Which material pairings, in terms of material and surface roughness, minimise abrasion with abrasive ceramic powders?

Question

Accepted Answer

How amixon® minimises abrasion and the ingress of foreign particles

amixon® reduces abrasion first of all by design: the mixing tools deliberately run at low speed (typically approximately 0.8–3.5 m/s), without crushing and impact zones, and the SinConvex® total flow mixes by forced restratification rather than by high-energy throwing and shearing movement. Wear is thereby kept small at source, before materials technology even comes into play. Building on this, material pairings are selected clearly according to the application: from wear-resistant base material through hard-alloy weld overlay with tungsten and chromium carbides and smoothly grindable armouring for highly abrasive powders, up to full-surface oxide ceramic coatings or hard metal and ceramic tiles for large, slow-rotating mixing tools, including on-site repair without dismantling and with a self-polishing effect in highly stressed zones.

Foreign particles are consistently excluded by design: mixing chambers welded free of joints and ground, without screw connections in the product chamber, mixing shafts supported exclusively at the top without a lower shaft passage, dynamic shaft seals – executed, depending on the industry and the mix, as a gland packing, a PTFE lip seal, a labyrinth or gap seal or as a mechanical seal (dry-running, gas- or liquid-flushed) – and the static OmgaSeal® seal at the inspection doors prevent ingress from seal and bearing wear just as they prevent contamination from outside. Realistically achievable abrasion and metal contamination values depend on product, geometry and operating regime; amixon® therefore determines them in the pilot plant with the original product, up to and including fully ceramic-coated mixing dryer synthesis reactors that can be operated from fine vacuum to overpressure and up to approximately 350 °C, and designs speed, mixing time and material pairing specifically for the target value before a production plant is specified and the investment made.

Verification in the amixon® pilot plant

Evidence comes before the investment: the amixon® pilot plant at the Paderborn headquarters has 35 test units of various sizes available, complemented by pilot plants in Japan, India, Thailand, China, South Korea and the USA. The trials run with the original product, with real fill levels and batch sizes, within the intended temperature and pressure range. Mixing quality, product protection, energy input, cleanability and reproducibility in later series operation are assessed; the results are evaluated and documented together with amixon® experts, as a robust basis for decision-making that removes technical and economic risks before purchase.

Answer

With abrasive ceramic powders, wear-resistant, dense and ideally low-porosity materials minimise abrasion above all. What is decisive is not only hardness, but also the combination of material pairing, surface condition, toughness and the constructional design of the contact-loaded components.

Suitable material pairings

Ceramic on ceramic can be very wear-resistant, for example based on aluminium oxide, silicon carbide or silicon nitride. Such pairings are particularly suitable where high hardness, good chemical resistance and low contamination are required. They only make sense, however, where the construction also takes account of the relatively low fracture toughness and the risk of brittle failure.

Hard metal on hard metal or hard metal on ceramic are likewise proven solutions where, alongside high hardness, a certain fracture toughness is also needed. WC-based materials in particular are frequently a robust choice for highly abrasive media, provided the binder phase and the material quality suit the loading.

Metals with hard coatings make sense where a metallic base body is required for constructional or economic reasons. Coatings such as TiN, CrN, TiAlN, DLC or wear-resistant thermal spray layers can reduce abrasion considerably, but they do not replace a sound basic construction.

Tougher materials with a wear-resistant design can also be suitable in individual cases, where the priority is not maximum hardness but rather impact resistance, damping or cost aspects. With highly abrasive ceramic powders, however, they usually make sense only where the loading is not primarily determined by pure sliding or flow abrasion.

Surface roughness

For contact surfaces with abrasive ceramic powders the following applies in principle: the smoother and denser the surface, the lower the abrasion usually is and the lower the particle adhesion. A very rough surface can retain particles, create local points of attack and thereby accelerate wear.

For many applications, finely ground or polished surfaces make sense, but the optimal roughness depends on the contact mechanism:

Very smooth surfaces are favourable at sealing faces, sliding contacts and closely toleranced gaps.
Medium roughness can be acceptable in some conveying or guiding areas, where lubrication, self-cleaning or flow play a role.
Surfaces that are too rough are usually unfavourable with abrasive powders, because particles can anchor themselves in the topography.

A rigid limit such as Ra ≤ 0.2 µm or Ra ≤ 0.4 µm is of little help. The surface should be as smooth as is functionally and technically sensible to manufacture, typically in the finely ground or polished range.

Porosity and microstructure

With ceramic materials, low porosity is also important. Open pores, micro-chipping or microstructural defects act as points of attack for abrasion and as starting points for crack formation. It is therefore not only the chemical composition that is decisive, but also the density and quality of the material microstructure.