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Mechanically activated synthesized zirconium carbide substrate to make ZrC-Mo cermets
Abstract
Mechanical activation by ball milling technology and reactive, vacuum sintering are used to synthesize zirconium carbide (ZrC) nanopowder from zirconia (ZrO2). The ZrC nanopowders subsequently underwent further high-energy ball milling with molybdenum (Mo) metal binder then vacuum sintered to form bulk cermets. Using ZrC nanopowder, synthesized from ZrO2 through mechanical activation technology, resulted in higher densification, increased Vickers hardness, and more Mo retained within the microstructure compared with using commercial grade ZrC nanopowder as a partnering substrate. Commercial ZrC, combined with Mo, showed a cermet with more Mo2C phase, yet yielded lower hardness and more structural porosity when vacuum sintered at 1800°C and 1900°C for 60 mins. The combination of low-energy ball milling and vacuum sintering of ZrC with Mo produced highest densification, even higher comparable hardness, smaller grain sizes, and a homogenous grain structure.
... Mechanical activation by ball milling technology and reactive vacuum sintering are used to synthesize zirconium carbide (ZrC) nanopowders from zirconia (ZrO 2 ) and graphite as described elsewhere [12] according to the reaction: ...
... However, the reaction could be favorable at temperature as low as ~1500 0 C under vacuum due to the lower partial pressure of CO, which was assumed to be the nominal furnace pressure. The required threshold sintering temperature needed to reactively synthesize ZrC from ZrO 2 mixed with carbon is estimated to be 1500 °C [12]. ...
The study is aimed at development, fabrication and preliminary mechanical characterization of ZrC-based composites for high temperature applications. The improvement in densification was achieved through introduction of second phase (molybdenum, zirconia, and titanium carbide). ZrC powder was produced by cost-effective in situ reactive sintering of nano-sized zirconium dioxide and graphite. The powder mixtures were pressureless sintered in vacuum at temperatures lower than 2000 0C. The final densities of the composites varied between 96 - 98%. Materials obtained were quite hard (HV10 > 17 GPa) while relatively tough (IFT > 6.6 MPa m1/2).
... Previous research by the authors of this paper [5,6] has focused on the synthesis of ZrC from zirconia, using high-energy ball milling and reactive, heat treatment to achieve ZrC yield at just 1500 °C. Furthermore, research from the same authors has been done on synthesising ZrC-Mo cermets using ball milling technology and vacuum sintering at 1900 °C producing a hardness of 17 GPa, densification to theoretical at 96%, and fracture toughness at ~6 MPa*m 1/2 . ...
... Subsequently based on results, we switched to only LEM mixing ZrC and TiC with decreasing TiC concentration from 50, 37.5 (1:1 molar ZrC-TiC), 30, and 20 (wt%). We compared two separate sources of ZrC: ZrC(CP), which is a commercially purchased nanopowder carbide (~100nm grain size, 99% purity, China) and ZrC(TUT), which is an in-lab synthesised powder [5]. LEM involved using a WC-Co rotary drum (1L), WC-Co balls (16mm Ø), a 6:1 ball-powder ratio, and ethanol to thoroughly mix substrate powders for 72 h. ...
Inherently, zirconium carbide (ZrC) suffers from low fracture toughness (~3 MPa*m1/2) and excessive porosity when sintered in vacuum. One way to improve ZrC’s sinterability and fracture toughness is the addition of binder metal or other carbides to increase densification. Using mechanical activated synthesis (MAS) to homogenously mix ZrC and titanium carbide (TiC) powders, followed by sintering at 1900 °C, produces a ZrC-TiC composite with hardness and fracture toughness at 20 GPa & ~7 MPa*m1/2, respectively. 80ZrC-20TiC (wt%) gave the highest fracture toughness value compared to other ratios. Varying TiC ratio from 20 - 50 wt% does little to affect mechanical hardness or densification of the composite. However, fracture toughness appears to increase marginally with decreasing TiC concentration down to ~20 wt%.
... Mechanical activated synthesis (MAS) is achieved during high-energy ball milling, a process that mechanically increases the specific surface area of particles as well as homogenously mixes substrates allowing for chemical reaction and sintering at lower temperatures [6][7]. Previous studies performed by the authors of this paper have shown ZrO 2 and graphite undergoing 10 hours of high-energy milling (HEM) and subsequent reactive vacuum sintering at 1500°C can yield cubic ZrC up to >98% purity [8]. ...
The high cost of commercial ZrC nanopowder has spurred the development of cost-efficient and low-energy approaches for carbide synthesis. Mechanical activation synthesis (MAS) by high-energy ball milling technology and reactive vacuum sintering was used to synthesize ZrC powder from zirconia (ZrO2) at the low temperature of 1500°C. However, a major drawback for structural ZrC powder made from MAS was the relatively low fracture toughness, and difficulty ensuring adequate hardness and densification when vacuum sintered below 2000°C. ZrC combined with 20% weight molybdenum was sufficient to yield a cermet of 96.1% densification, hardness of 17GPa, and fracture toughness of 5.23-6.08MPa∗m1/2. This present study was able to achieve a genuinely homogenous ZrC-Mo cermet in vacuum sintering temperatures not exceeding 1900°C.
The properties, performance, and reliability of a cermet material depend mostly on the intrinsic properties influenced by microstructure that evolves during processing and use. In this study, the effect of microstructure on reliability of multiphase materials in erosive media is analyzed with reference to WC-, TiC-, and Cr3C2 - based ceramic-metal composites. Microstructure of multiphase materials, fracture mechanisms, ability of energy dissipation, thermo-mechanical parameters and erosion resistance are examined. It has been shown that microstructural variables play a very important role in cermets performance. The energetic criterion of non-homogeneous materials selection has been presented.
TiC-based cermets have advantages such as lower friction coefficient and higher oxidation resistance compared with WC-based cemented carbides. However, the brittleness is still an unavoidable limitation for their utilizations. Microstructure of TiC–xZrC–15WC–14Mo–20Ni–1C cermets with different ZrC contents was studied by using X-ray diffractometry (XRD), scanning electron microscopy (SEM) in combination with energy dispersive spectrometer (EDS). The experimental results show that ZrC dissolved in the cermets and formed solid solutions of (Zr, Ti, Mo, W)C. The bright and gray coreless grains have been regarded as beneficial for the toughness of the cermets.
Pressureless sintering of ZrC–Mo cermets was investigated in a He/H2 atmosphere and under vacuum. A large density increase was observed for specimens with >20 vol% Mo after heating at 2150°C for 60 min in a He/H2 atmosphere. The increase in density was attributed to the formation of Mo2C during heating and its subsequent eutectic reaction with Mo, which produced rounded ZrC grains in a Mo–Mo2C matrix. Sintering in vacuum did not produce the same increase in density, due to the lack of Mo2C formation and subsequent lack of liquid formation, which resulted in a microstructure with irregular ZrC grains with isolated areas of Mo. Mechanical properties testing showed a decrease in Young's modulus with increasing Mo content that was consistent with the models presented. Flexure strength of ZrC–Mo cermets sintered in He/H2 atmosphere materials increased with increasing Mo content from 320 MPa at 20 vol% Mo to 410 MPa at 40 vol% Mo. Strength was predicted by adapting theories developed previously for WC–Co cermets.
The chemical compatibility of ZrC and Mo was investigated in carburizing and carbon-free environments at temperatures from 1700° to 2200°C. Heating in the carburizing atmosphere resulted in the complete reaction of Mo with C, while the carbon-free atmosphere resulted in retained metallic phase with a maximum of 13.8 mol% Mo2C formed. The presence of Mo2C was not detected at 2100°C in the carbon-free atmosphere, confirming the existing phase equilibria in the Zr–Mo–C system. Heat treatments in the carbon-free atmosphere also showed liquid formation at 2200°C, as evident from microstructure analysis. Liquid formation was consistent with the interaction between Mo and Mo2C. The liquid was found to comprise at least 7 vol% of the total component, based on a phase diagram for the Mo–C system. The formation of a liquid should allow for the processing of ZrC–Mo cermets by liquid-phase pressureless sintering.