CVD Diamond Coated Rotating Tools for Composite Machining

Niels Chris EngdahlCrystallume

ABSTRACT

The technology for the Chemical Vapor Deposition(CVD) of pure diamond coatings on rotating cutting toolshas developed rapidly during the past several years.Tungsten Carbide round tools are regularly being coatedwith continuous, well adherent, diamond films thatimprove machining performance and increase toollifetimes in composite materials by 15 to 30 times overuncoated tools. This paper reveals the process methodsinvolved in CVD diamond film growth on tungstencarbide tool surfaces and the unique properties ofdiamond thin films. The surface of the diamond films canbe controlled to create a faceted cutting edge that isideally suited to machining abrasive composite materialsand produces superior surface finishes. Applicationmaterials for diamond coated tools include carbon fiberreinforced composites, fiber reinforced plastics, greenceramics, advanced fiberglass structures and aluminummetal matrix composites.

INTRODUCTION

After more than ten years of commercialization, CVD(Chemical vapor deposition) of diamond-coated tooling isstill a novelty in some machining circles. Even some ofthe shops currently using more established diamondtooling (bonded grit, PCD) are not generally aware ofCVD Diamond’s capabilities. And some of the bestapplications for this new technology are not traditionaldiamond applications. Some of the problem can beconfusion over just what CVD diamond is and how it isdifferent from other diamond tooling. Some simpledefinitions will be useful to distinguish between thedifferent types of diamond tooling before going intodetails of how the CVD diamond coating is applied tocutting tools and a discussion of a few primaryapplications.

BONDED DIAMOND GRIT TOOLS

Bonded diamond grit tools have probably been aroundsince people first had diamond dust and glue. These arewhat most people think of when you say “diamondcoated tools”. Today’s diamond grit tools usesophisticated methods to bond natural and syntheticdiamond particles to a large variety of cutting, grinding

and polishing tools. These tools are used primarily in theconstruction and mining trades, but they are also verypopular for edge finishing of carbon fiber compositestructures and grinding hard materials like ceramics andglasses.

PCD DIAMOND TOOLS

Most engineers and machinists also know about PCD(Poly-crystalline diamond) tooling. PCD also starts withfine diamond particles but they are then formed into adense sintered material using cobalt as a binder. Thesecemented compacts of diamond and cobalt are cut intoshape and brazed into solid carbide tool bodies. Onepopular manufacturer forms the sintered material directlyinto a slot in the tool body, allowing them to create a widevariety of helixed tools and longer lengths of cut. ThePCD tool edge is then ground to a finished shape andcan be reground for repeated use. PCD tools have beenaround for over 50 years and are widely used in theaerospace industry for hole drilling in carbon fibercomposites. The automotive industry was the first toaccept this technology and uses a large number of PCDtipped inserts for machining highly abrasive castaluminum-silicone engine components. The toolsproduced today still have a limited variety of geometriesavailable and a high initial cost but they can be the bestoption in applications demanding a very sharp edge orhigh impact resistance.

CVD DIAMOND TOOLS

CVD diamond is something completely different. Usingthis new technology, diamond coatings are actuallygrown atom by atom onto the surface of the tool. Severaltechnologies are used to produce CVD diamond filmscommercially, but the primary method used for coatingcutting tools is known as Hot Filament Deposition. Insidethe typical coating system, an array of superheatedtungsten wires are used to activate hydrogen and acarbon-containing gas (usually methane). This reactivevapor mixture will condense onto the part to be coated,producing the diamond film over the entire surface of thetool. The diamond crystal growth is carefully controlled toproduce a high-purity diamond film with a microstructuresuited to the machining application. The coatings arefrom 4 to 40 microns in thickness depending on tooldiameter and application. These films are pure diamondwith no binder; they are both mechanically andchemically bonded to the tool surface. The result is the

ability to combine the latest in cutting tool geometry withthe superior mechanical properties of diamond.

CVD COATED DIAMOND TOOLING

ADVANTAGES OF DIAMOND TOOLS

Diamond has many unique physical properties and somethat make it an ideal material for cutting tool applications(Figure 1). Of course it is the hardest known material andso is extremely abrasion resistant. And the extremelyhigh thermal conductivity of diamond removes damagingheat from the cutting edge. Diamond’s low coefficient offriction (similar to Teflon) aids in chip formation andmaterial flow up the flutes of the coated tools. Thesethermal and wear properties mean diamond tools can berun at speeds that would destroy all other coatingmaterials. And when used at normal speeds diamondtools operate cooler than other tool materials, reducingdamage to heat sensitive work-piece materials.

Figure 1

So what does CVD diamond machine well? Many of theadvanced composite materials that are gaining inpopularity lately are perfectly suited to be machined withCVD diamond tooling. Carbon fiber composites, metalmatrix materials, green ceramics, fiber reinforced plasticand graphite/graphite composites are all materials thatrapidly wear out standard tooling, and they are alsomaterials where CVD diamond has proven itsperformance. Other suitable materials are graphite, highsilicone aluminum, and copper alloys.

Diamond is not for everything though; it will not machineany of the ferrous materials due to a chemical reactionbetween the iron in these metals and the carbon atomsmaking up the diamond film. And no, it is not the answerto all of your titanium problems, though it can machine it,but diamond films are a new solution to many otherdifficult to machine materials.

The ideal CVD Diamond tooling applications are thosewhere machining the material forms powder or small grit.These are situations where the primary operation at thecutting edge is basically abrasive wear rather than chipformation. Materials like graphite or fiberglass are perfectexamples. The best application is machining ceramics inthe green state (unfired), when machining these waxyabrasive materials, the CVD diamond coating will lastfrom 50 to 70 times longer than standard carbide.

CVD COATED DIAMOND TOOLING

TOOL MATERIAL

Selection of the proper tool material is crucial to thesuccess of the diamond coating process. The prolongedhigh temperature necessary during the coating processwill damage all but cemented tungsten-carbide andceramic cutting tool materials. And for the optimumcoating adhesion, a C-2 grade of tungsten-carbide mustbe used (6% cobalt binder, grain size above 1 micron).

Once you obtain the correct material, the tool grindingmust be carefully performed. Any overheating of thetungsten-carbide during grinding operations (burning) willdamage the carbide surface and cause the diamond filmto flake off of the burned areas.

SURFACE PREPERATION

Careful preparation of the tungsten-carbide tools beforediamond coating is another of the keys to consistentperformance. The parts to be coated are carefullycleaned and then typically put through a two-stepchemical preparation. The first step roughens thecarbide surface for improved mechanical adhesion(Figure 2) and the second step removes cobalt from thesurface to optimize chemical adhesion. Other methodsinvolve micro-blasting processes to optimize the physicalroughening, interlayer films and re-sintering stepsdesigned to improve chemical adhesion.

Property CVDDiamond

Comparison

Microhardness(HV)

7000-9000

CBN 5000

ThermalConductivity(w/cm-C)@25C

10-18 Copper 4.0

Coefficient ofFriction

0.05- .07 Teflon 0.05

Density(x1000 Kg/m3)

3.52 PCD 4.12

Cobalt Tungsten CarbideGrainsC-2 6%Cobalt

Carbide

Exposed TungstenGrains

Figure 2

COATING PROCESS

Next the parts are loaded into vacuum chambercontaining hydrogen and methane gasses at a pressurefrom 10 to 100 torr. A series of tungsten wires, heated toover 2300 degrees C, are used to provide the energynecessary to both break-up the gas molecules and heatthe tools to over 750 degrees C (Figure 3). When theproper conditions are achieved, the activated carbonatoms can recombine into a pure diamond film over theentire tool surface.

CVD Diamond Coating Process

Figure 3

During the initial growth phase the carbon atoms willform diamond crystallites in the crevasses between thetungsten carbide grains. These crystallites will grow toform a continuous film that is both mechanicallyinterlocked with the etched tungsten carbide structureand chemically bonded to the carbide grains. (Figure 4).

Figure 4

This coating process is very slow, with typical film growthrates of only .5 to 1.5 microns per hour. Since functionalcoating thicknesses range from 2 to 40 microns, this canmean very long growth cycles of 2 days or longer.

Another unique advantage of the CVD diamond coatingprocess is the ability to grow a wide range of surfacestructures and optimize the film for a given application.(See Figures 5-8) When tool edge sharpness is aprimary issue, the diamond film can be grown thin andvery smooth. If abrasion is the main wear, mechanismand thicker films are needed, growing a faceted surfacehas been shown to reduce the cutting forces andincrease lifetime.

Figure 5 Ultra Fine film 1000x

Figure 6 Smooth film 1000x

Figure 7 Standard film 1000x

Activated gasses

Part Temperature 700 – 900 C

99% H21%CH4

2350 CTungstenwires

Diamond Coated Carbide

Figure 8 Cubic film1000x

APLICATION EXAMPLES

CARBON FIBER COMPOSITES

Drilling and countersinking in carbon fiber compositesare just the type of abrasive machining operations thatare perfect for CVD diamond coatings. The first exampleis drilling and countersinking rivet holes in a 0.76cm thickcarbon fiber laminate. The tool used was a combination6.1mm diameter drill / 13mm diameter countersinkrunning at 3000 rpm and a feed rate of 28 cm/min.Results of testing conducted at Boeing Aircraft in Seattle,WA are shown in the graph. (Figure 9)

The coating used here has a faceted surface and cuttingedge (Figures 10 & 11) as compared to the ground edgeof the carbide and PCD tools. This faceting isresponsible for reducing the cutting forces that couldnormally be expected with such a thick coating (20microns).

Figure 9

The data (Figure 9) show how abrasive this material ison standard carbide tooling, typically wearing out a tool inless than 200 holes. Due to the sharp grind on theuncoated tool, the initial thrust is low but rises quickly asthe edge wears away. A PCD tool can do the job quitewell, but still has a shorter life. The CVD coated tool hasthe best performance with low thrust, very nice qualityholes and minimal backside delamination. Although endof life for this test was 50 lbs, the CVD tools were stillproducing high quality holes, with very graceful wear.The tools will last longer on the shop floor where end oflife is closer to 60 lbs.

Figure 10 Faceted edge 1000x

Figure 11 Faceted Edge 4000x0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000

# Of Holes

Thrust(Lbs)

DiamondCoated

PCDUncoatedCarbide

3000 RPMFR- 28 cm/min

HOLE QUALITY ISSUES

Often the most important criteria used for toolperformance in carbon fiber material will be the amountof fiber breakout created during the drilling operation.Drill lifetime is a key issue, but damage of the compositestructure will always be a very costly mistake.

A former Boeing company plant in Tulsa OK has beenusing CVD coated tools for several years. They aredrilling holes in a 12mm thick carbon fiber laminatematerial. The original tools used were PCD tipped drillsbut due to a modification in the carbon laminate materialthey begin to see unacceptable delamination of thecarbon fiber layer on the exit side of the hole.

Extensive testing revealed that tool lifetimes were theequivalent for both PCD and CVD drills, but micrographsof hole finish show the CVD coated drill created a muchhigher quality hole. The photos below (Figure 12 & 13)show cross sectioning of a hole drilled with a PCD tippedtool and one drilled with a CVD coated tool. The holesare both taken near the end of the test cycle of 1000holes.

Production Test Beam – PCD DrillBMS 8-276 Form 3 / 3000 RPM 9 IPM Feed

Figure 12

Figure 13

The reduction in fiber breakout was dramatic in thisapplication, and not just on the entrance and exit sideswhere visual inspection is normally done. Micrographs ofthe sectioned holes revealed the PCD tools were alsocreating a significant amount of fiber pullout throughoutthe depth of the hole (Figure 14). This damage was notapparent during normal visual inspection and onlydiscovered during this evaluation of relative tool life.

Figure 14

Trimming or side milling of this material also presentssignificant challenges. A standard 3/8” 2 flute carbidetool will wear out in less than 20 inches of operation.While the operation can be done effectively with abonded grit tool or even a burr-cut carbide router, a handfinishing operation is then required to clean up the cut. Inthe same operation, the CVD diamond tool produced aclean, burr free surface for over 250 inches. Typicallytrimming is not a tool intensive step in most productionoperations, but for long runs and large dimension parts,the advantage can be dramatic.

GRAPHITE

With the rapid development of electrical dischargemachining (EDM), which uses a graphite electrode tocreate molds for the casting industry, the machining ofgraphite has become commonplace. The very densegraphite materials used are extremely abrasive, andonce again ideal for using CVD diamond coated tooling.A CVD diamond tool will last 10 to 15 times longer thanuncoated carbide. And since the diamond tool can be runat much higher speeds, CVD coatings combined withmodern auto loading machining centers and high-speedspindles can produce a dramatic increase in productivity.Electrode machining can now be done around the clockwith tool lifetimes of 60 to 80 hours.

METAL MACHINING

High silicone aluminum and metal matrix materials arealso extremely difficult to machine with standard tooling.These materials are gaining in popularity due to theirlight weight and high strength, but they are typically veryabrasive to machine. One new aluminum based materialcalled SXA is used for space-based mirror mounts andother lightweight thermally sensitive structural

applications. This material will wear out a standardcarbide tool in less than 10 inches of machining. The 15ximprovement from the CVD coated tool is still a relativelyshort machining cycle, but it does make it possible toaccomplish something between tool changes.

Another application that illustrates the abrasionresistance of diamond films is drilling holes in highsilicone aluminum materials. The graph below (Figure15) shows a comparison with uncoated carbide drillsusing 390 aluminum (18% silicone). The conditions were10,000 RPM, 250 cm/min, and no coolant - using a6.1mm drill with a simple 4 facet point to drill a 2.5cmblind hole.

Figure 15

The test data show uncoated carbide wearing out veryquickly, failing in less than 5 minutes. The CVD diamond-coated tools typically completed 70 minutes of drillingwith the forces still below end of life (350 lbs. Force).Note that this is also another application where thecutting forces are lower for the CVD tool, in spite of thethick diamond coating (22 microns).

RECOMENDED MACHINING CONDITIONS

If your plant is using tungsten-carbide tooling already,you pretty much know how to use CVD diamond. Itrequires the same setup as tungsten-carbide foroptimum performance: rigid tool holding, high qualityspindles and solid work piece fixtures. But remember,the diamond cutting edge can survive a much higher

150

200

250

300

350

400

0 20 40 60 80

DiamondCoated Drill

CarbideDrills

Drilling Time, min.

Lbs.Thru

Thrust Data in 390Al

temperature and so the tool can be run at a higherspeed. The basic advice is to run at your highesteffective spindle speed and then increase your feed rateuntil you have the same chip load per rotation as youwould when running your carbide tools under full load.The full range of cutting speeds and feeds for specificmaterials are available on the web sites of all the majorCVD diamond tool coaters.

ECONOMICS

And so, is it worth it? CVD diamond coated tools aretypically priced at 4 to 6 times the cost of uncoatedtungsten-carbide tooling, but for that price you receive10x to 20x increase in lifetime. This translates into nettooling cost reductions from 40% to 80%. Additionally,the increased tolerance control and uninterruptedmachining can have a big impact on overall productivity.And if your spindle is capable of high speeds (+15,000rpm) you can take full advantage of CVD diamond’sproperties to increase your production rates.

If we are comparing to PCD tooling we have seen thatthe CVD tool will have equal or better performance inmany applications. The cost of the CVD tool isapproximately equal to the regrinding cost of the muchmore expensive PCD tool. A single use tool also hasadvantages related to the necessary inventory controland breakage loss during handling and regrinding of thePCD tooling.

CONCLUSION

Like any new product, CVD diamond tools will need toprove their value on your shop floor before they reallymake sense to you. But CVD diamond has become amature technology, which has the potential todramatically reduce tooling cost and perhaps solve someproblems at the same time. There are now severalcompanies producing diamond-coated tools for theaerospace industries. Many of the standard tools in usetoday are available in these company’s catalogs. If youare willing to provide carbide tools ground from a suitablegrade of carbide, the major diamond coaters will usuallycoat those tools as free samples for evaluation in yourparticular application.

ACKNOWLEDGEMENTS

This work was accomplished with the help of verytalented development engineers from Boeing Companyfacilities in Auburn, WA and Tulsa OK.

CONTACT

Chris EngdahlV.P. TechnologyCrystallumecengdahl@crystallume.com

Ed FrancisV.P. ImagineeringCrystallumeefrancis@crystallume.com