ICMCTF2009 Session D2-3: Diamond and Diamond-Like Carbon Materials

Due to its inert nature and carbon composition, diamond has long been presumed to be very biocompatible. Coupled with various exceptional properties such as strength, wear resistance, thermal conductivity and wide bandgap, it has long been considered to have high potential as a biomaterial. More recently, progress in surface functionalization and diamond electrochemistry have generated interest in diamond as a biosensor platform. The covalent bonding that is possible between carbon atoms at the diamond surface and carbon-based biochemicals implies robust and chemically specific surface modifications are possible. In particular, attaching specific surface molecular groups and utilizing electrochemical detection modes are expected to provide a wide array of versatile applications for diamond biosensors. These sensors have the potential to sense a variety of biochemicals from small molecules to proteins, thereby impacting metabolomics, genomics and proteomics. An increasing body of literature providing insight into the electrochemistry of diamond enables the design of sensing protocols, moving the sensors from laboratory curiosity to prototype.

This presentation will start with a general review of diamond biosensing followed by a discussion of more recent highlights in the literature. Specific results for the detection of a particularly important class of biomolecule, NO_x species, will then be discussed. In particular, it was found that NO (aqueous) oxidation at approximately 1V with hydrogen terminated boron-doped diamond in phosphate buffered solution was insufficiently distinct to electrochemically detect NO bubbled into the system. However, an amine-thiol functionalized Boron Doped Diamond surface provided a distinct oxidation peak from 25 nanomolar to 1 micromolar concentrations. Multiple cyclic voltammagrams with amine-thiol functionalized Boron Doped Diamond electrode indicated that the covalently attached surface molecules resisted decomposition during both anodic and cathodic potentials.

CVD-diamond coating on steel promise an enormous potential for a multiplicity of technical applications. The combination of the unique properties of diamond (e.g. highest hardness, low coefficient of friction) and steel, the material with a countless number of applications, seems to be the material matching. However, the deposition of crystalline diamond coatings on steel comes along with several problems. Under the occurring coating conditions during diamond deposition iron acts as a catalyst for graphite formation on the steel surface. The metastable iron carbide decomposes into iron and graphite. And so a graphite layer grows, on which diamond deposition occurs without any adhesion to the steel substrate. To solve this problem, we use a diffusion chromium carbide interlayer which is deposited at 900°C. The polycrystalline diamond layer shows excellent adhesion on this interlayer.A further problem is the great difference in the thermal expansion coefficients of diamond (1.05•10^-6K^-1at 20°C) and steel (~10•10^-6 K^-1 at 20°C). This generates high compressive stresses in the diamond layer, already at low coating temperatures which directs to low diamond growth speed. By using the austenite-ferrite transformation, which leads to a volume expansion of the steel during cooling down, lower compressive stresses and therefore well adherent diamond layers can be achieved. The deposition processes (interlayer and diamond layer) act simultaneously as incorrect heat treatment processes for the steel substrate and affect consequently the mechanical properties.

In this presentation the basics for adherent diamond deposition on steel and first industrial applications are shown.

In recent years the investigation and application of boron doped diamond (BDD) electrodes for wastewater treatment and electrosynthesis are increased. Usually the diamond films are deposited on niobium or silicon as substrate material. To lower the substrate costs the substitution of niobium or silicon with titanium is aspired.

The in-situ formation of titanium carbide during the diamond coating process on titanium deteriorates the properties of BDD coated titanium electrodes for wastewater treatment due to the electrochemical instability of the carbide. The titanium carbide layer on the other side prevents the diamond coating from flaking of the substrate. The formation of the carbide is favoured at high substrate temperatures because of the higher diffusion of carbon in the β-microstructure. Pure Titanium transforms above 882°C from hexagonal α- to a cubic β-microstructure. The hydrogen rich atmosphere of the CVD-coating process adjusts this allotropic transformation temperature to lower values. Therefore substrate temperatures wide below the transformation temperature of 882°C have to be reached in the coating process to stabilize the α-microstructure and decrease the titanium carbide thickness. At this low substrate temperature also the diamond growth rate is very slow, which raise the production costs of stable electrodes.

An oxygen pretreatment can stabilizes the α-phase at the titanium surface. According to the time and temperature of this oxygen pretreatment and the diamond coating process parameters, it is possible to achieve α-phase, β-trans and mixtures of these in the microstructure. The pretreatment enables a CVD diamond coating process at substrate temperatures of 770°C with reduced titanium carbide thickness and still sufficient diamond growth rate.

The hot filament chemical vapour deposition (HF-CVD) of crystalline diamond layers offers the possibility to bring highly wear-resistant coatings on heavily used technical surfaces. Besides the problem of adherence to the substrate the high process temperatures up to 900°C limit the spectrum of materials to be coated. A two step process offers the possibility to circumvent these difficulties:

1. High temperature deposition of diamond on adequate templates which do not build any carbide interlayers.

2. Joining of grown self-supporting diamond foils with thicknesses of up to 80 µm to substrates at considerably lower temperatures afterwards, allowing the coating of otherwise uncoatable materials like polymers or light metals.

Process parameters like filament temperature, pressure and gas-flow of methane and hydrogen determine the microstructure of the diamond foils. Low pressure and high methane concentration lead to the growth of crystallites with a grain size in the range of nanometers. The surface quality of the template is mapped onto the diamond foil, for which reason the preparation of the template's surface plays an immensely important role. By altering the process parameters and the condition of the template's surface the foils’ mechanical properties like Young’s Modulus, hardness and fracture toughness can be tuned. The structure of the foils and the inherent residual stresses also strongly depend on the process parameters and are investigated.

Altogether this talk is to show how self-supporting diamond foils can be manufactured in a HF-CVD process and how the process parameters and the preparation of the template's surface influence their structure and mechanical properties.

Coating of steel surfaces is a standard technique to adjust the tribological properties for the requirements of technical applications. Crystalline CVD diamond, due to its unique properties like the highest wear resistance and a marginal friction is an ideal coating material for severe applications. Especially in mechanical face seals CVD diamond is increasingly popular as engineered surface coating. Steel as substrate material with CVD diamond seems to be an extraordinary combination.

In the last years the process route for diamond coating of steel was developed. To prevent graphite formation during diamond deposition on the iron surface because of the decomposition of the metastable iron carbide we use a diffusion chromium carbide interlayer. The temporary volume expansion caused by phase transformation of the steel substrate during cooling down from coating temperature (~ 800°C) reduces the compressive stresses in the diamond layer and enables a homogeneous and well adherent deposition.

In this study we have investigated the tribological properties of diamond coated C35 (1.0501) specimens, that were differently mechanically treated before HF-CVD-coating and chromium-carbide-coating respectively to achieve different surface roughnesses. The tests were performed in a water lubricated ring-on-disc configuration to simulate conditions of mechanical face seals. The pre-treatment of the steel surface has high influence on the friction and wear of the CVD-diamond-coating, which is explained in a model for tribological behavior of diamond coated steels.

Detonation nanodiamond is presently used with high efficiency for processes of polishing, superfinishing and besides as adsorbents and catalysts of different processes.

Due to unique specific properties such as overdeveloped surface in combination with the original structure characteristics and nanodimensional effects, the nanodiamond is a perspective material for medicine to make a range of various new treatments.

The new use of nanodiamond is the instruments with the nanodiamond coating for effective cutting. Nanodiamond particles are also doped as carbon binding into the hard metal compositions in order to increase their strength, hardness and productivity. At that, the nanodiamond capacity to maintain the constant chemical structure and physical properties at high temperature conditions i.e. its thermostability is one of the questions of high importance. On that reason the investigation of nanodiamond powders thermostability is very important and perspective.

The aim of our work was to study the effects of various factors on nanodiamond powders thermostability and to establish the main regularities to their oxidizing process.

The experiments were conducted on samples of nanodiamond grades ASUD-75 and ASUD-99. The grades were prepared from a mixture synthesized by the Ukrainian company ALIT with detonation of oxygen-deficient explosives.

Thermooxidation of nanodiamond powders was analysed at different thermal conditions. At nonisothermal conditions thermooxidation of samples was assessed by derivatography at the temperature range from 273 to 1273K at a heating rate of 5 deg/min. The kinetic of thermooxidation of nanodiamond by atmospheric oxygen was studied by gravimetric control of the sample mass variation at the constant temperature for an hour and by the kinetic method using the “coal in cylindrical beaker” model as well as by methods with chromatography control for given reaction product.

The predominated influence of chemical composition and energetic state of nanodiamond surface on thermostability of nanodiamond powders at the temperature range from 273 to 1273K has been shown. The kinetic parameters of chemical reaction of diamond with atmospheric oxygen were determinate.

It was stated that the objective characteristic for thermostability of diamonds may be the rate constant of chemical reaction of diamond with components of aggressive environment.

Expanding DNA application field in technologies of materials-processing demands a comparison with the modern leader of finishing - chemical-mechanical polishing technology (CMP) by colloidal silica.

The work purpose consisted in the comparative analysis and experimental study of basic polishing components (composition, pad, machine) work in technology of production of materials surfaces with ultimate geometrical and structural properties by using polycrystalline DNA particles and amorphous SiO₂ compositions.

Analyzing manufacturing technology of these particles, their structural and superficial properties (size, phase condition, modified chemical layers, etc.), colloidal-chemical properties of their water suspensions and the general requirements to optimum polishing compositions are formulated.

Experimental results of studying of colloidal-chemical properties of DNA and SiO₂ water suspensions and their mixes at different ratio w ith chemical agents allowed to define intervals of their sedimentation stability in a range of рН 1-14 and optimum viscosity values.

Polishing rate of Si, Ge, ZnO and other crystals by DNA compositions is several µm in hour while colloidal SiO₂ CMP depending on the equipment gives from 0,2 to 1,5 µm in minute. After polishing by DNA and its mixes with SiO₂ AFM methods revealed on a crystals surface nanoscratches by width of hundreds of nanometers. After SiO₂ CMP abrasion traces were not found. Influence of modes of processing and pad type of two technologies on a relief roughness also determined.

TEM method indicated a processing dislocations absence in Si and Ge after CMP. RBS reveals the increasing of superficial areas structural perfection according to sequential reduction of size of diamond particles from 130 to 3-5 nanometers. The minimum amorphization depth (2-3 lattice parameter) corresponds to a surface after CMP.

Perspective areas of application of DNA compositions in comparison with CMP for achieve highly perfect surfaces of materials (semiconductors, dielectrics, conductors, composites) with high efficiency are defined.