Title:
Coating of carbon-containing substrates with titanium carbide
United States Patent 3916052
Abstract:
A method for providing a titanium carbide coating on a carbon-containing substrate. Titanium is electron beam evaporated in a vacuum-pumped enclosed chamber, to form an evaporant stream. The stream is deposited, at least partially as titanium, onto a carbon-containing substrate, such as a tungsten carbide tool bit. The substrate is thereafter heat treated to diffuse carbon, which is derived at least partially from the substrate, into the coating to form titanium carbide.
US Patent References:
Method for coating graphite with metallic carbides
Steinberg - March 1960 - 2929741
Method of forming carbon vapor barrier
Smith - January 1966 - 3230110
Method for forming a refractory metal or carbide coating on refractory materials and article
Lewis et al. - February 1967 - 3306764
/3721577.html
Woerner - March 1973 - 3721577
HIGH RATE DEPOSITION OF CARBIDES BY ACTIVATED REACTIVE EVAPORATION
Bunshah - February 1974 - 3791852
Method for coating graphite with metallic carbides
Steinberg - March 1960 - 2929741
Method of forming carbon vapor barrier
Smith - January 1966 - 3230110
Method for forming a refractory metal or carbide coating on refractory materials and article
Lewis et al. - February 1967 - 3306764
/3721577.html
Woerner - March 1973 - 3721577
HIGH RATE DEPOSITION OF CARBIDES BY ACTIVATED REACTIVE EVAPORATION
Bunshah - February 1974 - 3791852
Inventors:
Shattes, Walter J. (Bloomfield, NJ)
Marancik, William G. (Basking Ridge, NJ)
Marancik, William G. (Basking Ridge, NJ)
Application Number:
05/360706
Publication Date:
10/28/1975
Filing Date:
05/16/1973
View Patent Images:
Export Citation:
Assignee:
Airco, Inc. (Montvale, NJ)
Primary Class:
Other Classes:
427/596, 427/383.300, 427/250, 427/249.190, 427/399
International Classes:
C23C14/58; C23C30/00; C23C14/06; C23C14/14; C23C14/30; C23C14/28; B05D3/00
Field of Search:
117/118,107,93.3,169R,228,221,62,16C
Primary Examiner:
Martin, William D.
Assistant Examiner:
Bell, Janyce A.
Attorney, Agent or Firm:
Cassett, Larry Bopp Edmund Mathews Hume R. W. H.
Claims:
We claim
1. A method of providing a metal carbide armor coating on a substrate comprising tungsten carbide, comprising the steps of:
2. A method in accordance with claim 1, wherein the thickness of said deposited coating is about 5 microns.
3. A method in accordance with claim 1, wherein said heat treatment is conducted in the temperature range of from about 1150° to 1400°C.
4. A method in accordance with claim 3 wherein said heat treatment is continued until substantially all of the metal of the deposited coating is converted to metal carbide.
5. A method in accordance with claim 1 wherein said substrate is maintained at a temperature of between 400°C and 1000°C during deposition of the evaporated metal thereon.
1. A method of providing a metal carbide armor coating on a substrate comprising tungsten carbide, comprising the steps of:
2. A method in accordance with claim 1, wherein the thickness of said deposited coating is about 5 microns.
3. A method in accordance with claim 1, wherein said heat treatment is conducted in the temperature range of from about 1150° to 1400°C.
4. A method in accordance with claim 3 wherein said heat treatment is continued until substantially all of the metal of the deposited coating is converted to metal carbide.
5. A method in accordance with claim 1 wherein said substrate is maintained at a temperature of between 400°C and 1000°C during deposition of the evaporated metal thereon.
Description:
BACKGROUND OF INVENTION
This invention relates generally to a method for providing a hard protective layer on carbon-containing substrates, and more specifically relates to a method for providing a protective coating of titanium carbide upon the surface of a carbide cutting tool.
Carbide cutting tools of the type used, for example, in steel cutting operations, are often armored with a thin layer of a material intended to increase the resistance of the tool to wear, and thereby prolong the useful life of the tool. The tools in question, can, for example, comprise tungsten carbide, which usually is dispersed in a cobalt matrix, with small amounts of tantalum and/or titanium carbide also being present in the tool composition. More specifically, it has in the past been proposed, to increase the life of tools of this and similar types by applying thereto a thin surface-coated layer of titanium carbide. Reference may be had in this connection, for example, to page 30ff., of the MIT Report contained in "NSF Hard Materials Research," Volume 1, 1972. Both in this report, and elsewhere, it is contemplated that metallic titanium be deposited upon and diffused into the carbide tool surface, whereby titanium carbide is formed at the surface of the tool. This prior art, however, has contemplated that the titanium be provided either through electrolytic deposit from a molten salt electrolyte, or by chemical vapor deposit as, for example, by decomposition of titanium tetraiodide, or by other reaction capable of providing the desired chemical vapor.
The above-indicated prior techniques for yielding a titanium carbide armor coating on the said carbide tools, are undesirable in several highly important aspects. Firstly, from the important viewpoint of practicality, the cited techniques simply do not enable production of coated objects at volume or unit costs, which render such techniques commercially feasible. Of at least equal importance is the fact that such prior techniques, being basically chemical in nature, result in highly toxic and polluting by-products, such as for example, titanium chloride or other halogen compound vapors, or similar toxic gaseous matter. Such toxic emanations are dangerous and undesirable, both to the personnel engaged in utilizing the associated equipment, and to the public at large. Further tools coated by the present invention unexpectedly exhibit superior wear characteristics in metal cutting service.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for depositing and forming a titanium carbide armor coating upon carbon-containing substrates, such as carbide tool bits and the like, which method is readily and safely adaptable to high volume production operations at economically reasonable costs, and which method results in little or no toxic by-products.
It is a further object of the present invention, to provide a method for forming a titanium carbide protective layer upon carbide-based cutting tools and the like, which results in a product displaying superior wear resistance in comparison to products coated by methodology of the prior art.
It is a still further object of the present invention, to provide a method for forming a titanium carbide protective layer upon carbide-based cutting tools, which by limiting the amount of carbon diffused from the substrate during formation of the armor coating, has minimal effect upon the strength characteristics of volumes of the substrate adjacent the coating.
SUMMARY OF INVENTION
Now in accordance with the present invention, the foregoing objects, and others as will become apparent in the course of the ensuing specification, are achieved in a method whereby a carbon-containing substrate, such as for example, a carbide tool bit or the like, is positioned in a vacuum-pumped enclosed chamber, wherein an electron beam is utilized for evaporating titanium to form an evaporant stream. The evaporant stream is deposited, at least partially as titanium, onto the said substrate. Thereafter, the substrate carrying the deposited coating, is heat treated at a temperature of approximately 1100° to 1400°C, to diffuse carbon derived at least partially from the substrate, into the coating to form titanium carbide. The said stream may be deposited substantially all as titanium, in which case the carbon is substantially all derived from the substrate. In another embodiment of the method, the chamber may be provided with a bled gas atmosphere of a hydrocarbon such as acetylene or other suitable reactive gas, which reacts with the titanium evaporant stream to provide (at least partially) titanium carbide in the deposit forming on the substrate. In this instance the acetylene therefore serves as a source for at least part of the titanium carbide coating present after the heat treatment.
BRIEF DESCRIPTION OF DRAWINGS
The invention is diagrammatically illustrated, by way of example, in the drawings appended hereto, in which:
FIG. 1 is a diagrammatic cross-sectional view of typical apparatus utilizable in accordance with the method of the invention; and
FIG. 2 is a graph, setting forth wear rates for representative tool elements coated, respectively, by methodology of the prior art, and by the methodology of the invention.
DESCRIPTION OF PREFERRED EMBODIMENT
In FIG. 1 herein, a highly diagrammatic cross-sectional view appears of a typical electron beam furnace 10 which may be utilized in the practice of the invention. As the furnaces utilized herein are not per se of the present invention, these elements are not set forth in detail; but are merely illustrated to aid in an understanding of the manner in which the invention is carried out. The furnace 10 thus depicted includes an enclosed chamber 12, which may be maintained at suitable vacuum conditions by the schematically depicted vacuum diffusion pump 14. A crucible 16, constructed of a suitable inert material, is positioned at the bottom of chamber 12. An evaporant 18, constituting titanium, is present in crucible 16, and is heated to evaporating temperatures by an electron gun 20, energized by power supply 22. Any suitable electron gun may be utilized in accordance with known techniques; for example, in the depicted apparatus a 270° gun is used. The evaporant stream established in consequence of the heating effected by electron beam 24 impinging upon evaporant 18, is schematically suggested at 26. Stream 26 proceeds toward the substrate 28, upon which the film of said evaporant is to be deposited. Substrate 28 is supported in evaporant stream 26 by convenient stand means or the like, which are not explicitly shown in the Figure. The substrate is also heated to a suitable deposition temperature, by a radiant electrical heater 30, with the radiant energy being schematically suggested at 32. Electrical connections for heater 30 are conventional, and are not shown.
The substrate 28 to be coated in the present instance, typically comprises a carbide tool such as, for example, a tungsten carbide tool bit. The tungsten carbide in such material is commonly dispersed in a cobalt matrix which constitutes on the order of 7% of the material by weight. Small fractional percentages of tantulum and/or titanium carbides are usually present as well in the material of the tools. While substrate 28 is depicted diagrammatically as a single element in FIG. 1, it will be understood that one of the principal advantages of the present invention is that a relatively large number of appropriately mounted tool bits, or the like, may be coated at a single time; and more importantly (from a viewpoint of rendering the process commercially practical) the rate of coating may proceed largely in accordance with power input supplied to the said electron beam furnace, as is well known in this art.
During operation of apparatus as set forth in FIG. 1, electrical energy is supplied to heater 30 such that the substrate temperature is typically maintained at around 600°C, with a representative range of suitable operation being from about 400° to 1000°C. Utilizing a power input to gun 20 of from about 2 to 8 kw, a deposition rate upon the substrate of approximately 1 to 5 microns per minute of titanium may be obtained in a typical operation; e.g. a deposition rate of 4 microns per minute of titanium may be deposited utilizing a gun input power of 7 kw. During the deposition process subatmospheric pressure on the order of 10 -5 to 10 -6 Torr are maintained. Most commonly the deposition process is carried out to provide about 5 microns of the titanium coating. As the titanium layer begins to appreciably exceed this level, it is found to be of decreasing value, in that the subsequently formed layer of titanium carbide tends in such thicker depths to be increasingly subject to chipping and similar mechanical damage.
Following deposit of the titanium layer, the substrate (or substrates) are removed from the electron beam furnace 10, and thereupon subjected to a heat treatment at temperatures of the order of 1100° to 1400°C, with the range of 1150° to 1400° being preferred. This heat treatment serves to diffuse carbon from the underlying substrate into the titanium coating to convert substantially all of the titanium to titanium carbide and to make a strong diffusion bond between the substrate and the over layer. This heat treatment is preferably conducted in a standard vacuum heat treating furnace, such as for example, a furnace of this type available from the Richard D. Brewand Company, under the model designation 801. Heat treatment in an inert gas atmosphere is also suitable.
EXAMPLE I
In a typical example illustrating practices of the invention, a tungsten carbide tool bit was positioned as the substrate in a furnace generally of the type illustrated in FIG. 1. The substrate temperature was maintained at 600°C, and utilizing a gun input power of 7 kw the bit was coated at the rate of 4 microns/minute to a layer depth of about 5 microns. The coated tool bit was removed from the electron beam furnace and then heat treated in a vacuum furnace for 1 hour at 1300°C.
In accordance with a further aspect of the present invention, it has been found that instead of depositing substantially pure titanium upon the substrates heretofore discussed, one may suitably deposit upon the substrate a layer which as it deposits constitutes at least partially titanium carbide. This may be effected by electron beam evaporating titanium in an atmosphere including partial pressures of a suitable carbon containing reactive gas. The concept per se of depositing titanium carbide on a substrate by electron beam evaporation of titanium under such conditions, is not part of the present invention, but is, for example, reported in an article by A. C. Raghuram and R. F. Bunshah at page 1389 of J. Vac. Sci. Technol. Vol. 9, No. 6, for November-December, 1972. In carrying out the process of the invention in accordance with this aspect thereof, apparatus of the type shown in FIG. 1 may be utilized, with the addition of means for providing the desired pressure of the selected reactive gas. Preferably, however, apparatus such as that illustrated in the H. R. Harker and R. H. Hill article at page 1397 of the same technical journal just referenced, may be utilized. This latter type of apparatus differs from that illustrated in FIG. 1 in including a pressure barrier enabling maintenance of respectively lower and higher pressures at the electron beam gun, and in the vicinity of the supported substrate. A gas bleed means is provided enabling the reactive gas to leak into the upper part of the apparatus chamber, whereat interaction with the evaporant stream may occur.
Utilizing apparatus of this type the bled gas preferably comprises acetylene, which is bled at a rate such that a partial pressure of the order 5 × 10 -4 Torr is maintained. Depending upon the rate of reactive gas feed and/or the rate of evaporation determined by input power to the electron beam gun, the layer depositing on the substrate in accordance with this latter aspect of the invention, will contain varying proportions of titanium carbide. Typically, depending upon the said factors, from about 60 to 95% titanium carbide will result in the deposited layer, with the balance of the layer being titanium. It may be noted here that production of relatively high proportions of titanium carbide is promoted by utilizing a probe, operated at a potential under 100 volts, which probe projects into the evaporant stream in a manner taught in the Raghuram and Bunshah article, previously referenced.
EXAMPLE II
In a representative procedure incorporating a reactive gas atmosphere, the substrate temperature was maintained at 1000°C, and gun input power at 3kw. The aforementioned probe was operated at 60 volt potential, with a current input of 2 amps. With acetylene being fed in at about 3 cc/sec. to yield a pressure of 5 × 10 -4 Torr, a deposition rate of the order of 1 micron/min. was obtained. A 5 micron layer (about 80% TiC) was built up and the substrate was then removed from the electron beam furnace, placed in a standard vacuum heating furnace, and heat treated at 1300°C for 1 hour.
The coated substrate prepared by use of the reactive gas environment in conjunction with the titanium evaporant stream, in general, is subjected after coating to heat treatment in accordance with the techniques previously described for the pure titanium deposits. One of the principal advantages of utilizing the reactive gas atmosphere, is that less carbon is required to be withdrawn from the substrate during heat treatment for combining with unreacted titanium. This is advantageous, in that some evidence appears to indicate that diffusion of the carbon from the underlying substrate may reduce the strength characteristics of the carbon depleted zones, a phenomenon which may be avoided or minimized by the present technique.
Carbide cutting tools and similar substrates coated in accordance with the techniques of the invention, have been unexpectedly found to display superior wear characteristics, as compared to similar elements coated by the chemical vapor deposition techniques of the prior art. The reason for this is not completely understood at the present time, although it is hypothesized that the evaporant stream provided pursuant to electron beam deposition techniques, may result in deposits of superior physical or chemical characteristics, or may result in a superior bond being formed at the substrate surface. The effect cited, is illustrated in FIG. 2 herein, wherein a pair of curves respectively identified as A and B are set forth. Curve A has been produced by performing wear tests upon a representative tungsten carbide tool bit coated with titanium carbide by chemical vapor deposition techniques of the prior art. The curve represents a plot in millimeters of a linear dimension, measured inwardly from its initial value of 0 at the surface of the newly prepared tool bit, as a function of time of use of the tool in a cutting operation. The curve B shows the result where a tool otherwise similar, is provided with a titanium carbide armor coating in accordance with the present invention. More specifically, the curve B corresponds to a tool that was coated with a titanium layer utilizing an apparatus configuration similar to that set forth in FIG. 1 herein, the layer being deposited to an approximate depth of 5 microns, after which the tool was heat treated at 1300°C for a time period of 1 hour. The most significant point to note in comparing the two curves of FIG. 2 is that while the shape of each is approximately the same, the slopes differ, so that beyond the point 35 the two curves begin to diverge from one another. It is therefore evident that the curve B, representing the tool prepared in accordance with the invention, indicates a markedly decreased rate of wear as a function of time, and that the difference in wear between the two compared tools becomes more pronounced as the period of tool use is increased.
While the invention has been particularly set forth in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the instant teaching. While, for example, as has already been pointed out, the present invention has particular applicability to providing a protective armor coating of titanium carbide on a tungsten carbide cutting tool, or similar constituted substrate, the same basic methodology may be utilized for providing a coating of a carbide of zirconium, hafnium, or niobium, on suitable carbon bearing substrates. Accordingly, the invention is to be broadly construed and limited only by the scope and spirit of the claims now appended hereto.
This invention relates generally to a method for providing a hard protective layer on carbon-containing substrates, and more specifically relates to a method for providing a protective coating of titanium carbide upon the surface of a carbide cutting tool.
Carbide cutting tools of the type used, for example, in steel cutting operations, are often armored with a thin layer of a material intended to increase the resistance of the tool to wear, and thereby prolong the useful life of the tool. The tools in question, can, for example, comprise tungsten carbide, which usually is dispersed in a cobalt matrix, with small amounts of tantalum and/or titanium carbide also being present in the tool composition. More specifically, it has in the past been proposed, to increase the life of tools of this and similar types by applying thereto a thin surface-coated layer of titanium carbide. Reference may be had in this connection, for example, to page 30ff., of the MIT Report contained in "NSF Hard Materials Research," Volume 1, 1972. Both in this report, and elsewhere, it is contemplated that metallic titanium be deposited upon and diffused into the carbide tool surface, whereby titanium carbide is formed at the surface of the tool. This prior art, however, has contemplated that the titanium be provided either through electrolytic deposit from a molten salt electrolyte, or by chemical vapor deposit as, for example, by decomposition of titanium tetraiodide, or by other reaction capable of providing the desired chemical vapor.
The above-indicated prior techniques for yielding a titanium carbide armor coating on the said carbide tools, are undesirable in several highly important aspects. Firstly, from the important viewpoint of practicality, the cited techniques simply do not enable production of coated objects at volume or unit costs, which render such techniques commercially feasible. Of at least equal importance is the fact that such prior techniques, being basically chemical in nature, result in highly toxic and polluting by-products, such as for example, titanium chloride or other halogen compound vapors, or similar toxic gaseous matter. Such toxic emanations are dangerous and undesirable, both to the personnel engaged in utilizing the associated equipment, and to the public at large. Further tools coated by the present invention unexpectedly exhibit superior wear characteristics in metal cutting service.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for depositing and forming a titanium carbide armor coating upon carbon-containing substrates, such as carbide tool bits and the like, which method is readily and safely adaptable to high volume production operations at economically reasonable costs, and which method results in little or no toxic by-products.
It is a further object of the present invention, to provide a method for forming a titanium carbide protective layer upon carbide-based cutting tools and the like, which results in a product displaying superior wear resistance in comparison to products coated by methodology of the prior art.
It is a still further object of the present invention, to provide a method for forming a titanium carbide protective layer upon carbide-based cutting tools, which by limiting the amount of carbon diffused from the substrate during formation of the armor coating, has minimal effect upon the strength characteristics of volumes of the substrate adjacent the coating.
SUMMARY OF INVENTION
Now in accordance with the present invention, the foregoing objects, and others as will become apparent in the course of the ensuing specification, are achieved in a method whereby a carbon-containing substrate, such as for example, a carbide tool bit or the like, is positioned in a vacuum-pumped enclosed chamber, wherein an electron beam is utilized for evaporating titanium to form an evaporant stream. The evaporant stream is deposited, at least partially as titanium, onto the said substrate. Thereafter, the substrate carrying the deposited coating, is heat treated at a temperature of approximately 1100° to 1400°C, to diffuse carbon derived at least partially from the substrate, into the coating to form titanium carbide. The said stream may be deposited substantially all as titanium, in which case the carbon is substantially all derived from the substrate. In another embodiment of the method, the chamber may be provided with a bled gas atmosphere of a hydrocarbon such as acetylene or other suitable reactive gas, which reacts with the titanium evaporant stream to provide (at least partially) titanium carbide in the deposit forming on the substrate. In this instance the acetylene therefore serves as a source for at least part of the titanium carbide coating present after the heat treatment.
BRIEF DESCRIPTION OF DRAWINGS
The invention is diagrammatically illustrated, by way of example, in the drawings appended hereto, in which:
FIG. 1 is a diagrammatic cross-sectional view of typical apparatus utilizable in accordance with the method of the invention; and
FIG. 2 is a graph, setting forth wear rates for representative tool elements coated, respectively, by methodology of the prior art, and by the methodology of the invention.
DESCRIPTION OF PREFERRED EMBODIMENT
In FIG. 1 herein, a highly diagrammatic cross-sectional view appears of a typical electron beam furnace 10 which may be utilized in the practice of the invention. As the furnaces utilized herein are not per se of the present invention, these elements are not set forth in detail; but are merely illustrated to aid in an understanding of the manner in which the invention is carried out. The furnace 10 thus depicted includes an enclosed chamber 12, which may be maintained at suitable vacuum conditions by the schematically depicted vacuum diffusion pump 14. A crucible 16, constructed of a suitable inert material, is positioned at the bottom of chamber 12. An evaporant 18, constituting titanium, is present in crucible 16, and is heated to evaporating temperatures by an electron gun 20, energized by power supply 22. Any suitable electron gun may be utilized in accordance with known techniques; for example, in the depicted apparatus a 270° gun is used. The evaporant stream established in consequence of the heating effected by electron beam 24 impinging upon evaporant 18, is schematically suggested at 26. Stream 26 proceeds toward the substrate 28, upon which the film of said evaporant is to be deposited. Substrate 28 is supported in evaporant stream 26 by convenient stand means or the like, which are not explicitly shown in the Figure. The substrate is also heated to a suitable deposition temperature, by a radiant electrical heater 30, with the radiant energy being schematically suggested at 32. Electrical connections for heater 30 are conventional, and are not shown.
The substrate 28 to be coated in the present instance, typically comprises a carbide tool such as, for example, a tungsten carbide tool bit. The tungsten carbide in such material is commonly dispersed in a cobalt matrix which constitutes on the order of 7% of the material by weight. Small fractional percentages of tantulum and/or titanium carbides are usually present as well in the material of the tools. While substrate 28 is depicted diagrammatically as a single element in FIG. 1, it will be understood that one of the principal advantages of the present invention is that a relatively large number of appropriately mounted tool bits, or the like, may be coated at a single time; and more importantly (from a viewpoint of rendering the process commercially practical) the rate of coating may proceed largely in accordance with power input supplied to the said electron beam furnace, as is well known in this art.
During operation of apparatus as set forth in FIG. 1, electrical energy is supplied to heater 30 such that the substrate temperature is typically maintained at around 600°C, with a representative range of suitable operation being from about 400° to 1000°C. Utilizing a power input to gun 20 of from about 2 to 8 kw, a deposition rate upon the substrate of approximately 1 to 5 microns per minute of titanium may be obtained in a typical operation; e.g. a deposition rate of 4 microns per minute of titanium may be deposited utilizing a gun input power of 7 kw. During the deposition process subatmospheric pressure on the order of 10 -5 to 10 -6 Torr are maintained. Most commonly the deposition process is carried out to provide about 5 microns of the titanium coating. As the titanium layer begins to appreciably exceed this level, it is found to be of decreasing value, in that the subsequently formed layer of titanium carbide tends in such thicker depths to be increasingly subject to chipping and similar mechanical damage.
Following deposit of the titanium layer, the substrate (or substrates) are removed from the electron beam furnace 10, and thereupon subjected to a heat treatment at temperatures of the order of 1100° to 1400°C, with the range of 1150° to 1400° being preferred. This heat treatment serves to diffuse carbon from the underlying substrate into the titanium coating to convert substantially all of the titanium to titanium carbide and to make a strong diffusion bond between the substrate and the over layer. This heat treatment is preferably conducted in a standard vacuum heat treating furnace, such as for example, a furnace of this type available from the Richard D. Brewand Company, under the model designation 801. Heat treatment in an inert gas atmosphere is also suitable.
EXAMPLE I
In a typical example illustrating practices of the invention, a tungsten carbide tool bit was positioned as the substrate in a furnace generally of the type illustrated in FIG. 1. The substrate temperature was maintained at 600°C, and utilizing a gun input power of 7 kw the bit was coated at the rate of 4 microns/minute to a layer depth of about 5 microns. The coated tool bit was removed from the electron beam furnace and then heat treated in a vacuum furnace for 1 hour at 1300°C.
In accordance with a further aspect of the present invention, it has been found that instead of depositing substantially pure titanium upon the substrates heretofore discussed, one may suitably deposit upon the substrate a layer which as it deposits constitutes at least partially titanium carbide. This may be effected by electron beam evaporating titanium in an atmosphere including partial pressures of a suitable carbon containing reactive gas. The concept per se of depositing titanium carbide on a substrate by electron beam evaporation of titanium under such conditions, is not part of the present invention, but is, for example, reported in an article by A. C. Raghuram and R. F. Bunshah at page 1389 of J. Vac. Sci. Technol. Vol. 9, No. 6, for November-December, 1972. In carrying out the process of the invention in accordance with this aspect thereof, apparatus of the type shown in FIG. 1 may be utilized, with the addition of means for providing the desired pressure of the selected reactive gas. Preferably, however, apparatus such as that illustrated in the H. R. Harker and R. H. Hill article at page 1397 of the same technical journal just referenced, may be utilized. This latter type of apparatus differs from that illustrated in FIG. 1 in including a pressure barrier enabling maintenance of respectively lower and higher pressures at the electron beam gun, and in the vicinity of the supported substrate. A gas bleed means is provided enabling the reactive gas to leak into the upper part of the apparatus chamber, whereat interaction with the evaporant stream may occur.
Utilizing apparatus of this type the bled gas preferably comprises acetylene, which is bled at a rate such that a partial pressure of the order 5 × 10 -4 Torr is maintained. Depending upon the rate of reactive gas feed and/or the rate of evaporation determined by input power to the electron beam gun, the layer depositing on the substrate in accordance with this latter aspect of the invention, will contain varying proportions of titanium carbide. Typically, depending upon the said factors, from about 60 to 95% titanium carbide will result in the deposited layer, with the balance of the layer being titanium. It may be noted here that production of relatively high proportions of titanium carbide is promoted by utilizing a probe, operated at a potential under 100 volts, which probe projects into the evaporant stream in a manner taught in the Raghuram and Bunshah article, previously referenced.
EXAMPLE II
In a representative procedure incorporating a reactive gas atmosphere, the substrate temperature was maintained at 1000°C, and gun input power at 3kw. The aforementioned probe was operated at 60 volt potential, with a current input of 2 amps. With acetylene being fed in at about 3 cc/sec. to yield a pressure of 5 × 10 -4 Torr, a deposition rate of the order of 1 micron/min. was obtained. A 5 micron layer (about 80% TiC) was built up and the substrate was then removed from the electron beam furnace, placed in a standard vacuum heating furnace, and heat treated at 1300°C for 1 hour.
The coated substrate prepared by use of the reactive gas environment in conjunction with the titanium evaporant stream, in general, is subjected after coating to heat treatment in accordance with the techniques previously described for the pure titanium deposits. One of the principal advantages of utilizing the reactive gas atmosphere, is that less carbon is required to be withdrawn from the substrate during heat treatment for combining with unreacted titanium. This is advantageous, in that some evidence appears to indicate that diffusion of the carbon from the underlying substrate may reduce the strength characteristics of the carbon depleted zones, a phenomenon which may be avoided or minimized by the present technique.
Carbide cutting tools and similar substrates coated in accordance with the techniques of the invention, have been unexpectedly found to display superior wear characteristics, as compared to similar elements coated by the chemical vapor deposition techniques of the prior art. The reason for this is not completely understood at the present time, although it is hypothesized that the evaporant stream provided pursuant to electron beam deposition techniques, may result in deposits of superior physical or chemical characteristics, or may result in a superior bond being formed at the substrate surface. The effect cited, is illustrated in FIG. 2 herein, wherein a pair of curves respectively identified as A and B are set forth. Curve A has been produced by performing wear tests upon a representative tungsten carbide tool bit coated with titanium carbide by chemical vapor deposition techniques of the prior art. The curve represents a plot in millimeters of a linear dimension, measured inwardly from its initial value of 0 at the surface of the newly prepared tool bit, as a function of time of use of the tool in a cutting operation. The curve B shows the result where a tool otherwise similar, is provided with a titanium carbide armor coating in accordance with the present invention. More specifically, the curve B corresponds to a tool that was coated with a titanium layer utilizing an apparatus configuration similar to that set forth in FIG. 1 herein, the layer being deposited to an approximate depth of 5 microns, after which the tool was heat treated at 1300°C for a time period of 1 hour. The most significant point to note in comparing the two curves of FIG. 2 is that while the shape of each is approximately the same, the slopes differ, so that beyond the point 35 the two curves begin to diverge from one another. It is therefore evident that the curve B, representing the tool prepared in accordance with the invention, indicates a markedly decreased rate of wear as a function of time, and that the difference in wear between the two compared tools becomes more pronounced as the period of tool use is increased.
While the invention has been particularly set forth in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the instant teaching. While, for example, as has already been pointed out, the present invention has particular applicability to providing a protective armor coating of titanium carbide on a tungsten carbide cutting tool, or similar constituted substrate, the same basic methodology may be utilized for providing a coating of a carbide of zirconium, hafnium, or niobium, on suitable carbon bearing substrates. Accordingly, the invention is to be broadly construed and limited only by the scope and spirit of the claims now appended hereto.