GB2341603A

GB2341603A - Method of applying glass ceramic dielectric layers to metal substrates

Info

Publication number: GB2341603A
Application number: GB9820062A
Authority: GB
Inventors: Jeffery Boardman
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-09-16
Filing date: 1998-09-16
Publication date: 2000-03-22
Also published as: GB9820062D0

Abstract

A method of forming a dielectric layer of a glass ceramic consisting of a mixture of metallic oxides on to a metal substrate comprises (a) preparing the glass ceramic in the form of dry particles, (b) cleaning and roughening a metal substrate, and (c) thermally spraying the glass ceramic particles in molten or semi-molten state on to the prepared metal substrate.

Description

2341603

BACKGROUND OF THE INVENTION AND DESCRIPTION OF PRIOR ART

Mixtures of metal oxides commonly known by the generic term of "GLASS CERAMICS" are extensively used to provide electrically insulating dielectric coatings to metal substrates which then act to electrically insulate the metal substrate from any electrical energy applied to the surface of the dielectric glass ceramic coating.

The conventional method of applying these glass ceramic mixed metal oxide compounds to the required surface of the metal substrates is by a system known as screen printing, described as follows'.

(a) The surface of the metal substrate is scrupulously cleaned to remove all traces of grease, oils or other types of contaminants.

(b) The cleaned metal substrate surface is then prepared in such a fashion that the topmost layer will advantageously combine with the glass ceramic to be applied to it to ensure good adhesion one to the other.

The methods used to prepare the surface may include such techniques as the formation of a thin oxide layer on the substrate surface by heating in an oxidising atmosphere under the appropriate temperature/time conditions, or mechanical or chemical abrasion to produce a roughened substrate surface.

(c) A printing screen is next firmly fitted onto the prepared and cleaned metal substrate. The screens generally used consist of an area of woven wire mesh firmly held within a rigid frame and the number of perforations in the mesh and the wire thicknesses used may be varied according to the type of glass ceramic to be applied, the particle size distribution of the glass ceramic mixture, the final thickness of the glass ceramic insulating layer, and the required dielectric properties.

(d) With the printing screen and metal substrate firmly supported on a suitable flat surface a layer of glass ceramic compound is applied to the substrate surface by means of a roller which rot - ates to traverse across the whole area of the metal substrate to be coated, whilst simultaneously applying a thin layer of the glass ceramic compound to the Wre mesh screen and exerting such pressure that a thin layer of glass ceramic is deposited onto the prepared metal substrate surface.

The metal oxide compounds used as the glass ceramic insulating materials must be crushed and ground to extremely small particle sizes, generally in the sub micron range but usually no greater than 2-5 microns, and are combined with hydrocarbon solvents to produce liquid/particle mixtures generically known as "inks".

2 Due to the sensitivity of the glass ceramic dielectric layers to failure from nonuniformity of thickness and entrapped vapours in the form of bubbles, the printing process used to apply the glass ceramic dielectric compounds is done in two or more stages, usually allowing the solvents present to evaporate between each stage.

As an aid to ensuring that the glass ceramic deposit is of uniform thickness the orientation of the printing screen to the metal substrate is changed at each stage.

(e) At the point where a multiplicity of printing stages have produced the required thickness of the glass ceramic compounds onto the prepared metal substrate and the hydrocarbon solvents have mostly been evaporated off, the coated metal substrate pieces are passed through a furnace where they reach temperatures within the range at which the metal oxide glass ceramic compounds become molten.

This heating or furnacing operation is the final and probably most critical stage of the process. It is designed to raise the substrate and screen printed glass ceramic layer to the temperature at which the metal oxide compounds become sufficiently molten that the individual particles melt to form a liquid glass ceramic layer free from any voids and of uniform thickness over the whole of the metal substrate surface.

The glass ceramic coated substrate pieces are then cooled at a rate to produce the required form of solid glass ceramic layer at ambient temperature, which form may range from polycrystalline to amorphous but is generally in the amorphous state.

(f) Additionally the glass ceramic layer may be produced by separately pre- heating both the metal substrate and glass ceramic compounds to a temperature at which the glass ceramic becomes molten and then pouring the molten glass ceramic onto the rotating metal substrate surface, the thickness of the final glass ceramic insulating layer being determined by the speed of rotation of the substrate and the rate of application of the molten glass ceramic.

3 DISADVANTAGES OF THE CURRENT METHODOLOGY Whilst the aforementioned systems utilised to apply glass ceramic compounds to metal substrates are effective and widely employed, they have inherent disadvantages which are listed as follows:

(a) The metal oxides used in the glass ceramic compounds must be crushed and ground to very fine particle dimensions in the range of -5 microns to sub micron sizes, and the particle size range distribution must be closely controlled in the formulation and preparation of the "inks" used in the screen printing method. The material preparation and sizing processes are both expensive and time consuming.

(b) Due to the sensitivity of the glass ceramic dielectric layers to failure by uneven thicknesses, the presence of voids arising from air or vapour entrapment, or the inclusion of extraneous foreign materials, the screen printing deposition process must be done in multiple stages in a dust-free clean air environment.

This defect sensitivity imposes restrictions on the rate at which glass ceramic coated substrates may be produced and the locations within which the process may be utilised.

The result of the above impositions is a cost penalty on the unit price of the coated substrate items.

(c) The screen printing process is virtually restricted in its use to metal substrates which are completely flat, and it is extremely difficult to utilise the process to apply glass ceramic dielectric layers to curved or non-uniform substrate shapes. The same constraint applies to the process of rotating a substrate and dropping onto it molten glass ceramic.

(d) By virtue of the need to use mesh screens in the printing application of the metal oxide glass ceramic compounds it is extremely difficult to completely cover the entire surface of the metal substrate, and it is necessary to leave an area around the edge of the substrate.

Thi's uncoated peripheral area may be a source of defects for the dielectric layer in that an electrical charge applied to the glass ceramic surface may "leak" off the edge down to the uncoated metal surface resulting in a "short circuit", 4 (e) To ensure that the final glass ceramic layer is both uniform in thickness, free of any internal defects and correctly and sufficiently bonded to the metal substrate, both metal substrate and glass ceramic must be heated in a furnace to the temperature at which the glass ceramic becomes molten or liquid.

Dependent upon the type of glass ceramic/metal oxide mixture utilised, the temperatures at which the glass ceramics become molten are generally in the range of 7000C to 11 OOOC.

One property of glass ceramic materials is their brittleness and lack of ductility, rendering such layers liable to crack, shatter and delaminate from the metal substrate if subjected to inordinate tensile, sheer or compressive stresses.

Such stresses may be detrimentally induced into the glass ceramic layer by the metal substrate if there are dimensional differences between the sizes of the layer and the metal substrate as a result of their cooling from the liquidus temperature of the glass ceramic in the process furnace.

To eliminate the possibility of the cracking or delamination of the glass ceramic dielectric layer, combinations of glass ceramic compounds and metal substrates are used which have identical or closely matching coefficients of thermal expansion over the temperature range from that at which the glass ceramic becomes molten and the ambient or operating temperatures for the insulated metal component.

This essential requirement that the coefficients of thermal expansion of both the glass ceramic compound and the metal substrate are identical or closely matched imposes a restraint on both the metals, which may be used as the substrates and the glass ceramic compounds available for use as insulating dielectric layers.

For example, the glass ceramic compounds utillsed to provide insulating dielectric layers for use as elements in domestic kettles may only be combined with metal substrates composed of 430 or 440 type stainless steels.

A combination of the same glass ceramic compounds with metal substrates comprised of copper and its alloys, with greater coefficients of thermal expansion, would not be possible due to the cracking and deiamination of the glass ceramic layer on cooling from the molten ceramic temperature to ambient. An entirely different compositional form of glass ceramic mixture is required for use with copper and copper alloys.

It is possible to vary the melting point values and coefficients of thermal expansion of the metal oxide compounds comprising the glass ceramic mixtures to match other metals and alloys by the use of alkali metal oxides. However, the results of these additions is generally to deleteriously affect the dielectric properties of such compounds due to the ionic mobility and consequent charge carrying capacities of the alkali metal ions at elevated temperatures.

Additionally, several types of metals and metal alloys which would be suitable as substrate pieces are unusable in combination with the glass ceramic compounds due to their melting temperatures being below the melting temperatures of the glass ceramic compounds.

This requirement for the close matching of the coefficients of thermal expansion of both the metal substrates and the glass ceramic compounds and the need for the substrate metal melting or softening temperature to exceed that of the associated glass ceramic mixture imposes cost constraints on the material combinations which may be used and consequently on the unit price of glass ceramically insulated metal substrate pieces.

V 6 DESCRIPTION OF THE INVENTION

This present invention relates to the use of thermal spraying techniques to apply the glass ceramic compounds to metal substrate pieces, so providing such metal substrate pieces with an electrically insulating dielectric layer having the requisite dielectric and other properties required by the subsequent operating conditions to which the combination of metal substrate and glass ceramic may be subjected.

The process concerning this present invention is described in the following stages:

(a) The surface of a metal substrate is prepared by firstly cleaning it to remove all traces of oils, grease and other such contaminants, and then mechanically or chemically abraded to roughen it and increase the surface area of the said surface such that it provides a means by which molten particles of the glass ceramic impacting onto it will strongly and sufficiently adhere.

(b) Thermal spraying equipment is used to pre-heat the particles of the appropriate glass ceramic compound to a temperature at which the glass ceramic particles are molten or semi-molten, and to project the aforementioned molten or semi-molten glass ceramic particles onto the prepared metal substrate layer in such a way that a fully dense uniformly thick layer of the appropriate glass ceramic material is deposited and formed upon the whole area of the metal substrate surface, so constituting the requisite dielectric layer of the required thickness.

The area and thickness of the layer so formed is dependent upon the multiplicity of passes of the thermal spraying equipment over and across the metal substrate surface, the rate at which the glass ceramic powder particles pass through the heating source of the thermal spraying equipment and becomes molten or semi-molten, and the distance of the heating source from the metal substrate surface.

(c) The dielectric layer so produced upon the whole of the metal substrate surface may be mechanically machined by such techniques as surface grinding, lapping or polishing to improve the surface smoothness and amend the deposited thickness as required, or alternatively, roughened, grooved or engraved by such techniques as laser, mechanical or chemical equipment erosion to vary the thickness of localised areas of the aforementioned glass ceramic dielectric layer formed onto the metal substrate.

7 ADVANTAGES OF THE PRESENT INVENTION The advantages of the present invention over the current methodology previously described are numerous and substantial and include those subsequently listed.

(a) That the use of thermal spraying techniques allows glass ceramic dielectric layers to be applied to metal substrates which are curved or asymmetrically shaped and not constrained solely to flat surfaces.

In fact the use of thermal spraying techniques will apply or deposit glass ceramic dielectric layers onto any form of metal substrate surface whose shape may be described by a mathematical equation. Such equation would be used as part of a computer programme available to control the movements of a robot which progresses the thermal spraying equipment and associated stream of molten or semi-molten glass ceramic particles over and across the surface of an appropriate metal substrate to provide a uniform glass ceramic dielectric layer.

(b) That the use of thermal spraying equipment allows the glass ceramic layer to be applied to a metal substrate surface in one continuous operation comprised of a multiplicity of passes over the whole of the substrate surface in such a manner as to progressively build up the final layer thickness to the required dimension.

(c) The thermal spray equipment used to apply the molten glass ceramic particles may consist of either conventional flame spray guns operating with mixtures of oxygen and commercially available liquified petroleum gases, plasma equipment which utilised electrical energy to produce heat, or high velocity oxygen fuel systems.

These three types of thermal spray equipment all utilise the glass ceramic compounds in powder form. However, the particle sizes required for these three pieces of equipment are only in the range of +5 microns to -120 microns, magnitudes larger than the particle size ranges needed for the screen printing process.

Consequently the glass ceramic powder particles utillsed in the thermal spray processes are easier to produce and hence inherently cheaper than the same materials required for the screen printing techniques.

(d) In the utilisation of thermal spraying techniques to apply deposits of glass ceramic compounds to metal substrates there is no need to heat the metal substrate to the temperature of the molten glass ceramic. By the use of various cooling methods the temperature of the metal substrate may be maintained at relatively low levels even when being coated with molten glass ceramic particles at elevated temperatures approaching 1 OOOOC.

Consequently the need to use only glass ceramic compounds and metal substrate materials with identical or very closely matching coefficients of thermal expansion is reduced and it is possible to form glass ceramic and metal substrate combinations with widely differing coefficients of thermal expansion, previously considered unusable by conventional techniques.

In summary therefore, the advantages of this present invention of using thermal spraying techniques to apply glass ceramic insulating compounds to metal substrates will make the process more cost effective at an improved production rate, allowing new combinations of glass ceramic and metals to the whole of a metal substrate surface area which need not only be flat but may be of any shape definable by a mathematical equation, which may be utilised in a robotic control system.

9

Claims

1 A method of forming an electrically insulating dielectric layer of a glass ceramic compound consisting of mixtures of metal oxides onto a supporting metal substrate, the method comprising the steps of:

(a) preparing a glass ceramic compound in the form of dry powder particles-, (b) cleaning and roughening the surface of the supporting metal substrate surface onto which the glass ceramic dielectric is to be formed; (c) thermally spraying the dry glass ceramic powder particles in the molten or semi-molten state onto the prepared metal substrate surface such that a dense homogeneous dielectric layer is so formed.

2. A method as in claim one whereby the thermal spraying technique may be the simple flame spray process which uses the combustion of oxygen and some other hydrocarbon gas or liquid to pre-heat and propel the glass ceramic particles, or an electrically powered plasma system Whereby heating of the glass ceramic powder particles is achieved using electrical energy, or a system utilising the combustion of oxygen and some other hydrocarbon in liquid or gas form to both pre-heat the glass ceramic powder particles and to impart to them a high kinetic energy known generally as the high velocity oxyfuel system, or a system whereby a solid rod previously formed from glass ceramic particles is melted by the combustion of oxygen and hydrocarbon fuel.

3. A method as claimed in claims one and two whereby the glass ceramic dielectric layer is formed onto the supporting metal substrate in one continuous thermal spraying operation or stage, comprising a multiplicity of passes of the stream of molten or semi-molten glass ceramic particles over and across the surface of the said supporting metal substrate such that the whole of the metal surface is coated with a deposit of glass ceramic and the thickness of the deposit may be dependent upon the total number of passes.

4. A method as claimed in claims one to three inclusively whereby the prepared dry glass ceramic powders to be utilised in the thermal spraying process lie in the size range of +5 microns to 125 microns.

5. A method as claimed in claims one to four inclusively whereby the glass ceramic compounds to be thermally sprayed may be in the form of a solid rod or tube formed from the previously described dry glass ceramic powder particles.

6. A method as claimed in claims one to five inclusively whereby the supporting metal substrate may be cooled using a variety of methods such that glass ceramic compounds having widely different coefficients of expansion to those of the supporting metal substrates may be successfully deposited onto said supporting metal substrates, consequently permitting the use of glass ceramic compounds and supporting metal substrate combinations not producable from current techniques.

7. A method as claimed in claims one to six inclusively whereby the glass ceramic compounds may be thermally sprayed onto any shape or configuration of the supporting metal substrate which may be described by a mathematical equation, which in turn may be used to control a robotic operation utilised to direct the multiplicity of passes across and over the surface of the said supporting metal substrate.

8. A method as claimed in claims one to seven inclusively whereby the glass ceramic dielectric layer may be applied to supporting metal substrate surfaces which have previously had applied layers or substances such that the dielectric layer insulates not only the supporting metal substrate but also the previously applied layers or substances.

9. A method as claimed in claims one to eight inclusively whereby the layers or substances previously applied to the supporting metal substrate may consist of electrically resistive elements in a variety of forms-

10. A method as claimed in claims one to nine inclusively whereby a glass ceramic dielectric layer may be applied to a supporting metal substrate surface in the form of a cylinder or roller to a thickness whereby it will provide insulation between the said metal roller substrate and a very high voltage source of the order of 40,000 volts or more, such that a discharge Corona is set up between the insulating glass ceramic surface of the roller and the high voltage electrical source.

11. A method as claimed in claims one to ten inclusively whereby the glass ceramic layer applied to the supporting metal substrate may be further machined by such mechanical methods as grinding, lapping or polishing to improve the uniformity of thickness or surface finish of the said dielectric layer.

12. A method as claimed in claims one to eleven inclusively whereby the glass ceramic layer applied to the supporting metal substrate may be further machined by chemical or laser cutting techniques to engrave or form a pattern either regular or random into said dielectric layer.