Micromechanical Characterization of Scratch and Mar Behavior of

Chapter 27

Micromechanical Characterization of Scratch and Mar Behavior of Automotive Topcoats 1

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L.Lin ,G. S.Blackman ,and R. R. Matheson 1

Downloaded by COLUMBIA UNIV on June 15, 2013 | http://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch027

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DuPont Automotive, Marshall Research Laboratory, 3500 Grays Ferry Avenue, Philadelphia, PA 19146 DuPont CRD, Experimental Station E323/110B, Wilmington, DE 19880 DuPont Automotive, 950 Stephenson Highway, Troy, MI 48007 3

The automobile industry and its customers demand durable glossy appearance of automotive topcoats. A micro-scratch technique has been developed to quantitatively characterize scratch and mar behavior. With the micro-scratch instrument normal force, tangential force, penetration depth and permanent damage can be controlled or measured precisely as function of time or position during scratch experiments. These physical quantities allow us to evaluate scratch and mar resistance based on damage mechanisms. The micro-scratch instrument is stable enough to produce centimeters long scratches of various types and sizes so that the relationship between scratch morphology and appearance can be investigated. Results from statistical surveys on well-defined scratches suggest that a scratch with fracture is the most visible type of damage.

Automobile exteriors are coated with multi-layer polymeric coatings for protection and appearance. The electrical coat and primer layers provide corrosion protection and better adhesion to the steel substrate. The basecoat provides the color component. The topcoat protects the other layers from the environment and is responsible for the glossy appearance. The automobile industry and its customers have long demanded that these coatings retain their initial attractive appearance despite weathering and chemical and mechanical damage (J). Mar is one kind of physical damage that affects the appearance of topcoats. This type of damage occurs within a few micrometers of the surfaces of topcoats. The major contribution to mar is micro-scale scratches (2). Causes of mar include carwashing, in-plant finessing, keys and fingernails, building materials, branches, and blowing sand. There is general agreement that the major source of mar damage is a

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© 2000 American Chemical Society

In Microstructure and Microtribology of Polymer Surfaces; Tsukruk, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


429 car-washing since it is a normal periodic activity for many automobile owners (2). During a car-wash, dust and grit is rubbed against the topcoat surface resulting in many such micro-scale scratches. These micro-scale scratches have a dramatic impact on the appearance and cause a significant loss of gloss. It has been a problem for the automobile industry and paint suppliers to characterize the mar behavior. There is no well-established test to measure mar resistance. Existing methods used are the Taber test (3), Rub test (3,4), Scrub test (5), Falling abrasive test (6), and Bench-top car-wash test, all of which make simultaneous multi-point scratch damage during experiments. An Optical Imaging Method (3) was developed to work with the above tests to quantify the damage level. Contradictory conclusions often occur even when attempting a relative ranking between good and bad. Some of the problems of the multi-contact tests are poor reproducibility, poor discrimination, and most importantly no measurement of physical quantities. Several investigators have developed single point contact techniques (7-10) to study abrasive wear of polymeric materials. The macroscopic single point scratch tests make damages too severe compared to regular mar. Many of the other techniques have limited scratch length, penetration depth and poor long-range stability. Often important physical quantities are not measured adequately. We developed a technique that is sensitive enough to make scratches as small as a few nanometers yet has the long-range stability and large depth-range to make a single scratch as long as a few centimeters. Coupling this technique with Atomic Force Microscopy (AFM) and statistic surveys, the damage mechanism of mar and its effect on appearance of automotive topcoats was investigated. Mar resistance is characterized based on damage mechanisms. Experimental The principle of a scratch experiment is to move an indentor into a topcoat while the sample is translated perpendicular to the indentor motion. During the process, the force parallel to indentor motion (Normal force) and force parallel to sample motion (Tangential force), the displacements of indentor and sample are controlled or measured with high resolution and fast dynamic response. In addition, other controllable parameters are penetration rate, scratch rate, lubrication condition, cutting geometry and temperature. To implement the micro-scratch experiment described above, a unique instrument was developed at DuPont Marshall R&D Laboratory. As illustrated in the block diagram of Figure 1, the instrument mainly consists of a nano driving / probing system, tangential force / stages system, signal conditioning / motion controller system, data acquisition system, and microscope / video system. The driving / probing system provides vertical displacement and measures corresponding resistant normal forces or generates the force to drive the indentor into coatings and measures the corresponding penetration depth. The tangential force / stage system is used to move coating samples at a given constant rate, position the indentor tip, and measure tangential force during the scratch. The signal conditioning and motion controller system includes electronic elements for signal converting, servo-control, and signal amplifying to all mechanical components. The integrated optical microscope and video system is used to capture the real-time deformation and elastic recovery during



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PC, Data Acquisition, Software Signal Conditioning & Motion Controllers

Frame NanoDrive& Probe

'Video Microscope

Tangential Force Sensor & Staees Base

Figure 1. Block diagram of the micro-scratch apparatus.



431 the scratch and to characterize the permanent damage afterwards. In addition, the video system is very useful for indentor tip positioning and sample leveling with position stages and tilting stages, for setting-up the experiment. The design of the instrument with more details will be published elsewhere (//). For the micro-scratch experiments presented here, the instrument configured for a maximum of 100 milli-Newtons (mN) normal force with 5 micro-Newtons (μΝ) resolution, and 90 micrometers (pm) vertical displacement with 2 nanometer (nm) resolution. A variety of normal force and displacement ranges and system resolutions can be constructed depending on the application. The resolution for the sensor of tangential force measurement is 20 μΝ. Most commercial polymer topcoats have long-range height variation as much as a few microns for every millimeter distance. Since most scratch damage itself is not more than a few microns, displacement control is not adequate for a micro scratch experiment. Control of the normal force during the experiment is necessary. In the force control mode, the indentor automatically follows the coating surface to produce the necessary penetration from coating surface according to the applied force, thus correcting for any surface height variation. The instrument is capable of either force control mode or displacement control mode. However, all work described here is done in force control mode. A complete experiment consists of three separate "scratches", a pre-scratch to measure topography of the undamaged coating, a scratch to produce the damage, and a post-scratch to measure the permanent deformation of the coating. All pre-scratch and post-scratch measurements were done under an applied constant normal force 20 μΝ. Scratch experiments can be performed in many different patterns. Ramp scratches are used to precisely measure physical quantities corresponding to a given force and to probe transitions in damage mechanisms as will be discussed in a later section. Step scratches were used to check instrument reproducibility, and more importantly to make precise scratches of different types and sizes for studying the relationship to visual appearance. In a ramp scratch, the normal force starts at 20 μΝ constant, and after 5 seconds, increases at a rate of 20 μΝ/s to a predefined maximum. During the scratch the displacement, normal, and tangential loads are recorded at ~ 100 data point per second. The high data acquisition rate is important to capture abrupt or transient events. Other parameters of the experiments are 25μηι/8 scratch rate, 21°C temperature, ambient condition and 60° conical indentors with spherical tips. The radius of the tips will be specified as discussions of results. The AFM used in these experiments is a precision large stage M-5 instrument from Park Scientific Instruments. The stage motion is calibrated with NIST traceable standards and the absolute reproducibility is within a few microns. The micro-scratch instrument and the AFM are in perfect registry so that images and morphological calculations are precisely related to the micro-mechanical measurement (12). The material used in this study is a model styrenated -acrylic/melamine topcoat over a black basecoat. The thickness of the topcoat is ~40 μπι. The topcoat material is known to have long-term reproducible mechanical properties.


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Results and Discussion Figure 2 shows the results of a typical micro-scratch experiment. The indentor used here had a 1 μηι radius. The Curve A measured in pre-scratch shows the profile or height variation of the undamaged coating. Curve C, D and Ε were obtained during scratch damage process. Curve Ε and C are the applied normal force and the measured tangential force, respectively. Curve D is the displacement of the indentor. The total deformation under the load or penetration depth can be calculated from the indentor displacement (D) by subtracting the pre-scratch profile (A). Curve Β measured in post-scratch is the surface topography after the scratch damage made. The amount of unrecovered damage to the coating can be obtained from the post-scratch (B) profile by subtracting the pre-scratch (A). Figure 3 presents the total deformation (A) and the plastic deformation (B) as function of the applied normal force. As can be seen in Figure 2, the surface height variation within the 2.5 mm scratch distance is more than 100 nanometers, which is much better than average commercial coatings. Even with this sample, the variation could result in large error if the absolute displacement instead normal force was controlled. In the force control mode, all of the topographic details of the undeformed coating (A) are faithfully reproduced in both curves Β and D before a distance of 1.747 mm where large fluctuations begin to occur. However, as seen in Figure 3, the total deformation (A) under indentor during scratch and the unrecovered deformation (B) of scratch damage measured afterwards are not effected by topographic details but are only a function of applied force. To achieve this realistic measurement, it is necessary for the instrument to have a high sensitivity and fast dynamic response. In the early region of the scratch, Curves A and Β of figure 2 are superimposed on one another, signifying that the deformation is totally recovered, i.e. elastic. As the load increases, the two curves diverge, indicating the beginning of plastic deformation. Under video microscopy, a faint but smooth scratch is observed, which steadily increases in size as the load increases. The smooth increase of the unrecovered deformation as the normal force increases was also measured afterwards as shown in Figure 3. At a scratch distance of 1.747 mm or an applied normal force of 1.29 mN, (see Figures 2 and 3), the character of the curves abruptly change. The displacement curve (D) and the tangential force curve (C) start to show large and rapid fluctuations well above the local height or roughness variations in the coating. Cracks begin to appe.ar on the scratch observed by the video microscope. The starting point of the fluctuation indicates that fracture or rupture events occur in addition to plastic deformation. This is clearly confirmed by detailed AFM images near this transition point (12). As the normal load increases further, both the frequency and the magnitude of the rupture increases. However, at the early stage of the on and off fracture events, there is no material removal or debris observed through video microscopy. Wear rates calculated from high-resolution AFM images at the same locations confirm that no chips are generated (11). If the controlled normal force is increased further, debris from the surface is eventually generated, which clings to the indentor. At the same time the scratch morphology becomes very irregular. Based on this study as well as on field studies, scratch damage can be classified into two types: a) plastic flow (ploughing) where the damage is regular and



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Figure 2. Force and displacement vs. scratch distance during a micro-scratch experiment.

A p p l i e d N o r m a l F o r c e (mN) Figure 3. Total deformation and permanent deformation vs. applied normal force.



434 the surface topography changes smoothly; and b) plastic flow with rupture (fracture) where cracks and crevices open in the coating surface. In characterization of mar behavior of coatings, both plastic deformation resistance and fracture resistance, which in most cases are not proportional, must be considered. It is simply impossible to correctly evaluate mar resistance with a single number, as is currently done in the automotive industry. The visual impact from a collection of scratches depends on the damage morphology. The different types of damage result in different visual effects and are governed by different types of mechanical properties. Much of the confusion and controversy in evaluating mar resistance comes from a historical insistence on using visual rankings based on a single number without regard for the damage mechanisms. The result of visual evaluation is dramatically influenced by variables such as color, lighting condition, damage size, orientation of observation, duration of inspection, psychological factors, and so on. To our knowledge, there is no example that all of the variables were controlled during such evaluations. It can be argued that if a coating has better mechanical properties, both against plastic deformation and fracture, it should have a better mar resistance. As a first step, it is better to think of mar resistance as a purely mechanical problem and find a way to evaluate mar objectively based on damage mechanisms. A minimum of two parameters is required to characterize mar performance, one related to plastic flow and the other related to fracture. In a micro-scratch experiment with increasing applied normal force, deformation continuously changes from elastic behavior to visco-plastic and finally to fracture. The physical quantities at the transition points between two regimes of deformation are naturally the most important. They are the mechanical thresholds of the different types of deformation. As can be seen in Figure 2 and Figure 3, the fracture threshold is easily identified because the curves change their characters at this point. As the applied normal force increases to 1.29 mN, the tangential and total deformation reach 0.63 mN and 0.9 μπι, deformation changes from pure visco-plastic alone to visco-plastic with fracture. As results of the fracture, the tangential force and total deformation fluctuate to keep up with the normal forces through the closed-loop control. The applied force at the transition point is the minimum (or critical) force for fracture to occur. One of the tasks to improve mar resistance is to modify a topcoat to have a higher critical force against fracture. The threshold for plastic deformation is much more difficult to directly determine by a micro-scratch experiment because there is no dramatic change in the curve character. Although we can see that the curve Β deviates from curve A in Figure 2 or from horizontal axis in Figure 3, we can not locate the exact starting point. In fact, even with AFM the precise threshold of plastic deformation is difficult to determine. However, we know more deviation of curve Β from A in Figure 2 (or Β from horizontal axis) means more permanent damage made by a scratch, which further indicate lower mar resistance of the coating. To extract a single number to evaluate the plastic behavior, we calculate the area under curve Β in figure 3 to a predefined normal force. This predefined force should be as large as possible but smaller than the lowest of fracture thresholds in a group for comparison. The larger the calculated number, the larger the permanent damage from the scratch and the lower the mar resistance the material. The reason for using the area rather than the depth of permanent damage is to reduce possible error due to local noise of the measured data.



435 There may be other ways to evaluate plastic resistance of coatings based on the data of micro-scratch experiment. Nevertheless, mar resistance can be evaluated or ranked mechanistically and objectively with the area under the plastic deformation curve and the critical normal force before fracture. To completely understand the mar behavior of coatings, it is also important to understand the relationship between physical damage and visibility or appearance. This is a very complex problem since many variables are involved, including basecoat color, lighting, damage orientation, duration of inspection, and psychological factors, as well as size and type of damage. By means of the micro-scratch technique, different types and sizes of scratches can be produced with precision. This allows us to conduct statistical surveys to investigate size and type impact on visibility of micro scratches as described below. First, ten well-defined scratches were made on a 25mmx50mm sample with a 2 pm-radius indentor. These scratches are 10 mm long and parallel to each other 1 mm apart. Table 1 summarizes the normal forces used to produce the scratches, and morphology parameters of the scratches measured by AFM. Three of the ten scratches have only plastic deformation. Six scratches involved continuous fracture. One is on the transition, where fracture occurred intermittently. Figure 4 shows three AFM images of scratches near the fracture transition. Note the scales of the images are different. Other conditions for the survey were chosen to simulate the worst case scenario for visibility, a black basecoat and light intensity equivalent to the noonday sun. The following questions were asked to try to determine the nature of scratches and the critical "size" of the scratches that can be seen by human eye. How many scratches can you see? a) after 5 seconds; b) after 60 seconds; c) after being told there were twelve scratches on the panel. Participants were told there were twelve rather than ten scratches in an attempt to eliminate possible psychological factors. Seventythree people participated in this survey. The statistical results, however, are based on 72 people because there was one person who saw twelve scratches and his answer was removed from the survey. More than 60% of the participants observed five and six scratches after cursory examination of the sample (Figure 5). The percentage is much higher than the lines with pure plastic deformation (seven lines or higher). This indicates that scratches in continuous fracture are much more visible in very short observation time. The narrow distribution around five and six scratches suggests that once fracture occurs, the scratches become easily visible. When the participants were allowed sixty seconds to examine the panel, the result shifts to more scratches (smaller scratches) and broadens. Approximately 20% of people were able to see eight scratches. The result suggests that scratches with pure plastic deformation are visible if given longer observation time. After being told that there were twelve scratches present, the distribution shifts to even smaller scratches. Scratches as small as a few of nanometers in depth (Table 1) are visible by most people if they know what and where to look. The current survey does not provide a direct answer to what geometrical parameters govern the visibility of a scratch. However, from this and other studies, we believe neither width nor depth, but local geometrical changes are most important for visibility of plastic deformation.



Figure 4. A F M images of scratches near fracture threshold.


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Table 1. Scratch parameters for ten line statistical survey.


Load (mN) 1.0 2.0 3.0 4.0 5.0 6.0 7.0 10.0 15.0 20.0

Scratch Wear Rate Width Depth (nm) Roughness (nm) (pm 3/pm) (pm) 0.000 4.9 2.33 8.0 0.026 8.5 4.11 20.3 -0.018 7.3 4.54 42.5 0.085 21.5 51.8 6.11 55.4 0.381 9.12 99.6 0.504 104.7 8.95 153.8 0.597 162.1 10.05 273.0 1.176 173.3 13.18 354.0 1.545 254.1 16.38 652.1 1.430 301.8 21.53 903.9 A

Figure 5. Statistical survey results.


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Conclusions Mar damage of coatings can be classified into pure ploughing (plastic deformation) and ploughing with cracks (fracture). Characterization of mar behavior based on the damage mechanisms is objective and unambiguous. A micro-scratch technique with an instrument was developed to quantitatively evaluate the mar performance of polymer coatings. The instrument has high resolution, fast dynamic response and long-range stability, and is capable of collecting relevant mechanical values as a function of time at a high data acquisition rate. With the instrument, polymer surface deformation can be studied and important physical quantities can be obtained to evaluate both plastic and fracture resistance. Studying mar resistance of automotive topcoats is an example of such an application; however, the instrument can be used as research tool for many other applications. Fracture is the most serious damage mechanism on automotive topcoats. This type of damage is visible in a much shorter time and under a wider range of lighting conditions than plastic ploughing. Increasing the toughness of a topcoat may dramatically improve its scratch and mar resistance. Small scratch damage produced by plastic ploughing is also visible. In comparison with fracture scratches, brighter lighting conditions, longer duration of inspection, and correct angle of observation are required to observe plastic damage. It should be noted that significantly larger plastic deformation occurs in other type of coatings and in this case scratches by pure plastic ploughing may be more visible. Research is in progress to apply the same techniques to other type of topcoats. The new methodology shown will help to advance the understanding of wear of polymeric topcoats. Acknowledgments The authors would like to acknowledge B.V. Gregorovich for helpful conversations and S.A. Riggs for her AFM contributions. Literature Cited 1. Adamsons, K; Blackman, G.; Gregorovich, B.; Lin, L.; Matheson, R.R. Proc. XXIII International Conference on Organic Coatings 1997 23, 151.

2. Gregorovich,B.;Mcgonigal, P.J. SME technical paper, 1993. 3. Gregorovich, B.; Mcgonigal, P.J., Proc. of the ASM/ESD Advanced Coating Technology Conference 1992, 121-125.

4. ASTM D673-93a, D3389-94, D4060-95, D1630-94, and D1242-95a. 5. ASTM D2486-95. 6. ASTM D968-93. 7. Briscoe, B.J.; Evans, P.D. Composites Science and Technology 1989, 34, 73.

8. Yang, A.C.-M.; Wu, T.W. J. Materials Sci. 1993, 28, 955. 9. Ruan, J.; Bhushan, B. Trans. ASME 1994, 116, 378. 10. Shen, W.; Smith, S.M.; Jones, F.N.; Ji, C.; Ryntz, R.A.; Everson, M.P. J. Coatings Technol. 1997, 69, 123.

11. Lin, L.; Blackman, G.S.; Matheson, R.R. to be published. 12. G. S. Blackman, L. Lin and R. R. Matheson, ACS Symposium 1999.


Micromechanical Characterization of Scratch and Mar Behavior of

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