Hardness comparison table

Brinell HB
(10 mm Ball, 3000 kg load) Vickers HV
(120 kg) Rockwell C HRC
(120 degree cone 150 kg) Rockwell B HRB
(1/16" ball 100 kg) Leeb HLD[1]
800 - 72 - 857
780 1220 71 - 850
760 1170 70 - 843
745 1114 68 - 837
725 1060 67 - 829
712 1021 66 - 824
682 940 65 - 812
668 905 64 - 806
652 867 63 - 799
626 803 62 - 787
614 775 61 - 782
601 746 60 - 776
590 727 59 - 770
576 694 57 - 763
552 649 56 - 751
545 639 55 - 748
529 606 54 - 739
514 587 53 120 731
502 565 52 119 724
495 551 51 119 719
477 534 49 118 709
461 502 48 117 699
451 489 47 117 693
444 474 46 116 688
427 460 45 115 677
415 435 44 115 669
401 423 43 114 660
388 401 42 114 650
375 390 41 113 640
370 385 40 112 635
362 380 39 111 630
351 361 38 111 621
346 352 37 110 617
341 344 37 110 613
331 335 36 109 605
323 320 35 109 599
311 312 34 108 588
301 305 33 107 579
293 291 32 106 572
285 285 31 105 565
276 278 30 105 557
269 272 29 104 550
261 261 28 103 542
258 258 27 102 539
249 250 25 101 530
245 246 24 100 526
240 240 23 99 521
237 235 23 99 518
229 226 22 98 510
224 221 21 97 505
217 217 20 96 497
211 213 19 95 491
206 209 18 94 485
203 201 17 94 482
200 199 16 93 478
196 197 15 92 474
191 190 14 92 468
187 186 13 91 463
185 184 12 91 461
183 183 11 90 459
180 177 10 89 455
175 174 9 88 449
170 191 7 87 443
167 168 6 87 439
165 165 5 86 437
163 162 4 85 434
160 159 3 84 430
156 154 2 83 425
154 152 1 82 423
152 150 - 82 420
150 149 - 81 417
147 147 - 80 413
145 146 - 79 411
143 144 - 79 408
141 142 - 78 405
140 141 - 77 404
135 135 - 75 397
130 130 - 72 390
114 120 - 67 365
105 110 - 62 350
95 100 - 56 331
90 95 - 52 321
81 85 - 41 300
76 80 - 37 287

Knoop hardness test

The Knoop hardness test (pronounced kuh-nūp) is a microhardness test - a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed into the polished surface of the test material with a known force, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. The Knoop hardness HK or KHN is then given by the formula:

where:

L = length of indentation along its long axis

C_p = correction factor related to the shape of the indenter, ideally 0.070279

P = load

HK values are typically in the range from 100 to 1000, when specified in the conventional units of kg_f·mm^-2. The SI unit, pascals, are sometimes used instead: 1 kg_f·mm^-2 = 9.80665 MPa.

The test was developed by Frederick Knoop and colleagues at the National Bureau of Standards (now NIST) of the USA in 1939, and is defined by the ASTM D1474 standard.

The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter.

Sample values

Material HK

Gold Foil 69

Quartz 820

Silicon Carbide 2480

Diamond 8000