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Scaling behavior in the scratching of automotive clearcoats.

Abstract The scratch resistance of four clearcoat formulations was evaluated using a CSM nano-scratch tester, an AMTEC-Kistler simulated carwash tester, and a laboratory scale macro-scratching tester. Significant differences in the rank-order of all the clearcoats were found when comparing the scratch and mar behavior using macro-scratching, nano-scratching, and AMTEC-Kistler testing. Filed vehicles were also examined where the mean and median widths of scratches found on vehicles were 237 and 141 [micro]m, respectively. The rate of loads associated with events with the potential to create real-would scratches was found to be significantly higher (5-35 N) than the forces needed to make scratches of the same mean size as those seen in the field (7-10 N). These results indicated that a significant improvement in the scratch resistance of these clearcoats would be needed before customers would notice improvement in their paint finish's scratch resistance.

Keywords Scratch, Automotive coatings UV coating, Scratch resistance

Introduction

Automotive clearcoats were introduced to the market in the mid-1980s to improve the initial and long-term appearance of vehicles. Over the years, refinements have been made to the paint systems to reduce the risk of failures such as cracing and peeling. However, these and other failures such as gloss loss, color change, and acid etching are impossible to totally eliminate. In addition to these performance characteristics, consumers also desire a permanent, scratch-free finish on their vehicle. Scratch resistance has become one of the highest rated customer concerns for automotive paint systems. (1)

The scratches that occur on vehicles are difficult to classify into distinct categories. They occur along a continuum, from very small, micron size scratches that do not scatter light and can only be seen under specific viewing conditions, to very large, millimeter wide scratches that appear white due to significant light scattering. Most previous research has focused on the two extremes of this continuum when analyzing scratch behavior. (1-3)

One scratch extreme, which is typically caused by automatic car wash brushes and home car washing, is referred to as marring damage. These car wash scratches are typically only a few microns in width and depth, and do not fracture the clearcoat. (2), (4) The scratches can occur in large numbers, and can reduce the overall gloss of the clearcoat. finish, with the naked eye, due to their nature and size. However, as the paint system is exposed to continued car washing, their numbers increase, along with their visibility. Once a significant number of car wash scratches are present, they can be easily seen on bright, sunny days, while observing the paint finish at a shallow angle.

The other extreme on the scratch continuum are the large, easily visible scratches, which are classified as fracture scratches. These scratches are caused by more severe contact damage from keys, tree branches, grocery carts, and anything else that can cause the clearcoat to fracture along the scratch groove. The cracks in the clearcoat will scatter light and make the scratch appear white, causing the scratch to be easily visible to the naked eye. The scratch sizes can range from very small (under 25 [micro]m) to very large (over1 mm). These scratches can even damage the paint finish to the bare metal substrate, compromising the corrosion protection of the area.

Consumers in different geographical regions have different levels of expectation with regard to scratch performance. North American consumers are most concerned about large, fracture type scratches. In Europe, where automatic car washes often utilize stiff. plastic bristles, consumers are more concerned with car wash scratches and the associated loss of gloss. Unfortunately, each type of scratch has a unique damage mechanism, so improving a clearcoat's resistance to one type of scratch may not improve its resistance to the other type. Considerable effort has recently been put into formulating clearcoats with a noticeable improvement in scratch resistance. (6-11) Unfortunately, it is unclear as to how good is "good enough" when comparing laboratory scratch data to customer perception. Two different approaches have been pursued to improve scratch resistance: higher crosslink densities (UV curable and oligomeric clearcoats) and nanoparticle reinforced clearcoats.

Oligomeric clearcoats are thermoset clearcoat systems that utilize lower molecular weight precursors than standard thermoset clearcoats. (5) They are designed to achieve more regularly crosslinked networks. This regularity is thought to improve mar performance. UV curable clearcoarts have the potential to increase scratch resistance because they can achieve higher crosslink densities then their typical thermoset counterparts. (6), (7) Unfortunately, increased crosslink density can also lead to brittle coatings, which have a higher risk of cracking in-service. Therefore, a compromise between brittleness and scratch resistance must be made during formulation and development.

The addition of nanoparticles is another method thought to increase the scratch resistance of clearcoats. (8), (9) In these systems, hard nanoparticles are dispersed in a clearcoat to enhance the hardness of the surface layer. It is unclear if the nanoparticles provide sufficient enhancement to the clearcoat's scratch resistance to large fracture type scratches or only car wash marring scratches. Some work has even begun to combine UV curable coatings with nanoparticles to try and increase the abrasion resistance further. (10), (11)

To properly evaluate the scratch resistance of coatings, a reliable test method must exist. A number of test methods have attempted to measure the resistance of a clearcoat to either fracture scratches or car wash scratches, and sometimes both. Three of the most widely used tests are the AMTEC-Kistler car wash test, the five-finger scratch test. and the nano-scratch test.

The AMTEC-Kistler is a widely used European test method that simulates car wash scratches by running a paint panel under a large, rotating car wash brush. (12) After testing, the gloss of each sample is checked and ratioed to its initial value. A lower ratio corresponds to a less scratch resistant coating. Evaluation is restricted to either the subjective view of the operator, or a simple measure of gloss loss. Correlation to field date is debatable. In addition, this test is very dependant on the type of brush used (plastic vs foam/etc.). as well as the age of the brushes. This test imparts only small scratches across the coating and does not fracture the clearcoat.

The five-finger scratch test is a common test used by US automotive OEMs. particularly with painted plastics. (13) This test features five scratch tips with tip diameters that range from 1 to 7 mm. Different weights. are then applied to the tips as the panel is moved beneath them to produce an array of scratches. A drawing of the five-finger scratch machine is shown in Fig. 1. While this method simulates very large-scale scratches and mars, evaluation is typically done subjectively and not measured in a quantitative manner.

[FIGURE 1 OMITTED]

The nano-scratch test developed by Lin et al. is a quantitative test for analyzing both scratch and mar resistance. (14), (15) A stylus is used to produce scratch damage by applying a progressively higher load as the panel is moved under the stylus. This test allows the user to determine two quantities: the mar resistance and the fracture resistance of the clearcoat. After a test is run, the user can determine the fracture resistance of the coating by examining the scratch groove under the instrument's microscope. The force where the clearcoat visually begins to fracture is marked by the user and recorded as the "load to fracture" for the clearcoat. The mar resistance is determined by examining the residual depth profile of the scratch. Since marring occurs at relatively low loads, 5 mN is used as the load to record the residual depth for each clearcoat tested. This residual depth is used as a measure of mar resistance; the greater the depth, the poorer the performance. However, the fracture type scratches that occur in the field are thought to be caused by asperities that are one to two orders of magnitude larger than the tip of the nano-scratch tester. While a great deal of work has been done using nano-scratch methodologies, (2), (16-19) limited work has been published on using a larger scratch tip to create well-controlled scratches in coating systems. (3), (20-22)

This article focuses on understanding the scaling behavior of scratches caused by different size scratch tips on automotive clearcoats. The well-controlled scratches created in the laboratory are compared to those created using the AMTEC-Kistler method, as well as to scratches created on vehicles used by consumers. The loads to create the various scratches are compared to the scratch size (width and depth) for a variety of clearcoats that possess significantly different scratch and mar resistance, as measured by the nano-scratch method.

Experimental

Four different clearcoats were used in this study. Clearcoat A was a thermally cured acrylic/melamine/silane clearcoat. Clearcoats B-D were UV-curable clearcoats that cured without the application of heat (monocure). Full clearcoat formulations were proprietary.

Dynamic mechanical analysis

Crosslink densities and glass transition temperatures of Clearcoats A and C (Clearcoat B's information was provided by the manufacturer while Clearcoat D's information was not provided, but was formulated to be very highly crosslinked) were determined using a Rheometric Scientific DMTA IV equipped with a tension test fixture. The [T.sub.g] of each coating was taken as the maximum in the tan [delta] curve. Crosslink density was calculated using the following equation:

[v.sub.e] = E'/3RT (2)

where E' was the minimum in the storage modulus (E') curve, R was the gas constant, and T was the temperature at which this minimum in storage modulus was reached. (23)

Scratch resistance

Nano-scratch testing was performed on all clearcoats using a nano-scratch tester with a stylus of radius 2 [micro]m (CSM Instruments, Needham, MA). During nano-scratch testing, a force was applied to the coating normal to the coating's surface. As the sample was moved laterally, the normal force was increased, from 0 to 100 mN, at a constant rate until the clearcoat fractured and microcracks appeared in the clearcoat surface (viewed under a microscope at 100x magnification). This critical force was recorded as a measure of the "scratch resistance" of the clearcoat, as in the method of Lin. (14) The amount of plastic deformation done (residual penetration depth) at 5 mN of normal force was also recorded. This second number is thought to be inversely proportional to the mar resistance of a coating; that is the ability of the clearcoat to resist light scratches that do not fracture the surface.

AMTEC-Kistler testing was used to evaluate the resistance of the clearcoats to car wash scratches. (24), (25) The test utilized a plastic brush drum that was rotated over the surface of the samples. As the panels moved beneath the brush, the brush rotated the bristles opposite the direction of sample movement. A quartz dispersion (SH200, 1.5% concentration) was sprayed during the test to a simulate dirt and other abrasive particles. The samples were exposed to 10 cycles in the test apparatus to simulate car wash scratching. After a gloss measurement (GIs) was taken, the samples were allowed to recover in an oven set @ 70[degrees]C for 2 h. Gloss measurements were taken again (Glr) and compared to the initial gloss (Gli).

Macro-scratch

Laboratory simulations of macro-scratches were performed using a custom built scratch apparatus. Four different diamond scratch tips (The Quad Group, Spokane, WA) with tip radii of 27, 126, 281, and 460 [micro]m, respectively, were used to create scratches. Each tip had an inclusion angle of 90 degrees.

These tips were attached to load cells from Honey-well Sensotec (AL311, Columbus, OH) whose full-scale load capacity ranged from 2.45 to 45 N. The load cells were connected to a computer through a Transducer Techniques (Temecula, CA) TMO-2 signal conditioner. The load cell/tip assembly was held fixed by attachment to a rigid cross member that could be moved vertically. The paint panels were attached to 3-axis stage. An in-house data acquisition program utilized a PID controller to adjust displacement and load. The scratch-tip speed was set to 1 mm/s for all simulations performed. To make scratches, each clearcoat system was first loaded to a constant level. The paint panel was then moved along the y-axis to provide the motion for scratch damage, during which the load was kept constant (with an error of ~5%). Scratch length was set to 20 mm. Scratches were made with loads starting below those that caused clearcoat fracture and increased up through radial and longitudinal fracturing. The minimum and maximum load applied to each clearcoat system and corresponding tip sizes are shown in Table 1.
Table 1: Minimum and maximum loads applied to each clearcoat system for
each of the four stylus sizes. Forces in Newtons

Paint System, Tip Size      Minimum load (N)  Maximum load (N)

Clearcoat A, 27 [micro]m           0.4               1.0
Clearcoat A, 126 [micro]m          1                 5.5
Clearcoat A, 281 [micro]m          4                13
Clearcoat A, 460 [micro]m         14                37
Clearcoat B, 27 [micro]m           0.8               2.2
Clearcoat B, 126 [micro]m          2                 8
Clearcoat B, 281 [micro]m          7                18
Clearcoat B, 460 [micro]m         20                43
Clearcoat C, 27 [micro]m           0.5               1.4
Clearcoat C, 126 [micro]m          3                 6.5
Clearcoat C, 281 [micro]m          8                15
Clearcoat C, 460 [micro]m         20                38
Clearcoat D, 27 [micro]m           0.5               2
Clearcoat D, 126 [micro]m          1                 7
Clearcoat D, 281 [micro]m          6                14
Clearcoat D, 460 [micro]m         12                24


Scratch widths and depths were determined by using a UBM Microfocus Type UBC-14 optical profilometer. The scans were analyzed using UBSoft for Windows version 1.8. The standard conditions for performing the profilometer measurements were a scanning speed of 1 mm/s and a scanning rate of 2000 points/mm. The scan length was dependent on the number of scratches being analyzed. The method used for analyzing the surface scans started with leveling the scan data using a 4th order polynomial. This leveling provided a surface profile in which curvature, orange peel, and sample tilt were effectively eliminated. Scratch width was then measured between the two peak shoulders of each scratch groove while depth was measured from the top of the scratch shoulder to the bottom of the scratch valley.

Three-dimensional images of the scratch damage caused by the macro-scratch tests were taken using a Wyko NT3300 (Veeco, Tucson, AZ) profiling system. This device used vertical scanning interferometry (VSI) to determine the topography of the scratch damage. The scratch damage was focused on using the 2.5x objective, at which point the machine would scan a vertical range of 80 [micro]m, with a 10 [micro]m back-scan. All data were processed using Veeco's Vision32 [TM], which then created 3-D images of the scratch area.

Real-world data

In-service vehicles were examined for easily visible fracture scratches on painted metal parts. These scratches were analyzed using a Scalar DG-2 Digital Video Microscope (Tokyo, Japan) with a 200x lens attachment. Measurement calibration photos were also taken of marked reticules for scratch width analysis. Scratch photos were analyzed using the Image Analyzer software package.

Real-world scratch event forces were measured using a 45 N load cell from Honeywell Sensotec (AL311). The forces associated with a key scratching a vehicle, a shopping cart scraping against a vehicle, a person scraping their belt buckle against a vehicle, as well as a bag of groceries sliding along a vehicle were all measured. In each case, the load cell was positioned at the point of contact, and the force was then recorded using a Datastick (Campbell, CA) DAS-1254 data acquisition system (utilizing a Tungsten T3 Palm Pilot) connected to a TMO-1 amplifier/conditioner module by Transducer Techniques.

Results

Material properties

To determine any correlation between material properties and scratch resistance, the crosslink density and glass transition temperature were determined for each clearcoat. The values for each of these properties are shown in Table 2.
Table 2: Tabulated values of crosslink density and glass transition
temperature for each clearcoat system

             Crosslink Density  [T.sub.g] (C[degrees])
                 (mol/cc)

Clearcoat A       2.1 E-3                 72
Clearcoat B      2.31 E-3                 65
Clearcoat C      4.41 E-4                 56
Clearcoat D         N/A                   N/A


Scratch properties

The nano-scratch properties of each clearcoat system were measured to compare fracture and mar resistance for each of the three systems. The permanent deformation at 5 mN vs the normal force at fracture is plotted for each system in Fig. 2. Clearcoat A had the lowest fracture value (~15 mN) and best mar value (~0.225 [micro]N) and a mar value (~0.355 [micro]m @ 5mN). Clearcoat C had a fracture value (~44.16mN) and the worst mar value (~0.56 [micro]m @ 5 mN). Clearcoat D has the highest fracture value (~90.75 mN) and a mar value (~3.315 [micro]m @ 5mN).

[FIGURE 2 OMITTED]

The AMTEC-Kistler results for each of the clear coat systems are shown in Table 3. Clearcoat B showed the best results for gloss loss, both pre-and post-recovery. Clearcoat C showed the best recovery, showing an increase in gloss retention of ~10% after recovery. Clearcoat D was shown to have the worst gloss retention (53%) after testing in both the pre-and post-recovery measurements.
Table 3: Results from AMTEC-Kistler testing for Clear-coats A-D

Clearcoat  Glr/Gli  Gls/Gli   Gls    Glr    Gli

A          0.8142   0.7899   68.45  70.55  86.65
B          0.8881   0.8471   72.3   75.8   85.35
C          0.8533   0.7482   69.4   79.15  92.75
D          0.5298   0.5333   45.6   45.3   85.5


The load at fracture results from the macro-scratch tests are shown in Fig. 3. The plotted data are loads at which the clearcoat failed by longitudinal cracking occurred when the clearcoat fractures parallel to the scratch direction, causing obvious whitening.

[FIGURE 3 OMITTED]

The macro-scratch data is plotted in a similar way as the nano-scratch data in Fig.4. Force to longitudinal fracture (N) vs permanent deformation at N was plotted for each of the clearcoat systems when scratched with the 126 [micro]m tip. Unlike the nano-scratch data which showed a large separation between the four systems, this data was more clustered, with Clearcoat B showing the best fracture performance, but worst mar performance. Clearcoat D had the best mar performance, but worst fracture performance.

[FIGURE 4 OMITTED]

The (width) (2) of the macro-scratches produced are plotted vs load in Figs. 5-8. Each set of data showed a linear trend, indicating that the scratching load scaled with the area of the scratch. Thus, scratch loads can be used to estimate scratch widths, and vice versa for a constant tip geometry and material system. It should be noted that this data showed no obvious discontinuities when moving from one tip size to another. This indicates the width of the scratch is likely independent of tip size.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

A profilometry scan of the 460 [micro]m tip scratch created with 24 N of force is shown in Fig.9. This scratch had a drastically different profile from the one created by 14 N of force (also shown in Fig. 9), well below the critical fracture force.

[FIGURE 9 OMITTED]

Real-world scratches

Scratch widths measured on ~35 in-service vehicles located in Dearborn, Michigan with >5 years in service were plotted vs frequently and are shown in Fig.10. The scratch widths were binned into 100 [micro]m wide ranges. Each of these scratches was a fracture type scratch which could be easily seen from ~1 m away from the vehicle. The mean scratch width for this data was 237 [micro]m, while the median was 141 [micro]m.

[FIGURE 10 OMITTED]

Force profiles taken from simulated scratches (cart, key, belt buckle, and grocery bag) are shown in Figs.11-14. The average load maximums for eachdamage event are shown in Table 4. Average load maximums were the range of peak loads seen during the damage simulations.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]
Table 4: Damage mechanisms measured and the average load range for each
of the mechanisms

Damage Mechanism  Average Load Range (N)

Key                       15-25
Cart                      15-35
Belt buckle               20-30
Shopping bag               5-15


Discussion

The results above and previous research clearly show that the relationship between physical properties and scratch resistance is complex. This work demonstrates that the damage that occurs during scratching does not scale in a straightforward manner with different size scratch tips. Thus, to understand the scratch behavior at all size scales, the underlying differences between nano-scratching and macro-scratching must be under-stood. In addition, the expected impact on real-world behavior of utilizing "scratch resistant" coatings must be examined.

Nano-scratch vs macro-scratch results

Previous work has shown that different coatings can demonstrate significantly different behavior when subjected to nano-scratch testing. (14) Coatings that display high loads to fracture have been classified as "scratch resistant coatings." Those that possess small residual deformation after a 5 mN load is applied are though to display superior mar performance. However, conclusive evidence linking nano-scratch behavior to field results is lacking. The nano-scratch results shown in Fig. 2 rank Clearcoat D as the most resistant to fracture scratches. It possesses a force to fracture of over 90 mN. Clearly different conclusions can be drawn about scratch resistance depending on the sizw of the scratching tip.

In trying to connect the laboratory test to real-world performance, those lab scratches that mimic real-world scratches are preferable. When the same type of lest is run with the macro-scratch test, however, Clearcoat B is the superior fracture resistant clearcoat (Fig. 4). The scratches made by the nano-scratch tester cannot be seen by a casual observer. They are only visible under the appropriate lighting conditions, even though at the higher loads they fracture the clearcoat extensively. The scratches made by the macro-scratch tester mimic those on vehicles in both size and appearance. Thus, the scratches made on vehicles must be made by asperities of the same approximate size as the tips used during macro-scratching.

A number of different reasons can be proposed to explaing the differences between the performance of coatings during nano-scratching and macro-scratching. Since the volume of material impacted by the scratch tip is five orders of magnitude greater in the macroscratch test than in the nano-scratch test, the probability of the scratch tip encountering a flaw in the coating in the macro-scratch test is greater. While flaws can be placed in polymers by various extrinisic means, most polymers behave as if they have an intrinisic flaw size. The size of these intrinsic flaws is dependent on a number of molecular factors. For example, studies have shown the inherent flaw size to be on the order of 1 mm in polystyrene, while in PMMA the intrinsic flaw size has been measured to be approximately 70 [micro]m. (26) When using the larger sized tip, the likelihood of encountering a flaw is increased and, therefore, the force to fracture would be relatively smaller than if no flaw had been encountered. This also explains the smaller spread in the data for the larger tip sizes. As tip size is increased. the likelihood of encountering a flaw during scratching is increased for all of the coatings and therefore the scratch behavior is similar. When using the nano-scratch method, the chances of encountering a flaw are smaller and the chance of the affectedmaterial volume in some coatings being above and others below the intrinisc flaw size is greater, thus giving rise to the greater spread in the coatings' behavior at small tip sizes.

For the larger tip sizes, the interactions of the underlying layers cannot be ignored. In the case of the largest tip sizes, the thickness of the coating approaches that of the tip penetration. This likely partially explains the anomalous behavior of Clearcoat D, which shows a dramatic drop in scratch performance at the larger tips size. A loss of adhesion between the clearcoat and basecoat at the larger tip sizes would be interpreted as a fracture scratch. therefore giving he impression of a lower scratch resistance. While a complexity in interpreting the data. this phenomenon is likely to be of practical significance in the real-world where scratches of this depth can be expected.

With respect to marring. differences also exist between nano and macro-scratching. Clearcoat C shows the most residual deformation after the 5 mN load in the nano-scratch test. but is almost the best using the same metric in the macro-scratch test. In macro-scratching, Clearcoat B is clearly the worts in terms of mar performance yet in nano-scratching is roughly on par with Clearcoats A and D. As has been pointed out by previous workers, the residual deformation after scratching (in the non-fracture regime) is related to the proportion of deformation that is elastic vs plastic. (27) The reversal in rank-order in going from nano-to macro-scratching may again be due to the influence of the underlying basecoat. In particular, during macro-scratching, Clearcoat D is particularly hard to deform until it fractures. Thus, the transition from elastic deformation to fracture occures with little intervening plastic deformation, giving Clearcoat D superior mar resistance. In the macro-scratch tests, the basecoats will undoubtedly play a large role due to the larger depth of penetration, which approaches a significant proportion of the clearcoat thickness. AMTEC-Kistler testing, which is though to be the best representation real-world car wash scratch performance displays yet a different rank-order (B.C.A. D best to worst in Table 4) than either nano-or macroscratching (mar results). To fully understand which test best correlates with real-world car wash scratching. a long-term study of field vehicles must be undertaken.

In comparing macro-scratch results vs nano-scratch results, a final point to consider is the homogeneity of the clearcoat through its thickness. If the clearcoat had a significantly harder surface than the bulk, a proportionately higher nano-scratch (residual depth) value could be obtained compared to the macro-scratch results. Clearcost A possesses the best mar performance in the nano-scratch test, but the second worst performance in the macro-scratch test. In order to determine if the surface scratch hardness was the same as that of the bulk of the clearcoat, a sample of Clearcoat A was microtomed using a slab microtome fro Leica Instruments (SM2500E). The top 15 [micro]m ofthe clearcoat was removed, which was followed by macro-scratching using the 25 [micro]m scratch tip and the same forces that were used previously. The results did not show dramatic differences between the residual widths and depths of each location. This indicates that the surface of the clearcoat does not possess a different level of scratch resistance then that of the bulk.

Implications for real-world scratch performance

As previously noted, the (width) (2) of the macroscratches scales linearly with load (Figs. 5-8). If it is assumed that the damage done during real-world scratch events is governed by similar processes to those in the macro-scratching, then this data can be used to estimate the force that caused a scratch of a given width.

The results shown in Fig. 10 were obtained from measuring a number of real-world scratches that were easily visible to the naked eye. the mean scratch width of the sample set measured was 237 [micro]m, while the median of the sample set was 141 [micro]m. Using the linear trend line for the data from Clearcoat A (Fig. 5), which best represents the clearcoat chemistries on the vehicles tested, the average and median forces that caused these field scratches were calculated. Accordingly, a 237 [micro]m scratch would be caused by a contact force of 7.30 N, while a scratch with a width of 141 [micro]m would be caused by a contact force of 2.64 N. These loads are significantly less than the average loads (Table 4) seen in some typical paint scratch events, indicating that the scratch resistance of the coatings has deteriorated over time and/or that the asperities causing the scratches in the real-world are more damaging that the conical the real-world are more damaging that the conical indenters used in this study. The latter point is likely to be true as most asperities in the real-world are likely to be sharper than the conical indenters used in the laboratory scale testing.

Ultimately, the level of improvement that can be expected by changing to more "scratch resistant" clearcoats needs to be established. By combining the data taken from the macro-scratch tests and measurement of real-world fracture scratches, estimations of scratch resistance improvement can be made. Using the scratch width data taken from real-world vehicles, a mean scratch width of 237 [micro]m and a median scratch width of 141 [micro]m were found. As mentioned earlier, 7.30 N and 2.64 N of force would be needed to cause scratches of these widths in Clearcoat A with a conical indenter. Using the macro-scratch (width) (2) vs force data for Clearcoats B-D (Figs. 6-8), the forces required to create the same 237 and 141 [micro]m scratches can be determined. These values are shown in Fig. 15 for each clearcoat system. This data suggests that the improvement seen between the most scratch resistant clearcoat system and the system currently used on vehicles today is limited to 2.25 N (237 [micro]m scratch) and 1.04 N (141 [micro] scratch). A maximum load of fracture increase from 7.3 to 9.55 N is insignificant when comparing it to the real-world events whereloads can reach as high as 30 N. Customer perception will only be changed by a quantum leap in scratch resistance, not one limited to an improvement of < 10% of the maximum load required for scratching. Realistically, none of these clearcoats will resist the majority of fracture scratch events that happen in the field.

[FIGURE 15 OMITTED]

Conclusion

Four clearcoat formulations were evaluated for scratch resistance using a CSM nano-scratch tester, AMTEC-Kistler simulated carwash tester, and laboratory scale macro-scratching tester. Significant differences in the rank-order of all the clearcoats was found when camparing both the scratch and mar behavior using macro-scratching, nano-scratching, and AMTEC-Kistler testing. The range of loads associated with major scratch events, such as keys, belt buckles, grocery bags, and cart were found to be significantly higher than the threshold value for fracture type scratches in all of the clearcoats tested. This finding implies a significantly greater improvement in scratch resistance will be required before the customer perception of scratch performance is dramatically changed.

Acknowledgments The authors would like to thank Kevin Ellwood and Mike Debolt for creating the initial version of the macro-scratch testing machine.

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(18.) Nolhhelfcr-Richter, R. Klinke. E. Eisenbach. CD. "revaluation of the Scralch Resistance with Nano- and Muhiple Scratching Methods-" Maemmot Symp., 187 853-860 (2002)

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(20.) Demirci. I. Gauthier. C. Schirrer. R. "Mechanical .Analysis Of the Damage of a Thin Polymeric Coaling during Scratching: Role of the Ratio of the Coaling Thickness lo the Roughness of a Scratching Tip." Thin Solid Fihils. 479 207-215 (2005)

(21.) Ni. BY, Le Faou, A. "Scratching Behavior ol Polymer Films using Blunl Spherical Syli."J. Mat. Sci., 31 3955 3963 (1996)

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(23.) Hill. LW. Korzeniowski. HM. "Crosslink Density of High Solids MF-Cured Coalings." J. Coai. TechnoL. 59 (751) hill (1987)

(24.) ISO/FDIS 20566

(25.) DIN 55668

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C. M. Seubert, M. E. Nichols

Materials Research and Advanced Engineering Department, Ford Motor Company, Dearborn. MI 48121-2053. USA

e-mail: cseubert@ford.com
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Author:Seubert, Christopher M.; Nichols, Mark E.
Publication:JCT Research
Geographic Code:1USA
Date:Mar 1, 2007
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