Process for coating with ceramic vaporizing materials

Details Process for coating with ceramic vaporizing materials Process for coating with ceramic vaporizing materials

1999-02-16

2001-01-02

Meeks, Timothy (Department: 1762)

Coating processes

Coating by vapor, gas, or smoke

C427S249500, C427S255310, C427S255394

Utility Patent

active

06168833

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains to ceramic vaporizing materials (ingots) for the coating of surfaces by means of physical vapor deposition, a process for the preparation thereof and the use of the ceramic vaporizing materials for the coating of objects.
BACKGROUND OF THE INVENTION
The coating of materials using ceramics has recently gained much importance in many different fields of applications. In this way, well-aimed combinations of properties can be achieved which are not possible when using uncoated materials.
Different coating methods have become established for the different fields of applications, for example, thermal spraying or physical vapor deposition for the preparation of thick films. Physical vapor deposition (PVD) has advantages over the other thick film methods in the field of coating controlled microstructures of thick films. The material to be coated (substrate) is situated in a vapor plume forming over a superheated melt, and condensation from the vapor phase occurs due to the difference in temperature. A reaction with the gas being present can occur while still in the gas phase or during the condensation of the vaporized particles (reactive vaporization). This is used primarily in oxidic ceramics in order to achieve a stochiometric deposition of the oxides which in part highly dissociate. For producing the superheated melt, high-energy radiation, e.g. laser radiation or electron radiation, can be used.
The material to be vaporized must be present in a form suitable for the particular process. In smaller plants (laboratory scale), the material is frequently vaporized from granules (coarse powder) which are present in a crucible. This process variant hardly requires a special preliminary processing of the material to be vaporized, but has the decisive drawback that a uniform and continuous coating process over an extended period of time is not possible. However, these criteria are of critical importance in the industrial application and in the preparation of thick films.
Therefore, a different concept of material supply has been developed for industrial and semi-industrial plants. The material to be vaporized is processed into round rods (ingots), for example, having diameters of 2 inches (~50 mm) or 2.5 inches (~63 mm) and lengths of about 25 cm. These are molten at the surface thereof in a laterally arranged crucible, open at the bottom, and can be continuously supplied through a conveying mechanism which is integrated in the coating chamber. Thus, a homogeneous and constant coating of objects becomes possible over an extended period of time.
It has been shown, however, that the preparation of the ingots and the ingot properties resulting therefrom have a distinct influence on the vaporizing performance of the ingots themselves and the quality of the deposited films. A calm, non-sputtering melting bath generation is required for the vaporization process. The melt should be chemically homogeneous which can be achieved both by the use of chemically homogenized powders and, when inhomogeneous powders are used, by a homogenization taking place in the melt (by convection).
The non-steady operational phases (heating, cooling and transient power cutoff by flashovers) have proven particularly critical for the use of ingots. In such phases, disastrous failure by breakage of the ingots frequently occurs in conventional dense or partially sintered ceramic materials due to the pronounced difference in temperature. For this reason, a thermal shock resistance which is sufficiently high for such loads is defined as a requirement to be met by the ingot. This is generally realized by a sufficient residual porosity.
During the coating of the objects, further reactions take place in the ingot. In the PVD process, the vaporization rate is proportional to the temperature, and therefore, it is sought to superheat the melting bath as highly as possible (up to 4000° C.) in order to achieve high coating rates. Due to the low thermal conductivity of the ceramic materials, a steep temperature gradient forms in the zone beneath the melting bath. Correspondingly, different processes take place in a region narrowly limited in space. A corresponding local thermal expansion appears for the whole thermally strained range, corresponding to the respective local temperature. At temperatures above the onset temperature of sintering, a counter-acting volume shrinking takes place due to sintering processes. The interplay of these mechanisms can result in a failure of the ingot during the coating process.
A further requirement to be met by the ingot is a constant density throughout the height of the ingot. Further, for mechanical engineering reasons, certain requirements must be demanded from the accuracy of ingot geometry and the handling properties (especially green strength). The ingots are generally brought to the desired dimensions by mechanical processing (turning on a lathe or circular grinding).
Commercially available are only yttria (Y
2
O
3
) partially stabilized zirconia ingots which are prepared by a cumbersome processing and sintering method. DE 43 02 167 C1 describes a target made of zirconia and a process for the preparation thereof. For the preparation of heat-insulating layers on refractory materials by electron beam physical vapor deposition (EB-PVD), targets of zirconia are described which are to be thermal shock resistant. Such targets contain from 0.5 to 25% by weight of yttria besides zirconia, both with a purity of at least 99.8%. A fraction of 50 to 80% by weight of the zirconia must be present as a monoclinic phase, and the targets must have a sintered density of from 3.0 to 4.5g/cm
3
. The preparation of the targets is performed by compression-molding and sintering zirconia/yttria mixtures into corresponding molded articles. It has been described to be of advantage to use powders the average particle size, d
50
, of which is below 50 &mgr;m wherein more than 90% by weight of the particles must be 0.4 &mgr;m or larger in size, and more than 50% by weight must be 1 &mgr;m or larger in size.
U.S. Pat. No. 4,676,994 describes a dense ceramic film on substrates, the film having a density of more than 94% of the theoretical density. No further specifications are given with respect to the target to be used.
From DE 39 08 322 A1, sintered parts made of zirconia are known which serve as targets for the vapor deposition of optical layers. The zirconia used may contain from 50 to 2000 ppm of calcium oxide and must have a purity of at least 99%. In addition, the targets must have a density of more than 4.9 g/cm
3
(80% relative density). Lower densities are said to have a negative effect on the quality of the vapor deposited films. When 0.2% of calcium oxide is used, cracks will form in the target so that stable high-temperature phases of zirconia which are required for the preparation of heat-insulating layers cannot be produced with these targets.
The term “sintering” represents an important process step in powder metallurgy which is a thermal densification of powders or powder pellets by diffusion processes, usually without involving molten phases. A special case is liquid-phase sintering. As a rule, the sintering temperature is within a range of from half to three forths of the melting temperature of the lowest-melting material. Three phenomenological stages of sintering can be distinguished:
1. Growth of particle contacts by the formation of so-called sintering bridges.
2. Formation of a continuous pore skeleton. The original particles lose their identities, and shrinking occurs with the formation of new grain boundaries.
3. Pores become round or are eliminated with further shrinking. The remaining pore space increasingly becomes inaccessible from outside (closed pores). In the borderline case, complete densification occurs.
To avoid oxidations, this process can be performed under inert gas atmosphere or in vacuo. Sintering is a process step which is used with both metals and ceramics. It is different from densification by compression-molding in that sol

Affiliated with

Also associated with

Rating

Process for coating with ceramic vaporizing materials does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for coating with ceramic vaporizing materials, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for coating with ceramic vaporizing materials will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2534274