The Effect of TiC Additive on Mechanical and Electrical Properties of Al 2 O 3 Ceramic

Featured Application: The work aimed to develop a new class of electrically conductive ceramics, which can be recommended for the production of cutting inserts in tools for the machining of superhard hardened steels, hard-to-machine materials, composites and other materials used in mechanical engineering. Abstract: In this study the inﬂuence of TiC content on the mechanical and electrical properties of Al 2 O 3 -TiC composites containing 30 and 40 vol.% TiC were investigated. The Vickers hardness and fracture toughness of the composites increased with the addition of TiC phase. The composite with 40 vol.% TiC showed the highest ﬂexural strength (687 ± 39 MPa), fracture toughness (7.8 ± 0.4 MPa · m 1/2 ) and hardness (22.3 ± 0.3 GPa) with a homogeneous distribution of the second phase within the ceramic matrix. Besides enhanced mechanical properties, it was found that ceramic composites with more than 30 vol.% TiC fabricated by the spark plasma sintering possess sufﬁcient electrical conductivity for electrical discharge machining as well. Therefore, they do not limit the ﬂexibility of the shape, and any intricate parts can be easily made with these composites which can be recommended for the production of cutting inserts in the tools for machining of superhard hardened steels, hard-to-machine materials, composites and other materials used in mechanical engineering.


Introduction
Although WC-Co still dominates the cutting tools market, alumina-based composite materials may be a good alternative to improve cutting speed and lower production costs [1][2][3][4].Alumina-based ceramic tools are used due to their excellent operational properties in extreme conditions such as wear resistance, hardness and low chemical interactivity with surrounding materials [5][6][7][8].Meanwhile, the range of applications of alumina ceramics as a structural material is limited by their low fracture toughness [9][10][11].Therefore, cracks will propagate easily and can cause unpredictable failure of ceramic composites [12,13].Various techniques have been proposed to enhance the mechanical performance of alumina.For instance, the addition of reinforcing phases such as ceramics, metals, and intermetallic compounds into an Al 2 O 3 matrix, forming a composite material, has been proved to be an effective experimental route to improve the toughness of the ceramic matrix [14][15][16][17][18].
In addition, the widespread use of these materials requires a non-traditional way of manufacturing [19][20][21][22].Today, materials are usually machined by grinding using abrasive materials or hard tools.However, machining of ceramic materials with sophisticated shapes using traditional machining methods is a complicated and time-consuming process due to their exceptional physical properties, especially extreme hardness.
Secondary phase TiC was incorporated into a ceramic matrix to improve the fracture toughness and electrical conductivity of alumina-based composites.
Also, when traditional sintering methods are used to fabricate Al 2 O 3 -TiC composites, a high temperature is necessary to obtain nearly fully dense material.However, higher sintering temperature and sintering time lead to undesirable grain growth which may reduce the operational properties of the material [30][31][32].
Furthermore, the following chemical reactions between Al 2 O 3 and TiC [33,34], can occur at high temperatures and can cause porosity which also causes deterioration in the mechanical properties.Therefore, to decrease the sintering temperature spark plasma sintering (SPS) was used as an alternative consolidation method.Using this novel technology facilitates fast consolidation, allows fabrication of fully dense composites and avoids grain growth [35][36][37][38][39][40].
SPS also offers considerable advantages such as faster and shorter processing time compared to conventional sintering methods, due to simultaneous application of mechanical pressure and electric pulses [41][42][43].
Therefore, the goals of the present investigation are twofold: the fabrication of electrically conductive ceramic composites with full density, and analysis of the effect of TiC phase addition on their mechanical performance and electrical conductivity.
Powder mixtures with different amounts of TiC 30 and 40 vol.% were mixed in a multidirectional Turbula shaker mixer (Eskens B.V., Alphen aan den Rijn, Netherlands) in ethanol in a polyethylene container at 150 rpm for 24 h [17].
Lyophilizer FreeZone2.5 (LabConco, Kansas, MO, USA) was used for the drying of the obtained slurries to avoid agglomerates of TiC phase in the alumina matrix.The collector temperature was set at −50 ± 2 • C, while the shell temperature was +23 ± 2 • C and the chamber pressure was 0.02 ± 0.01 mbar during the entire process.Also, this method allows powders to be sintered without sieving [44].The powders were sintered with a KCE FCT-H HP D-25 SD spark plasma sintering machine (FCT Systeme GmbH, Rauenstein, Germany) at 1400 • C with a heating rate of 100 • C•min −1 , under a uniaxial pressure of 80 MPa in a vacuum.
The final temperature and pressure were maintained for 3 min and 20-mm-diameter, 2-4-mm-thick as-sintered sample disks were labeled depending on their TiC content.
XRD measurements were carried out in an Empyrean diffractometer (PANalytical, Almelo, Netherlands) ranging from 25 • to 70 • .The step size was 0.05 • with a scan speed of 0.06 • /min.The diffractometer used Cu Kα radiation (λ= 1.5405981), working at 60 kV and with an intensity of 30 mA.Density measurements (ρ) of the sintered specimens were done in distilled water using Archimedes' principle and compared with the theoretical values (3.89 g/cm 3 for Al 2 O 3 and 4.93 g/cm 3  for TiC, respectively) calculated from the rule of mixture.
A scanning electron microscope (SEM) VEGA3 (Tescan, Brno, The Czech Republic) was used for characterization of surfaces polished down to 1 µm.The energy-dispersive X-ray spectroscopy (EDS) spectra were conducted in specific regions at 20 kV voltage and 12 µA beam current for 5 min to obtain well-resolved Y-Kα and Zr-Kα peaks for the chemical microanalysis of sintered samples.
Ten Vickers impressions were carried out in the surfaces of each of the tested samples, which had previously been polished down 1 µm.The 9.8 N load was applied for ten seconds by a Vickers diamond indenter Leco 100-A (Leco Corp., St. Joseph, MI, USA).The HV magnitude was calculated as follows: where P is the applied load, N; d is the average length of the two diagonals, mm.SEM was used for evaluation of the size of the indentations.The values of fracture toughness (K 1c ) were also estimated by Vickers microindentation.The cracks were observed at 294 N and load holding time was set to 10 s.
At least 10 indentations tests were performed per specimen.The fracture toughness was calculated using the formula presented in Miranzo [45].The flexural strength (σ f ) was evaluated through a biaxial bending test.Each sample was placed onto a device with three balls of 3 mm in diameter that were made of hardened steel and disposed on a holder (10 mm diameter) at 120 • to each other.The load was applied with a 5 kN universal testing machine (Auto Graph AG-X, Shimadzu Corp., Kyoto, Japan) using a plain head of 1.2 mm in diameter at a speed of 1 mm•min −1 up to failure.The specimen thickness was measured at the breakage point.12 specimens were tested to obtain the average strength value.A review of the procedure is reported in Smirnov [46].
The electrical resistance of Al 2 O 3 -TiC ceramic composites sintered via SPS was determined by a four-wire Kelvin connection scheme using a separate current source, Keithley 6220, and a two-channel nanovoltmeter, Keithley 2182A (Keithley Instruments, Solon, OH, USA) [47,48].The four-point set-up comprises four equally spaced tungsten carbide electrodes.These electrodes have a diameter of 0.4 mm and are separated by a distance of 1 mm.A power source with a high internal resistivity sends a constant current through the two outer electrodes.An ammeter was used for measurement of the current output.The approbation of electrical discharge machining ability was conducted on 4-axis CNC-machine M500SG (Seibu Electric & Machinery Co., Fukuoka, Japan).A brass wire, CuZn35 with a diameter of d w = 0.25 mm, was used during the experiments.The conductivity of the deionized water was 0.1 µS•cm −1 ; electric voltage V o = 270 V; and current intensity I e = 0.05 A. The frequency of the working impulses was f w = 0.2 MHz [49,50].

Characterization of the Samples and their Mechanical Properties
A representative X-ray diffraction pattern corresponding to the sintered and polished Al 2 O 3 -TiC composites is shown in Figure 1.The pattern reveals that no contamination was detected, nor the presence of side reactions along the sintering of the powder.
The polished and fractured microstructure of dense Al 2 O 3 -TiC composites was studied by SEM on 1 µm polished surfaces, where the dark phases are for alumina grains, and the light ones are for titanium carbide grains (Figure 2).
The microstructure analyses confirmed that the density of composites was the same as the theoretical value since no pores were observed.The polished and fractured microstructure of dense Al2O3-TiC composites was studied by SEM on 1 μm polished surfaces, where the dark phases are for alumina grains, and the light ones are for titanium carbide grains (Figure .2).
The microstructure analyses confirmed that the density of composites was the same as the theoretical value since no pores were observed.Figure 3 shows energy-dispersive X-ray spectroscopy (EDX) maps of the polished surface of Al2O3-40 vol.%TiC composite, showing a homogeneous distribution of the elements throughout the sample and, consequently, indicating that the wet processing route was suitable for the fabrication of bulk materials.The main mechanical properties of sintered composites in comparison with a few properties of Al2O3-TiC (40 vol.%) cutting ceramic (K01, ISO) are shown in Table 1.The hardness of the composites increases as the TiC content increases because TiC is relatively harder than Al 2 O 3 .The fracture toughness of the samples increases as the proportion of TiC additive increase in the samples.The inclusion of second phase particles in a matrix can hinder crack propagation and consequently increase toughness.Different toughening mechanisms, induced by interaction between the crack and microstructure, such as crack deflection and grain bridging by TiC grains, were observed and are presented in Figure 4, where the dotted red lines and solid yellow lines indicate the area where close-up was taken.The red and yellow arrows show the crack bridging and crack deflection by TiC.These mechanisms are effectively increasing the crack propagation resistance, resulting in an improvement in the fracture toughness.
Appl.Sci.2018, 8, x FOR PEER REVIEW 6 of 13 The hardness of the composites increases as the TiC content increases because TiC is relatively harder than Al2O3.The fracture toughness of the samples increases as the proportion of TiC additives increase in the samples.The inclusion of second phase particles in a matrix can hinder crack propagation and consequently increase toughness.Different toughening mechanisms, induced by interaction between the crack and microstructure, such as crack deflection and grain bridging by TiC grains, were observed and are presented in Figure 4, where the dotted red lines and solid yellow lines indicate the area where close-up was taken.The red and yellow arrows show the crack bridging and crack deflection by TiC.These mechanisms are effectively increasing the crack propagation resistance, resulting in an improvement in the fracture toughness.

Electrical Properties and Electrical Discharge Machining of the Samples
The measured and calculated electrical properties of sintered composites in comparison with the Al 2 O 3 -TiC (40 vol.%) cutting ceramic (K01, ISO), TiC particles and brass wire material CuZn35 (426-1, ISO) are shown in Table 2.The closest value of specific electrical resistance for the produced sample of sintered Al 2 O 3 -TiC ceramic with 40 %vol. of TiC is for pure Titan (5.56 × 10 −7 Ω•m) [56][57][58][59].The EDM parameters for processing graphite and titan were chosen as more common in conditions of real manufacturing, to provide adequate values of the discharge gap and frequency and duration of the pulses on the industrial CNC-machine.
Figure 5 shows the surface of the samples after electrical discharge machining, where white and yellow arrows indicate microcracking in the recast layer and initial structure of the composite, respectively.
Appl.Sci.2018, 8, x FOR PEER REVIEW 7 of 13 The closest value of specific electrical resistance for the produced sample of sintered Al2O3+TiC ceramic with 40 %vol. of TiC is for pure Titan (5.56 × 10 −7 Ω•m) [56][57][58][59].The EDM parameters for processing graphite and titan were chosen as more common in conditions of real manufacturing, to provide adequate values of the discharge gap and frequency and duration of the pulses on the industrial CNC-machine.
Figure 5 shows the surface of the samples after electrical discharge machining, where white and yellow arrows indicate microcracking in the recast layer and initial structure of the composite, respectively.In this figure, two different process zones are observed.In EDM the material is machined by electrical discharges without direct contact between the electrode and the workpiece that leads to the formation of plasma in the neighborhood of the machined front and causes material fusion (Figure 5a).
Consequently, there are places where the wire has caused material fusion, and the presence of surface microcracks caused by EDM was determined, especially in the recast layer.Residual stresses formed at the surface are related to an excessive heating-quenching temperature difference and lead to the formation of microcracks during the EDM process.On the other hand, there are zones where an initial structure of the composite is still observed.Also, in contrast to EDX analysis of the polished surface (Figure 3) on the machined surface (Figure 5b), the presence of Cu and Zn was observed.This can be explained by the transfer of these elements from the brass wire.The micrograph of the machined surface (Figure 5a) shows that in most cases the tiny microcracks occur only within the recast layer [60].

Characterization of the Samples and its Mechanical Properties
The ceramic samples were entirely produced in the laboratories of MSTU "Stankin" with the maximum achievable homogeneity, using the spark plasma sintering method assisted by the electrical current [17,28].The main SPS parameters were chosen according to the recommendations of the equipment manufacturer and based on practical experience with the equipment.The samples were of 2-4 mm thickness and 20 mm in diameter.
As presented, the main mechanical properties of the samples showed that the samples have similar flexural strength as the sample of Al2O3-TiC cutting ceramic produced industrially.At the same time, the Vickers hardness of the samples is twice as high as for their industrial analogue.In this figure, two different process zones are observed.In EDM the material is machined by electrical discharges without direct contact between the electrode and the workpiece that leads to the formation of plasma in the neighborhood of the machined front and causes material fusion (Figure 5a).
Consequently, there are places where the wire has caused material fusion, and the presence of surface microcracks caused by EDM was determined, especially in the recast layer.Residual stresses formed at the surface are related to an excessive heating-quenching temperature difference and lead to the formation of microcracks during the EDM process.On the other hand, there are zones where an initial structure of the composite is still observed.Also, in contrast to EDX analysis of the polished surface (Figure 3) on the machined surface (Figure 5b), the presence of Cu and Zn was observed.This can be explained by the transfer of these elements from the brass wire.The micrograph of the machined surface (Figure 5a) shows that in most cases the tiny microcracks occur only within the recast layer [60].

Characterization of the Samples and its Mechanical Properties
The ceramic samples were entirely produced in the laboratories of MSTU "Stankin" with the maximum achievable homogeneity, using the spark plasma sintering method assisted by the electrical current [17,28].The main SPS parameters were chosen according to the recommendations of the equipment manufacturer and based on practical experience with the equipment.The samples were of 2-4 mm thickness and 20 mm in diameter.
As presented, the main mechanical properties of the samples showed that the samples have similar flexural strength as the sample of Al 2 O 3 -TiC cutting ceramic produced industrially.At the same time, the Vickers hardness of the samples is twice as high as for their industrial analogue.

Electrical Properties and Electrical Discharge Machining of the Samples
It should be noted that conductors have specific electrical resistance R < 10 −5 Ω•m as the specific electrical resistance of dielectrics is R > 10 8 Ω•m [61].Thus, the produced samples of sintered Al 2 O 3 -TiC ceramic are suitable for electrical discharge machining [62].The value of specific electrical resistance for produced conductive ceramic with 30 vol.% of TiC is close to the value for carbon in the form of graphite (8.0 × 10 −6 Ω•m) [63].
The TiC content in the samples makes them conductive since the electrical resistance of titanium carbide is 6.0 × 10 −7 Ω•m [21,22,64], which is higher than the value for the brass wire (Table 2).As the equivalent ratio of TiC content in the samples increases, the conductivity of the samples increases with no loss of mechanical properties.
However, TiC is highly resistant to melting in the presence of metals with a low-melting-point such as copper, aluminum, brass, cast iron, etc. as its stability temperature is 3140 It should be noted that the primary material of the matrix of samples was alumina, which is a dielectric [66,67] or an n-type semiconductor [68] with a melting point of 2044 • C and a boiling point of 2977 • C [22,69].
As the boiling point of any alloy corresponds to the highest value of the boiling point of its components, the melting point of brass will be ~900 • C [55,70], and the boiling point will be ~2590 • C. In the same way, it can be supposed that the melting point for samples produced of the conductive ceramic will be 3140 • C as for the TiC component.
This means that brass wire is not suitable for electrical discharge machining of the conductive ceramic or other hard-to-melt materials.Figure 5 showed that the brass sublimated and adsorbed on the surface of the opposing electrode.The quality of the machined surface is low and demonstrates the presence of chaotic overlapped wells with a diameter of 10-15 µm.It can be concluded that at the moment when the temperature in the discharge channel achieves the value which is suitable for sublimation of TiC, the brass has already evaporated, with the formation of a plasma cloud (this can be seen during processing as a series of tiny bubbles on the surface of the dielectric).Then, the discharge gap and series of impulses are disrupted [71].
The value of the discharge gap for the conductive ceramic was experimentally determined to be in the order of 0.005 mm, which is 15 times less than the recommended value for easy-to-melt alloys such as aluminum alloy (~0.075 mm).A value of 0.005 mm is difficult to control by adaptive control of EDM-machine based on monitoring electrical parameters [72].The environmental conditions and amplitude of the wire oscillation under the impulse current may affect the accuracy of processing even during roughing [73].
Hence, use of an electrode made of tungsten can be recommended for further research, as it has higher values for melting and boiling points (3410 • C and 5660 • C, respectively), and a higher value of electrical resistance (4.9 × 10 −8 Ω•m) [74].For electrical discharge machining with the profiled electrode, it may be suitable to use graphite instead of copper.

Conclusions
The results achieved in this work demonstrate that homogeneous and dense (>98th%) alumina-titanium carbide ceramic composites (with 30 and 40 vol.%TiC) were produced following a colloidal processing route, freeze drying method and spark plasma sintering.Al 2 O 3 -TiC composite with 40 vol.%TiC simultaneously improved fracture toughness (7.8 ± 0.4 MPa•m 1/2 ) and hardness (22.3 ± 0.3 GPa) of the composites, which is twice as high as the value for traditional Al 2 O 3 -TiC cutting ceramic (K01, ISO).The multiple actions of reinforcement in crack deflection and bridging were observed due to the presence of TiC particles.
Besides improved mechanical properties, it was found that ceramic composites also possess electrical conductivity (higher than 10 −2 S•cm −1 ) and consequently, this implies EDM machinability with the required complex shapes and high accuracy.This is more efficient than conventional machining operations such as diamond grinding, polishing, and lapping.Therefore, EDM allows the production of complex shapes and the machining of developed Al 2 O 3 -TiC composites, combined with a high degree of automation, which significantly increase the accuracy, speed, and cost-effectiveness of manufacturing.
These composites can be processed into any intricate shape that makes it possible for them to be used in various applications, including miniaturized structures or indexable inserts for machining of hardened steel, difficult-to-machine materials, composites and other materials used in mechanical engineering.

Figure 1 . 13 Figure 1 .
Figure 1.XRD pattern of the polished surface of Al 2 O 3 -TiC composites, where A is α-alumina, and T is TiC.

Figure 2 .
Figure 2. SEM micrographs of Al 2 O 3 -TiC composites: (a) polished surface of the sample with 30 vol.% TiC; (b) polished surface of the sample with 40 vol.%TiC; (c) fractured surface of the sample with 30 vol.%;(d) fractured surface of the sample with 40 vol.%.In the figure, alumina is dark, and TiC is light.

Figure 3
Figure 3 shows energy-dispersive X-ray spectroscopy (EDX) maps of the polished surface of Al 2 O 3 -40 vol.%TiC composite, showing a homogeneous distribution of the elements throughout the sample and, consequently, indicating that the wet processing route was suitable for the fabrication of bulk materials.The main mechanical properties of sintered composites in comparison with a few properties of Al 2 O 3 -TiC (40 vol.%) cutting ceramic (K01, ISO) are shown in Table1.
Figure 3 shows energy-dispersive X-ray spectroscopy (EDX) maps of the polished surface of Al 2 O 3 -40 vol.%TiC composite, showing a homogeneous distribution of the elements throughout the sample and, consequently, indicating that the wet processing route was suitable for the fabrication of bulk materials.The main mechanical properties of sintered composites in comparison with a few properties of Al 2 O 3 -TiC (40 vol.%) cutting ceramic (K01, ISO) are shown in Table1.

Figure 2 .
Figure 2. SEM micrographs of Al2O3-TiC composites: (a) polished surface of the sample with 30 vol.% TiC; (b) polished surface of the sample with 40 vol.%TiC; (c) fractured surface of the sample with 30 vol.%;(d) fractured surface of the sample with 40 vol.%.In the figure, alumina is dark, and TiC is light.

Figure 3 .Table 1 .1
Figure 3.The Al2O3-40 vol.%TiC composite: (a) polished surface of the sample; (b) combination map for phases in a composite; (c) distribution map of X-ray of individual element Ti Kα; (d) distribution map of X-ray of individual element Al Kα.

Figure 3 .
Figure 3.The Al 2 O 3 -40 vol.%TiC composite: (a) polished surface of the sample; (b) combination map for phases in a composite; (c) distribution map of X-ray of individual element Ti Kα; (d) distribution map of X-ray of individual element Al Kα.

Figure 4 .
Figure 4. Vickers indentation crack induced on the surface of the Al2O3-40 vol.%TiC composite.

Figure 4 .
Figure 4. Vickers indentation crack induced on the surface of the Al 2 O 3 -40 vol.%TiC composite.

Figure 5 .
Figure 5.The surface of Al2O3-40 vol.%TiC composite after electrical discharge machining: (a) image obtained on SEM; (b) (Energy-dispersive X-ray spectroscopy) EDX map of the surface.

Figure 5 .
Figure 5.The surface of Al 2 O 3 -40 vol.%TiC composite after electrical discharge machining: (a) image obtained on SEM; (b) (Energy-dispersive X-ray spectroscopy) EDX map of the surface.