Background Powder processing techniques for ceramics and metals have inherent features which restrict their application. Viscous plastic processing minimises defect sizes and widens the range of shaping options. The technology, available under license from Ceram Research, offers cost effective manufacturing routes and enhanced product performance and can be used to produce a range of ceramic shapes (figure 1).
Figure 1. A range of ceramic shapes produced by VPP.
Advantages of the VPP RouteAdvantages of VPP over other more conventional processing routes include:
· Enhanced material consistency
· Easier green handling
· Increased strength
· Improved creep resistance
· Increased fracture toughness
· Superior surface finish
· Rapid processing
· Environmentally friendly systems
What is VPP?Viscous plastic processing (VPP) is a technique which has been developed to produce ceramics with improved properties by eliminating microstructure defects. A major cause of these flaws is the agglomeration which occurs spontaneously in fine powders such as those used to produce ceramics. By carefully controlling the mixing conditions, VPP breaks down these agglomerates.
The doughs produced can be processed using a range of polymer forming techniques which extend the options for both simple and complex shapes. These technique have been applied to an extensive range of materials, including advanced refractory ceramics and tile bodies.
The VPP TechnologyIn VPP, ceramic powders are mixed into a viscous polymer solution under very high shear conditions, such as using a two-roll milling technique. The process is appropriate for most ceramic powders.
Ceram Research has established that the final ceramic will attain its ultimate properties only when the polymer/powder mix possesses a certain rheology. Visco-elastic behaviour is highly dependent on the types and amounts of polymer and solvent selected for use with fine ceramic powders.
Work at Ceram Research has determined the precise levels of various polymers and solvents required to give the optimum rheology for agglomerate breakdown and processing. The technology is available under licence from the Company.
The Benefits of VPPVPP is superior to alternative methods as it completely de-agglomerates the powder during mixing and maintains that state during the green forming and shaping stages through to sintering. The ceramic paste has high green strength so components in process can lie handled with low losses. Conventional or novel plastic forming processes may be used to shape the ceramic paste and the green material is easily machined offering economies for difficult shapes.
Mechanical PropertiesViscous plastic processing can result in products of much greater flexural strength than is achieved by powder pressing (table 1).
Table 1. Strength comparison of pressed and VPP manufactured components.
Material Powder Pressed(MPa) VPP & Extrusion(MPa) Titania 190 760 Alumina 290 980 Silicon Carbide 425 1087 Mullite/Alumina(kiln furniture) 7.1 18.4 Zirconia HSY-3 690 995 Tile Body 35 44
Products are much more reliable and offer improved creep resistance and increased fracture toughness compared with those produced conventionally. A particular benefit is the homogeneity of the product which results in consistency in strength from component to component.
Physical PropertiesDependent upon the grain size of the powder, a superior surface finish is obtained. VPP provides further advantages by improving other flaw-controlled properties such as translucency, electrical resistivity and dielectric loss.
ProcessingThe VPP technique is very rapid - raw powder can bee processed into extrudate or calendared tape in minutes. It also satisfies environmental conservation criteria as the systems used can be based on water and non-hazardous binders.
ApplicationsThe use of VPP is not limited by chemistry of the ceramic materials. Binder systems have been developed for oxide, non-oxide and metal powders; aqueous or non-aqueous solvent systems are available and have been applied to powders of submicron to millimetre sizes. Further to this, plastic forming techniques are sufficiently versatile to produce thin flat sheets or complex extrusions and mouldings (figure 2).
Figure 2. Industrial scale production of advanced technical ceramic cups by VPP.
The enhanced properties of VPP ceramics provide the opportunity to design components with reduced weight and thinner wall sections, without compromising performance. This means that they are ideal in applications where material reliability is critical, including aerospace components as well as precisely engineered items such as high specification kiln furniture.
Potential applications are still being explored, but opportunities for VPP in ceramic materials processing are proven it the following areas:
· Kiln furniture
· Specialist refractories
· Body armour
· Tableware
· Roof, Floor, and Wall tiles
· Composites
· Structural engineering ceramics
· Electroceramics
· Bioceramics
· Metal filters
· Heating elements