Download Sclerometric hardness measurement methods

УДК 006.91
SCLEROMETRIC HARDNESS MEASUREMENT METHODS
Sergey Romanchuk
Scientific Advisor – A. V. Grichchina
Kharkiv National Automobile and Highway University
Invention of new materials and coatings made it possible to evaluate
physical and mechanical properties of these materials with the help of nondestructive methods. Hardness measurement is the easiest and widely used method
of mechanical testing.
Hardness reflects the ability of a material to resist penetration of a harder
material - an indenter. The long history of the development of the method led to
appearance of dozens of varieties of methods of hardness measurement as well as
the instruments used to measure it. A variation of the method is the method of
scratching or sclerometry.
Scratch hardness testing method consists in making a scratch on the
surface of a sample with the help of a diamond indenter or some other indenter
which is put under constant load. In practice, various criteria of scratch hardness
measurement are used.
According to State Standard 21318-75 scratch microhardness with a
trihedral or tetrahedral diamond pyramid shall be expressed:
1) indentation microhardness numbers obtained by dividing the normal load
by the reference area of contact between the pyramid and the test surface according
to the following formulas:
- for a tetrahedral pyramid:
H
P

3,708  P
b2
(1)
- for a trihedral pyramid:
HP 
3,138  P
b2
(2)
where P - normal load, H (kgf), b - width of a groove, mm.
2) the number of the normal load which is required for obtaining a groove of
a predetermined width b which equals 5, 10 or 20 microns by scratching with an
edge or a face. By convention, the load value for a given groove width b is thought
to express Martens microhardness and is marked НМ (  a triangular base of the
pyramid), НМ (  a square base of the pyramid) indicating in the text the method
of scratching with the edge for a tetrahedral pyramid or with the edge or the face
for trihedral pyramid. As a rule, PMT-3 Microhardness tester is used to measure
scratch microhardness, and a measuring microscope is used to measure the width of
scratches.
Sclerometry method offers significant advantages while measuring
hardness at the submicron and nanometer scale before applying the method of
indentation which implies penetration of an indenter into the material surface. This
is connected with the high degree of elastic recovery of hardness indentation, as
deformation under the influence of the indenter is elastic-plastic for most hard and
superhard materials. Since hardness is determined as the ratio of the load and the
indentation area (or indentation projection), its elastic recovery results in greatly
overestimated values of hardness.
During scratching the test material is deformed first elastically and then
plastically, and when the voltage reaches the value corresponding to the failure
resistance, shear fracture of the material may occur. In this case the material is
extruded from the furrow and muckpiles are formed (Fig. 1). The width of the
scratch is taken as the distance b between the peaks of the muckpiles formed on the
sides of the scratch. According to the research [3] after removing the load the
scratch depth decreases due to elastic recovery.
a
b
Fig. 1. Sclerometric hardness measurement: a - SPM-image of the scratch, b scratch width definition diagram
However, the recovery of the scratch width is negligible. Thus, the
application of the sclerometrical method can allow to avoid errors caused by the
elastic recovery after unloading a print. That appears to be particularly important
for measuring the hardness at the nanoscale. Scanning probe microscopes (SPM)
are used at the present day for the purposes of measuring scratch parameters and
obtaining three-dimensional images of scratches.
Scratch method can be used, for example, to detect heterogeneity in
distribution of mechanical properties of the metal in weld connections. An indenter,
shaped as a tetrahedral pyramid, is indented into the surface of the section of the
welded joint. Then, the indenter is moved horizontally across the surface of the
section until the scratch will not pass through the entire welded joint (Fig. 2). In the
process the instrument produces a permanent record of the scratching effort and the
scratch length. These data are transmitted to the personal computer, wherein the
hardness value is calculated at each point of the scratch.
Fig. 2 Photo of the weld joint: 1 - weld metal area, 2 - heat affected area,
3 - base metal; arrows indicate the direction of scratching
According to the scratch hardness values obtained, the personal computer
plots a scratch diagram in coordinates “hardness  scratch length” which shows the
distribution of the mechanical properties of the metal at each point of the weld
joint.
References
1. Гоголинский К. В. Применение сканирующих зондовых микроскопов
и нанотвердомеров для изучения механических свойств твёрдых материалов
на наноуровне / К. В. Гоголинский, Н. А. Львова, А. С. Усеинов // Заводская
лаборатория. Диагностика материалов. ― 2007. ― № 6. ― Т. 73. ― С. 28-36
2. Измерение микротвёрдости царапанием алмазными наконечниками.
ГОСТ 21318-75. ― М. : Издательство стандартов, 1976. ― 30 с.
3. Мощенок В. И. Новые методы определения твёрдости материалов :
монография / В. И. Мощенок. ― Харьков : ХНАДУ, 2012. ― 324с.