Ceramic heat-insulating layers with club-structure

Details Ceramic heat-insulating layers with club-structure Ceramic heat-insulating layers with club-structure

2000-03-07

2001-12-25

Copenheaver, Blaine (Department: 1771)

Stock material or miscellaneous articles

Pile or nap type surface or component

Particular shape or structure of pile

C428S119000, C428S144000, C428S210000, C428S629000, C428S632000, C428S633000, C428S655000, C428S660000

Reexamination Certificate

active

06333090

ABSTRACT:

The present invention relates to a process for vapor-depositing zirconium dioxide on ceramic or metallic substrates, and the heat-insulating layers obtainable by such process.
A wide variety of ceramic heat-insulating layers for high-temperature applications, for example, on gas turbine components, which are prepared by electron-beam evaporation, preferably of zirconium dioxide having a columnar microstructure, have already been described and protected. The prior art comprises the three essential elements of such electron-beam vapor-deposited heat-insulating layer systems, consisting of a metallic substrate which can form a dense alumina layer (corundum) by oxidation, the alumina layer itself, and a discontinuous ceramic heat-insulating layer having a characteristic columnar structure which allows for the accomodation of lateral expansions of the substrate. Such expansions may arise from the different thermal expansions of the substrate and ceramic heat-insulating layer, or may be caused by elastic or plastic deformations in the substrate, and by external actions (e.g., particle erosion). The functionality of these layers is retained if, in addition to sufficient adhesion, the spaces between the columns which serve the function of expansion gaps are retained. Therefore, the latter must not be disturbed or even removed by sinter bridges or large upsets of the substrate.
The condensation of vapor particles on a solid substrate occurs in three partial steps:
loose binding of the condensating atoms and radicals as so-called “adatoms” with relatively high mobility along the surface;
diffusion of the adatoms to reach a low-energy site on the surface during their lives, or re-evaporation in the case of a vain search for such a site;
bulk diffusion of incorporated adatoms into the final atomic union to form crystal nuclei, and growth thereof to give crystal columns.
Now, the more the melting point of the substance and the substrate temperature during the vapor-depositing process approach, the greater is the mobility of the atoms, and the denser the growing layers thereby become. Depending on the homologous temperature of the layer substances, different characteristic layer structures are generated. These layer structures have been described many times. The best-known is the three-zone-structural model by B. A. Movchan et al., “Study of the structure and properties of thick vacuum condensates of nickel, titanium, tungsten, aluminium oxide and zirconium dioxide”, Fiz. Met. Metalloved. 28 (1969), p. 83-90. In zone 1, at homologous temperatures, T
s
, of up to about 0.26 (for oxides, and up to 0.3 for metals), there are formed very thin acicular discontinuous crystallites with a high defect density. In this form, they are unsuitable for high thermal and mechanical stresses. As already mentioned for the ZrO
2
growth of the heat-insulating layers, zone 2 (T
s
of up to 0.45) is characterized by predominating surface diffusion. The layers grow to contain continuous columns, the thicker stems growing at higher temperatures. Between the stems, there are points of contact as well as gaps which confer a certain pseudo-plasticity to the layers. However, the free volume of the gaps decreases as the temperature is increased. Above 0.45 T
s
, bulk diffusion predominates, resulting in dense structures.
Now, if the substrate is continuously rotated during the layer growth in the temperature range of predominating surface diffusion, for example, in order to provide it with an all-around coating, these layer structures are subjected to another modification of their shapes. Due to more effective growth selection mechanisms, the stem structure is driven to adopt a coarser shape and is thus imprinted a <001> texture. As a rule, the <110> direction or <100> direction is in the direction of the rotational axis, irrespective of a left-handed or right-handed sense of rotation. Repeated reversal has no marked influence, either. The stem diameters do not only become thicker through increasing temperatures, but the stems may similarly be rendered coarser by increasing the rotational speed as demonstrated in studies by U. Schulz, “Wachstum, Mikrostruktur und Lebensdauer von elektronenstrahlaufgedampften Wärmedämmschicht-Systemen für Turbinenschaufeln, Shaker Publishers, Aachen, Germany (1995), p. 1-133.
For example, the actually observed layer structures in heat-insulating layers of zirconium oxides exhibit mostly parallel stem structures in a fine design if vapor-deposition has taken place in the stationary mode at about 0.4 T
s
. Mostly parallel stem structures, although in a coarse design, occur upon rotation at considerably increased temperatures, e.g., when 0.55 T
s
and 12 rpm, or 0.46 T
s
and 30 rpm is achieved. At essentially lower temperatures/rotational speeds (for example, 0.4 T
s
and 12 rpm), branched lean stems will occur which do not extend through the entire thickness of the layer. In a medium range (e.g., 0.4 T
s
and 30 rpm, or 0.46 T
s
and 12 rpm), growth selection mechanisms will greatly prevail which will give rise to many thin stems at the beginning of the layer growth, with a drastical reduction of the number thereof in the further coarse of growth, and growth of the diameter of the remaining stems. This gives rise to a conical and in part slightly club-like structure, especially with those columns which have continuously grown from the base to the surface. The thin necks of the clubs adhere to the substrate. The great majority of the columns grown from nulcei in the adhesion region will not reach the surface of the layer, but terminate growth in an early or medium stage.
On a commercial scale, the adjustment of the desired layer structures is achieved by a well-aimed control of the main parameters: substrate temperature, total gas pressure in the vapor-depositing vessel, as well as oxygen partial pressure and rotational speed of the substrates during the coating process, as reported by Rigney et al., “PVD thermal barrier applications and process development for aircraft engines”, NASA conference publication, 3312 (1995), p. 135-149, and U.S. Pat. No. 5,350,599. Prior to coating, the surface condition of the samples is one of the decisive criterions for the adhesion and formation of the layer structures.
An improvement of the adhesion of the heat-insulating layer by a particularly even design (polishing) of the metallic substrate is described in U.S. Pat. No. 4,321,310. On this surface, a thin corundum layer is produced by thermal oxidation prior to coating which has a similarly even design as the subtrate. On top of it, heat-insulating layer is vapor-deposited which. must have a vertical orientation of the stems in accordance with the patent. Of course, on even substrates consisting exclusively of corundum, a well-adhering heat-insulating layer can also be applied by direct electron-beam physical vapor deposition on corundum, as shown by studies by Schmücker et al., “Haft-mechanismen in ausgewäahlten EB-PVD-Wäarmedämmschichtsystemen”, Fortschrittsberichte der Deutschen Keramischen Gesellschaft, 10 (1995), 4, p. 379-384.
The roughness of the substrate surface initiates the formation of differently ordered and disturbed layer structures, as shown by Rigney et al. (loc. cit.). That is, in the reverse case, parallel growth of the stems is promoted by a very even (polished) surface. However, the disturbances of the columns which may be useful, for example, under stress-geometric aspects, are detrimental to adhesion since they are bought at the expense of a greatly interlocked Me/MeO intermediate layer which functions as the source of additional exalted stress in the adhesion region under alternating heat stresses. This favors premature breakage failure within the ceramic layer near the Me/MeO transition. Thus, a certain amount of roughness must not be exceeded at this critical place for the sake of a good adhesion.
U.S. Pat. No. 5,087,477 describes a process for the coating of metallic substrates, such as gas turbine blades made of Ni-based super-alloys, with oxide-ceramic

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