eFunda: Convert Hardness: Mohs

Mohs Hardness Scale

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Devised by Austrian mineralogist Frederick Mohs in 1822, Mohs' Hardness is applied to non-metallic elements and minerals. In this scale, hardness is defined by how well a substance resists scratching by another substance. A scale of 1 to 10 with half-step increments is employed. Members with higher scale numbers will scratch the surface of members in equal or lower scales.

The reference minerals for the ten scales are:

1	2	3	4	5
Talc	Gypsum	Calcite	Flourspar	Apatite
	[CaSO₄· 2H₂O]	[CaCO₃]	[CaF₂]	[Ca₅ (PO₄)₃ (F, Cl, OH)]

6	7	8	9	10
Orthoclase	Quartz	Topaz	Corundum	Diamond
[KAlSi₃O₈]	[SiO₂]	[Al₂SiO₄ (F, OH)₂]	[Al₂O₃]	[C]

Common Mohs' testing kits consist of low cost specimens of the 10 minerals in the Mohs' scale (or 9 if the expensive diamond is absent), labeled and stored in a wooden box. Specimens are sometimes in the form of metal rods, each containing a fragment of the reference minerals at its tip.

The Mohs' Hardness for common materials are listed below:

Material	HM	Material	HM	Material	HM
Fingernail	2.5	Copper Penny	3	Chalk	3
Tooth Dentin	3 ~ 4	Tooth Enamel	5	Amalgam	4 ~ 5
Gold	2.5 ~ 3	Knife Blade	5.5	Glass	5.5
Pumice	6	Steel File	6.5	Floor Tile	6.5
Tungsten Carbide	9	Silicon Carbide	9 ~ 10	Boron Carbide	9 ~ 10

Since Mohs' Hardness Scale uses existing common minerals as reference measures, it is convenient to use but does not give a continuous range of measurements. For instance, diamond (10) is 140 times harder than corundum (9), whereas flourspar (4) is only marginally (~10%) harder than calcite (3). As a result, the hardness conversions from this calculator are not exact and should be used for reference purposes only.

HM 5 approximately* =

Hardness
Symbol	Amount	Name	Suggested Range
HB (3000)	497	Brinell 10 mm Standard 3000 kgf	80~445
HB (500)	>>	Brinell 10 mm Standard 500 kgf	89~189
HB (Tungsten 3000)	503	Brinell 10 mm Tungsten 3000 kgf	80~620
HB (Indentation)	2.76 mm	Brinell Indentation	6~2
HK	564	Knoop	97~920
HRA	77	Rockwell A-Scale	59~86
HRB	>>	Rockwell B-Scale	41~100
HRC	51	Rockwell C-Scale	19~69
HRD	64	Rockwell D-Scale	39~77
HRF	>>	Rockwell F-Scale	88~100
HR-15N	86	Rockwell Superficial 15N	69~94
HR-15T	>>	Rockwell Superficial 15T	77~93
HR-30N	70	Rockwell Superficial 30N	41~85
HR-30T	>>	Rockwell Superficial 30T	53~82
HR-45N	57	Rockwell Superficial 45N	19~76
HR-45T	>>	Rockwell Superficial 45T	28~71
HS	69	Shore Scleroscope	17~97
Approx. TS	1840 MPa	Tensile Strength (Approx.)	390~2450
HV	535	Vickers	20~1800

Legend
<<	The hardness value is below the acceptable range of the particular hardness scale.
>>	The hardness value is above the acceptable range of the particular hardness scale.
###	The hardness value is near the limit (within 15%) of the acceptable range of the particular hardness scale.

*	The many hardness tests listed here measure hardness under different experimental conditions (e.g. indenters made in different sizes, shapes, and materials, and applied with different loads) and reduce their data using different formulae. As a result, there is NO direct analytic conversion between hardness measures. Instead, one must correlate test results across the multiple hardness tests. This calculator is based on hardness data compiled from ASM Metals Reference Book 3rd ed, published by ASM International, and Machinery's Handbook 25th ed, published by Industrial Press. The calculator curve-fits multiple hardness data onto a common polynomial basis and then performs an analytic conversion. The accuracy of the conversion depends on the accuracy of the provided data and the resulting curve-fits, and on the valid ranges spanned by the different hardness tests. Converted hardness values should be used for comparative purposes only.