an improvement to the method of making a structure by superplastic forming , wherein portions of a preform are expanded , in the superplastic condition , against a forming member , is disclosed . the improvement comprises the step , prior to expanding the preform , of providing a lower coefficient of friction between the portions of the preform to be expanded and the forming member than which exists between the portions of the preform where expansion is intended to be minimal and the forming member . the two values of coefficient of friction are obtained by applying first and second release coatings to the portions of the preform to be expanded and those where expansion is to be minimal respectively , with the first release coating having a coefficient of friction less than the second release coating . alternately , the first and second release coatings can be applied to the forming member . the preferred first and second release coatings are boron nitride and yttria respectively .