Franck Tancret

Calcium phosphate cements for bone substitution: chemistry, handling and mechanical properties.

Since their initial formulation in the 1980s, calcium phosphate cements (CPCs) have been increasingly used as bone substitutes. This article provides an overview on the chemistry, kinetics of setting and handling properties (setting time, cohesion and injectability) of CPCs for bone substitution, with a focus on their mechanical properties. Many processing parameters, such as particle size, composition of cement reactants and additives, can be adjusted to control the setting process of CPCs, concomitantly influencing their handling and mechanical performance. Moreover, this review shows that, although the mechanical strength of CPCs is generally low, it is not a critical issue for their application for bone repair--an observation not often realized by researchers and clinicians. CPCs with compressive strengths comparable to those of cortical bones can be produced through densification and/or homogenization of the cement matrix. The real limitation for CPCs appears to be their low fracture toughness and poor mechanical reliability (Weibull modulus), which have so far been only rarely studied.

A novel injectable, cohesive and toughened Si-HPMC (silanized-hydroxypropyl methylcellulose) composite calcium phosphate cement for bone substitution.

This study reports on the incorporation of the self-setting polysaccharide derivative hydrogel (silanized-hydroxypropyl methylcellulose, Si-HPMC) into the formulation of calcium phosphate cements (CPCs) to develop a novel injectable material for bone substitution. The effects of Si-HPMC on the handling properties (injectability, cohesion and setting time) and mechanical properties (Youngs modulus, fracture toughness, flexural and compressive strength) of CPCs were systematically studied. It was found that Si-HPMC could endow composite CPC pastes with an appealing rheological behavior at the early stage of setting, promoting its application in open bone cavities. Moreover, Si-HPMC gave the composite CPC good injectability and cohesion, and reduced the setting time. Si-HPMC increased the porosity of CPCs after hardening, especially the macroporosity as a result of entrapped air bubbles; however, it improved, rather than compromised, the mechanical properties of composite CPCs, which demonstrates a strong toughening and strengthening effect. In view of the above, the Si-HPMC composite CPC may be particularly promising as bone substitute material for clinic application.

A simple and effective approach to prepare injectable macroporous calcium phosphate cement for bone repair: Syringe-foaming using a viscous hydrophilic polymeric solution

UNLABELLED In this study, we propose a simple and effective strategy to prepare injectable macroporous calcium phosphate cements (CPCs) by syringe-foaming via hydrophilic viscous polymeric solution, such as using silanized-hydroxypropyl methylcellulose (Si-HPMC) as a foaming agent. The Si-HPMC foamed CPCs demonstrate excellent handling properties such as injectability and cohesion. After hardening the foamed CPCs possess hierarchical macropores and their mechanical properties (Youngs modulus and compressive strength) are comparable to those of cancellous bone. Moreover, a preliminary in vivo study in the distal femoral sites of rabbits was conducted to evaluate the biofunctionality of this injectable macroporous CPC. The evidence of newly formed bone in the central zone of implantation site indicates the feasibility and effectiveness of this foaming strategy that will have to be optimized by further extensive animal experiments. STATEMENT OF SIGNIFICANCE A major challenge in the design of biomaterial-based injectable bone substitutes is the development of cohesive, macroporous and self-setting calcium phosphate cement (CPC) that enables rapid cell invasion with adequate initial mechanical properties without the use of complex processing and additives. Thus, we propose a simple and effective strategy to prepare injectable macroporous CPCs through syringe-foaming using a hydrophilic viscous polymeric solution (silanized-hydroxypropyl methylcellulose, Si-HPMC) as a foaming agent, that simultaneously meets all the aforementioned aims. Evidence from our in vivo studies shows the existence of newly formed bone within the implantation site, indicating the feasibility and effectiveness of this foaming strategy, which could be used in various CPC systems using other hydrophilic viscous polymeric solutions.

Design of a creep resistant nickel base superalloy for power plant applications. Part 3: Experimental results

Abstract A semi-industrial scale rolled bar of the creep resistant Ni base superalloy designed in Parts 1 and 2 has been fabricated. The influence of heat treatment on microstructure and phase formation has been investigated using both optical and electron microscopy, and X-ray diffraction. Mechanical properties at ambient and high temperature have been measured by hardness testing, compression testing, and tensile creep testing. These experimental results are in good agreement with the predictions of Parts 1 and 2 concerning phase formation, yield stress and creep rupture stress. The target of a lifetime of 100 000 h at 750° C under 100 MPa seems attainable. The design procedure is therefore mainly validated, and results in a promising new alloy for power plant applications.

Computational thermodynamics, Gaussian processes and genetic algorithms: combined tools to design new alloys

A new alloy design procedure is proposed, combining in a single computational tool several modelling and predictive techniques that have already been used and assessed in the field of materials science and alloy design: a genetic algorithm is used to optimize the alloy composition for target properties and performance on the basis of the prediction of mechanical properties (estimated by Gaussian process regression of data on existing alloys) and of microstructural constitution, stability and processability (evaluated by computational themodynamics). These tools are integrated in a unique Matlab programme. An example is given in the case of the design of a new nickel-base superalloy for future power plant applications (such as the ultra-supercritical (USC) coal-fired plant, or the high-temperature gas-cooled nuclear reactor (HTGCR or HTGR), where the selection criteria include cost, oxidation and creep resistance around 750??C, long-term stability at service temperature, forgeability, weldability, etc.

Multi-objective constrained design of nickel-base superalloys using data mining- and thermodynamics-driven genetic algorithms.

A new computational framework for systematic and optimal alloy design is introduced. It is based on a multi-objective genetic algorithm which allows (i) the screening of vast compositional ranges and (ii) the optimisation of the performance of novel alloys. Alloys performance is evaluated on the basis of their predicted constitutional and thermomechanical properties. To this end, the CALPHAD method is used for assessing equilibrium characteristics (such as constitution, stability or processability) while Gaussian processes provide an estimate of thermomechanical properties (such as tensile strength or creep resistance), based on a multi-variable non-linear regression of existing data. These three independently well-assessed tools were unified within a single C++ routine. The method was applied to the design of affordable nickel-base superalloys for service in power plants, providing numerous candidates with superior expected microstructural stability and strength. An overview of the metallurgy of optimised alloys, as well as two detailed examples of optimal alloys, suggest that improvements over current commercial alloys are achievable at lower costs.

Study and modelling of the thermo-mechanical behaviour of melt-textured ybacuo-composites containing Ag and/or 211 particles

Abstract YBa 2 Cu 3 O x high- T c ceramic superconductors have been elaborated by the melt-textured-growth method with additions of Ag particles and/or Y 2 BaCuO 5 (211 green phase) in order to study the fracture properties and the thermal shock resistance. The Vickers indentation technique, initially devoted to the sole determination of the toughness has been used in conjunction with a new thermal shock resistance parameter. It makes use of the initial stage of stable propagation of Vickers indentation cracks when submitted to a thermal stress, and results in a generalisation of the fourth parameter of Hasselman. Whereas the toughness increases with increasing 211 content, does the thermal shock resistance decrease. This is explained with respect of the strongly micro-cracked (along the 001 planes) microstructure which is seen as a stacking of dense laminates bound by the 211 particles and subjected to a linear temperature gradient. The higher the number of bounding particles, the stiffer the quenched material, and the higher the induced thermal stress. This micro-mechanical model is implemented into a finite element software. The first results of the calculations confirm the above approach.

EBSD Phase Identification and Modeling of Precipitate Formation in HP Alloys

Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used as reformer tubes in petrochemical industries. Their composition includes Nb and Ti as strong carbide formers. In the ascast condition, alloys exhibit an austenite matrix with intergranular MC, M23C6 and/or M7C3 eutectic carbides. During exposure at high temperature, phase transformations occur: chromium carbides of M7C3 type transform into the more stable M23C6 type, intragranular M23C6 carbides precipitate, and a silicide, the G-phase (Ni16Nb6Si7), forms due to the instability of MC carbides (NbC). Thermodynamic simulation is of great help for understanding precipitate formation and transformations. Thermo-Calc and Dictra are used to simulate the precipitation of carbides in the austenite matrix during service. However, from an experimental point of view, M23C6 and M7C3 are not easy to distinguish in bulk alloys. Indeed, backscattered scanning electron microscopy does not bring any contrast between the two phases, and energy dispersive spectroscopy (EDS) analysis does not lead to carbon content and consequently to the distinction between M23C6 and M7C3. With transmission electron microscopy (TEM), sample preparation is difficult and the observed area is extremely small. In the present work, HP alloys are investigated by electron backscatter diffraction (EBSD) coupled to EDS. Carbides are identified on the basis of crystal structure, in the bulk, within their microstructural context, and the experimental procedure is both simpler and cheaper than TEM. Precipitates (M23C6, M7C3) could be identified by orientation mapping and single spot analysis.

Mechanical Properties of Macroporous Biphasic Calcium Phosphate Bioceramics Fabricated Using a Porogen

Macroporous biphasic calcium phosphate bioceramics, for use as bone substitutes, have been fabricated by cold isostatically pressing and conventional sintering, using naphtalen or saccharose particles to produce macropores. The resulting ceramics, composite materials made of hydroxyapatite and b-tricalcium phosphate containing ~ 45% macropores and ~ 25% micropores, have been submitted to compression and three-point bending tests, toughness tests by single-edge-notched-bending, and spherical indentation tests. A new model is established to describe mechanical properties as a function of the amount and morphology of porosity, and propositions are made to optimise the fabrication procedure. Finally, those highly porous ceramics, although very brittle, exhibit a damage-tolerant contact behaviour, due to the compaction of the porous body under the indenter.

Finite Element Simulation and Optimisation of Ageing Precipitation in Nickel Base Superalloys with a Low Gamma-Prime (g’) Volume Fraction

A finite element approach is used to simulate the precipitation of Ni3(Al,Ti) intermetallics in nickel-based superalloys containing a low volume fraction of spherical g’ precipitates, in which precipitation occurs following nucleation and growth mechanisms. Classical differential equations of nucleation and growth are implemented in the software Comsol (formerly Femlab), to compute the number of precipitates per unit volume and their mean size. Another originality of the model is the use of thermodynamic quantities coming from phase diagram computations (Thermo-Calc), like the temperature variation of the equilibrium g’ volume fraction, and the evolution of the concentration of g’ forming elements (Al, Ti) in the matrix with the volume fraction of precipitates. Once adjusted to experiment in the case of isothermal ageings, the model can be used to simulate precipitation during complex thermal histories. Finally, automatic heat treatment optimisation procedures are proposed and tested, which can reduce heat treatment times by a factor of more than five.