Unit 8.7 Surface treatments and coatings

Surface engineering is a multidisciplinary activity intended to tailor the properties of the surfaces of materials so that their function and serviceability can be improved. The desired properties of surface-engineered components include:

• Improved corrosion resistance through barrier or sacrificial protection

• Improved oxidation and/or sulphydation resistance

• Improved wear resistance

• Reduced frictional energy losses

• Improved mechanical properties, for example, enhanced fatigue or toughness

• Improved electronic or electrical properties

• Improved thermal insulation

• Improved aesthetic appearance

Process of coating involves application of thin film of functional material to a substrate. The functional material may be metallic or non-metallic; organic or inorganic; solid, liquid or gas.

Coatings provide barriers to certain gases, liquids, radiation, or other substances. These may include oxygen, water vapour, aroma, light, oils and fats, and so on. The barrier is usually intended to prevent both penetration and loss from the package, which could otherwise cause advanced spoilage of the packaged product. The ultimate packaging barriers to oxygen and moisture barrier are metal cans and glass followed by aluminium foils. These materials provide zero gas and vapour transmission and, in the case of glass, also visibility of the product, which is often an important point of sale marketing consideration. The improvements to polymer film technologies, coupled with inorganic and organic surface treatments and coatings, continue to improve their barrier properties. Such coatings include organic solvent and aqueous applied polyvinylidene chloride (PVDC), aqueous or extrusion-coated ethylene vinyl alcohol (EVOH), inorganic and organic sol–gels, and aqueous poly(vinyl alcohol) (PVOH). Alternatives to aqueous or solvent-based coating applications include inorganic layers deposited either as vapour in a vacuum or by various electron-sputtering or plasma enhanced chemical vapour deposition (PECVD) processes. The most common vacuum-deposited barrier coating is aluminium, silicon oxide (SiOx) and aluminium oxide (AlOx) where the proportion of oxygen is nonstoichiometric. These coatings have one distinct advantage of being excellent barriers to both oxygen and water vapour.