MANUFACTURING A FREESTANDING CERAMIC TILE

Description

FIELD

[0001] The present invention relates to a method of producing a freestanding ceramic tile.BACKGROUND

[0002] Ceramics are often used in high temperature environments, such as gas turbine engines. Using a protective layer of ceramic material can help prevent engine components from being damaged by the extreme temperatures they are exposed to while the engine is operating. In order to test the protective qualities of a ceramic material, it is known to produce pieces of ceramic, such as ceramic tiles, which can be exposed to various simulated environments and then analysed to determine their suitability for use in such extreme environments.

[0003] There are a number of known methods for producing freestanding ceramic tiles for high temperature test work. One method is to spray salt solution on a steel substrate before drying it, then, after application of the ceramic coating, re-hydrating the salt to force the ceramic tile away from the steel. An alternative method is to spray the ceramic on to a substrate material such as aluminium, which can then be dissolved away using a strong acid or alkali material (dependant on the base alloy) over a period of time. A third method involves spraying the ceramic on to silicon mould, and a fourth method is to spray the ceramic onto a thin, unblasted metal sheet, that can then be peeled away from the ceramic tile.

[0004] However, each of the above methods has its drawbacks. Using a salt solution can be very inconsistent in terms of the ceramic adhesion and the ability to re-hydrate the salt, and therefore there is a high failure rate resulting in many broken ceramic tiles. The second method, whilst having a much lower mechanical failure rate, is extremely time consuming, and uses undesirable noxious chemicals that can contaminate the ceramic tile, meaning it cannot be used for testing. Using a silicon mould is very material dependant, with a low success rate, and if an appreciable amount of ceramic thickness is required (> 1.5 mm) the residual stresses in the coating tend to make the coating curl up at the edges, meaning it is no longer flat, which in most cases is undesirable. The fourth method again suffers with the issue of residual stress in the coating resulting in wavy or curved end products.

[0005] Japanese patent application JP S63280631 A discloses a method of forming a ceramic coated layer. The method involves forming a matrix that has a laminated surface, applying a ceramic material to the surface of the matrix to be coated, and using heat to remove the matrix thereby leaving a dense ceramic coated layer.

[0006] United States patent application US 2005 / 003172 A1 discloses a method of applying a profiled abradable coating onto a substrate in which an abradable ceramic coating composition is applied to a metal substrate using one or more coating application techniques to produce a defined ceramic pattern without requiring a separate web or grid to be brazed onto the substrate. JPH11165478 discloses making a ceramic free-standing layer by thermal spraying, by first blasting a substrate with glass spheres, then applying a graphite separation layer of up to 5 microns and then removing the sprayed layer by peeling.

[0007] There remains a need for an improved method of producing freestanding ceramic tiles or at least a useful alternative to known methods.SUMMARY

[0008] The present invention concerns a method of producing a freestanding ceramic tile. According to a first aspect there is provided a method of producing a freestanding ceramic tile as set out in claim 1.

[0009] This method has been found to repeatedly produce flat, free-standing ceramic tiles with a very low failure rate. It does not require the use of any noxious substances, and is faster than other known methods for producing ceramic tiles. In particular, the use of a carbon or graphite release layer is advantageous as carbon and graphite both allow some movement of the ceramic to accommodate stresses arising during the ceramic deposition process, but without allowing it to curl, thus reducing the failure rate compared to other known processes.

[0010] The substrate can comprise nickel or stainless steel. Nickel and stainless steel do not react with ceramic and are stable at elevated temperatures.

[0011] The grit can comprise one or more of alumina, metal shot, sand, or solid CO 2 . Such materials are not noxious, are capable of imprinting on the substrate, and are readily available.

[0012] The release layer can be applied using vacuum sputtering or physical abrasion. Such methods are suitable for creating thin and even layers upon the substrate.

[0013] The ceramic can be applied over the release layer by plasma spraying. Plasma spraying allows for a high rate of ceramic deposition.

[0014] The thickness of the ceramic layer can be 1.5 mm or more. Ceramic tiles with thicknesses of 1.5 mm or more are desirable for freestanding test applications.

[0015] The substrate can be heated using an air furnace. Air furnaces provide a means for evenly heating the substrate.

[0016] The substrate can be quenched using cold air or water. Both methods are cheap, effective, and readily available.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments will now be described by way of example only, with reference to the Figures, in which: Figure 1 is a flow chart illustrating the method of the present invention; and Figure 2 shows an isometric view of a release layer sandwiched between a substrate and a layer of ceramic.

[0018] The following table lists the reference numerals used in the drawings with the features to which they refer: Ref no.FeatureFigure100Method1110First step1120Second step1130Third step1140Fourth step1150Fifth step1160Sixth step1200Substrate2210Release layer2220Ceramic layer2 DETAILED DESCRIPTION

[0019] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art.

[0020] Figure 1 shows a flow chart for a method 100 according to the present invention. At the first step 110, a substrate 200 (see Figure 2) is grit-blasted using a grit size in the range from 36 mesh to 220 mesh. In this example the substrate comprises stainless steel, and the grit consists of alumina, but the skilled person will appreciate other substrate materials, such as nickel, and other grit materials, such as Alumina, metal shot, sand, or solid CO 2 , can be used. The purpose of the grit blasting is to both clean and abrade the surface of the substrate, so that further layers deposited upon the substrate will stick to it.

[0021] The second step 120 in the process is to deposit a release layer 210 (see Figure 2) on to the grit-blasted surface of the substrate 200.The release layer can consist of carbon or graphite. The deposition of the release layer 210 can be achieved by any process suitable for creating an even, thin layer from 2 to 10 microns thick across the entirety of the substrate surface; for example vacuum sputtering, or physical abrasion using a carbon or graphite block.

[0022] In the third step 130, the ceramic layer 220 (see Figure 2) is applied to the release layer until the desired thickness for the eventual freestanding tile (typically greater than 1.5 mm) is achieved. In this example the ceramic layer is applied using a standard air plasma spraying process, but the skilled person will appreciate any suitable method may be used. If using such a spraying process, it is important the substrate is mounted such that there is no shadowing of the substrate (i.e. that the path from the source of the spray to the substrate surface is not blocked at any point), as this will lead to uneven distribution of the ceramic, meaning be the ceramic tile created is ultimately unusable. Magnetic mounting of the substrate is one way to mount the substrate such that its surface is completely unobstructed with respect to the ceramic spray, but the skilled person will appreciate there are other ways to achieve the same result.

[0023] In a fourth step 140, the substrate 200 (and therefore the release layer 210 and ceramic layer 220) is heated to a temperature from 800 to 1000 degrees Celsius (1472 to 1832 degrees Fahrenheit). This can be achieved by placing the substrate into an air furnace for example, although the skilled person will appreciate alternative heating means could be used to heat the substrate.

[0024] In a fifth step 150, the substrate is held at a temperature from 800 to 1000 degrees Celsius for a period of time equalling from 10 to 20 minutes. By holding the carbon or graphite layer in the temperature range of from 800 to 1000 degrees Celsius for a period of time equalling from 10 to 20 minutes, the carbon or graphite layer will burn out, owing to the fact that carbon and graphite oxidise within this temperature range.

[0025] In the sixth and final step 160, the substrate is quenched to bring it rapidly back down to room temperature. In this context, "rapidly" equates to a cooling rate of at least 200 degrees Celsius (or 392 degrees Fahrenheit) per minute. This can be achieved using air or water that has been cooled, or other suitable means as will be familiar to the skilled person. The ceramic layer will separate from the substrate during the quenching process, due to the carbon or graphite release layer having been burnt out during the previous step of the method.

[0026] This method has been found to repeatedly produce flat, free-standing ceramic tiles with a very low failure rate. It also does not require the use of any noxious substances, and is faster than other known methods for producing ceramic tiles. In particular, the use of a carbon or graphite release layer is advantageous as carbon and graphite both allow some movement of the ceramic to accommodate stresses arising during the ceramic deposition process, reducing the failure rate compared to other known process where the ceramic layer curls away from the substrate, leading to curved or broken tiles unsuitable for use in testing.

Claims

1. A method of producing a freestanding ceramic tile, the method comprising the steps of: (a) grit-blasting a substrate using a grit size in the range from 36 mesh to 220 mesh; (b) depositing a release layer on the grit-blasted surface of the substrate, wherein the release layer is a layer of carbon or a layer of graphite, the release layer being from 2 to 10 microns thick; (c) applying ceramic over the release layer until a desired thickness of ceramic is achieved to form a ceramic layer; (d) heating the substrate, release layer, and ceramic layer to a temperature of from 800 to 1000 degrees Celsius; (e) keeping the substrate, release layer, and ceramic layer at a temperature from 800 to 1000 degrees Celsius for a time from 10 to 20 minutes so as to remove the release layer; and (f) cooling the substrate and ceramic layer via quenching at a rate of at least 200 degrees Celsius per minute, such that the ceramic layer separates from the substrate to produce a freestanding ceramic tile.

2. The method of claim 1, wherein the substrate comprises either nickel or stainless steel.

3. The method of claim 1 or 2, wherein the grit comprises one or more of alumina, metal shot, sand, or solid CO2.

4. The method of any preceding claim, wherein the release layer is applied using vacuum sputtering or physical abrasion.

5. The method of any preceding claim, wherein the ceramic is applied over the release layer by plasma spray.

6. The method of any preceding claim, wherein the thickness of the ceramic layer is 1.5 mm or more.

7. The method of any preceding claim, wherein the substrate is heated using an air furnace.

8. The method of any preceding claim, wherein the substrate is quenched using air or water.