melted in the arc and if the pressure is too low the powder will not be carried into the arc if the powder feed rate is too low the plasma spray torch will operate inefficiently and may overheat the substrate and if the powder feed rate is too great not all of the powder will be melted in the torch and the torch may become clogged a carrier gas which is preferably argon is required in operating the plasma spray torch to carry the powder into the arc in a plasma spray torch particles of the powder are entrained in a jet of the carrier gas which passes through the electric arc this melts the particles which then strike and adhere to the substrate the substrate is typically placed about 2 to about 3 inches away from the arc the process of this invention may be used to coat substrates of virtually any composition including silica glass metals and plastic the substrate may be of almost any shape but surfaces which are not accessible to the powder spray will not be coated while coatings of almost any thickness can be built up internal stresses in very thick coatings may result in cracking of the coating or in flaking of the coatings from the substrate coatings can be made more uniform in thickness if desired by grinding or other technique the resulting coated substrate may be used in fuel cells batteries laboratory test work and other applications two grades of u03b2 alumina were obtained an unground calcined u03b2 alumina having a particle size specification of 100 12 200 69 and 325 87 and a superground u03b2 alumina which did not have a mesh size specified but was said to be close to the individual crystallite size each grade of alumina was.