EN 843-4-2005 pdf download.Advanced technical ceramics – Mechanical properties of monolithic ceramics at room temperature – Part 4: Vickers,Knoop and Rockwell superficial hardness.
Inspect the shape of the indentations for regularity, and reject any that show suspected irregular shape, displacement of one or more corners, loss of one or more corners or excessive radial cracking (see Figure 1, schematics 6 to 14). If the indentations are asymmetric (see Figure 1, schematic 7), the surface is not adequately fiat or perpendicular to the axis of the mactime, and the test piece should be remounted or re-polished as appropriate. Repeat the tests until at least five acceptable indentations are produced. Measure the lengths of both diagonals of each indentation to the nearest 0,2 pm using the criteria appropriate to the test machine and the guidance shown in Figure 2.
NOTE 2 Most ceramics ace translucent under the conditions of observation of the indentations En Vckers (and Knoop) tests. This results in very poor contrast at the corners of the indentations compared wth metallic materials, and there are consequent difficulties an placement of measuring orosswires. Some expenence may be needed by an operator in order to develop a consistent criterion for measurement
If damage to the indenter has occurred during the course of the tests, replace the indenter and repeat the reference block tests and the tests on the test-piece.
Calculate the HV1,0 hardness in GPa or the hardness number for each Indentation according to equation (1). calculate the mean result and the standard deviation.
5.5 Accuracy and uncertainties
The principal errors arising in a Vickers hardness test on advanced monolithic technical ceramics vary in magnitude according the size of the indentation, and thus the indentation force used, The dickers diamond geometry was originally chosen because natural cJeavage planes of the diamond were employed, Variations in geometry between indenters are therefore small, and can usually be ignored except when indentations are of less than 20 pm diagonal length where ttie tip and edges near the tip may be variable between indenters. In particular, the edges may have flats up to 1 pm across on them, and this has the effect of cutting the corners off the indentation. The error that this introduces is insignificant if the indentation is Larger than about 30 pm, but increases rapidly in importance as the size is reduced.
Determination of the diagonal lengths using cross-wires or other device attached to the instri.aiaent relies on the operator positioning them at the “true” opposing corners of the indentation. There is a subjective element In performing this task which increases with poor optical contrast and reducing size of the indentation. The possible errors can be reduced by experience, and by consistent use of high-hardness, preferably ceramic or hardmetal, test blocks to familiarise the eye at the start of measurement sessions. In this way any systematic measurement bias can be reduced. In a round-robin exercise on high-alumina ceramics (reference L31. Bliography), it was found that when two individuals measure the same set of indentations on different measurement equipment, a poor correlation was obtained unless the true sizes of the indentations varied by more than ± 1 pm. It follows that, discounting differences between machines, It cannot be guaranteed that any two observers wi agree that one material is signiflcantiy harder than another unless the average indentation sizes are systematically smaller by more than 1 pm. Thus even if It is possible to measure the indentation diagonal length to an apparent precision of 0,1 pm, or the optical resolution limit if larger, the ability to discriminate between materials is limited to an order of magnitude greater in size. Errors of this size assume significance when the indentation size is less than about 20 pm. In addition there is the actual scatter in indentation sizes as a result of local microstructure variations such as grain size, grain orientation, secondary phase content, microcracking, porosity, etc.EN 843-4-2005 pdf download.