Protective layers by nickel plating

Abstract

The DURNI-COAT® process (DNC) has been developed for use in the space domain to protect the surface of metal components against wear, providing high hardness and high sliding ability. In particular, it has been used for the development of the Munich Space Chair (MSC), which was regularly used during the European MIR-mission EUROMIR 95 as well as for the coating of aerodynamic models tested in hypersonic wind tunnels against particles of high kinetic energy. Through the DURNI-COAT® process, a protective layer is created on the outer skin by chemically nickel plating and yields a thickness of only 30 μm. The deposition of the coating material is achieved without the need for any external source of electricity. Such functional coating can also provide high protection for technical applications outside the space domain such as in the energy sector, the automotive sector or mining industries. Applications are also possible for pharmaceutical and medical tool building as well as in the food and home appliance industry.

Description

DURNI-COAT® is a process for the functional coating of metals, where the properties of the protective layer range from chemical resistance, dimensional accuracy, optimum anti-friction properties, electrical conductivity to improved hardness. The coating process allows for the chemical deposition of a protective layer without resorting to any external power source. In this process the work piece is immersed into an aqueous solution with a defined content of nickel ions. During the process, these ions reduce to nickel metal. The chemical reactants and suppliers of the necessary electrons are the so-called Hypophosphite ions that are contained in the solution. They are transformed into so-called Orthophosphate by oxidation in the course of the reaction. As a result, a nickel-phosphorus alloy layer forms on the work piece’s surface. This layer effectively protects the work piece against wear and corrosion.

By varying of the electrolyte and process parameters, the protective layer’s properties may be customised to specific applications. The wear and corrosion resistance depends on the layer's phosphorous content. This content in turns varies depending on the electrolyte's composition as well as process conditions. Even the layer thickness affects the coating's resistance: Layers with a thickness in the range of 2 to 10 μm are resistant to mild corrosion loads, while those in the range of 5 to 10 μm are resistant to mild wear loads. A moderate degree of resistance requires layers with a thickness in the range of 10 to 25 μm, while severe and very severe loads require thickness values ranging from 25 to 50 μm and over 50 μm respectively (standard layer thicknesses according to DIN EN ISO 4527).

In addition, DURNI-COAT® ensures uniform layer formation true to the original contours with a narrow layer thickness tolerance of ± 3 μm. Therefore, such coating is an ideal solution for the plating of geometrically complex work pieces with sharp edges and ledges, accessible cavities or bores. Prior to the surface treatment, all parts that should not be covered by the protective layer have to be masked by means of masking lacquer or tape. Masked areas will not be coated during the dipping of the parts into the nickel-electrolyte. After the treatment the masking can easily be removed.

The chemically nickel-plated surface resisted the surface loading experienced in hypersonic wind tunnel tests very well. Fig.1 shows a typical wind tunnel model used in such test campaigns. Most industrially utilized metals may be coated by this procedure and applications in the non-space domain are manifold.

Innovations and advantages of the offer

The layers are particularly suitable for applications with strong requirements for:

  • accurately maintaining the original contours and dimensional accuracy
  • high sliding ability
  • protection against corrosion, erosion und cavitation
  • connectivity and solderability
  • surface hardness and wear resistance
  • magnetic properties
  • conductivity of the surface

Application

The surface processing described is a functional coating with a high-grade protection value for technical applications in the following industrial areas:

  • Aerospace
  • General Mechanical and Plant Engineering
  • Energy and reactor technology
  • Hydraulics
  • Automotive
  • Mining
  • Pharmaceutical and medical tool building
  • Food and home appliance industry
  • Instrumentation and control technology

Description of Space Heritage

Munich Space Chair (MSC): Working in space under zero gravity cannot be compared to working conditions on Earth. Up to now the problem of the missing force of gravity in manned space vehicles was solved mainly by foot loops attached to the floors of space laboratories. However, this fixation was not adequate for experiments that need to be performed precisely and for tasks that require secure footing. To address this the Munich Space Chair (MSC) was developed which was regularly used during the European MIR-mission EUROMIR 95. With this new fixation it became possible to work in a concentrated and exact manner with both hands under zero gravity without “floating away”. The astronaut is fixed between a seat plate, a thigh plate and a footrest in a so-called “Zero-g posture” natural for zero gravity. He props himself with the balls of his feet up off the foot rest to be pressed with the bottom to the seat plate due to the leverage at the thigh plate. For the MSC the steel guide beads that secure the adjustable aluminium profile of the backrest and the slidable connection to the thigh tube were chemically nickel plated by the technology provider with the process described here. This nickel-plating process perfectly met the requirements of high wear protection, high hardness and high sliding ability.

Design models for hypersonic wind tunnel tests: The example of an application of the coating process in Fig. 1 displays the wind tunnel model of a demonstrator (X-38) for a Crew Rescue Vehicle, which was meant to enable a fast evacuation of the crew of the International Space Station ISS in case of emergency. Aerodynamic experiments were conducted successfully with this 30 cm long design model, made of AlZnMgCu0,5 F35 (CERTAL) alloy.

Category
Materials
Reference No.
TDO0215
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