Acoustic　emission　(AE)　technique　was　adopted　to　monitor　the　damage　evolution　of　air　plasma-　sprayed　(APS)　yttria-partially-stabilized　zirconia　(YSZ)　thermal　barrier　coatings　(TBCs)　during　instrumented　indentation　testing,　and　then　the　failure　mechanisms　were　investigated　by　cluster　analysis　and　wavelet　transform　methods.　The　results　of　cluster　analysis　showed　that　there　were　three　classes　associated　with　distinct　failure　types　for　the　8YSZ　coatings　under　Vickers　indentation.　Based　on　wavelet　transform,　these　three　clusters　could　be　clearly　distinguished　from　their　dominant　frequency　bands,　which　were　concentrated　on　levels　A5　(0–156.25　kHz),　D5　(156.25–312.5　kHz)　and　D4　(312.5–625　kHz),　respectively.　Thus,　the　failure　mechanism　of　8YSZ　coatings　under　Vickers　indentation　could　be　clarified　by　the　distribution　of　different　failure　types　in　indentation　depth.　To　sum　up,　as　indentation　load　increases,　the　8YSZ　coatings　can　accommodate　the　indenter　by　elastic　or　little　plastic　deformation,　microcracks　propagation　and　then　debonding　at　the　splat　boundaries.　By　comparing　the　distribution　of　AE　signals　induced　by　different　failure　types　in　indentation　depth　for　samples　with　different　thermal　exposure　time,　it　can　be　inferred　that　thermal　exposure　treatment　can　accelerate　the　degradation　of　APS　8YSZ　TBCs.　©　2018　Chinese　Materials　Research　Society