Diamond-like carbon

A ta-C thin film on silicon (15 mm diameter) exhibiting regionsof 40 nm and 80 nm thickness.

A Co-alloy valve part from a producing oil well (30 mm diame-ter), coated on the right side with ta-C, in order to test for addedresistance to chemical and abrasive degradation in the workingenvironment.

Diamond-like carbon (DLC) is a class of amorphouscarbon material that displays some of the typical prop-erties of diamond. DLC is usually applied as coatingsto other materials that could benefit from some of thoseproperties.[1]

DLC exists in seven different forms.[2] All seven containsignificant amounts of sp3 hybridized carbon atoms. Thereason that there are different types is that even diamondcan be found in two crystalline polytypes. The usual onehas its carbon atoms arranged in a cubic lattice, while thevery rare one (lonsdaleite) has a hexagonal lattice. By

Dome coated with DLC for optical and tribological purposes.

mixing these polytypes in various ways at the nanoscalelevel of structure, DLC coatings can be made that at thesame time are amorphous, flexible, and yet purely sp3bonded “diamond”. The hardest, strongest, and slickest issuch a mixture, known as tetrahedral amorphous carbon,or ta-C. For example, a coating of only 2 μm thickness ofta-C increases the resistance of common (i.e. type 304)stainless steel against abrasive wear; changing its lifetimein such service from one week to 85 years. Such ta-Ccan be considered to be the “pure” form of DLC, sinceit consists only of sp3 bonded carbon atoms. Fillers suchas hydrogen, graphitic sp2 carbon, and metals are usedin the other 6 forms to reduce production expenses or toimpart other desirable properties.[3][4]

The various forms of DLC can be applied to almost anymaterial that is compatible with a vacuum environment.In 2006, the market for outsourced DLC coatings wasestimated as about 30,000,000 € in the European Union.In October 2011, Science Daily reported that researchersat Stanford University have created a super-hard amor-phous diamond under conditions of ultrahigh pressure,which lacks the crystalline structure of diamond but hasthe light weight characteristic of carbon.[5][6]

1 Distinction from natural andsynthetic diamond

Naturally occurring diamond is almost always found inthe crystalline form with a purely cubic orientation of sp3bonded carbon atoms. Sometimes there are lattice de-

1

2 3 PROPERTIES

fects or inclusions of atoms of other elements that givecolor to the stone, but the lattice arrangement of the car-bons remains cubic and bonding is purely sp3. The inter-nal energy of the cubic polytype is slightly lower than thatof the hexagonal form and growth rates from molten ma-terial in both natural and bulk synthetic diamond produc-tion methods are slow enough that the lattice structure hastime to grow in the lowest energy (cubic) form that is pos-sible for sp3 bonding of carbon atoms. In contrast, DLCis typically produced by processes in which high energyprecursive carbons (e.g. in plasmas, in filtered cathodicarc deposition, in sputter deposition and in ion beam de-position) are rapidly cooled or quenched on relatively coldsurfaces. In those cases cubic and hexagonal lattices canbe randomly intermixed, layer by atomic layer, becausethere is no time available for one of the crystalline ge-ometries to grow at the expense of the other before theatoms are “frozen” in place in the material. AmorphousDLC coatings can result in materials that have no long-range crystalline order. Without long range order thereare no brittle fracture planes, so such coatings are flexi-ble and conformal to the underlying shape being coated,while still being as hard as diamond. In fact this prop-erty has been exploited to study atom-by-atom wear atthe nanoscale in DLC.[7]

2 Production

There are several methods of producing DLC, which relyon the lower density of sp2 than sp3 carbon. So the ap-plication of pressure, impact, catalysis, or some combi-nation of these at the atomic scale can force sp2 bondedcarbon atoms closer together into sp3 bonds. This mustbe done vigorously enough that the atoms cannot simplyspring back apart into separations characteristic of sp2bonds. Usually techniques either combine such a com-pression with a push of the new cluster of sp3 bondedcarbon deeper into the coating so that there is no room forexpansion back to separations needed for sp2 bonding; orthe new cluster is buried by the arrival of new carbon des-tined for the next cycle of impacts. It is reasonable to en-visage the process as a “hail” of projectiles that producelocalized, faster, nanoscale versions of the classic com-binations of heat and pressure that produce natural andsynthetic diamond. Because they occur independentlyat many places across the surface of a growing film orcoating, they tend to produce an analog of a cobblestonestreet with the cobbles being nodules or clusters of sp3bonded carbon. Depending upon the particular “recipe”being used, there are cycles of deposition of carbon andimpact or continuous proportions of new carbon arrivingand projectiles conveying the impacts needed to force theformation of the sp3 bonds. As a result, ta-C may havethe structure of a cobblestone street, or the nodules may“melt together” to make something more like a sponge orthe cobbles may be so small as to be nearly invisible toimaging. A classic “medium” morphology for a ta-C film

SEM image of a gold-coated replica of a ta-C “diamond-like”coating. Structural elements are not crystallites but are nodulesof sp3-bonded carbon atoms. The grains are so small that thesurface appears mirror smooth to the eye.

is shown in the figure.

3 Properties

As implied by the name, diamond-like carbon (DLC),the value of such coatings accrues from their abilities toprovide some of the properties of diamond to surfacesof almost any material. The primary desirable quali-ties are hardness, wear resistance, and slickness (DLCfilm friction coefficient against polished steel ranges from0.05-0.20[8]). DLC properties highly depends on plasmatreatment (,[9] ([10]) deposition parameters, like effectof bias voltage ([11]), DLC coating thickness ([12]),([13]),interlayer thickness ([14])etc.However, which properties are added to a surface and towhat degree depends upon which of the 7 forms are ap-plied, and further upon the amounts and types of diluentsadded to reduce the cost of production. In 2006 the Asso-ciation of German Engineers, VDI, the largest engineer-ing association in Western Europe issued an authoritativereport VDI2840[15] in order to clarify the existing multi-plicity of confusing terms and trade names. It provides aunique classification and nomenclature for diamond-like-carbon (DLC) and diamond films. It succeeded in report-ing all information necessary to identify and to compare

3.1 Hardness 3

different DLC carbon films which are offered on the mar-ket. Quoting from that document:

These [sp3] bonds can occur not only withcrystals - in other words, in solids with long-range order - but also in amorphous solidswhere the atoms are in a random arrangement.In this case there will be bonding only betweena few individual atoms and not in a long-rangeorder extending over a large number of atoms.The bond types have a considerable influenceon the material properties of amorphous car-bon films. If the sp2 type is predominant thefilm will be softer, if the sp3 type is predomi-nant the film will be harder.

A secondary determinant of quality was found to be thefractional content of hydrogen. Some of the productionmethods involve hydrogen or methane as a catalyst anda considerable percentage of hydrogen can remain in thefinished DLC material. When it is recalled that the softplastic, polyethylene is made from carbon that is bondedpurely by the diamond-like sp3 bonds, but also includeschemically bonded hydrogen, it is not surprising to learnthat fractions of hydrogen remaining in DLC films de-grade them almost as much as do residues of sp2 bondedcarbon. The VDI2840 report confirmed the utility of lo-cating a particular DLC material onto a 2-dimensionalmap on which the X-axis described the fraction of hy-drogen in the material and the Y-axis described the frac-tion of sp3 bonded carbon atoms. The highest qualityof diamond-like properties was affirmed to be correlatedwith the proximity of the map point plotting the (X,Y)coordinates of a particular material to the upper left cor-ner at (0,1), namely 0% hydrogen and 100% sp3 bonding.That “pure” DLC material is ta-C and others are approx-imations that are degraded by diluents such as hydrogen,sp2 bonded carbon, and metals. Valuable properties ofmaterials that are ta-C, or nearly ta-C follow.

3.1 Hardness

Within the “cobblestones”, nodules, clusters, or“sponges” (the volumes in which local bonding is sp3)bond angles may be distorted from those found in eitherpure cubic or hexagonal lattices because of intermixingof the two. The result is internal (compressive) stress thatcan appear to add to the hardness measured for a sampleof DLC. Hardness is often measured by nanoindentationmethods in which a finely pointed stylus of naturaldiamond is forced into the surface of a specimen. Ifthe sample is so thin that there is only a single layerof nodules, then the stylus may enter the DLC layerbetween the hard cobblestones and push them apartwithout sensing the hardness of the sp3 bonded volumes.Measurements would be low. Conversely, if the probingstylus enters a film thick enough to have several layers

STM image of surfaces at the edge of a 1 μm thick layer of ta-C“diamond-like” coating on 304 stainless steel after various dura-tions of tumbling in a slurry of 240 mesh SiC abrasive. The first100 min shows a burnishing away from the coating of an over-burden of soft carbons than had been deposited after the lastcycle of impacts converted bonds to sp3. On the uncoated part ofthe sample, about 5 μm of steel were removed during subsequenttumbling while the coating completely protected the part of thesample it covered.

of nodules so it cannot be spread laterally, or if it enterson top of a cobblestone in a single layer, then it willmeasure not only the real hardness of the diamondbonding, but an apparent hardness even greater becausethe internal compressive stress in those nodules wouldprovide further resistance to penetration of the materialby the stylus. Nanoindentation measurements havereported hardness as great as 50% more than values fornatural crystalline diamond. Since the stylus is blunted in

4 4 APPLICATIONS

such cases or even broken, actual numbers for hardnessthat exceed that of natural diamond are meaningless.They only show that the hard parts of an optimal ta-Cmaterial will break natural diamond rather than theinverse. Nevertheless, from a practical viewpoint itdoes not matter how the resistance of a DLC materialis developed, it can be harder than natural diamond inusage. One method of testing the coating hardness is bymeans of the Persoz pendulum.

3.2 Bonding of DLC coatings

The same internal stress that benefits the hardness ofDLC materials makes it difficult to bond such coatingsto the substrates to be protected. The internal stressestry to “pop” the DLC coatings off of the underlying sam-ples. This challenging downside of extreme hardness isanswered in several ways, depending upon the particular“art” of the production process. The most simple is to ex-ploit the natural chemical bonding that happens in cases inwhich incident carbon ions supply the material to be im-pacted into sp3 bonded carbon atoms and the impactingenergies that are compressing carbon volumes condensedearlier. In this case the first carbon ions will impact thesurface of the item to be coated. If that item is madeof a carbide-forming substance such as Ti or Fe in steela layer of carbide will be formed that is later bonded tothe DLC grown on top of it. Other methods of bondinginclude such strategies as depositing intermediate layersthat have atomic spacings that grade from those of thesubstrate to those characteristic of sp3 bonded carbon. In2006 there were as many successful recipes for bondingDLC coatings as there were sources of DLC.

3.3 Tribology

DLC coatings are often used to prevent wear due to theirexcellent tribological properties. DLC is very resistantto abrasive and adhesive wear making it suitable for usein applications that experience extreme contact pressure,both in rolling and sliding contact. DLC is often usedto prevent wear on razor blades and metal cutting tools,including lathe inserts and milling cutters. DLC is usedin bearings, cams, cam followers, and shafts in the au-tomobile industry. The coatings reduce wear during the'break-in' period, where drive train components may bestarved for lubrication.DLCsmay also be used in chameleon coatings that are de-signed to prevent wear during launch, orbit, and re-entryof land-launched space vehicles. DLC provides lubric-ity at ambient atmosphere and at vacuum, unlike graphitewhich requires moisture to be lubricious.Despite the favorable tribological properties of DLC itmust be used with caution on ferrous metals. If it is usedat higher temperatures, the substrate or counter face maycarburize, which could lead to loss of function due to a

change in hardness. This phenomenon prevents the useof DLC coated machine tool on steel.

3.4 Electrical

If a DLC material is close enough to ta-C on plots ofbonding ratios and hydrogen content it can be an insulatorwith a high value of resistivity. Perhaps more interestingis that if prepared in the “medium” cobblestone versionsuch as shown in the above figure, electricity is passedthrough it by a mechanism of hopping conductivity. Inthis type of conduction of electricity the electrons moveby quantum mechanical tunneling between pockets ofconductive material isolated in an insulator. The resultis that such a process makes the material something likea semiconductor. Further research on electrical proper-ties is needed to explicate such conductivity in ta-C inorder to determine its practical value. However, a dif-ferent electrical property of emissivity has been shown tooccur at unique levels for ta-C. Such high values allow forelectrons to be emitted from ta-C coated electrodes intovacuum or into other solids with application of modestlevels of applied voltage. This has supported importantadvances in medical technology.

4 Applications

Applications of DLC typically utilize the ability of thematerial to reduce abrasive wear. Tooling components,such as endmills, drill bits, dies and molds often use DLCin thismanner. DLC is also used in the engines ofmodernsupersport motorcycles, Formula 1 racecars, NASCARvehicles, and as a coating on hard-disk platters and hard-disk read heads to protect against head crashes. Virtuallyall of the multi-bladed razors used for wet shaving havethe edges coated with hydrogen-free DLC to reduce fric-tion, preventing abrasion of sensitive skin. It is also beingused as a coating by some weapon manufacturers/customgunsmiths. Some forms have been certified in the EU forfood service and find extensive uses in the high-speed ac-tions involved in processing novelty foods such as "chips"and in guiding material flows in packaging foodstuffs withplastic wraps. DLC coats the cutting edges of tools for thehigh-speed, dry shaping of difficult exposed surfaces ofwood and aluminium, for example on automobile dash-boards.Medical applications: The wear, friction, and electri-cal properties of DLC make it an appealing material formedical applications. Fortunately, DLC has proved tohave excellent bio-compatibility as well. This has en-abled many medical procedures, such as Percutaneouscoronary intervention employing brachytherapy to benefitfrom the unique electrical properties of DLC. At low volt-ages and low temperatures electrodes coated with DLCcan emit enough electrons to be arranged into dispos-

5

able, micro-X-ray tubes as small as the radioactive seedsthat are introduced into arteries or tumors in conven-tional brachytherapy. The same dose of prescribed ra-diation can be applied from the inside, out with the addi-tional possibility to switch on and off the radiation in theprescribed pattern for the X-rays being used. DLC hasproved to be an excellent coating to prolong the life ofand reduce complications with replacement hip joints andartificial knees. It also has been successfully applied tocoronary artery stents, reducing the incidence of throm-bosis. The implantable human heart pump can be con-sidered the ultimate biomedical application where DLCcoating is used on blood contacting surfaces of the keycomponents of the device.The Space Black stainless steel Apple Watch[16] is coatedwith diamond-like carbon.

5 Environmental benefits ofdurable products

The increase in lifetime of articles coated with DLC thatwear out because of abrasion can be described by the for-mula f = (g)µ, where g is a number that characterizes thetype of DLC, the type of abrasion, the substrate materialand μ is the thickness of the DLC coating in μm.[17] For“low-impact” abrasion (pistons in cylinders, impellers inpumps for sandy liquids, etc.), g for pure ta-C on 304stainless steel is 66. This means that one-μm thickness(that is ~5% of the thickness of a human hair-end) wouldincrease service lifetime for the article it coated from aweek to over a year and two-μm thickness would increaseit from a week to 85 years. These are measured values;though in the case of the 2 μm coating the lifetime wasextrapolated from the last time the sample was evaluateduntil the testing apparatus itself wore out.There are environmental arguments that a sustainableeconomy ought to encourage articles not engineered tolower performance or to fail prematurely. This in turnwill reduce the need to support greater production of unitsand their frequent replacement, which might provide aneconomic disincentive to manufacturers of such devices.Currently there are about 100 outsource vendors of DLCcoatings that are loaded with amounts of graphite and hy-drogen and so give much lower g-numbers than 66 on thesame substrates.

6 See also

• Chemical vapor deposition

• Cathodic arc deposition

• poly(hydridocarbyne)

7 References[1] Robertson, J. (2002). “Diamond-like amorphous carbon”.

Materials Science and Engineering: R: Reports 37 (4–6):129. doi:10.1016/S0927-796X(02)00005-0.

[2] Name Index of Carbon Coatings

[3] Kržan, B.; et al. (2009). “Tribological behav-ior of tungsten-doped DLC coating under oil lu-brication”. Tribology International 42 (2): 229.doi:10.1016/j.triboint.2008.06.011.

[4] Evtukh, A.A; et al. (2001). “Silicon doped diamond-like carbon films as a coating forimprovement of elec-tron field emission”. Proceedings of the 14th In-ternational Vacuum Microelectronics Conference: 295.doi:10.1109/IVMC.2001.939770.

[5] Louis Bergeron (Oct 17, 2011). “Amorphous Diamond,a New Super-Hard Form of Carbon Created Under Ul-trahigh Pressure”. Science Daily. Retrieved 2011-10-21.An amorphous diamond -- one that lacks the crystallinestructure of diamond, but is every bit as hard -- has beencreated by a Stanford-led team of researchers. ... Thatuniform super-hardness, combined with the light weightthat is characteristic of all forms of carbon -- includingdiamond -- could open up exciting areas of application,such as cutting tools and wear-resistant parts for all kindsof transportation.

[6] Yu Lin, Li Zhang, Ho-kwang Mao, Paul Chow, Yum-ing Xiao, Maria Baldini, Jinfu Shu, and Wendy L. Mao.Amorphous diamond: A high-pressure superhard carbonallotrope. Physical Review Letters, 2011

[7] Achieving ultralow nanoscale wear

[8] DLC Coatings

[9] Argon plasma treatment on metal substrates and effects ondiamond‐like carbon (DLC) coating properties By AbdulWasy ZIA et al. DOI: 10.1002/crat.201300171

[10] Effect of Physical and Chemical Plasma Etching on Sur-face Wettability of Carbon Fiber–Reinforced PolymerComposites for Bone Plate Applications By Abdul WasyZIA et al. DOI: 10.1002/adv.21480

[11] Evaluation of bias voltage effect on diamond-like carboncoating properties deposited on tungsten carbide cobalt ByAbdul Wasy ZIA et al. DOI: 10.1002/sia.5400

[12] Thickness dependent properties of diamond-like carboncoatings by filtered cathodic vacuum arc deposition byAbdul Wasy Zia et al, DOI: http://dx.doi.org/10.1179/1743294414Y.0000000254

[13] Effect of Diamond like Carbon Coating Thickness onStainless Steel Substrate by Abdul Wasy Zia et al,

[14]

[15] Pressemitteilungen

[16] . Apple Inc https://www.apple.com/watch/craftsmanship/. Missing or empty |title= (help)