Chair of Materials Science and Engineering for Metals
University of Erlangen-Nuernberg
Prof. Dr.-Ing. habil. Carolin Koerner

German Flag
menuekopf menuefuss

Ultra Hard Coatings

Crystalline diamond coatings

Diamond films exhibit a large number of unique surface properties like extreme hardness and wear resistance, chemical inertness, low friction coefficients, high thermal conductivity and negative electron affinity. Therefore crystalline diamond coatings are very attractive for a wide range of technical applications. At the chair of WTM diamond thin films are generated mainly by the Hot-Filament Chemical Vapour Deposition (HF-CVD) technique which stands out by homogeneous coating thickness also on large substrates and is easy for scale-up. The research focuses on wear protective diamond coatings for sliding bearings and seals as well as on diamond/metal and diamond/ceramics composites e.g. for heat sinks with adjusted thermal expansion done by so-called Chemical Vapour Infiltration (CVI).

1. Diamond on steel (C. Bareiß)

Since many years diamond deposition on steel is of great industrial interest and therefore it has been in the focus of several research projects in the past. Aim of this project, which is funded by the Bavarian Joint Research Program for carbon-based materials (FORCARBON), is the deposition of adherent diamond films on steel substrates. Direct deposition of diamond on steel is not possible for several reasons. The main problem is the instability of the Fe 3C carbide phase during deposition process, which leads to great graphite formation on the surface and hinders deposition of adherent diamond films. For this reason, the use of interlayers is indispensable. Such interlayers have to fulfil several requirements.

 

In this project two types of interlayers show the described requirements and therefore have an extraordinary potential for succeeding in deposition of adherent diamond films on steel.

Chromium carbide interlayers, formed in a powder pack process at elevated temperatures show appropriate barrier properties and ensure deposition of adherent diamond films of high quality. The high deposition temperature and the reaction of carbon from the steel substrate with the deposited chromium layer leads to changes in microstructure and mechanical properties in the steel sample (grain coarsening, carbon reduced zone at the interface). Thin CVD-TiBN layers, consisting of the two phases TiB 2 and TiN fulfil the mentioned requirements as well and show less influence on the characteristics of the steel material (Fig. 1).

Differences in thermal expansion for diamond, chromium carbide or TiBN interlayer and steel lead to high compressive stresses in the diamond film during cooling down after deposition process. High residual stresses in the diamond film ensure a very good adhesion on the multilayer compound. Using a specific pre-treatment of the steel samples and employing a novel diamond deposition technique at high temperatures these stresses can be reduced and implementing of thick diamond layers is possible. Fig. 2 shows a high speed steel (HSS) cutter with a 2 µm diamond coating.

fig 1 fig 2
Fig. 1 Fig. 2

2. CVD diamond films as wear protective coating for sliding applications (A. Schade)

Within the scope of the project, funded by the Deutsche Forschungsgemeinschaft (DFG), one main focus is the growth of fibre textured diamond films on sintered SiC sliding rings.

 

Especially for mechanical seals or bearings that are applied in boundary lubrication regimes or, for short terms, even under dry running conditions the extraordinary material properties of diamond are beneficial. Nevertheless CVD diamond is also exposed to friction and wear when self-mated diamond films are in dry planar sliding contact.

 

In order to enhance the tribological behaviour of self-mating diamond coatings further, a promising idea is the generation of fibre textured diamond films. The so-called grinding hardness, which is the resistance of a diamond facet to abrasion and well known from polishing of single crystal diamonds for gems, indenters and so on, is much greater for {111} diamond faces than for {100} diamond faces.

 

Consequently a polycrystalline diamond film with an <111> diamond texture where the {111} diamond faces are aligned parallel to the substrate surface plane (Fig. 3) should achieve a better wear resistance than an <100> diamond texture. For this texture the diamond grains exhibit an octahedral shape after diamond deposition (Fig. 4) and the respective {100} diamond faces which are align parallel to the substrate surface plane are revealed not until the octahedral diamond tips are worn off.

fig 3 fig 4
Fig. 3: <111> diamond fibre texture on SSiC Fig. 4: <100> diamond fibre texture on SSiC

The different diamond fibre textures grown in a self-made Hot Filament CVD plant are characterised beside SEM by Raman spectroscopy and X-Ray diffraction in the as-deposited state and by surface profilometry after tribological testing. The dry sliding experiments are performed in a ring-on-ring configuration in a tribological test rig under ambient air.

The preliminary results achieved with these self-mating textured diamond films are promising in terms of comparably low coefficients of friction in combination with low diamond wear.

3. Fabrication of diamond composites via chemical vapor infiltration (A. Glaser)

Aim of this project, which is funded as a part of the Bavarian joint research program for carbon-based materials (FORCARBON), is the investigation of new possibilities in producing diamond/metal or diamond/ceramics composites. Because of the expected outstanding properties diamond/metal or diamond/ceramics penetration structures have a high potential as e.g. heat sinks in the microelectronic industry, for tribological applications (e.g wire drawing dies and spray nozzles) and as electrode material presuming electroconductive diamond (boron doped diamond). In general diamond/metal composites are produced via powder metallurgy by using a metal powder as a matrix with diamond particles embedded. In this case it is not possible to achieve a continuous network of diamond throughout the whole composite. Thus, advantage of the full capability of diamond like the high thermal conductivity is not taken. To achieve a continuous network of diamond in porous substrates a possible solution is the infiltration with diamond by chemical vapour infiltration (CVI).

 

Important for successful diamond infiltration are parameters like diamond growth rate, infiltration depth and diamond purity over the pore depth. High diamond growth rates are important for a rapid pore filling with diamond and a high purity of the deposited layers is necessary to achieve properties as close as possible to natural diamond. Ideal growth conditions for diamond in a pore can be achieved by variation of the process parameters gas pressure, feed gas composition, gas flow and substrate temperature. Complete infiltration is difficult as the pore mouth gets closed at first. This is caused by the overgrowing diamond coating from the surface since the concentration of pertinent diamond growth species decreases rapidly with increasing pore depth. Besides process window evaluation by using standard Hot-Filament-CVD and Microwave-Plasma-CVD the main focus of this project is diamond infiltration by using a new designed Hot-Filament-CVI plant.

 

Most characteristic for the new Hot-Filament-CVI plant (Fig. 5) is the possibility of a forced flow of activated diamond growth species through a porous substrate. Separate feeding of important precursors like hydrogen, methane and oxygen above or underneath the substrate ensures the production of pertinent diamond growth species directly at or in the substrate or rather in the pores. These new operating states should allow diamond infiltration with high growth rates combined with the deposition of high purity diamond layers. Fig. 6 shows a cross section image of a diamond infiltrated copper sheet, which was infiltrated by using the new designed Hot-Filament-CVI plant.

fig 5 fig 6
Fig. 5 Fig. 6

Top Start of page

last update 12/20/2011