The obtaining of high-density specimens and analysis of mechanical strength characteristics of a composite based on ZrO2-WC nanopowders
Mechanical Department, Ukrainian State Academy of Railway Transport (UkrSART)
, Feyerbakh square 7, Kharkov 61001,
Department of Physics and Technology, V.N. Karazin Kharkiv National University
, Kurchatov Av. 31, Kharkov 61108,
The structures, processes of shrinkage, and phase composition of the compact system ZrO2-WC, obtained by hot pressing with the transmission of high current, are considered in the article. We found that as a result of compaction, the ZrO2-WC-ceramics have uniform density distribution, with the following optimal mode consolidation values T = 1,350C, P = 30 MPa and t = 2 min. These conditions allow us to achieve the best combination of ceramic properties by criteria density and strength.
Recently, most binary systems were made based on
ZrO2 such as ZrO2-TiB2, ZrO2-TiCN, ZrO2-SiC,
ZrO2TiN, and ZrO2-TiC. Consequently, high mechanical
properties of the material can be expected when ZrO2 is
hardened by nanoparticles of the second phase (tungsten
carbide). It will allow extensive use of obtained ceramics.
It is known that tungsten carbide is widely used in the
manufacture of hard alloys based on WC-Co due to its
high resistance to wear and low temperatures during
use. However, the thermal stability of the cobalt binder
greatly limits its use as a structural component, where
high heat resistance, resistance to oxidation, and
corrosion are very important. Previously, attention was paid
to determine the optimum ZrO2 in the composite
materials based on WC made by high-energy FAST methods
[1,2]. Also, the authors in  reported that the addition
of 30% micron-sized WC to ZrO2-matrix significantly
increases the hardness and fracture toughness, but their
values were low.
Research on the possibility of compacting ZrO2-WC
composites via hot pressing with electric current
(electroconsolidation) is the purpose of this work. It is also
important to identify optimal regimes to obtain
highdensity samples having homogeneous microstructure
with high mechanical characteristics.
The nanopowders were mixed using a planetary milling
plant Pulverisette 6(Fritsch GmbH, Idar-Oberstein,
Germany with isopropyl alcohol for 2 h for a uniform
distribution of particles in the sample. The rotation
speed of planetary disk is 160 rpm. To break the
agglomerates, alumina milling balls were added to the
Installation for hot vacuum pressing, designed and
patented by the authors, was done to consolidate the
powders. This installation, in comparison with the
wellknown FAST method in Europe, differs mainly because
of the possibility that it uses a conventional AC power
frequency without special optional equipment pulse
generators. This method later in this article will be referred
to as electroconsolidation.
The nanopowders were sintered using a hot pressing
facility with a direct current under a pressure of 30 MPa
and held for 2 min at various temperatures. Further
studies were done on molded samples such as tablets of
20 mm in diameter. Sintering curve looks like this: at a
pressure of 10 MPa, the temperature was raised at
150C/min up to a temperature of 600C; then, at the
same pressure, the temperature was adjusted to a
holding temperature (1,200C to 1,400C). This temperature
was held for 2 min. At the same time, the pressure was
raised to 30 MPa. After the rise of the holding
temperature stopped, the sample cooled and formed.
Pressure is removed after the final cooling. Full-time
consolidation was 15 min.
The microstructure of the nanoceramic compositions,
obtained by electroconsolidation, was examined by
scanning electron microscopy; by the same method, the grain
sizes of the obtained samples were evaluated. The
samples for electron microscopic studies were prepared as
shear of sintered tablets.
Using a universal hardness tester, the Vickers hardness
(HV10) of the composite is evaluated with a load of
10 kg. The fracture toughness (KIC) calculations were
made based on the measurements of the radial crack
length produced by Vickers (HV10) indentations,
according to Anstis formula . The reported values are the
averages of the data obtained from five indentation tests.
Detailed microstructural characterization and phase
identification were carried out using a Quanta 200 3D
(FEI Co., Hillsboro, OR, USA) scanning electron
microscope (SEM) and a Rigaku Ultima IV X-ray diffractometer
(Rigaku Europe SE, Ettlingen, Germany) (CuK radiation,
Results and discussion
The commercially available high-purity WC (primary
crystallite size 30 nm, Wolfram, Salzburg, Austria) and
ZrO2 (3 mol% Y2O3) powders (primary crystallite size
20 nm, The Research Centre of Constructional Ceramics
and The Engineering Prototyping, Russia) were used as
starting powders. The sintering parameters and relative
density of the obtained ZrO2-WC composites are
presented in Table 1.
Table 1 The sintering parameters and relative density of
the obtained ZrO2-WC composites
Table 1 shows that the holding time is a
temperatureindependent parameter and slightly influences the
densification. The density data reveal that the maximum
density of approximately 99.5% th can be achieved in
composite sintered at 1,350C and holding time of 2 min
with 20 wt.% WC additive.
Microstructure of ZrO2-WC composites with 10% and
20% WC is shown in Figure 1. The WC phase (bright)
was uniformly dispersed in the ZrO2-matrix (dark)
except for a number of agglomerated particles. However, a
careful study using computerized color
cathodoluminescence (CCL) attached to the SEM allowed for the
determination of a significant amount of zirconia particles in
the light phase (Figure 1a). This fact indicates a rather
homogeneously mixed ZrO2-WC composition.
It was found that the maximum pressure is only
applied when the compact system reach its maximum
temperature (for complete degassing of adsorbed gases).
This mode results in the formation of finer structure of
material (Figure 2a), in which the pressure was applied
at the beginning of the sintering cycle and was remained
constant (Figure 2b). The application of the maximum
pressure at lower temperatures results in an increased
porosity due to the presence of adsorbed gases.
Shrinkage due to the evaporation of absorbed moisture and
burnt impurities competes with the process of thermal
expansion in the first stage of the sintering process.
Moreover, the high purity of the starting powder and
narrow particle size distribution were the cause of
avoidance of abnormal growth (exceeding some medium-sized
grains) and the homogeneity of the material microstructure.
The latter circumstance is also characterized by a uniform
distribution of density and, accordingly, the diameter of the
microhardness indentation of the sample that allows to
obtain materials with high mechanical properties and
longer service life extension of ceramic products. The
most uniform hardness distribution on the diameter of
the sample was indicated in ZrO2-20 wt.% WC that
was sintered at 1,300C and with a pressure of 30 MPa
with a holding time of 2 min.
Figure 3 shows the X-ray of the polished surface, and
Figure 4a shows the X-ray of the fracture pattern and of
the samples. The increasing number of monoclinic
zirconium oxide peaks indicates that there is a
tetragonalmonoclinic transformation during loading. The average
grain size of the sample is 350 nm. The structure is
homogeneous and contains no grains with sizes that
differ greatly from the others. That is, the addition of
20 wt.% tungsten carbide further hardened the material
based on zirconium oxide, while it demonstrated the
abnormal grain growth and formation of a fine structure
with a high content of tetragonal phase which is able to
transform into the monoclinic phase (under the
influence of stress) in the vicinity of the crack tip.
Figure 1 The ZrO2-WC composite microstructure in the different regimes. SEM-SE image of the composite microstructure based on ZrO2
with 10 wt.% (a) and 20 wt.% (b) WC and SEM images ZrO2-WC ceramics in regime CCL (c).
Figure 2 SEM-SE image of the microstructures of ZrO2-20 wt.% WC. WC was sintered at T = 1,350C and P = 30 MPa during the holding time
(a) and T = 1,350C and P = 30 MPa applied in the beginning of the sintering cycle (b).
Figure 3 XRD patterns of polished cross-sections of the
ZrO2-20 wt.% WC composites. T = 1,350C, P = 30 MPa,
and holding time = 2 min.
The microstructure of fracture surfaces of ceramics
obtained at 1,350C. One can distinguish two types of
areas: areas of intergranular fracture and the so-called
twinning topography (Figure 4b indicated by arrow).
The paper  stated that the presence of fracture surface
areas with relief twinning can indicated that the
structure undergoes a stress-induced martensitic
(tetragonalmonoclinic) transformation during fracture. We assume
that some of the grains with twin structure are zirconia
grains. However, to confirm this hypothesis, the
chemical analysis of the samples should be carried out.
Figure 5 Vickers hardness and fracture toughness of the
ZrO2-20 wt.% WC composites. Vickers hardness and fracture
toughness as functions of the sintering temperature.
The formation of W2C assumed to be a reaction
between ZrO2 and WC :
ZrO2x 2yWC yW2C ZrO2xy
where x is the oxygen vacancy concentration in the
ZrO2 as a result of the dopant concentration, and y is
the additional vacancy concentration created in the
ZrO2 due to the reaction with WC.
This reaction contributes to the formation of
additional oxygen vacancies and W2C. The occurrence of
additional oxygen vacancies leads to an increase of
nonstoichiometry ZrO2 phase. This can improve the
diffusion coefficient in a certain degree, whereby the mass
Figure 4 XRD patterns (a) and SEM-SE image of microstructure
(b) of fractured surfaces of the ZrO2-20 wt.% WC composites.
T = 1,350C, P = 30 MPa, and holding time = 2 min.
Figure 6 SEM-SE microstructure of fracture surface of WC-ZrO2
composite. T = 1,350C, P = 30 MPa, and holding time = 2 min.
transfer occurs quickly and, therefore, increases the rate
The Vickers hardness (HV10) and indentation fracture
toughness (KIC) of the ZrO2-20 wt.% WC composites
are graphically presented as a function of the sintering
temperature in Figure 5.
The hardness variation with sintering temperature is
closely related to the bulk density and microstructural
features. The hardness increased continuously with
increasing temperature from 1,200C to 1,350C (Figure 5),
due to an increased densification, reaching a maximum
hardness at full densification when temperature was at
1,350C. At higher sintering temperatures, the hardness
slightly decreased due to the increased WC and ZrO2
grain size, as well as the partial spontaneous
transformation of the ZrO2 phase.
The fracture toughness increased rapidly from 5.5 to
8.5 MPa m1/2 with increasing temperature from 1,200C to
1,350C (Figure 5), followed by a decreasing trend to
8.1 MPa m1/2 at 1,400C. The high value of fracture tough
ness may be due to the fact that a part of the tetragonal
phase of ZrO2 transforms to the monoclinic ZrO2 (Figure 4)
during electroconsolidation at a temperature of 1,350C.
Moreover, in the ZrO2-WC composites, crack
deflection is an effective toughening mechanism besides the
ZrO2 phase transformation toughening. The radial crack
pattern originating in the corners of the Vickers
indentations revealed that the propagating cracks were deflected
by the WC grains (Figure 6), which was also observed in
hot pressed ZrO2-WC composites .
Electroconsolidation provides a uniform density
distribution, without any plasticizers that are potential sources of
impurities and additional porosity in the sintered product.
The maximum density of the ZrO2-20 wt.% WC composite
was obtained at 1,350C for 2 min at 30 MPa.
The best combination of mechanical properties was
obtained for a 2 mol.% Y2O3-stabilized ZrO2 composite
with 20 wt.% WC, obtained by electroconsolidation at
1,350C, combining a hardness of 16.5 GPa and a
fracture toughness of 8.5 MPa m1/2.
EG and OM were the principal investigators of this study. EG investigated the
mechanical properties. OM investigated the structure and performed full factorial
experiment for technology of hot pressing with direct transmission of high
amperage current. VC prepared the experiment, carried out the X-ray analysis,
and analyzed the results. All authors read and approved the final manuscript.
We thank the Research Centre of Constructional Ceramics and The
Engineering Prototyping (Russia) for research assistance and for providing
the ZrO2 nanopowder synthesized from Ukrainian raw materials, using its