DESC:Complete Specification

“Chemical vapour deposition process, equipment for implementing the same, and surface-modified articles resultant thereof”

Cross references to related applications: This complete specification is filed pursuant to patent application No. 201721036636 filed on 16/10/2017 originally with provisional specification, the entire contents of which are incorporated herein by way of reference.

Copyright notice: It shall be generally noted that the applicant named herein holds copyrights in and to at least a major portion of this document and accordingly has no objection to the facsimile reproduction by anyone of this work, as it appears in records of the Patent Office, but otherwise reserves all copyright rights whatsoever.

Field of the invention
The present invention subject hereof is identified generally in relation to processes and equipment for surface modification of articles, and a surface-modified article resultant thereof directed specifically at applications requiring endurance / wear-resistance as a key attribute.

Background of the invention and description of related art
Medical treatments and procedures, especially those of invasive nature, require sophisticated tooling. Burs, drills / drill-bits, files, saws are some routinely encountered examples of such tools which mandate extremely defined surfaces for carrying out their intended functions. From a longevity as well as cost perspective, it is highly desirable that said tools are somehow rendered to have high endurance and yet capable of sustaining efficiency and precision across prolonged use with none or at least minimum wear.

Conventionally available tools for dentistry, odontological, orthopedic, neurosurgical, otorhinolaryngological and other medical applications are commonly observed to have finite service lives and moreover exhibit progressive decline in efficiency and ultimately loss in function specifically due to gradual dulling, fouling, or otherwise deterioration of their functional surfaces. It would be therefore advantageous to have some means for augmenting said functional surfaces for avoiding, or at least deferring, such eventualities to therefore factorize meaningfully cost-effective medical equipment and hence the treatments and procedures they help deliver.

Also, instruments for cutting, drilling, grinding, or filing of hard structures such as teeth and bones routinely require to be operated at very high intensity / speed. Pain management protocols, integrated systems for lubrication and cooling are indispensible hence to counter trauma consequent to the shear forces and heat produced while operating said instruments. Non-uniformly worn instruments are additionally prone to vibration which worsens the situation and may also lead to fragmentation of the functional surface. Probability of operator error and increased trauma therefore both increment exponentially with imprecise or deteriorated instruments deployed in such application environments. Needless to say, it is an acute need to have some way of developing new instruments, or treating existing ones, for increased precision, toughness and endurance and increased comfort on part of the patient without affecting their hardness, lubricity and heat transfer properties, sans undue complexities, high costs, unwieldy ergonomics, or restricted application, maneuverability, and dexterity on part of the operator.

Prior art, to the limited extent presently surveyed, lists some ways for addressing aforementioned wants. For example, US4190958 discloses an endodontic drill file having a drilling surface coated with diamond particles. Another reference, US4466795 discloses a helocoidally grooved dental bur having a working surface covered with diamond particles. Yet another reference, US4681541 discloses a dental bur having a metal nitride or carbide layer applied over diamond particles. FIGURE 1(a), 1(b), and 1(c) are microphotographic views of prior art bur tips prepared by such known techniques (M. J. Jackson et al. Journal of Medical Engineering & Technology, Vol. 31, No. 2, March/April 2007, 81 – 93).

From an overview of prior art, the concept of coating resilient or hard materials such as metal nitrides or carbides, or diamond on functional, that is the contacting or working surfaces of surgical equipments, is not new. Both physical and chemical vapour deposition techniques have been reported in state-of-art for surface modification of dental tools, called burs, with diamond. The accompanying microphotographic images at FIGURE 1(a) and FIGURE 1(b) showcase prior art bur tips in which surface modification with diamond has been achieved using physical vapour deposition technique and chemical vapour deposition technique respectively. FIGURE 1(c) is a magnified view of the surface of bur tip shown at FIGURE 1(b).

However the coating processes involved in prior art processes are not effective solution/s, prime reason being said resilient or hard materials are bonded or embedded into relatively soft substrate (working surface of the instrument), and as said substrate erodes, the resilient or hard materials are dislodged and at times lost as jagged shrapnel into the operative envelope. Also, another vice observed with conventional instruments is the build-up of debris on or within dead spaces of their functional surfaces which reduces function intended. Such occurrences are particularly traumatic and involve severe complications for the patient. Thus, the art needs an surface modification process and equipment implanting it which result in uniform layering of the resilient or hard material/s onto varied functional surfaces of surgical equipments and of which the resultant thin layer of resilient material is resistant to erosion, effects of dislodging or gathering debris into or from the operative envelope.

Surface coating or deposition or growth of diamond or diamond-like materials has seen increasingly sophisticated deposition devices and techniques in recent times. Examples to this effect are the teachings of US5749955 and US572204, which report a high deposition rate of up to about 1 mrn/hr. Particularly, the difficulty in attaining sufficient bonding strength between diamond coatings and substrates has hindered much of the usefulness and commercial applicability of such technologies. There is thus a need to provide a method for firmly attaching CVD diamond coatings and other hard material coatings onto relatively delicate and / or micro-scale fine substrates, such as working surfaces of dental burs.

Prior art, therefore to the extent surveyed, does not list a single effective solution embracing all considerations mentioned hereinabove thus preserving an acute necessity-to-invent in locus of the field hereof. The present inventors have identified that in-situ growth of diamond or diamond like materials selectively on specialty surfaces, such as regions designated to effect functionalities including cutting, drilling, filing and so on would be beneficial in addressing the issues mentioned in the foregoing narration.

Accordingly, the present inventors have therefore, through focused research, come up with a novel solution for resolving at least a major of said issues once and for all. Work of said inventors, specifically directed against the technical problems recited hereinabove and currently part of the public domain including earlier filed patent applications or research publications, is neither expressly nor impliedly admitted as prior art against the present disclosures which are maintained to be entirely novel as well as inventive.

A better understanding of the objects, advantages, features, properties and relationships of the present invention will be obtained from the following detailed description which sets forth an illustrative yet-preferred embodiment.

Objectives of the present invention
The present invention is identified in addressing at least all major deficiencies of art discussed in the foregoing section by effectively addressing the objectives stated under, of which:

It is a primary objective to provide a methodology for achieving a highly adherent, pure, uniform, and stable layer of a hard material (diamond) on an article (dental bur) to thereby enable significantly higher efficiency of drilling and cavity preparation.

It is another objective further to the aforesaid objective(s) that said methodology does not mandate inclusion of any additional material for embedding or otherwise providing mechanical interlocking between surface of the bur being coated and the hard material, that is diamond, being deposited thereon.

It is another objective further to the aforesaid objective(s) that said methodology lends easily to adaptation for achieving different diamond morphologies depending on end-application intended.

It is another objective further to the aforesaid objective(s) that said methodology lends easily to control over grain size distribution, uniformity and adherence of coating of the diamond layer over the receiving substrate.

It is another objective further to the aforesaid objective(s) that said methodology results in a uniform diamond layer that has a minimal shear off while in use.
It is another objective further to the aforesaid objective(s) that the bur resulted from said methodologyhas high endurance and is able to easily cut through tooth enamel accurately without risk of chipping the same.

It is another objective further to the aforesaid objective(s) that the bur resulted from said methodologyhelps in decreasing time and risk of infection in invasive dental procedures while causing minimal trauma to the patient.

It is another objective further to the aforesaid objective(s) that the bur resulted from said methodology has enhanced lubricity to therefore imply better heat transfer and reduced vibrations in use.

It is another objective further to the aforesaid objective(s) that the bur resulted from said methodology does not mandate undue costs or complexities on part of the dentist using the same.

The manner in which the above objectives are achieved, together with other objects and advantages which will become subsequently apparent, reside in the detailed description set forth below in reference to the accompanying drawings and furthermore specifically outlined in the independent claims. Other advantageous embodiments of the invention are specified in the dependent claims.

Summary
The present invention attempts to resolve the wants of art, by meeting the objectives stated hereinabove. Specifically, principles of the present invention are generally directed to the production of a dental bur in which a customized surface modification process is applied to thereby selectively augment, via micro-fabrication, its frictional surfaces involved in functions such as cutting, drilling, filing and so on. This invention is identified in growing of SP3 crystalline diamond coating on metallic substrate (tungsten carbide bur) by bombarding ionized carbon species created from a hydrocarbon gas source like methane in excess hydrogen.

More specifically, an isolated embodiment of the present invention to be recited herein under outlines aspects defining a hot filament chemical vapour deposition process for surface deposition / in-situ growth of thin films predominantly comprising a hard substance such as diamond, equipment for implementing such process, and as an illustrative example, highly efficient and durable burs prepared by said process.

Brief description of drawings
The present invention is explained herein under with reference to the following drawings, in which:

FIGURE 1(a) is a microphotographic image of a prior art bur tip in which surface modification with diamond has been achieved using laser sintering technique.

FIGURE 1(b) is a microphotographic image a prior art bur tip in which surface modification with diamond has been achieved using chemical vapour deposition technique.

FIGURE 1(c) is a magnified view of the surface of bur tip shown in FIGURE 1(b).

FIGURE 2 includes microphotographic images (a), (b), (c), and (d) showing the diamond-coated bur of the present invention photographed at 50X magnification, and surface topography of this bur when photographed at 250X, 800X, and 2500X magnification respectively.

FIGURE 3 is a photograph taken at 450X magnification showing ballas diamond formation on surface of the diamond-coated bur as per an alternative embodiment of the present invention.

FIGURE 4 includes graphs (a), and (b) showing Raman signatures of a prior art 1 micron film of CVD diamond on a 200mm silicon wafer at centre and edge of said wafer respectively.

FIGURE 4(c) is a graph showing Raman signature of another prior art CVD diamond layer deposited on silicon wafer recorded at an excitation wavelength of 488 nm.

FIGURE 5 is a graph showing Raman signature of the CVD diamond-coated bur of the present invention.

FIGURE 6 is a schematic representation of the equipment assembly used to produce the diamond-coated bur of the present invention.

FIGURE 7 is another schematic representation of the equipment assembly shown in FIGURE 6.

The above drawings are illustrative of particular examples of the present invention but are not intended to limit the scope thereof. The drawings are not to scale (unless so stated) and are intended for use solely in conjunction with their explanations in the following detailed description. In above drawings, wherever possible, the same references and symbols have been used throughout to refer to the same or similar parts. Though numbering has been introduced to demarcate reference to specific components in relation to such references being made in different sections of this specification, all components are not shown or numbered in each drawing to avoid obscuring the invention proposed.

Attention of the reader is now requested to the detailed description to follow which narrates a preferred embodiment of the present invention and such other ways in which principles of the invention may be employed without parting from the essence of the invention claimed herein.

Definitions and interpretations
Before undertaking the detailed description of the invention below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect, with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.

Also, as some technical terms are not used uniformly in this field, a few definitions are given in the following to clarify the meaning of terms as they are used in this patent application. Accordingly, the term ‘IID’ shall refer Ion Implantation and deposition; ‘CVD’ shall refer Chemical vapor Deposition; ‘SP3’ shall refer configuration and bonding structure of carbon atoms that are seen in diamond allotrope; ‘SP2’ shall refer Configuration and bonding structure of Carbon seen in Graphite, Coal, Carbon bucky tubes, carbon bucky balls and other amorphous forms; ‘WC’ shall refer Tungsten carbide; “hard material” refers any material harder (having Vickers hardness value greater) than material of the substrate on which said hard material is coated and includes diamond, diamond-like materials, carbon nitride, cubic boron nitride, titanium nitride, titanium carbonitride, silicon carbide, their equivalents and their combinations; “SCCM” refers standard cubic centimeter per minute; “FEG-SEM” refers Field Emission Gun Scanning Electron Microscopy; “PLC” refers programmable logic control. Other standard nomenclatures will have their recognized meanings.

Detailed description
Principally, general purpose of the present invention is to assess disabilities and shortcomings inherent to known systems comprising state of the art and develop new systems incorporating all available advantages of known art and none of its disadvantages. Accordingly, the disclosures herein are directed towards an inventive hot filament chemical vapour deposition process for surface deposition / in-situ growth of thin films predominantly comprising a hard substance such as diamond, equipment for implementing such process, and as an illustrative example, a highly efficient and durable bur prepared by said process.

According to a first aspect of the present invention, the process for surface deposition / in-situ growth of thin films predominantly comprising a hard substance such as diamond on functional surface/s of surgical tools, as will be particularly described hereinafter, essentially accomplishes deposition of synthetic diamond film on metallic substrates. More specifically, diamond deposition is arranged through bias-enhanced hot filament CVD of carbon atoms in hydrocarbon gases on burs made of stainless steel as well as on burs made of tungsten carbide.

Conventionally, the production of discrete particles or continuous, conformal, polycrystalline diamond films or coatings by CVD techniques 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 at elevated temperatures. Hydrogen, in its monatomic state, acts as an etchant and essentially removes all non-diamond phases condensed on the substrates, thereby leaving small diamond nuclei on the substrate’s surface which then grow and interlock into a polycrystalline, continuous film or coating. In this way, properties of pure diamond, including its extremely high hardness, wear resistance, chemical inertness, and biocompatibility can be engineered onto working surfaces of a variety of products.

Diamond has popularly been an ideal surface-coating material for coating dental burs for enhanced use in cutting hard tissues, namely tooth and bone. While the CVD process itself, and surface modification of cutting / abrasive tools with diamond or diamond-like materials is not new, the present inventors have set out to establish optimization therein to overcome lacunae and shortcomings from mechanical as well as chemical standpoints. These optimizations are tailored, at least in one perspective, for controlling / minimizing interfacial stresses that arise due to thermal contraction mismatches between diamond and substrate materials as they cool down from deposition temperatures; b) improving the adherence strength of CVD diamond coatings to their underlying substrates; c) improving on production cycle time of CVD diamond coated articles; d) allowing small, precisely ground articles such as dental burs to withstand harsh treatment conditions incidental to CVD diamond coating processes without appreciable dimensional distortions; e) allowing uniform seamless coating and precise control over morphology of CVD coated discrete diamonds.

Normal physical and chemical properties of metallic substrates do not allow diamond deposition such that diamond semi-embeds into the substrate, nucleates and finally forms a uniform and adherent film. Hence the present inventors have sought, in the process methodology to be described in detail hereafter, to alter the substrates’ acceptance to the diamond film to embed and stay adhered to it even when operating at high intensities / speeds (around 400000 RPM).

According to one aspect, as particularly seen in implementation of the inventive process hereof, particular emphasis is made on selection of resilient material / hard material to be formed in-situ (amorphous Carbon, nano and microcrystalline diamond and other related materials) for control over grain size distribution, uniformity and adherence of coating especially at cutting edges of the substrate (bur) being coated. Accordingly, plant parameters including pre-treatment method, filament temperature, pressure, time, choice of gases, residence time, pressure, hydrocarbon flow rates, heat distribution, material of construction for equipment claimed herein would be optimized.

Reference is now made to certain examples which showcase the best mode and alternatives thereto, of performing the present invention which may be regarded as illustrative in nature and not restrictive whatsoever. Accordingly, best mode of performing the inventive method proposed herein is identified in implementation of the following steps-

Preconditioning of raw tungsten carbide burs: Raw tungsten carbide burs are selected for processing. These typically contain 8 to 14% cobalt. Cobalt cements tungsten carbide particles but inhibits deposition of carbon atoms onto carbide surface. Hence, removal of surface cobalt content in an upstream preparatory step is critical for efficient surface deposition / in-situ growth of diamond. The present inventors propose sequentially using ultrasonic treatment with Murakami’s reagent (also known as Murakami's etch) and acid etching for this purpose. Accordingly, the burs selected for coating are first immersed in Murakami reagent (aqueous 10% KOH&10% K3Fe(CN)6) and preferably subjected to ultrasonication at 33khz for 1min.Thereafter, the burs are immersed in an acidic working solution comprising 33% hydrochloric acid, 33% Nitric acid and demineralized water taken in the ratio 1:1:1 v/v for 2 minutes

Once superficial cobalt content is removed, the burs are immersed in 80 micron diamond paste in acetone amidst ultrasound treatment at 33 kHz for 15 minutes. This creates superficial imperfections which act as active sites for diamond coalescence, nucleation and eventual crystal formation.

Preconditioning of equipment (modified hot filament chemical vapor deposition system): The tungsten filaments of a common art CVD station are heated to 1800 oC, methane is introduced at the rate of 7.5 SCCM, 2 hours, the tungsten filaments react with methane, forming a superficial tungsten carbide layer on the filaments, thus saturating the filament so that no further interplay between the carbon species and tungsten occur during the course of deposition.

Initial seeding: Pretreated burs are mounted on bur holders, which in turn, are placed into the CVD station. Pressure in the CVD machine is initially kept in the range of 1-3 x 10-2 mbar (near vacuum), filament temperature brought in the range of 1800-2250 oC, and at a distance between 6-15 cm from the substrate head, pure hydrogen is introduced at 500 SCCM for 15 minutes. This action scavenges the organic contaminants on bur surface, bur holder and the immediate environment in the vessel. Substrate temperature is then brought in the range of 500-800oC by auxiliary heat input through ceramic heaters placed below the substrate holder. Hydrogen flow is stopped, chamber is evacuated to a base pressure of 0.02 mbar, followed by flow of pure methane at 5 SCCM at a process pressure of 30 Torr for 5 minutes. This creates the carbon rich interface layer which creates necessary surface conditions and environment for diamond layer formation. A negative bias of 250 V is now given to the substrate bur and methane gas flow rate is reduced to 2.5 SCCM and pure hydrogen is simultaneously introduced at 500 SCCM at process pressure of 30 Torr. The bias conditions are maintained for 30 minutes. This creates nucleation sites on the bur surface.

Deposition: After 30 minutes, the bias is switched off, the gas flow rates maintained as above and For 8-14 hours following the seeding process, depending on the output morphology needed, the burs are kept at the same parameters as the seeding stage, but at a different methane gas concentration (0.5% - 2.5%v/v of hydrogen), varying filament temperatures between 1700-2100 oC and at no bias voltage as mentioned above.

Cooling down: Prolonged heating and morphological changes that the bur has been through, creates internal stresses. Cooling the burs through regulated steps helps to remove the internal stresses and reduce the brittleness. Cooling happens in the following steps :i) Deposition temperature: +2200 oC, pressure 25-50 Torr ii) 1 hr:1500 deg , 200 Torr iii) 1.5 hr: 1200 deg, 400 Torr iv) 2 hr 800 deg , 600 Torr v) 2.5 hr 300 deg 760 Torr vi) 3 hr-room temperature, 760 Torr.

Above protocol embodies the inventive method proposed herein through which highly adherent, very hard and yet smooth diamond can be grown onto substrates in order to make significantly better cutting tools as well as surfaces with very little friction.

End product achieved using the process disclosed hereinabove can be visualized in the accompanying FIGURE 2 which includes microphotographic images (a), (b), (c), and (d) showing the diamond-coated bur of the present invention photographed at 50X magnification, and surface topography of this bur when photographed at 250X, 800X, and 2500X magnification respectively.

Control over morphology of CVD-diamond is showcased in FIGURE 3 which is a microphotograph taken at 450X magnification showing ballas diamond formation on surface of the diamond-coated bur reached as per the foregoing narration. Accordingly, the alteration in process conditions is identified as a key to achieve user-selected morphology, identified in the scheme as follows-
a) To obtain crystalline morphology, the optimized process parameters included hydrogen flow at the rate of 150 to 250 SCCM, Methane flow at the rate of 2 to 4 SCCM, process pressure of 30 Torr, article temperature of 700 to 900 oC, and filament temperature of 1800 to 2000 oC; Or
b) To obtain ballas morphology, the optimized process parameters included hydrogen flow at the rate of 350 to 650 SCCM, Methane flow at the rate of 1.5 to 3.5 SCCM, process pressure of 30 Torr, article temperature of 775 to 975 oC, and filament temperature of 1070 to 2170 oC

Improvement in coating of burs with CVD-diamond was validated using Raman signatures. Accordingly, reference is made to the accompanying FIGURE 4 which includes graphs (a) and (b) showing Raman signatures of a prior art 1 micron film of CVD diamond on a 200mm silicon wafer at centre and edge of said wafer respectively (Reference: James Herlinger, sp3’s experience using Hot Filament CVD Reactors to grow diamond for an expanding set of applications, Written for presentation at ICNDST – 9, March 2004). Also, the accompanying FIGURE 4(c) is a graph showing Raman signature of another prior art CVD diamond layer deposited on silicon wafer recorded at an excitation wavelength of 488 nm (Reference: Mehedi H.-A., Achard a J, Rats D. et al, Low temperature and large area deposition of nanocrystalline diamond films with distributed antenna array microwave-plasma reactor, Diamond & Related Materials 47 (2014) 58–65).

In comparison to figures referred in the preceding paragraph, attention of the reader is requested to FIGURE 5, which is a graph showing Raman signature of the CVD diamond-coated bur of the present invention. Characteristic sharp peaks are observed in the range between 1300/cm to 1400/cm thereby indicating existence of a uniform layer of at least nearly monophasic CVD-diamond.

Equipment for implementing the diamond deposition process: An allied aspect of the present invention is establishment of a modified CVD station for implementing the diamond deposition process outlined in the foregoing narration. Accordingly, as may be referred in the accompanying FIGURE 6, a common art CVD station (007) including a gas manifold (001), filament supply (002), bias supply (003), ceramic heater (004), vacuum system (005), pyrometer (006) is adapted for implementing the diamond deposition process outlined in the foregoing narration by inclusion of a programmable logic control (implemented as IC or PCB, not shown in the drawings) so as to enable computerized generated time and quantity programming / precise automatic control over CVD process parameters to be implemented (as per their optimized values outlined in the foregoing narration) to thereby achieve desired output parameters like adherence, control over nano and microcrystal sizes, consistency by substrate (bur and by batch), quality of diamond coating, negligible graphite deposition and negligible internal stresses, thereby creating an absolutely automatic and manual effort-free run for the entire duration right from seeding to end of batch.

As may be further referred in the accompanying FIGURE 7, during the deposition stage, the process pressure at steady state is 30-50 Torr, temperature in the range of 2000 degrees centigrade and Hydrogen and methane gas flow is set steady at a fixed flow rate. Raw tungsten carbide burs (1) are placed in slots made to dimension (of the burs received) within a molybdenum holder / plate which serves as the bur table (2). This arrangement is placed atop a ceramic heater (3). In this arrangement, the burs (1) receive heat through three means of transfer - a) radiation from the filament (8), b) convection from surrounding gases and c) electrical heating applied to the bur table (2) through ceramic heater (3). However, despite receiving heat from different sources on different geographies of the bur, at steady state during the deposition process, the metal cohort consisting of molybdenum (bur holder) and tungsten carbide (burs) is in thermal equilibrium and uniformly heated due to good heat conducting properties of both these metals. The equilibrium temperature is monitored using a thermocouple (10) slotted into the bur holder (2).

According to another aspect of the present invention explained with continued reference to the accompanying FIGURE 7, is that filament temperature is set independently with the primary purpose of ionising the gas mixture input into the process. Similarly gas flow rates are governed and controlled for ionisation, nucleation and growth rates and is also based on capacity of the batch size of burs to processed at a time. Temperature of the bur tip, which receives the ions for vapour deposition needs to be controlled independently, the primary purpose being different than that of filament heating, that is, to create amenable conditions and environment for nucleation and growth of diamond.

Once the gas flow rates and filament temperature are set and controlled, the bur temperature is set to a desired value by the operator on the human machine interface of a PLC (9), The PLC (9) regulates power supply to the ceramic heater and maintains the bur and bur holder temperature to s steady and precise value as set by the operator.

Said modified CVD station is hence typified in having the following modifications-
a) Precision power supply (not shown in the drawings): 50V, 100Amp DC power supply having pulse frequency 2 KHz is used for filament temperature control for consistent and repeatable process. Pulse width controls temperature precisely and avoids arcing;
b) Novel design of bur holder for a) uniform heat transfer in order to maintain a constant temperature of bur during growth and ease of placement / change of diamond-coated burs; b) adjustable distance from filament so that any type of bur – whether friction grip enamel and dentin cutting short shanks (16.5 mm height) or Right angled shanks or then burs used in other faculties like neurosurgery, ENT, ortho up to 100 mm
c) Independent control of bur temperature during diamond growth to in turn control diamond crystallite size. Programmable logic for control over gas flow ratios to thereby control crystallite size and elimination of graphitic phase.
d) Complete process automation for setting a recipe to achieve full control on process parameter values as well as repeatability for reaching the resultant CVD-coated burs.
e) No internal stresses due to in-situ temperature ramp down driven under programmable logic control.

Industrial applicability
The present invention has been reduced to practice by the inventors named in this paper particularly in the manner recited in the foregoing narration, thereby establishing an able technology for surface deposition / in-situ growth of thin films on at least the working surface of said articles, wherein the coating predominantly comprises a hard substance such as diamond with improved throughput and efficiency than any of its closest peers in state-of-art. Resultant burs are intended to be used in conjunction with teeth, bone, porcelain or other dental ceramic materials during a dental preparation or dental treatment procedure.

Experimental data
Final product prepared as per the present invention (the “CVD diamond burs”) was tested in independent experiments conducted by the inventors named herein. Round headed CVD diamond burs prepared as per the foregoing narration were compared with commercially available sintered diamond tools for drilling vertical holes on glass blocks. It was observed that the velocity of drilling (mm depth achieved /min) was 3 times higher for CVD diamond burs than the sintered diamond burs. The CVD burs could drill 4 such holes of 5 mm before the cutting edges getting blunt, versus sintered diamond burs which give way within 1.5 holes2to 4l RPM pneumatically operated hand piece, which provides, without doubt, proof of concept of the present invention.

The present invention is intended to cover all head geometries of dental burs, most typically of which are chosen among round and pear shapes that are commonly used for creating access points, cavity preparation, splitting roots of small teeth, undercuts and channels, as well as finishing burs that are commonly used for used for finishing restorations, soft tissue recontouring, enameloplasty, odontoplasty, and alveolaplasty.

Accordingly, merit of the present invention is typified in following salient features-
a) Control of size of diamond from nano crystalline to microcrystalline which is not possible in sintered diamond burs;
b) Inherent sterilization of substrate being processed due to vacuum and high temperature involved
c) Ease of cutting, less heat generation, lesser patient pain and finally increase in life of the bur
d) Two unique morphologies achieved – one for cutting (Crystalline) and one for low friction (ballas).
e) Adaptability of being repurposed for different substrates like silicon, aluminum nickel titanium alloy.

A consequent aspect of the present invention by way of example is a highly efficient and durable bur prepared by said process. Said bur is majorly characterized in having-
a) Uniform coating over 95% of bur area with micro-sized diamonds;
b) Hardness and flank wear resistance significantly higher than nearest peer in the market;
c) Operability (precision and ability to withstand high intensity operation) significantly higher than nearest peer in the market;
d) Negligible residual stresses due to by in-situ processing; and
e) Ease of cutting, less heat generation, lesser patient trauma
f) Ability to withstand the conditions of chemical and/or steam sterilization without losing surface properties engineered via the present invention
g) Negligible, or non-existent discontinuities in the diamond deposition layer, thereby limiting surface shear off of said layer

As will be realized further, the present invention is capable of various other embodiments and that its several components and related details are capable of various alterations, all without departing from the basic concept of the present invention. The present inventors intend implementation of the present invention for production of dental burs, ortho burs, ENT burs, Neuro burs as well as root canal files, stents and so on.

Accordingly, the foregoing description will be regarded as illustrative in nature and not as restrictive in any form whatsoever. Modifications and variations of the system and apparatus described herein will be obvious to those skilled in the art. Such modifications and variations are intended to come within ambit of the present invention, which is limited only by the appended claims. ,CLAIMS:1) A process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material, the process comprising:
? preconditioning, in a first manual step, the article for receiving, via surface deposition, the hard material to be coated;
? preconditioning, in a second manual step, a modified hot filament chemical vapor deposition system to thereby avoid any interplay between carbonaceous species and said filament;
? placing the preconditioned article in the preconditioned modified hot filament chemical vapor deposition system, and causing controlled initial seeding and progressive growth of discrete moieties of the hard material having user-selected morphology on surface of said article; and
? in the preconditioned common art modified hot filament chemical vapor deposition system, implementing a staggered temperature cycle arranged to avoid internal stresses and brittleness of the deposited hard material to thereby result in the article having a highly uniform and strongly adhered layer of the in-situ formed hard material.

2) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in any one of the preceding claims, wherein the article is a dental tool, particularly a raw tungsten carbide bur having a head portion, a shank portion and a neck portion joining said head and neck portions among which the at least one portion selected for coating with the hard material is the working surface of said head portion.

3) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in claims 1 and 2, wherein the step of preconditioning the at least one portion of the article includes the steps of:
? immersing the article, being the raw tungsten carbide bur, in Murakami’s reagent for a period of 1 minute followed by immersing said article in an acidic working solution having 33% hydrochloric acid, 33% Nitric acid and demineralized water in a 1:1:1 ratio v/v for a period of 2 minutes to thereby result in removal of surface cobalt content necessary for formation of the uniform, strongly-adhered layer of the hard material; and
? immersing the article, being the raw tungsten carbide bur, in acetone containing 80 micron diamond paste for a period of 15 minutes to thereby result in surface imperfections necessary for coalescence, nucleation and formation of discrete moieties of said hard material.

4) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in claim 3, wherein the step of immersing the article in Murakami’s reagent is undertaken amidst ultrasonication at 33khz for accentuating the action of the Murakami’s reagent.

5) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in claim 1, wherein the step of preconditioning the modified hot filament chemical vapor deposition system includes heating its filament to 1800 oC and holding the same for two hours amidst introduction of methane at the rate of 7.5 SCCM which causes a superficial layer of tungsten carbide layer to form on said filament which thereby avoids any interplay between carbonaceous species and said filament.

6) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in claim 1, wherein control over morphology of the discrete moieties of said hard material is achieved via

7) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in claim 1, wherein the step of causing initial seeding and progressive growth of discrete moieties of the hard material on the conditioned article includes creation of an optimized reaction environment by executing the steps of:
? Arranging initial process conditions identified in a chamber pressure of 0.01 to 0.03 mbar; filament temperature of 1800-2250 oC;
? Introducing an scavenger, particularly pure hydrogen, at a distance between 6 cm to 15 cm from head of the article to remove all organic contaminants from the reaction chamber;
? Introducing auxiliary heat input, via ceramic heaters, thereby bringing substrate temperature in the range of 500-800 oC;
? Evacuating the reaction chamber to a base pressure of 0.02 mbar and, in parallel, introducing methane at the rate of 5 SCCM at a process pressure of 30 Torr for 5 minutes;
? Applying a negative bias of 250 V for 30 minutes to the article and in parallel, reducing methane input to a rate of 2.5 SCCM while simultaneously introducing pure hydrogen at the rate of 500 SCCM at process pressure of 30 Torr; and
? After 30 minutes, switching off the negative bias applied, while maintaining the methane and pure hydrogen inputs to rates of 2.5 SCCM and 500 SCCM respectively over subsequent 8 to 14 hours.

8) A system for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material, comprising:
In a common art hot filament chemical vapor deposition system, integrating:
? 50V, 100Amp DC power supply having pulse frequency 2KHz for powering the filament;
? a holder having a molybdenum stage including apertures therein sized according to dimensions and geometry of the article to be received, said holder being adapted for adjustable distance from the heating filament for uniform heat transfer and adjustability to accommodate articles of varying geometry;
? a ceramic heater positioned below the holder for supplying auxiliary heat to the articles held therein for coating with the hard material; and
? a programmable logic controller programmed to execute, in said common art hot filament chemical vapor deposition system, sequential implementation of stages identified in chamber temperature and pressure values of-
a) at least 2200 oC and 25 to 50 Torr respectively at the step of initial seeding and progressive growth of discrete moieties of the hard material;
b) 1500 oC and 200 Torr respectively for subsequent 1 hour;
c) 1200 oC and 400 Torr respectively for subsequent 1.5 hours;
d) 800 oC and 600 Torr respectively for subsequent 2 hours;
e) 300oC and 760 Torr respectively for subsequent 2.5 hours; and
f) Room temperature and 760 Torr respectively for subsequent 3 hours.
to thereby control crystallite size and elimination of graphitic phase.

9) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in any one of the preceding claims, wherein the hard material is diamond.

10) The process for coating an article with a highly uniform and strongly adhered layer of in-situ formed hard material as claimed in claim 9, wherein formation of said layer of hard material includes the in-situ formation of at least a majority of species of diamond conforming alternatively to:
? crystalline morphology, if the optimized process parameters included hydrogen flow at the rate of 150 to 250 SCCM, Methane flow at the rate of 2 to 4 SCCM, process pressure of 30 Torr, article temperature of 700 to 900oC, and filament temperature of 1800 to 2000 oC; and
? ballas morphology, if the optimized process parameters included hydrogen flow at the rate of 350 to 650 SCCM, Methane flow at the rate of 1.5 to 3.5 SCCM, process pressure of 30 Torr, article temperature of 775 to 975 oC, and filament temperature of 1070 to 2170oC.