The Vickers hardness test provides the hardness (HV) of a material by calculating the force over area of the indentation. The formula is HV = 1854.4 x Force/d2 where the force is in gf. And d is the diagonal length in microns. There are variations to change the formula for macro Vickers and the Knoop scale as well but the idea is similar. The Knoop formula uses the projected area while the Vickers scale uses the surface area calculation. Most digital testers do this calculation as well as averaging the diagonals for you and most non-digital testers are supplied with look up tables to convert the filar units to the correct hardness. There are 3 main components to the hardness tester itself and each one needs to be in the proper working order to get the most accurate measurements from your tester. The first component is the loading system of the tester. The tolerances of the forces and speeds at which indenter contacts the sample are all specified in the ASTM E384-10 document for Vickers and Knoop testing. The manufacturer should certify that all these criteria are met when the tester is produced and there is little that you as the operator can do to check these individual components. Therefore it is important to validate your tester on a regular interval as defined in the E384 document. The indenter itself is also very difficult for the user to validate other than to check for abnormalities in the indentation like rounded corners or a larger offset where the faces meet at the tip of the indentation. Sometimes the indenter becomes coated in oil and will leave a rainbow effect on the indentation making it difficult to measure correctly. A little alcohol on a cotton swab can be used to clean this but it is often easier to place a piece of paper between the sample and the indenter and make a test indentation. Of course this indentation should not be used for measurements. Assuming that the mechanics of the testers are working properly, the last and most commonly Overlooked component is the optical measuring system. And please include the operator in that system! The main component is the objective lens which should be chosen for magnification according to the recommendations listed in the ASTM specifications. These lenses usually are very close to the sample surface and may accidentally come into contact with the sample causing scratches or even cracking. This may only show up as a slightly fuzzy image or just not as bright. Care should be taken when removing any lens because lens position is important for centering and some manufactures use a spacer that determines both magnification and indenter travel. Lenses cannot generally be repaired. Cleaning either the lens or the eyepiece is not difficult but only a small amount of lens cleaner (1 drop or less) should be added to the lens tissue, not directly to the lens. Extremely dirty lenses might require two or more tissues. Lenses internal to the tester that are dirty are Probably best left to service or calibration technicians. Despite the flat field objectives lenses commonly in use, centering the indentation in the field of view will produce the most accurate readings. Check your manual but most testers allow the optical system or indenter mechanism to be adjusted to align to one another, usually with a target or crosshair and a zero mark. This will also save time hunting around for the correct indentation. The illumination system of the tester usually allows for alignment of the bulb to the optical path. Most testers use a ribbon filament halogen bulb that can be centered by gently removing the entire filar measuring unit, (the complete eyepiece assembly) and adjusting the filament image now visible in the open eye tube. Some instruments have lamp houses that are adjustable with thumb screws or set screws and some newer machines are non-adjustable due to improved manufacturing technology of the bulbs. A second way is to position the indentation in the very center of the field of view and look at the 4 faces of the indentation. They should be equally illuminated. If you have an adjustable aperture diaphragm, it should also be adjusted properly by stopping it down until a slight amount of the diaphragm is visible in the eye tube (with the entire filar eyepiece removed). The aperture technically should be adjusted every time a lens is changed but the lens used for the actual measurement is the only required setting. Most older testers are fixed to the highest power measuring magnification. Lastly and the most importantly is the operator. Each user has different eye sight and the eyepiece needs to be corrected accordingly. This difference often causes the largest discrepancy between user’s measurements because they are looking at the same indentation differently. Their eyepiece should be adjusted, such that the operator can see the filar lines in the eyepiece as clearly as possible. This is like a diopter for a camera or like the values listed on regular eyeglasses. For units with only 1 line or reference mark, the operator should rotate the eyepiece lens to focus on the line or Point. Remember to always use the same side of the line to measure. For units with 2 filar lines, open the lines so that just a small gap is visible between them and focus the eyepiece on the edges of the gap. In this case, remember that you are going to zero out the distance between the filar lines as your next step. Care should be taken not to force the filar lines into each other as damage can occur. Some testers remember this zero point even when the unit is turned off but it is wise to check the zero point frequently as well as the eyepiece correction. With the microscope portion of the hardness tester correctly aligned and the eyepiece tuned to the users’ eyesight, greater repeatability and accuracy are possible from just a bit more effort. The ASTM standard E384-10, available for purchase at www.ASTM.ORG , also has recommendations for other common problems encountered during the test method such as these: Minimum material thickness. The part should be 10 times the penetrated value of the indentation. For Vickers, penetrated value is about 1/7th of the diagonal length and approximately 1/30th of the diagonal for Knoop testing. Minimum diagonal length. Measurements below 20 microns in length require special precautions and equipment. Minimum space between indentations or edge of sample. Test piece surface requirements. The smaller the indentation, the better the surface needs to be. Walter Wardzala is the Corporate Material Testing Specialist at Mitutoyo America Corporation. He is actively involved with ASM and ASTM. If you have any questions regarding Micro Hardness Testing or any type of hardness testing, you can contact him at 847-682-1628 or email email@example.com.
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