Alumina Ceramic, a prime example of technical ceramics, is an outstanding material known for its remarkable mechanical and electrical qualities. This compound, comprised of aluminum and oxygen, is extensively employed in a myriad of industrial applications. Its crucial attributes like extreme hardness, high thermal conductivity, insulating capability, corrosion resistance, high melting point, and exceptional hardness make alumina ceramics a preferred choice for several manufacturing processes.
Chapter One – What is Alumina Ceramic?
Being a popular and commonly utilized fine ceramic material, Alumina Ceramic, also referred to as Al2O3, showcases an impressive combination of electrical and mechanical features. This material shares the same sintered crystalline body as sapphire and ruby. Alumina Ceramics have been utilized for years in electrical components due to their excellent electrical insulation. Moreover, its high strength, coupled with corrosion and wear resistance, makes this material ideal for mechanical parts.
Alumina Ceramics – A Closer Look
Alumina Ceramic is a well-regarded industrial oxide ceramic recognized for its extreme hardness and high thermal conductivity. It is produced from bauxite and can be shaped using various methods including injection molding, die pressing, isostatic pressing, slip casting, diamond machining, or extrusion. Alumina Ceramics possess the same sintered crystalline body as sapphire and ruby.
Use Cases for Alumina Ceramics
Alumina Ceramic, due to its excellent insulation characteristics, is predominantly used as an insulation material for electrical components. In addition, it finds extensive usage for its incredible strength and resistance to corrosion and wear in several applications.
Classification of Alumina Ceramics
Alumina ceramics are referred to as technical ceramics due to their properties and price-to-performance ratio. The classification of these ceramics is based on their alumina content, which can vary from 70% to 99.9%. The higher the purity of alumina, the stronger is its wear and corrosion resistance.
Chapter Two – Properties of Alumina Ceramics
Alumina ceramics are made from a white granular material similar to table salt or a fine, silky, dense white powder. The three types of alumina are hydrated, calcined, and tabular. Each type comes in a variety of grades.
Types of Alumina
The types of alumina vary according to the amount of soda(Na2O), iron (Fe2O3), and silica (SiO2) they contain as well as their chemical purity and the properties of the powder used in the production process.
Calcined Alumina
To produce calcined alumina, aluminum oxide is heated to 1050°C or 1900°F. The superheating eliminates all chemicals and water, creating a very pure alumina, 99.99% pure, with a 9 on the Mohs hardness scale.
Hydrated Alumina
Alumina hydrate, or alumina hydroxide, is used as a glaze because of its ability to stay in suspension in glaze slurries and adhesive qualities.
Tabular Alumina
Tabular alumina is produced by heating aluminum oxide to 1650°C or 3000°F. It has high heat capacity, excellent thermal temperature, strength, and volume stability.
Properties of Alumina
High Temperature Ability
Alumina is used in oxidizing and reducing atmospheres up to 1650°C or 2900°F as well as vacuum environments of 2000°C or 3600°F.
Abrasion Resistant
Abrasion wears down a material by rubbing it away by friction. The resistance to abrasion means a material will maintain its original structure even after mechanical wear. Alumina ceramics are high in abrasion resistance due to its hardness.
Chemical Resistance
Alumina is resistant to acids and alkalis at high temperatures because it is inert, not chemically reactive, which makes it resistant to the effects of chemicals such as solvents and salt solutions.
Density
The density of a material is its mass divided by its volume. This is read as grams per cubic centimeter (g/cm3), where grams is mass and cubic centimeters is volume. The volume and density of the material are inversely proportional.
Chapter Three – How are Alumina Ceramics Produced?
The powder material for alumina ceramics comes from the processing of bauxite, which is an aluminum-rich clay-like material located a few meters below the earth’s crust. The method for manufacturing alumina was discovered in 1887 and is called the Bayer process.
Alumina Ceramic Production
Grinding
The mined bauxite is taken to a processing plant, where it is ground to extract the aluminum oxide. The composition of the removed aluminum oxide looks like a white powder.
Washing and Drying
The ground bauxite is washed, dried, and dissolved in a mixture of caustic soda and lime to form a slurry that is heated in a digester to 300°F or 145°C and placed under 50 lbs. of pressure for several hours to dissolve the aluminum compounds.
Flash Tanks
The slurry is pumped through a series of flash tanks to reduce the pressure and heat on the material. The flash tanks cool the material at atmospheric pressure and flash off steam.
Settling Tank
The impurities in the slurry, like sand, iron, and other elements that do not dissolve, settle to the bottom of the tank. The liquor at the top of the tank moves through a set of filters and is washed to recover the alumina. It is filtered several times before moving on.
Precipitators
The filtered clear sodium aluminate is pumped into a series of precipitators. Particles of alumina are added to begin the process. The alumina grows around the seeds causing them to fall to the bottom of the tank to be removed and sent to thickening tanks. The material is filtered again.
Calcination
The final step in the extraction of alumina involves a heating process to remove water from the alumina hydrate. It is filtered and washed again to remove impurities and moisture. A conveyor, seen in the diagram, moves the hydrate to calcination, a gas fired kiln on an incline that rotates to ensure even heating.
Producing Alumina Ceramics
The powder from refining is mixed with other materials before being put through the forming process. The blend of alumina and other substances determines the grade of the ceramics. Three of the mixing processes are spray-dried powder, aqueous slip, and ceramic dough feedstock.
Spray Dried Powder
Spray drying produces a granulated powder for uniaxial and isostatic pressing. The raw powder is milled in a solvent, such as water. A binder is added before the spraying process, which gives the material strength for pressing.
Aqueous Slip
With aqueous slip processing, the raw materials are milled together using a ball mill in water with an organic dispersant that produces a concentrated slurry. The slip is placed under high pressure that produces green density parts.
Ceramic Dough Feedstock
In the ceramic dough feedstock process, water, alumina, a binder, and plasticizer are mixed to produce a clay-like material. The feedstock has to be of high quality to produce the necessary green density for consolidation, which lowers shrinkage during the sintering stage.
Forming Process
Consolidation is the forming of the ceramic part for handling and forming. Four of the many methods for consolidation are extrusion, uniaxial and isostatic pressing, electrophoretic deposition, and slip casting.
Injection Molding
In the injection molding process, fine powder is mixed with a binder to form the feedstock, which is heated to form a viscous slurry and injected molded to form the green part. The binder is removed to form the brown part, which is densified through sintering.
Extrusion
Extrusion uses dough feedstock that is deformed under pressure. The binders in the mix help the form keep its shape as it dries.
Isostatic pressing
Isostatic can be wet or dry bag pressing. Sprayed powder is placed in a bag of rubber or polyurethane and placed under isostatic pressure. Wet bag pressing produces simple shapes, while dry bag pressing can produce very complex shapes.
Slip Casting
Slip casting, or drain casting, uses a fluid aqueous slip that is poured into a mold made of plastic. Water is drained from the mixture leaving a consolidated powder mix on the sides of the mold. As the thickness of the cast increases, the remaining slip is drained. With pressure casting, pressure is applied to the slip creating thicker walled and more solid parts.
Uniaxial Pressing
Uniaxial pressing is a compacting of alumina powder by applying pressure in a single axial direction using a piston, punch, or plunger. Presses for uniaxial pressing can be hydraulic or mechanical that have a top punch, bottom punch, and die.
Electrophoretic deposition (EPD)
EPD used for several industrial coating and painting processes. For ceramic production, an electrostatic charge consolidates ceramic particles from suspension and deposits them on the surface of a mold. Once the necessary thickness is achieved, the mold is removed from the slip container or the slip is poured off.
Finishing Processes
Sintering
During sintering, the consolidated green ceramic part is fired to give it density. At the high temperatures of sintering, particle rearrangement, grain growth, and pore elimination take place. For alumina ceramics, this is known as solid-state sintering. One of the outcomes of sintering is shrinkage from up to 20% to 25%. However, the uniformity of the shrinkage and the quality of the final part depends on the density of the green material.
Green Machining
Alumina can be machined in green or fully dense states. In the green form, it can be easily shaped into complex geometries. However, though it is easy to shape alumina in its green form, it will shrink by 20% during sintering, changing the dimensions of the piece. Therefore, green-machined alumina will not hold its tolerances during sintering. In order for alumina to maintain its tolerances, it needs to be machined after sintering with a diamond-coated wheel.
Diamond Grinding
Diamond grinding is done after sintering to match tight tolerances, improve the surface finish, and remove any flaws. Diamond processes include grinding, cutting, honing, lapping, and polishing. The diamond tooling is necessary due to the hardness of alumina ceramics.
Chapter Four – Top Alumina Ceramic Machines
Several high-tech machines are used in the production of alumina ceramics. These machines have been perfected over the years and are essential in achieving the desired properties of alumina ceramics. Here are five leading companies that produce these machines, along with the specific models and their unique features:
CeraNova Corporation
Model: High-Speed Ceramics Milling Machine
Features: The High-Speed Ceramics Milling Machine by CeraNova Corporation is designed specifically for the precision milling of alumina ceramics. It utilizes advanced cutting technology to achieve high material removal rates while maintaining excellent surface finish. The machine offers high-speed spindle options, multiple-axis control, and specialized tooling for efficient and accurate ceramic machining.
CoorsTek
Model: CeraSurf™ Ceramic Grinding Media
Features: CoorsTek offers a range of ceramic grinding media, including alumina-based CeraSurf™. These grinding media are designed for various applications, such as ball milling and attritor milling. CeraSurf™ ceramic grinding media provide high wear resistance, chemical inertness, and uniform particle size distribution, resulting in improved grinding efficiency and product quality.
Sacmi USA Group
Model: PH Series Presses
Features: The PH Series Presses by Sacmi USA Group are hydraulic presses specifically designed for the production of alumina ceramics. These presses offer precise control over pressure, speed, and stroke, allowing for the formation of complex shapes with high density. These machines feature a user-friendly interface, advanced automation options, and the ability to integrate with downstream processes.
Harper International
Model: Pusher Furnaces
Features: Harper International manufactures advanced pusher furnaces used for sintering alumina ceramics. These furnaces provide high-temperature, controlled atmospheres necessary for the sintering process. They offer precise temperature control, uniform heating profiles, and adjustable residence times. Harper’s pusher furnaces are designed for high throughput and can accommodate various ceramic shapes and sizes.
LECO Corporation
Model: AMH43 Moisture Determination Analyzer
Features: The AMH43 Moisture Determination Analyzer by LECO Corporation is specifically designed for the analysis of moisture in alumina ceramics and other materials. It utilizes a precision balance, high-temperature drying, and advanced software algorithms to accurately determine moisture content. The analyzer offers rapid analysis, easy operation, and customizable test parameters.
Please note that the availability and specific models may vary over time, so it’s always recommended to check with the respective manufacturers for the latest product offerings.
Chapter Five – Alumina Ceramic Products
Alumina Ceramic is a major engineering material that has excellent mechanical and electrical properties, which makes it applicable for a wide range of uses. A few of its uses are listed below.
Alumina Ceramic Products
Insulation
One of the main uses of alumina ceramics is as an insulation material due to its dielectric and thermal shock properties. High frequency applications use extremely pure and dense alumina that has been fired at 1600°C. Television and satellite transmitter tubes, microwave generators, and lasers depend on the strength and durability of alumina ceramics for insulation.
Labware
Alumina ceramic labware is used for laboratory applications with high-temperature conditions that need to be contamination free. The chemical and corrosion resistance, high hardness, strength, durability, and wear resistance of alumina ceramics makes it a perfect solution for lab applications.
Electronics
Alumina ceramics are used in the electronics industry for interconnectors, resistors, and capacitors. It is an economical and durable material for substrates for hybrid integrated circuits, surface mount devices, and sensors.
Cutting Tools
Alumina ceramic cutting tools exhibit strength and thermal conductivity. Though alumina cutting tools were once very expensive, the material is now engineered with the use of composites to be cost-effective and are manufactured by sintering and die pressing. These composites are used as a substitute material for abrasive and cutting tools, extrusions, nozzles, friction parts in piston engines, and machinery when wear resistance is a concern. In addition, alumina ceramics are uniquely used in Olympic ice skaters’ blades.
Military Use
Alumina ceramic is used in ballistics due to its high shock resistance, making it ideal for body armor in tanks, helicopters, and bulletproof jackets. The use of alumina ceramics in armor is to withstand the initial impact of a projectile. It is the first line of defense and protection.
Biomedical Material
Since alumina ceramics are inert, they are insoluble in chemical reagents, have wear resistance, and can have a highly polished finish, which makes them useful as biomaterial. Consequently, alumina ceramics are used for artificial joints, bone spacers, cochlear implants, and teeth implants. Tubes and scientific products are also made from alumina ceramics. The positive properties of alumina ceramics makes it an ideal material for use in replacing body parts.
Chapter Six – Types and Grades of Alumina Ceramic Material
The grades and types of alumina ceramics are determined by the percentage of alumina the material contains, which can vary from 70% to 99.9%. The percentage of alumina fluctuates according to the amount of other elements that are added to the mixture.
Types of Alumina Ceramics Material
92% Alumina
Features electrical properties, mechanical strength, wear resistance, chemical and corrosion resistance, thermal stability, and is dense non-porous. It is used for electrical packaging, bushings, grinding media, wear resistant components, and industrial applications.
94% Alumina
Has low thermal expansion, high volume resistivity, abrasion and chemical resistance, dielectric constant, and accepts manganese metal coating for high-temperature brazing. It can be used as a pressure sensor, bearing coatings, electron tube, and laser components.
95% Alumina
Has similar properties to 94% alumina with added qualities of compressive and flexural strength and excellent hermeticity. It is good for ceramic to metal feedthroughs, X-ray component feedthroughs, high voltage bushings, and medical implants. This percentage of alumina can be molded into body armor for military applications.
96% Alumina
Is used for medical applications. It has a combination of mechanical, electrical, thermal, and chemical properties. It can be used as an excellent electrical insulator, particularly the higher purity grades, which offer better resistivity.
Chapter seven – Alumina Ceramic Products
Alumina is a major engineering material that has excellent mechanical and electrical properties, which makes it applicable for a wide range of uses.
Alumina Ceramic Products
Insulation
One of the main uses of alumina ceramics is as an insulation material due to its dielectric and thermal shock properties.
Labware
Alumina ceramic labware is used for laboratory applications with high-temperature conditions that need to be contamination-free.
Electronics
Alumina ceramics are used in the electronics industry for interconnectors, resistors, and capacitors.
Cutting Tools
Alumina ceramic cutting tools exhibit strength and thermal conductivity.
Military Use
Alumina ceramic is used in ballistics due to its high shock resistance, making it ideal for body armor in tanks, helicopters, and bulletproof jackets.
Biomedical Material
Since alumina ceramics are inert, they are insoluble in chemical reagents, have wear resistance, and can have a highly polished finish, which makes them useful as biomaterial.
Chapter eight – Types and Grades of Alumina Ceramic Material
The grades and types of alumina ceramics are determined by the percentage of alumina the material contains, which can vary from 70% to 99.9%. The percentage of alumina fluctuates according to the amount of other elements that are added to the mixture.
Types of Alumina Ceramics Material
92% Alumina
Features electrical properties, mechanical strength, wear resistance, chemical and corrosion resistance, thermal stability, and is dense non-porous.
94% Alumina
Has low thermal expansion, high volume resistivity, abrasion and chemical resistance, dielectric constant, and accepts manganese metal coating for high-temperature brazing.
95% Alumina
Has similar properties to 94% alumina with added qualities of compressive and flexural strength and excellent hermeticity.
96% Alumina
Is used for medical applications. It has a combination of mechanical, electrical, thermal, and chemical properties.
In conclusion, Alumina Ceramic, with its unique set of properties, is a crucial material in various industries. Its excellent mechanical and electrical properties make this ceramic material a preferred choice for numerous manufacturing processes. Whether it’s used as an insulation material, in labware, electronics, cutting tools, military armor, or biomedical materials, alumina ceramics have proven their value and versatility.
