Differential Effects of Hydrothermal Ageing on the Surface Fracture Toughness of Ceramics

Abstract

The in vivo wear of ceramic bearings used in total hip arthroplasty (THA) is a complex phenomenon which is not only affected by surface
mechanical properties, but also by tribochemical effects including frictional heating at the contact interface, material dissolution, protein deposition, and
biofilm formation. Wear debris is generated by the propagation and coalescence of surface microcracks. Ceramic wear is therefore highly dependent upon
surface fracture toughness.[1,2] Yet, fracture toughness measurements are typically determined using specimens derived from bulk as opposed to surface
material. Consequently the roles of environmental and tribochemical factors in determining surface fracture toughness are presently unknown. We report,
for the first time, on the effect of hydrothermal ageing on the surface fracture toughness of bioceramics. Previously, hydrothermal ageing was only used to
observe for changes in phase composition and flexural strength which is a standard and well-accepted practice.[3–5] In this study, we compared the surface
toughness of three commercially-available bioceramics before and after hydrothermal exposure, with the null hypothesis being that respective surface fracture
toughness values would not change after accelerated ageing.

Full text

Differential Effects of Hydrothermal Ageing on the Surface Fracture Toughness of Ceramics
Bryan J. McEntire1, Yuto Enomoto2, Wenliang Zhu2, Marco Boffelli2, Elia Marin2, B. Sonny Bal1,3, and Giuseppe Pezzotti2
1Amedica Corp., Salt Lake City, UT, 2Ceramic Physic Laboratory, Kyoto Institute of Technology, Kyoto, Japan, 3University of Missouri, Columbia, MO
Disclosures: Bryan J. McEntire (3A-Amedica Corporation), Yuto Enomoto (N), Wenliang Zhu (N), Marco Boffelli (N), Elia Marin (N), B. Sonny Bal (3A-
Amedica Corporation), and Giuseppe Pezzotti (N).
INTRODUCTION: The in vivo wear of ceramic bearings used in total hip arthroplasty (THA) is a complex phenomenon which is not only affected by sur-
face mechanical properties, but also by tribochemical effects including frictional heating at the contact interface, material dissolution, protein deposition, and
biofilm formation. Wear debris is generated by the propagation and coalescence of surface microcracks. Ceramic wear is therefore highly dependent upon
surface fracture toughness.[1,2] Yet, fracture toughness measurements are typically determined using specimens derived from bulk as opposed to surface
material. Consequently the roles of environmental and tribochemical factors in determining surface fracture toughness are presently unknown. We report,
for the first time, on the effect of hydrothermal ageing on the surface fracture toughness of bioceramics. Previously, hydrothermal ageing was only used to
observe for changes in phase composition and flexural strength which is a standard and well-accepted practice.[3–5] In this study, we compared the surface
toughness of three commercially-available bioceramics before and after hydrothermal exposure, with the null hypothesis being that respective surface frac-
ture toughness values would not change after accelerated ageing.
METHODS: Materials compared were: silicon nitride (Si3N4, MC2®, Amedica Corporation, Salt Lake City, UT), alumina (Al2O3, BIOLOX® forte) and
zirconia-toughened alumina (ZTA, BIOLOX® delta, CeramTec GmbH, Plochingen, Germany). Details on their composition and processing are known.[6,7]
Specimens were Ø36mm femoral heads tested in the “as-received” condition, and after accelerated ageing in an autoclave for 200 h at 121°C and 1 bar steam
pressure.[3] Vickers indentations (1 to 5 kgf) were imprinted on the finely polished surfaces of these femoral heads before and after autoclaving. Crack
lengths and crack-opening displacements (COD) were assessed by scanning electron microscopy. Raman spectroscopy [8] was used to differentiate between
residual stresses associated with the indentation imprints themselves (i.e., so-called Yoffe stress fields [9]) and the actual effects of the hydrothermal treat-
ment. A quantitative rationalization of the stress fields surrounding the Vickers imprints was performed by observing for Raman band shifts specific to each
material, which allowed for the determination of surface fracture toughness. A minimum of four cracks were measured for each biomaterial with up to 50
Raman assessments performed along the crack-length and ahead of the crack-tip.
RESULTS: For Si3N4, the results showed the formation of a slight surface layer of amphoteric SiO2 from the hydrothermal exposure, but otherwise no sur-
face structural changes. For Al2O3, nearly identical band shifts were observed for “as-received” and autoclaved samples. However, it was noted that inden-
tation imprints averaged ~8 µm larger for the hydrothermally treated Al2O3 samples, which may relate to release of oxygen from the material’s lattice struc-
ture, resulting in softening of the surface consistent with previous reports.[10] Conversely, significant structural changes were seen in the ZTA ceramic.
Prolonged autoclave exposure led to the polymorphic phase transformation of its zirconia, with the surface monoclinic content increasing by a factor of 4x,
on top of which the Vickers imprint introduced a strong gradient in monoclinic content with a maximum of ~73 vol.% at the indentation edge. With the
knowledge of the residual stresses developed along the crack-length, it was possible to compute the surface fracture toughness of each material as a function
of crack length. Figures 1(a) and (b) summarize the near-tip rising R-curve behavior of these different ceramic materials before and after autoclaving, re-
spectively. These results suggest that environmentally-driven effects induced by hydrothermal ageing were negligible in Si3N4 and Al2O3, while the ZTA
composite clearly underwent surface embrittlement with respect to short-crack propagation.
DISCUSSION: Different toughening mechanisms are responsible for the observed surface fracture toughness of the pristine and hydrothermally-aged ce-
ramic materials investigated in this study. The stability observed in Si3N4 was solely due to crack-face bridging, which is its dominant toughening mecha-
nism. In contrast, ZTA, which relies on transformation toughening, was markedly affected by stoichiometric and crystallographic alterations that were trig-
gered during exposure to the hydrothermal environment. A consistent embrittlement of its surface was observed, with its resulting toughness lowered to that
of monolithic Al2O3. This newly developed Raman-assisted indentation method for determining surface fracture toughness revealed that the transformation-
toughening observed in ZTA is not a durable in vitro (or likely in vivo) mechanism, whereas crack-face bridging in Si3N4 remains predictable and effective.
In particular, the latter material preserved its steeply rising R-curve behavior for short-cracks even after long-term hydrothermal exposure.
SIGNIFICANCE: The durability and reliability of ceramic articulations in prosthetic joints may well be determined by each material’s intrinsic surface
fracture toughness. In contrast to Si3N4, which demonstrated resilient toughness even in the presence of severe accelerated ageing, the observed embrittle-
ment of ZTA may foreshadow its lifetime limitations.
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4367-4410, (2014). Figure 1 – Near-tip rising R-curve behavior as obtained on indentation cracks for Si3N4 , Al2O3, and ZTA bio-
materials before (a) and after (b) autoclaving.