Hard tool ceramics

Electrodes for electric discharge machining are partially coated with pyrolytic boron nitride in order to prevent undesirable electric discharge between the electrode and a workpiece [330]. A boron nitride layer is used to prevent bonding of a conductive material, applied for indicating wear, to hard tool ceramics [331]. Equipment for applying pyrolytic boron nitride layers has been described [332], and also for applying boron nitride as protective sheets for immersion pyrometers for molten metals [333]. 

Technological Essentials for Producing Hard and Strong Tool Ceramics... 

In order to extead appHcatioas of cBN to iaclude machining of medium-hardness steels, modifications of the cBN were iatroduced. An example is the fabrication of siatered cBN tools by the same HP—HT process, but usiag biader and second phase (either metallic or nonmetaUic) such as TiN or TiC to iacrease toughness (171). In regard to phase distribution, cBN tools resemble cemented-carbide or alumina—TiC ceramic tools, but are tougher and have greater chemical stabUity. 

A hard, mst-resistant shaft of at least 0.25 micrometer finish is usually required. Common shaft surfaces are hardened tool steel, chrome plate, high strength bronze, and carbide and ceramic overlays. Test results over a broad speed range from 0.05 to 47 m/s (10 to 9200 fpm) iadicate that a coefficient of friction of 0.16—0.20 and a wear factor of 14 X 10 m /N(70x 10 ° in. min/ft-lb-h) are typical for dry operation of weU appHed grades of carbon—graphite (29). 

Though such capacitors usually lose their capacitance at high temperatures, their ESR typically improves. I picked a lOnF capacitor in Figure 4-13 and varied the temperature using Murata s design tool. The resonant frequency hardly changes, but the ESR does improve with temperature. That, like the previously reported dependence on frequency, is once again similar to an elko. But luckily, this does not cause the ceramic capacitor to evaporate as an elko would ... 

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