Abstract X-ray diffraction (XRD) and atomic force microscopy (AFM) were utilized to investigate the aging behavior of pressureless sintered monolithic ZrO 2 (TZ-3Y) and zirconia-toughened alumina composite with additions of 2 wt% m -ZrO 2 nanoparticles at 1520 °C for 1 h. Results show that TZ-3Y exhibited significant aging in water steam at 134 °C under pressure 2 bar compared to zirconia toughened alumina (ZTA) composite. The maximum fraction of monoclinic polymorph resulted after 10 and 15 h of aging for monolithic TZ-3Y (~ 85%) and ZTA (~ 2.3%) composite, respectively. The homogeneous dispersion of zirconia into the matrix and its fine grain size as well as the high Young's modulus of the Al 2 O 3 matrix which can provide a constraining effect on the zirconia could partially inhibit the phase transformation. In terms of monoclinic phase content, our ZTA composite fulfil ISO 1336–2008 requirements for implants for surgery, being thus an interesting alternative in this field.