Testing technology for surface characteristics of ceramic bearing parts

Abstract: Discusses the detection technology of surface characteristics of ceramic bearing parts, including optical method, ultrasonic method, acoustic emission, dyeing method and ray method, and introduces the advantages and disadvantages of these detection methods in detail, as well as the corresponding items that can detect the surface characteristics of ceramic bearing parts.

Keywords: ceramic bearing; spare parts; Surface characteristics; testing

Engineering ceramic materials such as silicon nitride used in rolling bearings have the advantages of wear resistance, high temperature resistance, corrosion resistance, nonmagnetism, low density, small thermal expansion coefficient and insulation. They are suitable for working in special environments such as high temperature and high speed, corrosion resistance, non-magnetic, oil-free lubrication, and light weight, and have attracted more and more attention. Due to the limitations of ceramic manufacturing technology and material characteristics, there are often many defects on the surface or inside of ceramic bearing parts (mainly rolling elements and rings), such as cracks, pores, inclusions and additive segregation, as well as changes in the physical mechanical properties of the surface layer. This will not only reduce the friction and wear properties, leading to a significant reduction in strength, but also seriously affect the service performance and fatigue life of the bearing. Therefore, the surface characteristics of bearing related parts shall be strictly checked during the processing and operation test, that is, in addition to the size, shape accuracy and surface roughness of the parts, the defect characteristics of their surfaces shall also be tested. In order to promote the research on the method of non-destructive testing of the surface characteristics of ceramic bearing parts, this paper discusses the corresponding main testing technologies according to the tests and studies on the rolling contact fatigue, wear, lubrication, machining and surface coating of related parts of ceramic bearing performance at home and abroad in recent years.

1 Detection of surface roughness, spherical error and hardness

1.1 Surface roughness and waviness

Like ordinary bearings, the measurement methods of surface roughness of ceramic bearing parts can be divided into contact type and non-contact type according to whether they are in contact with the measured surface. At present, the most commonly used contact roughness tester is the stylus profilometer (such as Talysurf - 6).

The non-contact detection technology is the interference microscope method and laser speckle method used to measure the steel ball. They do not contact the measured surface during the measurement process, so they will not affect the morphology of the measured surface. It is generally considered that Ra is not enough to evaluate the surface roughness, and Ry must also be added, because the latter has a significant impact on the surface of ceramic balls (such as scratches and pits). Therefore, in order to ensure the uniqueness of the measurement results, JB/T7051 93 stipulates that Ra of the steel ball is subject to the profiler and Ry is subject to the interferometer.

The spherical error of ceramic ball is usually measured by bearing tester or roundness meter (such as Talyrond 300 roundness meter). In order to reduce the measurement error, we should pay attention to the definition of football error as much as possible and the statistical law of data collection.

It should be pointed out that little attention has been paid to the waviness of spherical surface at present, which is usually used to describe the random or repeated surface waviness between the shape deviation and surface roughness. The waviness of the ball will cause high frequency vibration, which is very unfavorable to the use of high-speed ceramic bearings. Therefore, it is necessary to strengthen the research in this area.

1.2 Microhardness

Vickers or Knoop hardness tester is usually used to measure the hardness of ceramic parts. Compared with other measurement methods, the indentation obtained by Vickers hardness has similar geometric shape and high measurement accuracy. Knoop hardness has many characteristics compared with Vickers hardness: the long diagonal length of Knoop indentation is 2.8 times longer than Vickers, which can improve the accuracy of length measurement. Because the Knoop hardness indentation is very shallow, it is particularly sensitive to the sample surface, and can be used to detect the burning area, softening area, hardening deformation scar, uneven area of surface treatment and other suspicious defects on the sample surface. In addition, Knoop hardness can withstand greater indentation load without cracks, but the advantage of Vickers hardness is that it can determine the fracture toughness Kv and KIC values of ceramic materials in a single simple test process.

2 Defect detection

Optical method, ultrasonic method, acoustic emission, dyeing method and ray method can be used to detect the defects such as holes, peeling, scratches, micro scratches or microcracks on the surface of parts, as well as internal delamination, porosity and porosity.

2.1 Optical microscope (OM)

Optical microscope is the most commonly used and simplest tool to directly observe and analyze the microstructure and surface morphology of materials, with a resolution of about 200 nm and a magnification of about 1000 times. In order to make full use of the resolution, replica metallography (or replica metallography) is sometimes used, but it can only intuitively reflect the organizational morphology at the size of 100 nm, while it is powerless for smaller organizational structures and units (such as dislocations). At the same time, it can also be used to check the failure fracture morphology on the wear surface, observe and analyze the morphological characteristics, surrounding conditions, hardness measurement and fracture morphology of the cracks.

2.2 Ultrasonic testing, scanning acoustic microscope (SAM)

Ultrasonic testing is to collect and analyze the reflected sound wave from the depth of a given bearing part to display the image of its microstructure by using the directivity and other characteristics of the ultrasonic wave, so as to quantitatively determine the size, depth, shape and distribution of defects, and also measure the viscosity, temperature, hardness and other properties of the part materials. The most common frequency of ultrasound is between 500 kHz and 20 MHz.

The scanning acoustic microscope, developed on the basis of ultrasonic technology, is a nondestructive testing technology to reveal internal micro defects. Its working principle is to send a signal with an acoustic frequency of about 200 MHz, collect reflected waves and convert them into electrical signals to obtain images. It can not only detect defects, but also accurately determine the location of defects and give quantitative values of the size and depth of defects. It is applicable to the verification of optical microscope observation and quantitative detection of cracks and subsurface damage that cannot be observed by optical microscope. The scanning width is 0.5~3.75 mm, and the depth can be explored 5~30 μ M (beyond the limit of conventional ultrasonic testing), and can determine the characteristics of fiber matrix interface in composite ceramic materials.

2.3 Dyeing method (FDP)

With high resolution penetrant and advanced lighting technology and detection means, very small defects on the surface of parts (which have been successfully used in mass production of silicon nitride balls) can be effectively detected, but these defects are usually invisible to the naked eye. If ceramic bearing parts are immersed in highly sensitive fluorescent dye penetrant, it is easy to observe the penetrant that has penetrated under the surface of the element, so as to obtain the range and shape image of small surface cracks and peeling. The disadvantage of dyeing method is that it is unable to reliably detect tiny cracks, especially when detecting porous materials, its background difference may obliterate the difference of defects. In addition, it is not suitable for rapid automatic detection and chemical pollution problems.

3 Ray method

Also known as electron microscope technology or analytical electron microscope AEM, it is a comprehensive testing technology. In particular, high-energy electron beams are used as light sources to illuminate the sample to be analyzed, and magnetic fields are used as lenses to collect and analyze various signals (such as secondary electrons, Auger electrons, etc.) excited by the sample that reflect the local information of the area directly irradiated by the sample, which can not only obtain the high resolution and high magnification surface morphology of the small areas in the part sample, Relevant data of crystal structure and defects (crystallographic parameters) and chemical composition can also be obtained. It mainly includes the following types: scanning electron microscope (SEM), transmission electron microscope (TEM), Auger electron spectrometer (AES), X-ray photoelectron spectrometer (XPS), electron probe instrument (EPMA) and atomic force microscope (AFM).

4 Detection of residual stress

The surface residual stress is a very important index in the surface characteristics of ceramic bearing parts. At present, a non-destructive non-contact testing method widely used is X-ray diffraction (XRD). The disadvantage is that it is not as intuitive as optical microscope, so it is impossible to combine morphology observation with crystal structure analysis microcosmically; The measuring depth is only a few microns to tens of microns below the surface of the sample, and the residual stress at a certain depth can be measured by means of layer by layer erosion of the sample with appropriate means (such as electrolytic polishing, etc.); The test results are greatly affected by the surface state (such as collision), so the requirements on the tested surface are high (avoid damaging the original surface and keep it clean). The advantages are simple principle, convenient measurement, high accuracy and small measurement area (generally 5 mm × 10 mm to 2 mm × 2 mm), and the stress distribution map can be made with fine scale, and the contact stress and fracture stress can also be inferred, which is suitable for high temperature and other harsh environments, as well as qualitative and quantitative analysis of phase, structural analysis, crystallinity analysis, lattice parameter measurement, etc. It should be pointed out that, due to the particularity of ceramic materials and the shape of rolling bodies, there is no satisfactory method to measure the residual stress on the surface of ceramic balls at present. Therefore, various experimental studies such as indentation crack method and scanning electro acoustic microscopy (SEAM) have been carried out.

According to the above detection technologies, Table 1 lists the main items that can be used to detect the surface characteristics of ceramic bearing parts.

More about FRC Ceramic Bearings:

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