Ever since iron-bearing meteorites were discovered on earth, metal has been the material of choice for making almost anything requiring greater strength than wood. Raw materials, those found by mining or direct discovery, require manipulation of some kind — whether by heat, mechanical forming, or chemical reaction. Aluminum is a case-in-point.
Mined from a narrow band of resources on the equator, clay deposits containing bauxite are separated and mixed with hot caustic soda and lime-producing aluminum oxide. This oxide, electrically melted with carbon electrodes, forms the metal that is then cast with trace alloys into ingots, from which the final products emerge via further processing. Making steel is somewhat easier. Iron ore can be directly smelted into pig iron and refined with strengthening alloys in carbon arc furnaces, also producing cast ingots, which are formed into items like gears.
Steel used for transmitting motion requires added strength and wear resistance not found in the as-formed (green) item, thus heat treating is required — as it is for aluminum and most other metals. However, steel is heavy, and aluminum is light; but aluminum cannot be made to have the compressive strength, tensile strength, or hardness required for gears to transmit forces from one tooth to another. While aluminum has an excellent strength-to-weight ratio, its hardness is not relative; it’s finite and measurable. An additional problem with metals is distortion resulting from heat treating. The only remedy for this nearly unavoidable issue is the choice of the heat treat process used.
Metals expand and contract — not only from thermal applications affecting the lattice, but also due to phase changes as they proceed through the strengthening process. Metals also lose strength rapidly at elevated temperatures. Alloying different metals is intended to blend the best characteristics of each metal to achieve the desired performance. For example, adding chromium to steel in excess of 12 percent increases oxidation resistance, forming stainless steel. Nickel increases the temperature range of iron making it austenitic, avoiding thermal phase changes. Adding zinc to copper (creating brass) improves the overall strength beyond that of either metal individually. I could go on and on describing the performance benefits of alloys. But metals must always be heat treated after producing their final shape, mostly to force phase changes to enhance the properties. But there are alternatives to metal that are slowly achieving long-desired application in the real world — ceramics and ceramic composites.
Ceramics possess unparalleled thermal resistance, superior hot strength, and excellent wear properties. But there’s one major roadblock – thermal shock sensitivity. You’ve likely heard of a hot ceramic dish cracking when placed in cold water. Low ductility, low thermal conductivity, high expansion coefficient, and high density spell thermal shock issues. You also may have heard of Pyrex® (borosilicate glass) kitchenware. Basically ceramics and glass like Pyrex are composed of oxides of minerals and/or metals. Pyrex, for example, is comprised of 80.6% SiO2 (silicon dioxide), 12.6% B2O3 (boron trioxide), 4.2% Na2O (sodium oxide), 2.2% Al2O3 (aluminum oxide), and trace amounts of iron oxide, calcium oxide, and magnesium oxide.
Thermal shock is a function of thermal expansion and thermal conductivity. In the case of cracking, if a non-Pyrex glass or ceramic with high thermal expansion is exposed to cold water, the surface contracts while the adjoining core can’t. This results in tensile stresses, which cause the material to crack. If the ceramic was more ductile like a metal, with even higher thermal expansion and higher thermal conductivity, the material would bend under the tensile stress rather than crack. Subsequently, there are many R&D efforts underway attempting to marry the ductility of metals with the high-temperature strength of ceramics. This is where the emergence of composites (combining dissimilar material/properties) into one homogeneous material is the goal.
One of the methods employed in creating ceramic composites is combining nonmetallic fibers with a ceramic matrix and sintering the component in a mold or supporting structure. In this method, these fibers provide the same strengthening effect that rebar (iron rods) does for reinforced concrete. Although not ceramic, carbon-carbon or carbon fiber composites (CFC) are made this way. CFC trays and other components — even golf club shafts — are made by layering (plywood-style) alternating woven carbon fiber sheets, followed by polymer impregnation and heating to meld the assembly in a preform mold. GE Aviation has developed a proprietary ceramic matrix composite (CMC) made from silicon carbide fiber and ceramic resin with a special coating for lightweight turbine blades. Much of the ceramic composite development is funded by government and/or aerospace money, as the required manufacturing techniques are both too expensive for private or consumer entities and primarily for structural applications. Someday, probably within the next 20 years, gears and bearings will be made with ceramic composites, eliminating lubricants and allowing operation at much higher temperatures and rotation speeds, resulting in improved drivetrain efficiency. No doubt when new composite ceramic drivetrain composites do materialize, they will likely first see application in military aircraft and space exploration.