Self Reporting Intelligent Coatings

What are Functional Coatings?

In the field of coating technology, many coatings have been developed over decades of research, leading to high-quality coatings which have:

• Good adhesion
• Mechanical properties, and
• Other important functions which are defined by the final application and use.

However, next to good mechanical and chemical properties additional functionalities have been introduced to further increase the added value of these coatings. Well-known examples of such functions, just to name a few are for instance:

• Anti-reflection
• Anti-fingerprint
• Corrosion protection, or
• Superhydrophobicity

Smart Coatings Versus Functional
Coatings

Next to functional coatings, the term smart coating is often misused. The question remains: when is a functional coating also smart?

Even though there is no fixed definition of when a coating is smart, the consensus is that the smart coating has the added functionality that it actively reacts to an external or internal trigger, whereas the functional coating is a passive layer which is insensitive to environmental conditions.

Stimuli
As it was mentioned before, smart coatings react to an external or internal stimulus. The main categories that may induce the response of the smart coating are:

• UV Light
• Temperature
• pH
• Moisture
• Damage

Chemical Conversion Coating
In the specific case of corrosion protection, chemical conversion can be used as the smart function within the paint or coating. These chemical conversion coatings contain inorganic compounds which rely on the principle of the dissolve – reprecipitation effect in local defects.

In the figure below we can see the principle of a chemical conversion coating. The active protection of the magnesium alloy takes place in 4 stages:

• The dissolution of magnesium (and evolution of hydrogen gas at the AlMn particles)
• The formation of magnesium phosphate
• The nucleus formation of calcium phosphate and zinc phosphate, and finally
• The growth of the crystalline deposit, giving again protection to the magnesium alloy.

Chemical conversion coatings have the important advantages of having excellent mechanical-stability and the simple preparation method of the coatings. Their limitation lies in the low-efficiency self-healing performance, which limits their application in certain ways.