Hardness testers should look like big industrial presses right? Not so fast. The Vickers method uses basic geometry, the simplicity of which make it applicable across all materials from soft aluminum to hard ceramic. By applying the same type of indenter (a diamond pyramid) at the exact same angle (136 degrees), one tester can measure any material’s hardness. Engineers know that by loading up the tester properly, they only need one tester to span all material strengths. You do not need a special tester for coatings or a special tester for steel. You only need one machine if you does it right. It’s all about the geometry, Most folks don’t know that it doesn’t really matter what size indenter is used as long as the geometry are the same (in this case, the indenter is a pyramid which imparts a geometry that is similar no matter what size). So you can get away with using tiny loads when testing very thin film and very large loads when testing large volume forgings all while getting meaningful comparisons.
What you’ll see in the chart above are the diagonals read out in terms of hardness numbers based off load, but the key is the load categories themselves. For things like thin coating you need to test at Micro Vickers loads expressed in grams of force. Testing a very small coating with a macro load means punching through into the underlying material. That screws up your quality control run. Match the load to the feature size.
How to Use Vickers Hardness Testing Correctly
A gram to a thousand-gram range of loads is good for thin coatings such as diamond-like carbon and titanium nitride coatings. Those micro indentations will show you the variation in hardness from weld heat affected zones, all the way through to the case hardened portion of the piece, while not harming the component. On coated surfaces, the load needs to create an indentation no deeper than one tenth the thickness of the coating. Anything deeper and you’re indenting down into the substrate. That is not what you wanted to do here, that’s wasted effort instead of an accurate result.
For the heavier materials (carbides and tool steels), the size of the shift is significant. Hardened high speed steels are roughly 870 HV and tungsten carbides is more like 2400 HV. This chart also allows for a comparison between old specs using older testing methods and new testing. For this reason, it includes conversions into other common measurement scales such as Brinell and Rockwell C. Often you’ll find a formula that takes the Vickers hardness and multiplies it by 3.4, yielding an approximate tensile strength expressed in megapascals for steels. This is a convenient number to use for quick estimates, but it is only an approximation. When accuracy matters, always use actual testing results.
Materials are grouped by their visual reference which shows a pattern in their metallurgical properties. The ones at the top like carbides and ceramics has high hardness but low toughness; those in the mid-range such as titanium and alloy steels strike a good balance between strength and ductility. This grouping helps you to anticipate what behavior to expect from a material when stressed. Remember, this is more than simply finding a number. It’s about finding the right property profile that matches your intended application. Though they may appear alike on paper, a turbine blade doesn’t need the same characteristics as a bearing race.
There are always variables in testing; the unknown variable here is sample preparation. How clear are the edges of the indentation? Is the surface rough? Are you measuring diagonally accurately? Polish the test area. That is the basis for quality test data. Also, make sure there’s enough space between each set of indents so as to avoid influence on the next. The common guideline is 2.5 x the diagonal distance apart. Fail here and you’ll have permanent bending of adjacent tests which skews your results.
The Vickers hardness test is based on the idea of applying a specific amount of force to a surface to see how much it can absorbs before it deforms. The nice thing about this test is that no matter what material you have you use the same language. So a huge casting has the same data as a medical implant. Apply your force. Let it sit for its dwell time. Read the diagonal, and crunch the numbers. It seems easy enough but taking the time with details makes a big difference. The diamond pyramid gives you nothing but objectivity, so sloppiness means wrong answers. You should of taken the time to learn it, and you’ll discover the data will tell you everything you want to know about part integrity.
