Heating rate effects on the thermal and mechanical properties of ZrB₂

Abstract

Zirconium diboride ceramics were densified by hot pressing and spark plasma sintering with heating rates varying from 5 to 300℃/min. Slower heating rates produced larger grains due to the longer times spent at temperatures between 1500 and 1900℃, which is the temperature range in which ZrB2 grains coarsen. Heating rates above 50℃/min resulted in rapid densification, but this led to the retention of up to 3.3 vol.% of ZrO2 particles in the ceramics. After densification, changes to the microstructure were evaluated to interpret the effects of heating rate on thermal and mechanical properties. The flexure strength of ceramics processed by hot pressing up to 80℃/min was proportional to the inverse square root of the maximum grain size based on the Griffith criteria. Conversely, densification by spark plasma sintering, which had heating rates of up to 300℃/min, resulted in microcracks that decreased the elastic modulus from >500 GPa for pristine specimens to <485 GPa for microcracked materials. The use of heating rates >20℃/min also reduced the thermal conductivity due to the presence of retained ZrO2, but improved the strength by reducing the maximum grain size.

Mechanical properties of ZrB2 ceramics determined by two laboratories

“…The measured elastic modulus was 529 ± 17 GPa, the shear modulus 231 ± 7 GPa, and Poisson's ratio .14. The elastic property values are in excellent agreement with previously published values for hot‐pressed ZrB 2 14–18 …”

“…The elastic property values are in excellent agreement with previously published values for hot-pressed ZrB 2 . [14][15][16][17][18] The average flexure strength and fracture toughness values obtained by the two laboratories along with the pooled data from both sets are shown in Table 1. A Weibull analy-sis † † † was performed on the individual strength data sets.…”

“…When pooled, the average strength across both sets of strength bars was 458 ± 77 MPa, which is about 15% higher than previously published values for a hot-pressed ZrB 2 that were obtained using the same specimen size and loading configuration. 14,15 Strengths for ZrB 2 of 480 MPa, 16 508 MPa 18 , and 565 MPa 17 have been reported, but these values were obtained using the smaller "A" size flexure bar (1.5 × 2 × 25 mm) and associated 4-point loading configuration outlined in ASTM C1161. Strength size-scaling, according to ASTM C1683 ‡ ‡ ‡ , of the pooled average flexure strength and Weibull modulus from this study to the smaller specimen dimensions yields a value of 560 MPa.…”

Ultrafast high‐temperature sintering of ZrB2

“…No evidence of melting and recrystallization, glassy phases, or decomposition was observed in the SEM images (Figure 2). Furthermore, the heat‐up time to the estimated temperature of 2630°C was only ∼30 s, which is notably faster (10 3 –10 4 °C/min) than prior studies (300°C/min) 23 . These results indicate that UHS at temperatures notably greater than T GBD , and close but below T m for just seconds, may yield the best sintering conditions by which to favor lattice diffusion over surface diffusion, without incurring notable grain growth, melting, or recrystallization.…”

“…Furthermore, the heat-up time to the estimated temperature of 2630 • C was only ∼30 s, which is notably faster (10 3 -10 4• C/min) than prior studies (300 • C/min). 23 These results indicate that UHS at temperatures notably greater than T GBD , and close but below T m for just seconds, may yield the best sintering conditions by which to favor lattice diffusion over surface diffusion, without incurring notable grain growth, melting, or recrystallization. A detailed study will be conducted in the future to understand the densification mechanism in UHS.…”