Huge market potential for stress-reduced harder and upscaled ta-C coatings. Article by Stephane Neuville.
Ta-C coating materials similar to amorphous diamond (a-D) is the only harder carbon material (> 60GPa) which has the combined outstanding properties originally erroneously thought Diamond like Carbon (DLC) would have.
Developments of revised carbon material fundamentals allow design for corresponding up-scaled coating equipment’s and understood process enabling stress to be reduced and adhesion secured without significant ta-C degrading. Thus explaining why ta-C is now to be considered for many new technologic application and often in substitution to many elder more common less performing coatings and which will correspond to an important future market concerning both specific equipment manufacturing and subcontractor coating.
I. HARDER ta-C COATINGS (AMORPHOUS DIAMOND)
It has to be emphasized that hard carbon coatings does not correspond to a single homogeneous category but to several different species having much different structure, hardness, optical, electric chemical and mechanical properties. Harder, stress reduced, much more performing ta-C coating is a diamond-like carbon (DLC) [1].
It combines outstanding properties, which no other known material can simultaneously produce (including polycrystalline diamond and graphene, whenever they have some higher specific properties interesting for some particular applications). Properties of ta-C correspond actually to what was often claimed for more common less performing DLC, which in fact never achieved the predicted combination of better properties erroneously believed with first superlative description. Therefore, harder stress reduced ta-C, is expected to correspond to first elder marketing figures elaborated at beginning of the nineties) which are about x100 higher than presently achieved with DLC.
Those properties include highest homogeneity and atomic packing density (because of low size carbon atom and highest interatomic bonding energy ~7eV) providing best diffusion barrier properties and chemical stability (anticorrosion), surface smoothness and surface passivation (high antisoiling and hydrophobic properties), hardness ~ 60 to 80 GPa and high elasticity, low friction coefficient (<0.1) and anti-wear properties (down to less than 10-10 mm3/N.m), similar to polished diamond (last one however, requesting higher coating thickness in consequence of isolated crystallite nucleation and high surface roughness.
Main hard ta-C coating material properties
– Extreme hardness, close to diamond, over 60 GPa, with associated surface smoothness – Low friction f = 0, 1 (even in wet environment)
– Wear rates down to less than x1000 (compared to early DLC generation)
– Application with coating thickness down to less than 100 nm (x10 lower than others) – Better mechanical properties (lower thickness, new function)
– Optical properties. High refraction. Close to diamond. New possible interference colors – Optoelectronic properties (adjustable gap from 1 –3 eV)
– Combined electric properties (catalytic residual doping)
– Thermal stability.650°C (instead of 300°C for usual less hard DLC and a-C:H)
– Thermal conductance similar to degraded diamond (x10 better than usual DLC)
– Chemical inertness and higher diffusion barrier properties
– Low electric work function (,1 eV) (6 eV for graphitic DLC)
– Low internal stress (achievable by catalytic atomic rearrangement effects) and low bias – Depositing rates,2μm /h, partial 3D geometry (lower cost for coating thickness~100 nm) – Possible larger reactors (substrate holder size starting at 1 m2, instead of 0.2 m2).
– Possible adjustments of gradients of mechanical, optical, electrical properties, etc.
COMPARISON WITH GRAPHENE
– Doped ta-C has better electric and thermal conductance than hydrogenated /oxidized graphene (so-called graphane, which is in fact a dielectric hexagonal diamond)
– ta-C is homogeneous and smooth with stable optoelectronic and electrochemical properties (different from graphene containing many defects and discontinuities)
– ta-C has combined higher wear resistance with low friction
– Easier preparation, and possibility for stronger and more stable adhesion
II. CONSIDERING TODAY ta-C FOR LARGER APPLICATIONS.
Severe limitation to industrial implementation of ta-C used to exist up to recent past [5] which can now be overcome.
On one hand these limitations have been owing to poorly adapted equipment design depositing principle and limited size, relatively reduced growth rates (~ 0.1 to 1 μm/h) and limited throughput [6-7], and on other hand to the very important internal stress which dramatically affect the adhesion [8-9]. This had obliged to make use of less performing graphitic degraded ta-C which has lower internal stress, thus, allowing easier and stronger adhesion. These degraded ta-C used to be obtained with lower depositing ion flux and higher temperature, and have significant lower hardness (~30 to 40 GPa) than a-D and which no longer produces better performing surface multi-functions (optoelectronic gap ~1 eV, wear rates ~ 10-8 mm3/N.m, lower thermal and chemical properties).
Notwithstanding, that arc evaporation – one of the most performing ta-C depositing devices – can produce dust particles needing to be filtered in order to avoid embedded particles in the film (which otherwise will harm to tribological and optical thin film material properties). Ion filtering device can then much reduce the coating growth rates [6-7]. Other means such as laser arc avoiding either dust particles, will also limit the upscaling with the laser geometry [3, 9].
Altogether, which explains why it was for long time not possible in practice to get much profit of ta-C multifunction (higher hardness, superior smoothness, better optical properties etc. ) and why ta-C degraded coating material had been used and only for some reduced number of applications.
III. RECENT PROGRESS IN CARBON FUNDAMENTALS.
In 1999 hard carbon (diamond-like DLC) coating technology was erroneously believed to be mature and would not have new important improvement possibilities [10]. Note that it is now understood [11] that hardness corresponds to density of cohesion energy (depending from atomic interlinking binding strength and atomic packing density) and that wear rates is ~ f.E/H (f the friction coefficient, E the elasticity and H the hardness). However, they are also much depending on adhesion strength and thermal stability of the considered interfacing materials. Therefore, superior antiwear properties will also depends on internal stress (which affects the adhesion strength) and on coating surface
roughness. Many common DLC with significant graphitic content, with lower chemical and thermal stability have only reduced elasticity and will not have superior surface smoothness, explaining why common DLC has relatively reduced wear rates (~10-6 to 10-8 mm3/N.m in comparison to up to 10-11 mm3/N.m for better performing ta-C [2]). Last points explain the important role of surface polishing and running in procedure for coarse polycristallin diamond coatings [12] and for more graphitic DLC (which generally have higher surface rugosity) [9, 13-14], and which can limit the practical application interest for them.
More performing ta-C used up to now to be exploited to relatively reduced extent, in consequence of several practical limitation owing to very high stress [5] for which was for long not known how to reduce them more significantly without degrading the material [3-4]. More performing ta-C could only be produced at relatively reduced scale with particular more complex substrate coating interface with which some relative low and medium strength adhesion could be provided [3, 6, 9] or in form of more graphitic degraded version which has no longer combined superior properties, but lower internal stress enabling the coating to be better and easier adherent [5].
Image: DLC coatings on drill bits.
Some improvements have been achieved with Laser-laser technology (however with limited throughput) [3] and with the use of denser plasma and better optimized plasma surface interaction. This can be achieved with specific systems (for instance combination of Arc and different Microwave plasma or DC or RF Magnetron sputtering etc.) [14-15] and with High Power Impulse Magnetron Sputtering (HIPIMS) [16]. Meanwhile, usual cold plasma, being insufficiently ionized (high content of neutrals), achieves generally only limited DLC improvement (hardness ~30 GPa, whenever with higher depositing rates) [5].
No decisive progress could be achieved before huge progress recently achieved in carbon material science (not only for graphene and carbon nanotube) [17]. Those are also concerning revised and updated fundamentals on carbon material characterization [18] and growth mechanisms of diamond and more diamond- like ta-C carbon material. They correspond to quantum electronic (QE) activated sp3 atomic rearrangement (in competition with thermal graphitic degrading) [14] caused by several effects [19-20] (duly confirmed with any produced former
experimental results). These can now be used for definition of new coating
equipment and for the engineering of coating process for more performing ta-C coatings. In contrast to elder descriptions [5,10] which consider only some few such as thermal spikes [21-22] and which for instance give no account for diamond growth without ions or only reduced ion energy [14, 23-24].
Difficult in the past to master the ta-C coating process without easy possibility to clearly characterize the multiple carbon material structures of composite materials [17]. Elder carbon Raman spectra interpretation were not satisfactory, because of misfits on basic theory bringing confusion on disorder effects and substructure identification and what nowadays can be achieved [18, 25-28].
III. ta-C COATINGS FUTURE MARKETS.
III.1. First estimations. With demonstration of ta-C combined outstanding very low wear rates, diffusion barrier, chemical, electrochemical, thermal and optical properties (optoelectronic gap ~3 eV and higher antireflecting optical property) many new and elder applications have been identified for them [1]. They correspond first to applications covered with common DLC for which better performing and cheaper solutions are requested. New identified applications correspond often to combined surface functions which could not be covered with less performing coating materials or being too much expensive.
Therefore, much higher future marketing figures have to be considered for 60 GPa ta-C than presently reported with common hard carbon coatings, including merely degraded less homogeneous ta-C coatings (hardness ~ 30 to 40GPa with lower optoelectronic gap (~ 0.5 eV up to 1.5 eV). This is not to be confused- as often in the past [5]- with ta-C:H and a-C:H which have higher optoelectronic gap however with less performing mechanical and chemical stability [14, 20].
Diamond coatings (polycrystalline) have to be considered at more reduced scale [29], considering they corresponds generally to quite hot depositing processes (not always compatible with substrate to be coated), lower adhesion strength depending on difference of thermal dilatation between diamond coating material and substrate material. It has also to be considered that relative high coating thickness is requested in consequence of its discontinuous structure and coarse rugosity which cannot secure anticorrosion at reduced coating thickness.
Marketing figures for more performing ta-C (a-D) will correspond for the first part to exploited common DLC (hardness 15 to 20 GPa) corresponding to the bulk of hard carbon coating business, with in addition the part of degraded ta-C coatings and to some parts of the diamond coating market. Many applications covered with DLC can be substituted to the better multifunction ta-C all the more that those can generally be used at nearly x10 reduced coating thickness.
Those corresponds to yearly turnover figures ~200 M$ achieved with at least 10 major companies from which the world leader IHI has produced for DLC nearly 50 M$ turnover in 2016 [30]. These figures appear to be very small in comparison to what was estimated beginning of the nineties (~ up to 20 billion $) and which had been established on originally believed superlative DLC properties (reproduced in reference [31]) and which in fact corresponds to the better performing ta-C. Therefore, a corresponding market which at least can be estimated to 2 billion $ (~ 10 times more than produced with common DLC).
Must be added all considered applications which originally have been thought to be covered with believed superlative properties of DLC (and which in reality can only be covered with ta-C) and all identified applications for which surface multifunction and different specific better properties need to be used and which can be estimated to ~ 20 billion $ and expected to grow to ~ 200 billion $.
III.2. Example of applications.
– Medical prosthesis antiwear, gliding and biocompatible (~20M$).
– Roll and gliding bearings (~ 500 M$).
– Engine friction parts for any transportation system (~ 200M$).
– Anti-scratch, antisoiling glass sheets (~ 50 M$).
– Optical lenses hydrophobic properties with antierosion (~ 50M$).
– Anti-icing coatings (aircraft) (~ 70M€).
– Dry friction (~300M$) (weapons, ammunitions, space and nuclear energy seeking for combined low friction, anticorrosion and gliding function.
– Application to energy storage and energy conversion (300M$).
To be adapted as specific modified material for interlayers of existing fuel cell design in order to increase efficiency and life time. Carbon coatings to be used in different porosity, work function and appropriate specific electric conductivity.
– Packaging and anti-moisture encapsulating (~ 30M€) (power LED, transparent foils)
– Anti-wear and combined tribological properties (~ 400 M$). for drills, wood cutting tools, razor blades, threading taps.
– Water treatment. Depollution, and drinkability (~ 6 billion$).
It is known for long that low gap semiconducting material can generate transient electric transversal electric field with physisorbtion of organic material and which can have antibacterial effect. Low physisorbtion energy and stable chemical stability can insure reversibility and self-cleaning effect.
– Heat exchangers and sea water desalinization (~4 billion $). Ta-C has high chemical stability, high thermal conductivity and hydrophobic properties.
– Solar energy (~ 10 billion $ expected to grow to ~100 billion $).
*Antireflecting ta-C encapsulating. (~ 2 billion $) To be compared with well-known less performing optical property of TiOx and SiNx coatings which are more expensive and less erosion and corrosion resistant. Ta-C can be deposited on glass with strong and stable adhesion, or directly on transparent conductive electrodes, which can then have lower thickness (less expensive). Considering 100 million m2 of solar cells to be encapsulated per year (among 200 million new one to be installed). Coating business to be achieved with ~100 specific large coaters.
Additional cost per m2 for antireflection with combined other properties corresponds approx. to additional harvested solar energy gain and of same order of magnitude than unit cost per m2 thin glass sheet.
*Carbon PV materials (~8 billion $). Already been reported with ~ 12% efficiency, with a-C:H and which can be improved with appropriate diode design which by principle is much superior than with Si in combining higher electric conductivity, gradient gap and higher front side optical transparency and stronger work-function differences. Observing that PV application can be combined with antireflective encapsulating.
* Photocatalytic for hydrogen production. (~ 3 billion $). Considering that doped ta-C can be degraded to higher electric conductivity and to the requested optimized 1.8 eV gap. Same order of magnitude business than with carbon PV diodes.
– Solar reflectors ( ~ 2 billion $).
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