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The role of Al 2 O 3 impurities on the microstructureand properties of Y-TZPS.N.B. Hodgson a,* , J. Cawley b , M. Clubley ca Loughborough University, I.P.T.M.E., Loughborough, Leicestershire, UKb Sheffield Hallam University, Materials Research Institute, Sheffield, UKc MEL Chemicals, Manchester, UKAbstractAl 2 O 3 is commonly present in, or introduced to, tetragonal zirconia polycrystal (TZP) ceramics as either an impurity or an additiveduring powder manufacture and/or subsequent processing, although its effects are not well understood. A study has been carried out into theeffects of variously dispersed Al 2 O 3 at the 0–1 wt.% level on the sintering characteristics, microstructure, phase composition, andmechanical properties of yttria stabilised, tetragonal zirconia polycrystal (Y-TZP). The results show that these levels of Al 2 O 3 can enhancesintering at low temperatures, but can also cause exaggerated grain growth and reduction in sintered density for higher sinteringtemperatures, with corresponding changes in the mechanical properties. # 1999 Elsevier Science S.A. All rights reserved.Keywords: Al 2 O 3 ; Ceramic; Zirconia1. IntroductionTransformation toughened zirconia ceramics such asYttria stabilised, tetragonal zirconia polycrystal (Y-TZP)are of considerable interest as engineering ceramic materialswith unusually high values of strength and toughness. Thetransformation toughening phenomena responsible for theseunusual properties arises from volume changes and pseu-doplasticity associated with the martensitic tetragonal tomonoclinic phase transformation in these materials [1,2].There are a range of commercially produced TZP ceramicpowders, which are manufactured by a variety of differentprocess routes, giving rise to large differences in cost andproperties. Generally, these materials contain between 0.01and 0.1 wt.% Al 2 O 3 impurity. However, products manufac-turedfromtheseceramicsfrequentlycontainhigherlevelsofAl 2 O 3 as a result of contamination during processing of thematerials, during milling, comminution, and heat treatmentusing Al 2 O 3 media and refractories. In addition, Al 2 O 3additions are sometimes deliberately made to modify theproperties and behaviour of stabilised zirconia powders [3].Whilst it has been recognised for some time that Al 2 O 3does produce significant changes to the behaviour of zirco-nia, the nature of these effects and the processes responsibleare not well understood, with a number of workers reportingapparently contradictory findings [4–6]. In addition, littlepublished information is available regarding the relationshipbetween the effects of Al 2 O 3 impurities and the processingconditions used on the properties and behaviour of thematerial.This work reports the results of an investigation, into theeffects of Al 2 O 3 at the 0 to 1 wt.% level, on the micro-structure and properties of sintered Y-TZP ceramic compo-nents produced under a variety of heat treatment conditions.The aims of the study were to identify the role of, andinteractions between, Al 2 O 3 content, distribution and pro-cess conditions, with a view to determining the optimumprocessing conditions of Al 2 O 3 -contaminated or-doped Y-TZP.2. Experimental procedure2.1. Production and characterisation of doped powdersThe study was carried out by introducing appropriatelevels of Al 2 O 3 as dopants to a commercial Y-TZP ofhigh purity. The material chosen as the basis for this study(TOSOH TZ3Y) contained 5.2 wt.% Y 2 O 3 stabiliserJournal of Materials Processing Technology 92–93 (1999) 85–90*Corresponding author. Fax: +44-1509-223949E-mail address: S.N.B.Hodgson@lboro.ac.uk (S.N.B. Hodgson)0924-0136/99/$ – see front matter # 1999 Elsevier Science S.A. All rights reserved.PII: S0924-0136(99)00164-8