# 4.1 Mechanical Interlocking Theory

> From: LOCTITE®X Digital Manual for Bonding (https://www.loctitex.com/en/manuals/bonding/4.1-mechanical-interlocking-theory/)

### 4.1 Mechanical Interlocking Theory

The mechanical interlocking theory states that the adhesion between the adhesive and the substrate is the result of the penetration of the adhesive product through the irregularities or pores that the substrate may have. The surfaces are bonded to each other by the locking effect of liquid flowing into the undercuts and the additional surface area created by the irregularities and then solidifying. Increasing the surface roughness to add more cavities can increase this effect.

_Figure 74 Increased surface area and undercuts created by an irregular surface_

![Figure 74 Increased surface area and undercuts created by an irregular surface](/en/manuals/bonding/assets/img/surface-undercuts-increased-area-roughness-adhesive.jpg)

![Figure 74 Increased surface area and undercuts created by an irregular surface](/en/manuals/bonding/assets/img/surface-undercuts-increased-area-roughness-adhesive-1.jpg)

![Figure 74 Increased surface area and undercuts created by an irregular surface](/en/manuals/bonding/assets/img/surface-undercuts-increased-area-roughness-adhesive-2.jpg)