Ceramic tools have high resistance to heat and to wear and can be used to machine metals that are extremely hard, and they are chemically stable. These attributes allow them to be used to machine metals at high cutting speeds and in dry machining conditions because it is not necessary to reduce temperatures on the cutting edges of these tools.
However, these favorable properties are exchanged in machining for lower toughness when these tools are compared with carbide tools. This deficiency can be offset by selecting an appropriate ceramic cutting grade and the type of tool.
Ceramic tools are based primarily on alumina (Al2O3) and silicon nitride (Si3N4) compounds and are available in a variety of grades that include ceramics mixed with other materials and reinforcing whisker materials that make them harder.
Reinforced or whiskered, ceramics use extremely fine-grained silicon-carbide crystals that are called “whiskers” because they resemble small hairs under a microscope to reinforce and toughen basic ceramic compounds.
In ceramic tool materials, single-crystal silicon carbide whiskers, on the order of one micron in diameter and 0.003937 in. (100 microns) in length, are intertwined within the alumina-matrix structure. These whiskers have a tensile strength of about 1 million psi and dramatically improve the fracture toughness of the tool material. They also effectively block and prevent propagation of cracks.
Cutting with ceramic inserts requires high surface speed and balanced feedrates. High speed is necessary to generate the high temperature in the shear zone and to ensure that the heat propagates into the chip-forming zone immediately ahead of the cutter. When cutting speeds are too slow, insufficient heat is generated to soften the material in this zone, and the cutting forces are raised and insert failure occurs.
A strategy for using ceramic inserts is to program fewer, but deeper cuts that bury the insert deep in the workpiece. This moves the notch formation further up the face of the insert to an area that has a larger, stronger cross section. Ramping cuts should be programmed to accommodate these tools and fixed depths of cut should be avoided to spread wear over a larger section of the insert.
When machining interrupted cuts with reinforced ceramics, it is important to keep the speed of the cutter high. A rule of thumb is to estimate the percentage of voids in the workpiece surface and increase cutting speed by that percentage. This increase in surface speed offsets the loss of heat generation created by the voids.
Whiskered ceramics work best on hard ferrous materials and difficult-to-machine nickel-base alloys, including Inconel, Waspoloy and Hastelloy. They do not work well on ferrous alloys below Rc 42 hardness because of the chemical reaction that occurs between iron and the carbon that is part of the silicon carbide reinforcing material.
When used with powerful machine tools, silicon nitrides enable high cutting speeds (more than 800 in./min.) and feeds (0.2 to 0.3 in./min.) for rough boring cast iron.
Silicone nitride tools offer fracture resistance, but their relatively low resistance to chemical wear limits their use in machining nodular cast irons. However, wear-resistant, chemical vapor deposit (CVD) alumina coatings have expanded the range of applications for silicon nitride-based tools to include these difficult-to-machine irons. Gray cast iron and nodular graphite cast iron can be milled at cutting speeds of 500 to 800 in./min or sometimes faster than 1,000 in./min.
Five Types of Ceramic Cutting Tools.
There are five types os ceramic cutting tools. Here are the basic differences:
ALUMINIUM OXIDE OR WHITE CERAMIC
NTK GRADES: HC1, CX3, HW2
Pure alumina strengthened by Zirconium.
MIXED OR BLACK CERAMICS
NTK GRADES: HC2, HC4, HC5, ZC4
Alumina with metallic phase of Titanium Carbide and Titanium Nitride which improves thermal conductivity.
SILICON NITRIDE BASED CERAMICS
NTK GRADES: SX1, SX2, SX8, SP2
WHISKER REINFORCED CERAMICS
NTK GRADES: WA1
Pure alumina structurally strengthens from introduction of Silicon Carbide in the form of whiskers.
NTK GRADES: SX5
Alumina substrate strengthened by Silicon Nitride.
The above text is from American Machinist's Cutting Tool Central. For up to the minute information follow them on Twitter @AmericanMachnst
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