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Department of coating technology - Thermal Spraying, PVD, CVD

Thermal spraying

Thermal spraying involves processes in which spray additives inside or outside the spraying equipment are melted, or warmed up for a suitable plasticization, and spin-coated onto prepared surfaces. The treated surfaces are not melted in the process (according to DIN-EN 657).

[Translate to Englisch:] Einteilung der thermischen Spritzverfahren nach DIN EN 657
[Translate to Englisch:] Einteilung der thermischen Spritzverfahren nach DIN EN 657
Thermal spraying principles
Thermal spraying principles

The thermal spray processes can be classified according to various criteria, such as the type of spray material (wire, rod, cord, powder, or weld pool), the production type (manual, mechanized, or automatic), or the type of energy source (flame , electric arc, plasma). Almost all materials can be coated with thermal spray processes: metal, ceramic, plastic, textiles, as well as wood or glass. Mainly used substrate is metal. Some examples of thermal spray coating applications are turbine blades, sealing rings, liners, screw conveyors, engine valves, or pressure rollers.

Thermal spraying processes with corresponding layer morphology (copper substrate)
Thermal spraying processes with corresponding layer morphology (copper substrate)

Specific layer properties can be further achieved by additional thermal or mechanical treatment or by sealing.

The main research subjects of the thermal spraying group at the institute of materials science are:

  • (Applied or hybrid) plasma processes
  • High-speed processes
  • Development of functional layers
  • Thermal spraying simulation

Research projects, such as numerical simulations of plasma-particle interactions, have brought up a broader understanding of the different possibilities to improve particle properties. Furthermore, current projects also investigate, for example, the influence of substrate roughness on the layer adhesion. Other are concerned with the development of coatings for light metal alloys such as magnesium and aluminum substrates.

Current research projects

  • SFB871: Near-net shape turbine blade repair using joining and coating hybrid processes (Subproject B1)

The aim of the research project is the combination of repair soldering with hot gas corrosion protection coating in an integrated process to shorten the state of the art process chain for turbine blade repair. Both the solder application as well as the hot gas corrosion protection coating are done by means of thermal spraying. For this, the material deposition should thereby be implemented as close to the final contour as possible. The thermal spraying processes are to be conducted in such a way that the soldering process can be integrated in the CVD (Chemical Vapor Deposition) diffusion annealing process as a TLP (transient liquid phase) bonding process, and thus, be omitted as an independent process.

Therefore, the main hypothesis of the research project is to be able to combine a thermal coating and a joining process into a common integrated hybrid process, gaining both qualitative and economic advantages. The importance of this combination of methods lies in the reduction of grinding efforts and in the avoidance of previously needed vacuum soldering processes, thus reducing production costs.

Process chain (current and planned) for turbine blade regeneration
Process chain (current and planned) for turbine blade regeneration
  • Transplantation of structured thermally sprayed coatings by die casting (DFG)

A particularly promising method of coating in this area is the layer transplantation. While in conventional production, large finished components are subsequently coated, here, thermal spraying is integrated into the die casting process by layer transplantation. Therefore, this eliminates the need for pre and post-processing steps. The main idea is that the component itself is not coated but an insert in a die-casting mold is. In the next step of the die casting process, the layer on the insert is infiltrated by the melt, creating a positive-locking connection. Then, the finished component is removed together with the now fixed coating layer from the insert, completing the transplantation process.

Process chain of the transplantation of thermal sprayed coatings
Process chain of the transplantation of thermal sprayed coatings

Equipment:

  • Visuarc 350 arc spraying system, Sulzer Metco OSU
  • K2 high-speed flame spraying system, GTVmbH
  • G-Gun high-speed flame spraying system, GTVmbH
  • DELTA atmospheric three-anode plasma spraying system, GTVmbH
  • F4 atmospheric single-cathode plasma spray, Sulzer Metco AG
  • TRIPLEX II atmospheric three-cathode plasma spraying system, Sulzer Metco AG
  • Kinetics 4000 cold gas spraying system, CGT

PVD and CVD (Thin Film Technology)

In the area of "PVD - thin-film technology" current topics and projects in the field of thin-film technology are investigated and new applications are developed. Currently, at the IW, PVD coatings can be applied using two PVD systems. Another PVD system is under construction with which future reactive oxide coatings (e.g. for medical applications) will be applied. In addition, the IW is a founding member of the CZM (Clausthaler Zentrum für Materialtechnik), where the department has another state-of-the-art PVD facility at its disposal.

At the IW, PVD coatings are being developed for a wide range of applications starting with wear protection of forging dies to innovative implant surface treatments. For example, PVD multilayer systems are being developed as exothermic solder layers. In addition, the department is working on the development of a spray system for resorbable implant coatings on permanent implants.

Basics of PVD technology

DIN 8580: 2003-09 summarizes different coating methods in main group 5 - this is further differentiated into 8 groups and their corresponding subgroups. Group 5.8 shows the PVD coating processes.

Main group 5 listed in DIN 8580:2003-09

The PVD coating processes used at the IW differ in the type of atomization technique. In addition to the arc evaporator, where the material to be evaporated is vaporized with a high-energy arc, plasma-based evaporators are also used. These differ in the type of plasma generation:

DC (direct current) plasma: a DC plasma is a direct voltage plasma, in which an electrical voltage is applied between two electrodes, igniting the plasma. The cathode is continuously bombarded with ions by the plasma, and therefore, evaporated. The material to be vaporized must be electrically conductive.

RF (radio frequency) plasma: a radio frequency plasma (with 13.56 MHz). Because of the high frequency, the energy required for the plasma is transmitted inductively. As a result, materials with poor electrical conductivity properties can also be evaporated.

MF (middle frequency) plasma: a so-called medium-frequency plasma (usually in the kHz range). oxidic coatings can be applied to different substrates with a suitable electrical control.

HiPIMS or HPPMS (high power pulse magnetron sputter) plasma: is a very short pulse-based plasma (pulse duration ~ 50 μs). The average electrical power is comparable to that of a DC plasma, but this is delivered in very short and powerful pulses with a power up to a MW range.