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 °C. 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.