Hardness and Indentation Fracture Toughness of Slip Cast Alumina and Alumina-Zirconia Ceramics

materials

Article Hardness and Indentation Fracture Toughness of Slip Cast Alumina and Alumina-Zirconia Ceramics

Irena Žmak 1 , Danko Corić´ 1,*, Vilko Mandić 2 and Lidija Curković´ 1

1 Department of Materials, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, HR-10000 Zagreb, Croatia; [email protected] (I.Ž.); [email protected] (L.C.)´ 2 Department of Inorganic Chemical Technology and Non-Metals, Faculty of Chemical Engineering and Technology, University of Zagreb, Maruli´cevtrg 20, 10000 Zagreb, Croatia; [email protected] * Correspondence: [email protected]; Tel.: +385-98-137-9401

Received: 1 October 2019; Accepted: 23 December 2019; Published: 26 December 2019

Abstract: Alumina (Al2O3) and zirconia (ZrO2) have good overall properties and thus are widely used oxide technical ceramics. The biggest drawback of Al2O3 is its low fracture toughness. In contrast, ZrO2 is relatively tough, but is also much more expensive. In order to improve the alumina toughness, composite ceramics are being developed. Slip casting technology has economic advantages over the conventional hot isostatic pressure technology, but problems may arise when preparing stable highly-concentrated suspensions (slip) for filling the mold. The purpose of this study is to prepare aqueous suspensions using 70 wt. % α-Al2O3, with 0, 1, 5 and 10 wt. % of added t-ZrO2. Suspensions were electrosterically stabilized using the ammonium salt of polymethylacrylic acid, an alkali-free anionic polyelectrolyte dispersant. Also, magnesium oxide in form of magnesium aluminate spinel (MgAl2O4) was used to inhibit the abnormal alumina grain growth during the sintering process. Minimum viscosities were used as stability estimators, where an increase in ZrO2 content required adding more dispersant. After sintering, the Vickers indentation test was used to determine the hardness and the indentation fracture toughness from the measurement of the crack 1/2 length. Also, the brittleness index (Bi, µm− ) was calculated from values of Vickers hardness and the Vickers indentation fracture toughness. It was found that with increasing ZrO2 content the fracture toughness increased, while the hardness as well as the brittleness index decreased. Zirconia loading reduces the crystallite sizes of alumina, as confirmed by the X-ray diffraction analysis. SEM/EDS analysis showed that ZrO2 grains are distributed in the Al2O3 matrix, forming some agglomerates of ZrO2 and some pores, with ZrO2 having a smaller grain size than Al2O3.

Keywords: alumina; zirconia; slip casting; Vickers hardness; fracture toughness

1. Introduction

Aluminum oxide (Al2O3) is the most important technical material of the oxide ceramics group, suitable for various applications in the electrical, electronic, chemical and medical industries. Densely sintered Al2O3 ceramic is characterized by low fracture toughness and high hardness, temperature stability, good wear resistance, corrosion resistance at elevated temperatures and excellent biocompatibility. A major demerit of aluminum oxide is its pronounced brittleness, that is, its relatively 1 low fracture toughness which is 4–6 MPa m 2 . After the start of cracking, its propagation does not stop by plastic deformation, but continues until fracture. This phenomenon is usually caused by individual defects on the surface or very close to the surface of the material, since it is the site of greatest stress [1–3]. Pure zirconium oxide (ZrO2) also belongs in the oxide ceramics group. It has almost ideal 1 properties: high fracture toughness (up to 15 MPa m 2 ), high ﬂexural and tensile strength, high wear

Materials 2020, 13, 122; doi:10.3390/ma13010122 www.mdpi.com/journal/materials Materials 2020, 13, 122 2 of 17 resistance and corrosion resistance, low thermal conductivity, good thermal shock resistance, resistance to high temperatures and excellent biocompatibility [3,4]. Pure ZrO2 occurs in three polymorphic modifications: monoclinal, m (from room temperature up to 1170 ◦C), tetragonal, t (1170 ◦C–2370 ◦C) and cubical, c (from 2370 ◦C to melting point). During cooling, the transition from tetragonal to monoclinic phase takes place at a temperature of about 100 ◦C below 1170 ◦C, whereby the volume increases by 3–5%. Because of the stress resulting from the phase transformation, while cooling from sintering temperature (1500–1700 ◦C), cracking occurs in the final product. To avoid phase transitions and thus cracking, ZrO2 is stabilized by the addition of metal oxides (CaO, CeO2, MgO, Y2O3)[4]. By mixing ceramic powders in the initial phase of forming it is possible to produce Al2O3–ZrO2 composite ceramics, which exhibit better properties compared to monolithic Al2O3, respectively ZrO2 ceramics. Since the addition of ZrO2 increases the fracture toughness of Al2O3 ceramics, such composite ceramics are referred to as zirconia toughened alumina–ZTA in literature [5,6]. Stress that occurs under increased strain conditions can cause cracks in the ceramic material. In case of Al2O3–ZrO2 ceramic, crack formation is mitigated on behalf of a phase transition. Namely, ZrO2 grains that are found in the cracking zone undergo phase transformation from tetragonal into monoclinic phase, where the corresponding volume change facilitates closure of the cracks and prevents further propagation. These favorable mechanical and tribological properties of Al2O3–ZrO2 composite ceramics make it suitable for use in many areas, including cutting tools and implants. Al2O3–ZrO2 is a perspective biomaterial also, since, apart from biocompatibility and mechanical properties, it also meets the aesthetic criteria [5–9]. Contemporary trends in material development focus on improving the properties of existing materials as well as developing new ones. In order to lower their production costs and make them more environmentally friendly, the interest in the production of technical ceramics by slip casting has increased in the last few years. This technology is inexpensive, simple, fast, environmentally friendly and flexible. It enables production of ceramic products of different sizes and form complexities but requires an adequate understanding of colloidal solutions in order to optimize process parameters for the final ceramic product to have the required mechanical and other properties [10]. The properties of ceramic products obtained by slip casting depend on particle size of the ceramic powders and their proportion in the suspension. Generally, smaller particles and higher suspension concentration ultimately result in better properties. However, high suspension concentrations and particle diameter less than 1 mm cause enhanced interactions between particles, which significantly increases viscosity and makes it difficult to cast the suspension into a gypsum mold [3,10–12]. Many studies are focused on observing the influence of certain kinds and amounts of additives on the viscosity of ceramic suspensions and consequent properties of the final ceramic product [13–18]. For reasons mentioned above, highly concentrated suspensions are stabilized by addition of various additives (dispersants). The difference between the ceramic product obtained by casting a stable and an unstable suspension is best illustrated in Figure1. If an unstable suspension containing irregular aggregates of particles (agglomerates) is poured into a mold, the particles are arranged into an irregular structure when dry (draining of water into mold walls), leaving cavities (voids) and irregularities in the microstructure of the raw material (Figure1a). By sintering, these irregularities are further enhanced, resulting in a ceramic product of unsatisfactory properties. If the suspension is stable, by drying the particles become densely arranged, which after sintering gives a ceramic product of the appropriate mechanical and other properties (Figure1b). Agglomeration and sedimentation should be prevented by enhancing the rejection forces between particles of the ceramic powder. These forces must be strong enough to overcome the attractive, van der Waals force [7]. These interaction can be controlled with chemical additives in three different ways: electrostatic, steric, and electrosteric (a combination of the first two) stabilization. The influence of additives (different dispersants) on the ceramic suspension stability has been extensively researched [13–19]. Materials 2020, 13, x FOR PEER REVIEW 3 of 17

These interaction can be controlled with chemical additives in three different ways: electrostatic, Materialssteric,2020 , and13, 122 electrosteric (a combination of the first two) stabilization. The influence of additives 3 of 17 (different dispersants) on the ceramic suspension stability has been extensively researched [13–19].

Bad dispersion Agglomeration = weak material Good dispersion Close packing = strong material (a) (b)

FigureFigure 1. The 1. The influence influence of of particle particle dispersiondispersion on on properties properties on sintered on sintered ceramics ceramics:: (a) bad dispersion (a) bad dispersion and (b) good dispersion and (b) good dispersion Some of the dispersants which have been used as stabilizing agents for preparation of stable Someaqueous of alumina the dispersants suspension which include have ammonium been polymethacrylate used as stabilizing (“Darvan agents C”) [19 for–21 preparation], 4,5- of stabledihydroxy aqueous-1,3 alumina-benzenedisulfonic suspension acid include disodium ammonium salt (“Tiron”) polymethacrylate [21–23], triammonium (“Darvan salt C”) of [19–21], 4,5-dihydroxy-1,3-benzenedisulfonicaurintricarboxylic acid (“Aluminon”) [21], acid (Darvan disodium C-N) [23], salt sodium (“Tiron”) pyrophosphate, [21–23], triammonium diammonium salt of aurintricarboxylichydrogen citrate acid [24], (“Aluminon”) citric acid [23], [ammonium21], (Darvan polyacrylate C-N) [23 (],“Seruna sodium D- pyrophosphate,305”) [25], carbonic diammonium acid hydrogensalt (“Dolapix citrate [ 24CE], 64”) citric [20,26,27], acid [23 polycarbonic], ammonium acid polyacrylatesalt (“Dolapix PC (“Seruna 33”) and D-305”) carbonic [ 25acid], carbonicester acid (“Dolapix ET 85”) [27]. salt (“DolapixThe stability CE 64”) of the [20 suspension,26,27], polycarbonic is tested by sedimentation acid salt (“Dolapix tests, by measuring PC 33”) the and zeta carbonic potential acid ester (“Dolapixand the ET particle 85”) [27 size]. in the suspension and by determining rheological parameters [6,8]. The goal of Thethe presented stability ofstudy the was suspension the preparation is tested of stable by sedimentation Al2O3 and Al2O3- tests,ZrO2 suspension by measuring by electrosteric the zeta potential and thestabilization particle sizesuitable in thefor slip suspension casting. In andaddition, by determining the main interest rheological of this research parameters is the effect [6 of,8 ].ZrO The2 goal of content (0, 1, 5 and 10 wt. %) on the microstructure and mechanical properties (hardness, fracture the presented study was the preparation of stable Al2O3 and Al2O3-ZrO2 suspension by electrosteric toughness and brittleness index) of sintered Al2O3-ZrO2 composite ceramics. stabilization suitable for slip casting. In addition, the main interest of this research is the effect of ZrO2 content2. Materials (0, 1, 5 and and 10Methods wt. %) on the microstructure and mechanical properties (hardness, fracture toughness and brittleness index) of sintered Al2O3-ZrO2 composite ceramics. 2.1. Ceramic Powder and Reagents 2. Materials and Methods Samples of monolithic Al2O3 and composite Al2O3-ZrO2 were prepared by the slip casting technique. For preparation of highly concentrated aqueous suspensions (slips) following components 2.1. Ceramicwere used: Powder and Reagents

Samples High of-purity monolithic Al2O3, with Al av2Oerage3 and particle composite size of 300 Al2–O4003-ZrO nm (Alcan2 were Chemicals, prepared Stamford, by the CT, slip casting technique.USA) For preparation of highly concentrated aqueous suspensions (slips) following components were used: High-purity ZrO2 stabilized with 3 mol % of yttria (Y2O3), with average particle size of 25 nm (SkySpring Nanomaterials Inc., Houston, TX, USA)  High-purityAn alkali Al-free2O 3anionic, with polyelectrolyte average particle dispersant size of 300–400Dolapix CE nm 64 (Alcan (Zschimmer Chemicals, & Schwarz Stamford, GmbH CT, USA) • &Co KG Chemische Fabriken, Lahnstein, Germany)–70 wt. % aqueous solution of the High-purity ZrO2 stabilized with 3 mol % of yttria (Y2O3), with average particle size of 25 nm • ammonium salt of polymethacrylic acid (PMAA-NH4) (SkySpring Nanomaterials Inc., Houston, TX, USA)  Magnesium oxide added as magnesium aluminate spinel (MgAl2O4) made by Alfa Aesar, An alkali-freeHaverhill, MA, anionic USA polyelectrolytewas used to inhibit dispersant the abnormal Dolapix alumina CEgrain 64 growth (Zschimmer during the & sintering Schwarz GmbH • &Co KGprocess Chemische [22]. Magnesium Fabriken, spinel Lahnstein, is segregated Germany)–70 on the grain wt. boundaries % aqueous of alumina solution grains of the and ammonium reducing the mobility of the grain boundaries salt of polymethacrylic acid (PMAA-NH4)  Deionized water Magnesium oxide added as magnesium aluminate spinel (MgAl O ) made by Alfa Aesar, • 2 4 Haverhill,Chemical MA, composition USA was of used the Al to2O inhibit3 and ZrO the2 abnormalpowders, according alumina to grain the manufacturer’s growth during data, the is sintering processgiven in [Table22].s Magnesium 1 and 2, respectively. spinel is segregated on the grain boundaries of alumina grains and reducing the mobility of the grain boundaries Deionized water •

Chemical composition of the Al2O3 and ZrO2 powders, according to the manufacturer’s data, is given in Tables1 and2, respectively.

Table 1. Chemical composition of Al2O3 powder.

Component MgO Fe2O3 SiO2 Na2O CaO Al2O3 wt. % 0.066 0.015 0.02 0.05 0.013 balance Materials 2020, 13, x FOR PEER REVIEW 4 of 17

Materials 2020, 13, 122 4 of 17 Table 1. Chemical composition of Al2O3 powder.

Component MgO Fe2O3 SiO2 Na2O CaO Al2O3 wt. % Table 2.0.066Chemical 0.015 composition 0.02 of the0.05 ZrO 2 powder.0.013 balance

Component Y2O3 ZrO2 Table 2. Chemical composition of the ZrO2 powder. mol. % 3 97 Component Y2O3 ZrO2 mol. % 3 97 In the present study, powder X-ray diffraction (PXRD) analysis was used in order to determine In the present study, powder X-ray diffraction (PXRD) analysis was used in order to determine phase compositions of the raw Al2O3 and ZrO2 powder and heat treated Al2O3-ZrO2 composites phase compositions of the raw Al2O3 and ZrO2 powder and heat treated Al2O3-ZrO2 composites at at 1650 ◦C. The device used was Shimadzu XRD6000 (Shimadzu Corporation, Kyoto, Japan) X-ray 1650 °C. The device used was Shimadzu XRD6000 (Shimadzu Corporation, Kyoto, Japan) X-ray diffractometer with CuKα radiation. The fixed step scans were collected in the 2θ range 20–60◦ with diffractometer with CuKα radiation. The fixed step scans were collected in the 2θ range 20–60° with steps of 0.02 2θ and counting time 0.6 s under accelerating voltage of 40 kV and current of 30 mA. steps of 0.02°◦ 2θ and counting time 0.6 s under accelerating voltage of 40 kV and current of 30 mA. 2.2. Suspension Preparation and Characterization 2.2. Suspension Preparation and Characterization Four groups of alumina-zirconia aqueous suspensions were prepared. All suspensions contained Four groups of alumina-zirconia aqueous suspensions were prepared. All suspensions 70 wt. % of dry ceramic powder and 30 wt. % of deionized water. The dry powder composition was contained 70 wt. % of dry ceramic powder and 30 wt. % of deionized water. The dry powder as follows: composition was as follows:

 100 wt. % Al2O23 3 • 100 wt. % Al O  99 wt.99 wt. % Al% AlO2Oand3 and 1 1 wt. wt. % % of of ZrOZrO2 • 2 3 2  95 wt.95 wt. % Al% AlO2Oand3 and 5 5 wt. wt. % % of of ZrOZrO2 • 2 3 2  90 wt.90 wt. % Al% AlO2Oand3 and 10 10 wt. wt. % % of of ZrOZrO2.. • 2 3 2 DOLAPIX CE 64 dispersant was used for the electrosteric stabilization of all ceramic DOLAPIX CE 64 dispersant was used for the electrosteric stabilization of all ceramic suspensions. suspensions. The structure of the functional group of the dispersant is shown in Figure 2. The structure of the functional group of the dispersant is shown in Figure2.

CH3

C CH3

C O

O NH 4 n Figure 2. The molecule structure of dispersant Dolapix CE 64. Figure 2. The molecule structure of dispersant Dolapix CE 64. The optimal amount of the dispersant was determined for each suspension separately. The optimal amount of the dispersant was determined for each suspension separately. The The composition of the diﬀerent suspensions for determining the optimal amount of DOLAPIX composition of the different suspensions for determining the optimal amount of DOLAPIX CE 64 are CEgiven 64 are in given Table in 3. Table The optimal3. The optimal amount amount of dispersant of dispersant was determined was determined by measuring by measuring the apparent the apparentviscosity viscosity as a function as a function of the ofamount the amount of dispersant of dispersant added. added. In previous In previous research, research, it was itfound was foundthat thataddition addition of of magnesium magnesium aluminate aluminate spinel spinel (MgAl (MgAl2O4)2 doesO4) doesnot affect not a theﬀect apparent the apparent viscosity viscosity of the high of the highconcentrate concentrate alumina alumina suspension suspension [22]. [22 The]. The viscosity viscosity is isat at minimum minimum value value when when the the dispersion dispersion of of ceramicceramic particles particles is optimal.is optimal.

TableTable 3. The 3. The composition composition of of suspensions suspensions for for determining determining thethe optimal amount of of dispersant dispersant DOLAPIX DOLAPIX CECE 64. 64.

wt.wt. (Ceramic (Ceramic wt. (MgAl2O4) *, wt.wt. (Dolapix (Dolapix CE CE SampleSample CeramicCeramic Powder Powder Composition Composition wt. (MgAl O ) *, % Powder),Powder), % % %2 4 64)64) *, *, % % 1 70 100 wt. % Al2O3 0.2 0.15 to 1.0 1 70 100 wt. % Al2O3 0.2 0.15 to 1.0 22 7070 9999 wt. wt. % Al% 2AlO23O+3 1+ wt.1 wt. % % ZrO ZrO2 2 0.20.2 0.2 0.2 to to 1.0 1.0 33 7070 9595 wt. wt. % Al% 2AlO23O+3 5+ wt.5 wt. % % ZrO ZrO2 2 0.20.2 0.4 0.4 to to 1.2 1.2 44 7070 9090 wt. wt. % Al% 2AlO23O+3 10+ 10 wt. wt. % % ZrO ZrO2 2 0.20.2 0.8 0.8 to to 1.4 1.4 * *weight weight percentage percentage basedbased onon the the amount amount of of dry dry ceramic ceramic powder. powder.

For determining the apparent viscosity all suspensions were prepared by adding deionized water containing dissolved DOLAPIX CE 64 into the grinding jar of a planetary ball mill, after which ceramic Materials 2020, 13, x FOR PEER REVIEW 5 of 17

Materials 2020, 13, 122 5 of 17 For determining the apparent viscosity all suspensions were prepared by adding deionized water containing dissolved DOLAPIX CE 64 into the grinding jar of a planetary ball mill, after which powdersceramic were pow added.ders were The added. grinding The jar grinding and ten ballsjar and used ten for balls homogenization used for homogenization were made ofwere alumina made of ceramicsalumina in orderceramics to prevent in order the to contamination prevent the contamination of suspensions. of Each suspensions. of the prepared Each of suspensions the prepared weresuspensions homogenized were for homogenized 90 min at a ratefor 90 of 300min rpm at a inrate the of planetary 300 rpm ballin the mill planetary (PM 100, ball Retsch, mill (PM Haan, 100, Germany).Retsch, Haan, After mixing,Germany). the After ceramic mixing, balls werethe ceramic separated balls from were the separated suspensions. from Prior the suspensions. to the apparent Prior viscosityto the measurement apparent viscosity and forming measurement of the green and bodies, forming suspensions of the green were ultrasonicallybodies, suspensions treated inwere theultrasonically ultrasonic bath treated BRANSONIC in the ultrasonic 220 (Branson bath BRANS UltrasonicsONIC Corp., 220 (Branson Danbury, Ultrasonics CT, USA) Corp., with 50Danbury, kHz frequencyCT, USA) and with power 50 of kHz 120 W frequency to remove and trapped power air bubblesof 120 W and to agglomerates, remove trapped as well air as bubbles tempered and at 25agglomerates,1 C with as the well assistance as tempered of the at thermostatic25 ± 1 °C with bath the assistance Lauda Eco of REthe 415thermostatic (LAUDA-Brinkmann, bath Lauda Eco ± ◦ Delran,RE 415 NJ, (LAUDA USA). -Brinkmann, Delran, NJ, USA). TheThe apparent apparent viscosity viscosity of eachof each suspension suspension was was determined determined by by means means of theof the rotational rotational viscometer viscometer DV-IIIDV- UltraIII Ultra (Brookfield (Brookfield Engineering Engineering Laboratories, Laboratories, Middleboro, Middleboro, MA, USA) MA with, USA) small with sample small chamber sample 1 −1 andchamber SC4-18 spindle.and SC4 Viscosity-18 spindle. was Viscosity determined was atdetermined the shear rate at the of 50shear s− ,rate which of 50 is thes , exactwhich shear is the rate exact of gravityshear rate slip of casting. gravity slip casting. AfterAfter completing completing the rheological the rheological measurements measurements and finding and the finding optimum the amount optimum of the amount dispersant, of the sedimentationdispersant, testssedimentation were performed. tests were Four performed. groups of suspensions Four groups were of suspensions additionally were prepared additio withnally theprepared optimum with amount the ofoptimum dispersant. amount The pH-valuesof dispersant. of these The preparedpH-values suspensions of these prepared were determined suspensions onwere the FE20 determined/EL20 pH on meter the FE20/EL20 (error range pH 0.01) meter manufactured (error range by 0.01 Mettler) manufactured Toledo GmbH by Mettler (Greifensee, Toledo Switzerland).GmbH (Greifensee, In this caseSwitzerland). the pH value In this of case the the samples pH value did of not the change, samples it did was not done change, only it forwas the done confirmationonly for the of confirmation the stability ofof thethe optimizedstability of suspensions. the optimized suspensions.

2.3.2.3. Sintering Sintering of Monolithicof Monolithic Alumina Alumina and and Composite Composite Alumina-Zirconia Alumina-Zirconia Ceramics Ceramics

MonolithicMonolithic Al2 AlO32Oand3 and composite composite Al 2AlO32O-ZrO3-ZrO2 ceramics2 ceramics were were prepared prepared by by conventional conventional sintering sintering of greenof green bodies bodies formed formed by byslip slip casting casting forming forming method. method. Therefore, Therefore, after after the the apparent apparent viscosity viscosity measurementmeasurement the the suspensions suspensions were were poured poured into into previously previously prepared prepared gypsum gypsum molds molds and and air-dried. air-dried. TheThe gypsum gypsum mold mold draws draws water water from from the the poured poured slip slip and and gives gives a form a form to the to the green green body. body. Afterwards,Afterwards, dried dried samples samples were were removed removed from from molds. molds. The The green green bodies bodies were were sintered sintered in thein the high-temperaturehigh-temperature furnace furnace P 310 P 310 (Nabertherm, (Nabertherm, Lilienthal, Lilienthal, Germany) Germany) by by the the following following regime: regime: initial initial heatingheating at aat rate a rate of 3of◦ C3 /°C/minmin up up to theto the temperature temperature of 500of 500◦C, °C, holding holding at 500at 500◦C °C for for 1 h, 1 followedh, followed by by heatingheating at aat rate a rate of 5of◦ C5 /°C/minmin up up to theto the temperature temperature of 1650of 1650◦C, °C, holding holding at 1650at 1650◦C °C for for 2 h 2 and h and ﬁnally finally slowslow cooling cooling in thein the furnace furnace to roomto room temperature temperature (Figure (Figure3). After3). After sintering, sintering, the the Archimedes Archimedes density density waswas measured measured for for all samples.all samples.

FigureFigure 3. Schema 3. Schema of sintering of sintering regime regime of monolithic of monolithic Al2O Al3 2andO3 and Al2 OAl32–ZrOO3–ZrO2 composite2 composite ceramics. ceramics.

MaterialsMaterials 20202020, ,1133, ,x 122 FOR PEER REVIEW 66 of of 17 17

2.4. Characterisation of Monolithic Alumina and Composite Alumina-Zirconia Ceramics 2.4. Characterisation of Monolithic Alumina and Composite Alumina-Zirconia Ceramics Sintered samples were prepared for the following tests according to the standard ceramographic techniqueSintered [28]. samples Surface weremorphology prepared of forthe the sintered following ceramic tests samples according was to determined the standard by ceramographic the scanning electrontechnique microscope [28]. Surface (SEM), morphology Tescan Vega of the Easy sintered Probe ceramic 3, Brno- samplesKohoutovice, was determined Czech Republic by the operating scanning atelectron 10 kV, microscopeadditionally (SEM), equipped Tescan with Vega energy Easy Probedispersive 3, Brno-Kohoutovice, spectrometer (EDS), Czech Oxford Republic Instruments, operating Abingdon,at 10 kV, additionallyUK. Distribution equipped of the with elements energy aluminum dispersive (Al), spectrometer zirconium (Zr) (EDS), oxygen Oxford (O) Instruments,and yttrium (Y)Abingdon, on fracture UK. surface Distribution of sinte ofred the samples elements was aluminum determined (Al), by zirconium EDS mapping. (Zr) oxygen (O) and yttrium (Y) on fractureVickers surfacehardness of sintered(HV30) of samples sintered was ceramic determined samples by was EDS measured mapping. under 294 N indentation load byVickers means hardness of hardness (HV30) oftester sintered 5030 ceramic TKV (Indentec samples was Hardness measured Testing under Machines294 N indentation Ltd., West load Midlands,by means of UK). hardness The “Vickers tester 5030 indenta TKV (Indentection fracture, Hardness (VIF)” Testing or “Vickers Machines indentation Ltd., West Midlands,crack length” UK). methodThe “Vickers was used indentation for fracture fracture, toughness (VIF)” determination or “Vickers indentation of all ceramic crack samples. length” methodThis method was used uses fora Vickersfracture indenter toughness to determinationmake a hardness of allindentation ceramic samples. on a polished This method ceramic uses sample a Vickers surface. indenter The toindenter make a chardnessreates a plastically indentation-deformed on a polished region ceramic underneath sample the surface. indenter The as indenter well as createscracks that a plastically-deformed emanate radially outwardregion underneath and downward the indenter from the as vertices well as cracksof the thatVickers emanate indentation. radially Besides outward radial and- downwardmedian cracks, from Palmqvistthe vertices cracks of the can Vickers also occur indentation. (Figure Besides4). A simple radial-median way to differentiate cracks, Palmqvist between cracks the two can types also occuris to polish(Figure the4). surface A simple layers way away: to di theﬀerentiate median betweencrack system the two will types always is remain to polish connected the surface to the layers inverted away: pyramidthe median of the crack indentation, system will while always the Palmqvist remain connected cracks will to become the inverted detached, pyramid as shown of the in indentation, Figure 4. while the Palmqvist cracks will become detached, as shown in Figure4.

Figure 4. Palmqvist and median crack system developed from the Vickers indents, before and after Figure 4. Palmqvist and median crack system developed from the Vickers indents, before and after polishing [29]. polishing [29]. In the order to calculate Vickers indentation fracture toughness, the lengths of these cracks were In the order to calculate Vickers indentation fracture toughness, the lengths of these cracks were measured. Fracture toughness is calculated on the basis of the crack lengths, the indentation load, measured. Fracture toughness is calculated on the basis of the crack lengths, the indentation load, the the hardness, the elastic modulus, the indentation diagonal size, and an empirical ﬁtting constant. hardness, the elastic modulus, the indentation diagonal size, and an empirical fitting constant. Nine Nine equations based on the Palmqvist, radial-median cracks and both were found to be applicable for equations based on the Palmqvist, radial-median cracks and both were found to be applicable 1fo/2r the the fracture toughness determination of the ceramics (Table4). The brittleness index ( Bi, µm− ) was fracture toughness determination of the ceramics (Table 4). The brittleness index (Bi, m−1/2) was calculated from the ratio of Vickers hardness and Vickers indentation fracture toughness. calculated from the ratio of Vickers hardness and Vickers indentation fracture toughness.

Materials 2020, 13, 122 7 of 17

Table 4. Models by diﬀerent authors for calculation of Vickers indentation fracture toughness (KIC) values for diﬀerent crack types [29].

Crack Type Model Author(s) of Model 0.5 Palmqvist K = 0.024 F E Casellas [2,30] IC × c1.5 × HV 0.5 Palmqvist K = 0.0028 HV0.5 F Palmqvist [31] IC × T = F Palmqvist KIC 0.0319 a l0.5 Shetty et al. [32] × · 0.4 K = 0.0089 E F Palmqvist IC × HV × a l0.5 Niihara et al. [32] for 0.25 < l/a < 2.5 · 0.5 Median K = 0.016 F E Anstis [1,2,33] IC × c1.5 × HV Median K = 0.0752 F Evans and Charles [34] IC × c1.5 Median K = 0.0725 F Tanaka [34] IC × c1.5 0.4 Niihara, Morena and Hasselman Median K = 0.0309 E F IC × HV × c1.5 (NMH) [35]

KIc = 0.0782 Any kind 0.4 × 1.56 Lankford [32] HV a0.5 E c − × HV × a F, applied load during Vickers test (N); c, the crack length from the center of the indentation to the crack tip (m); E, Young’s modulus (GPa); HV, the Vickers hardness (GPa); l, the crack length measured from vertices of the indentation to the crack tip (m); T, the total crack length (m): T = l1 + l2 + l3 + l4; a, half of the indentation diagonal (m).

3. Results and Discussion From the diffractogram in Figure5, the qualitative crystalline composition determination was possible for all samples. The diffractogram in Figure5 indicates that the raw Al 2O3 consists of only α-Al2O3 crystalline phase (corundum) (ICDD PDF#46-1212). On the other hand, for raw ZrO2 powder two phases were assigned to the main phase; zirconia tetragonal phase (t-ZrO2) (ICDD PDF#42-1164) and the minor phase; zirconia monoclinic phase (m-ZrO2) (ICDD PDF#37-1484). Amorphous phase or residuals are not present. Qualitatively, sintered zirconia-toughened alumina composite shows the presence of α-Al2O3 (ICDD PDF#46-1212) as the main phase, t-ZrO2 (ICDD PDF#42-1164) as the minor phase and m-ZrO2 (ICDD PDF#37-1484) in traces. With increase of the zirconia content, both t-ZrO2 and m-ZrO2 intensity increased (the intensity increase of the t-ZrO2 phase peak is shown in Inset). Semiquantitatively, the loading of the t-ZrO2 is also shown in Inset (having in mind Al2O3 and ZrO2 show different absorption coefficients, the loading should only point out to a trend between different values and not to exact values). The intensity of the t-ZrO2 strongest peak and m-ZrO2 strongest peak were compared to allow an insight in the mutual dependence of the zirconia phases (t-ZrO2 to m-ZrO2 ratio) as a function of the zirconia loading. With the introduction of 1 wt. % of zirconia, the relative content of the m-ZrO2 is about 15 wt. % (85 wt. % t-ZrO2). However, the calculation of the ratio of phases that are present in levels of about 1 % is questionable. Upon an increase in the zirconia loading to 5 wt. %, the relative presence of the m-ZrO2 is reduced to about 7 % (93 % t-ZrO2), and the similar ratio remains for further increase of the zirconia loading up to 10 wt. %. Basically, the ratio between m-ZrO2 and t-ZrO2 zirconia phases remains the same. This actually makes sense as there is no clear reason why the increased zirconia content would affect the ratio between m-ZrO2 and t-ZrO2 zirconia. The only reason could be the consequence of the phase transformation from tetragonal into monoclinic phase in the cracking zone (closure of the crack due to the volume changes). However, such microeffects are not statistically observable using a method like XRD. The important issue is that for the healing of the crack there is plenty of the main zirconia phase, the t-ZrO2 available [36,37]. As the microstructure is considered, the use of raw α-alumina yields crystallites of about 262 nm in size, while raw zirconia yields crystallites of about 32 nm. Crystallite size was calculated by applying the Scherrer equation on the XRD patterns. Materials 2020, 13, x FOR PEER REVIEW 8 of 17 Materials 2020, 13, 122 8 of 17 XRD patterns. Some growth occurs in the subsequent process, as ceramic with pure alumina yields 324 nm, where 1 and 5 wt. % of zirconia in composites marginally affect the crystallite size (370 and Some growth occurs in the subsequent process, as ceramic with pure alumina yields 324 nm, where 1 369 nm). 10 wt. % of zirconia definitively impacts the microstructure of composites, reducing the and 5 wt. % of zirconia in composites marginally affect the crystallite size (370 and 369 nm). 10 wt. % of crystallite size to 314 nm. zirconia definitively impacts the microstructure of composites, reducing the crystallite size to 314 nm.

FigureFigure 5. 5.XRD XRD patterns patterns of raw of raw Al2 O Al3,2 ZrOO3, 2 ZrOand2 thermaland thermal treated treated (1650 ◦C) (1650 Al2O °C)3-ZrO Al22Ocomposite3-ZrO2 composite powders. powders. Viscosity measurements were used for the suspension stability estimation. Rheological measurements showedViscosity that measured measurements apparent were viscosity used increases for the with suspension the increasing stability zirconia estimation. content. The Rheological optimal amountmeasurements of Dolapix showed CE 64 that also measuredincreases with apparent the increasing viscosity zirconia increases content. with The the diagramincreasing in Figure zirconia6 1 showscontent. the The apparent optimal viscosity amount atof theDolapix shear CE rate 64 of also app. increases 50 s− , whichwith the is theincreasing shear rate zirconia of the content. gravity slipThe casting.diagram in Figure 6 shows the apparent viscosity at the shear rate of app. 50 s−1, which is the shearThe rate obtained of the gravity minimum slip castin viscosityg. values represent the most stable suspension, which served as theThe guideline obtained for minimum the preparation viscosity of values the most represent suitable the ceramic most stable suspensions suspension, to be which poured served into the as moldthe guideline to prepare forgreen the preparation bodies and of to the be most sintered, suitable in order ceramic to get suspensions the samples to ofbe monolithic poured into Al the2O 3moldand Alto 2prepareO3-ZrO 2greencomposite bodies ceramics. and to be sintered, in order to get the samples of monolithic Al2O3 and Al2O3- ZrO2By composite comparing ceramics. the results in Figure6 it is evident that the optimum amount of the dispersant increasesBy comparing with increased the results ZrO2 content.in Figure The 6 it reasonis evident for thisthat is the probably optimum the amount fact that of this the component dispersant hasincreases ﬁner particles.with increased Ceramic ZrO particles2 content. of smallerThe reason dimensions for this is have probably a larger the speciﬁc fact that surface. this compo Therefore,nent thehas areafiner that particles. the macromolecules Ceramic particles of the of dispersantsmaller dimensions must cover have to ensurea larger the specific stability surface. of the Therefore, system is larger.the area Hence, that the a higher macromolecules amount of of the the dispersant dispersant is neededmust cover to stabilize to ensure such the suspensions stability of the [18 ].system is larger. Hence, a higher amount of the dispersant is needed to stabilize such suspensions [18].

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FigureFigure 6. 6.Inﬂuence Influence of of amounts amounts ofof DolapixDolapix CE CE 64 64 dispersant dispersant (wt (wt.. %) %) on on apparent apparent viscosity viscosity (η) of (η )the of 70 the

70wt. wt. % % suspensions suspensions with with ceramic ceramic powder powder composition composition of sample of sample 1: 100 1: wt. 100 % wt.Al2O %3; Alsample2O3; sample2: 99 wt. 2: 99% wt. Al %2OAl3 +2 1O wt.3 + %1 wt.ZrO %2; ZrOsample2; sample 3: 95 wt. 3: % 95 Al wt.2O %3 + Al 5 2wt.O3 %+ ZrO5 wt.2; sample % ZrO2 4:; sample 90 wt. % 4: Al 902O wt.3 + %10 Al wt.2O %3 + 2 10ZrO wt. %. ZrO2.

TableTable3 shows 3 shows the the composition composition of of the the tested tested suspensions, suspensions, whilewhile thethe optimal compositions (i.e. (i.e.,, optimaloptimal dispersant dispersant amount) amount) are are shown shown in in Table Table5 .5.

Table 5. The compositions of stable suspensions for preparation of monolithic Al2O3 and Al2O3–ZrO2 Table 5. The compositions of stable suspensions for preparation of monolithic Al2O3 and Al2O3–ZrO2 compositecomposite ceramics. ceramics.

wt. (Ceramic Ceramic Powder wt. (MgAl2O4) Optimal Amount of wt. (Ceramic Optimal Amount SampleSample Ceramic Powder Composition wt. (MgAl O ) *, % Powder),Powder), % % Composition *, % 2 4 Dispersantof Dispersant * * 1 70 100 wt. % Al2O3 0.2 0.25 wt. % 1 70 100 wt. % Al2O3 0.2 0.25 wt. % 99 wt. % Al2O3 + 1 wt. % 22 7070 99 wt. % Al2O3 + 1 wt. % ZrO2 0.2 0.20.30 0.30wt. wt.% % ZrO2 3 70 95 wt. % Al2O3 + 5 wt. % ZrO2 0.2 0.70 wt. % 4 70 90 wt.95 %wt. Al %O Al+2O103 + wt. 5 wt. % ZrO% 0.2 1.00 wt. % 3 70 2 3 2 0.2 0.70 wt. % * wt., weight percent basedZrO2 on the applied ceramic dry powder. 90 wt. % Al2O3 + 10 wt. % 4 70 0.2 1.00 wt. % ZrO2 In this study no typical sedimentation tests were performed [14,38], meaning, the sedimentation * wt., weight percent based on the applied ceramic dry powder. rate of the suspension was not observed depending on the pH value. Only the sedimentation rates for suspensionsIn this with study the no optimum typical sedimentation proportion of tests the dispersant were performed at their [14,38], "natural" meaning, pH value the sedimentation were measured. Therate measured of the suspension pH values was of the not real observed samples depending and their on mean the valuespH value. of three Only measurements the sedimentation are shownrates in Tablefor suspensions6. with the optimum proportion of the dispersant at their "natural" pH value were measured. The measured pH values of the real samples and their mean values of three measurements Table 6. Measurement results of pH values on real samples of stable suspensions of monolithic Al O are shown in Table 6. 2 3 and Al2O3-ZrO2 composite ceramics (x-mean, s-experimental standard deviation).

Table 6. Measurement results of pH values on real samplesSample of stable suspensions of monolithic Al2O3 pHand Value Al2O3-ZrO2 composite ceramics (푥̅-mean, s-experimental standard deviation). 99 wt. % Al2O3 + 95 wt. % Al2O3 + 90 wt. % Al2O3 + 100 wt. % Al2O3 1 wt. % ZrO2 Sample 5 wt. % ZrO2 10 wt. % ZrO2 pH Value 99 wt. % Al2O3 + 95 wt. % Al2O3 + 90 wt. % Al2O3 + x s 1008.92 wt.0.06 % Al2O3 8.36 0.05 8.52 0.09 8.26 0.10 ± ± 1 wt. ±% ZrO2 5 wt. %± ZrO2 10 wt. % ±ZrO2 풙̅ ± s 8.92 ± 0.06 8.36 ± 0.05 8.52 ± 0.09 8.26 ± 0.10 The prepared suspensions did not show any indication of phase separation after 3 days and the The prepared suspensions did not show any indication of phase separation after 3 days and the suspensions were still homogeneous. Full separation of the solid and liquid phase was observed after suspensions were still homogeneous. Full separation of the solid and liquid phase was observed after more than seven days in all four suspension groups. For this reason, the prepared suspensions can be more than seven days in all four suspension groups. For this reason, the prepared suspensions can considered stable [14,38], without the need for pH control. be considered stable [14,38], without the need for pH control. The results of SEM-EDS analysis of surface fracture of sintered monolithic Al2O3 and Al2O3–ZrO2 The results of SEM-EDS analysis of surface fracture of sintered monolithic Al2O3 and Al2O3–ZrO2 compositecomposite ceramics ceramics are are shown shown in Figuresin Figure7–s11 7.–11. It can It can be seenbe see (Figuren (Figure7b–d,s 7 Figuresb–d, 9, 910–11 and) that 11) the that ZrO the 2 particles (the brighter phase) are distributed in Al O matrix, with some agglomerates of ZrO and ZrO2 particles (the brighter phase) are distributed2 in3 Al2O3 matrix, with some agglomerates of ZrO2 2 someand pores. some Thesepores. observationsThese observations were additionally were additionally conﬁrmed confirmed by the by SEM-EDS the SEM mapping-EDS mapping of the surfaceof the fracturesurface of fracture sintered of samples sintered (Figures samples9 (Figure–11). s 9–11).

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MaterialsMaterials 20 2020, 1,3 1, 3x, FORx FOR PEER PEER REVIEW REVIEW 1010 of of 17 17

The composites with 5 and 10 wt. % ZrO2 have shown larger agglomerates of ZrO2 in Al2O3 2 2 2 3 TheThe composites composites with with 5 5and and 10 10 wt. wt. %% ZrOZrO22 havehave shownshown larger agglomerates ofof ZrOZrO2 2 inin Al Al2O2O3 3 matrix compared to 1 wt. % ZrO2 (Figure7b–d)). The pores are mostly distributed around ZrO 2 2 2 matrixmatrix compared compared to to 1 1 wt. wt. % % ZrO ZrO2 2(Figure (Figure 7 7bb––d)).d)). The The pores pores are mostly distributed distributed around around ZrO ZrO2 2 agglomerates. Also, the increasing of ZrO2 content has resulted in a reduction of Al2O3 grain size. agglomerates.agglomerates. Also, Also, the the increasing increasing of of ZrO ZrO22 content content hashas resultedresulted in a reduction ofof AlAl22OO3 3grain grain size. size. All All Allagglomerates. these microstructural Also, the increasing characteristics of ZrO typically2 content ahasﬀect resulted the mechanical in a reduction properties, of Al2 suchO3 grain as hardness size. All thesethese microstructural microstructural characteristics characteristics typically typically affectaffect thethe mechanicalmechanical properties, suchsuch as as hardness hardness and and theseand fracture microstructural toughness. characteristics typically affect the mechanical properties, such as hardness and fracturefracture toughness. toughness.

Figure 7. Surface fracture of Al2O3–ZrO2 composite ceramics with different content of ZrO2: (a) 0 wt. Figure 7. Surface fracture of Al O2 3–ZrO 2composite ceramics with diﬀerent content of ZrO :(2a) 0 wt. %; Figure 7. Surface fracture of Al22O33–ZrO22 composite ceramics with different content of ZrO2 2: (a) 0 wt. b%; (b) 1 wt.c %; (c) 5 wt. d%; (d) 10 wt. % prepared by slip casting and sintering at 1650 °C. (%;) ( 1b wt.) 1 wt. %; %; ( ) ( 5c) wt. 5 wt. %; %; ( )( 10d) 10 wt. wt. % prepared% prepared by by slip slip casting casting and and sintering sintering at 1650at 1650◦C. °C.

Figure 8. (a) EDS spectra and (b) SEM micrographs of surface fracture of Al2O3 ceramics. Figure 8. (a) EDS spectra and (b) SEM micrographs of surface fracture of Al2O3 ceramics. Figure 8. (a) EDS spectra and (b) SEM micrographs ofof surfacesurface fracturefracture ofof AlAl22O3 ceramics.ceramics.

Figure 9. (a) EDS spectra of surface fracture of Al2O3–ZrO2 ceramics with 1 wt. % of ZrO 2 and

corresponding element mappings: (b) Aluminum (Al);2 (3c) Oxygen2 (O); (d) Zirconium (Zr); (e) Yttrium2 Figure 9. (a) EDS spectra of surface fracture of Al2O3–ZrO2 ceramics with 1 wt. % of ZrO2 and Figure 9. ((aa)) EDS EDS spectra spectra of surface fracturefracture of of Al Al2OO3–ZrO–ZrO2 ceramics with with 1 1 wt. wt. % of ZrO2 and corresponding(Y). element mappings: (b) Aluminum (Al); (c) Oxygen (O); (d) Zirconium (Zr); (e) Yttrium corresponding element element mappings: mappings: (b ()b Aluminum) Aluminum (Al); (Al); (c )(c Oxygen) Oxygen (O); (O); (d) ( Zirconiumd) Zirconium (Zr); (Zr); (e) Yttrium(e) Yttrium (Y). (Y).

Materials 2020, 13, x FOR PEER REVIEW 10 of 17

The composites with 5 and 10 wt. % ZrO2 have shown larger agglomerates of ZrO2 in Al2O3 matrix compared to 1 wt. % ZrO2 (Figure 7b–d)). The pores are mostly distributed around ZrO2 agglomerates. Also, the increasing of ZrO2 content has resulted in a reduction of Al2O3 grain size. All these microstructural characteristics typically affect the mechanical properties, such as hardness and fracture toughness.

Figure 7. Surface fracture of Al2O3–ZrO2 composite ceramics with different content of ZrO2: (a) 0 wt. %; (b) 1 wt. %; (c) 5 wt. %; (d) 10 wt. % prepared by slip casting and sintering at 1650 °C.

Figure 8. (a) EDS spectra and (b) SEM micrographs of surface fracture of Al2O3 ceramics.

Figure 9. (a) EDS spectra of surface fracture of Al2O3–ZrO2 ceramics with 1 wt. % of ZrO2 and Materialscorresponding2020, 13, 122 element mappings: (b) Aluminum (Al); (c) Oxygen (O); (d) Zirconium (Zr); (e) Yttrium11 of 17 (Y).

Materials 2020, 13, x FOR PEER REVIEW 11 of 17

Figure 10. (a) EDS spectra of surface fracture of Al2O3–ZrO2 ceramics with 5 wt. % of ZrO2 and

Figurecorresponding 10. (a) element EDS spectra mappings: of surface (b) Aluminum fracture of (Al); Al2O (c3)–ZrO Oxygen2 ceramics (O); (d) withZirconium 5 wt. (Zr); % of (e ZrO) Yttrium2 and corresponding(Y). element mappings: (b) Aluminum (Al); (c) Oxygen (O); (d) Zirconium (Zr); (e) Yttrium (Y).

Figure 11. ((aa)) EDS EDS spectra of surface surface fracture fracture of of Al Al22OO33–ZrO–ZrO22 ceramics with 10 10 wt. wt. % of ZrO22 and corresponding element element mappings: mappings: (b (b) Aluminum) Aluminum (Al); (Al); (c )(c Oxygen) Oxygen (O); (O); (d) ( Zirconiumd) Zirconium (Zr); (Zr); (e) Yttrium(e) Yttrium (Y). (Y). The results of measuring the density of sintered samples (Al2O3 and Al2O3-ZrO2 composite) conﬁrmThe that results the density of measuring increases the with density increasing of sintered the ZrO samples2 content (Al (Table2O3 and7). TheAl2O theoretical3-ZrO2 composite) density 3 3 ofconfirm pure Althat2O the3 is density 3.97 g/cm increasesand of with pure increasing ZrO2 6.10 the g/ cmZrO.2 content From these (Table theoretical 7). The theoretical densities ofdensity pure ceramics,of pure Al weight2O3 is content3.97 g/cm of3 components,and of pure andZrO from2 6.10 the g/cm measured3. From bulkthese densities, theoretical the densities relative densities of pure ofceramics, each sample weight were content calculated of components, (Table7). Theand highestfrom the relative measured densities bulk densities, were recorded the relative for pure densities Al 2O3 andof each for sample the composite were calculated with 1 wt. (Table % ZrO 7).2 The. When highest ZrO relative2 content densi wasties increased were recorded to 5 and for 10 pure wt.%, Al the2O3 relativeand for densitythe composite decreased with and 1 wt. consequently % ZrO2. When the porosity ZrO2 content has increased. was increased Similar to results 5 and were 10 wt. published %, the previouslyrelative density [39]. decreased and consequently the porosity has increased. Similar results were publishedThe Vickers previously indentation [39]. method was used for the determination of hardness (HV30), fracture toughnessThe Vi (ckersKIC) and indentation brittleness method index (wasBi) ofused sintered for the samples determination of monolithic of hardness Al2O3 and(HV30), Al2O fracture3-ZrO2 compositetoughness ceramics.(KIC) and Thebrittleness results (Tableindex7 ()B showedi) of sintered that the samples hardness of ofmonolithic the prepared Al2O samples3 and Al decreased2O3-ZrO2 withcomposite the increased ceramics. ZrO The2 content. results There (Table are 7) several showed reasons that forthe this hardness phenomenon. of the prepared One of them samples is the factdecreased that zirconia with the has increased lower hardness ZrO2 content. than Al There2O3. Also,are several as previously reasons described,for this phenomenon. the microstructure One of analysisthem is theof composites fact that zirconia has shown has the lower coarsening hardness of than ZrO 2 Algrains2O3. Also, and consequently as previously the described, formation the of porosity.microstructure These analysis ﬁndings of are composites in correlation has sh withown other the coarsening publications of ZrO [2,402 ,grains41]. Higher and consequently relative density, the henceformation higher of porosity.hardness, These may be findings achieved are when, in correlation for example, with hot-pressing other publications (HP) is [2, used40,41 to]. prepare Higher Alrelative2O3-ZrO density,2 composites hence higher [42], hot hardness, isostatic may pressing be achieved [43] or by when, spark for plasma example, sintering hot-pressing (SPS) [44 ].(HP) is used Theto prepare fracture Al toughness2O3-ZrO2 composites of the tested [42 samples], hot isostatic (Table pressing8) also increases [43] or by with spark the plasma increase sintering of the ZrO(SPS)2 amount[44]. for all applied mathematical models. These results may be assigned to possible phase transformations,The fracture formationtoughness ofof microcracksthe tested samples or crack (Table branching 8) also [2 ].increases Composite with ceramics the increase with of ZrO the2 grains,ZrO2 amount exhibit for the all martensitic applied mathematical phase transformation: models. Thes whene theresults stress may is applied, be assigned tetragonal to possible ZrO2 grainsphase transformtransformations, into monoclinic formation ZrO of 2 microcracksat the crack or tip crack (Figure branching 12). [2]. Composite ceramics with ZrO2 grains, exhibit the martensitic phase transformation: when the stress is applied, tetragonal ZrO2 grains transform into monoclinic ZrO2 at the crack tip (Figure 12).

Figure 12. Transformation toughening mechanism in zirconia [29].

Materials 2020, 13, x FOR PEER REVIEW 11 of 17

Figure 10. (a) EDS spectra of surface fracture of Al2O3–ZrO2 ceramics with 5 wt. % of ZrO2 and corresponding element mappings: (b) Aluminum (Al); (c) Oxygen (O); (d) Zirconium (Zr); (e) Yttrium (Y).

Figure 11. (a) EDS spectra of surface fracture of Al2O3–ZrO2 ceramics with 10 wt. % of ZrO2 and corresponding element mappings: (b) Aluminum (Al); (c) Oxygen (O); (d) Zirconium (Zr); (e) Yttrium (Y).

The results of measuring the density of sintered samples (Al2O3 and Al2O3-ZrO2 composite) confirm that the density increases with increasing the ZrO2 content (Table 7). The theoretical density of pure Al2O3 is 3.97 g/cm3 and of pure ZrO2 6.10 g/cm3. From these theoretical densities of pure ceramics, weight content of components, and from the measured bulk densities, the relative densities of each sample were calculated (Table 7). The highest relative densities were recorded for pure Al2O3 and for the composite with 1 wt. % ZrO2. When ZrO2 content was increased to 5 and 10 wt. %, the relative density decreased and consequently the porosity has increased. Similar results were published previously [39]. The Vickers indentation method was used for the determination of hardness (HV30), fracture toughness (KIC) and brittleness index (Bi) of sintered samples of monolithic Al2O3 and Al2O3-ZrO2 composite ceramics. The results (Table 7) showed that the hardness of the prepared samples decreased with the increased ZrO2 content. There are several reasons for this phenomenon. One of them is the fact that zirconia has lower hardness than Al2O3. Also, as previously described, the microstructure analysis of composites has shown the coarsening of ZrO2 grains and consequently the formation of porosity. These findings are in correlation with other publications [2,40,41]. Higher relative density, hence higher hardness, may be achieved when, for example, hot-pressing (HP) is used to prepare Al2O3-ZrO2 composites [42], hot isostatic pressing [43] or by spark plasma sintering (SPS) [44]. The fracture toughness of the tested samples (Table 8) also increases with the increase of the ZrO2 amount for all applied mathematical models. These results may be assigned to possible phase transformations, formation of microcracks or crack branching [2]. Composite ceramics with ZrO2 Materialsgrains,2020 exhibit, 13, 122 the martensitic phase transformation: when the stress is applied, tetragonal12 ZrO of 172 grains transform into monoclinic ZrO2 at the crack tip (Figure 12).

FigureFigure 12. 12.Transformation Transformation toughening toughening mechanismmechanism inin zirconiazirconia [[29].29].

This transformation induces a volume expansion from 3–5 %, which prevents the crack propagation due to induced compressive stress. This phenomenon is known as the transformation toughening. The larger the ZrO2 content in the composite ceramics, the higher the possibility of crack closure [29]. Besides the zirconia phase transformation, it was found that the alumina grain bridging is also a toughening mechanism which contributes to an increase in toughness of similar alumina zirconia composites [30]. The ratio of the Vickers crack length and half of the Vickers indentation diagonal (c/a, Table7) indicates the crack type and the crack depth. The crack type is an indirect indicator of material toughness [1,29,35,45]. According to the obtained results (Table7) and interpretation from literature sources [1,29,35,45], median cracks occurred in the first three groups of samples, while shallow, Palmqvist cracks appeared in the fourth group (90 wt. % Al2O3 + 10 wt. % ZrO2). It should be emphasized that the c/a ratio for the first three groups of samples is slightly above the limit value of 2.5, so it can be concluded that even low ZrO2 content reduces the crack depth, i.e., increases the toughness. According to Tang et al. [45], samples with a c/a ratio between 2.5 and 3.5 indicate the presence of both types of cracks (both Palmqvist and median), therefore represent the transition between the two types of cracks. According to this interpretation, it is possible to determine the presence of the transitional crack shape (between median and Palmqvist type) in the first three groups of samples, while in the fourth sample group only the Palmqvist cracks occur. According to the obtained c/a values, from 2.3 to 3.9, the most adequate model for the indentation fracture toughness is the Langford model, since it is applicable to both crack types (median and Palmquist).

Table 7. The density, porosity and hardness of Al2O3 and Al2O3-ZrO2 samples and the c/a ratio. Where, c is the crack length from the center of the indentation to the crack tip in m and a is a half of the indentation diagonal.

Bulk Density, Total Porosity, Sample Composition Relative Density, % HV30 c/a g/cm3 %

1 100 wt. % Al2O3 3.882 98.04 1.96 1679 3.89 99 wt. % Al O + 2 2 3 3.920 98.21 1.79 1447 2.82 1 wt. % ZrO2 95 wt. % Al O + 3 2 3 3.931 96.43 3.57 1328 2.59 5 wt. % ZrO2 90 wt. % Al O + 4 2 3 3.938 94.14 5.86 1153 2.28 10 wt. % ZrO2

Since not all groups of samples developed the same crack type, not all selected models were applicable to all four sample groups [32]. Nevertheless, all models showed the same trend, i.e., an increase in fracture toughness with increasing ZrO2 content (Table8 and Figure 13). The obtained results show that the toughness of monolithic Al2O3 ceramics can be improved by the addition of ZrO2 nanoparticles. Materials 2020, 13, 122 13 of 17

Table 8. Values of the Vickers indentation fracture toughness for diﬀerent crack type and models.

1/2 KIC, MPa m Crack Type Author(s) of Model Sample 2 Sample 3 Sample 4 Sample 1 (99 wt. % Al2O3 + (95 wt. % Al2O3 + (90 wt. % Al2O3 + (100 wt. % Al2O3) 1 wt. % ZrO2) 5 wt. % ZrO2) 10 wt. % ZrO2) Palmqvist Casellas [2,30] 5.23 8.24 9.09 10.50 Palmqvist Palmqvist [31] 5.94 6.87 6.90 6.92 Palmqvist Shetty et al. [32] 6.36 7.27 7.30 7.32 Palmqvist Niihara et al. [32] 6.36 7.69 7.93 8.33 MaterialsMedian 2020, 13, x FORAnstis PEER [1 REVIEW,2,33] 3.48 5.49 6.06 7.00 13 of 17 Median Evans and Charles [34] 3.31 4.88 5.21 5.67 MedianNiihara, Tanaka Morena [34] and 3.20 4.70 5.02 5.47 Niihara, Morena and MedianMedian 4.894.89 7.607.60 8.338.33 9.51 9.51 HasselmanHasselman (NMH)(NMH) [35 ][35] Any kindAny kind Lankford Lankford [32 [32]] 5.295.29 8.228.22 9.069.06 10.4110.41

1/2 8

, MPa m MPa, Ic

K 4

0 sample 1 sample 2 sample 3 sample 4 Palmqvist/Casellas 5.23 8.24 9.09 10.50 Palmqvist/Palmqvist 5.94 6.87 6.90 6.92 Palmqvist/Shetty et al. 6.36 7.27 7.30 7.32 Palmqvist/Niihara et al. 6.36 7.69 7.93 8.33 Median/Anstis 3.48 5.49 6.06 7.00 Median/Evans i Charles 3.31 4.88 5.21 5.67 Median/Tanaka 3.20 4.70 5.02 5.47 Median/NMH 4.89 7.60 8.33 9.51 Median/Lankford 5.29 8.22 9.06 10.41

Palmqvist/Casellas Palmqvist/Palmqvist Palmqvist/Shetty et al. Palmqvist/Niihara et al. Median/Anstis Median/Evans i Charles Median/Tanaka Median/NMH Median/Lankford

Figure 13. Comparison of Vickers indentation fracture toughness values for diﬀerent crack types, Figure 13. Comparison of Vickers indentation fracture toughness values for different crack types, models and all samples (NMH-Niihara, Morena and Hasselman). models and all samples (NMH-Niihara, Morena and Hasselman). For each sample, the brittleness index (B , µm 1/2) was calculated from the ratio of values of i −1/2 For each sample, the brittleness index (Bi, m ) was calculated from the ratio of values1/2 of Vickers hardness (HV30, GPa) and the Vickers indentation fracture toughness (KIC, MPa m ) using Vickers hardness (HV30, GPa) and the Vickers indentation fracture toughness (KIC, MPa m1/2) using Equation (1) [46]: Equation (1) [46]: HV Bi = (1) K퐻푉IC 퐵i = (1) Calculated values of the brittleness index for퐾 allIC sintered samples are shown in Figure 14. All models showed that the brittleness index decreases with increasing ZrO content. Calculated values of the brittleness index for all sintered samples are 2shown in Figure 14. All models showed that the brittleness index decreases with increasing ZrO2 content.

Materials 2020, 13, 122 14 of 17 Materials 2020, 13, x FOR PEER REVIEW 14 of 17

1/2 -

m 3



, i B 2

0 sample 1 sample 2 sample 3 sample 4 Palmqvist/Casellas 3.15 1.72 1.43 1.08 Palmqvist/Palmqvist 2.77 2.07 1.89 1.63 Palmqvist/Shetty et al. 2.59 1.95 1.78 1.54 Palmqvist/Niihara et al. 2.59 1.85 1.64 1.36 Median/Anstis 4.73 2.58 2.15 1.62 Median/Evans i Charles 4.97 2.91 2.50 1.99 Median/Tanaka 5.15 3.02 2.59 2.07 Median/NMH 3.37 1.87 1.56 1.19 Median/Lankford 3.11 1.73 1.44 1.09 Palmqvist/Casellas Palmqvist/Palmqvist Palmqvist/Shetty et al. Palmqvist/Niihara et al. Median/Anstis Median/Evans i Charles Median/Tanaka Median/NMH Median/Lankford

−1/21/2 FigureFigure 14 14.. ComparisonComparison of of brittleness brittleness index index (B (Bi,i ,µmm− ) values) values for for different diﬀerent crack crack types, types, models models and and all all samplessamples (NMH (NMH-Niihara,-Niihara, Morena Morena and and Hasselman). Hasselman).

4.4. Conclusions Conclusions

InIn this this paper, paper, monolithic monolithic Al Al22OO3 3andand composite composite Al Al2O2O3–3ZrO–ZrO2 2ceramicsceramics samples samples were were formed formed by by slipslip casting casting stable stable suspensions suspensions in in gypsum gypsum molds. molds. The The following following conclusions conclusions can can be be drawn drawn as as a aresult result ofof the the research: research:  XRDXRD analysis analysis confirmed confirmed that that alumina alumina powder powder consists consists of of αα-phase-phase alumina alumina (corundum). (corundum). The The XRD XRD • resultsresults confirm confirm the the changing changing levels levels of alumina andand zirconiazirconia in in the the composites, composites, and and point point out out to to the thestable stable ratio ratio between between main main tetragonal tetragonal phase phase and monoclinic and monoclinic zirconia zirconia phase in phase traces. in In traces. addition, In addition,the change the ofchange the crystallite of the crystallite sizes because sizes because of the zirconia of the zirconia loading loading was quantified. was quantified.  WithWith the the addition addition of of the the commercial commercial dispersant dispersant DOLAPIX DOLAPIX CE CE 64, 64, it itis is possible possible to to prepare prepare stable stable • 7070 wt. wt. % % aqueous aqueous suspensions suspensions of of monolithic monolithic Al Al22OO3 3andand composite composite Al Al22OO3-3ZrO-ZrO2 2ceramicsceramics using using commercialcommercial powders. powders.  0.250.25 wt. wt. % %of ofDOLAPIX DOLAPIX CE CE 64 64 dispersant dispersant is isrequired required to to stabilize stabilize the the 70 70 wt. wt. % % aqueous aqueous suspensions suspensions • ofof monolithic monolithic Al Al2O23O ceramics.3 ceramics.  0.30.3 wt. wt. % %of ofDOLAPIX DOLAPIX CE CE 64 64 dispersant dispersant is isrequired required to to stabilize stabilize the the 70 70 wt. wt. % % aqueous aqueous suspensions suspensions • ofof composite composite Al Al2O23O–ZrO3–ZrO2 ceramics,2 ceramics, composed composed of of99 99wt. wt. % Al %2 AlO32 Oand3 and 1 wt. 1 wt.% ZrO % ZrO2. 2.  0.70.7 wt. wt. % %of ofDOLAPIX DOLAPIX CE CE 64 64 dispersant dispersant is isrequired required to to stabilize stabilize the the 70 70 wt. wt. % % aqueous aqueous suspensio suspensionsns • of composite Al2O3–ZrO2 ceramics, composed of 95 wt. % Al2O3 and 5 wt. % ZrO2 of composite Al2O3–ZrO2 ceramics, composed of 95 wt. % Al2O3 and 5 wt. % ZrO2  1 1wt. wt. % %of of DOLAPIX DOLAPIX CE CE 64 64 dispersant dispersant is isrequired required to to stabilize stabilize 70 70 wt. wt. % % aqueous aqueous suspensions suspensions of of • composite Al2O3–ZrO2 ceramics, composed of 90 wt.% Al2O3 and 10 wt.% ZrO2. composite Al2O3–ZrO2 ceramics, composed of 90 wt.% Al2O3 and 10 wt.% ZrO2.  ApparentApparent viscosity viscosity and and the the required required amount amount of of Dolapix Dolapix CE CE 64 64 increase increase with with increasing increasing the the • zirconiazirconia content. content.  Green bodies of monolithic Al2O3 and composite Al2O3-ZrO2 ceramics were formed by slip casting process in plaster molds. After drying, the green bodies were sintered at a temperature of 1650 °C.

Materials 2020, 13, 122 15 of 17

Green bodies of monolithic Al O and composite Al O -ZrO ceramics were formed by slip • 2 3 2 3 2 casting process in plaster molds. After drying, the green bodies were sintered at a temperature of 1650 ◦C. SEM-EDS analysis of prepared composite ceramics showed that ZrO particles are dispersed in • 2 Al2O3 matrix with some agglomerates of ZrO2, and pores. The obtained c/a values ranging from 2.3 to 3.9 indicate that the Langford model is the most • appropriate model for the indentation fracture toughness, because this model can be applied to both median and Palmquist crack types. By adding ZrO nanoparticles in alumina matrix, the hardness has decreased because the hardness • 2 of tetragonal zirconia is lower than alumina. Also, the addition of ZrO2 nanoparticles has caused an increase in total porosity, hence lowering the hardness. On the other hand, the fracture toughness of alumina matrix has increased by adding • ZrO2 nanoparticles as a result of the synergistic eﬀect of transformation toughening and the microstructural changes.

Author Contributions: Conceptualization, L.C.,´ I.Ž. and V.M.; Methodology, I.Ž., D.C.´ and L.C.;´ Formal analysis, L.C.,´ D.C.´ and V.M.; Investigation, L.C.,´ V.M. and D.C.;´ Resources, L.C.;´ Writing—original draft preparation, L.C.,´ I.Ž. and D.C.;´ Writing—review and editing, L.C.,´ I.Ž., D.C.´ and V.M.; Visualization, I.Ž., L.C.,´ D.C.´ and V.M.; Supervision, I.Ž.; Project administration, I.Ž., L.C.´ and D.C.;´ Funding acquisition, L.C.´ All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Croatian Science Foundation under the project Monolithic and composite advanced ceramics for wear and corrosion protection (WECOR) (IP-2016-06-6000). Acknowledgments: The authors would like to thank the Rea Veseli, Master of Applied Chemistry for proofreading the English language. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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