Some complementary scratch resistance characterization methods.

In the characterization of mar/scratch resistance of coatings/materials with single-probe techniques, the probe usually scrapes the tested surface under a constant load or increasing load, followed by dimensional measurements of the scratch, identification of the critical force at which cracking/fracture occurs, or evaluation with optical instruments. Due to a variety of coatings/materials properties and various application requirements, the single-probe method may not provide pertinent characterization of coatings/materials in their applications in some cases. Three complementary test methods are presented in this article: i.e., a crack density measurement used for fragile thermoplastic olefin (TPO), a repeated scratching test, and a cross-scratching test used for glazing materials for automobile windows.

Keywords: Atomic force microscopy, dynamic mechanical properties, hardness, scratch resistance, surface analysis, adhesion, chip resistance, mechanical properties, physical properties, automotive-OEM, coatings-substrate interface, plastics

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Mar/scratch resistance is an important and highly desired property of coatings in many applications, e.g., the polymeric clear topcoats used on automobiles and the hard protective coatings used in information storage systems. Consequently, reliable laboratory test methods that can predict the performances of coatings against marring/scratching in their real-field applications and direct their further improvement have been sought. With the development of micro and nano instruments, starting in the mid-90s, single-probe testing techniques have been widely used. Some researchers used the atomic force microscope (1-9) to carry out the tests, and others used the commercial Nano-indenters and nano-scratchers, such as Nano-Indenter XP made by MTS Systems Corporation and Nano/Micro Scratch Tester made by CSM Instruments, (10-15) or a variety of customized devices. (16-29) In these tests, the single probe scratches the surface of the tested samples under preselected conditions, followed by an evaluation with scanning probe microscopy, optical instruments, or other methods, which makes it possible to study marring/scratching mechanisms under different test conditions, and correlates the mar/scratch resistance of the tested coatings/materials with their physical and chemical properties.

Morphology of scratch damage is diverse. It is classified and characterized by researchers in different ways. For example, ASTM Subcommittee D01.23 suggested classifying the scratch damage of coatings into two categories: plastic flow and fracture. In the former case, a scratch is made under a 5 mN constant load, and then the depth is measured and used to characterize the coating's resistance to plastic flow. In the latter case, a scratch is made under an increasing load, and the critical force at which the plastic flow transitions to the fracture is used to characterize the coating's resistance to fracture. (30,31) Some researchers also use critical load I, at which the first crack occurs, and critical load II, at which the first severe cracking and delamination occur, to characterize the scratch resistance of coatings. (28,32) A more comprehensive classification may divide the damage into five modes: mars, rough troughs, cracking, delamination, and chipping. (33) Mars are light damage, i.e., narrower and shallower scratches. They are usually fairly neat, consisting of a ditch with a smooth bottom and two, if any, well-shaped shoulders on both sides of the ditch. Thus, the micro mar resistance (MMR), defined as normal load applied during the scratching divided by the cross-section area of the ditch, can be used to characterize the coatings' ability to resist light damage. Scraping the surface with an increasing normal force, the bottom of the ditch as well as the ridges of the two shoulders, becomes rough. The neat mar becomes a rough trough. MMR is no longer an appropriate characterization since the cross-section area of the ditch, used in the calculation of MMR, begins to change erratically along the rough trough. As the normal force increases further, cracking may show up in the surface of the coatings. Under the continuously increasing normal force, delamination may take place if the penetration depth of the tip reaches the interface and the stress generated by the scraping tip exceeds the interfacial adhesion strength. Increasing the force further may result in the delaminated top layer being chipped off, piece by piece, from the surface. To characterize the coatings' ability to resist medium to severe damage, the critical force for rough trough, i.e., the load at which a neat mar transitions to a rough trough, as well as the critical forces for cracking, delamination, and chipping, if any, are used. Combined, the measurements of MMR and critical forces may provide a full spectra of mar/scratching resistance characterization of the materials. However, due to the variety of materials' properties and application conditions, the above-introduced characterization methods might not be appropriate in some cases. Some complementary test methods are described below.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Crack Density Measurement

Thermoplastic olefin (TPO) has experienced increased use as an interior and exterior material in automobiles in recent years. The mar/scratch resistance is important in its applications. In the beginning, the TPO components used on automobiles were coated with primer, colored basecoat, and topcoat. The mar/scratch resistance of the topcoats on TPO was extensively studied, and several useful characterization methods were proposed. (7,25,26,34) To simplify the process, mold-in-color TPO was developed later to replace the coated TPO. Testing the mar/scratch resistance of the mold-in-color TPO is a challenging task. The surface of the molded TPO is fairly rough, when compared with the surface of the clear topcoats, with a height fluctuation up to several hundred nanometers, which is equivalent to the depth of a majority of mars. Due to the roughness of the surface, it is not always feasible to make a neat mar (shallower and narrower scratch), measure its cross-section area, and calculate the micro mar resistance, as discussed above. On the other hand, TPO is a fragile material and is easily damaged. (21,35) Due to its low tensile strength and modulus, the edges of the indentation or plowed up region often crack as soon as the tip sticks into the surface, sometimes even before the scraping, which makes identifying the critical force impossible.

To characterize the mar/scratch resistance behavior of TPO, a complementary method was proposed, in which a Nano-indenter, equipped with a 90[degrees] conical-shaped diamond tip with a radius of 1 [micro]m at its apex, was used to scrape the surface of TPO under a constant normal load of a couple of millinewtons. After the scraping, a scanning probe microscope (SPM) was used to examine the scratch. Under the applied tensile stress on both sides of the ditch generated by the scraping tip, cracks initiate from the sides of the scratch, and propagate backward at an angle. Depending on the properties of the material, the density of the cracks varies, which is an indication of the material's toughness against fracture. Figure 1 is a 25 x 25 [micro][m.sup.2] image of a scratch made under 3 mN normal force at a scraping speed of 5 [micro]m/sec on the surface of a commercial mold-in-color interior automotive polypropylene, showing the cracks distributed on both sides of the ditch. About two dozen TPO samples with various components, and prepared with different processing procedures, have been examined using this method. The average density of cracks of the samples, obtained from about a dozen images examined for each, varied from about eight cracks per 25 [micro]m to about 16 cracks per 25 [micro]m under the above test conditions.

The measured density of the cracks depends on the properties of the samples, as well as the test conditions. A study of the effect of applied normal load and scraping speed on the density of cracks was carried out at the surfaces of three interior TPO samples of varying color, with colorant A labeled as E, colorant B labeled as F, and natural labeled as G. Three normal loads, 2 mN, 3 mN, and 4 mN, and three scraping speeds, 1 [micro]m/sec, 5 [micro]m/sec, and 25 [micro]m/sec, were used in the study.

As the normal load increased, the density of cracks increased. For example, as the normal force increased from 2 to 4 mN while the scraping speed was fixed at 1 [micro]m/sec, the density of cracks at the surface of sample E increased and the cross-sectional area of the ditch almost doubled under the larger normal force of 4 mN. Figure 2 shows a pair of images of the scratches made at the scraping speed of 1 [micro]m/sec on the surface of sample E, under a normal load of 2 mN and 4 mN, respectively. The preliminary explanation is that as the normal load increases, the scraping tip penetrates deeper, pushing more material in front of it, thus creating deeper cracks. The tensile elongation to failure is likely exceeded more readily under the higher applied stress generated by a 4 mN load than for the shallow cracks made at the top layer of the surface under the lower applied stress generated by a 2 mN load.

As the scraping speed increased, the density of cracks increased significantly for all three tested samples. For example, as the scraping speed was increased from 1 [micro]m/sec to 25 [micro]m/sec while the normal load was fixed at 2 mN, the density of cracks at the surface of sample E increased about 35%, although the cross-section area of the ditch was decreased by 30% at the higher speed of 25 [micro]m/sec. Figure 3 shows a pair of images of the scratches made under a normal load of 2 mN on the surface of sample E, at a scraping speed of 1 [micro]m/sec and 25 [micro]m/sec, respectively. The preliminary explanation is that as the speed increases, the strain rate applied to the material is increased. At higher strain rates the polypropylene material at the surface has lower tensile elongation at failure under the same applied stress, and it will tend to crack more readily than it will flow. Since the scraping tip "resides" at each point less at the higher speed and elastic modulus of the material is also increased, a shallower and narrower ditch is formed, which is the same as in the indentation test. The longer the tip holds under the maximum load before unloading, the deeper the indent depth will be. Rate dependent deformation of polypropylene, as well as other semicrystalline polymers and glassy polymers, has been characterized by several researchers. (36) It was found that the slower strain rate resulted in more plastic deformation, which was consistent with the results found here.

Table 1 lists the widths and depths of the scratches made at the surface of sample E under the different loads and at the different speeds.

When comparing the mar/scratch resistance of tested samples using the crack density method, the test conditions have to be considered, the same as using the other single-probe techniques.

[FIGURE 3 OMITTED]

Repeated Scraping Test

In the development of a new glazing system to replace glass in automobile side windows, consider the fact that hard dust particles may be trapped between the window and the rubber seals, resulting in repeated mar and scratch in the same trench at the surface when the window is rolled up and down, causing major damage to the side windows. This repeated scraping test method was proposed as a complementary way to characterize mar/scratch resistance of the samples of side windows.

[FIGURE 4 OMITTED]

In the test, the Nano-indenter, equipped with a 90[degrees] conical-shaped diamond tip with a radius of 1 [micro]m at its apex, was used to scrape the sample surface along the same trench with a relatively low and constant load for a selected number of times. After scrapings, images of the scratches were taken using the SPM to characterize the morphology, and the depths of the scratches were measured. The test results of two glazing materials, labeled as A and B, potential candidates for windows, are presented here as an illustration of the test method. They are polycarbonate-based, multilayered composites. Transpar-ent polycarbonate is much lighter than glass, thus reducing the weight of vehicles and improving fuel efficiency as well as lowering the center of gravity of vehicles and improving the stability. In addition, polycarbonate possesses high impact resistance. To prevent the polycarbonate from turning yellowish in color under UV, a siloxane coating was used in the composites. To improve the mar/scratch resistance, an inorganic-organic hybrid coating about 3 [micro]m thick, containing Si, O, H, and C, was applied by plasma-enhanced chemical vapor deposition (PECVD) to the top of the composites. Samples A and B had slightly different components in their PECVD coating, and were prepared in slightly different procedures. The test was used to distinguish the difference in their performance and facilitate further improvements.

In the test, the surfaces of sample A and sample B were scraped along the same trench under a constant load of 5 mN repeatedly for 5, 10, 20, 40, and 60 times, respectively, at a scraping speed of 20 [micro]m/sec. After the scrapings, the images of the scratches were taken by SPM for a morphology study, and the depths of the mars were measured. The depths of the mars after 5 and 10 scrapings at the surface of A were 60 nm and 90 nm, respectively; the depths of the mars after 5, 10, and 20 scrapings at the surface of B were 100 nm, 125 nm, and 135 nm, respectively, before the damages transitioned to a rough trough. The depths of the mars at the surface of A were shallower than those at the surface of B after the same number of scrapings, which was an indication that sample A was harder than B at the top layer. It was confirmed by the micro indentation hardness test performed afterwards that A, under a light normal force of 1.5 or 3.0 mN, was harder than B by about 35%.

The images of the scratches showed that the damage to sample A belonged to the category of "mar" after 5 or 10 scrapings, then became a "rough trough" after 20 scrapings, and then chipping took place after 40 or 60 scrapings. In contrast, the damage to B stayed in the category of "mar" beyond 20 scrapings. However, when the chipping occurred after 40 and 60 scrapings, the damage at the surface of B was more severe than that at the surface of A and the chipped spots were deeper and wider. The selected images for illustration are shown in Figure 4. The results indicate that while B is softer, it is more ductile, so it stayed in the "mar" category for more cycles. On the other side, sample A is hard, but it may be brittle and was vulnerable under the repeated scrapings, and the damage transitioned to rough trough earlier. The results are consistent with the results of the micro mar resistance and critical force measurements of A and B. Under the light loads, MMR of sample A is better than B, since B is soft and ductile, and the mars in the surface of B were deeper and wider. However, the critical force of transition from mar to rough trough of sample A was lower than B, since A is brittle, and its surface is easily broken under the increasing load.

[FIGURE 5 OMITTED]

Examining the dimensions of the chipping further suggests that the vulnerable layer of sample A, where the cohesion was weak and was easily broken and chipped off, might be in the depth of 600-800 nm, while B might be in the depth of 1.4-1.8 [micro]m. When the chipping took place in both surfaces, B suffered more severe damage than A.

Cross-Scratching Test

For materials that will be subjected to the cross-scraping in applications, the cross-scratching test can provide a proper measurement of their mar/scratch resistance behavior in the applications. For example, if the glazing materials mentioned above are used for wiped automobile windows (backlites), the area swept by rubber wipers may be cross-scratched by other abrasive actions like hard dust particles trapped between the rubber blade of a second wiper and the material's surface, or by independent scratches not parallel to the wiping pattern.

The scraping pattern of the cross-scratching test is similar to the Taber abrasion test, which is commonly used in industry. In the Taber test, the abrasion is produced by the contact of a test sample, turning on a vertical axis, with the sliding rotation of two abrading wheels, and an abraded ring-shaped area formed by crossed scratches results. The most severe damage occurs at the intersections of the crossed scratches. (37) Thus, the cross-scratching test with the Nano-indenter and SPM could be useful in providing more detail regarding surface and layer properties which influence the results of the Taber abrasion test.

[FIGURE 6 OMITTED]

Samples A and B were again used as examples to describe the cross-scratching test. The 1000 [micro]m long scratches were made with the Nano-indenter, equipped with a 90[degrees] conical-shaped diamond tip with a radius of 1 [micro]m at its apex, under a linearly increasing force from 0 to 30 mN at a scraping speed of 20 [micro]m/sec. Two groups of a total of 20 scratches were made in a pair of orthogonal directions, which made a matrix with a spacing of 100 [micro]m, as shown in Figure 5. After the scratching, all the intersections were examined by the SPM, and a mark "x" was used to indicate the observation of chipping at the intersections.

There were more chipped intersections in the surface of A than in the surface of B. The chipping began to take place under lighter loads in surface A than in surface B. However, as soon as the chipping began to take place at the intersections in surface B, the damage was more severe than that of surface A. Figure 6 shows two pairs of images. One was taken at the intersection of (12 mN, 15 mN), where the chipping took place only in the surface of A; the other was taken at the intersection of (27 mN, 24 mN), where the chipping occurred in both surfaces, and the chipped spot in the surface of B was bigger and deeper. The results give further support for the repeated scraping test described above. Due to the brittleness of the top layer of sample A and relative ductility of sample B, the intersections in the surface of A began to chip first. Under the increasing loads, chipping took place at the intersections of surface B, as well. The vulnerable layer of B is deeper, thus the chipped pieces in the surface of B are thicker and bigger than in A. Again, the chipping resulted from the cohesive failure of the 3 [micro]m thick top coating in both sample A and B.

SUMMARY

To better characterize the mar/scratch resistance of coatings/materials that possess different properties and are used in different environments, it is necessary to continuously develop new test methods to fit application needs. The presented crack density measurement method, in which the tested sample surface is scraped by a single probe under a constant load and a subsequent crack density measurement is made, is a better way to characterize mar/scratch resistance of fragile materials, such as TPO. The density gives a reliable measurement of the materials' toughness against fracture during scratching. Repeated scratching tests and cross-scratching tests are appropriate for the coatings/materials that will be subjected to repeated scratching and cross-scratching in their applications. It must be noted that the test conditions, i.e., applied load, scraping speed, geometry of the probe, etc., affect the test results primarily due to the rate dependence of polymer properties. In comparison of mar/scratch resistance of different samples, the test method selected should be similar to the particular application and the test conditions, i.e., applied load, scraping speed, geometry of the probe, etc., should be kept the same for the different samples. The scratching damage essentially depends on the applied stress, and the resulting strain and its rate, generated by the scraping probe. However, applying the same controllable test conditions mentioned above in the tests is an objective way for comparison.

While the nano instruments provide accurate measurements in forces, displacements, etc. in micro and nano scale, one of the critical shortages in the mar/scratch tests is their limited scraping speed, which is much slower than the encountered scraping speed in the real application field. We used a speed of 20 [micro]m/sec in the repeated scratching and cross-scratching tests based on the data acquisition rate of our indenter.

ACKNOWLEDGMENTS

This work was supported by Visteon Corporation and Exatec, LLC, as members of the National Science Foundation University/Industry Cooperative Research Center in Coatings at Eastern Michigan University (NSF U/I CRCC at EMU).

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Lan Mi, Hao Ling, and Weidian Shen ([double dagger]) -- Eastern Michigan University*

Rose Ryntz and Beth Wichterman -- Visteon Corporation ([dagger])

Alex Scholten -- Exatec GmbH & Co. KG**

* Surface Science and Nanotribology Laboratory, Ypsilanti, MI 48197.

([dagger]) 401 Southfield Rd., Dearborn, MI 48121. Dr. Ryntz is now affiliated with Collins and Aikman, Southfield, MI.

** Friedrich-Ebert-Strasse, D-51429 Bergisch Gladbach, Germany.

([double dagger]) Author to whom correspondence should be addressed. Email: [e-mail]; fax: [phone].

Table 1 -- Dimensions of the Scratches Made at the Surface of Sample E
Under Three Different Loads and Three Different Speeds

Width ([micro]m) and Depth ([micro]m)
of Scratches Under Different
Loads and Different Speeds             2 mN      3 mN       4 mN

 1 [micro]m/sec                        W = 8.44  W = 10.31  W = 11.88
                                       D = 1.30  D = 1.58   D = 1.92
 5 [micro]m/sec                        W = 8.13  W = 10.00  W = 11.38
                                       D = 1.13  D = 1.37   D = 1.65
25 [micro]m/sec                        W = 7.50  W = 9.38   W = 10.94
                                       D = 0.97  D = 1.14   D = 1.40