JOURNAL OF MATERIALS SCIENCE LETTERS 20, 2001, 1819 – 1821
The effect of indenter heating on indentation creep testing of MgO
X. J. REN, R. M. HOOPER, C. GRIFFITHS
School of Engineering and Computer Science, University of Exeter, Exeter EX4 4QF, UK
E-mail: x.ren@exeter.ac.uk
J. L. HENSHALL
Mechanical and Manufacturing Engineering Department, Nottingham Trent University,
Nottingham, NG1 4BU, UK
Indentation testing is an important method for assess-
ing the properties of materials, particularly ceramics
and ultrahard materials, which are difficult to characte-
rise by other means. Elevated temperature and extended
time indentation testing of such materials provides an
experimental means of investigating the deformation,
which can be interpreted in terms of the relevant mecha-
nisms [1]. The influence of various experimental pa-
rameters, such as specimen orientation [2, 3], applied
load [4], sample/surface preparation [5], vibrations [6]
and operator dependence [7] have been thoroughly in-
vestigated previously. This is not the case for the exper-
imental procedure for the investigation of indentation
testing at elevated temperatures, and indeed Everitt [8]
has noted that for the magnesium oxide (MgO) hardness
results obtained in her experimental apparatus there was
a very marked influence of heating method.
Conventionally, the heating is supplied either from
beneath or around the specimen, with no separate heat-
ing supplied to the indenter itself [9–12]. In the method-
ology developed and used by the present authors and
collaborators over a number of years, the indenters were
preheated by placing them in contact with the surface of
the test specimen for 30 min prior to commencing the
actual indentations to be measured [13], so that ther-
mal equilibrium is attained. At relatively low temper-
atures, ≤700
◦
C, a thermal insulator is used between
the loading mechanism and the indenter and holder. In
the majority of cases concerning elevated temperature
hardness testing reported in the literature this level of
experimental detail is not reported. The purpose of this
note is to compare the influence of variations in indenter
heating/pre-heating on the resultant indentation hard-
ness values in MgO for temperatures up to 600
◦
C and
dwell times up to 1800 s.
A {001} surface was prepared by cleaving a MgO
single crystal (Spicer Ltd., London, UK) to a thickness
of 2 mm. The samples were chemically polished in
orthophosphoric acid at 110
◦
C and rinsed in warm dis-
tilled water. The crystal was then ultrasonically cleaned
in an acetone bath prior to testing.
All the indentations were made in air using a modi-
fied Leitz miniload microhardness tester, incorporating
a hot stage, with a Knoop diamond indenter fitted to
a pyrophyllite extension rod, which was mounted on a
stable vibration free plinth. The indenter diagonal was
aligned along 110 in all cases reported herein. The
applied load was 4.90 N. The specimen was placed on
a small heating platform, which was contained within a
thermally insulating holder. When used, the separate
indenter heating was achieved by using coiled wire
around the indenter and its ceramic extension. The tem-
perature of the sample surface and the indenter tem-
perature were controlled by K type thermocouples to
within 2
◦
C. The indentation diagonal lengths were sub-
sequently measured optically at ambient temperature
with a precision of 0.2 micrometer.
Four different preheating conditions have been used
as shown in Fig. 1. In condition 1 (cold indenter), the
indenter was brought to the indenting position (0.5 mm
above the sample surface) from room temperature with-
out any preheating. Between each test, the indenter was
held away from the heating source until its tempera-
ture reached ambient air temperature. In condition 2
(warm indenter), the indenter was held 0.5 mm above
the sample for 30 min. The indenter temperature would
be higher than that for condition 1, but still lower than
that of the sample surface. In condition 3 (hot indenter),
the indenter was left in contact with the MgO sample
with the measuring load on for 30 min to equilibrate the
indenter and sample surface temperatures. In condition
4 (heated indenter), the indenter was heated separately
prior to and during the testing. This is analogous to the
method used by Everitt [8]. The indenter heater con-
sisted of nichrome wire element wound on a glass tube,
in a similar arrangement to that used by other work-
ers [14]. The temperature, as measured at a point on
the shank of the indenter, close to the tip (1 mm) was
maintained within 2
◦
C of the specimen’s surface tem-
perature.
Fig. 2 shows the results for 12 s and 1800 s dwell
time microindentation hardness tests. Each point rep-
resents the mean of at least six measurements of hard-
ness; the error bars represent ±2 standard errors. As
shown in Fig. 2, the conventional hardness (12 s dwell
time) values decreased continuously between 100
◦
C
and 600
◦
C. The results obtained using the hot or heated
indenter techniques, conditions 3 and 4 above, were
the same at all temperatures. However, for the cold and
warm indenters there is a significant difference from the
hot/heated indenter hardness values at all temperatures,
even though the trends are similar. The differences
vary between 83 and 60 kg/mm
2
at 100
◦
Cto35and
27 kg/mm
2
at 600
◦
C for the cold and warm indenters,
0261–8028
C
2001 Kluwer Academic Publishers
1819