The probe tack test

6. Experimental setup and image

Chapter 6. Experimental setup and image treatment

Force

Time Approach

Contact Debonding

Plateau

Final debonding

(a) The force as a function of time in the probe tack test.

Stress

Strain s_max

e_max

(b) The stress as a function of strain in the probe tack test.

Figure 6.1.: Typical force-time and stress-strain curves in the probe tack test.

formed and stretched over a long distance. The final debonding can be adhesive, meaning a complete detachment from the probe without any residues on the probe surface, or cohesive, meaning a break up in the middle of the fibrils with residues on the probe surface. This final debonding marks the end of the test. By means of the stress-strain curves, the adhesion energyW_adh of a material can be determined under well-defined conditions. W_adh is the energy per unit area needed to debond the probe from an adhesive layer with thicknessb₀,

W_adh =b₀ Z _²max

0

σ(²)d² . (6.1)

The force and displacement is not the only information gathered during the exper- iment. The whole debonding process is visualized from above via a camera. This additional information allows for a qualitative characterization of the pattern for- mation during the debonding. Furthermore, the 2Dprojection the contact area can be determined.

Setup

The “µ-tack” setup has been developed in the laboratory by Josse et al. [58]. It has a slightly different approach concerning the testing protocol. In this set up, the contact of probe and sample is established in the conventional way by moving a circular indenter towards an adhesive layer at a constant speedv_approach untilF_max is reached. After the contact timetthough, the probe is not lowered but instead, the table holding the sample is moved upwards. Of course the relative motion of probe and tested layer is the same in both protocols. Yet the “alternative” protocol has two advantages. First, the motion is realized here with step motors, which have a certain slack. Moving the motors only in one direction prevents uncertainties in the displacement measurement caused by the mechanical slack when the motor changes

6.1. The probe tack test

Figure 6.2.: Schematical view of theµ-tack setup.

direction. The second advantage of moving the sample holder during the debonding is that the camera focus remains fixed on the probe surface. In that way, sharp pictures are provided during the whole test. Figure 6.2 shows a schematical view of the apparatus. It features a stepping motor in the middle that moves the probe and three stepping motors that move the table on which the sample is fixed. An optical fibre measures the relative displacement of probe and table, and a load cell the force on the probe. Finally, a camera mounted on a microscope ensures good visualization quality. The table with the fixed sample can be tilted via three screws to align the probe surface and the adhesive layer. Good alignment is crucial for having a maximum contact area between probe and adhesive.

The displacement is measured by an optical fibre that measures intensity vari- ations in the reflection of infrared light. In the present setup, the optical fibre is fixed to the sample holder, and the reflecting silicone waver is fixed directly to the probe. In this way, the measured displacement corresponds to the distance between probe and sample table. The displacement has three contributions: the deformation of the adhesive layer, the deformation of the glass slide, and the deformation of the sample holder. The net displacement of the adhesive can be calculated measuring the compliance of the apparatus and the bending of the glass slide. More details of the apparatus and in-depth considerations concerning its compliance are given in references [57] and [19]. We shall only recall here the specifications of the material:

Chapter 6. Experimental setup and image treatment

Figure 6.3.: The probe edges are protected with small metal discs during the polishing.

• microscope glass slides (10 cm×2.6 cm×0.2 cm) with a bending compliance of 0.3^µm/Npurchased atPreciver,

• a Zeiss microscope with a Zeiss Epiplan Neofluar x1.25 objective,

• a Philtec D63 LPT optical fibre (equipped with an infrared laser) with a resolution of 0.4µm,

• Physik Instrumentestep motors with optical encoders and a resolution of about 100 nm,

• aEntranload cell working between−50 N and +50 N with a precision of 0.02 N,

• two different cameras: a digitalMarlin Allied Vision Technologiescamera with an acquisition rate of 12^frames/s and an image resolution of 1392 x 1040 pixel, and a Pulnix CCD camera with 25^frames/s and an image resolution of 768 x 567 pixel.

Probes

We used probes made of stainless steel and mostly with a diameter of 6 mm. In Chapter 10, we also used probes with a diameter of 3 mm and 10 mm. A crucial part in the probe preparation process was the polishing. The probes were polished in a Mecapol P 220U polishing machine with sandpaper with roughnesses going gradually down to a mirror surface. The last grain size was about 5µm. The probes have to fulfil several requirements. First, the surface has to be normal to the probe axis and absolutely flat. The flatness is important for a good contact to the adhesive layer. In Chapter7we study the pattern formation that takes place at the very edge of the probe. Therefore, a high quality of the probes was crucial. We needed very sharp edges and borders with no roughnesses to prevent any influence on the pattern formation. The sharp edges are important as the probe is illuminated from above through the microscope. Any curvature at the borders leads to a reflection of the light at an angle 6= 90^◦, the light cannot be collected by the microscope lens, and the visual information right at the borders is lost. Polishing the probes without protection entails rounded edges as the polishing plate with the sandpaper is not perfectly rigid, see figure6.3. For protection we glued small metal discs around the