Ceramic Composite

1. Fracture toughness
of Alumina–zirconia
composites
Ishan Kossambe 13MCD0038

2. INTRODUCTION
• Ceramic as a structural material
• Low fracture toughness
• Can be overcome by designing and preparing composites

3. Mechanisms to increase fracture
toughness
1. T -> M phase transformation (tetragonal to monoclinic)
2. Crack deflection
3. Micro cracking of the matrix

4. Alumina Zirconia Composite
• Efficient phase transformation
• Development of micro cracks
• Compressive stress fields on alumina particles

5. Model Description
• Consider a representative volume element
• Converting square into spherical element of same volume

6. • 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.

7. 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

8. 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.

10. 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.

12. Experiment
• Stress intensity factor is calculated by vickers indentation
technique using the formula:
P-applied load
C-length of the crack
H- Vickers hardness

13. 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

14. • 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%.

16. • 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.

17. Behavior of ratio KI/K0 as the volume functionof zirconia

18. • 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.

19. • 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

20. • 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.

21. High temperature analysis

22. • 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

23. Thank You