Chemistry: analytical and immunological testing – Process or composition for determination of physical state... – Surface area – porosity – imperfection – or alteration
Patent
1996-01-11
1999-06-29
Alexander, Lyle A.
Chemistry: analytical and immunological testing
Process or composition for determination of physical state...
Surface area, porosity, imperfection, or alteration
436 56, 422202, G01N 3320
Patent
active
059168110
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of determining the level of spallation of protective surface layers or coatings from an article, such as an engine component for example, as a result of repetitive heating and cooling. The invention also relates to an apparatus suitable for use in the above-mentioned method.
In view of the desirability of providing components for various engineering applications which have improved wear resistance and corrosion resistance, much attention has been focussed in recent years on the provision of protective surface coatings for such components. There are a variety of coatings currently available which reduce attack on the component material by aggressive reactive species such as oxygen, sulphur, carbon, nitrogen, halogens or sodium and/or reduce corrosive damage which results from exposure to high temperatures.
One of the simplest forms of protective coating is an oxide scale which naturally forms on many metal alloys as a result of exposure to air or other oxidising media especially at high temperatures. A scale such as Cr.sub.2 O.sub.3, Al.sub.2 O.sub.3 or SiO.sub.2 isolates the bare alloy from the environment and thereby slows down further corrosive attack. Other known coatings include those based on aluminium, in particular a simple aluminide applied by an aluminizing process or a modified aluminide in which the aluminium is combined with another element such as chromium, platinum or palladium or a more complex metal overlay coating of the general formula MCrAlY where M is another metallic element. Protective coatings based on ceramic materials have also been developed, based particularly on zirconia.
These latter coatings are useful in high temperature applications and form a part of the known thermal barrier coatings which consist of an MCrAlY underlay or "bond" coat and a zirconia overlay.
Apart from the above, protective surfaces can also be generated on a substrate by exposure of the surface to a high energy beam such as high energy ions, an electron beam or a laser beam.
Regardless of the type of coating, protective layers have, to a variable degree, the tendancy to flake off or spall from the coated article over time when the article is subjected to high temperatures and particularly when subjected to repeated temperature cycles, as would normally be the case with an engine component for example.
To determine the resistance to spalling of any particular coating under thermal cycling a coated article or specimen must be subjected to repeated heating and cooling cycles and the loss of coating due to spalling measured at various time intervals during the test. Such cyclic testing has been described by Singh Raman and Gnanamoorthy in Materials at High Temperatures 10 No 3 1992, 171-176, Probst and Lowell in Journal of Metals, October 1988, 18-21, Lowell et al in Oxidation of Metals 36 Nos 1/2 1991, 81-111 and by Wortman et al in Materials Science and Engineering A121 (1989), 433-440.
In the first three of the above-mentioned reports the amount of spalling is evaluated by determining the weight change (generally the specific weight change defined as weight change per unit of surface area) at various time intervals during the test e.g. after 10, 20 or 50 thermal cycles. Such weight change values can be compared with weight change in tests of similar duration at constant temperature without temperature cycles which give an indication of the effect due to high temperature exposure e.g. corrosion. However results obtained in this way can be misleading. For example, although spalling might result in weight loss, corrosion will usually result in weight gain due to the uptake of species from the environment. Therefore the relationship between weight change and material loss, the actual variable one wants to measure, can be very complicated. In addition the response to exposure of a spalled area may be different to that of an intact area and the effect of exposure at low temperature during the temperature cycle may not be negligible. Other complications arise when samples are not
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Macchi Giovanni Michele
Stroosnijder Marinus Freder
Alexander Lyle A.
European Economic Community
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