1 Introduction.
The adhesion properties of the surface, whether or not it meets the requirements, can be used as an indicator of surface preparation (cleanliness, wettability, polymerization state, etc.). Surface adhesion is particularly important when developing adhesive joints, seals or depositing protective or decorative layers. The adhesion of the surface coating of the substrate is usually measured by a scratch test, while the adhesion of the top surface is usually measured by a "stickiness" test. The nanoindentation method is ideal for measuring surface adhesion properties in parallel with the mechanical properties of thin films. In this application report, nanoindentation will be presented as a tool for measuring surface adhesion to flexible and adhesive films and materials. Recently, we have seen a great deal of interest in measuring the adhesion properties of many materials, mainly packaging and various electronic components.
One of the main applications in this field is optically clear adhesives. Since the advent of touch screens, OCA has become very important in mobile phone production. There are very few methods available to determine the adhesion and mechanical properties of this adhesive film. Nanoindentation is considered a candidate method for measuring both bonding properties and mechanical properties. Both of these properties depend on temperature, UV radiation, and time, so it is important to test different types of OCA films. This application note describes a nanoindentation method for simultaneously measuring the adhesion and mechanical properties of various materials. Adhesion theory is briefly introduced [1], followed by typical adhesion measurements for soft materials and OCA films.
2 Basic Theory.
For many years, it has been known that glues and other adhesives have macroscopic adhesion on many materials, but research on microcosmic adhesion and its rationale has only recently begun. The now widely accepted adhesion model was developed in the 70s of the 20th century by Derjaguin, Muller, and Toporov [2], as well as by Johnson, Kendall, and Roberts [3], both of which are well summarized in [1]. The DMT theory is more suitable for harder materials, whereas the JKR theory is more suitable for the contact of hard and soft materials. Therefore, the JKR theory is generally applicable to the indentation of soft materials. Typical indentation curves obtained during the indentation process of a soft-bonded material are shown in Figures 2 and 3. When a spherical indenter of radius r acts on a pair of materials with a decreasing modulus er and a surface energy w12, the contact radius a with force f is given by equation (1):
From Eq. (1), the contact radius at f=0 can be calculated:
FAD can be easily determined by recording the entire indenter contraction until contact with the surface is completely lost. FAD is the minimum force (pull-off adhesion) achieved at this stage. The indentation software analyzes the negative forces recorded during the indentation process and automatically selects the lowest force corresponding to the FAD. If the indentation is carried out with a spherical indenter of radius r, the surface energy w12 can be calculated using the formula (3).Taking Figure 3 as an example (FAD -102 N, radius 100 m), the surface energy W12 is about 216 mJ m2. Adhesion and adhesion energy depend on the state of the two materials (material surface and indenter surface). functionalized surfaces, dirty or dirty.
Soft materials, due to their low stiffness, produce a large contact area when in contact. Under dry conditions, the adhesion of hard materials is often too small to be detected by current nanoindentation testers. 3 = 12 = (3) FAD can be easily determined by recording the entire indenter contraction until contact with the surface is completely lost. FAD is the minimum force (pull-off adhesion) achieved at this stage. The indentation software analyzes the negative forces recorded during the indentation process and automatically selects the lowest force corresponding to the FAD. If the indentation is carried out with a spherical indenter of radius r, the surface energy w12 can be calculated using the formula (3).Taking Figure 3 as an example (FAD -102 N, radius 100 m), the surface energy W12 is about 216 mJ m2. Adhesion and adhesion energy depend on the state of the two materials (material surface and indenter surface). Functionalized surfaces, dirty or contaminated surfaces can cause different FAD and W12 values. Another fact to consider is that adhesion is mainly observed in dry conditions.
When the sample is completely submerged in the liquid, it is less noticeable – if at all. This suggests that adhesion is related to the capillary force generated by the condensation of water vapor at the tip-specimen interface [4]. Once the tip-specimen interface is in a liquid state, these forces are non-existent because no condensation occurs. It has also been shown that the adhesion force can change the indentation force-displacement curve, mainly for very soft gels, and that the Hertz model is not suitable, and the JKR model must be used for the calculation of the elastic modulus [5]. Surface tension is also considered to be an important factor influencing indentation testing when indentation tests are performed on soft materials (e < 50 kPa) with small needle tips (r < 30 m) and low loads [6]. However, the above contact conditions are rarely met in ordinary nanoindentation experiments.
3 Applications. 3.Measurement of 1 m elastomers and hydrogels The first application of nanoindentation for measuring adhesion was the measurement of soft gels (Figure 4). These gels can be used for a variety of purposes, from human tissue gels modelled with soft implants to decorative gels, they are stable in the air and adhesion phenomena are easily measurable due to their high flexibility (very low stiffness). The elastic modulus of these gels varies greatly, from the softest gel with an elastic modulus of up to 10 kPa and the hardest gel with an elastic modulus of several MPa. Bioindentation instruments are used to measure these gels and are typically equipped with spherical indenters (radii 100 m to 500 m). As many gels are very soft. Therefore, the indentation parameters must be adjusted. The applied force is relatively low (0.).01 mn 1 mn), otherwise the indentation depth will be too large to be analyzed (usually a Hertzian elastic contact model). Compared to indentations in hard materials, the indenter approach and retraction speeds can be set higher and the measurement process is faster. To measure pull-out adhesion, set a longer retraction time to record the force during head removal. Typical PS-6C elastomer indentation measurements of two decorative gels (white and blue) are shown in Figure 5, and Table 1 summarizes the properties obtained from indentation measurements.
Another example of adhesion testing is the indentation test on polydimethylsiloxane (PDMS), using a procedure similar to the previous example (ruby spherical indenter with a radius of 100 m, drying conditions). Figure 6 plots 5 indentation curves; The results show excellent weight, both in the indentation part of the curve (load applied) and in the adhesion (negative force) part.
Refolding. It is important to note that the adhesion of hydrogels is much lower, if any, compared to gels. This is due to the fact that the hydrogel is almost always immersed in the liquid during the measurement process and the adhesion due to moisture and capillary effects is greatly reduced. Some bioindentation measurements of hydrogels under dry conditions confirm that the hydrogels are shrinking, so they must be at least hydrated, even if they are not fully immersed in the liquid.
3.2 Optically clear film.
Optically Clear Adhesive (OCA) is a thin film of adhesive that is used in the smartphone industry to install large** screens. Not only are the adhesive properties of these films important, but so are their mechanical properties, as they largely determine how OCA is used. Anton Paar's bioindenters have been used to measure this adhesive film. The bioindenter senses adhesion and can also probe the film to measure its stiffness (elastic modulus) and time-dependent (creep) properties (by calculating relaxation time and creep modulus). Special attention must be paid to sample mounting, as adhesive films can be very soft, with thicknesses ranging from tens to hundreds of microns.
For harder films, use cyanoacrylate glue or epoxy resin to attach them to a solid support (e.g., a glass slide or a flat, smooth block of aluminium as shown in Figure 7). For flexible films that are adhered on both sides, the film is usually placed directly on a solid support (no glue required) as they will stick automatically. It is critical to ensure that the film is firmly attached to the substrate to avoid errors due to film bending during indentation measurements. In this example, three different adhesive films were tested: a soft film (a) with an elastic modulus (e) of 035 MPa and two rigid (b, c) with elastic modulus of 208 MPa and 80 MPa. Due to the low thickness of these films, the maximum indentation depth must be limited to 10 to 20 percent of the film thickness, especially for stiffer films.
A spherical indenter with a radius of 500 m was used in the experiment. However, it is possible to use indenters with smaller half-diameters, i.e. for thinner films where the depth of penetration must be limited. The maximum force is 500 m, the maximum penetration depth is 1 m 16 m. Hold time at maximum force for 30 seconds. If creep properties are to be determined, holding time is necessary: the creep modulus and relaxation time are calculated by fitting the depth data obtained during this period [7].
Figure 8 shows a comparison of the three indentation curves of the three OCAs tested. NOTE: Record the adsorption force near the surface of the indenter. To facilitate the comparison of materials, the adhesion and subsequent indentation curves on the same sample are very similar.
4 Conclusion. While Anton Paar's nanoindentation instruments are primarily used for indentation testing of hard materials, the instrument (i.e., bioindentation instruments) can also be used to measure the adhesion properties of gels or other soft-adhesive materials. The main advantage of nanoindentation measurements is that both adhesive properties (indentation and withdrawal) and mechanical properties (including creep) can be determined. While spherical indenters are mostly used, other indenter geometries can simulate different contact conditions.
In addition to adhesive and mechanical properties, surface energy can also be calculated. Due to the easy exchange of tips, different contact conditions (indenter radius or shape) can also be simulated. Adhesion measurements are best done using a bioindentation instrument or MCT3 without the need for a reference ring. For all other instruments (NHT3, HIT 300, UNHT3, MCT3 with reference), there is a risk of sticking to the reference ring, which may result in damage to the tip of the instrument when returned. Excessive indenter adhesion can also lead to instrument damage (this applies to all indentation instruments).