Self-healing ceramic working at low temperature

A team of researchers recently published an article in the journal Materials which demonstrated the healing capacity of ceramic composites at low temperatures.

Study: Self-healing of SiC-Al2O3-B4C ceramic composites at low temperature. Image Credit: A Mazhor/Shutterstock.com

context

Ceramic is widely used in some complex environments due to its high flexural strength and hardness. However, the fragility is a major drawback of the material. Different curing mechanisms and systems used to solve the problem produce micro cracks in the material and degrade the overall performance of the ceramic composite. Thus, attention shifted to the self-healing mechanisms of ceramic materials, where cracks could heal spontaneously before causing serious damage.

(a) X-ray diffraction pattern, backscattered electron micrographs of phase composition (b) Vickers indentation (c) and fissure (d) of SiC-Al2O3-B4C ceramic composite.

(a) X-ray diffraction pattern, backscattered electron micrographs of phase composition (b) Vickers indent (vs) and crack (D) of SiC-Al2O3-B4C ceramic composite. Image Credit: Wang, B et al., Materials

Appropriate healing agents have been applied in ceramic composites to introduce crack healing capability. However, wound healing agents were mainly applied at moderate and high temperatures until now. Studies on the application of healing agents at low temperatures (ohC) were rare. In this study, the effect of healing time and healing temperature of B4C (15% mass fraction) on crack healing behavior in SiC/Al3O2 the basic ceramic was studied. The healing behavior was analyzed on the macro-properties and the microstructure of the ceramic.

The study

The raw materials used in the preparation of the sample for the study include glass powder, B4C,Al3O2, and SiC. A spark plasma sintering (SPS) process was used to prepare a sintered sample. The precision cutting machine was used to cut the sample into rectangular strips of size 3 × 4 × 20 mm. The cracks of the sample were prefabricated using a Vickers hardness test to simulate the cracks on the surface of the ceramic material, and the length of the cracks was measured by controlling the load.

The cracks in the sample were healed for different temperatures (Th = 600-800ohC) and times (th=0-300 min) with a heating rate of 10 ohC/min in air. The electronic universal testing machine was used to measure the strength of the specimen after and before three-point bending healing. Three-point flexion was performed at a loading rate of 0.5 mm/min with a span of 20 mm.

X-ray diffraction (XRD) was used to identify the phase composition of the ceramic composite using Cu Kα radiation. A field emission scanning electron microscope (FE-SEM) was used to observe the surface morphology of the cracks in the sample.

A spectrometer attached to the FE-SEM was used to perform energy dispersive X-ray spectroscopy (EDS) analysis of the sample. The chemical compositions of the surface oxides were studied using X-ray photoelectron spectroscopy (XPS), while the thermodynamic chemistry software HSC was used to calculate the thermodynamic data during the stoichiometric phase.

SEM morphology of Vickers indentation and fissures after healing at (a,b) 600°C, (c,d) 700°C and (e,f) 800°C for 60 min.

SEM morphology of Vickers indentation and fissures after healing at (a,b) 600°C, (vs,D) 700°C and (and,F) 800°C for 60 min. Image Credit: Wang, B et al., Materials

Comments

Apart from Al crystal phases2O3 and SiC, no other side reactions were observed in the XRD pattern of sintered ceramic composites during high temperature sintering. Healing Agent B4C was uniformly dispersed in the SiC/Al2O3 matrix, indicating the prerequisite for the healing of cracks.

Observations of XRD patterns after sample cure at 600-800ohC for 60 min indicated that Al2O3 and SiC did not participate in the oxidation process, as their diffraction peak intensity remained the same at each temperature. The microstructure of the cracks heals at different temperatures for 60 min. The length of cracks reduced after annealing the sample to 600ohC, indicating partial cure.

The cracks disappeared completely and merged with the matrix when the specimen was healed at 700-800ohC. However, observations of the Vickers indentation morphology indicated that the cracks annealed at 700ohC at different times. Although the cracks disappeared completely after the sample was healed for 30 min, several pores were produced after the sample was healed for 300 min. Thus, curing the sample for a long time resulted in multiple defects and multiple pores in the sample, adversely affecting the properties of the ceramic material.

The flexural strength of the pre-crack specimens was healed at different temperatures for 60 min. After healing of the sample at 600ohC, the flexural strength was partially recovered, while at 700ohC the flexural strength recovered significantly. However, the flexural strength decreased at 800ohC due to increased rate of oxidation. At 700ohC, the flexural strength substantially recovered when the specimen was healed for 30 min. B4C showed optimal self-healing behavior at 700-800ohVS

As the oxidation reaction of healing agent B4It was mainly responsible for the crack healing behavior, the recovery of the flexural strength was mainly dependent on temperature and healing time. Observations from the XPS analysis indicated that B4C participated in oxidation-induced crack healing, while observations of SEM morphologies also indicated that B4It is effective in repairing cracks.

Schematic diagram of the three-point bending of the sample.

Schematic diagram of the three-point bending of the sample. Image Credit: Wang, B et al., Materials

Significance of the study

This study demonstrated that rapid healing of precast cracks at low temperatures improves the mechanical properties, service life and utilization rate of ceramic materials. Cracks in the composite can be completely healed at 700ohC for 30 min, resulting in substantial recovery of flexural strength. The good healing capacity of SiC-Al2O3-B4The low temperature C ceramic composite can make the material very useful in several applications in the future.

Source

Wang, B., Tu, R., Wei, Y. et al. Self-healing of SiC-Al2O3-B4C ceramic composites at low temperature. Materials 202215, 652. https://www.mdpi.com/1996-1944/15/2/652

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