T

Figure 5. Comparison of (a) SFM and (b) SEM images of TiAl6Nb7 alloy treated with anodic oxidation under galvanostatic conditions in 1 m H2SO4 at room temperature with a current density of 10 mA/cm2 up to a final potential of 40 V. The SEM image shows the surface on a larger scale exhibiting the differences of the a and 3 phase of the alloy. The small black dots that cannot be further identified with this technique are characterized further with SFM to yield topographical information of the structure. SEM image reprinted with permission from [245], C. Leinenbach et al., Mater. Sci. Eng. Technol. 8, 442 (2002). © 2002, Wiley-VCH.

Figure 5. Comparison of (a) SFM and (b) SEM images of TiAl6Nb7 alloy treated with anodic oxidation under galvanostatic conditions in 1 m H2SO4 at room temperature with a current density of 10 mA/cm2 up to a final potential of 40 V. The SEM image shows the surface on a larger scale exhibiting the differences of the a and 3 phase of the alloy. The small black dots that cannot be further identified with this technique are characterized further with SFM to yield topographical information of the structure. SEM image reprinted with permission from [245], C. Leinenbach et al., Mater. Sci. Eng. Technol. 8, 442 (2002). © 2002, Wiley-VCH.

Friction force microscopy is done by monitoring the twist of the cantilever while scanning over the surface [73]. With this nanomechanical measurement the shear strength can be determined. An overview on friction on the atomic scale is given in [74]. After chemical modification, the tip of the cantilever can be used to gain information about the chemical properties, respectively forces between a biomolecule and a biomaterial surface. Images are produced that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Surfaces can be imaged in friction mode down to a resolution of about 0.5 nm [21]. An overview over several different chemical interactions of model surfaces with different chemical modifications is given in [75]. An image contrast is achieved between regions terminated by different functional groups [76] but can be influenced by topographical features. Hydrophobic and hydrophilic parts of the surface can be distinguished [77]. It is even possible to gain information about chirality [78], which in this case was done, on immobilized molecules by investigation of the friction and adhesion force. The load dependency is another important parameter. The atomic stick slip mechanism and interfacial friction, adhesion, and elastic properties measured by friction force microscopy are reviewed in [30].

In scanning spreading resistance microscopy (SSRM), different conductivity of the substrate surface is monitored by applying a voltage and measuring the resistance while scanning in contact mode with DLC-coated tips for better wear properties [79].

In force modulation mode (FM), the force between the cantilever and the sample surface is modulated by vibrating the sample respectively the tip in vertical direction while scanning in contact mode. The modulation frequency is in the range of 5 to 30 kHz with a cantilever resonance frequency of more than 200 kHz. The lateral distribution of the hardness can be obtained by monitoring the amplitude changes [80].

Imaging at the resonance frequency of the tip in contact with the surface is called contact resonance imaging (CRI) [81] and can be used for imaging soft and sticky surfaces like proteins on surfaces.

The modulation of the probe can also be carried out just above the resonance frequency of the cantilever to monitor viscoelastic and anelastic properties of submicron phases. This mode, which can also be done with temperature ramping, is called scanning local-acceleration microscopy (SLAM) [82].

A new technique called atomic force acoustic microscopy (AFAM) uses ultrasonic actuation of the cantilever while scanning in contact mode above its resonance frequency. It is possible to measure the contact stiffness and to distinguish between different orientations of a silicon wafer surface [83].

Besides the actuation of the sample in the normal direction, a high-frequency lateral vibration can be applied. By doing so, local measurements of shear strength and Young's modulus can be undertaken. The indentation of the tip into the surface is reduced and accordingly the possible damage. This method was introduced as harmonically modulated lateral force microscopy (HM-LFM) in [84, 85].

0 0

Post a comment