Properties and Applications of Alumina Ceramics
Alumina ceramics are ceramic materials with a-Al2O3 as the main crystal phase. It can be used in harsh environments because of its properties like high melting point, high hardness, heat resistance, corrosion resistance, electrical insulation, etc. Alumina ceramics are inexpensive, and the manufacturing process is well-established. Currently, it is one of the ceramic materials with the highest output and the broadest range of applications. Knives, wear-resistant components, and bioceramics are some of its most common applications. Additionally, alumina is extensively used in the fields of energy, aerospace, chemical engineering, electronics, etc. Alumina 95 and 99 porcelain, which are both used in structural and electronic ceramics, are among ceramic materials with the broadest range of applications and the highest consumption.
Quality Issues of Alumina Ceramics
Alumina ceramics will go through a number of manufacturing procedures. There are numerous chances for raw materials to contact with the environment, tools, machinery, equipment, and utensils during the production process. Consequently, a variety of factors contribute to the pollution and quality issues with porcelain parts. After alumina ceramics have been fired, the following are the most frequent color-related quality issues:
(1) Porcelain components have spots on their surface, including black, brown, and red spots;
(2) Plaques, such as dark spots, black cloud spots, bright spots, etc., appear on the surface of porcelain components;
(3) Porcelain components appear a phenomenon of chromatic aberration, like an overall yellow or gray tone;
(4) The porcelain appears light yellow.
Causes of Alumina Ceramic Spots
During the production of alumina ceramics, there are frequently black, brown, and pink spots on the surface of the porcelain components, as well as internal spots that are visible through close inspection. These spots are one of the primary reasons why alumina ceramic products fail. Fe is the primary impurity found in black and brown spots, while Fe, Cr, and Ni are the primary impurities found in pink spots. It is possible to make a preliminary determination that the black and brown spots are primarily brought on by the mixing of mechanical iron particles in the production processes of raw materials and ceramics by analyzing the spot composition and monitoring the production process. And composition analysis indicates that the pink spots are due to tiny stainless steel material particles. The two main sources of mechanical impurities are the raw material and the manufacturing process.
Measures to Prevent Alumina Ceramic Spots
(1) Select high-quality raw materials with low iron content, and remove iron using magnetic separation if necessary.
(2) Pay close attention to whether the lining brick of the ball mill falls off during grinding, and repair it in a timely manner if it does.
(3) During granulation, devices like magnetic separator, hot blast furnace, and hot blast filter are installed in the slurry transportation to prevent rust from entering the material from the hot blast system. Before being used, the granulated powder undergoes magnetic separation to remove iron.
(4) All pipelines shall be lined with polyurethane as much as possible.
Alumina Dark Spots
It is possible for 95 porcelain to occasionally develop a dark spot defect, also named cloud spot, oil spot, etc. Although the cause of the 95 porcelain's dark spots is more complicated, it is not due to impurities in the raw materials. It should have some sort of connection to the ceramics formula, and the main reason for the appearance of spots is that the production technology and process for ceramics have some issues.
As seen in the image below, the dark spot varies in size, with the largest even covering more than half of the porcelain object. It gives the impression that there are ink or oil drip on the paper. These dark spots are commonly found on tubular porcelain pieces, as well as thicker and larger porcelain pieces, particularly those produced by granulation and isostatic pressing. Dark spots are typically concealed inside porcelain components and only visible with a light source. In severe cases, they will also show up on the surface of porcelain parts.
The following are the dark spots on alumina porcelain parts:
(a picture)
Prevention of Dark Spots
Although the specific conditions of the production site should be taken into consideration when analyzing the prevention of dark spots, the following basic measures should be taken:
(1) Build a firing curve that is reasonable, and allow enough time and temperature for porcelain pieces to debind during firing;
(2) Strengthen the kiln exhaust and maintain a proper oxidizing atmosphere.
(3) Within the firing temperature range, increase the firing temperature as much as possible, or moderately lengthen the residence time in the high temperature period.
Alumina Bright Spots
Alumina 95 and 99 porcelain flakes, particularly substrates made by tape casting, occasionally generate bright spots that measure 1-2 mm in size and are more noticeable in the light. Using a microscope with a magnification of more than 100 times, it is possible to see that the surface of porcelain pieces is concave where the bright spot appears. Because the concave part of the surface is thinner and the light transmission is greater, bright spots are created there. The structure and shape of the concave surface should be the result of mechanical damage. For instance, during the tape casting process, there can be mechanical damage brought on by the adhesion between the substrate green body and the base tape, as well as other causes. As a result, precautions should be taken to avoid mechanical damages during the manufacturing, storage, and movement of the green body.
Bright spots are displayed in the following figure:
(a picture)
Chromatic Aberration of Alumina Ceramics
Alumina 95 porcelain is typically white, though the whiteness can vary slightly due to different formulas. During the manufacturing process, however, it is possible for some porcelain components to acquire a grayish hue. The contamination of impurities introduced during the production process is the main cause of the color variation in alumina ceramic parts.
Reasons for the Yellowing of Alumina Ceramics
Alumina ceramics are typically white after firing, but they frequently appear light yellow. The higher the alumina content and the greater the compactness of the porcelain, the more noticeable this phenomenon will be. Additionally, alumina ceramics undergo color change when exposed to sunlight; the color is slightly yellow compared to unirradiated porcelain, and the color darkens as the exposure time increases. There are no appreciable differences in internal trace elements, according to numerous tests that have been conducted on this. At the moment, no specific reasons for the yellow color of alumina ceramics have been identified, but the following aspects are widely held:
(1) The color will change after being exposed to sunlight because the internal composition undergoes an irreversible change.
(2) Alumina ceramics may deliquescence, resulting in the formation of hydrated alumina. In fact, other materials besides ceramics also possess this quality. Therefore, the only way to prevent the discoloration of alumina precision ceramics is to reduce the impact of external factors.
(3)Ceramics with a high alumina content of 99% or more are primarily sintered by solid phase sintering, and the sintering aid is solid-fused to the oxide crystal and largely accumulated on the grain boundary, resulting in a decrease in crystal purity and a yellow-based color. In addition, due to surface lattice defects and other reasons, the surface is in a metastable state, and it readily absorbs various fine substances, resulting in discoloration.
Measures to Prevent Yellowing of Alumina Ceramics
(1) Modify and improve the grinding process, select the least wearable grinding medium, determine the best grinding conditions, such as material-to-ball ratio, grinding time, mill working parameters, etc. to reduce impurities in the grinding process as well as the wear on the porcelain lining and ceramic balls.
(2) To prevent contaminating the porcelain body during the wax discharge process, the wax discharge powder used in hot die-casting should be changed regularly and the selection of special wax discharge powder is essential.