Microstructural Investigation of the Aging Behavior of (3Y‐TZP)–Al₂O₃ Composites

Abstract

The low temperature autoclave aging behavior of zirconia toughened alumina composites processed by a classical powder mixing processing route was analyzed using atomic force microscopy (AFM), scanning electron microscopy and X-Ray diffraction. The transformation was evaluated in terms of nucleation and growth, assessed by XRD. The time-temperature equivalency of the transformation was used to measure an apparent activation energy of the nucleation stage of the transformation of 78 kJ/mol. The microstructural features influencing the transformation were identified, and the influence of the alumina matrix on the transformation was investigated. Transformation progression grain by grain was observed by AFM. Transformation does not only occur in zirconia agglomerates but also in isolated zirconia grains. The matrix could partially inhibit the transformation. This behavior could be rationalized considering the constraining effect of the alumina matrix, shape strain accommodation arguments and microstructural homogeneity effects. INTRODUCTIONZirconia toughened alumina (ZTA) composites are of raising interest as far as biomedical applications are concerned, as an alternative to biomedical grade alumina and zirconia. Indeed, alumina is somewhat brittle, and recent serious problems have been reported 1,2 with yttria-stabilized zirconia aging degradation used in total hip replacement (THR). Aging is related to the tetragonal to monoclinic (t-m) phase transformation of zirconia 3,4 , 2 transformation promoted by mechanical and hydrothermal stresses 3,5,6 . This transformation is accompanied by a 4 vol.% increase and 16% shear, leading to microcracking, surface degradation and eventually grain pop-out. and concluded that either the martensitic theory needed to be modified to take into account bulk effects, or that twins formed subsequently as deformation twins to minimize the shape change of the particle due to transformation. It has since been demonstrated that twins in zirconia were self-accommodated martensitic variants 19 . This study raised the interest for taking into account bulk conditions, as opposed to the behavior of thin foils. Transformation was nonetheless stress-induced, and not autoclave-induced.Among the identified factors influencing the transformation behavior, the zirconia grain size is of prime importance. By limiting zirconia grain growth, it seems possible restricting the transformation. However, this was identified only for transformation occurring during heat treatment at high temperatures (>1200K) or during cooling after sintering 16 , which is different to autoclave aging conditions.Some of the microstructural features that could influence the autoclave aging behavior of zirconia remain ambiguous. Further understandings of these phenomena require local observation of the transformation at the 3 surface of bulk specimens. Recent developments of atomic force microscopy allow observing the transformation-induced surface relief modification at a nanometer scale 20 . The purpose of this paper i...

Ceramics and ceramic coatings in orthopaedics

“…This size for Y-TZP under hydrothermal conditions is reportedly less than 360 nm [204]. Deville et al confirmed that small ZrO 2 grains <500 nm are also required in ZTA ceramics [205], with the Al 2 O 3 matrix influencing their critical transformation size. Typically, the t-ZrO 2 grains are located at alumina grain boundaries or at triple-point grain junctions [206]; but unique intragranular composites have been developed having submicron Al 2 O 3 /t-ZrO 2 grains embedded within larger Al 2 O 3 grains [207].…”

“…Remarkably, they reported no increase in monoclinic content and no observable degradation for either composition. Later, Deville et al published two studies on the low-temperature ageing of ZTA [205,459]. Their first paper examined a range of Al 2 O 3 -ZrO 2 compositions from 2.5 wt.% ZrO 2 in Al 2 O 3 to pure 3Y-TZP.…”

“…Adequate intermixing of the Al 2 O 3 and ZrO 2 ceramic powders prior to densification is essential [208]. For ZTA composites, where Al 2 O 3 is the dominant phase, keeping the ZrO 2 concentration below the theoretical percolation limit (∼16 vol.%) prevents linking-up of t-ZrO 2 grains throughout the Al 2 O 3 matrix [205]. Their isolation is important in inhibiting intergranular H 2 O diffusion which leads to premature transformation.…”

“…The most degradation was observed for 22 wt.% 3Y-TZP in Al 2 O 3 , which corresponded to the percolation limit. Their second study focused on ZTA degradation using only Y-TZP/Al 2 O 3 composites [205]. Compositions from 10 to 20 vol.% Y-TZP were aged for up to 80 hours in an autoclave at between 110 • C to 140 • C. All compositions showed surface transformation even for short exposures.…”

Alumina/Zirconia Micro/Nanocomposites: A New Material for Biomedical Applications With Superior Sliding Wear Resistance

“…In some applications, like the femoral head implants, the increase of the surface implant roughness and microcraking due to a transformation of specific agglomerated zirconia grains can have a dramatic effect on their life span. 22 Therefore, the presence of zirconia aggregates, especially if the zirconia is stabilized with yttria, should be avoided. 23 Experiments carried out by many researchers 24,25 have shown that the mechanical properties and wear of alumina can be significantly enhanced by the dispersion of ceramic particles in the nanometer size range.…”