1.
Introduction
Alumina ceramics are common for structural applica-tions due to their high hardness, abrasion resistance and moderate price. The moderate fracture resistance can be improved by dispersions of second phases such as par-ticles of YAG and silicon carbide parimproved by dispersions of second phases such as par-ticles, plate-shaped particles (platelets) or whiskers [1]. In these material sys-tems, toughening is based on crack deflection and crack bridging [2–3]. The most widely used alumina dispersion ceramic in engineering and biomedical applications is zirconia toughened alumina (ZTA). In the ZTA microc-racking, residual stress and phase transformation are the dominant toughening mechanisms [4]. Mechanical proper-ties of ZTA can be excellent with bending strength values as high as 1200 MPa and fracture resistance values of 7–8 MPa∙m1/2 [5].
Introduction of transition metal carbides, nitrides or borides to make the material electrically conductive and thereby ED-machinable typically imposes some limita-tions on strength and fracture resistance. Landfried re-ported strength values of 845 MPa (3-pt) and KIC values of
4.22 MPa∙m1/2 (ISB-method) for composites containing 17
vol.% zirconia and 24 vol.% titanium carbide in an alumina matrix [6]. Other conductive dispersions such as TiN, TiCN, WC and TiB2
added in the same volume fraction lead to in-crementally better mechanical properties but at a significant deterioration of ED-machinability [7].
In situ platelet reinforcement of ZTA with cerium hex-aaluminate can be carried out under reducing conditions by reacting cerium dioxide with alumina (2CeO2 + 11Al2O3 + C
→ 2CeAl11O18 + CO) [8]. In hot pressing, the oxygen reacts
with the carbon die or carbon monoxide contained in the atmosphere. As shown recently this leads to an enhanced
In situ
cerium hexaaluminate platelets reinforcement
of ED-machinable ZTA-TiC ceramics
Andrea Gommeringer, Ulrich Schmitt-Radloff, Frank Kern*, Rainer Gadow
Institute for Manufacturing Technologies of Ceramic Components and Composites (IFKB), University of Stuttgart, Allmandring 7b, 70569 Stuttgart, Germany
*e-mail: frank.kern@ifkb.uni-stuttgart.de
Abstract
Electrical discharge machinable ceramics open new applications fields by combining good mechanical properties such as high strength, hardness, abrasion resistance and high temperature resistance with the possibility to produce complex shape customized components. Alumina based ceramics with dispersions of zirconia and a conductive refractory carbide offer enhanced hardness and abrasion resistance compared to other ED-machinable ceramics based on silicon nitride or Y-TZP based compounds, however only a moderate toughness. In the present study cerium hexaaluminate (CA6) platelets were introduced in the material system of zirconia toughened alumina with 24 vol.% titanium carbide by in situ reaction sintering to further improve the fracture resistance. With addition of CA6 the fracture resistance and strength increases while hardness and electrical conductivity were incrementally reduced. CA6 addi-tion leads to a slight reduction in material removal rate in die sinking EDM and reduction of roughness in machined surfaces. Keywords: Zirconia, Alumina, Electrical discharge machining, Platelet, Cerium hexaaluminate
WZMOCNIENIE W POSTACI PŁYTEK SZEŚCIOGLINIANU CERU POWSTAJĄCYCH IN SITU W ELEKTROEROZYNIE OBRABIALNEJ CERAMICE ZTA-TiC
Elektroerozyjnie obrabialna ceramika otwiera nowe pola zastosowań poprzez połączenie dobrych właściwości mechanicznych ta- kich jak wysoka wytrzymałość, twardość, odporność na ścieranie i wysoka odporność temperaturowa z możliwością wytwarzania zło-żonego kształtu komponentów stosownie do wymagań klienta. Ceramika oparta na tlenku glinu zawierająca dyspersje tlenku cyrkonu i przewodzącego węglika ogniotrwałego oferuje podwyższoną twardość i odporność na ścieranie w porównaniu z pozostałą ceramiką obrabialną elektroerozyjnie opartą na azotku krzemu lub Y-TZP, ale jedynie umiarkowaną odporność na pękanie. W prezentowanych badaniach do układu materiałowego tlenku glinu wzmocnionego tlenkiem cyrkonu zawierającego 24% obj. węglika tytanu wprowadzo-no płytki sześcioglinianu ceru (CA6) drogą in situ spiekania reakcyjnego, aby poprawić odporność na pękanie. Wraz z dodatkiem CA6 zwiększyły się odporność na pękanie i wytrzymałość, podczas gdy twardość i przewodność elektryczna zostały przyrostowo zreduko-wane. Dodatek CA6 prowadzi do nieznacznego zmniejszenia szybkości usuwania materiału w trakcie elektrodrążenia wgłębnego EDM i redukcji chropowatości obrabianej powierzchni.
fracture resistance of ZTA materials [9]. The reinforcing effect in ZTA can however not be traced back to crack deflection such as in platelet reinforced zirconia [9] but is predominantly linked to a change in transformability of the zirconia dispersion coming along with a change in the state of residual stress [10]. In case of ZTA materials with 10–24 vol.% zirconia dispersion the addition of CA6 was found to improve fracture resistance while strength and hardness were slightly reduced [9, 11].
In the present study it was therefore attempted to try the in situ platelet reinforcement concept for ED-machinable ZTA-TiC and quantify the effects on microstructure, me-chanical properties and ED-machinability.
2.
Materials and methods
The starting powders for the material synthesis were submicron size alumina (HP-DBM, Baikowski, France, SBET = 8 m²/g), partially stabilized zirconia (TZ-3YSE, SBET = 8 m²/g, Tosoh, Japan), unstabilized zirconia (TZ-0, SBET = 17 m²/g, Tosoh, Japan), titanium carbide (STD 120,
particle size d50 = 2.8 µm, HC Starck Germany) and
nanoscale cerium dioxide which was synthesized from cerium(IV) ammonium nitrate (Cer-IV-ammoniumnitrat >98% purity, Roth, Germany). Besides a plain ZTA-TiC reference containing 17 vol.% zirconia stabilized with 1.5 mol.% yttria was made by blending 3Y-TZP and mono-clinic zirconia, 24 vol.% TiC and alumina. Four batches were prepared containing 0.6, 1.2, 1.8 and 2.4 mol.% CeO2. At full conversion, these ceria fractions should
lead to the formation of approximately 2.5, 5, 7.5 and 10 vol.% CA6 replacing the same amount of alumina. Five different batches of 200 g each of commensurate amounts of the starting powders were blended and attri-tion milled at 400 rpm for 2 h in 300 ml 2-propanol using 870 g of Y-TZP milling balls. The titanium carbide was added to the milled oxide blend 30 minutes before the end to prevent tribo-oxidation of the carbide phase. After separation of the milling media the resulting dispersions were dried at 50 °C and screened through a 200 µm mesh. The samples were consolidated by hot pressing in vacuum in graphite paper clad dies at 1525 °C for 2 h at 40 MPa axial pressure. Two disks of 45 mm diameter
and 2.5 mm thickness were pressed of each composition for mechanical testing. One thicker disk of 5 mm height was made for die sinking ED-machining. The disks were lapped and polished to a mirror like finish with 15, 6 and 1 µm diamond suspension. Mechanical testing included measurement of Vickers hardness HV10 (Bareiss, Germany, ten indents), Young’s modulus, bulk modulus and Poisson’s ratio (acoustic method, IMCE Belgium, 2 entire disks), 4-point bending strength (Zwick, Germany, outer span 20 mm inner span 10 mm, crosshead speed 0.5 mm/min, > 10 samples each) and fracture resistance by indentation strength in bending (ISB) method (the residual strength of a HV10 indented sample was measured in the same 4-point setup at 2.5 mm/ min crosshead speed, three samples each). SEM images were taken of polished and untreated as well as hydrogen etched samples (1350 °C, 5 min) at 3 kV and 10 kV accel-eration voltage (Zeiss Gemini, Germany, in lens). Phase composition of the samples was determined by XRD using the calibration curve of Toraya [12] (Bruker D8, Germany, Bragg Brentano setup, 2-theta 27–33°, integration of (1¯11)m, (111)m and (101)t reflections). A standardized ED-machin-ing test (400 V, Elotherm Eloplus 30, AEG, Germany) with fixed parameters was performed by die sinking using cop-per electrodes of 5×5 mm diameter in oil based electrolyte (Oelheld, IonoPlus IME-MH, Germany).
3.
Results
3.1. Microstructure
Measurement of density by buoyancy method and SEM images revealed that samples were fully dense. Fig. 1 shows SEM images of the reference material and ZTA-TiC containing 5 vol.% CA6 thermally etched in hydrogen and non-etched one. While the non-etched sample appears blurry, the etching at high temperature reveals the grain boundaries of all phases; TiC grains were strongly attacked by the hydrogen atmosphere. In the SEM images the grains of the zirconia phase have the brightest grey. Grains with a slightly darker grey but also with a smooth surface rep-resent the alumina phase. Fig. 1b shows two large CA6 platelets forming a “λ”. a) b) c)
formation toughness increment is however very moderate (0.6–1 MPa∙m1/2)
3.3. Mechanical properties
Fig. 3 shows the Vickers hardness HV10, bending strength σ4pt and fracture resistance KISB of samples
con-taining various amounts of CA6.
Evidently, HV10 declines with increasing CA6 fraction from 1860 to 1750. Young’s modulus (not shown) forms a flat maximum of 380 GPa at 2.5 vol.% CA6 before it declines linearly to 330 GPa at 10 vol.% CA6. Bending strength in-creases linearly from 705 MPa to 810 MPa between 0 and 5 vol.% CA6 and retains this level up to the highest pre-cipitate content. Fracture resistance of CA6-containing composites is higher than for the reference (4.7 MPa∙m1/2). Toughness shows a distinct maximum of 6.3 MPa∙m1/2 at 2.5 vol.% CA6 then drops to a plateau value of 5.3 MPa∙m1/2 at higher CA6 fractions. Against the trend of the transforma-tion toughness increment, there is no toughness maximum in plain ZTA-TiC. Evidently, other toughening mechanisms must exist besides transformation toughening which are responsible for the enhancement of the fracture resistance.
3.4. EDM
experiment
Die sinking ED-machining tests were performed with copper-electrodes with a square base of 5×5 mm2 for15 minutes. Material removal rate (MRR) and electrode wear ratio were measured. The roughness was determined by white light interferometry. The ED-machinability of all samples could be proven. The trend of the electrical con-ductivity, which decreases with increasing CA6 content, has only a minor effect on material removal rate. Fig. 4 shows electrical conductivity, material removal rate and surface roughness as a function of CA6 content. The material re-moval rate slightly decreases from 0.3 to 0.25 mm³/min with increasing CA6 content. The electrode wear remains al-most negligible in all cases (not shown). At the highest MRR found in the reference samples, the roughness shows the highest values. With increasing CA6 content, the roughness decreases which indicates a mechanistic change in the eroding process. The images of the corresponding cross The grain sizes of TiC (1–2 µm) and alumina (0.5–1 µm)
remain unaltered, when compared to the reference, while the zirconia grains in the CA6 containing samples seem to grow from 300–400 nm in the reference to 400–600 nm in the materials containing > 5 vol.% of CA6. The CA6 pre-cipitates are plate-shaped and rather large, aspect ratios range between 5:1 and 8:1. The length of the CA6 precipi-tates ranges between 1–2 µm, the thickness between 200– 350 nm. As the formation of CA6 proceeds very fast during the hot pressing cycle (heating rates ~ 50 K/min), the CA6 grains may contain intragranular zirconia or grow through smaller TiC grains. Composites containing 7.5 vol.% CA6 or more often show conjoined or multilayer precipitates. In all cases CA6 grains were well embedded into the micro-structures and no microcracks were visible at adjacent grain boundaries.
3.2. Phase transformation
Phase compositions of the polished surfaces Vm,pol and
fracture faces Vm,fract of the composites are shown in Fig. 2
together with the calculated transformation toughness in-crements ΔKT IC according to McMeeking [13]: h V ε E v X f K T f T IC= − ∆ 1 (1)
Here f is the zirconia fraction (17 vol.%: f = 0.17), Vf the
transformed volume fraction (Vf = Vm,fract - Vm,pol), E - the
measured elastic modulus, ν - the measured Poisson’s ratio, εT =
0.05 volume change upon transformation and transfor-mation efficiency X = 0.27 (Y-TZP, predominantly dilatoric [14]). Transformation zone sizes h were calculated accord-ing to Kosmac [15] from XRD data.
The monoclinic phase content measured at the fracture face declines with the addition of CA6 from 51 vol.% for the reference to 38 vol.% for 2.5 vol.% CA6 and stays on this level up to higher contents of CA6. Despite the lower mono- clinic phase content at the fracture faces, the transformabil-ity of the tetragonal phase hardly changes. According to the difference between the phase composition of the polished surfaces and fracture faces, the transformation toughness increment ΔKT IC has a distinct minimum at 5.0 vol.% CA6
and the highest value for the reference material. The trans-Fig. 2. Monoclinic contents Vm and transformation toughness ΔKTIC
sections through machined surfaces show that the smooth-er surfaces of samples with high CA6 content are covered by a thin layer of glassy phase produced by melting and resolidification. This layer is absent in plain ZTA-TiC. Fur-ther investigations of the surface-near regions showed that with decreasing roughness the number of cracks near the surface with a length of up to 50 µm parallel to the surface increases. This recast layer in the CA6 containing samples evidently causes tensile stress in the adjacent region below and induces the formation of perpendicular cracks. Pure ZTA-TiC features a clean but slightly foamy sur-face and a material removal by ED, which leaves the bulk structure unchanged.
4.
Discussion
4.1. Toughening mechanism
As shown in Fig. 3 the bending strength increases with increasing CA6 content. While there is no rise in the phase transformability there has to be another toughening mecha-nism. As Guo and Rejab noted, the addition of platelets can improve the fracture behaviour by crack deflection and crack bridging [3, 11]. Fig. 5 and 6 show SEM images of crack propagation of Vickers indentation cracks. The crack propagation direction is always from the right bottom to the left top. Compared to optimized biomedical grade ZTA, the ternary ZTA-TiC composite with identical zirconia content shows more brittle behaviour. The reason is probably the presence of large TiC grains. As shown in Fig. 5a the crack propagation through large TiC grain is transgranular without any crack deviation or crack bridging. The interaction of the crack with CA6 platelets shows the similar behaviour. Contrary to the conclusions of Guo and Rejab [3, 11], there is little evidence of crack deflection at CA6 precipitates. Depending on the orientation of the platelet, the crack runs either straight through the platelet (in most cases). Slight crack deflection can only be observed if the crack hits the platelet close to its end (see Fig. 5b, where the residual stress concentration due to misorientation to the surround-ing matrix is highest) or if the crack hits the platelet under a very flat angle (Fig. 5c, this however contributes no ad- ditional toughness). This is caused by the only slight differ-ence in the Young’s modulus of the matrix material. This transgranular cracking of CA6 platelets can however be expected due to the fact that Young’s modulus of the CA6 is lower than the modulus of both TiC and alumina [16]. Fig. 6 shows other detected cracks through CA6 platelets in detail. The platelet in the middle of Fig. 6a has been split parallel to the plane direction because of the layer-like struc-ture of the CA6 phase. The layer-like structure of the platelets also appears in Fig. 6b. There the crack has been lightly devi-ated at one of the layer surfaces in the CA6 platelet. Fig. 6c shows, that if the crack tip hits the platelet parallel to the orientation of the grain the crack propagates intergranular. Investigation of the crack propagation behaviour provided no evidence on toughening mechanism by crack deflection or bridging. So most probably the mechanism responsibleFig. 4. Electrical conductivity, material removal rate and roughness vs. content of CA6. Inserts show images of surfaces of relevant samples.
for the rise in bending strength and fracture toughness is the influence of the residual stress. Gregori [10] calculated and verified the residual stresses in plain ZTA due to the CTE mismatch. The stress in the alumina matrix <σ>A depends on the volume fraction of zirconia and its monoclinic content according to the following equation:
[
Z A sf mz tm]
Z Z Z Z A A A Z Z Z A A Kf nKnKf KnKf f Kf α α T T X e σ> = + + + − − + − − < 3 ( )(0 ) (2) where K is the bulk modulus and f is the volume fraction. The subscripts A and Z refer to alumina and zirconia respec-tively. α is the CTE. T0 is the room temperature and Tsf is thetemperature below which elastic stresses are frozen [10]. Xm-z stands for the amount of monoclinic zirconia relating
to the total amount of zirconia fraction measured by X-ray diffraction and calculated by the formula of Toraya [12]. et-m
represents the linear transformation strain from the tetrago-nal to the monoclinic phase (et-m = 0.016) and n is given by:
A A v v n= +− 1 ) 2 1 ( 2 (3)
with νA as Poisson modulus of alumina (νA = 0.27).
For the present study, equation 2 has been used to com-pare the residual stress in zirconia to the residual stress in the remaining matrix, which consists of alumina, titanium
carbide and the CA6 phase. For the calculations some vari-ables have been modified. KA, νA and αA are replaced by KMatrix, νMatrix and αMatrix with values calculated according to the rule of mixture. The terms are listed in Table 1.
As the material in total is stress-neutral, the stress in the matrix <σ>Matrixand the stress in the zirconia phase <σ>Z,
which are given by: − > < − < Z Z Matrix
f
f
σ σ>= 1 (4) The results for the residual stress are visualized in Fig. 7 and confirm the assumption that the equilibrium of stress influences the mechanical behaviour. Tensile stress in the zirconia phase should increase its transformability and cause a rise in toughness. Compared to the results for fracture re-sistance (see Fig. 3) the present study confirms this trend. By the addition of 2.5 vol.% CA6 <σ>Z reaches a maximum of 454 MPa tensile stress. At the same point the matrix, which contains the remaining (brittle) phases such as alumina, tita-nium carbide and CA6, is loaded by compressive stress by a maximum of <σ>A = -93 MPa, which should be beneficial in terms of strength as the residual stress adds to the applied stress. Addition of CA6 in excess of 2.5 vol.% leads to de-creasing values <σ>Z between 219 MPa to 324 MPa, whichcoincides with a decrease in fracture toughness.
a) b) c)
Fig. 5. Crack propagation through 1.5Y-17ZTA-24TiC (a), 1.5Y-17ZTA-24-TiC-2.5CA6 (b) and 1.5Y-17ZTA-24TiC-5CA6 (c). Crack propagation direction is always from the right bottom to the left top.
a) b) c)
Fig. 6. Detailed SEM-images of crack propagation through CA6 platelets: a) platelet split parallel to the plane direction because of the layer-like structure of the CA6 phase, b) deviation of the crack at one of the layer surfaces in the CA6 platelet; c) intergranular crack propagation when the crack tip hits the platelet parallel to the orientation of the grain.
The interpretation of the values for the residual stress should be handled carefully. Gregori [10] investigated a two phase ceramic (ZTA) and the transfer to a four-phase ce-ramic leads to a very complex situation as locally also the stresses between the matrix phases and the anisotropy of the CA6 have to be considered. This rather global estima-tion of the distribution of hydrostatic stress provides an idea of the magnitude of stress and its meaning for the changes in strength and toughness observed. Further investigations will be necessary to provide more evidence.
5.
Conclusions
Summarizing the above shown results it can be stated that the introduction of cerium hexaaluminate precipitates into ED-machinable ZTA-TiC materials is a promising so- lution to improve the mechanical properties of the materi-als and open up new application fields inaccessible by the standard material, which suffers from limited strength and fracture resistance. The reinforcement effect of the CA6 cannot be clearly assigned, neither to crack deflection – which was not observed - nor to a change in transformation toughness as was detected in case of plain ZTA reinforced by CA6. With regard to recent findings on the influence of residual stress on toughness in ZTA, there is however reason to believe that the CA6 precipitates shift the point of stress neutrality, i.e. the point where residual stress by CTE mismatch is offset by transformation strain [10]. The change in material removal mechanism induced by CA6 addition will have to be studied in detail in order to identify suitable ED parameters for the modified materials in order to be able to exploit the full potential of the material in machined components.Acknowledgements
The authors would like to thank Mrs. Felicitas Predel of the Max-Planck-Institute in Stuttgart for the preparation of SEM images.References
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Fig. 7. Residual stress in zirconia (<σ>z) and the remaining matrix
(<σ>Matrix) as a function of CA6 content.
♦
Table 1. Applied terms for residual stress calculation.
Variable by
Gregori the present studyConditions for Value
T0 – Tsf T0 – Tsf -1150 K
KA KMatrixf=(fAKA + fTiCKTiC + CA6KCA6)/(1-fZ)
263 – 270 GPa
KZ KZ 150 GPa
νA νMatrix=(fAνA + f(1-fTiCνTiC + fCA6νCA6)/ Z) ~0.25 αA αMatrix =(fAαA + fTiCαTiC + fCA6αCA6)/(1-fZ) 8.63∙10–6 - 8.66∙10–6 K-1
