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Ultra-hard Coatings

Bild zum Forschungsbereich

The research group “Ultra-hard coatings” investigates PVD and CVD coatings with special focus on crystalline CVD-Diamond layers. The successful research work enabled the founding of a company called DiaCCon in Fürth, Germany in 2002 (http://www.diaccon.de).

Mitarbeiterfoto Stefan Rosiwal

Stefan Rosiwal, Dr.-Ing.

Mitarbeiterfoto Rudolf Borchardt

Rudolf Borchardt, M.Sc.

Mitarbeiterfoto Timo Fromm

Timo Fromm, M.Sc.

Mitarbeiterfoto Hanadi Ghanem

Hanadi Ghanem, Dr.-Ing.

Mitarbeiterfoto Thomas Helmreich

Thomas Helmreich, M.Sc.

Mitarbeiterfoto Maximilian Göltz

Maximilian Göltz, M.Sc.

Ziel der geplanten Arbeiten sind die Erforschung, Entwicklung und Anwendung von neuen verschleißfesten Elektrodendiamantdünnschichten für die Mikrosenkerosion. Der Vorteil solcher Diamantbeschichtungen liegt in der effizienten Mikrostrukturierung von großflächigen Elektroden mit spanenden Fertigungsverfahren wie z. B. Mikrofräsen und dem anschließenden Beschichten mit einer verschleißfesten Beschichtung, welche einen effizienten Materialabtrag während…

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Das Aufgabengebiet „Innovative solarthermische Energiegewinnung“ hat sich zum Ziel gesetzt, die Kombination von Nickelbasisstrahlungsabsorbern mit thermoelektrischen Materialien zur Stromerzeugung zu untersuchen. Dabei werden in additiven Fertigungsverfahren neuartige offenzellulare Receiverstrukturen aus hochtem­peratur­beständigen Superlegierungen (Nickel- und Cobaltbasis) entwickelt und getestet. Weiterhin werden p- und n-leitende Diamantstrukturen für den Bau…

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Thermoelectric generators (TEG) based on the Seebeck Effect can directly produce electrical current from the waste heat generated by cars or power plants for instance. The established TEG materials currently enable no economical use due to its toxicity, its rare availability (Bismuth- and Lead Tellurides) or its low efficiency (Silicon Germanium).Single crystalline and microcrystalline diamond have a very high thermal conductivity (ca. 2000W/mK) and a very low electrical conductivity, therefore diamond seems to be unsuitable as thermoelectric material. Nanocrystalline diamond foils can be produced by chemical vapour deposition of boron doped diamond on temperature stable templates. After deposition a controlled delamination of the complete nanodiamond layer as nanodiamond foil is possible. It is a promising thermoelectric material, due to its good electrical conductivity and low thermal conductivity (ca. 2 W/mK). Nano diamond is very stable at elevated temperatures (ca. 600°C in air, ca. 1100°C without oxygen). These properties should enable high thermoelectric efficiencies (ZT- value > 2-3). In this project we want to produce boron-dope Nanocrystalline diamond foils with by variation of the HF-CVD process parameters (pressure, methane content, boron content, coating temperature). The thermoelectric properties (Seebeck coefficient, thermal and electrical conductivity) will be measured. Furthermore a thermoelectric generator should be built and characterized by using the new boron-doped foils (p-conductivity) and "poor" (low effiency) N-doped carbon foils (n-conductivity).

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Research of hot-filament CVD technology constantly continues to provide new scientific developments that are transferred into economically functioning products. This business model enabled the establishment of the worldwide largest experimental CVD facility.

Development details:

  • Upscaling of the hot-filament diamond surface area up to 10.000cm2.
  • Flexible chamber set up to coat small (weight < 1 g) as well as large components (weight > 40 kg) via CVD.
  • Reduction of energy input (electric power/carat) for hot-filament CVD.
  • Homogenisation of diamond growth rate and boron doping for a 2D and 3D substrates.
  • Reproducible substrate temperature ranging from 650 °C to 950 °C.
  • Integration of heat treatment during the hot-filament process.
  • Development of in-situ measurements, e.g. online measurement of diamond growth rate.

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The addition of Titanium or Vanadium during the CVD diamond coating process is expected to enable new electrical states in diamond crystal structures. The effect of alloying on the mechanical properties is also being investigated.

Current research topics:

  • Evaporators for organometallic compounds with Titanium or Vanadium.
  • Combination of sputtering processes with hot-filament diamond CVD.

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Boron doped diamond (BDD) electrodes have a very broad application potential. This is due to the high over potential at the cathode (hydrogen formation at -1,2 V) and anodic (oxygen formation at 2.5 V) water electrolysis. This makes BDD electrodes suitable for applications, like: Efficient disinfection by killing bacteria, water treatment via direct chemical oxidation of all carbon types and cathodic reduction of CO2 in hydrocarbons.

We offer different electrochemical reactors with diamond electrodes for research partners and industries.

Electrochemical reactors with diamond electrodes:

  • Batch systems with CVD-diamond for expanded metals or plate-like electrodes for water treatment up to 1m3 water volume.
  • Experimental flow reactors.
  • Mini water disinfection systems on USB-basis.

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Different steels can be CVD-diamond-coated via high temperature chrome-carbide diffusion interlayers or CVD titanium-boron-nitrides. An adjusted diamond coating temperature and heat treatment is necessary to maintain the functionality (strength) of the diamond coated steel components. The research focus lies on the expansion of the spectrum of diamond coatable steels, the optimisation of necessary steel strength and strongly adherent diamond layers with a thickness higher than 10 µm.

An important application of diamond coated steel is the processing of aluminium, since aluminium does not react with the diamond surface at temperatures higher than 500 °C.

Examples:

  • Gas pressure spring of 41Cr4.
  • Screw taps made of tool steel HS 2-9-2.
  • Tools for aluminium high pressure die casting made of hot-working steels X37CrMoV5-1 (12343) and X46Cr13 (1.4034).
  • Ultrasound welding sonotrodes for aluminium and copper cables e.g. out of powder metallurgical steel SPM10.

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Adherent CVD Diamond layers on hard metals are possible with titanium-boron-nitride intermediate layers. The diamond layer thickness can be higher than 100 µm. The removal of the cobalt binding phase via etching is no longer necessary, which improves the mechanical strength in the intermediate zone.  

Examples:

  • Piston rings and ball bearings made of hard metals
  • Hard metal tools
  • Hard metal erosion protection components

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In 1995, we started research and development of CVD diamond coatings on silicon carbide ball bearings and piston rings.

In 2002 the know-how gained in Bavarian research projects was successfully transferred into industrial applications by the start-up of DiaCCon (Fürth, Germany).

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Since Iron and Cobalt catalyse graphite formation during the CVD diamond deposition, high temperature intermediate layers are developed, which inhibit graphite forming on steel or hard metal surfaces. A special surface microstructure of the intermediate layer alloys a good mechanical adhesion with the subsequently growing diamond layer.

In the temperature range from 500 °C to 1100 °C, CVD deposition of metallic Titanium or Tantalum layers and their carbides, nitrides or borides are possible.

Coating examples:

  • TiNB-intermediate layer on X46Cr13 for optimal diamond adhesion.
  • TiB2 on graphene fibres for stabilization after Zirconium-melt infiltration.
  • Ta on graphene to increase the chemical stability.

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The production of the CVD diamond layer can take place separately from the components’ surface. To achieve this, a CVD diamond coating is deposited on silicon or copper based substrates. Free-standing diamond foils with a layer thickness of 20 µm and above can be peeled off the substrate. Laser cutting allows the adequate tailoring of the diamond foils. Bonding and soldering processes are currently under research and are developed for „cold” application onto the component surface.

Application examples:

  • Diamond foils on steel to avoid damage caused by high pressured erosion by a water-sand mixture.
  • Diamond foils on steel to reduce friction (no aluminium adhesion) and wear during aluminium processing.

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Doping and varying the grain sizes via manipulation of CVD process parameters allow the production of diamond foils with application specific properties. On the one hand, micro crystalline diamond foils with a very high heat conductivity (around 2000 W/mK)  and a very low electrical conductivity can be produced, while on the other hand boron doped (p-conduction) diamond foils  with corresponding micro and nano grain sizes can have electrical conductivities of up to 40.000 S/m and a thermal conductivity of significantly less than 100 W/mK.

It was already possible to measure Seebeck coefficients above 350 µV/K. These completely different diamond foils are being further developed to improve thermoelectrical properties.

The doping of Titanium and Vanadium for the n-conduction of diamond is another topic of research.

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In tribometer experiments CVD-diamond coated silicon carbide piston rings allow dry runs up to 100 km. The diamond wear in these experiments is below 3 µm. Using adjusted process parameters, a textured diamond layer in the <111> direction can be created on the piston ring surface. Therefore, the surface is the most wear resistant when exposed to friction. The diamond coating of piston rings, which has been under investigation since 1995, is now used in industry.

DiaCCon, a company founded by the chair of WTM, is the leading research entity for this application worldwide. A new research topic is the expansion of the diamond piston ring applications to metallic ring materials such as steel or hard metals.

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The durability of diamond titanium electrodes is limited especially due to the anodic load of the electrochemical dissociation of the titanium carbide interface between diamond and titanium substrate. With the use of tantalum or niobium as more stable electrode material, the lifetime of diamond electrodes can be increased significantly. A new research effort aims to develop economic diamond electrodes, for which firstly a tantalum or titanium intermediate layer is deposited on titanium or steel plates. After this, the boron doped diamond coating will be deposited.

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For Aluminium high pressure die casting, different diamond coated steel tools with TiNB-intermediated layers are applied at industrial partners. The lifetime of bending tools can be increased from a few weeks to several months.

Diamond coated steel sonotrodes for the ultrasonic welding of Aluminium cables are currently also being tested by industrial partners.

The CVD diamond layer can permanently inhibit the reaction of the tool surface with liquid or solid Aluminium. The necessary strength of the steel must be adjusted after every heat treatment, which is steel specific, to avoid material failure due to insufficient fatigue strength. The CVD diamond coating of Tungsten tools is being researched as an alternative or as an addition to the high temperature intermediate layer to avoid special heat treatments.

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