Available online at www.sciencedirect.com ScienceDirect Journal of the European Ceramic Society 34 (2014) 4311–4319 Assessment of low-temperature degradation of Y-TZP ceramics based on Raman-spectroscopic analysis and hardness indentation Christian Schröder ∗ , Alexander Renz, Christof Koplin, Andreas Kailer Fraunhofer Institute for Mechanics of Materials IWM, Business Unit Tribology, Freiburg, Germany Received 5 May 2014; received in revised form 3 July 2014; accepted 15 July 2014 Available online 16 August 2014 Abstract This paper focuses on the analysis of hydrothermally induced phase transformation of yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) and its influence on the hardness. Due to the hydrothermal exposure and the accompanied low-temperature degradation (LTD), a micro-cracked transformation zone is generated at the surface and progresses into the subjacent material. Raman-spectroscopic analysis of hydrothermally loaded and cross-sectioned samples revealed complete phase transformation within this zone. Its depth as well as its temperature-dependent growth rate was verified. Raman-spectroscopic measurements at the surfaces were correlated with the progression of the transformation zone. An efficient model, which assumes one extinction coefficient for tetragonal and monoclinic microstructure, enables to determine the depth of the transformation zone from the measured Raman signals. Furthermore, an exponentially decreasing Vickers hardness with increasing depth was determined. Finally, a differently sintered Y-TZP ceramic revealed enhanced resistance against LTD for the same hydrothermal loading conditions. © 2014 Elsevier Ltd. All rights reserved. Keywords: Y-TZP; Low-temperature degradation; Vickers hardness; Phase transformation; Raman-spectroscopy 1. Introduction Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics are promising materials for many technical applica- tions as they provide a very high hardness and fracture toughness and mechanical strength. 1–4 Furthermore, due to their corro- sion and wear resistance and biocompatibility, they are utilized for artificial femoral heads and dental restorations. 1,5 The reli- ability of some zirconia ceramics is still limited by undesired phase transformation in contact with aqueous solutions which is known as “low-temperature degradation” (LTD). This degrada- tion is characterized by nucleation and growth of a transformed and destabilized zone from the hydrothermally loaded surface into the bulk material. 6 Due to the accompanied intergranu- lar micro-cracking, LTD causes a loss of mechanical strength. 7 Extensive research on LTD of zirconia ceramics has been done in recent years. 6,8 In this context, research mainly focused on ∗ Corresponding author. Tel.: +49 7615142468. E-mail address: christian.schroeder@iwm.fraunhofer.de (C. Schröder). microstructural features, which affect the resistance to degrada- tion. The addition of alumina to zirconia is for instance a suitable method to decelerate the aging process. 9 Further promising approaches are the decrease of both, the grain size and ten- sile residual stresses within the material. 6 An excellent review was published by Basu, 2 which generally summarizes different microstructural features that have an impact on the phase trans- formation. In addition, Chevalier 6 investigated the characteristic features of LTD. In summary, it can be stated that the produc- tion of aging-resistant zirconia ceramics requires an accurate setting of relevant process stages like powder mixing, sintering and machining. In general, the experimental examination of LTD is carried out using hydrothermal ageing experiments. Subsequent inves- tigations enable the quantification of the ratio of tetragonal and transformed monoclinic phases. Here, -Raman-spectroscopy has become a widely used method as it is very sensitive to monoclinic zirconia and offers a high spatial resolution. 10,11 Based on previous research, mathematical correlations were identified between the transformed monoclinic phase content V m and both the absolute hydrothermal loading temperature T and http://dx.doi.org/10.1016/j.jeurceramsoc.2014.07.018 0955-2219/© 2014 Elsevier Ltd. All rights reserved.