Mechanical surface pretreatment

Grinding, brushing, sanding or blasting are the most important methods. Prior degreasing is required at any rate, since otherwise probable grease residues could be spread on the surface or even pressed into fine pores or other depressions. 

The roughnesses achievable during blasting depend on the jet pressure and the grain size of the shot they range between 50 and 100 pm (1 pm = 1 micrometer = 0.001 mm). 

If the dimensions of the surfaces to be prepared allow, blasting is carried out in closed steel cabins equipped with a collecting and recirculation device for the repeated application of the shot. Since in these cabins the emission of dust particles cannot be excluded, it is recommendable at any rate to install them in rooms separated from bonding work. For larger surfaces so-called back-suction jet systems are recommendable, where the shot will be fed back into the jet circuit by means of a suction device concentrically arranged around the outlet nozzle. 

Due to the abrasive grains striking the adherend surfaces with high energy by means of compressed air, surface densification with ensuing development of tension may occur, entailing a deflection, especially in the case of thin adherends (sheet metal up to 2 mm thick). This phenomenon may be avoided by clamping 

As the compressed air required for blasting is generated in compressors, the presence of small quantities of oil cannot be excluded, which remain on the surface after blasting. For this reason, ensuing degreasing is essential, with the additional advantage that possibly existing shot residues in the surface finish will also be removed. 

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The adsorption and contamination layers shown in Figure 7.7 are removed during the mechanical surface pretreatment. Such layers, for example, adsorbed moisture or dusts, quickly develop again on clean surfaces. For this reason, the time between the surface pretreatment and the application of the adhesive should... 

In contrast to this, mechanical surface pretreatment methods, as described in Section 7.1, are universally applicable. With the process steps... 

Base metal, that is, if stored, layers of different chemical compositions (oxides, hydroxide, oxydhydrates, carbonates) cover the surfaces their adhesion to the base metal does not then guarantee sufficient strength for a bonded joint. Mechanical surface pretreatment is also required. 

Mechanical surface pretreatment, followed by very careful degreasing. Carry out bonding immediately, since especially silver surfaces can change due to silver sulfide formation (dark staining). 

Mechanical surface pretreatment, advantageous with SACO-method (Section 7.1.2.1). 

Brown SR, Turner IG, Reiter H (1994) Residual stress measurement in thermal sprayed hydroxylapatite coatings. J Mater Sci Mater in Med 5 756-759 Brown WE, Chow LC (1983) A new calcium phosphate setting cement. J Dent Res 63 672 Browne M, Gregson PJ (2000) Effect of mechanical surface pretreatment on metal ion release. Biomaterials 21 385-392... 

Compensation behavior occurs in the decomposition of hydrogen peroxide on Ag-Au alloys (25) and, unlike most other alloy systems, there is a systematic change in the Arrhenius parameters with proportions of metals present. This behavior is ascribed to the progressive transformation, with alloy composition, of the reaction mechanism from that characteristic of one metal to that which occurs on the other. In contrast, decomposition of hydrogen peroxide on Pd-Au alloys (27) does not correlate with ratios of metals present in the catalyst, and kinetic parameters are sensitive to surface pretreatment. 

Adhesion promoters are the substances that improve adhesive strength of paints in terms of its resistance against mechanical separation from the painted surface. A large number of different chemical adhesion promoters are available. These include silanes, silicones, titanium compounds, zirconates, amides, imines, phosphates, and specially modified polymers. Furthermore, there are binders, plasticizers, and additives, which have the secondary effect of providing good adhesive strength. Adhesion promoters can be used as additives to the paint formulation, or can be employed in the form of a surface pretreatment. 

In conclusion, we have discussed the use of C clusters as diamond nucleation sites on Si substrates. This nucleation method substitutes the current practice of polishing surfaces with diamond grits. We have also demonstrated how C clusters can be used to selectively grow diamond on Si surfaces. In addition, our process provides a means of better understanding the mechanism of diamond nucleation. From our experiments, we can also speculate the reason why surface pretreatment is not necessary in the case of flame torch diamond deposition methods. We postulate that C clusters formed by the torch are helping to nucleate diamond on surfaces. The use of C clusters for diamond growth on other substrate materials, and the details of diamond nucleation will be reported elsewhere. 

Since hydroxyl groups and element-oxygen bonds are key active sites on the Si, Ti and A1 oxides surface at moderate temperature of a surface pretreatment [1-4,6,22], the most organic compounds interact with these sites during chemisorption according to the following mechanisms. 

Finally, an important difference between mechanical and chemical surface pretreatment methods should be pointed. Concerning their effects, the former are only limited to the adhesive surface area, that is, they are not able to protect the neighboring areas of the adhesive surface against possible stress from the ambience. 

Mechanical removal of adhesive layers (sanding, rotating steel brushes, etc.) with ensuing degreasing has to be carried out as surface pretreatment. 

In spite of these studies and results, the relative importance of the gas-phase nucleation compared to the surface nucleation is unclear as yet. In fact, the number of diamond particles collected from the gas phase is very small compared to the typical surface nucleation densities, thus the homogeneous nucleation mechanism cannot account for the large variety of nucleation densities observed on different substrate materials and from different surface pretreatments. It is speculated and also supported by a recent experimentl l that the nuclei formed in the gas phase may reach the growing surface and increase the surface nucleation density. However, how the diamond particles formed in the gas phase could serve as seeds on the substrate surface for the subsequent growth of a diamond film remains unknown. 

SURFACE PRETREATMENT METHODS AND NUCLEATION ENHANCEMENT MECHANISMS... 

Figures 32(a and b) show typical microscopic pictures of FFC on polymer-coated iron, and aluminum. FFC develops in the presence of pores, mechanical defects, unprotected cut edges, or residual salt crystals underneath the organic coating. The corrosion filaments start growing perpendicular from a defect into the polymer-coated area. FFC occurs only at moderate humidity (60-95%) and therefore, not under full immersion conditions. FFC has been found to be triggered by anions such as chloride, bromide, and sulfate. The filament growth rate increases with temperature. Like for cathodic delamination on iron and zinc the corrosion kinetics depend strongly on the surface pretreatment and coating composition.

The purpose of surface preparation is to remove contamination and weak surface layers, to change the substrate surface geometry, and/or introduce new chemical groups to provide, at least in the case of metals, an oxide layer more receptive to the adhesive. An appreciation of the effects of pretreatments may be gained from surface analytical or mechanical test techniques. Experimental assessments of the effects of surface pretreatment, even when using appropriate mechanical tests, are of limited value unless environmental exposure is included. Self-stressed fracture mechanical cleavage specimens, as discussed in Chapter 4 and in the texts edited by Kinloch(2,5) for example, are therefore referred to wherever possible. 

Some comparisons of the effect of surface pretreatment on mechanical joint strength, or measurable adhesion, are reproduced in Tables 3.5-3.7. It is stressed again that the important consideration is the effect on long-term bond integrity, and not on short-term strength. 

Experimental assessments of the effects of surface pretreatment are of limited value using mechanical tests unless environmental exposure is included. It is very sound policy to collect and examine information on joints loaded and exposed to natural weathering conditions rather than depend solely on laboratory experiments. It is clear that water is the substance which causes most problems in attaining environmental stability of bonded joints interfacial failure generally indicates that a better surface pretreatment would impart improved joint performance. 

The control of surface pretreatment procedures may be done optically, by assessment of surface wettability, by surface analytical means, or by the use of simple mechanical test procedures. (See Adhesion control .)... 

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