Knoop hardness number

The Knoop hardness number is computed from the measured long diagonal by the following formula where HK = Knoop hardness,... 

The Knoop test is a microhardness test. In microhardness testing the indentation dimensions are comparable to microstructural ones. Thus, this testing method becomes useful for assessing the relative hardnesses of various phases or microconstituents in two phase or multiphase alloys. It can also be used to monitor hardness gradients that may exist in a solid, e.g., in a surface hardened part. The Knoop test employs a skewed diamond indentor shaped so that the long and short diagonals of the indentation are approximately in the ratio 7 1. The Knoop hardness number (KHN) is calculated as the force divided by the projected indentation area. The test uses low loads to provide small indentations required for microhardness studies. Since the indentations are very small their dimensions have to be measured under an optical microscope. This implies that the surface of the material is prepared approximately. For those reasons, microhardness assessments are not as often used industrially as are other hardness tests. However, the use of microhardness testing is undisputed in research and development situations. 

According to ASTM D1474 the Knoop hardness number is measured with the pyramidal Knoop diamond. According to ISO 2815 the Buchholz instrument is used. This method cannot detect elastic deformation of the coating that disappears immediately after the load is removed. 

Other common hardness tests involve the use of diamond pyramids. In the Vickers hardness test, a square pyramid is used and in the Knoop hardness test, the pyramid is elongated. The area term in the former test is the actual indentation area and in the latter, the projected area. From the impression geometries, shown in Fig. 6.30, the Vickers Hardness Number (VHN) and Knoop Hardness Number (KHN) can be shown to be VHN=1.854F/a and KHN=14.2F/L, respectively. A common hardness test in the USA is the RockweO hardness test, which uses various indenter types and loads. The result of these tests is a dimensionless number and leads to the use of various hardness scales (e.g., Rockwell B, Rockwell C). 

Knoop Elongated pyramid Knoop hardness number (KHN) KHN = 14.2F/L ... 

Hardness scale See Mohs, scale, knoop hardness number, pencil hardness, and rocker hardness tester. 

See knoop hardness number andpfund hardness number. 

Knoop hardness number, KHN n. The indentation hardness determined with a Knoop indenter, and calculated as follows ... 

CH Cold work hardened HK Knoop hardness number... 

KHN. KNOOP HARDNESS NUMBER (q.V.). Kibbler Rolls. Toothed steel rolls of a type frequently used in the crushing and grinding of brick clays from an old word... 

Knoop Hardness. A pyramidal-diamond indentation test. The result is expressed in kg/mm2, the applied load being indicated, eg. Knoop 100 or KlOO a US abbreviation is KHN(Knoop Hardness Number) The test was introduced by F. Knoop, C G Peters and W. B. Emerson (/. Res. Nat. Bur. Stand., 23,39,1939). ASTM C849 describes its application to ceramic whitewares C730 to glass. 

L. Tarasov and N. Thibault, Determination of Knoop Hardness Numbers Independent of Load, Trans. A.S.M., 38 331 353 (1947). 

Hardness -nos n (before 12c) (1) The resistance to local deformation. (2) Ability of a coating film, as distinct from its substrate, to resist cutting, indentation, or penetration by a hard object. An arbitrary scale of hardness is based upon ten selected minerals. For metals the diameter of the indentation made by a hardened steel sphere (Brinnell) or the height of rebound of a small drop hammer (Shore Scleroscope) serve to measure hardness. See Barcol Hardness, Brinell Hardness, Durometer, Indentation Hardness, Knoop Hardness Number,... 

Hardness Scale See Scale, Knoop Hardness Number, Pencil Hardness, and Rocker Hardness Tester. 

Figure 1. Influence of load on Knoop hardness number of composites. Mean values and standard deviations for 12 measurements for Estilux (full line) and Nuva-Fil (broken lines).

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