Fabrication of SiC-B4C–Si and SiC-B4C–Al Cermets by Powder Metallurgy Route

Abstract

Cermets are unique that combines the characteristic properties of metals and ceramics. These superior properties make cermet a promising material for use in several aero-engine components, turbo pumps, heat exchangers, fusion reactors, armour plates, wear resistant components as blast nozzles and wheels dressing tools, seal rings, valve components and bearings, oxidation resistant material as refractory lining, coating to protect materials from corrosive environments and combustion engines. SiC-B4C based cermets are being developed as remarkable engineering materials for their outstanding physical and mechanical properties, including high hardness, lightweight, high wear resistance, high melting point, oxidation resistance and great resistance to chemical attack. Fabrication of highly dense SiC-B4C based ceramic composite is quite difficult due to its high melting point and strong covalent bond. Properties of these materials can be improved by reducing initial particle size of powders and incorporation of metallic elements into ceramic matrix. The fine and disordered structure produced by mechanical milling increases the molecular diffusion channel during sintering and enhance the densification. Incorporation of Si and Al into SiC-B4C ceramic matrix improves the properties since it acts as a binder, enhances densification, and lowers the sintering temperature.
In the present study, SiC-B4C (60:40 wt. %) ceramic composite powder, SiC-B4C–Si (2, 5, 10 and 20 wt. % of Si) and SiC-B4C–Al (2, 5, 10 and 20 wt. % of Al) cermet powders of various compositions were prepared separately by mechanical milling for 10 h in a planetary mill. An average particle size of 3–4 μm was obtained after 10 h milling for all the compositions. XRD, SEM and particle size analysis reveal the particle size reduction and homogeneous distribution of Si, Al, in the respective SiC-B4C composite matrix.
All the compositions were consolidated by both conventional sintering at 1950 C and spark plasma sintering (SPS) methods at different temperatures respectively. The SPS was carried out at 1500 and 1600 °C for high melting SiC-B4C ceramic composite. SPS temperature was optimized to 1350 °C and 1300 °C for SiC-B4C–Si and SiC-B4C–Al cermets. Higher density, microhardness, indentation fracture toughness, and compressive strength values were achieved for all the compositions consolidated by SPS than conventional sintering. Cermets containing 10 wt. % Si and 10 wt. % Al exhibit optimum mechanical properties than other compositions. There is an enhancement of Vickers microhardness from 18±2.23 to 28±2.12 GPa for the cermet sample containing 10 wt. % Si when the sintering condition was changed from conventional sintering to SPS. The relative density, microhardness, indentation fracture toughness and compressive strength of SiC-B4C–10 wt. % Si composition fabricated by SPS are 98%, 28±2.12 GPa, 3.8 MPa m1/2, and 1387 MPa respectively. Similarly, the optimum relative density, microhardness and indentation fracture toughness of SiC-B4C–10 wt. % Al consolidated by SPS are 97%, 23±2.29 GPa and 3.28 MPa.m1/2, respectively.
Un-lubricated sliding wear behavior of SPS and conventional sintered samples against diamond indenter were compared at different loads (20, 40, 60 and 80N for SPS samples; 20 and 40N for conventional sintered samples). It has been observed that wear resistance decreases with increasing applied load for both cases. Minimum wear depth with high wear resistance, lowest wear rate and lowest volume of wear debris were obtained for the SPS samples containing 10 wt. % Si and 10 wt. % Al. The possible wear mechanisms are abrasive grooves, delamination, and formation of crack.
High temperature oxidation study was performed to understand the tendency of varying Si amount (0–20 wt. %) in SiC-B4C ceramic matrix and the effect of different oxidation temperatures. Isothermal oxidation study was performed at 800, 1000 and 1200 °C for all the compositions. The cermets with 10 and 20 wt. % Si show more oxidation resistance than the other compositions. At 1200 °C, the SiO2 glassy phase formed as an oxidation protective layer over the sample surface and prevented from further oxidation.
Finally, after performing all characterizations and analyzing results it was concluded that mechanical milling followed by SPS technique was more efficient and advanced technique to produce SiC-B4C based cermet with better physical, mechanical, wear and oxidation properties. Moreover, metallic reinforcement improved the key properties needed for structural and oxidation applications.