Fereydoon
Namavar
Spire
Corporation
Bedford,
MA0173
fnamavar@spirecorp.com
Alternative
surfaces are sought to increase durability and life of metallic and plastic
biomedical and industrial components. Although hard ceramic and diamond-like
coatings offer significant potential for addressing this need, their promise
has gone largely unrealized due to poor adherence of dissimilar materials
and brittleness of the films.Increasing
hardness and strength are concomitant with decreasing toughness, ductility
and adherence.
Films
comprised of nanocrystalline materials often have physical properties that
differ from those of materials having larger grains.One
fundamental reason for this is that, for grain sizes smaller than 100 nm,
the volume of material influenced by proximity to a grain boundary becomes
significant.At a grain size of
about 5 nm, approximately 50% of the volume is ?near grain boundary? material.Such
structural changes have a strong influence on the most fundamental of material
properties such as conductivity, and elastic modulus (E). (The benefit
of small grains for increased hardness of metals, the Hall-Petch relation
is given byH~d-1/2,
where H is hardness and d is grain size).On
the other hand, the superplasticity observed in nanocrystalline ceramics
or glasses is attributed to the sliding of grains past one another, as
compared to conventional ceramics, in which large grains do not slide easily
and are brittle.
This
presentation will address the development of functionally graded, nanocrystalline,
multiphase ceramic coatings using ion-beam-assisted deposition (IBAD).Ion-enhanced
processes employ energetic ionic bombardment to control grain size as well
as ?stitch? ceramic or metallic films to the substrate.We
will discuss the formation and characterization of nanocrystalline AlN,
TiN, CrN, Cr, Co-Cr and (diamonized) C60 buckyballs films.
We will also report our latest results on deposition of functionally graded,
Ti/TiN/Ti-B-N nanocrystalline multiphase (TiN, TiB2 and BN)
films with hardness greater than 42 GPa. The graded structure provides
a gradual transition from metallic to covalent bonding throughout the thickness
of the film, enhancing adhesion. Wear resistance evaluation (pin-on-disk)
for 5 million cycles (25 days) at 1 GPa contact stress showed no wear,
indicating the potential for the coating to extend the lifetime of substrate
materials by several orders of magnitude. These
graded, nanocrystalline,
multiphase ceramic films have potential for use in broad applications ranging
from orthopedic prostheses to geothermal drilling components.