Technical ceramic materials are made up of inorganic and non-metallic phases, with essentially ion-covalent bonds, not completely glassy and generally consolidated by sintering a powdery compact in the form of the desired object.
These compounds which contain both metallic (Al, Mg, Zr, etc.) and non-metallic (O, N, C, B, etc.) elements, thus give oxides, carbides, nitrides, borides, etc.
Ceramic materials are generally hard and brittle at break while sometimes being characterized by very high mechanical strengths.
“Technical” ceramics are prepared from synthetic materials or purified natural materials. We can distinguish “oxide” ceramics and “non-oxide” ceramics.
They are characterized, compared to metals, by a very high melting temperature and great chemical stability even at high temperature.
Alumina Al2O3 is made from bauxite (hydrated alumina), mainly used for its properties of stability, purity, refractoriness, chemical inertia , etc.
A quarter of the alumina produced passes through the refractories.
Silica SiO2 is a vitreous silica, due to its low coefficient of expansion and the absence of change in crystalline phases in temperature, it has an excellent resistance to thermal shock.
Zirconia ZrO2 exists in one of the three crystalline forms (allotropic forms) monoclinic –1100 ° C - quadratic - 2300 ° C - cubic –2700 ° C (fusion). It is necessary to stabilize the zirconia in one of the high temperature structures in order to avoid fragmentation during cooling. The addition of a few percent of MgO, CaO, Y2O3 or CeO2 leads to this result.
Oxide ceramics are also widely used for electronics and electrical engineering because of the great diversity of their electrical properties (ceramics for electronics represent 70% of the world market for technical ceramics):
The most common are carbides such as SiC, the transition metal carbides TiC, ZrC, HfC as well as VxC, NbxC, TaxC, MoxC, WxC and nitrides such as Si3N4, AlN, TiN, etc.
Overall , these materials will have high hardness (B4C, TiC, SiC, WC, etc.), low toughness compared to that of metals and alloys (generally <10 MPa.m2) reflecting their brittleness and very good resistance to corrosion and wear.
At high temperature, they have a mechanical resistance which can be higher than that of common metals and alloys, good resistance to creep and to oxidation (especially Si3N4). From the point of view of thermal characteristics, they have a very low coefficient of thermal expansion (especially SiC) and a more or less high thermal conductivity depending on the type of material (that of AlN is high).
The properties depend on the processing methods which control grain size, porosity, characteristics of the grain boundaries and for which the table on the previous page gives some orders of magnitude.
Some applications of non-oxide ceramics: abrasives (SiC), valves and fittings (Al2O3, SiC), bearings and friction parts for the automobile (Si3N4), grinding parts (SiC, Si3N4, WC), sandblasting nozzles ( SiC, W2C, Al2O3), dispersing agents in polymer matrix composites for anti-abrasive applications (SiC, Si3N4), shields (B4C, SiC, BN-SiC, ZrC-TiB2), high temperature components in nuclear reactors (B4C, SiC, ZrC), large space mirrors (SiC) with a weight gain of 5 compared to glass, high temperature heat exchangers (Si3N4, SiC), cosmetics industry (BN), power electronics (SiC , AlN), cutting tools (cermets - ceramic-metal - based on WC or TiN, SiC, SiAlON, etc.).
Because of their properties (high melting temperature, high hardness, lack of ductility at low temperatures, brittleness, low toughness), ceramic objects are generally obtained by consolidation at high temperature (sintering) of a granular structure (part raw or green piece) developed by implementing a shaping process.
The most used shaping processes are casting, pressing, injection, extrusion. The deposition methods (vapor phase, plasma spraying, etc.) should be added to this. The organic liquids and auxiliaries used during these stages are removed (drying, debinding) before sintering.
The shaping of traditional ceramics can be done using a suspension (wet process), plastic paste (semi-wet process) or granules (dry process). These are essentially criteria of size and shape of the parts and production cost that govern the choice between these three paths.
From suspensionThe aqueous suspensions of mineral raw materials used for the preparation of ceramics are called slurries. They generally contain a significant fraction of large grains (> 40 μm). They are used in particular in the porous mold casting and pressure casting processes, for the shaping of objects of complex shape (decorative pieces or dishes) and / or of large size (sanitary).
In all cases, consolidation, called setting, must occur before the mold is removed and the part is handled. It occurs most often after extraction of part of the water from the slip and formation against the walls of the mold of a layer of wet material, called cake or cake. The shaping is always followed by a drying step.
In the case of mold casting, the water is transferred into the porosity of the mold. If the pores of the mold are significantly smaller than those of the cake, the transfer occurs without external stress under the effect of the capillary suction. The use of a degreaser made up of large grains (> 40 μm), which stabilize large pores within the cake, can therefore be very favorable. In practice, the pore size is centred on 1 μm for plaster molds and on 15 μm for resin molds (need to apply pressure in the latter case).
From plastic pastePlastic pastes behave like non-Newtonian fluids with a high threshold stress. They are used for shaping by injection, pressing or extrusion (parts of simple geometry and / or axial symmetry). Their water content depends on the nature of the clay contained in the mineral mixture and the shaping conditions. It varies between 18% (hard paste for extrusion) and 30% (soft paste for injection) of the mass of dry matter. In all cases, the products must be dried before cooking.
From pelletsThe shaping of parts of simple geometry can be carried out by pressing. To obtain a homogeneous filling of the pressing matrix, it is customary to use the raw materials in the form of spheroidized granules (generally between 300 and 600 μm in diameter) having a high flowability. Obtained by mechanical granulations or by spraying / drying, these granules contain only the water (or the binder) necessary for their cohesion (a few % by mass). It is therefore easier to dry the part. This route is widely used for shaping floor or wall tiles.
Sintering can be defined as the set of transformations which lead, by heat treatment and without total melting of the material, from an assembly of disjoint grains (the raw part) to a consolidated part (the sintered part). The welds which occurred between the grains may or may not be accompanied by densification and / or growth of the grains.
We can thus have:
- consolidation without densification (production of ceramic filters) - densification alone (sought for obtaining very fine microstructures);
- densification associated with growth (the most frequent situation).
If no liquid phase appears, sintering is said to be “in solid phase” with two cases:
- non-reactive sintering: a chemical constituent at the start, a part composed of the same constituent at the end,
- reactive sintering: densification is accompanied by one or more chemical reactions between the constituents.
If a liquid phase appears, the sintering is said to be “in the liquid phase”: the liquid phase (minority to maintain the mechanical strength of the part) can come from the simple fusion of a second constituent present (addition of sintering or impurity) or of an eutectic reaction between different constituents.
Depending on whether an external mechanical stress is applied or not, a distinction is made between natural sintering and sintering under load.