CT-07 - Coating thickness gauge

In engineering, a coating is classified as a material that is deposited onto the surface of another material (commonly referred to as a substrate) to specifically enhance the properties of the surface for functional or decorative purposes.

A number of factors dictate the type of coating that would be applied to the material surface which include cost, life of service of the component, substrate the coating is applied to, size and shape of the component, component environment and most importantly the function of the component that is to be coated. Coatings are applied to surfaces according to specific standards, such as ISO, BS, ASTM, DIN etc., additionally, the thickness of the applied coating is dictated by its function and cost. Typical coating thicknesses can range from a few nanometres to several centimetres. As an example, Titanium Zirconium (TiZr) coatings can be 1nm to 5nm thick, while anodic films can range from 0.5μm to 150μm, paint coatings can be up to a couple of millimetres in thickness.

Coatings fall into two distinct categories: layers formed by reactions involving the modification of the surface of the substrate material and those that are deposited on the surface of the substrate material.

Coatings formed by reactions involving the surface of the substrate tend to form a layer at the substrate surface by chemically acceleration reactions that naturally take place in order to protect the substrate from for example corrosion. Anodising is one of the most notable of these methods which can be used to protect the underlying substrate material.

The latter types of coatings are more varied and offer different properties depending on the coating that is applied to the substrates.

Most common types of coatings applied to substrates are those applied in solution by a reduction reaction of ions and include electro-deposited coatings (chrome, gold, silver etc) or by electroless deposition methods (eg. Electroless nickel plating).

Why measure coatings?

Coatings that have functional roles, are usually highly regulated by standards BA, ASTM, DIN, ISO, QUALICOAT, QUALINOD etc. Coatings performing specific functions have to be regulated. In the example of the TiN coating in the engine, the coatings are applied to strict tolerance limits. Above the specified limits, the coatings would impede the function of the engine components, while below these limits the components may fail without adequate protection from the frictional stresses it is applied for. When components are manufactured the coatings are checked to ensure they are within these parameters; alternatively, coatings can be measured at various intervals during the life of a component, to ensure that they have not been worn away. Coatings wearing away can cause corrosion to occur in the substrate material potentially causing catastrophic failure, especially if that component is used in the automotive, aerospace, rail or marine industries.

A second reason to measure coatings is strictly from a financial standpoint. Current prices of gold, platinum and silver for example, mean that companies that manufacture components with these coatings can be losing money if they add too much of the coating to the components. An example is the electroplated silver on copper wires used in cabling for the aerospace sector. In order to ensure that wires meet the strictest standards for this sector, it may be common to apply more silver to the copper wire, to ensure the wire is in specification. For manufacturers this can translate into an accumulated cost of $250,000 per year in just over-depositing the silver coating. Another example is the automotive industry applying paint and lacquer to the car bodies. By saving 1μm on the paint/ lacquer that is over applied, the manufacturers can save £1 per car. In 2019 approximately 2.3 million cars were registered in the UK, suggesting that a potential of £2.3 Million can be saved per year (Society of Motor Manufacturers and Traders, 2020). Thus, the need to measure the coatings becomes clear.