MnO2与纳米TiO2对MFAl2O3复合材料力学性能影响英文.docx

rare metal materials and engineeringvolume 40, suppl.1, june 2011effect of mno2 and nano-tio2 on the mechanicalproperties of mf/al2o3 compositesding yinsen,wang zhi,liu jianfei,xing guohong,shi guopuuniversity of jinan, jinan 250022, chinaabstract: effect of mno2 and nano-tio2 on the mechanical properties of mf(mullite fiber)/al2o3 composites was studied. the results show that the flexural strength and fracture toughness of the materials, which were sintered at 1250 for 40 min, increasefirst and then decrease with more mno2 and nano-tio2. when the contents of mno2 and nano-tio2 are 3wt% and 1.8wt% respectively, the flexural strength and fracture toughness reach up to 568 mpa and 6.8 mpam1/2, respectively. the diffusion of mn4+ and ti4+ to the interface between mf and al2o3 matrix and thin interfacial layer of mnoal2o3 and 3mnoal2o3sio2 could improve the bonding between mf and al2o3 matrix. pull-out, debonding and bridging of fibers, and cracks deflection consuming large fracture energy should be the main reasons for increasing the strength and toughness of composites.key words: alumina; mullite fiber; microstructure; mechanical propertieslow flexural strength and fracture toughness limitwere passed through a 50-mesh sieve for use. the powderswere formed by applying a pressure of 50 mpa after doping3% pva as agglomerant. the bodies were sintered to final1250 for 40 min with a heating rate of about 10 /min. the composition and microstructure of the compositeswere investigated by sem and eds.the specimens were cut to a dimension of 2.5 mm5 mm25 mm and polished to a surface roughness of about ra=1 m. the flexural strength and fracture toughness were measured by three-point bending test with a span of 20 mm at a crosshead speed of 0.5 mm/min.2results and discussion2.1mechanical properties of the ceramic compositesthe mechanical properties of the composites by different fabricating processes are listed in table 1. as can be seen from the table, when the contents of mno2 and tio2 are3wt% and 1.8wt%, respectively, the flexural strength andfracture toughness can reach up to 568 mpa and 6.8mpam1/2 respectively, which are increased by 12.7% and52.4% compared with mf/al2o3 composites comprised only of mf and al2o3 with flexural strength of 504 mpa andalumina matrix ceramics1. mf(mulliteapplications offiber), which has high modulus, resistance to high temperatureand to chemical corrosion has been identified as an candidate material for toughening ceramics2. but the composite is limited due to the fiber degradation either during processing or at high temperature. however, mno2 combined withnano-tio2 can lower sintering temperature3 because they4dissolve in al2o3 . the addition of mno2 and nano-tio2 tomf/al2o3 composites can alleviate mullite damageimprove the bonding between mf and alumina matrix.1experimentandraw materials were al2o3 powders (purity 99%) of 1.5 mmean size, mullite fibers of 5 mm average length and 12 m diameter, nano-tio2 (purity 99%) of 59 nm median size, and mno2 (purity 98%) of 5.026 g/cm3 density.the powders were dispersed by peg and absolute ethyl alcohol. firstly mno2 and tio2 were dispersed by supersonic wave for 30 min, then ball-milled with al2o3 for 12 h, and fi- nally continued ball-milling for 3 h after mf was added. the mixed slurry was dried in vacuum, and the dried powdersreceived date: november 1, 2010foundation item: shandong provincial natural science foundation (y2006f53); doctorial foundation of university of jinan(b0514)corresponding author: ding yinsen, candidate for master, school of materials science and engineering, university of jinan, jinan 250022, p. r. china, e-mail:; wang zhi, professor, tel: 0086-531-82767660, e-mail: copyright 2011, northwest institute for nonferrous metal research. all rights reserved.cite this article as: rare metal materials and engineering, 2011, 40(s1): 244-246articleding yinsen et al. / rare metal materials and engineering, 2011, 40(s1): 244-246fracture toughness of 4.46 mpam1/2. the mechanical proper- ties of specimen 7 are greatly increased because optimal content of mno2 and tio2 can improve interfacial bonding between fiber and matrix which will be explained in the fol- lowing section.fiber also consumes a lot of fracture energy to strengthenand toughen alumina ceramic composites.in order to study the bonding between fiber and matrix, the elements around interface were analyzed. as can be seen in fig.2a, where the upper two curves represent al and si while the lower three curves represent o, mn and ti, mn and ti elements have diffused into the mullite fiber. in particular, the interfacial bonding can be strengthened because of the solution of mn4+ and ti4+ in al2o3 which exists in both matrix and mullite fiber(3al2o32sio2).table 1 design of the experiment and properties of the ceramicsmno2content, wt/%tio2content, wt/%relativedensity/%flexuralstrength/mpafracturetoughness/mpam1/2no.1234567891011121314152.53.03.03.03.02.02.02.092.994.794.995.194.495.395.695.594.995.995.795.695.195.795.995.54265245355414855365685564565235455514525125235275.806.106.306.406.406.506.806.706.306.356.506.406.206.286.406.30abcdfig.1 fracture morphologies of the composites2.2 microstructure analysis and toughening mechanismbetter mechanical properties of mullite fiber toughening alumina ceramics need good physical and chemical compati- bility between fiber and matrix5. when the interface is physical bonding, the bonding between fiber and matrix is mechanical joining-up. the interfacial bonding strength is lower and the fiber is liable to pull-out. when the interface is chemical bonding, bonding strength is higher because there is a thin layer reaction product which can improve mechanical properties of composites. but if interface bonding is excessive, the fiber is liable to damage which can result in brittle fractureof fiber. so the fiber loses its toughening effect.fig.1 is the fracture surface morphology of specimen 7. as can be seen from fig.1a and 1b, there are a lot of matrix par- ticles adhereing to mullite fiber which indicates that the bonding between matrix and fiber is proper6. when the composites break, mullite fiber can play an important role in inhibiting crack propagation by transferring stress imposed on matrix. it can be observed in fig.1c that crack deflection occurred because of high strength of mullite fiber and stress field generated by thermal expansion maladjustment between matrix and fiber. the crack propagation can consume muchfracture energy to alleviate stress at crack tip.the surface of mullite fiber pulled out is smooth as shown in fig.1d which illustrates that the interfacial bond- ing is proper, neither strong nor weak. pull-out of mullitethermodynamics calculation has shown that the trans- formation from mno2 to mno is feasible at 1250 . thechemical equation is as follows:no = n + 1 2(s)2（g )2(1)the relation between free energy and temperature can beobtained by looking up thermodynamic data in ref.7. the relationship isg 0 = 140.9 + 4.89103t ln t + 0.83106 t 2 508 t 0.133tr(2)when the temperature is 1523 k, free energy isg 0 = 5.5 kj/mol0(3)rit indicates that the reaction can occurs spontaneously. it shows that the composites sintered at 1250 should containmno.in order to study the component lying in interface, raw materials mixed homogeneously are supposed. in small zone between matrix and fiber, system of mno-3al2o32sio2- al2o3 is converted in proportion to system mno-sio2-al2o3(2.86 wt/% mno2, 2.64 wt/% sio2 and 94.5 wt/%al2o3). it can be discovered that the constituted site located in triangle of mnoal2o3, 3mnoal2o3sio2 and al2o38. so it can be known that the interfacial products includeding yinsen et al. / rare metal materials and engineering, 2011, 40(s1): 244-246mnoal2o3, 3mnoal2o3sio2 and al2o3. and the newproducts of mnoal2o3 and 3mnoal2o3sio2 should im- prove the interfacial bonding. this may improve the me- chanical properties of alumina ceramic composites.structure of mullite fiber which leads to fiber losting itshigh strength performances.3conclusions1) the mf/al2o3 composite is prepared with the addition of mno2 and nano-tio2. its flexural strength and fracture toughness can reach up to 568 mpa and 6.8 mpam1/2 which have increased by 12.7% and 52.4% respectively comparingwith mf/al2o3 composites comprised only of mf and al2o3.2) microstructure observations suggest that mn4+ and ti4+ diffused to interface as middle carrier improve interfacial bonding between mullite fiber and alumina matrix. the inter- facial products of mnoal2o3 and 3mnoal2o3sio2 play an important role in interfacial bonding. with crack propagation, fiber pull-out and debonding consumed much fracture energy to improve mechanical performances of mf/al2o3 composite.referencesabalcdsimg1hanlian liu, chuanzhen huang, jun wang et al. 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