Damaged surface layer depth

XPS is very useful for the study of surface layers and corrosion films. In the case of corrosion films and oxides it is important to do depth profiling by coupling XPS with ion milling of the surface. Another important aspect of XPS is that the incident X-rays cause negligible damage to the surface. 

Figure 8.4 Static SIMS. Primary ion bombardment results in surface damage and emission of ions and neutral atoms. The damaged area is quantified by the damage cross-section (a ). In static SIMS the total primary ion dose should be limited so that only small portion of the surface is damaged, a, surface layer b, bulk c, damaged zone d, depth of damage zone r, radius of damage zone.

We can estimate the time scale in which the whole surface layer is affected by the primary ions. The lifetime of a surface may be simply estimated from the primary ion flux (Ip) and damage cross-section (er) generated by each impact. Ip is commonly measured in A cm-2 (1 A = 6.2 x 1018 charged particles per second). Assume that each primary ion generates a = 10-13 cm2. Then, 1013 primary ions cm-2 will affect the whole surface area of 1 cm2. It means that the lifetime of a surface with the flux density Ip= 1 pA cm-2 (= 6.2 x 1012 ions cm-2) is less than 1 second. Apparently, 1 p A cm-2 of flux density for primary ions is too high for static SIMS. Since it is commonly accepted for the static SIMS condition to limit the total amount of primary ions up to 1013 ions cm-2, for a 10-min duration of static SIMS examination a primary flux density of about 2.7 nA cm-2 is required to preserve the chemical structure of the surface top layer where the secondary ions are emitted. This flux is extremely low compared with that of dynamic SIMS, which requires a flux density of greater than 1 pA cm-2 to ensure a reasonable erosion rate of surface for depth profiling. 

RBS can provide absolute quantitative analysis of elemental composition with an accuracy of about 5%. It can provide depth-profile information from surface layers and thin films to a thickness of about 1 pm. In some cases, however, the high-energy beam can damage the surface. This is particularly a problem with insulating materials, such as polymers, alkali halides, and oxides. The Mars Pathfinder mission in 1997 contained an alpha proton X-ray spectrometer (APXS). In its RBS mode, the spectrometer bombarded samples with alpha particles and determined elemental composition via energy analysis of the backscattered particles. In addition to RBS, the APXS instrument was designed to carry out proton emission and particle-induced X-ray emission (PIXE) experiments. Soil and rock compositions were measured and compared to those from the earlier Viking mission. 

Thompson, Wadsworth e Louat [52] testing mirror-polished specimens of polycrystalline high purity copper, found that if they would interrupt tests, from time to time, and electro polish a surface layer 2 pm depth, most of slip lines and bands would disappear with the exception of some that, instead, would persist. This was the sign of a surface damage that, though microscopic, was deeper than 2 pm. 

As structures made of concrete and allied materials are involved in approximately half of the repair and renovation work, it is important to understand and evaluate the performance of the materials to be used to make good damaged/eroded concrete,and their composite behaviour with the parent structures. While there are many cases where replacement of material in depth is required these are likely to be individual rather than general cases. However, a high proportion of repairs involve the application of surface layers or patches varying in thickness from a few mm to 50 or 100 mm. 

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