Surface Analysis Techniques: STM, AFM, and Tribocorrosion

Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM)

Scanning Tunneling Microscopy (STM)

Scanning Tunneling Microscopy (STM) is an instrument used for imaging surfaces at the atomic level, revealing the topography of conductive specimens. It measures tunneling current and can be used in air, water, and various gas environments, at temperatures ranging from near zero Kelvin to a few hundred degrees Celsius. STM operates in two modes: constant interaction mode and constant height mode.

Advantages of STM

• High resolution, not limited by diffraction
• Small sample size
• Can modify samples to create small structures
• Does not require a partial vacuum

Disadvantages of STM

• Difficult to determine detailed shapes
• Slow scanning speed
• Small image sizes
• Not suitable for buried solid-solid or liquid-liquid interfaces

Examples of STM Applications

• Roughness measurement
• Searching for fractal behavior
• Catalysis research
• Atom manipulation

Atomic Force Microscopy (AFM)

Atomic Force Microscopy (AFM) is a very-high-resolution type of scanning probe microscopy (SPM), with resolution on the order of fractions of a nanometer.

Advantages of AFM

• 3D surface profiling
• No special sample treatments needed
• Works in air or liquid environments
• Higher resolution than SEM

Disadvantages of AFM

• Limited image and scanning area
• Slow scanning speed
• Images can be affected by piezoelectric material
• Not suitable for measuring steep walls or overhangs

Examples of AFM Applications

• Biochemistry
• Chemistry
• Materials science
• Nanotechnology
• Physics and biophysics

Techniques Using Micro-Tips for Electrochemical Surface Studies

Several techniques utilize micro-tips to study the electrochemical behavior of surfaces. Here are some examples:

Capillary-Based Technique

This technique involves a micropipette filled with electrolyte. The tip is polished, and the micropipette is mounted on a microcell, which is fixed on a microscope for precise positioning. It is used to observe the influence of microstructures, deformations, and crystallographic orientations.

Scanning Reference Electrode Technique (SRET)

Scanning Reference Electrode Technique (SRET) is used to determine the distribution of current. It employs a reference micro-electrode to map potential distribution. SRET is connected to engines for surface mapping, revealing information about potential distribution.

Scanning Vibrating Electrode Technique (SVET)

Scanning Vibrating Electrode Technique (SVET) measures the voltage drop in a solution to image the current at the sample surface. The tip vibrates in the Z-direction, the potential is measured between two points, and the current is calculated using Ohm's law. It is used for the observation of oxides and inclusions.

Resolution: Micropipette > SVET > SRET

Tribocorrosion: Mechanisms and Impact

Tribocorrosion is an irreversible material degradation process resulting from the combined effects of corrosion and wear. The term tribocorrosion reflects the underlying disciplines of tribology and corrosion.

Main Mechanisms Involved in Tribocorrosion

• Wear
• Corrosion
• Friction
• Fatigue

Surface Modification by Welding

Surface modification by welding involves the addition of metal to a surface to restore the component.

Methods of Surfacing by Welding

• Arc Welding: Uses a welding power supply to create an electric arc between an electrode and the base material. Types include SMAW, GMAW, and GTAW.

• Gas Welding: Employs a focused, high-temperature flame generated by gas combustion. An example is oxy-fuel welding.

• Resistance Welding: Generates heat by passing a high current through the resistance caused by the contact between metal surfaces. Types include spot welding and seam welding.

• Energy Beam Welding: Uses a focused, high-energy beam.

• Solid-State Welding: Does not melt the materials being joined. Types include ultrasonic welding, explosion welding, and friction welding.