WO1997003776A1

WO1997003776A1 - Composite powders

Info

Publication number: WO1997003776A1
Application number: PCT/CA1996/000474
Authority: WO
Inventors: Montasser Wael
Original assignee: Westaim Technologies Inc.
Priority date: 1995-07-17
Filing date: 1996-07-15
Publication date: 1997-02-06
Also published as: AU6350896A

Abstract

Composite powders useful for thermal spray coatings or for powder metallurgy parts are provided. The spherical powder particles are characterized by the presence of a fully alloyed metallic phase and a second non-metallic phase uniformly dispersed therein.

Description

COMPOSITE POWDERS Field of the Invention

The present invention relates to novel composite powders exhibiting exceptional mechanical properties and to a process for the production thereof. Background of the Invention

Composite powders are broadly defined as any powder wherein each particle comprises an intimate mixture of metal and ceramic or polymeric particles. These powders may be engineered to attain particular properties for specific applications.

Composite powders are commonly utilized to form protective coatings having good wear, anti-seizing, corrosion resistant and scuffing resistant properties. Typically, the coatings are applied using thermal spray techniques. Other applications for composite powders include the fabrication of powder metallurgy parts, such parts being produced using powder injection moulding, hot isostatic pressing or other powder consolidation techniques.

Examples of composite powders known in the prior art for the thermal spray applications described supra generally comprise mechanical blends of molybdenum - nickel - chromium - chromium carbide powders, or nickel - chromium clad chromium carbide or spray dried and sintered molybdenum - nickel - chromium - chromium carbide powders. The current commercial processes for the production of such composite powders utilize mechanical blending or spray drying techniques. Mechanical blending is a process wherein the constituents remain as separate particles. In spray drying methods, there may be some agglomeration of the constituents into a single form, however the metallic phase does not become fully alloyed. Both processes, deleteriously are expensive processes and the composite powders thus formed are of two separate solid phases. As a result, during the application of these powders, significant undesirable burn-out of carbides takes place. Additionally, the resultant coating is not homogeneous and the metal phase is not fully alloyed.

U. S. patent 2,853,398 issued to V. N. Mackiw et al., and assigned to Sherritt Gordon Mines Ltd. discloses a method of producing composite powders comprising a non - metallic core encapsulated by a metal coating. U. S. patent 3,914,504 to D. A. W. Fustukian describes the production of composite alloy coated particles, such as nickel coated graphite or cobalt coated tungsten carbide, with finely divided particles of at least one metal. U. S. patents 4,291,089 and 4, 374,173 are illustrative of composite powders sprayable by thermal spraying onto a substrate to thereby form an abradable seal coating.

A more modern process contemplates the preparation of composite powders which involves melting the metal component and subjecting the molten metal to gas atomization with the injection of a second phase taking place at the point of atomization. An exemplary process is described in Japanese Patent SHOU 64-56810 assigned to Daido Special Steel Company, Aichi. However, the composite powders formed by this process typically comprise a central metal alloy core with some peripheral second phase particles on the outer surface.

In U. S. patent 4,876,158 issued to Onuki et al., there is disclosed a piston ring having a wear resistant layer, which layer is composed of 40 - 70 wt.% cobalt or a cobalt-based alloy as a binder with the balance being substantially chromium oxide particles. It is evident from the teachings of this reference that the wear resistant coatings for piston rings are still being prepared using mechanical mixing techniques with all the drawbacks associated therewith. Therefore, one seeks to find a process for producing composite powders which is simpler and less costly, but at the same time avoids the problems associated with the prior art whilst yielding powders exhibiting superior performance characteristics. Summary of the Invention

The present invention encompasses two types of composite powders wherein the commonality therebetween resides in the existence of a fully alloyed metallic phase and a uniformly dispersed non - metallic phase in each discrete particle which is attained by the use of starting materials which are in the powder form.

In the first class of composite powders, the non-metallic phase is formed during the preparation thereof. Furthermore, the composite powders are derived from the elemental constituents being in a well mixed powder form. It has been determined that when the mixture is subjected to heating, so that one of the metal constituents reaches its melting temperature, other metals present which have higher melting points dissolve in the molten metal. The molten alloy then reacts with the non-metallic constituents to form the non-metallic phase. As a result it is possible to attain a uniform, fully alloyed composition in the metallic phase with discrete aggregates of the non- metallic phase being uniformly dispersed in the melt. The melt is then atomized to form the composite powder. The atomized composite powders exhibit exceptional hardness, wear resistance and other beneficial mechanical properties.

The second class of composite powders are those wherein preformed non - metallic aggregates are uniformly dispersed in a metallic phase or vice versa and wherein the non-metallic phase is a constituent of the feed mixture and does not change during processing.

SUBSTITUTESHEET(RULE26} In accordance with one aspect of the present invention the first class of composite powders is formed of particles wherein each particle comprises a metallic phase, formed by the dissolution of one or more metals within another metal, at the melting temperature of the latter, said metallic phase comprising one or more alloys; and a second non- metallic phase, said second phase being substantially uniformly dispersed within said metallic phase.

This first class of composite powders finds particular utility as coatings sprayable by thermal spraying but it is to be understood that the utility of the powders is not limited to this single application. Preferred composite powders would be selected from MoNiCr/Cr₃C2 or NiCr/Cr3C2 or the like.

Advantageously, the products formed exhibit uniformity of the fine non - metallic phase, for example the carbide phase in the preferred powders above, in the powder particles thereby providing a uniform composition when utilized as coatings.

As stated earlier, the invention also extends to a second class of composite powders wherein in each particle, a non - metallic phase is trapped inside a metal alloy matrix, or vice versa without any reaction or dissolution between the metal constituents having taken place. Such powders as aluminum-silicon-silicon carbide typically find use in the powder metallurgy parts market.

In accordance with this aspect of the invention there is provided a composite powder formed of particles in which each particle comprises an alloyed metallic matrix; and a non - metallic phase, said non- metallic phase being uniformly dispersed within said metallic matrix or wherein each particle comprises a non - metallic matrix having an alloyed metallic phase uniformly dispersed therein.

Advantageously, again in commencing with starting materials which are in powder form and well admixed, a homogenous composition results because no segregation can take place during the filling of molds, as occurs in the case of mechanically mixed powders, during the manufacture of the parts in powder injection and other powder consolidation processes.

In a process aspect, the invention involves: admixing the powders to be utilized in predetermined quantities; optionally, compacting said powders into forms; melting the powder mixture; and atomizing the molten feed to thereby break down the feed melt with gas jets into particles having a pre-engineered composition and microstructure.

Using this process for the production of the first class of composite powders it is possible to dissolve high melting point metals (such as molybdenum and tungsten) which otherwise would not easily melt, thus permitting operation of the process at a much lower temperature. Description of the Drawings

The products of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a photomicrograph showing a perspective view a MoNiCr/Cr3C2 atomized powder particle; Figure 2 is a photomicrograph showing a cross- sectional view of a NiCr/Cr3C2 composite powder particle;

Figure 3 is a photomicrograph showing a cross- sectional view of an AlSi/SiC composite powder particle; .

SUBSTITUTESHEEKBULE26) Figure 4 is a photomicrograph showing a cross- sectional view of an AlMg/A^O composite powder particle; and

Figure 5 is a photomicrograph showing a cross- sectional view of a MoNiCr/Cr3C2 composite powder particle wherein the carbide has been formed in- situ from a carbon-containing feed material. Description of the Preferred Embodiment

Having reference to the accompanying figures, the products of the invention and the process for their production will now be described.

Suitable powders functional to form the metallic phase are selected from molybdenum, chromium, nickel, zinc, aluminium, copper, silicon or cobalt. The particle size ranges from 5 to 200 microns. Typically one metal having a lower melting temperature would be used in conjunction with at least one metal which is soluble in the lower melting temperature metal. The quantity/quantities of metal powder would be selected to provide the desired composition.

The non - metallic phase of the composite powder is selected from a ceramic or a preformed carbide, or from a carbide formed in situ. The second phase should be particulate and may be either reactive or non-reactive. Suitable compounds would include such ceramics as alumina or aluminium nitride or zirconia or silicon carbide. Alternatively, a stoichiometric amount of carbon may be added to form the carbide. In the cases wherein the non - metallic phase does not react with the metal phase, it is evident that, depending upon the quantities of constituents it is possible to obtain a metallic matrix having a non - metallic second phase or a non - metallic matrix having a metallic phase therein. The starting material powders are admixed in the desired amounts using a "V" type blender for a time in the range of about 3 to 4h.

Optionally, at this stage the mixed powders may be compacted into shaped forms using a press at a pressure in the range of about 50,000 psi. at ambient temperature.

The blended powder mixture is then heated to its melting point, the temperature varying with the component powders. Generally, the powders will melt at about the lowest melting point of the powders contained therein. For example nickel containing powders will melt at between about 1440^C - 1450^C irrespective of the fact that they may contain molybdenum which melts at 2760^C. The heating stage is conducted in a furnace with an atmosphere of argon at atmospheric pressure for times ranging between 2 to 4h.

Once melted, the molten mixture is atomized in a conventional gas atomizer and broken down using gas jets into particles having a particle size ranging from between 10 - 150 microns and having the desired composition and microstructure, and is then cooled.

The product and process of the invention will now be described with reference to the following non- limitative examples. Example I

A comparison of the properties of a coating made from a MoNiCr/Cr3C2 composite powder prepared using the prior art process, namely a physical blend of Mo and NiCr/Cr3C2 powders, with a coating made from powders prepared using the process of the present invention, wherein the Mo, Ni, Cr and C are in powder form at the commencement of the process, indicates that the Vickers Hardness for the coating made from the prior art composite powder is 700 - 900, whereas that for the coating made from the composite powder of the present invention is 1800.

Claims

WE CLAIM;

1. A composite powder formed of particles wherein each particle comprises a fully alloyed metallic phase, formed by dissolution of one or more metals within another metal at the melting temperature of the latter, said metallic phase comprising one or more alloys; and a second non - metallic phase, said second phase being substantially uniformly dispersed within said metallic phase.

2. A composite powder as set forth in claim 1 wherein said metal powders forming said metallic phase are selected from molybdenum, or chromium, or nickel, or zinc or aluminium or cobalt or copper or magnesium.

3. A composite powder as set forth in claim 1 wherein said non - metallic phase comprises chromium carbide or alumina or zirconia or silicon carbide.

4. A composite powder as set forth in claim 3 wherein said non - metallic phase is formed by the addition of carbon to form carbides.

5. A composite powder as set forth in claim 1 wherein said metal powders forming said metallic phase are selected from molybdenum, or chromium, or nickel, or zinc or aluminium or cobalt, or copper, or magnesium and wherein said non - metallic phase comprises chromium carbide or alumina or zirconia or silicon carbide.

6. A composite powder comprising a metallic phase of a molybdenum nickel alloy and a non - metallic phase comprising chromium carbide.

7. A composite powder comprising a metallic phase of a nickel chromium alloy and a non - metallic phase of chromium carbide.

8. A composite powder which comprises: a fully alloyed metallic matrix; and a non - metallic phase, said non - metallic phase being uniformly dispersed within said metallic matrix.

9. A composite powder which comprises: a non - metallic matrix; and a fully alloyed metallic phase, said metallic phase being uniformly dispersed within said non- metallic matrix.

10. A composite powder as set forth in claim 8 wherein said metallic matrix comprises aluminium, or silicon, or magnesium.

11. A composite powder as set forth in claim 8 wherein Osaid non - metallic phase is selected from alumina, aluminium nitride, or zirconia or silicon carbide.

12. A composite powder as set forth in claim 9 wherein said non - metallic matrix comprises alumina, aluminium nitride, or zirconia or silicon carbide.

13. A composite powder as set forth in claim 9 wherein said metallic phase comprises aluminium, or silicon, or magnesium.

14. A process for producing composite powders which comprises: admixing the powders to be utilized in predetermined amounts; optionally, compacting said powders into forms; melting the powder mixture; and atomizing the molten feed to thereby break down the feed melt with gas jets into particles having a pre-engineered composition and microstructure.