Coated Cemented Carbide Elements

1. Field of the Invention

This invention is concerned with protective coatings for wear-resistant elements intended for severe operating conditions and with methods of depositing such coatings on the wear-resistant substrates or inserts.

2. Prior Art

Cemented carbides, also known as sintered carbides, are solid and coherent masses made by pressing and sintering a mixture of powders of one or more metallic carbides and a much smaller amount of an iron group metal, such as cobalt, to serve as a binder metal. These carbides may be used as tools or inserts for machining, milling, blanking, and drawing operations; also as shaping dies, spinning dies, and for many other applications. Such tools are capable of performance beyond the limits of high speed steels and in many cases are suitable for use on very hard abrasive materials and on tough alloys. Cemented carbides are widely used for structural parts and tools subject to intense wear or other service loads that require high compressive strength. Suitable carbides for such purposes include the carbides of tungsten, titanium, tantalum, columbium, molybdenum, vanadium, chromium, zirconium, and hafnium.

Cemented carbides because of their unique strength and hardness are frequently subjected to conditions of intense wear. The manner and rate of wear depends very largely on the use and service conditions of the cemented carbide part. Wear is frequently caused by mechanical abrasion which can be greatly intensified by diffusion processes when operating at elevated temperatures. Corrosion is also a cause in the wear of such parts, for example, when cemented carbide is used in corrosive media or when oxidation in the air takes place at elevated temperatures. When cemented carbides are used for shaping of other parts, either by machining and nonchipping shaping operations or by other processes, the cemented carbide parts are not only subjected to heavy mechanical stresses but there is frequently also intense heating which greatly accelerates the wear.

In the machining of long-chipping and short-chipping materials, as for example, steel or cast iron, respectively, wear on the cutting edge can our as a result of cratering of the rake face and abrasion of the clearance face. Furthermore, the chips can adhere or weld onto the cutting edge of the part and cause chipping or splintering of the edge. In interrupted cuts, crack formation may be caused by rapid temperature changes and splintering can occur from heavy impacts.

It has now been discovered that wear-resistant stratified coatings composed of a number of layers of certain different compositions provide a number of significantly improved results of an unpredictable nature as described hereinafter, particularly when these coatings are employed as protective wear-resistant coatings on cemented carbide substrates and other hard material bases. The coated articles have many uses for metal working and other purposes that involve exposure to mechanical wear and abrasion.

The present invention relates to a hard-wear resistant multilayer coating of nonuniform composition in adjoining layers and wherein said coating contains at least two different wear-resistant materials and includes at least two elements of the group consisting of carbon, nitrogen, boron and silicon in chemical combinations with titanium. It also encompasses articles with such coatings on hard material bases or substrates of the type described hereinafter, and especially cemented carbides substrates having an adherent coating that contains at least two different wear-resistant materials and includes carbon and nitrogen in chemical combination with titanium.

This invention also encompasses a method of making the aforesaid articles by decomposition onto substrates from reactive gaseous mixtures under controlled reaction conditions, including the composition of the gaseous mixtures.

Other aspects of the invention involve one or more of such features as the structure of the multilayer (i.e., more than one layer) article and the thickness and disposition or location of its layers, especially carbon-rich layers composed of material of a substantial or high carbon content and a lower nitrogen content, as well as nitrogen-rich layers of material containing less carbon than nitrogen.

The coated articles of the invention may employ as a base or substrate any hard solid material to which the coatings will adhere. These hard materials are generally hard metals or alloys thereof, including metal-like alloys, compositions or materials. Thus, the substrates may be cermets and like products of powder metallurgy, as well as hard metal carbides and/or nitrides, etc. and alloys, such as cobalt-chromium-tungsten, superalloys and other alloys employed in high speed cutting tools. In general, sintered materials are preferred as substrates, especially the wear-resistant cemented carbides. Among the many suitable base or substrate materials, cemented tungsten carbide compositions containing about 5-30% of a binder metal of the iron group (cobalt, iron, and/or nickel) have been used with excellent results, and up to a total of 40% of the tungsten carbide content may be replaced by one or more other carbides, including those of titanium, tantalum, niobium, vanadium or chromium. Cemented carbides containing from 60-90% titanium carbide with the balance being nickel or a nickel-molybdenum alloy binder are also used and up to half of this titanium carbide may be replaced by titanium nitride.

A coating of the present invention is characterized by a multilayer structure, often so thin that the total thickness of the coating is less than 10 or 20 microns, as well as the fact that its chemical composition is not uniform or homogeneous although its physical appearance is typically homogeneous with no strata visible even under a microscope. These coatings are made up of two or more layers, and at least two of the layers are of different composition in respect to having either different chemical components or different proportions of the same components or elements. The principal constituents of the coatings are metal compounds of at least two nonmetals of the group consisting of carbon, nitrogen, boron and silicon, of which carbon and nitrogen are generally preferred. But also compounds of nonmetals, such as boron carbide, boron nitride and silicon carbide, can be used. Titanium may be utilized as the sole metallic component of such compounds; also it is contemplated that it may be associated with considerable amounts of compounds of one or more other metals in Groups III, IV, V and VI of the Periodic Table of Elements that have atomic numbers in the range of 21 to 74. Thus, one may also employ compounds of one or more of such metals as chromium, molybdenum, tungsten, scandium, yttrium, lanthanum, zirconium, hafnium, vanadium, niobium and tantalum, usually in lesser total amount so that the titanium constitutes a major portion or more that 50% of the total weight of the chemically combined metals in the coatings. In some cases it may be also useful to substitute more than 50% of the titanium. If coatings have particularly great ductility are required, then additions of iron, cobalt, or nickel, or molybdenum, or mixtures thereof up to about 20% of the total coating weight are advantageous. While the specific description hereinafter is directed entirely at coatings composed of titanium compounds and their decomposition for the purposes of a full and detailed disclosure of materials that have provided greatly improved results, such description is generally applicable to coatings which also contain the compounds of the aforementioned other metals and nonmetals.

In respect to the contents of nonmetals in the titantium compounds in the coatings of this invention, a nitrogen or carbon content of more than 14% by weight is regarded as a high nitrogen or high carbon content. Amoumts of these elements below 6% are considered low contents, and the intermediate range of 6-14% may be called an average or medium content of carbon or nitrogen. On the other hand, the expressions "carbon-rich" and "nitrogen-rich" herein are intended to denote coating deposits wherein carbon or nitrogen, respectively, constitutes a major proportion of the total weight of combined nonmetals in the particular deposits and also to denote gaseous mixtures containing enough carbon-containing or nitrogen-containing gas material to produce such solid deposits. In general, this means that a solid deposit has a carbon or nitrogen concentration, respectively, that is either a high content or in the upper half of the intermediate content range, say above 10%.

In general, the chemical compositions of each of the various layers or strata are substantially uniform in their planar dimensions, that is across the entire width and length of their areas, by reason of the manner of their deposition from the vapor state onto solid bases or substrates as described hereinafter; instead of the desired and significant differences in composition occur through the depth or thickness of the coating, and the composition of one principal layer is different from the adjoining overlying or underlying layer; and this difference in composition may be slight and progressive in gradations through a transition zone having many ultra-thin strata in some instances or a more pronounced change in other cases.

The arrangement, thickness and composition of the individual layers will depend upon the surface conditions of the wear part. For instance, these may consist of several different layers, namely pure titanium carbide, titanium carbonitrides of medium to high carbon content, pure titanium nitride or titanium carbonitrides with a high nitrogen content. In some cases, it may be advantageous to have five or more layers while in other cases, two principal layers may be sufficient, for example, a carbon-rich layer and a nitrogen-rich layer of titanium compounds. The individual layers can be either contiguous or there may be an intermediate stratified transition zone wherein the intermediate layers or strata have variable carbon and nitrogen contents; that is, the combined carbon content is decreasing and the combined nitrogen content increasing in one direction through the thickness of that zone (e.g., away from the substrate and toward the exterior surface of the composite coating), and the carbon is increasing and nitrogen decreasing in the other direction (e.g., toward the substrate). Such changes in composition may be essentially continuous or very gradual in such a transition zone wherein the composition is changed from a composition similar to that of a principal layer on one side of the zone to a composition similar to that of a different principal layer on the other side of the zone. On the other hand, the change in composition may be very marked as at the single interface of two contiguous carbon-rich and nitrogen-rich coating layers.

The thickness of a transitional carbonitride layer or zone may be about 0.1 to 10 microns. The individual or principal layers of titanium nitride and titanium carbonitride having medium to high nitrogen content, are usually present in thicknesses of about 0.3 to 30 microns. The individual or principal layers of titanium carbide and titanium carbonitride having a low nitrogen content are usually present in thicknesses of 0.2 to 20 microns.

In many cases, it has been found advantageous to have a layer structure in which the layer or stratum with the maximum carbon content is located close to the base or substrate surface and remote from the external surface of the coating, whereas the layer or stratum of maximum nitrogen content is located close to the surface of the coating, and remote from the substrate. For example, the layer near the base may consist of titanium carbide or a titanium carbonitride and the layer near the surface of the coating of titanium nitride. The coating can be formed in such manner that a pure titanium carbide layer is joined to a pure titanium nitride layer by means of a series of carbonitride layers in a transition zone that provide a gradual and continuous change in composition from carbide to nitride.

The formation of the coating layers in accordance with the present invention can be most simply carried out by use of a gas phase deposition. However, other well known methods such as plasma spraying, sputtering, or pack cementation have also been found useful.

Gas phase deposition has the particular advantage that the layer composition can be readily controlled by regulation of the composition, the pressure and temperature of the gas phase. The composition of the gas phase can easily be adjusted by addition of suitable gaseous compounds. Furthermore, the mixing ratio of the gas phase may also be varied during the deposition process in a simple manner which causes a corresponding change in the deposited coating composition. The cemented carbide parts may be cleaned by conventional techniques prior to applying the coating material. Surface carburization of the cemented carbide substrates may also be carried out where the carbon content of the cemented carbide is too low for effective coating with the hard materials of this invention.

Atmospheric pressure may be employed for the gas deposition of all the coating materials as in the examples that follow, but it is also contemplated that it may be desirable to employ higher or lower pressures in some instances. For example, it may be advantageous to employ subatmospheric pressures for depositing coatings of titanium carbide, and it is also contemplated that titanium nitride deposits may be formed with a gaseous mixture at superatmospheric pressures. Suitable operating temperatures for the preferred titanium compounds of carbon and nitrogen are in the 700.degree. to 1200.degree. C. range.

The gas mixture is varied depending upon the composition of the layer desired to be deposited. Generally, in order to form a pure titanium nitride layer, the gas mixture comprises a titanium halide, for example, titanium tetrachloride; nitrogen or gaseous nitrogen compounds such as ammonia, and similar nitrogen compounds. The gas mixture may also contain inert gases, or hydrogen, or a mixture thereof.

In order to form titanium carbonitride layers, a gas mixture preferably comprises a titanium halide, a hydrocarbon gas, and nitrogen or a gaseous nitrogen compound, such as, aniline, pyridine, aliphatic and aromatic amines and similar organic nitrogen compounds. The gas mixture may also contain inert gases, or hydrogen, or a mixture thereof.

In order to form a pure titanium carbide layer, the gas mixture may comprise a titanium halide, and hydrocarbon gas. The gas mixture may also contain inert gases, or hydrogen, or a mixture thereof. Other sources of carbon for the processes are the vapors of organic halides of either the aliphatic or aromatic type.

According to the present invention, there is provided a new coating for cemented carbide parts which reduces abrasion and cratering of the surfaces of the cemented carbide tool inserts to a surprising and outstanding degree. This is in marked contrast with results that have been obtained with tool inserts bearing a coating of uniform composition. For instance, special cutting disposable inserts and tips made of cemented carbides for machining parts have recently been developed, and these have a wear-resistant surface coating of titanium carbide. Such disposable cutting inserts, e.g., coated with titanium carbide, generally have had two to three times the service life of similar uncoated disposable tips when used under comparable machining conditions. However, their service life is limited by the cratering type of wear although the clearance face wear is relatively slight.

It has been proposed in the U.S. Pat. No. 3,717,496 to coat cemented carbide inserts with titanium nitride or titanium carbonitrides in order to reduce wear. In practice it was found that titanium carbide produces better protection for the clearance face, whereas titanium nitride provides higher resistance to cratering, but the clearance face wear is 2 or 3 times greater than with a comparable insert coated with titanium carbide. Accordingly, in the case of titanium nitride coatings, the great clearance face wear frequently causes failure of the inserts before marked cratering has occured; hence, the improved cratering resistance could not even be utilized.

It has now been discovered that the coatings of the present invention which may consist of different or alternating layers rich in titanium combined with nitrogen and carbon, respectively, impart a much greater wear-resistance in cutting tools than a uniform coating composition of either alone. While the cause for this is not completely understood, this phenomenon is probably due to the fact that each of the layers react differently to the different types of wear. As previously mentioned, the clearance face wear in cutting tools is predominantly due to abrasion, while the cratering wear is primarily due to diffusion. If part of a tool coated according to this invention is subjected to one of such types of wear, it is thought that if the uppermost layer contains a composition with little resistance to this type of wear, it will be rapidly abraded and thus expose the next layer having a different composition which is more likely to be highly resistant to this type of wear. Moreover, the combination of the different layers provides surprising and improved results as mentioned hereinafter.

An important advantage of the combination of layers of the new coatings is that they have a less pronounced epitaxial structure and a finer grain structure, and this reduces possibility of cracking and chipping of the layers.

Additional advantages are found when the new coated tool elements are subjected to interrupted cuts, particularly in milling, for they display greater resistance to cyclical temperature changes and to impact effects.

The structure of the coating layers of the present invention is not only of significance for cutting tools, but is equally advantageous for tools used for non-chipping, shaping operations, e.g., for drawing, swaging, or forging.

When drawing thick steel wire using drawing dies coated with titanium carbide alone it has been found that the titanium carbide is highly resistant to the large tensile forces exerted on the surface of the drawing die bore; however, galling of the wire was observed relatively early. The formation of undesirable grooves in the wire was found to be due to peeling of the titanium carbide coating.

When drawing dies were equipped with the combined titanium carbide-titanium nitride coating of this invention, there appeared to be a substantial reduction in the coefficient of friction, for the drawing force was reduced; consequently the service life was increased compared with the comparable drawing dies having only a pure titanium carbide coating. This reduction of the drawing force also makes it possible to effect larger reductions in a single pass.

The following examples illustrate the practice of the invention.

Cemented carbide plates (84% tungsten carbide, 10% titanium and tantalum carbides with 6% cobalt as a binder metal) or drawing dies (tungsten carbide containing 6% cobalt as a binder metal) which are to be coated are first cleaned and then carburized in order to enrich the surface with carbon. The cemented carbide parts are then annealed at 850.degree. to 1200.degree. C. in a gas mixture consisting of 92.3 volume % of hydrogen, 0.4% titanium tetrachloride, 7% methane and 0.3% nitrogen. The gas mixture reacts at the surface of the cemented carbide part and forms an initial titanium carbonitride deposit with a high carbon content. After 5 to 20 minutes, the flow of methane is slowly reduced and that of the nitrogen increased over a period of 10 to 40 minutes. After this operation, the parts are coated for 20 to 60 minutes in a gas mixture consisting of 0.5% titanium tetrachloride, 69.5% hydrogen and 30% nitrogen, which yields a final layer of pure titanium nitride.

Metallographic examination of the coatings showed that, depending upon the process parameters (temperature, time and gas composition), the coatings comprise a 0.5 to 3 microns thick initial layer of titanium carbonitride of high carbon content (80% titanium 18% carbon and 2% nitrogen) in contact with the carburized cemented carbide substrate, and the composition of the deposited material changes gradually to provide a 3 to 20 microns thick surface layer of titanium nitride. Since titanium carbide and titanium nitride are miscible in any proportions, sharp boundaries between the individual layers in the coating cannot be observed.

Cemented carbide tips (71% tungsten carbide, 20% titanium and tantalum carbides with 9% cobalt) or drawing dies (tungsten carbide containing 9% cobalt binder) to be coated are first cleaned and may be subsequently carburized in order to enrich the surface with carbon. The parts are then annealed at 850.degree. to 1200.degree. C. in a gas mixture consisting of 0.4 volume % of titanium tetrachloride, 10% methane, 0.2% nitrogen and the remainder of hydrogen. The gas mixture reacts at the surface of the cemented carbide parts resulting in the deposition of a titanium carbonitride of high carbon content. After 5 to 20 minutes, the proportion of methane is slowly reduced, while that of nitrogen is increased during a transition deposition period of 30 to 60 minutes. The parts are further coated in a gas mixture consisting of 0.4% by volume of titanium tetrachloride, 1.2% methane, 74% hydrogen and 25% nitrogen, with the nitrogen content now higher than the methane content, for another 20 to 60 minutes, with the result, that a titanium carbonitride layer of high nitrogen content is now deposited. Atmospheric pressure is employed in all of the foregoing treatments and in all other detailed examples herein.

Metallographic examination of the coatings showed that, depending on the process conditions, each includes a 0.5 to 3 microns thick, hich carbon titanium carbonitride initial layer (80% Ti, 19% C and 1% N), and the composition changes continuously with the content of combined carbon decreasing and nitrogen increasing until a 3 to 20 microns thick final or surface layer of titanium carbonitride with a high nitrogen content (77.5% Ti, 21.5% N and 1% C) is reached.

The disposable inserts coated in accordance with Examples I and II were subjected to machining tests under different cutting conditions and showed an increase in service life of approximately 10-30% compared with inserts coated with pure titanium nitride only. Because of the approximately 25 to 50% reduction in clearance face wear compared with the titanium nitride coated inserts, superior surface quality was displayed by the workpieces produced by the inserts of the present invention.

In thick wire drawing with steel wire (e.g., C60) at high drawing speeds, it was found that the drawing dies equipped with the combined coating of titanium carbide-titanium nitride has approximately 2.5 to 3 times longer service lives than uncoated dies whereas the drawing dies coated with titanium carbide only had approximately twice the service life of the uncoated dies.

Clean cemented tungsten carbide cutting inserts or drawing dies with a 6% content of cobalt are heated in an oven at approximately 850.degree. to 1200.degree. C. in a gas mixture consisting of 0.6 volume % of titanium tetrachloride, 0.6% benzene, 60% argon and 39% hydrogen. The gas mixture reacts at the surface of the cemented carbide parts and titanium carbide is deposited. After 5 to 20 minutes, the supply of benzene and argon is discontinued, the charging of nitrogen is started and the proportions of hydrogen is increased. All of these adjustments are effected quickly so that no significant transition period occurs. The coating is continued with the gas mixture, which now consists of 0.5% titanium tetrachloride, 69.5% hydrogen and 30% nitrogen, for another 20 to 60 minutes, with pure titanium nitride being deposited as the surface layer.

Metallographic study of the coating revealed that depending on the operating parameters, it is made up of a 0.5 to 2 microns thick initial titanium carbide layer contiguous with a 3 to 20 microns thick final layer of titanium nitride.

When subjected to machining tests and depending on the cutting conditions, disposable inserts equipped with this coating display an increase in service life of approximately 10-20% compared with disposable inserts coated with pure titanium nitride only.

Clean cemented carbide inserts (77% tungsten carbide, 10% titanium and tantalum carbides, with 13% cobalt) or tungsten carbide drawing dies of 12% cobalt content are heated in a furnace at approximately 850.degree. to 1200.degree. C. in a gas mixture consisting of titanium tetrachloride, benzene, argon and some hydrogen. The gas mixture reacts at the surface of the cemented carbide parts and titanium carbide is deposited. After approximately 5 to 15 minutes, the supply of benzene and argon is slowly diminished, nitrogen is introduced and the proportions of hydrogen and nitrogen are slowly increased during an interval of 10 to 30 minutes until benzene is no longer present. After 20 to 60 minutes, the proporation of nitrogen is slowly reduced and the charging of benzene and argon resumed in 10 to 30 minutes; finally coating is continued for another 1 to 10 minutes in a mixture now containing only titanium tetrachloride, hydrogen, argon and benzene. The steady gas mixtures are the same as in Example 3.

Metallographic examination of the coating showed that depending on the chosen conditions, the coating comprises a 0.5 to 3 microns thick titanium carbide initial layer in direct contact with the cemented carbide element, then the composition changes gradually and continuously with combined carbon decreasing and nitrogen increasing until a 3 to 20 microns thick layer of titanium nitride is reached; and overlying this are further layers of gradually decreasing combined nitrogen and increasing carbon content, and finally an approximate 0.1 to 0.5 micron thick layer of titanium carbide at the external surface.

Disposable inserts equipped with these coatings gave an important service life in milling applications of approximately 10-30% compared with inserts coated with pure titanium nitride or pure titanium carbide. Furthermore, the surface quality of the milled parts was superior to that of inserts coated with titanium nitride or carbide, because the inserts with the combined coating has less tendency to splintering of the cutting edges.

This invention is not restricted to the above-described embodiments which are intended for purposes of illustration. It may be used to advantage not only in all types of tools for machining and for drawing dies but also for other tools used in chipless forming. In principle, it is possible by means of the invention to increase the wear resistance of all parts which are subject to mechanical wear. Accordingly, the present invention should not be construed as limited in any particulars except as may be stated in the appended claims or required by the prior art.