Fernando Guiberteau

Improving the compressive strength of bioceramic robocast scaffolds by polymer infiltration

The effect of polymer infiltration on the compressive strength of β-tricalcium phosphate (TCP) scaffolds fabricated by robocasting (direct write assembly) is analyzed in this work. Porous structures consisting of a tetragonal three-dimensional mesh of interpenetrating rods were fabricated from concentrated TCP inks with suitable viscoelastic properties. Biodegradable polymers (polylactic acid (PLA) and poly(ε-caprolactone) (PCL)) were infiltrated into selected scaffolds by immersion of the structure in a polymer melt. Infiltration increased the uniaxial compressive strength of these model scaffolds by a factor of three (PCL) or six (PLA). It also considerably improved the mechanical integrity of the structures after initial cracking, with the infiltrated structure retaining a significant load-bearing capacity after fracture of the ceramic rods. The strength improvement in the infiltrated scaffolds was attributed to two different contributions: the sealing of precursor flaws in the ceramic rod surfaces and the partial transfer of stress to the polymer, as confirmed by finite element analysis. The implications of these results for the mechanical optimization of scaffolds for bone tissue engineering applications are discussed.

A model for microcrack initiation and propagation beneath Hertzian contacts in polycrystalline ceramics

Abstract A fracture mechanics model of damage evolution within Hertzian stress fields in heterogeneous brittle ceramics is developed. Discrete microcracks generate from shear faults associated with the heterogeneous ceramic microstructure; e.g. in polycrystalline alumina, they initiate at the ends of intragrain twin lamellae and extend along intergrain boundaries. Unlike the well-defined classical cone fracture that occurs in the weakly tensile region outside the surface contact in homogeneous brittle solids, the fault-microcrack damage in polycrystalline ceramics is distributed within a subsurface shear-compression zone below the contact. The shear faults are modelled as sliding interfaces with friction, in the manner of established rock mechanics descriptions but with provision for critical nucleation and matrix restraining stresses. This allows for constrained microcrack pop-in during the loading half-cycle. Ensuing stable microcrack extension is then analyzed in terms of a K-dfield formulation. For simplicity, only mode I extensions is considered specifically here, although provision exists for including mode II. The compressive stresses in the subsurface field constrain microcrack growth during the loading half-cycle, such that enhanced extension occurs during unloading. Data from damage observations in alumina ceramics are used to illustrate the theoretical predictions. Microstructural scaling is a vital element in the microcrack description: initition is unstable only above a critical grain size, and extension increases as the grain size increases. Internal residual stresses also play an important role in determining the extent of microcrack damage. Implications of the results in the practical context of wear and fatigue properties are discussed.

Contact fracture of brittle bilayer coatings on soft substrates

Contact-induced fracture modes in trilayers consisting of a brittle bilayer coating on a soft substrate were investigated. Experiments were performed on model transparent glass/sapphire/polycarbonate structures bonded with epoxy adhesive, to enable in situ observation during the contact. Individual layer surfaces were preferentially abraded to introduce uniform flaw states and so allowed each crack type to be studied separately and controllably. Fracture occurred by cone cracking at the glass top surface or by radial cracking at the glass or sapphire bottom surfaces. Critical loads for each crack type were measured, for fixed glass thickness and several specified sapphire thicknesses. Finite element modeling (FEM) was used to evaluate the critical load data for radial cracking, using as essential input material parameters evaluated from characterization tests on constituent materials and supplemental glass/polymer and sapphirse/polymer bilayer structures. The FEM calculations demonstrated pronounced stress transfer from the applied contact to the underlying sapphire layer, explaining a tendency for preferred fracture of this relatively stiff component. Factors affecting the design of optimal trilayer structures for maximum fracture resistance of practical layer systems were considered.

Designing damage-resistant brittle-coating structures: II. Trilayers

A FEA study of coating/substrate bilayers is conducted as a foundation for damage analysis. Attention is focused on the stresses along the contact axis immediately adjacent to the bilayer interface, where radial cracking or yield in the coating, or yield in the substrate, tend to occur. The stress analysis is used to determine critical loads to initiate each damage mode in terms of basic material properties and coating thickness. Controlling material parameters are strength (brittle mode) and yield stress or hardness (plastic mode). The critical loads are shown to have a simple quadratic dependency on coating thickness, but more complex dependencies on elastic modulus mismatch ratio. Simplified explicit modulus functions afford a route to prediction of the critical loads for design purposes. Implications concerning the design of bilayers for specific applications are discussed.

Role of flaw statistics in contact fracture of brittle coatings

Abstract A flaw statistics analysis is here developed to account for systematic differences between experimentally observed and theoretically predicted critical loads for the initiation of contact-induced radial cracks in brittle coatings on compliant substrates. Specific attention is drawn to deviations in critical load ( P R ) data from ideal quadratic dependence on coating thickness ( d ), i.e. P R ∝ d 2 , especially at low d values. It is postulated that these deviations are attributable to the existence of distributions in flaw size and location, in relation to the bell-shaped tensile stress fields responsible for initiation of the radial cracks at the coating lower surface. A statistics-based expression is derived for the mean values of P R in terms of flaw density and size distribution. Data from model bilayers consisting of glass plates of different thicknesses d bonded to polycarbonate substrates are used as an illustrative case study. Controlled pre-abrasion flaws are introduced into the lower glass surfaces before joining into the bilayer configuration, to enable a priori characterization of distribution parameters by image analysis. Finite element modelling is used to determine the tensile stress distribution at the coating lower surface. The predicted statistics-based P R ( d ) function is shown to fit the data within uncertainty bounds. Implications concerning the continued usefulness of the ideal, P R ∝ d 2 relation for designing ceramic coatings for failure resistance are considered.

Finite element modeling as a tool for predicting the fracture behavior of robocast scaffolds

The use of finite element modeling to calculate the stress fields in complex scaffold structures and thus predict their mechanical behavior during service (e.g., as load-bearing bone implants) is evaluated. The method is applied to identifying the fracture modes and estimating the strength of robocast hydroxyapatite and beta-tricalcium phosphate scaffolds, consisting of a three-dimensional lattice of interpenetrating rods. The calculations are performed for three testing configurations: compression, tension and shear. Different testing orientations relative to the calcium phosphate rods are considered for each configuration. The predictions for the compressive configurations are compared to experimental data from uniaxial compression tests.

Joining of yttria-tetragonal stabilized zirconia polycrystals using nanocrystals

Superplastic flow has been successfully used to join fully dense 3 mol% Y{sub 2}O{sub 3}-tetragonal ZrO{sub 2} polycrystals (Y-TZP) at temperatures as low as 1,350 C, a temperature at which direct diffusional bonding would be unlikely to produce a strong, pore-free joint. The objective of the present work was to determine whether bonding temperatures could be further reduced. To achieve lower bonding temperatures, the authors have investigated the bonding of conventional Y-TZP in which a nanocrystalline Y-TZP with a 20-nm grain size is used as the interlayer between two pieces of 0.3 {micro}m grain sizes Y-TZP. Little is known about the deformation of fully dense nanocrystalline Y-TZP, but recent work indicates that above a threshold stress, the principal deformation mechanism would be grain boundary sliding that results in superplastic flow. Questions on the deformation mechanism in the nanocrystalline Y-TZP are being addressed as part of a larger investigation of high-temperature compressive creep behavior, currently in progress.

Effect of Polymer Infiltration on the Flexural Behavior of β-Tricalcium Phosphate Robocast Scaffolds

The influence of polymer infiltration on the flexural strength and toughness of β-tricalcium phosphate (β-TCP) scaffolds fabricated by robocasting (direct-write assembly) is analyzed. Porous structures consisting of a tetragonal three-dimensional lattice of interpenetrating rods were impregnated with biodegradable polymers (poly(lactic acid) (PLA) and poly(ε-caprolactone) (PCL)) by immersion of the structure in a polymer melt. Infiltration increased the flexural strength of these model scaffolds by a factor of 5 (PCL) or 22 (PLA), an enhancement considerably greater than that reported for compression strength of analogue materials. The greater strength improvement in bending was attributed to a more effective transfer of stress to the polymer under this solicitation since the degree of strengthening associated to the sealing of precursor flaws in the ceramic rod surfaces should remain unaltered. Impregnation with either polymer also improved further than in compression the fracture energy of the scaffolds (by more than two orders of magnitude). This increase is associated to the extraordinary strengthening provided by impregnation and to a crack bridging toughening mechanism produced by polymer fibrils.

HETEROGENEOUS JUNCTION OF YTTRIA PARTIALLY STABILIZED ZIRCONIA BY SUPERPLASTIC FLOW

Layers of different composition and/or grain size of yttria partially stabilized zirconia (YPSZ) have been compressed, with the stress perpendicular to the interface of the layers and temperature of 1400 °C, in order to produce a joint using the microstructural feature of superplasticity found in fine-grained ceramics. The pieces joined have been characterized by scanning electron microscopy (SEM), showing a clean interface with no cavitation. The stiffness of the junction was checked using Vickers indentation at room temperature at the interface and compression tests at the same conditions ( T = 1400 °C in air) used for the joining and the stress parallel to the interface. The observation and comparison between the cracks developed around the indents at the interface and in the bulk of the pieces joined as well as the absence of cavities along the interface in the samples compressed parallel to the interface shows that this technique is useful to produce a joint with a clean and strong interface.

Reinforcing bioceramic scaffolds with in situ synthesized ε‐polycaprolactone coatings

In situ ring-opening polymerization of ε-caprolactone (ε-CL) was performed to coat β-tricalcium phosphate (β-TCP) scaffolds fabricated by robocasting in order to enhance their mechanical performance while preserving the predesigned macropore architecture. Concentrated colloidal inks prepared from β-TCP commercial powders were used to fabricate porous structures consisting of a three-dimensional mesh of interpenetrating rods. Then, ε-CL was in situ polymerized within the ceramic structure using a lipase as catalyst and toluene as solvent, to obtain a highly homogeneous coating and full impregnation of in-rod microporosity. The strength and toughness of scaffolds coated by ε-polycaprolactone (ε-PCL) were significantly increased (twofold and fivefold increase, respectively) over those of the bare structures. Enhancement of both properties is associated to the healing of preexisting microdefects in the bioceramic rods. These enhancements are compared to results from previous work on fully impregnated structures. The implications of the results for the optimization of the mechanical and biological performance of scaffolds for bone tissue engineering applications are discussed.