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CHAPTER 5

M E T R O L O G Y F O R C H A R A C T E R I Z I N G THE SCRATCH RESISTANCE OF P O L Y M E R I C COATINGS T H R O U G H OPTICAL S C A T T E R I N G

LI-PIIN SUNG l' *, PETER L. DRZAL 2, MARK R. VANLANDINGHAM 3, and AARON M. FORSTER I

1National Institute of Standards and Technology, Building and Fire Research Laboratory, 1O0 Bureau Dr, Mail Stop 8615, Gaithersburg, Maryland 20899-8615 2pPG Industries, Inc., Resin and Coatings Research and Development, 4325 Rosanna Drive, Allison Park, Pennsylvania, 15101 3Multifunctional Materials Branch, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069 *E-mail: lipiin.sung@nist, gov ABSTRACT Scratch and mar resistance is an attribute of great practical importance to polymeric materials. Physical measurements of scratch and mar resistance have been conducted, but the ability to link these resistance to other properties of a polymer and to the customer-perceived appearance of a surface has remained elusive. Thus, optimization of material properties enhancing scratch and mar resistance is not possible. Over the years, a multitude of scratch test devices and protocols has been developed, but the large amount of data generated has made it very difficult to compare and pool data, and ultimately to standardize any test protocol. Also, the relevancy of physical measures of scratch and mar are continually being raised, since a link between physical measurements of scratch and mar and consumer perceived failure caused by scratch and mar has not been established. Establishing a connection between physical and appearance characterizations of scratch and mar are difficult, since appearance measures of scratch and mar are almost always based on qualitative, as opposed to quantitative, assessments of damage. These appearance measurements are made through visual inspection, gloss changes, and changes in gray scale level or lightness. These qualitative assessments are seldom either repeatable or reproducible, and thus a reliable standardized test method for assessing scratch and mar from a consumer's perspective is also not currently available to the polymeric materials community. In this chapter, progress is reported toward the development of a scientifically based standardized test method for quantifying scratch resistance based on optical scattering metrologies. A scratch protocol to impart a standard scratch deformation is described. Optical scattering measurements are conducted to identify the onset of plastic deformation by analyzing specular and off-specular intensities. The angular-resolved optical scattering reflectance profiles at various incident angles are measured using a custom-designed optical scattering instrument at the National Institute of Standards and Technology (NIST). Angular-resolved optical scattering from surfaces having a single or multiple scratches are compared to gloss measurements taken

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at 20 ~ and 60 ~ with a commercial glossmeter. By analyzing the specular and off-specular scattered intensities, scratch damage can be quantitatively assessed, even when the results from gloss measurements are indistinguishable. Additionally, the correlation between surface roughness and gloss measurements is presented. Finally, there is commentary herein on future research directions that includes implementing metrologies for linking appearance-based scratch resistance measurements to nanomechanical material properties measured through instrumented indentation.

KEYWORDS Appearance, gloss, laser scanning confocal microscopy, mechanical properties, optical scattering, scratch and mar, surface morphology

INTRODUCTION Damage to the appearance of polymeric materials by surface deformation, such as a finger nail scratch or gouge, remains a major challenge for commercial applications of these materials. As a result, considerable scientific and engineering efforts have been expended to assess, improve, and predict the appearance durability or "scratch resistance" of plastic materials. Typically, surface deformation that negatively affects appearance can be categorized as either scratch or mar. Marring usually occurs under less severe conditions compared to scratching, and the depth of mar damage is less than that of a scratch. Scratching, on the other hand, is associated with a much lower density of larger, deeper scratches, sometimes even a single scratch. In both cases, the result is a decrease in the appearance of the polymeric surface, the extent of which qualitatively depends on a customer's perception of what he or she considers a defect. To understand scratch and mar damage, a number of commercial and custom instruments have been developed to impart scratch or mar damage. These instruments can be quite sophisticated, providing instrumented signals such as displacement, scratching force, frictional force, and many others. Coinciding with many of these instruments [1-7] are numerous test methods to quantify or rank the scratch resistance of polymeric materials. The wide variety of scratch methods and instrumentation presents many challenges in the standardization of a scratch protocol, because the same material scratched under different tip geometries, normal force, and velocity will give dramatically different damage morphologies. Equally difficult to standardize is the assessment and measurement of scratch resistance. Scratch resistance is commonly measured by assessing appearance changes brought about by scratch damage. Scratch damage can range from plastic grooving in a ductile material, to cracking and chipping in a brittle material. The severity of the scratch appearance will not only depend on the damage morphology of the scratch region but also on other variables such as surface roughness, color of the plastic part, and index of refraction of the surface. Therefore, a scratch resistance ranking methodology based on a single damage event, like cracking, does not adequately capture the materials resistance to scratch visibility. In the coating industry, specular gloss measurements using commercially available glossmeters remain the mainstream measurement tool for assessing the appearance and durability of coated objects. Commercial glossmeters can be used to measure the change in gloss (decrease in specular reflection) due to scratch or mar damage occurring in laboratory or field testing of a

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material. As will be demonstrated, specular gloss measurements have significant limitations relative to assessing scratch and mar damage. For example, a severe scratch or damage from multiple scratches on a coating surface results in a strong suppression of the specular gloss. A commercial glossmeter is only sensitive to the overall reduction in specular gloss and not the underlying scratch morphology that actually reduced the gloss value. Additional assessment methods such as gloss change, change in gray scale level, or lightness have been created as useful adjuncts to visual inspection to help distinguish between two highly damaged surfaces. However, these techniques often provide only a relative answer, such as whether the surface is scratched or not. For example, a single scratch in a coating or surface produces only a slight change in specular scattering, but it produces a large increase in diffusive scattering intensity (off-specular). Commercial glossmeters, gray scale level, or lightness measurements are not sensitive to these minute decreases in specular gloss nor are they sensitive to the rise in diffusive scattering intensity. Thus, specular techniques cannot be used to quantitatively describe the scratch damage morphology and its resulting scratch visibility for a single scratch surface. More quantitative approaches, such as described in a recent study by Rangarajan et al. [8], have used optical imaging techniques to quantify the optical contrast of a scratch on a glossy polymer surface. The total optical contrast was a function of scratch dimensions and the contrast in both specular and off-specular scattering. Therefore, it provided a more robust measurement of the relationship between damage and visibility. Similarly, a strong correlation between the total optical contrast and scratch visibility was proposed by the industrial appearance perception study [9]. However, this study did not include discussion of the relationships between scratch appearance and the material properties or surface characteristics (e.g., elastic modulus, frictional coefficient, yield stress, and surface roughness). A full angular-resolved optical scattering profile along with characterization of the surface mechanical properties is needed to fully understand the structure-property relationships that govern the scratch durability over time. Currently, relationships between appearance attributes and surface deformation associated with scratching and marring are ambiguous. This lack of connectivity is one of the major barriers to the development and acceptance of standard measurement techniques for determining scratch and mar resistance. In order to successfully implement a scientifically based standardized test method for quantifying scratch resistance, it is vital to understand the relationships between material/mechanical properties, morphology, and appearance (optical properties) of surface and sub-surface deformation. A three-step methodology is proposed to provide the information required to draw conclusions about structure-property relationships that govern scratch resistance. A scratch is first generated in a material using a well-controlled scratch measurement protocol [ 10]. Second, an optical imaging technique is utilized to identify the "onset" of plastic deformation. At this point, the onset may be linked to the mechanical properties. Finally, the scratch damage is characterized with respect to the background signal from the undamaged surface by measuring both the specular and off-specular scattering intensities. The ratio of these two scattering signals includes information on surface roughness, substrate color, incident wavelength, and angle, and is used to evaluate the visibility of the scratch. The combination of absolute physical and optical measurements permits the quantitative evaluation of scratch resistance for each material. Therefore, we may objectively relate performance (durability, appearance) to material/mechanical properties.

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This chapter describes an optically-based scratch test methodology developed through a National Institute of Standards and Technology (NIST) partnership with industry (Polymer Interphase Consortium (PIC)) and its application to a series of model polymeric materials. The reader is expected to gain a better understanding of (1) how scratch variables like tip geometry, scratch speed, and method of force application (constant or progressive) influence the onset of scratch visibility, (2) the contribution of surface roughness to scratch visibility, (3) the importance of off-specular scattering and the limitations of specular gloss measurements, and (4) the effect of multiple scratches and a single-scratch on surface appearance.

EXPERIMENTAL SECTION*

Materials Materials used in this study included a set of model amine-cured epoxy (EP, prepared at NIST) coatings, 2 component polyurethane clear coatings (PU, provided by PPG), black pigmented poly(methyl methacrylate) (PMMA, a commercial product), and black pigmented highly crystalline poly(propylene) (PP, provided by Dow Chemical). A brief description of sample preparations along with mechanical characterization is described below.

Influence of Roughness:

A set of epoxy (EP) coatings, each having a different surface roughness, were fabricated to study the link between surface morphology and optical reflectance properties such as gloss [11,12]. The epoxy coatings were cast between a smooth black glass substrate (Schott Glass NG-1, 3 mm thickness) and a mold embossed with a well-characterized surface roughness. Mold fabrication and sample preparation details are reported elsewhere [ 11 ]. The final coatings, designated as EPI5 through EP45, had decreasing RMS roughness values ranging from 800 nm (EP15) to 100 nm (EP45), as estimated from mechanical (stylus) profiling data [13].

Multiple Scratch Visibility: Two different polyurethane clear coat formulations (PU-A and PUB, coating thickness --40 ~m, prepared by PPG) were cast on NG-1 black glass substrates. Multiple scratches were generated on the cured coatings using 5 cycles from an AMTEC Kistler (AK) scratch tester (performed by PPG). Non-scratched polyurethane samples were also prepared for reference.

Single Scratch Visibility: Injection-molded plaques of highly crystalline poly(propylene) (PP) (3 mm thickness) were provided by Dow Chemical. Poly(methyl methacrylate) PMMA samples (3.8 mm thickness) were obtained from a commercial source. An instrumented indentation and scratch testing system (MTS Nanoindenter XP equipped with lateral force measurementinstrument described elsewhere [14]) was used to create and characterize single scratches on these materials. Scratch tests were performed at NIST using 45 ~ semi-apical angle diamond cone indenters with tip radii of 1 lam and 10 ~tm. Scratches were generated by either progressive-force or constant force scratch methods. In a progressive force scratch test, the applied normal force linearly increases over the length of the scratch to a maximum applied force. In a constant-force scratch test, the applied normal force is maintained at a constant value over the length of the scratch. Regardless of the test method, the scratch and residual depths,

Certain instruments or materials are identified in this paper in order to adequatelyspecify experimentaldetails. In no case does it imply endorsement by NIST or imply that it is necessarily the best product for the experimental procedure.

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friction coefficient, and residual roughness are measured during scratch testing. Scratch lengths were either 500 ~tm or 1000 ~tm. For all testing, the samples described above were used as received, with no further annealing or modification. A representative indentation modulus was obtained for each material using an MTS DCM Nanoindenter, and the results are listed in Table 1.

Table 1" Modulus values of samples measured using a MTS DCM Nanoindenter and evaluated at an indentation depth of 1 lam. Each average modulus value is followed by a + symbol and another value representing plus or minus one standard deviation (k = 1), estimated from 10 individual indentations. Specimen Modulus (GPa)

EP-Series 4.16__.0.06

Pu-A 3"88 +0.03

PU-B 4.00 + 0.01

" PMMA "5.11 +0.08

PP " ! 1.33 +0'.07 1

Surface Roughness and Scratch Morphology Characterization A Zeiss model LSM510 reflection laser scanning confocal microscope (LSCM) was employed to characterize surface morphology (topographic profile) and scratch damage. A detailed description of LSCM measurements can be found elsewhere [15, 16]. The laser wavelength used in this study was 543 nm. LSCM images presented in this paper are 2D intensity projections (an image formed by summing the stack of images over the z direction, 512 pixel x 512 pixel), or 3D topographic profiles of the coating surface. The 2D intensity projection images are effectively the sum of all the light scattered by different layers of the coating, as far into the coating (approximately 100 lam in the 5X configuration) as light is able to penetrate. The pixel intensity level represents the total amount of back-scattered light. Darker areas represent regions scattering less light than lighter colored areas. Figure 1 shows an example of (a) epoxy coating surface morphology, and (b) a scratch profile generated by a constant-force scratch test method. The scratch width was defined as the peak-to-peak distance perpendicular to the scratch length, and is indicated in Figure 1b. The Root-Mean-Square (RMS) surface roughness Sq is calculated using a surface tilt correlation across an automatic plane fit of the 3D topographic image. The plane fit is a single polynomial fit to the intensity data across the image. The polynomial fit is subtracted from the image to remove any tilt in the image. Since the subtraction occurs over the entire image, the plane fit does not corrupt the topographic data. The RMS surface roughness is calculated without a numerical filter according to the following formula:

Sq-

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Fig. 1. LSCM images of (a) a rough surface- 2D intensity projection (top, left) and 3D topological presentation (bottom, left); (b) a scratch generated by a constant-force scratch method: 2D intensity projection (top, right) and height profile (bottom, fight).

Angular-resolved Optical Scattering Measurements Optical scattering measurements using a newly constructed optical scattering instrument were conducted at various incident angles in the specular, off-specular, and out-of-plane scattering configurations on each polymer sample with and without scratches. The apparatus consists of a laser light source, a five-axis goniometric sample stage, and a two-dimensional (2D) detector mounted in a concentric ring around the sample stage (see Figure 2a). The incident laser wavelength was 633 nm, and the beam was vertically polarized (90 ~ and focused on the sample with a spot size diameter of 1 mm. The sample rotation stage and the detector ring position determine the incident angle of the beam on the sample and the viewing angle of the detector. The bottom illustration of Figure 2a presents the optical geometry, where 0~ and 0s are the incident and scattered angles measured with respect to the sample normal. The quantity, ct (omitted for clarity here), is the out-of-plane scattering angle, which is important for characterizing the scattering profile due to the scratch damage. Specular reflection is measured when Os = 0,-. A detailed description of the instrument will be reported elsewhere [ 17]. Results are presented in terms of the 2D angular distribution of light scattered from a scratched surface at incident angles of 20 ~ 45 ~ and 60 ~ One-dimensional (1D) angular-resolved optical scattering profiles are obtained by circularly integrating the 2D scattering intensity (Figure 2a) as a function of the scattering angle. The scratch scattering signal is compared to the undamaged coating background signal, and the ratio of these two signals is used to evaluate the visibility of the scratch. Figure 2b illustrates the angular distribution of scattered light with respect to gloss (specular angle) for smooth and rough surfaces in surface appearance measurements. The surface roughness contributes more off-specular intensity and reduces the specular intensity. Since commercial glossmeters measure only specular scattering, information related to roughness or scratch morphology that is contained within the off-specular scattering is absent.

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Fig. 2. (a) Photo of the incident laser beam, goniometric sample stage, and 2D detector, and the top view of the layout and optical geometry for the incident, 0~, and scattering, 0s, angles, respectively. A 2D scattering image is also presented here. (b) A diagram illustrates the surface appearance measurements for smooth and rough surfaces. The collection angular-range of scattered intensity in the gloss and haze measurements is also shown.

Gloss Measurements Gloss measurements were made using a hand-held commercial glossmeter (Minolta, MultiGloss model 268). Measurements conform to the ASTM D 523 standard measurement protocol. The reflectance area for 20" and 60 ~ gloss measurements was 9 mm x 9 mm, and 9 mm x 18 mm, respectively. The collection angle is + 0.9" from the specular angle, as illustrated in Figure 2b. All data presented in this chapter are the average of 6 measurements obtained from each sample. The estimated uncertainties in the gloss measurement presented are one standard deviation from the mean value for these 6 measurements.

APPEARANCE-BASED SCRATCH RESISTANCE STUDIES Measurement Protocol for Scratch Test Researchers from NIST and industry have proposed a methodology to quantitatively relate surface deformation (scratch morphology) to appearance attributes and, eventually, to quantitatively evaluate the scratch resistance of polymer coatings and plastics. This work was conducted through a NIST partnership with a number of industrial partners under the auspices of the Polymer Interphase Consortium (PIC) [18], and the research conducted within this consortium has generated a measurement protocol, called the Polymer Interface Consortium Scratch Test Protocol (PICSTP). A brief description of this measurement protocol follows: (1) A series of progressive-force scratch tests are used to impart a number of scratches coveting a range of severities; (2) LSCM (or a high-resolution optical microscopy) is used to characterize the resulting surface deformation and identify the chosen onset of scratch visibility; (3) constant force scratch tests are conducted at force levels both above and below the previously determined onset force (from Step 2) for plastic deformation; (4) LSCM is used to analyze the constant force scratches to precisely identify the force that corresponded to the onset of scratch visibility; (5) important scratch features such as scratch width, yield coefficient of friction, scratch depth,

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and residual depth at the onset of plastic deformation are identified from LSCM and scratch profilometer data; and (6) this quantitative assessment of scratch damage is then correlated with visual inspection and optical scattering (appearance) measurements.

Scratch Test for the PMMA System Figure 3 is an example of the PICSTP applied to PMMA. Figure 3a corresponds to the scratch profile produced by a progressive-force test using the 1 jam cone. The scratch load ranged from 0 mN to 30 mN over a total scratch length of 500 lam using a scratch velocity of 1 lam/s. Plastic deformation and a concave cracking pattern were observed along the scratch direction. This cracking pattern was typical of materials that have undergone brittle failure [ 19]. The onset of scratch visibility was determined to be very close in proximity to the onset of cracking. This deformation behavior is a consequence of the small diameter tip producing large contact pressures under relatively small forces. Nevertheless, the onset of the readily perceivable cracking pattern in PMMA can be determined both optically and through the instrumented scratch apparatus. The optically determined cracking onset is highlighted in Figure 3a. In Figure 3b, the scratch topography was determined using the 1 ~tm cone as a stylus, and the onset of cracking was defined as the initial increase of the residual roughness level above the undamaged surface roughness. Both methods resulted in similar onset points for the PMMA sample. Constant force tests (Figure 3c) at force values slightly above and slightly below the "critical load" were conducted to identify the onset point more precisely. The critical load was determined to be (3.8 + 0.2) mN.

Fig. 3. (a) LSCM image of a scratch on PMMA produced by a progressive-force test; the onset is defined (arrow) as the point where cracking occurred. (b) The residual roughness level measured from the progressive-force test in (a) along the scratch; the onset is defined by the significant increase in the residual roughness level above the undamaged surface. (c) Two scratches produced by constant force tests below (3 mN) and above (5 mN) the onset load.

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Scratching of materials and applications

Figure 4 shows the scratch penetration data generated by the instrument during progressiveforce scratch tests on PMMA. Variations in the penetration profiles resulted from "stick-slip" behavior corresponding to the formation of the cracking pattern that had been previously identified through optical inspection. The corresponding residual depth and scratch width that signal the onset of cracking were estimated to be (500 + 20) nm and (6 + 1) l.tm, respectively. At the onset point, the elastic recovery was determined to be 56 %.

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Fig. 4. Plot of penetration data recorded by the indentation instrument during the progressiveforce scratch tests described in Figure 3 for PMMA. The lower curve represents the penetration depth during the scratch, whereas the upper curve represents the unrecovered depth (residual depth) remaining just after scratching. The estimated uncertainty is one standard deviation (k= 1) in the data, which is about 3 %.

Scratch velocity is known to be a critical parameter in the creation of scratch surface damage (depth, width, and the onset force). Figure 5a shows the scratch damage obtained with a conical indenter at a constant 4 mN of normal force but at different scratch velocities. The measured scratch width was approximately 30 % wider at 1 larn/s than it was at 100 larn/s. The velocity dependence of scratch deformation demonstrates how the viscoelastic properties of a polymer influence scratch resistance. At the high velocities, the material is stiffer, and, thus, resulting plastic deformation is less. Similar trends were observed in the scratch and residual depth measurements when varying scratch velocities under different constant-force values. Figure 5b and 5c show the semi-log plot of scratch/residual depth and scratch width as a function of scratch velocity, respectively. The residual depth decreased from = 570 nm at 1 ~m/s to = 400 nm at 100 larn/s, and the elastic recovery changed from 56 % at 1 l.trn/s to 63 % at 100 ~m/s, for a constant scratch force of 4 mN. In both plots, a linear relationship (inversely correlated) was observed in the semi-log plot, i.e. depth (or width) oc log (velocity). The complex interplay between polymer viscoelasticity and the velocity dependence of scratch resistance illustrated the challenge in comparing scratch rankings when the scratch velocity is not constant between laboratories. Scratch rankings can be further convoluted when different tip geometries or an onset of deformation (cracking) is used to compare dissimilar materials. These issues are explored on the model PP materials in the next section.

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Fig. 5. (a) The scratch profiles of PMMA samples at 4 mN for five different scratch velocities. Semi-log plots of (b) scratch and residual depths and (c) scratch width vs. scratch speed for two different scratch forces. The lines are the linear fit to data in the semi-log plots. The error bars represent one standard deviation (k= 1).

Scratch Test for the PP System The PICSTP methodology applied to the PP samples using the same 1 ~tm radius conical indenter at a velocity of 1 larn/s is summarized in Figure 6. Noticeably, the scratch morphology of the PP system was quite different from the PMMA. In this case, a convex cracking pattern was observed. This scratch pattern is typical for tough materials such as polyolefins. When compared to the PMMA, the initial scratch damage was more severe and occurred at lower normal force values. The cracking onset force obtained from the LSCM image (Figure 6a) and the residual roughness level data (Figure 6b) of a progressive-force scratch test (0 mN to 30 mN) were difficult to determine, and were estimated at 1.2 mN and 1.8 mN, respectively. Subsequent constant-force scratch tests conducted at normal forces below the critical load, shown in Figure 6c, continued to generate both significant plastic deformation and cracking within the PP. Again, the high contact stresses due to the small diameter cone made the identification of the cracking onset force difficult to isolate given the available force resolution. A larger radius cone, 10 p,m radius, was used to better resolve the forces at which plastic deformation occurred and is shown in Figure 7. Although force resolution improved with this tip geometry, the onset of plastic deformation remained difficult to isolate from a progressive force scratch. The onset of cracking was shit~ed to longer scratch lengths with the larger radius tip. Onset values determined from LSCM images and the residual roughness level were restricted by instrumental limitations and are shown in Figure 7a and Figure 7b. Ultimately, a series of constant-force scratches using the 10 ~tm cone, shown in Figure 7c, provided better identification of the cracking onset near 5 mN. The critical force is affected not only by scratch velocity, but tip geometry. Therefore, it is reasonable to have deformation occur at a higher force for a larger radius tip. Additional information describing the elastic recovery of the PP was also measured. The deformation measured during testing with the 10 ~tm radius cone and the residual damage from both progressive and constant force tests are shown in Figure 8. The

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elastic recovery at low scratch loads (less than 3 mN) was almost 100 %. The recovery drops to 86 % at 4 mN and further to 48 % at 30 mN. Note that the progressive and constant force deformation data overlap quite well at the same scratch velocity.

Fig. 6. (a) LSCM image of a scratch (on PP) produced by a progressive-force test using 1 ~m indenter tip. (b) The residual roughness level corresponding to the scratch progressive-force test in (a) as a function of scratch distance. (c) Three scratches produced by constant force tests, below and near the onset load.

A comparison of the critical loads determined with the two different tips demonstrates the advantage of the constant-force scratch test method and the correct tip geometry selection to better identify the elastic-plastic transition. Determination of this transition is a challenge for experimentalists, because the onset of deformation should be unambiguous. In light of better force and displacement identification corresponding to the deformation onset, we have selected the 10 ktm 90 ~ cone as our standard scratch cone. This tip size and geometry affords us the greatest versatility for most polymeric materials evaluated to date. Further, we have designated the onset of scratch visibility, as determined by LSCM, to be our critical scratch criterion. We define this onset as the point at which 500 nm of residual depth exists in the scratch deformation. This has been selected for a number of reasons. Primarily, we are interested in appearance criteria, and 500 nm provides a general limit of what is perceivable to the human eye. Secondarily, we have selected this criterion because it provides an onset point that is independent of material-dependent transitions such as elastic-plastic or ductile-brittle. Severe deformation events such as fracture are highly variable depending on scratch parameters such as tip geometry, scratch velocity, force application, and polymer type. Lastly, the displacement, force, and cone tip geometry corresponding to the onset of scratch visibility presents an opportunity to define a critical strain or critical stress for appearance loss as other researchers have suggested [20].

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Fig. 7. (a) LSCM image of a scratch (on PP) produced by a progressive-force test at a velocity of 1 ~m/s using the 10 ~tm cone indenter tip. (b) The residual roughness level corresponding to the scratch progressive-force test in (a) as a function of scratch distance. (c) Three scratches produced by constant force tests: 2 mN, 4 mN, and 5 mN of scratch force. Scale bars represent 10 ~tm. Onset !

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Appearance Measurements The scratch protocol described previously was utilized to identify the general conditions under which scratch deformation is optically perceivable. Optical perception is the metric that coating suppliers and academic researchers have invested considerable effort in quantifying and predicting to improve a material's resistance. Coating researchers wish to answer the questions: "When is a scratch visible?" and "How may mechanical properties be tailored to minimize scratch and mar damage?" Currently, there has been no definitive link established between material properties, scratch geometry, and appearance. Efforts to develop this link are

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complicated because appearance is a visual measurement affected by many physical variables such as illumination (incident angle and wavelength, orientation of perception) scratch geometry, surface roughness, scratch density, gloss, and coating color. Recent efforts at NIST [11, 12, 21] to link surface morphology and subsurface microstructure to optical reflectance properties (appearance) in coated materials have shown promise. This appearance measurement methodology utilizes a ray scattering model and precise, multi-angle illumination of a coating to quantify the optical reflectance (related to gloss values) for a given surface morphology/microstructure. Applying this methodology, we can directly link the scratch morphology to optical reflectance properties, and ultimately to the visibility of a scratch. This section on appearance measurements demonstrates how light scattering techniques, such as these developed at NIST, are objectively able to characterize scratch damage by considering the above variables.

Surface Topography, Optical Scattering, and Gloss Measurements Figure 9 shows the 2D intensity projections of LSCM images taken from an EP coating series. The RMS surface roughness, Sq, calculated from the LSCM generated 3D topographic data are listed in Table 2. The RMS roughness value measured by LSCM was consistent with RMS roughness values determined from interferometric microscopy measurements [ 11 ].

Fig. 9. LSCM 2D intensity projection images of EP-coatings of two different magnifications: 5X (top row, 1842.7 ~tm x 1842.7 ~tm) and 20X (bottom row, 460.7 p.m x 460.7 ~tm).

One-dimensional (1D) angular-resolved optical scattering reflectance data at an incident angle of 20 ~ are presented in Figure 10. The gray shaded region indicates the specular scattering angular range measured by commercial glossmeters. Clearly, the rougher surface (EP 15) has a broader angular scattering profile and a lower specular scattered intensity. The surface roughness has been shown to be a major source of off-specular scattering [11 ]. Similar optical scattering results were obtained at the incident angle of 60~ for these two coatings, and the specular reflectance values were higher at 60 ~ than those values at 20 ~ [ 11 ]. The corresponding 20~ and 60 ~ gloss values of EP series are also listed in Table 2. Here the gloss values were obtained using a commercial handheld glossmeter, which collected only specular reflectance and excluded off-specular components. Therefore, the gloss measurements do not represent the total scattering from a rough surface as shown in Figure 10.

Metrology for characterizing the scratch resistance of polymeric coatings

115

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Table 2. The Root Mean Square (RMS) surface roughness, Sq, measured by LSCM for the EP coating series and their corresponding 20 ~ and 60 ~ gloss values. Error bars represent one standard deviation (k=l), estimated from 5 measurements for surface roughness data and 6 measurements for gloss measurements. Specimen

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Correlation between Surface Roughness and Gloss Measurements Figure 11 displays the 20 ~ and 60 ~ gloss values as a function of RMS surface roughness for two different measured surface areas: 5X objective (1842.7. ~m x 1842.7. lam) and 20X objective (460.7. ~tm x 460.7 ~tm). Figure 11 clearly demonstrates that specular 20 ~ gloss values correlate linearly to RMS surface roughness at a 20X field of view, see Figure 1 lb. This linear relationship is similar for the larger 5X field of view, although it fails for the roughest surfaces measured (EP15). Note that the larger 5X field of view is much closer to the area measured using a commercial glossmeter. Typically, 20 ~ gloss measurements are applied to high gloss surfaces (low surface roughness), which have a strong specular reflectance. In the steep slope of the curve (the linear part of Figure 1 l a), the differences between glossy samples are clearly measured, while in the fiat part (EP 15, higher surface roughness) the measurement geometry no longer correlates with visual observation. Therefore, the 20 ~ gloss measurement is not sensitive enough to distinguish between semi-gloss surfaces (when the 20 ~ gloss value is less than 20), and a 60 ~ measurement geometry is selected for a semi-gloss surface due to the higher specular reflectance at this angle. This phenomenon reflects a linear correlation between 60 ~ gloss measurements and RMS surface roughness throughout the EP series at the larger field of view (Figure 1 l c). A deviation from linearity for the 60 ~ measurements at the smaller field of view (5X) and smoothest EP surface is observed. This result highlights how the correlation between

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116

surface roughness and gloss measurements is a function of the size of the area measured and the optical geometry. 100

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Fig. 11. Comparison between RMS surface roughness values and 20 ~ and 60~ gloss values of EP-coatings for two different surface measurement sizes: 5X objective (1842.7 gm x 1842.7 gm) and 20X objective (460.7 gm x 460.7 gin). The lines represent the best fits for the data: polynomial fit to 20~ gloss values at 5X LSCM magnification and linear fit for the rest of plots.

However, gloss measurements quantify only the specular contribution to the roughness scattering intensity. There is still information about roughness present in the off-specular scattering intensity. The off-specular scattering contribution may be quantified by the ratio of off-specular intensity to specular intensity. For example, if the area under the off-specular scattering intensity (the intensity outside the gray area in Figure 10) is integrated and normalized by the specular scattering intensity (the equivalent 20 ~ gloss value from inside the gray area in Figure 10, a ratio of 69 % for EP45 (smooth) and 37 % for EP 15 (rough) coatings is calculated. Note that for these samples the 20 ~ gloss values decrease more than 30 %. Therefore, gloss values may represent the differences in surface roughness when the offspecular contribution is weak. When considering scratch damage on a surface, the off-specular scattering contribution becomes significant, and should be considered to evaluate surface damage.

Characterization of Damage from Mar or Multiple Scratches A high density of shallow scratches on a coating causes marring. Marring results in an overall reduction of surface gloss and, correspondingly, an increase in haze. Traditionally, mar is only quantified as either a damaged or undamaged surface from gloss measurements. This is an acceptable methodology for two main reasons. First, due to the inherent diffusive scattering from a marred surface, it is difficult to link an optical gloss value to the type and degree of mar damage (RMS roughness). Therefore, it is only possible to rank material performance after gross marring is observed. Second, for a coating exposed to light marring, the only method to visualize the scratches is by evaluating the off-specular reflection, as an observer would do by tilting a sample. Commercial glossmeters are not sensitive to this information, and therefore

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cannot discern light marring from an undamaged surface. In this section, the technique for characterizing mar and off-specular scattering is demonstrated to show that off-specular scattering is not required to characterize mar damage.

Fig. 12. LSCM images of PU coatings: (a) non-scratched surface (same for coating PU-A and PU-B), (b) PU-A, and (c) PU-B with multiple scratches created by 5 cycles of the AK scratch test. The bottom row represents the 2D optical scattering profiles (at 0~= 20 ~ 0s = 23~ and the numerical values under the scattering graphs are the 20 ~ gloss values measured using a commercial glossmeter. The boxes indicate the specular reflection region.

For this measurement, two different polyurethane coatings were tested. The modulus for PU-A is slightly lower than PU-B (see Table 1) as measured from instrumented indentation experiments. Single scratch measurements show that 500 nm of residual roughness occurs at 4.5 mN and ~ 6 mN of force using a 10 ~tm conical tip. Also, the friction coefficient of PU-A is higher (0.47) than PU-B (0.22) at 6 mN normal force. Figure 12 shows the surface morphologies (LSCM) and optical scattering profiles for two polyurethane clearcoats with and without multiple scratches. Scratches were generated through 5 cycles of an AMTEC Kistler (AK) scratch tester. LSCM scratch density images and 20~ gloss values confirm that the PU-A coating exhibited less damage than did the PU-B coating, i.e. the scratch resistance of PU-A is higher than PU-B coating. The correlation of surface roughness from multiple scratch damage to the gloss value for the PU coatings was examined. The RMS roughness at 5X of non-scratch PU, scratched PU-A, and scratched PU-B are (0.08 + 0.01) ~tm, (0.08 + 0.01) ~tm, and (0.11 + 0.01) lam, respectively. There was little change in RMS surface roughness and no strong correlation was found between RMS surface roughness, and 20 ~ or 60~ gloss values in the PU scratched coatings. For the PU coatings, the scratches do not affect the RMS surface roughness because they are small and shallow. Gloss loss is more likely due to the optical grating effects, which can shift scattering off-specularly and generate destructive interference and incoherent diffusive scattering at certain viewing angles. Diffusive and destructive intensity is observed in these conditions, as shown in the optical scattering profiles in Figure 12 and Figure 13 (the same offscattering intensity profiles at longer exposure time). It clearly indicates that PU-A has a higher

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scratch resistance ranking than PU-B, and the specular gloss measurements (20 ~ or 60 ~ gloss values for PU-A: 81.3 vs. PU-B: 64.8) are sufficient to properly characterize the scratch damage for a multiple scratch surface. While gloss is useful for mar measurements, a single, deep scratch on a surface provides unique challenges for the traditional gloss measurement.

Fig. 13. Off-specular optical scattering (at 0i = 20 ~ 0~= 24.5 ~ from scratched (a) PU-A and (b) PU-B at higher exposure time. (c) The relative optical geometry of the laser with respect to the scratch profile and orientation.

While gloss measurements were used to detect the specular reflectance changes in the multiple scratch surface (similar to a current field test), when the scratch intensity decreases and/or the scratches are shallow (ie. marring), the scattering intensity is weak in the specular direction, as shown in Figure 12. Thus, the specular gloss measurements are not able to differentiate mar resistance at low scratch densities. In the next section, the extreme case of a light, single scratch, which causes minimal changes in the specular reflection, is evaluated. In order to detect this type of scratch, the observer must tilt the sample to recognize the out-of-plane scattering intensity at a particular viewing angle.

Single Scratch Measurement At low scratch densities or for scratches that minimally scratch damage the surface of a coating, the full scattering profile caused by the damage is evaluated to accurately quantify scratch resistance. In this section, the scratch behavior of PP is discussed. Figure 14a shows the LSCM image of a scratch (only the initial 200 ~m) on PP produced by a progressive-force test using a 10 lam radius conical indenter tip. The scratch damage (visibility) increases with scratch force. As discussed previously, the LSCM scratch profile provides an estimate of the onset of plastic deformation, as shown in the circle of Figure 14a. As discussed earlier, the onset of plastic deformation was difficult to isolate for a progressive scratch test. A series of constant-force scratch tests was required to identify the onset of plasticity at ~. 4 mN at a velocity of 10 lam/s for a 10 ~m radius conical tip. A similar onset value is estimated from the scratch depth and residual depth profiles (Figure 14b). If the shaded area represents 100 % elastic recovery, then the onset of plastic deformation is clearly visible on the plot.

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Fig. 14. (a) LSCM image of a single scratch (only the first 200 ~m portion) on PP produced by a progressive-force test from 0 mN to 30 mN at a velocity of 10 ~tm/s using 10 ~tm indenter tip. The total scratch length was 500 lam. The visibility of the scratch increases with increasing scratch force, and (b) the scratch and residual depths as a function of scratch force for a progressive scan. The shaded area indicates the 100 % recovery area.

To link the onset of the deformation to the visibility of the scratch, the scattering profiles from two scratches were compared against the original non-scratched surface. Figure 15a shows the scratch morphology measured by LSCM of the unscratched surface (ns) and two 3 mm-long single scratches (sl and s2) in the PP sample. The corresponding optical scattering measurements are also included as Figure 15b. Scratch s l was made with a scratch force of 1 mN less than the onset load, while scratch s2 was made at a scratch force of 2 mN greater than the onset load. The relative optical geometry of the laser with respect to the scratch profile and its orientation are illustrated in Figure 13, for an incident angle of 45 ~. By visual inspection, scratch s l was hardly visible, while scratch s2 was readily visible. The 20 ~ specular gloss measurements of all three surfaces using a commercial glossmeter were indistinguishable, 56.4 + 1.0 for all three surfaces. However, the scattering profiles from the unscratched surface and two scratches are distinguishable (Figure 15b is specular, and Figure 15c is off-specular). Table lists the calculated scattered intensity for specular gloss intensity, and the total intensities from the scattering profiles shown in Figure 15b and Figure 15c. The specular gloss intensity was obtained by integrating the scattered intensity within the angular range of 45~ 0.9 ~. Similar to the results from the commercial glossmeter, there was little difference in specular gloss intensity for the three surfaces. In order to distinguish the difference in visibility of two scratches, the non-specular intensity must be considered. As shown earlier, scratch parameters, such as size, shape, depth of the scratch, pile-up, and roughness of the unscratched surface have a strong impact on the total scattered intensity distribution. The total scattered intensities listed in Table 3 of scratch s2 are greater than the values of ns and sl for both near-specular (0s = 43 ~ ct = 0.5 ~ and off-specular (0s = 40 ~ a = 0.5 ~ configurations. Here, 0~ and ct are the scattering angle (as defined in Figure 2) and the out-of-plane scattering angle, respectively. This preliminary

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Scratching of materials and applications

result indicates that the onset of a visible scratch may be resolved from optical scattering experiments that account for both specular and off-specular scattering intensity. Current research has been dedicated to replicating these measurements on scratches with different features that affect scratch visibility, such as surface roughness, subsurface microstructure, and color.

Fig. 15. (a) LSCM images of the non-scratch surface (ns) and two single scratches (sl, s2) of a PP sample and their corresponding scattered intensity patterns from (b) at near-specular (0s = 43 ~ a = 0.5 ~ configuration and (c) at off-specular (0s = 40 ~ or= 0.5 ~ configuration. Here 0s and ct are the scattering angle and the out-of-plane scattering angle, respectively.

Table 3: The total scattered intensity from the unscratched surface (ns) and two single scratches (sl, s2) in Figure 15b and Figure 15c. Error bars represent an estimated standard deviation (k=l Gloss intensity* Total intensity from Total intensity from (b) (specular angle __+ (c) (x 106 counts) Location 0.9 ~ (x 106 counts) (x 10 6 counts) 2.20 + 0.06 6.91 __+0.06 0.15 _ 0.04 ns sl 2.16 + 0.06 6.92 + 0.06 0.10 + 0.05 0.38 + 0.05 s2 2.14 + 0.06 8.80 + 0.06 *Gloss intensity was obtained by integrating the scattered light intensity from the scattered profile in Figure 15b within the angular range 45 0__+0.9~ This value is similar to a specular gloss measurement. ..,

,

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SUMMARY AND FUTURE DIRECTIONS The purpose of this chapter was to demonstrate how optical scattering provides the critical information needed to determine the appearance-based scratch resistance of polymeric materials. Characterization of the scratch resistance of polymeric materials included optical scattering experiments that distinguished the severity of the damage while accounting for the impact of surface roughness, gloss, damage type (mar or scratch) over the full 2D angular space. The optical evaluation, including deformation pattern, scratch width/depth, and the transition from elastic to permanent deformation, was determined for PMMA and PP commercial samples using a proposed measurement protocol (PICSTP). The deformation patterns observed in each material were quite different. The critical forces for plastic deformation and cracking were determined to depend on both the scratch velocity and the tip shape. The use of a 10 p.m 90 ~ cone provided the best onset force and displacement resolution for the model materials evaluated. LSCM was used to identify the position along the scratch that yielded a residual depth of 500 nm. The defined onset of scratch visibility provided a perception-based limit that was independent of deformation mode and provided the critical force required to distinguish the severity of the scratch using optical scattering.

Fig. 16. Schematic of the iterative methodology used in the appearance based scratch resistance metrology under development through the NIST/Industry Polymer Interphase Consortium.

The research efforts of the NIST/Industry Polymers Interphase Consortium continue with a second phase and the correlation of appearance-based metrology with mechanical properties and modeling. Figure 16 illustrates a schematic of the iterative design used to determine the material structure-property relationships that govern the appearance-based scratch resistance. Material and mechanical characterization utilizing instrumented indentation have been used to measure material properties such as elastic modulus, hardness or yield stress, visco-elastic properties [22,23], and frictional coefficients over length scales that are relevant to the scratch resistance

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and material heterogeneity. These research results will be reported in the literature at a later date. Future research will expand to include polymeric materials and their appearance-based scratch resistance as a function of environmental aging.

ACKNOWLEDGEMENTS The authors gratefully acknowledge funding support from the NIST-Industry Polymer Interphase Consortium (PIC). PIC Industrial members include: Visteon Corporation, Dow Chemical, PPG industries, MTS system Corporation, Arkema Inc., and Eastman Chemical. The authors give special thanks to PPG industries and Dow Chemical for providing the samples reported here.

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