a pattern of depressions in a sealing surface on a cmc wall of gas turbine ring segment allows minimum clearance against turbine blades tips , and thus maximizes working gas sealing . an array of depressions on the surface increases abradability of the surface by blade tip contact during zero clearance conditions and reduces blade tip damage . the depressions are unconnected , preventing bypass of the working gas around the blade tips . a desired abradable surface geometry may be formed in a stacked laminate wall construction by staggered laminate edge profiles or by machining of depressions after construction .