Am Cera Sn.86|121225-272003 journa Martensitic Relief Observation by Atomic Force microscopy in Yttria-Stabilized Zirconia Sylvain Deville and Jerome Chevalier National Institute of Applied Science, Materials Science Department(INSA-GEMPPM) Associate Research Unit CNRS 5510, 69621 Villeurbanne. france The tetragonal to monoclinic(I-m) phase transformation of I. Experimental Procedure zirconia has been the object of extensive investigations of the past 20 years and is now recognized as being of martensitic Samples were processed using yttria-stabilized zirconia powder nature. However, martensitic transformation has only been (3Y-TZP, Tosoh TZ-3YS. Tosoh Corp, Tokyo, Japan).Green observed by transmission electron microscopy or indirect bodies. after uniaxial cold pressing at 10 bar. were isostatically methods, Though the benefit on the fracture toughness and cold pressed at 1300 bar and finally sintered at 1773 K for 5 h in crack resistance was the main interest, the transformation is air. The obtained plates were machined to small bars. The side of now considered for its consequences on the degradation of the each bar on which analysis was to be performed was mimor material. The use of atomic force microscopy reported lier polished with diamond slurries and pastes, reaching surface allowed the observation of the first stages of martensite r roughness (Ra) values as low as 2 nm. Samples were thermally growth and of new martensitic features. etched for 18 min at 1673 K, i.e., 100 K below the sintering temperature to form slight thermal graves of grain boundaries without affecting the physical properties. The agi Ing treatments L. Introduction were conducted in an autoclave (Fisher Bioblocl France)at 383 K, ensuring a 100%o water vapor atmosphere. The tetragonal to monoclinic (I-m) phase transformation of surface was observed zirconia has been probably the most studied phase transfor- surface was observed using AFM(D3100, Digital Instruments)in contact mode with a scanning speed of 10 ums mation among ceramics, since the transformation toughening There is now regained interest from a biomedical point of view since degradation and rupture of hip joint prostheses related to the IlL. Results and Discussion transformation have been reported recently. Phase transformation A transformed zone is shown in Fig. 1. The aFm deflection occurring with time at the surface of zirconia implants is now referred to as aging. Zirconia is indeed retained in its tetragonal image provides a very clear image of the surface, showing typical metastable structure at ambient temperature after processing, and martensitic self-accommodating variants within almost each grair of the zone. after I h of aging treatment. Most of the theories phase transformation to its stable monoclinic phase might occur developed so far predict the apparition of a regular pattern of derlying chemical mechanism is still a point of debate, the orientation of the gran ma n, related to the crystallographic transformation is now commonly recognized as being of marten- sitic nature. i.e., diffusionless and athermal, involving shape change dominated by shear and occurring at high speed. Indirect methods such as X-ray diffraction(XRD) or differential thermal analysis(DTA) provide information on the global behavior of the material, Further progress into understanding the nucleation and growth(NG) mechanism was accomplished by using optical interferometry, and martensitic relief observations were also reported with use of interferometry and Nomarski interference One atomic force microscopy(AFM)observation, performed at high scanning speed and in tapping mode, has confirmed the NG nature of the transformation. Transmission electron microscopy (TEM) on thin foils has confirmed". 5.I6 the presence of mono- clinic laths within the grains. The next step in the mechanistic understanding is the observation of the transformation initiation Thus, AFM observations brought new features to light, associated with the relief, that were not accessible by other conventional D. Marshall contributing editor inscript No, 10219 Received May 16, 2003: approved July 17, 2003 devilleeins, Fig. I. AFM deflection image. The observed relief is undoubtedly characteristic of a martensitic transformation
Communications of the American Ceramic Societv Vol. 86. No, I consistent with the crystallography, since two perpendicular planes of the tetragonal system are crystallographic Another point of debate was whether a grain would transform all at once. due to high-speed transformation, or if the martensitic plates would progressively invade the grain. Some TEM observa- ions.of partially transformed grain have already been reported However, due to the intrinsic nature of TEM, the observed samples might be considered as being in a modified physical and chemical ing grains, neither on their side nor above or below them. Thus, the extrapolation to massive samples might be questionable. This henomenon is reported here for the first time on the surface, by means of direct observations, as shown in Fig 3, where the first stage of growth of some monoclinic plates is observed initially at 500nm the grain boundary and propagated later on to the adjacent part of the grain. This clearly proves that the transformation of a single grain does not occur instantaneously, completing previous TEM Fig. 2. AFM height image of a grain(vertical scale 80 nm), Martensitic observations. It was also expected from calculations and stress variants crossing at right angles are observed. Arrows indicale the junction analysis , that the transformation would start at a triple junction of grain boundaries. due to higher residual stresses, though no observations have been reported so far. This is observed in Fig. 3 Further statistical analysis will be performed to confirm this tions of the martensitic relief for each grain. The image of Fig 2n Finally, several theoretical studies have shown the propagation observations here clearly show the apparition of different orienta- echanism of martensitic plates from one grain to another one. was acquired on a single grain and at very low scanning speed considering that high local stresses appear at the junction of the re.The very regular martensitic laths due to shear strain accommodation. Thus. the provide maximum definition of the pictu and different variants apparition of a single martensitic variant within a grain might of various orientations and sizes are present. Though the grain is provoke the transformation of another variant of the neighboring not very large, several laths within just a grain appeared during the grain, providing enough stresses-related energy to overcome the aging treatment. Moreover, it is worth noticing that the three main nucleation barrier. Considering this mechanism, the orientation of variant pairs are intersecting at a right angle. This observation is most of the transformed laths is different from that of the initial 300nm Fig. 3. AFM deflection images at four different aging times (20 25 30, and 40) min at 383 K). The growth and progressive invasion (arrows)of martensitic variants within a single grain are clear. DO nm Fig. 4. AFM deflection image. A detailed zone is extracted on the right, showing a transgranular martensitic variant
December 2003 Communications of the American Ceramic Sociery 2227 one. However, an interesting feature is characterized here, in Fig F F Lange, G L. Dunlop, and B side of the grain boundary. Theoretically, it has been shown/7 4. A transfor med lath starts at one gra d goes through the oth Transformation Toughened / -,O, Materials at 250C./,Am. 21237-40019 al domains. This would explain very nicely the observed Zirconia at 370 K under a Low Applied Stress, Mater. Sci. Eng- A297. 26-30 "R Guo. D, Guo, D. Zhao. Z. Yang, and Y. Chen, "Low Temperature Ageing in IV. Conclusion Water Vapor and Mechanical Properties of ZTA Ceramics, Mater. Left, 86 III Observations of martensitic relief of a few dozens of nanome. ers in yttria-stabilized zirconia by atomic force microscopy in Its Role in transformation“PmM.4146357 contact mode are reported here for the first time. The different F. Lange. Transformation Toughening. Part 1: Size Effects Associated wi features showed here fit very well the model of martensitic the Thermodynamics of Constrained Transformations. "J. Mater. Sci. 17,255-62 transformation. The junction planes angle correspondence and the (1982) progressive invasion of martensite laths with different orientation J. Chevalier. B, Cales, and J M Drouin. "Low-Temperature Aging ef Y-TZP was observed. Transgranular monoclinic variants also appeared, as Ceramies,J. Am. Ceram. So, 82 1812150-54(1999) M. Hayakawa. K. Adachi,and predicted theoretically by some authors. Using AFM should bring bservations of martensitic features that were not accessible by M例x22m,A other techniques uroda. and M. Nihe,"Surface Relief Associated with Isothermal Martensite in Zirconia-3-moler-Yttria Ceramies Ohserved by Atomie References Force Microscopy, J, Am. Ceram Soc., 82 1ID12921-23(1999) L W: Chen and Y. H. Chiao, "Martensitic Nucleation in /ro). Acta Metall, 31 D. J. Green, R H J Hannink, and M V, Swain, Trnsformation Toughening of 01627-38(1983 cronies. CRC Press, Boca Raton FL 198 I Y. H Chiao and I. w. Chen, ""Martensitic Growth in Z,-An in Situ, Small A Gi. Evans and A H. Heuer. "Review-Transformation Toughening in Ceram- Particles, TEM Study of a Single Interlace Transformation." Acta Metall. Mater. 38 Martensite Transformation in Crack-Tip Stress Fields, J, Am, Ceram, Soc. 6.3 611163-7401990 6]241-48(1980 E P. Butler and A. H. Heuer, " X-ray Micmanalysis of Zro, Particles in ZrO,- Toughened AL-O,, "J, An. Cerim. So, 65 1121C-206-C-207(1982) "L Gtemillar. T. Epicier, J. Chevalier, and G, Fantozzi. "Microstructural Study 3|61485-502(1995) ot Silica-Doped Ceramics, Acta Mater, 48. 4647-52 200 H. Tsubakino, M. Hanamoto, and R Notato. "Tetragonal to Monocline Phase S. Schmatder and H. Schubert."Sign of Intemal Stresses for the Transformation dunng Thenmal Cycling and Iso Ageing in Yttria-Partially Martensitic Transformation in Yitria- Stabilized Tetragonal Zirconia polycrystals during Degradation. "J. An. Ceran. Sec., 69 [71 534(1986)
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