AU718396B2 - Coating substrates with high temperature ceramics - Google Patents

Description

I I i 1

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant/s: Actual Inventor/s: Address of Service: Invention Title: The BOC Group plc Geoffrey Kenneth CREFFIELD and Mark Andrew COLE SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "COATING SUBSTRATES WITH HIGH TEMPERATURE

CERAMICS"

The following statement is a full description of this invention, including the best method of performing it known to us:- (File: 19960.00) -la- COATING SUBSTRATES WITH HIGH TEMPERATURE CERAMICS The present invention relates to methods of coating a substrate with high temperature ceramics.

Throughout this specification the expression "high temperature ceramics" is intended to encompass oxides, carbides and nitrides of metals such as chromium, aluminium and zirconium having a melting point above 18000C.

Chromium oxide has been plasma-sprayed on substrates for many years for applications in such industries as the aerospace and automobile industries.

A further need for relatively thick, high hardness and low porosity chromium oxide 10 coatings is in the print roller industry. In this industry, coatings are usually laser engraved thereby producing indents which are designed to hold ink. The harder and thicker the coating, the greater the density of holes that can be achieved.

o:i Atmospheric plasma spraying has produced coating densities between 90 theoretical values, but this allows aggressive gases to penetrate the open porosity 15 and damage both the coating and substrate material. Considerable work has been

I.

attempted to reach gas tightness in plasma-sprayed chromium oxide coatings using vacuum plasma spraying, post heat treatments and capsule hot isostatic pressings, **but with little success.

Some work has been carried out on the use of acetylene in the high velocity oxyfuel 20 thermal spraying of chromium oxide. However, acetylene is a fuel gas well-known for its tendency to decompose with violence and has to be used at relatively relatively low pressures.

Other pressurised gas fuels have been used in high velocity oxyfuel thermal spraying processes including a stabilised mixture of methylacetylene and -2propadiene (MPS). MPS is a mixture of methylacetylene and propadiene together with diluents or stabilisers such as propane and propylene. Other diluents can be present for example, methane, butane or ethane but in small percentage amounts. MPS is used extensively particularly in the United States as a safer and more economic substitute for acetylene. However, difficulties have been experienced since it is customary to store liquid MPS in a pressurised cylinder and eject the liquid MPS as a gas under vapour pressure to employ the same in high pressure, high flow rate oxyfuel thermal spraying applications. It has been found that high vapour withdrawal rates effectively results in a ii fractional distillation of the MPS gas components resulting in composition changes as the cylinder content decreases. This has been found to cause fluctuations in flame 0% 0 temperature and a need to adjust the flow rate of oxygen to avoid excess carbon build-up i" or excess oxygen.

0 pp Maintaining a constant pressure and flow rate presents further problems. This is S•because as the level of the liquid fuel in the pressurised cylinder decreases, the temperature likewise decreases due to the latent heat of vaporization. A reduction in 0i temperature within the pressurised cylinder results in a reduction of pressure which adversely affects both the pressure and flow rate of the vaporized fuel stream.

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

According to the present invention, a method of coating a substrate with a high temperature ceramic comprises the steps of: a) injecting an inert gas under pressure into a container containing liquid methyl acetylene propadiene stabilised (MPS) sufficient to generate a stream of liquid; -3b) removing said stream of liquid MPS from the container; c) vaporising said stream of liquid MPS; d) delivering the vaporised MPS to a mixing chamber of a high velocity oxygen fuel spray gun where it is mixed with oxygen under pressure; e) introducing said mixture into a combustion chamber of the high velocity oxygen fuel spray gun together with a powdered high temperature ceramic entrained in a stream of an inert gas; and f) spraying the heated particles of the high temperature ceramic on to the o surface of a substrate.

S 10 Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an 1o inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense S•of"including, but not limited to".

Advantageously, at least in a preferred form, the present invention may provide an improved method of coating a substrate with a high temperature ceramic using a high velocity oxyfuel thermal spraying technique in which the fuel gas is MPS gas.

A preferred embodiment of the invention will now be described, by way of example only, with reference being made to the Figures of the accompanying drawings in which: Figure 1 is a diagrammatic sketch of an apparatus for producing MPS gas having a substantially constant pressure, flow rate and gas composition; Figure 2 is a diagrammatic sketch of an apparatus for coating a substrate with a high temperature ceramic and incorporating the apparatus illustrated in Figure 1; and -3a- Figure 3 is a diagrammatic sketch of a high velocity oxyfuel gun forming part of the apparatus of Figure 2.

Referring first to Figure 1 which illustrates an apparatus 2 for generating a vaporised stream ofMPS gas having a substantially constant pressure, flow rate and composition. The apparatus 2 includes a storage container 4 for liquid MPS having an inlet 6 for receiving an inert gas, for example, nitrogen via a conduit 8 extending -4from a pressurised nitrogen gas cylinder 10. The storage container 4 contains a tube 12 which extends almost to the floor 14 of the container and which provides a pathway for the flow of liquid MPS when pressure is applied by the incoming nitrogen. The storage container 4 includes an outlet 16 through which pressurised liquid MPS can pass into a conduit 18 via a flexible delivery tube 20. The flow of MPS liquid through the conduit 18 is controlled by a valve 22.

The conduit 18 is connected to a vaporiser 24 operating at a temperature sufficient to vaporise the liquid MPS. One such example of a suitable vaporiser is a hot water/glycol vaporiser maintained at a temperature sufficient to vaporise each component of the liquid MPS typically between 30 and 1000C by means of a thermastatically controlled immersion heater 26.

The flow of liquid MPS from the conduit 18 into the vaporiser 24 is controlled by a S: temperature sensitive shut off valve 28 which includes a thermal probe 30 which S. detects the temperature of the water bath within the vaporiser. The valve 28 is 15 operated to prevent the flow of the liquid MPS into the vaporiser 24 until the water •oo• bath has attained a minimum desired temperature sufficient for vaporisation of the liquid MPS. The valve 28 therefore prevents flooding of the vaporiser 24 with the ~liquid MPS before it has reached operating temperature and thus avoids any liquid carry over into the vaporising portion of the apparatus 2.

The vaporiser 24 transforms the liquid MPS into a super heated high pressure, high flow rate vaporised fuel stream having a temperature of typically up to about The vaporised MPS stream exits the vaporiser 24 through a conduit 32 controlled by a valve 34. The conduit 32 may be heated and/or insulated to prevent condensation of the vaporised MPS stream. For example, the conduit 32 may be wrapped in a heating tape for this purpose.

I

In use, nitrogen from the gas cylinder 10 is fed to the storage container 4 via the conduit 8. A pressure regulator 36 is provided to ensure that the nitrogen is fed into the storage container 4 at a suitable pressure, typically from about 115 to 190 psig, preferably from about 140 175 psig.

A safety valve 38 is provided in the conduit 8 to allow the release of the nitrogen through a vent 40 when the storage container 4 has been substantially relieved of the liquid MPS.

The nitrogen enters the head space 42 of the container 4 thereby exerting a downward force against the surface 44 of the liquid MPS. The MPS is therefore forced upwardly through the tube 12 and out of the outlet 16 and thus eventually into the vaporiser 24.

:Referring now to Figure 2, there is illustrated an apparatus 50 for coating a substrate with a high temperature ceramic such as chromium oxide. The apparatus includes a high velocity oxyfuel gun 52 (see also Figure 3) having a gas mixing 15 chamber 54, a combustion chamber 56 and a nozzle 58 extending outwardly from the combustion chamber 56. The chambers 54, 56 are divided by a partition provided with holes 57.

As shown, communicating with the gas mixing chamber 54 is a first conduit connected to a source of oxygen under pressure and a second conduit 62 connected to the conduit 32 extending from the vaporiser 24.

o Extending through the mixing chamber 54 and communicating directly with the combustion chamber 56 is a third conduit 64. Conduit 64 extends from a chromium oxide powder reservoir 66. A pipe 68 extends from a source of argon under pressure into the upper (as shown) end of the reservoir 66.

-6- The gun 52 is provided with channels 70 for a coolant, for example water.

In use, MPS vapour is supplied to the gas mixing chamber 54 from the vaporiser 24 via conduits 32, 62 at a substantially constant pressure, flow rate and gas composition. Simultaneously, a stream of oxygen is supplied via the conduit 60 into the gas mixing chamber 54. The oxygen and the MPS vapour are mixed in the mixing chamber and exit the mixing chamber to enter the combustion chamber 56 of the gun 52 via the holes 57 where they are ignited. Simultaneously, argon under pressure passes through the pipe 68 into the reservoir 66 where it entrains chromium oxide powder and thereafter passes through the conduit 64 directly into the combustion chamber 56. Exhaust flames and heated powdered chromium oxide particles leave the combustion chamber through the nozzle 58 and are deposited on the substrate S. Examole Chromium oxide coated test samples were produced using a Miller Thermal HV2000 15 High Velocity Oxyfuel Gun, having a 22 millimetre combustion chamber designed for high melting point powders. Sulzer Metco's Amdry 6417 high purity chromium oxide, powder size range between 5 and 22 p.m was used to spray all test samples at a powder feed rate of 25 grams per minute using high purity argon carrier gas at 11.5 litres per minute. The MPS vapour was introduced into the combustion chamber at a pressure of 85 psi and a flowrate of 70 I/min and the oxygen was introduced into the combustion chamber at a pressure of 150 psi and a flow rate of 233 I/min.

Surface treatment of all test samples with 40 grit alumina gave a minimum sample :..!surface roughness (rc) of 7 10 Lrm. All test pieces were coated to a thickness between 200 260 J.m, keeping coating temperatures below 1500C. Thicknesses greater than 380 [m were achievable using many of the conditions.

-7- It has been found that the deposition on a substrate of a high temperature ceramic such as chromium oxide or zirconium oxide using high velocity oxyfuel thermal spraying where the fuel gas is MPS delivered at a substantially constant pressure, flow rate and composition results in a coating of high quality having very little porosity and high hardness.

o o* *~o

Claims (7)

1. A method of coating a substrate with a high temperature ceramic comprising the steps of a) injecting an inert gas under pressure into a container containing liquid methyl acetylene propadiene stabilised (MPS) sufficient to generate a stream of liquid; b) removing said stream of liquid MPS from the container; c) vaporising said stream of liquid MPS; d) delivering the vaporised MPS to a mixing chamber of a high velocity 10 oxygen fuel spray gun where it is mixed with oxygen under pressure; o e) introducing said mixture into a combustion chamber of the high velocity oxygen fuel spray gun together with a powdered high temperature o ceramic entrained in a stream of an inert gas; and f) spraying the heated particles of the high temperature ceramic on to 1 5 the surface of a substrate.

2. A method as claimed in Claim 1, in which the high temperature ceramic is chromium oxide which is applied to the substrate up to a thickness of 380 Lm.

3. A method as claimed in Claim 1, in which the high temperature ceramic is zirconium oxide. -9-

4. A method as claimed in Claims 1, 2 or 3 in which the injected inert gas is nitrogen.

A method as claimed in any one of Claims 1 to 4 in which the high temperature ceramic is entrained in a stream or argon.

6. A method as claimed in any one of Claims 1 to 5 in which the vaporised methyl acetylene propadiene is supplied to the mixing chamber at a substantially constant pressure, flow rate and composition.

7. A method of coating a substrate with a high temperature ceramic substantially as herein before described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. 10 DATED this 15th day of February, 2000 THE BOC GROUP plc SAttorney: JOHN D. FORSTER Fellow Institute of Patent and Trade Mark Attorneys of Australia 1_of BALDWIN SHELSTON WATERS