Materials - RHV Technik

Materials

Coatings

Materials for thermal coatings.

Special requirements call for different solutions

Essentially, we use these materials for thermal coatings.

The requirements of a thermal coating can be very different. That naturally also applies to the properties of the base material being coated. That is why we use a wide variety of materials for thermal coating.

Coating materials that are often used are different metals (e.g. chrome-plated steel, bronze, nickel base alloys and many others), oxide ceramics (chromium oxide, zirconium oxide) or carbide coatings (e.g. tungsten carbide and chromium carbide).

The usual coating thicknesses are in a range from 50 µm to 500 µm, but they may also be several millimetres, depending on the coating material. As varied as the range of materials is, which may be applied on the provided component, the areas of application for thermal coating are equally varied.

The applications for thermal surface coating range from drive shafts from the mechanical engineering sector, which are made more wear-resistant by means of a coating of chromium oxide, or bearing bushings, which have better gliding properties when coated with bronze, through to corrosion protection against gases and moisture. Due to this variety, companies from all sectors of industry are among the customers of rhv-Technik.

Ceramics

Wherever high wear resistances and hardnesses, chemical resistances to acids and alkalis, high temperature resistance or insulation against thermal or electrical conductivity are required, ceramics are particularly well suited. For example, ceramic coatings outperform hard chrome coatings in terms of wear resistance and hardness many times over. We use different ceramic spray materials for ceramic coating:

  • Aluminium oxide titanium dioxide (Al2O3TiO2)
  • Aluminium oxide (Al2O3)
  • Zirconium dioxide (ZrO2)
  • Impressive properties, for example:
  • tolerance to temperature changes
  • enormously pressure resistant
  • can be used in a vacuum or in an environment with different gases
  • prevents the so-called “pick-up effect” on drums and rollers
  • resistant to friction and abrasion

The service life of machine parts and equipment can be significantly increased by being coated with chromium oxide ceramics, which in turn reduces material usage and machining costs. A possible application area is: pumps, especially their shafts, housings and impellers. The ceramic coating protects against corrosion, erosion and also wear at high temperatures. Another objective is to create electrical insulation.

The coatings can be applied to almost any carrier material and feature the following properties:

  • very high melting point up to approx. 500 °C
  • great hardness
  • resistance to chemical influences
  • good adhesion and elasticity
  • low coefficients of expansion
Metals

Metallic materials are usually sprayed in wire form during thermal coating. The spray material can be processed as a solid or flux-cored wire, which means almost any combination of materials is possible. The common coating methods are wire or arc flame spraying. In the case of the arc method, two metallic materials can be combined, which then melt together to make a so-called pseudo alloy. The advantage of this method of procedure is that two properties can be brought together in one coating.

If flux-cored wires are used, even materials with a carbide content can be processed in wire form. Metallic materials are ideally suited and highly recommended especially for repairs, maintenance work or scrap salvage operations.

This type of repair is very economical and in addition, unlike with the other methods, the coatings can be sprayed on up to several millimetres thick.

The range of sprayable metals is as wide as the possible application range of the components being coated. Here is a selection:

  • When it comes to corrosion protection, zinc is a standard coating material. In case of corrosion attacks by salt water or seawater, aluminium is even more suitable.
  • Molybdenum offers a high resistance to abrasion wear and at the same time forms outstanding protection to friction or cold welding due to its high dry-running properties. Typical cases of application are e.g. piston ring coatings. If the molybdenum coating is not subsequently mechanically reworked, this leads to a higher coefficient of friction.
  • When it comes to plain bearings, bearing metal coatings using bronze, white metal or brass are highly suitable.
  • If the base material of the original component has a lower bonding capacity, the coefficient of expansion can be bridged by means of a so-called bonding material, for example nickel aluminium.
  • If the components come into contact with chemically aggressive media, such as shafts, pump parts or bearings, then Hastelloy, an alloy of nickel, chromium and molybdenum, is the material of choice.
  • Components that have been made out of a relatively cheap base material can thus be enhanced by a thermally applied Hastelloy coating. In these cases, components are only suitable for the intended application after the finishing process by means of thermal coating.

Components that really ought to be made of 1.4404 can be produced using a more cost-effective material and then coated with 316L. Particularly when repairing components made of stainless steel, which must have good resistance to acid and corrosion, a solution can be found thanks to thermal spraying.

Other metallic materials may be MIG35, which is characterised by a boron content and therefore reaches a high level of hardness.

Repairs can also be carried out with chromium steel (13 % / 17 % / 19 %) or carbon steel (without chromium). The coating is characterised by a high level of hardness, as is required e.g. for shaft bearing seats.

Carbides

Carbide materials expand the application range in the surface technology of thermal spraying many times over. Carbide materials are applied using the high velocity oxygen fuel method, also referred to as HVOF for short.

Carbides, compounds of carbon and metal, come in many combinations.

Tungsten carbide-cobalt chrome, WC-CoCr, and chromium carbide-nickel chromium, CrC-NiCR, are among the common carbides in the field of thermal surface technology. Any other combination based on nickel or cobalt is also possible, as the so-called matrix of carbide shows, such as WC-Co or WC-Ni.

So-called MCrAlYs are often used when resistance to oxidation, hot gas and sulphur corrosion is required, and thus constitute a particularly corrosion-resistant bonding coating. Operating temperatures up to 850 °C are conceivable.

Main applications for carbides for surface optimisation purposes:

  • abrasion
  • erosion
  • corrosion
  • sliding wear

Advantages of carbide coatings:

  • very high hardness (up to 1600 Vickers HV0.3)
  • temperature resistance
  • low porosity
  • corrosion resistance
  • temperature resistant up to 850 °C

In addition, thermally sprayed carbide coatings are an optimal alternative to conventional hard chrome coatings. A tungsten or chromium carbide coating is particularly ideal for screw conveyors and rotors. If wear is incurred on the rotor due to the conveyed medium, the carbide coating, unlike the chrome-plated layer, is gentle on the stator.

Another advantage of carbide coatings is the ability to vary the surface roughness. If these coatings are mechanically reworked, polished surfaces can be produced. In addition, rhv-Technik can produce surfaces “as sprayed” to generate certain roughness levels if particular conveying functions of media need to be fulfilled.

Self-fluxing alloys

Materials based on Ni-B-Si represent the group of self-fluxing alloys. Self-fluxing alloys are applied in thermal surface technology using the powder flame spraying method and subsequently melted.

During the fusing process, the coating is so compressed that it is possible to talk of gas tightness, where there is a major application. The coating is also characterised by an extremely high wear resistance, which can even be subject to punctiform or linear loading. Particularly when they come into contact with aggressive media, molten thermal spray coatings are the first choice.

Unlike the previously described spraying methods, a microstructural change takes place in the base material when molten, as this is melted on in order to produce a substance-to substance bond with the self-fluxing alloy.

A wide range of carbides can be mixed with the spray material, which develop coating hardnesses of over 60 HRC after the melting process.

Special developments for individual requirements during thermal coating.

As an experienced specialist for surface technology and thermal coatings, we naturally also develop material combinations based on the requirements specified by our clients. Where conventional materials reach their limits, rhv-Technik enables new ways and possibilities. We have three critical pillars for achieving very special coating solutions.

Firstly, we bring almost unique expertise to any project in the field of surface technology.

Secondly our laboratory and development staff are seasoned professionals, who enjoy a first-class reputation across all industries when it comes to individual thermal coatings.

Thirdly, not only do we have efficient production with a high vertical range of manufacture in house, but also a first-class laboratory.

Special developments for you and your application are always possible.

We combine various materials and create the basis for very special product properties, which are oriented towards your individual requirements profile.

 

Is quality as important to you as it is to us? Do you also have high and individual coating requirements?

 

Then get in touch with us with just one click.

Martin Dauner, Leader F+E and Laboratory

 

 

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