Microhardness testing (argon). The later must be dry and oxygen-free. A

5.2.2.4 Microhardness testing (argon). The later must be dry and oxygen-free. A

The measurement of hardness with a microscope Pt:Pt –10Rh thermocople is inserted in the specimen.

attachment, comprising a diamond indentor and The furnace should have a low thermal inertia and

means for applying small loads, dates back more

be capable of heating or cooling the specimen at than 50 years. Initially used for small components controlled rates; temperatures of up to 1800 °

(watch gears, thin wire, foils), microhardness testing possible in some designs. The presence of a window,

C are

was extended to research studies of individual and possibly cooling devices, drastically reduces the

phases, orientation effects in single crystals, diffusion available working distance for the objective lens,

gradients, ageing phenomena, etc. in metallic and particularly when a large numerical aperture or high

ceramic materials. Nowadays, testing at temperatures magnification are desired. One common solution is to

C is possible. In Europe, the pyramidal use a Burch-type reflecting objective with an internal

up to 1000 °

Vickers-type (interfacial angle 136 ° ) indentor, which mirror system which gives a useful working distance of

produces a square impression, is generally favoured. 13–14 mm. The type of stage unit described has been

Its counterpart in general engineering employs test used for studies of grain growth in austenite and the

loads of 5–100 kgf: in microhardness testing, typical formation of bainite and martensite in steels, allotropic

test loads are in the range 1–100 gf (1 gf D 1 pond D transformations, and sintering mechanisms in powder

1 p D 9.81 mN). A rhombic-based Knoop indentor of compacts.

American origin has been recommended for brittle and/or anisotropic material (e.g. carbides, oxides,

When polished polycrystalline material is heated, glass) and for thin foils and coatings where a shallow individual grains tend to reduce their volume as

depth of impression is desired. The kite-shaped Knoop

a result of surface tension and grain boundaries impression is elongated, with a 7:1 axial ratio. appear as black lines, an effect referred to as thermal

Microhardness tests need to be very carefully con- etching or grooving. If a grain boundary migrates,

trolled and replicated, using as large a load as possible. as in the grain growth stage of annealing, ghost

The surface of the specimen should be strain-free (e.g. images of former grooves act as useful markers. As

electropolished), plane and perpendicular to the inden- the melting point is approached, there is often a

tor axis. The indentor is lowered slowly at a rate of noticeable tendency for grain boundary regions to fuse

< 1 mm min 1 under vibration-free conditions, even- before the bulk grains; this liquation effect is due

tually deforming the test surface in a manner analogous to the presence of impurities and the atomic misfit

to steady-state creep. This condition is achieved within across the grain boundary surface. When interpreting

15 s, a test period commonly used. the visible results of hot-stage microscopy, it is

The equations for Vickers hardness (H V ) and Knoop important to bear in mind that surface effects do

hardness (H K ) take the following forms: not necessarily reflect what is happening within the

bulk material beneath the surface. The technique can

H V D 1854.4⊲P/d 2 ⊳ kgf mm 2 ⊲ 5.1⊳ produce artefacts; the choice between evacuation and

H K gas-purging can be crucial. For instance, heating in D 14228⊲P/d 2 ⊳ kgf mm 2 ⊲ 5.2⊳ vacuo can favour decarburization and grain-coarsening

In these equations, which have the dimensions of in steel specimens.

stress, load P and diagonal length d are measured in The classic method for studying high-temperature phases and their equilibria in oxide systems was

upon the surface area of the impression; the second based upon rapid quenching (e.g. silicates). This

is based upon its projected area and the length of the indirect method is slow and does not always

long diagonal.

preserve the high-temperature phase(s). A direct microscopical technique uses the U-shaped notch of

1 Developed by W. Gutt and co-workers at the Building

a thermocouple hot junction as the support for a

Research Station, Watford.

The characterization of materials 131 The main potential difficulty concerns the possi-

a standard set of three diagonal d-values, as shown in ble dependence of microhardness values (H m ) upon

Figure 5.5. The approximate values for the set shown test load. As the test load is reduced below a cer-

D 120. When tain threshold, the measured microhardness value may

are H

D 160, H

D 140, H

the anisotropy ratio for elastic moduli is high, micro- tend to decrease or increase, depending upon the mate-

hardness values can vary greatly from grain to grain rial. In these circumstances, when measuring absolute

in polycrystalline material.

hardness rather than relative hardness, it is useful to Combination of the Vickers equation with the Meyer consider the material’s behaviour in terms of the Meyer

equation gives the following expression: equation which relates indenting force P to the diag- onal length of the Vickers-type impression produced,

H V D constant ð d 2 (5.4)

d , as follows: Accordingly, if n D 2, which is true for the conven- P D kd n

tional Vickers macrohardness test, the gradient of the

H m line becomes zero and hardness values are conve- The Meyer exponent n expresses the strain-hardening

niently load-independent.

characteristics of the material as it deforms plastically during the test; it increases in value with the degree