New materials would facilitate the development of new technologies. The invention, for instance, of the cast iron process made possible the steam engine.
Engineering systems and products are becoming increasingly sophisticated. They are required to perform more complex tasks than ever before as well as perform more and more under severe operational conditions. Such systems and products are demanding more of the materials from which they are being made.
As most metals are approaching the limits of their capability, engineering ceramics are emerging as the most desirable alternative for various high performance applications.
Advanced or engineering ceramics could be ideally suited for such high performance applications where a combination of properties such as wear resistance, hardness, stiffness and corrosion resistance are important. In addition to these properties, engineering ceramics have relatively high mechanical strength at higher temperatures.
What are Engineering Ceramics?
The word ceramics comes from the Greek work keramos which means pottery. The word keramos which, in turn, has its roots in the Sanskrit word "to burn". The Greeks used to refer to pottery as "things made from burnt earth".
Like the clays used for making pottery, engineering ceramics are mainly formed in the wet plastic state, dried and then sintered ("fired") at high temperatures.
Monolithic engineering ceramics are usually derived from inorganic materials and often have non-metallic properties. They are generally good thermal and electrical insulators. Examples include ionic-covalent materials such as Alumina (Al2O3), Zirconia (ZrO2) and Magnesia (MgO) as well as covalent materials such as Diamond, Cubic Boron Nitride (CBN), Silicon Nitride (Si3N4) and Silicon Carbide (SiC).
Engineering ceramics are distinguished from metals and some alloys by their exceptional properties:
Engineering ceramics are very hard materials and are highly wear-resistant. Indeed when compared to their metal counterparts, engineering ceramics components are more durable and have longer life spans under given operational conditions. Ceramic blades, for instance, in their lifetime often require no sharpening or replacement due to wear, and will last at least 60 to 100 times longer than steel blades (references 3 and 7).
Engineering ceramics are high temperature materials. Whilst metals weaken rapidly at temperatures above 8160C (15000F), engineering ceramics retain a good degree of their mechanical properties at much higher temperatures. For example, the maximum service temperature of Zirconia is 20770C (38120F), and for Alumina is 19490C (35400F), while that of Silicon Carbide is 16490C (30000F). In contrast, the nickel alloys are seldom serviceable above 8160C (15000F). Thus the reputation of engineering ceramics for heat resistance is well justified (reference 7).
Most engineering ceramics can withstand highly aggressive and corrosive environments. They are chemically resistant to most acids, alkalis and organic solvents.
Why use Zirconia Engineering Ceramics?
When compared to other ceramics (such as silicon nitride, silicon carbide, CBN and alumina), zirconia ceramics offer a combination of properties that make them attractive candidates for wear-resistant applications under higher load-bearing conditions (references 5, 7).
Applications using zirconia ceramics include cutting blades, valve trains in engines (e.g. components limited by wear such as cams, tappets and exhaust valves), seals in valves, slurry pump components and cutting tools (references 5,7,12).
Of the various zirconia ceramics currently available, CeTZP often does not suffer from hydrothermal degradation; that is, of the various zirconias, CeTZP is the most stable against degradation under humid conditions and is also more stable against leaching in aqueous environments (see below).
UHM's CeTZP Engineering Ceramics
A comparison of the mechanical and thermal properties between UHM's CeTZP engineering ceramic and various materials are given below: