Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-08-03
2001-10-23
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S472000, C416S24100B, C416S24100B
Reexamination Certificate
active
06306517
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to air plasma spray (APS) thermal barrier coatings (TBCs) such as are commonly applied to articles for use in high temperature environments. More specifically, the present invention comprises APS TBCs having a coherent, continuous columnar grain microstructure and a preferred vertical crack pattern which enhance the physical and mechanical properties of these coatings in ways which are intended to improve their resistance to spalling in cyclic high temperature environments.
APS TBCs are well known, having been used for several decades. They are typically formed from ceramic materials capable of withstanding high temperatures and are applied to metal articles to inhibit the flow of heat into these articles. It has long been recognized that if the surface of a metal article which is exposed to a high temperature environment is coated with an appropriate refractory ceramic material, then the rate at which heat passes into and through the metal article is reduced, thereby extending its applicable service temperature range, service longevity, or both.
Prior art APS TBCs are typically formed from powdered metal oxides such as well known compositions of yttria stabilized zirconia (YSZ) These TBCs are formed by heating a gas-propelled spray of the powdered oxide material using a plasma-spray torch, such as a DC plasma-spray torch, to a temperature at which the oxide powder particles become momentarily molten. The spray of the molten oxide particles is then directed onto a receiving metal surface or substrate, such as the surface of an article formed from a high temperature Ti-based, Ni-based, or Co-based superalloy, thereby forming a single layer of the TBC. In order to make TBCs having the necessary thicknesses, the process is repeated so as to deposit a plurality of individual layers on the surface of interest. Typical overall thicknesses of finished TBCs are in the range of approximately 0.010-0.055 inches.
The microstructure of a typical prior art TBC formed by APS deposition is described now by reference to
FIGS. 1
a
and
1
b.
FIGS. 1
a
and
1
b
are scanning electron microscope (SEM) photomicrographs of fracture surfaces through the thickness of a prior art TBC taken at magnifications of 50× and 3000×, respectively. The TBC has been removed by acid dissolution of the metal article on which it was deposited, and fractured to expose the characteristics of the resulting microstructure.
In order to make the TBC of
FIGS. 1
a
and
1
b,
the TBC was deposited using an apparatus comprising an air plasma spray torch positioned adjacent to a rotatable cylindrical metal drum for holding the articles to be coated. The plasma spray torch was positioned at a distance from the drum and perpendicular to its axis such that it could be moved along a line parallel to the axis. A TBC was deposited by rotating the drum containing a metal article, comprising an approximately 0.125 inch thick coupon of a Ni-based alloy, while the plasma spray torch was moved in a path parallel to the drum axis, so as to make one pass across the exposed top surface of the metal coupon. Each rotation of the drum carried the plasma-spray torch onto, across and off the top surface of the coupon and resulted in the deposition of what is termed herein as a “single sub-layer” or simply a “sub-layer” of the TBC. The “spray pattern” or “footprint” of the torch deposit as termed herein, is a cross-section of the spray pattern of molten particles having a finite size, e.g. one-half inch in diameter. The footprint may be circular or other shapes depending on the shape of the plasma-spray stream, the angle of the surface of the article being deposited to the stream, and other factors. The size of the footprint is largely a function of the distance of the article from the plasma-spray gun and the shape of the plasma-spray stream. Depending on the combination of drum rotation rate and torch traverse rate, multiple sub-layers may be deposited at a given spot as the torch footprint passes over in a single pass. Therefore, a “primary layer”, as termed herein, comprises the thickness of TBC of coating deposited in a single pass of the torch and may, and most often does, consist of a plurality of sub-layers. A “torch holiday”, as termed herein, occurs when the plasma-spray torch from which a TBC is being deposited moves away from the area on the article on which the TBC is being deposited so that cooling of the surface occurs, or when the article is moved out from under the plasma-spray torch, or when the motion of both the article and the torch causes the area being deposited to be moved away from the stream of plasma-sprayed particles.
Referring to
FIGS. 1
a
and
1
b,
the TBC was deposited in multiple passes, wherein the plasma spray torch was translated back and forth across the top surface of the coupon. During the passes, the drum upon which the coupon was secured was also rotated at a speed such that each area of the coupon being deposited with the TBC passed under the plasma-spray torch footprint a plurality of times during each pass, for example 4 to 5 times. This method of deposition produced layers in two respects, a primary layer resulted from each repeated translation of the torch across the surface of the substrate, secondary or sub-layers resulted from multiple rotations of the drum. In
FIGS. 1
a
and
1
b,
the TBC includes about 150 primary layers resulting from the combination of the rotation of the drum and the translation of the torch.
The TBC shown in
FIGS. 1
a
and
1
b
was made from −120 mesh YSZ powder having a composition of 8% yttria by weight with a balance of zirconia, and was deposited using a perimeter feed DC plasma spray torch, Model 7MB made by Metco Inc. The torch current was approximately 500 A, and the distance of the plasma spray flame to the surface of the article was approximately 3-5 inches. The deposition temperature measured at the back surface of the coupon was less than 260° C. The resulting TBC was approximately 0.050 in. thick. Applicants believe that the TBC shown in
FIGS. 1
a
and
1
b
is representative of prior art TBC generally.
FIG. 1
a
reveals a rough and irregular fracture surface, the reasons for which are more readily apparent from examination of
FIG. 1
b.
The fracture surface of
FIG. 1
b
is made up of what appears to be a stack of many discrete particles which do not share a common fracture plane, but which are rather fractured jaggedly along a path of what appears to have been weaker points within and between the individual particles. This jagged fracture path explains the rough appearance at the lower magnification of
FIG. 1
a.
The explanation for the appearance of this fracture surface is given below.
As noted above, the TBC comprises a plurality of layers as a result of the combination of rotation of the drum and translation of the torch and area of the torch footprint. These layers are formed from the stream of individual molten particles of YSZ, which impact either the surface of the coupon, or particles from a previously deposited TBC layer. Upon impact, molten particles are joined to the metal article in part by a physical mechanical interlocking of the molten particles within the features provided by the surface roughness of the article, or to previously deposited particles by a process known as micro-welding, which is described further below. Applicants have observed in
FIG. 1
b,
and in the examination of similar prior art TBCs, that the majority of these particles appear to be weakly bonded to particles in prior and subsequent sub-layers, and that micro-welding between sub-layers appears to be very limited; as evidenced by the distinct surfaces which still appear as demarcations between these sub-layers, such as are shown in
FIG. 1
b.
Referring to
FIG. 1
b,
the particles appear as irregularly shaped platelets, and exhibit internally a fine-grained, columnar structure which is formed in a direction generally perpendicular to the contact surface of the underlying platelet
Borom Marcus Preston
Gray Dennis Michael
Johnson Curtis Alan
Lau Yuk-Chiu
Nelson Warren Arthur
General Electric Company
Johnson Noreen C.
Jones Deborah
Stoner Douglas E.
Young Bryant
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