Densification behavior of dynamically shock compacted AI 2 O 3 /ZrO 2 powders synthesized through rapid solidification
- PDF / 1,721,611 Bytes
- 7 Pages / 598 x 778 pts Page_size
- 93 Downloads / 156 Views
I.
INTRODUCTION
T H E incorporation of submicron zirconia crystallites into ceramic matrices has proven to be an effective method for improving the fracture toughness of structural ceramic materials.tl.2,3] One mechanism which operates in these materials is transformation toughening541 A martensitic phase transformation from t-ZrO2 to m-ZrO2 is initiated by the shear stresses which are generated by a crack propagating in the material. The shear strain which is a product of this transformation places the surface of the crack in a compressive state which opposes propagation of the fracture. A second mechanism which operates in zirconia-containing materials is microcrack tougheningY1 Upon cooling from peak sintering temperatures, larger zirconia crystallites will transform to the thermodynamically stable m-ZrO2 polymorph. The shear and dilatational stresses which accompany this transformation generate microcracks in the material surrounding the transformed particle. These microcracks have the ability to absorb fracture energy and, in turn, enhance the fracture toughness of the material. Zirconia-toughened materials have been developed with significantly enhanced values of fracture toughness. The critical stress intensity factor (K~c)of Mg partially stabilized zirconia (Mg-PSZ), where t-ZrO2 crystallites are incorporated in a c-Zr02 matrix, has been reported in excess of 15 M P a ~ .[61The K~c of zirconia-toughened alumina (ZTA),
JOHN FREIM and J. McKITTRICK are with the Department of Applied Mechanics and Engineering Sciences and Materials Science Program, University of California, San Diego, La Jolla, CA 92093-0411. W.J. NELLIS is with the Lawrence Livermore National Laboratory, Institute of Geophysics and Planetary Physics and H. Division, University of California, Livermore, CA 94550. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICALAND MATERIALSTRANSACTIONSA
where t-ZrO2 crystallites and microcrack-generating m - Z r O 2 crystallites are incorporated in an A1203 matrix, has been reported in excess of l0 MPa~/m .t7] These values are significantly higher than those which are inherent in singlep h a s e A1203 (3.5 MPa~/m ) and m - Z r O 2 (1.1 MPa~-m )581 The PSZ and ZTA materials possess inherent limitations, however. Fabrication of Mg-PSZ requires intricate sintering cycles, and the metastable c-ZrO 2 matrix is subject to degenerative subeutectoid reactions.[g] Considering ZTA, the microcracks which are generated by the t ~ m-ZrO2 phase transformation impose a limitation on the amount of mZrO 2 which can be successfully implemented into ZTA materials, tx~ The maximum diameter at which t-ZrO2 is thermodynamically stable ranges from - 1 0 nm in unconstrained crystallites, to - 8 0 nm in crystallites constrained by c-ZrO2 (PSZ), to - 0 . 4 / x m in crystallites
Data Loading...
