Ján Dusza

SiC/Si3N4 nano/micro-composite — processing, RT and HT mechanical properties

Two SiC/Si3N4 nano/micro composites were prepared from a starting mixture of crystalline α-Si3N4, amorphous SiNC, Y2O3 and/or Al2O3. The composite material for room temperature (RT) application has high strength of 1200 MPa, Weibull modulus of 19 and moderate fracture toughness of 7 MPa m1/2. The composite for high temperature (HT) application, without Al2O3 has RT strength of 710 MPa and is able to keep 60% of its RT strength up to 1300°C. The creep resistance of composite material is approx. 1 order higher compared to relative monolith up to 1400°C.

Reinforcement of silicon nitride ceramics by β-Si3N4 Whiskers

Abstract The silicon nitride based ceramics are reinforced by β-Si3N4 whiskers. β-Si3N4 whiskers do not influence the sinterability of the composite up to an addition of 10 wt%. The improvement of fracture toughness at room temperature of silicon nitride ceramics reinforced by β-Si3N4 whiskers is observed.

Layered Si3N4 composites with enhanced room temperature properties

Four silicon nitride multilayer composites were prepared by hot pressing. Internal stress distribution along the layer boundary dictates the anisotropy of mechanical properties of the composite. The room-temperature bending strength and fracture toughness of all layered composites was higher than the bending strength of related monoliths. Layered ceramic materials exhibited higher tolerance to flaws in comparison to the monolithic ceramic.

Bending creep behaviour of pressureless sintered MoSi2

Creep behavior in bending of the hot pressed MoSi{sub 2} was studied in the temperature and stress intervals from 1,100 C--1,200 C and from 20 to 100 MPa, respectively. In spite of the fact that the MoSi{sub 2} was not reinforced with a second particle/whisker phase the creep resistance of the material was comparably high because of the clean character of the MoSi{sub 2}/MoSi{sub 2} grain boundaries. The resulting data, the creep exponent from n = 1.3 to 2.4 and the apparent activation energy from Q = 159 to 634 kJ mol{sup {minus}1} are comparable with the data achieved in compressive creep tests for similar materials and together with TEM results they prove that the principal creep mechanism at 1,200 C is probably dislocation climbing. The bending creep test seems to be a good technique for the characterization of the high temperature mechanical properties of MoSi{sub 2} based materials, but similarly as in the case of structural ceramics it is limited to the low-deformation regimes.

Scratch behaviour of graphene alumina nanocomposites

The scratch resistance behaviour of alumina-graphene nanoplatelet (GNP) (0.5, 2 and 5 vol.-%) composites was investigated using a Rockwell indenter with normal applied loads ranging from 1 to 200 N. The alumina-GNP composites behaved differently during scratch testing depending on the normal applied load. The coefficient of friction of the composites did not change much at low normal loads but increased with increasing amount of GNP in the alumina matrix for high normal loads. The addition of GNP contributed to improved scratch resistance of alumina nanocomposites only for low loads below ∼97 N. This correlates with the mechanical properties of the composites. As the applied load increased, the scratch resistance of the GNP composites decreased due to the presence of weakly bonded grain boundaries in the alumina matrix, which enhanced chipping of material.

Mechanical properties of Si3N4 + β-Si3N4 whisker reinforced ceramics

Abstract The mechanical properties of Si 3 N 4 + β-Si 3 N 4 whisker reinforced ceramics were studied at room and elevated temperatures up to 1200°C. The four-point bend strength, Weibull modulus and fracture toughness values were analysed in connection with technological defects and fracture mechanisms. The main fracture origins were clusters of Si 3 N 4 whiskers and the main toughening mechanisms were crack deflection, whisker/matrix debonding and to a lesser extent crack branching and whisker pull-out. The experimentally achieved toughening was compared with those theoretically predicted. The strength degradation was caused by subcritical crack growth at elevated temperatures.

Low-cycle fatigue strength under step loading of a Si3N4 + SiC nanocomposite at 1350°C

The low-cycle fatigue behaviour of a hot pressed silicon nitride/silicon carbide nanocomposite and a reference monolithic Si3N4 have been investigated in 4-point bending at 1350°C in air using stepwise loading. The nanocomposite was prepared using 20% of SiCN amorphous powder as an additive, together with 5% yttria, to crystalline α-silicon nitride powder. Two types of specimen have been tested, with and without a sharp notch (notch tip radius ∼10 μm) at applied loads from 50 N with steps of 25 N and from 50 N with steps of 50 N, respectively. Five cycles have been performed at all applied load levels with an amplitude of 50 N for both types of specimen. The deflection of the specimens has been recorded up to specimen failure. The failure load of the unnotched nanocomposite was significantly higher than that of the monolithic material whereas for the notched specimens only a small difference has been found between the failure loads of the monolithic and the composite. Notched specimens of both materials exhibited a similar size of the slow crack growth area at catastrophic fracture, whereas for unnotched specimens the size of the slow crack growth area was significantly larger for the monolithic ceramic. The nanocomposite exhibits higher fatigue strength due to its higher resistance against stress corrosion damage and stress corrosion crack growth.

Carbon Nanofibers Reinforced Ceramic Matrix Composites

Modern ceramic materials have, thanks to their crystallographic structure and strong atomic bonds, many excellent properties, such as extremely high hardness, strength, high thermal and chemical stability, high corrosion resistance, and wear resistance. Their weakness is low fracture toughness and crack growth resistance and hence high brittleness and lower reliability. One of the ways how to overcome these drawbacks is preparation of composite materials, where the base ceramic matrix is reinforced by secondary phases in forms of particles/whiskers and in recent years increasingly in a form of fibrous structures. In advanced fine grained ceramics these usually take form of nanofibers and/or nanotubes. Among the most promising candidates are carbon-based filamentous nanomaterials such as carbon nanotubes (CNTs) and also carbon nanofibers (CNFs), which attracted a lot of attention due to their outstanding mechanical properties, excellent thermal performance and useful electrical characteristics (high electrical conductivity). Nowadays, new ceramic/carbon nanotube composites are being developed mostly with two aims: to improve the mechanical properties of the ceramic materials by reinforcing with carbon nanofibers and to develop functionalized ceramics with improved magnetic and electric properties. Studies show that CNTs (both single-wall and multi-wall) should be ideal reinforcing/functionalizing elements for composites due to their small size, low density and good electrical and thermal conductivity. This work focuses on investigations of ceramic matrix composites based on alumina, zirconia and silicon nitride reinforced by carbon nanofibers and nanotubes. The basic characteristics of commercially available nanofibers/nanotubes are studied by various techniques. The chapter then focuses on mechanical properties of reference monolithic and experimental composite materials. The effect of volume fraction of carbon nanofibers on hardness and fracture toughness is illustrated. Further, the possibilities of improving the tribological and wear properties are discussed. The chapter concludes with the section that explores important aspect of functionalization of ceramics composites by improving their electrical properties, namely electrical conductivity.

Wear Behavior of ZrO2-CNF and Si3N4-CNT Nanocomposites

Tribological behavior of ZrO2 and Si3N4 based nanocomposites with addition of carbon nanofibres and nanotubes has been studied by the pin-on-disc technique. Friction coefficients were measured and recorded, wear rates were calculated in terms of material volume loss per load and sliding distance. The wear damage was studied using optical and electron microscopy and its mechanisms were identified. In monolithic materials the dominant wear mechanism was abrasion, in composites with CNF and with higher volume fraction of CNTs (5 and 10%) fiber pull-out and lubricating by the carbon phases occurred.

Dynamic fatigue of a Si3N4+SiC nanocomposite at 1350°C

Abstract The dynamic fatigue strength of a hot pressed silicon nitride/silicon carbide nanocomposite and reference monolithic Si 3 N 4 have been investigated in four-point bending at 1350°C in air using different loading rates from 0.01 to 1 mm min −1 . The flexure strength significantly decreased with decreasing stress rate in both ceramics, however at all stress rates the strength of the composite was lower than that of the monolithic. The formation and growth of a stress-oxidation damage zone are primarily responsible for the strength degradation of the monolithic ceramic. In the composite intrinsic flaws were the main fracture origins, which extend by oxidation assisted slow crack growth and decrease the strength of the specimens significantly.