WO2001036711A1

WO2001036711A1 - Adherent hard coatings for dental burs and other applications

Info

Publication number: WO2001036711A1
Application number: PCT/US2000/026710
Authority: WO
Inventors: Bernardo N. Nobrega
Original assignee: Kerr Corporation
Priority date: 1999-11-12
Filing date: 2000-09-28
Publication date: 2001-05-25
Also published as: AU7729900A

Abstract

A method is provided for forming strongly adherent hard material coatings on various article substrates (10). A plurality of discrete particles (12) are attached to a substrate surface (14) such that the particles (12) are at least partially embedded in the substrate surface (14) and protrude outwardly relative to the substrate surface (14). The substrate surface (14) having the outwardly protruding particles (12) is thereafter coated with a hard material (16), including generally those materials having a hardness greater than the hardest steel. There is formed thereby an article (10) having a substrate surface (14) coated with a hard material (16) that is strongly adhered to the substrate surface (14) by discrete particles (12) that form a mechanical interlock between the substrate (10) and the hard material coating (16).

Description

ADHERENT HARD COATINGS FOR DENTAL BURS AND OTHER APPLICATIONS

Field of the Invention

This invention relates to the manufacture of various articles having

adherent hard coatings, and more particularly to the application of hard coatings

on dental burs.

Background of the Invention

The production of discrete particles or continuous, conformal,

polycrystalline diamond films or coatings by CVD (chemical vapor deposition)

techniques is a well established technology. It typically involves the breakdown

of carbon-containing fluids (gases and/or liquids) in a high energy environment

to which superabundant concentrations of hydrogen (and, sometimes, minor

amounts of other gases) are added. Carbonaceous radicals and other species

released from the fluids condense on top of substrates normally kept in the 700 - 6711

-2- 1,000°C temperature range. The hydrogen acts as an etchant in its monoatomic

state and essentially removes all non-diamond phases condensed on the

substrates. Thus, small diamond nuclei are left on the substrate surface, which

grow and interlock into a polycrystalline, continuous film or coating. In this

way, many of the outstanding properties of pure diamond can be engineered into

a variety of new products coated with CVD diamond layers. Some of these

properties, such as the extremely high hardness, wear resistance, chemical

inertness, and biocompatibility of diamond, make it an almost ideal coating

material for dental burs.

Despite the impressive progress made in this field in recent years, which

has seen increasingly sophisticated deposition devices and techniques, for

example, as described in U.S. Patent Nos. 5,749,955 and 5,722,046, and

correspondingly higher deposition rates (up to about 1 mrn/hr), an inherent

difficulty in attaining sufficient bonding strength between diamond coatings and

substrates has hindered much of the usefulness and commercial applicability of

this technology.

From a mechanical standpoint, the problem stems from high interfacial

stresses that arise due to thermal contraction mismatches between diamond and

most substrate materials as they cool down from the elevated deposition

temperature range. Some of the approaches that have been tried to overcome

this aspect of the problem include: (a) limiting substrate choices to materials

with low thermal expansion coefficients similar to that of diamond, for example,

molybdenum, silicon nitride, or cemented carbides of low binder content; (b) -3- employing stress relieving thermal treatments as, for example, in U.S. Patent

No. 5,701,578; (c) managing stress/strain levels in the coatings as, for example,

in U.S. Patent Nos. 5,633,087 and 5,286,524; and (d) applying intermediate

layers of softer materials to accommodate interfacial stresses as, for example, in

U.S. Patent No. 5,688,557.

From a chemical standpoint, poor adhesion strength of diamond coatings

derive from chemical interactions with substrates that show high affinity for

carbon at elevated temperatures. Cobalt and nickel, two of the most widely

used binders for cemented carbides, and iron, the base material of steels and

other ferrous alloys, are prime examples. These elements can dissolve large

amounts of carbon at high temperatures and tend to graphitize the substrate-

diamond interface. Some of the reported methods to reduce or eliminate this

effect include: (a) removal of Co or Ni from a thin superficial layer in cemented

carbide substrates by means of chemical etching as, for example, in U.S. Patent

Nos. 5,713,133, 5,700,518, 5.567,526 and 5,236,740, or by means of thermal

treatments as, for example, in U.S. Patent Nos. 5,716,170, 5,701,578, 5,068,148,

5,648,1 19 and 5,585,176, or by means of a combination of chemical etching and

thermal treatment as, for example, in U.S. Patent Nos. 5,660,881, 5,618,625,

5,415,674 and 5,204,167; and (b) use of interlayers or chemical barriers as, for

example, in U.S. Patent Nos. 4,992,082, 4,988,421 , 4,919,974 and 4,734,339.

While the removal of the Co or Ni binder may be effective in improving

the diamond adherence to cemented carbides, it is believed that such removal

weakens a thin substrate surface layer, which can lead to coating failure. Rather than breaking at the interface, the coating is lost due to mechanical failure of the

substrate material a few microns into the Co- or Ni-depleted and weakened

carbide layer.

Current trends toward improving the adherence strength of CVD

diamond coatings to their underlying substrates are based on a combination of

mechanical and chemical effects.

U.S. Patent Nos. 5,716,170, 5,701,578, 5,648,119 and 5,585,176

describe a successful method applicable to cemented carbide substrates. Prior to

the CVD diamond coating, the carbide parts are subjected to heat treatments at

very high temperatures and under subatmospheric pressure of nitrogen for

several hours. Such treatments not only promote a very superficial Co or Ni

binder removal but also cause considerable carbide grain growth at the substrate

surface. The heat treatment conditions are tailored to obtain a certain minimum

surface density of very large and protruding carbide grains which have been

found to become effective mechanical anchoring sites for the subsequently

grown CVD diamond coating. This method, however, is applicable only to

cemented carbide parts and only to those that can withstand the harsh thermal

treatment conditions without appreciable dimensional distortions. It is not

suited for small, precisely ground articles such as dental burs.

U.S. Patent No. 5,776,355 describes a method of preparing substrate

materials for diamond deposition based on a similar principle. Patterns of

parallel or cross hatched grooves are scored onto the substrate surface to create -5- the mechanical interlocking effect. This method, however, fails to address the

chemical interaction problem.

U.S. Patent No. 5,772,336 discloses strongly adherent CVD diamond

coatings obtained on cemented carbide tool surfaces having a plurality of wave¬

like projections on their surfaces. This method also fails to address the chemical

interaction problem.

Other hard coatings, besides diamond, also suffer from adherence

problems. Any material harder than the hardest steel is generally classified as a

"hard material". The following diagram shows the relative Vickers hardness

values of some hard materials:

VICKERS HARDNESS NO.

10,000

Diamond

c-BN

5,000

B₄C

TiC

TiN

WC

1,000 - Hard Chrome

Hardened Steel

Except for c-CN, which has been theoretically predicted to be harder

than diamond, no other known material matches the hardness and wear

resistance of diamond.

When applied by PVD (physical vapor deposition) methods, hard

coatings show inherently lower adhesion strengths to substrates than CVD

produced films. This is explained by the typically low PVD deposition -6- temperatures which do not allow any appreciable atomic thermal interdiffusion

to take place between the coatings and substrates. Hence, PVD coatings are not

suitable for high temperature applications (above 500 °C) and are generally

limited to only about 5μm in thickness.

Although CVD hard coatings are generally more adherent and may be

grown to thicker layers, they still suffer from the thermal expansion mismatch

problem that hinders most of the usefulness of diamond coatings. The

following table presents thermal expansion coefficients at room temperature for

some hard materials:

Another critical parameter affecting the adherence strength of hard

coatings is their ability to yield and deform under external loading and/or

interfacial, residual stresses without cracking or peeling off. In general, the harder the material, the less compliant it is. Therefore, well adherent CVD

diamond coatings are among the most difficult ones to be obtained.

The bonding strength may also be severely impaired by chemical

interactions between hard coatings and substrates. Some of the most important

cases occur in the CVD deposition of carbides onto parts that have a large

affinity for carbon at high temperatures (e.g., steels, cemented carbides). CVD

coatings of TiC, VC, HfC, or diamond applied onto WC-Co or steels usually

require some sort of pre-treatment of the substrate surface to avoid undesirable,

bulk carburization of the parts before the hard coatings can actually start to

form.

There is thus a need to provide a method for firmly attaching CVD

diamond coatings and other hard material coatings onto substrates, such as

dental burs.

Summary of the Invention

The present invention provides a method for forming strongly adherent

hard material coatings on various article substrates. To this end, and in

accordance with the principles of the present invention, a plurality of discrete

particles are attached to a substrate surface layer such that the particles are

partially embedded in the substrate, or in an intermediate layer coated on the

substrate, and protrude outwardly relative to the substrate surface layer. The

substrate surface layer having the outwardly protruding particles is then coated

with a hard material. There is formed thereby an article having a substrate

surface coated with a hard material that is strongly adhered to the substrate -8- surface by discrete particles that form a mechanical interlock between the

substrate and the hard material coating. The hard material coating is a material

having a hardness greater than the hardest steel, such as diamond, diamond-like

materials, carbon nitride, cubic boron nitride, titanium nitride, titanium

carbonitride, silicon carbide, and other X_aY_bZ_c compounds and/or

mixtures/multilayers of uniform/graded compositions where X, Y and Z are

elements selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Al, Si, C, N,

O or B; and a, b and c are non-negative integer numbers.

These and other objects and advantages of the present invention shall

become more apparent from the accompanying drawings and description

thereof.

Brief Description of the Drawings

The accompanying drawings, which are incorporated in and constitute a

part of this specification, illustrate embodiments of the invention and, together

with a general description of the invention given above, and the detailed

description given below, serve to explain the principles of the invention.

FIG. 1 is a cross-sectional view of a first embodiment of an

article made by the method of the present invention;

FIG. 2 is a cross-sectional view of a second embodiment of an

article made by the method of the present invention;

FIG. 3 is a cross-sectional view of a third embodiment of an

article made by the method of the present invention; and FIG. 4 is a cross-sectional view of a fourth embodiment of an

article made by the method of the present invention.

Detailed Description

The present invention provides a method for promoting strong adherence

between a variety of substrate materials and hard coatings by the use of an

intermediate layer of outwardly protruding anchoring particles bonded to, and

extending into, the surface layer of the substrate. This method is applicable to a

large number of products and articles, and is not limited or restricted to the

preferred conditions or examples outlined below in regards to CVD diamond

coated dental burs. For example, in addition to dental burs, other applications

of the present invention include coatings on dental files, pliers, cutting/abrasive

wheels, drills and other dental instruments; on dental implants; on cutters, files,

markers, drills, engraving tools and other articles used in the jewelry and glass

industries; on medical/surgical knives, scalpels, bone implants and other

medical articles; on cutting tools for the metal forming and wood cutting

industries, including indexable inserts, drills, taps, reamers, cutting/abrasive

wheels/stones, etc.; for wear resistant surfaces in micrometers, callipers,

bearings, nozzles and wire transport guides; for chemically inert surfaces used

in laboratory hardware and appliances or in lithographic maskings; on extruding

dies, wire drawing dies and other metal forming tools; on electronic heat sinks,

electronic packaging devices and other electronic/power components; and for

radiation resistant surfaces for nuclear instruments, satellite components and

other space bound devices. 711

-10- As used herein, the term "hard" in the context of "hard coatings" is

intended to encompass a wide variety of materials including but not limited to

diamond. Generally speaking, the "hard" coatings contemplated by the present

invention include coatings of materials having a Vickers hardness value greater

than the hardness value of the hardest steel, which said steel has a Vickers

hardness value on the order of about 1,000.

The first step in the method of the present invention is selecting an

appropriate substrate material. The material should be capable of withstanding

temperatures used in the coating process, up to about 1100° C for diamond

CVD, without degradation of properties. Further, the material should have an

average linear thermal expansion coefficient in the 25-1000 °C range not greater

than 12xlO^"6oC^"', and preferably in the range less than 8xl0^"6°C For example,

cemented carbides, such as straight cemented carbide having 6% or less Co

binder (WC-Co<6%); steel; silicon nitrides; Ti/Al/V alloys, such as Ti/6A1/4V

(Ti-grade 5 per ASTM specification); Ti/Al/V/Sn alloys; Fe/Ni/Co alloys, such

as KOVAR® (having an approximate composition of Fe/29Ni/17Co (available

from Carpenter Alloys, Reading, PA); Mo; Cu; Ti; W; Ta; Va; and low thermal

expansion coefficient materials are suitable substrate materials for dental burs

and a variety of other applications.

Before coating, the article may need to be manufactured observing all

specifications and dimensional tolerances for the particular article, but allowing

up to 10-150 m for linear dimensional increase in the region to be coated. The linear dimensional increase needed is dependent on the size of the discrete

particles, which vary from about lOμm up to about 250μm.

The second step in the method of the present invention is to firmly attach

to the substrate area to be coated a distribution of discrete particles in

appropriate sizes and shapes so as to form a densely packed and uniform

distribution of protruding anchoring sites on the substrate surface. These

particles may be directly embedded into the substrate surface to be coated or

they may be attached to it by means of an intermediate binding layer of a

suitable material. Thus "substrate surface layer" as used herein includes the

surface portion of the substrate itself and intermediate binding layers on the

substrate. The surface density of the protruding particles should be in the range

of about 2% to about 100% of the total surface area to be coated, and preferably

in the range of about 50% to about 85% of the total surface area to be coated.

The particles may vary in size from about lO m up to about 250 m and should

be locked into place so that about 1 %-90% of their average sizes,

advantageously about 50%-75%, are either directly embedded into the substrate

surface or into an intermediate binding layer on the substrate. Thus, significant

portions of the particles protrude outwardly to provide the desired anchoring

sites. Further, particles of any shape may provide adequate mechanical

interlocking to achieve strong adherence, but jagged particles are preferred to

smooth, round particles.

These particles may be of ceramic and/or metallic materials, comprising

one or more phases (for example, single crystals, coated particles, or -12- composites) and may be irregularly (for example, crushed grits) and/or

smoothly (for example, spheroids) shaped. Typical materials are natural or

synthetic diamond, silicon carbide, silicon nitride, cubic boron nitride, oxides,

such as metal oxides and ceramic oxides, reinforced composite materials, such

as fiberglass or silicon carbide reinforced metal matrices, and other superhard

particles.

Particles of different materials and/or different sizes and shapes may be

mixed together to obtain desired properties, such as mechanical toughness and

compliance in terms of thermal expansion, and may be attached as a continuous

or discontinuous monolayer or as a thick multilayer, as long as the outermost

surface to be coated possesses the required distribution of protruding anchoring

particles.

Methods of positioning the anchoring particles onto the substrate surface

or onto an intermediate binding layer, prior to permanently locking them in

place, may involve the use of volatile adhesives such as ethylene glycol, methyl

cellulose, hydroxypropyl cellulose, polyethylene, polypropylene and

polystyrene. The adhesive is subsequently burned off or vaporized, typically in

a vacuum furnace at temperatures below 700 °C. The particles may be randomly

positioned onto the substrate surface layer by immersing the article to be coated

in a colloidal solution containing the particles and the volatile adhesive or by

painting or spraying the volatile adhesive onto the article surface and then

blasting the surface with a gaseous jet containing the particles. Alternatively,

random positioning of electrically conductive particles on the substrate surface -13- layer may be accomplished by electrophoretic means, in which the particles are

"dragged" toward the article by electromagnetic forces. The protruding

particles also may be positioned in preferred configurations by magnetic fields

and/or by any other suitable aligning method. For example, where surface

textures and/or patterns (such as symbols or marks) are desired on the final

coated surface, they can be created by selectively positioning the particles by

masking the areas not to be coated, such as in lithographic techniques.

Methods of permanent attachment of the particles to the substrate

surface layer may include direct surface bombardment or blasting, usually aided

by thermal softening of the surface layer, such as by laser-induced surface

softening or melting. Alternatively, the substrate surface layer may include an

intermediate binding layer into which the anchoring particles are permanently

attached. This intermediate binding layer may include a brazing alloy with

adequate bonding and wetting properties. The intermediate binding layer may

be brazed or plated to the article or may be chemically and/or electrochemically

co-deposited together with the particles. Alternatively, the intermediate binding

layer may be plasma sprayed onto the part using appropriate mixtures

containing the particles and metallic alloys, or the mixture may be deposited by

a number of different CVD and/or PVD techniques.

Suitable brazing alloys for permanently securing protruding particles

onto a substrate depend on the specific substrate and protruding particle

materials being used. In general, Ni/Cr, Ag/Cu and Au alloys may be

considered for brazing alloys. By way of example, for a WC substrate with -14- protruding diamond or WC particles, suitable brazing alloys would include

Ni/5-26wt.%Cr/<15wt%P,B,Fe; Ag/27wt.%Cu/4.5wt.%Ti; or

Au 3wt.%Ni/<lwt.%Ti. For a KOVAR® substrate with protruding diamond or

WC particles, a suitable brazing alloy would be Ni/5-26wt.%Cr/<10wt.%

Si,B,Fe.

The brazing alloy is typically provided in powder or tape form. The

brazing powder is mixed with a volatile binder, as described above, which is

then vaporized. In one method, the discrete particles are mixed in with the

brazing powder and binder, applied to the substrate, followed by vaporization of

the binder. Upon cooling, a hard brazed surface layer is achieved on the

substrate, with discrete particles protruding outwardly from the brazed surface

layer. Due to the wetting properties of the brazing alloy, some brazing alloys

will coat the protruding surfaces of the discrete particles, but a protruding

morphology will still be achieved. In another method of brazing, the brazing

alloy powder in a binder is applied to the substrate surface, the binder is

vaporized, and the brazing alloy is allowed to cool to form a solid surface layer.

The discrete particles in a binder are then applied to the substrate surface brazed

layer, and the article is heated to both vaporize the binder and melt the brazed

layer. Upon melting of the brazed layer, the discrete particles sink into the

brazing alloy, thus providing anchoring particles protruding outwardly from the

braze coated substrate. To ensure the protruding morphology, the brazed layer

or other coating layer should have a thickness of about 5% to about 75% of the

average size of the anchoring particles. Finally, particle attachment of the 6711

-15- present invention may involve any other method or combination of processes

that result in attaining the desired surface distribution of firmly attached

protruding anchoring particles.

By way of example, but not intended to limit the scope of the present

invention, irregularly shaped particles of natural or synthetic diamond, type A,

having an average particle size of 51 m are brazed to a carbide dental bur head

surface with a layer of a suitable NiCr brazing alloy having a thickness of about

38 m to produce a closely packed and uniformly dense monolayer of diamond

particles firmly embedded in the brazed layer and protruding outwardly an

average height of about 25 m.

If necessary, a thin and continuous chemical barrier coating of suitable

material(s) may be applied over the entire particle-attached substrate surface.

The chemical barrier coating may be about O.Ol m to about lO m thick, and

preferably between about O.Sμm and 5 m. The purpose of the chemical barrier

coating is to prevent harmful chemical interactions between the hard coating

and the underlying materials. In the case of CVD diamond coatings, appropriate

interlayers must create a barrier to C absorption into the particular underlying

materials. Further, the barrier material must be capable of withstanding the

deposition temperature of the hard coating without deleterious chemical

interactions and degradation of properties. Finally, the chemical barrier layer

must be compliant, not stiff or brittle, so that the layer does not fracture.

Typical materials include boron, titanium boride, titanium nitride, and titanium

carbonitride. The chemical barrier layer may be applied by conventional PVD 6711

-16- and/or CVD techniques in one or more layers of one or more compounds, as a

single, homogeneous structure or as a graded composition barrier. It is

important, however, that the discrete particles, now coated, still protrude

outwardly from the substrate surface layer to provide the necessary anchoring

sites.

Again, by way of example, and not intended to limit the scope of the

invention, a stoichiometric TiN coating less than 3 m thick is applied by a PVD

technique over an entire carbide bur head, covering the attached diamond

anchoring particles and spaces in between them, but keeping the desired

protruding morphology undisturbed.

The third step in the method of the present invention is to coat the

prepared surface to achieve the desired and/or required total hard coating

thickness. Hard coating thicknesses may range from less than l m to about

300μm, and preferably between about 30 m and about 150 im. Any available

hard coating technique may be employed in the method of the present invention.

Such techniques include, by way of example: chemical or electrochemical

coating of hard chrome; CVD or plasma enhanced CVD for diamond or

diamond-like materials, c-BN, TiN, TiC or TiCN; or cathode arc sputtering,

magnetron sputtering, electron beam evaporation, ion plating or plasma

spraying for TiN, HfC, VC, TiCN, TiB₂, Cr or WC. For example, adherent

diamond coatings may be deposited by CVD onto dental burs. Hard coating

materials contemplated by the method of the present invention include those

materials having a hardness greater than the hardest steel, which has a Vickers -17- hardness value of about 1,000. Examples include diamond, diamond-like

materials (which have some properties similar to those of diamond, but contain

various amounts of non-diamond phases), carbon nitride, cubic boron nitride,

titanium nitride, titanium carbonitride, silicon carbide, and other X_aY_bZ_c

compounds and/or mixtures/multilayers of uniform graded compositions where

X, Y and Z are elements selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn,

Fe, Al, Si, C, N, O or B; and a, b and c are non-negative integer numbers.

Various embodiments of the resulting articles produced according to the

method of the present invention are shown in FIGS. 1-4. FIG. 1 shows an

article substrate 10 having discrete particles 12 embedded or partially extending

into the substrate surface layer 14, with the substrate surface layer 14 and

protruding discrete particles 12 coated with a hard coating 16. FIG. 2 shows a

chemical barrier coating 18 applied over the substrate surface layer 14 and

protruding particles 12 of FIG. 1, with the hard coating 16 applied over the

chemical barrier layer 18. FIG. 3 shows a brazing alloy layer 20 applied to the

substrate 10, with the discrete particles 12 embedded or partially extending into

the surface layer 14 of the brazed layer 20 and the hard coating 16 applied over

the brazed layer 20 and protruding particles 12. FIG. 4 shows a chemical barrier

layer 18 applied over the brazed layer 20 and protruding particles 12 of FIG. 3,

with the hard coating 16 applied over the chemical barrier layer 18. -18- Examples

Example 1

Dental burs with cylindrical heads were made of cemented carbide with

6% cobalt content. The average linear thermal expansion coefficient of the

material is 5.6xlO^"6oC^"' in the 25-1000°C temperature range. Some of these bur

samples, in accordance with the principles of the present invention, had

synthetic diamond grit particles in the 230/325 mesh size range (50-70 m

average particle size) brazed to the cylindrical heads with a braze material

containing 14% Cr, 9% P, balance Ni. Brazing was conducted in a furnace at

950°C for 2 minutes in a dry hydrogen atmosphere. This resulted in closely

packed and uniformly dense monolayers of diamond particles firmly attached to

the bur heads and protruding an average height of 25-30 m above the braze.

Titanium nitride layers of 3-5μm thickness were then PVD deposited onto the

brazed bur heads in a chamber filled with a nitrogen gas and titanium vapor

mixture at a temperature of about 350-400°C. Other bur samples, for

comparative purposes, had no diamond particles attached to the cylindrical

head, prior to being PVD coated with titanium nitride to the same final

thickness of 3-5μm. The bur samples of the present invention and the

comparative bur samples were then placed together in a CVD reactor and coated

with diamond at 850 °C in a methane-hydrogen gas mixture to produce a 50-

60μm thick CVD diamond coating on each sample.

The CVD diamond layers on the comparative bur samples having no

brazed anchoring particles cracked and flaked off the bur heads 711

-19- immediately upon cooling down from the 850 °C deposition temperature. By

contrast, the CVD diamond coatings on the burs of the present invention having

the brazed anchoring particles showed no signs of delaminating. The bur

samples of the present invention were individually examined under high

magnification in a scanning electron microscope (SEM) and weighed to 0.000 lg

accuracy both before and after being submitted to extensive cutting trials. Each

CVD diamond coated bur of the present invention was evaluated in numerous

cutting tests performed with a high speed dental turbine cutting through glass

and MACOR® bars, which is a ceramic material having hardness and texture

similar to that of dental enamel. In all cases, no detectable differences were

seen from the visual inspection of the SEM data and from comparison of the

weight data collected before and after the cutting tests. Such results

demonstrate the excellent adhesion between the CVD hard coatings and

substrates prepared according to the principles of the present invention, despite

the unusually high thickness of the CVD diamond layers and the high interfacial

stresses resulting therefrom.

Example 2

The procedures of Example 1 were repeated with the exception that the

230/325 mesh diamond anchoring particles were nickel plated to the carbide bur

heads of the present invention, rather than brazed. The plating also resulted in

well-packed and uniformly dense monolayers of diamond particles firmly

attached to the bur heads and protruding an average height of 25-30 m.

Titanium nitride layers of 3-5 m thickness were then PVD deposited onto the -20- plated bur heads. The CVD diamond coating was deposited at 950 °C with the

resulting layers having a thickness of 25-35μm. Again, the CVD diamond

coatings spontaneously flaked off the comparative bur heads not having the

plated anchoring particles, but remained well adhered to the bur heads of the

present invention having the nickel plated diamond particles. SEM

investigations, weighing, and extensive glass/MACOR® cutting tests as

described above were also carried out and again confirmed the excellent

bonding strength of the CVD diamond coatings on the bur samples of the

present invention.

Example 3

The procedures of Example 2 were followed, with the exception that the

230/325 mesh diamond anchoring particles were replaced by coarse 60/80 mesh

diamond particles (208 m average particle size), and the average height of the

protruding crystals was 150μm. The CVD diamond coatings were deposited to

a thickness of about 50 m. Upon cooling from the deposition temperature of

950°C, only the bur samples of the present invention with the plated anchoring

particles retained their CVD diamond coatings. Again, the comparative

coatings not having the plated particle layer suffered from cracking and flaking

of the diamond coating. The SEM analysis, weight data and glass/MACOR®

cutting tests with the dental rotary turbine confirmed the extremely good

adherence of the CVD diamond layers for the bur samples of the present

invention. -21-

Example 4

Dental burs with cylindrical heads were made of KOVAR®, a metallic

alloy containing 17%> Co, 29%o Ni, balance Fe, with the mean linear coefficient

of thermal expansion of KOVAR® being H xlO^'^C¹ in the 25-900°C

temperature range. As in Example 2, 230/325 diamond mesh anchoring

particles were nickel plated to the KOVAR® bur heads with the same

protruding morphology, and titanium nitride was then PVD deposited onto the

plated bur heads at a thickness of 3-5 m. CVD diamond coating was performed

at 950°C to achieve a deposited layer of 25-35 m thickness. Similar results

from SEM, weighing and cutting tests were obtained to that of the previous

examples, further demonstrating that CVD diamond coatings adhere very well

even to metallic substrates when prepared according to the present invention.

Example 5

Bur samples with a tapered, fine-pointed geometry were manufactured

from cemented carbide containing 11%> Co and 12%o titanium/tantalum/niobium

carbides. Such a high cobalt content in cemented carbides is prohibitive in any

other known technique developed to promote well-bonded CVD diamond

coatings to carbide parts. The average linear thermal expansion coefficient of

this carbide material is 6.4xlO^"6oC^"' in the 25-1000°C temperature range.

Diamond anchoring particles 40μm in size were vacuum brazed to the bur heads

at 1040°C for 5 minutes with a braze material containing 7% Cr, 4.5% Si,

3% Fe, 3.2% B, <1% Co, balance Ni. The required closely packed monolayers

of diamond particles were firmly attached to the bur heads protruding an average of 20μm above the braze. Comparative samples of the same carbide

and geometry but with no brazed anchoring particles were also prepared, and

both types of samples were PVD titanium nitrided and CVD diamond coated at

950 °C. The resulting diamond layers were 50μm thick.

As in the previous examples, the CVD coatings on the comparative

samples having no anchoring diamonds peeled off spontaneously upon cooling

from the deposition temperature, whereas samples of the present invention were

extensively tested in glass/MACOR® cutting trials, with no measurable weight

losses nor observable microscopic changes in the CVD diamond coatings on

any of the samples of the present invention.

It is thus demonstrated by the above examples that hard coatings on

samples prepared with a protruding particle morphology as described herein

exhibit very high bonding strength to their substrates. It has therefore been

demonstrated that the present invention does provide a method for promoting

strong adherence between a variety of substrate materials and hard coatings by

the use of an intermediate layer of outwardly protruding anchoring particles

partially embedded in the substrate surface layer.

While the present invention has been illustrated by the description of one

or more embodiments thereof, and while the embodiments have been described

in considerable detail, there is no intention to restrict or in any way limit the

scope of the appended claims to such details. Additional advantages and

modifications will readily appear to those skilled in the art. The invention in its

broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described.

Accordingly, departures may be made from such details without departing from

the scope or spirit of applicant's general inventive concept.

WHAT IS CLAIMED IS:

Claims

711-24-

1. A process for coating a substrate with an adherent hard coating,

comprising the steps of:

attaching to a surface layer of a substrate to be coated a

distribution of discrete particles that partially extend into the substrate surface

layer and partially protrude outwardly from the substrate surface layer; and

coating the substrate surface layer having attached outwardly

protruding particles with a hard material.

2. The process of claim 1, wherein the hard material has a Vickers

hardness value of greater than about 1,000.

3. The process of claim 1, wherein the hard material is selected

from the group consisting of: diamond, carbon nitride, cubic boron nitride,

titanium nitride, titanium carbonitride, silicon carbide, and X_aY_bZ_c where X, Y

and Z are elements selected from the group consisting of: Ti, Zr, Hf, V, Nb, Ta,

Cr, Mo, W, Mn , Fe, Al, Si, C, N, O and B; and a, b and c are non-negative

integer numbers.

4. The process of claim 1, wherein the substrate to be coated is

comprised of a material selected from the group consisting of: cemented

carbide, steel, silicon nitride, Ti/Al/V alloys, Ti/Al/V/Sn alloys, Fe/Ni/Co

alloys, Mo, Cu, Ti, W, Ta and Va.

5. The process of claim 1, wherein the discrete particles attached to

the substrate surface layer are comprised of a material selected from the group

consisting of: natural diamond, synthetic diamond, silicon carbide, silicon

nitride, cubic boron nitride, ceramic oxides, metal oxides, reinforced

composites, and mixtures thereof.

6. The process of claim 1, wherein the distribution of discrete

particles creates a surface density of the outwardly protruding particles of about

50%) to about 85%o of the substrate surface layer to be coated.

7. The process of claim 1, wherein the discrete particles to be

attached to the substrate surface layer have an average size of about 1 O m to

about 250 m.

8. The process of claim 7, wherein the discrete particles are

attached to the substrate surface layer such that about 10%> to about 90%> of each

discrete particle protrudes outwardly from the substrate surface layer.

9. The process of claim 1, wherein the discrete particles are

attached by bombarding the substrate surface layer with the particles.

10. The process of claim 9, wherein the substrate surface layer is first

softened.

11. The process of claim 9, further comprising coating the substrate

surface layer with a volatile adhesive prior to attaching the particles.

12. The process of claim 11, wherein the substrate surface layer is

coated with an adhesive selected from the group consisting of: ethylene glycol,

methyl cellulose, hydroxypropyl cellulose, polyethylene, polypropylene,

polystyrene, and mixtures thereof.

13. The process of claim 1, wherein the discrete particles are

attached to the substrate surface layer by mixing the particles with a volatile

adhesive, coating the substrate surface layer with the particle-containing

adhesive, and vaporizing the adhesive.

14. The process of claim 13, wherein the particles are mixed with an

adhesive selected from the group consisting of: ethylene glycol, methyl

cellulose, hydroxypropyl cellulose, polyethylene, polypropylene, polystyrene,

and mixtures thereof.

15. The process of claim 1, wherein the substrate surface layer is an

intermediate binding layer and the discrete particles are attached to the binding

layer.

16. The process of claim 15, wherein the intermediate binding layer

is applied to the substrate by a method selected from the group consisting of:

brazing, plating, chemically depositing, electrochemically depositing, plasma

spraying, CVD and PVD.

17. The process of claim 16, wherein the discrete particles are

attached to a brazed intermediate binding layer selected from the group

consisting of: Ni/Cr alloys, Ag/Cu alloys and Au alloys.

18. The process of claim 1 further comprising coating the substrate

surface layer and outwardly protruding particles with a chemical barrier prior to

coating with the hard material, wherein the chemical barrier coated particles

protrude outwardly from the chemical barrier coated substrate surface layer.

19. The process of claim 18, wherein the chemical barrier is

comprised of a material selected from the group consisting of: boron, titanium

boride, titanium nitride and titanium carbonitride.

20. The process of claim 18, wherein the chemical barrier is coated

to a thickness of about O.Olμm to about lOμm.

21. The process of claim 1, wherein the hard material is coated to a

thickness of less than about lμm up to about 300μm. -28-

22. The process of claim 21 , wherein the hard material is coated to a

thickness between about 30μm and about 150μm.

23. The process of claim 1, wherein the hard material is coated by

chemical vapor deposition.

-29- 24. A process for coating a substrate with an adherent hard coating,

comprising the steps of:

attaching to a surface layer of a substrate to be coated a

layer and partially protrude outwardly from the substrate surface layer, the

discrete particles comprising a material selected from the group consisting of:

natural diamond, synthetic diamond, silicon carbide, silicon nitride, cubic boron

nitride, ceramic oxides, metal oxides, reinforced composites, and mixtures

thereof; and

coating the substrate surface layer and outwardly protruding

particles with a hard material having a Vickers hardness value of greater than

about 1,000.

1

-30-

25. A process for making a dental bur having an adherent diamond

coating, comprising the steps of:

attaching to a surface to be diamond coated a distribution of

discrete particles that partially extend into the surface and partially protrude

outwardly from the surface;

coating the surface having attached outwardly protruding

particles with a diamond coating by chemical vapor deposition.

-31-

26. A dental bur having an adherent diamond coating, comprising:

a substrate;

a plurality of discrete particles partially embedded in the

substrate; and

a diamond coating on the discrete particles and the substrate,

wherein the discrete particles form a mechanical interlock between the substrate

and the diamond coating.

27. The dental bur of claim 26, wherein the substrate is comprised of

WC-Co<6%.

28. The dental bur of claim 26, wherein the discrete particles are

comprised of a material selected from the group consisting of: natural diamond,

synthetic diamond, silicon carbide, silicon nitride, cubic boron nitride, ceramic

oxides, metal oxides, reinforced composites, and mixtures thereof.

29. The dental bur of claim 26, wherein the distribution of discrete

particles has a surface density of about 50%> to about 85% of the substrate.

30. The dental bur of claim 26, wherein the discrete particles have an

average size of about lOμm to about 250μm. -32-

31. The dental bur of claim 30, wherein about 10% to about 90% of

each discrete particle is embedded in the substrate.

32. The dental bur of claim 26, wherein the diamond coating has a

thickness of less than about lμm up to about 300μm.

33. The dental bur of claim 26, wherein the diamond coating has a

thickness between about 30μm and about 150μm.

34. A dental bur having an adherent diamond coating, said dental bur

made by the process of claim 9, and said dental bur comprising:

a substrate;

a plurality of discrete particles partially embedded in the

substrate; and

a diamond coating on the discrete particles and the substrate,

and the diamond coating.

711

-33-

35. A dental bur having an adherent diamond coating, said dental bur

made by the process of claim 13, and said dental bur comprising:

a substrate;

a plurality of discrete particles partially embedded in the

substrate; and

a diamond coating on the discrete particles and the substrate,

and the diamond coating.

36. A dental bur having an adherent diamond coating, said dental bur

made by the process of claim 17, and said dental bur comprising:

a substrate having a brazed intermediate binding layer thereon;

a plurality of discrete particles partially embedded in the brazed

intermediate binding layer; and

a diamond coating on the discrete particles and the brazed

intermediate binding layer, wherein the discrete particles form a mechanical

interlock between the brazed intermediate binding layer and the diamond

coating.

711

-34- 37. A dental bur having an adherent diamond coating, comprising:

a substrate;

a plurality of discrete particles partially embedded in the

substrate, wherein the discrete particles have an average size of about lOμm to

about 250μm;

a chemical barrier coating on the discrete particles and the

substrate, wherein the chemical barrier coating has a thickness of about 0.0 lμm

to about 1 Oμm; and

a diamond coating on the chemical barrier coated discrete

particles and substrate having a thickness of less than about lμm up to about

300μm, wherein the discrete particles form a mechanical interlock between the

substrate and the diamond coating.

-35-

38. A dental bur having an adherent diamond coating, comprising:

a substrate;

a braze alloy layer on the substrate;

a plurality of discrete particles partially embedded in the braze

alloy layer, wherein the discrete particles have an average size of about lOμm to

about 250μm;

a chemical barrier coating on the discrete particles and the braze

alloy layer, wherein the chemical barrier coating has a thickness of about

O.Olμm to about lOμm; and

a diamond coating on the chemical barrier coated discrete

particles and braze alloy layer having a thickness of less than about lμm up to

about 300μm, wherein the discrete particles form a mechanical interlock

between the braze alloy layer and the diamond coating.

-36-

39. A dental bur having an adherent diamond coating, comprising:

a substrate;

a braze alloy layer on the substrate;

a plurality of discrete particles partially embedded in the braze

about 250μm; and

a diamond coating on the discrete particles and the braze alloy

layer having a thickness of less than about lμm up to about 300μm, wherein the

discrete particles form a mechanical interlock between the braze alloy layer and

the diamond coating.

40. An article having an adherent hard coating, comprising:

a substrate;

a plurality of discrete particles partially embedded in the

substrate; and

a hard coating on the discrete particles and the substrate, wherein

the discrete particles form a mechanical interlock between the substrate and the

hard coating.

41. The article of claim 40, wherein the hard coating is comprised of

a material selected from the group consisting of: diamond, carbon nitride, cubic

boron nitride, titanium nitride, titanium carbonitride, silicon carbide, and X_aY_bZ_c

where X, Y and Z are elements selected from the group consisting of: Ti, Zr, Hf,

V, Nb, Ta, Cr, Mo, W, Mn , Fe, Al, Si, C, N, O and B; and a, b and c are non-

negative integer numbers.

42. The article of claim 40, wherein the substrate is comprised of a

material selected from the group consisting of: cemented carbide, steel, silicon

nitride, Ti/Al/V alloys, Ti/Al/V/Sn alloys, Fe/Ni/Co alloys, Mo, Cu, Ti, W, Ta

and Va. 711

-38-

43. The article of claim 40, wherein the discrete particles are

comprised of a material selected from the group consisting of: natural diamond,

oxides, metal oxides, reinforced composites, and mixtures thereof.

44. The article of claim 40, wherein the distribution of discrete

particles has a surface density of about 50% to about 85% of the substrate.

45. The article of claim 40, wherein the discrete particles have an

average size of about lOμm to about 250μm.

46. The article of claim 45, wherein about 10% to about 90%> of each

discrete particle is embedded in the substrate.

47. The article of claim 40, wherein the hard coating has a thickness

of less than about lμm up to about 300μm.

48. The article of claim 40, wherein the hard coating has a thickness

between about 30μm and about 150μm.

49. The article of claim 40, wherein the article is a dental article

selected from the group consisting of: file, plier, cutting wheel, abrasive wheel

and drill.

-39- 50. The article of claim 40, wherein the article is a dental implant.

51. The article of claim 40, wherein the article is selected from the

group consisting of: file, cutter, marker, drill and engraving tool used on jewelry

and glass.

52. The article of claim 40, wherein the article is a medical article

selected from the group consisting of: knives, scalpels and bone implants.

53. The article of claim 40, wherein the article is selected from the

group consisting of: extruding die, drawing die, indexable insert, drill, tap,

reamer, cutting wheel, cutting stone, abrasive wheel and abrasive stone used for

forming and cutting metal and wood.

54. The article of claim 40, wherein the substrate is a wear resistant

surface on an article selected from the group consisting of: micrometer, calliper,

bearing, nozzle and wire transport guide.

55. The article of claim 40, wherein the article is an electronic

component.

56. The article of claim 40, wherein the article is a space bound

device.

57. An article having an adherent diamond coating, comprising:

a substrate;

a metal binder layer on the substrate;

a plurality of discrete particles partially embedded in the binder

layer;

a chemical barrier coating on the discrete particles and the binder

layer; and

a diamond coating on the chemical barrier coated discrete

particles and binder layer,

wherein the discrete particles are of a size sufficient to protrude

outwardly from the binder layer to provide mechanical anchoring for the

diamond coating to the substrate and binder layer, and

wherein the chemical barrier coating has a thickness sufficient to

substantially prevent harmful chemical interactions between the diamond

coating and the substrate and binder layer.

58. The article of claim 57, wherein the metal binder layer includes

nickel.

59. The article of claim 58, wherein the substrate includes tungsten

carbide.