GB1584371A

GB1584371A - Deposition of protective coatings on workpieces

Info

Publication number: GB1584371A
Application number: GB27753/77A
Authority: GB
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1976-07-07
Filing date: 1977-07-01
Publication date: 1981-02-11
Also published as: DE2630507B2; FR2357656A1; DE2630507C3; FR2357656B1; IT1079751B; DE2630507A1

Description

(54) IMPROVEMENTS IN OR RELATING TO DEPOSITION OF PROTECTIVE COATINGS ON WORKPIECES (71) We, MOTOREN-UND TURBINEN-UNION MUNCHEN GMBH., a German Company of Postfach 500640, 8000 Munchen 50, Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a method for depositing protective coatings on workpieces to combat hot gas corrosion and/or mechanical wear, wherein the protective coating is deposited by the so-called "plasma spraying technique" in which a coating powder essentiallv consisting od an alloy is injected into an ionised hot gas stream and is sprayed onto the workpiece.

Such a method, which is referred to hereinafter as "a method of the kind referred to", is described in an artide entitled "THERMISCHES SPRITZEN IM FLUG TRIEBWERKBAU" (thermal spraying in aircraft engine construction) which is published in the journal "Industrielle Fertigung", No. 65, Vol. 1975, pp. 619 to 624 by the Springer Publishing House. Protective coatings deposited on workpieces by such a method serve to protect the workpieces from undesirable effects of thermal and mechanical loads to which they can be subjected concurrently in practical service as components in aircraft engines or rockets, it aften being impossible to find materials for such components which would satisfy the requirements for both mechanical strength and resistance to corrosion and wear. Components which satisfy similar requirements of both mechanical strength and resistance to corrosion and wear can be required in fields other than the construction of aircraft engines or rockets, for example in the construction of chemical apparatus.

While plasma spraying, as described in the abovementioned article has provided substantial improvements in the manufacture of components for such purposes, there has been a tendency for the useful properties of the coating materials to be inferior to what might have been expected and for the adhesion of the protective coatings on the workpiece to be reduced to a point where the specified requirements were not met.

One object of the present invention is to provide an improvement over known plasma spraying methods such that the useful properties of the resultant protective coatings will be nearer to what might have been expected than has tended to be the case with the coatings deposited by the known techniques and such that the adhesion of the coatings on the sprayed workpiece are improved.

According to one aspect of this invention there is provided a method for depositing protective coatings on workpieces to combat hot gas corrosicn and/or mechanical wear, in which the protective coating is deposited by plasma spraying a coating powder essentially consisting of an alloy under sub-atmospheric pressure conditions, and wherein the workpiece is smoothed by scouring, polishing and/or wet blasting for a finish of Ra (CLA) = 0.8 ,am or better before the spraying.

The cardinal benefit provided by a method in which this invention is embodied is that the risk of oxidation is eliminated both for the workpiece and the coating powder.

Hence the tendency for oxides to be formed embedded in the protective coatings that are deposited, which leads to an impairment of the useful properties of the protective coatings, is minimised, as is the tendency for oxide layers to be formed upon the surface of the workpiece upon which the protective layer is being deposited which reduce the adhesion of the protective layer so formed to the surface of the workpiece upon which it is depostied. Another advantage is that the high velocity of the gas and of the particles under sub-atmospheric pressure conditions leads to an improvement in the density and adhesion of the protective coatings. Furthermore it has been shown that plasma spraying, when performed under sub-atmospheric pressure conditions leads to the formation of an especially uniform coating, a feature which may well be of particular significance to the dimensional accuracy of the workpiece to be coated. It has also been shown that excellent adhesion properties are achieved only on very smooth surfaces such as are characterised by a finish of the degree defined above.

Preferably the coating powder for coatings to inhibit hot gas corrosion consists of one or another of the alloys A-H listed below.

Alloy Cr Al Ni Y C Other metals A 23 13 | 0.7 3.02 0.5 metallic remainder oxide B 23 13 0.02 0.5 Hf remainder C1, 2 33 rem.

c2 23 13 0.7 0.02 remainder D 2 33 rem.

E,' 20 4 rem.

E,' 23 13 0.7 0.02 remainder F 25 3 10 0.5 ~ 5 Ta remainder G 25 10 10 0.5 10 Ta remainder H 22 13 0.7 0.D14 remainder The alloys C, and C2 are mixed together mechanically at 1:1 ratio as are the alloys E, and E2.

Although a number of metals have led to the protective coatings having very beneficial properties, cobalt is the preferred metallic remainder for the coating powders.

Preferably a tungsten carbide-cobalt material of the composition 4.1%C, 11% Co, 2% Fe and the remainder W is used to deposit a coating having wear inhibiting characteristics. It has been shown that the method of this invention gives a coating of extreme hardness when this coating powder is used, with the hardness reading exhibiting maximum uniformity over the entire coating. Conveniently, the grain size of the coating powder is in the 15--30,um range.

The degree of vacuum in the sub-atmospheric pressure conditions may be 50-100 Torr while the pressure of the plasma gas and of the coating powder is about 18 atmg.

It has been shown that this-combination of negative pressure on the one hand and positive pressure of the plasma spraying stream on the other is an optimum compro- mise between the factors that influence the coating thickness to be achieved and the effort needed to maintain them.

The workpiece may be heated additionally during the coating process, which in many cases again improves the adhesion of the protective coating to the workpiece.

Preferably the workpiece is heated by setting up an electric arc between the plasma spraying torch and the workpiece, the arc been generated without much added complexity of construction to produce a considerable heating effect.

The workpiece may be subjected to homogenising at a temperature in the vicinity of 10000 C, after the coating powder has been deposited on it, in order to achieve complete integration between the protective coating and the base material of the workpiece.

The coating surface may be scoured or polished for a finish of Ra (CLA) = 1.2 ,um after the coating powder has been deposited, a finish of that order leading to an increase in the resistance of the protective coating to corrosion.

Conveniently the coating method in which this invention is embodied is used specifically for coating compressor or turbine blades of a gas turbine engine. This application of the invention deserves special mention because the requirements for compressor and turbine blades of gas turbine engines relative to strength and hot gas corrosion alike are extremely stringent and because these workpieces need a maximum of dimensional accuracy and a minimum of surface roughness so as not to impair the flow conditions and, thus, the efficiency of the gas turbine engine.

According to another aspect of this invention there is provided plasma spraying equipment to implement a method according to the preceding aspect of this invention, the equipment including: a) a vacuum chamber to accommodate the workpiece and a plasma spray torch, the vacuum chamber comprising a double-walled cylindrical component having a removable flat cover at its front face and the plasma spray torch being arranged on the removable cover of the vacuum chamber for sliding movement along an axis and for rotation about the axis; b) a holder for the workpiece which is loaded horizontally into the chamber through a lock in the cylindrical casing and which is movable pivotally about a horizontal axis; c) means for establishing sub-atmospheric pressure conditions within the vacuum chamber; d) linlet lines for an electrical supply, the plasma gas supply and a coolant supply which are combined in a chimney-like structure on top of the cover on the vacuum chamber; e) an inlet line for the coating powder supply; and f) means for injecting the powder into the plasma.

Making the vacuum chamber a double-walled cylindrical component helps cooling purposes in that the space between its walls can be used to accommodate a water jacket. Also constructional measures for evacuation are economised since the arrange ment of the plasma spray torch in the removable cover of the chamber makes it possible to unite all connections in the cover and thus minimise sealing problems by making the cover a face-side flat closure for the cylindrical chamber. The ability ob the torch to be moved transversely and concurrently of the part holder for rotation makes for uniform deposition of the coating material in a single operation. Accordingly, the flow of the coating material through the torch can be precipitated and the total time for coating is correspondingly short.

One embodiment of this invention will be described now by way of example with reference to and as illustrated in the accompanying drawing which shows plasma spraying equipment in which this invention is embodied.

The drawing shows a double-walled cylindrical component which forms a vacuum chamber 1. In operation, the space between the double wall serves as a cooling water jacket. The vacuum chamber 1 is evacuated by a vacuum pump 2 connected to the vacuum chamber 1 by a line 3. A filter 4 in line 3 prevents the entry of spraying medium into the vacuum pump. A plasma torch 5 is arranged on a flat circular cover 6 on the face of vacuum chamber 1 for movement along a straight horizontal guide member 7 and simultaneously for rotation about said horizontal guide member 7. These movements are caused by electrical motors 8 and 9, respectively, which are connected such that the two movements are superimposed one upon the other. The supply lines 10, 11, 12 and 13 for power, for coolant, tor current and for plasma gas are combined in a chimney-like structure 14 attached to the cover 6. The locks 15 and 16 are provided in the cylindrical wall of the vacuum chamber 1 for access to the interior of the vacuum chamber A part holder 17 is inserted through lock 15. Part holder 17 takes the shape of a long arbor and is rotated by suitable means such that the work 18 at its front end can be infinitely rotated in front of the plasma torch 5. In the cover 6 of the vacuum chamber 1 two inlet means 20 and 21 are provided additionally for admitting the coating powder, with lines 22 and 23 extending from the inlet means to the torch 5. The work 18 here represents a turbine rotor blade, to which refer all essential coating process data cited hereafter: Turbine rotor blade material Inconel (Registered Trade Mark) 100 (In 100) Preparation of specimen Wet blasting with Al203 80 mesh Coating medium CoCrAIY of the composition 22,77% Cr, 13.14% Al, 0.7% Y, 0.0 14% C, remainder Co Pressure (degree of vacuum) 50 - 100 Torr Coating temperature 10500C Post-heating Homogenize at 10400C for approximately 4 hours Post-treatment Scour Plasma equipment settings Current 800 amps Voltage 70 volts Plasma gas pressure 17.6 atmg Torch-to-work distance 37 cm Plasma gas flow 198 dm3/min Powder flow 26.4 dm3/min Spraying time per blade about 20 secs.

Plasma gas argon WHAT WE CLAIM IS:- 1. A method for depositing protective coatings on workpieces to combat hot gas corrosion and/or mechanical wear, in which the protective coating is deposited by plasma spraying a coating powder essentially consisting of an alloy under sub-atmospheric pressure conditions, and wherein the workpiece is smoothed by scouring, polishing and/or blasting for a finish of Ra (CLA) = 0.8 ,am or better before the spraying.

2. A method according to Claim 1, wherein. the coating powder for hot gas corrosion inhibiting coatings consists of one or another of the following alloys A to H:

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

of the vacuum chamber A part holder 17 is inserted through lock 15. Part holder 17 takes the shape of a long arbor and is rotated by suitable means such that the work 18 at its front end can be infinitely rotated in front of the plasma torch 5. In the cover 6 of the vacuum chamber 1 two inlet means 20 and 21 are provided additionally for admitting the coating powder, with lines 22 and 23 extending from the inlet means to the torch 5. The work 18 here represents a turbine rotor blade, to which refer all essential coating process data cited hereafter: Turbine rotor blade material Inconel (Registered Trade Mark) 100 (In 100) Preparation of specimen Wet blasting with Al203

80 mesh Coating medium CoCrAIY of the composition 22,77% Cr, 13.14% Al, 0.7% Y, 0.0 14% C, remainder Co Pressure (degree of vacuum) 50 - 100 Torr Coating temperature 10500C Post-heating Homogenize at 10400C for approximately 4 hours Post-treatment Scour Plasma equipment settings Current 800 amps Voltage 70 volts Plasma gas pressure 17.6 atmg Torch-to-work distance 37 cm Plasma gas flow 198 dm3/min Powder flow 26.4 dm3/min Spraying time per blade about 20 secs.

Plasma gas argon WHAT WE CLAIM IS:- 1. A method for depositing protective coatings on workpieces to combat hot gas corrosion and/or mechanical wear, in which the protective coating is deposited by plasma spraying a coating powder essentially consisting of an alloy under sub-atmospheric pressure conditions, and wherein the workpiece is smoothed by scouring, polishing and/or blasting for a finish of Ra (CLA) = 0.8 ,am or better before the spraying.
2. A method according to Claim 1, wherein. the coating powder for hot gas corrosion inhibiting coatings consists of one or another of the following alloys A to H:

Alloy Cr Al . Ni Y C Other metal A 23 13 0.7 p.02 0.5 metallic remainder oxide B 23 13 0.02 0.5 Hf remainder C,' 2 33 rem. C,' 23 13 0.7 0.02 remainder D 2 33 rem. E,' 20 4 rem. Ea 23 13 0.7 0.02 remainder F 25 3 10 0.5 5 Ta remainder G 25 10 10 0.5 10 Ta remainder H 22 13 0.7 0.014 remainder

The alloys C, and C, are mixed together mechanically at 1:1 ratio as are the alloys

E, and E,.
3. A method according to Claim 1, wherein the coating powder for wear inhibiting coatings consists of a tungsten carbide material of the composition 4.1%C, 11% Co, 2% Fe, remainder W.
4. A method according to Claim 1, Claim 2 or Claim 3, wherein the grain size of the coating powder is in the 15-30 ssm range.
5. A method according to any one of Claims 1 to 4, wherein the vacuum is 50 to

100 Torr while the pressure ob the plasma gas and of the coating powder is approxi mately 18 atmg.
6. A method according to any of Claims 1 to 5, wherein the workpiece is addi tionally heated during the coating operation.
7. A method according to Claim 6, wherein heating is achieved by an electrical arc generated between a plasma spraying torch and the workpiece.
8. A method according to any one of Claims 1 to 7, wherein the workpiece is subjected to homogenising at a temperature of about 10000C after deposition of the coating powder by spraying on the workpiece.
9. A method according to any one of Claims 1 to 8, wherein the coated surface is scoured and/or polished for a finish of Ra (CLA) = 1.2 pm or better after deposition of the coating powder by Spraying.
10. A method according to any one of Claims 1 to 9 wherein the workpiece is a compressor or turbine blade of a gas turbine engine.
11. Plasma spraying equipment for implementing a method according to one or more of the preceding claims, including: a) a vacuum chamber to accommodate the workpiece and a plasma spray torch, the vacuum chamber comprising a double-walled cylindrical component having a removable flat cover at its front face and the plasma spray torch being arranged on the removable cover of the vacuum chamber for sliding movement along an axis and for rotation about that axis; b) a holder for the workpiece which is loaded horizontally into the chamber through a lock in the cylindrical casing and which is movable pivotally about a horizontal axis; c) means for establishing sub-atmospheric pressure conditions within the vacuum chamber; d) inlet lines for an electrical supply, the plasma gas supply and a coolant supply which are combined in a chimney-like structure on top of the cover on the vacuum chamber; e) an inlet line for the coating powder supply; and f) means for injecting the powder into the plasma.
12. A method for depositing protective coatings on workpieces substantially as described hereinbefore with reference to the accompanying drawing.
13. Plasma spraying equipment substantially as described hereinbefore with reference to and as shown in the accompanying drawing.