Metals having wear-resistant surfaces and their fabrication

[57] ABSTRACT

A metal article, such as a machine element or tool, composed of a metal base and an integral wearresistant surface layer or coating consisting of one or more finely divided refractory metal oxides (e.g. zirconia and spinel) dispersed in a matrix of a refractory metal (e.g. molybdenum and alloys) with the matrix metal preferably constituting 40 to 70 percent of the volume of the components employed in the coating; optionally, a solid lubricant such as lead oxide or molybdenum sulfide is also dispersed in the coating layer.

These articles are fabricated by depositing the coating components onto the metal base from a plasma jet while the matrix metal is in the molten state in order to thermally bond the surface layer to the metal base or substrate. Various techniques are used to facilitate feeding the components of the surface coating to plasma guns in convenient form, for example by the inclusion of a powdered metal oxide within an elongated tube of the matrix metal or by binding or cementing the powdered oxide to a rod of that metal.

Claims, No Drawings

3,844,729

METALS HAVING WEAR-RESISTANT SURFACES AND THEIR FABRICATION

The present invention concerns metal machine elements having coatings that are characterized by excellent wearing properties as well as methods for manufacturing such articles.

Relative motion of the sliding, rolling and/or revolving types may occur between the surfaces of machine elements that contact one another. Due to functional requirements, such contact surfaces are frequently subjected to heavy unit pressures, cyclical loads and high temperatures as well as high sliding speeds. For these reasons, maintaining the lubrication of the contact surfaces is frequently extremely difficult, and in some instances those surfaces are almost continuously operating within the range of mixed friction or even the solid friction state wherein no lubricant is present between the contacting frictional surfaces. Hence, the machine elements are subjected to the adhesive wear resulting from adhesion of sliding parts to one another as well as abrasive wear and corrosion wear at the contact surfaces resulting in early functional failure and the disadvantages involved in shutting down machines or an entire plant.

Wear-resistant coatings, such as electroplated chromium, thermally sprayed hard alloys and wear-resistant metals, surface diffusion coatings of the type disclosed in U.S. Pat. No. 3,037,883 and electrostatically applied cemented carbide coatings have been applied in the past to the contact surfaces of such machine elements. Unfortunately, many of these coatings were unable to withstand the stresses or failed for other reasons. Although sprayed molybdenum coatings have given good results even under conditions of inadequate lubrication, these coatings also failed at high temperatures.

However, coatings consisting of hard granular materials, such as carbides, borides or nitrides embedded in various metallic matrices have proved useful in improving the wear resistance of steel parts, such as vehicle tracks, gears, etc. For example, cemented titanium carbide coatings increase the service life of tools.

U.S. Pat. No. 3,378,392 discloses spray applications with an oxygen-acetylene powder flame spray gun and plasma guns that deposit wear-resistant and loadresistant surface coatings on bearings, rolls, etc. Almost all of the descriptive matter is concerned with the twocomponent coatings prepared from powder components, and it is stated that such mixtures are preferably sprayed per se. All of the detailed examples in the patent are concerned with depositing coatings derived from the two-component mixtures and general statements as to the results. One essential component is a high melting powder containing one or more of a specified group of refractory metals which include tungsten and molybdenum and their alloys. The other essential component is a “self-fluxing” metal powder which contains at least one fluxing constituent (e.g., boron or preferably boron and silicon) in addition to certain metals. However, there is a brief mention of an optional third component namely other spray materials, such as aluminum, refractory carbides or refractory oxides, such as aluminum oxide or zirconium oxide, and in ad- ₆„ dition the compound molybdenum disilicide, etc. The ° third component may be sprayed in admixture or in conjunction with the two essential powder components, but there is no disclosure as to effects produced by the optional additive.

U.S. Pat. No. 3,016,311 describes rocket nozzles fabricated by spray deposition as zirconia dispersions in 5 tungsten per se or as coatings of these dispersions on graphite, and these articles were found to display good resistance to severe high velocity flame conditions.

U.S. Pat. Nos. 2,570,649, 2,875,043 and 2,936,229 disclose various procedures for spraying coatings using 10 heat fusible materials in wire or rod form, as powders and as rods constructed of powders consolidated by means of synthetic resin binding agents.

The instant invention is directed at the problem of eliminating the aforementioned disadvantages of the 15 frictional contact surfaces of such prior art machine elements and considerably increasing the service life of such parts to thereby substantially improve the economics of operating such machines.

The present invention relates to machine elements 20 with hard and wear-resistant surfaces on metal bases and their production. These surfaces are capable of not only withstanding large stresses and the friction resulting from high sliding speeds but also of sliding contact with one another at elevated temperatures and under 25 conditions of inadequate lubrication. These improved results are obtainable with a metal machine element having a frictional contact surface coated with a thermally bonded wear-resistant composition which consists essentially of at least one refractory metal oxide ³⁰ finely dispersed in a flux-free metal matrix. It has been discovered that no “self-fluxing alloy” or other fluxing agent is necessary in the coating compositions involved in the instant invention. Also, the inclusion of the metal oxide produces unexpectedly less wear on a comple³⁵ mentary uncoated contact surface in engagement with the coated contact surface as is described hereinafter.

The term “metal” is employed herein in its broad sense of referring to alloys as well as elemental metals; ⁴⁰ and the expression “metal oxides” is used as encompassing metal-oxygen complexes, such as the spinels, as well as simple refractory oxides containing a single metal.

In many embodiments of the invention, the metal of the tool or machine element or component may preferably be made of steel. According to the invention, the coating or contact layer consists of a mixture of metals with metal oxides and/or spinels. The metals are preferably molybdenum, tungsten, chromium, nickel, cobalt, ^υ steel and their alloys. Molybdenum and its alloys are particularly preferred and recommended molybdenum alloys are those containing one or more of such metals as copper, nickel, cobalt, chromium, iron, titanium, aluminium, hafnium or zirconium. The metal oxides preferably include zirconia, chromia, alumina, titania, aluminum-magnesium spinel with the formula Mg(AIO₂)₂, or chromium-iron spinel, that is Fe(CrO₂)₂; and zirconia is especially preferred. In general, desirable oxides neither melt nor decompose at temperatures below 1,400°C. and these may be further exemplified by thoria, beryllia, hafnia, magnesia and ceria. According to the invention, the simple metal oxides and spinels may also consist of mixtures, e.g., of zirconia and chromia.

The optimum proportion of the oxide component of such coatings depends on the desired high temperature strength with the amount of oxide being increased with

3,844,729 an increase in the operating temperature. The proportion of metal in the coating should be from 20 to 98 volume percent (preferably 40-70 vol. percent), and the balance is essentially the metal oxide constituent, except for those instances in which a minor proportion of a solid lubricant is present as described hereinafter. On a weight basis the matrix metal may amount to between 20 and 95 percent of the total weight of the coating composition.

The formation of the coatings in accordance with the present invention may utilize certain known techniques, especially plasma spraying and flame spraying (flame plating) as described in U.S. Pat. No. 3,378,392, and those disclosures are incorporated herein by reference. In one embodiment, powder mixtures containing the coating components may be sprayed onto the part to be coated by means of a plasma gun. In order to prevent segregation of the components of the mixture, the powdered and mixed components may be presintered and this product may then be ground to a powder. Segregation may also be averted by addition of a binder compound (e.g., a wax, paraffin or an aqueous solution of methyl cellulose) which desirably provides frangible type of adhesion or consolidation of the powder particles. Finally, coatings having compositions in accordance with the invention may be produced by using two plasma guns simultaneously, one for the matrix metal and one for the refractory oxide component.

Rods or wires containing the components may also be used in forming the coatings. By means of these rods or wires, the coatings in accordance with the invention may be applied by such methods as plasma gun spraying or melting in an oxyacetylene flame or electric arc. The fabrication of such rods or wires may be carried out by the sintering of powdered material and subse- ³⁵quent shaping or forming at elevated temperature, e.g., by forging, rolling or drawing. A sintered rod with a high metal oxide content (i.e., above 15 vol. percent) is usually difficult to work, but it can be encased in a ductile metal sheath which may subsequently be removed, if necessary (e.g., by dissolving the sheath).

Another method of producing such a rod or wire with the desired composition is to coat a matrix metal wire with the oxide material, for example, by dipping the wire in a suspension in which the oxide component is finely dispersed, for example, in an aqueous solution of methyl cellulose, sugar or an aliginate which serves as a thickener for stabilizing the suspension as well as a binding agent. After drying the coating, the dry binding agent cements the oxide powder to the metal core, and the coated wire can be used immediately or it may be further drawn out. Coatings resulting from melting such wires contain the components in a suitably uniform distribution.

Another embodiment is using tubes, e.g., molybdenum tubes, filled with the oxide component or a mixture of metal and oxide components in powdered form, and these tubes may be used immediately or they may be drawn to finer gauges.

A similar method consists in filling a metal block having a bore with powdered oxides or a mixture of oxides and metals and then shaping the block into an elongated tube consisting of a metal jacket on the outside around a core of powdered material having the selected oxide content.

Another method of converting the coating components into the elongated form of rod or wire is to add an extrusion assistant to the original powder and to extrude the plastic mixture. The extrusion assistant is a highly viscous solution either of method cellulose in water or of a wax or paraffin in a suitable organic solvent. Its solvent may be subsequently evaporated and the material may be further consolidated by sintering. Of course, none of this organic matter remains after plasma or flame spraying the coating material.

Another method of producing coatings in accordance with the present invention is to use two plasma guns, of which one sprays the metallic components in the form of wire and the oxide component in the form of powder is sprayed from the other gun.

A further improved embodiment of the invention consists in adding solid lubricants, such as lead oxide, sulfur, molybdenum sulfide or carbon, to the coating materials during the spraying process. Organic lubricants, such as polytetrafluoroethylene, may also be used. The distribution of the lubricant in the coating should be uniform, and this can be accomplished by maintaining uniform rates of supplying the lubricant, the metal oxide and the matrix metal in the aforementioned application methods. Also, the solid lubricant concentration should be restricted to minor contents which do not reduce the mechanical strength of the coating below the level required for any particular application.

In one specific embodiment, a coating consisting of a finely dispersed mixture of 50 vol. percent zirconia and 50 vol. percent molybdenum, applied by means of a plasma gun, has been found to increase considerably the service life of piston rings of internal combustion engines without deleterious effects on the cylinder linings. Although coatings of pure molybdenum have provided approximately the same long service life for the piston ring, the complementary surface of the cylinder sleeve was subjected to increased wear. Thus the zirconia addition evidently causes a considerable inhibition of the metallurgical reactions between the contact sur⁰ faces; such reactions are regarded as predominantly of a physical nature.

The coatings in accordance with the invention have proved suitable for machine parts subject to all types of friction but particularly sliding friction. The coating protects not only the coated parts but also the complementary surfaces.