Method of toughening diamond coated tools

Reference Number: N 03-03

Inventors: Drawl, William R.; McCune, Robert C.

Owner: NISTAC

USPTO Link: 4988421

Invention Summary

The method of this invention provides an economical process for toughening diamond or diamond-like coatings as applied to tools. The method comprises making a composite coated tool by the steps of: (a) depositing, by low pressure CVD, a plurality of layers of separated diamond or diamond-like particles onto a nondiamond or nondiamond-like tool substrate which is selected to facilitate such diamond or diamond-like deposition and to retain its strength-related properties after such CVD; and (b) interposing a mechanically adherent, planarized binding material between and on each of such particle layers, the binding material extending across the separated particles within each particle layer and being substantially devoid of diamond graphitizing agents. This method achieves what the prior art has not provided, a thick (greater than 15 microns), adherent, and robust diamond-containing coating system for cutting tools. Such coated tools wear better because of improved diamond crystal positioning and integrity; such tools cut smoother because of a planarized exposed cutting surface.

A planarized coating is one that is smoother and has less conformity to the contours of its supporting surface. Planarization of the binding material is effected by use of physical vapor deposition at temperatures below 500.degree. C. in which binding material particles are impelled for greater impact to cause back scattering of the deposited binding material and thereby erode the peaks and fill in the valleys of the morphology of the binder deposition. Preferably, particles of the binder material can be sputtered from a target and impelled for greater impact by negatively biasing the tool substrate or workpiece to constitute a secondary cathode (i.e., 25-500 electron volts). The ion bombardment from sputtering creates surface defects which increases nucleation sites for diamond, increases density of diamond deposit, and increases mechanical stress of binding material on diamond and substrate.

The binding material is mechanically adhered to the tool substrate by (i) the force of ion impact or bombardment, and (ii) the shrink force resulting from heating previously deposited binding material layers to the CVD growth temperature for diamond coating and thence cooling to room temperature. This heating and cooling cycle is repeated several times in accordance with the number of layers employed.

A continuous barrier film or layer is preferably deposited by low pressure CVD onto the tool substrate prior to any deposition of diamond particles to provide a platform on which diamond particles will more effectively nucleate and which prevents egress of chemicals contained by the substrate that poison diamond nucleation. The substrate will often be a metal bonded carbide, which metals (i.e., Co, Ni, Fe) are catalytically active to diamond, causing graphitization and attendant reduction of properties. The barrier film or layer is preferably selected from the group consisting of transition metals, transition metal carbides, boron, boron carbide, silicon, silicon carbide, and silicon nitride.

The diamond or diamond-like particles are deposited by a two-stage technique to ensure separation of the particles. The first stage comprises initiating chemical vapor deposition of such separated diamond or diamond-like particles onto the substrate by use of low pressure metastable deposition of carbon in the presence of atomic hydrogen and at a temperature that favors the nucleation of such Particles, the substrate being selected to facilitate such diamond or diamond-like deposition (i.e., substrate constituted of a carbide or carbide former). The particles are formed from a nucleus of four-coordinated carbon atoms with sp.sup.3 bonding. The second stage comprises substantially suppressing nucleation of additional particles before formation of a contiguous film of such particles takes place, while permitting the existing particles to grow to a predetermined maximum crystal size consistent with separated crystals.

Chemical vapor deposition of the diamond or diamond-like particles comprises metastable thermal decomposition of a hydrocarbon gas containing hydrogen (such as 0.5% by volume methane and the remainder essentially hydrogen). Such deposition may be carried out with the use of heterogeneous diamond seeding on the surface or by homogeneous carburizing nucleation directly onto the surface. The presence of atomic hydrogen from such gas is facilitated such as by microwave plasma discharge heating or by filament heating. Nucleation is assured by maintaining the substrate temperature in the range of 600-950.degree. C. with the deposition chamber pressure in the range of 50-100 Torr. Suppression of nucleation is carried out by raising the substrate temperature to at least 1000.degree. C. The density of nucleation can be varied by varying the temperature of the substrate within such range or by varying the roughness and defect sites in the substrate surface.

The tool substrate is preferably selected to retain strength related properties (i.e., fracture toughness and thermal shock resistance) after exposure to temperatures of up to 1050.degree. C. for 20 hours; the substrate preferably is selected from the group consisting of SiAlON, Si.sub.3 N.sub.4, SiC, W, Si, Ti, Co cemented WC, TiC, Ni-Mo alloy cemented TiCN. The binding material is a refractory carbide former and is preferably selected from the group consisting of Ti, Ni, Co, Mn, Cr, Mo, W, Zr, Ta, Si, Hf, and Fe.