A scratch tester is a materials testing device for determining the amount of stress a coating can endure before it fails. In a scratch test, the operator sets the machine to drag a stylus across a surface at a controlled rate, and determines when the coating starts to crack, chip, or delaminate. All coatings will eventually fail under pressure, and the goal of scratch testing is to make sure a coating will meet the requirements of a given application.
Typically, a scratch tester has a diamond stylus. Industrial diamonds are extremely hard and well suited to this type of testing. Depending on how the technician programs the machine, it can pull the stylus at a consistent rate of pressure or it can vary the pressure. Using incremental pressure increases can allow the technician to more accurately track the behavior of the material under stress.
People must view scratch testing results under a microscope. Failure is often invisible to the naked eye in the early stages. The technician uses the scratch tester and microscope evaluation to identify the precise level of pressure required to make the coating start to fail, and writes up the result of the testing to show how higher pressures break the bond between the coating and the underlying substrate. If the failure pressure is unusually low, it may be necessary to redesign the coating or think of a different application method to make sure it will maintain its integrity.
A scratch tester can vary in size. Some have hand cranks, and others have computer-controlled systems for changing pressure and direction. Companies specializing in industrial testing usually have such machines available for use, along with a variety of other equipment to strain and stress materials. Some companies may maintain their own scratch tester for frequent testing if they have concerns about quality control or the security of product development. Sending materials out for testing can create a weak point in the security chain, potentially allowing people to release confidential information about products currently in development.
The scratch tester needs periodic calibration to make sure it works accurately. This can be done by the technician who operates it or a specialist from the manufacturer can perform testing and any necessary repair. For materials testing facilities, logs are available to show how and when people most recently performed service. This allows customers to determine if the facility is using appropriate quality control measures to keep their testing accurate and reliable.
