US2899338A

US2899338A - Thermal element

Info

Publication number: US2899338A
Authority: US
Publication date: 1959-08-11
Anticipated expiration: 1976-08-11

Description

Aug. 11, 1959 c. G. GOETZEL ET AL 2,899,338

THERMAL ELEMENT 2 Sheets-Sheet 1 Filed Jan. 28, 1955 C DATING METAL.

INFEQIOR fiONP CARBIDE. BASE.

. MATERIAL FIG. 2-.

CARBIDE BASE.

[ ATER IAI- FIG. 5.

L m E M MM w m C CARBIDE. BASE HATER IA L.

niww A.

4 K W? N 4 T mi? N m6 $0M GJP/ "m 5 5 L WWW/Jed 460A C MVZ w free THERMAL ELEMENT Claus G. Goetzel, Yonkers, N.Y., Nicholas J. Grant, Winchester, Mass, Leonard P. Skolnick, New York, N.Y., and Jack A. Yoblin, Roseville, Mich, assignors to Sintercast Corporation of America, Yonkers, N.Y., a corporation of New York Application January 28, 1955, Serial No. 485,568

12. Claims. (Cl. 117--65) The present invention relates to coated refractory metal compound composites and more particularly to a method for obtaining adherent ductile coatings upon refractory metal carbide materials characterized by improved resistance to oxidation, improved resistance to thermal and mechanical impact or shock, and generally improved properties at elevated temperatures.

The advent of modern jet engines, rockets and other types of prime movers involving heat engines operating at elevated temperatures of up to about 1000 C. and higher have provoked intensive research in the development of high temperature materials, particularly in the development of thermal elements for use as turbine blades, buckets, nozzles, vanes, guides, partitions, etc. which in use are exposed to cororsive gaseous atmospheres. The use of special fuels containing lead compounds, vanadium compounds and other compounds either present as additives or inherent in fuel composition have been particularly troublesome in view of harmful vapors of lead oxide, vanadium pentoxide, etc. which are given off during combustion of the fuel and which readily chemically attack and corrode unprotected component parts of heat engines at elevated temperatures.

In an attempt to solve the foregoing problem, certain wrought and cast heat resistant alloys of special corrosion resistant compositions were developed. However, these alloys were limited in their application because of their melting points which range in the neighborhood of about 1300 C. to 1500 C. As more powerful jet engines were designed to operate at higher temperatures, additional burdens were placed on these alloys which had to be replaced by more chemically stable materials of higher melting point.

An outstanding material which was developed and proposed to meet this need was a refractory carbide material consisting of about equal proportions by weight of titanium carbide and an alloy of the so-called superalloy type usually containing nickel or cobalt as major alloying constituents, and chromium tungsten, molybdenum, titanium, iron, aluminum, etc. as other alloying constituents. This material could either be prepared by the well-known cementing method employed in the powder metallurgical production of carbide tools, that is by mixing refractory carbide particles with a given amount of binder metal, for example nickel or cobalt or an alloy based on these metals, followed by pressing the mixture into a desired shape and thereafter sintering it; or the material could be produced by forming a sintered porous skeleton comprising a refractory metal carbide and thereafter infiltrating it with a heat resistant metal or alloy to form a strong composite structure. The latter method was preferred over the former as it enabled the consistent production of superior materials. It was found that thermal elements produced by infiltration comprising about 50% by weight of titanium carbide and about 50% by weight of a nickel-base alloy containing about 13% to 15% chromium and about 6% to 7% iron as alloying constituents could withstand a stress of about 42,000 p.s.i. at a temperature of about 875 C. for hours in an oxidizing atmosphere before it would rupture. It was also found that the same material at 985 C. could withstand a stress of 16,000 psi. before it would rupture after 100 hours of testing. At the lower temperature the thermal elements after testing exhibited an elongation of the order of about 2% to 3% and an impact strength of about 10 ft. lbs. for a Charpy-type unnotched bar While at the higher temperature they exhibited an elongation of about 4% to 8% and an impact strength of about 6.5 ft. lbs.

The foregoing properties have been found satisfactory for parts to be used in short-time applications in jet engines, and performance records of up to about 100 hours have been established in actual service tests. However, present demands for increased service life under more aggravated service conditions prevailing in the latest jet engine and rocket designs have necessitated the development of even better high temperature materials. It was found that it was necessary to improve oxidation resistance, strength, thermal shock and impact resistance of refractory metal carbide composites at temperatures in the range between 950 C. and 1050 C. and bring these properties in closer accord with the extremely good properties which were established for these same materials at 875 C.

It was observed that while these materials were a substantial improvement over wrought or cast heat resistant alloys, they tended to fail during prolonged service as a result of surface deterioration due to oxidation and corrosion at temperatures above 900 C. and of the order of 950 C. to 1050 C. It was found that surface deterioration would occur due to corrosion which markedly deleteriously affected the impact resistance and strength of the material. The failure was usually of a type characteristic of brittle materials. vIt was felt that, in order to inhibit such surface attacks and sustain the properties of the material, it would be necessary to provide a protective surface coating around the exposed working surfaces of the metal carbide composite so as to protect it from hot corrosive atmospheres. Methods proposed for applying such protective coatings included (1) electroplating, (2) deposition from the vapor phase of a metal constituent, (3) deposition of the metal coating resulting from a reaction between the surface to be protected and a halide containing the coating metal, (4) deposition of the coating metal resulting from a reaction of the surface to be protected with a gaseous metal carbonyl, (5) hot dipping the base material in a molten metal coating bath, (6) coating the base material by solid diffusion with a heat resistant cladding or sheathing material, (7) electroless coating from chemical solutions, (8) application of a protective layer of coating metal by spraying a fused wire or a fused powder, (9) casting of a protective metal coating around or onto the base material, etc. Generally speaking, the results Were promising and indicated a step in the right direction. While the service life of the protected material was improved to a certain extent, it was found that when failure did occur it was usually attributed to the quality of the coating and/ or the bond of the coating to the base.

Tests indicated that in order to get consistently improved results, the bonding had to be adequate and the coating produced on the refractory base material had to be substantially non-porous so as to set up a barrier against diffusion of corrosive gases which would otherwise attack the underlying base material. At the same time the coating had to be ductile to enable the equal distribution of applied stress during service and to inhibit brittle failures characteristic of uncoated materials.

For example, when nickel, or chromium, or both were electroplated upon the surface of a titanium carbide composite there was generally IZL high gas pick-up which would result in a porous coating which could only be densified by heating at very high temperatures. The bond formed between the coating material and the base material was generally discontinuous with large zones of inferior bond or no bond at all.

In order to insure a satisfactory bond, it was necessary to heat both the refractory carbide-base material and the coating to a relatively high temperature close to the melting point of the coating itself and in many instances above the melting point of the lowest melting phase of the refractory carbide material. While, in this case, the coating adhered quite strongly to the base material, certain of its properties, particularly its ductility, were frequently affected by excessive high temperature diffusion of carbon and other embrittling agents from the underlying carbide. Thus, while the coating provided requisite resistance to chemical attack in a corrosive environment, it did not always provide adequate resistance to thermal and mechanical impact or shock due to embrittlement of the coating. When failure occurred due to shock, it was usually of the brittle type.

When other coating methods were employed which also utilized very high bonding temperatures above the melting point of the lowest melting phase of the refractory carbide base material, or coating material, similar embrittlement of the coatings occurred. It became apparent that in order to avoid the difficulties inherent in high temperature dififusion bonding, the bonding temperature would have to be controlled and maintained at lower levels during coating to inhibit excessive diffusion of carbon and other embrittling agents from the base material into the overlying coating material. Thus, in View of the limitations imposed by the bonding temperature, it was not always possible to obtain consistently a strong bond while still maintaining a relatively ductile coating.

A method has now been discovered for producing a strongly bonded relatively ductile coating on refractory carbide and other refractory compound materials, whereby thermal elements made of the coated material have improved resistance to oxidation and thermal and mechanical impact and shock at elevated temperatures and are capable of prolonged use under high stresses in corrosive gaseous atmospheres.

The objects of the invention will be apparent from the description when taken in conjunction with the drawings in which:

Fig. l is a reproduction of a photomicrograph taken at 200 magnification showing the cross-section of a coated product produced in accordance with the prior art;

Fig. 2 shows a reproduction of a photomicrograph also taken at 200 magnification showing the lack of bonding obtained with a coating produced by electroplating;

Fig. 3 is a reproduction of a photomicrograph taken at 200 magnification showing the satisfactory bond obtained with the invention; and

Figs. 4 to 6 are diagrams comparing the properties of a refractory carbide body, with and without coating.

Broadly speaking, the invention comprises providing, without materially embrittl'ing it, a strongly bonded, relatively ductile coating on the surface of a material comprising a refractory metal compound (hereinafter referred to as the base material). Because diffusion is most rapid when occurring in the liquid phase, one of the components, either the base material or the coating, must remain solid and preferably be maintained at a temperature ranging up to 100 C. below its lowest melting point. Under the aforementioned conditions, diffusion of carbon and other embrittling agents into the coating will be greatly inhibited. One phase of either the refractory body or of the coating may however be at or above its melting point at the bonding temperature. Thus, if the bonding is to be achieved by utilizing the low melting phase of the 'base material as the bonding agent, then the temperature of the coating material it casting.

should be maintained at a level at which excessive ditfusion of carbon and other embrittling agents into the coating is greatly inhibited. Such a temperature may range up to about C. below the melting point of the coating material. Depending upon the relative melting temperatures of the base and coating materials, the time for achieving a strongly bonded structure may range up to about minutes, generally about 10 to 120 minutes, the shorter time being employed when the melting temperatures of the materials being joined are close together and when the bonding temperature is close to the melting point of the materials. When the aforementioned precautions are taken, excessive diffusion of carbon and other embrittl'ing agents into the coating is greatly inhibited, thus maintaining to a large extent its ductility.

Alternately, the bonding may be achieved by controlling the temperature of the base material so that it does not exceed the melting point of its lowest melting phase. In this case the coating material may be kept at a higher temperature, provided it does not exceed the temperature at which excessive diffusion of carbon or other embrittling agents occur from the base material into the coating. Generally, the temperature of the coating will range from just below the melting point of the lower melting phase of the base material up to about 250 C. above the melting point of the coating material. By maintaining the base material temperature below its lowest melting point, the coating can be cast onto the suface of the base material without excessive difiusion of carbon and other embrittling agents into it so that its ductile properties are not substantially aifected. In the above coating method the bonding time is quite short ranging from about 2 minutes to about a few seconds just sutficient to achieve complete bonding.

One method of quickly producing a cast-on coating completely covering the base material is by centrifugal This is achieved by locating a heat resistant metal infiltrated refractory compound body of desired shape within a mold so that its surface is a predetermined distance from the inner surface of the mold. The mold is disposed radially to the axis of spinning, and may be heated to a temperature below the melting point of the lowest melting phase of the composite refractory compound body. A protective atmosphere such as hydrogen, nitrogen, helium or argon, or a vacuum of between 1 and 200 microns may be provided for the refractory body. The mold is caused to rotate and when coating metal is poured into the mold it is thrust to the outside surface of the body by centrifugal force to fill the space between it and the mold. Thus, the surface of the refractory compound is enriched and is covered by a continuous protective layer of coating material.

In still another method where the temperature of both the refractory metal compound composite and the coating is maintained as low as possible commensurate with obtaining a good bond without sacrificing substantially the ductility of the coating, a bond promoting or precoat alloy is employed as an intermediate transitory coating or interface between the base material and the principal coating. The precoat alloy is generally similar to the coating alloy in its main alloying constituent but diifers in that it contains alloying ingredients which impart bond-promoting properties thereto. Thus, if a ductile metal coating material of the nickel-chromium type in the proportion of 4 parts nickel to one part of chromium is employed to protect the base material, the precoat alloy will generally comprise nickel as the main ingredient, preferably some chromium, and also contain one or more of such ingredients as boron, phosphorous, magnesium, silicon, manganese, carbon, etc. to impart desired bond-promoting properties to the alloy. An important feature of the precoat alloy is that it enables the obtaining of a strong bond at temperatures below the melting point of any of the low melting phases which may be present in the binder metal matrix of the refractory compound composite or base material. Thus, high bonding temperatures of either material are not required to achieve adequate bonding.

An important characteristic of the precoat alloy is its high degree of wettability and flowability in the molten state. Another important characteristic of the precoat alloy is its high degree of diffusibility which enables it in the molten state to diifuse rapidly into both the base material and into the principal coating at relatively low temperatures. However, it is important that the amount of embrittling agents, such as carbon, be limited by the thickness of the precoat layer as well as its proportion in the precoat composition so that diffusion of these agents into the principal coat does not result in its harmful embrittlement.

Normally, the precoat alloy contemplated in the present invention would not be employed as a permanent bonding layer or as the principal coat because of its inherent brittleness. However, when employed in the invention its presence is not apparent because it can be employed in very thin layers to achieve the desired amount of bonding. In this respect, it is transitory and avoids leaving a residual embrittling layer. It is preferred that the precoat layer does not exceed one quarter of the thickness of the principal coat.

As has already been stated, the precoat alloy is usually similar to the coating alloy in its main alloying ingredient and, depending on whether the coating comprises a nickelbase, cobalt-base or iron-base heat resistant alloy of high melting point, the precoat alloy will generally comprise an alloy of lower melting point based on at least one metal of the iron group, i.e., nickel, cobalt and iron. The iron group-base, precoat alloy is one which, for the purposes of the invention, contains a low melting phase, such as an eutectic phase, and has a melting point lower than the melting point of any of the low melting phases which may prevail in the binder metal matrix of the refractory compound composite material. In the case of a nickelbase, or cobalt-base, or iron-base bond-promoting alloy, the low-melting phase may be on involving phosphorous, or boron, or silicon, or magnesium, or combinations of these, etc. Such precoat alloys enable both materials being bonded to be maintained at temperatures not exceeding the melting point of the lowest melting phase of the system which will generally be in the binder metal matrix of the refractory compound composite. The temperature of both bodies being bonded will range from about 300 C. below the melting point of the low melting phase of the binder metal matrix and up to its melting point. Thus, both the surfaces being joined are maintained at a temperature below that at which excessive diffusion of carbon and other embrittling agents into the coating occurs.

The precoat alloy may comprise an iron group metal, generally similar to the principal coating alloy, and may contain at least one of the following ingredients:

Up to 20%magnesium Up to 5% boron Up to 12% phosphorous Up to 4% silicon Up to 2% manganese Up to 2% carbon the total amount of these alloying ingredients usually not exceeding about 20% of the total alloy composition, althrough this will depend upon the relative embrittling effect of the alloying agents and the thickness of the precoat layer relative to that of the principal coat. Alloys of the aforementioned type with to 15% of alloying ingredients will generally have a melting point below 1100 C. Where a nickel-chromium alloy is employed as the coating material (for example, 80% nickel and chromium), it is preferred that the precoat alloy comprise nickel and chromium in the proportion of 4 parts nickel to 1 part chromium. A nickel-chromium precoat alloy containing boron has been found very satisfactory. When the alloy is applied as the intermediate layer, a thickness of about one-tenth to one-fourth the thickness of the final coating has been found satisfactory. The principal coating may be applied over the intermediate layer by either spraying, by sheathing or by electroplating or other suitable means, and the whole heat treated at the appropriate temperature to obtain the requisite bond.

Where the coating is obtained by spraying a ductile heat resistant alloy (e.g. a nickel-chromium alloy) over the bond-promoting layer, the bonding heat treatment is carried out at a temperature above the melting point of the bond-promoting alloy in a reducing atmosphere, preferably of subatmospheric pressure, while maintaining the base and coating materials in the solidus state. A nickelchromium base alloy spray-coating, which is characteristically porous because of the spraying technique, will be infiltrated by the previously applied precoat during liquid phase bonding and consequently densified. The bonding can be performed in high vacuum (0.001 micron to 0.5 micron of mercury column) or moderate subatmospheric pressure (1 micron to 200 microns of mercury column) provided the atmosphere is reducing in nature. A good reducing atmosphere at atmospheric pressure, such as purified and dried hydrogen or carbon monoxide, will also be satisfactory. However, moderate vacuum or subatmospheric pressure without a reducing atmosphere present, or tank hydrogen or any reducing gas which has not been carefully deoxidized and dried, will tend to oxidize the porous sprayed coating and the surface of the base material. This condition can completely inhibit bonding or, at best, yield a coating with zones of weak bonds.

Fig. 3 which is a reproduction of a photomicrograph taken at 200 magnification illustrates the improved bond which is obtained when a precoat iayer is employed to promote bonding between a coating metal and a refractory carbide composite according to the invention, i.e. below temperature at which excessive diffusion of embrittling agents occurs to the detriment of the principal coating. It will be noted that there is a relatively sharp line of demarcation between the coating metal 10 (about nickel and 20% chromium) and the refractory carbide base material 11 (about 50% titanium carbide and 50% of a nickel-base alloy) showing practically no diffusion depth of carbon into the coating metal and practically no trace of the precoat material. The coated product was produced in accordance with Example 1 given hereinafter.

In contradistinction, a coating similarly produced but without the benefit of a bond-promoting interface (note Fig. 1) did not exhibit a good bond. It will be noted from the photomicrograph of Fig. 1 that the bonding is incomplete, if there is any bonding at all. The poor bonding which results from electroplating one or more layers of nickel and/ or chromium onto a titanium carbide base material followed by heat treatment below the temperature at which excessive diffusion of embrittling agents occurs and without the use of a bond-promoting precoat alloy is illustrated by the photomicrograph of Fig. 2 which shows practically no adherence of the coating to the base material.

As illustrative of the invention, the following examples are given:

Example 1 A body of titanium carbide bonded with a high temperature resistant nickel alloy (weight ratio approximately 5050) is grit blasted at p.s.i. air pressure with silicon carbide particles. This is followed by a degreasing operation using carbon tetrachloride and ether. The specimen is then coated with a low melting point precoat, approximately 0.002 inch thick per side. The precoat bonding alloy contains from 3% to 5% boron, up to 1% carbon, 1% to 3.5% silicon, less than 5%. total of iron,

7 manganese and magnesium, and the balance nickelchromium in the ratio of 4 parts by weight of nickel to 1 part of chromium. The precoat material is applied with a power spray torch. The precoat is bonded at 1050 C. to 1100 C. for fiftee'n minutes in a. carbon tube induction heat vacuum furnace at to 50 microns pressure. After cooling, the specimen is removed from the furnace and again is blasted using a moderate pressure (50 psi.) and silicon carbide grit. The specimen is then spray-coated with the principal coating material to a thickness of 0.008 inch to 0.010 inch per side. The principal coating material is greater than 95% nickelchromium (80% nickel, 20% chromium), and the balance iron, magnesium, carbon and silicon. The melting point of this alloy is higher than 1150 C. The specimen is then re-treated in the above vacuum furnace at 1100 C. for one hour. The intermediate precoat alloy melts during bonding and infiltrates the sprayed-on principal coating, thus densifying it. The precoat alloy then'diffuses throughout the material leaving practically no trace (Fig. 3).. The finished specimen can be belt ground or polished to accurate dimensions, if necessary.

Example 2 Same as Example 1 above, except that the principal coating material is applied directly over the intermediate precoat alloy before the former has been diffusion treated. The entire composition is now processed in the carbon tube induction-heated vacuum furnace for one hour at 1100" C.

Example 3 A body consisting of titanium carbide grains bonded together with a high temperature resistant nickel-base alloy (weight ratio approximately 50/50) was grit blasted at 100 p.s.i. air pressure with silicon carbide particles. This was followed by a degreasing operation using carbon tetrachloride and ether. .A sheet of nickel-base alloy comprising about 14% chromium, 6% iron and the balance substantially nickel and having a thickness of about 0.010 inch was treated similarly. The sheet material was placed around the carbide body and the entire assembly placed into an alumina coated graphite mold having an internal configuration simulating a turbine nozzle vane. The two halves of the mold were held together by means of a dead weight providing a low pressure of approximately one pound per square inch.

The mold containing the carbide body and the sheet with the weight on top was placed into a vacuum furnace and heated to 1290 C. for one hour. The vacuum was approximately 100 microns of mercury column. During this operation the sheet material became strongly bonded to the refractory carbide by means of the low melting phase in the carbide body. Yet the temperature of the sheet material was sufficiently low (more than 100 C. below its melting point) so as to avoid excessive diffusion of carbon and other embritt-ling agents into it. Such a body exhibited excellent resistance to thermal and mechanical shock and to oxidation and corrosion.

Example 4 A turbine blade form of a titanium carbide skeleton infiltrated with a nickel-chromium alloy of approximately 80% nickel and 20% chromium was manufactured with a graded carbide concentration. A porous blade of varying porosity was first produced having a high porosity at the foot section and a low porosity at the air foil section, the porosity gradually decreasing from about 45% at the foot section to about 20% at the tip of the air foil section. The porous titanium carbide skeleton body thus produced was then infiltrated in a vacuum of about 50 to 150 microns mercury column at approximately 1500" C. The higher concentration of titanium carbide produced in the foil section provided the blade with additional strength and rigidity and resistance to deformation in this portion. The section nearer to the foot of the blade contained a :greater proportion of'the nickel chromium alloy infiltrant, thus providing a tougher and more ductile portion and a less brittle fastening memher. The infiltrated body was placed in a refractory mold of a centrifugal casting machine with the longitudinal axis of the blade disposed radially to the axis of spinning; A protective atmosphere of hydrogen was provided in the mold.

A coating material comprising approximately 52% cobalt, 27% chromium, 12% nickel and 9% tungsten was melted and heated to 1400 C. The metal was poured into the mold and allowed to fill the space between the carbide body and the .mold by centrifugal force. The

temperature of the infiltrated body was maintained below I provided which was filled with excess coating metal and this excess metal portion was machined and used as the root section.

The protection afforded to a high temperature base material by a good coating alloy, properly treated and bonded, is illustrated in Fig. 4, where even under drastic. conditions of oxidation and thermal shock, the coated material yields properties which are relatively constant and unaffected by the treatment. After the material was coated in accordance with the invention, it and comparable uncoated material were subjected to high temperature oxidation at 1000 C. for 20 hours and then subjected to a shock test by quenching rapidly in water. After the quench, the specimens were tested to determine the effect of high temperature oxidation and shock treatments on the modulus of rupture. Referring to Fig. 4, it will be noted that the rupture properties of the coated material was substantially unaffected while the uncoated material deteriorated in the first oxidation-thermal shock treatment.

Figures 5 and 6 show the protection afforded to thin specimens of 50-50 nickel-base alloy bonded titanium carbide composites to various types of property-weakening factors. These thin sections simulate the leading and trailing edges of gas turbine blades and buckets.

Thus, it will be noted from Fig. 5 that when the coating thickness for thin sections (e.g. the leading and trailing edges of a turbine bucket or blade) is con-trolled up to about 0.01 inch thickness (corresponding to about 15% of the thickness of the uncoated section), the coated section sustains high transverse rupture strength at room temperature even after being subjected to oxidation for hours at 1000 C. (note curves B and C), and even when the oxidized coated sections are subjected to five drastic water quenches from 1000 C. (note curves 2 and 3). If no coating at all is used (not curve A and curve 1), the strength properties are generally inferior with or without drastic quenching. This is also generally true of high temperature strength properties where similar trends are noted. It is preferred that when the refractory carbide composite comprises about 55% to 70% by volume of the refractory carbide with the balance substantially the binder metal matrix, the coating thickness at the regions of the leading and trailing edges should range from about 1% to about 30% of the uncoated thickness, and generally at least about 0.0005 inch thick. The thickness of the principal coating may range from about 0.0005 inch to about 0.010 inch. Best results are obtained when the coating is employed on a base material having a refractory composition ranging from about 60% to 70% by volume for thin sections, such as are common for the leading and trailing edges of turbine blades and similar thermal elements.

9 As has been indicated above, Fig. also illustrates the necessity of having a surface coating to impart resistance to thermal shock. It will be noted from the figure (note curve 1) that the uncoated titanium carbide composite was detrimentally affected by 5 water quenches from 1000 C.

The ability of the coated product to resist chemical attack and retain its ductility is also indicated by its ability to bend at 1000 C. after being subjected to oxi dation for 100 hours at an elevated temperature of 1000 C. Thus, Fig. 6 shows that the uncoated body (curve C) exhibited very poor high temperature bending after prolonged oxidation as compared to the good bending properties exhibited by the coated bodies (curves D and E).

Good room temperature impact strength after oxidation is also dependent upon maintaining an adequate corrosion resistant coating. This is shown in Fig. 6 by comparing

curves

5 and 6 of the coated body with curve 4 of the uncoated body.

The refractory compound material referred to herein includes such high melting point materials as the carbides, borides, nitrides, silicides, etc. of titanium, zirconium, chromium, molybdenum, tungsten, vanadium, columbium, tantalum, etc., and mixtures thereof, the expression refractory metal meaning a metal having a melting point above the melting point of iron which is about 2800 F. The invention is preferably applicable to refractory carbide compositions and more particularly applicable to titanium-base carbide composition. The expression titanium-base carbide covers carbides containing substantial amounts of titanium and includes titanium carbide per se. As has previously been stated, the refractory compound composite may be produced by pressing and sintering a mixture of the refractory compound, e.g. titanium carbide, and a binder metal as is well known in the art; or, if desired, the composite may be produced by forming a porous skeleton of titanium carbide which is thereafter infiltrated by a binder metal.

The skeleton body may be of graded porosity so as to provide a component having certain preferred properties. This is particularly desirable in producing turbine blades where the foot section of the porous skeleton is produced with a high porosity of the order of about 40% to 50% and where the tip of the airfoil section is produced with a low porosity of about 15% to 25 the intermediate part of the turbine blade skeleton having a density intermediate the two densities. Thus, when such a graded turbine blade skeleton is infiltrated, the foot section will be highly saturated with the infiltrant metal and consequently capable of being easily machined and welded. The airfoil section of the infiltrated blade with lower amounts of infiltrant metal will have considerable strength and rigidity and thus be capable of resisting the high deformation stresses resulting from the high speed of rotation of the turbine rotor.

The binder metal may comprise an iron group metal, i.e. iron, cobalt and nickel, mixtures of these metals, or heat resistant alloys based on these metals, the alloying ingredients including such metals as chromium, tungsten, molybdenum, tantalum, columbium, titanium, zirconium, etc. Examples of heat resistant binder metals and alloys are: an alloy containing about 13% to 15% chromium and 6 to 7% iron and the balance substantially nickel; an alloy containing about 80% nickel and 20% chromium; an alloy comprising about 13% to 16% chromium. about 15% to 19% molybdenum, about 3.5% to 5.5% tungsten, about 4% to 7% iron and the balance substantially nickel; an alloy comprising about 4 to 6% aluminum and the balance substantially nickel; an alloy comprising about 16% to 20% chromium, about 6% to nickel and the balance substantially iron; an alloy comprising about 25% nickel, 16% chromium, 6% molybdenum and the balance substantially iron; an al' by comprising about 25 chromium, 6% molybdenum '10 and the balance substantially cobalt; and an alloy eoni prising about 26% chromium, 10% nickel, 7.5% tungsten and the balance substantially cobalt, etc.

The coating material may comprise any one of the foregoing alloys or alloys falling within the group of heat resistant alloys as defined herein. The heat resistant alloys contemplated as coating materials are generally nickel-base, cobalt-base, or iron-base heat resistant alloys.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. In a method for producing a strongly coated thermal element by contacting a ductile heat resistant metal coating material with a composite base material comprising substantially refractory metal compound grains selected from the group consisting of carbides, borides, nitrides and silicides of titanium, zirconium, chromium, molybdenum, tungsten, vanadium, columbium and tantalum, and mixtures thereof, distributed through a matrix of a heat resistant binder metal containing a low melting phase based on the binder metal and the refractory metal compound and wherein the coating and composite materials in contact with each other are heated to an elevated temperature to effect the bonding of one with the other, the improvement which comprises maintaining the solidus temperature of at least one of the materials not higher than C. below its lowest melting point for a time not exceeding minutes, shorter times being employed when the melting points of the materials being joined are close together, whereby excessive diffusion of embrittling agents into the coating material is greatly inhibited.

2. The coating method as defined in claim 1 as applied to a composite base material comprising refractory metal carbide grains distributed through a matrix of a heat resistant binder metal, the improvement wherein the coating temperature is maintained at not more than 100 C. below the melting point of the coating material while being maintained above the melting point of the lowest melting phase of the composite base but below the melting point of said composite.

3. The method of claim 2 wherein the refractory carbide material comprises titanium-base carbide.

4. The coating method as defined in claim 1 as applied to a composite base material comprising refractory metal carbide grains distributed through a matrix of a heat resistant binder metal, the improvement wherein the coating temperature is maintained at not more than 100 C. below the melting point of the lowest melting phase of the composite base while maintaining the liquidus temperature of the coating material to not over 250 C. above its melting point.

5. The method of claim 4 wherein the refractory carbide material comprises titanium-base carbide.

6. The method of claim 5 wherein the coating material is applied to the composite base material by casting.

7. In a method for producing a strongly coated thermal element by contacting a ductile heat resistant metal coating material with a composite base material comprising refractory metal carbide grains distributed through a matrix of a heat resistant binder metal, said base material having on the surface thereof a precoat layer of a highly diffusable bond-promoting alloy of melting point below the lowest melting point of the composite base and coating material, the improvement wherein the coating temperature is maintained at not more than 100 C. below the melting point of the lowest melting phase of the composite base and coating material but above the melting point of said precoat layer, whereby the precoat alloy wets and diffuses into both the base material and the coating, thus promoting a strong bond therebetween without any sub- ;stantial a mount of diffusion of carbon and other embritw 'tling agents occurring to the detriment of .the :coating material.

8. The improvement as defined in claim 7 wherein the precoat layer is comprised substantially of an iron group base metal. 7 1

9. The method as defined in claim 8 wherein the retrace tory carbide comprises titanium-base carbide.

'10. The method as defined in-claim 9 wherein'thecoab,

ing material comprises a ductile heat resistant nickelchromium alloy and wherein theprecoat layer comprises ahighlydifiusible bond-promoting nickel-base alloy.

1 1. A'heat resistant turbine blade for heat-engines 'et engines and the like having a leading edge and a trailing edge, said blade comprising a composite base material containing as a major constituent a refractory metal compound selected from the group consisting of carbides, borides, nitrides and silicides of titanium, zirconium,

chromium, molybdenum, tungsten, vanadium, columbium and tantalum, and mixtures thereof, with a 0on tinuous protective coating of a ductile heat resistant metal.

strongly bonded thereto, the thickness ofithecoating in the regions of the leading and trailing edges ranging .up to about 0.010 inch and constituting up to 30% t of the thickness of the base material in said regions, said coating being substantially free from :a brittle difiusion zone.

1 2. A heat resistantturbine blade as defined in claim 1-=1, said blade comprising a refractory metal carbide composite, the thickness of the coating in the regions of the leading andtrailing edges ranging from about 0.0005 inch to 0.010 inch'zand constituting about 1% to 3.0% of the thickness of carbide base material in said regions.

References'Cited in the file of this patent UNITED STATES PATENTS 2,612,442 Goetzel vSept. 30, 1952

Claims

1. IN A METHOD FOR PRODUCING A STRONGLY COATED THERMAL ELEMENT BY CONTACTING A DUCTILE HEAT RESISTANT METAL COATING MATERIAL WITH A COMPOSITE BASE MATERIAL COMPRISING SUBSTANTIALLY REFRACTORY METAL COMPOUND GRAINS SELECTED FROM THE GROUP CONSISTING OF CARBIDES, BORIDES, NITRIDES AND SILICIDES OF TITANIUM, ZIRCONIUM, CHROMIUM, MOLYBDENUM, TUNGSTEN, VANADIUM, COLUMBIUM AND TANTALUM, AND MIXTURES THEREOF, DISTRIBUTED THROUGH A MATRIX OF A HEAT RESISTANT BINDER METAL CONTAINING A LOW MELTING PHASE BASED ON THE BINDER METAL AND THE REFRACTORY METAL COMPOUND AND WHEREIN THE COATING AND COMPOSITE MATERIALS IN CONTACT WITH EACH OTHER ARE HEATED TO AN ELEVATED TEMPERATURE TO EFFECT THE BONDING OF ONE WITH THE OTHER, THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE SOLIDUS TEMPERATURE OF AT LEAST ONE OF THE MATERIALS NOT HIGHER THAN 100*C. BELOW ITS LOWEST MELTING POINT FOR A TIME NOT EXCEEDING 120 MINUTES, SHORTER TIMES BEING EMPLOYED WHEN THE MELTING POINTS OF THE MATERIALS BEING JOINED ARE CLOSE TOGETHER, WHEREBY EXCESSIVE DIFFUSION OF EMBRITTLING AGENTS INTO THE COATING MATERIAL IS GREATLY INHIBITED.