FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

1. INTRODUCTION
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Ceramics such as silicon nitride, silicon carbide, and zirconia have excellent
corrosion resistance, low thermal conductivity, high wear resistance, and good
strength at elevated temperatures. These properties have generated considerable
interest in the use of these ceramics in energy conversion systems, e.g., diesel
engines, turbines, and heat exchangers.
The hard and brittle nature of ceramics, however, inhibits the release of
stresses at flaws under load by plastic deformation. This implies that a very
small flaw can cause a catastrophic failure in ceramics. Therefore, flaws in the
size range of 20-100 vim are considered as "critical" in ceramics for such
applications as turbine blades.
The reliable use of ceramics as structural parts requires nondestructive
evaluation techniques which are capable of detecting and characterizing such
small defects. Microfocus x-radiography and high frequency ultrasonics have been
found^ ' to be the most suitable NDE methods for ceramics.
This paper describes a high frequency ultrasonic system with computer controlled
data acquisition and analysis capability and its application in the detection and
characterization of microscopic surface cracks in silicon nitride.
2. EXPERIMENTAL STUDIES
2.1 Material and standard cracks
Hot pressed silicon nitride is one of the most promising materials for high
temperature structural applications. The majority of failures in this material
originate from surface cracks introduced during grinding and machining operations.
These produce arrays of semi-elliptical cracks with random inclination to the
surface, but a preferred alignment parallel to the direction of motion of the
abrading particles ,. A Knoop indentation technique was used to simulate
surface machining cracks. In silicon nitride the Knoop indentation produces
sharp semi-circular cracks with a mouth opening of the indentation less than one
fifth of its depth, a, and its surface length about 2a.
Knoop indentations in the depth range of 10 to 300 ym were introduced on a
polished surface of a hot pressed silicon nitride (NCI32) plate. These cracks
were carefully measured under an optical microscope prior to ultrasonic
examination.
2.2 High frequency ultrasonic system
Since the critical flaw size in ceramics is very small and the high stiffness to
density ratio generally gives comparatively high values of acoustic velocity and
hence long wavelength at conventional ultrasonic frequencies, it is necessary to
use high frequency ultrasonic waves.