Understand your test material better and decide what hardness test to perform on it.
Hardness refers to a material’s level of resistance when force or abrasion is applied. There isn’t one singular measurement that represents the idea of hardness though because it isn’t an intrinsic property of materials. Tests use different indentor points that are usually diamond tips or metal balls. These are pressed into the test material with force and the resulting indentation is measured. There are a number of different tests that use relative indentation scales to measure hardness levels. Knowing which type of material you have and its physical properties will determine the best indentor point to use.
1. Determine the type of material you are using for the test. Even though you may not know the hardness measurement on a particular tested scale, you can still categorize the material into an expected range. If you aren’t sure the exact type of material being used you can perform a Mohs hardness test to get a general range. Mohs uses different tools of relative hardness to scratch the surface of your test material. Talc is the lowest level on the 10-point scale and diamond is the highest. Use a Mohs tool kit to scratch your test material. Use each tool starting from Level 2, Gypsum, and move up until you find a tool that will scratch the surface of the test material. When scratched, the test material is softer than that tool and you can place it on the Mohs scale.
2. Use either the Mohs relative hardness level you determined earlier or the type of material to determine which hardness test to perform. For instance, the Rockwell B test is best for materials slightly softer than calcite to slightly softer than orthociase — Levels 2 to 6 on the Mohs scale. This also corresponds to materials from brasses and aluminum alloys to easily machined steels. Likewise, the Rockwell C test starts at Mohs Level 4, flourite, and can be used for materials as hard as topaz. The Knoop hardness test can handle materials that are slightly softer than calcite to slightly softer than topaz and includes some aluminum alloys, easily machined steels and cutting tools. The Brinell test can be used on virtually all metals and even softer materials like wood, making it an option for fragile materials. This is because the metal or diamond points on other tests can crack more fragile materials, but the larger spherical point on the Brinell test distributes the load on a wider surface area.
3. Look at the different recommendations for points and load weight for the test you have chosen. Many materials can be tested on different scales and with different points, but the best one to use depends on the material itself. For instance, if you are using the Rockwell B scale on nonferrous metals you would use a 1.6 mm ball indentor with a load of 100 kg. Meanwhile, the Rockwell C scale calls for a diamond cone indentor point and a 150 kg load and is used for ferrous metals and steel tools
4. Determine potential surface area that can be tested. For instance, Rockwel steel ball indentors come in a variety of sizes such as 1/16, 1/8, ¼ and ½. The area measured from the center indentation point to the material edge should be a clean, flat surface that is perpendicular to the indentor point. Measure this diameter starting from the center point of the indentation to the edge of your material. This should be 2 1/2 times the diameter of your indentation. The larger the indentor point used, the more accurate the hardness test; however, make sure you have the width available.
5. Determine the thickness of your material. You want a depth of 10 times the indentation for diamond indentors and 15 times the depth for ball indentors. Be sure there are no known deformations in the material as this can affect the test results. Once you know the depth of your material, you will know the load that can be used and the indentor points that are possible.