Oxide ceramics are alumina-based and are generally used in roughing and finishing applications of the cast and gray irons. Increasingly, oxide ceramics are being used in dry machining applications at high speeds. This is due to the advancements within materials science, including the incorporation of whisker-based materials.
Nitride ceramics exhibit extremely high fracture toughness and are used for roughing and semi-roughing of cast irons under harsh conditions, such as when there is considerable starting and stopping that would typically increase fracturing.
CMCs contain ceramics mixed with other hard materials like cemented and titanium carbides. They can also contain reinforcing whiskers and other materials to increase wear resistance and toughness under hot machining conditions. These are commonly referred to as SiC-whisker reinforced. Alumina reinforced with SiC whiskers is the toughest and most resistant to thermal shock of the oxide-based ceramics due to the extremely high tensile strength of the SiC whiskers. It offers the additional benefit of being able to be run without coolant, thus making dry machining a common application while also being able to offer an increase in machining rates up to 800%.
All ceramic cutting tools have excellent wear resistance at high cutting speeds.
There is a range of ceramic grades available for a variety of applications.
Oxide ceramics are aluminum oxide-based (Al2O3), with added zirconia (ZrO2) for crack inhibition. This generates a material that is chemically very stable but lacks thermal shock resistance.
(1) Mixed ceramics are particles reinforced through the addition of cubic carbides or carbonitrides (TiC, Ti(C, N)). This improves toughness and thermal conductivity.
(2) Whisker-reinforced ceramics use silicon carbide whiskers (Siew) to dramatically increase the toughness and enable the use of coolant. Whisker-reinforced ceramics are ideal for machining Ni-based alloys.
(3) Silicon nitride ceramics (Si3N4) represent another group of ceramic materials. Their elongated crystals form a self-reinforced material with high toughness. Silicon nitride grades are successful in gray cast iron, but a lack of chemical stability limits their use in other workpiece materials.
These benefits lead to increases in efficiency. For example, many machinists would slow cutting speed if temperatures were to approach maximum carbide limits. It is the opposite for ceramic tools, which excel in higher temperature conditions.
Advantages of silicon nitride ceramic cutting tools
Compared with carbide cutting tools, silicon nitride ceramic cutting tools have obvious advantages, which are shown in the following aspects.
● The HRA value of silicon nitride ceramic cutter is usually 91-93 (HRA is used to represent the hardness index of soft and hard materials), so it has good wear resistance and can be used to process high hard materials that are difficult to be processed by traditional tools or cannot be processed at all, such as various kinds of hardened steel and hardened cast iron with the processing hardness up to HRC65 (60HRA = 20HRC).
● It can be used for rough and finish machining of high hardness materials, as well as for high-impact machinings such as milling, planning, and intermittent cutting.
● The cutting tools made of silicon nitride ceramics can be cut at high speed to realize the function of “milling for grinding” for high hard materials, simplify the processing technology and improve the processing efficiency, so as to save time, electricity, number of machine tools and plant area and other effects.
● The silicon nitride ceramic blade has less friction with metal when cutting, which makes it difficult to stick to the blade and the roughness of the workpiece is low.
● The tool’s durability is several times or even dozens of times higher than that of traditional tools, which reduces the number of tool changes during machining, ensures the uniformity of the workpiece, and also ensures small taper and high accuracy.
● The cutting tool consumes the most abundant element silicon in the crust as raw material, which can save a lot of strategic metal materials such as tungsten, cobalt, and titanium.
Applications of silicon nitride ceramic cutting tools
Auto parts processing
A series of parts such as milling cylinder cover, cylinder sleeve, and automobile brake disc are processed with composite silicon nitride ceramic blade, which can improve the efficiency. For example, a milling cutter can increase its speed from 92r/min to 275r/min.
The rolling bearing adopts the technology of rough, fine, quenching, rough grinding, and fine grinding for a long time. For example, after hardening to 63HRC, the blank of the GCr15 precision forging bearing will be deformed and uneven and will get stuck in the rough grinding of the automatic grinding machine directly, and even burn the spindle of the grinding machine in serious cases. The finishing process of the bearing ring before quenching and the rough grinding process after quenching are combined into the finishing process of the ceramic cutter after quenching, which will make the workpiece fully meet the required size and geometric accuracy, realize the substitution of grinding by car and greatly improve the processing efficiency.
Hard cast iron processing
If the composite nitride ceramic tool is used to process the cold cast iron roll, the tool life is increased four times than that of cemented carbide; it can be used to process superhard nickel alloy spray welding roller bushing, which can greatly improve the cutting efficiency and speed, save time and cost and improve economic efficiency.
In the fields of machinery, metallurgy, mining, railway locomotive, aerospace, precision instruments, molds, etc., the technology can be reformed by using silicon nitride ceramic cutting tools to greatly improve production efficiency, save energy and reduce cost.