Why Spots and Discoloration Occur in Alumina (Al₂O₃) Ceramics

Alumina ceramic is a kind of ceramic material with α-Al2O3 as the main crystal phase. Because of its high melting point, high hardness, heat resistance, corrosion resistance and electrical insulation properties, it has been widely used in many fields, can be used under more severe conditions. Alumina ceramics is one of the most widely used ceramic materials with the largest output and mature production process. It is mainly used in the fields of cutting tools, wear-resistant parts and bioceramics. In addition, it is also widely used in energy, aerospace, chemistry, chemical electronics and so on. In particular, 95% and 99% alumina are one of the most widely used ceramic materials in both structural ceramics and electronic ceramics. Recently, some customers—and even industry peers—have asked why the alumina ceramics they produce or purchase elsewhere sometimes show spots, discoloration, or color variations. In a previous article, we discussed the causes of alumina ceramic turning yellow. Today, we will address the common types of spots found in alumina ceramics.

Spots

Common phenomena
During the production of alumina ceramics, black, brown, reddish-brown, and pink spots often appear on the surface of the ceramic parts. Upon careful examination, these spots can also be found inside the material. Such spots are one of the main reasons why alumina ceramic products fail to meet quality standards.

Cause analysis
These spots are generally quite noticeable, making sample collection and analysis relatively easy. Techniques such as scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), or chemical analysis can be used, and usually, a qualitative analysis is sufficient. The main impurities producing black, brown and brown spots are Fe, and the main impurities producing pink spots are Fe, Cr and Ni. By analyzing the composition of the spots and tracking the production process and production process, it can be preliminarily determined that the black, brown and brown spots are mainly caused by the mixing of mechanical iron particles in the raw materials and ceramic production process. The pink spots can be judged from the composition analysis is caused by fine particles of stainless steel material. There are two main aspects of mechanical impurity mixing: one is raw materials, the other is the production process.

Prevention and solution
Note when molding:

        1. For raw materials, it is recommended to select high-quality materials with low iron content, and, if necessary, apply magnetic separation to remove impurities.

        2. Grinding: Watch out for “brick shedding.” Regularly check whether the lining bricks of the ball mill are detached or damaged, and repair them promptly to ensure stable grinding quality.

        3. Granulation: Install magnetic separators during slurry transportation to remove iron impurities. Use clean hot air from the furnace and filtration system to prevent rust from entering the material. Before final product packaging, perform magnetic separation again to ensure powder purity.

        4. Piping: all piping shall be lined with polyurethane as far as possible.

Dark Spot (also called cloudy spot, cloud spot, oil spot, etc)

Common phenomena

During the production of 95% alumina ceramics, a defect known as a dark spot (also referred to as shadow spot, cloud spot, or oil spot) may occasionally occur. The appearance resembles ink or oil drops on paper, varying in size, with the largest ones covering up to half of the ceramic surface area.

These dark spots are commonly found in 95% alumina ceramic tube parts or thicker and larger alumina components, particularly those produced by granulation or isostatic pressing processes.

The dark spots are usually hidden inside the ceramic body and can only be observed under strong or directed light. In severe cases, they may also appear on the surface of the ceramic part.

Cause analysis
The reasons for the dark spots on 95% Al2O3 ceramic are complex, but it can be confirmed that this phenomenon is not caused by impurities in the raw materials, and it should have a certain relationship with the ceramic formulation, and more importantly, ceramic production process and processing conditions.

Prevention and solution
According to the specific situation of the production site to analyze. The basic measures are as follows:

        1. Establish a reasonable firing curve: Ensure sufficient temperature and holding time during the binder burnout stage of the firing process.

        2. Enhance kiln ventilation: Maintain adequate airflow and a proper oxidizing atmosphere inside the kiln.

        3. Optimize the firing parameters: Within the appropriate firing temperature range, increase the firing temperature as much as possible, or moderately extend the soaking time at the high-temperature stage.

Bright spots

Common phenomena
For alumina ceramics (95% and 99%), especially thin sheet components produced by the tape casting process, bright spots sometimes appear on the surface. These spots are typically 1–2 mm in size and become more visible under light illumination.

Cause Analysis
Observation under a microscope (at magnifications above 100×) shows that the bright spot areas are slightly concave on the ceramic surface. Because these concave regions are thinner, the light transmittance increases, resulting in the bright spot effect. The shape and structure of these concave areas indicate mechanical damage, possibly caused during the tape casting process — for example, adhesion between the ceramic green sheet and the carrier tape, or other forms of mechanical contact damage.

Prevention and Solutions
Attention should be paid to avoid mechanical damage during green body production, storage and handling.