Mechanical Properties of Boron Doped Diamond Films Prepared by MPCVD
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Mechanical Properties of Boron Doped Diamond Films Prepared by MPCVD Qi Liang, Shane A. Catledge, and Yogesh K. Vohra Department of Physics, University of Alabama at Birmingham (UAB) Birmingham, AL 35294-1170, U.S.A. ABSTRACT A chemical vapor deposition hydrogen/methane/nitrogen feedgas mixture with unconventionally high methane (15% CH4 by volume) normally used to grow ultra-hard and smooth nanostructured diamond films on Ti-6Al-4V alloy substrates was modified to include diborane (10% B2H6 in hydrogen) for boron-doping. The flow rate for B2H6 was varied to investigate its effect on plasma chemistry, film structure, and mechanical properties. It was found that boron in the plasma can easily be incorporated into diamond films and change the lattice parameter and affect film structure as measured by Raman spectroscopy. Grazing angle x-ray diffraction shows a strong dependence of diamond lattice parameter with diborane flow rate and B2H6:CH4 flow rate ratio. Thermal stability of these films was evaluated by heating in an oxygen environment above 700 ºC. Nanoindentation measurements show that the films have high hardness close to that of nanostructured diamond. High film hardness and toughness, combined with good thermal stability and low surface roughness indicate great potential as wear resistant coatings able to withstand high temperature oxidizing environments. INTRODUCTION Open-air thermal stability of diamond coatings is crucial for some applications such like industrial abrasives and cutting tools. It was reported that nanostructured diamond films undergo slow surface graphitization when exposed to temperatures in excess of 600 ºC in air and, as a consequence, their mechanical properties are degraded [1]. Although the field of boron doped diamond films has been studied with respect structure, growth rate and electrical properties [2-5], the open-air thermal stability of these films has not yet been well investigated. In this study, we will study the dependence of boron incorporation level, lattice parameter and crystallinity for various boron and nitrogen concentrations in the plasma by glancing-angle x-ray diffraction and micro-Raman spectroscopy, and we also will investigate the effect of boron-doping on the thermal stability of nano-structured diamond films. EXPERIMENTAL DETAILS Boron doped diamond films were prepared by microwave plasma-assisted CVD on mirror polished 7mm diam and 3 mm thick titanium alloy (Ti-6Al-4V) substrates, which were initially seeded by ultrasonic agitation in a diamond powder/water solution. The flow rate rates for H2, CH4, and N2 remained constant at 500, 88, and 8.8 sccm respectively. One film was grown without diborane and seven more films were grown using diborane mixture (10% B2H6 in hydrogen) flow rates from 0.5 sccm to 3.5 sccm, with an increment of 0.5 sccm. For each experiment, the operating pressure was 35 Torr, the microwave power was 850 ± 15 W, the average substrate temperature was 810 ± 20 ºC, and the deposition time was 2 hours. The thickness of each film was
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