4.3.3 Surface Preparation: Mechanical Abrasion and Roughening

4.3.3 Surface Preparation: Mechanical Abrasion and Roughening

Increased bond strength is a beneficial side effect of cleaning by mechanical abrasion. Substrate surfaces are frequently roughened to take advantage of this, regardless of any cleaning requirements.

Figure 88 Corroded steel, prepared by grit blasting
Figure 88 Corroded steel, prepared by grit blasting Figure 88 Corroded steel, prepared by grit blasting

Roughening a surface increases bond strength in two ways. The roughened surface presents a larger bond area and adhesive, which flows into microscopic cracks and porosity of the surface, cures solid and mechanically locks into the surface.

The beneficial effect of roughening substrates is measurable, as shown in Table 14.

Table 14 Butt joint tensile strengths with abraded aluminium substrates and an epoxy adhesive. From C.W. Jennings 1972
Adherend Surface Treatment Tensile Strength N/mm2 (psi) Strength Difference from Polished Surface Finish
Polished, 1 µ diamond dust 28.8 ± 7.0 (4,180 ± 1,020) n/a
Abraded Through 600 paper 30.9 ± 7.7 (4,480 ± 1,120) + 7.2%
Abraded Through 280 paper 39.0 ± 6.8 (5,660 ± 0,990) + 35.4%
Abraded Through 180 paper 36.7 ± 7.5 (5,320 ± 1,090) + 27.3%
Sandblasted (40-50 grit) 48.5 ± 7.0 (7,030 ± 1,020) + 68.2%

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