Abstract
Introduction
The in vivo evolution of surface material properties is important in determining the longevity of bioceramics. Fracture toughness is particularly relevant because of its role in wear resistance. Some bioceramics, such as zirconia (ZrO2) undergo in vivo phase transformation, resulting in a marked reduction in toughness and commensurate increased wear. Here, we investigated the effect of accelerated aging on the surface toughness of alumina (Al2O3), zirconia-toughened alumina (ZTA), and silicon nitride (Si3N4) femoral heads, in order to identify the optimal ceramic material for in vivo implantation and long-term durability.
Materials
A newly developed Raman microprobe-assisted indentation method was applied to evaluate and compare surface fracture toughness mechanisms operative in Si3N4 (Amedica Corporation, Salt Lake City, UT, USA), Al2O3 and ZTA (BIOLOX® forte, and delta, respectively, CeramTec, GmbH, Plochingen, Germany) bioceramics. The Al2O3 and ZTA materials have long established histories in total hip arthroplasty; whereas Si3N4 has been newly developed for this purpose. The improved method proposed here consisted in coupling the “traditional” indentation technique with quantitative assessments of microscopic stress fields by confocal Raman microprobe piezo-spectroscopy. Concurrently, crack opening displacement (COD) profiles were also monitored by Raman spectroscopy. Toughness measurements were determined using both as-received and hydrothermally exposed (100–121°C for up to 300 hours) femoral heads.
Results
The Raman microprobe visualized two main toughening mechanisms operative in Si3N4 and ZTA bioceramics, namely crack-face bridging by acicular Si3N4 grains and polymorphic transformation of ZrO2 dispersoids. Both mechanisms elevated the resistance to crack propagation above the brittle behavior of Al2O3, which experienced a low crack resistance of ∼1.5 MPaâ��m1/2 independent of crack length. The as-received Si3N4 showed a sharply rising R-curve, up to ∼7 MPaâ��m1/2 within a propagation distance of ∼110 µm. A rising R-curve was also observed in the as-received ZTA, although its increase was less pronounced, ∼4 MPaâ��m1/2 within ∼120 µm.
After hydrothermal exposure, surface toughness values decreased by ∼5%, ∼10%, and ∼42% for Si3N4, Al2O3, and ZTA, respectively. Substantial embrittlement was particularly noted at the surface of the ZTA material, with its toughness value reduced to the level of Al2O3. At the micromechanical scale, such embrittlement is obviously related to decreased availability of transformable (metastable) tetragonal ZrO2dispersoids at the surface. In ZTA, the hydrothermal attack annihilated any rising R-curve effect; whereas this degradation mechanism was not present in either Al2O3 or Si3N4 (Fig. 1).
Discussion
Empowered by the Raman microprobe, the indentation micro-fracture method was shown capable of providing reliable surface toughness measurements in dissimilar biomaterials. Different from bulk toughness, surface toughness is the most relevant parameter in designing bioceramic microstructures for use as hip arthroplasty bearing couples for improved tribological performance. It was demonstrated that the environmentally driven phase transformation in ZTA is detrimental to surface toughness. On the other hand, Si3N4 experienced surface toughness values conspicuously independent of hydrothermal attack.
Conclusions
Unlike transformation toughening (which is operative in ZTA), crack-face bridging (which is the toughening mechanism in Si3N4) proved to be the most durable surface toughening effect for a biomaterial to be employed in joint arthroplasty.
