a method of forming a coating deposits a material onto a substrate with high velocity thermal spray apparatus . the method comprises the steps of mixing of an oxidizer gas and a gaseous fuel in the mixing unit , igniting and combusting the oxidizer and gaseous fuel mixture in the combustion chamber , feeding products of combustion to the accelerating nozzle , introducing selected spraying material into accelerating nozzle to form a supersonic stream of hot combustion product gases with entrained particles of spray material , and spraying at high velocity onto a surface positioned in the path of the stream at the discharge end of the nozzle ; and forming a non - clogging convergent - divergent gas dynamic virtual nozzle in the accelerating nozzle by annularly introducing a coaxial gas flow , through a narrow continuous slot of circumferential ring geometry in the vicinity of the entrance to the diverging outlet bore of the accelerating nozzle .