Centre for Non-Oxide Ceramics (CNOC)
SiC Technology For High Tech Reliable Ceramics
Silicon carbide (SiC) is one of the most attractive ceramic materials because of its excellent mechanical properties. SiC parts made through solid sate sintering and hot pressing comprised of single phase, homogenous microstructure, and superior strength at ambient as well as elevated temperature. ARCI has developed pressure less sintered and hot pressed silicon carbide processing technology to produce parts for wear and abrasion resistant applications through selection of suitable binder and additive system. Raw SiC powder can be shaped through either by uniaxial or cold iso-static pressing (CIPing) technique followed by heat treatment in a controlled atmosphere vacuum furnace or directly hot pressing the powder. Primary limitation for structural ceramics is their strength reliability as they undergo catastrophic failure due to their inherent brittleness. ARCI has developed technology to produce reliable silicon carbide ceramics with characteristics strength and Weibull modulus for SiC > 400 MPa and 8.24 respectively.
Silicon Carbide ( SiC ) Chemical Vapour Deposition ( CVD ) Technology For Extremely Smooth Reflecting Surface For Optical Applications
SiC based substrates with CVD SiC as reflecting surface are used for laser mirrors, solar collectors, solar concentrators and astronomical telescopes etc. These substrates show outstanding physical, mechanical, thermal and optical properties in combination with excellent polishability (<0.1 nm). However, properties of CVD SiC largely depend on the specific process parameters used e.g. deposition temperature, furnace pressure, chemical reagents /their composition and flow geometry of the reactants to achieve defect- free coatings with uniform thickness. CNOC at ARCI has optimised the CVD process for coating of large area substrates through pyrolysis of appropriate silane precursors in a sub atmospheric CVD reactor. Defect-free SiC components of 1.2 meter diameter were successfully produced through SiC CVD coating on sintered SiC substrate. Coated surface has been polished to extremely low surface finish (5-7 A) to obtain a defect-free polished surface of CVD SiC coating.
Development Of SiAlON Products Through Gelcasting Technique
b-SiAlON is an outstanding engineering grade material characterised by high strength, fracture toughness, hardness, chemical and thermal stability , low dielectric constant. They are used for making crucibles for molten metal handling, cutting tools, and RF windows etc. However, the material development and shaping of SiAlON based ceramic component is a great challenge. Among all the ceramic fabrication techniques (viz. die-pressing, injection molding, freeze casting), gel casting is one of the emerging methods to fabricate near-net shape ceramic based component. Process parameters to fabricate b-SiAlON based near net shape ceramic products through judicious selection of additives and gel casting technique have successfully been optimized at CNOC. Green gel cast part of b-SiAlON of 250 mm height, 122 mm outer dia and 6 mm wall thickness was produced using optimized processing parameters.
Establishment of Solar Thermal Power Technologies At ARCI
In order to meet the growing scarcity of power supply, ARCI has taken initiatives to develop alternate energy resources using concentrated solar power (CSP) technology in collaboration with private industries. CSP plants produce electricity using steam to drive a turbine, utilizing the energy from solar radiation. Four main elements are required: a concentrator, a receiver, some form of transport media or storage, and power conversion. ARCI is aiming for consolidate development and demonstration of solar thermal receiver (key element of the solar thermal power plant), for operating temperature upto 1000oC compatible for pressurized water/air/oil/molten salts, having excellent solar thermal absorption (>98%), low emissivity (<0.4), and high thermal conductivity. In the overall high temperature solar thermal power plant, the central absorbing receiver is the most complex and critical technology. Silicon carbide, with appropriate surface modification through titanium di-oxide ( TiO2 ) coatings, has been found to have excellent solar absorbance, moderate emittence. This coupled with SiC inherent physical, chemical, thermal, and mechanical properties shall make it a potential candidate as central solar absorber in CSP. The key components of this project consist of (i)development of the process for drawing SiC tubes upto 1 M length and 50mm diameter with wall thickness of 3 to 6mm, (ii) development and optimization of the process for welding/joining of SiC tubes to realize single tubes with length of 3M, (iii) development and optimization of suitable process technology through sol-gel/spray/dip coating compatible for SiC to achieve high solar absorbance(>95%) and low emittence (<0.4). (iv) draw plan of action for test and evaluation for the above three steps and conduct integrated test with the procured solar reflector, tracking system, pressurization system for water/oil/molten salt upto temperature range of 900oC-1000oC and also accelerated life test for its stability,(v) establishment of necessary test facilities such as humidity, salinity, thermal loss, adhesion and abrasion test.
Development of Non Oxide Ceramic Based Nano Composites
Investigations are being carried out to develop nano structured silicon carbide, boron carbide based ceramics with improved properties compared to their conventional counterpart, for high impact resistance applications. Silicon carbide or boron carbide with sub-micron or micron size grains shows less fracture toughness. It is evident that such type of ceramics with nanometer grain sizes (<100 nm) exhibits higher mechanical properties like higher hardness and fracture toughness. In addition to this, research have been pursued towards the development of nano particle reinforced ceramics such as nano-SiC reinforced SiAlON, nano-SiC reinforce TiB2 ceramics to obtain superior mechanical properties than those processed using micron size powders.
Development of Reaction Bonded Boron Carbide Ceramics
Boron carbide based composites produced by reaction bonding process is an alternative to hot-pressed monolithic boron carbide. Reaction Bonded Boron Carbide (RBBC) microstructure consists of boron carbide (major phase), reaction formed silicon carbide, and residual silicon. Presently, RBBC is being used as potential candidate material for abrasion and wear resistant applications because of its unique combination of physical property (e.g. bulk density 2.52gm/cc) and mechanical property (hardness 2800-3400Kg/mm2, Young's modulus 350GPa). Due to the low processing temperature (1450-1600oC) and capability to retain exceptional dimensional stability, reaction bonding process was adopted at CNOC to produce near net shape product. In this process, molten silicon infiltrates into porous boron carbide perform by capillary action and spontaneously reacts with carbon and forms SiC. Silicon carbide, formed through this reaction, linked with pre-existing boron carbide grains, occupy fraction of pores, and leftover pores are filled with residual silicon.
Development of Silicon Carbide Foam Through Aqueous Gel Casting Process Followed By Liquid Phase Sintering Technique
Porous silicon carbide (SiC) ceramics find application as catalytic supports, hot-gas and molten metal filters, light-weight structural parts for high temperature applications etc. ARCI has developed SiC foams through direct foaming and gel casting technique. The final porosity was directly controlled by incorporating varying amount of gas into the slurry by generating gas bubbles inside liquid slurry of SiC powder followed by setting the slurry in order to maintain its porous morphology in the green stage. Water soluble gel casting monomers and liquid phase sintering additives Y2O3/Al2O3 were used in formulation of SiC powder suspension in water. Defect free sintered foams with porosity in the range of 60-82% were produced using this technique while maintaining sufficient strength.
Indigenous Development of Ready To Press ( RTP ) Silicon Carbide Powder
Spray dried ready to press (RTP) SiC powder with proper addition of sintering additives and binder is the basic raw material to produce high quality sintered SiC parts, when processed through conventional powder metallurgy route. Although, ready to press (RTP) SiC powder is available in overseas market, the same is not available in India at present, hence processing of parts using imported powder is highly cost intensive. Therefore, CNOC at ARCI has taken initiative to develop RTP silicon carbide powder through judicious selection of sintering additives and binder. ARCI has already optimized the powder properties and the sintered properties of the components produced using optimized SiC powder, which has shown results at per with those produced from commercially available powder from imported sources.