Strength Improvement in Transformation Toughened Ceramics using Compressive Residual Surface Stresses

We have made a theoretical study of the bend strength of flat, multilayered alumina−zirconia composites. We used the linear theory of elasticity to calculate the stresses in materials subjected to a three‐point bending test, including the thermal residual stresses that result from the fabrication process. We studied symmetrically and non‐symmetrically layered composites with a variable number of layers, with the focus on the optimization of individual layer thicknesses to get the highest composite strength. Using typical strength data for particulate composites from the literature, we found that it is sensible to optimize the composite with up to seven layers as taking even more layers does not further improve the composite strength significantly. Furthermore, there is a qualitative difference in the total stress distribution through the sample thickness in the case of optimized and non‐optimized structure, respectively.

Section: Introductionmentioning

confidence: 99%

Optimization of the Bend Strength of Flat‐Layered Alumina−Zirconia Composites

Ambrožič

Kosmač

2007

“…The strengthening of oxide ceramics by phase-transformationinduced stresses has been studied in a trilayer system of Al 2 O 3partially stabilized ZrO 2 (PSZ). [1][2][3][4] However, this approach has the disadvantage of instability of the ZrO 2 monoclinic phase above 1000°C; i.e., structural benefits above 1000°C are marginal. Damageresistant, layered composites with surface compressive stresses have been synthesized in ceramic systems.…”

Section: Introductionmentioning

confidence: 99%

Failure Prediction Maps for a Model Al₂O₃|c‐ZrO₂/Al₂O₃|Al₂O₃ Brittle Polycrystalline Trilayer Composite

Barnett-Ritcey

Nicholson

2003

Four failure zones are defined for the three‐layer model, Al2O3|c‐Zr(8Y)O2/Al2O3|Al2O3 ceramic composite in flexure as a function of inner‐layer composition and geometry. An optimum flexural strength is predicted based on thermal expansion coefficients and elastic moduli. Tunnel‐crack threshold and flexural strength predictions are supported by experimental evidence and displayed in failure‐map form. Many three‐dimensional surface graphs and cutaways of such are used to explain the ramifications (in terms of laminate strength) of choosing various layer thickness values and inner‐layer compositions for a representative trilayer laminate system.

Measurement of Residual Stresses in Oxide–ZrO₂ Three‐Layer Composites

Virkar

Jue

Hansen³

et al. 1988

Zr02 and MgO-Zr02 layered composites were fabricated in such a way that the outer layers of bar-shaped specimens consisted of the oxide (A1203 or MgO) and unstabilized Zr02, while the bulk consisted of the oxide and partially stabilized Zr02. During cool-down from the sintering or the annealing temperature, the outer layers expanded because of the tetragonal-monoclinic transition in Zr02, thereby creating compressive stresses in the outer layers and tensile stresses in the bulk. Residual stresses were determined using a strain gage technique in which a strain gage was mounted on one face while the opposing face was incrementally ground off. Measurement of the strain as a function of thickness permitted the evaluation of residual stresses using pertinent equutions,from simple beam theory. 84112 84119 T Is well-known that many transformaItion-toughened zirconia-based ceramics exhibit an increase in strength after surface grinding. This strength increase is attributed to the increase in volume which occurs because of the tetragonal .+ monoclinic transition in zirconia particles in the near surface regions. Specifically, upon transformation to the monocfinic polymorph, the surface region suffers a net increase in volume which is constrained by the bulk. As a result, large surface compressive stresses are developed, leading to an enhancement in strength. Green' was able to show that surface compressive stresses can also be developed by an annealing procedure in which stabilizing oxides from the near surface regions are removed by a diffusion step. As the diffusion distances are typically on the order of a few micrometers for a few hours anneal at -1400°C, the zone of compression is also quite small.