AU617563B2

AU617563B2 - Process to produce a surface coating from molybdenum by thermal spraying

Info

Publication number: AU617563B2
Application number: AU46185/89A
Authority: AU
Inventors: Peter Heinrich; Heinrich Kreye
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 1988-12-15
Filing date: 1989-12-13
Publication date: 1991-11-28
Anticipated expiration: 2009-12-13
Also published as: DE58906109D1; EP0374585B1; ATE96849T1; ES2047646T3; EP0374585A1; DE3842263C1; AU4618589A

Abstract

The invention relates to a method for applying a molybdenum coating by thermal spraying. In known methods, substantial restrictions in carrying out the method and hence also quality defects in the layer produced must be accepted because of the high melting point of molybdenum. In the method according to the invention, in which the coating is produced by flame spraying or high-speed flame spraying with a pre-oxidised or partially oxidised molybdenum spraying material, the said disadvantages are overcome and higher qualities are achieved.

Description

617563 COMMONWEALTH aUSTRALIA PATENTS Ac r 1952 COMPLETE SPECIFICATION NAME ADDRESS OF APPLICANT: Linde Aktiengesellschaft Abraham-Lincoln-Strasse 21 D-6200 Wiesbaden Federal Republic of Germany NAME(S) OF INVENTOR(S): Heinrich KREYE Peter HEINRICH ADDRESS FOR SERVICE: DAVIES COLUSON SPatent Attorneys SI Little Collins Street, Melbourne, 3000.

COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: Process to produce a surface coating from molybdenum by thermal spraying S The following statement is a full description of this invention, including the best method S of performing it known to me/us:- S ta a a

C

The invention refers to a process to produce a surface coating from molybdenum by thermal spraying.

Molybdenum is known as an exceptionaly wear resistant material, and therefore in case of many metals it is the aim to provide these with a high-grade molybdenum surface coating. For this purpose particularly thermal spray processes are used. The difficulties of thermal spraying of molybdenum coatings are particularly due to the fact, that the molybdenum has a very high melting point (26100 C).

Hitherto basically two methods have been used for the thermal spraying of molybdenum: one of them is known to produce the o molybdenum layer by means of plasma spray, i.e. powdery molybdenum is introduced with the aid of a carrier gas into a plasma jet of a plasma burner, it melts there and the molten molybdenum particles are further transported in the existing gas flow onto the to-be-coated surface of the workpiece.

The high melting temperature of the molybdenum makes it necessary, that the spraying particle has to dwell for a certain minimum time in the melting zone, i.e. in the plasma jet.

Consequently, the conveying speed of the molybdenum powder particle is limited. This leads to limitation in the development of the process and limits the quality of the to-be-applied coating. Moreover, in case of plasma spraying there is no quality enhancing fusion of the sprayed particle during the melting d.ue to insufficient oxygen in the melting zone. This also leads to a deterioration of the coating quality.

Besides the plasma spraying with a powdery base material it is also known to produce molybdenum coatings by means of flame spraying with wire or rod-shaped base material. In this case the melting of the molybdenum from a wire is brought about by a combustible gas flame and the transporting of the melting -2particle from the wire to the workpiece takes place also with a carrier gas flow. Due to the high melting point of the molybdenum the flame spraying of molybdenum cannot be carried out as with other materials in a powder form, but the flame spraying of the molybdenum so far has been feasible only with wire and rod-shaped base material, as in this manner the dwell time of the molybdenum in the melting flame is controllable and an adequate melting will be achieved. This, however, means again a considerable limitation as far as the flame spraying technology is concerned, which will also affect the quality of the applied surface coating. For example, due to the feeding of the spraying material in the form of a wire only one relatively low speed of the melting ~15 particle can be achieved when striking the surface of the workpiece, as the acceleration of the particles can take so place only between the melting operation and the striking of the surface of the workpiece, leading to low adhesive strength values and to larger porosity of the applied

~I

20 coating.

On the other hand, regarding the molybdenum material !which is applied in this present invention it is known S"that it can be produced in oxidised or partially oxidised 25 form, where these configurations in comparison with the pure form have a reduced melting point (up to below 1000 0 oc).

A requirement accordingly exists for a thermal spray process to produce molybdenum coatings which overcomes the disadvantages of the known process, and particularly provides in comparison with the known processes a greater flexibility of the process parameters with a consequent better quality of the molybdenum coating.

According to the present invention there is provided a process to produce a surface coating from molybdenum by S$ 910910,dbdatO079,46185.spe,2 rn 1%C thermal spraying by using pre-oxidised or partially preoxidised molybdenum spraying material which is oxidised before the spraying, characterised in that the coating is produced by autogenous flame spraying or high velocity flame spraying with said spraying material in a powdery state.

Because low melting, fully or partially pre-oxidised molybdenum is used as spraying material, particular advantages are achieved especially when using flame spraying or high velocity flame r~r o r~a ti 01 r o a r r r

I

1 r T Mr, 7~~ It:v~ xB 910910,dbdat079,46185.spe,3 spraying as well as a considerably increased flexibility of the process. First of all there is now the possibility to use powdery spraying material even in the case of flame spraying and r high velocity flame spraying. Moreover, it is now feasible to use higher propellent gas and/or combustible gas pressures when using either wire-shaped or powdery pre-oxidised molybdenum than in case of the known process. In addition, when using the flame spraying method the basic advantages present when using an autogenous flame will result, namely the diffferent adjustability of the flame and the consequent feasibility to influence the spraying operation. For example, in contrast to the plasma spraying with the autogenous spraying process an oxidisation of the spraying material can be achieved, leading to a greater hardness of the produced sprayed coating. A reducing affect can be also regulated. In this regard the already preoxidised spraying material proves itself as an advantage.

t Particularly advantageous is the execution of the process according to the invention with powdery spraying material. This development permits a high degree of flexibility with regards the to-be-selected operating pressure for the combustion gas and the carrier gas as well as the possibility to use various grain sizes for the spraying material.

A refinement of the process, which leads to particularly high quality, consists of that, that the operating pressures are selected so, that melting particle velocities greater than 250 m/sec, preferably greater than 300 m/sec are achieved.

tO0 Based on a schematic drawing an example of the process according to the invention is described in detail as follows.

In the drawing a cross-section of a flame spraying burner head E is illustrated, without showing in detail the combustion gas and the material supplies. The burner head has a centrally arranged combustion chamber 21, into which supply channels 22 open for the combustion gas mixture, consisting for example of combustion gas and oxygen, so that the entering burning gas streams cross in the combustion chamber 21 and form a flame 24. Into this flame is introduced the carrier gas stream through a central channel 23. The carrier gas stream contains the pre-oxidised molybdenum spraying material. The spraying material is melted in the flame 24 created by the combustion gas and the oxygen and the entire mixture of burned gas, carrier gas and molten spraying material will be guided further through a main channel adjoining the combustion chamber 21 and will leave the burner head finally in the form of a spraying stream 27. The burner head is cooled through a channel 29 which is formed between an external casing 28 of the burner head and the main channel wall as well as the combustion chamber wall.

This burner head is operated according to the invention as follows: Powdery, fully pre-oxidised molybdenum is introduced through the central channel 23 in a nitrogen flow into the combustion S'o chamber 21. In this connection the nitrogen carrier gas is in an i advantageous design approx. 5 bar. On the other hand hydrogen 2p and oxygen is supplied to the combustion chamber through the S" supply channel 22 in an approximately stoichiometric composition. This takes place at a pressure of approx. 4.5 bar.

By these process parameters a high spraying velocity, as a rule above 300 m/sec, will be produced and on the other hand an adequate melting of the spraying material is also achieved (grain size of the spraying material is approx. 50 to 100 i The pre-oxidised, powdery molybdenum is simultaneously partially reduced and melted while passing through the flame 24 and it strikes the to-be-coated surface of the workpiece in this form.

"0O A particularly hard surface coating is created with an adhesive I strength while having a slight porosity.

0* @0 0 The flame spraying process 'in accordance with the invention may be carried out with other combustible gases, like propane, propylene, acetylene, etc.

Summarising, it should be emphasised once again, that the autogenous thermal spraying process in accordance with the invention, in contrast to the plasma spraying, has the 4 T advantages of an autogenous flame, where simultaneously a high flexibility in the process parameters is achieved. Particularly the advantages of using higher processing pressures at high speed flame spraying, like the advantageously oxidised spraying particles at high velocity, are to be emphasised.

*|e vi f t I l t e ,1 t IS.

Claims

1. A process to produce a surface coating from molybdenum by thermal spraying by using pre-oxidised or partially pre-oxidised molybdenum spraying material which is oxidised before the spraying, characterised in that the coating is produced by autogenous flame spraying or high velocity flame spraying with said spraying material in a powdery state.

2. A process according to c.'tim 1, characterised in that the operating pressures for the combustion gas and the carrier gas are selected so that the spraying particle velocities are greater than 250 m/sec.

3. A process according to claim 1 or 2, characterised in that the spraying particle velocities are greater than 300 m/sec.

4. A process to produce a surface coating from molybdenum according to any one of the preceding claims, substantially as hereinbefore described with reference to the drawings. *i DATED this 10th day of September 1991 Linde Aktiengesellschaft By Its Patent Attorneys DAVIES COLLISON 910910,dbdat.079,46185.spe,6