Composites

Ceramic Matrix Composites

Ceramic Matrix Composites (CMC’s) comprise a ceramic matrix reinforced by a refractory fiber such as silicon carbide, alumina or carbon. More readily available CMC’s today include alumina or zirconia as the base with silicon carbide fibers that can be designed as either short whiskers, or longer filaments.

Technical ceramics have been developed over a long time to offer outstanding properties that far surpass metals and certainly also plastics for hardness, chemical resistance, high temperature strength and electrical resistance. Common examples of technical ceramics include alumina (Al2O3), zirconia (ZrO), silicon carbide (SiC), silicon nitride (Si3N4) and Aluminum Nitride (AlN). However, one of the limiting drawbacks for technical ceramics in general is their relatively low tensile strength and subsequent likelihood for sudden/catastrophic failure under tension or impact.

In recent decades, producers have sought improvement against this drawback by incorporating high strength fibers into the matrix to gain fracture toughness, thermal shock resistance and improved dynamical load capability. This was first done with polymer bases to great benefit.  However, polymers themselves are limited in high temperature applications compared to technical ceramics. The role of the fiber is to fight the stress imposed by micro-cracks, and then eventually to bridge micro-cracks along the fiber to avoid sudden catastrophic failure. The mechanisms for fighting crack propagation are complex and are affected by fiber material, size, orientation and boundary effects.

So it has become possible and practical to produce technical ceramic composites with ceramic fibers to offer excellent high temperature strength properties as well as thermal and shock resistance. This has led to applications in gas turbine liners and shrouds as well as thrust control flaps for military jet engines. Brake disks and slide bearings have also benefited due to sudden and repeated high temperature cycling. Further, the incorporation of controlled porosity establishes a lower density that allowed applications where reduced weight is critical such as heat shield systems for space vehicles or some projectile and missile designs.

One additional benefit of some of these CMC’s is ability to fabricate larger sizes and incorporate shapes and features that extend beyond normal capability for technical ceramic production and firing. Limitations of the near-net shape might still require final machining to incorporate tight tolerance and part reproducibility. The parts being of technical ceramic materials cannot be successfully machined to tight tolerance with conventional single point cutting.  Diamond grinding is still the preferred method to achieve tight tolerances on final features although specialized techniques including lasers might be employed also for fine features.

Insaco has both the experience and capability to diamond grind CMC’s to precise tolerance. We have fabricated both civilian and series production military applications for many years and welcome the opportunity to discuss manufacturability issues with customer proposals.

Zirconia Oxide when alloyed with Yttria or Magnesium becomes an extremely hard technical ceramic. These materials are best known as the toughest of ceramics against impact and with inherent fine grain size can be fabricated to allow very fine edges and features. High density and low thermal conductivity are consistent, but electrical properties can vary with high temperature.