GB2264718A

GB2264718A - Vapour deposited coatings having at least three transitions in structure

Info

Publication number: GB2264718A
Application number: GB9204807A
Authority: GB
Inventors: Allan Matthews
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1992-03-04
Filing date: 1992-03-04
Publication date: 1993-09-08
Anticipated expiration: 2012-03-04
Also published as: GB2264718B; GB9204807D0

Abstract

A coating process comprises forming on a substrate by physical vapour deposition a layered coating of a material, the layers including at least three transitions in structure, the process comprising varying continuously or semi-continuously the structure of each layer through the structure to prevent discontinuities between successive layers. The composition of the material may be the same or generally the same throughout the layers. The problem of inter-lamellar debonding and separation in such structures is thus overcome by the modulation of the process parameters to produce layers whose composition varies gradually to avoid shock transitions between structural zones. This can be achieved by, for example, varying continuously or semi-continuously the degree of ionization; the substrate bias voltage (Fig 2); or the chamber pressure. The degree of ionization may be controlled by ranging the electron emission from a thermionic emitter in plasma- assisted PVD; or by varying the amount of radio frequency power coupled into the plasma in a radio frequency plasma system. <IMAGE>

Description

COATINGS PRODUCED BP VAPOUR DEPOSITION The invention relates to coatings produced by vapour deposition and more particularly to coatings formed on a substrate formed by three or more layers of material, the layers being formed by physical vapour deposition (PVD) and varying in their structure to provide improved thermo-mechanical properties.

In general, layered coatings are well known. A review of coating techniques and their production by plasma assisted PVD is contained in A Matthews "Developments in Ionisation Assisted Processes" J.Vac.Sci.Technol.A3(6) Nov/Dec 1985 pages 2345-2363 ("Reference 1"). The specific use of such structured layering to provide improved thermal barrier coatings is known from J.Prater and E.L.Courtright "Ceramic Thermal Barrier Coatings with Improved Corrosion Resistance" Surface Coatings and Technology 32(1987) 389-397 ("Reference 2").

In the case of layered coatings acting as thermal barrier coatings, a substrate such as a turbine blade can be provided with a number of layers by PVD with the layers being alternately a layer having good thermal barrier properties and then a layer with less effective thermal barrier properties. The thermal barrier provides a layer having a thermal conductivity significantly lower than the thermal conductivity of the substrate and the protective barrier prevents the diffusion into the thermal barrier of gases such as O2 and provides a tough strong "binder" and outer coating.

These successive layers of different structures or materials can be formed by a variety of PVD processes, and Reference 1 discloses a number of plasma assisted PVD processes. In plasma assisted PVD, the source of material to be deposited is placed in a chamber with a substrate on to which material from the source is to be deposited The chamber is evacuated to a low pressure (for example 10 5Torr) and a gas is introduced into the chamber capable of supporting an electric discharge. Material from the source is vaporized and ionized and neutral species are deposited at the substrate.

In Reference 2, a negative d.c. bias is applied to the substrate to produce the denser protective layers and is removed to produce columnar thermal barrier layers.

In this case (Reference 2), the resulting layered structure has discreet layers. This can give rise to probems of inter-lamellar debonding and separation due to this discontinuity in the - structure and/or composition and/or stress state at the interface.

According to the invention, there is provided a coating process comprising forming on a substrate by PVD a layered coating of a material, the layers including at least three transitions in structure, the process comprising varying continuously or semi-continuously the structure of each layer through the structure to prevent discontinuities between successive structures.

Since there is a gradual change of structure between the layers, the problems of inter-lamellar debonding and separation are mitigated or eliminated.

Preferably, the composition of the material is the same or generally the same throughout the layers.

Where the PVD is plasma-assisted PVD, the variation is preferably achieved by continuously or semi-continuously changing the plasma process parameters. For example, the degree of ionization and/or the substrate bias voltage and/or the chamber pressure may be varied. This provides independent control of the degree of ionization of the bombarding species and hence the number and the type of energetic particles hitting the surface and controlling the structure.

Where the plasma-assisted PVD is a thermionically enhanced d.c. plasma system, the degree of ionization may be controlled by varying the electron emission from the thermionic emitter.

Where the plasma system is a radio frequency plasma system, the degree of ionization may be controlled by varying the amount of radio frequency power coupled into the plasma.

The following is a more detailed description of some embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a schematic view of a thermionically enhanced d.c. plasma PVD system, and Figure 2 is a graph of time against current density at a substrate in a plasma PVD system of the kind shown in Figure 1.

Referring to Figure 1, the system comprises a chamber 10 containing a vapour source 11 arranged opposite a substrate 12 on which material from the source is to be deposited. The chamber also contains a thermionic emitter 13. The emitter 13 and the-substrate 12 are connected to respective negative biases.

In use, the chamber is evacuated to, for example, a pressure of 10 STorr . It is then filled with an inert gas such as argon to produce a pressure which supports ionization in the chamber. A suitable pressure may be 5-15mTorr. The emitter 13 enhances ionization independently of other process variables such as the evaporation rate of the source 11 and the pressure within the chamber 10. Ionized particles and vapour from the source 11 are deposited on the surface of the substrate 12 and produce a structure determined by the operating characteristics of the system.

The bias supply to the emitter 13 is varied to provide a variation in the degree of ionization (or the ionization efficiency) in the chamber 10. This influences the current measured at the substrate 12 and Figure 2 is a graph showing a typical variation in this current with time. As will be seen, the current varies continuously from a maximum of 2mA/cm 2 to a minimum of lmA/cm 2 and back again every minute. When the current is at a maximum, the deposition is more dense to produce a protective barrier layer whereas when the current is at a minimum, the layer is less dense to produce a better thermal barrier layer. Thus, in the four minute cycle shown in Figure 2, there will be four protective barrier layers arranged alternately with four thermal barrier layers.

However, due to the gradual change in the current, there will be no distinct interface between the layers. Rather, each thermal barrier layer will gradually merge into a protective barrier layer. This will eliminate or reduce inter-lamellar debonding and separation.

The above description is of a continuous change in the parameters. A semi-continuous variation may, however, be provided where, for example, the current is held at the maximum or minimum for a specified period of time to produce a more lamellar structural pattern, but without distinct boundaries where delamination is likely to occur.

There are a number of substances which can be used in the source. Two examples will now be given.

The first is a nitride ceramic material such as zirconium nitride or titanium nitride. These substances, if deposited in a highly dense form, exhibit build-up of stress which in turn leads to debonding and at the substrate coating interface. By using the system described above, the stress build-up is reduced, since the degree of strain relaxation occurs in the less dense layers. Thus the coating can be grown thicker without debonding and thus has a wider range of possible applications.

A second example is an oxide ceramic such as aluminium oxide or zirconium oxide. These ceramics are often used in high temperature applications, but temperature cycling can lead to inter-lamellar debonding in a conventionally layered structure with discreet layers. By using the system described above, the thermo-mechanical properties of the layers are considerably enhanced.

A typical deposition would evaporate a partially yttria stabilized zirconia material using an electron beam gun, in an appropriate partial vacuum environment as described above so that a glow discharge can be generated by applying a negative bias voltage to the substrate 12. By applying the bias at a radio frequency (for example, 13.56MHz) the level of ionization and thus the intensity of bombardment can be varied by varying the power linked into the plasma from the supply. If this is varied in a sinusoidal manner, with a period of one minute, a modulated layer structure of varying density is produced on the substrate 12. Since the degree of bombardment also influences the substrate temperature, this can be further controlled by the addition of a separate heating system, such as by the use of radiant heating elements.

Claims

1. A coating process comprising forming on a substrate by physical vapour deposition a layered coating of a material, the layers including at least three transitions in structure, the process comprising varying continuously or semi-continuously the structure of each layer through the structure to prevent discontinuities between successive layers.

2. A process as claimed in claim 1 wherein the composition of the material is the same or generally the same throughout the layers.

3. A process as claimed in claims 1 or 2 wherein the physical vapour deposition is plasma assisted and the variation of layer structure is achieved by continuously or semi-continuously changing the plasma process parameters.

4. A coating process substantially as hereinbefore described with reference to, and as illustrated by, the drawing.

4. A process as claimed in claim 3 wherein the plasma assisted physical vapour deposition is a thermionically enhanced d.c. plasma process in which the degree of ionization is controlled by varying an electron emission from a thermionic emitter.

5. A process as claimed in claim 3 wherein the physical vapour deposition is radio frequency plasma assisted in which the degree of ionization is controlled by varying the amount of radio frequency power coupled into the plasma.

6. A coating process substantially as hereinbefore described with reference to, and as illustrated by, the drawing.

Amendments to the claims have been filed as follows CLAIMS 1. A coating process comprising forming on a substrate by plasma assisted physical vapour deposition a layered coating of a material, the layers including at least three transitions in structure and having a composition of material which is substantially the same throughout the layers, the process comprising varying continuously or semi-continuously the plasma process parameters to vary continuously or semi-continuously the structure of each layer through the structure to prevent discontinuities between successive layers.

2. A process as claimed in claim 1 wherein the plasma assisted physical vapour deposition is a thermionically enhanced d.c. plasma process in which the degree of ionization is controlled by varying an electron emission from a thermionic emitter.

3. A process as claimed in claim 1 wherein the physical vapour deposition is radio frequency plasma assisted in which the degree of ionization is controlled by varying the amount of radio frequency power coupled into the plasma.