Surface chemical analysis failure surfaces

Table 1.3 Surface Chemical Analysis (ESCA) in a Cohesive Failure of Adhesive Bond...

Ion beams provide useful information either as a diagnostic tool or as a precision etching method in adhesion research. The combination ISS/SIMS method used along with other techniques such as SEM provides a powerful tool for elemental analysis of surface composition. These results, as well as earlier work in this laboratory, indicate that the surface composition can be significantly different from the bulk due to contamination, selective chemical etching and segregation. These same techniques also provide an analysis of the mode of failure in adhesive joints. Many failures classified as "adhesive" on the basis of visual inspection are frequently mixed mode failures or failures at a new interface containing elements of both adhesives and adherend. 

As in the case of corrosion failures, the sequence of steps involved in analyzing wear failures are initial examination of the failed component including service conditions to establish the mode or combination of modes of wear failure, metallographic examination to check if the microstructure of the worn part met the specification, both in the base material and in the hardened case or applied surface coatings, existence of localized phase transformations, shear or cold worked surfaces, macroscopic and microscopic hardness testing to determine the proper heat treatment, X-ray and electron diffraction analysis to determine the composition of abrasives, wear debris, surface elements and microstructural features such as retained austenite, chemical analysis of wear debris surface films and physical properties such as viscosity and infrared spectral determination of the integrity of lubricants and abrasive characteristics of soils or minerals in the cases of wear failures of tillage tools. 

Chemical analysis of scale deposits present on the surface of the failed clamp by X-ray diffraction revealed the presence of predominantly sodium iron oxide, sodium carbonate sodium chloride ( 10%), iron oxide and iron sulfide. The scale composition was consistent with the evaporated residue from the 80% quality steam, which had been leaking from the joint prior to the failure. The high sodium concentration in the scale was attributed to the zeolite ion exchange system used to soften the boiler feedwater, while the chlorides and sulfides were naturally present in the feedwater. 

The pipe joint had been leaking steam and water prior to the failure, and chemical analysis of the scale deposits on the clamp surface after the failure confirmed the presence of a number of sodium-based mineral compounds from the leaking steam, including approximately 10% sodium chloride. The presence of high concentrations of moist, hot chloride salts on the highly stressed austenitic stainless steel surface, particularly with concurrent exposure to atmospheric oxygen, created an ideal chloride stress-corrosion cracking (SCC) environment. 

Sheppard, W.L., Jr., Failure Analysis of Chemically Resistant Monolithic Surfacings, Chem. Engr. (July 23,1984). 

The following material is from "Failure analysis of chemically resistant monolithic surfacings." Reprinted by special permission tom Chemical Engineering, July 23,1984 by McGraw-Hill, Inc., New York. 

Historically, electron spectroscopy has matured In two separate but related areas. One has been the use of electron spectroscopy as applied to analytical problems, especially those that relate to surfaces, such as failure analysis, corrosion, catalysis, or tribology. In such studies, the technique Is often used In conjunction with other techniques such as low energy electron diffraction (LEED), secondary Ion mass spectrometry (SIMS), or Ion scattering spectroscopy (ISS). Another related area Is the use of electron spectroscopy to examine the electronic structure of materials or chemical species. 

Tables 1.3-1.5 show the result of analyses of several bonds between a substrate and a polyvinyl fluoride him using an acrylic adhesive. All surfaces were analyzed by electron spectroscopy for chemical analysis (ESC A). ESC A yields chemical analysis of organic surfaces in atomic percentage, with the exclusion of hydrogen, which is undetectable by this technique. To determine the type of bond failure, ESCA results for the failed surfaces are compared with those of the adhesive and the polyvinyl fluoride him.

Electrocatalytic reactions occur on catalyst surfaces. The catalyst surface structure and chemically bonded or physically absorbed substances on the catalyst surface exert strong influences on catalyst activity and efficiency. X-ray photoelectron spectroscopy (XPS) (also known as electron spectroscopy for chemical analysis (ESCA), auger emission spectroscopy (AES), or auger analysis) is a failure analysis technique used to identify elements present on the surface of the sample. For instance, this can be used to identify Pt and carbon surface chemical species that may present histories of chemical reactions or contamination in the catalyst layer. AES and XPS can also provide depth profiles of element analysis. Wang et al. [41] studied XPS spectra of carbon and Pt before and after fuel cell operation. They observed a significant increase in O Is peak value for each oxidized carbon support, the result of a higher surface oxide content in the support surface due to electrochemical oxidation. However, sample preparation in AES and XPS analysis is critical because these methods are very sensitive to a trace amount of contaminants on sample surfaces, and detect as little as 2-10 atoms on the sample surface. 

Further, the techniques of Scanning Electron Microscopy (SEM), Specular Reflectance Infrared Spectroscopy (SRIS), and Electron Spectroscopy for Chemical Analysis (ESCA) have proved complementary in this investigation of the relationships between adherend surfaces and adhesive properties. As SRIS results have shown the presence of adhesive on all fracture samples, ESCA and SEM results further clarified the nature of the fracture surface through the presence or absence of a Ti ESCA spectrum and the observation or lack of observation of the substrate structure in the SEM photomicrographs. It is concluded from the results of the three techniques that for the Set I samples, cohesive failure was noted for 219D2 whereas adhesive failure was noted for 220D3. Cohesive failure was noted for samples lm2-517 and lmp2-516 and adhesive failure was noted for 2m2-515 in Set II. 

As water molecules can easily penetrate through the adhesive and oxidize/hydrate the bonding surfaces, ICA joints with different metallizations have different reliability performances in high humidity environments. Several investigations (Ref 10-12) showed that joints with noble Au and Ag-Pd metallizations had much lower resistance increases, compared with those with non-noble Sn-Pb and Cu metallizations. With transmission electron microscopy (TEM) and electron spectroscopy for chemical analysis (ESCA), the detailed failure mechanisms were investigated by Liu et al. (Ref 10). 

The use is demonstrated of microscopic analysis techniques for the investigation of adhesive failure. PVC sheet was bonded to glass using a M35R hybrid UV curable adhesive based on epoxy resin. Atomic force microscopy and X-ray induction photoelectron spectroscopy were used for the chemical characterisation of of failure surfaces. 11 refs. 

In aluminum alloy (2024-T3)-epoxy joints, for exanqrle, the initial oxide produced on the aluminum substrate is usually amorphous AI2O3. Upon exposure to moisture, AI2O3 is converted to aluminum hydroxide with a chemical composition between that of boehmite (AI2O3H2O) and pseudoboehmite (A1203-2H20). Failure surface analysis reveals that the hydroxide layer is normally attached to the adhesive side, suggesting that adhesion of the hydroxide to aluminum is very weak. Thus, once a hydroxide is formed, it is separated easily from the substrate, causing failure of the joint. 

The features of these layers were validated by Pauschitz et al. [79,21] using a series of systematic but comprehensive experiments. The formation of these layers and their failure mechanisms are presented in Fig. 6.16 schematically [79]. Their presence can explain all the observed behaviour of elevated temperature sliding wear of metallic materials [20]. Furthermore, by carrying out systematic chemical analysis of the worn surface and the wear debris it is possible to identify the various types of layers that form during elevated temperature wear. Further, systematic study by optical interferometer can also identify the types of layers that form and their failure mechanisms. By using both methods a complete failure analysis of the worn surface can be carried out without destroying the wearing sample. 

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