Sep 1, 2009 - on Hard and Soft Substrates ... Springer Science+Business Media, LLC 2009 ... study the scratch behavior of polymeric coatings on soft and.
Tribol Lett (2010) 37:159–167 DOI 10.1007/s11249-009-9505-8
ORIGINAL PAPER
Mechanical Modeling of Scratch Behavior of Polymeric Coatings on Hard and Soft Substrates Han Jiang Æ R. Browning Æ J. D. Whitcomb Æ M. Ito Æ M. Shimouse Æ T. A. Chang Æ H.-J. Sue
Received: 22 December 2008 / Accepted: 23 August 2009 / Published online: 1 September 2009 Ó Springer Science+Business Media, LLC 2009
Abstract An ASTM standard scratch test is utilized to study the scratch behavior of polymeric coatings on soft and hard substrates. Depending on the different combination of polymeric coatings and substrates utilized, various damage modes can take place, which include coating delamination, transverse cracking, and buckling failure. A soft coating on a hard substrate will give rise to an entirely different scratch damage pattern from those of a hard coating on a soft substrate. The stress and strain responses of scratch on polymeric coating are analyzed using three-dimensional finite element (FE) simulation. The analysis provides mechanistic insights for the observed polymer coating deformation mechanisms and failure modes. Usefulness of the ASTM scratch method and FE modeling to evaluate polymer coating scratch behavior is discussed. Keywords Polymers Coating Scratch Finite element modeling
H. Jiang R. Browning H.-J. Sue (&) Polymer Technology Center, Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA e-mail: hjsue@mac.com J. D. Whitcomb Department of Aerospace Engineering, Texas A&M University, College Station, TX 77843-3123, USA M. Ito M. Shimouse Japan Polypropylene Corp., Products Technical Center One, Yokkaichi, Mie 510-0848, Japan T. A. Chang PolyLab LLC, Houston, TX 77042, USA
1 Introduction Polymeric coating systems are currently used to improve the product surface performance and enhance surface esthetics. The rising concern over the scratch performance of polymer coating has come about with its extensive usage in the electronic, optics, and automotive industries. The scratch phenomenon involves a tribological contact between two surfaces sliding against each other under load, and poses significant challenges to the material and mechanics research community. The material properties discontinuity between coating and substrate leads to scratch damage mechanisms totally different from the bulk material. Various coating scratch damage modes based on metallic and ceramic coatings have been observed to include coating debonding, through-thickness cracking and plastic deformation or cracking in the coating or substrate [1–3]. Due to the complex material behavior and damage mechanisms of polymeric materials, it is much more difficult to understand the scratch behaviors and damage modes when polymer coatings are involved. The large scale deformation and time-dependent properties of polymers, such as viscoelasticity and viscoplasticity, will significantly complicate the analysis. Consequently, no analytical solutions are available for modeling the polymer scratch behavior. To date, the design and development of polymer coatings have been a matter of trial and error without a clear understanding of the scratch phenomena. Although analytical equations are available both in bulk material and bilayer systems for purely elastic contact [3, 4], the stress field around the scratch tip is generally too complicated to be analytically described for polymeric materials due to its nonlinear material characteristics. Especially in the case of polymeric bilayer systems, both coating and substrate could be extremely sensitive to strain,
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strain rate, and temperature, in addition to the classical mechanics complexities. Furthermore, it is difficult to experimentally determine the coating performance and relate it to material parameters, and because of these limitations, much work is still needed. Thus far, numerous research efforts have been carried out to study fundamental scratch behaviors of materials. The relationship between scratch behavior and various polymers’ material properties has been discussed [5, 6]. 2-D elastic and plastic material behaviors have been employed to study the coating scratch behaviors of various material systems [7–11]. Finite element (FE) modeling has been carried out to study the scratch behavior of bulk polymers [11–15]. Molecular dynamics simulation has been conducted to understand polymer tribological behavior [16, 17]. The effect of scratch testing rate on scratch performance has been studied [18, 19]. FEM modeling has been adopted to study the material and surface parameters effect on polymer scratch [14, 15]. Surface roughness and friction coefficient have been shown to impose significant effect on polymer scratch behavior [14, 15, 20–22]. The above findings provide important insights in aiding fundamental understanding of the observed polymer scratch phenomena. However, research on polymer coating surface scratch behavior is still largely lacking [23–27]. The recently established ASTM and ISO standards [28, 29] for polymer scratch, which have been shown to be effective for the study of bulk polymer scratch [30], has been demonstrated to exhibit good potential as a tool for investigating scratch behavior of polymeric coatings [27]. The same methodology will be chosen for this study. For bilayer systems, it is well known that a given coating can exhibit different mechanical behaviors during indentation as a function of the underlying substrate. Using the combination of the ASTM/ISO scratch test and threedimensional (3-D) FE modeling, this project studied how the substrate can affect the scratch behavior of polymeric coatings. Scratch tests are performed on (1) an acrylic coating on a steel substrate and (2) a polyurethane coating on a polypropylene substrate. Three-dimensional FE modeling is employed to analyze the stress and strain responses of the polymer coatings during the scratch. The failure mechanisms observed experimentally are well correlated with the numerical modeling results.
2 Method and Procedure 2.1 Model Coating Systems To study the scratch behavior of polymer coatings, two model coating systems have been investigated. One is a soft coating on a hard substrate, i.e., an acrylic coating on
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steel substrate with Ec/Es = 0.015 and ryc/rys = 0.33. Here, Ec, Es, ryc, and rys are Young’s modulus and yield stress of coating and substrate materials, respectively. The thickness of the acrylic coating layer is approximately 60 lm and the thickness of the steel substrate is 813 lm. The details of the sample preparation can be found elsewhere [27]. The second model coating system is a hard coating on a soft substrate, i.e., a hard polyurethane coating on a polypropylene substrate with Ec/Es = 1.52 and ryc/ rys = 2.08. The thickness of the polyurethane coating and the polypropylene substrate are approximately 70 lm and 3 mm, respectively. The samples were provided by Japan Polypropylene Corp., Yokkaichi, Japan. 2.2 Scratch Testing and Analysis The ASTM standard (D7027-05) for polymer scratch testing has been shown to be effective for bulk polymers. Although the scratch damage features for polymer coating systems are significantly different from the bulk, the standardized scratch test is still expected to be useful for the study of coating scratch behavior. Scratch tests were performed at room temperature using a custom-built scratch machine with a 1 mm diameter stainless steel spherical tip [27]. The scratch length was set at 150 mm. A linearly increasing normal load from 1 to 50 N was imposed on the scratch tip. The scratch velocity of 100 mm/s was adopted in this study, as recommended by the ASTM and ISO (19252:2008) test standards. After the scratch tests were performed, the scratched samples were scanned using Epson 4870 Perfection Photo flatbed PC scanner at 3200 dpi resolution in grayscale mode to promptly identify the various damage modes. The more detailed damage mechanisms were investigated with optical microscopy (OM, Olympus BX60) and scanning electron microscopy (SEM, JEOL JSM-6400). 2.3 Numerical Modeling Numerical modeling can provide insight into the mechanics that correspond to the experimentally observed phenomena. 3-D FE modeling is used here to understand the polymer coating scratch and to explore how the soft and hard substrates influence the scratch damage mechanisms. ABAQUS/ExplicitÒ 6.4.5 was employed to simulate the scratch process of a polymeric coating [31]. The 3-D FE models with substrate dimensions of 50 9 10 9 2 mm and coating thickness of 60 and 70 lm were used for acrylic coating and polyurethane coating, respectively, (Fig. 1). The spherical stainless steel tip is assumed to be a rigid surface since it is much stronger and more rigid than the investigated polymer coatings. The mesh is assumed to be symmetrical about the 2-axis (Fig. 1). The typical size of a
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True Stress (MPa)
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40 20 0 0
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Fig. 1 Finite element analysis model
3-D brick element along the scratch path is 0.05 9 0.05 9 0.05 mm and was chosen to give converging simulation results and assure numerical accuracy. Eight-node tri-linear elements with three nodal displacement degrees of freedom and reduced integration (refereed as C3D8R in ABAQUS) were used to model the coating and substrate layers. To avoid excessive distortion of elements during the scratch process, the adaptive remeshing methodology provided in ABAQUS was carried out to preserve mesh quality throughout the analysis [31]. To simulate the progressively increasing normal load scratch condition as suggested in the ASTM standard, the normal load applied on the indenter was linearly increased from 1 to 50 N during the scratch process. Perfect bonding between the coating layer and substrate was assumed before the debonding happens. The friction coefficient between the indenter and coating was set to be 0.25, which was experimentally obtained by a sliding test [22]. The elastic-pure-plastic material type was adopted for the steel substrate, which is adequate since no yielding is expected. The acrylic coating, polyurethane coating and polypropylene substrate were described by piecewise linear elastic–plastic stress–strain curves similar to the one shown in Fig. 2 [32]. The von-Mises yield criterion and isotropic hardening were adopted. The key material properties, which were provided by material manufacturers, are shown in Table 1.
3 Results and Discussion 3.1 Acrylic Coating on Steel Substrate 3.1.1 Experimental Observation The scratch behavior for acrylic coating on steel substrate was experimentally investigated by Browning et al. [27]. A typical scratched surface is shown in Fig. 3. The
Fig. 2 Typical piece-wise linear stress–strain curve used for FEM modeling [32] Table 1 Material Properties for FEM modeling Hard substrate
Soft substrate
Acrylic
Polyurethane
Steel
Polypropylene
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1.65
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0.35
0.4
ry (MPa)
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50
q (g/cm3)
1.25
7.8
1.2
0.90
discontinuity of material properties at the interface of coating system leads to scratch damage modes different from the bulk material. Three damage features, i.e., delamination, transverse cracking, and buckling-induced damage can be identified. In zone 1 (Fig. 3a), the coating layer begins to delaminate. In zone 2 (Fig. 3b), the coating layer cracks under the center of the indenter tip and is propagated outward at an angle. As the normal load increases, the buckling-induced delamination takes over as the main damage mechanism (zone 3). Subsequently, the coating layer is removed and the substrate is exposed directly to the contact with the indenter (Fig. 3c). The apparent frictional force versus linearly increasing normal load for the acrylic–steel coating system is shown in Fig. 4. The fluctuation of friction force at an early stage is due to the inertia effect when the scratch tip experiences sudden speed change from 0 to 100 mm/s. The apparent frictional force comes from two parts: the traditional surface contact friction, which mainly depends on the selected material pair and their surface characteristics, and additional frictional force resulting from material resistance [22]. Material resistance is induced by various scratch deformation mechanisms that occurred during the scratch process, such as delamination, coating cracking, and buckling. The latter part of frictional force can be considered negligible at low load level, but will become more dominant as scratch deformation becomes more severe. It
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Fig. 3 Typical scratch damage modes of the acrylic–steel coating system [27]: a three damage zones (zone 1 delamination, zone 2 transverse cracking, zone 3 buckling damage), b onset of transverse cracking, and c buckling damage
forces. Therefore, the apparent frictional force curves can be also utilized to identify the scratch damage mechanism transitions. It is important to note that the occurrence sequence of the transverse crack and adhesive delamination depends not only on coating ductility and adhesive strength, but also on the ratios of Ec/Es, ryc/rys, and the coating thickness.
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The variation of the von-Mises stress field for the acrylic– steel coating system is shown in Fig. 5. The spherical tip is
Scratch Distance (mm)
Fig. 4 Apparent friction force versus applied normal load for the acrylic–steel and the polyurethane–polypropylene coating systems
is observed that the increasing rate of apparent frictional force is faster than that of the normal load, which indicates the important of material resistance during scratch. The slope change of the apparent frictional force can also be observed at the normal load level about 20 and 30 N, which are related to the occurrences of transverse cracking and buckling-induced damage, respectively. These additional damage mechanisms introduce additional material resistance forces, and increase the apparent frictional
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Fig. 5 The von-Mises stress field for the acrylic–steel coating system
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removed to clearly display the stress field. The initial position of interface between the coating layer and substrate is illustrated by the dashed line, which does not change since there is no significant deformation of the steel substrate. The coating is plastically deformed; the thinning of coating and side groove pile-up become significant with the increase of the applied normal load level. It is apparent that the size of the plastic deformation zone increases with the increased normal load while the severe plastic deformation of coating layer concentrates in the area under the tip and around it. The substrate deformation is negligible since steel is much stronger than the coating layer. The maximum principal stress (r1) is used as the main failure indicator in this study. The stress fields at different scratch locations, i.e., normal load levels of 13, 32 and 45 N, are extracted from the FEM data and plotted in Figs. 6, 7 and 8, respectively. For clear illustration, only the direction of the corresponding peak maximum principal stresses is shown. Under the normal load of 13 N, the peak value of r1 is located behind the scratch tip and is away from the middle of the scratch path (area A in Fig. 6a). The direction of the r1 is almost perpendicular to the interface plane (Fig. 6b). If it is higher than the interfacial adhesive strength, the coating will be peeled off to cause delamination. The directionality of r1 also shows that the debonding induced by scratch is mixed mode damage other than a simple Mode I or Mode II failure.
Under the normal load of 32 N, the second peak value of r1 develops behind the tip (area B in Fig. 7a). It is closer to the middle of the scratch path. Accordingly, possible damage is expected to occur near the scratch path. As illustrated in Fig. 7b, through the thickness of the coating layer right behind the indenter, r1 tilts at an out-of-plane angle of about 88 and at a transverse angle to the direction of scratch. This could promote inlayer failure via transverse cracks which will propagate outward once this scratch occurs. At the normal load of 45 N, a complex stress state exists in the region close to the scratch tip where bucklinginduced damage of the coating occurs. There are two peak values of r1, areas A and B, at which r1 exhibits a small out-of-plane angle tilt. Two possible damage initiation locations are expected (Fig. 8a). Area A is at the rear side of scratch path and area B is in front of the scratch tip. The r1 also has a secondary peak (area C). The coating material at points A and B will crack and the damage will develop from point A and will propagate until arriving at area C, which shows an arc shape. Once transverse cracks and delamination occur concurrently and are fully developed, the buckling-induced delamination is most likely to become the main failure mode. With a stronger interfacial adhesion, the transverse cracking in the coating layer could occur before debonding takes place. Furthermore, delamination and transverse cracking will occur before the formation of buckling damage. If the interfacial adhesion is strong, delamination
Fig. 6 Maximum principal stress for the acrylic–steel coating system at 13 N: a the contour plot (top view) and b the direction of peak stress (side view)
Fig. 7 Maximum principal stress for the acrylic–steel coating system at 32 N: a the contour plot (top view) and b the direction of peak stress at area B (top view)
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Fig. 8 Maximum principal stresses for the acrylic–steel coating system at 45 N: a the contour plot (top view) and b the direction of peak stresses (top view)
is prevented, or if the coating layer is tough, transverse cracking is prevented, and buckling may not occur. 3.2 Rigid Polyurethane Coating on Polypropylene Substrate 3.2.1 Experimental Observation For the polyurethane–polypropylene system, i.e., a hard coating on a soft substrate, scratch damage modes totally different from the acrylic coating on steel have been observed (Fig. 9). Smooth indentation is observed early in the scratch process (Fig. 9b). Neither delamination nor buckling-induced damage is observed. Two types of cracks, i.e., hair-line radial cracking and severe cracking (Fig. 9c), are observed before the ultimate failure of the coating system. As the load is further increased, the scratch
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tip will be ploughed into the substrate and subject it to scratch deformation and damage (Fig. 9d). The apparent frictional force versus linearly increasing normal load for the acrylic–steel coating system is also shown in Fig. 4. It is observed that the increasing rate of apparent frictional force changes gradually with the normal load in the early scratching process where the tip smooth indentation is the dominant deformation. Then, the coating layer cracking introduces much more frictional force which speeds up the increase of apparent frictional force at the normal load level of 35 N. After that, the ultimate failure of the coating system, i.e., the scratch tip penetration of the coating layer, occurs to introduce large fluctuation. The apparent frictional force curve here also shows the correlation with the occurrence of various scratch damage modes. As illustrated in Fig. 10, coating thinning occurs under the scratch tip, which moves outward, while the side groove pile-up exists due to the extrusion of the coating materials. For a soft coating on a hard substrate (Fig. 10a), only the coating layer experiences significant deformation. With the increase of applied normal load, the adhesive interface between the coating and substrate will be severely strained and cause debonding. As shown in Fig. 10b, the absence of delamination here can be attributed to the combination of hard coating and soft substrate scenario. Here the underneath substrate significantly deforms and the stress can be easily dispersed underneath. Even under a high loading level, the strain magnitude of the interface between the coating and substrate is much smaller than those of a soft coating on a hard substrate. As a result, delamination of coating layer cannot easily occur. 3.2.2 Finite Element Simulation The von-Mises stress fields of the polyurethane–polypropylene coating system at various load levels are shown in Fig. 11. The dashed line marks the initial position of interface between the coating layer and substrate. Other than the acrylic–steel coating system, plastic deformation of the polypropylene substrate is quite significant.
Fig. 9 Typical scratch damage modes of the polyurethane-polypropylene coating system: a scanned image of scratched sample; b the smooth indentation (zone 1); c the cracking (zone 2); and d the tip penetration and scratch onto the substrate (zone 3)
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SoftCoating Soft Coating Hard Substrate Hard Substrate
(a)
HardCoating Hard Coating Soft SoftSubstrate Substrate
(b) Fig. 10 Illustration of the coating thinning and pile-up under scratch. a soft coating on a hard substrate and b hard coating on a soft substrate
Fig. 11 von-Mises stress field for the polyurethane–polypropylene coating system at the normal load of (side view); a 13 N, b 32 N, and c 45 N
At a low loading level, the residual scratch depth is dominated by the deformation of coating layer. One can actually find that the stress distribution of polyurethane coating at a low load level is similar to that of the scratch on bulk polymers [12–14] since the scratch tip cannot sense the substrate underneath the coating layer yet. At a higher load level, more contribution for scratch residual depth comes from the soft substrate plastic deformation since the load is transferred through the hard coating layer and dispersed into the substrate. Figure 12 shows the top view of the r1 field at different scratch load levels, i.e., 13, 32 and 45 N, respectively. The location of the peak r1 migrates with the increase of normal load. The peak area formed in front of the tip contact (area A in Fig. 12a and b) will induce hair-line radial cracks. The second peak area of r1, which is tilted at a small out of plane angle, gradually develops behind the scratch tip as the normal load increases (area B in Fig. 12b). At a high load level (45 N), the second peak area of r1 becomes dominant with a direction shown in Fig. 13. This
Fig. 12 Maximum principal stress field for the polyurethane–polypropylene coating system at the normal load of (top view): a 13 N, b 32 N, and c 45 N
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Fig. 13 Direction of the maximum principal stress of the polyurethane–polypropylene coating system at a normal load of 45 N (top view)
large tensile stress behind the indenter will promote severe inlayer failure through the thickness of the coating layer. Subsequently, the scratch tip will penetrate through the coating layer and scratch the underneath substrate directly. It is worthwhile to mention that the possible damage modes of the polyurethane–polypropylene system were first predicted by the modeling work and then validated by the scratch experiment. 3.3 Towards a Quantitative Method to Evaluate Polymer Coating Scratch Resistance Since the stress distribution and deformation obtained from numerical simulation are well correlated with the scratch experiment observation, it is possible to quantitively evaluate the polymer coating performance with the help of the scratch test and the FE simulation. For a linearly increasing normal load scratch test, once the onset of the specific scratch damage transition is
located, the critical load value of that damage mode can be determined experimentally [27]. While it is not possible to experimentally determine the local critical stress values, FE modeling can be utilized to indirectly estimate the corresponding stresses based on the specific experimentally observed damage modes and their critical load values. As an illustration, the calculated stresses for various damage modes of the acrylic–steel coating system are shown in Fig. 14. The stress values of the onset of scratch damage calculated via FE modeling appear to be reasonable. It is important to note that there are still many factors that should be considered for the above approach to become quantitatively accurate. These factors include: (1) refined constitutive equations and accurate material properties for the coating and substrate, (2) an appropriate algorithm to describe interfacial debonding, and (3) valid failure criteria for the coating and substrate. Additional experimental and modeling work is underway to address the above concerns.
4 Conclusion The linearly increasing normal load scratch test, based on ASTM and ISO standards, has been shown to be effective for studying polymer coating scratch behaviors. Two polymeric coating systems, i.e., acrylic–steel (soft coat on hard substrate) and polyurethane–polypropylene (hard coat on soft substrate), were studied. Significantly different scratch damage modes have been observed. With the aid of FE modeling, the scratch damage mechanisms are shown to be correlated with the material properties and the corresponding stress fields, which are related to the geometry of
Fig. 14 Critical load and strength of various damage modes for the acrylic–steel coating system
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the scratch tip and coating thickness. Combining the ASTM scratch test method and FE modeling, a quantitative evaluation methodology of polymer coating system is proposed. Acknowledgments The authors would like to thank the financial support and valuable insights provided by the Texas A&M Scratch Behavior Consortium members (Advanced Composites, Atlas-MTS, Braskem, Cabot, Ciba Specialty Chemical, Clorox, Dow Chemical, Japan Polypropylene, Kaneka, Rio Tinto, Phillips-Sumika, Solvay Engineered Polymers, Sumitomo Chemical, Surface Machine Systems, and Visteon) in this research endeavor. The authors would also like to acknowledge partial financial support of the Department of Transportation (DTPH56-06-T-000022).
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