Role of Y2O3 and Al2O3 Additive during Fabrication of Si3N4-Mo, Si3N4-Mo-Si and Si3N4 Mo-Al Cermets by Powder Metallurgy

Abstract

Ceramics are well known for their high hardness, strength, refractoriness and good chemical resistance but have poor fracture toughness and are brittle in nature. The addition of some metallic phases improves the ductility in the ceramics and hence enhances the toughness. Cermets have the combined characteristic properties of both metals and ceramics and thus, have superior properties to the constituents. Due to this, the cermets are a promising material for use as turbopumps, heat exchangers, high-temperature parts of combustion engines, wear-resistant components as blast nozzles, aero-engine components, fusion reactors, cutting tools, valve components, bearings, refractory lining and coating to protect materials from corrosive environments. Silicon nitride is a refractory ceramic known for its structural applications. As a cutting tool, Si3N4 is more durable than coated carbide tools and has a better surface finish. The performance and machinability of Si3N4 are so good that it can machine difficult to cut materials like hardened steel and nickel-based super alloys. But the only drawback of Si3N4 is its low fracture toughness compared to other engineering materials like steel. Properties of Si3N4 can be improved by reducing the particle size and incorporating some metallic elements into the matrix. Moreover, Si3N4 has poor sinterability due to its covalent nature. The sinterability can be improved by adding suitable additives. In the present study, Si3N4-Mo, Si3N4-Mo-Si and Si3N4-Mo-Al cermet powders have been prepared by mechanical milling of constituents for 2 hours with the help of a high-speed dual-drive planetary mill. Average particle size of 1-2 μm has been achieved after milling. The reduction in particle size has been confirmed by particle size distribution analysis, XRD, SEM and TEM. All the prepared compositions of milled powders have been consolidated by conventional pressure-less sintering in argon (Ar) gas and hydrogen (H) gas atmosphere with Y2O3 and Al2O3 as sintering additives. The individual and combined effects of Y2O3 and Al2O3 additives have been studied, and the amount of additive has been optimized. The sintering temperature was maintained at 1500 °C with a holding time of 1 hour. The optimal compositions of cermets have been chosen for spark plasma sintering (SPS) at 1350 °C with 15 min holding. The relative density, Vickers micro-hardness, indentation fracture toughness (IFT), and wear resistance have been enhanced in the cermets consolidated by SPS and with a combined additive content of 5 wt. % Y2O3 and 10 wt. % Al2O3. Maximum relative density, hardness and IFT of 96.66 %, 15.72 ± 1.23 GPa and 11.12 ± 1.34 MPa.m1/2, respectively, have been obtained in SMS5Y10A-SPS. The addition of Si has enhanced the densification and hardness of the material by forming the phases like Mo3Si, Mo5Si3, MoSi2, YSi2, Y2Si2O7 etc. The reaction between the oxide additives and Si3N4 has resulted in formation of Y3Al5O12 phase which has resided in the molybdenum silicide grain boundaries and hence, have improved the densification and hardness. Presence of Y3Al5O12 phase has resulted in crack deflection through the grain boundaries. Hence the uniform distribution of the molybdenum silicide phase throughout the matrix has enhanced the physical and mechanical properties of the fabricated cermets. The un-lubricated sliding wear resistance of the cermets against a diamond indenter has been examined with the normal applied loads of 20 N and 40 N. It has been observed that the wear rate, weight loss, and wear depth has been increased with the increase in load. The dominant wear mechanisms are observed to be delamination, abrasion and formation of microcracks. Infiltration of the molten Si/Al metals into the cermet preforms has been performed to investigate the extent of infiltration and the effect on the properties of the cermets and to correlate with the properties of the cermet powders prepared by milling followed by conventional sintering at 1500 °C. Aluminium and Silicon have chosen to see the effect of low melting ductile metal (Al) and high melting brittle metal (Si) on the properties of the infiltrated surface. Due to its low melting point, Aluminium has been formed the melt at low temperature and have the maximum time for the reaction by capillary action and, hence, shown better properties than Si infiltrated cermets. After the detailed investigation of all the above factors, it has been concluded that (Si3N4)75-(Mo)15-(Si)10 is the best composition among the discussed cermets due to the presence of Si and SPS is the best method to achieve high density, hardness, fracture toughness and wear-resistance among the discussed cermets. An amount of 5 wt. % Y2O3 and 10 wt. % Al2O3 is the best combination of additives to achieve the optimal mechanical and tribological properties for practical applications.