Alida Bellosi

Processing and properties of zirconium diboride-based composites

Two zirconium diboride-base composites were produced and characterised. The chosen starting compositions were: 55 wt.% ZrB2+41 wt.%TiB2+4 wt.% Ni and 83 wt.% ZrB2+13 wt.% B4C+4 wt.% Ni. The microstructure and properties of these composites were compared to those of a monolithic ZrB2+4 wt.% Ni material. In all cases, metallic Ni as the sintering aid promoted the formation of the liquid phase which improved mass transfer mechanisms during sintering. From the powder mixture ZrB2+TiB2, two solid solutions of Zr–Ti–B were obtained. In the case of the other mixture, B4C particles were dispersed in the ZrB2 matrix. The composite materials have better mechanical properties than those of the monolithic ZrB2 ceramic; in particular the fracture toughness and the flexural strength were almost doubled at room temperature. Long term oxidation tests indicated that the ZrB2-based composites, particularly the composite containing B4C as the second phase, were more resistant to oxidation than the monolithic ZrB2 due to the formation of surface oxide products which were protective against the complete degradation by oxidation observed for the ZrB2 matrix material.

Effect of the addition of silicon nitride on sintering behaviour and microstructure of zirconium diboride

Abstract Using Si3N4 as a sintering aid in ZrB2 greatly improved densification and microstructure compared to additive-free zirconium diboride. Nearly fully dense material was obtained by hot pressing at 1700 °C. The microstructure consists of fine ZrB2 grains and of grain boundary phases (BN, ZrO2, ZrSi2, B–N–O–Si–Zr glassy phase) mainly located at triple points.

Development and characterization of electroconductive Si3N4-TiN composites

Abstract Dense Si 3 N 4 -TiN composites, with the second phase ranging from 20 to 40 vol.%, were produced by hot pressing, gas-pressure sintering and pressureless sintering under nitrogen gas atmosphere. The influence of densification technique and parameters and of amount and grain size of TiN particles on microstructure, mechanical properties, electrical resistivity and oxidation resistance was evaluated. The addition of TiN particles increases the stiffness and the fracture toughness of the base material. For TiN content higher than 30 vol.% the electrical resistivity of the composites is less than 10 −3 ωcm. An evident effect of the grain size distribution of TiN powders on some mechanical properties was ascertained. The thermal stability of the composites is strongly affected by the amount of the second phase.

Oxidation behavior of a pressureless sintered ZrB 2 –MoSi 2 ceramic composite

Ultra-refractory ceramic composites of composition ZrB 2 + (5 to 20) vol% MoSi 2 were produced by pressureless sintering at 1830 °C under argon atmosphere.Sintering cycles and microstructural analysis point out that at least 20 vol% molybdenum disilicide is necessary for obtaining a dense material. Thereafter, the composite 80 vol% ZrB 2 + 20 vol% MoSi 2 was used to test the thermal stabilityunder oxidizing environment. Oxidation tests were carried out in flowing syntheticair in a thermogravimetric analyzer from 700 to 1400 °C with exposure time of30 h. In the low-temperature range (700–1000 °C), the oxidation of the composite resembles that of monolithic ZrB 2 ceramics, for temperatures >1200 °C the silica resulting from oxidation of molybdenum disilicide seals the sample surface, preventing zirconium diboride from fast degradation.

Bonding of zirconia to super alloy with the active brazing technique

Abstract Zirconia stabilized with yttria was bonded to a Ni-based superalloy by active brazing. An interlayer with the composition of the silver–copper eutectic plus 1.75 wt.% titanium was used as brazing material. Various brazing heating cycles were performed using a furnace with graphite heating elements and under flowing argon. The microstructure of the bonded couples is related to the chemical reactions occurring during the brazing cycles, to the properties of the specific compounds formed and to the interdiffusion phenomena occurring across the interfaces. Critical feature for the success of the joint is the nature of the interfaces that the brazing interlayer formed with the superalloy and the zirconia. At the interface with the superalloy adhesion is obtained under a relatively wide range of experimental conditions. On the contrary, at the interface with zirconia, a good wetting and adhesion occurred only when a titanium oxide sublayer with a specific thickness formed. The best results were obtained with a maximum temperature between 870 and 900°C, a soaking time of 10 min and a fast heating rate (10°C/min). The optimal thickness of the TiO x sublayer was less than 1 μm.

On the possibility of silicon nitride as a ceramic for structural orthopaedic implants. Part I: processing, microstructure, mechanical properties, cytotoxicity

Notwithstanding the good combination of mechanical and tribological properties, the suitability of silicon nitride for application as prosthesis in bone reconstruction or in articular joints replacements is still controversial. This study aims to design and produce three different silicon nitride-based ceramics and to test the materials. In this Part I the microstructure and mechanical properties evidence outstanding characteristics and the cytotoxicity studies confirm that all the materials are extremely inert and biocompatible. In Part II, the wear performance and the wettability and chemical stability against different aqueous media and physiological solutions are investigated and discussed.

Efficacy of HfN as sintering aid in the manufacture of ultrahigh-temperature metal diborides-matrix ceramics

HfB 2 and (ZrB 2 + HfB 2 )-based ceramics containing 19.5 vol% SiC particulate were developed from commercially available powders by hot-pressing. With the assistance of 3 vol% HfN as sintering aid, after hot-pressing at 1900 °C and 50 MPa of applied pressure, full density in both the composites was successfully achieved. The materials revealed a homogeneous microstructure, characterized by faceted diboride grains(2 μm average size) and SiC particles regularly dispersed. Limited levels of secondary phases were found. The thermomechanical properties of the composites were promising: about 22 GPa microhardness and 500 GPa Young’s modulus for both. The HfB 2 –SiC composite showed values of strength of 650 ± 50 and 465 ± 40 MPa at 25 and 1500 °C, respectively. Likewise, the (ZrB 2 –HfB 2 )–SiC composite exhibited values of strength of 765 ± 20 and 250 ± 45 MPa at 25 and 1500 °C, respectively. The excellent response at high temperature in air was attributed to the refractoriness of the phases constituting the composites and to the resistance to oxidation enhanced by the presence of the SiC particulate.

Effect of annealing treatments on microstructure and mechanical properties of liquid-phase-sintered silicon carbide

Dense hot pressed SiC based materials were produced with alumina and yttria as sintering aids in different percentages. The microstructure and mechanical properties were evaluated on as-sintered materials and related to the starting compositions. Annealing treatments were carried out at different temperatures and holding times to evaluate the possibility of a further improvement of the material properties, above all indentation toughness and high temperature properties. The attention was focused on the microstructural modifications induced by thermal treatments, i.e. formation of elongated grains, reduction and modification of secondary amorphous and crystalline phases. Mechanical properties of the annealed materials were explained on the basis of microstructural changes and compared to those of the as-sintered materials. Annealing treatments were found to be highly beneficial for improving mechanical properties and grain boundary phase microstructural changes were found to be the main factor affecting such improvements.

Microstructure and properties of porous β-SiC templated from soft woods

Abstract Porous β-SiC with a multimodal porosity preferentially oriented along one direction is obtained by infiltration of pyrolyzed wood with Si at T > T Si Melting . Hard woods (obece, poplar and assembled poplar) are used as starting materials. The microstructure of the starting wood, of the wood after pyrolysis and after Si infiltration, is characterised in terms of overall porosity, pore size distribution and of crystallographic phases. The process is optimised for obtaining porous templates of only β-SiC with a microstructure that replicates the original wood microstructure. Features such as the presence of unreacted carbon, or conversely, of Si within the open pores of the infiltrated materials are minimized by a careful control of the amount of Si in contact with the carbon preform during the infiltration cycle. Compression tests on cubic samples are performed along the axial and longitudinal direction.

Oxidation of monolithic TiB2 and of Al2O3-TiB2 composite

In view of the susceptibility of TiB2 to oxidation, the thermal stability of monolithic TiB2 and Al2O3-TiB2 composite was investigated. The temperature at which TiB2 ceramic starts to oxidize is about 400°C, oxidation kinetic being controlled by diffusion up toT≈900°C and in the first stage of the oxidation at 1000 and 1100°C (up to 800 and 500 min, respectively), and by a linear law at higher temperatures and longer periods. Weight gains of Al2O3-TiB2 composite can be detected only at temperatures above ≈700°C and the rate-governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism atT=700 and 800°C and by two-dimensional diffusion at higher temperatures. The composition and morphology of the oxidized surfaces were analysed.