Plasma test on Industrial Diamond Powder in Hydrogen and Air for Fracture Strength Study
Creators: name: Asuri Sudharshana Chary, Rohit Asuri Sudharshana
Publisher: name: University of Nevada, Las Vegas
Year: 2012

description: Diamonds are the most precious material all over the world. Ever since their discovery, the desire for natural diamonds has been great; recently, the demand has steeply increased, leading to scarcity. For example, in 2010, diamonds worth $50 billion were marketed. This increased demand has led to discovering alternative sources to replace diamonds. The diamond, being the hardest material on earth, could be replaced with no other material except another diamond. Thus, the industrial or synthetic diamond was invented. Because of extreme hardness is one of diamond's properties, diamonds are used in cutting operations. The fracture strength of diamond is one of the crucial factors that determine its life time as a cutting tool. 
Glow discharge is one of the techniques used for plasma formation. The glow discharge process is conducted in a vacuum chamber by ionizing gas atoms. Ions penetrate into the atomic structure, ejecting a secondary electron. The objective of this study is to determine the change in fracture strength of industrial diamond powder before and after plasma treatment. This study focuses mainly on the change in crystal defects and crushing strength (CS) of industrial diamond powder after the penetration of hydrogen gas, air and hydrogen-air mixture ions into the sample powder. 
For this study, an industrial diamond powder sample of 100 carats weight, along with its average fracture strength value was received from Engis Corporation, Illinois. The sample was divided into parts, each weighing 10-12 carats. At the University of Nevada, Las Vegas (UNLV), a plasma test was conducted on six sample parts for a total of 16 hours on each part. The three gas types mentioned above were used during plasma tests, with the pressure in vacuum chamber between 200 mTorr and 2 Torr. The plasma test on four sample parts was in the presence of hydrogen-air mixture. The first sample had chamber pressures between 200 mTorr and 400 mTorr. The remaining three samples had chamber pressures between 600 mTorr and 1 Torr. The fifth sample part underwent plasma in the presence of atmospheric air, due to accidental closing of the hydrogen valve, at pressure levels 200 mTorr to 400 mTorr. On the last sample, the plasma test was carried out in hydrogen gas at pressures of 1 and 2 Torr. X Ray Diffraction (XRD) was conducted at UNLV before and after plasma tests. The samples were shipped back to Engis Corporation to test the difference in CS after plasma treatment. 
The XRD results revealed that plasma experiment had no significant change in lattice constants on diamond powder. However, the crystal defect concentrations increased due to change in full width half maximum (FWHM), but the samples are in a highly crystalline state since the change is very small. FWHM after plasma is higher than that before plasma. This confirms a change in defect concentration after plasma. The plasma test conducted in hydrogen gas increased the CS of diamond powders by 0.4%. The CS of the plasma treated sample in air was 1.4% above the average crushing strength. From the four plasma samples of hydrogen-air mixture, the plasma performed at pressures of 200 mTorr to 400 mTorr had a higher CS than the average value by 4.4%. The other three samples had CS between 49-50% which was lower than the average value. 
These experiments suggest that the plasma test conducted in pure hydrogen gas proved to increase the crushing strength of diamond powders. However, it is possible that limiting the pressure values below 400 mTorr during plasma experiment in pure hydrogen could give better crushing strength results. Plasma test performed in hydrogen- air mixture yielded better CSI results compared to plasma tests conducted in only hydrogen or only air.