Preparing refractory metal boride powder

In the process for preparing refractory metal boride powder selected from the borides of the metals titanium, zirconium and hafnium by gas phase reaction of the corresponding metal halide reactant and boron source reactant selected from the group consisting of boron halide and boron hydride in the presence of hydrogen in a reactor wherein said reactants are introduced into a reactant mixing zone through reactant inlet assembly means and reacted within the reactor to form solid refractory metal boride powder and wherein the metal boride powder tends to deposit and accumulate on the surfaces of the assembly means exposed to said reactants, the improvement which comprises establishing a hydrogen-containing gas stream in close proximity to but spaced from the exposed surfaces of the reactant inlet assembly means, introducing boron source reactant mixed with inert carrier gas into said hydrogen-containing gas stream, introducing metal halide reactant and substantially anhydrous hydrogen halide into said hydrogen-containing gas stream upstream of the boron source reactant, the amount of hydrogen halide introduced being sufficient to retard growth of said metal boride powder on said exposed surfaces of the assembly means, the heat content of the hydrogen-containing gas stream and reactants being sufficient to establish refractory metal boride forming temperatures in said reactor.