Guy Anné

Functionally graded ceramic and ceramic-metal composites shaped by electrophoretic deposition

Because of the increasing interest in colloidal processing methods for shaping advanced technical ceramics and hardmetals, the potential of electrophoretic deposition as a shaping process is investigated. The mechanisms of deposition, which have been proposed in literature, and the relation between the evolution of the current, yield and the electric field strength are evaluated. Special attention is given to the electrophoretic characteristics of the constituent powders, such as the point of zero charge, the natural pH, the electrophoretic mobility, the effective particle charge and the resistance of the suspension. The investigated materials comprise alumina, zirconia and their composites as well as various formulations of hardmetals. The fabrication of homogeneous as well as continuously graded plate-shaped components is highlighted. The main feature of a functionally graded material (FGM) is the position-dependant composition and related mechanical properties. Moreover, a mathematical model of the EPD process for FGMs is presented that allows to predict the composition and slope of the gradient in the sintered material from the starting composition of the suspensions, the EPD process parameters and the powder specific EPD characteristics. The correlation between the experimental results and the predicted gradient profile is excellent.

Electrophoretic deposition for coatings and free standing objects

A simulation of the electrophoretic deposition process is presented, which allows one to predict the yield of deposition as a function of time for different values of the electrical properties of the liquid and the powders used. It is shown that with special precautions, conditions can be defined where the resistance of the deposit will limit its thickness. Under most conditions however, particularly if a liquid with reasonable electrical conductivity is used, the resistance of the deposit should not limit its growth. Other issues which are of importance in the shaping of free standing objects are also discussed, in particular the electrode shape and electric field distribution, separation of the deposit from the electrode and drying of the object.

Thick Plate-Shaped Al2O3/ZrO2 Composites with Continuous Gradient Processed by Electrophoretic Deposition

This paper concerns the processing of free standing objects with a gradient composition. The possibility to shape 5 mm thick ZrO2-Al2O3 FGM discs by means of electrophoretic deposition (EPD) from suitable stable suspensions was explored. The EPD processing parameters and sintering schedules are described, and the densified FGM composites are evaluated. A mathematical model was established for the EPD process, allowing to calculate the composition gradient in plate-shaped FGM materials from the starting composition of the suspensions, the EPD operating parameters and the powder specific EPD characteristics. The model was essential to engineer the gradient profiles in the green deposits. Defect free Al2O3/ZrO2 FGM discs with a diameter of 55 mm and a thickness of 5 mm could be successfully processed.

Laminated and Functionally Graded Ceramics by Electrophoretic Deposition

Electrophoresis is the effect that when an electric field is applied to a suspension of a powder in a liquid, the powder particles move under influence of this field. Frequently the powder particles also deposit at one of the electrodes. The form of the electrode determines the form of the deposit, hence shaping is possible. The current insights into the science and technology of electrophoretic deposition (EPD) will be summarized. EPD is well suited for shaping layered microstructures (laminates), by simply changing repeatedly between two or more suspensions during deposition. Tubular laminates consisting of silicon carbide layers and crack deflecting graphite interlayers have been produced. These tubes demonstrate an enhanced fracture energy and a gradual mode of failure. Another area of advanced ceramics where the use of EPD makes sense are functionally graded materials (FGM) in which one tries to combine in one component high hardness and high toughness. EPD allows the formation of FGM by depositing from a powder suspension to which a second suspension is continuously added during the process. An example will be shown of a graded WC-Co hardmetal.

Processing of a Graded Ceramic Cutting Tool in the Al2O3-ZrO2-Ti(C,N) System by Electrophoretic Deposition

In this study, the development of a functionally graded material (FGM) with hard outer surfaces and a tougher inner core was envisaged. The applicability of electrophoretic deposition (EPD) for the processing of FGM materials by continuously changing the suspension composition is shown. Optimisation of the colloidal processing technique was combined with hot pressing experiments on homogeneous composites in the Al2O3-ZrO2-Ti(C,N) system in order to create a very hard functionally graded material with beneficial residual stresses. Finally, the residual stress distribution was briefly discussed using an existing analytical model.

Electrophoretic Deposition as a Novel Near Net Shaping Technique for Functionally Graded Biomaterials

Complex shaped functionally graded alumina and zirconia based femoral ball-heads for biomedical applications were shaped by electrophoretic deposition (EPD). A composition gradient in alumina and zirconia was engineered to obtain a pure alumina surface region and a homogeneous alumina/zirconia core with intermediate continuously graded regions to generate appropriate thermal residual stresses after sintering. The gradient profiles were designed to obtain maximum compressive surface stresses and minimal tensile stresses in the core of the component to increase the strength and wear properties when compared to pure alumina components.

Throwing Power during Electrophoretic Deposition

The deposit can induce an extra potential drop near the electrode, depending on the suspension composition. This can result in a levelling off of the deposition rate in a constant-voltage deposition process. The magnitude of the extra voltage drop determines the uniformity of the deposit as function of the uniformity of the electric field present at the deposition electrode. It was experimentally proven that a uniform Al2O3 coating thickness was obtained in a non-homogeneous electrical field in ethanol with addition of HNO3, while the coating thickness varied uniformly with the E-field strength for a MEK with n-butylamine based suspension. The uniformity of the coating deposited from these suspensions was related to the measured potential drop over the deposit during electrophoretic deposition.

Influence of electrostatic interactions in the deposit on the electrical field strength during electrophoretic deposition

A model was developed to explain the magnitude of the potential drop over the deposit for non-conductive powders during electrophoretic deposition (EPD). The magnitude of the potential drop over the deposit is explained in terms of a reduced ion transport through the deposit, as controlled by the pore potential that is related to the thickness of the electrostatic double layer relative to the pore radius and the magnitude of the surface potential of the powder particles. This model was validated for EPD of Al2O3 powder from ethanol-based suspensions with HNO3 addition. The specific resistivity of the deposit could be related to the calculated potential in the pores of the deposit.

Neutron Diffraction Studies of Functionally Graded Alumina/Zirconia Ceramics

New functionally graded Al2O3/Y-ZrO2 ceramics are developed as a new material for production of hip prosthesis, especially all-ceramic hip bearings. The used concept of graded ceramics is expected to provide better long-term performance based on improved mechanical properties and lower clinical risks. Mechanical properties are strongly dependent on the distribution of residual stresses resulting mainly from phase specific stresses after cooling from the sintering temperature. However, these stresses could be optimized by an appropriate selection of a concentration profile of constituent phases. The non-destructive neutron diffraction mapping of residual stresses has been used to investigate this problem experimentally. In addition, the smallangle neutron scattering technique was applied to study the porosity in the ceramics as function of the production parameters.

Mechanical Properties of Alumina/Zirconia Functionally Graded Material Prepared by Electrophoretic Deposition

The mechanical properties of a quasi-symmetrical planar functionally graded material (FGM) of Al2O3 / Al2O3+20%ZrO2 / Al2O3+5%ZrO2 prepared by electrophoretic deposition and pressureless sintering has been studied. Hardness and fracture toughness were measured using indentation methods on cross sections of samples. From the difference between lengths of cracks parallel and perpendicular to layers the residual stresses (arisen due to the thermal expansion coefficient mismatch) were calculated, their profile along the cross section has been estimated and compared with values obtained using a finite element model.