The last few years have witnessed the extensive development of methods used for the characterization of mechanical properties of automotive polymeric clearcoats, [1-3] eventually resulting in the establishment of a dedicated ASTM standard  which is based on the Nano Scratch Tester.
This standard follows on from earlier methods employed in the evaluation of mar and scratch, where the objective was to physically scratch a surface and visually inspect it to assign a ranking. Although useful, some techniques do not have the reproducibility and accuracy of the nano scratch test method, which allows investigation of the relationship between damage shape and size, and external input (such as applied load, indenter geometry, speed, etc.)
Mar resistance characterizes the ability of the coating to resist damage caused by light abrasion. The difference between scratch and mar resistance is that mar is related only to the relatively fine surface scratches which deteriorate the appearance of the coating. Therefore, mar resistance can be directly related to the level of gloss retention in the coating after exposing it to harsh environmental conditions. Typical conditions include salt and acid rain exposure, daily and seasonal fluctuations of humidity and temperature, road grit and car washing. Standard nano scratch testing techniques have focused on simulation of mar-type damage, which comprises of shallow depths (typically < 10 µm) and small width scratches (typically < 10 µm), which both require small indenter radii (typically < 5 µm) to achieve.
The scratch test method is very useful for the characterization of mar-type damage, but can also be employed to simulate other types of damage experienced by an automotive topcoat during the lifespan of the vehicle. Such types of damage might be occurred when keys or jewelry are scraped across the surface, or due to large particle impact (gravel from the road), both of which lead to large-scale damage that can be easily noticed regardless of paint color and light level. Larger sized spherical indenter tips and higher applied loads are required for simulation of larger scale damage.
This article discusses the influence of indenter tip radius on the scratch resistance by changing tip radius and examining the resultant differences in measured signals and the effect on the physical damage of the polymeric coating, especially the critical point at which fracture of the coating takes place. It is a widely known fact that the critical point can rely on the temperature, scratch speed, the amount of deformation (strain) and the deformation history. The indenter tip produces a complex distribution of strains and stresses around its contact which are directly affected by the shape and size of the tip and the rheological properties of the polymer.
ASTM D7187 describes the onset of fracture as the point where the tangential force, normal force and penetration depth start to fluctuate uncontrollably, which is also commonly referred to as the critical failure load (Lc1). However, it is a challenging task to determine the exact fracture point in some polymer topcoats due to the particular formulation and the way that fracture initiates. Observing the normal load measured during the post-scan phase of the test is one way to “amplify” this transition from plastic deformation to fracture.
The post-scan comprises of running the indenter along the scratched track with a very low load to determine the residual depth remaining after the scratch test. This normal load signal, called FnP, can serve as a good indicator of fracture even when no marked transition is observed for the depth signals. Figure 1 illustrates a progressive load scratch on an automotive polymer topcoat where the FnP signal is a good indicator of coating fracture.
Figure 1. Nano Scratch Tester results for a progressive load scratch (0.1 – 15 mN) on a GEN III automotive polymer topcoat showing the penetration depth (Pd), residual depth (Rd) and normal load during post-scan (FnP) signals. The onset of fracture (Lc1) is clearly visible (shown here as a dotted line) and corresponds exactly with the optical micrograph shown of the scratch around the fracture point.
Experiment and Results
To study the effect of indenter tip radius on the scratch resistance, a polymeric topcoat material of known mechanical properties needs to be selected. Here, a common automotive polymeric topcoat called GEN III was used. Upon scratching with a 2 µm radius indenter, this material shows a very reproducible critical fracture point at ~ 9 mN, thus making it an ideal candidate for comparative testing. It is interesting to note that at Anton Paar Applications Lab, a sample of GEN III stored under ambient conditions for more than 10 years still provides exactly the same critical fracture point. Therefore, it can be concluded that this material remains stable after initial curing if not exposed to significant temperature fluctuation and ultraviolet radiation.
It was decided to cover indenter tip radii from 2 µm up to 50 µm, which is the widest range of radius possible with a single machine (here, the instrument was the Nano Scratch Tester with an applied load range of 0.01 – 1000 mN). By trial and error method, the ideal load range was found to be 1 – 600 mN for the 5 indenter geometries and a complete scratch made with each is illustrated in Figure 2. Initial observation of these scratch tracks indicates the presence of two different modes of failure: chevron-cracking along the sides of the scratch and the onset of full fracture within the scratch. The best example of these two modes is the 10 µm indenter radius scratch in Figure 2C, where cracking initiates at 51 mN and full fracture at 184 mN.
Figure 2. Scratches made on a GEN III polymer topcoat with the following spherical indenter radii; (a) 2 µm; (b) 5 µm; (c) 10 µm; (d) 20 µm; (e) 50 µm. Scratch test conditions were identical in each case, namely applied load 1 – 600 mN, speed 0.5 mm/min., loading rate 300 mN/min. and scratch length 1 mm.
Both failure modes can also be observed in a 5 µm indenter radius scratch, where cracking initiates at 28 mN and full fracture at 52 mN. This transition cannot be observed for the smallest radius (2 µm) due to the occurrence of fracture soon after the initiation of the scratch. However, standard testing of this material with a 2 µm indenter revealed that failure (both cracking and fracture) takes place at ~ 9 mN. Only cracking can be seen for the larger radii indenters: at 146 mN for the 20 µm indenter radius and 492 mN for the 50 µm indenter radius.
It is interesting to note that the cone angle for all the indenters used in the range of 2-50 µm was 90°, meaning once the penetration depth of the scratch is more than the indenter radius then the influence of the sidewalls of the cone will be more on the material deformation than the actual tip radius. However, the sidewalls did not show any influence on the first critical failure point (Lc1) because the penetration depth at Lc1 did not exceed the indenter radius (Figure 4). The plotted data in this graph shows a linear dependence between indenter radius and penetration depth at Lc1.
During a scratch test, the strain rate can be described as the ratio V/a, where a = Scratch width and V = Scratch speed. Since the scratch speed was kept constant in all tests (0.5 mm/min., or 8.33 µm/s), the strain rate decreased from 2.16 s-1 for the 2 µm indenter, to only 0.11 s-1 for the 50 µm indenter. This evolution is also plotted as a function of indenter radius in Figure 4.
This significant reduction in strain rate helps to explain why there is less damage with the 50 µm indenter compared to the 2 µm indenter. This knowledge is helpful to understand one of the key characteristics of automotive polymer coatings, which is the dependence of their behavior on the applied strain rate.
Figure 3. Cross-section through a spherical indenter of radius r which is scratching through a material to a depth h. The width of the scratch is therefore given by the distance 2R.
These viscoelastic characteristics of the polymer have a great influence on how the material deforms under the indenter tip, and how it recovers after the deformation has taken place. The scratch width (a) is derived using the following equation where a = 2R:
R2 = r2 - (r - h)2
Figure 3 shows the parameters, which are defined based on the spherical indenter radius (r) and the scratch depth at the first critical load (Lc1), given by h.
Figure 5 presents overlay plots for the Pd, Rd and Ft signals for the 5 scratches shown in Figure 2. The penetration depth overlays (a) seem to scale with the indenter radius with the minimum depth from the 50 µm and the maximum depth from the 2 µm radius. The same trend is detected with the residual depth plots (b) with the sharpest indenter producing the noisiest residual profile due to the drastic increase in damage caused to the surface.
Figure 4. Penetration depth at the first critical failure point (Lc1) plotted against indenter radius for the 5 tested geometries. The corresponding strain rate at Lc1 is shown as a dotted line.
These plots make the most sense compared to the micrographs presented in Figure 2, where the onset of damage correlates well between observed fracture and the recorded signals. The friction force overlays (c) rank the 2 µm indenter with the highest frictional component and the 50 µm indenter with the lowest. This makes senses when it is considered that the sharper indenters are scratching far deeper than their actual radius and thus the influence of the cone above the actual radiused tip is significant. For example, for the 2 µm radius, the penetration depth at maximum load is approximately 40 µm so there is a much larger contact area from the indenter sidewalls than from the tip. Incidentally, all 5 indenters used in this study had a cone angle of 90°, which is the standard angle for scratch testing on polymeric coatings as described in ASTM D7187 .
Figure 5. Overlays of (a) Penetration depth (Pd), (b) Residual depth (Rd) and (c) Friction Force (Ft) for the 5 tested indenter geometries. These data sets correspond to the scratches shown in Figure 2.
Figure 6. Scratches made on a GEN III polymer topcoat with the following spherical indenter radii and contact conditions; (a) 50 µm radius, 0.02 - 5 N applied load; (b) 100 µm radius, 0.02 - 5 N applied load; (c) 200 µm radius, 0.9 - 30 N applied load; (d) 500 µm radius, 0.9 - 100 N applied load.
To support the scratch tests illustrated in Figure 2, the same GEN III sample was subjected to some additional tests using indenters with radii from 50 - 500 µm and loads large enough to induce failure in the coating. Both the Revetest (load range up to 200 N) and the Micro Scratch Tester (load range up to 30 N) were utilized for such a broad range of contact conditions. Adequate load was applied with the 200 µm and 500 µm indenters to delaminate the polymer coating stack (topcoat and primer) from the underlying steel substrate. Figure 7 shows a typical example of a data set for the 200 µm indenter, where Lc1 corresponds to the onset of cracking in the coating and Lc2 corresponds to full delamination of the stack. This Lc2 value is in line with a sudden decrease in the penetration depth and an increase in the friction force. Since the indenter has reached the steel substrate, the Pd signal flattens off at higher applied loads (around 23 N).
Figure 7. Revetest scratch tester results for a progressive load scratch (0.9 – 30 N) on a GEN III automotive polymer topcoat showing the penetration depth (Pd), residual depth (Rd) and friction force (Ft) signals. The onset of fracture (Lc1) and full delamination (Lc2) points are clearly visible (dotted lines) and correlate with the optical micrograph shown of the entire scratch in Figure 6 (c). Indenter radius is 200 µm.
- J. L. Courter, Mar Resistance of Automotive Clearcoats: Relationship to Coating Mechanical Properties, J. Coating Tech., 69 (1997) 57-63
- L. Lin, G. S. Blackman and R. R. Matheson, A new approach to characterize scratch and mar resistance of automotive coatings, Progress in Organic Coatings, 40 (2000) 85-91
- M. Osterhold and G. Wagner, Methods for characterizing mar resistance, Progress in Organic Coatings, 45 (2002) 365-371
- ASTM D7187: Test method for measuring mechanistic aspects of scratch/mar behavior of paint coatings by nanoscratching
This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.
For more information on this source, please visit Anton Paar TriTec SA.