Having confirmed the formation of passably pure titanium nitride by heating the mixture of graphite and titanium oxide in the current of pure nitrogen (1956) the authors have carried out a series of investigations on the reaction of nitrogen with metallic titanium (99.5% or higher).
A titanium plate was heated in a graphite tube furnace to 1100°-1600°C for 1, 2, and 3 hours in the current of pure nitrogen under the pressures 7, 76, and 760mmHg.
The products so obtained were investigated by X-ray, and by microscope. Also the refractive indices were measured by immersion method.
Weight increase by nitrogenation proceeded rapidly from 1400°C, on which the temperature exerted larger influence than furnace pressure. Moreover, larger increase in wight was found by the heating under reduced pressure.
The lattice constant of the surface layer varied from 4.26₂ to 4.30₅ Å, which is higher than the value for TiN and lower than that of TiC. The examination under microscope revealed the existence of the small amount of graphite crystals, from which the authors attributed the origin of the carbon to the heating element of graphite furnace.
By X-ray analysis it was confirmed that the surface layer was composed of the TiN-TiC solid solution having the lattice of NaCl type, while in the inner layer was formed a solid solution of hexagonal crystals containing less amount of carbon and nitrogen. The surface layer, composed almost entirely of the crystals of NaCl type, was obtained by the heat treatment at a temperature higher than 1400°C.
Observation under microscope has disclosed that the crystals of the surface layer may be divided into two main classes, isotropic and biaxial. And a small amount of biaxial crystals were identified in the specimen which was heated at 1600°C.
The refractive index of high refractive isotropic crystals was found to be n=1.72₈, while that of low refractive ones gave the value n=1.49₂. The refractive indices of biaxial crystals ranged from 1.50₀ up to 1.78₆.