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Ta ceramic-metal composites
Accepted Manuscript Wire electrical discharge machining of 3Y-TZP/Ta ceramic-metal composites A. Smirnov, P. Peretyagin, J.F. Bartolomé PII:
S0925-8388(17)34417-1
DOI:
10.1016/j.jallcom.2017.12.221
Reference:
JALCOM 44307
To appear in:
Journal of Alloys and Compounds
Received Date: 25 October 2017 Revised Date:
19 December 2017
Accepted Date: 21 December 2017
Please cite this article as: A. Smirnov, P. Peretyagin, J.F. Bartolomé, Wire electrical discharge machining of 3Y-TZP/Ta ceramic-metal composites, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.221. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Wire electrical discharge machining of 3Y-TZP/Ta ceramic-metal composites
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A.Smirnova,b*, P. Peretyaginb and J.F. Bartoloméa
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Científicas (CSIC), C/ Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain
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Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones
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101472 Moscow, Russian Federation
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Moscow State University of Technology STANKIN, Vadkovskij per. 1, Moscow Oblast,
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Corresponding author; e-mail: [email protected] Tel.: +372 55 82 827
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Abstract
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Dense (>98 th%) and homogeneous ceramic/metal composites were obtained by spark plasma
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sintering (SPS) using ZrO2 and lamellar metallic powder of tantalum (20 vol.%) as starting
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materials. Composites showed a fracture toughness value of 16 MPa·m1/2 mainly due to crack
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bridging of the elastic–plastic deformations of ductile metal particles. This fracture toughness
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was accompanied by a simultaneous enhancement in damage tolerance and fatigue resistance of
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3Y-TZP/Ta composites.
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Besides exceptional mechanical properties, SPS sintered 3Y-TZP/Ta composites also showed the
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electrical conductivity suited to wire electrical discharge machining (WEDM). Therefore, they
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are suitable to be produced in mechanically performant, complex shape components with the
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required tolerance while reducing machining costs. The aim of this work was the study of the
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electrical properties of the materials as well as the characteristics of the machining process and
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the machined surfaces influence on the bending strength of composites. The results show that
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workpieces can be machined with high accuracy and without a drop in mechanical strength.
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1. Introduction
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Stabilized zirconia ceramics have been demonstrated to display the highest strength and
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toughness amongst oxide ceramics due to a polymorphic phase transformation that occurs in
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doped zirconia and known as phase transformation toughening [1].
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Besides exceptional mechanical properties, stabilized zirconia also showed excellent resistance
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against erosion, friction and thermal shock that makes this material very attractive for a wide
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range of applications [2] in the fields of manufacturing and cutting tools [3–5], punches [6],
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biomedical applications [7, 8] and even automobile and aerospace [9].
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Nevertheless, the application of these ceramic materials is still remained in some cases limited
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by their high sensitivities to crack propagation. One of the most effective methods to increase
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fracture toughness in monolithic ceramics is to incorporate ductile metal particles reinforcement
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[10, 11]. The mechanical properties of the ceramic matrix toughening by means of various
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mechanisms, for example, ductile-phase toughening, transformation toughening, or crack
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blunting allow design composite with improved mechanical performance [12-16].
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In the previous studies, it was reported that addition of tantalum lamellar shape metal particles
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(20 vol.%) allow design zirconia-metal composite with improved mechanical performance due to
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the interactions between transformation toughening of zirconia and crack bridging mechanisms
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[17, 18]. Moreover, spark plasma sintered zirconia based ceramic-metal composites were
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evaluated from the point of view of their mechanical behavior under monotonic and cyclic
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loading with artificial induced flaws. 3Y-TZP/Ta composites have demonstrated simultaneous
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enhancement in damage tolerance and fatigue resistance [19]. Furthermore, in our earlier results
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obtained in an in vitro study [20] suggest that a new zirconia-Ta biocomposite displays
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biocompatibility. Despite the presence of high crack resistance of 3Y-TZP/Ta composites,
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another factor limiting the application of these ceramics is the inability to attain sufficient
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dimensional control during net-shape fabrication. Common machining methods for producing
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ceramic parts rely on finish machining using diamond tools. However, the high hardness and
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brittleness of ceramics makes conventional machining very difficult or even impossible.
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Therefore, for manufacturing of advanced ceramics many different shaping technologies can be
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used. Such shaping technologies can be pressing (e.g. cold and hot isostatic pressing, uniaxial
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pressing), casting (pressureless casting, pressure casting, tape casting), extrusion, injection
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moulding, green machining, up to high sophisticated processes like rapid prototyping or
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manufacturing, freeze gelation, printing technologies and many others [21, 22]. However, due to
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ceramic shrinkage it extremely difficult to fabricate product to final net shape, hence their final
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cost will be increased. Thus, in order to obtain sophisticated shaped products advanced
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machining methods should be used instead of traditional machining techniques, because of
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associated advantages that include reduced processing costs, reduced waste, high precision,
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versatility and degree of automation [23].
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Wire electrical discharge machining (WEDM) is such a technique that can be successfully
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applied to machining of single-phase ceramics, cermets and ceramic matrix composites [24]. An
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important feature to remember with WEDM is that it will only work with materials that are
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electrically conductive. For being electrical discharge machinable, the materials’ electrical
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resistivity should be lower than 100–300 Ω·cm [25]. In case of a composite, a mixture of
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conducting and insulating phases becomes conducting when the volume fraction of conducting
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phase exceeds a ‘percolation threshold’ of 16%, the minimum amount to give a continuous path
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across the whole sample. This threshold is independent of the size and shape of the conducting
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phase, as long as its particles are equiaxed. If the conducting phase consists of long thin particles,
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the chance of contact increases, this reduces the percolation threshold so that conduction occurs
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at much lower loading [26-28]. Consequently, electrical discharge machining offers production
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of intricate ceramic composite parts regardless of their high hardness and is therefore ideally
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suited for machining ceramic based composites, requiring only sufficient electrical conductivity.
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In comparison with conventional machining techniques, WEDM achieves higher removal rates
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for these materials with respect of surface integrity and tolerances of below 1 µm have been
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achieved as well [25]. However, difficulties also arise with respect to the surface finish
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conditions [29, 30], the corrosion of these materials during machining [31], and the influence the
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machining parameters in the surface damage, i. e. cracks produced within the thermally affected
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zone (recast layer and adjacent regions) beneath the shaped surface.
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The aim of this study was to assess the WED-machinability of 3Y-TZP/Ta composite obtained
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by Spark Plasma Sintering (SPS) and their surface microstructural quality and final strength.
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Experimental Procedure
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2.1
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Commercially available powders were used as raw materials: (1) t-ZrO2 polycrystals (3Y-TZP, 3
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mol% Y2O3; TZ-3YE, Tosoh Corp., Tokyo, Japan), with an average particle size d50=0.26±0.05
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µm, and (2) tantalum (99.97% purity, Alfa Aesar, Karslruhe, Germany) with an average particle
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size d50=44 µm.
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Tantalum raw powder (Fig. 1A) was milled by high-energy milling that was performed using 3
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mm diameter zirconia balls and teflon vial using isopropilic alcohol as liquid media. A ball-to-
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powder weight ratio (BPR) of 8:1 was used. Rotational speed and milling time were kept
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constant during all milling operations as 1350 rpm and 4 h, respectively. The aspect ratio of
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flake-shaped milled metallic particles was obtained from Scanning Electron Microscopy (SEM)
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image analysis. The results showed that flake-like deformed Ta particles had a mean size of 42
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µm with a high aspect ratio around 50:1 (Fig. 1B).
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2.2
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To fabricate the zirconia matrix reinforced with lamellar Ta particles, 3Y-TZP powder was wet
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mixed with 20 vol.% of the ball-milled Ta powder. Details of the ceramic/metal slurry 5
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Starting materials
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Powder processing
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processing were reported elsewhere [18]. The obtained powder was was placed in a die-punch
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setup made from isostatic graphite (grade С4, DonCarb Graphite, Rostov, Russia) and
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compacted by spark plasma sintering (SPS, FCT Systeme GmbH, HPD25, Effelder-Rauenstein,
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Germany) in vacuum (≈ 1x10-2 mbar) at 1400°C, applying a heating rate of 200°C/min and an
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uniaxial pressure of 80 MPa. The final temperature and pressure were maintained for 3 min. The
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temperature was controlled during sintering by a pyrometer situated at the top of the machine
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and focused at the center of the blank (3 mm over the top surface). The as-sintered sample disks
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showed diameters of 50 mm and a thickness of approximately 3-4 mm.
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2.3
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The tantalum powder before and after milling was analyzed by X-ray diffraction (XRD). The
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specimens for the XRD (D8 diffractometer, Bruker AXS Inc., Madison, WI, USA, Cu-Kα
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radiation, wavelength 1.5405981 Å, accelerating voltage 40 kV, beam current 30 mA)
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measurements were prepared by suspending a small volume of Ta powder in acetone directly on
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a Si (510) single-crystal wafer within a specially made supporting assembly and drying the
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suspension by evaporating the acetone. An evenly spread distribution on Ta powder was
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observed.
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The X-ray diffraction powder and sintered composites patterns were collected at diffraction
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angles 2θ ranging from 20° to 70°. Qualitative analyses of the crystal phases were found using
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the following Powder Diffraction Files (PDF) from the International Centre for Diffraction Data
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(ICDD): ICDD-PDF 01-083-0113 (t-ZrO2), ICDD-PDF 00-024-1165 (m-ZrO2), ICDD-PDF 00-
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035-0789 (tantalum) and ICDD-PDF 00-35-1193 (Ta2O5). The compounds formed on the
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EDM’ed surface were determined through XRD as well. The amount of m-ZrO2 after WED
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machining was evaluated from the XRD diffractograms (2θ range 27°–33°) according to Garvie
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and Nicholson method [32]:
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XRD characterization
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Xm =
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I m (111) + I m (111)
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−
I m (111) + I m (111) + I t (101)
where It and Im represent the integrated intensities (areas under the reflections) of the tetragonal
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(101)t as well as the monoclinic (111)m and (-111)m reflections. Its volume fraction Vmtot was
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calculated as proposed by Toraya et al. [33]:
Vmtot =
1.311X m 1 + 0.311X m
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2.4
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The density of the sintered samples was measured using the Archimedes’ method. The
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measurements were carried out in distilled water on an analytical balance (Sartorius YDK01,
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Goettingen, Germany). Scanning electron microscopy (SEM) characterization was carried out on
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polished down to 1 µm surfaces by a microscope (Phenom G2, Eindhoven, The Netherlands).
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The bending strength, σf, was determined by using the three-point bending test as specified by
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the ASTM specification C1161–13 [34]. The tests were performed at room temperature using a
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universal testing machine (Shimadzu AutoGraph AG-X 5kN, Japan). The specimens in the form
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of bars with dimensions of 3.0 × 4.0 × 45 mm3 were loaded to failure with a crosshead speed of
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0.5 mm/min and a span of 40 mm. Some of WEDM cut bars were directly used as bending test
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samples, but some others were traditional prepared and then went to bending test for comparison
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purpose. Traditional preparation means that SPS sintered discs were cut into bars using diamond
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saw, ground and polished with 1 µm diamond paste. The strength was calculated from the failure
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loads, F (N), and the specimen dimensions, using the equation:
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Microstructural and mechanical characterization
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σf =
3FL 2bh 2
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Where N is the failure load, L is the span, b is the width and h is the height. The strength results
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were averaged over 10 specimens. The formulas and calculation procedures used in the
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measurements of fracture toughness (KIc) have been reported in previous publications [35].
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Measurement of electrical resistance and wire-electroerosion machining (WEDM)
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To measure the electrical resistivity, four point probe method was used. The four point probe
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setup consists of four equally spaced tungsten carbide electrodes. These electrodes have a
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diameter of 0.4 mm and separated by a distance of 1 mm. A power source with a high internal
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resistivity sends a constant current through the two outer electrodes. The current output can be
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obtained by an ammeter. The second set of electrodes is used for sensing and since negligible
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current flows in these probes – only voltage drop – thus accurate resistance is measured. A
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resistance of the sample between inner electrodes is the ratio of the voltage registering on the
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voltmeter to the value of the output current of the power supply. In the present work, the
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electrical resistance of sintered ceramic-metal composites was measured with a four-wire
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(Kelvin) connection method using a separate current source (Keithley 6220, Cleveland, OH,
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USA) and a two-channel nanovoltmeter (Keithley 2182A, Cleveland, OH, USA).
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Wire EDM experiments on the commercial machine (Seibu M500SG, Seibu Electric &
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Machinery Co., Koga, Japan) have been performed. Commercial thin brass wire electrodes were
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used (Osaka Brasscut A500, 0.25 mm diameter). Metallic wire and sintered disk are both
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electrodes separated by a deionized water with conductivity of 0.1 µS/cm and subjected to an
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electric voltage (270 V) and intensity (0.05 A). The machining via EDM is a multi-stage process.
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In order to find optimal working parameters the system database which includes various
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parameters (workpiece material, thickness, wire diameter, etc.) was used. However, machine
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manufacturer’s manual has not provided enough information for machining of ceramic-metal
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composites. For this reason, three cutting conditions of the wire EDM of the ZrO2-Ta composites
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were chosen (Table 1). The input parameters such average working voltage, average working
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current and pulse off time were considered for analysis, while other parameters remain constant.
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Optical microscope (Leica, Wetzlar, Germany) was used to study WEDM machined pieces and
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detailed analysis of heat affected regions has been done by using SEM equipped with the 3D
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Roughness Reconstruction application. The field of view area for the height map calculation was
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490 µm2 (×550).
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181 3. Results and Discussion
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3.1. XRD study
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The ball-milled Ta powder can be textured by the milling process. This texture of the deformed
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metal particles is revealed by XRD (Fig. 2A and 2B).
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A strong preference for the deformed crystallites to have their (200) reflection parallel to the
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specimen surface occurred. A strong preference for the {100} type of lattice planes to be parallel
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to the surface was reported for cold-rolled polycrystalline bcc metals [36]. Ball-milling and cold-
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rolling imply that the material is flattened in between compressing surfaces; that is, the balls in
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the ball-milling equipment used here and both rolls in rolling apparatus. The texture can be
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explained as follows. The most particles that deformed were flattened into flakes, which, after
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being suspended in acetone (in the procedure for specimen preparation for XRD analysis),
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position on to the Si substrate with strong tendency for their flat sides to be parallel to the surface
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of the Si substrate. On the other hand, the measurements of the Ta powders showed that the XRD
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lines become substantially broadened upon ball-milling (Fig. 2B). This broadening can be
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ascribed to the introduction of lattice microstrain.
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Fig. 3 shows diffractograms obtained from wet mechanically mixed 3Y-TZP/Ta powder (A) and
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from polished surfaces of sintered composite (B) where the complete conversion of the
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monoclinic ZrO2 into tetragonal ZrO2 is revealed.
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There were no other or new phases than initial powders' phases observed. In addition, graphite
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die-punch setup can act as planar sources of graphitic carbon, the presence of which can affect
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the composition of the subsurface layers of the sintered material. However, the X-ray diffraction
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patterns do not show the presence of any carbon-containing phase (carbides) in sintered and
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polished samples, as can be seen in Fig. 3B. It is likely that carbon-affected layer with negligible
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thickness was removed by polishing from the sintered compact.
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3.2. Microstructural characterization and toughening mechanism
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Dense (>98 th.%) and homogeneous ceramic–metal composites were initially fabricated from
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3Y-TZP and tantalum (20 vol.%) powders. Electron scanning micrographs of polished surface of
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3Y-TZP/Ta composites are shown in Fig. 4A. Different surface morphologies can be observed
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depending on the analyzed orientation: perpendicular or parallel to the pressure direction applied
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in SPS (Fig. 4A and B, respectively).
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The metallic particles are preferentially oriented due to the effect of the applied pressure during
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spark plasma sintering process. Moreover, during flexural test the polished surface of the
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composites was used as the tensile side, i.e. was perpendicular to the SPS-direction. The
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tantalum particles are uniformly dispersed in the matrix and no porosity is observed. It was
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found that the addition of 20 vol. % of Ta particles to the 3Y-TZP matrix produces reinforcement
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effect which appears as increase of fracture toughness up to 16 MPa·m1/2. The SEM observations
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of the fracture surfaces showed the major toughening mechanism is the plastic deformation with
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further partial debonding of tantalum particles (Fig. 4B). In earlier studies, have been shown that
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addition of Ta particles to the zirconia matrix produces reinforcement effect due to the
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anticipated contributions between crack bridging of ductile phase and 3Y-TZP stress-induced
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phase transformation, which appears as increase of fracture crack growth resistance [19].
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3.3. Electrical resistivity measurements and WEDM results
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The electrical resistivity of composites was equal to 2 x 10-4 ± 1 Ω·cm. This value is about five
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orders of magnitude lower than the limit of 100–300 Ω·cm, consequently, the addition of 20
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vol.% of conductive phase makes the material suitable for WEDM. Optical photo of WEDM
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machined piece with complex geometry is presented in Fig. 5A.
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The machined surface of ceramic-metal composite has been analyzed by using tabletop SEM and
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XRD. WEDM process is based on thermoelectric energy between the work piece and an
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electrode. A pulse discharge occurs in a small gap between the work piece and the electrode and
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removes the unwanted material from the workpiece through melting and vaporizing. The
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machined surface after third cut was examined using a SEM for detailed analysis. Fig. 5B-D
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shows the effects of the wire over the material surface after cutting. When the distance between
235
electrodes is reduced, the discharge flows between the two electrodes. Once the discharge starts,
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plasma is formed in the neighborhood of the machined front and there are places where the wire
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has caused materials fusion (Fig. 5D). The surface topography of the machined composite is a
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recast layer with discharge craters.
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The SEM cross-sectional view and corresponding XRD spectrum image of the EDM surface
240
layer for the 3Y-TZP/Ta ceramic-metal composite is presented in Fig. 6A and 6B, respectively.
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The XRD pattern showed that the WEDM surface layer consists of tetragonal and monoclinic
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zirconia, Tantalum and a minor amount of Ta2O5. The monoclinic volume fraction for EDM’ed
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surface was about 68 vol.%. Unlike the polished sintered samples in which the negligible amount
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(6 vol.%) of m-ZrO2 was found, after WEDM this amount was higher due to the presence of
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solid solution of Ta2O5 which enhance destabilization of the zirconia from the tetragonal to the
246
monoclinic phase, resulting in the formation of microcracks [37, 38]. Therefore, transformed
247
zirconia leads to a volume increase, stressing the neighboring grains and produces the micro
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crack formation on the wire-electrical discharge machined surface. In addition, cracking could
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also be attributed to the field of tensile residual stresses resulting from thermal effects due to the
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rapid heating and quenching cycles induced during the WEDM process (Fig. 5D). However, the
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occurrence of surface cracks on the machined surface after wire-EDM was only evidenced in the
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recast layer (Fig. 6A).
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The values of average bending strength of WEDM specimens with surface roughness are
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presented in Table 2. From these values, it is obvious that polished samples have the highest
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bending strength. Meanwhile for WEDM cut bars strength increase as the machining parameters
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and surface roughness decrease. However, despite that fine polishing can give materials higher
257
strength and high reliability the difference between strength values is not distinct due to high
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damage tolerance of ceramic-metal composite. Therefore, it could be concluded that initial flaw
259
size (≈ 100 µm) is smaller than critical crack size [19]. Consequently, the WEDM technique
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could be a suitable method to obtain ZrO2-Ta composites with complicated shape for a wide
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range of high-performance applications, such as inserts for cutting tools, biomedical implants or
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microdevices, among many others.
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4. Conclusions
The obtained results clearly point out that besides exceptional mechanical properties of 3Y-
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TZP/Ta composites obtained by Spark Plasma Sintering also possess the electrical conductivity
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and consequently is suitable for fabrication components with sophisticated shapes by electrical
268
discharge machining technique with the required tolerance while reducing machining costs.
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Although the surface quality was modest after WEDM cutting, with the formation of a thermally
270
induced recast layer, exhibiting many resolidified droplets and voids as well as surface
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microcracks, the surface roughness was about 1.1 µm and did not to significantly decreased
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values of strength compared with polished samples due to very high flaw tolerance of
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zirconia/Ta composites.
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Acknowledgements
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This work was supported by the Ministry of the Russian Federation by contract 14.B25.31.0012,
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26 June 2013.
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toughened by 5–20 vol% Ta metallic particles fabricated by pressureless sintering, Ceram. Inter.
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40 (2014) 1829-1834.
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J.F. Bartolomé, Unprecedented simultaneous enhancement in damage tolerance and fatigue
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resistance of zirconia/Ta composites, Sci. Rep. 7 (2017) 44922.
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[20] L. Esteban, A. Smirnov, C. Prado, J.S. Moya, R. Torrecillas, J.F. Bartolomé,
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Multifunctional ceramic-metal biocomposites with Zinc containing antimicrobial glass coatings,
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ceramics by electro, chemical and physical processes, Mfg. Tech. 65 (2016) 761–784.
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removal mechanisms in EDM of composite ceramic materials, J. Mater. Process. Tech. 149
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[24] W. König, DF. Dauw, G. Levy, U. Panten, EDM-Future Steps towards the Machining of
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Torrecillas, J.S. Moya, Electroconductive Alumina-TiC-Ni nanocomposites obtained by Spark
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Plasma Sintering, Ceram. Inter. 37 [5] (2011) 1631-1636.
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[26] S. Hussain, I. Barbariol, S. Roitti, O. Sbaizero, Electrical conductivity of an insulator matrix
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(alumina) and conductor particle (molybdenum) composites, J. Eur. Ceram. Soc. 23 (2003) 315-
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[27] J.S. Moya, S. Lopez-Esteban, C. Pecharroman, The challenge of ceramic/metal
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microcomposites and nanocomposites, Prog. Mater. Sci. 52 (2007) 1017-1090.
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[28] J. Robertson, Realistic applications of CNTs, Mater. Today 10 (2004) 46-52.
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[29] I. Puertas, CJ. Luis, L. Álvarez, Analysis of the influence of EDM parameters on surface
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quality, MRR and EW of WC–Co, J. Mater. Process. Tech. 154 (2004)1026-1032.
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[30] AA. Khan, M. Ali, M. Shaffiar, Relationship of surface roughness with current and
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voltage during wire EDM, J. Applied. Sci. 6 (2006) 2317-2320.
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[31] H. Obara, H. Satou, M. Hatano, Fundamental study on corrosion of cemented carbide
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during wire EDM, J. Mater. Process. Tech. 149 (2004) 370-375
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[32] R. C. Garvie and P. S. Nicholson, Phase analysis in zirconia systems, J. Am. Ceram. Soc. 55
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[6] (1972) 303–305.
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[33] H. Toraya, M. Yoshimura, and S. Somiya, Calibration curve for quantitative analysis of the
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monoclinic tetragonal ZrO2 system by X-ray diffraction, J. Am. Ceram. Soc. 67 [6] (1984) 119–
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121.
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[34] ASTM C1161 – 13, Standard Test Method for Flexural Strength of Advanced Ceramics at
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Ambient Temperature.
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[35] A. Smirnov, J.F. Bartolomé, H.D. Kurland, J. Grabow, F.A. Müller, Design of a new
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zirconia–alumina–Ta micro-nanocomposite with unique mechanical properties, J. Am. Ceram.
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Soc. 99 (2016) 3205–3209.
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[36] Y.B. Park, D.N. Lee, G. Gottstein, The evolution of recrystallization textures in body
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centered cubic metals, Acta Mater. 46 (1998) 3371-3379.
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[37] D.-J Kim, Effect of Ta2O5, Nb2O5, and HfO2 alloying on the transformability of Y2O3-
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stabilized tetragonal ZrO2, J. Am. Ceram. Soc. 73 (1990) 115–120.
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[38] A.Smirnov, J.F. Bartolomé, Microstructure and mechanical properties of ZrO2 ceramics
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toughened by 5–20 vol% Ta metallic particles fabricated by pressureless sintering, Ceram. Inter.
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40 [1] (2014) 1829-1834.
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Figure captions
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Fig.1. SEM micrograph of tantalum powder before (A) and after wet ball-milling for 4 h (B).
Fig.2. XRD analysis of Ta raw powder before (A) and after milling (B). The 200 and 110 reflections of Ta for an as-received and milled powder. Upon ball-milling, pronounced texturing
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and broadening occurs as a result of deformation and the increase of lattice strain.
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Fig.3. XRD analysis of the 3Y-TZP/Ta powder as prepared (A) and polished surface of sintered composite (B). Labelling “t” and “m” denotes tetragonal and monoclinic zirconia, respectively. “■” marks tantalum metal reflections.
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Fig.4. SEM micrographs of a polished (A) and a fractured (B) surface of 3Y-TZP/Ta composite. Zirconia: dark-gray; tantalum: light-gray inclusions. White arrows show decohesion between the
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matrix and the metallic particles. Yellow arrows show plastic deformation of Ta.
Fig.5. Optical photo of WEDM’em 3Y-TZP/Ta composite (A) with complex geometry. SEM
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micrograph showing 3Y-TZP/Ta surface finishing (B, C (topographic view) and D). White arrows show cracks formed on composite’s surface machined.
Fig.6. SEM cross-sectional view (A) and corresponding XRD pattern of the EDM’ed surface (B) after third cut.
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Tables
Table 1. Wire electrical discharge machining parameters Average working
Off time (µs)
voltage (V)
current (A)
1 cut
120
2.1
8
2 cut
100
1.2
4
3 cut
85
0.4
2
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Average working Machining conditions
Table 2. The WEDM cutting parameters influence on surface roughness and strength values Surface roughness, Sa (µm)
Flexural strength, σf (MPa)
Traditional preparation
0.3 ± 0.1
967 ± 8
3.1 ± 0.1
903 ± 22
2.4 ± 0.1
921 ± 17
1.1 ± 0.1
942 ± 14
1 cut 2 cut
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Machining conditions
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ACCEPTED MANUSCRIPT Highlights
Fully dense and homogeneous ceramic-metal composites were successfully fabricated.
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Composites showed outstanding value of fracture toughness of 16 MPa·m1/2.
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3Y-TZP/Ta composites suited to wire electrical discharge machining (WEDM).
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WEDM cut specimens showed insignificant degradation in the bending strength.
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S0925-8388(17)34417-1
DOI:
10.1016/j.jallcom.2017.12.221
Reference:
JALCOM 44307
To appear in:
Journal of Alloys and Compounds
Received Date: 25 October 2017 Revised Date:
19 December 2017
Accepted Date: 21 December 2017
Please cite this article as: A. Smirnov, P. Peretyagin, J.F. Bartolomé, Wire electrical discharge machining of 3Y-TZP/Ta ceramic-metal composites, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.221. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Wire electrical discharge machining of 3Y-TZP/Ta ceramic-metal composites
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A.Smirnova,b*, P. Peretyaginb and J.F. Bartoloméa
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Científicas (CSIC), C/ Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain
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Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones
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b
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101472 Moscow, Russian Federation
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Moscow State University of Technology STANKIN, Vadkovskij per. 1, Moscow Oblast,
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Corresponding author; e-mail: [email protected] Tel.: +372 55 82 827
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Abstract
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Dense (>98 th%) and homogeneous ceramic/metal composites were obtained by spark plasma
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sintering (SPS) using ZrO2 and lamellar metallic powder of tantalum (20 vol.%) as starting
17
materials. Composites showed a fracture toughness value of 16 MPa·m1/2 mainly due to crack
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bridging of the elastic–plastic deformations of ductile metal particles. This fracture toughness
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was accompanied by a simultaneous enhancement in damage tolerance and fatigue resistance of
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3Y-TZP/Ta composites.
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Besides exceptional mechanical properties, SPS sintered 3Y-TZP/Ta composites also showed the
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electrical conductivity suited to wire electrical discharge machining (WEDM). Therefore, they
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are suitable to be produced in mechanically performant, complex shape components with the
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required tolerance while reducing machining costs. The aim of this work was the study of the
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electrical properties of the materials as well as the characteristics of the machining process and
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the machined surfaces influence on the bending strength of composites. The results show that
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workpieces can be machined with high accuracy and without a drop in mechanical strength.
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1. Introduction
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Stabilized zirconia ceramics have been demonstrated to display the highest strength and
32
toughness amongst oxide ceramics due to a polymorphic phase transformation that occurs in
33
doped zirconia and known as phase transformation toughening [1].
34
Besides exceptional mechanical properties, stabilized zirconia also showed excellent resistance
35
against erosion, friction and thermal shock that makes this material very attractive for a wide
36
range of applications [2] in the fields of manufacturing and cutting tools [3–5], punches [6],
37
biomedical applications [7, 8] and even automobile and aerospace [9].
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Nevertheless, the application of these ceramic materials is still remained in some cases limited
39
by their high sensitivities to crack propagation. One of the most effective methods to increase
40
fracture toughness in monolithic ceramics is to incorporate ductile metal particles reinforcement
41
[10, 11]. The mechanical properties of the ceramic matrix toughening by means of various
42
mechanisms, for example, ductile-phase toughening, transformation toughening, or crack
43
blunting allow design composite with improved mechanical performance [12-16].
44
In the previous studies, it was reported that addition of tantalum lamellar shape metal particles
45
(20 vol.%) allow design zirconia-metal composite with improved mechanical performance due to
46
the interactions between transformation toughening of zirconia and crack bridging mechanisms
47
[17, 18]. Moreover, spark plasma sintered zirconia based ceramic-metal composites were
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evaluated from the point of view of their mechanical behavior under monotonic and cyclic
49
loading with artificial induced flaws. 3Y-TZP/Ta composites have demonstrated simultaneous
50
enhancement in damage tolerance and fatigue resistance [19]. Furthermore, in our earlier results
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obtained in an in vitro study [20] suggest that a new zirconia-Ta biocomposite displays
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biocompatibility. Despite the presence of high crack resistance of 3Y-TZP/Ta composites,
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another factor limiting the application of these ceramics is the inability to attain sufficient
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dimensional control during net-shape fabrication. Common machining methods for producing
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ceramic parts rely on finish machining using diamond tools. However, the high hardness and
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brittleness of ceramics makes conventional machining very difficult or even impossible.
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Therefore, for manufacturing of advanced ceramics many different shaping technologies can be
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used. Such shaping technologies can be pressing (e.g. cold and hot isostatic pressing, uniaxial
59
pressing), casting (pressureless casting, pressure casting, tape casting), extrusion, injection
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moulding, green machining, up to high sophisticated processes like rapid prototyping or
61
manufacturing, freeze gelation, printing technologies and many others [21, 22]. However, due to
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ceramic shrinkage it extremely difficult to fabricate product to final net shape, hence their final
63
cost will be increased. Thus, in order to obtain sophisticated shaped products advanced
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machining methods should be used instead of traditional machining techniques, because of
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associated advantages that include reduced processing costs, reduced waste, high precision,
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versatility and degree of automation [23].
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Wire electrical discharge machining (WEDM) is such a technique that can be successfully
68
applied to machining of single-phase ceramics, cermets and ceramic matrix composites [24]. An
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important feature to remember with WEDM is that it will only work with materials that are
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electrically conductive. For being electrical discharge machinable, the materials’ electrical
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resistivity should be lower than 100–300 Ω·cm [25]. In case of a composite, a mixture of
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conducting and insulating phases becomes conducting when the volume fraction of conducting
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phase exceeds a ‘percolation threshold’ of 16%, the minimum amount to give a continuous path
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across the whole sample. This threshold is independent of the size and shape of the conducting
75
phase, as long as its particles are equiaxed. If the conducting phase consists of long thin particles,
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the chance of contact increases, this reduces the percolation threshold so that conduction occurs
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at much lower loading [26-28]. Consequently, electrical discharge machining offers production
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of intricate ceramic composite parts regardless of their high hardness and is therefore ideally
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suited for machining ceramic based composites, requiring only sufficient electrical conductivity.
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In comparison with conventional machining techniques, WEDM achieves higher removal rates
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for these materials with respect of surface integrity and tolerances of below 1 µm have been
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achieved as well [25]. However, difficulties also arise with respect to the surface finish
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conditions [29, 30], the corrosion of these materials during machining [31], and the influence the
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machining parameters in the surface damage, i. e. cracks produced within the thermally affected
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zone (recast layer and adjacent regions) beneath the shaped surface.
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The aim of this study was to assess the WED-machinability of 3Y-TZP/Ta composite obtained
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by Spark Plasma Sintering (SPS) and their surface microstructural quality and final strength.
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88 2.
Experimental Procedure
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2.1
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Commercially available powders were used as raw materials: (1) t-ZrO2 polycrystals (3Y-TZP, 3
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mol% Y2O3; TZ-3YE, Tosoh Corp., Tokyo, Japan), with an average particle size d50=0.26±0.05
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µm, and (2) tantalum (99.97% purity, Alfa Aesar, Karslruhe, Germany) with an average particle
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size d50=44 µm.
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Tantalum raw powder (Fig. 1A) was milled by high-energy milling that was performed using 3
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mm diameter zirconia balls and teflon vial using isopropilic alcohol as liquid media. A ball-to-
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powder weight ratio (BPR) of 8:1 was used. Rotational speed and milling time were kept
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constant during all milling operations as 1350 rpm and 4 h, respectively. The aspect ratio of
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flake-shaped milled metallic particles was obtained from Scanning Electron Microscopy (SEM)
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image analysis. The results showed that flake-like deformed Ta particles had a mean size of 42
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µm with a high aspect ratio around 50:1 (Fig. 1B).
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2.2
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To fabricate the zirconia matrix reinforced with lamellar Ta particles, 3Y-TZP powder was wet
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mixed with 20 vol.% of the ball-milled Ta powder. Details of the ceramic/metal slurry 5
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Starting materials
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Powder processing
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processing were reported elsewhere [18]. The obtained powder was was placed in a die-punch
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setup made from isostatic graphite (grade С4, DonCarb Graphite, Rostov, Russia) and
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compacted by spark plasma sintering (SPS, FCT Systeme GmbH, HPD25, Effelder-Rauenstein,
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Germany) in vacuum (≈ 1x10-2 mbar) at 1400°C, applying a heating rate of 200°C/min and an
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uniaxial pressure of 80 MPa. The final temperature and pressure were maintained for 3 min. The
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temperature was controlled during sintering by a pyrometer situated at the top of the machine
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and focused at the center of the blank (3 mm over the top surface). The as-sintered sample disks
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showed diameters of 50 mm and a thickness of approximately 3-4 mm.
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2.3
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The tantalum powder before and after milling was analyzed by X-ray diffraction (XRD). The
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specimens for the XRD (D8 diffractometer, Bruker AXS Inc., Madison, WI, USA, Cu-Kα
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radiation, wavelength 1.5405981 Å, accelerating voltage 40 kV, beam current 30 mA)
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measurements were prepared by suspending a small volume of Ta powder in acetone directly on
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a Si (510) single-crystal wafer within a specially made supporting assembly and drying the
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suspension by evaporating the acetone. An evenly spread distribution on Ta powder was
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observed.
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The X-ray diffraction powder and sintered composites patterns were collected at diffraction
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angles 2θ ranging from 20° to 70°. Qualitative analyses of the crystal phases were found using
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the following Powder Diffraction Files (PDF) from the International Centre for Diffraction Data
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(ICDD): ICDD-PDF 01-083-0113 (t-ZrO2), ICDD-PDF 00-024-1165 (m-ZrO2), ICDD-PDF 00-
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035-0789 (tantalum) and ICDD-PDF 00-35-1193 (Ta2O5). The compounds formed on the
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EDM’ed surface were determined through XRD as well. The amount of m-ZrO2 after WED
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machining was evaluated from the XRD diffractograms (2θ range 27°–33°) according to Garvie
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and Nicholson method [32]:
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XRD characterization
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Xm =
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I m (111) + I m (111)
(1)
−
I m (111) + I m (111) + I t (101)
where It and Im represent the integrated intensities (areas under the reflections) of the tetragonal
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(101)t as well as the monoclinic (111)m and (-111)m reflections. Its volume fraction Vmtot was
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calculated as proposed by Toraya et al. [33]:
Vmtot =
1.311X m 1 + 0.311X m
(2)
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2.4
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The density of the sintered samples was measured using the Archimedes’ method. The
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measurements were carried out in distilled water on an analytical balance (Sartorius YDK01,
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Goettingen, Germany). Scanning electron microscopy (SEM) characterization was carried out on
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polished down to 1 µm surfaces by a microscope (Phenom G2, Eindhoven, The Netherlands).
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The bending strength, σf, was determined by using the three-point bending test as specified by
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the ASTM specification C1161–13 [34]. The tests were performed at room temperature using a
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universal testing machine (Shimadzu AutoGraph AG-X 5kN, Japan). The specimens in the form
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of bars with dimensions of 3.0 × 4.0 × 45 mm3 were loaded to failure with a crosshead speed of
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0.5 mm/min and a span of 40 mm. Some of WEDM cut bars were directly used as bending test
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samples, but some others were traditional prepared and then went to bending test for comparison
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purpose. Traditional preparation means that SPS sintered discs were cut into bars using diamond
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saw, ground and polished with 1 µm diamond paste. The strength was calculated from the failure
148
loads, F (N), and the specimen dimensions, using the equation:
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Microstructural and mechanical characterization
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σf =
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Where N is the failure load, L is the span, b is the width and h is the height. The strength results
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were averaged over 10 specimens. The formulas and calculation procedures used in the
152
measurements of fracture toughness (KIc) have been reported in previous publications [35].
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153 2.5.
Measurement of electrical resistance and wire-electroerosion machining (WEDM)
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To measure the electrical resistivity, four point probe method was used. The four point probe
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setup consists of four equally spaced tungsten carbide electrodes. These electrodes have a
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diameter of 0.4 mm and separated by a distance of 1 mm. A power source with a high internal
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resistivity sends a constant current through the two outer electrodes. The current output can be
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obtained by an ammeter. The second set of electrodes is used for sensing and since negligible
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current flows in these probes – only voltage drop – thus accurate resistance is measured. A
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resistance of the sample between inner electrodes is the ratio of the voltage registering on the
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voltmeter to the value of the output current of the power supply. In the present work, the
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electrical resistance of sintered ceramic-metal composites was measured with a four-wire
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(Kelvin) connection method using a separate current source (Keithley 6220, Cleveland, OH,
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USA) and a two-channel nanovoltmeter (Keithley 2182A, Cleveland, OH, USA).
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Wire EDM experiments on the commercial machine (Seibu M500SG, Seibu Electric &
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Machinery Co., Koga, Japan) have been performed. Commercial thin brass wire electrodes were
168
used (Osaka Brasscut A500, 0.25 mm diameter). Metallic wire and sintered disk are both
169
electrodes separated by a deionized water with conductivity of 0.1 µS/cm and subjected to an
170
electric voltage (270 V) and intensity (0.05 A). The machining via EDM is a multi-stage process.
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In order to find optimal working parameters the system database which includes various
172
parameters (workpiece material, thickness, wire diameter, etc.) was used. However, machine
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manufacturer’s manual has not provided enough information for machining of ceramic-metal
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composites. For this reason, three cutting conditions of the wire EDM of the ZrO2-Ta composites
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were chosen (Table 1). The input parameters such average working voltage, average working
176
current and pulse off time were considered for analysis, while other parameters remain constant.
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Optical microscope (Leica, Wetzlar, Germany) was used to study WEDM machined pieces and
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detailed analysis of heat affected regions has been done by using SEM equipped with the 3D
179
Roughness Reconstruction application. The field of view area for the height map calculation was
180
490 µm2 (×550).
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181 3. Results and Discussion
184
3.1. XRD study
185
The ball-milled Ta powder can be textured by the milling process. This texture of the deformed
186
metal particles is revealed by XRD (Fig. 2A and 2B).
187
A strong preference for the deformed crystallites to have their (200) reflection parallel to the
188
specimen surface occurred. A strong preference for the {100} type of lattice planes to be parallel
189
to the surface was reported for cold-rolled polycrystalline bcc metals [36]. Ball-milling and cold-
190
rolling imply that the material is flattened in between compressing surfaces; that is, the balls in
191
the ball-milling equipment used here and both rolls in rolling apparatus. The texture can be
192
explained as follows. The most particles that deformed were flattened into flakes, which, after
193
being suspended in acetone (in the procedure for specimen preparation for XRD analysis),
194
position on to the Si substrate with strong tendency for their flat sides to be parallel to the surface
195
of the Si substrate. On the other hand, the measurements of the Ta powders showed that the XRD
196
lines become substantially broadened upon ball-milling (Fig. 2B). This broadening can be
197
ascribed to the introduction of lattice microstrain.
198
Fig. 3 shows diffractograms obtained from wet mechanically mixed 3Y-TZP/Ta powder (A) and
199
from polished surfaces of sintered composite (B) where the complete conversion of the
200
monoclinic ZrO2 into tetragonal ZrO2 is revealed.
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There were no other or new phases than initial powders' phases observed. In addition, graphite
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die-punch setup can act as planar sources of graphitic carbon, the presence of which can affect
203
the composition of the subsurface layers of the sintered material. However, the X-ray diffraction
204
patterns do not show the presence of any carbon-containing phase (carbides) in sintered and
205
polished samples, as can be seen in Fig. 3B. It is likely that carbon-affected layer with negligible
206
thickness was removed by polishing from the sintered compact.
207
3.2. Microstructural characterization and toughening mechanism
208
Dense (>98 th.%) and homogeneous ceramic–metal composites were initially fabricated from
209
3Y-TZP and tantalum (20 vol.%) powders. Electron scanning micrographs of polished surface of
210
3Y-TZP/Ta composites are shown in Fig. 4A. Different surface morphologies can be observed
211
depending on the analyzed orientation: perpendicular or parallel to the pressure direction applied
212
in SPS (Fig. 4A and B, respectively).
213
The metallic particles are preferentially oriented due to the effect of the applied pressure during
214
spark plasma sintering process. Moreover, during flexural test the polished surface of the
215
composites was used as the tensile side, i.e. was perpendicular to the SPS-direction. The
216
tantalum particles are uniformly dispersed in the matrix and no porosity is observed. It was
217
found that the addition of 20 vol. % of Ta particles to the 3Y-TZP matrix produces reinforcement
218
effect which appears as increase of fracture toughness up to 16 MPa·m1/2. The SEM observations
219
of the fracture surfaces showed the major toughening mechanism is the plastic deformation with
220
further partial debonding of tantalum particles (Fig. 4B). In earlier studies, have been shown that
221
addition of Ta particles to the zirconia matrix produces reinforcement effect due to the
222
anticipated contributions between crack bridging of ductile phase and 3Y-TZP stress-induced
223
phase transformation, which appears as increase of fracture crack growth resistance [19].
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3.3. Electrical resistivity measurements and WEDM results
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The electrical resistivity of composites was equal to 2 x 10-4 ± 1 Ω·cm. This value is about five
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orders of magnitude lower than the limit of 100–300 Ω·cm, consequently, the addition of 20
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vol.% of conductive phase makes the material suitable for WEDM. Optical photo of WEDM
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machined piece with complex geometry is presented in Fig. 5A.
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The machined surface of ceramic-metal composite has been analyzed by using tabletop SEM and
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XRD. WEDM process is based on thermoelectric energy between the work piece and an
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electrode. A pulse discharge occurs in a small gap between the work piece and the electrode and
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removes the unwanted material from the workpiece through melting and vaporizing. The
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machined surface after third cut was examined using a SEM for detailed analysis. Fig. 5B-D
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shows the effects of the wire over the material surface after cutting. When the distance between
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electrodes is reduced, the discharge flows between the two electrodes. Once the discharge starts,
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plasma is formed in the neighborhood of the machined front and there are places where the wire
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has caused materials fusion (Fig. 5D). The surface topography of the machined composite is a
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recast layer with discharge craters.
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The SEM cross-sectional view and corresponding XRD spectrum image of the EDM surface
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layer for the 3Y-TZP/Ta ceramic-metal composite is presented in Fig. 6A and 6B, respectively.
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The XRD pattern showed that the WEDM surface layer consists of tetragonal and monoclinic
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zirconia, Tantalum and a minor amount of Ta2O5. The monoclinic volume fraction for EDM’ed
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surface was about 68 vol.%. Unlike the polished sintered samples in which the negligible amount
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(6 vol.%) of m-ZrO2 was found, after WEDM this amount was higher due to the presence of
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solid solution of Ta2O5 which enhance destabilization of the zirconia from the tetragonal to the
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monoclinic phase, resulting in the formation of microcracks [37, 38]. Therefore, transformed
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zirconia leads to a volume increase, stressing the neighboring grains and produces the micro
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crack formation on the wire-electrical discharge machined surface. In addition, cracking could
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also be attributed to the field of tensile residual stresses resulting from thermal effects due to the
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rapid heating and quenching cycles induced during the WEDM process (Fig. 5D). However, the
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occurrence of surface cracks on the machined surface after wire-EDM was only evidenced in the
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recast layer (Fig. 6A).
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The values of average bending strength of WEDM specimens with surface roughness are
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presented in Table 2. From these values, it is obvious that polished samples have the highest
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bending strength. Meanwhile for WEDM cut bars strength increase as the machining parameters
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and surface roughness decrease. However, despite that fine polishing can give materials higher
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strength and high reliability the difference between strength values is not distinct due to high
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damage tolerance of ceramic-metal composite. Therefore, it could be concluded that initial flaw
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size (≈ 100 µm) is smaller than critical crack size [19]. Consequently, the WEDM technique
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could be a suitable method to obtain ZrO2-Ta composites with complicated shape for a wide
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range of high-performance applications, such as inserts for cutting tools, biomedical implants or
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microdevices, among many others.
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4. Conclusions
The obtained results clearly point out that besides exceptional mechanical properties of 3Y-
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TZP/Ta composites obtained by Spark Plasma Sintering also possess the electrical conductivity
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and consequently is suitable for fabrication components with sophisticated shapes by electrical
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discharge machining technique with the required tolerance while reducing machining costs.
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Although the surface quality was modest after WEDM cutting, with the formation of a thermally
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induced recast layer, exhibiting many resolidified droplets and voids as well as surface
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microcracks, the surface roughness was about 1.1 µm and did not to significantly decreased
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values of strength compared with polished samples due to very high flaw tolerance of
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zirconia/Ta composites.
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Acknowledgements
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This work was supported by the Ministry of the Russian Federation by contract 14.B25.31.0012,
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26 June 2013.
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Figure captions
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Fig.1. SEM micrograph of tantalum powder before (A) and after wet ball-milling for 4 h (B).
Fig.2. XRD analysis of Ta raw powder before (A) and after milling (B). The 200 and 110 reflections of Ta for an as-received and milled powder. Upon ball-milling, pronounced texturing
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and broadening occurs as a result of deformation and the increase of lattice strain.
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Fig.3. XRD analysis of the 3Y-TZP/Ta powder as prepared (A) and polished surface of sintered composite (B). Labelling “t” and “m” denotes tetragonal and monoclinic zirconia, respectively. “■” marks tantalum metal reflections.
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Fig.4. SEM micrographs of a polished (A) and a fractured (B) surface of 3Y-TZP/Ta composite. Zirconia: dark-gray; tantalum: light-gray inclusions. White arrows show decohesion between the
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matrix and the metallic particles. Yellow arrows show plastic deformation of Ta.
Fig.5. Optical photo of WEDM’em 3Y-TZP/Ta composite (A) with complex geometry. SEM
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micrograph showing 3Y-TZP/Ta surface finishing (B, C (topographic view) and D). White arrows show cracks formed on composite’s surface machined.
Fig.6. SEM cross-sectional view (A) and corresponding XRD pattern of the EDM’ed surface (B) after third cut.
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Tables
Table 1. Wire electrical discharge machining parameters Average working
Off time (µs)
voltage (V)
current (A)
1 cut
120
2.1
8
2 cut
100
1.2
4
3 cut
85
0.4
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Average working Machining conditions
Table 2. The WEDM cutting parameters influence on surface roughness and strength values Surface roughness, Sa (µm)
Flexural strength, σf (MPa)
Traditional preparation
0.3 ± 0.1
967 ± 8
3.1 ± 0.1
903 ± 22
2.4 ± 0.1
921 ± 17
1.1 ± 0.1
942 ± 14
1 cut 2 cut
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ACCEPTED MANUSCRIPT Highlights
Fully dense and homogeneous ceramic-metal composites were successfully fabricated.
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Composites showed outstanding value of fracture toughness of 16 MPa·m1/2.
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3Y-TZP/Ta composites suited to wire electrical discharge machining (WEDM).
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WEDM cut specimens showed insignificant degradation in the bending strength.
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