This article will take an in-depth look at ceramic machining.
The article will bring more detail on topics such as:
This chapter will discuss what ceramic machining is, how it is performed, and other considerations regarding ceramic machining.
Ceramic machining refers to the manufacture of ceramic materials into finished usable products. Machining involves the continual removal of material from the workpiece, in this case, ceramic material. This is done to change the size or shape of the material and is carefully controlled.
A ceramic object exhibits various characteristics such as hardness, brittleness, heat resistance, corrosion resistance, high strength, low density, high stiffness, wear resistance, and thermal stability. Ceramic is made by shaping and firing a nonmetallic mineral, such as clay, at a high temperature. Common examples include stoneware, porcelain, and earthenware.
Ceramics are hard materials that require much specialization and specifications in their machining. The processes performed in ceramic machining include mixing, forming, firing, and finishing. Mixing is a step where the ceramic ingredients are made into slurries through the addition of water or other specific liquid chemicals. After mixing, the forming process begins. This is where the base material is produced through a number of processes, like slip casting, extrusion, injection molding, and pressing, for dry powders. During forming, the basic shape of the ceramic is created.
After the forming process, sintering (also known as firing) is performed. Firing is done after forming and drying to allow the ceramic to dry out and solidify. Firing is when the formed clay is hardened through exposure to very high temperatures of up to 1832 °F (1000 °C), thus reaching its most favorable melting point. The melting occurs on the molecular level, meaning that the formed shape is not decomposed. This process is usually completed in two steps, bisque firing and glaze firing.
Bisque firing is the first time formed clay is put through high-temperature heating. Items are heated in a kiln with great care until they turn glass-like. The temperature is allowed to rise slowly; this gradual increase is important because this is where the last of the water is driven out of the material. If the heat rises too quickly, the water turns into a gaseous state inside the body and causes it to burst. When the kiln reaches 662 °F (350 °C), the water molecules begin to vaporize away. When the temperature reaches about 932 °F (500 °C), all the water should have been driven away, leaving the clay dehydrated and changed forever into hard ceramic material.
The bisque firing process continues until about 1742 °F (950 °C). At this temperature, the clay has been transformed to a point where it is less fragile and yet still porous enough to accept glazes. The kiln is then turned off after the required temperatures are reached, and the ceramic is cooled slowly to avoid breakage as a result of rapid temperature changes. Finally, the bisqueware is formed.
Glaze firing is the next step. At this step, the coating is applied to the new bisqueware to color it, decorate it, or improve some material qualities such as water resistance. The kiln is then slowly heated to the required temperatures to apply the glaze and slowly cooled again.
After firing is completed, the typical machining steps are finished. These include grinding, cutting, milling, turning, and etching.
Some difficulties could arise in the ceramic manufacturing and machining processes. This section introduces some of the difficulties that one may face and some solutions to assist the ceramic machining processes.
Unintended alteration of the product is the most common and detrimental defect in the ceramic industry. The main cause of deformation is improper kiln-drying tactics. Temperature changes can also cause deformation if they are done by rapidly heating or cooling the kiln during the firing process. Therefore, the temperature control should be watched carefully.
Rapid temperature changes can also cause cracks on the product surface. This causes uneven shrinking inside and outside the ceramic product.
Foaming occurs when insufficient oxidation during the decomposition of the porcelain tile causes the glaze to form bubbles on the ceramics. In billet glaze, sulfate and organic impurities are what cause the foaming of the product.
This chapter will discuss the methods of ceramic machining based on their types.
Abrasive machining is not necessarily the same as precision grinding. Neither extreme accuracy nor high-luster surface treatments are the goal of abrasive machining. The primary result of abrasive machining is significant stock removal. Although abrasive machining isn't considered a precision grinding technique, it doesn't mean it isn't accurate.
The "large-chip" machining techniques, including milling, planing, broaching, and turning, can be replaced by abrasive machining. The surface quality and accuracy obtained by abrasive machining are superior to those obtained through large-chip procedures. The fact that little to no burr is produced is an added benefit. One further significant characteristic of abrasive machining is that it provides a method for making difficult-to-machine materials, whether they be nonmetals or metals, machinable.
There are numerous types of grinding processes, including reciprocating, internal-, external-, centerless, creep-feed, and more. Nevertheless, all grinding processes are united in their use of a circular wheel or tool that is spun about its axis of symmetry while parts of its periphery slide against the workpiece.
Abrasive grains are linked together to form the entire tool, which consists of a layer that may be only a few millimeters thick, or only one layer as in the case of plated tools. The instrument might be as small as a submillimeter drill with an abrasive coating or as large as grinding wheels with a diameter of over a meter. There are many different types of abrasives utilized.
The most prevalent grinding materials for metals are silicon carbide and aluminum oxide. However, only the super abrasives, diamond and cubic boron nitride (CBN), are employed for application to advanced ceramics, with diamond being the material of choice. Despite being harder than other modern ceramics, diamond has been proven to wear out far more quickly than cubic boron nitride.
Like grinding, honing also uses fixed abrasives, and diamond is the abrasive of choice for use with sophisticated ceramics. The topics of honing and grinding are frequently discussed together. The substantially slower surface speeds used in honing are the primary distinction between it and grinding. In general, honing is not used to remove a lot of material. Its main use is to repair dimensional tolerance flaws caused by other, quicker machining techniques while achieving the appropriate surface polish. Polishing internal cylindrical surfaces is the most common use for honing, with the cylinder walls of car engines serving as a particularly notable example.
Applications like the latter can be fully automated with total control over every step of the process, including automatic part dimension gauging. Honing is also used to finish external surfaces, such as the outer surfaces of ball and roller-bearing races, valves, and gear teeth. Since honing is a lower-speed process, it produces less heat than grinding. As a result, the harm and workpiece distortion brought on by heating is reduced. The quantity of cutting fluid used is also influenced by how little heating occurs during honing. The necessity for cooling is less of an issue, but lubricity must be achieved using chemicals. Unlike in grinding, it doesn't seem like any significant attempts have been made to create honing fluids expressly for use with sophisticated ceramics.
Both rotary ultrasonic machining and ultrasonic impact machining fall within the category of ultrasonic machining. Both techniques involve vibrating the tool at a frequency of typically 20 kHz along its axis, which is set parallel to the surface of the workpiece. Otherwise, the two approaches diverge greatly. Diamond-coated grinding tools, most frequently core drills, are used in drilling or vertical milling configurations during rotary ultrasonic machining. Only the fact that the rotating tool vibrates with an amplitude of 0.025 to 0.05 mm while pressed on the workpiece separates it from regular milling and drilling operations.
The vibration reduces friction, aids in reducing fluid access, and makes it easier to remove swarf. The end result is an increase in the machining rate. The tools typically have a diameter of no more than 50 mm. Rotating ultrasonic machining has also used small grinding and thread-cutting wheels in addition to core and solid drills. Cutting fluids, bond and abrasive qualities, and the impact of grinding parameters should all be given equal weight when considering this process. Therefore, advancements in the ceramic grinding technique ought to be transferable to rotary ultrasonic machining.
The tool does not contain abrasive materials and does not make contact with the workpiece when using ultrasonic impact machining. This technique involves circulating an abrasive-containing slurry between the vibrating tool and the workpiece. The fluid and abrasive particles are vibrated by the tool. Particles striking the workpiece cause indentation and fracture, which lead to material removal. The distance between the workpiece and the vibrating tool significantly impacts the pace of removal. Ultrasonic impact grinding is likely the most popular machining technique for advanced ceramics, second only to grinding. Higher vibration frequencies and specialized fluids may enable faster machining through ultrasonic impact grinding.
Similar to honing, lapping is mostly a finishing procedure used on objects that have already been machined to near their final dimensions. However, in contrast to honing, lapping uses a loose or free abrasive technique. With lapping, the workpiece is pressed against a stiff surface, frequently cast iron coated in a slurry of abrasive particles. Some of these particles cause a cutting action on the workpiece when they become embedded in the surface of the lapping tool. The elimination procedure also involves other particles that roll between the two surfaces.
Lapping and polishing are occasionally distinguished from one another as separate processes. However, this is not always the case. Lapping simply uses a firmer, more rigid surface, whereas polishing uses a softer, more flexible one. Lapping is used to acquire accurate tolerances, eliminate damage, and enhance surface smoothness. In contrast, polishing is mostly used to repair damage and create extremely smooth surfaces.
Instead of shaping or surface finishing, liquid jet systems are typically used for cutting. Porous materials with very high cutting rates are regularly subjected to this technique. However, liquid jet cutting is ineffective on advanced ceramics, which are hard and dense with substantially lower cutting rates.
The use of liquid jet cutting on modern ceramics has been made possible by the addition of abrasive grit to the fluid stream. This method combines localized fracture, brought on by liquid cavitation, with slurry erosion removal procedures. However, the inclusion of these abrasive particles will create another type of machine damage. Components needing a close tolerance and fine surface quality are not something for which the liquid abrasive jet process is particularly well-suited.
Some of the non-abrasive machining methods include:
Electrical energy can also be used for ceramic machining. This is known as electrical discharge machining (EDM) and has found many applications for advanced ceramic machining in the past years. EDM requires the ceramic workpieces to have an electrical resistivity of less than 100Ω-cm. This implies that Electrical Discharge Machining cannot be used for machining glasses and some ceramics.
EDM has been successfully applied on silicon-infiltrated silicon carbide, siliconized silicon carbide, and hot-pressed silicon carbide materials. In order to extend the application of EDM to other ceramics, the resistivity of the materials needs to be reduced to meet the required range. It is not clear, however, whether EDM is a low-damage machining method since it typically leaves a surface layer of melted or heat-affected material containing a high level of residual stress and numerous microcracks.
Focused laser beams have been widely used for the precise cutting of a variety of materials such as metal, wood, and even ceramics. Innovative approaches have been applied, such as using two laser beams intersecting within the workpiece to cut blind kerfs. This technique uses certain cutting geometries, especially when some parts of the workpiece need to be removed.
Friction cutting uses a circular blade rotating at a speed where it produces more heat than the material needing to be cut. For friction cutting, alumina and silicon nitride are sliced into small slots using a spinning, mild steel disk that is then water cooled. For the microwave cutting process, sintered alumina is penetrated by localized 17 microwave heating. In this procedure, local melting and an explosive discharge of the molten material occurs from below the surface when penetrated by a wafer of sintered alumina. These techniques work well for cutting and slicing tasks, but they are typically ineffective for producing a contoured surface with a tight tolerance.
Some of the combined methods include:
Some engineered ceramics claim to operate well when heated with a plasma torch while being turned. The workpiece material is heated up to a maximum temperature of 1000°C using this technique before being cut using a polycrystalline diamond compact (PDC) or CBN cutting tool. Where machinability is enhanced, it is due to a change in the deformation and removal processes from more rigid to more plastic ones at the higher temperature.
In spite of an 8-fold reduction in tool wear when turning silicon nitride, the level of tool wear is still too high. By heating with a plasma torch, materials susceptible to thermal shock, such as alumina and zirconia, still do not exhibit increased machinability. Engineers are able to make ceramic material more pliable in reaction to mechanical forces by heating it with a laser source first, then cutting it with a diamond tool. Due to the limited tool life and subpar surface smoothness, thermally assisted turning does not lend itself to high volume production.
As documented, it is now possible to combine electrical discharge with ultrasonic machining techniques. After much experimentation, engineers now know how to use a metal-bonded diamond tool to manufacture titanium diboride. Under selected conditions, they achieve higher material removal rates and a higher removal ratio of 110 (workpiece loss/wheel loss). It is still necessary, however, to investigate whether this technology can be used with additional materials that have adequate conductivity for EDM.
Some engineers described chemical-electrical discharge as combining electrochemical reactions in an electrolyte with wire electrical discharge machining. This technique is used on silicon carbide, non-conducting glass, alumina, and silicon nitride specimens that are submerged in an appropriate electrolyte solution to allow for conduction.
This technique enables efficient cutting of surface contours without having to make direct contact between the tool and the workpiece. According to reports, the ceramics can be cut at a rate between 0.12 and 0.14 mm/min. However, the viability of this technology for mass manufacturing sophisticated ceramics requires more investigation.
This chapter will discuss the types of ceramics based on their categories.
Both the phrases pottery and ceramic refer to items made out of clay, fired to hardness, and then decorated or glazed. Rock that has weathered naturally produces clay. It is a useful material for producing dinnerware since it is pliable, flexible, and will permanently solidify if baked at high temperatures. There are three major types of ceramic and pottery materials: porcelain, stoneware, and earthenware.
Potters have fired earthenware in ovens for countless years. When the Roman Empire was at its height, ceramics were employed as amphorae to ship wine and olive oil to the furthest reaches of the realm. However, liquids may leak through these containers, allowing goods like oil to go rancid after repeated use for an extended period. Earthenware, however, can be fired at lower temperatures than other conventional ceramics like stoneware and porcelain, reaching as low as 1200 °F (648 °C).
Some earthenware potters coat their works with varnish to seal in the moisture. However, due to the reduced firing temperature, one may still scratch and harm it with a knife. These days, terracotta planters, other kitchenware, and most of the bricks used in construction are made of earthenware. Earthenware is a common choice for new potters. The ease of working with earthenware clay contributes to its appeal. It is less flexible and fragile than other types of pottery.
The popularity of porcelain increased in Europe and North America in the 1700s, making it the last type of pottery to reach the West. It has been prized for its toughness and durability in China for long before that point. In the past, porcelain was fired at temperatures considerably higher than stoneware. Typically, the final firing temperature was in the range of 2600 °F (1426 °C).
The main difference between stoneware and porcelain today is that porcelain is typically manufactured with white clay. Almost any white clay or bone ash will do, although the white mineral kaolin is usually used for porcelain. However, kaolin is less forgiving and more difficult to deal with than other clays. It is also more vulnerable in modern society.
Sculptors can also carve porcelain into more delicate shapes than they can with stone or earthenware. Since the 18th century, porcelain has been prized by collectors, and it has been used to recreate anything from the flowing mane of a horse to the folds of a robe. For contemporary potters, the lines between porcelain and stoneware are becoming less apparent as new technologies and ideas are developed.
Stoneware came into production after the invention of earthenware. Compared to earthenware, it takes a long time to fire. Most stoneware is fired at temperatures between 2000°F and 2400° F (1093 and 1315 °C), which is hotter than volcanic lava. Stoneware is vitrified at these extremely high temperatures, turning the exterior glazes into glass. Stoneware can currently be produced using a variety of clay shades, unlike porcelain, which is now almost exclusively white. Additionally, some stoneware has different clay colors blended in for a distinctive twist.
Stoneware exhibits many favorable qualities over traditional ceramics; it is sturdy, hardy, and nonporous. As a result of its durable, fashionable, and versatile nature, it can be used for everything from a personalized trophy to a baking dish. It can also survive the heat from a microwave, dishwasher, or even oven under the right conditions. Stoneware also holds and distributes heat more evenly than other materials, making it perfect for drinking coffee and tea.
Sanitary equipment like sinks and baths are made of stoneware. Stonewares are also used in the chemical sector to create pumps, valves, absorption towers, drainage pipes, underground cable sheathings, sewerage pipes, and residential pipes. Despite being more affordable than many other building supplies, however, they are fragile and have little market value once damaged.
Some advanced ceramics include:
Bricks are typical ceramics because they are usually made by heating materials that resemble clay, like sand. Many homes include this kind of ceramic. It has a wide range of properties based on how it was manufactured. This ceramic is tough, heavy, and generally able to tolerate high temperatures. Fire bricks can be used in chimneys, fireplaces, and on walls. Additionally, they are commonly used in landscaping because of these qualities.
Tungsten carbide is a thick and durable material composed of the same amounts of carbon and tungsten. These ceramics are strong, thick, hard, long-lasting, and exhibit little electrical resistance. Because of these qualities, many items are made from this material ranging from various cutting tools to golf clubs.
By introducing powdered bone ash to the standard ceramics recipe, ceramicists created a less-brittle variety of porcelain known as bone china. As a bonus, it looked ivory-white, similar to traditional porcelain. Today's world has largely replaced pure porcelain with this material. Bone china is considered by many to be a stronger type of porcelain.
Bone china is naturally colorless or white, whereas unfired porcelain clay can be cream or white. Bone china is fired twice. The first time (bisque firing) allows the material to form into a translucent, glass-like form, and the second firing (glaze firing) is performed at a lower temperature to melt this material into more specific forms with a protective shell.
The shell is physically strong and affords bone china a strong resistance to chipping wear. The presence of feldspar, kaolin, phosphates, quartz, and other materials helps provide this strength.
Glass ceramics are one of the many different types of ceramics made by controlling crystallization, resulting in properties akin to glass but with ceramics' strength and hardness. Modern manufacturing techniques are used to create glass ceramics, producing materials with desirable qualities that include zero porosity, mechanical strength, durability, high temperature resistance, transparency, and biocompatibility.
Glass ceramics have excellent superconductivity and chemical resistance. They are used to make cookware, bakeware, and stovetop accessories. Additionally, this ceramic is frequently used in industrial, scientific, and medical equipment.
Another common ceramic substance viewed as superior because of its chemical characteristics is silicon. This ceramic is widely distributed and makes up around 90% of the earth's crust. Additionally, silicon is typically present in the clays used to make conventional pottery. For instance, silicate minerals like kaolinite and silica ceramic are used to manufacture porcelain and burnt bricks, respectively.
Silicon is the preferred choice for manufacturing semiconductors due to its ability to bond atoms, strength, and abundance. Crystalline silicon, which is related to polycrystalline silicon, is used to make exceptionally high purity semiconductors, such as integrated circuits and solar panels. High-quality silicon minerals are also used to make cement aggregate, glass, and ceramics. Silicon, therefore, is the most widely utilized raw material in the construction sector.
Another type of ceramic material is silicon carbide, a superior semiconductor made of silicon and carbon. It naturally occurs as a rare stone called moissanite. Ceramics made of silicon carbide are strong and incredibly hard. There are around 250 distinct crystalline materials that contain this semiconductor.
Although this porcelain is naturally white, additional substances, like iron, sometimes color it. It also exhibits a low thermal conductivity. Examples of applications for this ceramic include cutting tools, furnaces, braking discs, abrasives, heating elements, lights, and electrical power systems. The natural form of silicon carbide is prized as a jewel because it resembles diamonds in appearance and toughness. It is a more durable substitute for synthetic zirconia.
Silicon nitride is a high-performing ceramic that consists of silicon and nitrogen (Si3N4). It is characterized by having exceptional strength, toughness, hardness, and outstanding chemical and thermal stability. The five types of silicon nitride ceramic result from their fabrication, which determines their properties and applications.
The five types of silicon nitride are:
Titanium carbide is a ceramic material that is very strong, heat-resistant, and dark in color. This material is extremely durable, wear-resistant, heat-resistant, and corrosion-resistant. It is frequently found in watch movements, heat shields, machine parts, and tool bits.
Boron carbide ceramics consist of boron and carbon, which makes it one of the hardest known substances. The extreme hardness of boron carbide is between 9.5 and 9.75 on the Mohs hardness scale. It is resistant to chemical reactions and provides shielding against neutrons.
Known as the black diamond, boron carbide has been shown to be a p-type semiconductor. Its extreme hardness makes it wear-resistant. Boron carbide’s good mechanical properties and low specific gravity make it ideal for producing lightweight armor.
Boron carbide is produced using fusion with carbon or by a magnesiothermic reaction. It can also be manufactured using pressureless sintering at temperatures of 4172 to 4352 °F (2300 to 2400 °C) using various sintering aids.
Structure ceramics are often made of clay and are pressed into the desired shape. Their effective insulating qualities can be changed by adjusting their density. The ceramic's insulating characteristics decrease with increasing density. Bricks, dinner bricks, dinner plates, and statues are all examples of structural ceramics.
Refractory ceramics maintain their strength and shape under extreme heat. They are employed in furnaces and kilns for this reason. They are produced utilizing a variety of oxides, including zinc oxide, titanium dioxide, and silicon dioxide.
Excellent electrical qualities are a hallmark of electrical ceramics, often known as electroceramics. They are useful for various applications because they have good mechanical, thermal, and electrical qualities. Electrical ceramics become more conductive as the temperature rises. Examples of electrical ceramics include ceramic rapid ion conductors and dielectric ceramics.
Oxide materials with a certain form of permanent magnification (ferrimagnetism) are called magnetic ceramics. Ferrites, crystalline minerals formed of iron oxide combined with another metal, are the basis of magnetic ceramics. Transformers, telephony, and data storage are just a few applications of magnetic ceramics.
Ceramic abrasives, which are used to cut or grind away other softer materials, can be either natural or synthetic. They are extremely durable, wear-resistant, and hard; a diamond would be the most notable abrasive ceramic.
This chapter will discuss the applications, benefits, and disadvantages of ceramics as used in ceramic machining.
Ceramic materials are produced into useful items using a process known as ceramic machining. The process of machining includes continuously removing material – in this case, ceramic material – from the workpiece. This is carefully managed to alter the size or form.
Because they are hard materials, ceramics require extensive, specialized, and precise machining. Ceramic machining involves several steps, including mixing, shaping, firing, and finishing. Ceramic elements are blended in a variety of ways during the mixing stage and become slurries by the addition of water or other particular liquid chemicals. The process of formation starts after mixing. For dry powders, this refers to the production of the basic material using a variety of techniques such as slip casting, extrusion, injection molding, and pressing. The fundamental form of the ceramic is produced during the forming process. Thus, choosing the right ceramic for the right ceramic machining process is critical.
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Acid etching, also known as chemical etching or photo etching, is the process of cutting a hard surface like metal by means of a specially formulated acid for the process of etching in order to allow for the creation of a design onto the metal...
Glass cutting is a method of weakening the structure of glass along a score line that can be broken by applying controlled force; this separates the glass into two sections along the score line or fissure. Regardless of the application, the cutting of glass is...
Metal etching is a metal removal process that uses various methods to configure complex, intricate, and highly accurate components and shapes. Its flexibility allows for instantaneous changes during processing...
Photochemical etching, also known as photochemical machining or metal etching, is a non-traditional, subtractive machining process in which photographic and chemical techniques are used to shape the metal workpiece...
Quartz is one of the most abundant and widely distributed minerals in nature. Quartz is the only stable polymorph of crystalline silica on the Earth‘s surface. It is found in all forms of rocks: igneous, metamorphic and sedimentary. It becomes concentrated in...