Technique de traitement de surface

Surface and coating technology

The industry is continuously forced by economic and environmental reasons to improve the service life time of machines and components using optimized surface properties. Extremely wear and corrosion resistant coatings can be applied by thermal spraying processes without changing the properties of the base material.

Thermal Spraying

Thermal spraying processes (classified in the standards EN 657 and ISO 14917) offer numerous applications within modern surface technologies. Components made of different basic materials can be coated with protective layers, for example against wear and corrosion, made of high melting-point metals or ceramics. At the same time, thermally conductive or in contrast heat-insulating layers can be applied to components subjected to heavy thermal loads. Virtually all coating materials which are in powder or wire form can be processed this way.

The coating materials are added and melted onto an energy-rich heat source (fuel gas oxygen-flame, electric arc or plasmas made of noble gases such as argon, hydrogen, nitrogen, or helium). The particles which are softened or melted are accelerated in the direction of the work piece and collide there at a high speed (40–600 m/s). Following heat transmission to the base material, they harden and form a layered coating. The desired thickness of the coating is achieved by passing over the layer repeatedly with the burner.

The base materials

Virtually all base materials can be coated, no matter if they are metals, ceramics, even polymers, fiber compounds or natural materials such as stone, wood etc. As a result, the thermal spraying processes offer a high level of flexibility in terms of base and coating material combinations.

The coating thickness

Optimum coating thicknesses, which can heavily vary from application to application, are a prerequisite for good results in operation. Depending on the materials and the processes used, layer thicknesses ranging from several 10 µm up to some millimeters can be achieved. Structural layers can be sprayed on worn parts for which the total layer thickness cannot be freely determined.

The component temperature

Work pieces are generally heated up to temperatures below 150°C during the coating process and the surface temperatures are monitored. Structural changes in the base material are normally excluded with the exception of self-fluxing alloys, which are later melted or re-melted at temperatures above 1000°C.

Post-processing

As much importance must be paid to the rational post-processing of spayed layers and the required surface quality as for optimum material selection and material quality. Nova Werke AG has the modern equipment for turning, grinding, lapping, honing and polishing.

Metallographic laboratory for micro-structure analysis

The coating quality can be directly assessed on the cross sections in our own in-house metallographic lab with modern equipment. The coating porosities can, for example, have a strong effect depending on the material selection and spraying method and have to be adjusted to the application. With the spray equipments used by Nova Werke AG, coating porosities of less than 1 vol-% up to approx. 40 vol-% can be achieved. Fused layers consisting of self fluxing alloys, on the other hand, are absolutely dense. It is also possible to seal sprayed layers by filling the residual pores and capillaries with a corrosion-proof medium.

Quality assurance

Thermal spraying is not only a question of trust but is based instead on a consistently implemented awareness of quality on four levels, the so-called 4 M rule: Materials, machines for production, men (i.e. trained people) and measurements (i.e. quality testing). Nova Werke AG has at its own disposal state-of-the-art testing devices for three-dimensional tolerance monitoring and a metallographic laboratory, where beside cross sections also roughness measurements with roughness profiles can be recorded in addition to hardness measurements and adhesion tests. The QA measurements are coordinated with the customer for each order based on relevant standards.

Thermal spraying of NOVA SWISS®

It strongly depends on the specific application which of the thermal spray processes is used. Considerations of cost effectiveness play always an important role.