O. Van der Biest

Impulse excitation apparatus to measure resonant frequencies, elastic moduli, and internal friction at room and high temperature

This paper presents a new apparatus to measure elastic properties and internal friction of materials. The apparatus excites the test specimen by a light mechanical impact (impulse excitation) and performs a software-based analysis of the resulting vibration. The resonant frequencies fr of the test object are determined and, in the case of isotropic and regular shaped specimens, the elastic moduli are calculated. The internal friction value (Q−1) is determined for each fr as Q−1=k/(πfr) with k the exponential decay parameter of the vibration component of frequency fr. A furnace was designed and equipped with automated impulse excitation and vibration detection devices, thus allowing computer-controlled measurements at temperatures up to 1750 °C. The precision of the measured fr depends on the size and stiffness of the specimen, and varies from the order of 10−3 (that is ±1 Hz at 1 kHz) in soft, high damping materials or light specimens, to values as precise as 10−5 (that is ±0.1 Hz at 10 kHz) in larger or ...

Sol-gel preparation of high-Tc Bi-Ca-Sr-Cu-O and Y-Ba-Ca-O superconductors

High‐Tc superconducting oxides have been prepared by a liquid‐mix technique using etylene‐diamine‐tetra‐acetic acid (EDTA) as a complexing agent. Bi‐Sr‐Ca‐Cu oxides and Y‐Ba‐Cu oxides were made using this technique giving a better compositional homogeneity, a more precise control of the cation stoichiometry and a decreased firing temperature as compared to conventional material produced by a solid‐state reaction. This technique extends the amorphous citrate process to those systems where no citrate complex exists. EDTA binds with most metallic elements of the periodic table, making this technique a versatile tool in the production and study of these new ceramic materials. Therefore, the method is easily adapted to the preparation of new superconducting oxides.

HYBRID SINTERING WITH A TUBULAR SUSCEPTOR IN A CYLINDRICAL SINGLE-MODE MICROWAVE FURNACE

Abstract Usage of a tubular susceptor for hybrid sintering in a cylindrical single-mode microwave furnace is studied experimentally. The new approach allows an immediate, smooth and repeatable heating of room-temperature-transparent materials such as ZrO2 and Al2O3. The sintering tests show that the microwaves can partially penetrate the thin susceptor and give a hybrid sintering of the ceramics. A variety of ceramics were sintered successfully by the hybrid sintering with tubular susceptor.

Functionally graded WC-Co materials produced by electrophoretic deposition

Abstract Functionally graded WC–Co materials can be fabricated using electrophoretic deposition from a suspension of hard metal powder in acetone, with variable cobalt content. The deposits can be sintered to closed porosity at 1290 and 1340 °C. The cobalt gradient disappears if sintering is carried out at 1400 °C.

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.

Fretting wear behavior of TiB2-based materials against bearing steel under water and oil lubrication

Lubricated fretting tests in water and paraffin oil were performed with a monolithic TiB 2 ,aT iB 2-based cermet with 16 vol.% Ni3(Al, Ti) binder, a sialon–TiB2 (60/40) composite and a ZrO2–TiB2 (70/30) composite against ball bearing grade steel. Based on the measured friction and wear data, the ranking of the investigated fretting couples was evaluated. Furthermore, the morphological investigations of the worn surfaces and transfer layers are carried out and the wear mechanisms for the investigated friction couples are elucidated. While fretting in water, experiments revealed that tribochemical reactions, coupled with mild abrasion, played a major role in the wear behavior of the studied material combinations. ZrO2–TiB2 (70/30)/steel wear couple has been found to have the highest fretting wear resistance among the different tribocouples under water lubrication. Under oil lubrication, extensive cracking of the paraffin oil at the fretting contacts, caused by tribodegradation, leads to the deposition of a carbon-rich lubricating layer, which significantly reduced friction and wear of all the investigated tribosystems.

Influence of humidity on the fretting wear of self-mated tetragonal zirconia ceramics

Abstract The influence of different humidity levels on the tribological behaviour of polycrystalline tetragonal zirconia polycrystals (Y-TZP) sliding against commercial Y-TZP balls has been investigated. Fretting experiments (mode I) have been conducted at three different relative humidity (RH) levels, namely 5%, 50% and 85% RH. Based on the obtained wear and friction data as well as the morphological investigation of the worn surfaces, the wear mechanisms in different humidity conditions are proposed. The morphology of the wear pits suggests that plastic deformation coupled with delamination is the wear mechanism in dry sliding (5% RH) and in ambient humidity (50% RH) conditions. Additionally, Raman measurements showed that the phase transformation of tetragonal zirconia phase to monoclinic symmetry also plays a role in the fretting wear under all the investigated humidity conditions. XPS analysis of the wear debris formed at 85% RH indicates that tribochemical wear is the dominant wear mechanism at high humidity conditions.

Gradient profile prediction in functionally graded materials processed by electrophoretic deposition

Electrophoretic deposition (EPD) allows the formation of plate-shaped binary functionally graded materials (FGM) by depositing from a powder suspension to which a second suspension is continuously added during the process. The deposition yield is described as a function of time and starting composition of both suspensions, resulting in a model of the EPD process that allows predicting the composition gradient in the green deposit as well as in the sintered material. The model presented enables calculation of the composition gradient in the FGM material from the starting composition of the suspensions, the EPD operating parameters and the powder-specific EPD characteristics. The powder-specific parameters, i.e. the effective charge, the electrophoretic mobility, the specific conductivity of the intermicellar liquid and the specific resistance of the deposit, incorporated in the model were experimentally determined from EPD of the individual homogeneous powder suspensions. The model was verified for the ZrO 2-Al2O3 system by the actual preparation and analysis of two different FGM materials. Because of the excellent correlation between the predicted and the measured concentration profiles, the described model allows to precisely engineer and design the composition profile in FGM materials produced by EPD.

Development of ZrO2-ZrB2 composites

Abstract Yttria-stabilised tetragonal zirconia (Y-TZP) composites with 30 vol.% ZrB 2 are prepared by hot pressing in vacuum for 1 h at 1450°C. Different commercial zirconia starting powders as well as a range of ZrO 2 ‘powder mixture’ grades, based on co-precipitated powders were used for the composite production. The measured differences in the mechanical properties of the obtained composites is explained in terms of the microstructure, the residual stresses due to the presence of ZrB 2 , and the stabiliser content and its distribution. While the stabiliser content and distribution remained significant, the residual stress in the zirconia matrix is found to be an important additional factor influencing the tetragonal zirconia transformability. Crack deflection by the ZrB 2 phase was identified as an active toughening mechanism in the composites.

Miscibility of amorphous ZrO2–Al2O3 binary alloy

Miscibility is a key factor for maintaining the homogeneity of the amorphous structure in a ZrO2–Al2O3 binary alloy high-k dielectric layer. In the present work, a ZrO2/Al2O3 laminate thin layer has been prepared by atomic layer chemical vapor deposition on a Si (100) wafer. This layer, with artificially induced inhomogeneity (lamination), enables one to study the change in homogeneity of the amorphous phase in the ZrO2/Al2O3 system during annealing. High temperature grazing incidence x-ray diffraction (HT-XRD) was used to investigate the change in intensity of the constructive interference peak of the x-ray beams which are reflected from the interfaces of ZrO2/Al2O3 laminae. The HT-XRD spectra show that the intensity of the peak decreases with an increase in the anneal temperature, and at 800 °C, the peak disappears. The same samples were annealed by a rapid thermal process (RTP) at temperatures between 700 and 1000 °C for 60 s. Room temperature XRD of the RTP annealed samples shows a similar decrease in...