Ceramic components have a reputation for handling the heat that would melt steel, and for being relatively easy to form if precision is not critical. But a recent family of ceramics is said to add to those features high purity, durability, and precision manufacturing that can produce components with clearances (between piston and cylinders) as tight as 0.000075 in. In addition, says technical-ceramics manufacturer CoorsTek Inc., Golden, Colo. (coorstek.com), its CeraPure line of materials are USP Class VI compatible for medical and surgical applications. These materials include alumina (Al203), zirconia, zirconia-toughened alumina (ZTA), and a yttrium (Y2O3) partially stabilized zirconia (YTZP).
These materials are relatively light compared to stainless steel (densities of about 4 to 6 gm/cc versus 8 gm/cc). Many of the ceramics are said to work well in electro-surgical devices because of their high strength, stiffness, and dielectric strength. And in other applications, low friction and long-wear characteristics let them work longer than metal versions would.
An advantage of ceramics over traditional metallic components and polymers, says the company, is that its recent ceramics have greater thermal stability, strength, hardness, and insulating capabilities. The firm adds that the material characteristics can be adjusted to particular tasks.
Product manager Perry Goldberg says the design guidelines he can offer are somewhat temporary because the company is finding new ways to manufacture ceramic parts. “So far, we can produce diameters down to 100 microns and typical tolerances are as low as 0.5% of the required dimensions, without secondary machining. When tighter tolerances are needed, they can be produced with conventional diamond grinding or, in some cases, nonconventional methods. An erosion method we are experimenting with, for instance, shows promise,” says Goldberg. He adds that his group often makes design for manufacturing recommendations to proposed parts that trim costs, simplify production, and improve product performance.
While CoorsTek produces ceramic components for medical devices used in instruments and short-term implantables (less than 30 days), applications requiring permanent implantable ceramic components are the focus of the company's subsidiary, C5 Medical Werks, Grand Junction, Colo. Manufacturing operations at that ISO-13485 certified facility include ceramic forming by isostatic and dry pressing, extrusion, and injection molding for orthopaedic, spinal, and electro-implantable devices. After green forming (before firing), parts can be CNC milled and turned. Firing methods include gas and electric kilns, and hot isostatic pressing. Finishes are applied with spherical grinding and polishing, 5-axis CNC grinding, along with ID, OD, and surface grinding.
Another line of ESD (electro-static discharge) safe ceramics are intended more for electronics, but applicable to medical tasks as well. “CeraPure ceramics are intended to accommodate many of the market's next-generation requirements,” says CoorsTek Executive VP Mark Chenoweth.
A few ceramic properties
|Property, units||Test||CeraPure Alumina FG (fine grain)||CeraPure Alumina Plus (Higher purity)||CeraPure ZTA (Zirconia toughened alumina)||CeraPure YTZP (Yitria stabilized zirconia)||316 Stainless|
|Density, gm/cc||ASTM C20||3.8||3.92||4.01||6.07||7.9|
|Flexural strength (Mor) at 20C, MPa (103 psi)||ASTM F417||375 (54)||375 (54)||450 (65)||1,720 (250)|
|Elastic moldulus at 20C, GPa (106 psi)||ASTM C848||350 (51)||370 (54)||360 (52)||210 (30)||193 (tension)|
|Compressive strength at 20C, MPa (103 psi)||ASTM C773||2,500 (363)||2,500 (363)||2,900 (421)||2,500 (363)||(30.4)|
|Hardness, GPa (kg/mm2)||Knoop 100 gm||13.7 (1,400)||14.1 (1,440)||14.4 (1,475)||14.1 (1,440)||79 RB|
|Fracture toughness Klc, MPa-m0.5||Notched beam||4 to 5||4 to 5||5 to 6||13|
|Thermal conductivity at 20C, W/mK||ATSM C408||27.5||30.0||27.0||2.2||16.2|
|Coefficient of thermal expansion from 25 to 1,000C, 10-6/C||—||8.2||8.2||8.3||10.3||16.2 to 19.9|
|Thermal shock resistance, ΔTc, C||—||200||200||300||200|
|Max use temperature, C||No load condition||1,700||1,750||1,500||1,500||Melts about 1,371C|
|Volume resistivity at 25C, Ohm-cm||ASTM D1829||<1014||<1014||<1014||<1013|
|Ceramic values are from CoorsTek. Values for 316 stainless are approximate.|