Influence of Carbon Nanotubes‐Based Coatings on the High‐Temperature Compression Strength of Al₂O₃‐C Foam Filter Structures

Abstract

Materials for steel‐melt filtration have to fulfill different thermomechanical requirements and especially withstand the thermal shock attack in the beginning of casting. In particular, carbon‐bonded alumina materials provide an excellent thermomechanical performance and have been used in functional components such as submerged entry nozzles, stoppers, and sliding gates for several years. The present study investigates the mechanical high‐temperature behavior of Al2O3‐C foam filter structures with coatings based on carbon nanotubes+Al2O3‐nanosheets (CNT‐ANS) and carbon nanotubes+Al2O3‐nanospheres (CNT‐ANB) at 1100 and 1450 °C. The foam filters are tested in an electromechanical testing machine under argon atmosphere in quasi‐static compression. The tests show a brittle material behavior at 1100 °C with higher compressive strength of the coated filter samples. At 1450 °C, a loss of strength occurs, accompanied by an increase in plastic deformation. Rupture of struts and behavior of the coatings are microstructurally analyzed by scanning electron microscopy. As the mechanical load during the tests is much longer than in real immersion tests, the high‐temperature performance is considered trustworthy.

Compression Behavior of Carbon‐Bonded Alumina Spaghetti Filters at Room and High Temperatures

“…[11] The hollow strut thickness of an uncoated Al 2 O 3 -C foam filter analyzed by CT exhibited the following values: d 50 ¼ 1.34 mm and d 90 ¼ 1.82 mm. [30] With an additional AC6.3 coating, d 50 and d 90 increased to 1.62 and 3.04 mm, respectively. If an additional alumina coating was applied instead, d 50 and d 90 changed to 1.85 and 2.90 mm, respectively.…”

“…Compression tests for cylindrical Al 2 O 3 -C foam filters of %Ø 20 mm  25 mm height at high temperatures were reported previously. [28][29][30] The compositions as well as some parameters and testing conditions of these filters are shown in Table 2. All these filters were prepared through the replica method using ten pores per inch polyurethane foam templates.…”

“…[31] As regard to foam filters, a higher coking temperature may increase the mechanical strength of coated foam filters due to a stronger shrinkage of the coating materials compared with the bulk. [13] For uncoated foam filters, no significant difference was found within scatter for both coking temperatures comparing the compression test results at 1100 C [28,30] as well as at 1300, 1400, and 1500 C. [28,29] These previous studies revealed that Al 2 O 3 -C foam filters were brittle below 1300 C and ductile at and above 1400 C. For uncoated foam filters, Solarek et al [28] showed that the average compressive strength of filter samples with 20 mm diameter and 25 mm height at 800 C was %120 N. The values at 1000, 1100, 1200, and 1300 C were even lower (%90-110 N). The uncoated foam filters tested by Ranglack-Klemm et al [30] exhibited compressive strength of 100-190 N at 1100 C, whereas the CNT-ANS-and CNT-ANB-coated ones were 100-300 and 130-220 N, respectively.…”

“…[13] For uncoated foam filters, no significant difference was found within scatter for both coking temperatures comparing the compression test results at 1100 C [28,30] as well as at 1300, 1400, and 1500 C. [28,29] These previous studies revealed that Al 2 O 3 -C foam filters were brittle below 1300 C and ductile at and above 1400 C. For uncoated foam filters, Solarek et al [28] showed that the average compressive strength of filter samples with 20 mm diameter and 25 mm height at 800 C was %120 N. The values at 1000, 1100, 1200, and 1300 C were even lower (%90-110 N). The uncoated foam filters tested by Ranglack-Klemm et al [30] exhibited compressive strength of 100-190 N at 1100 C, whereas the CNT-ANS-and CNT-ANB-coated ones were 100-300 and 130-220 N, respectively. According to a recent study, [29] uncoated foam filters as well as those coated by AC6.3 and alumina had, considering the high scatters, compressive strengths less than %400 N. All these values are significantly lower than the compressive strength of the spaghetti samples tested at 800 C in this study (i.e., 1532 AE 390 N).…”

“…All these factors also apply to the previously studied foam filters which exhibited such kind of scatter as well. [28][29][30] One reason for the superior compressive strength of the spaghetti filters, either as brittle material at lower temperatures or as ductile material at a higher temperature, should be the stronger full struts which are considered as one of the major advantages against the foam filters with hollow struts. The hollow space with a triangular cross section in the middle of a strut was inevitable in the case of foam filters prepared by the replica method.…”