This work reports the oxidation and crack healing behavior of a fine-grained (~2 lm) Cr2AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C– 1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin a-Al2O3 layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X-ray tomography. Three-point bend tests showed the dependence of strength recovery at 1100° Ca s a function of initial crack length and healing time.

Oxidation and Crack Healing Behavior of a Fine‐Grained Cr2AlC Ceramic

Recently, the authors developed Si 3 N 4 , Al 2 O 3 and mullite ceramics with good self-crack-healing abilities It was shown that the optimized crack-healing condition to get high temperature strength was: 1573 K, 1 h, in air, and the healed zone exhibited the same strength as the base material up to about 1573 K (Si 3 N 4 and Al 2 O 3 ) and 1473 K (mullite), respectively Using this good crack-healing ability, a new methodology to guarantee the reliability of ceramic components [crack-healing + proof test] was proposed It was shown that reliability could be guaranteed before service by this technique, using about 200 samples However, if a crack initiated during service, reliability would be severely impaired Therefore, if a material can heal a crack during service, and if the healed zone has enough strength at the temperature of healing, it would be very desirable for structural integrity From the above points of view, a new methodology to guarantee the structural integrity of ceramic components using in situ crack-healing ability was proposed and the usefulness is discussed using the test results in terms of crack-healing behavior and proof test theory by the authors

Crack-healing ability of structural ceramics and a new methodology to guarantee the structural integrity using the ability and proof-test

Crack-healing behavior of zirconium diboride composite reinforced with silicon carbide whiskers

Structural ceramics, including ultrahigh-temperature ceramics such as zirconium diboride (ZrB2) and hafnium diboride (HfB2), are brittle and sensitive to flaws. As a result, the structural integrity of this ceramic component may be affected seriously. In this paper, a novelty technique namely preoxidation was proposed to overcome this problem. A ZrB2-based composite containing 20 vol% SiC whisker was produced by hot pressing. The composite was preoxidized at temperatures of 800° and 1200°C for various times ranging from 10 to 180 min in air. The flexural strength and microstructure of specimens were investigated systematically under each preoxidation condition. The strength was improved after preoxidation, with a maximum increase of strength (15%) for samples after preoxidizing at 800°C for 180 min. Microstructure analysis suggested that the increase in strength was due to the self-crack-healing behavior of the composite by surface oxide glass, such as B2O3-, SiO2-rich liquids, and borosilicate glass. However, oxide evaporation and formation of gas (i.e., B2O3 and CO) were found to be detrimental to for the crack-healing ability. Oxidation behavior during the preoxidation process followed parabolic-law kinetics.

Preoxidation and Crack‐Healing Behavior of ZrB2–SiC Ceramic Composite

In recent years, numerous studies have been conducted on the crack-healing behavior of ceramic materials. Design of ceramic structures with the ability of crack-healing in service is one of the most important objectives of many scientists. Development of such structures in fuel cells, aerospace, automotive and military industries can save a lot of money from a maintenance cost viewpoint. This paper addresses the most recent findings from researchers in the field of crack-healing in ceramics and discusses future perspectives in this research area.

Crack-healing in ceramics

Si 3 N 4 /SiC composite ceramic specimens, made to JIS standard, were sintered and subjected to three-point bending. Semi-elliptical surface cracks of 50–400 μm in diameter were made on the tensile side of each specimen. Crack-healing behavior as a function of environment, temperature, time, and crack size, and oxidation behavior as a function of temperature and time were studied. The main conclusions are as follows: (1) Cracks healed completely in air, but did not heal in Ar gas, N 2 gas nor in a vacuum. (2) This Si 3 N 4 ceramic has the ability to heal a crack at temperature from 900 to 1400 °C completely. (3) The maximum surface crack size that can be healed completely was 200 μm in diameter. (4) The activation energies for crack-healing and oxidation were 150 and 131KJ/mol, respectively.

Sung-Po Liu

Papers

Crack-healing and oxidation behavior of silicon nitride ceramics