Mechanical Properties of Ultrananocrystalline Diamond (UNCD) Films with an Influence of the Nucleation Density on the Structure

Diamond is the hardest of all single-phase materials with hardness values around 100 GPa, depending on the crystalline direction. The hardness values for polycrystalline diamond (PCD) films range from 80 to 100 GPa, i.e. rather close to the single crystalline values.

The same is true even for ultrananocrystalline diamond (UNCD) and nanocrystalline (NCD) films, for which the reported hardness values were 95 GPa. Although there is a significant spread of hardness values for this type of films, systematic studies on the impact of important deposition parameters as well as the morphology and structure are missing.

While the UNCD films are a candidate for applications biomedicine and tribology, it is essential to know how the hardness as well as other tribological properties such as friction and adhesion depend on the deposition process and other film properties.

This article reveals the impact of the nucleation density on the morphology of the films, as well as the tribological and mechanical properties of the films. The nucleation density varied between 1 x 10⁸ and 1 x 10¹⁰ cm^-2 due to variation of the quantity of ultradisperse diamond powder (3-5 nm) added to the nanocrystalline diamond powder (250 nm) used for the ultrasonic pre-treatment of the silicon substrates. Scanning electron microscopy and atomic force microscopy were used to investigate structure and morphology of these films, while Nano Scratch and Nanoindentation tests were used for tribological and mechanical properties.

Deposition Set-Up and Conditions

Ultrananocrystalline diamond/amorphous carbon (UNCD/a-C) composite films were made through microwave plasma chemical vapor deposition (MWCVD) from 17% CH₄ /N₂ mixtures in a deposition set-up. The experiments were carried out at a substrate temperature of 600 °C, a MW plasma input power of 800 W, a working pressure of 22 mbar and the duration of the deposition was 390 min. The films were made from monocrystalline (100) silicon wafers, etched in NH4 F/HF and then pre-treated ultrasonically in a diamond powder suspension in n-pentane in order to enhance the nucleation density. The pre-treatment suspension contained 50 mg of NCD powder with a mean grain size of 250 nm, to which variable amounts (up to 80 mg) of ultradisperse diamond (UDD) powder with a mean grain size ranging from 3 to 5 nm were added.

Figure 1. Deposition principle

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

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