New Structural Biomaterial

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Biomaterial for prosthesis
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  1 SCIENTIFIC  REPORTS  | 7:44922 | DOI: 10.1038/srep44922 www.nature.com/scientificreports Unprecedented simultaneous enhancement in damage tolerance and fatigue resistance of zirconia/Ta composites A. Smirnov 1 , 2 , J. I. Beltrán 1 , T. Rodriguez-Suarez 3 , C. Pecharromán 1 , M. C. Muñoz 1 , J. S. Moya 1 , 4  & J. F. Bartolomé 1 Dense ( > 98 th%) and homogeneous ceramic/metal composites were obtained by spark plasma sintering (SPS) using ZrO 2  and lamellar metallic powders of tantalum or niobium (20 vol.%) as starting materials. The present study has demonstrated the unique and unpredicted simultaneous enhancement in toughness and strength with very high aw tolerance of zirconia/Ta composites. In addition to their excellent static mechanical properties, these composites also have exceptional resistance to fatigue loading. It has been shown that the major contributions to toughening are the resulting crack bridging and plastic deformation of the metallic particles, together with crack deection and interfacial debonding, which is compatible with the coexistence in the composite of both, strong and weak ceramic/metal interfaces, in agreement with predictions of ab-initio calculations. Therefore, these materials are promising candidates for designing damage tolerance components for aerospace industry, cutting and drilling tools, biomedical implants, among many others. 3Y-ZP is an extensively used material or many structural applications due to its good mechanical perormance, which is related to the tetragonal to monoclinic phase transormation o ZrO 2  and it is associated to the volume expansion, 3–5%, and shear strain ≅  7% 1 . Tis volumetric expansion generates stresses in the ceramic matrix, which hinders the crack propagation. Nonetheless, zirconia based ceramic materials are not suitable or applica-tions under severe loading conditions due to insufficient surace finish or cracking induced during service and mishandling, being the main reasons or unpredictable ailure o these ceramic components. Tereby, the pres-ence o any type o bulk discontinuities or tiny deects may reduce their reliability.Brittle ceramics matrix can be toughened by adding a ductile second phase to them. Cermets are ideally designed to combine the optimal properties o both, high wear resistant ceramics and ductile metals, which pos-sess the ability to reduce crack propagation and prevent catastrophic ailure.However, these improvements are ofen achieved at the expense o strength. In these structural materials strength and toughness are usually considered mutually exclusive 2 . In terms o microstructural design, require-ments or high strength are ofen different or even contradictory to those or high racture toughness.Several studies on zirconia reinorced with metals such as nickel 3–6 , stainless steel 7–9 , molybdenum 10 , tita-nium 11 , tungsten 10,12 , chromium 13 , iron 13  have been reported in the literature. Te mechanical properties o these ZrO 2 -based cermets reported by diverse scientific studies are substantially scattered. Exemplarily, the flexural strength ( σ    f  ) and racture toughness ( K  Ic ) o zirconia/metal composites described in the literature reach values up to 1200 MPa and 5.9 MPa∙m 1/2  or ZrO 2 /Ni cermet 4  and 310 MPa and 5.1 MPa∙m 1/2  or ZrO 2 /i cermet 11 , respec-tively. Tat could be explained by the variations in initial compositions, distribution o reinorcement phase, processing and sintering conditions. Furthermore, the thermal expansion and elastic modulus o the different phases 14,15 , the particle-matrix interaces 16,17  and their structural anisotropy are important parameters. Moreover, 1 Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior deInvestigaciones Cientícas (CSIC), C/Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain. 2 Moscow State University of Technology “STANKIN”, Vadkovskij per. 1, Moscow, 101472, Russian Federation. 3 Element Six UK Ltd, Global Innovation Centre, Fermi Avenue, Harwell Oxford, Didcot, OX11 0QR, UK. 4 Nanomaterials and Nanotechnology Research Center (CINN), CSIC-University of Oviedo (UO), Avda de la Vega 4-6, El Entrego, 33940 San-Martín del Rey Aurelio, Spain. Correspondence and requests for materials should be addressed to J.F.B. (email:  jbartolo@icmm.csic.es) Received: 11 July 2016 A ccepted: 14 February 2017 P ublished: 21 March 2017 OPEN  www.nature.com/scientificreports/ 2 SCIENTIFIC  REPORTS  | 7:44922 | DOI: 10.1038/srep44922 it has been shown in many previous studies that the mechanical response o transition metals depends primarily on its purity, particularly regarding the oxygen, nitrogen, hydrogen and carbon content. Te purity o the metal powder, the manuacturing process and the level o deormation are other actors that influence the final material properties 18–22 .his suggests that a more reined understanding o the interrelations between dierent strengthening and toughening mechanisms is needed to optimize ceramic/metal composite microstructures or structural applications.Additionally, embedding a ductile metallic phase into a brittle matrix increases not only their racture tough-ness and damage tolerance, but also influences the atigue perormance 23,24 . However, to date, studies on the mechanical properties o such ductile-particle reinorced brittle materials have mainly ocused on strength and racture toughness behaviour; in particular on the contributions to toughening under monotonic loading condi-tions 25–29 . Conversely, very ew investigations have been ocused on the composite behaviour under cyclic load. Tese studies have shown that toughening is ar less effective in atigue simply because the ductile phase ails pre-maturely. Indeed, the atigue-crack growth properties are ofen similar, and sometimes worse, than those corre-sponding to the unreinorced matrix 30–32 . Properties such as toughness (damage tolerance) and atigue resistance are generally mutually exclusive. Tereore, atigue analysis and atigue strength prediction are highly required especially in the case o ail sae or damage tolerance design or components in spacecrafs and rocket engines, cutting and drilling tools, uselage o supersonic planes, biomedical implants, among many others. Accurate pre-diction o atigue lie is a challenge in ceramic/metal composites due to the complicated nature o atigue crack initiation and propagation, interaces and complex material behaviour under loading and unloading regimes.Recently, we have demonstrated that wet mixing route and hot press sintering tailored suits the abrication o zirconia/Nb 16  and zirconia/a 23  composites with a flexural strength o 800 MPa and 992 MPa, and racture toughness o 15 MPa∙m 1/2  and 16 MPa∙m 1/2 , respectively. In the present work, we ocus on the interplay between the damage tolerance and atigue resistance phenomena within zirconia/Nb and zirconia/a composites. Results Microstructure. Scanning electron micrographs corresponding to ZrO 2 /metal composites are shown in Fig. 1. In these micrographs, the darker and bright phases correspond to zirconia and metal grains, respectively. Te metallic particles are uniormly dispersed in the matrix and no porosity is observed. In the present work, the larger metallic particles are preerentially oriented due to the effect o the applied pressure during spark plasma sintering process. Tese flake shaped metal particles will be textured normal to the crack propagation. As can be seen, ZrO 2 /a and ZrO 2 /Nb interaces are well bonded and no microcracks are observed. It has only been observed that a solid solution o Nb 2 O 5  and a 2 O 5  in ZrO 2  takes place 33,34 . Tis solid solution o niobia and tantala in the zirconia matrix were calculated rom the EDX spectra. Tese ractions were estimated to be 0.9 ±  0.4 and 1.4 ±  0.4 mol. %, respectively. Tereore, we assume that the entire passivation layer o a 2 O 5  and Nb 2 O 5  that is always present on the particle suraces o the metal starting powder is dissolved in a solid solution in the zirconia matrix afer sintering. On the other hand, it may well be that the oxygen is dissolved and distributed statistically in the metal 35 . However, no new suboxide has been detected.A representative high resolution transmission electron micrograph corresponding to ZrO 2 /a interace is shown in Fig. 2. Tis micrograph reveals a direct contact between both grains at the interaces without any addi-tional phases. Mechanical properties. Te results related to the mechanical evaluation o the composites are enclosed in able 1. Figure 1.  Scanning electron images corresponding to the microstructure o ( A ) zirconia/a and ( B ) zirconia/Nb composites. Darker and lighter phases are zirconia and the corresponding metal, respectively.  www.nature.com/scientificreports/ 3 SCIENTIFIC  REPORTS  | 7:44922 | DOI: 10.1038/srep44922 Te mean biaxial flexural strength values corresponding to ZrO 2 , ZrO 2 /a and ZrO 2 /Nb were ound to be 1217 ±  10 MPa, 970 ±  18 MPa and 850 ±  20 MPa, respectively. As a direct consequence o the smaller critical grain size in ZrO 2 , the bending strength o the monolithic ceramic is higher than the strength corresponding to both zirconia/metal composites. Te Young’s moduli o the composites ( ≈  200 GPa) were ound to be very close to the  values predicted by the rule o mixtures by Voigt and Reuss models. Te average racture toughness o ZrO 2 /Nb and ZrO 2 /a composites was ound to be 15 ±  1 and 16 ±  0.9 MPa·m 1/2 , respectively; much higher than the value obtained or the monolithic zirconia (6 ±  0.3 MPa·m 1/2 ).In Fig. 3, a chart o the indentation load versus the strength o indented samples is plotted. Each data point represents the mean value o about twelve specimens tested at a given load. Linear fitting was applied and it was ound that the slopes o monolithic zirconia and ceramic/metal composites were 0.30 and 0.06, respectively.Fatigue is undoubtedly a very important type o loading or many components containing dissimilar systems. In a atigue loading regime, a structure may ail at a small percentage o its racture strength. Te results or cyclic atigue lie or the specimens tested with a disc geometry are presented in a semi logarithmic orm in Fig. 4 as peak stress versus cycles to ailure ( N  ). Te tests were interrupted at N =   10 7  cycles, or the unailed samples, which are marked with an arrow symbol. Tree maximum stress levels ( σ  max ) were selected in relation to the initial strength obtained under static tests (able 2). For the interpretation o materials atigue data the expo-nential model was used. Te estimation o the model parameters was based on linear regression analysis. It was ound that the atigue limit or ZrO 2 , ZrO 2 /a and ZrO 2 /Nb is 1200 ±  15 MPa, 860 ±  30 MPa and 370 ±  30 MPa, respectively (able 2). Discussion A detailed analysis o the micrograph in Fig. 2 allowed us to determine the presence o (111) crystallographic planes corresponding to the equivalent zirconia crystals parallel to the a (110). Additionally, a (10–1) planes were also detected (see the F image inset in Fig. 2), so it can be concluded that <  111 >  direction o a crystal is perpendicular to the picture plane. In this regard, a perpendicular plane to the {110} and {111} amilies is the plane a(0-1-2). Considering the geometry in Fig. 2, the only plausible ZrO 2  planes parallel to this one are the ZrO 2 {110}. So that, we can assign the interace in the micrograph to be a(1-1-2)/ZrO 2 (3–30). Tis interace has a mismatch o 16% and has not been previously observed in the similar system ZrO 2 /Nb 16 . In this sense, the pres-ence o (012) cleaving planes in milled a grains, could be the srcin o such interace orientations 36 . In addition, Figure 2.   High resolution transmission electron micrograph o the ZrO 2 /Ta interace. SpecimenDensity [% ϕ th ]Elastic modulus [GPa]Flexural strength σ   f   [MPa]Hardness HV   [GPa]Fracture toughness K  Ic   [MPa·m 1/2 ]Volume ractions o t- and m-ZrO 2  [vol%]Transormability o t-ZrO 2   V  trans PolishedFracturedtmtm ZrO 2 99198 ±  51217 ±  1013 ±  0.36 ±  0.39919732ZrO 2 /a98194 ±  7970 ±  189 ±  0.716 ±  0.9946772317ZrO 2 /Nb98179 ±  6850 ±  2010 ±  0.815 ±  1982792119 Table 1.   Densities and mechanical properties o the all studied specimens as well as volume ractions o tetragonal “t” and monoclinic “m” zirconia in polished and ractured suraces and the resulting transormabilities o tetragonal zirconia.  www.nature.com/scientificreports/ 4 SCIENTIFIC  REPORTS  | 7:44922 | DOI: 10.1038/srep44922 due to the lamellar shape o a particles, it is expected that different ZrO 2  crystallographic orientations orm interaces with the a (100) surace, as it occurs in ZrO 2 /Nb composites 16 . In act, ab-initio calculations o the bonding strength and stability o several plausible ZrO 2 /a interaces indicate that the experimentally observed (3–30)/(1-1-2) interace is thermodynamically stable and its bonding strength, measured by the work o separa-tion (Wsep) 37,38 , is moderated (see Fig. S1 and able S1 o Supplementary Inormation, SI). While the (100) ZrO 2 /(100) a polar interace presents the largest bonding strength, although its stability is lower. Te Wsep o the rep-resentative investigated interaces ranges rom high (~10 J/m 2 ) to medium values (~2 J/m 2 ) (see SI or a discussion o the ab-initio calculations). Tereore, ZrO 2 /metal composites show diverse interaces with different strength, being the strongest less stable and, consequently, the composites show a moderate average interace strength. As a result, the metal particle-matrix bond should be strong enough or load transer and energy dissipation by metal Figure 3.   Indentation load versus strength plots o 3Y-TZP/Ta, 3Y-TZP/Nb composites and zirconia ceramic. Te indentation-strength data to the P − 1/3  strength response is shown by the diagonal dashed line. Figure 4.   Fatigue resistance S–N curves with values o slopes or un-indented polished specimens.
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