Optics: measuring and testing – Inspection of flaws or impurities
Reexamination Certificate
1999-06-11
2001-09-04
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
C356S369000
Reexamination Certificate
active
06285449
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an optical instrument and method for the inspection of ceramic materials and is particularly directed to the automated, real-time detection of defects and microstructural changes in ceramic and ceramic composite materials in bulk form or as a coating.
BACKGROUND OF THE INVENTION
Ceramic and ceramic composite material components and ceramic coated components are attractive candidate materials for improved efficiency power systems over their metal counterparts because of the lighter weights and higher strengths at elevated temperatures of the ceramic-based components. For example, monolithic silicon nitride vanes and turbine blades and silicon carbide composite combustor liners are currently being evaluated in high performance gas turbine engines. Highly stressed areas of ceramic components, such as turbine blades, include the near-surface regions, particularly the blade trailing edges. Accumulation of elevated temperature damage and microstructural changes over time in these near surface regions may lead to premature failure.
When ceramic and ceramic composite materials and ceramic coatings are loaded at temperatures above approximately one-third of their melting/ablation temperatures, these materials and coatings undergo creep phenomena. These creep phenomena may include: development of cavities or voids along grain boundaries, development of voids in grain boundary glassy phases, formation of wedge cracks at grain boundary triple points, linkage of cavities to form grain facet sized cracks which then link to form microcracks, and linkage of microcracks to form macrocracks until failure occurs when a critical flaw size is reached. Microstructural changes such as grain growth, phase changes, dissolution and precipitation phenomena, oxidation, scaling and spalling may also occur. Coatings subjected to thermal and mechanical stresses may develop internal cracks as well as delaminations at the coating substrate interfaces, leading ultimately to spalling.
Structural ceramics, e.g., silicon nitride and silicon carbide ceramic and ceramic composite materials, are increasingly being used for high temperature gas turbine applications in not only vanes and turbine blades, but also combustor liners because of their high thermal stabilities and elevated temperature strengths. In these types of applications, the critical regions of a component experiencing the highest stresses are frequently the surface and near-surface regions to a depth on the order of 200-300 microns. Increased depths to approximately 500 microns are also of interest in the structural analysis of these ceramic-based materials. The presence of large stresses on the surface and in near-surface regions can lead to the development of creep cavities and wedge cracks which ultimately link to form microcracks until a dominant microcrack results in rupture. The detection of damage accumulation in structural ceramics, particularly Si
3
N
4
and SiC creep tested in air is extremely difficult. Periodic surface inspection of Si
3
N
4
components undergoing high temperature loading reveals that oxidation products and the formation of glassy phases frequently obscure the presence of surface flaws. Indeed, attempts to use hardness indentations to measure the development of creep strains in creep flexure bars have proved unsuccessful due to glassy phases obscuring the reference indents during the creep test.
The present invention addresses the aforementioned limitations of the prior art by allowing for the periodic “imaging” of a ceramic component in its operating environment, i.e., as installed in an operating component of a machine, and allows the material at the surface as well as below the surface to be assessed for damage. This inventive method and apparatus is particularly adapted for use in detecting defects and microstructural changes in ceramic and ceramic composite materials in bulk form as well as in ceramic coatings such as used in nozzles, vanes, rotor blades and combustor lines used in high temperature gas turbine applications.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to detect defects and microstructural changes in ceramic and ceramic composite materials on a real-time basis using an automated approach.
It is another object of the present invention to detect surface and subsurface defects and microstructural changes that develop in ceramic and ceramic composite materials either in bulk form or in the form of a coating when exposed to high temperatures and constant, cyclic, random, residual or shock loading.
Yet another object of the present invention is to use a combination of optical microscopy and elastic optical scattering to detect surface and subsurface creep damage in ceramic and ceramic composite materials and coatings during on-site inspection of the ceramic component without removing the component from the item in which it is installed.
A further object of the present invention is to increase the accuracy and reliability of detection of the presence and distribution of small surface and subsurface flaws in ceramic bulk compositions and ceramic coatings using plural detectors and statistical image processing employing quantative discrimination.
This invention contemplates apparatus for detecting and characterizing surface and subsurface defects and microstructural changes in translucent ceramic-based materials and coatings, the apparatus comprising a tunable wavelength laser providing an output beam of high intensity light; optical means for directing the output beam onto a specimen of a ceramic-based material or coating and for receiving light scattered from the specimen, the optical means including polarizing means for controlling the polarization of the output beam prior to incidence upon the specimen, where the output beam wavelength allows the beam to penetrate the specimen from its surface to a given depth of the specimen, and wherein the given depth corresponds to the upper wavelength limit; a detector system with means for receiving light scattered over two scattering angles from the localized area of the specimen's surface or subsurface being investigated and providing an output signal representing light scattered from the localized area; two detectors for receiving light scattered over two scatter angles for providing two output signals representing light scattered from two scatter angles; and display processing means coupled to the first and second detectors for processing the first and second output signals and visually displaying defects and microstructural changes on and below the surface of the specimen.
REFERENCES:
patent: 4286293 (1981-08-01), Jablonowski
patent: 4725139 (1988-02-01), Hack et al.
patent: 5149978 (1992-09-01), Opsal et al.
patent: 5426506 (1995-06-01), Ellingson et al.
patent: 5689332 (1997-11-01), Ellingson et al.
Wave Optics Inc. informational report on “Polarization Maintaining Fiber,” pp. 12-13, from www.Waveoptics.com website, undated.
Ellingson William A.
Sun Jiangang
Todd Judith A.
Emrich & -Dithmar
Pham Hoa Q.
University of Chicago
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