Article Mechanical Properties and Electrical Discharge Machinability of Alumina-10 vol% Zirconia-28 vol% Composites

Andrea Gommeringer * and Frank Kern

Institute for Manufacturing Technologies of Components and Composites (IFKB), University of Stuttgart, Allmandring 7b, D-70569 Stuttgart, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-711-685-68234

 Received: 31 March 2020; Accepted: 16 April 2020; Published: 18 April 2020 

Abstract: Electrical discharge machinable ceramics provide an alternative machining route independent on the material hardness which enables manufacturing of customized ceramic components. In this study a composite material based on an alumina/zirconia matrix and an electrically conductive titanium nitride dispersion was manufactured by hot pressing and characterized with respect to microstructure, mechanical properties and ED-machinability by die sinking. The composites show a combination of high strength of 700 MPa, hardness of 17–18 GPa and moderate fracture resistance of 4.5–5 MPa√m. With 40 kS/m the electrical conductivity is sufficiently high to ensure ED-machinability.

Keywords: ED-machinable ceramics; die sinking; zirconia; titanium nitride

1. Introduction Zirconia toughened alumina materials (ZTA) are ceramics with alumina as a majority phase and zirconia as a homogeneously dispersed secondary phase. This results in mechanical properties combining high hardness provided by the alumina matrix and enhanced toughness and strength introduced by the zirconia dispersion [1]. Toughening mechanisms are typically transformation toughening, microcracking and residual stress [2]. The enhanced mechanical properties qualify ZTA materials for mechanically and tribologically loaded applications in mechanical engineering or biomedical applications [3,4]. However, the same properties limit the machinability of sintered ZTA so that net-shape or at least near-net-shape processing cycles (injection molding or pressing/green machining) should be designed to limit machining effort to the minimum. For customized components with complex geometries and filigree structure this approach is very often not possible. Component manufacturing may then be either impossible or uneconomical. Non-conventional machining operations such as laser machining or electrical discharge machining (EDM) could fill this gap as here the machining speed is independent on material hardness and toughness. The application of these processes requires purpose-made ceramic materials. Oxide ceramics can be made electrically conductive by addition of a conducting third phase such as transition metal , or borides or by nanocarbon compounds such as nanotubes or graphene [5,6]. Rak investigated alumina–titanium nitride ceramics with 5–25 vol% conductive dispersion and obtained materials which were electrically conductive and ED-machinable at a content of 25 vol% [7]. Landfried carried out a screening study on ZTA materials with different conductive phases and ended up with the assumption that ZTA-TiC was the composite with the best combination of mechanical properties and machinability [5]. The latest results of Schmitt-Radloff on ZTA-NbC materials led the authors to the conclusion that the ZTA-TiN system should be revisited and characterized in more detail [8]. Especially as the initial study was carried out with respect to limitation

Ceramics 2020, 3, 199–209; doi:10.3390/ceramics3020018 www.mdpi.com/journal/ceramics Ceramics 2020, 3 200 due intellectual property which restricted the compositional range to TiN contents below 25 vol.% [9]. In the present study, a very simple ZTA-system with 10 vol% unstabilized zirconia was selected in order to circumvent the stabilization issue which was known to inflate the set of experiments. With regard to the latest results, a fine grain TiN-dispersion of 28 vol% was chosen to ensure sufficient electrical conductivity. Processing by hot-pressing ensures full densification, temperatures were varied in order to ensure a certain robustness of the composition which is required for the scale up of the hot-pressing process to make larger blanks.

2. Materials and Methods The starting powders for the study were submicron-size α-alumina (APA0.5, Ceralox, USA; 2 2 SBET = 8 m /g, d50 = 300 nm), nanoscale monoclinic zirconia (UEP, DKKK, Japan, SBET = 5–20 m /g) and fine titanium nitride (Höganäs, Germany, d50 = 1.5 µm). A 300 g powder batch containing 10 vol.% zirconia (calculated as tetragonal ρ = 6.1 g/cm3), 28 vol% TiN and 62 vol.% alumina was dispersed in 200 ml 1-propanol and attrition milled for 2h at 799 rpm using 3Y-TZP milling balls of 2 mm diameter. The milling media were separated, and the slurry was dried overnight at 60 ◦C in flowing air. The dried powder was subsequently screened through a 250 µm mesh. Hot pressing was carried out in cylindrical graphite dies of 45 mm diameter clad with hexagonal suspension. For mechanical testing two disks of 18 g powder separated by a graphite spacer were hot pressed in simultaneously. Heating to final temperatures (1475 ◦C–1550 ◦C in 25 K increments) under a pre-load of 3 kN (~2 MPa) was carried out at 30 K/min. The pressure was then increased within 5 min to 63 kN = 40 MPa. Temperature and pressure were kept for 2h dwell. Then the load was released the compartment was filled with argon up to ambient pressure and the sample was left to cool in the press (cooling rate ~5–10 K/min). For ED-machining experiments two disks of 4 mm thickness were sintered under identical conditions at 1525 ◦C/2h/40 MPa. The sintered disks were then machined. Disks for mechanical testing were lapped with 15 µm diamond suspension on one side and lapped and polished with diamond suspension of 15–1 µm grain size to a mirror like-finish on the second side. The final thickness of the disks was ~2.2 mm. Disks for EDM were lapped on both sides. Prior to sample cutting the ρ was determined by buoyancy measurement and the Young’s modulus E was determined by acoustic emission measurement (IMCE, Belgium). The Vickers hardness HV10 (five indents) was measured 8 mm from the edge of the disk on the polished side, the wing cracks of the indents were measured in order to calculate fracture resistance by direct crack length measurement (DCM). Disks were the cut into bending bars of 4 mm width with a diamond wheel. The cutting grooves were then removed by lapping, finally the edges were beveled carefully with a 40 µm diamond grinding disk. Electrical conductivity of the bending bars was measured by 4-point measurement (4 samples each, self-built equipment). Four bending bars were indented with 98.1 N load on the polished side with cracks parallel and perpendicular to the sides. The residual strength of these notched bars was measured immediately after notching in 4-point setup with 20 mm outer and 10 mm inner span at a crosshead speed of 2.5 mm/min. 4-point bending strength σ4pt was measured on the remaining 10 bars at a crosshead speed of 0.5 mm/min in the same setup (Zwick, Ulm, Germany). Considering measured hardness and Young modulus values, indent size and wing crack length (DCM) the indentation fracture resistance KDCM was calculated using the model of Evans [10]. Based on residual strength data the fracture resistance KISB (ISB = indentation strength in bending) was calculated according to the model of Chantikul [11]. The phase composition of polished surfaces and fracture surfaces after ISB-test was measured by X-ray by integrating the intensity of the tetragonal and monoclinic peaks in the 27–33◦ 2θ range (X’Pert MPD, Panalytical, NL, Bragg-Brentano-Setup, CuKα1, Ge-monochromator). The fractions of tetragonal and monoclinic phase were calculated according to the calibration curve of Toraya [12]. Transformation zone sizes were calculated based on X-ray data according to Kosmac [13]. The transformation toughness Ceramics 2020, 3 201 increments were estimated according to McMeeking [14] assuming a transformation efficiency of 0.49 (dilatationCeramics and 2020, shear).3 FOR PEER REVIEW 3 The microstructure of polished and thermally etched (hydrogen atmosphere, 1250 ◦C, 5 min) samples wasThe investigatedmicrostructure by of SEM polished to evaluate and thermally the homogeneity etched (hydrogen of phase atmosphere, distribution 1250°C, and the 5 grain min) sizes. samples was investigated by SEM to evaluate the homogeneity of phase distribution and the grain EDM die sinking experiments were carried out in oil based dielectric fluid (IonoPlus IME-MH, sizes. Oelheld, Germany) using copper electrodes of 5 5 mm2 cross-section and proven parameter sets EDM die sinking experiments were carried out× in oil based dielectric fluid (IonoPlus IME-MH, (ElothermOelheld, Typ Germany) 400, AEG, using Germany). copper electrodes The machining of 5 × 5 speedmm² cross-section and electrode and wearproven were parameter determined sets by weighing(Elotherm the samples Typ 400, andAEG, electrodes Germany). before The machinin and afterg speed the machiningand electrode experiments wear were determined (3 15 min). by The × followingweighing table the shows samples the and machine electrodes parameters before and used. after Thethe machining workpiece experiments was set as (3 cathode × 15 min). ( )The and the − electrodefollowing was table set as shows anode the ( +machine). The machineparameters allows used. The no directworkpiece adjustment was set as of cathode electrical (-) and parameters, the so thatelectrode the actual was set currents as anode were (+). measuredThe machine inductively allows no direct and displayedadjustment onof electrical an oscilloscope parameters, and so listed in Tablethat1 .the The actual machined currents substrates were measured were inductivel cleaned. They and surface displayed structure on an oscilloscope was studied and by listed SEM, in cross sectionsTable through 1. The themachined machined substrates areas were cleaned. polished The and surface studied structure by optical was microscopystudied by SEM, and SEM.cross The sections through the machined areas were polished and studied by optical microscopy and SEM. The surface quality was measured by white light interferometry. The compositional changes in pristine surface quality was measured by white light interferometry. The compositional changes in pristine and machinedand machined surfaces surfaces were were checked checked by by X-ray. X-ray.

TableTable 1. 1.EDM EDM characteristics characteristics for die die sinking sinking experiments. experiments.

Process DischargeDischarge Current [A] ImpulseImpulse Duration [Offµs] Time O ff Time [µs] Process Roughing 10.0Current [A] Duration 9.9[µs] [µs] 34.8 Trimming 1Roughing 7.010.0 9.9 7.8 34.8 44.2 Trimming 2Trimming 1 3.5 7.0 7.8 5.0 44.2 46.5 Trimming 2 3.5 5.0 46.5 3. Results 3. Results 3.1. Microstructure 3.1. Microstructure Figure1 shows SEM images of thermally etched ZTA-TiN materials sintered at the four di fferent Figure 1 shows SEM images of thermally etched ZTA-TiN materials sintered at the four different temperatures.temperatures. Evidently Evidently the etchingthe etching process process in hydrogen in hydrogen is very is everyfficient efficient to reveal to reveal the grain the boundariesgrain in theboundaries oxide matrix. in the oxide matrix.

FigureFigure 1. SEM 1. SEM images images of of polished polished and andthermally thermally etched ZTA-TiN ZTA-TiN composites composites sintered sintered at different at different temperatures, dark grains are Al2O3, small light grains are ZrO2 and medium grey is TiN. temperatures, dark grains are Al2O3, small light grains are ZrO2 and medium grey is TiN.

Ceramics 2020,, 3 FOR PEER REVIEW 2024

However, the etching also leads to significant re-crystallization of the titanium nitride, thus the titaniumHowever, nitride the grains etching appear also leads to toprotrude significant from re-crystallization the surface. The of the alumina titanium tends nitride, to thuscoarsen the titaniumprogressively nitride at grains higher appear sintering to protrude temperatures. from the Grain surface. sizes The increase alumina from tends ~0.8 to coarsenµm at 1475°C progressively to ~1.5 atµm higher at 1550°C. sintering The temperatures. zirconia initially Grain occupies sizes increase the from4-junctions ~0.8 µ mbetween at 1475 ◦theC to larger ~1.5 µ grains.m at 1550 With◦C. Theprogressive zirconia coarsening initially occupies of the the alumina 4-junctions grains between some migration the larger grains.and coalescence With progressive of zirconia coarsening can be ofobserved the alumina which grains lead to some a visible migration coarsening and coalescenceeffect. of zirconia can be observed which lead to a visibleAt coarsening 1475 °C the eff ect.zirconia grains typically have a size of ~200 nm, which is too small to be transformable.At 1475 ◦C At the 1525 zirconia °C and grains 1550 typically °C the have zirconia a size grain of ~200 size nm, rises which to a is range too small of 400–500 to be transformable. nm, which Ataccording 1525 ◦C to and Heuer 1550 is◦C ideal the zirconia for stress grain induced size rises transfor to a rangemation of [15]. 400–500 At the nm, same which time according the larger to Heuerzirconia is idealgrains for are stress no longer induced numerous transformation enough [to15 block]. At theall 4-junctions same time theand larger prevent zirconia matrix grains grain are growth. no longer The numerousprimary titanium enough tonitride block grains all 4-junctions are larger and than prevent the grains matrix of grain the growth.oxide phases. The primary Moreover, titanium they nitride tend grainsto form are aggregates larger than of the2–4 grainsµm size. of theNevertheless, oxide phases. the Moreover,volume fraction they tend is sufficient to form aggregatesto form a percolation of 2–4 µm size.electrically Nevertheless, conducting the volume network. fraction The issize sufficient and shap to forme of athe percolation titanium electrically nitride does conducting not differ network. with Thechanges size andin sintering shape of temperature. the titanium nitride does not differ with changes in sintering temperature.

3.2. Mechanical Properties The densitydensity ofof all all samples samples reaches reaches values values between between 99.5–99.7% 99.5–99.7% of theoretical of theoretical (not shown (not shown in detail). in Asdetail). the theoreticalAs the theoretical density wasdensity calculated was calculated assuming as asuming tetragonal a tetragonal phase content phase ofcontent 100% inof the100% zirconia in the dispersion,zirconia dispersion, the materials the materials can be considered can be consider fully dense.ed fully The dense. small The density small increments density increments are probably are closelyprobably related closely to related the changes to the in changes monoclinic in monoclinic content (see content Section (see 3.3). paragraph The Young’s 3.3). modulus The Young’s of all materials is almost identical, it ranges between 383–387 GPa with a scattering of 2 MPa (not shown in modulus of all materials is almost identical, it ranges between 383–387 GPa with± a scattering of ±2 detail).MPa (not Figure shown2 shows in detail). the VickersFigure 2 hardness shows the HV10 Vickers and hardness the bending HV10 strength and the of bending ZTA-TiN strength materials of vs.ZTA-TiN sintering materials temperature. vs. sintering The Vickers temperature. hardness The shows Vickers a maximum hardness shows at 1800 a HV10 maximum at 1500 at ◦1800C. Further HV10 risingat 1500 sintering °C. Further temperatures rising sintering lead totemperatures a continuous lead reduction to a continuous of hardness reduction to 1710 of HV10hardness at 1550 to 1710◦C. SuchHV10 a at decrease 1550 °C. of Such hardness a decrease is an indication of hardness of coarsening is an indication of the microstructure.of coarsening of The the bending microstructure. strength increasesThe bending slightly strength with increasingincreases sinteringslightly with temperature. increasing At sintering 1550 ◦C thetemperature. maximum averageAt 1550 strength °C the amountsmaximum to average 734 MPa strength with aamounts small standard to 734 MPa deviation with a ofsmall 49 MPa.standard At deviation 1475 ◦C–1525 of 49◦ MPa.C individual At 1475 samples°C–1525 somewhat°C individual reach samples higher strengthsomewhat than reach for 1550high◦erC, strength the scattering than for of the1550 data °C, is, the however, scattering much of larger.the data This is, however, indicates much the existence larger. This of larger indicates defects the andexistence a broader of larger defect defects size distributionand a broader at defect lower sinteringsize distribution temperatures. at lower sintering temperatures.

1850 900

1800 850

1750 800 [MPa]

1700 750 4pt σ

1650 700

1600 650

1550 600 Vickers hardness HV10 [-] HV10 bending strength 1500 σ 550 4pt 1450 500 1475 1500 1525 1550 sintering temperature [°C]

4pt Figure 2.2. VickersVickers hardness hardness HV10 HV10 and 4-point bending strength σ4pt ofof ZTA-TiN depending on sintering temperature.

Figure 33 shows shows the the fracture fracture toughness toughness K KDCMDCM determined by direct crack length measurementmeasurement evaluated withwith the the Evans’ Evans’ model model [10 ][10] and and the fracturethe fracture toughness toughness KISB calculated KISB calculated from residual from residual strength strength measurements using the model of Chantikul [11]. Both methods show very similar results

Ceramics 2020, 3 203

Ceramics 2020, 3 FOR PEER REVIEW 5 measurements using the model of Chantikul [11]. Both methods show very similar results in the intoughness the toughness range range between between 4.5–5.3 4.5–5.3 MPa MPa√m.√m. The The ISB-toughness ISB-toughness probably probably provides provides moremore objective evidence than the DCM DCM method method as the the material material was was difficult difficult to to polish polish and and crack crack length length measurements measurements may be subject to individual errors. According According to to the the ISB-values ISB-values there there is is a a very moderate rise in toughnesstoughness with sintering temperature (and(and graingrain size).size). The The DCM-values DCM-values show show an intermediate toughnesstoughness maximum atat 15251525 ◦°C.C. AA tentativetentative interpretationinterpretation on on the the di differentfferent response response of of the the methods methods is isgiven given in in the the discussion discussion section. section.

6.0

KDCM m] m] K √ √ ISB 5.5 [MPa [MPa Ic Ic

5.0

4.5 fracture resistance K fracture resistance K

4.0 1475 1500 1525 1550 sintering temperature [°C]

Figure 3. Fracture resistance KDCM (Evans model) and KISBISB of ZTA-TiN vs. sintering temperature. FigureFigure 3.3. FractureFracture resistance resistance K KDCM (Evans(Evans model) model) and and K K ISBof ZTA-TiNof ZTA-TiN vs. vs.sintering sintering temperature. temperature.

3.3. Phase Phase Composition Composition and and Transformation Transformation Toughness The monoclinic phasephase contentcontent ofof thethe zirconia zirconia fraction fraction in in polished polished surfaces surfaces and and fracture fracture phases phases of ofZTA-TiN ZTA-TiN sintered sintered at di atff differenterent temperatures temperatures is shown is shown in Figure in Figure4a. The 4a. calculated The calculated transformation transformation zone zonesizes sizes and values and values of transformation of transformation toughness toughness are plotted are plotted in Figure in Figure4b. 4b.

0.6 4 1.00 m] m] [-] [-] √ √ m m 0.5 [MPa [MPa 3 0.75 [MPa T T Ic Ic K 0.4 K Δ Δ

0.3 2 0.50

0.2 V 1 0.25 m,p h

transformation zone size h [µm] h 0.1 V transformation zone size h [µm] m,f Δ T KIcIc transformation toughness transformation toughness monoclinic zirconia phase content V monoclinic zirconia phase content V Vt = Vm,f - Vm,p t m,f m,p 0 0.00 0.0 1475 1500 1525 1550 1475 1500 1525 1550 sintering temperature [°C] sintering temperature [°C]

(a) (b)

Figure 4. ((aa)) monoclinic monoclinic zirconia zirconia phase contentscontents inin polishedpolished surfaces surfaces V Vm,pm,p, in fracturefracture surfacessurfaces VVm,fm,f andand transformabilitytransformability V ff ,,( (bb)) transformation transformation zone zone size size h h and and transformation transformation toughness toughness increment Δ∆KICT.

InIn the polished surfaces thethe monoclinicmonoclinic zirconiazirconia phasephase contentcontent VVm,pm,p rangerange between between 32–36%, the maximum at at 1525°C 1525 ◦C sintering sintering temperature temperature is isnot not very very pronounced. pronounced. This This monoclinic monoclinic content content in the in polished state indicates that the alumina and titanium nitride phases are already under slight hydrostatic tension [16].

Ceramics 2020, 3 204 the polished state indicates that the alumina and titanium nitride phases are already under slight Ceramics 2020, 3 FOR PEER REVIEW 6 hydrostatic tension [16]. In the fracture surfaces the monoclinic zirconia phase contents V range between 48–56%, the In the fracture surfaces the monoclinic zirconia phase contents Vm,fm,f range between 48–56%, the maximum value is again observed at 1525 C. The transformability V , which is the difference between maximum value is again observed at 1525◦ °C. The transformabilityf Vf, which is the difference V and V , shows an increase at higher temperatures from 17% at 1475–1500 C to 21% at 1550 C. betweenm,f Vm,pm,f and Vm,p, shows an increase at higher temperatures from 17% at 1475–1500◦ °C to 21%◦ at transformability1550 °C. transformability values show values a good show correlation a good corre tolation the ISB totoughness the ISB toughness (see Figure (see3 ).Figure Based 3). on Based the T phaseon the composition phase composition the transformation the transformation zone size h and zone the size transformation h and the toughness transformation increments toughness∆KIC were calculated. As ∆K T correlates to the product of V and √h an identical trend is evident. It can be increments ΔKICT wereIC calculated. As ΔKICT correlates fto the product of Vf and √h an identical trend statedis evident. that transformationIt can be stated that toughening transformation provides toughening a significant provides toughness a significant increment toughness of 0.6–0.9 increment MPa√m despiteof 0.6–0.9 the MPa low√ zirconiam despite content the low in zirconia the composites content of in only the composites 10 vol%. of only 10 vol%. 3.4. Electrical Conductivity and EDM Characteristics 3.4. Electrical Conductivity and EDM Characteristics The electrical conductivity of the samples (not shown in detail) shows only very small changes The electrical conductivity of the samples (not shown in detail) shows only very small changes with variation in sintering temperature. The conductivity values vary between 39–41 kS/m with a with variation in sintering temperature. The conductivity values vary between 39–41 kS/m with a standard deviation of 600 S/m. This level of electrical conductivity is several orders of magnitude standard deviation of ±±600 S/m. This level of electrical conductivity is several orders of magnitude higher than the threshold of 1 S/m for EDM defined by Iwanek [17]. higher than the threshold of 1 S/m for EDM defined by Iwanek [17]. Electrical discharge machining experiments of the material sintered at 1525 C showed an excellent Electrical discharge machining experiments of the material sintered ◦at 1525°C showed an machinability of the materials. Material removal rates and electrode wear ratios for the roughing and excellent machinability of the materials. Material removal rates and electrode wear ratios for the trimming steps are plotted in Figure5. As expected, the MRR scales with the discharge current level roughing and trimming steps are plotted in Figure 5. As expected, the MRR scales with the discharge (i.e., the energy input/time). In the roughing step the MRR of >4 mm3/min is extremely high compared current level (i.e., the energy input/time). In the roughing step the MRR of >4 mm³/min is extremely to earlier studies on ZTA-TiC materials (0.5–2 mm3/min) [18]. Even the trimming steps at reduced high compared to earlier studies on ZTA-TiC materials (0.5–2 mm³/min) [18]. Even the trimming energy input show attractive MRR. The relative electrode wear (weight loss of the electrode/weight steps at reduced energy input show attractive MRR. The relative electrode wear (weight loss of the loss of the workpiece–EW) of <1% is very low. Landfried already reported material transfer to the electrode/weight loss of the workpiece–EW) of < 1% is very low. Landfried already reported material copper electrode [18]. Still, weight gain of the working electrode in the slow trimming step should not transfer to the copper electrode [18]. Still, weight gain of the working electrode in the slow trimming be overstressed as here the relative measuring error is considerably higher. step should not be overstressed as here the relative measuring error is considerably higher.

5 MRR EW 4

3

2

1

electrode wear EW [%] wear electrode 0

material removal rate MRR [mm³/min] MRR rate removal material -1

roughing trimming 1 trimming2 machining steps

FigureFigure 5. 5. MaterialMaterial removal removal rates rates MRR MRR an andd electrode electrode wear wear ratios ratios EW EW of of different different machining machining steps. steps.

FigureFigure6 6shows shows overview overview top top view view on on machined machined surfaces surfaces in in the the roughing roughing and and trimming trimming steps. steps. EvidentlyEvidently thethe dominantdominant materialmaterial removalremoval mechanismmechanism isis melting.melting. TheThe surfacesurface isis coveredcovered withwith aa veryvery thinthin glass-likeglass-like layer.layer. MaterialsMaterials machinedmachined atat veryvery highhigh energy inputinput (roughing) are extremely smooth,smooth, whilewhile thethe trimmingtrimming operationsoperations with with lower lower energy energy input input lead lead to to rougher rougher surfaces surfaces (see (see Table Table2). 2).

Ceramics 2020, 3 205 Ceramics 2020, 3 FOR PEER REVIEW 7

Ceramics 2020, 3 FOR PEER REVIEW 7

FigureFigure 6. SEMSEM images images showing showing an an overview overview of of ZTA- ZTA-TiNTiN surfaces surfaces ED-machined ED-machined with with different different parameterparameter set. set.

At higher magnificationTable more 2. Roughness interesting values details for ED-machinedare revealed surfaces. (Figure 7). The crack network is somewhat more pronounced at higher energy input as can be expected. The glassy layer resulting from the roughingProcess steps shows Average wavy Roughness patterns Ra which [µm] are Averageless pronounced Roughness in Depth the trimming Rz [µm] steps. Holes resultingRoughing from discharge of gaseous 1.86 0.28 decomposition products are 8.61 smaller1.29 and more evenly ± ± Trimming 1 2.21 0.30 10.1 1.36 distributed in the surfaces derived from ±the roughing operation. The trimming± operations especially Trimming 2 2.25 0.33 10.3 1.46 at the lowestFigure energy 6. SEM inputimages haveshowing a foamyan overview± aspect. of ZTA-Moreover,TiN surfaces in the ED-machined trimming± withsurfaces different some larger particles parameterseems to set. be embedded into the glassy layer and some globular machining debris is loosely attachedAt higher Atto thehigher surface. magnification magnification more more interestinginteresting details details are are revealed revealed (Figure (Figure 7). 7The). Thecrack crack network network is is somewhatsomewhat more more pronounced pronounced at at higher higher energyenergy input input as as can can be be expected. expected. The Theglassy glassy layer layerresulting resulting fromfrom the roughingthe roughing steps steps shows shows wavy wavy patterns patterns which which are are less less pronounced pronounced in in the the trimming trimming steps. steps. Holes resultingHoles fromresulting discharge from discharge of gaseous of decompositiongaseous decomposition products products are smaller are smaller and more and evenlymore evenly distributed in thedistributed surfaces in derived the surfaces from derived the roughing from the operation. roughing Theoperation. trimming The trimming operations operations especially especially at the lowest at the lowest energy input have a foamy aspect. Moreover, in the trimming surfaces some larger energy input have a foamy aspect. Moreover, in the trimming surfaces some larger particles seems to be particles seems to be embedded into the glassy layer and some globular machining debris is loosely embedded into the glassy layer and some globular machining debris is loosely attached to the surface. attached to the surface.

Figure 7. SEM images showing details of ZTA-TiN surfaces ED-machined with different parameter sets.

Table 2. Roughness values for ED-machined surfaces.

Average Roughness Ra Average Roughness FigureFigure 7. SEM 7. SEM images imagesProcess showing showing details details of of ZTA-TiN ZTA-TiN surfaces surfaces ED-machined ED-machined with different different parameter parameter sets. sets. [µm] Depth Rz [µm] Roughing 1.86 ± 0.28 8.61 ± 1.29 TrimmingTable 1 2. Roughness 2.21 values ± 0.30 for ED-machined surfaces. 10.1 ± 1.36 Trimming 2 Average 2.25 Roughness ± 0.33 Ra Average 10.3 Roughness ± 1.46 Process [µm] Depth Rz [µm] Roughing 1.86 ± 0.28 8.61 ± 1.29 Trimming 1 2.21 ± 0.30 10.1 ± 1.36 Trimming 2 2.25 ± 0.33 10.3 ± 1.46

Ceramics 2020, 3 206 Ceramics 2020, 3 FOR PEER REVIEW 8

The observation of an increasing surface roug roughnesshness with decreasing discharge energy coincides with the integral values of surface roughness determined by white light interferometry shown in Table2 2.. Figure 88 showsshows cross-sectionscross-sections ofof machinedmachined ZTA-TiNZTA-TiN ceramics.ceramics. TheThe imagesimages ofof thethe cross-sectioncross-section confirmconfirm the previous observations. Both Both samples samples from from roughing and trimming operations show a wavy surface with a glass-like layer on the surface of variable thickness. In case of the roughing step the glassy layer has a thicknessthickness of ~5–6~5–6 µm.µm. Some ve verticalrtical cracks are stopped at the interface been boundary layer and unaunaffectedffected substate material. However, However, some lateral cracks are formed which indicates a considerable level of residual stress in thethe surface layer. In case of the trimming operation the process zone is thinner in average (1–2 µµm).m). An accumulation of glassy material is found at the bumps ofof thethe surface, surface, here here the the layer layer may may be 5 µbem thick.5 µm Evidently,thick. Evidently, the larger the bright larger particles bright embedded particles embeddedinto the glassy into surfacethe glassy layers surface are titanium layers are nitride titanium grains. nitride Vertical grains. as wellVertical as lateral as well cracks as lateral are shorter. cracks areDelamination shorter. Delamination of the outer of shell, the outer as foreshadowed shell, as foreshadowed in the case in of the the case boundary of the boundary layer of the layer roughing of the roughingstep, cannot step, be observed.cannot be The observed. structure The of non-etched structure substrateof non-etched material substrate is very homogeneousmaterial is very and homogeneousconfirms the observations and confirms in the Section observations 3.1. in Section 3.1.

Figure 8. SEMSEM images images showing showing cross cross sections sections of of ZTA-TiN ZTA-TiN surfaces ED-machined with different different parameter sets (left roughing,roughing, right trimming 2).

It2500 was tried to identify the composition of the boundary layer by X-ray. Figure9 shows the spectra of the pristine surface Polished and of the surface after the roughing step. As the thicknesspeak ofmax. the 3270 process counts zone is TiN very low (5–6 µm) ED-machined the spectrum is dominated by the substrate. Still, some interesting changes can be 2000 3 found. The intensity of tetragonal zirconia2 is reduced radically in machined surfaces which can be 2 O either a hint of selective removal of this phase or – which is more likely – that the2 remaining zirconia TiN Al on the surface transformst-ZrO into monoclinic zirconia during ED-machining. Furthermore, the alumina

1500 t-ZrO + and zirconia phases seem to be removed faster3 than titanium nitride (see differences in peak altitudes). O

On the diffractogram of the ED-machined2 surface the peaks are broader. It can be expected that any new formed peaks are very broad as the crystallineAl phases within the re-solidified layers are nanoscale 3

1000 θ O

and/or poorly crystallized (see e.g., m-ZrO -peak at 2 =2 45.7 ). Counts [-] 2 ◦ Al 2 2 2 2 3

500 O m-ZrO 2 m-ZrO m-ZrO m-ZrO Al

0 28 30 32 34 36 38 40 42 44 46 48 θ 2 [°]

Ceramics 2020, 3 FOR PEER REVIEW 8

The observation of an increasing surface roughness with decreasing discharge energy coincides with the integral values of surface roughness determined by white light interferometry shown in Table 2. Figure 8 shows cross-sections of machined ZTA-TiN ceramics. The images of the cross-section confirm the previous observations. Both samples from roughing and trimming operations show a wavy surface with a glass-like layer on the surface of variable thickness. In case of the roughing step the glassy layer has a thickness of ~5–6 µm. Some vertical cracks are stopped at the interface been boundary layer and unaffected substate material. However, some lateral cracks are formed which indicates a considerable level of residual stress in the surface layer. In case of the trimming operation the process zone is thinner in average (1–2 µm). An accumulation of glassy material is found at the bumps of the surface, here the layer may be 5 µm thick. Evidently, the larger bright particles embedded into the glassy surface layers are titanium nitride grains. Vertical as well as lateral cracks are shorter. Delamination of the outer shell, as foreshadowed in the case of the boundary layer of the roughing step, cannot be observed. The structure of non-etched substrate material is very homogeneous and confirms the observations in Section 3.1.

CeramicsFigure2020, 38. SEM images showing cross sections of ZTA-TiN surfaces ED-machined with different 207 parameter sets (left roughing, right trimming 2).

2500 Polished peak max. 3270 counts ED-machined TiN 2000 3 2 2 O 2 TiN Al t-ZrO

1500 t-ZrO + 3 O 2 Al 3

1000 O 2 Counts [-] Al 2 2 2 2 3

500 O m-ZrO 2 m-ZrO m-ZrO m-ZrO Al

0 28 30 32 34 36 38 40 42 44 46 48 θ 2 [°]

Figure 9. X-ray diffractogram of polished and ED-machined surfaces. 4. Discussion ZTA-TiN materials with 10 vol% zirconia reinforcement and 28 vol% titanium nitride as conductive phase show attractive mechanical properties comparable to ZTA-TiC and ZTA-NbC materials studied previously [8,19]. Moreover, a far superior machining speed compared to these materials was observed. The attractive strength may be attributed to the fully dense microstructure and high homogeneity. Replacement of the brittle carbides by the tougher nitride [20,21] seems to be beneficial to both strength and toughness. Compared to other investigations on alumina-TiN composites the addition of zirconia enhances the toughness and enables lower sintering temperatures so that the microstructure remains fine grained [7]. Transformation toughening seems to be the dominant reinforcement mechanism. Transformability values of up to 21% are only slightly below the value of 25% reported for plain 10ZTA made from zirconia coated powders reported by Naglieri [22]. The powder choice and compounding procedure, therefore, seems to provide a very homogeneous distribution with a significant size fraction in the “correct” grain size range between ~400–500 nm. As the monoclinic content in the polished samples is above ~25% the alumina and TiN fractions are under tensile hydrostatic stress [16]. As tensile stress in the matrix weakens the grain boundaries, microcracking may also provide a certain toughness increment. This assumption is supported by the observation that the materials were difficult to polish due to grain breakout. The maximum monoclinic content at an intermediate sintering temperature—though not very pronounced – is an interesting detail. A priori, we would expect a rise in monoclinic content with an increase of sintering temperature and grain size. As the materials are sintered under reducing conditions, we may expect a higher amount of thermal oxygen vacancies at higher temperature [23,24]. Moreover, it is known that interdiffusion of oxygen and in zirconium and titanium compounds may lead to stabilization via the anion lattice of zirconia [25]. Replacing of oxygen by nitrogen results oxygen vacancies for charge neutrality and in the same stabilizing effect as addition of yttria [26]. These combined effects seem to offset the effect of . The nitride fraction is obviously sufficient to form a percolating network which provides high electrical conductivity of 40 kS/m. ≈ The machining results are at first sight very surprising as the roughing step provides the smoothest surface. Typically, in machining of or other ceramics the roughness is reduced by subsequent machining steps with lower energy input. In the present case trimming steps therefore seem inefficient to increase the surface quality. However, the investigation of the sub-surface structure indicates that the Ceramics 2020, 3 208 lower energy input of the trimming operations leads to a thinner boundary layer, shorter cracks and to a reduction of residual stress. The reduction of the energy input leads to a shift in material removal mechanisms. At high energy input the predominant mechanism is melting and gas bubbles indicate a contribution by evaporation. The ripples on the surface are apparently frozen structures resulting from Rayleigh surface waves propagating through the molten surface layer. The melt therefore seems to have a very low viscosity especially at high energy input. The low viscosity of the melt provides a smooth surface and facilitates material removal. A reduction of the energy input leads to rougher and foamier surfaces as molten material re-solidifies. The detail images (see Figure7) indicate that the alumina and zirconia fraction as well as the smaller titanium nitride particles are completely molten while larger titanium nitride particle do not melt completely and are incorporated into the re-solidified surface layer.

5. Conclusions The investigated ZTA-TiN materials with a moderate amount of 10 vol% unstabilized zirconia reinforcement and 28 vol.% commercially available fine grain titanium nitride show a combination of good mechanical properties in terms of strength, toughness and hardness. This qualifies these materials for many mechanically demanding engineering applications. The excellent machinability, especially the higher material removal rate compared to previously studied ED-machinable ZTA-TiC and ZTA-NbC materials offers a large economic benefit as roughing operations can be significantly accelerated. The effect of the material removal process and resulting surface properties on mechanical properties are yet to be determined. The fact that only short cracks are produced and that these cracks are terminated in the interface between boundary layer and substrate hint at a moderate strength reduction by machining operations.

Author Contributions: Conceptualization and methodology, F.K.; sample manufacturing and mechanical testing, F.K.; X-ray and EDM-experiments, A.G. writing, F.K. and A.G. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to thank Willi Schwan for sample preparation and SEM-images. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Wang, J.; Stevens, R. Zirconia-toughened alumina (ZTA) ceramics. J. Mater. Sci. 1989, 24, 3421–3440. [CrossRef] 2. Lange, F.F. Transformation toughening: Part 4 Fabrication, fracture toughness and strength of Al2O3-ZrO2 composites. J. Mater. Sci. 1982, 17, 247–254. [CrossRef] 3. Marinescu, I.D.; Hitchiner, M.; Uhlmann, E.; Rowe, W.B.; Inasaki, I. Handbook of Machining with Grinding Wheels; CRC Press: Boca Raton, FL, USA, 2006; ISBN 9780429171055. 4. Kurtz, S.M.; Kocagöz, S.; Arnholt, C.; Huet, R.; Ueno, M.; Walter, W.L. Advances in zirconia toughened alumina biomaterials for total joint replacement. J. Mech. Behav. Biomed. Mater. 2014, 31, 107–116. [CrossRef] [PubMed] 5. Landfried, R.; Kern, F.; Burger, W.; Leonhardt, W.; Gadow, R. Development of Electrical Discharge Machinable ZTA Ceramics with 24 vol% of TiC, TiN, TiCN, TiB2 and WC as Electrically Conductive Phase. Int. J. Appl. Ceram. Technol. 2013, 10, 509–518. [CrossRef] 6. Echeberria, J.; Rodríguez, N.; Vleugels, J.; Vanmeensel, K.; Reyes-Rojas, A.; Garcia-Reyes, A.; Domínguez-Rios, C.; Aguilar-Elguézabal, A.; Bocanegra-Bernal, M.H. Hard and tough carbon nanotube-reinforced zirconia-toughened alumina composites prepared by spark plasma sintering. Carbon 2012, 50, 706–717. [CrossRef] 7. Rak, Z.S.; Czechowski, J. Manufacture and properties of Al2O3–TiN particulate composites. J. Eur. Ceram. Soc. 1998, 18, 373–380. [CrossRef] Ceramics 2020, 3 209

8. Schmitt-Radloff, U.; Kern, F.; Gadow, R. Spark plasma sintering and hot pressing of ZTA-NbC materials—A comparison of mechanical and electrical properties. J. Eur. Ceram. Soc. 2018, 38, 4003–4013. [CrossRef] 9. Suzuki, K.; Morishita, T.; Yogo, T. Sintered Ceramic Bodies and Ceramic Metal Working Tools. U.S. Patent 09/112,456, 7 November 1995. 10. Evans, A.G.; Charles, E.A. Fracture Toughness Determinations by Indentation. J. Am. Ceram. Soc. 1976, 59, 371–372. [CrossRef] 11. Chantikul, P.; Anstis, G.R.; Lawn, B.R.; Marshall, D.B. A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: II, Strength Method. J. Am. Ceram. Soc. 1981, 64, 539–543. [CrossRef] 12. Toraya, H.; Yoshimura, M.; Somiya, S. Calibration Curve for Quantitative Analysis of the Monoclinic-Tetragonal ZrO2 System by X-Ray Diffraction. J. Am. Ceram. Soc. 1984, 67, C119–C121. [CrossRef] 13. Kosma´c,T.; Wagner, R.; Claussen, N. X-Ray Determination of Transformation Depths in Ceramics Containing Tetragonal ZrO2. J. Am. Ceram. Soc. 1981, 64, C72–C73. [CrossRef] 14. McMeeking, R.M.; Evans, A.G. Mechanics of Transformation-Toughening in Brittle Materials. J. Am. Ceram. Soc. 1982, 65, 242–246. [CrossRef] 15. Heuer, A.H. Transformation Toughening in ZrO2-Containing Ceramics. J. Am. Ceram. Soc. 1987, 70, 689–698. [CrossRef] 16. Gregori, G.; Burger, W.; Sergo, V. Piezo-spectroscopic analysis of the residual stresses in zirconia-toughened alumina ceramics: The influence of the tetragonal-to-monoclinic transformation. Mater. Sci. Eng. 1999, 271, 401–406. [CrossRef] 17. Iwanek, H.; Grathwohl, G.; Hamminger, R.; Brugger, N. Machining of ceramics by different methods. In Ceramic Materials and Components for Engines: Proceedings of the 2nd International Symposium, Lübeck, Germany, 14–17 April 1986; Bunk, W., Ed.; Deutsche Keramische Gesellschaft: Cologne, Germany, 1986; pp. 417–423. ISBN 3-925543-01-5. 18. Landfried, R.; Kern, F.; Gadow, R. Electrical Discharge Machining of Alumina-Zirconia-TiC Composites with Varying Zirconia Content. Key Eng. Mater. 2013, 554, 1916–1921. [CrossRef] 19. Schmitt-Radloff, U.; Gommeringer, A.; Assmuth, P.; Kern, F.; Klocke, F.; Holsten, M.; Schneider, S. Effects of Composition on Mechanical and ED-machining Characteristics of Zirconia toughened Alumina—Titanium (ZTA-TiC) Composite Ceramics. Procedia CIRP 2018, 68, 17–21. [CrossRef] 20. Carbide, Nitride and Boride Materials Synthesis and Processing, 1st ed.; Weimer, A.W. (Ed.) Chapman & Hall: London, UK, 1997; ISBN 978-94-010-65251-4. 21. Kuwahara, H.; Mazaki, N.; Takahashi, M.; Watanabe, T.; Yang, X.; Aizawa, T. Mechanical properties of bulk sintered titanium nitride ceramics. Mater. Sci. Eng. A 2001, 319, 687–691. [CrossRef] 22. Naglieri, V.; Palmero, P.; Montanaro, L.; Chevalier, J. Elaboration of Alumina-Zirconia Composites: Role of the Zirconia Content on the Microstructure and Mechanical Properties. Materials 2013, 6, 2090–2102. [CrossRef] 23. Hillert, M. Thermodynamic Model of the Cubic—Tetragonal Transition in Nonstoichiometric Zirconia. J. Am. Ceram. Soc. 1991, 74, 2005–2006. [CrossRef] 24. Bartolome, J.F.; Montero, I.; Diaz, M.; Lopez-Esteban, S.; Moya, J.S.; Deville, S.; Gremillard, L.; Chevalier, J.; Fantozzi, G. Accelerated Aging in 3-mol%-Yttria-Stabilized Tetragonal Zirconia Ceramics Sintered in Reducing Conditions. J. Am. Ceram. Soc. 2004, 87, 2282–2285. [CrossRef] 25. Vleugels, J.; van der Biest, O. Development and Characterization of Y2O3-Stabilized ZrO2 (Y-TZP) Composites with TiB2, TiN, TiC and TiC0.5N0.5. J. Am. Ceram. Soc. 1999, 82, 2717–2720. [CrossRef] 26. Cheng, Y.-B.; Thompson, D.P. Role of Anion Vacancies in Nitrogen-Stabilized Zirconia. J. Am. Ceram. Soc. 1993, 76, 683–688. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).