In 1925, two British men developed the Vickers hardness test (HV), which uses mechanical force to determine the hardness of metals or other materials. The force generally leaves a visible impression, which technicians measure to obtain the Vickers Pyramid Number, a number from zero to 900. The higher the number obtained on a Vickers hardness test, the harder the material.
The first part of the Vickers hardness test consists of using a diamond-tipped bit that leaves an impression on the material being tested. The tip of the bit has a microscopic, square-based pyramid shape. Some people refer to the test as the diamond pyramid hardness test, or DPH, because of the shape of the tip. In larger test devices, the bit is positioned in a drill press-like tool that is lowered onto the test object. The tip of the bit makes contact with the material and technicians apply a controlled amount of force for 10 to 15 seconds.
Testers use a digital gauge to indicate the amount of force being applied. After removing the bit, the test object has an inverted pyramid shaped indentation. The press applies anywhere from 2.2 to 220.5 pounds (1 to 120 kilograms) of sustained pressure, and industries use this force for testing the hardness of anything from gems to metals. The Vickers microhardness test typically uses 0.022 to 2.2 pounds (10 to 1,000 grams) of sustained force. Industries use the microtest for determining the hardness of thinner materials and special coatings.
Using a specially designed microscope, technicians measure the length of both diagonals of the impression and average the numbers. They determine the hardness number by using conversion tables or with Vickers hardness testing software. The number is obtained by dividing the amount of force by the squared diagonal average and is written in a specific format, such as 500HV/15. The number 500 indicates the hardness level and the 15 indicates that force was applied for 15 seconds.
Vickers hardness test devices can be handheld devices or free-standing computerized machines. Portable, handheld testers have a force application device attached to a small digital readout monitor. Bench-top versions resemble large microscopes with the diamond tipped bit located on the revolving objective. These instruments apply the desired amount of force, enabling technicians to measure the impression by positioning microscopic lines on the opposing diagonals. The internal software in most modern Vickers hardness test devices automatically performs the hardness calculations.