On-Line NDE for Control and Modeling of Ceramic Processing
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Introduction The high-temperature structural properties of ceramics make them unique candidates for application in such systems as advanced gas turbines and other heat engines. Of concern, however, is the variability in fast fracture strength of structural ceramics which is due, in part, to the sensitivity of ceramics to very small (e.g., 20-50 /im) critical flaws and the difficulty in detecting and characterizing this type of flaw by nondestructive examination (NDE) techniques. The flaw sensitivity of ceramics and the typically wide variation in flaw sizes result in the situation illustrated in Figure 1, which is a frequency distribution of fast fracture strengths for a hypothetical structural ceramic with characteristic strength of 350 MPa and Weibull modulus1,2 of 5. The strength requirement, 250 MPa, for a particular application is shown. In this illustration, a sig0.008 >
0.006
100 200 300 400 500 600 TENSILE STRENGTH (MPa) Figure 1. Weibull frequency distribution for hypothetical ceramic materials with characteristic strength o z
0.015
s
0.010
OQ = 4 5 0 /
o
w 0.005 Surface finish — See green ceramics — Optical microscopy and holography — Dye penetrants — Ultrasonics
' Mechanical properties — Destructive tests — Ultrasonics — Acoustics • Porosity size and distribution — X-radiography — Ultrasonics — Small-angle neutron scattering — Metallography — Permeability
• Mechanical properties — Destructive tests — Proof tests — Ultrasonics — Acoustics 1
* Surface finish — Profilometry — Light scattering
Elemental analysis and distribution — See powders
Figure 3. General processing flowsheet and inspections considered.
these inspections should be performed in a given product line, but all have been considered in assessing the relative priorities for the inspection techniques. It is anticipated that a limited number of key inspections would be identified for any given product line. This article addresses the role of NDE as a tool for process control to achieve the desired high quality. Traditionally, the role of NDE in structural materials has been to detect (and more recently to characterize) flaws, generally in finished products and components. Under optimum conditions, the NDE information gained from such final examination is fed back into the production system to correct problems that lead to rejection of materials and components. Such an application is important and should be strengthened as a significant element of process control. However, an equally important (if not more important) role for NDE technology is materials characterization during and after processing as well as flaw detection (for feedback to process control) at interim stages of fabrication. Judicious use of NDE in each of these areas can do much to assure the economical fabrication of high-reliability ceramics with reduced rejections. As shown in Figure 3, the several stages of fabrication include preparation and evaluation of raw material (powMRS BULLETIN/APRIL 1988
ders, sintering aids, and processing aids), body preparation and f
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