Nano and microhardness testing of heterogeneous structures.
Baskutis, S. ; Vasauskas, V. ; Zunda, A. 等
1. Introduction
Indentation methods, as remarkably flexible mechanical tests, are
finding increasing use in the study of mechanical properties of bulk and
thin film nonhomogeneous materials over a wide range of size scales [1].
Most engineering materials are of heterogeneous structure, different
phase properties and facing dynamic changes. Therefore, the control of
the local microstructure is especially important aiming to attain the
macro scale properties. Instrumented indentation is an important tool
that can be used to evaluate the mechanical properties of a wide range
of engineering materials across the nano and micro length scales. By
using the different shapes of indenters tip geometry and varying the
applied load, indentation techniques can be used to probe different
volumes of materials [2]. Correlation between the hardness and the
microstructure in weld joints has been established for engineering
materials [3].
The nanoindentation technique allows rather small regions in grains
to be investigated and different phase structures are distinguished
using this technique [4]. For low load nano and microindentation, the
area of contact between the indenter and material varies during testing
and is indirectly dependent on the measured depth of penetration. Micro
and nanoindentation tests, i.e. indentation depths of 0.1 to 100 ^m and
less than 0.1 ^m, respectively, proved to be the most cost-efficient, as
well as fast, precise and non-destructive insert. However, there are
numerous indentation tests at scales on the order of a micron or a
submicron that have shown that the measured hardness increases
significantly with decreasing the indentation size [5, 6]. This
phenomenon is commonly referred to as the indentation size effect (ISE)
[7]. The behaviour of the ISE in single crystals for nano and
microindentation was investigated by Manika et.al. [8]. Association of
the ISE with friction and with strain hardening was confirmed by M.
Atkinson [9].
According to [10], ranges of hardness testing are defined: macro
scale--test force diapazon 2 N--30 kN, micro scale--test force less than
2 N, indentation depth more than 0.2 [micro]m, nano scale--test force
less than 0.3 N, indentation depth less than 0.2 [micro]m. The micro
scale distinguished by the test force in relation to the indentation
depth. For the nano scale, the mechanical deformation strongly depends
on the real shape of indenter tip. Homogeneity in the material could be
an important issue particularly in composite, where the indenter tip may
be or may not be hit the particle and this could change the values of
material properties.
The effects on the localization of deformation at various scales
ranging from the microscales down to the nanoscales are discussed for
heterogeneous structures. At the nanoscale, a dominant mechanism of
deformation is the rearrangement of free nano volume and exchange of
momentum between bulk and grain boundary space. At the microscale, a
most common mechanism of deformation is dislocation motion [11].
Investigations developed by L.Qian et.al. [12] showed that
nanoindentation and microindentation hardness tests have similar load
effect and their differences are between 10% and 30%. Correlation
aspects between nanoindentatrion hardness tests results and Vickers
hardness was described by T. Sawa [13]. It should be noted, that
nanoindentation hardness and corresponding Vickers hardness values are
different. This difference scale is because of the different manner the
hardness values are defined. Nanoindentation hardness is determined
using the indenter under load, while Vickers hardness is calculated
after the load is removed.
The presented work deals with relatively more reliable and accurate
hardness analysis in heterogeneous structures engineering. The nano and
microindentation of polycrystalline copper thin films of different
thickness and approximately the same grain size and carbon steel
specimens, welded by gas tungsten arc welding (TIG), were used for the
experiments.
2. Background
Nano and microindentation methods often are presented together and
it seems to be simple and similar. In fact, process of indentation is
very complicated regarding the deformation mechanism as well as the
changes in the material structure under the indenter. The trend of
nanohardness and microhardness profiles in the intermediate area between
the inner layers with constant hardness and abnormal area gives
information on structural heterogeneity [14]. This is the area where the
ISE in indentation measurements can be demonstrated [15, 16]. The idea,
which enables the assessment of the homogeneity of the samples, was
proposed and described in this article. Furthermore, many materials and
especially structural ones exhibit phase heterogeneity and mechanical
differences of the phases on different length scales. In order to model
heterogeneous material systems, multi-scale approach that allows for
separation of scales based on some characteristic dimension of a
material microscopic feature for each level is often utilized. Averaged
(effective) composite properties can be found if the indentation depth
is much larger than the characteristic phase dimension (h [much greater
than] D). In this case averaged properties are obtained. If the (h [much
less than] D), intrinsic properties of the distinct phase are obtained.
Since Vickers tips cause relatively negligible strain in the material
during indentation, the hardness results obtained from test using the
different tip geometries can be directly compared. However, care must be
taken to correctly perform the conversion of the results. There is a
definition change between the hardness measured using microindentation
to the hardness used in nanoindentation. While both hardness values are
calculated as the peak force divided by the area of contact, the
definition of the contact area differs between the test techniques [15].
For microindentation, the contact area is the surface area of the tip
that area in contact with the sample. While, for nanoindentation the
contact area defined as the projected area between the sample and tip.
In nanoindentation the hardness is determined as the mean contact
pressure. The nanoindentation hardness of the specimen can be analysed
by the curves with the Oliver-Pharr method [1]. The conventional
hardness:
H(h) = F(h)/[A.sub.c](h) (1)
and the differential hardness:
[H.sub.d](h) = dF/d[A.sub.c] (2)
where F is load, [A.sub.c] is indentation area, are calculated as
continuous functions of the depth h and compare to each other in this
paper. It turns out that [H.sub.d] describes the momentary material
resistance to deformation, whereas H integrates our deformation states
from first tip sample contact to current penetration h. This difference
is important for materials not homogeneous in depth, e.g. layer systems.
[FIGURE 1 OMITTED]
The nanoindentation hardness ([H.sub.nano]) can be calculated using
following formula:
[H.sub.nano] = [F.sub.max]/[A.sub.c]([h.sub.c]), (3)
where in equation [F.sub.max] is the peak indentation load,
[A.sub.c] is the cross-sectional area and [h.sub.c] represents the
contact indentation depth between the indenter and the specimen (Fig.
1). The determination of the contact indentation depth [h.sub.c] is
obtained from equation:
[h.sub.c] = [h.sub.max] - 0.75 - [F.sub.max]/S, (4)
where S is the contact stiffness and can be obtained by:
S = dF/dh (5)
where dF/dh is the stiffness at the upper portion of the unloading
data calculated from the slope of the depth-force curve.
The microindentation of tested samples had a larger scatter due to
the influence of several factors: hardness of grains ([H.sub.nano]),
ISE, microstructure and grain boundary phase and at higher loads by
"mix-phase" volume below the indenter.
3. Experimental
Two types of heterogeneous materials were used in present study. At
first, carbon steel was welded by using gas tungsten arc welding method,
in the Ar/C[O.sub.2] protective gas environment, with the non-fusible
electrode from tungsten, and second, copper (Cu) films were made by
electrospark deposition (ESD) on steel substrates. The specimens were
deposited using 3A current and sparking voltage of 60 V. Film thickness
was varied by changing the deposition time. In this paper we examined
copper films of thickness 60 [micro]m and 120 [micro]m.
Welding experiments were carried out under different welding
regimes: current and voltage. The welding with non-fusible electrode
required relatively large density welding current, therefore, small
diameter (0.8 mm) welding wire, which was fed into electric arc by a
relatively high feed rate. The welds were cross sectioned for
microexamination under the microscope. The process of preparation of
microsections involved three steps: cutting, mechanical polishing and
etching. The abrasive water jet technology, which provides the
possibility to eliminate the heat load, was used for the specimens'
preparation.
Vickers microhardness and depth sensing indentation tests were
performed using the prepared metallographic specimens. The properties in
both nanolevels and microlevels were characterized using Nano-Hardness
Tester developed by CSM Instruments, Switzerland. Indents were performed
over a range of loads from 7 to 250 mN. We used a constant strain rate
setting 1 / P (dP / dt) to be 2 [min.sup.-1] and a pause of 10 s was set
at maximum load.
The layered substrate film system is not completely equivalent to
the disordered structural multiphase materials but it can be
successfully used as the first estimation. Microstructure was analysed
using optical microscope Nicon Eclipse 1000 with magnification from x 50
to x 1000.
For microhardness and nanohardness investigations, identical
specimens and areas have been used. However, it should be noted, that
exactly the same grains of heterogeneous structures can't be
investigated, but similar properties in adjusted regions of the
microstructure are assumed. Microhardness has been measured on the fine
and coarse-grained structures of both samples which were compared to the
nanohardness measurements. Scratch test was made by CSM Nano-Scratch
Tester using continuous ly increasing load from 5 to 2500 mN. A scratch
length of 4 mm has been used.
The specimen showed high plasticity and excellent adhesion (Fig. 2)
with no adhesion cracks.
[FIGURE 2 OMITTED]
Neither failure to a bigger extent nor complete exposure of
substrate occurred within the predetermined range of normal load.
4. Results and discussion
The area of heat affected zone (HAZ) of welded joints basically
depends on heat input and thermal conductivity of base metal. The
boundary between the fuse zone and HAZ is clear to identify (Fig. 3) as
the crystals in the first are dendrites and in the second globular. Due
to recrystallization process, two different zones of HAZ can be
observed, viz., HAZ1 and HAZ2. HAZ1 has coarse grains while HAZ2 has
fine grains. Size and shape difference of grains is due to the distance
from the weld.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Cross section image of Cu electro-sparking coating with thickness
of 60 gm on steel substrate, layers zones as well as indents of hardness
testing are shown in Fig. 4.
Different indentation data may be obtained analysing different
perpendicular cross sections of the weld. Approximately 10% variation of
the nano and microindentation hardness data measured on the longitudinal
cross section was observed. This fluctuation of the hardness values
along the weld structure centreline is not uniform. Microhardness values
were influenced by elastic recovery whereas nanohardness readings were
not affected by plasticity. Microhardness was also more affected by
grown of grains, than nanohardness.
Table 1, Fig. 5, Fig. 7 and Fig. 8 show the relationship between
microhardness and nanohardness. The linear regression equation for this
relationship is [H.sub.nano] = 1.22 HV, where [H.sub.nano] is the
nanohardness value in MPa and HV is the microhardness value.
As follows from Table 1, the fuse zone has a smaller indentation
depth, i.e. higher hardness because it contains larger amounts of
alloying elements such as silicon, carbon, and manganese.
[FIGURE 5 OMITTED]
HAZ2 zone also distinguished by the microhardness values, thus
confirming that the microhardness reduction was influenced by
self-tempering.
Fig. 5 shows a nano and microhardness profiles obtained across the
welded region from the fuse zone through the unaffected base metal.
Tests were carried on three specific locations: fuse zone at A, base
metal at B and HAZ regions at C with reference to Fig. 5. A reduction in
hardness (softening) with respect to base metal (avg. 300 HV) was
clearly revealed in the sub-critical HAZ.
Apparent Vickers microhardness values HV were calculated at each
load using conventional approach
HV = 1.8544P/[d.sup.2], (6)
where d is the average diagonal length of the Vickers indentation
impression and P is the indentation test load.
Clear ISE was observed in all of the samples that were tested. The
ISE means the increase of indentation hardness with the decrease of
indentation depth, which is close related with load. Particularly it was
clear for determining hardness values of base metal as homogeneous
structure (Fig. 6).
[FIGURE 6 OMITTED]
The hardness increases while decreasing penetration depth at both
micro and nano-scales. However, the extent of the increase is much more
dramatic in nano-scale regime than in micro-scale regime. Hence, as the
indentation depth decreases from about 1600 to 200 nm, the hardness
increases by a factor of about 0.90-1.22. Independent zones also have
quite similar hardness values for indentation depths between the
micro-scale to the nano-scale.
Microhardness, as well, was measured as the function of the
residual area (contacted area) after load removal. However, the area
function is not considered to be ideal because of blunting in the
nanoindentation experiments and such a correction is not considered
here. This is sufficient for a relative comparison of the data. The
microindentation with a diagonal length about 14 pm always crosses a
grain boundary. It was found in both microstructures that an indent,
which crosses several boundaries, shows lower hardness values. Indents,
which cross grain boundaries and precipitations as well as grain
boundary of carbides, indicate higher hardness values, corresponding to
the fact that an interface increases the hardness. Therefore, the
coarse-grained structure (existing among other phases of needle-shaped
ferrite) has a higher hardness than the fine-grained structure.
Measurements revealed smaller hardness in the centre and higher towards
the grain boundary. Typical load progression (load-displacement or P-h)
curves during indentation of Cu coating, outer and inner layers and
steel substrate are shown in Fig. 7.
In the case of cladded materials, the elastic part is recovered
upon load removal causing a decrease in the size of the residual area.
Consequently, the resulting hardness value (Fig. 8) will appear larger.
The nanoindentation trials were therefore specifically designed to
investigate tempering induced of facts on individual phases-zones. The
indentation measurements along the coating cross-section showed that
hardness values were lower at the coating side than at the
coating-substrate interface (Fig. 7).
[FIGURE 7 OMITTED]
At a low load of 7 mN, the coating demonstrated a hardness value of
about 94 MPa at a depth of about 170 nm which dropped by 15%, e.g., near
10 MPa at a depth of about 2000 nm for a higher load of 250 mN (Fig. 8).
[FIGURE 8 OMITTED]
Obtained data showed the presence of a strong ISE on the
nanohardness behaviour of the coatings. At low indentation load (7 mN),
the size of indent was relatively small, whereas the measured hardness
values were higher. Under the higher load (250 mN), indentation size was
bigger and hardness values smaller, while measuring in the identical
areas.
It was defined experimentally that nanoindentation hardness and
microindentation hardness have relationship described using a
coefficient k\
[H.sub.nano] = [k.sub.1] HV, (7)
where [k.sub.1] = 0.90 - 1.22. However, the interpretation of the
data is very difficult due to ISE. The ISE is clearly present in single
grains, but is absent in fine-grained volumes as indentation size
exceeds or is comparable to the grain size.
5. Conclusions
1. It was found that for Cu coating samples the nanohardness values
were highest for the lowest indenter load of 7 mN. At higher indenter
loads in the range of 50250 mN, the hardness values decreased. The main
difference concerns the comparison between the size of the indentation
surface and the size of the heterogeneities of the material. The hetero
ratio of the nanohardness ([H.sub.nano]) to the microhardness (HV) was
smaller in the HAZ, indicating that there is a significant
grain-boundary effect.
2. It was found that nanoindentation hardness and microindentation
hardnesses are related by the coefficient [k.sub.1]. By measuring
nanohardness and microhardness in automatic regimes the ISE was not
clearly expressed if compared to the conventional hardness testing
method.
3. Microhardness is also more sensitive to changes in the
microstructure, e.g. recrystallization, than nanohardness. This scale of
testing minimizes the potential variations caused by local heterogeneity
in submicron structural features. The results show that hardness can no
longer be simply compared based solely on the same indenter type and
indentation load.
4. Problems associated within the indentation process in
nonhomogeneous materials remain a problem that is still under
investigation.
Received November 23, 2015
Accepted March 15, 2016
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S. Baskutis, Kaunas University of Technology, Studenty str. 56,
51424 Kaunas, Lithuania, E-mail: saubask@ktu.lt
V. Vasauskas, Kaunas University of Technology, Studenty str. 56,
51424 Kaunas, Lithuania
A. Zunda, Aleksandras Stulginskis University, Studenty str. 11,
53361 Kaunas region, Lithuania, E-mail: audrius.zunda@asu.lt
crossref http://dx.doi.org/10.5755/j01.mech.22.2.13690
Table 1
Summary of the average values of nanohardness and
microhardness in different zones of the specimens
Speci- Nano-
men Zones hardness Microhardness
Wel-ding 7 mN 50 mN 250 mN
(TIG)
Base metal 308 339 269
HAZ2 298 335 264
HAZ1 316 352 299
Fuse zone 361 402 340
Clad- Cu coating 94 81 105
ding Outer layer 254 230 208
(ESD) Inner layer 313 300 295
Substrate 259 256 238
Speci-
men Relation
Wel-ding [H.sub.nano]/ [H.sub.nano]/
(TIG) HV50 HV250
0.91 1.14
0.89 1.13
0.90 1.06
0.90 1.06
Clad- 1.16 0.90
ding 1.10 1.22
(ESD) 1.04 1.06
1.01 1.09
