A method of controlling a thermal spray apparatus comprises using a thermal spray gun 12 to conduct a plurality of sprays to produce a coating on at least one test substrate, altering the variables of the gun and sampling acoustic emissions for each spray, testing the coatings to determine at least one property of each and linking variations in coating properties to gun variables, spraying production articles 14 with the gun whilst using a processor 28 to sample acoustic emissions and alter gun variables depending on the sampled emissions to vary or maintain the properties of the coating. The quality of a coating can thus be maintained. The linking of variations in properties may be undertaken by Fourier transform analysis or by neural network data analysis of the acoustic emissions. The spray gun is preferably a high velocity oxy-fuelled (HVOF) gun. The gun variables may comprise at least one of gas flow rates, gas pressure, gun to substrate standoff distance, particle feed rate, substrate temperature, substrate angle and pass number. The properties of the coatings may comprise hardness, microhardness, porosity, cracking and carbide content.
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