Scratch testing from nano to macro scale

H. Rojacz*, A. Nevosad, C. Tomastik, U. Cihak-Bayr

AC2T research GmbH, Wiener Neustadt, AUT

INTRODUCTION: Scratch testing is a fundamental abrasive wear test, simulating a hard particle abrading the surface¹. The focus of this study is to understand ongoing wear processes at various stress levels and indenter radii. It may bridge the gap between macro and nanoscale scratch testing and its influence on the apparent coefficient of friction (COF) and wear mechanism in dependence of various factors. Therefore, data and a thorough discussion shall be provided on surface conditions and the resulting changes of wear mechanisms. A scheme of ongoing processes during scratching is presented in Figure 1².

Figure 1 – Scheme of deformation processes during scratching.

As seen, in the figure, the load, velocity, surface conditions and materials chemistry and state (mechanical properties) have influence on the materials response (deformation / cracking) and the apparent COF. A study of elastic and plastic proportions of deformation and their influence on the apparent COF shall be given; further statistical methods are applied to correlate all relevant parameter.

MATERIALS AND METHODS: Prior to the experiments, metallographic samples of an austenitic steel (1.4571) were prepared. To ensure a constant microstructure and certain roughness, samples surface was grinded, fine grinded, polished with a 1 µm diamond dispersion and finish polished with a neutral, colloidal oxide dispersion (0.2 µm). The surface roughness, depth and appearance of performed scratches were measured utilising a Leica® confocal microscope. To evaluate governing stresses, a calibration curve for each indenter was calculated via indentation depth at certain loads. Load-stress curves were then calculated and enabled the selection of constant stresses and the correlation of different COF and the changes of wear mechanisms at chosen contact stresses; calculated according to Fisher-Cripps ³.

Scratch tests were performed with diamond indenters at various stresses at two different devices: A Hysitron® Triboindenter T900 with indenter radii 0.652 µm and 100 µm was used for nano scratch, a modified TriboTech® scratch tester and 10, 50 and 200 µm radii were utilised for the macro scratch tests.

For a thorough analysis, scanning electron microscopic images were taken with a Zeiss® Gemini Supra 40 VP SEM, to reveal surficial changes and wear mechanisms in dependence of load (stresses) and indenter geometry.

RESULTS: As given in Figure 2, two loads were chosen for evaluation to show the influence of the indenter radii at constant loads. At macroscopical scratching, higher indentation depth leads to higher deformation and therefore to a higher COF. The influence of the ratio of elastic and plastic deformation shall be discussed at constant stresses, and will be published in the full-version publication of this study and will be further performed for the nano scratches as well.

Figure 2 – Friction data and indenter depths – Macro scratching

Figure 3 shows the influence of the different indenter radii at nano scratching conditions performed at 100 µN and 1000 µN. At 1000 µN, a behaviour similar to the macro scratches can be observed. At smaller loads and higher radii the coefficient of friction increases. At these conditions deformation adhesive mechanisms become dominant and cause the COF rise.

Figure 3 – Friction data of nano scratches performed at 1000 µN.

DISCUSSION: The influence of the indenter size cannot be neglected in scratch testing, since it crucially changes the outcome of the measurements. The shear forces can be influenced by different effects, e.g. materials deformation, microstructural changes, the change of wear mechanisms due to changes in the contact situation and stresses at macroscopical abrasive wear ^4-6. For nanoscopic wear, adhesion effects can act in addition to the previous effects ⁷. Threshold parameter where adhesional effects dominate the apparent COF will be discussed.

The detailed parameter study will be completed by a thorough statistical analysis and a quantification of the key parameters

REFERENCES: 1. Zum Gahr, Elsevier (1987), 2. Rojacz, EuroMat - Thessaloniki. (2017), 3. Fisher-Cripps, Nanoindentation (2004), 4. Varga, Wear. (2017), 5. Bull, Surf. Coat. Technol (1991), 6. Holmberg, Coatings Tribology. (2009), 7. Tomastik , EPJ Web. Conf. (2013).

ACKNOWLEDGEMENTS: This work was funded by the Austrian COMET Program (Project K2 XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology”. Stefan Krenn is acknowledged for data evaluations.