Effect of LaB6 additions on densification, microstructure, and creep with oxide scale formation in ZrB2-SiC composites sintered by spark plasma sintering

Sala

2021

Metall Mater Trans A

Self Cite

The effect of LaB 6 (7, 10, and 14 vol pct) addition on oxidation behavior of spark plasma-sintered ZrB 2 -20 vol pct SiC composites involving heating under non-isothermal condition till 1400 °C, and isothermal and cyclic exposures at 1300 °C to 1500 °C have been examined. The mass gain of the composite with 14 vol pct LaB 6 is found as the highest during non-isothermal oxidation, whereas it appears as the least on isothermal exposure for 24 hours at 1400 °C or 1500 °C. The mass gain after 24 cycles of 1-hour exposure at 1300 °C to 1500 °C is found to be lower than that recorded under isothermal condition for all the composites, with the difference decreasing with LaB 6 content. The oxidation exponent (n) is found to decrease to near parabolic rate law with the increase in cyclic exposure temperature. The parabolic rate constant (k p ) is found to decrease with the increase in LaB 6 content after a few cycles of exposure at all the investigated temperatures. The oxide scales formed on isothermal exposure at 1400 °C or 1500 °C have shown a thin La 2 Si 2 O 7 layer with borosilicate glass (BSG) as the outer layer, followed by layers containing BSG along with coarse and fine ZrO 2 , and a SiC-depleted ZrB 2 layer. The oxide scales formed during cyclic exposure contain an outer compact layer comprising BSG, La 2 Si 2 O 7 , and ZrSiO 4 , followed by ZrO 2 + BSG. Additionally, a SiC-depleted layer is found at the oxide-composite interface on cyclic exposure at 1500 °C for 24 hours, and at 1400 °C for 100 hours. The outer compact layer appears to have a critical role in protection against oxidation, with k p decreasing with the increase in its thickness.

Section: A Microstructurementioning

confidence: 99%

Oxidation Resistance and Evolution of Multi-layered Oxide Scale During Isothermal and Cyclic Exposure of ZrB2–SiC–LaB6 Composites at 1300 °C to 1500 °C

Sala

2021

Metall Mater Trans A

Self Cite

“…% LaB 6 composites containing B 4 C and C as additives has shown the formation of an outer continuous layer comprising a mixture of La 2 Zr 2 O 7 and La 2 Si 2 O 7 , followed by ZrSiO 4 , ZrO 2 + SiO 2 + SiC, and then an inner layer of SiO 2 at the oxidecomposite interface (Fig. 15) [55]. A similar layered oxide scale has been also observed in the case of the creep-tested ZrB 2 -SiC composite containing 10 vol.% LaB 6 , with the outermost La-rich layer being discontinuous.…”

Section: Oxidation Behavior During Creepmentioning

confidence: 99%

“…The thickness of the oxide scales formed during creep tests under 47 MPa stress or isothermally exposure for same duration without stress at 1300 C[55] …”

mentioning

confidence: 99%

High-Temperature Environmental Degradation Behavior of Ultrahigh-Temperature Ceramic Composites

Mallik

Handbook of Advanced Ceramics and Composites

2019

“…In a recent study on the ZrB 2 ‐20 vol% SiC‐(7, 10, or 14 vol%) LaB 6 composites prepared by SPS at 1600°C with B 4 C and C as the additives, the oxidation resistance and the protective character of the multilayered oxide scale formed on the surfaces during isothermal or cyclic exposure in the range of 1300–1500°C have been found to scale with the LaB 6 content 28 . Further, the effect of LaB 6 content on the compressive creep behavior at 1300°C with simultaneous oxide scale formation in the ZrB 2 ‐20 vol% SiC composites prepared by SPS at 1800°C has been previously studied, and the results have been published elsewhere 18 . As the steady‐state creep rates were determined only at a single temperature (1300°C), it was not possible to determine the activation energy in the earlier study.…”

Section: Introductionmentioning

confidence: 99%

Effect of LaB₆ addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB₂‐SiC composites

2022

J Am Ceram Soc.

Self Cite

A study has been carried out to examine the effect of LaB6 addition on the compressive creep behavior of ZrB2‐SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB2‐20 vol% SiC composites containing LaB6 (10% in ZSBCL‐10 and 14% in ZSBCL‐14) besides 5.6% B4C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL‐14 composite has exhibited a lower steady‐state creep rate at 1300°C than the ZSBCL‐10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB2 grain boundaries and ZrB2‐SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL‐10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL‐14 composite at 1300–1400°C suggest that ZrB2 grain boundary diffusion is the rate‐limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation‐induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep‐based deformation and damage at the inner core of samples.