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Scratching of polymers: Deformation mapping and wear modeling

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is more pronounced in the case of pol)'Tilers as they are thermal insulators and their melting/softening temperatures are much lower in comparison to those for metals and ceramics.

Summarizing, there is a clear indication that scratching characteristic of pol)'Tilers is a very complex phenomenon and the concept of scratching map can be used to present such data which are otherwise difficult to comprehend. Surface deformation characteristics can be an initiator of a whole range of tribological phenomena such as friction, wear and surface fatigue in addition to the visual aesthetic aspect (gloss) of a pol)'Tiler surface.

SCRATCHING AS A MEANS TO UNDERSTAND ABRASIVE WEAR

A number of attempts have been made in the past, both for metals and pol)'Tilers, to devise means of predicting wear performance (mainly abrasive). Scratching basically models one hard asperity of a surface if the shape and size of the scratching tip is well defined and they are similar to those of the asperities on the surface [12, 13].

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Fig. 4. Specific wear rate in abrasive wear tests as a function of the scratch hardness (Data for PMMA is from ref [15] whereas all other data are from ref. [6]). UHMWPE is among the softest pol)'Tiler with very low wear rate while PS is among the harder pol)'Tilers with very high wear rate (Reprinted from ref [16) with publisher's permission).

The work on modeling wear by scratching for pol)'Tilers have been prompted by several evidences that the indentation or normal hardness does not correlate with the wear performance for pol)'Tilers. One example may be cited here. PMMA is roughly 5-6 times harder in normal indentation than UHMWPE but the wear rate ofPMMA is approximately 85 times greater when compared to that of UHMWPE [14]. Similarly, Budinski's [6] work has also shown that the wear rate of pol)'Tilers furnishes no correlation with the scratch hardness as seen in Fig. 4. The general conclusion is that a harder pol)'Tiler is not necessarily more wear resistant than a softer pol)'Tiler when they are slid against the same metallic surface and under the same experimental

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131

The classical work on the modeling of polymer wear have looked at the macroscopic wear tests and then plotted wear rate as a function of some combination of material properties. The most useful work in this area has been carried out by Ratner and coworkers [ 18] and then by Lancaster [19]. Ratner and coworkers, in their classic work of sliding polymers against metal gauges, looked at the wear of polymers as a surface fatigue phenomenon and related specific wear rate to the mechanical properties by a relation [ 18],

Specific wear rate= const. [(1-1/H crr e)] (I}

where 1-1 is the coefficient of friction, H is the indentation hardness, crr is the breaking stress and e is the % elongation to break. Equation (1} includes indentation hardness in the form of inverse proportionality. Interestingly, many later workers have not included indentation hardness as one variable in their wear models [16]. One other important parameter, which is the roughness of the counterface, is not included in equation (1). The roughness of the counterface has been found to affect the wear behavior of polymers whether it is adhesive or abrasive wear [20]. The actual effects of asperities, their shapes, sizes and of other characteristics are very complex and the reader is referred to ref. [21] for further explanation on these effects.

Recognizing the importance of tensile strength, the counterface roughness parameter and fatigue property of polymers, Hollander and Lancaster gave a model for wear as [21 ],

W a crr-n r-2n/3

(2)

where crr is the failure stress of the polymer, r is the mean tip radius of the asperities on the abrasive surface and n is the fatigue exponent, a material property that can be derived from fatigue test on bulk material. One important finding in the work by Hollander and Lancaster is the realization that it is important to consider a suitable roughness parameter such as, r, which takes into account the actual tip shape (especially the attack angle), rather than a statistical mean value of roughness such as R.. Many further studies on the wear modeling for polymers have provided equations for wear, however, a large majority of them have been tested on only one polymer system which limits the general applicability of the models. One notable observation, perhaps as a way of data presentation, has been found that the specific wear rate when plotted against the reciprocal of the product of ultimate tensile strength (crurs) and % elongation at failure (e) furnishes a linear relationship [22]. Figure 6 gives such a plot where the data are obtained from literature. We may observe here that, regardless of the counterface roughness condition and the prevailing coefficient of friction for individual polymer, a linear relation is obtained.

132							Scratching of materials and applications
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Fig. 6. A plot of wear rate (mm3 mm·1 kg-1) as a function of the reciprocal of the product of ultimate tensile stress and elongation to fracture. The data are taken from literature. A - poly(ethylene) [20]; B - Nylon 66 [20]; C - PTFE [20]; D - poly(propene) [20]; E - High Density Poly(ethylene) [23]; F- Acetal; G- poly(carbonate) [20]; H- poly(propylene) [23]; I- poly(ethyleneterephthalate glycol) [23]; J - poly(vinyl chloride) [23]; K - PMMA [23]; L - poly(styrene) [19]; M - PMMA [23]. The plot shows reasonable linearity regardless of the polymer type. (Reprinted from ref[l6] with publisher's permission)

Thus, several previous works have pointed out that the wear rate of bulk polymers is, in some way, strongly related to the ultimate tensile strength and bulk toughness related parameter such as % elongation at failure. This is valid when the counterface is a hard surface. The above macroscopic observations have been further proven by conducting experiments at nanoscale scratching using single diamond tip [24,25]. The polymers were scratched by a diamond tip of 90° included angle and tip radius of 1 micron. The scratching was performed in cyclic fashion on a single track or on intersecting tracks. It was observed that for a very wear resistant polymer, such as ultra-high molecular weight poly(ethylene) (UHMWPE), there were no wear particle formation during scratching performed on a single track. Even repeated scratching on the same track had almost no effect on the wear particle generation. This is because most of the work-done was accommodated in the plastic deformation of the polymer making groove and side pileups in the very first scratching pass. Further scratching passes on the same track only helped in expanding the scratch width but only by a slight margin in the case of UHMWPE. We did not observe any wear particle even after 10 cycles of scratching on the same track. In contrast, for a harder polymer, such as PMMA, even single pass scratching gave many loose wear particles which were seen either along the scratch or attached to the tip surface. When UHMWPE is subjected to intersecting scratching, where the scratching direction is changed by

134	Scratching of materials and applications

hardness. Further work is now underway to show that by estimating the wear particle surface area or volume during cyclic scratching (single track or intersecting) we can rank polymers according to their wear resistance. This ranking has almost linear correlation with the specific wear rate data obtained from a pin-on-disk test for the same polymers against a rough (Ra - 1.35 micorn) steel counterface [14].

The above explanations on the use of scratching for modeling abrasive wear behavior of polymers has shown the capability of this versatile test. Though scratch hardness as a mechanical property parameter does not relate to wear rate, the quantification of the wear debris produced during scratching is related to wear performance. It may be clarified here that scratch hardness is calculated based on the width of the scratch which is not necessarily produced by wearing of the material. For polymers scratched by low attack angle tip, most of the scratch width is formed as a result of the plastic deformation leading to side pileups. This type of plastically deformed pileup is not wear particle as it is still part of the bulk of the materials. This is the main reason why scratch hardness value does not relate to wear performance. Thus, the actual wear debris particles produced during scratching is the real indicator of wear and should be taken into account while using scratch test for abrasive wear modeling.

CONCLUDING REMARKS

This chapter has looked at the usefulness of scratch testing for the tribological characterizations of bulk polymers. The examples drawn here are for two important aspects, scratch deformation mapping and abrasive wear modeling. It is shown that the scratch mapping of polymers can be used as a tool for understanding deformation characteristics of polymers for a wide range of scratching conditions of normal loads, scratching velocity, imposed strain (or indenter attack angle) and bulk temperature. This information can be particularly important where polymers' surface optical characteristic such as glossiness is important. Surface related properties such as friction, wear and fatigue are also related to the surface deformation characteristics of the polymer.

The abrasive wear of polymers and scratch hardness do not show any correlation. However, by the new technique of conducting cyclic (single track or intersecting) scratch test, where the amount of wear debris generated in scratching is taken into account, it is possible to rank polymers according to their abrasive wear resistance using scratching data. This ranking correlates well with the specific wear data obtained in a pin-on-disk type wear test. Further, the nanoscratching technique has clearly demonstrated why the macroscopically observed wear rate for polymers shows strong relation to the ultimate tensile stress and the % elongation at failure. The abrasive wear process for polymers, when investigated at micron to nano scale, has shown that wear particle formation is a localized low cycle fatigue process.

ACKNOWLEDGMENTS

The author would like to acknowledge the works carried out by many of his former and current coworkers that helped understand the fascinating area of scratch testing. Many of the stimulating discussions were extremely helpful. In particular, I would like to thank Professor Brian J. Briscoe, Dr. P. D. Evans, Dr. E. Pelillo, Mr. Brian K. P. Wong, Dr. Kaiyang Zeng, Ms. Joyce, P. Y. Tan, Mr. Rosli B. M. Sani, Mr. Mark C. W. Lup and ProfessorS. C. Lim.

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REFERENCES

I.Mohs, F. Grundriss der Mineralogie, 1824 (English translation by W. Haidinger:

Treaties on Mineralogy, Constable, Edinburgh, 1825).

2.Briscoe, B. J., Evans, P. D., Pelillo, E. and Sinha, S. K. (1996) Wear, 200, 137.

3.Jardret, V., Zahouani, H., Loubet, J. L. and Mathia, T. G. (1998) Wear, 218, 8.

4.Briscoe, B. J. (1998) Tribology International, 31, No. 1-3, 121.

5.Williams, J. A. (1996) Tribology International, 29, No. 8, 675.

6.Budinski, K. G. (1997) Wear, 203-204,302.

7.Felder, E. and Bucaille, J. L. (2006) Chapter 2 in this book.

8.Briscoe, B. J., Pelillo, E. and Sinha, S. K. (1997) Polymer International, 43,359.

9.Sung, L-P., Drzal, P. L., Vanlandingham, M. R. and Forster, A.M. (2006) Chapter 5 in this book.

10.Briscoe, B. Pelillo, E. and Sinha, S. K. (1996) Polym. Eng. and Sci., 36(24), 2996.

11.Bonne, M., Briscoe, B. J., Manimaaran, S. and Allan, A. (2003) Wear, 254, 55.

12.Zum Gahr, K. H. (1987) in Chapter 5, Microstructure and Wear of Materials, Elsevier Science Publishers B. V.

13.Briscoe, B. J. and Sinha, S. K. (2003) Materialwissenschaft und Werkstoffiechnik, 10-11, 989.

14.Sinha, S. K., Lup M. C. W. and Lim, S.C. (data to be published elsewhere).

15.Trezona, R.I. and Hutchings, I. M. (1999) Wear, 233-235, pp. 209-221.

16.Sinha, S. K. and Briscoe, B. J. (2006) in Encyclopedia of Polymer Science &

Technology, John Wiley & Sons, NY.

17.Archard, J. F. (1953) J Applied Physics, 24,981.

18.Ratner, S. N., Farberoua, I. I., Radyukeuich, 0. V. and Lure, E. G. (1964) Soviet Plastics, 7, 37.

19.Lancaster, J. K. (1969) Tribology Conv. 1969, Institute of Mechanical Engineers, London, pp. 100.

20.Briscoe, B. J. (1981) Tribology International, 231.

21.Hollander, A. E. and Lancaster, J. K. (1973) Wear, 25, 155.

22.Lancaster, J. K. in Encyclopedia of Polymer Science and Engineering (editor-in-chief: Jacqueline I. Kroschwitz), 1990, Wily, NY, 2"d Edition, I.

23.Shipway, P. H. and Ngao, N. K. (2003) Wear, 255, 742.

24.Wong, K. W. P., Sinha, S. K., Tan, J.P. Y. and Zeng, K. Y. (2004) Trib. Letters, 17(3), 613.

25.Sani, R. B. M., Sinha, S. K., Tan, J. P. Y. and Zeng, K. Y. (2005) Phil. Mag., 85(19), 2101.

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CHAPTER 7

AN OVERVIEW OF THE POTENTIAL OF QUANTITATIVE COATING ADHESION

MEASUREMENT BY SCRATCH TESTING

Originally published in Tribologv International vol 39, February 2006

S.J. BULL and E.G-BERASETEGUI

School ofChemical Engineering and Advanced Materials

University ofNewcastle, Newcastle-upon- Tyne

NEJ 7RU, U.K. E-mail: [e-mail]

ABSTRACT

The scratch test has been used to assess the adhesion of thin hard coatings for some time now and is a useful tool for coating development or quality assurance. However, the test is influenced by a number of intrinsic and extrinsic factors which are not adhesion-related and the results of the test are usually regarded as only semi-quantitative. The stress state around a moving indenter scratching a coating/substrate system is very complex and it is difficult to determine the stresses which lead to detachment. Furthermore the interfacial defect state responsible for failure is tmknown. However, by a careful analysis of the observed failure modes in the scratch test (not all of which are related to adhesion) it is possible to identify adhesive failures and in some cases these occur in regions where the stress state is relatively simple and quantification can be attempted.

Ideally engineers would like a material parameter (such as work of adhesion or interfacial toughness) which can be used in an appropriate model of the coating-substrate system stress state to determine if detachment will occur under the loading conditions experienced in service. This data is not usually available and the development of such models must be seen as a long term goal. In modem indentation and scratch systems the work of friction (or indentation) can be directly measured and the relationship between this parameter and adhesive failure can be demonstrated in some cases. This chapter reviews the main adhesion-related failure modes and the stresses responsible for them and indicates where quantification is possible iiiustrating this with results from hard coatings on steel, thermally grown oxide scales and optical coatings on glass. The use of empirical calibration studies, directly measured work of friction and quantification by finite element methods is discussed.

KEYWORDS

Coatings, thin films, adhesion, critical load.