Ceramic coating

Coating

Surface coating for metal and plastic

Based on our already developed systems, we can offer you a wide range of different surface coatings for ceramics. Since each system – from the choice of the precursor to the finished sol – was developed and synthesized in-house, a functional optimization of our sols to your special requirements is possible. In the past we have often succeeded in adapting laboratory processes for the synthesis and coating of brines to industrial applications. Within the scope of feasibility studies we offer to carry out first coating screenings on your samples.

In addition, we also take on new challenges to make ceramic coatings possible. Ceramic coatings based on ceramic slurries or geopolymers can lead to completely new coating properties. We work daily to make our CerCoat® range more versatile.

Ceramic layers

When are ceramic coatings used?

Ceramic coatings are always used when other materials reach their material limits. For example, the conditions and requirements at the interfaces of metallic constructions are often different from those in the volume of the material itself. These can be thermal stress, excessive wear, electrical conductivity or contact with melts. In these cases, a ceramic coating can achieve an improved and/or additional functionalization of the surface, extend the service life of components and save costs. The application of ceramic coatings thus leads to a functional separation of the surface and the actual material, with the aim of optimally adapting the surface properties of the component to the environmental influences.

What properties do ceramic coatings offer?

The possible properties of ceramic coatings are as versatile as the material-specific properties of ceramic materials themselves:

  •  Electrical isolation
  •  Low adhesion / Non-stick properties
  •  Low thermal conductivity
  •  High thermal isolation (TBC)
  •  Improved sliding properties
  •  High hardness
  •  Resistance to acidic and basic mediums
  •  Resistance to organic solvents
  •  Piezoelectric properties
  •  Heat and cold resistance
  •  Biocompatibility
  •  Corrosion resistance
  •  Increased wear resistance

No ceramic material can cover all the above properties at the same time. The final property of the ceramic coating, however, depends largely on the starting raw materials used. Therefore, the choice of raw materials must be carefully matched to the respective application. 

Procedure

Which processes are used to produce ceramic layers?

In the meantime, a whole range of processes for the production of ceramic coatings has become established. A distinction is usually made between thick and thin-film techniques. For the production of thick layers (>30 µm), thermal spraying processes such as flame and plasma spraying are preferably used, in which the material to be coated is first melted or fused on and applied to the substrate surface with the aid of a spraying device. For the production of thin layers (<30 µm), on the other hand, processes such as PVD, CVD, galvanic processes and, more recently, increasingly the sol-gel process are used. In the PVD process (Physical Vapour Deposition), the coating material is generally put into a vaporous state in order to then be deposited as a thin layer on the component surface. In the CVD (Chemical Vapour Deposition) process, a gas with the coating elements reacts with the substrate surface in a reaction chamber under the influence of heat, resulting in the desired coating.

Sol-Gel

What is the principle of sol-gel coating?

The advantage of the sol-gel process over all known coating processes is primarily the significantly reduced production costs of the ceramic layers. The process does not require complex equipment or closed chambers, nor does it require a high energy input. Therefore, the sol-gel process is the most sustainable and environmentally friendly method of applying ceramic coatings. In addition, unlike others, the process does not pose a safety risk for its user. Since several processes (dip, spin or spray coating) can be used for sol-gel coating, it is possible to produce both relatively thick (up to 50 µm) and very thin layers (approx. 500 nm), thus covering a very wide range of layer thicknesses.

Advantages

What are the advantages of sol-gel coating over PVD, CVD and thermal spray processes?

The advantage of the sol-gel process over all known coating processes is primarily the significantly reduced production costs of the ceramic layers. The process does not require complex equipment or closed chambers, nor does it require a high energy input. Therefore, the sol-gel process is the most sustainable and environmentally friendly method of applying ceramic coatings. In addition, unlike others, the process does not pose a safety risk for its user. Since several processes (dip, spin or spray coating) can be used for sol-gel coating, it is possible to produce both relatively thick (up to 50 µm) and very thin layers (approx. 500 nm), thus covering a very wide range of layer thicknesses.

What advantages do thin films offer over thick ones?

A ceramic coating on a metallic substrate not only offers advantages, but also harbors dangers. Due to the brittle material behaviour, ceramic coatings can easily flake off when bending stresses occur. Differences in thermal expansion can also lead to failure. Basically, it can be said that the danger of flaking increases with the thickness of the layer. Therefore, a major advantage of thin coatings is their better resistance to spalling.

Coating

What coating processes do we offer?

We apply sol-gel layers using two different methods:

In the dip-coating process, the component is automatically immersed in the sol. Pulling it out then causes a layer to be applied to the component. It is important in this process to work at a continuous speed both during dipping and pulling out, and to avoid irritation of the sol during coating in order to avoid inhomogeneities and defects in the resulting layer. The thickness of the coating can be controlled by the speed of the pull-out. If it is adjusted slowly, the result is a thinner layer. If it is set faster, a thicker layer is obtained. Dip-coating allows layers of approx. 500 nm to a maximum of 2 µm to be applied, although the process itself can be used for simpler geometries.

In contrast, the spray coating process can be used to coat much more complex geometries. Here, the sol is applied to the component in finely atomized form with the aid of a spray gun and air pressure. The process allows multiple applications, allowing the desired layer thickness to be set very precisely. In this way, layer thicknesses of approx. 1-50 µm can be achieved.

In the case of both processes, the components with the applied layers are first dried after coating and must then be cured in the oven at moderate temperatures, whereby the final properties of the coating are created.

CERAMICS COATINGS

What ceramic coatings do we offer?

Our coatings are mainly based on SiO2 systems, for whose production different precursors are used. The choice of the precursor used for the synthesis of a sol has a decisive effect on the final properties of the coating. We have therefore developed various systems with different properties, which are known as CerCoat®.

CerCoat® - H forms a wafer-thin, glass-like, transparent coating that is characterized by outstanding hardness. Additionally, coated metal surfaces are optically enhanced. The characteristics of CerCoat® - H are

  • increased scratch resistance (hardness can be adjusted according to requirements)
  • very low film thickness (< 1µm)
  • Dishwasher resistant Solvent resistant Facilitates the cleaning process
  • Curing temperature > 180°C

CerCoat® - H is applied by us as standard using the dip coating process.

By coating with CerCoat® - Y, hydrophobic properties can be achieved on a component surface. The system dispenses with the usual fluorine modification, making it significantly more environmentally friendly. CerCoat® - Y has the following properties:

  • Hydrophobic properties
  • Contact angle from 90° to 110° against water
  • Reduction of the adhesion of Plastic melting in the injection molding process
  • Facilitates the cleaning process
  • Resistant to chemicals and solvents
  • Curing temperature >200°C
  • Transparent layer

CerCoat® - P is a system in which ceramic particles are added to the sol. Thus a structuring of the surface is achieved. The layer thickness can be varied by the filling degree and/or the number of layers applied. The wear resistance is significantly increased compared to unfilled sols. Additionally, the electrical insulation and dielectric strength are increased. CerCoat® - P has the following properties:

  • By adding ceramic particles a layer thickness up to ≥ 80 µm is achieved
  • Wear resistance, depends on the degree of filling with particles
  • Electrical isolation ≥ 1,5kV
  • Hydrophobic properties
  • Contact angle from 90° to 110° against water
  • Transparent to coloured coating, depending on the type and filling level of the particles
  • Coring temperature > 200°C to 300°C

CerCoat® - P is applied by spray coating.

TBC Coatings

High temperature resistant coatings

Since 2019, WZR has been working on the topic of TBC systems (Thermal Barrier Coating). The aim of such coatings is to protect the substrate (typically metals) from high temperatures. This is achieved by using temperature-resistant materials with very low thermal conductivity. The application temperature of such systems is significantly higher than previously developed coating systems, which WZR calls CerCoat®-H, -Y and -P. All these systems are based on sol-gel processes that require a minimum curing temperature of 190°C and have a maximum application temperature of approx. 450°C.

The newly developed high temperature resistant coatings are now called CerCoat®-HT. The basis is a ceramic suspension that enables adhesion to metallic substrates with special binders. A temperature-resistant bonding is achieved by a heat treating process between 500°C and 650°C, which ensures the subsequent application temperature up to at least 650°C.

Due to its low thermal conductivity and special structure, yttria-stabilised zirconia (YSZ) is the most commonly used material for TBC systems. WZR follows the approach of mixing alumina with special hollow spheres and thereby achieving high thermal insulation with simultaneous high temperature resistance.

The new coating system can be applied by dip coating as well as by spray coating. Spray coating is often preferred because it enables the coating of complex geometries and the layer thickness can be varied as required.

Our TBC coatings are characterised by the following properties:

  • Low thermal conductivity of app. 0,4 W/m K @ 700°C
  • Layer thickness from 100 to 200 µm
  • Heat treatment tested up to 650°C
  • Very good adhesion measured by cross-cut test: Rating 4 to 3 according to ASTM D3359
  • Very good thermal shock resistance determined in thermal shock tests
  • Ageing of samples for 100h at 600°C: without findings

Both dip coating and spray coating are suitable coating processes. Spray coating is often preferred because it allows the coating of complex geometries and the layer thickness can be applied as desired.

The cross-cut test was evaluated according to DIN EN ISO 2409 and ASTM 3359. The following figure shows the GS tester used on the left. The illustration in the middle shows an overview of a tested sample, the illustration on the right shows the surface magnified 25 times. The very good adhesion of the coating to the substrate can be seen clearly.

Contact person

Tatjana Wiens
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