We report on the bond strength at the interfaces of Adhesive-Free-Bonded (AFB®) single crystal sapphire
composite bars as deduced from the fracture toughness. Fracture toughness (K<sub>IC</sub>) characterizes the resistance
of a bulk brittle material to fractural failure caused by unstable crack propagation. Correlation between the
well-defined initial flaw size and the apparent failure strength gives the fracture toughness that is a function of
the average bond strength at the fractured interfaces. To ensure that the fracture interface coincides with the
AFB® interface, we make a pre-notch at the AFB® interface to be the initial surface flaw using a 532 nm
wavelength marking laser. The measured failure strength yields the fracture toughness of the AFB® composites.
We have found in this study that the average fracture toughness of AFB composite samples is 2.51 MPa*√m
and that of non-composite control samples is 2.39 MPa*√m. The correlating fracture energy is 13.1 J/m<sup>2</sup> and
11.9 J/m<sup>2</sup>, respectively. The apparently greater fracture toughness of composite samples compared to that of
non-composite samples conforms with the hypothesis that attributes the origin of the bonding forces at the
AFB interfaces to the Lodon-Van der Waals interaction between two solid bodies.
(100) oriented polycrystalline diamond films are of benefit to applications such as optical windows and coatings due to
its smoother as-grown surface and potentially better optical performance than randomly oriented diamond films. (100)
Highly Oriented Diamond (HOD) films have been successfully grown on polished silicon wafer using the plasmaenhanced
chemical vapor deposition technique (PECVD). The seeding procedure was based on Bias Enhanced
Nucleation (BEN), which provided a high-density and uniform diamond nucleation on the entire two inch diameter
silicon wafer. During the diamond deposition step subsequent to the BEN process, nitrogen gas was added in the
standard methane/hydrogen processing gas mixture. The addition of small amount of nitrogen has three effects: 1) It
increases the growth rate almost 3 times. 2) It stabilizes and enhances the (100) orientation growth. 3) It makes the HOD
growth possible at high pressures (over 100 Torr) and high temperatures (over 1300 K). The diamond film has been
characterized by confocal Raman and SEM, and an optimal temperature window for HOD growth has been identified.
The growth rate of the (100) HOD growth rate reached > 16 μm/hour.