Pressureless sintering of chromium diboride using spark plasma sintering facility

Pressureless sintering of chromium diboride using spark plasma sintering facility

Int. Journal of Refractory Metals and Hard Materials 58 (2016) 165–171 Contents lists available at ScienceDirect Int. Journal of Refractory Metals a...

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Int. Journal of Refractory Metals and Hard Materials 58 (2016) 165–171

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM

Pressureless sintering of chromium diboride using spark plasma sintering facility K. Sairam ⁎, J.K. Sonber, T.S.R.Ch. Murthy, A.K. Sahu, R.D. Bedse, J.K. Chakravartty Materials Group, Bhabha Atomic Research Centre, India

a r t i c l e

i n f o

Article history: Received 20 January 2016 Received in revised form 25 April 2016 Accepted 9 May 2016 Available online 10 May 2016 Keywords: CrB2 Boride Spark plasma sintering Pressureless sintering Microstructure Grain coarsening

a b s t r a c t Sinterability of monolithic CrB2 was investigated under pressureless sintering condition using spark plasma sintering facility (SPS). Monolithic chromium diboride (CrB2) was sintered in a modified die setup instead of traditional/conventional plunger and die assembly. This kind of assembly creates pressureless sintering conditions in spark plasma sintering unit. The main objective of this modified setup is to couple the combined aspect of conventional pressureless sintering with fast heating. Densification experiments using CrB2 were conducted at temperatures in the range of 1600 °C to 2000 °C with different dwelling period (1–30 min) under no load condition. The effect of multi-step sintering treatment on the densification behaviour of CrB2 was also investigated. The density and the resulted microstructures of the sintered samples are presented and discussed in this paper. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Spark plasma sintering (SPS) or field assisted sintering technique (FAST) is an advanced sintering technique which is used to sinter materials that are difficult by conventional powder processing techniques [1, 2]. The advantage of SPS compared to conventional hot pressing is mainly realized due to the Joule's heating of the sample (direct heating) [3–5]. Wang et.al has studied the synthesis of nano-HfB2 powder using SPS apparatus [6]. Bradbury et.al synthesized SiC based composites by freepressureless spark plasma sintering concept using modified T-shaped graphite punches [7]. In the present study, an attempt has been made to sinter chromium diboride (CrB2) in SPS facility under no load condition. Diboride-based ceramics can be categorized under difficult to sinter materials as it possesses high melting points and strong covalent bonding [8–13]. In recent past, SPS has been extensively used to study the densification behaviour of diboride based ceramics, due to its fast heating rates and short holding time coupled with application of high mechanical pressures [4,9,11,14]. But, this method is mainly limited to fabricate simple shaped components as load has to be applied to keep components intact in order to close the electric circuit during sintering operation. On the other hand, conventional pressureless sintering process (PLS) offers fabrication of near net shaped components but at higher sintering temperatures and longer holding time compared to SPS. It is often difficult to ⁎ Corresponding author. E-mail address: [email protected] (K. Sairam).

http://dx.doi.org/10.1016/j.ijrmhm.2016.05.002 0263-4368/© 2016 Elsevier Ltd. All rights reserved.

retain the fine grain microstructures in conventional pressureless sintering due to the dominance of grain coarsening compared with densifying mechanism at those long duration elevated temperature operations [15,16]. Hence, in general, sintering of boride like ceramics is difficult with conventional pressureless sintering. The present study is aimed at synergizing the advantages of both PLS and SPS process, to sinter CrB2. 2. Experimental procedure In-house synthesized CrB2 (purity 99%, 0.5% O, 0.5% C; 5 μm mean particle diameter) was used as a starting material in this study [10]. XRD pattern and particle size distribution of the starting material is presented in Figs. 1 and 2 respectively. The conventional graphite plunger and die setup of SPS was replaced with a simple graphite crucible. The dimensions of the graphite crucible used in the present study were 40 mm outer diameter, 5 mm wall thickness and 25 mm height. In the present study, the sample was directly loaded on the heating element (crucible) and hence the transfer of heat energy is governed by conduction from the bottom and radiation from the sides of the crucible. CrB2 powder was cold compacted into 12 mm diameter disks by applying 250 MPa mechanical pressure using hydraulic mechanical press. A cold compacted CrB2 pellet of green density ~ 58% was loaded in the graphite crucible (Fig. 3) and then, the entire assembly was placed inside the spark plasma sintering chamber. The actual image of in-house built spark plasma sintering facility (specification: 200 kN, 5000 A, 8 V) is shown in Fig. 4. The 1 mm thick graphite sheet was placed in between cold compacted pellet and

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room temperature and measured for density using Archimedes principle. The surface of as-sintered compacts (without metallographic preparations) was characterized for microstructures using scanning electron microscopy (FESEM).

3. Results and discussion

Fig. 1. XRD pattern of starting material and 93%ρth dense compact showing the presence of single phase CrB2.

the graphite crucible surface so as to avoid the reaction of material with the graphite crucible and extend the life of the same. As shown in Fig. 3, thick graphite plates were placed one above and below the graphite crucible to avoid the direct exposure of steel rams to high temperatures. The temperature of the operation was measured using an optical pyrometer by focusing at the surface of the graphite crucible. SPS chamber was evacuated to 10− 3 mbar and maintained throughout the course of the experiment. Under no load/pressureless condition, sintering was performed at temperatures between 1600 °C to 2000 °C and its dwell time was set to 15 min. The effect of dwell time (varied between 1 and 30 min) on densification of CrB2 was investigated at temperatures of 1900 °C, 1950 °C and 2000 °C. The samples were also subjected to multi-step sintering treatment at selected temperature regimes for different durations as illustrated in Fig. 5. The heating rate during sintering treatment was maintained as 100 °C/min. The sintered samples were allowed to cool down to

In the initial set of sintering experiments, development of circumferential cracks was observed in the sintered compacts. This was probably due to the fast cooling rate (800 °C/min) followed from processing temperatures up to 1400 °C and a cooling rate of 200 °C/min from 1400 °C to 1100 °C. Further experiments were carried out at a controlled cooling rate of 100 °C/min in order to suppress the sample cracking and delamination effects. This fact demonstrates the necessity of slow cooling rate to obtain crack free samples, when sintering was performed under pressureless condition. A set of 3 experiments were carried out under each sintering condition. The variation in the density measured under the set condition was within 1%. The average of 3 results is presented. Fig. 1 is showing the XRD pattern of starting raw material and final sintered compact which indicates the retainment of single phase structure of CrB2 even after sintering.

3.1. Effect of sintering temperature Table 1 shows the sinter density of CrB2 processed at temperatures between 1600 °C to 1900 °C for 15 min holding period. The density of the sample sintered at 1600 °C was measured as 65%ρth and at 1800 °C, the density was 82%ρth. Fig. 6(a)–(b) showing the scanning electron micrograph of 65% dense CrB2 indicating porous microstructure with predominance of interconnected porosities between the bonded particles. Marginal increase in density (~ 1%) was observed, when the sintering temperature was increased from 1800 °C to 1900 °C. Fig. 7 shows the microstructure of 83% dense sample. CrB2 sample sintered for 15 min duration at 2000 °C was found to be melted (melting point of CrB2: 2200 °C) [10]. This could be due to the temperature difference (~ 200 °C) between the recorded temperature on the crucible surface and the actual sample temperature inside the crucible.

Fig. 2. Particle size distribution of starting material (CrB2) showing the bi-modal distribution with mean diameter as 5 μm.

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Fig. 3. Schematic of pressureless spark plasma sintering assembly. Note: processing temperature was measured by focusing the pyrometer at the outer surface of the modified graphite crucible setup.

3.2. Effect of dwell time Further experiments were carried out to understand the role of dwelling time on densification of CrB2. Table 2 shows the experimental results obtained at different time intervals at temperatures between 1900 °C to 2000 °C. At 1900 °C, CrB2 sintered for 1 min duration yielded a density of 65%ρth, whereas the sample sintered for 15 and 30 min duration results in higher densities of 84%ρth and 88.8%ρth respectively.

Fig. 4. Photograph of in-house built 200 kN spark plasma sintering system, available at Materials Group, BARC, Mumbai.

Fig. 5. Schematic illustrates (a) two and (b) four step of multi-step sintering schedule adopted in the present study.

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K. Sairam et al. / Int. Journal of Refractory Metals and Hard Materials 58 (2016) 165–171 Table 1 Effect of sintering temperature on densification of CrB2 at a fixed dwelling period (15 min). Temperature (°C)

Relative density (%)

1600 1800 1900 2000

65 82 83.3 Sample melted

This implies that there is a significant rise in density (~19%) during the first 15 min of sintering operation; upon further increase of holding period to 30 min, the densification was found to be sluggish. Similar behaviour was observed when the holding time was varied between 3 and 10 min at 1950 °C. This sluggish nature of densification could be due to the activation of non-densifying mechanism (i.e., grain growth process) competing with densifying mechanisms. The maximum sample density achieved was 88%ρth after sintering at 1950 °C for 10 min. Microstructure of sintered CrB2 (88%ρth) shows the densified region with interconnected porosities at the grain boundaries (Fig. 8(a)–(b)). Experiment carried out at 2000 °C for 1 min resulted a density of 84.2%ρth and upon increasing the dwell time to 2 min, the sample was observed to be melted. Microstructure of 84.2%ρth sample sintered at 2000 °C for 1 min reveals the discrete densified regions with intergranular porosities as shown in Fig. 9. The discrete densified region was identified to contain sharp grain boundaries with faceted grains structures (Fig. 10a). Some grains were observed to exhibit terraced structure which could have caused by faceting process (Fig. 10b). Scanning

Fig. 7. Microstructure of 83% dense CrB2 sample processed at 1900 °C for 15 min duration.

electron micrographs of melted CrB2 shows the presence of dendritic structure and river flow pattern which are the characteristics of solidified melt (Fig. 11(a)–(b)). 3.3. Role of multi-step sintering treatment CrB2 samples were subjected to two-step sintering schedule at temperatures between 1900 °C and 2000 °C with different holding period (Fig. 5(a)). The sample which was subjected to sintering at 2000 °C for 1 min followed with second step sintering treatment at 1900 °C for 15 min resulted a relative density of 84%ρth. Upon extending the dwelling period of second step sintering to 30 min, the sinter density achieved was 87.6%ρth which is lower than the density (88.8%ρth) that was obtained when the sample subjected to single step sintering at 1900 °C for 30 min. The reason for this behaviour is explained as follows: While sintering at 1900 °C, the diffusivity in the partially sintered mass (which was initially sintered at 2000 °C) is observed to be slower compared to the sample which has undergone complete sintering at 1900 °C. This two-step sintering requires higher energy for the mass transport compared to the single step sintering. While sintering of cubic-Y2O3 by two step sintering method [17], Chen et.al observed the slower kinetics nature in the frozen microstructure especially during the second step sintering treatment, which is in good agreement with the present study. Further, Chen et.al has successfully adopted two step sintering treatment to obtain high dense material and suppressed

Table 2 Effect of dwell time on densification of CrB2 at different temperatures (1900 °C, 1950 °C and 2000 °C). Temperature: 1900 °C Dwell time (min)

Relative density (%)

1 15 30

65 84 88.8

Temperature: 1950 °C Dwell time (min)

Relative density (%)

3 6 10

81.2 86.6 88

Temperature: 2000 °C

Fig. 6. (a)–(b). CrB2 (65%ρth) sintered at 1600 °C indicating porous microstructures with predominance of interconnected porosities.

Dwell time (min)

Relative density (%)

1 2

84.2 Sample melted

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Fig. 8. (a)–(b). Microstructures of CrB2 sample (88%ρth) sintered at 1950 °C for 10 min showing porosities at the grain boundaries.

the gain growth [17]; whereas in the present study, this two-step sintering logic helped in achieving high density compacts but with exaggerated grain growth. The reason for this different observation about the dwelling period effect (during second step sintering) on grain growth was mainly lies with the operational temperature regime adopted for sintering. Chen et.al [17] sintered Y2O3 at 1200 °C which is

Fig. 9. Microstructure of 84% dense CrB2 sample sintered at 2000 °C for 1 min showing discrete densified regions with intergranular porosities.

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Fig. 10. (a)–(b). Higher magnification of discrete densified regions of 84% dense sample indicates the predominance of faceted grain structures.

~0.5Tm (Tm: melting temperature) whereas in the present study, CrB2 was processed at 2000 °C which is ~0.9Tm. During multi-step sintering treatment, the sample was subjected to heat treatment at 2000 °C for 1 min followed with dwelling at 1900 °C for 15 min and the similar cycle was repeated again as illustrated in Fig. 5(b). The total dwelling time given for the sample during multistep treatment was 32 min (excluding heating and cooling time). CrB2 of maximum density ~ 93%ρth was achieved during the multi-step sintering operation. On comparing the density results of two step and multi-step sintering treatment (Table 3), the reason for increase in density especially during multi-step operation was primarily because of step 3 of sintering treatment that have accelerated the mobility of the atoms similar to the step 1 of sintering treatment. Sonber et.al reported that the CrB2 processed at 2000 °C for 1 h exhibits a density of 88%ρth by conventional pressureless sintering [10]. Pressureless sintering of CrB2 at 1800 °C for 6 h is reported to exhibit a density 93%ρth [10] whereas in the present study, by adopting non-conventional pressureless sintering methodology, sample of similar density (93%ρth) was achieved in just 32 min of dwell time which was very quick compared to conventional pressureless sintering process. The scanning electron microstructure of 93%ρth dense CrB2 sample clearly reveals the presence and distribution of sub-micron porosities (~ 0.7 μm) that are entrapped within the grains (Fig. 12(a)–(c)). The grains of CrB2 were measured to be ~50 μm in size which is one order higher than the initial starting powder size (~ 5 μm). This is evident due to the dominance of grain coarsening behaviour. When the rate of grain coarsening exceeds the rate of sintering, then it would result in

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Fig. 11. (a)–(b). Scanning electron micrographs of melted CrB2 showing the dendritic structures and river flow pattern.

the formation of entrapped porosities. In certain cases, it is reported that the interactions between pore and grain boundary would also contribute for exaggerated grain growth [18,19]. Sintering of alpha-alumina by pressureless SPS resulted in more than 1 order increase in grain size compared with the starting material; this observation is in line with our experimental results [20]. Though, the sample of high density was achieved in shorter duration compared with conventional pressureless sintering, this kind of processing results in enormous grain coarsening and entrapped porosities in the final structures which are considered deleterious from structural application point of view. 4. Summary 1. The modified graphite crucible assembly assisted to perform pressureless sintering operation in spark plasma sintering facility. Table 3 Multi-step sintering parameters and its corresponding sinter density of CrB2 sample. Multi-step sintering experiment

Sintering step

Temperature (°C)

Dwell time (min)

Relative density (%)

1

1 2 1 2 1 2 3 4

2000 1900 2000 1900 2000 1900 2000 1900

1 15 1 30 1 15 1 15

84

2 3

87.6 93

Fig. 12. (a)–(c). Microstructures of 93% dense CrB2 processed by pressureless spark plasma sintering showing the presence and distribution of (a) inter-granular porosities, (b) diffused grain boundary pattern and (c) submicron intra-granular porosities.

2. CrB2 of densities in the range of 88% to 89% was obtained by sintering at temperatures of 1900 °C and 1950 °C for 30 min and 10 min respectively. 3. A higher density close to 93%ρth was achieved by multi-step sintering between 1900 °C to 2000 °C with one order increase in grain size. 4. Presence of sub-micron pores within the grains of CrB2 indicates the dominance of grain coarsening mechanism.

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Acknowledgments The author wishes to thank Shri. C. Subramanian, Ex-scientist and Raja Ramanna Fellow, BARC for sharing the valuable inputs during SPS experimentation and Shri. Mahesh B, Post Graduate Student, NIT Warangal, India for his involvement in CrB2 cold pellet preparation and SPS operation during his project work at MPD, BARC.

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