Keywords

hardness; surface; carbonisation; friction; scratch resistance

Applications of polymers benefit from improved surface hardness

Hardness is an important parameter in many industrial applications of polymers, and is sometimes a key requirement. Examples of this include optical elements made of polycarbonate resins, such as CR39, whose scratch resistance is essential. These must satisfy a test in which a sharp point is drawn over the surface as it is being subjected to an increasing normal force. The onset of damage on the surface is detected either through the measurement of the friction coefficient or by the measurement of acoustic emission. It has been shown that scratch resistance can be improved by using ion implantation to increase the hardness of a surface layer of the polymer [1]. The ion-implanted surface layer becomes carbonized and, thus, has an indentation hardness higher than the hardness of the underlying polymer and a different force-indentation curve. The thickness of the carbonized layer depends on the ion penetration depth. The carbonized polymer surface layer may be thicker than the ion penetration depth, due to the diffusion of radicals into the material beyond the end of the range of the ions [2].

An increase in the hardness of polyethylene terephthalate (PET) sheet, as used for manufacturing of plastic bottles, was observed after plasma immersion ion implantation (PIII) with nitrogen ions [3]. Ion implantation has been used also for increasing the hardness of bimorphic humidity sensors made from polyimide and polyethersulfone [4], and for increasing the hardness of high-grade transparent optical lenses made from polycarbonate resin (CR39) [5,6]. The ion treatment of polymers often also induces darkening, which may compromise the lenses for some applications.

Medical devices for use in the human body are another important application in which polymers are extensively used. Polymer materials have high biocompatibility, low toxicity and a range of mechanical properties that are compatible with human tissue. Surface hardness is an important mechanical characteristic of polymers [7–9] used in replacement of joints in the body. Many biomedical applications require improved scratch resistance of the polymer surface. Ion implantation has been used to enhance the hardness and the friction characteristics of polyetheretherketone (PEEK) for applications in ventricular assist devices [10]. Components made out of polymers usually operate at low mechanical load, far from breaking stresses; however, even a low mechanical load can cause defects in the microstructure, which can progress when under higher loads into macro cracks, leading to the destruction of the component. Prevention of defect formation has such a strong influence on the functional lifetime of devices. This is of great value, especially for medical devices where reliability is mandatory. An increase of surface layer hardness can prevent the formation of structural defects during low load scratching.

A low friction coefficient is important in some applications involving sliding contacts. For example, ion implantation improved the tribological properties of ultrahigh molecular weight polyethylene (UHMWPE), HDPE and LDPE, used for total joint replacement (TJR) prostheses (e.g., acetabular cups and tibial components) [11–13].

Another example of an application requiring high scratch resistance is a stent delivery system, as used for placing a stent in coronary or peripheral blood vessels [14]. A thin-walled polymer balloon is part of this stent delivery system. The balloon is folded, after which the metallic stent is crimped around the folded balloon to a very small diameter to enable the delivery of the system to very stenosed regions. Once the delivery system has been guided through the vasculature to its destination, increasing the pressure inside the balloon expands the balloon, opening up the stenosed blood vessel and enabling simultaneous deployment of the stent.

The balloon wall must be soft and thin, to allow the medical device to be guided through tortuous vascular systems without damaging the vessel wall; the balloon wall must be strong enough to withstand the high inflation pressure and withstand potentially very high point forces caused by a possible high degree of calcification in hard lesions. Therefore, it is essential that the metal stent crimped on a folded balloon will not scratch the balloon, thereby ensuring that the balloon will not fail under these circumstances. In order to satisfy these apparently contradictory requirements, a very thin surface layer of increased hardness was produced without significant change to the bulk properties of the balloon wall. Ion modification was found to be successful for increasing the hardness of the surface of Pebax [15] without affecting the interior of the thin wall of the balloon.

Mechanisms for hardness improvement by ion implantation

The improvements in hardness are a result of structural transformations in a thin modified surface layer of the polymer. Infrared and Raman spectroscopies are useful for observing the changes in the structure of polymers after surface modification by ion implantation. For example, the structural changes in Pebax after ion beam implantation were observed by micro-Raman spectroscopy in which the modified material is excited by a laser beam confined to a thin surface layer of Pebax. The Raman lines at 1645, 1446, 1381, 1305, 1121, 1074 cm⁻¹, corresponding to the vibrations of polyamide/polyether Pebax macromolecules, were observed in the spectra of the unmodified Pebax and found to gradually disappear with increasing ion fluence (Figure 7.1), and a wide peak in the 1300–1600 cm⁻¹ region appeared. The 1300–1600 cm⁻¹ region of the Raman spectrum contains information about the carbon-rich network that is produced by the modification. The peak at 1540–1580 cm⁻¹ is named the G-peak and corresponds to carbon in the form of graphitic structures. The peak at 1330–1360 cm⁻¹ is named the D-peak and the intensity of this peak increases with the degree of disorder of the graphitic structure. The high intensity of these lines in modified polymers can be explained by a resonance effect in the Raman spectra of graphitic structures coupled with basic breathing vibrations of aromatic rings.

For surface layers modified with 20 keV ions, the 1542 cm⁻¹ peak corresponding to carbon vibrations in the graphitic phase has a strong intensity. This means that a graphite structure appears as a result of the carbonization of the polymer surface layer. Following treatment with 30 keV ions at low fluence, the micro-Raman spectra of Pebax samples show the same strong peak (Figure 7.2), with the addition of an observed decrease in intensity of the vibrational lines in unmodified Pebax and an increase of the ordered graphite carbon peak. For high-fluence treatments (from 10¹⁶ to 10¹⁷ ions/cm²), the Raman spectra contain two distinct peaks at 1590 and 1360 cm⁻¹. At this treatment fluence, the spectral lines of unmodified Pebax macromolecules have disappeared completely. The ion fluence of 10¹⁶ ions/cm² induces complete carbonization of the polymer surface layer. Subsequent ion implantation occurs into this carbon layer, not into the polymer layer. At fluences higher than 10¹⁶ ions/cm², the surface layer treated by nitrogen ions is transformed into a layer of nano-crystalline graphitic structures containing high concentrations of crosslinking between graphite planes. Such materials are known to have a high hardness.

The structural changes in the ion-modified Pebax surface layer are also observable by FTIR ATR spectroscopy. A wide band appears in the 3600−3200 cm⁻¹ wave number region in the spectra of ion-treated Pebax (Figure 7.3). This broad band contains the vibrational lines of hydroxyl and amine groups. In comparison to the narrow amide A line at 3306 cm⁻¹ of the unmodified Pebax amide group, the new band is significantly wider and contains some separated maxima (see Figure 7.3B). In the middle region of the FTIR spectrum, the line at 1638 cm⁻¹ and the Amide 1 doublet at 1720/1737 cm⁻¹ overlap, with new broad bands in the range 1650–1750 cm⁻¹. These new bands are interpreted as vibrations of carboncarbon unsaturated groups and carbonyl groups. The new carbonyl, hydroxyl, and amine groups appear as a result of depolymerization reactions in Pebax macromolecules due to the action of radicals produced by the ion bombardment. In the process of depolymerization, the carbonyl, hydroxyl, and amine groups are formed at the ends of fragmented macromolecules and a large number of carbonyl, hydroxyl, and amine groups appear in the treated Pebax. Similar depolymerization processes are also observed in polyethers and polyamides under high-energy treatments with γ- and UV-light irradiations. In the case of ion beam implantation, the depolymerization process occurs at depths greater than the ion penetration depth, where free radicals from the highly carbonized layer propagate deep into the untreated region of the polymer and cause depolymerization reactions, as also observed under γ- and UV-light irradiations.

Based on the analysis of Pebax after ion implantation, the structure of the modified polymer at high ion fluence can be represented by the schematic diagram shown in Figure 7.4. The surface layer is highly carbonized. The structure of the carbon layer depends on the ion fluence and ion energy used. High energy and high fluence form graphite-like structures with a high concentration of crosslinks between graphitic planes. The thickness of this layer is typically close to the ion penetration depth. The layer beneath contains partly a depolymerized and partly crosslinked polymer. This layer is below the ion penetration depth and is typically thicker than the top layer. Its thickness depends on the depth of propagation of radicals from the surface layer into the bulk-unmodified layers of polymer. The mechanical behavior of such a layered structure can be quite complicated and should be investigated for each case. Such an investigation can be carried out with an indentation test.

The indentation measurement of the thin surface layer was performed by AFM in contact mode with a silicon wafer used as a reference (Figure 7.5). The force curve shows the deformation of the cantilever in contact with a hard surface. The curve of the unmodified Pebax is less steep, corresponding to the deformation of the Pebax surface layer under load of the tip. The penetration depth of the tip into the Pebax film is equal to the difference between the silicon wafer curve and the Pebax curve. At the point where the maximum force is applied, the depth of penetration is about 50 nm. The asymptotic slope of the load curve is proportional to the elastic modulus (also known as Young’s modulus or tensile modulus) of Pebax. The unloading curve shows that hysteresis occurs in the surface layer. This corresponds to reference data for the mechanical properties of Pebax at a load lower than the load that leads to failure [15]. The hysteresis is caused by mechanical energy loss due to movement and conformational transitions of the polymer macromolecules under the load.

The curve of measured on the ion beam modified Pebax has a different shape (Figure 7.6). The asymptotic slope of the linear part of the curve is significantly steeper than that for the unmodified Pebax. The unloading curve traces out almost the same path as the load curve. The hysteresis has disappeared after the ion implantation treatment. This behavior is typical for a hard material with low molecular mobility and high elastic modulus. As indicated also by the spectroscopic investigation, the modified Pebax surface layer treated at high fluences is highly carbonized, with a high elastic modulus.

For intermediate ion fluences, carbonization is not complete and the graphite structure is not fully formed and, therefore, is not as hard as for a treatment with a high fluence. The depolymerization under the surface layer is also decreased. Therefore, the polymer has a top surface layer with increased hardness and a layer beneath it with decreased hardness relative to the unmodified polymer. The load curve shows two stages: at first, the tip penetrates into the hard layer, the hardness is quite close to that of the silicon reference surface; subsequently, the tip penetrates the soft layer, where the elastic modulus (or stiffness) is reduced relative to the unmodified polymer (Figure 7.7). The presence of the weak depolymerized layer causes a complicated mechanical behavior.

The elastic modulus of the polymer surface layer was calculated from the AFM load curves [16]. The elastic modulus increases with ion implantation fluence and with the energy of the ions (Figure 7.8). The highest elastic modulus was observed for 30 kV PIII, in which a graphitic structure with a high concentration of crosslinks was observed. The elastic modulus of the surface layer was found to increase by a factor of 10 compared to that of the untreated surface layer when PIII treated at 30 kV to a fluence of 10¹⁶ ions/cm².

We observed similar curves for different kinds of polyurethanes. Before modification, the load curve is characteristic of a soft material with large hysteresis (Figure 7.9A). After ion implantation at high fluences (up to 10¹⁶ ions/cm²), the hysteresis disappears (Figure 7.9B), and the elastic modulus of the polymer surface layer increases (Figure 7.10). The improvement of the surface layer hardness reduces the deformation of the surface layer and also reduces the friction coefficient of the polyurethane. This is particularly important in the case of many medical applications, for example in urinary catheters with a polyurethane coating, where friction of the catheter in the urinary tract causes significant pain and patient discomfort.

Some applications need a combination of improved mechanical properties with, for example, specific optical properties. We used ion implantation to improve polycarbonate and polyimide windows for aircraft that fly under conditions of high dust particle flux [17]. Such conditions exist at desert airports where frequent winds lift off dust clouds, and aircraft take off with high speed in dense clouds of dust. The dust’s abrasive action is weak but the ageing of the windows occurs significantly more quickly due to the collection of scratches. In time, this results in a decrease in optical transparency of the windows.

In our experiments, plates of polycarbonate and polyimide of 2 mm thickness were treated by an ion beam implanter with 40 keV nitrogen ions. Ion beams both in pulsed and continuous operation modes were applied. The current density did not exceed 20 μA/cm² in either mode. A fluence of 10¹³–10¹⁵ ions/cm² was applied.

The carbonization induced by ion beam implantation at high fluences (more than 10¹⁵ ions/cm²) darkens the polycarbonate and polyimide surfaces. The high degree of carbonization at high fluence reduces the transparency of the windows to levels that are not permitted for aircraft windows. The decrease of transparency starts from 5×10¹⁴–10¹⁵ ions/cm², which is therefore an upper limit to the fluence that can be applied for the window modification. In our case, the polycarbonate and polyimide windows were treated with a fluence of 2.5×10¹⁴ ions/cm², which does not increase the light absorbance of windows in the visible wavelength range and is, therefore, suitable for window modification.

Scratch testing was carried out with a diamond indenter. The indenter was loaded with different weights and moved about on the sample surface with constant speed. After scratching, the width and depth of the scratch were measured by optical microscopy. The experimental data are presented in Table 7.1. The indenter scratches the polymer to different depths corresponding to the penetration of the indenter, which is a function of the indenter load. The scratch hardness of the surface layer can be calculated according to the following equation:

$H_{\mathrm{scratch}}=\frac{F}{d^2}·k$ (Eq. 7.1)

(Eq. 7.1)

where H_scratch is the scratch hardness, F is the indenter load, d is the depth of the indenter penetration, and k is a coefficient that depends on the indenter geometry.

At high loads of 100 and 25 g, the indenter penetrates through the carbonized layer. The depth of indenter penetration is 20–50 µm for polycarbonate and 15–30 µm for polyimide. It corresponds to a scratch hardness of about 60 kgf/mm² for polycarbonate and about 100 kgf/mm² for polyimide, which equals the scratch hardness of the unmodified polymers. The indenter penetration depth is much higher than the thickness of the carbonized layer at these loads. Therefore, the hardness improvement of the surface layer cannot protect the polymer from scratching at such high loads.

At a low loads, such as 5 g, the indenter penetrates to a reduced depth but the depth of indenter penetration is still higher (200–900 nm) than the ion penetration depth in ion beam implantation (about 100 nm). The scratch hardness increases from 60 to 945 kgf/mm² for polycarbonate, and from 100 to 6173 kgf/mm² for polyimide.

Therefore, ion beam implantation improves the scratch hardness of the polycarbonate and polyimide surfaces. If only low loads are expected during application, then the carbonized surface layer can protect the polymer against scratching. However, if high loads are typical, the carbonized layer will be damaged and the effect of ion implantation will not result in increased resistance to scratching. In summary, the ion beam modification is only effective when the expected abrasive loads are sufficiently low. In such a case, the high hardness of the modified polymer, which corresponds to a highly carbonized surface layer, will be effective in prolonging the life of polymer devices, for example for aircraft windows under dusty atmospheric conditions.

Finally, we would like to note once more that the hardness improvement occurs in a thin surface layer of the polymer. If during mechanical loading (scratching, wearing) the tips (or abrasive particles, etc.) penetrate through the modified layer, the increased surface hardness will not improve the apparent hardness, and the ion beam modification will not effectively reduce wear and damage to the surface.