Frédéric Monteverde

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.

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.

Oxidation behavior of titanium carbonitride based materials

The oxidation behavior of two different Ti(C,N)–WC based materials (a ceramic type containing 0.9 wt.% Co and a cermet type containing (6.2Ni+2.9 Co) wt.%), was tested at 900 and 1000 °C in static laboratory air. The values of the mass gain (∼50 mg/cm2 after 100 h at 1000 °C) and extended microstructural modifications demonstrated that these materials are rather unstable under oxidizing conditions. After an initial transitory period where the mass change resulted from the competition between the mass gain due to formation of solid TiO2 and the mass loss due to released volatile tungsten oxides that were formed, the oxidation process obeyed a linear law in function of the exposure time, i.e. it is governed by a chemical reaction at the interface of the titanium based carbonitrides transformed into TiO2. Oxidation kinetics and mechanisms were discussed in terms of the phase composition, presence of metal binder at grain boundaries, and of the residual porosity of the starting materials.

High Oxidation Resistance of Hot Pressed Silicon Nitride Containing Yttria and Lanthania

Oxidation tests were carried out on Si 3 N 4 -La 2 O 3 -Y 2 O 3 hot pressed ceramics up to 1500°C. Morphological and analytical characterizations were performed on surfaces and reaction scales after oxidation and correlated with the oxidation kinetics. (Near)-parabolic behaviour was observed at temperatures < 1450°C for short periods, while for higher temperatures and longer exposures the kinetics shifted to a linear behaviour. Moreover the excellent oxidation resistance (as demonstrated by extremely low weight gains), particularly up to 1450°C, was related to the high refractoriness of the grain boundary phases in this additive system. Strength degradation after oxidation at several temperatures was also studied and discussed.

Plasma Torch Test of an Ultra-High-Temperature Ceramics Nose Cone Demonstrator

An ultra-high-temperatureZrB2–SiC ceramic nose cone was tested in an arcjet plasma torch facility for 10min at temperatures above 2000 C. The nose cone model was obtained from a hot-pressed billet via electrical discharge machining.The relevant portions of themodels directly exposed to the hot streamwere analyzedby scanning electron microscopy and energy-dispersive spectroscopy. The posttest cross sectioning of the model showed a nonnegligible surface recession on the tip of the nose. Nonetheless, the material exhibited a promising potential to withstand severe reentry conditions with temperatures exceeding 2000 C in a single-use application. Spectral directional emissivity evaluationswere performed on the fly during the test bymeans of thermography coupledwith dual-color pyrometer. The numerical calculations, which simulated the chemical nonequilibrium flow around the model assuming a low catalytic surface behavior, are in good accordance with the experimental results.

Carbon reduction reaction in the Y2O3–SiO2 glass system at high temperature

Abstract In order to assess the role of carbon with respect to the grain boundary chemistry of Si3N4-based ceramics model experiments were performed. Y2O3–SiO2 glass systems with various amount of carbon (from 1 to 30 wt.%) were prepared by high-temperature treatment in a graphite furnace. High carbon activity of the furnace atmosphere was observed. EDX analysis proved the formation of SiC by the carbothermal reduction of SiO2 either in the melt or in the solid state. The melting temperature of the Y2O3–SiO2 system is strongly dependent on the amount of reduced SiO2. XRD analysis of the products documented the presence of Y2Si2O7, Y2SiO5 and Y2O3 crystalline phases in that order with an increasing amount of free C in the starting mixture. The reduction of Y2O3 was not confirmed.

Factors influencing the crystallization and the densification of ultrafine Si/N/C powders

Abstract The employment of nanocomposite Si/N/C laser-formed powders to produce high performance ceramics results in several technological problems related to their nanometer particle size and to their high affinity for oxygen, which influence phase composition and densification. This study has focused on several aspects; the improvement of experimental methodologies for processing of ultrafine powders in a compact green body and for the addition of sintering aids; the evaluation of the starting composition of the Si/N/C ultrafine amorphous powders and of the thermal treatment conditions (temperature, time, atmosphere) on phase composition, thermal stability, grain size, specific surface area and crystallite size; and the production and characterization of dense Si 3 N 4 SiC composites. Above 1400 °C the amorphous Si/N/C powders crystallize in α- and β-Si3N4, SiC and Si2N2O, their relative amounts and grain sizes depending on processing conditions. The phenomena are discussed on the basis of a series of reactions involving the formation of intermediates in the system Si-C-N-O. After densification by hot pressing, a very fine microstructure ( ≈ 100 nm) was observed in the dense Si 3 N 4 SiC composites. High values of hardness (Hv ⩾ 21 GPa) and good values of fracture toughness (KIC ≈ 4.8 MPa m 1 2 ) were measured.

Ultra-Refractory Ceramics: The Use of Sintering Aids to Obtain Microstructure Control and Properties Improvement

The present study focuses on innovative processing for the densification of refractory diborides (ZrB2, HfB2) based ceramics, suitable to fabricate UHTC components for structural applications and thermal protection systems. The addition of small quantities (2-5 vol%) of sintering aids allows to overcome the intrinsic low sinterability of zirconium diboride and to improve properties through the microstructural refinement. The introduction of second refractory phases, for instance 20 vol% of SiC in the diboride matrices improves strength, toughness and oxidation resistance. Outstanding properties were measured: fracture toughness 3-5 MPam, hardness 9-20 GPa, elastic modulus 350-420 GPa, flexural strength 300 to 750 MPa at R.T and up to 300 MPa at 1500°C. The resistance to oxidation of different materials was compared.

Densification, Microstructure Evolution and Mechanical Properties of Ultrafine SiC Particle-Dispersed ZrB2 Matrix Composites

The densification behavior along with the microstructure evolution and some mechanical properties of four ultrafine SiC particle-dispersed ZrB2 matrix composites were studied. The SiC–ZrB2 composites, with a SiC content of 5, 10, 15 and 20 vol%, were densified to near full density by vacuum hot pressing at 1,900°C under a maximum uniaxial pressure of 45 MPa. The presence of SiC greatly improved the sinterability of ZrB2. Grain growth of the diboride matrix was increasingly inhibited for larger amounts of SiC added. Elastic modulus, Poisson ratio, microhardness, flexural strength and fracture toughness were measured at room temperature. Unexpectedly, no obvious effect of the increasing amount of SiC on flexural strength and fracture toughness was found. The former property ranged from 650 to 715 MPa but was actually affected by the exaggerated size of several tenths of micrometers of sintered SiC clusters which acted as dominant critical defects. Also fracture toughness did not receive a marked contribution from the increase of the SiC content. As for the matrix, the prevailing fracture mode of the composites was intragranular, regardless of the SiC content.