Wyniki wyszukiwania - BazTech

Ograniczanie wyników

1 Kompozyty

1 2001

Znaleziono wyników: 1

Liczba wyników na stronie

Wyniki wyszukiwania

Wyszukiwano:
w słowach kluczowych: SiC-C system

Sortuj według:

Ogranicz wyniki do:

Kompozyty SiC-C

Gubernat A., Stobierski L., Rudnik T.

Kompozyty

2001

R. 1, nr 1

100-105

Do uzyskania gęstych jednofazowych spieków węglika krzemu niezbędne jest aktywowanie proszku SiC niewielkimi dodatkami boru i węgla. Zmiany ilości aktywatorów wywołują istotne zmiany mikrostruktury spieków SiC, lecz tylko w niewielkim stopniu zmieniają ich właściwości. Przekroczenie granicznych ilości 0,5% boru i 3% węgla prowadzi do wystąpienia w spieku ziaren obcej fazy, odpowiednio węglika boru lub węgla. Skutkiem jest otrzymanie kompozytów ziarnistych w osnowie SiC, których właściwości odbiegają znacznie od właściwości tworzyw jednofazowych. W przypadku kompozytów SiC-C szczególnie korzystny jest wzrost odporności na kruche pękanie. Tym samym poprawiona zostaje ta cecha, która w największym stopniu ogranicza zakres przemysłowego wykorzystania wyrobów z SiC.

Materials based on silicon carbide have many useful properties such as: high hardness, high strength, very good high-temperature resistance and very good thermal conductivity, but SiC materials are fragile. Increase of fracture toughness could be achieved incase of composites. It is well known that dense polycrystalline SiC can be prepared by introducing some amounts of carbon and boron. Effective amounts of these activators are: 1.5 to 3 wt.% of carbon and 0.2 to 0.5 wt.% of boron (Fig. 1). It was noticed that changes of amounts of activators cause changes of microstructure of the resulting materials. The aim of the present study was studies of the influence of carbon additive on the microstructure of the sintered body. Carbon concentration ranged from 1 to 16 wt.%. Fig. I and 2 demonstrate that: porous single-phase materials occur in the carbon range of 1:1.5 wt.%, dense polycrystalline single-phase materials are observed when content is from 1.5 to 3 wt.%, composites occur if carbon content is from 3 to 16 wt.%. Changes of microstructure, mechanical properties and electrical properties these materials were investigated. Also computers simulations of internal thermal stresses were performed. Bending strength increase in the group of single phase materials was observed. Porosity and grain sizes with carbon content (Fig. 1, Table 1). Simultaneously strength increase occurs (Fig. 4). SEM observations indicated inclusions of a second phase in the materials of carbon content from 3 to 16 wt.%. The inclusions were identified as graphite by X-ray diffraction. Microscopic studies showed that carbon inclusions slowed down the grain grow (Table 1, Fig. 2). Within this group of materials (composites) fracture toughness increase of about 50% in comparison with the single-phase materials was observed. Increase of fracture toughness is not related to the grain size but depends on internal stresses due to the mismatch of coefficients of thermal expansion of the composite constituents (Table 2). Microscopic observations suggested that crack deflection mechanism is most probably responsible for the fracture toughness increase (Fig. 5). This conclusion seems to be corroborated by the state of stresses shown by the computer simulations (Fig. 6). Measurements of electrical conductivity show (Fig. 7) that in the group of composites particular composites (3:8 wt.% of carbon's additive) and materials where both phases (SiC and graphite) are continous duplex microstructure (8:10 wt.% of carbon's additive) can be distinguished. In the group of materials with carbon content from 6 to 16% mechanical properties are worse compared to the range of particular compositions (carbon content is from 4:6%) and single phase materials (Fig. 4).