ASTM D5178) [6]. Since all the above tests are mostly static in nature, they do not provide enough data on coating durability relative to a plastically deformed substrate. Thus, there is a need to directly address coating failures in deformation processes where contact mechanisms and plastic deformation play significant roles. To meet this need, this study presents a new testing methodology based on the key characteristics of coating adhesion tests and sheet metal formability tests. Experiments were conducted to assess the performance of sheet metal primed with polyester and polyurethane and
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then coated with polyvinylidene fluoride (PVDF). Based on the concept proposed by Wang et al. [7], the data collected in the experiments was used to construct the durability limit diagram (DLD) for a given coating system. The feasibility of using a DLD to predict coating performance during and after sheet metal forming is discussed. 2. Evaluation of coating durability in forming coated sheet metal As a result of the growing interest in deformable coated materials, work is being conducted on the deformation of polymer-coated sheet metal. Mechanical stress imposed on the coated material is a key factor that leads to coating failure. Various aspects of continuum plasticity (deformation and rupture of coating and substrate), adhesive bonding, and surface contact damage are being investigated. Adhesion between a solid and a stretched elastic material was investigated by Gay [8]. For such materials, it was concluded that the adhesion at a molecular scale is not affected by stretching. However, adhesion at a macroscopic scale is changed due to the elastic response of the material. Chang et al. [9] devised a test method called the notched coating adhesion (NCA) test to estimate the adhesive performance and durability by using special specimens that accelerate humidity conditioning. The specimens were subjected to uniaxial tensile strain and the strain at which the coating de-bonded was used to calculate the critical strain energy release rate. The NCA test method was used to measure the interfacial fracture toughness of certain coatings. Later, Dillard et al. [10] conducted finite element analysis to determine the critical strain energy release rates for the NCA specimen. Even though plastic deformation is developed when the NCA specimens are placed in tension, the NCA test cannot predict the adhesion failure of coatings on deformed substrates in practical situation of complex metal forming. The application of polymer-coated metal to strip ironing was studied by the Jaworski et al. [11]. The experiments demonstrated that the coating integrity was affected by tooling design and processing conditions. Huang [12] and Huang et al. [13] further explored the effects of temperature on the survivability of coating in strip ironing. Coating adhesion after deep drawing was investigated in relation to the pre-treatment of aluminum by Hatanaka et al. [14]. It was concluded that the deterioration of adhesion by drawing was due to the cohesive failure of films resulted from pre-treatment and the change in the underlying surface topography. The performance of polyurethane- and polyester-coated stainless steel after Erichsen cup drawing was evaluated by Deflorian et al. [15]. The adhesion was evaluated through electrochemical impedance spectroscopy. Braunlich et al. [16] evaluated the formability of polymer-coated steels in production conditions. Bending, folding, embossing, deep drawing, and other tests were conducted. Parts were then inspected for gloss loss, coating tear-off, etc. The work provided a means to evaluate coating performance.
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However, use of this approach for coating and application development is costly and time consuming. In sheet metal forming, failure normally occurs by the development of a localized neck in the metal. The strain states at which localized necks are first observed can be experimentally measured for various materials. By conducting the limit dome height (LDH) tests and plotting the measured strain states, the forming limit diagram (FLD) of the material can be established as shown in Fig. 1 [28,29]. Numerous attempts have also been made to predict the FLDs of the materials analytically [17–27]. To evaluate a forming operation, strain states from numerical calculations are compared with the FLD of the material. This widely used strain-based analysis technique deals with plastic deformation of sheet metal only. The coefficient of friction between tooling and sheet metal is an input parameter in finite element analyses. No consideration is given to the durability of the polymer coating. The strain states in sheet forming can be biaxial (e14e240), plane strain (e140, e2 ¼ 0), or deep drawing (e140, e2o0) as characterized in the LDH test. For coated sheet metal, these strain states may result in an increase or decrease of surface area at the coating–primer–substrate interface. The deformation may also create interfacial shear. How the changing and shaping of the interface affects the coating performance requires a thorough investigation. An attempt was first made by Wang et al. [7] to evaluate the coating durability in sheet metal forming using the concepts of FLD and DLD. Experiments were conducted to evaluate the performance of thermoset- and thermo-
Fig. 1. Limit dome height test and forming limit diagram [28,29].
plastic-coated steel under deformation. The methodology combined the NCA test, the sheet metal LDH test, and the ASTM cross-hatch tape test. It was shown that the coating adhesion can be affected by deformation. There was no significant debonding when the material was subjected to biaxial stretching, whereas the adhesive bond deteriorated and coating pickup was observed in the tape test for the deep drawing mode. Since the coating failure was observed only in the deep drawing deformation mode, this mode is considered to be detrimental for coating survival in manufacturing processes. Hence, it is the authors’ concern that there should be in-depth investigation for coated sheet metal in plane strain and deep drawing deformation modes. In order to fill this need, this study focuses on using the tests that would subject the specimens to the required deformation modes. 3. The proposed coating durability evaluation method Based on the concept of the FLD, the present study evaluates coating performance in coated sheet metal under plastic deformation by means of the notch coating adhesion (NCA) test, accelerated conditioning, the crosshatch tape test (ASTM D3359), uniaxial tensile test, and rectangular stretch bend test. Most coatings are susceptible to moisture ingression, but the time required for adhesive bond degradation and moisture equilibrium for a given coating may be few or several years depending on the type of coating, its use, and its exposure to the environment [9]. Thus, the use of notched, conditioned specimens becomes an excellent means for accelerated testing of adhesion. The notch serves as the initial debonding site that could lead to crack propagation and further damage as the material is strained. Instead of the single notch used by Chang et al. and Dillard et al., cross-hatched grids similar to (ASTM D3359) tape test are created in the present test specimens. The grids serve two purposes. First, they accelerate the conditioning of the specimen in different locations by shortening the diffusion path and reducing the time required to saturate the adhesive bond and the interface region. Secondly, similar to the etched or printed circular grids in LDH test, the notched grids can be used for strain measurement. The cross-hatched specimens are placed in a humidity cabinet for the conditioning that will cause coating adhesion to deteriorate and lead to debonding during deformation process. The specimens are subjected to different durations of conditioning. Tape tests are conducted after each conditioning to inspect coating adhesion. To investigate the effects of substrate strain on coating, the specimens are subjected to deformation using the uniaxial tensile test and rectangular stretch bend test. In both tests, the applied displacement determines the strain states obtained from the specimens. As the displacement can be controlled in a universal tensile tester as well as the hydraulic press, various strain states can be created during the tensile tests and stretch bend tests. Post-forming
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Experiments were conducted on two different types of coil-coated sheet metals supplied by a roll coating company. Let us designate the sheet metal materials as A and B. The composition of each is as shown in Table 1. As one can see, both materials contain the same type of coating but the primer and substrate are different.
The first cut was made along the length of the gauge section. The second cut was made at 901 to the first cut along the width of the gauge section creating a cross-hatch grid pattern with 210 squares of 1.5 1.5 mm. Care should be taken to ensure that the cut is made in one steady motion using just sufficient pressure on the blade to have the cutting edge reach the substrate. For the rectangular stretch bend test, the specimen was prepared by cutting the coil-coated sheet metal into a rectangular shape of 180 100 mm. Cross-hatch grids were created on the top surface of the specimen according to ASTM D3359-97. Following the procedure mentioned in the previous paragraph, a grid with 1600 squares of 1.5 1.5 mm was created. The location and number of cuts in the specimen are as shown in Fig. 3. For evaluation of coating under the environmental factors such as moisture and salt, the specimens were subjected to conditioning tests according to modified ASTM D870-97 [32]. Each specimen was submerged in a corrosion resistant closed container filled with a solution of water and salt at ambient temperature. The concentration of salt used was 6 g/l. Batches of specimen were tested for different durations of conditioning. The specimens were conditioned for a period of 1, 4, 7 and 14 days.
4.2. Specimen preparation
4.3. Deformation process
For the uniaxial tensile test, the standard dog bone shape specimen was prepared from coil-coated sheet metal according to ASTM A370-97a [30]. The dimensions of the specimen are as shown in Fig. 2. Cross-hatch grids were created according to ASTM D3359-97 [31] in the gauge section of the specimen. A blade with multiple cutting edges spaced 1.5 mm apart was used to create the grids.
4.3.1. Uniaxial tensile test In this test, the tension test specimens were tested on a universal testing machine according to ASTM A370 and ASTM E8. An electromechanical, testing system with a load frame capacity of 10 kN supplied by United Testing Systems Inc. was used. The system was equipped with a computer and software to monitor the extension, applied
evaluation of the specimens consists of inspection of coating adhesion by means of the tape test and by strain measurement. For a measured strain state, conclusions about coating performance can be made based on various criteria. Once the coating failures are identified, results of the evaluation are plotted according to the major and minor strains to depict the coating durability. The constructed durability limit diagram can then be used in conjunction with the FLD to determine the feasibility of a complex forming operation. Comparative results are presented in terms of percentage of the coating area that is debonded for different applied strain, and different durations of conditioning. 4. Experimental procedure 4.1. Materials
Table 1 Material data Sheet thickness (mm)
Material A Material B
0.584 0.572
Coating
Primer
Substrate material
Material
Thickness (mm)
Material
Thickness (mm)
PVDF PVDF
0.0180 0.0180
Polyester Polyurethane
0.0051 0.0046
HDG, hot dip galvanized; CQ, commercial quality. Galvalumes is an aluminum–zinc alloy metallic coating.
Fig. 2. Uniaxial tensile test specimen (all dimensions in mm).
Galvalume (Grade 50) HDG (CQ)
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Fig. 3. Rectangular stretch bend test specimen (all dimensions in mm).
load, and cross-head displacement. The experimental procedure is summarized as follows: 1. The tension test specimen is clamped from the grip section in the upper and lower jaws of the tensile testing machine. 2. An extensometer is attached to gauge section of the specimen to monitor strain. 3. The cross-head is displaced at the rate of 2.54 mm/min. 4. The movement of the cross-head is stopped when the extensometer indicates yielding in the gauge section. 5. The jaws are unclamped, and the specimen is removed from the machine. In order to evaluate the effect of higher strain states on coating adhesion, several specimens were tested beyond the yield point by increasing the displacement loading up to specimen failure or necking point. 4.3.2. Rectangular stretch bend test In this test, rectangular specimens were tested on a LDH testing machine supplied by MTS Systems Corporation. The system consists of a hydraulic power supply with pressures up to 35 MPa. It was equipped with a computer, display panel and software to monitor the clamping force, punch displacement and reaction force exerted on the punch. The experimental procedure is summarized as follows: 1. In this test, the specimen is clamped with a force of 700 kN by a circular clamp. 2. Displacement loading is applied on the specimen from bottom with a rectangular punch. 3. When the punch is moved upwards, a reaction force is exerted on the punch by the specimen in order to prevent deformation. This force increases until the material yields. It is measured by a sensor and displayed on the display panel.
4. The upward movement of the punch is stopped when a significant drop in the reaction force is observed. The drop in the reaction force signifies a failure in the specimen i.e. the substrate. 5. The punch is brought down at its original position, the clamp pressure is released, and the specimen is removed from the machine. 4.4. Tape pull adhesion measurement In this test, as per ASTM D3359, a pressure-sensitive adhesive tape was applied over the grids on the top of the specimen. Within 2 min of application, the tape was peeled away rapidly from the specimen by seizing the free end and pulling at an angle close to 1801. The grids were inspected for removal of coating. Coating adhesion was qualitatively determined from the severity of the pickup. The number of damaged elements in each sample was reported after the tape test. The tape pull test was conducted at three stages in the entire experimental procedure: 1. After cross-hatching the specimen, tape test was conducted to determine the possibilities of coating pickup due to initial debond. 2. After the accelerated conditioning, tape test was conducted to determine the loss of coating adhesion due to salt and moisture penetration. 3. After the application of strain i.e. after the uniaxial tensile test and rectangular stretch bend test, tape test was conducted to evaluate coating durability under plastic deformation of substrate. 4.5. Strain measurement The strain in the deformed specimen was measured using the Automated Strain Analysis and Measurement
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Environment (ASAME) Target Model (TRM), a digital camera-based system that measures surface geometry and strain distribution over an area of a deformed part that has been grided with a pattern of squares. The measurement procedure is as follows: 1. Two photographs of the measurement area (region with grids) with the target cube (a cube for reference) in the view are taken from different positions. 2. The photos are then processed to correct any defects in the grids. Defects can occur due to grid application error, excessive peeling during forming or uneven lighting conditions. 3. The digital photographs are then used to determine the three-dimensional coordinates of the intersections on a square grid by the ASAME software. 4. The measured three-dimensional coordinates, along with the size of the original undeformed square, are used to determine the surface strains. A measurement accuracy of 2.5% strain is obtained with this system. 5. The results are plotted in the form of a durability limit diagram showing the values of effective, major and minor strains at which debonding is observed in each specimen.
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to detect the type and amount of elements present in the debonded spots. A JEOL JSM-6400 scanning electron microscope and a Princeton Gamma-Tech unit equipped with a prism digital spectrometer and Spirit program (version 1.07) were used. The EDS was performed at 15 KV accelerating voltage for 60 s with working distance of 15 mm and a take off angle of 57.21. The detector material was Si (Li). The analyzed thickness was approximately 1 mm. The debonded spots were examined at different magnifications and the images were recorded photographically. 5. Results and discussions Based on the proposed methodology, tensile and stretch bend specimens were prepared from both materials A and B. Qualitative observation of adhesion test was made for specimens subjected to different durations of conditioning and different levels of straining. Samples (three specimens each) were taken from each conditioning variable (0, 1, 7, and 14 days) to count the number of debonded grids and to construct the durability limit diagram. The results and discussions are presented as the followings.
4.6. Optical microscopy and image processing Olympus 3M microscope was used to analyze small and large debonded spots on the specimen using an optical magnification ranging from 50 to 500 . The main purpose of conducting optical microscopy was to understand the physical nature of debonded spots for the two different types of materials used in this study. 2D and 3D images were produced using ImageJ [33], a java-based image analysis program, to obtain a clear picture of debonded locations. Multiple images of each debonded spot were taken along the optical axis. Using the ‘‘Extended Depth of Field’’ plugin available with ImageJ, infocus images were created by merging the stack of photographs taken at different focal lengths into a single totally focused composite image. 4.7. Energy-dispersive X-ray spectroscopy (EDS) The debonded surfaces on the specimen were characterized using EDS. The purpose of conducting the EDS was
5.1. PVDF coating, polyester primer on Galvalume steel substrate 5.1.1. Effect of conditioning and rectangular stretch bend test No damage was observed on the coating for a conditioning duration of up to 4 days. But when the samples were subjected to conditioning for 7 days some of the square grid elements showed scattered, swelled spots adjacent to the grids. Fig. 4 shows a swelled spot of about 0.5 mm in diameter. This was an indication that the adhesive bond within the primer and the coating had deteriorated due to penetration of salt and water. But the spots did not peel away when subjected to tape tests. Although the adhesive bond had deteriorated, it was strong enough to overcome the peel force and keep the coating attached to the substrate. When tape tests were conducted on the specimens subjected to straining, the swelled spots peeled off. Fig. 5 shows a peeled spot of approximately 0.5 mm in diameter. That is, strain
Fig. 4. Swelled spot from sample with polyester primer: (a) 2D image and (b) 3D image.
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caused complete failure of the swelled and partially debonded spots. It was observed that peeled spots contained polymer residue adhered to the metal substrate. In order to confirm the presence of polymer on the peeled surface, EDS was performed on some spots from few specimens. As shown in
Fig. 6, it was found that the major composition (more than 90% by weight) of the spots was carbon, which suggested the presence of polymer. Other significant elements found were aluminum and zinc, which showed the presence of material from the galvanized coating layer. Since the spots were affected by conditioning, chlorine (an element of salt)
Fig. 5. Debonded spot from sample with polyester primer: (a) 2D image and (b) 3D image.
Fig. 6. EDS of peeled spot from sample with polyester primer.
Fig. 7. Durability diagram for samples with polyester primer subjected to rectangular stretch bend test.
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was also detected in the spots. Other elements such as boron, chromium and molybdenum were also found in trace amounts in the spots. The major and minor strain values of the grids collected from the samples are shown on the durability diagram in Fig. 7. The color contour shows the effective strain at different locations in the sample. The damaged grids are selected by white dots at its corners in each specimen. The triangular marks on the graph represent the major and minor strain values of the damaged grids. The maximum effective strain in the specimens for rectangular stretch bend test was around 35% at substrate failure. One can see that most of the peeling occurred in grids with low values of major (between 5% and 20%) and minor (between 2% and 6%) strains. This was because moisture penetrated at these spots and weakened the adhesive bond. When the spots were subjected to straining, the adhesive bond failed leading to debonding of the coating. Many grids subjected to high strain states were found to be undamaged. This suggested that the coating and primer
Table 2 Number of damaged grids for samples with polyester primer subjected to rectangular stretch bend test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 17 14 24
10–20
20–30
30 and 4
0 0 15 21
0 0 3 5
0 0 0 0
487
in those grids were not affected by strain. Thus it can be concluded that this coating has the quality of sustaining high strain states without being damaged if it is not subjected to severe conditioning. When the samples were subjected to conditioning for 14 days, the size of the scattered, swelled spots increased to 1 mm in diameter. The number of spots remained approximately the same as for the conditioning of 7 days. Some of the spots debonded and peeled off when subjected to peel tests. This indicated that a period of 14 days of conditioning caused the adhesive bond to deteriorate completely, leading to coating failure in certain grids prior to application of strain. When the specimens were subjected to strain, all the swelled spots peeled from the substrate. This showed that the application of strain caused complete failure of all the swelled and partially debonded spots. The results for samples subjected to rectangular stretch bend test in terms of applied strain and duration of conditioning were summarized in the form of number of damaged grids. From Table 2 it can be concluded that the coating performed well at all values of effective strain when subjected to 0 and 1 days of conditioning. Damaged grids were found in samples subjected to 0–10% and 10–20% effective strain and a conditioning duration of 7 days. Most of the grids at 20–30% effective strain and above were able to survive the substrate deformation. This was because these grids showed good resistance to conditioning. 5.1.2. Effect of conditioning and uniaxial tensile test Similar results were obtained when the samples were subjected to uniaxial tensile test. The debonding was more severe in tensile samples. The numbers of failed grids were higher per specimen as well as for the overall results when
Fig. 8. Durability diagram for samples with polyester primer subjected to uniaxial tensile test.
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compared with samples subjected to rectangular stretch bend tests. Also higher values of major and minor strains were achieved before substrate failure occurred in the tensile tests. The durability diagram for the tensile experiment is shown in Fig. 8. The maximum effective strain in the samples for tensile test was around 40% at substrate failure. Table 3 shows the number of damaged grids. At 0–10% and 10–20% applied strain, almost double the numbers of grids were damaged in the tensile samples compared to samples in rectangular stretch bend test. For 20–30% strain and above, the number are same for both the tests. Also it is clear from the results that the longer the duration of conditioning the higher the number of damaged grids. Thus, for the PVDF-coated samples with polyester primer, it can be concluded that although the applied strain caused complete failure of the adhesive bond, the major damage was caused by moisture and salt penetration. This coating showed good performance at high values of strain but was susceptible to severe damage when subjected to conditioning. So one should take precautions before using this coated sample in humid and corrosive environment.
5.2. PVDF coating, polyurethane primer on hot dip galvanized steel substrate 5.2.1. Effect of conditioning and rectangular stretch bend test The samples were subjected to a conditioning duration from 1 to 14 days. No damage was observed on the coating Table 3 Number of damaged grids for samples with polyester primer subjected to uniaxial tensile test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 32 14 48
10–20
20–30
30 and 4
0 0 21 27
0 0 1 1
0 0 0 0
after any of these conditioning durations. The samples did show some moisture penetration through the grids but there was no swelling or debonding of the coating on the grids. Fig. 9(a) shows grids with moisture penetration after 7 days of conditioning duration. This was an indication that the grids were subjected to penetration of salt and water, even though the coating did not peel away when subjected to tape tests. Although the grids were affected, the adhesive bond within the coating and primer was strong enough to overcome the peel force and keep the coating attached to the substrate. When tape tests were conducted on the samples subjected to straining, the affected spots on the grids debonded. Fig. 9(b) shows debonded grids after the application of strain. On comparing Fig. 9(a) and (b), one can see that only the locations that show moisture penetration within the grids debonded after the application of strain. Strain-induced damage to the adhesive bond in the grids that were affected by salt and moisture penetration and caused debonding. Other grid locations on the sample did not show any signs of damage. The type damage observed in coating with polyurethane primer was different compared to the coating with polyester primer. There was debonding of the coating but no peeling was observed during the tape test. Similar to the samples with polyester primer, the debonded spots in this sample also contained polymer residue adhered to the metal substrate. Fig. 10 shows a debonded spot with polymer residue and exposed substrate. In order to confirm the presence of polymer on the debonded surface, EDS was performed on few spots. As shown in Fig. 11, it was found that the major composition (more than 89% by weight) of the spots was carbon which suggested the presence of polymer. Other significant elements found were boron, oxygen and iron. Aluminum and zinc were also found in trace amounts which showed the presence of galvanized coating layer. Chlorine (an element of salt) was also detected in the spots. The values of major and minor strains of the grids are shown on the durability diagram in Fig. 12. The triangular spots on the graph represent the major and minor strain values of the damaged grids. The maximum effective strain
Fig. 9. Sample with polyurethane primer: (a) moisture penetration in grids due to conditioning and (b) debonding in grids due to strain.
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Fig. 10. Debonded spot from sample with polyurethane primer: (a) 2D image and (b) 3D image.
Fig. 11. EDS of debonded spot from sample with polyurethane primer.
Fig. 12. Durability diagram for samples with polyurethane primer subjected to rectangular stretch bend test.
in the specimens with polyurethane primer subjected to rectangular stretch bend test was around 35% at substrate failure. One can see that most of the peeling occurred in grids with high values of major (between 15% and 30%) and minor (between 6% and 10%) strains. During conditioning, moisture penetrated at several locations in
the grids but debonding occurred only in grids that were subjected to high strain states. Many grids subjected to low strain states were found to be undamaged. This suggested that the adhesive bond within the coating and primer in those grids was not affected by strain or conditioning. Thus, it was noted that
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this coating performed well against conditioning and low strain states but was not able to sustain high strain states. When the samples were subjected to strain after a conditioning duration of 14 days, the number of damaged grids was slightly higher compared with the number of damaged grids after conditioning of 7 days. This indicated that a period of 14 days of conditioning resulted in deterioration of the adhesive bond in a higher number of grids, leading to failure after the application of strain. The results for samples subjected to rectangular stretch bend test in terms of applied strain and duration of conditioning were summarized in the form of number of damaged grids. From Table 4, it can be concluded that the coating performed well at all values of strain when subjected to 0 and 1 days of conditioning. But for a conditioning duration of 7 and 14 days, some damaged grids were noted in the samples. Most of the grids at 0–10% effective strain were able to survive the deformation of the substrate. This was because these grids showed good resistance to conditioning as well as straining. But many
Table 4 Number of damaged grids for samples with polyurethane primer subjected to rectangular stretch bend test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 0 14 4
10–20
20–30
30 and 4
0 0 20 29
0 0 46 41
0 0 1 6
girds subjected to 10–20% and 20–30% effective strain were damaged. Thus this coating was susceptible to damage at high strain states. 5.2.2. Effect of conditioning and uniaxial tensile test Different results were obtained when the samples were subjected to uniaxial tensile test. Girds were found to be damaged at low as well as high strain states. The debonding was more severe in tensile samples. The numbers of failed grids were higher per specimen as well as for the overall results when compared with samples subjected to rectangular stretch bend tests. The durability diagram for the tensile samples is shown in Fig. 13. The maximum effective strain in the samples for tensile test was around 50% at substrate failure. As one can see, the damaged grids are present all the way from 5% to 50% effective strain. Table 5 shows that at 0–10% and 10–20% effective strain almost double the numbers of grids were damaged in the tensile samples compared to samples in rectangular stretch bend test. For 20–30% strain and above, the number is slightly lower for the tensile tests. Many grids were found to be damaged at 30% and higher values of effective strain. Thus low as well as high strain states were detrimental for the coating survival in this (tensile) deformation mode. Thus for the PVDF-coated samples with polyurethane primer, it can be concluded that the applied strain caused complete failure of the adhesive bond. The coating showed resistance to damage due to conditioning but it was unable to withstand high substrate deformation. So one should take precautions before using this coated material in conditions where high strain states are involved.
Fig. 13. Durability diagram for samples with polyurethane primer subjected to uniaxial tensile test.
ARTICLE IN PRESS R. Vayeda, J. Wang / International Journal of Adhesion & Adhesives 27 (2007) 480–492 Table 5 Number of damaged grids for samples with polyurethane primer subjected to uniaxial tensile test Applied true effective strain (%) 0–10 Conditioning (days) 0 0 1 0 7 13 14 41
10–20
20–30
30 and 4
0 0 45 51
0 0 20 28
0 0 36 16
491
coated sheet metals for suitable applications. It is also believed that additional work is needed to develop analytical or numerical models to investigate the stress states at the polymer–metal interface under different deformation conditions. Acknowledgments
6. Conclusions The commonly used tests for adhesion of coatings are not effective in evaluating the coating performance in the presence of plastic deformation of the substrate. On the other hand, the formability test used in the sheet metal industry does not evaluate coating durability. In this paper, a testing methodology combines the key attributes of various experimental techniques is presented to effectively address the concern. Based on the proposed methodology, experimental investigations showed that coating adhesion can be affected by plastic deformation. Two coated sheet metal with different primer and metallic coating were tested for durability. In samples with polyester primer, a number of scattered, swelled spots were observed on the coating adjacent to the grids after conditioning. Some of the spots peeled off prior to application of strain. The durability diagram showed that debonding occurred in grids that subjected to low values of major and minor strain whereas many grids with high strain states were still found to be unaffected. It was concluded that moisture penetrated at certain spots through the grids and weakened the adhesive bond. When these spots were subjected to straining, the adhesive bond failed, leading to coating failure. Thus, the coating with polyester primer showed better performance under plastic deformation of substrate but was susceptible to damage by humid and corrosive environment. In samples with polyurethane primer, no significant damage patterns were observed in the coating after conditioning. But when subjected to straining, the coating showed debonded spots adjacent to the grids. The durability diagram showed that debonding occurred in grids subjected to high values of major and minor strain, whereas the areas with low strain states were found intact. The higher substrate strain caused the adhesive bond to deteriorate, leading to coating failure. Thus, the coating with polyurethane primer showed better resistance to humid and corrosive environment but was susceptible to damage under plastic deformation of the substrate. The assessment methodology presented in this paper can be used to investigate coating adhesion under conditions of substrate deformation. The experimental results suggested that the proposed testing method is effective in evaluating
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