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.
The use of natural diamond in wear components, such as in cutting and grinding tools, is quite old. In addition to extreme hardness, diamond""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.
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., xe2x80x9cStructure And Morphology Of Oriented Diamond Filmsxe2x80x9d, Proc. NIRIM Int. Symp. Adv. Mat., Tsukuba (Japan), Mar. 13, 1994; Wild et al., xe2x80x9cChemical Vapor Deposition And Characterization Of Smooth {100}-Faceted Diamond Filmsxe2x80x9d, Diamond And Related Materials, 2, 158-168, 1993; Clausing et al., xe2x80x9cTextures And Morphologies Of Chemical Vapor Deposited (CVD) Diamondxe2x80x9d, Diamond And Related Materials, 1, 411-415, 1992; Kohl et al., xe2x80x9cOriented Nucleation And Growth Of Diamond Films On xcex2-SiC and Sixe2x80x9d, 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 xcexcm. 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 xcexcm. 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 xcexcm 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 xcexcm.
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.
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