Metal Finishing Guide Book


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Calibrate With a Stage Micrometer Take several readings and average the results to determine any correction factors that need to be used in measuring the indents on test pieces. Magnification of the testing machine should be a minimum of 400×; higher magnification can be a benefit. This correction factor can be different for different people using the same machine, but it should not change for each individual as long as nothing is changed in the equipment. This calibration should be repeated every few months or whenever there is any reason to expect that there has been a change. Calibrate by Checking a Standard Test Block It is preferred that the test block be of the same general hardness as the deposit being tested, within reason. (Avoid using a hard nickel or steel test block when the deposit is to be soft gold.) Standard hardness test blocks are available from various laboratory and testing equipment supply houses. Certified standards for microhardness made from electroformed, hard nickel are available from the NIST Office of Reference Materials (Gaithersburg, MD 20899). Using standard test blocks of the same general material as the deposit may minimize potential errors due to differences in resilient properties possible with some metals. (To verify the testing machine, ASTM E 384 calls for 20 indentations on the standard test block.) Measure the Microhardness of the Sample Following the instructions of the testing machine manufacturer, make the indentations with the long axis of the indenter parallel to the test surface when using the Knoop indenter. The Vickers indentation should be placed so that one diagonal is perpendicular to the edge of the coating. Indentations should be made in the center of the deposit and spaced at least about one and one-half times the length of the diagonal from each other. For best results, indentations should be symmetrical. When using the Vickers scale, both diagonals are measured and averaged; each diagonal should be of equal length within 5%. Knoop indentations only require measurement of the long diagonal; however, each leg should be of equal length within 20%. (Less than 20% would be better.) Report the Results ASTM B 578 asks that results be based on at least five measurements and that the report include range of hardness units, average (arithmetic mean), estimated standard deviation, and coefficient of variation. ISO 4516 asks that the report include the range, with the average optional, whether the tests were done on the surface or a cross-section, and any deviations from normal conditions such as temperature or force-retention time. HOW MANY READINGS SHOULD BE TAKEN ASTM B 578 and ISO 4516 call for "at least five measurements." During an extensive series of round-robin tests done over several years, better results were obtained taking five measurements on each of five indentations. This work was done by a section of Committee BO8 in ASTM. From one series of some 6,500 test readings, covering both Knoop and Vickers indenters, it was found that the total average microhardness readings were very close to the certified readings of NIST, even though some ranges were somewhat broad. It is recommended that given the possible variables in the technique, this practice of taking five measurements of each of five indentations be used. Use this for calibrating the machine with the standard test block and when testing sample deposits. 522

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