Scratch Test: How to Test and Evaluate Results?

Scratch Test: How to Test and Evaluate Results?

Buy cheap, buy twice: this proverb is especially true for tools. That is why professional cutting tools are coated with hard materials to reduce friction and wear. Be it gold-plated watches or jewelry, products with coatings have to be tested for their coating’s adhesive and cohesive properties to ensure they retain their decorative or functional qualities. 

Scratch testing is an important quality control process which simulates the stress a coating undergoes in everyday use under a laboratory setting. In a typical experiment, a diamond indenter with a rounded tip (Rockwell) is pulled across the sample at a constant speed. The resulting scratch on the surface provides information about how the coating will behave in real life. 

Constant and Progressive Loads

The FISCHERSCOPE® ST200 and ST30 are Fischer’s best-selling scratch testers. These instruments offer multiple measurement modes to cover a wide range of applications. For example, the test force exerted by the indenter on the surface can be constant or progressive depending on the application.

A scratch test with constant force applied is typically used to determine the scratch hardness or scratch resistance. For most applications, the progressive mode is used. In this case, the force acting on the surface is increased either incrementally or linearly.

In progressive mode, the indenter begins scratching with a very low initial load. As the force increases, it gradually penetrates into the layer. As the penetration depth increases, the material stress increases as well. This happens until a certain force – the critical load Lc – is reached, this is the force that causes material failure.  At this point, cracks form and the coating may chip off.

For the test to be meaningful, the load must be chosen correctly. If it is too low, the coating is not sufficiently stressed. If the load is too high, the indenter could potentially be damaged.

The ST 200 and ST30 offer a wide range of possible test loads from 0.1 to 200N and 0.1 to 30N respectively to measure hard and soft material layers. With hard material layers such as Titanium Nitride or Diamond like Carbon (DLC), test loads higher than 30 – 50 N are often necessary to trigger large-scale chipping or spallation. With softer materials like paints, low test forces are used since these coatings usually fail at lower forces.

Evaluating results: Comparative and Visual

The scratch test is a comparative procedure that relies on reference measurements. Initial evaluations are always performed visually. Under a high-resolution microscope, the examiner determines at which force the first cracks formed, or when the coating chipped off.

In addition to the microscopic evaluation, the ST 200 and ST30 also record acoustic emissions (the noise the material makes when scratched) and the tangential force (the resistance that the material exerts against the indenter). Signs of material failure include a higher acoustic emission or a change of color when the coating is removed. 

The intuitive user interface of the WIN-SCU software summarizes all three parameters in a single evaluation step: the microscopic image, the acoustic signal and the change in tangential force. This allows critical loads to be defined in just a few mouse clicks.

Diamond like Coating (DLC)

diamond like coating dlc material failure scratch direction
  • Typical layer thickness: 1 – 5 μm on steel, other metals and semiconductors
  • LC1: First cracks usually occur at forces between 10 and 15 N
  • Frequent patterns of damage: Cracks open to the direction of the scratch

Titanium Nitride (TiN)

titanium nitride tin material failure scratch direction
  • Typical layer thickness: 1 – 5 μm on various kinds of steel
  • LC1: First cracks usually occur at forces around 8 N
  • Frequent patterns of damage: The cracks develop at the edge of the scratch track and point mostly outwards

Car Paint

car paint material failure scratch direction
  • Typical layer thickness: 50 μm, with several superimposed layers of paint on fillers and car bodies
  • LC1: First cracks usually occur at forces between 5 and 10 N
  • Frequent patterns of damage: The cracks run along the scratch track and are shaped like a pine tree

Fischer offers a complete quality assurance Solution

The FISCHERSCOPE® ST200 and ST30 are the ideal solutions for testing cohesive and adhesive properties of coatings. Together with our nanoindentation systems and X-ray fluorescence instruments, Fischer offers a complete quality assurance solution for Titanium Nitride (TiN), Diamond like Coatings (DLC) coatings and car paints.

Contact us today for a free demonstration!

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