Environmental Performance Limits of Ultrananocrystalline Diamond Films

Recent improvements in growth methodologies have decreased the grain sizes and thicknesses of polycrystalline diamond films to the nanometer range, while also increasing the film uniformity and growth rate and preserving the outstanding mechanical properties of diamond. This is rendering such films more technologically and commercially viable. Ultrananocrystalline diamond (UNCD) are the thinnest (<200 nm) and smoothest (Rq < 10 nm) diamond films available.[1] These films demonstrated self-mated friction coefficients as low as near frictionless carbon (μ < 0.007) in environments with sufficient humidity, and the corresponding wear rates could not be measured using scanning white-light interferometry. However, their response to environmental conditions (e.g. relative humidity, ambient species, velocity, and temperature) had not been systematically explored in the past. This study focused on identifying conditions that contribute to favorable tribological performance. We find low friction performance at humidity levels below 1.5% in both nitrogen and argon environments. INTRODUCTION Since their development, diamond films have demonstrated a variety of favorable properties. These films have hardness values close to 100 GPa with modulus values approaching 1000 GPa.[2] The thermal conductivity of diamond films is nearly 5 times that of copper. This combination of properties makes it ideal for coating machine tools where hardness and thermal conductivity dictate cutting performance. However, as a solid lubricant diamond has shown poor performance in low humidity environments. Also, micro and nanocrystalline diamond films have roughness values comparable to their grain sizes (1-10μm and 100-300nm respectively). Hayward et al. showed the roughness of these films dictated friction performance.[3] As a result, diamond like carbon films have been commonly used as solid lubricants because of their extremely low roughness (Rq < 1 nm). More recently improvements in the growth processes for diamond films have reduced the grain sizes in diamond films to below 10 nm. These films are termed ultrananocrystalline diamond (UNCD), and have roughness values comparable to the diamond grain size. Konicek et al. reported friction coefficients of 0.03 to 0.01 for these films in a humidified argon environment.[4] This paper reports on the investigation of these films in humidified nitrogen environments. The response of these films to varying humidity and varying exposure time at fixed humidity is reported. Experimental All friction testing was performed using a linear reciprocating tribometer described in detail by Dickrell et al. The sample geometry was pin-on-flat using a 1/8” diameter silicon-nitride sphere and silicon flat. Both the pin and the flat were coated with UNCD creating a self-mated interface. A smaller environment chamber was constructed to reduce the time constant for changing humidity and allow for more precise