ADVANCED CERAMIC TECHNOLOGY:
Why choose ceramic materials?
Advanced ceramic materials are often utilised in extreme environments in which other materials simply cannot cope, such as: ballistic armour, joint replacements, high speed cutting tools, high-temp engine and brake parts, electronic components, thermal insulation.
The principal advantages of advanced ceramics are: high compressive strength and stiffness, extremely high melting point, high hardness and therefore wear resistance, chemical inertness and low weight. The main disadvantages are: weakness in tension, relatively poor shock resistance and the difficulty to obtain complex shapes with close tolerances (which drives up the cost of manufacture). Nevertheless, by altering the design of parts and preferentially expose them to compressive forces, it is entirely possible to produce very reliable ceramic parts exhibiting vastly superior performance than those made from traditional materials.
Aren’t ceramics brittle?
That is usually the question which people ask first! I answer in different ways, but usually along the lines of “there are ceramics… and then there are ceramics” or “you’re thinking about your old grandma’s fragile porcelain teacups!”…
The simple answer is yes they are inherently brittle materials, but in the last few decades, there have been significant advances in ceramic technology. Advanced ceramic materials now exist (such as “cerium tetragonal zirconia polycrystals” or Ce-TZP for short) which surpass the toughness of some wrought magnesium alloys, and approach the toughness values of typical aluminium alloys.
How do “advanced ceramic materials” differ from clay and porcelain?
Ceramics for engineering applications can be broadly broken into “traditional” and “new” materials. We define traditional materials as those produced from minerals mined directly from the earth. The newer ceramic materials, those with well defined and controlled properties are produced from nearly chemically and phase pure starting materials.
What are the advantages and disadvantages of ceramics materials compared with traditional metals & composites?
> Much stiffer than conventional structural metals.
> Incredible compressive strength.
> Extreme hardness.
> Relatively low density.
> Very low coefficient of friction.
> Excellent corrosion resistance.
> Extremely high melting point.
> Dimensional tolerances difficult to control during processing (therefore expensive to manufacture)
> Poor shock resistance.
> Weak in tension.
How are ceramic parts produced?
Engineering ceramic materials first start out as very fine & chemically pure powders. These powders are formed into net shapes using one of the following processing methods:
Ceramic products are either cast (in liquid suspensions), extruded, or pressed (similar to forging) in what is called the “green” state, similar to the way metals are manipulated. After these initial shape-forming processes, they are fired (or sintered) at high temperatures to achieve their final consolidated shape. The temperature and sintering time is strictly controlled; it is dependent on the type of ceramic, the particle size and shape. The aim is to produce a product with close tolerances to the original desired shape in order to minimise (or even eliminate) the need for machining (which is expensive).
There are exceptions, such as “reaction bonded” ceramics which undergo an in-situ chemical reaction with a gas, forming the final product with minimal dimensional change (this process is sometimes used with Si3N4).
What gives ceramics these qualities?
A very good question. Actually I am convinced that the majority of product engineers today do not actually know what imparts the inherent qualities of the materials they are dealing with!
The answer is covalent bonding! Each atom is bonded to several others in a 3D interlocking macromolecular network. Metals have many “slip planes” which impart toughness.
If we could significantly reduce or eliminate all the imperfections, most materials would be orders of magnitude stronger than they are! This is in fact the reason that composite fibres are so strong. As objects become smaller, there is less chance of imperfections being introduced during the processing stage…
Why did you select High Purity Alumina as the structural component in the vértebræ housing?
The graph below shows the compressive strength to weight ratio of the highest strength materials traditionally used in the bike industry, compared with the type of Aluminium Oxide used in Vertebrae Ceramic cable outers:
96% Al2O3 ceramic 60% T1000/epoxy Bidirectional Carbon Fibre Composite Reaction Bonded Silicon Nitride (RBSN) Transformation Toughened Zirconia (TTZ) Aluminium 7067 Titanium 6Al/4V Reynolds 953 Maraging steel Magnesium ZK60A-T4 4129 Chromoly Steel
Alumina is a versatile material that offers excellent mechanical properties. Alumina is used in the following extremely demanding applications: ballistic armour, grinding media, prosthetic implants, spark plug bodies, cutting tools & abrasives.
> Alumina has a very high compressive strength – the grade of material we use has a compressive strength of over 2000 MPa (300,000 psi). This is 2 – 4x stronger than reaction bonded silicon nitride.
> Alumina has a lower density than other common engineering ceramics. The density is 3.65 g/cm3, 60% lower than zirconia.
> Alumina has a very high Young’s Modulus (degree of stiffness) – our grade of 96% alumina possesses a Young’s Modulus of 300-400 GPa (or 43-58,000 ksi). In comparison, Steel has a Young’s modulus of 210 Gpa, Titanium 120 GPa, and aluminium alloys only 70 GPa. Alumina is significantly stiffer than zirconia with comparable toughness KIC values.
> Alumina also has a very high hardness… harder than most ceramics, and any metal. In fact, most engineering ceramics are so hard, they normally require different testing procedures to metals as it is not practical to make the same economical test specimens! In comparison with all metals, hardness is best described as “off the chart!”. Each vértebra can theoretically support ~ 3.6 tonnes of force in the vertical direction. In comparison, the gear cables themselves break at approximately 160kg of tensile force.
Ceramic materials selection – Case study:
One of the best examples of the application of an advanced ceramic material is the manufacture of spark plugs bodies, which are slip cast from aluminium oxide. There are 4 of them inside every four cylinder internal combustion engine. The body of each and every spark plug must insulate the surrounding engine block from the strong electrical current which passes through its core. They are are continuously exposed to very high combustion temperatures inside the engine and must also maintain the high pressure which is generated by this severe environment. Yet spark plugs most often wear out or ‘break’ when the tungsten electrode erodes due to electrical arcing, not because the ceramic portion fails. Most people don’t stop to think about the widespread applications of ceramic materials such as this.
Do I think we will see the next generation of cyclists riding ceramic bicycles?
No, at least not homogeneous, or bulk, ceramic articles. While advanced ceramics are an excellent class of materials, there have a very limited application for bicycles.
The parts of a bicycle must be able to meet certain design parameters / restraints or requirements, for example adequate strength and toughness without being astronomically expensive.
Before ceramic materials can be safely applied, their physical properties must first be taken into account. For example, utilising ceramic materials almost always means redesign of that component to ensure that they are never subjected to large tensile forces.
They must also be made with accurate dimensions to fit with other standardised parts. As the components of a bicycle interact with each other to form an integrated system, it is often difficult to redesign just one component without needing to redesign the surrounding components.
For example, advanced ceramic materials would make excellent drivetrain parts, with higher stiffness & significantly longer lifetimes. To make a ceramic cassette sprocket is possible with today’s technology, but it is just not currently economically viable. This is because the shapes of such pieces are quite intricate, and since ceramic bodies undergo significant shrinkage after sintering, a lot of post-production machining is needed to achieve the final net shape. While this is possible with CNC’d aluminium alloys, it is difficult, and therefore expensive, to machine ceramics in the same way.
I am surprised that ceramic coatings haven’t boomed in the last decade, because of their superior hardness properties, but I suspect that people are generally pretty wary of all types of coatings (and incidentally – so was my professor!).