Fracture toughness of Alumina–zirconia compositesIshan Kossambe 13MCD0038

INTRODUCTION• Ceramic as a structural material

• Low fracture toughness

• Can be overcome by designing and preparing composites

Mechanisms to increase fracture toughness

1. T -> M phase transformation (tetragonal to monoclinic)

2. Crack deflection

3. Micro cracking of the matrix

Alumina Zirconia Composite

• Efficient phase transformation

• Development of micro cracks

• Compressive stress fields on alumina particles

Model Description• Consider a representative volume element

• Converting square into spherical element of same volume

• Volume fraction is given by

2b- inclusion spacing rf= radius of inclusion

• Analysis is done for determining stress intensity factor for different ratio of rm/rf when an uniform load P is applied.

Types of analysis• Two types of analysis are carried out

1. Mechanical load i.e. tensile load is applied on various samples

2. Thermal source, induced by different field temperature imposed to matrix and inclusion

Materials & Experiment

• Samples are prepared form SM8 Powder and zirconia stabilized by 3%mol Y2O3

• The composites were prepared using the powders TZ3Y20A, TZ3Y40A, TZ3Y60A and TZ3Y80A containing 20, 40, 60 and 80 wt% of alumina, respectively and denoted in the following as 8Z2A, 6Z4A, 4Z6A and 2Z8A.

• Samples are prepared by slip casting, then dried and sintered in air at various temperature between 1500oC to 1600oC.

• Alumina is present in α form and zirconia in tetragonal phase.

Experiment• Flexural strength of the material was determined by four point

bending.• Testing specimen in the form of bar obtained by cutting the

disc with rectangular c/s.Size= 2mm x 2.5mm x 25mm

• According to the recommendation reported in standard EN843-1

• Longitudinal edges of the specimen chamfered to 45o and then polished to eliminate edge cracks induced by cutting.

• The surface which is subjected to tensile stress was polished to eliminate edge cracks induced by machining.

• Finally to relax residual stress specimen is heat treated at 1200oC for 1hr.

Experiment• Stress intensity factor is calculated by vickers indentation

technique using the formula:

P-applied loadC-length of the crackH- Vickers hardness

RESULTS

• The radius rm of the sphere matrix was kept constant, while the radius rf of the inclusion and the crack length 2a was changed.

• Value rf is determine for every inclusion percentage using the formula

• While the rm is kept constant about 100µm

• Linear thermal expansion coefficient for two materials is

• Analysis for 2 phases• Material 1 = alumina as matrix, zirconia as a inclusion• Material 2= zirconia as matrix, alumina as a inclusion

• Analysis carried out for 3 different values of the crack length 2a

(rm/100, rm/50, and rm/25)

• And nine different percentages referred to the total volume of inclusions, in the range 5–45%.

• For marerial 1• KI increases with increase in the inclusion percentage• KI decreases(negligible) with increase in the crack length.• Young’s modulus and Vicker’s hardness decreases.

Behavior of ratio KI/K0 as the volume function of zirconia

• The thermal stress field due to the thermal expansion coefficient of the alumina lower than that of zirconia and simulated by imposing a field temperature T, both to the matrix and the inclusion, determines an increase of KI.

• Matrix subjected to radial compressive stress increase in the hoop stress increase in KI

• At 25°c, increase in KI is about 1%, which is negligible for higher % inclusion.

• Toughening effect associated with t m phase transformation• Uniform expansion in all direction• Not anisotropic• Neglecting tangential component of deformation

• Also because of the thermal component, radial compressive stress is induced which in comparison with the case of only tensile stress applied, it increases the KI value by 5 to 9% with increase of inclusion percentage.

• setting the volume of inclusion after transformation equal to that obtained when expansion is due to a uniform increase in temperature T

• For material 2• Setting the volume of the matrix after expansion equal to that

obtained when the expansion of the matrix is due to an uniform increase in temperature T

• KI decreases with increase in the inclusion percentage, and consequently, bending strength increases, furthermore, fracture toughness increases.

• Young’s modulus and vicker’s hardness increases. • On Increase in crack length, increase in the KI, but negligible.

High temperature analysis

• Phase transformation, thermal expansion coefficient considered.

• Young’s modulus constant.• For material 1• for same inclusion % (20 %)and crack length (rm/50), • for increase in temperature, KI increases with 1% , reducing the

fracture toughness.• For material 2• for increase in temperature, KI becomes practically independent.

• Analysis is not conducted at temperature higher than 750°c, Because monoclinic phase of zirconia becomes unstable at 800°c

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