Sintering properties of zirconia-based ceramic composite

Sintering properties of zirconia-based ceramic composite W. J. Kelvin Chew 1, S. Ramesh*1, Y. H. Wong 1, H. Misran 2, F. Yusuf 1, C. Y. Tan 1, M. Amiriyan 3, R. Tolouei 3 and W. D. Teng4 This study examines the effects of different ZrB2 content on various mechanical properties and electrical conductivity of ZrB2/Y-TZP composite. Composites with ZrB2 content of up to 20 wt-% were particularly beneficial at the lower sintering temperature range by achieving greater densification and better hardness than Y-TZP monolith. In contrast to the trends estimated from rule of mixture, the increment of ZrB2 content did not result in any significant improvement in the elastic modulus and hardness of the zirconia composites. Nevertheless, all composites showed tremendous improvement in fracture toughness compared with monolithic Y-TZP and thus, suggested that other toughening mechanisms were operative besides transformation toughening of zirconia. Incorporation of ZrB2 up to mass fraction of 20 wt-% into Y-TZP generally did not affect the tetragonal phase stability of zirconia. Significant reduction of electrical resistivity of the composites was achieved with ZrB2 content of 20 wt-% and sintering temperature of 1400°C. Keywords: Y-TZP, Zirconia, ZrB2, Composite, Sintering, Density, Mechanical properties, Resistivity Introduction While the high hardness of advanced ceramics such as zirconia is a desirable property for the finished product, such material characteristic presents a drawback for its manufacturing process. Traditional machining methods, which are generally abrasion based such as grinding, lapping and polishing, are time consuming and require harder materials as tooling or abrasives such as diamond. Moreover, the inherent brittleness of ceramics makes it prone to brittle failure and restricts the product design to simple geometries and without intricacies. These compounding difficulties result in the high cost of machining advanced ceramics, which accounts up to 80% of the production cost, and remain an obstacle to large-scale ceramic industrial manufacturing.1 Soaring energy and material costs of late necessitate new machining techniques such as electro-discharge machining (EDM), laser-assisted machining and ultrasonic machining to be explored as a more efficient manufacturing process with lower cycle times and waste generation. EDM is an alternative machining method that requires the work piece to be electrically conductive, typically of less than 100 Ω cm resistivity, but zirconia is highly insulative. One possible approach to improve the electrical 1 Department of Mechanical Engineering, Centre of Advanced Manufacturing & Material Processing (AMMP), University of Malaya, 50603 Kuala Lumpur, Malaysia 2 University of Tenaga Nasional, 43000 Kajang, Selangor, Malaysia 3 Faculty of Science and Engineering, Laval University, Quebec, G1 V 0A6 Canada 4 Ceramics Technology Group, SIRIM Berhad, 40911 Shah Alam, Malaysia *Corresponding author, email [email protected] © W. S. Maney & Son Ltd 2014 DOI 10.1179/1432891714Z.000000000939 conductivity of zirconia is to introduce hard and electro-conductive particulate such as zirconium diboride, ZrB2, into the ceramic matrix to produce composite with high conductivity, improved hardness and toughness.2 ZrB2 is a non-oxide ceramic which belongs to the family of ultra-high temperature ceramics. The strong covalent bonds of the transitional metal boride within hexagonal crystal structure renders the ceramic with attractive refractory properties such as high melting point, hardness, strength, thermal conductivity and chemical inertness.3 Despite its good electrical conductivity, the low toughness of ZrB2 restricts the ceramic from a wider range of applications.4 On the other hand, yttria-stabilized tetragonal zirconia polycrystals or Y-TZP is an oxide ceramic known for its high strength and fracture toughness. However, Y-TZP has comparatively poorer wear resistance owing to its modest hardness compared with other ceramics such Al2O3, Si3N4 and SiAlON.5 Therefore, reinforcing zirconia ceramic matrix with ZrB2 particulates can potentially produce an electrically conductive zirconia composite with good mechanical properties. Nevertheless, the fraction of reinforcing phase in the composite remains a crucial factor in determining the overall properties of the composite. Although the introduction of TiN led to substantial reduction in the electrical resistivity of the zirconia composite, Salehi et al.6 also observed the decline of hardness and strength owing to grain growth when the TiN content in the composite was increased from 35 to 95 vol.-%. Another form of boride, TiB2, has more stable intermediate states than ZrB72 and its phase instability was reported to have hindered any improvement in the hardness8 of 30 vol.-% TiB2–ZrO2 composite. Materials Research Innovations 2014 VOL 18 SUPPL 6 S6-105 Chew et al. Sintering properties of zirconia-based ceramic composite In this work, ZrB2 content in the composite was varied between 10 and 30 wt-% to study the effects of different ZrB2 content and sintering temperatures on the densification, phase content, mechanical properties and electrical resistivity of sintered ZrB2-reinforced Y-TZP composites. Experimental methods Fine ceramic powders of high purity for zirconia stabilized with 3 mol-% yttria, Y-TZP (grade KZ-3YF, KCM Corporation, Japan) and zirconium diboride, ZrB2 (Wako Pure Chemical Industries Ltd., Japan) were measured to obtain ratios of Y-TZP with 10, 20 and 30 wt-% ZrB2. These powder compositions were mixed in ethanol, ultrasonicated and ball milled for 2 hours with zirconia milling media. The slurry was then dried in oven for 12 hours before crushing and sieving through a 212-μm-mesh sieve to obtain fine ZrB2/ Y-TZP composite powders. Green compacts of the composites in the shape of disc and bar pellets were obtained by uniaxially pressing the powders using a bench press with applied pressure of 3·7 MPa followed by cold isostatic press at 200 MPa. All pellets were subjected to pressure-less sintering in a tube furnace under argon gas flow at temperatures of 1300–1500°C for 1 hour. Before characterization, each sintered sample was ground with SiC paper of several grit sizes and polished to 1 μm surface finishing with colloidal silica and diamond paste. The bulk density of the ceramic composites was determined through the water immersion technique or Archimedes principle by using an analytical balance and distilled water as the immersion liquid. Relative density of the composites reported were computed based on theoretical density of the composites derived from theoretical density of respective constituents of the composite – Y-TZP: 6·10 g cm−3, ZrB2: 6·09 g cm−3. A type non-destructive technique – impulse excitation technique, was applied to determine the Young’s modulus of the composites. The bar pellets were subjected to a light mechanical impulse and the resulting resonant frequency is measured by a commercial grade instrument (Grindo Sonic MK5 ‘Industrial’, Belgium). The resonant frequency of the sample is then used to compute the elastic modulus as per ASTM E1876-97 standard test method.9 The Vickers hardness of the composites were determined from the dimensions of indent generated by indentation of sample surface with a pyramidal diamond indenter (Matsuzawa Seiki: MV-1, Japan) with applied load of 10 kg. Subsequently, the fracture toughness of the composites were determined via indentation method by using an equation proposed by Niihara et al.10 which utilizes the length of the propagating cracks from the four corners of the same indent made for hardness measurement, as shown in equation (1). A total of five indents were made per sample and the average values were reported. In order to ascertain the electrical conductivity of the composites, the resistivity of the samples were measured at room temperature using 4-point probe technique (Everbeing SR-4 Resistivity, Taiwan, coupled with Keithly 3706 Multimeter, USA):
KIc φ Hv a1/2 
Hv Eφ S6-106 Materials Research Innovations 2/5
−1/2 L = 0.035 a 2014 VOL 18 SUPPL (1) 6 where KIc is the fracture toughness, Hv is the Vickers hardness, E is the Young’s modulus, a is the half length of the average diagonal, L is the average crack length = (L1 + L2 + L3 + L4)/4 and f the constraint factor which is taken as 3. The phase content of both powder and sintered composites were determined from X-ray diffraction (XRD) measurement (Pan Analytical Empyrean, The Netherlands) using a Cu-Kα radiation and operating at 45 kV and 40 mA with 0·02° step size. From the acquired XRD spectra, the peak intensities of tetragonal phase zirconia at (111) reflection and monoclinic phase zirconia at (−111) and (111) were used to determine the monoclinic and tetragonal phase content of zirconia based on the tetragonal–monoclinic relationship developed by Toraya et al.11 Microstructure examination of the composite samples was done by using a light microscope and scanning electron microscope (SEM) (FEI Quanta 250 FEG, USA). Before SEM analysis, all polished samples were thermally etched at 50°C below sintering temperature for 30 minutes under argon gas to delineate the grain boundaries. Results and discussion Powder XRD analysis revealed that the as-received monolithic Y-TZP powder contained 14% of monoclinic zirconia. The presence of secondary conductive phase of ZrB2 up to 30 wt-% showed marginal increase in monoclinic zirconia content of between 2 and 4%. This finding implied that the powder synthesis route for the composite has little effect on the phase stability of tetragonal phase zirconia. Nevertheless, the ZrB2 content in the composite is crucial after sintering. While composites with up to 20 wt-% ZrB2 only showed traces of monoclinic zirconia content at all sintering temperatures, as shown in Fig. 1, the ZrB2/Y-TZP composite of 30 wt-% ZrB2 content consistently recorded significantly higher monoclinic content. Greater destabilization effect on the tetragonal zirconia phase was observed when the sintering temperature was increased from 1300 to 1500°C. In general, the densification of ZrB2/Y-TZP composites proceeded rapidly with increasing sintering temperatures from 1300°C before saturating at approximately 91% relative density at 1450°C, as depicted in Fig. 2. Both 10 and 20 wt-% ZrB2 contents were clearly beneficial at the lower sintering temperatures as these zirconia composites attained significantly higher density compared with monolithic Y-TZP. However, 30 wt- 1 Monoclinic zirconia phase content of ZrB2/Y-TZP composites sintered at different temperatures Chew et al. 2 Densification of Y-TZP composites with different ZrB2 content and sintering temperatures %/Y-TZP composite exhibited the poorest densification behaviour among all composite and achieving only 80% relative density after sintered at 1500°C. Contrasting results were obtained by other researchers who reported relative densities greater than 97% for 30 wt-%/Y-TZP composite after sintering by various methods such as pressure-less sintering,12 hot pressing,13 sinter-hot isostatic pressing5 and spark plasma sintering.14 Sintering of ZrB2 to full density without pressure assistance, very high temperatures or prolonged soaking time is known to be especially challenging15 since the non-oxide ceramic has a high melting point, strong covalent bonding and low self-diffusion rate.3 Moreover, ZrB2 is also highly susceptible to oxidation at high temperatures, which leads to the formation of boria (B2O3) and zirconia scale. Boria has been known to substantially hinder densification in alumina16 at trace amounts and also capable of destabilizing a fully cubic zirconia phase in an 8 mol-%-yttria-stabilized zirconia into a fully monoclinic phase.17 It is possible that some ZrB2 content in the composites of this study had oxidized during sintering and thus, resulting in the lower overall density of the composite and monoclinic zirconia phase formation. These effects intensified with higher ZrB2 content and were very pronounced for 30 wt-% ZrB2/Y-TZP. In spite of the higher Young’s modulus and hardness of ZrB2 compared with Y-TZP, no significant improvement in the properties of the composites were noted when the ZrB2 content was increased up to 30 wt-%. The trend of these mechanical properties was opposite to that of estimated from rule of mixture, which is a simple rule that estimates the property of composite based on the property and volume of its constituents. Marginal change in the 3 Effect of different ZrB2 content on the fracture toughness of Y-TZP composites Sintering properties of zirconia-based ceramic composite properties of the composites was evident when the sintering temperature was increased from 1300 to 1500°C with the hardness gradually rising from 10 to 13 GPa and the elastic modulus of fluctuated between 150 and 200 GPa. Although composites containing 10 and 20 wt-% ZrB2 achieved about 4 GPa higher hardness than monolithic Y-TZP, the presence of 30 wt-% ZrB2 appeared to be detrimental. Thirty weight per cent /Y-TZP composite showed comparatively poor Young’s modulus of 105–127 GPa and low hardness of 4–6 GPa in the sintering regime studied. The poor densification behaviour of the composite possibly led to the low elastic modulus while the low hardness could be attributed to the significant monoclinic phase zirconia after sintering. Figure 3 shows the variation of fracture toughness of the composites with different ZrB2 content and sintering temperatures. All composites achieved better fracture toughness than monolithic Y-TZP. Such observation indicates that other toughening mechanisms besides transformation toughening are also afforded in the composites. Y-TZP is known to possess relatively higher fracture toughness compared with other ceramics owing its unique ability of transformation toughening whereby stress-induced martensitic phase transformation of zirconia from tetragonal to monoclinic symmetry results in compressive strains that arrest a propagating crack.18 One possible contribution to toughness improvement is crack deflection. Propagating cracks resulting from hardness indentation were frequently found deflected by larger ZrB2 particles, especially in the microstructure of composites with 20 and 30 wt-% ZrB2 content. Another contributing factor is possibly the thermal residual stress generated in the ceramic matrix composite arising from the different coefficient of thermal expansions of Y-TZP (10·0 × 10−6 K−1) and ZrB2 (7·4 × 10−6 K−1).13 Upon cooling from sintering temperature, the lower coefficient value of ZrB2 leads to thermal expansion mismatch and the development of tensile stresses in zirconia matrix which in turn increases the propensity of tetragonal to monoclinic zirconia phase transformation by lowering the magnitude of required critical stress. Nevertheless, there is apparently a limit to the ZrB2 content that is beneficial to the composite. In the ZrB2-rich region (≥70 vol.-%) of the ZrB2/Y-TZP composite, lesser volume fraction of zirconia resulted in lower tetragonal phase retention of zirconia and poorer fracture toughness.4 Moreover, for Y-TZP composites reinforced with Ni particles,19 the fracture toughness of the composites 4 Electrical resistivity of sintered Y-TZP composites with 0–30 wt-% ZrB2 content Materials Research Innovations 2014 VOL 18 SUPPL 6 S6-107 Chew et al. Sintering properties of zirconia-based ceramic composite gradually decreased as the volume fraction of Ni was increased from 0 to 40 vol.-%. In order to elucidate the electrical behaviour of the ZrB2/Y-TZP composites, the resistivity of the sintered composites were determined using the 4-point probe measurement at room temperature. The results of the measurement are as presented in Fig. 4. Both Y-TZP monolith and 10 wt-% ZrB2/Y-TZP composite appeared to be good insulators or having very high resistivity irrespective of the sintering temperature applied. The onset of electrical conductivity began when composite has ZrB2 mass fraction of 20 wt-% and sintered at 1400°C and above. As for alumina matrix composite, Postrach and Potschke16 reported higher ZrB2 threshold (∼28 wt-%) to achieve electrical resistivity below 0·25 Ω cm. Despite exhibiting relatively poor mechanical properties and ∼80% dense, the remarkably low resistivity range of 0·0005–0·01 Ω cm achieved by the 30 wt% ZrB2/Y-TZP composite indicates that there is good connectivity of the secondary conductive phase of ZrB2. The low density of this composite was unlikely to have influenced its electrical conductivity since De Souza et al.20 remarked that there is no influence of porosity on the electrical conductivity when the level of porosity is less than 15%. Although ZrB2/Y-TZP composites in this study obtained electrical conductivity well beyond the 100 Ω cm threshold required for EDM process,6 further optimization of sintering parameters needs to be done to improve the sintered mechanical properties and tetragonal phase stability of zirconia. Conclusion The effects of different sintering temperatures and ZrB2 content on the sintered properties of ZrB2/Y-TZP composite were investigated. Higher densification and better hardness compared with monolithic Y-TZP were achieved by composites with up to 20 wt-% ZrB2 content when sintered at 1300°C. Enhanced densification of the composites occurred with increase in sintering temperature, especially at the lower end of the sintering range studied. Despite the incorporation of secondary electro-conductive phase of ZrB2, which has higher elastic modulus and hardness than Y-TZP, no distinct improvements in these mechanical properties were observed in all ZrB2/Y-TZP composites. Contrary to the prediction from rule of mixture, all composites recorded higher fracture toughness than Y-TZP monolith, implying that besides transformation toughening, other toughening mechanisms such as crack deflection and thermal residual stress were operative. Thirty weight per cent/Y-TZP composite attained the highest electrical conductivity among all composites in spite of its comparatively poorer mechanical properties and significant monoclinic zirconia content that warrants further optimization in the sintering parameters employed. S6-108 Materials Research Innovations 2014 VOL 18 SUPPL 6 Acknowledgements The authors acknowledge University of Malaya for providing the necessary facilities and resources for this research. This research was supported by the PPP Grant No. PV098-2012A and UMRG Grant No. CG022-2013. References 1. E. C. Bianchi, P. R. Aguir, E. J. da Silva, C. E. da Silva Jr. and C. A. 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