A, frontier

    Honeycomb ceramics, one of the three pillars of modern materials, has been widely used in modern industry because of its special physical and chemical properties. However, because of all sorts of man-made or natural, honeycomb ceramic often produces in the process of manufacture or use of this or that defects, these defects exist more or less affect the performance of the honeycomb ceramic limits the popularization and application of honeycomb ceramics, some even have disastrous consequences, even regular honeycomb ceramic products also have the existence of defects, especially the presence of surface defects, so seriously affect the product quality and grade.

    In modern industrial production, the production process is gradually automated, which also requires continuous and rapid testing of products. At present, in the honeycomb ceramics industry (especially RTO honeycomb ceramics), it is often used to judge whether there are defects in the products by human eye observation and tapping and hearing. It is necessary to study a new nondestructive testing method which is not suitable for modern industrial production.

    So-called nondestructive testing (NDT), as the name implies, it is without damage (damage) materials and products under the condition of the material means of internal and surface with and without defects, that is, it USES the internal structure of the material caused by abnormal or the presence of defects such as the sound, light, electricity, magnetic, heat, radiation response characteristics of the changes, such as evaluation of structural abnormalities and flaws and its degree. The main purpose of NDT is product quality management, product quality appraisal and equipment maintenance testing.

Defects of honeycomb ceramics

    The nondestructive testing of honeycomb ceramics is generally divided into defect testing and deformation testing.

    Honeycomb ceramics is a typical brittle material, and young's modulus is generally high, even small defects or light deficiency strain will lead to great mechanical stress. It quickly causes damage, especially near the outer surface. Therefore, the actual size of defects detected should be l-2 orders of magnitude smaller than that of metal or compound materials.

Defects as test objects can be divided into:

    1. Internal defects and surface defects mainly introduced in the manufacturing process, such as surface and internal cracks, surface slagging and shrinkage shaft, surface and internal pores, etc. Surface defects are the most harmful, not only affecting the mechanical properties and physical and chemical properties of the products, but also affecting the appearance quality of the products.

    2. Surface defects mainly introduced in the process of mechanical processing and treatment, such as surface cracks and surface missing edges and angles, etc.

    3. Defects in the use (provided for use) of products or caused by the environment. Among these defects, crack-like defects (internal or surface cracks) have the greatest influence on the mechanical properties of materials, which is also a common defect in honeycomb ceramics. The internal porosity and interlayer are also the defects frequently appeared in honeycomb ceramics.

    Nondestructive testing methods of honeycomb ceramics at home and abroad

    At present, the nondestructive testing methods of honeycomb ceramics at home and abroad are summarized as follows:

1. Ultrasonic picking (UT method)

    Ultrasonic detection is to detect whether there are defects in the interior or surface of materials by measuring the changes of reflection cloak. This method is especially suitable for revealing the area type defects, such as cracks, delamination, etc. It is generally difficult to detect crack-like internal defects below 100 microns.

2. Ray picking and measurement

    Ray detection is a method to detect the internal defects of materials by the penetrating force of ray to various substances. X-ray detection includes X-ray detection, gamma detection, neutron ray detection, alpha ray detection, beta ray detection, etc. At present, X - ray and gamma - ray detection are widely used in non - destructive testing. In order to improve the detection accuracy, ray detection is generally used in conjunction with other technical means. X-rays can detect internal cracks of more than 750um, while experienced personnel can find internal defects of 150um.

3. Penetration testing

    Also known as liquid penetration detection method. It is an old nondestructive testing method, which is commonly used in conventional nondestructive testing at present. This test USES some properties of the liquid and makes use of these properties to conduct good penetration of material surface defects. When the imaging solution is sprayed on the surface of the workpiece, the residual penetrant in the defect will be sucked out. The defect traces are formed to achieve the purpose of flaw detection. Liquid penetration detection is generally limited to surface cracks above 150um.

    The above three methods are traditional nondestructive testing methods. In recent years, with the development of related technologies, new nondestructive testing methods keep appearing, especially the combination of ultrasonic testing and ray testing with other technologies, which greatly promotes the progress of nondestructive testing technology. Main new detection methods are described as follows:

4. Computer-assisted ultrasonic testing

    This detection technique has been used in Japan to detect defects in honeycomb ceramic combustion Chambers. It is the whole system automatic operation, the detection frequency is controlled by the computer, can detect the diameter of defects in more than 50 microns.

5. Computer aided tomography

    CT technology has also been introduced into the detection field of honeycomb ceramics. For example, in 1987, atomic energy Canada used CT system to detect advanced honeycomb ceramics such as ZrO2, SiC and A12O3, which can accurately detect defects of l0 to 100um and extremely small density distribution. The Argonne national laboratory in the United States used CT to detect cracks, gaps, and fake solder joints in ceramic wafers only 50um thick used for fuel cells. In 1988, the laboratory also used a 25-200kv X-ray CT system to detect gaps and directions in SiC fiber composites. The X-ray tomography (CT) method used at present can form tomography images, and can detect the crack openings of 30 microns, the minimum crack openings of 10 microns and the body density difference of 0.025‰. But the method is not sensitive enough to detect plane cracks with small openings. The plate-like cracks are said to be between 300 and 400 microns in size, and the equipment is very expensive.

6. Ultrasonic microscopy

    The ultrasonic wave of l0o-1000mhz is reflected and scattered on the surface of the sample. By detecting and analyzing the ultrasonic wave, the high resolution of nearly one micron can be obtained quantitatively. Because of high frequency ultrasonic attenuation, this method is only suitable for detecting defects under the skin. At present, the detection of skin defects of tens of microns is the limit, and it is difficult to apply to the detection of honeycomb ceramics. At present, people expect this method to be used as the research method of honeycomb ceramics in laboratory.

7. Laser scanning ultrasonic microscopy (SLAM method)

    The transmitted ultrasonic information is changed into visual information by laser incident medium as non-contact sensor. This kind of ultrasonic display mirror is a new type of ultrasonic display mirror. At present, the commonly used frequency is 100MHz, and the image resolution is said to be 15 microns. The effectiveness of the evaluation technique for honeycomb ceramics is constantly being proved. In particular, it has the feature of real-time observation with the inner eye. Compared with X-ray, it can detect plane defects, and it is possible to identify inclusions, pores and punctated plane defects. However, it is inconvenient to operate and difficult to apply to actual components.

8. Synchronous microscopic focusing X-ray fluoroscopy

    The system USES video probes instead of film to display X-ray images of honeycomb ceramics. Synchronization system has the advantage of energy of the X ray can be adjusted to match the target object of transmission, workers can adjust the source voltage optimization, the accept exploration area of view rift with "bright/dark spots", according to greatly improve the reliability of recognition, the method to detect the critical area of the cracks is ideal.

9. Acoustic emission (AE)

    Acoustic emission technology detection is accompanied by solid deformation, phase change and elastic wave generated by fault surface. This technology has the following characteristics: it can detect microcracks, determine the location of cracks, and is applicable to honeycomb ceramics with complex shapes. Quantitative data of microcrack area and cracking time can be obtained by analyzing the original waveform. Because of these advantages, acoustic emission detection has developed rapidly in recent years and is a promising detection method. It is said that microcracks of one to five meters can be detected by conventional acoustic emission measurements, but the development of such detection technology is not mature at present.

10. Surface acoustic wave technology (SAW)

    The technique USES the propagation properties of ultrasonic waves generated by laser beams to detect objects. When a defect is encountered, the ultrasound produces a higher amplitude shift, which can be detected by a laser interferometer. The detection technology is far from practical.

11. Thermal wave imaging technology

    It USES the heat transfer characteristics of parts to diagnose and detect surface fractures and defects close to the surface. The presence of defects can be judged by locally heating the surface with a modulated light source such as a focused light or electron beam, and by measuring the effect on the surface temperature noncontactively using optical or acoustic means. The system is very sensitive to small cracks. Broadly speaking, infrared nondestructive testing technology also belongs to thermal wave imaging technology. This technique has been used to detect the adhesion of ceramic thermal insulation coating on the piston of diesel engine.

12. Optical microscope

    Also known as photoacoustic imaging. Focusing and scanning a laser of 10 to several MHz on a honeycomb ceramic surface, the heat generated by absorbing the laser is detected by microphones as a change in gas pressure in the low frequency range and by lead zirconate titanate (PTZ) as a thermal deformation wave in the high frequency range. There are few reported instances. However, it is said that micron-scale defects and non-uniform parts on the surface of silicon nitride (Si3N4) samples have been observed by photoacoustic images. Although it is still far from being practical, people have great hopes to detect the stereoscopic distribution of defects in solids.

    So far, honeycomb ceramic detection methods, in addition to manual detection, basically these 12. It can be divided into two categories: nondestructive testing of internal defects and nondestructive testing of surface defects.