Abhishek Kothari

Tailoring nanocrystalline diamond coated on titanium for osteoblast adhesion.

Diamond coatings with superior chemical stability, antiwear, and cytocompatibility properties have been considered for lengthening the lifetime of metallic orthopedic implants for over a decade. In this study, an attempt to tailor the surface properties of diamond films on titanium to promote osteoblast (bone forming cell) adhesion was reported. The surface properties investigated here included the size of diamond surface features, topography, wettability, and surface chemistry, all of which were controlled during microwave plasma enhanced chemical-vapor-deposition (MPCVD) processes using CH4-Ar-H2 gas mixtures. The hardness and elastic modulus of the diamond films were also determined. H2 concentration in the plasma was altered to control the crystallinity, grain size, and topography of the diamond coatings, and specific plasma gases (O2 and NH3) were introduced to change the surface chemistry of the diamond coatings. To understand the impact of the altered surface properties on osteoblast responses, cell adhesion tests were performed on the various diamond-coated titanium. The results revealed that nanocrystalline diamond (grain sizes <100 nm) coated titanium dramatically increased surface hardness, and the introduction of O2 and NH3 during the MPCVD process promoted osteoblast adhesion on diamond and, thus, should be further studied for improving orthopedic applications.

Intrinsic stress evolution in nanocrystalline diamond thin films with deposition temperature

The stress evolution in nanocrystalline diamond (NCD) films deposited at different temperatures (from 800to400°C) was investigated. Results showed that the intrinsic stress gradually changed from tensile to compressive with decreasing deposition temperature. Most importantly, the intrinsic stress can be tailored to zero by adjusting the deposition temperature, which is critical to many applications. It has been proven that more H as well as sp2 bonded carbon was incorporated into the grain boundaries, which was responsible for the evolution of stress and other mechanical properties with deposition temperature. Moreover, all the NCD films showed excellent mechanical properties.

Stress evolution in nanocrystalline diamond films produced by chemical vapor deposition

Nanocrystalline diamond films were grown on silicon substrates by microwave plasma enhanced chemical vapor deposition with 1% methane, 2%–10% hydrogen, and argon. High resolution transmission electron microscope images and selected area electron diffraction patterns confirm that the films consist of 10–20nm sized diamond grains. The residual and intrinsic stresses were investigated using wafer curvature. Intrinsic stresses were always tensile, with higher H2 concentrations generally leading to higher stresses. Annealing the films in a hydrogen plasma significantly increased these stresses. These hydrogen induced changes also appear to alter stress levels and stress gradients during the growth process itself. Raman spectra revealed subtle changes in the chemical bonding that were correlated with some of the stress variations. These results suggest that grain boundary bonding and hydrogen induced reactions at the grain boundaries can influence the intrinsic stresses in nanocrystalline diamond films.

The impact of nanocrystalline diamond grain boundary chemistry on frictional response in sliding contact with 319Al alloy

Due to its exceptional mechanical and tribological properties, nanocrystalline diamond (NCD) has the potential to be used for tool coatings that can enable dry machining of Al alloys. This study explores the friction response of NCD coatings in sliding contact with 319 Al. NCD coatings were grown using microwave plasma CVD with a range of growth conditions to explore key growth modulators governing the tribological response of this material. These coatings were then characterized with Raman spectroscopic analysis at three different wavelengths. Pin-on-disc friction response of these coatings with 319 Al showed that the trans-polyacetylene content as determined by the respective Raman spectrum in NCD is the critical factor controlling friction behavior. This finding can have significant implications in other similar applications where friction response of NCD is a key design factor.