8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING” 19-21 April 2012, Tallinn, Estonia
REACTIVE SINTERING OF ZIRCONIUM CARBIDE BASED SYSTEMS Yung, D.; Kollo, L.; Hussainova, I.; & Zikin, A.
Abstract: 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 (ZrO 2 ) 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 5.236.08MPa*m1/2. This present study was able to achieve a genuinely homogenous ZrC-Mo cermet in vacuum sintering temperatures not exceeding 1900°C. Key words: mechanical activation, reactive vacuum sintering, cermets, carbides, ZrCMo
electric-arc furnace at temperatures >2500°C [5]. However, densification, hardness, and fracture toughness are evident weaknesses in eutectic ZrC. Various research attempts have signalled the use of molybdenum as a viable binder agent for liquid sintering in the effort to boost the mechanical properties of ZrC. Research into ZrC-Mo cermets synthesised in pressureless or non-carburizing environments have used temperatures in excess of 2000°C [1-5]. Flexure strength and fracture toughness increased with increasing Mo content from 1.0 to 6.6MPa*m1/2 respectively [5]. 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].
1. INTRODUCTION Zirconium carbide (ZrC) belongs to a class of ultra-high temperature ceramics, which possesses high melting temperature (~3400°C), decent refractory hardness (~25GPa), and good corrosion and oxidation resistance [1-4]. ZrC is typically produced by fusing zirconia with carbon in an
This study aims to further optimise the initial findings of synthesising ZrC from zirconia in an effort to increase the yield and efficiency of the process involving MAS and vacuum reactive sintering. In regards to HEM, the milling time and ball-powder (BP) ratios are factors taken into account. And then to improve upon the mechanical quality
of ZrC, this study produces a series of experiments to determine the adequate amount of Mo needed to amend the densification, hardness, and fracture toughness of ZrC-Mo cermets at temperatures no more than 1900°C. In this case, the percent weight of Mo and sintering temperatures are factor taken into account. The experimental methods of high-energy (HEM) and low energy (LEM) ball milling will be compared as these proven methods have shown to decreased the required sintering temperature for bulk samples.
homogenously mix the reactant substrates where reactive sintering will continue to yield quality ZrC. To evaluate milling time, the BP ratio was reset to 12:1 and using the original 40 grams of starting materials, milling time was set to increase from 3 hours to 10 hours. All samples underwent vacuum, reactive sintering at 1500°C for at least 1 hour. XRD and SEM/EDX of bulk samples were used to analyse the resulting ZrC.
2. EXPERIMENTAL
The second phase of experiments centred on mechanical enhancing the ZrC cermets. Just as in our previous research, we compared ZrC(TUT), synthesised from zirconia by MAS and HEM undergoing reactive sintering, with commercial grade ZrC(CP). With specific focus on ZrC-Mo composites, the goal was to improve upon the brittle, low fracture toughness starting material of ZrC as well as densification and hardness.
2.1 Optimising ZrC Synthesis We used un-stabilised zirconia (with the absence of yttrium) to characterise HEM milling parameters regarding the milling time and BP ratio. The first parameter was BP ratio during HEM. In our milling container, the amount of starting substrates was increased by 50% in each of three experiments. The original starting material amount was 40g weight at 1:1 molar ratio of ZrO 2 and graphite (see equation 1), which was subsequently increased to 60g and then 80g. ZrO 2 + C ZrC +CO 2
(1)
All stoichiometric weights of the zirconia and graphite were held constant as well as the milling time set to 10 hours and the number of milling balls. The milling liquid was topped off proportionally. The BP ratio consequently decreased from 12:1 to 9:1 to 6:1 as only more substrate was added. In all experiments, the ideal milling time was set to 10 hours. However, given the wear and tear inflicted on the machines with such long milling times, the goal was to reduce the needed time, but still
2.2 Creating ZrC-Mo
The first set of experiments looked at the stand point of using stoichiometric 1:1 molar weights in the following reaction scheme. ZrC + Mo ZrC-Mo
(2)
A series of LEM involved using only ZrC(CP) and Mo at decreasing Mo concentrations from 54.5% - 20% wt. All samples were added ~1% binding agent, a mixture of 1:1 weight organic liquid rubber and paraffin wax during milling. This would ensure quality mould pressing at 100 MPa for making green bulk samples. All green samples were pre-sintered at 1500°C for at least 1 hour before final sintering step. Final sintering up to 1900°C took place at 4°C/min and held for 30 minutes at final temperatures. Sintered samples were polished and examined with scanning electron microscopy energy (SEM) and
dispersive X-ray spectroscopy (EDX); density measured with Archimedes’ method; and fracture toughness determined using Palmqvist & Median crack equations based on Vickers’ indentation under light microscope [9]. 3. RESULTS AND DISCUSSION 3.1 ZrC Synthesis Changing the BP ratio or the milling time for HEM powders of ZrO 2 and graphite and then undergoing subsequent reactive sintering up to 1500°C show no discernable difference in the yield of ZrC(TUT).
Fig 1: XRD of ZrC(TUT) due to varying BP ratio
XRD patterns (Fig 1) show strong peaks for cubic ZrC even when BP ratio is dropped from 12:1 to 6:1. Fig 2 shows promising results to streamline the synthesis and Sintered Cermets elements ZrC(TUT) ZrC(TUT) ZrC(CP)+Mo ZrC(CP)+Mo ZrC(TUT)+Mo ZrC(TUT)+Mo
Fig 2: XRD ZrC(TUT) due to increasing milling time
manufacturing of ZrC, potentially decreasing the amount of energy and time needed to make ZrC. Ideally, the results could indicate ZrC can be manufacture using a parameter of BP ratio 6:1 and in as little as 3 hours HEM. ZrC(TUT) has densified up to ~88% at 1900°C and hardness ~13GPa (Table 1). Adding 1:1 molar Mo does not improve the densification when using HEM, although the results do show evidently higher hardness. This factor may be attributed to the titanium carbide (TiC) and tungsten contaminations from using WC-Co milling container and TiC balls during high-energy milling. However, ZrC(TUT) shows more stability at higher temperatures as it exhibited 25% more densification compared to ZrC(CP). The propensity of Mo to react with the carbon molecule in ZrC makes predicting densification more difficult.
Composition Sintering Measured Theoretical Densification Fracture Vickers' ZrC - Mo Temp Density Density Strength Hardness 3 3 wt% (°C) % GPa g/cm g/cm MPa*m 1/2 100% 1800 5.68 6.56 86.5 -13.9 100% 1900 5.76 6.56 87.8 -12.7 45.5% : 54.5% 1800 6.51 8.15 79.8 3.69 , 4.62 16.2 45.5% : 54.5% 1900 5.06 8.15 62.1 -4.0 45.5% : 54.5% 1800 4.36 8.15 53.4 -3.0 45.5% : 54.5% 1900 6.65 8.15 81.6 1.33 , 1.90 14.4
Table 1: High-energy milled ZrC and ZrC-Mo bulk samples comparing ZrC(TUT) and ZrC(CP); note that fracture toughness is given as palmquist and median crack measurements respectively
Sintered Cermets elements ZrC(CP)+Mo ZrC(CP)+Mo ZrC(CP)+Mo ZrC(CP)+Mo ZrC(CP)+Mo ZrC(CP)+Mo ZrC(CP)+Mo ZrC(CP)+Mo
Composition Sintering Measured Theoretical Densification Fracture Vickers' ZrC - Mo Temp Density Density Strength Hardness 1/2 3 3 wt% (°C) GPa % MPa*m g/cm g/cm 45.5% : 54.5% 1800 7.48 8.15 91.8 -10.1 60% : 40% 1800 7.27 7.66 94.8 3.55 , 4.32 11.8 70% : 30% 1800 6.95 7.35 94.5 -14.2 80% : 20% 1800 6.67 7.07 94.3 2.98 , 3.70 13.6 45.5% : 54.5% 1900 7.41 8.15 90.9 -12.8 60% : 40% 1900 7.29 7.66 95.2 3.27 , 4.50 15.6 70% : 30% 1900 7.08 7.35 96.3 -14.7 80% : 20% 1900 6.79 7.07 96.1 5.23 , 6.08 16.9
Table 2: Low-energy milling using commercial grade ZrC and decreasing Mo wt% concentrations; note that fracture toughness is given as palmquist and median crack measurements respectively
3.2 ZrC-Mo Synthesis Since HEM samples have proven to give inconsistent results mostly due to contamination from the milling media, switching to LEM gave more consistent results. Table 2 indicates densification of ZrC-Mo actually increases with decreasing Mo concentration where no more than 40% wt Mo should be used for optimal densification. As well, final sintering temperature also plays a role in densification, hardness, and fracture toughness with the higher 1900°C setting improving all three parameters by 2%, 40%, & 20% respectively. The inability to achieve full densification is most likely attributed by the lack of liquid formation during sintering [2], which
Fig 3: SEM image ZrC-Mo (80%, 20% wt respectively) sintered at 1800°C
Mo
ZrC
Mo2C
Zr Fig 4: SEM image of ZrC-Mo (70%, 30% wt respectively) sintered at 1900°C
requires temperatures in excess of 2000°C. Fig 3 shows that only 20% wt Mo is enough to create an impressive microstructure with high hardness, 17GPa and densification reaching >96%. Adding more Mo results in a more porous microstructure and lowers overall hardness (Fig 4). Depending on the integrity of ZrC, Mo can compete for the carbon molecule in ZrC to form Mo 2 C creating a third phase and leaving spots of free zirconium. Contrary to previous studies where researchers show fracture toughness increases up to 6.6MPa*m1/2 with increasing Mo concentration [4], results in this study seem to conclude the opposite effect. Mo concentration at 20% wt gives a mid-range
fracture toughness of ~5.7MPa*m1/2. Fracture toughness would actually decrease due to increasing Mo concentration as evident by macro-fractures appearing on all polished samples surface with Mo greater than 20% wt.
process after sintering leading to macrofractures visible to the naked eye.
200µm
Fig 7: Zoomed in picture of Fig 6. The fractures created by Vickers’ indentation extend and terminate at preexisting boundaries Fig 5: ZrC-Mo (80%, 20% wt respectively) sintered at 1800'C only exhibited microfractures only after indentation impact testing
Fig 5, with ZrC 80% and Mo 20%, displays a typical, expected fracture lines that terminate before reaching a pre-existing grain boundary. However, raise the concentration Mo by just half to 30% wt and macro-fractures become prevalent (Fig 6).
Fig 6: ZrC-Mo (70%, 30% respectively) sintered at 1800'C with easily visible macro-fracture cracks under light microscope with 5X.
It maybe the formation of Mo 2 C and resulting remnants of Zr that causes grain boundaries to enlarge during the cooling
4. CONCLUSIONS Based on the study within this work, the following conclusions can be drawn: 1. Synthesising ZrC from ZrO 2 and graphite can be done using highenergy milling and reactive sintering using parameters of 6:1 ball-powder ratio, 3 hours milling time, and 1500°C. 2. Synthesising ZrC-Mo, using lowenergy milling and sintering at 1900°C improves densification, hardness, and fracture toughness by 2%, 40%, and 20%, respectively, from 1800°C. 3. To avoid macro-fracture and improve fracture toughness and hardness, using 80%wt ZrC and 20%wt Mo gives the most optimal result. 5. ACKNOWLEDGEMENTS Estonian Ministry of Education and Research (targeted finance project nos. SF0140062s08) and Estonian Science
Foundation (ETF Grants 8211, 8850, 8472) and the strategic research project 102850199 (advanced multiphase tribo-functional materials) under “Austrian Comet-Program” (governmental funding program for precompetitive research) via the Austrian Research Promotion Agency (FFG) and the TecNet Capital GmbH (Province of Niederösterreich) are acknowledged for supporting this research. Special thanks to Tallinn University of Technology and staff for logistic and technical support. 6. REFERENCES [1] Landwehr, S.E., Hilmas, G.E., Fahrenholtz, W.G.; Processing of ZrC–Mo Cermets for High-Temperature Applications, Part I: Chemical Interactions in the ZrC–Mo System; J. Am. Ceram. Soc., 2007, 90, 1998–2002. [2] Landwehr, S.E., Hilmas, G.E., Fahrenholtz, W.G.; Processing of ZrC–Mo Cermets for High Temperature Applications, Part II: Pressureless Sintering and Mechanical Properties; J. Am. Ceram. Soc., 2008, 91, 873–878. [3] Landwehr, S.E., Hilmas, G.E., Fahrenholtz, W.G., Talmyc. I.G., Wang, H.; Thermal properties and thermal shock resistance of liquid phase sintered ZrC–Mo cermets; Mater. Chem., Phys., 2009, 115, 690–695. [4] Landwehr, S.E., Hilmas, G.E., Fahrenholtz, W.G., Talmyc. I.G., Wang, H., DiPietroc S.G.; Microstructure and mechanical characterization of ZrC–Mo cermets produced by hot isostatic pressing, Mater. Sci., Eng., 2008, 497, 79–86. [5] Zhao, L., Jia, D., Duan, X., Yang, Z., Zhou, Y.; Pressureless sintering of ZrCbased ceramics by enhancing powder
sinterability; Int. J. Refract. Met. Hard Mater., 2011, 29, 516–521. [6] Juhani, K; Reactive Sintered Chromium and Titanium Carbide - Based Cermets; PhD - Tallinn University of Technology, Estonia, 2009. [7] Suryanarayana, C.; Mechanical alloying and milling; Prog. Mater Sci., 46, 2001, 1184. [8] Yung, D., Kollo, L., Hussainova, I., Zikin, A.; Mechanically activated synthesised Zirconium Carbide substrate to make ZrC-Mo cermets; EURO PM2011, 28, #149. [9] Strecker, K., Ribeiroa, S., Hoffmann, M.J.; Fracture Toughness Measurements of LPS-SiC: A Comparison of the Indentation Technique and the SEVNB Method; Mater. Res., 2005, 8, 121-124. [10] Pierrat, B.; Oxidation of Ultra-High Temperature Ceramics – Zirconium Carbide; MSc Thesis - Lulea University of Technology, Sweden, 2010. [11] Tsuchida, T., Yamamoto, S.; MA-SHS and SPS of ZrB2–ZrC composites; Solid State Ionics 172, 2004, 215–216. 7. CORRESPONDING ADDRESS BSc Der-Liang Yung Department of Materials Engineering, Tallinn University of Technology (TUT), Ehitajate 5, 19086 Tallinn, Estonia Phone: +372-620-3371 Fax: +372-620-9136 Email:
[email protected]
