SURFACE LAYERS CHARACTERIZATION METHODS BASIC AND TUTORIALS

Numerous surface analytical techniques that can be used for the characterization of surface layers are commercially available (Buckley, 1981; Bhushan, 1996).

The metallurgical properties (grain structure) of the deformed layer can be determined by sectioning the surface and examining the cross section by a high magnification optical microscope or a scanning electron microscope (SEM).

Microcrystalline structure and dislocation density can be studied by preparing thin samples (a few hundred nm thick) of the cross section and examining them with a transmission electron microscope (TEM). The crystalline structure of a surface layer can also be studied by X-ray, high-energy or low-energy electron diffraction techniques.

An elemental analysis of a surface layer can be performed by an X-ray energy dispersive analyzer (X-REDA) available with most SEMs, an Auger electron spectroscope (AES), an electron probe microanalyzer (EPMA), an ion scattering spectrometer (ISS), a Rutherford backscattering spectrometer (RBS), or by X-ray fluorescence (XRF). The chemical analysis can be performed using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS).

The thickness of the layers can be measured by depth-profiling a surface, while simultaneously conducting surface analysis. The thickness and severity of deformed layer can be measured by measuring residual stresses in the surface.

The chemical analysis of adsorbed organic layers can be conducted by using surface analytical tools, such as mass spectrometry, Fourier transform infrared spectroscopy (FTIR), Raman scattering, nuclear magnetic resonance (NMR) and XPS. The most commonly used techniques for the measurement of organic layer (including lubricant) thickness are depth profiling using XPS and ellipsometry.

ISOTOPES OF HYDROGEN AND OXYGEN BASIC INFORMATION AND TUTORIALS

The heaviest water must fall first. Aristotle

In ordinary natural water the hydrogen and oxygen each consist of three different isotopic forms with different masses; this fact is of some geochemical significance.

The isotopes present in water are:
1H, 2Hor D, 3Hor T, 16O, 17O, 18O:

The first isotope of hydrogen has a nucleus consisting of only a single proton, and accordingly the mass number is 1. The second isotope has both one proton and one neutron in the nucleus, and the mass number is 2; this isotope has a special name: “deuterium,” hence the common use of the letter “D” to represent the isotope.

The third isotope has one proton and two neutrons, a mass number of 3, and also has a special name: “tritium.” Tritium is radioactive.

The nuclei of oxygen have 8 protons, and 8, 9, or 10 neutrons. These isotopes are present in all possible combinations, so that (not including molecules with tritium) all natural water contains nine kinds of water molecules.

The ratios of the different isotopes, one to another, can be measured with remarkably high precision using an isotope ratio mass spectrometer.

The different isotopic forms of water each have different vapor pressures and freezing points; these physical differences are important because they make it possible to use the isotopes as tracers of geochemical processes.

The differences in vapor pressure lead to a fractionation of the isotopes whenever water evaporates or condenses. The heavier isotopes are, in every case, concentrated into the liquid phase. This leads to differing isotopic compositions in water from different sources.

When water freezes, there is also a fractionation such that the heavier isotopes are concentrated into the solid phase, although the effect is less than in the
gas–liquid exchange processes.