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EVALUATION AND QUANTIFICATION OF SCRATCH
The great attention paid on aesthetics of polymer surfaces in recent years has led to significant interests in the scratch resistance and visibility of polymers. To this end, reliable methods to quantify scratch resistance and visibility become the top priority. The methods used in the quantification of scratch properties are mainly grouped under two categories depending on the parameter that is being measured. They are the measurement of physical dimensions, i.e., scratch depth and width, and surface reflectivity. This chapter will focus on discussing the methods available to achieve the above measurements. Current developments on quantification of surface damage and scratch evaluation will also be addressed.
The Surface Phenomena of Scratch：
A paper titled “The hardness of poly(methylmethacrylate)” (PMMA) published by Briscoe et al. [9] in 1996 proposed the basic theoretical background in analyzing scratch on polymers. Using the work done on metals by Bowden and Tabor [50],
Briscoe et al. investigated the scratch properties of PMMA. Conical steel indenters of semi-angle ranging from 60° to 150° and at different loads were used to perform scratch on PMMA. The paper presented a scratch map of PMMA, whereby the different damage mechanisms were delineated in the map. The following mechanisms were recognized: 1) ductile ploughing, 2) viscoelastic-plastic ploughing, 3) brittle cracking, 4) brittle deformation and 5) machining.
The ploughing process is also sensitive to many other factors, such as rate and temperature that further complicate the effort to predict such phenomena. Cutting and fragmentation are modes of material removal. Cutting produces ribbons of material in front of the scratching tip and is associated with ductile failure; whereas machining or fragmentation1 produces fragmented debris from the substrate and is associated with brittle failure [51].
By measuring scratch hardness and indentation hardness, the authors were able to
discern a linear relationship when conical angle is high. The effect of lubrication was also investigated and was found that lubrication increased the scratch hardness but decreases the indentation hardness. The scratch map that was the result of this work is useful in predicting the type of damage that might occur under different conical angles and load.
Other attempts by researcher to classify the different scratch behaviors that polymers exhibit have resulted in the construction of different scratch maps [16,38]. The scratch maps allow prediction of scratch behavior of specific polymers at different conditions such as cone angle, normal load, scratch width and tip geometry. It should be worth noting that Bertrand-Lambotte et al. [38] used the fracture energy and sample size criteria to predict ductile/brittle transition in nanoscratch of automotive clear-coats. A scratch map was constructed based on this work seemed to explain scratch behavior reasonably well.
Researchers have also sought to analyze scratch by classifying the many different types of surface damage features observed. Ironing denotes the scratch behavior which results in smooth featureless grooves that are due to plastic or viscoelastic/viscoplastic deformation. When the scratching process moves into the ploughing regime, wave-like pattern [21], cracking [16], plastic drawing [52] and bamboo-like feature [53] are some of the damage features observed in experiments. The cause(s) for each type of damage feature can be due to brittle or ductile mode of deformation, or both. Clearly, a wide range of surface damage phenomena can be observed during scratching of polymers, making it a major obstacle in fundamental understanding and prediction of scratchinduced damage in polymers.
At present, there is no definitive way to evaluate scratch resistance or surface damage of polymers. Analytical models for scratch has been developed that are based on concepts analogous to indentation hardness [54]. Quantities such as scratch and ploughing hardnesses have been used to characterize the scratch resistance of metals.
Briscoe and his colleagues [31] redefined the ploughing hardness as tangential hardness to include the adhesive contribution and specified another new hardness parameter called dynamic hardness for the purpose of their study or some authors refer to it as specific grooving energy [33]. To understand the terms given above, a list of definitions is given below in Table。
Thus, determining the scratch depth or width of a scratch groove will enable one to obtain the desired hardness value by calculating the relevant contact area. This provides an important tool for quantifying the scratch resistance of a polymer. Scanning electron microscopes (SEM) and optical microscopes are the common instruments used in inspecting the scratch surface of a material. This allows minute deformation mechanism(s) to be observed. Scratch widths can be measured using these methods.
Scratch width measurement is by far the most popular method because of its ease of observation. On the other hand, precise depth measurements are not possible.
To overcome this, 3D laser profilometry and laser scanning confocal microscopy(LSCM) allow the 3D imaging of the sample surface. A huge advantage is gained by being able to analyze damage feature in 3-D. Not only can we make physical measurements (such as depth and heights), any pattern that can be observed using conventional SEM and optical microscopes can also be observed using this method.
Atomic force microscopy (AFM) and scanning probe microscopy (SPM) are also used to obtain surface imaging. However, this method is limited to the nanometer and micrometer ranges.
The Visibility of Scratch
A scratched surface will reflect light in a different manner from an unscratched surface. By measuring the difference in the average intensity of the light reflected (reflectivity), scratch damage and visibility can be quantified [55].
Kody and Martin [28] used polarized light on a reflective optical microscope (Nikon Optiphot) to measure the reflectivity of surfaces. A Sony DXC-101 video camera was used to capture the reflected light after it has passed through the polarizer.
The signal from the camera was digitized using a Scion Video Image 1000 8-bit frame grabber board and analyzed with NIH Image version 1.37 software. Figure 3.1 shows the basic principle of this method. The intensity of the reflected light was measured when the scratch direction is parallel to the incident polarized light (where β = 0°) and when it is at 45° (where β = 45°). The two quantities were named B and D, respectively. Using the following definitions .
A recent paper by Rangarajan et al. [56] describes using bidirectional reflectance distribution function (BRDF) experiments in quantifying scratch visibility. The experiments involve a laser light source to bounce off a specimen surface at -30° to the normal. A black and white charge coupled device (CCD) is placed +30° to the normal above the scratch surface to collect the specular reflected light. Another CCD is placed at -10° to measure off-specular scattering. The former measurement gives information on the scratch size and surface specular reflectance. The latter measurement gives information on color and gloss of the scratch.
A third method used by Wang et al. [20] measured the light intensity reflected off a scratched surface using an optical flatbed scanner (ASTRA 1200S), then processed the information by digitizing the data using Scion Image 2 software and plotted a gray level profile across the scratches. The imaging software would assign values from 0 to 255 for each level of intensity. A profile plot over the scratch can be obtained. This method can be applied over a certain point on the scratch or over the whole length. It was found that scratch visibility was largely due to stress-whitening, and increases with normal load and addition of talc. This is a convenient way of comparing the scratch visibility level, and it can be done simultaneously over many scratches. Different variations of this method were also used by Chu et al [24-26] and Grasmeder [27] to obtain gray level plots.
Issues Concerning Evaluation and Quantification of Scratch：
Polymers present a unique case in scratch. Unlike metals and ceramics, viscoelastic effects allow polymers to recover quickly after scratch. Scratching of polymer surfaces can often produce different surface features concurrently or sequentially [21,53]. Fillers and additives can add to the complexity of the surface damage features observed, where stress-whitening often occur due to the formation of voids and exposure of filler particles [20,58,59].
Polymers undergo ironing, ploughing, cutting and fragmentation like metals do. Determination of types of damages occur during scratch is of great concern to tribologists. Ability to identify a criterion or a set of criteria to predict the type of damage feature during a scratch process is of paramount importance to polymer scientists today. This knowledge has implications in applications where polymers are used as structural or coating materials. Introducing scratches on the surface can result in a drop in fracture toughness of the polymer. In coating materials, delamination will occur if the scratch extends too deep into the coating layer. The severity of the scratch is dependent on the type of scratch damage that occurs, thus the ability to predict scratch behavior will allow polymer scientists to greatly extend the utilization of polymers for new engineering and value-added applications.
An additional problem in the study of scratch on polymers is the multitude of test methods employed. Differences in test conditions and methodology will produce very different scratch behavior and damage features. This concern has been raised by Wong et al. [52]. It has been proposed that the progressive load test be employed as a standardized scratch test, which allows for a better link to material parameters and for easier comparison of results. The present work will thus follow the newly proposed test method to study the scratch behavior of polymers.
Another major concern to polymer scientists and engineers is the visibility of scratches on polymer surfaces. Polymers in automotive interior and exterior parts are susceptible to mars and scratches that vastly degrade their appearances. Polymers that exhibit high scratch resistance are highly desirable. Visibility is a complex issue as it involves many different unquantifiable parameters that can affect how a viewer perceives a scratch. Many attempts have been made to quantify scratch visibility by measuring the surface reflectivity of the scratch [20,27,28,55,60]. Due to the diverse techniques employed and the lack of a systematic study to correlate scratch features with visibility [61], the results obtained for one set of study is often valid only within a set of narrowly defined conditions [55]. It remains to be seen which of these methods, if any,will prove to be the most useful in characterizing scratch visibility.
Summary ：
A comparison of the different evaluation techniques discussed is given in Table 3.2. The merits and disadvantages of each method are also briefly discussed. In summary, scratch hardness is the most relevant technique in quantifying scratch resistance because it can be measured easily and applied to any material. Various techniques can be employed to acquire force and scratch width and depth data to obtain scratch hardness. Scratch visibility is a much more complicated issue, as there is no simple relationship to link reflectance of a surface to human perception of a scratch groove. For PP, it has been shown that stress-whitening is the major contributing factor
to scratch visibility, thus VIEEW? , which is especially useful in quantifying stresswhitening, will be used in this work.
2019-04-10 16:08
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