Paint Tribology

The Technology of Paint

Paint is more complex and precisely engineered than is often appreciated.  It often consists of many different layers - each chosen to give a precise function.

Automotive Paint

Automotive paint has been engineered to withstand

• Fading by sunlight
• Impact from stones and road chippings
• Damage from rain, oil and petrol
• Damage from car washing

The UK car market is dominated by the fleet car market (~59% of the market) where aesthetic appearance is critical.  The UK automotive industry bought coatings worth £100 million in 1998, and the refinish market bought another £140 million.  This is therefore, an area of significant industrial interest.
 

Automotive paint systems comprise of up to five different layers of paint.  The system typically consists of:

• a galvanized steel substrate with a thin crystalline tri-cation phosphate treatment to
 enhance adhesion and improve corrosion protection;
• a ~25µm electrocoat is then added which is the first protective layer.  This is electrodeposited and then cured;
• lower panels have an antichip layer, which is a thick layer designed to give protection against impact from
 stones;
• this is then followed by a primer layer (~25µm), a basecoat layer (which is the colour layer, thickness ~25µm)
 and a clearcoat (50µm).
 

The basecoat layer includes the pigments which give the car its colour whilst the clearcoat layer contains UV absorbers to prevent UV degrading the colour of the paint below.  The clearcoat is also designed to give a glossy finish and provide scratch and mar resistance.

The mechanical properties of the system, and the ability of the system to withstand damage, depend on the mechanical properties and thickness of each layer.

Research Programme

The programme of research is using scratch and indentation testing to investigate the mechanical properties of the paint layers.

Why use Scratch Testing?

Traditionally, the paint industry has used simple tests such as the pencil test, which involves seeing which "hardness" of pencil will scratch a coating.  This is not particularly quantitative, and is dependent on the operator.

Other tests, have involved preparing thin coatings on glass and then measuring properties in a tensile test.  This allows the yield stress and fracture toughness to be determined.  Specimens are difficult to prepare, especially as harder and more brittle clear-coats are produced.

Scratch testing allows us to perform tests which measure the critical load at which cracks initiate in a repeatable way.  The micrographs below
show some of the results obtained.  These results are being used with contact mechanics models to determine the critical failure stresses for the paint layers.
 
 

Scanning electron micrograph showing initiation of cracks, the critical load, Lc is 8N.  The scratch direction is from left to right.	Scanning electron micrograph showing propagation of cracks, and "unzipping" of the clear-coat of the paint system.  The scratch direction is from left to right.
Back-scattered scanning electron micrograph of a scratch in a paint system.  The light contrast indicates high atomic numbers and the darker contrast indicates lower atomic number materials.  This allows us to determine whether the cracks have propagated through a complete layer.  The light areas here indicate that the scratch has penetrated through to the primer layer. The scratch direction is from top to bottom.

 

Anyone interested in further details of the research in progress should contact me at:
 

Department of Engineering
University of Leicester
University Road
Leicester
LE1 7RH

Telephone: 0116 252 5692
Fax: 0116 252 2525
 
 

E mail: svh2@le.ac.uk
 
 

Author: Sarah Hainsworth, last updated 17/3/2004.