Abstract:
A method is set forth for making a diamond film coated wear part, including the following steps: providing a part; depositing a first layer of polycrystalline diamond film on the part with non-{100} crystallographic faceting; and depositing on the first layer a second layer of polycrystalline diamond film having {100} crystallographic faceting, the second layer having a thickness sufficient to overgrow the roughness of said surface of said first layer with a continuous film.

Description:
FIELD OF THE INVENTION 
     This invention relates to improvements in wear components having synthetic diamond wear surfaces and, more particularly, to improved diamond-coated wear components and to a method of making such diamond coated wear components. 
     BACKGROUND OF THE INVENTION 
     The use of natural diamond in wear components, such as in cutting and grinding tools, is quite old. In addition to extreme hardness, diamond&#39;s superlative thermal conductivity, thermal stability, and inertness are unsurpassed for wear applications. In recent times synthetic polycrystalline diamond films have been successfully produced, such as by chemical vapor deposition (CVD), and used commercially in wear applications. The synthetic diamond films can be deposited directly on the base of a wear component as a film. For example, a cutting tool with a hard face or insert formed of tungsten carbide or of a ceramic, such as silicon nitride, can be coated with a thin film of synthetic diamond. Alternatively, the diamond can be produced separately, for example as a thick film, and mounted on the base of a wear component, such as by brazing. 
     After a diamond film has been deposited for use as a wear surface, it is generally necessary to polish the diamond coating in order to obtain a low friction surface. However the same hardness which makes diamond ideal for cutting tools, renders it difficult to polish. Polishing can be achieved with diamond abrasive, but it is expensive. It is among the objects of the present invention to provide diamond film for cutting tools which addresses and solves the problem caused by the difficulty of polishing diamond film cutting surfaces. 
     SUMMARY OF THE INVENTION 
     The present invention uses a layered diamond film structure, for cutting tool application and other wear applications, which reduces or eliminates the need for polishing the diamond surface after deposition of the diamond film. 
     The growth of {110} and {111} crystallographic orientation diamond films, which are the surface textures typically obtained when diamond film is produced by CVD, usually results in a film having a high profile, rough surface. As the diamond film thickness increases, the roughness generally increases for diamond films terminated by these textures since the films grow in a columnar fashion and the grain size of the growth surface increases with film thickness. 
     It is known that {100} orientation diamond film tends to have its facets parallel to the substrate on which it is deposited, thereby resulting in a relatively smooth surface. It has been shown that for {100} textured diamond films, although grain size does scale with film thickness, surface roughness does not. [See Koidl et al., “Structure And Morphology Of Oriented Diamond Films”, Proc. NIRIM Int. Symp. Adv. Mat., Tsukuba (Japan), Mar. 13, 1994; Wild et al., “Chemical Vapor Deposition And Characterization Of Smooth {100}-Faceted Diamond Films”, Diamond And Related Materials, 2, 158-168, 1993; Clausing et al., “Textures And Morphologies Of Chemical Vapor Deposited (CVD) Diamond”, Diamond And Related Materials, 1, 411-415, 1992; Kohl et al., “Oriented Nucleation And Growth Of Diamond Films On β-SiC and Si”, Appl. Phys. Lett. 63 (13) September, 1993.] Since {100} orientation diamond has coplanar facets generally parallel to the substrate surface, the result is known to be a relatively smooth surface, even without polishing. However, the {100} orientation diamond has the least wear resistance as bulk material of the various diamond crystallographic orientations. 
     In the present invention, the smooth surface characteristic of {100} orientation diamond is used to advantage, without the accompanying low wear resistance property, by depositing a different orientation diamond [for example, without limitation, {110} or {111} orientation] for most of the total film thickness, and then depositing an exceedingly thin top layer of {100} orientation diamond. Preferably, the surface layer will have the minimum thickness required to overgrow the rough underlayer and provide a smooth free-surface of the diamond film. The thickness of the {100} faceted top layer required is therefore dependent on the roughness of the underlying film. The resultant diamond film structure has the attribute of toughness resulting from the non{100} faceted material used for the bulk of the film, and the attribute of surface smoothness resulting from the {100} faceted material on the surface. 
     In accordance with an embodiment of the invention, a method is set forth for making a diamond film coated wear part, comprising the following steps: providing a part; depositing a first layer of polycrystalline diamond film on the part with non-{100} crystallographic faceting; depositing on the first layer a second layer of polycrystalline diamond film having {100} crystallographic faceting, the second layer having a thickness sufficient to overgrow the roughness of said surface of said first layer with a continuous film. While the thickness of the first layer is dependent on nucleation density, in practice it may be of the order of 0.5 to 10 μm. The second layer of polycrystalline diamond film having a {100} texture at the free surface has thickness in the range of 0.5 to 25 μm. The thickness is dependent on the application. Cutting tools used for machining metals for example would require a film with a total thickness on the order of 15-45 μm while other wear parts such as a pump seal or bearing surface may require a film with a thickness on the order of 1-10 μm. 
     In a form of the invention, the second layer is deposited using biased nucleation chemical vapor deposition to obtain diamond film having {100} crystallographic orientation the biased nucleation can comprise applying a silica powder impurity on the first layer before deposition of the second layer. Alternatively, the second layer of diamond film can be deposited by chemical vapor deposition using a set of deposition conditions that is different than the set of deposition conditions that were used to deposit the first layer. 
     Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a CVD plasma jet deposition system of a type which can be utilized in practicing an embodiment of the invention. 
     FIG. 2 is a schematic diagram of a CVD microwave plasma deposition system of a type which can be utilized in practicing an embodiment of the invention. 
     FIG. 3 is an operational flow diagram of a procedure in accordance with an embodiment of the invention for producing a synthetic diamond coated wear part. 
     FIG. 4 is an operational flow diagram of a procedure in accordance with another embodiment of the invention for producing a wear component in which a diamond film is made and then attached to a base surface. 
     FIG. 5 is a top view of a wear component of a type typically utilized as an insert for a milling machine cutting tool, and which can employ features of the invention. 
     FIG. 6 is a side view of the wear component of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown a diagram of a chemical vapor deposition (CVD) plasma jet deposition system  200  of a type which can be utilized in practicing an embodiment of the invention. The system  200  is contained within a housing  211  and includes an arc-forming section  215  which comprises a cylindrical cathode holder  294 , a rod-like cathode  292 , and an injector  295  mounted adjacent the cathode so as to permit injected fluid to pass over the cathode  292 . A cylindrical anode is represented at  291 . In the illustrated system the input fluid may be a mixture of hydrogen and methane. The anode  291  and cathode  292  are energized by a source of electric potential (not shown), for example a DC potential. Cylindrical magnets, designated by reference numeral  217 , are utilized to control the plasma generated at the arc forming section. The magnets maintain the plasma within a narrow column until the plasma reaches the deposition region  60 . Optional cooling coils  234 , in which a coolant can be circulated, can be located within the magnets. 
     In operation, a mixture of hydrogen and methane is fed to the injector  295 , and a plasma is obtained in front of the arc forming section and accelerated and focused toward the deposition region. The temperature and pressure at the plasma formation region are typically in the approximate ranges 1500-15,000 degrees C and 100-700 torr, respectively, and in the deposition region are in the approximate ranges 800-1100 degrees C and 0.1-200 torr, respectively. As is known in the art, synthetic polycrystalline diamond can be formed from the described plasma, as the carbon in the methane is selectively deposited as diamond, and the graphite which forms is dissipated by combination with the hydrogen facilitating gas. For further description of plasma jet deposition systems, reference can be made to U.S. Pat. No.s 4,471,003, 4,487,162, and 5,204,144. 
     The bottom portion  105 A of the chamber has a base  106  on which can be mounted a substrate  10  on which the synthetic diamond is to be deposited. If desired, the substrate can be a tool surface or tool insert on which diamond film is to be deposited. The base can include a temperature controller. The substrate may be, for example, molybdenum, tungsten, or graphite, with molybdenum (and its alloys such as TZM, which contains relatively small percentages of titanium and zirconium) being presently preferred. Reference can be made, for example, to U.S. Pat. No. 5,314,652, assigned to the same assignee as the present Application, which describes considerations of roughness of the substrate with regard to appropriate holding and release of the diamond during and after deposition, and also describes the advantageous use of an interlayer (e.g. illustrated at  30  in FIG.  2 ), such as a titanium nitride interlayer, for coating the substrate on which the synthetic diamond is to be deposited and ultimately released. 
     The invention can also be practiced using other deposition equipment, for example a microwave plasma CVD equipment of the type illustrated in FIG. 2. A metal container  310  defines the walls of a microwave cavity  315 , the top of which can be a plate  320 , with finger stock  325 , that serves as an adjustable sliding short. An excitation probe  314  is provided, and its position within the cavity can be made adjustable. A quartz bell jar  335 , which is used to contain a plasma  340 , is mounted on a ringshaped base  350  to which a vacuum chamber  305  and the microwave cavity  310  are attached. A gas injector  357  is used to feed a hydrocarbon gas (e.g. methane) and hydrogen mix into the plasma forming region through apertures indicated at  358 . A cooling line  359  can be used to circulate a coolant to cool the base, or cooling coils (not shown) can be provided. A mounting insert  115 , with a substrate  131  thereon (which, if desired, can be a tool surface or a tool insert), is positioned on a support  392 , and a disc-shaped metal grid  380  can be used to define part of the bottom of the microwave cavity, as shown. In operation, as the mixture of hydrogen and hydrocarbon gas is fed in, microwave energy in the cavity  315  creates the plasma  340 , and polycrystalline diamond will be deposited on the surface of substrate  131 . In a microwave plasma apparatus it is generally beneficial to control temperature at the substrate, and this may be implemented by any suitable known means, for example a heat exchanger. Also, if desired, an apertured BN plate and/or cover can be used on the insert (or on the entire holder) to protect edges from overheating in the microwave field. 
     Referring to FIG. 3, there is shown a flow diagram representative of a procedure in accordance with an embodiment of the invention. The block  410  represents providing of the part, such as the tool surface or tool insert on which a synthetic polycrystalline diamond surface is to be applied. The block  420 , which is optional, represents preparation of the surface of the tool or the insert for the diamond deposition. As one example, if the insert is comprised of Si 3 N 4 , the surface can be prepared by ultrasonic seeding using a suspension of diamond powder in alcohol. The block  430  represents establishing the deposition conditions for depositing polycrystalline diamond which does not have a {100} crystallographic orientation; that is, for example, {110} or {111} diamond that will have a relatively good wear characteristic. This is described further below. The block  440  represents depositing the non-{100} diamond film to a thickness of at least 10 microns. Then, the block  450  represents establishing deposition conditions for depositing polycrystalline diamond having {100} crystallographic faceting. The {100} faceted film is deposited to a prescribed thickness, as represented by the block  460 . The thickness is application dependent but may be in the range 10 to 25 μm. The diamond coated part or tool insert can then be used with no polishing or minimal polishing because of the smooth surface resulting from the thin {100} faceted diamond film. The non-{100} orientation film below the surface (e.g. a {110} or {111} faceted film) provides the desired toughness for the wear part. 
     FIG. 4 is a flow diagram representative of a procedure in accordance with a further embodiment of the invention in which a diamond film is separately made, and then applied to a base surface to make a wear component. The block  510  represents providing a substrate on which the diamond film is to be deposited. The blocks  520 ,  530 ,  540 ,  550  and  560  correspond to the blocks  420 ,  430 ,  440 ,  450  and  460 , respectively, in FIG.  3 . Briefly, the block  530  represents establishing the deposition conditions for depositing polycrystalline diamond which does not have a {100} crystallographic orientation, the block  540  represents depositing the non-{100} diamond film to a thickness of at least 0.5 μm, the block  550  represents establishing deposition conditions for depositing polycrystalline diamond film which is terminated at the free surface by coplanar {100} facets, and the block  560  represents deposition of {100} faceted film to a thickness that is preferably in the range 1 to 10 μm. The block  570  represents removal of the diamond film from the substrate, which will typically occur upon cooling of the substrate. If necessary, the film can then be divided into pieces, such as by laser cutting, and then, for example, attached to the base surface of a cutting tool (as represented by the block  580 ), such as by brazing. 
     FIGS. 5 and 6 illustrate an example of a wear component  20  of a type typically utilized as a cutting insert for a milling machine. A tungsten carbide element or body  24  is provided in a generally rectangular shape with a chamfered corner having a depression  26  that receives a piece of diamond  28  that serves as a cutter. As is known in the art, the diamond  28  can be synthetic polycrystalline diamond film. The diamond can be deposited directly on the element  24  or, more typically for diamond thick films, a piece of synthetic diamond film that is mounted, such as by brazing, on the element  24 . The diamond film (directly deposited, or formed and mounted) is the advantageously surface textured film hereof. 
     As noted above various techniques are known for deposition of diamond film having a {100} crystallographic orientation. Some of the techniques are summarized next. 
     Koidl et al., “Structure And Morphology Of Oriented Diamond Films”, Proc. NIRIM Int. Symp. Adv. Mat., Tsukuba (Japan), Mar. 13, 1994, describes growth of {100} orientation diamond films by bias enhanced nucleation using microwave plasma assisted CVD from CH 4 /H 2  gas mixtures. The deposition temperature and the methane concentration are shown to be selectable to obtain {100} orientation diamond film growth. 
     U.S. Pat. 5,380,349 discloses a mold having a diamond layer, for molding optical elements. An example is presented in this Patent in which a diamond film is grown by microwave plasma with a {100} orientation, which diamond film was shown to polish faster than {111} or {110} orientation film from a comparative example. 
     U.S. Pat. 5,240,740 discloses that a microwave plasma can be used for CVD deposition of a {100} orientation faces on diamond like ball particles. 
     Accordingly, it is seen that, for example, biased-enhanced nucleation and microwave plasma CVD with controlled deposition conditions, have been used to obtain {100} crystallographic orientation diamond film, and these or any other suitable technique can be used herein to obtain the {100} crystallographic orientation diamond film surface texture used on the surface of the wear components of the present invention.